WO2004019875A2

WO2004019875A2 - Non-invasive methods to identify agents for treating pain

Info

Publication number: WO2004019875A2
Application number: PCT/US2003/026890
Authority: WO
Inventors: Kenneth M. Hargreaves; Karl Keiser
Original assignee: Board Of Regents, The University Of Texas System
Priority date: 2002-08-30
Filing date: 2003-08-27
Publication date: 2004-03-11
Also published as: AU2003273247A8; AU2003273247A1; WO2004019875A3

Abstract

Disclosed are improved methods, compositions and kits for use in testing pain responses and identifying analgesic and anesthetic substances. The non-invasive screening assays described use compositions and kits comprising peripheral neurons, particularly those including neurons previously sensitized to pain in an in vivo setting, as in vitro models for identify agents for inhibiting pain signaling and for improved efficacy in the clinical treatment of pain.

Description

NON-INVASIVE METHODS TO IDENTIFY AGENTS FOR TREATING PAIN

BACKGROUND OF THE INVENTION

The present application claims priority to U.S. provisional application Serial No. 60/407,025, filed August 30, 2002, the entire text and figures of which are incorporated herein by reference without disclaimer. The U.S. Government owns rights in the present invention pursuant to grant number DEI 2888 from the National Institutes of Health.

1. Field of the Invention

The present invention relates generally to the fields of neurobiology and analgesic agents. The invention provides non-invasive methods, compositions and kits for use in testing pain responses and in identifying analgesic and anesthetic substances. In particular, the invention concerns the use of in vitro compositions and kits comprising peripheral neurons, including neurons previously sensitized to pain in vivo, in improved screening assays to identify agents for treating pain.

2. Description of Related Art

Screening assays to identify agents that cause pain are necessary to assess the safety or potential risk to humans of raw materials and finished products, such as components of food stuffs and pharmaceuticals, cosmetics and personal care items, as well as household and industrial products. A number of model systems using experimental animals are available for conducting such studies. In recent years, there has been an increasing emphasis on the desire to develop in vitro assays, which could reduce the use of animal testing.

In addition to the analysis of agents that could potentially cause pain during intended or accidental human use or exposure, assays concerning the pain response are integral in the development of analgesics and anesthetics. The objective of such studies is the identification of agents that can effectively inhibit the induction of pain and/or reduce the severity or duration of pain after the onset.

Again, a number of model systems for analyzing potential analgesics and anesthetics rely on experimental animals. This dependence on animal testing is a limitation in the field, not only for humane reasons and the high cost of such studies, but also due to concerns regarding the predictive value of such animal tests to the clinical situation.

Commonly used animal models of pain assessment include the rat paw-lick model (Cleozzi et al, 1980) and the tail flick model (Lichtenberger et al, 1998; Ong et al, 1980). The effectiveness of these models is limited due to the lack of reliability and sensitivity inherent in the qualitative and/or subjective nature of these assays. Moreover, the global limitation of animal testing, i.e., that the species chosen may not accurately model the human system, has been proven in this particular field.

The foregoing types of obstacles prompted the search for in vitro test systems and drug screening assays. Certain of these systems measure cytotoxicity, cell injury or cell death. However, the endpoints from such in vitro studies do not correlate well with in vivo results. In fact, such assays do not form a sound basis for testing sensitivity and pain, which are not co-extensive with cytotoxicity.

In response to such concerns, U.S. Patent 5,811,256 suggested the use of cultured neonatal neurons from rats to measure sensory irritation, preferably in co-culture with cells from other tissues in a complex co-culture system with separate chambers. Other assays using a cell culture system of spinal cord explants containing dorsal root ganglia (DRG) from fetal mice have been proposed as a more specific screen, designed to identify opioids for use as low-addictive drugs (U.S. Patent 5,624,932).

The scientific literature also includes reports concerning the isolation and culture of neuronal cells from dorsal root ganglia (dorsal root ganglion or DRG cells), typically from rodents. Cultured DRG cells have been subjected to pain-producing compounds and their electrochemical responses analyzed by patch-clamp techniques, which can detect parameters such as membrane conductance (Liu and Simon, 1996a; 1996b; Wood et al, 1988).

Although human DRG cells have been isolated and analyzed in culture (Baumann et al, 1996), the nature of this material - - the dorsal-root ganglion is the site of the cell bodies that form the peripheral neurons - - precludes the development of routine screening assays using human tissues. This is a significant limitation in any DRG screening methods because, as observed in whole animal studies, effects produced in neuronal tissue from other species are not necessarily reproduced in human tissues. Notably, there are documented examples of drugs, such as NKl antagonists, which function in rat cells but do not block the human form of the target receptor.

In an attempt to combat the species limitations inherent in DRG cell assays, cultured cells may be transfected with a human target gene, such as a gene that encodes a receptor involved in pain signaling. Neurons capable of transmitting pain are termed "nociceptive", and receptors involved in pain signaling can therefore be referred to as "nociceptors". Expression of a human nociceptor in a recombinant host cell can be used to provide a basic level screen for agents that bind to the receptor. However, such assays typically lack the fidelity required to select agents with appropriate physiological or pharmacological effects.

Therefore, despite attempts to develop in vivo and/or in vitro methods to analyze pain, there remains in the art a need for screening assays suitable for identifying analgesic and anesthetic substances. The development of in vitro model systems is particularly desirable, to overcome the many drawbacks associated with animal testing. Unfortunately, the limitations of methods involving recombinant expression, and the inability to apply the basic research from rodent DRG cells to screening assays using human cells, has left the art with a long-felt need that has yet to be satisfied.

SUMMARY OF THE INVENTION

The present invention solves the foregoing long-felt needs in the art by providing improved non-invasive methods, compositions and kits for use in testing pain responses and for identifying analgesic and anesthetic substances. The invention particularly provides in vitro compositions comprising peripheral neurons, preferably human neurons, including neurons previously sensitized to pain in vivo, for use in improved screening assays. These assays are ideally suited for identifying agents that inhibit sensory irritation and pain in human peripheral neurons and for developing pharmacological substances for use as anti-irritants, analgesics and anesthetics.

All aspects of the invention have the advantage of providing in vitro screening methods, which avoids the political and economic cost and lack of predictability associated with animal testing. In preferred embodiments, the use of in vitro compositions comprising peripheral human neurons, particularly those sensitized to pain in vivo, provides important new assays for use in testing agents for human consumption and for identifying analgesic and anesthetic agents for clinical use. Various embodiments of the invention further include particular advantages, as described herein.

As used throughout the entire application, the terms "a" and "an" are used in the sense that they mean "at least one", "at least a first", "one or more" or "a plurality" of the referenced components, except in instances wherein an upper limit is thereafter specifically stated or would be clearly understood by those of ordinary skill in the art. The operable limits and parameters of combinations, as with the amounts of any single agent, will be known to those of ordinary skill in the art in light of the present disclosure. The "a" and "an" terms are also used to mean "at least one", "at least a first", "one or more" or "a plurality" of steps in the recited methods, except where specifically stated.

Throughout the present specification and claims, the term "or" is used in the sense that it means "and/or" in reference to the disclosed and claimed components and steps, except in instances wherein a different meaning is thereafter specifically stated or would be clearly understood by one of ordinary skill in the art. Thus, unless otherwise expressly stated or clearly known by those of ordinary skill in the art, the term "or" is simply used as a succinct reference term to cover each recited component or step and all combinations thereof.

Unifying aspects of the invention concern methods, compositions and kits for testing sensory irritation and pain responses to agents, testing the ability of an agent to induce or reduce irritation or pain, and for analyzing the sensory effects or "pain profile" of a test substance. These compositions, kits and methods include those for use in analyzing whether a test substance will cause irritation or pain when administered to a human, and generally comprise:

(a) contacting a first isolated tissue sample that contains peripheral neurons, preferably human peripheral neurons, with the test substance in vitro; and

(b) assessing the effect of the test substance on pain signaling by the peripheral neurons or human peripheral neurons in the tissue sample, thereby determining the pain profile of the test substance. In the practice of such methods, a test substance that substantially alters the pain signaling by the peripheral neurons in the tissue sample is a "pain modulating substance". On the other hand, a test substance that does not substantially alter the pain signaling by the peripheral neurons in the tissue sample is a "pain neutral substance".

Within the pain modulating substances, a test agent or substance that increases or significantly increases the pain signaling by the peripheral neurons in the tissue sample is a "pain inducing substance". Testing for possible pain inducing agents may be used in analyzing agents that are not intended for human contact or consumption, such as in testing samples for safety, to detect contaminants and such like. These types of tests are applicable to testing samples suspected of containing unwanted substances, such as environmental toxins and/or pathogenic agents.

Testing for possible pain inducing agents may be used in the context of substances or agents that are intended or designed for potential human consumption or administration. In this context, a pain inducing substance is indicative of a substance that is not widely suitable for human administration in typical, although not in all, circumstances. Exceptions may include, e.g., pain inducing substances with therapeutic effects, wherein the downside of any pain induced is outweighed by the benefits of the therapeutic response obtained.

A test substance that "substantially maintains or lowers" the pain signaling by the peripheral neurons in the tissue sample will typically be a substance suitable for human administration. The phrase "suitable for human administration" in this context refers to the "pain inducing capacity" of the substance, not to all parameters, such as short and long term toxicity and adverse side effects, necessary for assessment before human administration.

These methods, compositions and kits of the invention are suitable for testing candidate substances prior to human consumption and/or administration, such as candidate pharmaceutical carriers, candidate pharmaceutical diluents and candidate components of injectable formulations. Candidate pharmaceutical carriers, diluents or injectable formulations will not generally be sought to reduce pain signaling themselves, but rather tested for the ability not to increase, or not to substantially increase, pain signaling by the peripheral neurons in the tissue sample. Candidates that do not "increase or substantially increase pain signaling" will be indicative of substances suitable for use as carriers, diluents or injectable formulations, subject to other requirements for human administration.

The existence of pharmaceutical carriers and injectable formulations that cause pain or irritation on administration is a significant limitation in the field of drugs and medicaments. The use of the present invention therefore allows pharmaceutical formulations to be readily tested, refined and improved. The invention thus further provides methods, compositions and kits for testing candidate substances in the form of modified, attenuated, improved and/or "second generation" version of known irritants, pain-inducing substances and candidate therapeutic agents with reduced side effects.

Further test and candidate substances that may be assessed by the methods, compositions and kits of the invention are food substances and cosmetic substances. The candidate substances may thus be ingestible substances or agents intended for topical application.

Important aspects of testing for pain modulating substances are assays to identify a test agent or substance that decreases or significantly decreases the pain signaling by the peripheral neurons in the tissue sample. An agent that decreases or significantly decreases the pain signaling is a "pain reducing substance". An agent or substance that modestly inhibits pain signaling by the peripheral neurons in the tissue sample is indicative of a candidate anti-irritant or itch medication. An agent or substance that significantly or markedly inhibits pain signaling by the peripheral neurons in the tissue sample indicative of a candidate painkiller, analgesic and/or anesthetic substance.

As such, the invention provides methods, compositions and kits for identifying a candidate substance with anti-irritant, painkilling, analgesic and/or anesthetic activity, preferably with peripheral analgesic activity, comprising:

(a) contacting a first in vitro composition that comprises peripheral neurons, preferably human peripheral neurons, with a candidate substance; and

(b) determining the ability of the candidate substance to inhibit pain signaling in the peripheral neurons or human peripheral neurons, thereby identifying a candidate substance with anti-irritant, painkilling, analgesic and/or anesthetic activity, preferably with analgesic or anesthetic activity, and more preferably with peripheral analgesic activity.

Further methods, compositions and kits of the invention are those for identifying a candidate neurosensory therapeutic agent, comprising:

(a) contacting a first isolated tissue sample that contains peripheral neurons, preferably human peripheral neurons, with a candidate agent in vitro; and

(b) determining the ability of the candidate agent to inhibit pain signaling by the peripheral neurons or human peripheral neurons in the tissue sample, thereby identifying a candidate neurosensory therapeutic agent.

Many sources of in vitro compositions may be used to provide the peripheral neurons, such as compositions comprising porcine, bovine, canine or feline peripheral neurons, or rodent peripheral neurons, such as mouse, rat, rabbit, hamster or guinea pig peripheral neurons. However, it is an advantage of the invention, particularly in regard to testing agents for human administration and clinical development, that in vitro compositions comprising human peripheral neurons can now be used.

The in vitro composition that comprises the peripheral neurons, preferably human peripheral neurons, may comprise partially or substantially purified peripheral neurons or human peripheral neurons. However, in many aspects of the invention, it is also an advantage that partial or substantial purification is not necessary, which reduces the complexity and cost of practicing the invention.

Therefore, the invention encompasses the use of in vitro compositions that comprise an isolated tissue sample that comprises the peripheral neurons or human peripheral neurons. Many tissue samples that comprise peripheral neurons can be employed. A "suitable or selected" tissue sample is a tissue sample, preferably a human tissue sample, that comprises peripheral neurons and that can be maintained in culture under conditions suitable and for a period of time effective to both substantially maintain peripheral neuron function in vitro and to permit the execution of an in vitro screening test. One advantage of using a suitable or selected tissue sample, preferably a human tissue sample, that comprises the peripheral neurons is that candidate substances may be tested and identified that indirectly modulate pain signaling by the peripheral neurons. Candidate substances that "indirectly modulate pain signaling by the peripheral neurons" are typically those that cause irritation or pain by stimulating or activating operatively connected and/or nearby cells in the tissue sample, which release factors that in turn activate the peripheral neurons. Such operatively connected and/or nearby cells include fibroblasts, mast cells and various cells of the immune system.

The partially or substantially purified peripheral neurons or human peripheral neurons, or preferably an isolated tissue sample that comprises such peripheral neurons, are preferably maintained in vitro in a pH buffered environment in the presence of biologically effective amounts of salts and neurotrophic factors.

The ease of obtaining the tissue sample is one important factor in the selection of a suitable tissue sample, so long as the tissue is amenable to culture that maintains peripheral neuron function in vitro as required for the screening method. Examples of suitable or selected tissues therefore include cadaver tissue samples and umbilical vein or umbilical vein tissue samples. Biopsy tissue samples are preferred, such as a biopsy sample from skin, respiratory, muscle, cardiac, hepatic, renal, gastrointestinal, nasal, corneal, oral, immune, bone, ligament or tendon tissue.

The effectiveness and simplicity of the invention applies to the selection and use of in vitro compositions or tissue samples that comprise normal or substantially normal peripheral neurons isolated from, or comprised within, a range of normal or substantially normal tissue samples, preferably human tissue samples.

However, the invention provides further advantages in terms of methods, compositions and kits for use in identifying candidate substances with analgesic or anesthetic activity, preferably with peripheral analgesic activity. In these aspects, the art is currently limited by the lack of in vitro models and screening assays that effectively represent the pain signaling process. These drawbacks are overcome by the aspects of the invention that utilize pain- sensitized peripheral neurons or peripheral human neurons. Accordingly, in certain preferred embodiments, the invention provides methods, compositions and kits for identifying candidate substances with analgesic or anesthetic activity, preferably with peripheral analgesic activity, and most preferably with peripheral analgesic activity in pain-sensitized tissues, which generally comprise:

(a) applying a candidate substance to a first in vitro composition that comprises pain-sensitized peripheral neurons, preferably human peripheral neurons, isolated from or comprised within a tissue sample or human tissue sample that was sensitized to pain in vivo; and

(b) determining the ability of the candidate substance to inhibit pain signaling in the pain-sensitized peripheral neurons or human peripheral neurons, thereby identifying a candidate substance with analgesic activity in pain-sensitized tissues.

"Pain-sensitized" peripheral neurons or peripheral human neurons, as used herein, typically means peripheral neurons isolated from or comprised within a tissue sample or human tissue sample that was sensitized to pain in vivo, such as peripheral neurons isolated from or comprised within a diseased, infected, inflamed, neuropathic or necrotic tissue sample. By conducting in vitro screening assays using such pain-sensitized peripheral neurons, the invention is better suited to identify drugs that have meaningful analgesic and peripheral analgesic activity.

The selection and use of a sample of a tissue that was exposed to chronic pain in vivo provides in vitro tests for identifying agents pre-selected for the ability to treat chronic pain, an area of significant clinical need, and/or relieve pain for a long duration. Using samples of tissues that caused acute pain in vivo similarly provides in vitro tests for identifying agents pre-selected for the ability to treat acute pain and/or quickly relieve pain.

When substantially normal tissue samples are used, tissue samples that are readily accessible will be preferred. When using pain-sensitized peripheral neurons, the methods can be carried out by obtaining a sample of any tissue that was sensitized to pain in vivo. However, it will again be convenient to use samples of tissues that are readily obtainable and/or routinely removed from the body when causing pain, such as samples from skin (dermatology punch), nasal, oral and immune tissues (e.g. tonsils) or an appendix. Another advantage of using diseased, infected or inflamed tissue samples is that the tissue sample can be obtained from the same biopsy process that would be conducted diagnostically in relation to the disease, disorder or infection.

The invention further provides assay methods, compositions and kits wherein the candidate substance is tested in a parallel assay using pain-sensitized peripheral neurons and normal or substantially normal peripheral neurons. As such, separate first and second in vitro compositions are used, one that comprises pain-sensitized peripheral neurons, preferably human peripheral neurons, isolated from or comprised within a tissue sample or human tissue sample that was sensitized to pain in vivo, and another that comprises normal or substantially normal peripheral neurons isolated from or comprised within a substantially normal tissue sample. Such embodiments are suitable for identifying a candidate substance with peripheral analgesic activity, and comprise:

(a) applying a first sample of a candidate substance to a first in vitro composition that comprises pain-sensitized peripheral neurons isolated from or comprised within a first sample of a selected tissue, or a human tissue, that was exposed to pain in vivo before isolation from an animal or preferably a human subject;

(b) applying a second sample of the candidate substance to a second in vitro composition that comprises normal or substantially normal peripheral neurons isolated from or comprised within a second, normal sample of the selected tissue, or human tissue, that was not exposed to pain in vivo before isolation from an animal or preferably a human subject; and

(c) determining the ability of the candidate substance to inhibit pain signaling in the pain-sensitized peripheral neurons and the normal or substantially normal peripheral neurons of the first and second in vitro compositions, thereby identifying a candidate substance with analgesic or anesthetic activity, preferably with peripheral analgesic activity, and more particularly, a substance with analgesic or anesthetic activity in normal and pain-sensitized tissues. Preferred tissues and tissue samples for use in the invention are teeth, a tooth or samples from a tooth. Preferred methods, compositions and kits are thus those for analyzing whether a test substance will cause pain when administered to a human, which comprise:

(a) exposing a first human tooth or tooth tissue sample that comprises peripheral neurons to a pain activating process or pain activating agent in vitro and to the test substance in vitro; and

(b) assessing the effect of the test substance on pain signaling by the peripheral neurons, wherein a test substance that increases the pain signaling is indicative of a test substance that will cause pain when administered to a human.

Further preferred methods, compositions and kits are thus those for identifying a candidate analgesic substance, which comprise:

(a) exposing a first tooth or tooth tissue sample, preferably a human tooth or a sample from a human tooth, that comprises peripheral neurons to a pain activating process or pain activating agent in vitro and to a candidate substance in vitro; and

(b) determining the ability of the candidate substance to inhibit pain signaling in the peripheral neurons or human peripheral neurons from the tooth or tooth tissue sample, thereby identifying a candidate analgesic substance.

Suitable teeth include normal teeth, although a tooth or a sample from a tooth sensitized to pain in vivo and comprising pain-sensitized peripheral neurons will often be preferred, such as a diseased, infected, inflamed, neuropathic or necrotic tooth or tooth tissue sample. A human tooth or a sample from a human tooth is another preferred embodiment, as exemplified by a tooth extracted from a patient, or a biopsy tissue sample thereof, such as an apical portion of the root of a human tooth.

Certain preferred methods, compositions and kits of the invention are therefore those for identifying a candidate substance with peripheral analgesic activity in pain-sensitized tissues, comprising: (a) applying a pain activating process or pain activating agent and a candidate substance to the sensory nerve endings of a human tooth or tooth tissue sample in vitro, wherein the tooth or tooth tissue sample is obtained from a patient sensitized to pain in vivo, has inflamed or necrotic pulp or root tissue and comprises pain-sensitized human peripheral neurons; and

(b) determining the ability of the candidate substance to inhibit pain signaling in the pain-sensitized human peripheral neurons, thereby identifying a candidate substance with peripheral analgesic activity in pain-sensitized tissues.

Irrespective of the source of the peripheral neurons, human peripheral neurons, pain- sensitized peripheral neurons, or tissue samples thereof, the "applying" and "determining" steps of the invention are straightforward to conduct. Preferably, the peripheral neurons, pain- sensitized peripheral neurons or tissue containing the peripheral or sensitized neurons is collected and the neurons are exposed to the candidate substance or agent ("test drug") and, in addition, are exposed to a pain activating process or pain activating agent.

"Exposure" to a pain activating process or pain activating agent may occur substantially simultaneously with contact of the candidate substance. Alternatively, the in vitro composition may be exposed to the pain activating process or agent "prior to" contact with the candidate substance.

Wherein the peripheral neurons or tissue sample is exposed to pain by application of a "pain activating process", such a process may be any process that activates or significantly activates pain fibers in the peripheral neurons. Suitable pain activating processes or treatments include the application of protons, heat, cold, pressure and/or an electrical stimulus. Wherein the peripheral neurons or tissue sample is exposed to pain by provision of a "pain activating agent", the agent may again be any agent or chemical that activates or significantly activates pain fibers in the peripheral neurons.

Exemplary "pain activating agents" include fatty acids, carbon dioxide, trophic factors and inflammatory mediators, such as nerve growth factor or an inflammatory mediator selected from the group consisting of arachidonic acid, an inflammatory prostaglandin, histamine, serotonin and bradykinin. In certain aspects of the invention, the pain activating process or agent may be selected for combined use with a particular type of peripheral neurons or tissue sample, such as for use with normal vs. pain-sensitized peripheral neurons, or for use with partially purified peripheral neurons vs. tissue samples containing such neurons. Such preferred combinations will be known to those of ordinary skill in the art in light of the present disclosure. However, an advantage of the invention is that many agents can be used. Capsaicin is a convenient, cost-effective and currently preferred pain activating agent.

The peripheral neurons or tissue sample may be exposed to the candidate substance, preferably with the pain activating process or agent, for a time period of less than about two minutes. In such embodiments, the method pre-selects for candidate substances that inhibit pain signaling by modulating an ion channel in the peripheral neurons. Wherein the peripheral neurons or tissue sample are exposed to the candidate substance, preferably with the pain activating process or agent, for a time period of between about two and about 20 minutes, the method pre-selects for candidate substances that inhibit pain signaling by affecting a metabotropic receptor in the peripheral neurons. Where the peripheral neurons or tissue sample are exposed to the candidate substance, preferably with the pain activating process or agent, for a time period of between about 1 and 2 hours, the method pre-selects for candidate substances that inhibit pain signaling by altering protein synthesis in the peripheral neurons.

In the determining or measuring embodiments, any suitable method for detecting and quantifying a pain signal from the peripheral neurons may be used. For example, pain may be measured by detecting an electrochemical response from the peripheral neurons. Suitable electrochemical responses are those that can be measured by standard electrophysiological techniques, such as by detecting a change in membrane potential, action potential, ion movement, ion uptake, inward current or whole-cell conductance in the peripheral neurons. Voltage sensitive markers such as fluorescent dyes may be used to detect changes in membrane potential.

An electrochemical response may be measured by detecting a change in movement of an ion marker of membrane current flow, such as by detecting a change in movement, or an influx, of cations such as K⁺, Na⁺ or Ca²⁺ ions. Cation influx, by way of example, may be measured by using cobalt ions in the culture medium in combination with a silver precipitation enhancement technique. Changes in ion movement can also be measured by fluorescent staining, e.g., using fluorescent calcium-binding dyes and techniques such as fluorescence microscopy. Ion movement may also be measured using an electrode specific for that ion, such as an electrode to detect K⁺, Na⁺ or Ca²⁺ ions.

In other preferred embodiments, the pain signaling in the peripheral neurons is measured by detecting and quantifying the release of a pain-associated agent or neurotransmitter from the peripheral neurons. Suitable pain-associated neurotransmitters that can be measured are glutamate, aspartate, calcitonin gene-related peptide and substance P. The release of neurotransmitter substances indicative of pain, such as calcitonin gene-related peptide and substance P, may be conveniently measured by an immunological-based assay.

The technical ability to perform immunological-based assays has long existed in the art and various useful immunodetection methods have been described in the scientific literature (e.g., Nakamura et al, 1987, specifically incorporated herein by reference). In general, immunological-based assays for use with the present invention will comprise contacting the sample suspected of containing the released pain-associated agent or neurotransmitter with at least a first antibody that has immunospecificity for the pain-associated agent or neurotransmitter under conditions effective and for a period of time sufficient to allow the formation of immune complexes, and detecting, and preferably quantifying, the immune complexes so formed.

Contacting the sample with the antibody "under conditions effective and for a period of time sufficient to allow the formation of immune complexes" (primary immune complexes) is generally a matter of simply adding the antibody to the sample and incubating the mixture for a period of time long enough for the antibodies to form immune complexes. After this time, the sample-antibody composition will generally be washed to remove any non-specifically bound antibody species, allowing only those antibodies specifically bound within the primary immune complexes to be detected.

The detection of immunocomplex formation is well known in the art and may be achieved through the application of numerous approaches. These methods are generally based upon the detection of a label or marker, such as any radioactive, fluorescent, biological or enzymatic tags or labels known in the art, including the use of enzymes that generate a colored product upon contact with a chromogenic substrate. Secondary binding ligands may also be used in the immunological-based assays. Primary immune complexes can thus be detected by means of a second binding ligand that has binding affinity for the primary antibody, i.e., the antibody bound within the primary immune complexes. In such cases, the second binding ligand is preferably linked to a detectable label. The second binding ligand is itself often an antibody, and may thus be termed a "secondary" antibody. The primary immune complexes are contacted with the labeled, secondary binding ligand, or antibody, under conditions effective and for a period of time sufficient to allow the formation of secondary immune complexes. The secondary immune complexes are then generally washed to remove any non-specifically bound labeled secondary antibodies or ligands, and the remaining label in the secondary immune complexes is then detected. Tertiary immune complexes can also be formed and detected, but this should not be required in the present invention as signal amplification is not a limitation of these methods.

Aside from the particular determining or measuring method chosen, the ability of the candidate substance to inhibit pain signaling in peripheral neurons is preferably determined in controlled studies. Certain methods, compositions and kits of the invention are those wherein the ability of the candidate substance to inhibit pain signaling in the peripheral neurons is determined by:

(a) identifying a measured level of pain signaling from the peripheral neurons in the first in vitro composition, such as a tooth or tooth tissue sample, contacted with the candidate substance; and

(b) comparing the measured level to a standardized level of pain signaling compiled by determining the level of pain signaling in a series of equivalent samples of the first in vitro composition in the absence of the candidate substance.

Other methods, compositions and kits of the invention are those wherein the ability of the candidate substance to inhibit pain signaling in the peripheral neurons is determined by: (a) individually measuring the level of pain signaling from the peripheral neurons in a control sample of the in vitro composition, such as a tooth or tooth tissue sample, in the absence of the candidate substance;

(b) measuring the level of pain signaling from the peripheral neurons in an equivalent test sample of the in vitro composition in the presence of the candidate substance; and

(c) comparing the levels of pain signaling in the control and test samples, thereby quantifying the ability of the candidate substance to inhibit pain signaling in the peripheral neurons.

In high throughput screening, the methods, compositions and kits will preferably comprise:

(a) establishing a control high level of pain signaling from the peripheral neurons in a control sample of the in vitro composition, such as a tooth or tooth tissue sample, in the absence of the candidate substance;

(b) testing a panel of candidate substances by measuring the level of pain signaling from the peripheral neurons in a series of equivalent test samples of the in vitro composition separately contacted with members of the panel of candidate substances; and

(c) selecting from the panel tested in step (b) a candidate substance that reduces the level of pain signaling to significantly below the control high level established in step (a).

In certain embodiments, the ability of the candidate substance to inhibit pain signaling in the peripheral neurons may be compared to the ability of a known analgesic or anesthetic substance to inhibit pain signaling in a second sample of the in vitro composition that comprises peripheral neurons. Such aspects of the invention comprise: (a) establishing a control high level of pain signaling from the peripheral neurons in a first control sample of the in vitro composition, such as a tooth or tooth tissue sample, in the absence of any analgesic or anesthetic substance;

(b) establishing a control low level of pain signaling from the peripheral neurons in a second control sample of the in vitro composition in the presence of a known analgesic or anesthetic substance;

(c) testing a panel of candidate substances by measuring the level of pain signaling from the peripheral neurons in a series of equivalent test samples of the in vitro composition separately contacted with members of the panel of candidate substances; and

(d) selecting from the panel tested in step (c) a candidate substance that reduces the level of pain signaling to substantially the control low level obtained in the presence of the known analgesic or anesthetic substance.

As preferred tissues and tissue samples for use in the invention include a tooth and tooth sample, certain preferred methods, compositions and kits of the invention are therefore those for analyzing whether a test substance will cause pain when administered to a human, which comprise:

Further preferred methods, compositions and kits are thus those for identifying a candidate substance with analgesic or peripheral analgesic activity, comprising (a) applying a first sample of a candidate substance to the sensory nerve endings in a first in vitro human tooth tissue sample that has inflamed pulp or root tissue;

(b) applying a second sample of the candidate substance to the sensory nerve endings in a second in vitro human tooth tissue sample that has substantially normal pulp and root tissue;

(c) stimulating the sensory nerve endings in the first and second human tooth tissue samples with a pain activating agent;

(d) measuring the release of a pain-associated neurotransmitter from the sensory nerve endings; and

(e) determining the ability of the candidate substance to inhibit the release of the pain-associated neurotransmitter from the sensory nerve endings in the first and the second human tooth tissue samples, thereby identifying a candidate substance with peripheral analgesic activity.

A wide range of candidate substances may be tested using the invention. The duration of exposure to the candidate substance may be varied to pre-select for candidate substances with particular profiles, such as inhibit pain signaling by modulating ion channels, receptors and/or gene expression. However, the invention is not limited to the pre-design of the assay to identify only particular types of agents.

Accordingly, the candidate substance may be designed or discovered to be a modulator of an ion channel or a metabotropic receptor or an antagonist of a known peripheral neuron receptor, such as an antagonist of a capsaicin-sensitive receptor or antagonist of Vanilloid receptor 1 (VR1). The candidate substance may also be designed or discovered to be an antagonist of an NMDA, AMPA, kainite, bradykinin, prostaglandin, serotonin, endothelin, histamine or trophic factor receptor. Particular examples are antagonists of bradykinin receptor Bl, bradykinin receptor B2, prostaglandin receptor EP1, prostaglandin receptor EP3, trophic factor receptor trkA and trophic factor receptor p75. The candidate substance may further be designed or discovered to be an agonist of a known neurotransmitter, such as an agonist of an adrenergic agent, a muscarinic agent or neuropeptide Y. Other candidate substances may be designed or discovered to be a cannabinoid or opiate, i.e., an opioid receptor agonist. This is validated by the invention, which successfully tests μ, δ and K opioid receptor agonists.

The invention may be applied to random screening, such as applied to candidate substances that are members of a library prepared by combinatorial chemistry or members of a library of naturally occurring biological compounds obtained from microbial, plant or marine sources. The candidate substances tested may otherwise be designed or pre-selected, such as designed by computer-based modeling and/or designed to modify a known agent to reduce the pain induction.

In addition to the valuable information from the present invention, before developing a selected candidate substance for veterinary or clinical use, the analgesic or anesthetic activity of the candidate substance identified will preferably be further quantified, such as using an in vivo pain perception test in a mammal. Accordingly, candidate substances with analgesic or anesthetic activity identified by the invention may be disposed in a pharmaceutically acceptable solution, formulation or vehicle.

Exemplary kits of the invention are those for use in testing a substance for human consumption, comprising:

(a) a tissue culture medium that maintains peripheral neuron function in vitro, such as maintaining peripheral neuron function in a tooth or tooth tissue sample in vitro;

(b) at least a first in vitro diagnostic component that detects a biological pain signal from a peripheral neuron; and (c) instructions for comparing the ability of a test substance to alter pain signaling in peripheral neurons in vitro to a control value in the absence of the test substance, thereby quantifying the pain-inducing or pain-relieving properties of the test substance and thus determining whether it is suitable for human consumption.

Further kits of the invention are those for use in identifying an analgesic or anesthetic agent, which comprise:

(b) at least a first diagnostic component effective to detect a biological pain signal from a peripheral neuron; and

(c) instructions for correlating the ability of a test substance to inhibit pain signaling in peripheral neurons in vitro with standardized values for known analgesic or anesthetic substances, thereby allowing the identification of new analgesic or anesthetic agents.

Either of such kits may comprise all the components of a diagnostic system for detecting a biological pain signal from a peripheral neuron. The kits may be fabricated to comprise at least a first tissue culture plate, dish, apparatus or system. The kits may also be supplied with at least a first pain activating agent and/or at least a first known analgesic or anesthetic substance, or an aliquoted series of one or more of such agents.

The invention further provides a substance, agent, anti-irritant, painkiller, analgesic or anesthetic compound identified by any of the methods disclosed herein, particularly an analgesic substance with peripheral activity for use in humans. Another aspect of the invention is therefore a pharmaceutical composition comprising a pharmacologically acceptable vehicle and a therapeutically effective amount of an analgesic or anesthetic substance identified by any of the methods of the present invention. Further aspects are methods and uses of a substance, agent, anti-irritant, painkiller, analgesic or anesthetic compound identified by the invention in treating, or in the preparation of a medicament for use in treating, an animal or human patient in need of treatment for irritation or pain. Such methods comprise providing to an animal or human patient in need thereof a biologically or therapeutically effective amount of a substance, agent, anti-irritant, painkiller, analgesic or anesthetic compound identified by any of the methods of the invention as disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein.

FIG. 1A, FIG. IB and FIG. 1C. Effect of the Yi selective agonist, [Leu₃ Pro₃₄]- NPY, on inhibiting VR1 neurotransmission from central terminals (FIG. 1A), somata (FIG. IB) and peripheral terminals (FIG. 1C) of peptidergic neurons. FIG. 1A, Yi inhibition of capsaicin-evoked immunoreactive calcitonin gene-related peptide (iCGRP) release from rat spinal dorsal horn slices is blocked by pre-treatment with a Yi antagonist (BIBP3226). FIG. IB, Y_\ inhibition of capsaicin-evoked iCGRP release from rat trigeminal ganglion slices. FIG. 1C, Yi inhibition of capsaicin-evoked iCGRP release from rat dental pulp.

FIG. 2. Effect of the Yi agonist, [Leu ι, Pro ₄]-NPY, on capsaicin-evoked mechanical allodynia in rats. Allodynia was measured using an automated device (Ugo Basille) with observers blinded to treatment allocation.

FIG. 3. Total releasable pool of immunoreactive substance P (iSP) from inflamed vs. control periradicular tissues (total releasable pool = baseline + capsaicin + recovery). **p<0.01, n=77.

FIG. 4A, FIG. 4B, FIG. 4C, FIG. 4D and FIG. 4E. An exemplary embodiment for identifying a candidate substance with analgesic activity. FIG.4A, collect either normal tooth, e.g. wisdom tooth, or one with chronic inflammation in the pulp or root of the tooth.

FIG. 4B, section the apical portion of the root that contains sensory nerve endings, which are then isolated and available for chemical stimulation. FIG. 4C, apply the test drug to the nerves surrounding the root biopsy. FIG. 4D, stimulate the pain neurons with chemicals, e.g. capsaicin or inflammatory mediators. FIG. 4E, measure neurotransmitter substances released from the pain neurons.

FIG. 5. Effects of capsaicin on evoking iSP release from periodontal ligament (PDL) tissue attached to isolated 6 mm apical sections of freshly extracted human teeth. Teeth were extracted, canals sealed with rope wax (to prevent pulpal origin of iSP from normal teeth) and the terminal 6mm apical section was removed and placed in 1 ml of Locke-Ringers buffer for 20 min. The root sections were then placed into sequential concentrations of capsaicin (5-500 μM for 20 min; N = 5 to 7 per group). Aliquots were acidified (IN HOAC, 0.02N HCl, 0.1% β mercaptoethanol), heated (10 min at 90°C), lyophilized and then assayed for iSP by radioimmunoassay (RIA). The two groups of teeth had clinical diagnoses of normal control teeth or painful teeth with a diagnosis of necrosis with an acute exacerbation of a chronic apical periodontitis *p<0.05 vs. normal control teeth.

FIG. 6. Effects of pre-treatment with either vehicle or a Yj receptor subtype-selective agonist, [Leu₃j, Pro₃ ]-NPY, on altering capsaicin-evoked release of iSP from PDL tissue attached to isolated 6mm apical sections of freshly extracted human teeth. Teeth were extracted, canals sealed with wax (to prevent pulpal origin of iSP from normal teeth), and the terminal 6mm apical section was removed and placed in 0.6ml of Locke-Ringers buffer for 30 min. The root sections were then placed in sequential concentrations of vehicle (Veh) or [Leu₃ι, Pro₃₄]-NPY 100 nM (30 min) and then stimulated with capsaicin (50 μM at 30 min). Aliquots were acidified (IN HOAC, 0.02N HCl, 0.01% β mercaptoethanol), heated (10 min at 90°C), lyophilized and then assayed for iSP by RIA. *P<0.05 vs. indicated comparison. N=22.

FIG. 7. Effects of pre-treatment with μ, δ and K opioid agonists and Yi agonist on capsaicin-evoked iSP release from biopsies of control and inflamed periradicular tissues. Teeth were extracted, canals sealed with wax, and the terminal 6mm apical section was removed and placed in 0.6ml of Locke-Ringers buffer for 30 min. The root sections were then contacted with vehicle alone, with a μ (DAMGO), δ (DPDPE) or K (U69,593) opioid agonist, or the Yi agonist [L,P]-NPY, and then stimulated with capsaicin. Aliquots were acidified, heated, lyophilized and then assayed for iSP by RIA. DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

An important need in the art solved by the present invention is the provision of high- throughput screening assays to identifying analgesic and anesthetic substances. The invention is based, in large part, on the use of in vitro compositions that comprise peripheral neurons, preferably human peripheral neurons, from which pain signaling can be measured. In these aspects of the invention, candidate analgesic and anesthetic substances are applied to the composition containing the peripheral neurons and their ability to inhibit pain signaling is measured. Pain inhibitory agents are thus identified as candidates for use as analgesics or anesthetics.

The methods of the invention have the advantage of using peripheral neurons as the targets in the screen, thus allowing the identification of analgesic drugs that have peripheral activity. The use of peripheral or sensory neurons is important as these neurons have unique features. For example, they preferentially transport neuropeptides, such as substance P, to peripheral endings of nerve terminals. Differential transport of receptors by sensory neurons (peripheral vs. central terminals vs. somata) is also known to occur, e.g., in glutamate (NMD A and AMP A), opioid (μ, δ, and K), neurokinin (NKi), glycine, serotinergic (5HT _A and 5HT c) and neuropeptide Y (Yi) receptors.

The use of human peripheral neurons, in particular, is preferred and advantageous as it overcomes those problems in the prior art associated with compounds that bind to rodent receptors, but not to the human counterpart of the receptor. For example, certain NKl antagonists are known to operate in rat cells, but do not function as antagonists in human cells.

These and other aspects of the invention have the further benefit, stemming from the use of peripheral neurons, that artifacts resulting from the use of cells transfected with a target gene are avoided, notably that recombinant cell expression and functional interactions may not resemble the levels or interactions in the natural tissue. As the precise levels and profile of proteins expressed in sensory neurons remain unknown, the use of recombinant cells does not permit the design of recombinant cell culture systems with the appropriate levels of expression or pattern of molecules required for an accurate screening assay. Thus, the prior art permits the identification of test agents that work in recombinant cell culture, but that do not have similar activity in treating pain in vivo. The preferred uses of peripheral human neurons also overcome the drawbacks in the prior art concerning DRG cells which, due to their intimate connection with the spinal cord, are confined to studies of DRG cells derived from experimental animals. Moreover, even if human DRG cells could have been used to a limited extent, the preferential transport of neuropeptides and receptors to specific subcellular locations suggested to the present inventors that studies conducted in DRG cell cultures would not anyway be a suitable model for identifying analgesic or anesthetic agents. Simply put, DRG cells lack adequate predictability for identifying candidate substances with peripheral analgesic activity. Thus, the present use of peripheral terminals of sensory neurons has many advantages.

U.S. Patent No. 5,624,932 and U.S. Patent No. 5,811,256 are each specifically incorporated herein by reference for purposes of even further describing and enabling certain technical steps associated with neuronal assays, notwithstanding that the present invention provides improved starting materials for use in the assays. The culture under conditions, in solutions and buffers, and for periods of time effective to substantially maintain neuronal function in vitro, and the execution of standard electrophysiological measurement techniques are particularly incorporated herein by reference from U.S. Patent Nos. 5,624,932 and 5,811,256, notwithstanding the various advantages of the invention.

In yet other aspects of the invention, the present inventors have both identified and solved a problem inherent, but not fully appreciated, in the art. In the prior art assays, even those not relying on recombinant cells, the inventors realized that the expression levels and profiles of cell surface and intracellular components may not resemble the levels and profiles that occur when pain is experienced. The present invention therefore provides preferred screening methods that address and solve this problem.

Peripheral tissue injury changes the proteins that are expressed in sensory neurons. For example, neuropeptide Y (NPY) has almost no expression in normal sensory neurons and is elevated 10-100 fold after injury. Despite these and like observations in the field, the changes in levels and types of proteins expressed in sensory neurons during injury remain largely unknown. Therefore, a screening assay for use in identifying agents that will be effective during injury could not be designed prior the present invention. The inventors recognized and solved this problem by developing certain preferred aspects of the overall invention. In particular, these aspects of the invention concern the sampling or biopsy of tissues containing "pain-sensitized peripheral neurons", preferably "pain-sensitized peripheral human neurons", i.e., peripheral neurons isolated from or comprised within a tissue sample that was sensitized to pain in vivo, such as peripheral neurons isolated from or comprised within a diseased, infected, inflamed, neuropathic or necrotic tissue sample. By conducting in vitro screening assays using such pain-sensitized peripheral neurons, the invention can identify analgesic drugs that have meaningful pain fiber inhibiting activity.

These aspects of the invention therefore compensate for the lack of precise knowledge concerning changes to sensory neurons during peripheral tissue injury. By using peripheral neurons collected from patients experiencing pain, which sensory neurons have already had phenotypic changes, even in genes not currently characterized, the invention permits the in vitro identification of analgesic drugs that will function in vivo.

Equally, by using isolated peripheral sensory terminals innervating native tissue, the present invention overcomes drawbacks from prior art cell culture studies, which can prove artifactual as in vitro culturing conditions can alter neuronal phenotype. Thus, overall, the invention better reflects the in vivo environment, particularly the environment to be encountered in the treatment of pain.

In addition, as the phenotypic changes in the peripheral neurons from pain patients can now be characterized, the invention provides methods for use in identifying and evaluating novel targets for rational drug design. Moreover, these methods can be used to study the mechanisms of pain in humans, thus furthering the development of novel analgesic strategies and yet other new agents.

The invention permits the selection and use of a sample of any tissue that was exposed to pain or chronic pain in vivo. Thus, the invention provides an in vitro model for identifying agents pre-selected for the ability to treat chronic pain, an area of significant clinical need.

In certain preferred embodiments, the present invention provides screening assays that take advantage of a common form of pain associated with inflammation and neuronal degeneration: odontalgia (toothache) with pulp necrosis. The most common form of pulpal inflammation results from bacterially-induced immunological and inflammatory responses, which lead to a substantial sprouting of peptidergic nociceptors in dental pulp. Following necrosis of pulp tissue together with degeneration of intrapulpal neurons, the periradicular tissue near the apex of the root displays extensive sprouting of peptidergic nociceptors, and nearby afferent fibers undergo substantial up-regulation of NPY.

Pulpal necrosis with associated periradicular inflammation therefore represents a clinical model of pain associated with inflammation, neuronal degeneration and neuronal plasticity and thus shares features of NPY plasticity observed in models of neuropathic pain. Moreover, odontalgia is the most common form of orofacial pain in the United States, allowing the analgesic effects of the substances identified to be directly confirmed in vivo in otherwise healthy patients.

In addition to the identification of pharmacological compounds, including peptidyl and non-peptidyl compounds, the invention also provides for the identification of active combinations of compounds that are otherwise inactive when used alone. Moreover, the invention can identify agents that are counter-indicated for use alone or in combination with other particular agents. Still further, agents that induce pain can be identified by the invention (e.g. capsaicin-like agents), which can then be used as positive controls in other pre-clinical development studies.

The invention further permits comparison between sensory neurons from normal tissues and those neurons collected from inflamed or diseased tissues, where an unknown number of genes may have elevated or reduced levels of expression, leading to altered drug sensitivity. This is important as it provides a simple in vitro test for a complex in vivo environment, wherein the pain being treated is typically localized, but the preferred drugs are delivered systemically. The model thus permits a determination of how a drug will function under normal vs. injured conditions that is applicable to clinical treatment.

In addition to their important predictive value, the methods of the present invention are also inexpensive and amenable to high-throughput screening. These sensitive and quantitative in vitro screening methods thus represent a significant breakthrough in the search for new or improved analgesic and anesthetic compounds for use in clinical embodiments. Pharmaceutical Formulations

The analgesic and anesthetic agents identified by the present invention will generally be formulated for use as pharmaceutical compositions. Such pharmaceutical compositions will typically comprise an effective amount of any of the agents identified by the invention, dissolved or dispersed in a pharmaceutically acceptable carrier or aqueous medium. Certain types of combined therapeutics are also contemplated, and the same type of underlying pharmaceutical compositions may be employed for both single and combined medicaments.

The phrases "pharmaceutically or pharmacologically acceptable" refer to molecular entities and compositions that do not produce an adverse, allergic or other untoward reaction when administered to an animal, or a human, as appropriate. Veterinary uses are included within the invention and "pharmaceutically acceptable" formulations include formulations for both clinical and/or veterinary use.

As used herein, "pharmaceutically acceptable carrier" includes any and all solvents, dispersion media, coatings, antibacterial and antifungal agents, isotonic and absorption delaying agents and the like. The use of such media and agents for pharmaceutical active substances is well known in the art. Except insofar as any conventional media or agent is incompatible with the active ingredient, its use in the therapeutic compositions is contemplated. For human administration, preparations should meet sterility, pyrogenicity, general safety and purity standards as required by FDA Office of Biologies standards. Supplementary active ingredients can also be incorporated into the compositions.

"Unit dosage" formulations are those containing a dose or sub-dose of the administered ingredient adapted for a particular timed delivery. For example, exemplary "unit dosage" formulations are those containing a daily dose or unit or daily sub-dose or a weekly dose or unit or weekly sub-dose and the like.

1. Injectable Formulations

The analgesic and anesthetic agents identified by the present invention can be formulated for parenteral administration, e.g., formulated for injection via the intravenous, intramuscular, sub-cutaneous, transdermal, or other such routes, including peristaltic administration and direct instillation into a tumor or disease site (intracavity administration). The preparation of an aqueous composition that contains an analgesic or anesthetic agent as an active ingredient will be known to those of skill in the art in light of the present disclosure. Typically, such compositions can be prepared as injectables, either as liquid solutions or suspensions; solid forms suitable for using to prepare solutions or suspensions upon the addition of a liquid prior to injection can also be prepared; and the preparations can also be emulsified.

The pharmaceutical forms suitable for injectable use include sterile aqueous solutions or dispersions; formulations including sesame oil, peanut oil or aqueous propylene glycol; and sterile powders for the extemporaneous preparation of sterile injectable solutions or dispersions. In all cases, the form should be sterile and fluid to the extent that syringability exists. It should be stable under the conditions of manufacture and storage and should be preserved against the contaminating action of microorganisms, such as bacteria and fungi.

The analgesic and anesthetic agents identified by the present invention can be formulated into a sterile aqueous composition in a neutral or salt form. Solutions of agents as free base or pharmacologically acceptable salts can be prepared in water suitably mixed with a surfactant, such as hydroxypropylcellulose. Pharmaceutically acceptable salts, include the acid addition salts (formed with the free amino groups of the protein), and those that are formed with inorganic acids such as, for example, hydrochloric or phosphoric acids, or such organic acids as acetic, trifluoroacetic, oxalic, tartaric, mandelic, and the like. Salts formed with the free carboxyl groups can also be derived from inorganic bases such as, for example, sodium, potassium, ammonium, calcium, or ferric hydroxides, and such organic bases as isopropylamine, trimethylamine, histidine, procaine and the like.

Suitable carriers include solvents and dispersion media containing, for example, water, ethanol, polyol (for example, glycerol, propylene glycol, and liquid polyethylene glycol, and the like), suitable mixtures thereof, and vegetable oils. In many cases, it will be preferable to include isotonic agents, for example, sugars or sodium chloride. The proper fluidity can be maintained, for example, by the use of a coating, such as lecithin, by the maintenance of the required particle size in the case of dispersion and/or by the use of surfactants.

Under ordinary conditions of storage and use, all such preparations should contain a preservative to prevent the growth of microorganisms. The prevention of the action of microorganisms can be brought about by various antibacterial and antifungal agents, for example, parabens, chlorobutanol, phenol, sorbic acid, thimerosal, and the like. Prolonged absorption of the injectable compositions can be brought about by the use in the compositions of agents delaying absorption, for example, aluminum monostearate and gelatin.

Prior to or upon formulation, the agents should be extensively dialyzed to remove undesired small molecular weight molecules, and/or lyophilized for more ready formulation into a desired vehicle, where appropriate. Sterile injectable solutions are prepared by incorporating the active agents in the required amount in the appropriate solvent with various of the other ingredients enumerated above, as desired, followed by filtered sterilization. Generally, dispersions are prepared by incorporating the various sterilized active ingredients into a sterile vehicle that contains the basic dispersion medium and the required other ingredients from those enumerated above.

In the case of sterile powders for the preparation of sterile injectable solutions, the preferred methods of preparation are vacuum-drying and freeze-drying techniques that yield a powder of the active ingredient, plus any additional desired ingredient from a previously sterile-filtered solution thereof.

Suitable pharmaceutical compositions in accordance with the invention will generally include an amount of the analgesic or anesthetic agent admixed with an acceptable pharmaceutical diluent or excipient, such as a sterile aqueous solution, to give a range of final concentrations, depending on the intended use. The techniques of preparation are generally well known in the art as exemplified by Remington's Pharmaceutical Sciences, 16th Ed. Mack Publishing Company, 1980, incorporated herein by reference. For human administration, preparations should meet sterility, pyrogenicity, general safety and purity standards as required by FDA Office of Biological Standards. Upon formulation, the agents will be administered in a manner compatible with the dosage formulation and in such amount as is therapeutically effective.

2. Sustained Release Formulations

Formulations are easily administered in a variety of dosage forms, such as the type of injectable solutions described above, but other pharmaceutically acceptable forms are also contemplated, e.g., tablets, pills, capsules or other solids for oral administration, suppositories, pessaries, nasal solutions or sprays, aerosols, inhalants, liposomal forms and the like. Pharmaceutical "slow release" capsules or compositions may also be used. Slow release formulations are generally designed to give a constant drug level over an extended period. The slow release formulations are typically implanted in the vicinity of the disease site, for example, at the site of a tumor.

Suitable examples of sustained-release preparations include semipermeable matrices of solid hydrophobic polymers containing therapeutic agents, which matrices are in the form of shaped articles, e.g., films or microcapsule. Examples of sustained-release matrices include polyesters; hydrogels, for example, poly(2-hydroxyethyl-methacrylate) or poly(vinylalcohol); polylactides, e.g., U.S. Patent No. 3,773,919; copolymers of L-glutamic acid and γ ethyl-L- glutamate; non-degradable ethylene-vinyl acetate; degradable lactic acid-glycolic acid copolymers, such as the Lupron Depot™ (injectable microspheres composed of lactic acid- glycolic acid copolymer and leuprolide acetate); and poly-D-(-)-3-hydroxybutyric acid. While polymers such as ethylene-vinyl acetate and lactic acid-glycolic acid enable release of molecules for over 100 days, certain hydrogels release proteins for shorter time periods.

3. Liposomes and Nanocapsules

In certain embodiments, liposomes and/or nanoparticles may also be employed with the analgesic and anesthetic agents identified by the present invention. The formation and use of liposomes is generally known to those of skill in the art, as summarized below.

Liposomes are formed from phospholipids that are dispersed in an aqueous medium and spontaneously form multilamellar concentric bilayer vesicles (also termed multilamellar vesicles (MLVs). MLVs generally have diameters of from 25 nm to 4 μm. Sonication of MLVs results in the formation of small unilamellar vesicles (SUVs) with diameters in the range of 200 to 500 A, containing an aqueous solution in the core.

Phospholipids can form a variety of structures other than liposomes when dispersed in water, depending on the molar ratio of lipid to water. At low ratios the liposome is the preferred structure. The physical characteristics of liposomes depend on pH, ionic strength and the presence of divalent cations. Liposomes can show low permeability to ionic and polar substances, but at elevated temperatures undergo a phase transition which markedly alters their permeability. The phase transition involves a change from a closely packed, ordered structure, known as the gel state, to a loosely packed, less-ordered structure, known as the fluid state. This occurs at a characteristic phase-transition temperature and results in an increase in permeability to ions, sugars and drugs.

Liposomes interact with cells via four different mechanisms: Endocytosis by phagocytic cells of the reticuloendothelial system such as macrophages and neutrophils; adsorption to the cell surface, either by nonspecific weak hydrophobic or electrostatic forces, or by specific interactions with cell-surface components; fusion with the plasma cell membrane by insertion of the lipid bilayer of the liposome into the plasma membrane, with simultaneous release of liposomal contents into the cytoplasm; and by transfer of liposomal lipids to cellular or subcellular membranes, or vice versa, without any association of the liposome contents. Varying the liposome formulation can alter which mechanism is operative, although more than one may operate at the same time.

Nanocapsules can generally entrap compounds in a stable and reproducible way. To avoid side effects due to intracellular polymeric overloading, such ultrafine particles (sized around 0.1 μm) should be designed using polymers able to be degraded in vivo. Biodegradable polyalkyl-cyanoacrylate nanoparticles that meet these requirements are contemplated for use in the present invention, and such particles may be are easily made.

4. Ophthalmic Formulations

Conditions associated with the eye and can be treated with the analgesic and anesthetic agents identified by the present invention. The agents may thus be advantageously employed in the preparation of pharmaceutical compositions suitable for use as ophthalmic solutions, including those for intravitreal and/or intracameral administration.

The ophthalmic preparations will contain an analgesic or anesthetic agent in a concentration from about 0.01 to about 1% by weight, preferably from about 0.05 to about 0.5% in a pharmaceutically acceptable solution, suspension or ointment. Some variation in concentration will necessarily occur, depending on the particular compound employed, the condition of the subject to be treated and the like, and the person responsible for treatment will determine the most suitable concentration for the individual subject. The ophthalmic preparation will preferably be in the form of a sterile aqueous solution containing, if desired, additional ingredients, for example preservatives, buffers, tonicity agents, antioxidants and stabilizers, nonionic wetting or clarifying agents, viscosity-increasing agents and the like.

Suitable preservatives for use in such a solution include benzalkonium chloride, benzethonium chloride, chlorobutanol, thimerosal and the like. Suitable buffers include boric acid, sodium and potassium bicarbonate, sodium and potassium borates, sodium and potassium carbonate, sodium acetate, sodium biphosphate and the like, in amounts sufficient to maintain the pH at between about pH 6 and pH 8, and preferably, between about pH 7 and pH 7.5. Suitable tonicity agents are dextran 40, dextran 70, dextrose, glycerin, potassium chloride, propylene glycol, sodium chloride, and the like, such that the sodium chloride equivalent of the ophthalmic solution is in the range 0.9 plus or minus 0.2%.

Suitable antioxidants and stabilizers include sodium bisulfite, sodium metabisulfite, sodium thiosulfϊte, thiourea and the like. Suitable wetting and clarifying agents include polysorbate 80, polysorbate 20, poloxamer 282 and tyloxapol. Suitable viscosity-increasing agents include dextran 40, dextran 70, gelatin, glycerin, hydroxyethylcellulose, hydroxmethylpropylcellulose, lanolin, methylcellulose, petrolatum, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, carboxymethylcellulose and the like. The ophthalmic preparation will be administered topically to the eye of the subject in need of treatment by conventional methods, for example in the form of drops or by bathing the eye in the ophthalmic solution.

5. Topical Formulations

In the broadest sense, formulations for topical administration include those for delivery via the mouth (buccal) and through the skin. "Topical delivery systems" also include transdermal patches containing the ingredient to be administered. Delivery through the skin can further be achieved by iontophoresis or electrotransport, if desired.

Formulations suitable for topical administration in the mouth include lozenges comprising the ingredients in a flavored basis, usually sucrose and acacia or tragacanth; pastilles comprising the active ingredient in an inert basis such as gelatin and glycerin, or sucrose and acacia; and mouthwashes comprising the ingredient to be administered in a suitable liquid carrier. Formulations suitable for topical administration to the skin include ointments, creams, gels and pastes comprising the ingredient to be administered in a pharmaceutical acceptable carrier. The formulation of therapeutic agents for topical use, such as in creams, ointments and gels, includes the preparation of oleaginous or water-soluble ointment bases, will be well known to those in the art in light of the present disclosure. For example, these compositions may include vegetable oils, animal fats, and more preferably, semisolid hydrocarbons obtained from petroleum. Particular components used may include white ointment, yellow ointment, cetyl esters wax, oleic acid, olive oil, paraffin, petrolatum, white petrolatum, spermaceti, starch glycerite, white wax, yellow wax, lanolin, anhydrous lanolin and glyceryl monostearate. Various water-soluble ointment bases may also be used, including glycol ethers and derivatives, polyethylene glycols, polyoxyl 40 stearate and polysorbates.

Formulations for rectal administration may be presented as a suppository with a suitable base comprising, for example, cocoa butter or a salicylate. Formulations suitable for vaginal administration may be presented as pessaries, tampons, creams, gels, pastes, foams or spray formulations containing in addition to the active ingredient such carriers as are known in the art to be appropriate.

6. Nasal Formulations

Local delivery via the nasal and respiratory routes is contemplated for treating various conditions. These delivery routes are also suitable for delivering agents into the systemic circulation. Formulations of analgesics and anesthetics in carriers suitable for nasal administration are therefore also included within the invention, for example, nasal solutions, sprays, aerosols and inhalants. Where the carrier is a solid, the formulations include a coarse powder having a particle size, for example, in the range of 20 to 500 microns, which is administered, e.g., by rapid inhalation through the nasal passage from a container of the powder held close up to the nose.

Suitable formulations wherein the carrier is a liquid are useful in nasal administration. Nasal solutions are usually aqueous solutions designed to be administered to the nasal passages in drops or sprays and are prepared so that they are similar in many respects to nasal secretions, so that normal ciliary action is maintained. Thus, the aqueous nasal solutions usually are isotonic and slightly buffered to maintain a pH of 5.5 to 6.5. In addition, antimicrobial preservatives, similar to those used in ophthalmic preparations, and appropriate drug stabilizers, if required, may be included in the formulation. Various commercial nasal preparations are known and include, for example, antibiotics and antihistamines and are used for asthma prophylaxis.

Inhalations and inhalants are pharmaceutical preparations designed for delivering a drug or compound into the respiratory tree of a patient. A vapor or mist is administered and reaches the affected area. This route can also be employed to deliver agents into the systemic circulation. Inhalations may be administered by the nasal or oral respiratory routes. The administration of inhalation solutions is only effective if the droplets are sufficiently fine and uniform in size so that the mist reaches the bronchioles.

Another group of products, also known as inhalations, and sometimes called insufflations, comprises finely powdered or liquid drugs that are carried into the respiratory passages by the use of special delivery systems, such as pharmaceutical aerosols, that hold a solution or suspension of the drug in a liquefied gas propellant. When released through a suitable valve and oral adapter, a metered does of the inhalation is propelled into the respiratory tract of the patient. Particle size is of major importance in the administration of this type of preparation. It has been reported that the optimum particle size for penetration into the pulmonary cavity is of the order of 0.5 to 7 μm. Fine mists are produced by pressurized aerosols and hence their use in considered advantageous.

The following examples are included to demonstrate certain preferred embodiments of the invention. It will be appreciated by those of skill in the art that the techniques disclosed in the examples that follow represent techniques discovered by the inventors to function well in the practice of the invention, and thus can be considered to constitute certain preferred modes for its practice. However, those of skill in the art will, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments that are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. EXAMPLE 1 Pain Signaling in Peripheral Neurons In Vitro

The present example shows that peripheral neurons can be maintained in vitro under conditions effective for manipulating and measuring pain signals from peripheral neurons. The data show that peripheral neurons in vitro express functional receptors, respond to pain- inducing stimuli by secreting neurotransmitters and that agonists function to modulate the activity of peripheral neurons in in vitro systems.

A. Expression of Receptors

NPY is a potentially important regulator of pain and allodynia. Studies were conducted to investigate whether trigeminal VR1 -positive nociceptors expressed the Yi receptor under basal conditions.

Using immunohistochemistry, co-localization of Y_\ receptors was identified in a subpopulation of VR1 -positive neurons in the rat trigeminal ganglion. In these studies, guinea pig anti-VRl antisera was used to identify VR-1 positive neurons (green staining), and rabbit anti-Yi antisera was used to identify Y_\ positive neurons (red staining). Superimposed orange images were detected, showing the co-localization of Y_\ receptors on the VR-1 positive subclass of trigeminal sensory neurons. These studies were conducted for a cell count of greater than 2,000.

These immunohistochemical studies demonstrated that about 25% of trigeminal VR1 -positive nociceptors expressed the Y_\ receptor under basal conditions. Thus, there is evidence at the cellular level that Y_\ agonists may directly modulate activity of the VR1 -subclass of nociceptors.

B. Human Tissues

Human trigeminal ganglia were obtained from tissue bank and prepared for immunohistochemistry. 20 μm sections were incubated with primary rabbit antiserum against human TRPVl (capsaicin receptor), then prepared for DAB (diaminobenzidine) staining and visualized with light microscopy. These studies showed positive immunohistochemical staining of human trigeminal ganglion neurons using the TRPVl receptor antibody and DAB staining. The apical 6mm of human roots and attached periodontal ligament (PDL) tissues were fixed, decalcified and prepared for immunohistochemistry. 20 μm sections were incubated with primary mouse antiserum against the pan-neuronal marker N52 (200kD neurofilament proteins, N52). Sections were then incubated with anti-mouse antibody that had been conjugated with Alexa-Fluor 488 and visualized by fluorescence microscopy using a Nikon E600 microscope. Select sections were stained only with hemotoxylin and eosin (H&E) to demonstrate gross anatomy.

The H&E stained section of human apical root tissue showed the periradicular tissue and dentin of the human root. Immunohistochemical staining using the pan-neuronal marker N52 demonstrated the innervation pattern of human periradicular tissue.

C. Pain Signaling and Function of Agonists

The Yi receptor for NPY was believed to be restricted to neuronal somata (Hokfelt et al, 1997). However, more recent studies have demonstrated that the Yi and Y receptors undergo axonal transport to both central and peripheral terminals (Brumovsky et l, 2001; Marchand et α/,, 1999).

Contact with capsaicin evokes release of immunoreactive calcitonin gene-related peptide (iCGRP) from central terminals, neuronal somata and peripheral terminals. Superfusion studies demonstrate that Yi agonists inhibit capsaicin-evoked release of iCGRP from central terminals (FIG. 1A), neuronal somata (FIG. IB) and peripheral terminals (FIG. 1C) of the VR1 -positive subclass of nociceptors.

EXAMPLE 2 In Vitro Pain Signaling Correlates with In Vivo Models

The present example shows that the results from the in vitro studies of Example 1 correlate with results from standard in vivo tests.

The demonstration that Yi receptors are co-expressed on a portion of the VR-1 -positive nociceptor population and that Yj agonists inhibit capsaicin-evoked neurosecretion (FIG. 1A, FIG. IB and FIG. IC) suggests that Yi agonists modulate activation of the VRl subclass of nociceptors.

The in vitro results are shown to correlate with in vivo tests by injecting (ipl) rat hindpaws with either capsaicin/vehicle or the combination of capsaicin and the Yi agonist, [Leu₃j, Pro₃₄]-NPY. In FIG. 2, the scale shows a reduction in mechanical thresholds as a negative score (i.e. it plots the difference from pre-drug to post-drug, with increasingly negative scores indicating greater mechanical allodynia). As shown in FIG. 2, the Yj agonist blocked capsaicin-evoked mechanical allodynia by about 50% upon injection.

EXAMPLE 3 Responses from Peripheral Neurons in Inflamed Tissues

This example shows that peripheral neurons from inflamed tissues can also be effectively maintained in vitro and used to measure the response to pain-inducing stimuli. Moreover, it is shown that inflamed tissue exhibits a greater response to pain in vitro than the corresponding normal tissue.

The immunoreactive substance P (iSP) release assay was conducted as follows. Following informed consent, teeth were anesthetized, then extracted and placed into cold modified Locke-Ringers (MLR) buffer for transport. The apical 6mm of the roots and attached PDL tissue were resected, the pulp canal systems sealed with wax and the specimens placed into wells containing MLR and capsaicin for 20 min.

At the completion of the studies, the cells were lysed by two freeze-thaw cycles to release total iSP and the superfusion and lysis samples were heated to denature proteases in IN HOAc, 0.1N HCl and 0.02% beta mercaptoethanol at 90°C for 10 min., frozen and then lyophilized. Samples were resuspended in ddH20 and incubated in the radioimmunoassay (RIA) with antisera for 24 hours. 100 μl of [ Ij-substance P was then added to the solution, vortexed and allowed to incubate for an additional 24 hours. The RIA was stopped by the addition of 50 μl of goat anti-rabbit antisera coupled to ferric beads with immunomagnetic separation. Capsaicin was evaluated separately and shown not to interfere with the RIA. These studies showed a statistically significant increase in capsaicin-evoked iSP release from inflamed periradicular tissues as compared to the corresponding normal tissue biopsies (FIG. 3).

EXAMPLE 4 Non-Invasive Methods to Identifying Analgesic Agents

The present example describes the development and successful execution of non- invasive methods to identifying agents for treating pain. Functional peripheral neurons from various sources, including those from healthy and inflamed tissues, are maintained in vitro under conditions in which the response to pain stimuli can be routinely monitored. Under appropriately controlled conditions, these systems allow testing of candidate substances to identify potential analgesic agents. Results from screening assays using peripheral neurons from inflamed tissues are particularly informative in evaluating agents for peripheral pain control in humans.

A. Peripheral Neurons from Inflamed Tissues

Pulpal necrosis is one suitable model of inflammation with neuronal degeneration (FIG. 4A, FIG. 4B, FIG. 4C, FIG. 4D and FIG. 4E). In addition to providing a useful model system, the following data also exemplify advantages of the invention that stem from the use of peripheral neurons that have been sensitized to pain - - as occurs in a patient. The results show the differential expression of neuropeptides when a sample of a given tissue is maintained under basal (normal) conditions and when the same tissue is exposed to a painful environment.

In a root apical PDL sample, no afferent fibers express detectable levels of immunoreactive NPY (iNPY) under basal conditions (Wakisaka et al, 1996a). However, pulpal necrosis evokes an extensive up-regulation of iNPY afferent fibers innervating the apical PDL. At the EM level, the iNPY reaction product is contained within unmyelinated fibers. A similar increase in iNPY is observed in the medium-large neurons in the trigeminal ganglia under these conditions. B. Neurosecretion from Tissue Samples In Vitro

Samples of periodontal ligament attached to the apices of freshly extracted human teeth were used to confirm that neurosecretion can be readily measured from peripheral neurons maintained in vitro. Neurosecretion was evaluated from peripheral terminals of VRl -expressing fibers innervating the PDL tissue attached to the apices of freshly extracted human teeth (FIG. 5).

The ability of different concentrations of capsaicin to evoke iSP release was analyzed using isolated 6mm sections of normal human root apices and 6mm sections of apices from teeth with necrotic pulps and an acute exacerbation of a chronic apical periodontitis. Teeth were anesthetized by nerve block injection, to avoid anesthetizing periradicular neurons, and extracted. The apical 6mm of the root and attached tissue was resected and placed into wells containing 1 ml of modified Locke-Ringers (MLR) buffer and maintained for 20 min.

The assays were conducted essentially as described in Example 3, although the root sections were placed into sequential concentrations of capsaicin. The concentrations were selected on the basis of studies using the recombinant human form of VRl in over-expressing cell systems that indicates an EC ₀ of about 1-10 μM (Cortright et al, 2001; Mclntyre et al, 2001).

At the completion of the incubation, aliquots were acidified (IN HOAc, 0.1N HCl, β mercaptoethanol), heated at 90°C for 10 min. to denature proteases and lyophilized. Samples were analyzed for iSP by RIA. Aliquots were pre-incubated with antisera for 48 hours, then 100 μL of [¹²⁵I]-SP (approximately 20,000-25,000 cpm) was added to the solution, vortexed and allowed to incubate an additional 24 hours. The RIA was stopped by the addition of 50 μl of goat anti-rabbit antisera coupled to ferric beads with immunomagnetic separation (Advanced Magnetics, Inc.). The minimal detection limit in the RIA is about 3-6 fmol.

The concentration-dependent effects of capsaicin on iSP release from inflamed vs. control periradicular tissues are shown in FIG. 5. These results indicate that not only does capsaicin evoke iSP from isolated human root apices, but that the painful teeth with inflamed and/or necrotic pulps and periradicular tissues had significantly greater responsiveness (iSP release) to lower capsaicin concentrations as compared to control teeth (FIG. 5). In other studies, the effects of a combination of bradykinin (BK) and prostaglandin E₂ (PGE₂) were tested in normal and inflamed periradicular tissues. These studies showed that an equimolar solution of bradykinin and prostaglandin E₂ stimulated a concentration-dependent release of iSP in normal, but not inflamed periradicular tissue.

C. Identifying Candidate Substances with Analgesic Activity

Once the ability of pain-inducing agents to stimulate a measurable pain response had been confirmed in healthy and inflamed samples of the same tissue (FIG. 5), a screening assay to identify potential analgesic substances was validated (FIG. 4A, FIG. 4B, FIG. 4C, FIG. 4D and FIG. 4E).

The assays were conducted essentially as described in Example 3, although the root sections were placed into wells containing capsaicin and the selected test agent. The test agents and capsaicin were evaluated separately and shown not to interfere with the RIA.

Pre-treatment with the Y_\ receptor subtype-selective agonist [Leu₃ι, Pro₃₄]-NPY was shown to inhibit capsaicin-evoked release of iSP from isolated 6mm sections of normal human root apices compared with 6mm sections of apices from teeth with necrotic pulps and an acute exacerbation of a chronic apical periodontitis (FIG. 6).

Scientifically, these results indicate that: 1) pre-treatment with a Yi agonist inhibits capsaicin-evoked iSP release from normal tissue; 2) pre-treatment with a Yi agonist inhibits capsaicin-evoked iSP release from painful teeth with necrotic and/or inflamed pulps and periradicular tissue; and that 3) painful teeth with necrotic and/or inflamed pulps and periradicular tissue had greater responsiveness to capsaicin as compared to iSP release from the normal control teeth. These data thus also provide the first evidence that Yi agonists significantly inhibit activation of isolated peripheral terminals of human VRl -expressing nociceptors and show that, in human pain patients, Yi agonists likely act in the periphery to modulate activity of the VRl subclass of nociceptors.

In addition to the Yi agonist, the effects of pretreatment with μ, δ and K opioid receptor agonists on capsaicin-evoked iSP release from biopsies of human periradicular tissue were next evaluated. These assays were conducted as described above, using the same Yi agonist, [L,P]-NPY, and the selected opioid receptor agonists as test agents (none of which interfered with the RIA).

The opioid receptor agonists tested included the μ (mu), δ (delta) and K (kappa) opioid agonists: DAMGO (μ selective opioid agonist); the cyclic enkephalin analogue, DPDPE (δ selective opioid agonist); and U69,593 (K selective opioid agonist). In addition to the Y_\ agonist (FIG. 6; FIG. 7), this study also showed that pretreatment with each of the μ, δ, and opioid agonists was effective in inhibiting capsaicin-evoked iSP release from periradicular tissues, particularly from inflamed tissues (FIG. 7).

In regard to the objective to develop a non-invasive screening assay to identifying agents for use in treating pain, these results validate the inventors' choice of model system. The use of readily accessible peripheral human neurons, particularly pain-sensitized peripheral human neurons, provide ideal screening assays to identify analgesic drugs. The peripheral neurons used in these methods have the phenotypic profile of the neurons that will be encountered in the treatment of pain. The methods are also inexpensive, reliable and reproducible, with about 2,000 samples being analyzed by the inventors in an eight month period.

The data in FIG. 7 also indicate that, in human pain patients, opioids and Yi agonists likely act in the periphery to inhibit a major subclass of nociceptors. Therefore, these agents may serve as a prototype for novel peripherally acting analgesics. Such candidate analgesics may be synthesized, either by design or randomly generated, and tested to select the effective and optimized structures using the effective non-invasive screening assays of the present invention.

These types of in vitro superfusion methods and screening assays can also be conducted with peripheral neurons from other tissues, such as buccal mucosa and skin (Flores et al, 2001; Garry et al, 1994; Goodis et l, 2000; Hargreaves et al, 1992; Kilo et al, 1997; Ulrich et al, 2001). In addition to the isolated human root apices detailed in the present example, the methodology therefore allows the pharmacological regulation of neurosecretion from terminals of nociceptors to be evaluated using neurons innervating a variety of other tissues. Thus, analgesic agents can now be pre-selected for effectiveness in a given tissue. EXAMPLE 5 Acute Sensitization of Peripheral Terminals To Inflammatory Mediators

The present example analyzes the effects of inflammatory mediators on the sensitization of peripheral terminals of sensory neurons in vitro. In addition to the information provided on the effects of inflammatory mediators on sensory neurons in vitro, this example also validates the inventors' insight that dorsal root ganglion (DRG) cell cultures are not an effective model for analyzing the effects of agents on sensory neurons in order to identify analgesic drugs.

The effects of the inflammatory mediators bradykinin, prostaglandin E , histamine and serotonin, their duration of exposure, and the interaction between these inflammatory mediators and capsaicin on evoking the release of iCGRP is analyzed using a model of isolated peripheral terminals of sensory neurons innervating dental pulp evaluated.

The results indicate that inflammatory mediators evoke iCGRP release in a concentration-dependent fashion (1-1,000 nM) and that the exocytotic mechanism is transient, with release rates returning to basal levels upon cessation of the chemical stimulus. A continuous exposure of inflammatory mediators (1 μM) over a one hour period produced a sustained release of iCGRP without evidence of desensitization. In contrast, a single 7 min exposure to capsaicin (30 μM) significantly reduced neuronal responsiveness to a subsequent 7 min application of capsaicin given one hour later. In addition, the combination of inflammatory mediators (1 μM) sensitized the terminals, as measured by augmented neuropeptide release, to a subsequent capsaicin-evoked iCGRP release (30 μM). Collectively, the results indicate that a combination of inflammatory mediators sensitizes terminals of sensory neurons as evidenced by enhanced responsiveness to a subsequent chemical stimulus. This mechanism requires continuous exposure of mediators without evident desensitization over the study period.

A. Introduction

Peripheral tissue injury induces a substantial increase in tissue levels of inflammatory mediators including bradykinin, prostaglandin E₂, histamine and serotonin (Hargreaves and Costello, 1990; Roszkowski et al, 1997; McArdle et al, 1999; Nielsen et al, 2001). Behavioral studies conducted in animals (Hong and Abbott, 1994; Abbott et al, 1996; Kato et al, 2002) and psychophysical studies conducted in humans (Babenko et al, 1999; Lischetzki et al, 2001) demonstrate that peripheral administration of these inflammatory mediators evokes hyperalgesic or allodynic responses. These findings are consistent with results from electrophysiologic studies demonstrating that inflammatory mediators such as bradykinin and prostaglandin E₂ activate or sensitize certain sensory neurons (Habelt et al. , 2000; Koppert et al, 2001; Rathee et al, 2002).

In vitro studies using cultured sensory neurons from either dorsal root or trigeminal ganglion have been employed to study the cellular basis for the actions of inflammatory mediators. Sensory neurons have been demonstrated to express receptor subtypes for bradykinin (Bi, B₂), prostaglandins (EP₃c, EP₄), histamine (Hi) and serotonin (5HT_{!B/ID/I F}, 5HT_2A 2c 5HT_3A 3B, and 5HT₇) (Chen et al, 1998; Kashiba et al, 1999; Southall and Vasko, 2001; Donaldson et al, 2001; Ma, 2001a; 2001b; Morales et al, 2001; Terron et al, 2001).

Studies indicate that either inflammatory mediators, or agents that activate the protein kinase A (PKA) and protein kinase C (PKC) signal transduction pathways, evoke neuropeptide release from sensory neurons (Averbeck et al, 2000; Bolyard et al, 2000; Southall and Vasko, 2001), and facilitate the effects of capsaicin on activating the vanilloid receptor type 1 (VRl) (Premkumar and Ahern, 2000; Vellani et al, 2001; Chuang et al, 2001; Southall and Vasko, 2001; Rathee et al, 2002; but see Lee et al, 2000). In addition, prolonged exposure of inflammatory mediators produces a continuous release of neuropeptides from sensory neuron cultures that does not desensitize and appears to be mediated at least in part by activation of G protein-coupled receptors that activate the PKA pathway (Bolyard et al, 2000; Southall et α/,, 2002).

However, prior to the present invention the actions of inflammatory mediators on peripheral terminals of sensory neurons were not adequately understood. This was a pertinent gap in knowledge, since sensory neurons preferentially transport neuropeptides such as substance P to peripheral endings of nerve terminals (Brimijoin et al, 1980). Selective transport appears also to occur with receptors expressed by sensory neurons. For example, differential transport of receptors by sensory neurons (peripheral vs. central terminals vs. somata) has been proposed recently for glutamate (NMDA and AMPA), opioid (μ, δ, and K), neurokinin (NKi), glycine, serotinergic (5HT_2A and 5HT₂c) and neuropeptide Y (Yi) receptors (Elde et al, 1995; Coggeshall and Carlton, 1997; Hokfelt et al, 1997). Demonstration that neuropeptides and receptors may be preferentially transported to specific subcellular locations suggested to the inventors that studies conducted in DRG cell cultures may not effectively model the impact of inflammatory mediators on peripheral terminals of sensory neurons. In contrast to experiments of basic research, the inventors realized that the use of cells and tissues other than DRG cells is particularly important in the design and execution of screening assays to develop analgesic agents, which led to important aspects of the present invention.

In this example, the effects of an equimolar combination of inflammatory mediators (bradykinin, prostaglandin E₂, histamine, and serotonin) on neuropeptide release were evaluated using a model of isolated peripheral terminals of sensory neurons innervating dental pulp. It was determined whether the length of time the peripheral terminals was exposed to the combination of inflammatory mediators modulated iCGRP release, and if there was any interaction between the inflammatory mediators and the selective stimulant of certain nociceptive sensory neurons, capsaicin. The evoked release of iCGRP from these peripheral sensory terminals was used as a dependent measure. The release of iCGRP from isolated superfused dental pulp is a selective marker for activation of peripheral sensory neuron terminals innervating this tissue because transection of the trigeminal inferior alveolar nerve abolishes tissue levels of iCGRP in dental pulp, while removal of the superior cervical ganglion has no effect (Wakiksaka et al, 1987; Silverman and Kruger, 1987).

B. Materials and Methods

1. Materials

Mandibular incisor teeth were obtained from a slaughterhouse (Long Prairie Packing Company, South St. Paul, MN). All chemicals and test drugs were purchased from Sigma Chemical Co. (St Louis, MO), with the exception of PGE₂ which was purchased from Cayman Chemical Co. (Ann Arbor, MI). The CGRP antiserum (MI-C2) was obtained from Dr. M. Iadarola (NIDCR, NIH).

2. Peptide Release

The superfusion method was employed as described by Hargreaves et al. (1992). The mandibular incisor teeth were collected from freshly killed 2-4 yr old Holstein cows at the local slaughterhouse, placed on ice, and transported to the laboratory. Pulp tissue was removed, sectioned into 1 mm slabs, and then chopped into 200 μm² slices with a Mcllwain tissue chopper (Mickle Lab. Eng. Co. Ltd.). The tissue was weighed, placed into 0.75cc chambers and superfused at 37°C with oxygenated Krebs buffer (420 μL/min). The Krebs buffer (pH 7.4) comprised NaCl (135 mM), KC1 (3.5 mM), MgCl (1.1 mM), NaH₂PO₄ (1 mM), CaCl₂ (2.5 mM), dextrose (3.3 mM), bovine serum albumin (0.1%), bacitracin (3 mg%) and 0.1 mM ascorbic acid. Samples of superfused buffer were collected over 7 min periods.

3. Study Conditions

After approximately 60 min of superfusing dental pulp tissue, a stable level of spontaneous iCGRP release is achieved (Hargreaves et al, 1992; Jackson and Hargreaves, 1999). Once this stable condition of peptide release was achieved, the equimolar combination of inflammatory mediators (bradykinin, prostaglandin E₂, histamine, and serotonin) was added to the Krebs buffer superfusing the preparation of isolated peripheral terminals of sensory neurons. Different studies varied the concentration (1-1,000 nM) of these mediators or the duration of their exposure (7-60 min). In other studies, tissue was also stimulated with capsaicin (30μM).

4. Immunoreactive CGRP Measurements

Levels of iCGRP were measured by radioimmunoassay as described by Richardson et al. (1998). Separate standard curves were prepared with aliquots of Krebs buffer containing relevant drug concentrations to permit respective comparison to study treatments.

5. Statistical Analysis

The data were analyzed by one-way or two-way ANOVA with repeated measures followed by Duncan's multiple range test to determine differences between groups. A difference was accepted as significant if the probability that it occurred due to chance alone was less than 5% (p<0.05). Release data were normalized by calculating the percent increase over baseline rates of iCGRP release using the formula 100 X (peak release - baseline)/(baseline). This reduced variability due to interexperimental sources. Data are presented as mean ± standard error of the mean (SEM). C. Results

Administration of an equimolar (1 μM) combination of bradykinin, prostaglandin E₂, histamine, and serotonin produced approximately a two-fold increase in iCGRP released from sensory neuron terminals in the isolated superfused dental pulp. In absolute rates of release, this combination of mediators increased iCGRP levels released into the superfusate from a baseline value of 11.1 ± 1.1 fmol/G/7 min to 28.3 ± 4.5 fmol/G/7 min (p<0.0\). This stimulatory effect was transient, with rates of iCGRP release returning to basal levels within 21-28 min of removal of the inflammatory mediators.

The equimolar combination of inflammatory mediators stimulated the release of iCGRP from isolated sensory neuron terminal in a concentration-dependent manner. The combined administration of bradykinin, prostaglandin E₂, histamine, and serotonin produced a linear increase in iCGRP release over the concentration range of 1-1,000 nM (r = 0.90; p<0.05). The 1,000 nM equimolar concentration of the inflammatory mediators evoked iCGRP release of a magnitude similar to that observed with a 30μM concentration of capsaicin.

Since a short administration (7 min) of the combined inflammatory mediators evokes iCGRP release, it was evaluated whether a longer duration of mediator perfusion (63 min) would reduce the rate of iCGRP release in a manner similar to what is observed with repeated administration of the stimulant capsaicin. Continuous exposure of the combined inflammatory mediators (1 μM) for more than one hour produced a sustained rate of iCGRP release that was similar to the release rate observed in the first 7 min of mediator exposure.

It was next determined whether pre-treating the peripheral sensory terminals from bovine dental pulp with the inflammatory mediator combination would alter the capsaicin- evoked release of iCGRP. Pretreating the peripheral sensory terminals with vehicle immediately before capsaicin produced approximately a two-fold increase (84% over basal) in iCGRP release. In contrast, the peripheral sensory terminals treated with the combined inflammatory mediators (lμM) and capsaicin (30 μM) resulted in approximately a three-fold increase (199% over basal) in iCGRP release (p<0.05 vs vehicle/capsaicin group at 7-14 min time point). The rates of iCGRP release returned to near basal levels for both groups by 35 min after stimulation with capsaicin. The effects of pretreatment with combined inflammatory mediators (1 μM) on altering desensitization to repeated applications of capsaicin was then determined. Pre-treatment with vehicle (at 0-7 min) followed by capsaicin (30 μM) application 53-60 min later produced about a two-fold increase in the rate of iCGRP release. In contrast, pre-treatment with capsaicin (30 μM at 0-7 min) followed by a second application of capsaicin (30 μM) application 53-60 min later produced little-to-no detectable increase in iCGRP release (- 10% over basal release). Finally, pre-treatment with inflammatory mediators (1 μM) and capsaicin 30 μM) (both at 0-7 min) followed by capsaicin (30 μM) application 53-60 min later produced a iCGRP release that was significantly (p<0.0\) greater than that measured in the group receiving capsaicin twice.

D. Discussion

The present example evaluated the effects of inflammatory mediators on neuropeptide release from isolated peripheral terminals of sensory neurons innervating dental pulp. The results indicate that inflammatory mediators evoke iCGRP release in a concentration- dependent fashion and that the exocytotic mechanism is transient, with release rates returning towards basal levels upon cessation of the chemical stimulus. A continuous exposure of inflammatory mediators over a one hour period produced a sustained release of iCGRP without evidence of desensitization. In contrast, a single 7 min exposure to capsaicin significantly reduced neuronal responsiveness to a subsequent 7 min application of capsaicin given one hour later. In addition, the combination of inflammatory mediators enhanced neuronal responsiveness to a subsequent capsaicin-evoked iCGRP release. Collectively, these results indicate that a combination of inflammatory mediators sensitizes terminals of sensory neurons as indicated by enhanced neuropeptide release. This mechanism requires continuous exposure of mediators without evident desensitization over the study period.

Electrophysiological studies have reported that the acute axotomy and ligation of a cutaneous nerve trunk induces sensitivity to inflammatory mediators applied to the ligated stump (Michaelis et al, 1997; 1998). This raises the possibility that the effect of inflammatory mediators in the present model of isolated nerve terminals may be due in part to exocytosis from neuronal terminals and in part due to neuropeptide release from the axotomized section of the nerve trunk (i.e. a non-physiologic site of release). However, this latter possibility is unlikely for the following reasons. First, the control studies evaluating neuropeptide release from axotomized sections of trigeminal nerve trunk fail to reproduce the iCGRP release rates observed from isolated sensory nerve terminals (Ulrich et al, 2001). Second, these prior electrophysiologic studies used whole-animal recordings on ligated nerves conducted up to 33 hours after transection, conditions previously shown to result in redistribution of ion channels to the ligated nerve stump (Nokavic et al, 1998). Third, application of a higher concentration (10 μM) of the same combination of inflammatory mediators as used in the present study depolarized only 3-4% of all C-frbers (Michaelis et al, 1997). Fourth, electrophysiological studies are influenced by ion channel distribution along the neuronal membrane and this does not necessarily reproduce the distribution of intracellular proteins and vesicles required for neuropeptide exocytosis. Accordingly, these results represent neuropeptide exocytosis from peripheral terminals of sensory neurons.

These studies demonstrate that inflammatory mediators sensitize neurons to various chemical stimuli and this effect required continuous exposure to inflammatory mediators. The present results extend prior studies using neonatal sensory neurons (Bolyard et al, 2000; Southall et al, 2002) by demonstrating that application of inflammatory mediators significantly, although incompletely, reduces the development of capsaicin-evoked desensitization at the terminals of sensory neurons. Moreover, the results indicate that peripheral sensory terminals have sufficient neuropeptide stores to sustain a prolonged release of neuropeptides following neuronal activation.

Several signal transduction pathways appear to mediate the effects of inflammatory mediators on sensory neurons. Prostaglandin-induced sensitization involves activation of the PKA pathway, and agents that directly activate this pathway (e.g., forskolin) mimic the effects of prostanoids (Bolyard et al, 2000; Southall et al, 2002). However, the effects of bradykinin are more complex. Bradykinin directly evokes neuropeptide release, and sensitizes sensory neurons, in part via release of endogenous prostanoids (Bolyard et al, 2000; Petho et al, 2001). Bradykinin also facilitates VRl activation and reduces desensitization of this receptor; these effects appear to be mediated by PLC-evoked reduction in phosphotidylinositol-4,5- bisphosphate levels and/or by PKC phosphorylation of the VRl receptor (Vellani et al, 2001). The effects of histamine on neuropeptide release are less well characterized, although about 40% of histamine-responsive sensory neurons are also capsaicin-sensitive, suggesting a direct site of action (Nicolson et al, 2002). The lack of desensitization to continuous exposure of a combination of inflammatory mediators is likely to contribute to a persistent afferent barrage during tissue inflammation. Moreover, the sustained release of neuropeptides from these peripheral terminals is likely to mediate the development of neurogenic inflammation and associated immune responses associated with tissue injury.

The present invention therefore provides groundbreaking non-invasive methods to identifying agents for treating pain. Example 4 from U.S. provisional application Serial No. 60/407,025, filed August 30, 2002, and all references cited therein, are specifically incorporated herein by reference, particularly in regard to the U.S. national stage of the present PCT application, for purposes including describing addition studies that may be used in conjunction with the invention, including in vivo tests for refining information on analgesic function.

All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods, and in the steps or in the sequence of steps of the methods described herein, without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents that are both chemically and physiologically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims. REFERENCES

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Claims

CLAIMS:

1. A method for identifying a candidate analgesic substance, comprising:

(a) exposing a first tooth or tooth tissue sample that comprises peripheral neurons to a pain activating process or pain activating agent in vitro and to a candidate substance in vitro; and

(b) determining the ability of said candidate substance to inhibit pain signaling in said peripheral neurons, thereby identifying a candidate analgesic substance.

2. The method of claim 1, wherein said first tooth or tooth tissue sample is a substantially normal tooth or tooth tissue sample comprising substantially normal peripheral neurons.

3. The method of claim 1, wherein said first tooth or tooth tissue sample is a tooth or tooth tissue sample that was sensitized to pain in vivo and comprises pain-sensitized peripheral neurons.

4. The method of claim 3, wherein said first tooth or tooth tissue sample is a diseased, infected, inflamed, neuropathic or necrotic tooth or tooth tissue sample.

5. The method of claim 1, wherein said first tooth or tooth tissue sample is a rodent, porcine, bovine, canine or feline tooth or tooth tissue sample.

6. The method of claim 1 , wherein said first tooth or tooth tissue sample is a human tooth or tooth tissue sample.

7. The method of claim 1, wherein said first tooth or tooth tissue sample is a tooth extracted from a patient or a biopsy tissue sample thereof.

8. The method of claim 7, wherein said first tooth tissue sample is an apical portion of the root of a human tooth.

9. The method of claim 1, wherein said first tooth or tooth tissue sample is exposed to said pain activating process or pain activating agent and to said candidate substance substantially simultaneously.

10. The method of claim 1, wherein said first tooth or tooth tissue sample is exposed to said pain activating process or pain activating agent prior to said candidate substance.

11. The method of claim 1 , wherein said first tooth or tooth tissue sample is exposed to pain by application of a pain activating process.

12. The method of claim 11, wherein said pain activating process comprises the application of protons, heat, pressure or an electrical stimulus.

13. The method of claim 1, wherein said first tooth or tooth tissue sample is exposed to pain by provision of a pain activating agent.

14. The method of claim 13, wherein said pain activating agent is a trophic factor or an inflammatory mediator.

15. The method of claim 13, wherein said pain activating agent is capsaicin.

16. The method of claim 1, wherein the ability of said candidate substance to inhibit pain signaling in said peripheral neurons is determined by:

(a) individually measuring the level of pain signaling from the peripheral neurons in a control sample of said tooth or tooth tissue in the absence of said candidate substance;

(b) measuring the level of pain signaling from the peripheral neurons in an equivalent test sample of said tooth or tooth tissue in the presence of said candidate substance; and

(c) comparing the levels of pain signaling in said control and test samples, thereby quantifying the ability of said candidate substance to inhibit pain signaling in said peripheral neurons.

17. The method of claim 1, wherein said method comprises the steps of:

(a) establishing a control high level of pain signaling from the peripheral neurons in a control sample of said tooth or tooth tissue in the absence of said candidate substance;

(b) testing a panel of candidate substances by measuring the level of pain signaling from the peripheral neurons in a series of equivalent test samples of said tooth or tooth tissue separately contacted with members of the panel of candidate substances; and

18. The method of claim 1, wherein said pain signaling in said peripheral neurons is determined by measuring an electrochemical response from said peripheral neurons.

19. The method of claim 1, wherein said pain signaling in said peripheral neurons is determined by measuring the release of a pain-associated neurotransmitter from said peripheral neurons.

20. The method of claim 19, wherein said pain-associated neurotransmitter is calcitonin gene-related peptide or substance P.

21. The method of claim 19, wherein said pain-associated neurotransmitter is measured by an immunological assay.

22. The method of claim 1 , wherein said method comprises the steps of:

(a) establishing a control high level of pain signaling from the peripheral neurons in a first control sample of said tooth or tooth tissue in the absence of any analgesic substance;

(b) establishing a control low level of pain signaling from the peripheral neurons in a second control sample of said tooth or tooth tissue in the presence of a known analgesic substance;

(c) testing a panel of candidate substances by measuring the level of pain signaling from the peripheral neurons in a series of equivalent test samples of said tooth or tooth tissue separately contacted with members of the panel of candidate substances; and

(d) selecting from the panel tested in step (c) a candidate substance that reduces the level of pain signaling to substantially the control low level obtained in the presence of said known analgesic substance.

23. The method of claim 1, wherein said method comprises:

(a) applying a first sample of said candidate substance to a first substantially normal tooth or tooth tissue sample comprising substantially normal peripheral neurons;

(b) applying a second sample of said candidate substance to a second tooth or tooth tissue sample that was sensitized to pain in vivo and comprises pain-sensitized peripheral neurons; and

(c) determining the ability of said candidate substance to inhibit pain signaling in said peripheral neurons in said first and second tooth or tooth tissue samples, thereby identifying a candidate substance with analgesic activity in normal and pain-sensitized tissues.

24. The method of claim 1 , wherein said candidate substance is a candidate modulator of an ion channel, a metabotropic receptor or a peripheral neuron receptor.

25. The method of claim 24, wherein said candidate substance is a candidate antagonist of a capsaicin-sensitive receptor.

26. The method of claim 1, wherein said candidate substance is a candidate agonist of a neurotransmitter.

27. The method of claim 26, wherein said candidate substance is a candidate agonist of neuropeptide Y or a candidate opioid receptor agonist.

28. The method of claim 27, wherein said candidate substance is a candidate μ, δ or K opioid receptor agonist.

29. The method of claim 1, wherein said candidate substance is designed by computer- based modeling.

30. The method of claim 1, wherein said candidate substance is a member of a combinatorial chemistry library or a member of a library of naturally occurring biological compounds.

31. The method of claim 1 , further comprising formulating the candidate substance with analgesic activity thereby identified in a pharmaceutically acceptable solution or vehicle.

32. A method for identifying a candidate substance with peripheral analgesic activity in pain-sensitized tissues, comprising:

(a) applying a pain activating process or pain activating agent and a candidate substance to the sensory nerve endings of a human tooth or tooth tissue sample in vitro, wherein said tooth or tooth tissue sample is obtained from a patient sensitized to pain in vivo, has inflamed or necrotic pulp or root tissue and comprises pain-sensitized human peripheral neurons; and

(b) determining the ability of said candidate substance to inhibit pain signaling in said pain-sensitized human peripheral neurons, thereby identifying a candidate substance with peripheral analgesic activity in pain-sensitized tissues.

33. A kit for identifying an analgesic agent, comprising:

(a) a tissue culture medium that maintains peripheral neuron function in tooth or tooth tissue samples in vitro;

(b) at least a first in vitro diagnostic component that detects a biological pain signal from a peripheral neuron; and (c) instructions for correlating the ability of a test substance to inhibit pain signaling in peripheral neurons in a tooth or tooth tissue sample in vitro with standardized values for known analgesic substances, thereby allowing the identification of a new analgesic agent.

34. The kit of claim 33, wherein said kit comprises all components of an in vitro diagnostic system for detecting a biological pain signal from a peripheral neuron.

35. The kit of claim 33, wherein said kit further comprises at least a first tissue culture plate, dish, apparatus or system.

36. The kit of claim 33, wherein said kit further comprises at least a first pain activating agent.

37. The kit of claim 33, wherein said kit further comprises at least a first known analgesic substance.

38. A method for analyzing whether a test substance will cause pain when administered to a human, comprising:

(a) exposing a first human tooth or tooth tissue sample that comprises peripheral neurons to a pain activating process or pain activating agent in vitro and to said test substance in vitro; and

(b) assessing the effect of said test substance on pain signaling by said peripheral neurons, wherein a test substance that increases said pain signaling is indicative of a test substance that will cause pain when administered to a human.

39. The method of claim 38, wherein a test substance that significantly increases said pain signaling is indicative of a test substance that is not widely suitable for human administration.

40. The method of claim 38, wherein said test substance is a candidate pharmaceutical carrier, pharmaceutical diluent or component of an injectable formulation.

41. The method of claim 38, wherein said test substance is a candidate modified attenuated version of known irritant or pain-inducing substance.

42. The method of claim 38, wherein said test substance is a candidate substance for human ingestion or topical administration.

43. The method of claim 38, wherein said test substance is a candidate therapeutic agent, food substance or cosmetic substance.

44. The method of claim 38, wherein said test substance is a suspected environmental toxin or pathogenic agent.

45. A kit for testing a substance for human consumption, comprising:

(b) at least a first in vitro diagnostic component that detects a biological pain signal from a peripheral neuron; and (c) instructions for comparing the ability of a test substance to alter pain signaling in peripheral neurons in a tooth or tooth tissue sample in vitro to a control value in the absence of said test substance, thereby quantifying the pain-inducing or pain-relieving properties of said test substance.