WO1999025416A1 - Method for creating and rupturing atherosclerotic plaques - Google Patents

Abstract

The present invention provides a method for creating and rupturing an atherosclerotic plaque in vivo, and thus provides an animal model for atherosclerotic plaque rupture. The present invention also provides a method of screening chemical compounds for their ability to inhibit atherosclerotic plaque rupture or ameliorate the effects of atherosclerotic plaque rupture.

Description

METHOD FOR CREATING AND RUPTURING

ATHEROSCLEROTIC PLAQUES

FIELD OF THE INVENTION

The present invention provides a method for creating and rupturing atherosclerotic plaques. The present invention also provides a method of screening chemical compounds for their ability to inhibit atherosclerotic plaque rupture or ameliorate the effects of atherosclerotic plaque rupture.

BACKGROUND OF THE INVENTION

Cells in every organ and tissue need oxygen and nutrients to support life. Blood delivers these ingredients to and removes waste from the cells. The heart pumps blood toward peripheral tissues through a system of tubes, named arteries. Arteries are made up of cells (endothelial cells lining the lumen of a tube, and smooth muscle cells making up the bulk of the arterial wall) and extracellular fibers. Normally, arteries are wide open, and blood does not clog them because special properties of the endothelial surface prevents clot formation.

Atherosclerosis is a disease characterized by formation of plaques on the luminal surface of arteries. Plaques comprise a central soft core and a fibrous cap, which separates the core from the blood. The core contains fat and remnants of dead cells. The core is highly thrombogenic; that is, the core induces clot formation when in direct contact with blood. The fibrous cap comprises cells and extracelluar fibers. Cells comprising the bulk of the fibrous tissue are arterial wall derived smooth muscle cells and blood-born macrophages. Endothelial cells cover the plaque's luminal surface.

Plaques extend into the arterial lumen and restrict blood delivery to peripheral tissues. However, when plaques are mechanically stable, the restriction does not usually pose any life threat because the arteries are not completely blocked and blood circulation is sufficient to maintain viability of peripheral tissues. When plaques become unstable, the fibrous cap can rupture and allow direct contact between the highly thrombogenic core material and the blood, which leads to clot formation. Such a clot can totally occlude the artery and halt blood supply to the peripheral tissues. The occlusion of arteries results in deprivation of oxygen and nutrients .and eventually leads to cells death. If this happens in the arteries that supply the heart, a heart attack occurs. If this happens in the arteries that supply the brain, a stroke occurs. Thus, plaque rupture is a key event in the life-threatening process of atherosclerosis.

Therapy preventing plaque rupture can significantly decrease the incidence of heart attack and stroke, while treatment reducing blood clot formation associated with an already occurred rupture can reduce the mortality after such catastrophic events. Currently, there is no specific therapy for plaque rupture. One of the main reasons that no suitable therapy has been identified is that there is a lack of animal models for plaque rupture. The present invention provides an animal model for plaque rupture in which a plaque can be formed and ruptured at will and under controlled conditions that allow for the investigation of procedures and therapies that will ameliorate the undesired effects of plaque rupture.

SUMMARY OF THE INVENTION

The present invention provides a method for creating and rupturing atherosclerotic plaques in an animal, the method comprising: a. providing a catheter having an expandable region and a means for expanding the expandable region; b. substantially covering the expandable region of the catheter with a plaque-forming substrate; c. contacting the covered expandable region of the catheter with the luminal wall of a blood vessel of an animal; d. allowing sufficient time for the attached, covered expandable region of the catheter to induce formation of an atherosclerotic plaque, whereby the expandable region of the catheter is substantially enclosed within the plaque; and e. expanding the expandable region of the catheter to rupture the plaque. In a preferred embodiment of the invention, the plaque-forming substrate is a portion of a blood vessel.

In another preferred embodiment of the invention, the energy required to rupture the plaque is measured. In another embodiment, the present invention provides a method of screening chemical compounds for their ability to inhibit atherosclerotic plaque rupture or ameliorate the effects of atherosclerotic plaque rupture, the method comprising: a. providing a catheter having an expandable region and a means for expanding the expandable region; b. substantially covering the expandable region of the catheter with a plaque-forming substrate; c. contacting the covered expandable region of the catheter to the luminal wall of a blood vessel of an animal; d. allowing sufficient time for the contacted, covered expandable region of the catheter to induce formation of an atherosclerotic plaque, whereby the expandable region of the catheter is substantially enclosed within the plaque; e. administering to the animal an amount of a chemical compound to be screened; and f. rupturing the atherosclerotic plaque and observing the results.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for creating and rupturing atherosclerotic plaques in vivo, and thus provides an animal model for atherosclerotic plaque rupture. The method comprises the steps of providing a catheter having an expandable region and a means for expanding the expandable region; contacting the expandable region of the catheter to the luminal wall of a blood vessel of an animal; allowing sufficient time for the contacted expandable region of the catheter to form an atherosclerotic plaque around it, whereby the expandable region of the catheter is substantially enclosed within the plaque; and expanding the expandable region of the catheter to rupture the plaque.

The catheter may be selected from those catheters readily available to those skilled in the art. In particular, the catheter should be designed to be easily inserted into a blood vessel of an animal.

The catheter contains an expandable region. The expandable region is typically located at one end of the catheter and is inserted into the blood vessel and is located adjacent to and in contact with the luminal wall of a blood vessel. In a preferred embodiment, the expandable region of the catheter is a balloon. The catheter also contains or is associated with a means for expanding the expandable region.

In general, a catheter is inserted into a blood vessel such that a portion of the catheter remains outside the animal's body. This portion typically contains a means for expanding the expandable region. For example, when the expandable region contains a balloon, the balloon can include a small tube that can transport a gas to inflate the balloon. Alternatively, a plunger can be used to increase the pressure in a tube connected to the expandable region causing the region to expand. The expandable region of the catheter can also comprise devices besides balloons. For example, the expandable region may consist of a plurality of jaws which can be opened using, for example, a guide wire.

The catheter containing the expandable region is inserted into a blood vessel of an animal. As used herein, the term "animal" includes humans as well as other animals such as rabbits, dogs, sheep, rats, pigs, and the like. The expandable region of the catheter is positioned in the blood vessel so that it contacts the luminal surface of the blood vessel. The contact of the catheter and the luminal surface allows for the formation of an atherosclerotic plaque around the catheter, and in particular around the expandable region. If the catheter does not contact the luminal surface of a blood vessel, then an atherosclerotic plaque is not formed around the catheter. One way of insuring that the expandable region of the catheter contacts the luminal surface of the blood vessel is to design the catheter to have a C-shape. The expandable region is then situated on the top or bottom of the C-shaped catheter. The width of the C-shaped portion of the catheter is slightly larger that the diameter of the blood vessel so that the expandable region is firmly pressed against the vessel wall. The catheter can also be designed to have other shapes that provide for contact between the expandable region and the vessel wall. For example, the catheter can have a coil, or can be S-shaped.

To speed the formation of atherosclerotic plaque formation, the luminal wall of the blood vessel that is to receive the catheter containing an expandable region may be physically disrupted. For example, when a balloon catheter is inflated inside a blood vessel and then dragged along the vessel for a distance, some of the luminal cells are disturbed. This disturbance results in the growth of luminal plaques. In general, a vessel is more likely to develop atherosclerotic plaques if the luminal cells have been damaged. Other ways of damaging the luminal cells of a blood vessel to promote plaque formation are known to those skilled in the .art, and include but are not limited to, infusion of distilled water, air or nitrogen; application of an electric current; changes in temperature; and application of chemical agents such as iron chloride or silver nitrate. Once the expandable region of the catheter is contacted with the luminal wall of the blood vessel, a period of time must be allowed for the atherosclerotic plaque to be formed. Preferably, the atherosclerotic plaque that forms surrounds the expandable region of the catheter. The time necessary for the formation of an atherosclerotic plaque can be easily determined for a particular species of animal by inspecting the implanted catheter at various times.

Once the atherosclerotic plaque has formed around the expandable region of the catheter, the expandable region can be expanded using the means for expanding the expandable region. When the expandable region is expanded, the plaque ruptures. The amount of energy required to rupture the plaque can be measured. For example, a pressure transducer can be connected to a strip chart recorder, and the pressure of the gas or liquid used to inflate the balloon in a balloon catheter can be measured. Other methods of measuring the amount of energy required to rupture the plaque can also be used.

In a preferred embodiment of the invention, the expandable region of the catheter may be covered with a plaque-forming substrate that is then contacted with the luminal wall of the blood vessel. A plaque-forming substrate is any material that promotes the formation of an atherosclerotic plaque or speeds the formation of an atherosclerotic plaque. A preferred plaque-forming substrate is a portion of a blood vessel. Other plaque-forming substrates include, but are not limited to, collagen, laminin, fibronectin coating, or cotton threads loosely arranged around the expandable region. The present invention also provides a method for screening chemical compounds for their ability to inhibit plaque rupture or ameliorate the effects of atherosclerotic plaque rupture. The method comprises creating an atherosclerotic plaque as described above, administering to the animal in which the atherosclerotic plaque has been formed an amount of a chemical compound to be screened, and then rupturing the plaque and observing the results. The chemical compound may be any chemical compound that is expected to inhibit plaque rupture or ameliorate an effect of plaque rupture, including but not limited to, compounds used in the treatment of thrombosis, stroke, or heart attack. Examples of suitable compounds to screen include, but are not limited to, matrix metalloproteinase inhibitors, lipid lowering agents, and anti-inflammatory compounds.

The examples set forth below are intended to illustrate particular embodiments of the invention and are not intended to limit the scope of the specification, including the claims, in any manner.

EXAMPLES

Male New Zealand white rabbits were meal fed a rabbit chow supplemented with 0.5% cholesterol, 3% peanut oil, and 3% coconut oil. The diet started 1 week prior to surgery and continued until the end of the experiment. Rabbits were anesthetized, a skin incision was made on the neck, and the left jugular vein was surgically exposed. The vein segment was isolated by two ties. This segment was frozen using a metal applicator cooled in liquid nitrogen. The vein was then allowed to thaw. An indwelling catheter was inserted into the isolated vein segment via a small incision. A catheter was advanced so that the vein segment covered the balloon. The frozen vein segment was then attached to the catheter by two ties and separated from the rest of the vein. As a result, a vein-derived fibrous bag was sealed over the expandable balloon. Via the same neck incision, the left common carotid artery was surgically exposed. A cerebral catheter (4. IF cerebral catheter) was inserted into the carotid artery and advanced under fluoroscopic guidance to a position in the descending thoracic aorta. A guide wire (0.014 inch SCEPTOR™ "EnTre-style") was passed through the cerebral catheter into the descending thoracic aorta. The cerebral catheter was then removed leaving the guide wire in place. An embolectomy catheter (4F) was introduced over the guide wire with the tip placed at the level of the diaphragm. The balloon was inflated and then pulled back to remove endothelial cells in the thoracic aorta. This process was repeated several times, and the catheter was removed.

A catheter with a vein-covered balloon was placed into the injured segment of thoracic aorta over the guide wire. The wire was removed, the ports of the catheter were implanted in the subcutaneous pocket on the neck, and the incision was closed.

Four weeks after catheter placement, the balloon was firmly attached to the aortic wall and completely plaque covered. Rabbits were anesthetized and perfused with saline via abdominal aorta to remove blood. Thoracic aorta was opened longitudinally, and the segment of the aorta containing the plaque was excised as one block with attached catheter. The aortic segment was pinned to a cork board so that the plaque was fully exposed en face. The catheter was attached to an inflation device. Air pressure within the plaque-covered balloon was measured in pounds per square inch using a PSI test gauge and recorded using an electronic pressure transducer, and a strip chart recorder. Digital images of the plaque were obtained using a video camera attached to a frame grabber and computer. A laser video disk recorder that collects 30 video frames a second was used to record balloon inflation. Using a hand-held inflation device, the pressure within the intraplaque balloon was gradually increased until the plaque ruptured.

The moment of visible plaque rupture was coordinated with a sudden drop of pressure on a recorded pressure-time curve. This pressure is considered the rupturing pressure and is viewed as an index of the overall mechanical strength of the plaque tissue.

In addition, an intraplaque balloon was inflated in vivo and rabbits were sacrificed 3 hours after. Macroscopic red thrombi associated with the sites of plaque rupture were detected. The size and makeup of the thrombi can be evaluated.

Claims

CLAIMSWhat is claimed is:

1. A method for creating and rupturing atherosclerotic plaques in an animal, the method comprising: a. providing a catheter having an expandable region and a means for expanding the expandable region; b. substantially covering the expandable region of the catheter with a plaque-forming substrate; c. contacting the covered expandable region of the catheter with the luminal wall of a blood vessel of an animal; d. allowing sufficient time for the contacted, covered expandable region of the catheter to form .an atherosclerotic plaque, whereby the expandable region of the catheter is substantially enclosed within the plaque; and e. expanding the expandable region of the catheter to rupture the plaque.

2. The method of Claim 1 wherein the animal is a rabbit.

3. The method of Claim 1 wherein the expandable region contains a balloon.

4. The method of Claim 1 wherein the plaque-forming substrate is a portion of a blood vessel.

5. The method of Claim 1 wherein the energy required to rupture the plaque is measured.

6. A method of screening chemical compounds for their ability to inhibit atherosclerotic plaque rupture or ameliorate the effects of atherosclerotic plaque rupture, the method comprising: a. providing a catheter having an expandable region and a means for expanding the expandable region; b. substantially covering the expandable region of the catheter with a plaque-forming substrate; c. contacting the covered expandable region of the catheter with the luminal wall of a blood vessel of an animal; d. allowing sufficient time for the contacted, covered expandable region of the catheter to form an atherosclerotic plaque, whereby the expandable region of the catheter is substantially enclosed within the plaque; e. administering to the animal an amount of a chemical compound to be screened; and f. rupturing the atherosclerotic plaque and observing the results.