WO2008045998A2

WO2008045998A2 - Enantiomerically enriched iminopyrrolidone aziridine compositions

Info

Publication number: WO2008045998A2
Application number: PCT/US2007/081085
Authority: WO
Inventors: Philip J.A. Kuehl; William A. Remers
Original assignee: The Arizona Board Of Regents On Behalf Of The University Of Arizona; Amplimed Corporation
Priority date: 2006-10-12
Filing date: 2007-10-11
Publication date: 2008-04-17
Also published as: WO2008045686A2; WO2008045686A3; WO2008045998A3

Abstract

Enantiomerically enriched iminopyrrolidone aziridine compositions are disclosed herein. Embodiments of the present invention include enantiomerically enriched compositions, aqueous solutions of, and methods of increasing the aqueous solubility of, enantiomerically enriched compositions, and methods of formulating drug products comprising enantiomerically enriched iminopyrrolidone aziridine compounds for use in the treatment of cancer.

Description

ENANTIOMERICALLY ENRICHED IMINOPYRROLIDONE AZIRIDINE COMPOSITIONS

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. provisional patent application number 60/851,358, filed October 12, 2006, the disclosure of which is incorporated herein by reference in its entirety for all purposes.

FIELD OF THE INVENTION

[0002] This disclosure relates generally to enantiomerically enriched iminopyrrolidone aziridine compositions, and more particularly, but not exclusively, to enantiomerically enriched iminopyrrolidone aziridine compositions having improved aqueous solubility compared to racemic mixtures, and to methods of drug product formulation utilizing enantiomerically enriched or enantiomerically pure iminopyrrolidone aziridine compositions.

BACKGROUND INFORMATION

[0003] Cancer is a leading cause of death in humans and animals, and as such, considerable resources are invested in the discovery and clinical investigation of therapies that may be effective in treating the many forms of the disease. The challenges of investigating potentially life-saving therapies are frequently compounded by difficulties associated with the formulation of the active pharmaceutical ingredient into a stable dosage form suitable for both storage and administration to patients. Although various dosage forms of chemotherapeutic agents are marketed for use in the treatment of cancer, antineoplastic drugs are commonly administered intravenously. This route of administration is often chosen to overcome disadvantageous bioavailability profiles associated with other routes of administration (e.g., oral), but the use of intravenous dosing also creates unique formulation challenges in the manufacture of the drug product.

[0004] Imexon ((5AS>4 amino- 1 ,3-diazabicyclo[3.1.0]hex-3-en-2-one) is an iminopyrrolidone aziridine compound that has been investigated extensively for its therapeutic effectiveness in the treatment of many types of cancers. Previously published studies have shown that imexon possesses cytotoxic activity against a variety of fresh human tumors in vitro, including breast cancer, colon cancer, kidney cancer, lung cancer, lymphoma, melanoma, myeloma, ovarian cancer, pancreas cancer, and acute promyelocytic leukemia. Hersh et al., J Natl Cancer Inst. 84: 1238-1244 (1992); Salmon et al, J Natl Cancer Inst. 86: 228-230 (1994); Dvorakova et al, Blood 97: 3544-3551 (2001); and Dorr et al., MJ Gastrointest Cancer 36: 15-28. In addition, the antitumor effect of imexon has been shown in vivo on the development of large cell lymphoma in SCID mice (Hersh et al., J Immunother 13: 77-83 (1993)), in P-388 and L-1210 leukemia in mice, in mast cell tumors in dogs (Dorr et al., Invest New Drugs 13: 113-116 (1995)), and in a variety of hematological and solid tumors in SCDD mice bearing human tumor xenografts, including myeloid leukemia, myeloma, promyelocytic leukemia, lymphoma, breast cancer, ovarian cancer, non-small cell lung cancer, melanoma, fibrosarcoma, and prostate cancer. Pourpak et al., Anticancer Drugs, 17(10): 1179-1184 (2006). Moreover, in limited human studies conducted with imexon in the 1970s, a complete remission was observed in one patient with metastatic non-small cell lung cancer, and stable disease was achieved in several patients with breast cancer, non-small cell lung cancer, and liver cancer. Sagaster et al., J Natl Cancer Inst. 87: 935-936 (1995).

[0005] Imexon continues to be the focus of clinical investigations for the treatment of various forms of cancer, including malignant melanoma, pancreatic adenocarcinoma, breast cancer, non-small cell lung cancer, prostate cancer, multiple myeloma, and lymphoma. As is common with many cancer chemotherapeutic agents, the clinical investigation of imexon has employed an intravenous method of administration to treat patients in various clinical studies. As such, the use of imexon in clinical practice relies on the production of a sterile dosage form for administration to patients.

[0006] The production of sterile dosage forms for parenteral administration is well known in the pharmaceutical industry, and several sterilization processes are described in detail in the literature. However, because of stability concerns associated with thermal sterilization, as well as with the storage of a ready-to-use imexon formulation, a lyophilized dosage form has been regarded as the most desirable form of an imexon drug product. Lyophilization is often used in the formulation of pharmaceutical products that are thermal-labile and that would otherwise be susceptible to physical and/or chemical degradation when stored as a ready-to- use formulation. The lyophilization process may summarily be described as a "freeze- drying" process, in which a pharmaceutical agent is dissolved with or without excipients in a suitable solvent, and then sterilized by passing the bulk solution through a bacteria-retentive filter. Following sterilization, the solution is then filled into individual sterile containers and frozen in a freeze-drying chamber. Finally, application of a vacuum to the freeze-drying chamber results in sublimation (primary drying) and desorption (secondary drying) of the solvent from the individual containers, which are then sealed to maintain sterility of the drug product.

[0007] The lyophilization process is time intensive and may take several days to complete, even under optimized conditions. The stability of the drug during the lyophilization process and the duration of the freeze-drying cycle are two major considerations impacting the time and manufacturing expense associated with the production of a lyophilized drug product. Both the freezing time and the primary drying time, which consumes the largest fraction of the freeze-drying cycle time, are directly related to the fill volume, i.e., the quantity of solution delivered into each individual container to achieve the desired dose in the final drug product. Complete freezing of the drug product solution requires significant time, and the larger the fill volume, the longer it takes to fully freeze. Similarly, the primary drying time is directly related to the ice sublimation rate, which is determined by numerous factors, including fill volume. Xiaolin et ύ., Pharmaceutical Research 21: 191-200 (2004). The most commonly-used solvent for the lyophilization of pharmaceutical agents intended for parenteral administration is sterile water for injection. The solubility of racemic imexon in sterile water is limited to about 23.6 mg/ml ± 0.5 mg/ml (Den Brok et al., J Pharm Sci 94: 1101-1114 (2005)), and the aqueous stability of imexon is of only limited duration. The present invention improves the solubility of iminopyrrolidone aziridine compounds such as imexon.

SUMMARY OF THE INVENTION

[0008] The present invention is directed to enantiomerically enriched compositions, aqueous solutions of enantiomerically enriched compositions, and to methods of increasing the aqueous solubility of iminopyrrolidone aziridine compounds and of formulating drug products comprising iminopyrrolidone aziridine compositions of the present invention for use in the treatment of cancer. Surprisingly, it has been discovered that enantiomerically enriched iminopyrrolidone aziridine compounds in accordance with the present invention exhibit dramatically enhanced aqueous solubility characteristics, as compared to racemic mixtures of the compounds that are not enantiomerically enriched or enantiomerically pure.

[0009] In one aspect, the present invention provides a composition including an enantiomeric excess of an enantiomer of a compound of the formula:

[0010] In Formula (I), R¹ and R² are independently selected from hydrogen, or substituted or unsubstituted (C₁-Ci_O) alkyl or cycloalkyl. In one embodiment, the composition has an aqueous solubility in excess of 40 mg/ml. In another embodiment, the composition comprises either an (i?)-stereoisomer or an (^-stereoisomer of the compound of Formula (I) in at least about 60% enantiomeric excess. In still another embodiment, the (i?)-stereoisomer or the (^-stereoisomer of the compound of Formula (I) is in enantiomeric purity.

[0011] In another aspect, the present invention provides an aqueous solution comprising a composition having an enantiomeric excess of an enantiomer of a compound of Formula (I) in which R and R comprise substitutents as described hereinbefore, and wherein the composition is soluble to a concentration in excess of 40 mg/ml. In one embodiment, the composition may comprise either an (i?)-stereoisomer or an (^-stereoisomer of the compound of Formula (I) in at least about 60% enantiomeric excess. In another embodiment, the (i?)-stereoisomer or the (^-stereoisomer of the compound of Formula (I) may be enantiomerically pure.

[0012] In yet another aspect, the present invention provides a method of formulating a drug product comprising a drug having at least about a 60% enantiomeric excess of an enantiomer of a compound of Formula (I) in which R¹ and R² are as described hereinbefore. In one embodiment, the method comprises dissolving the drug in an aqueous solvent to form a solution having a drug concentration in excess of 40 mg/ml, sterilizing the solution by passing it through a bacteria-retentive filter, filling the solution into one or more individual containers, freezing the solution in a freeze-drying chamber, and applying a vacuum to the chamber to remove substantially all of the aqueous solvent from the one or more individual containers. In one embodiment, the drug comprises either an (i?)-stereoisomer or an (S)- stereoisomer of the compound of Formula (I), and may, in an embodiment, comprise one stereoisomer in enantiomeric purity.

[0013] In still another aspect, the present invention is directed to a method of increasing the solubility of imexon by contacting a composition containing at least about a 60% enantiomeric excess of one imexon enantiomer with an aqueous solvent to yield a solution having an imexon concentration in excess of 40 mg/ml. In one embodiment, the imexon enantiomer in enantiomeric excess is (5i?)-4-amino-l,3-diazabicyelo[3.1.0]hex-3-en-2-one. In another embodiment, the imexon enantiomer in enantiomeric excess is (5ιS)-4-amino-l,3- diazabicyelo[3.1.0]hex-3-en-2-one.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0014] Embodiments of compositions, aqueous solutions, and methods of improving the aqueous solubility of, or of formulating drug products comprising, iminopyrrolidone aziridine compounds having an enantiomeric excess of either an (R)- or an (^-stereoisomer of a compound of Formula (I) are disclosed herein. In the following description, numerous specific details are provided, such as the identification of various components and structures, to provide a thorough understanding of embodiments of the invention. One skilled in the art will recognize however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, and the like, hi still other instances, well-known components, materials, or processes are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention.

[0015] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, component, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, components, or characteristics may be combined in any suitable manner in one or more embodiments.

I. Definitions

[0016] As used herein, "imexon" refers to an unsubstituted (5i?5)-4-amino-l ,3- diazabicyclo[3.1.0]hex-3-en-2-one, or a pharmaceutically acceptable salt or a solvate thereof.

[0017] As used herein, the term "enantiomer" refers individually to the (R)- or the (S)- stereoisomer of a compound having the formula:

with reference to the chiral center indicated by the dashed circle. As will be appreciated, other chiral centers may exist in the R-substituents of compounds in embodiments of the present invention, but all such stereoisomers shall, unless otherwise indicated, be encompassed by the term "enantiomer" with reference to the hereinbefore-identified chiral center, whether or not the absolute configuration of the chiral center is identified.

[0018] As used herein, the term "enantiomeric excess" is defined as |F(+) — F(-)|, and may be expressed as a percentage by multiplying |F(+) - F(-)| by 100. F(+) and F(-) represent mole or weight fractions of (+)- and (-)-enantiomers. The terms "enantiomerically pure" and "enantiomeric purity" refer to a compound that is present in 100% enantiomeric excess. The term "enantiomerically enriched" refers to a composition comprising one enantiomer in greater than 50% enantiomeric excess. Indications of enantiomeric enrichment expressed as a percentage, or indications of enantiomeric purity, as used herein, are intended to encompass a narrow range corresponding to variability resulting from experimental uncertainty.

[0019] As used herein, the term "alkyl," by itself or as part of another substituent, means, unless otherwise stated, a straight (i.e., unbranched) or branched chain hydrocarbon radical which may be fully saturated, mono- or polyunsaturated and can include di- and multivalent radicals, having the number of carbon atoms designated {e.g., Ci-C_1O means one to ten carbons). Examples of saturated hydrocarbon radicals include, but are not limited to, groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, t-butyl, isobutyl, sec-butyl, homologs and isomers of, for example, n-pentyl, n-hexyl, n-heptyl, n-octyl, and the like. An unsaturated alkyl group is one having one or more double bonds or triple bonds. Examples of unsaturated alkyl groups include, but are not limited to, vinyl, 2-propenyl, crotyl, 2-isopentenyl, 2- (butadienyl), 2,4-pentadienyl, 3-(l,4-pentadienyl), ethynyl, 1- and 3-propynyl, 3-butynyl, and the higher homologs and isomers. Similarly, the term "alkylene" by itself or as part of another substituent means a divalent radical derived from alkyl, as exemplified, but not limited by, methylene (-CH₂-), ethylene (-CH₂-CH₂-), propylene (-CH₂-CH₂-CH₂-), and isopropylene (-CH₂(CHs)-CH₂-).

[0020] The term "heteroalkyl," by itself or in combination with another term, means, unless otherwise stated, a stable straight or branched chain hydrocarbon radical consisting of at least one carbon atom and at least one heteroatom selected from the group consisting of O, N and S, and wherein the nitrogen and sulfur atoms may optionally be oxidized and the nitrogen heteroatom may optionally be quaternized. The heteroatom(s) O, N and S may be placed at any interior position of the heteroalkyl group or at the position at which the heteroalkyl group is attached to the remainder of the molecule. Examples include, but are not limited to, -CH₂- CH₂-O-CH₃, -CH₂-CH₂-NH-CH₃, -CH₂-CH₂-N(CHs)-CH₃, -CH₂-S-CH₂-CH₃, -CH₂-CH₂,- S(O)-CH₃, -CH₂-CH₂-S(O)₂-CH₃, -CH=CH-O-CH₃, -CH₂-CH=N-OCH₃, -CH=CH-N(CH₃)- CH₃, -0-CH₃, -0-CH₂-CH₃, -NH-CH₂-OH, -CH(OH)-CH₃, -C(O)-CH₂-OH, -C(O)-CH₂-O- C(O)-CH₂-CH₃, -O-C(O)-C(CH₃)₃, and -0-C(O)-CH₂-CH₃. Up to three heteroatoms maybe consecutive, such as, for example, -CH₂-NH-OCH₃ and -N=N-N(CH₃)₂. Similarly, the term "heteroalkyl ene" by itself or as part of another substituent means a divalent radical derived from heteroalkyl, as exemplified, but not limited by, -CH₂-CH₂-S-CH₂-CH₂- and -CH₂-S- CH₂-CH₂-NH-CH₂-. For heteroalkylene groups, heteroatoms can also occupy either or both of the chain termini (e.g., alkyleneoxo, alkylenedioxo, alkyleneamino, alkylenediamino, and the like). Still further, for alkylene and heteroalkylene linking groups, no orientation of the linking group is implied by the direction in which the formula of the linking group is written. For example, the formula -C(O)OR'- represents both -C(O)OR'- and -R⁵OC(O)-. Where "heteroalkyl" is recited, followed by recitations of specific heteroalkyl groups, such as - NR'R" or the like, it will be understood that the terms heteroalkyl and -NR⁵R" are not redundant or mutually exclusive. Rather, the specific heteroalkyl groups are recited to add clarity. Thus, the term "heteroalkyl⁵⁵ should not be interpreted herein as excluding specific heteroalkyl groups, such as -NR⁵R⁵' or the like.

[0021] The terms "cycloalkyl" and "heterocycloalkyl", by themselves or in combination with other terms, represent, unless otherwise stated, cyclic versions of "alkyl⁵⁵ and "heteroalkyl⁵⁵, respectively. Additionally, for heterocycloalkyl, a heteroatom can occupy the position at which the heterocycle is attached to the remainder of the molecule. Examples of cycloalkyl include, but are not limited to, cyclopropymethyl, cyclopentyl, cyclohexyl, cyclohexylmethyl, 1-cyclohexenyl, 3-cyclohexenyl, cycloheptyl, and the like. Examples of heterocycloalkyl include, but are not limited to, 1-piperidinyl, 2-piperidinyl, 3-piperidinyl, A- morpholinyl, 3-morpholinyl, tetrahydrofuran-2-yl, tetrahydrofuran-3-yl, tetrahydrothien-2-yl, tetrahydrothien-3-yl, 1 -piperazinyl, 2-piperazinyl 1-pyrrolidinyl, 2-pyrrolidinyl, and the like.

[0022] The terms "halo⁵⁵ or "halogen,⁵⁵ by themselves or as part of another substituent, mean, unless otherwise stated, a fluorine, chlorine, bromine, or iodine atom. Additionally, terms such as "haloalkyl," are meant to include monohaloalkyl and polyhaloalkyl. For example, the term "halo(Ci-C₄)alkyl" is meant to include, but not be limited to, trifluoromethyl, 2,2,2 -trifluoroethyl, 4-chlorobutyl, 3-bromopropyl, and the like. [0023] The term "aryl" means, unless otherwise stated, a polyunsaturated, aromatic, hydrocarbon substituent which can be a single ring or multiple rings (preferably from 1 to 3 rings) which are fused together or linked covalently. The term "heteroaryl" refers to aryl groups (or rings) that contain from one to four heteroatoms selected from N, O, and S, wherein the nitrogen and sulfur atoms are optionally oxidized, and the nitrogen atom(s) are optionally quaternized. A heteroaryl group can be attached to the remainder of the molecule through a carbon or heteroatom. Non-limiting examples of aryl and heteroaryl groups include phenyl, 1-naphthyl, 2-naphthyl, 4-biphenyl, 1-pyrrolyl, 2-pyrrolyl, 3-pyrrolyl, 3- pyrazolyl, 2-imidazolyl, 4-imidazolyl, pyrazinyl, 2-oxazolyl, 4-oxazolyl, 2-phenyl-4- oxazolyl, 5-oxazolyl, 3-isoxazolyl, 4-isoxazolyl, 5-isoxazolyl, 2-thiazolyl, 4-thiazolyl, 5- thiazolyl, 2-furyl, 3-furyl, 2-thienyl, 3-thienyl, 2-pyridyl, 3-pyridyl, 4-pyridyl, 2-pyrimidyl, 4- pyrimidyl, 5-benzothiazolyl, purinyl, 2-benzimidazolyl, 5-indolyl, 1-isoquinolyl, 5- isoquinolyl, 2-quinoxalinyl, 5-quinoxalinyl, 3-quinolyl, and 6-quinolyl. Substituents for each of the above noted aryl and heteroaryl ring systems are selected from the group of acceptable substituents described below. The terms "arylene" and "heteroarylene" refer to the divalent derivatives of aryl and heteroaryl, respectively.

[0024] For brevity, the term "aryl" when used in combination with other terms (e.g., aryloxo, arylthioxo, arylalkyl) includes both aryl and heteroaryl rings as defined above. Thus, the term "arylalkyl" is meant to include those radicals in which an aryl group is attached to an alkyl group (e.g., benzyl, phenethyl, pyridylmethyl and the like) including those alkyl groups in which a carbon atom (e.g., a methylene group) has been replaced by, for example, an oxygen atom (e.g., phenoxymethyl, 2-pyridyloxymethyl, 3-(l- naphthyloxy)propyl, and the like). However, the term "haloaryl," as used herein, is meant to cover only aryls substituted with one or more halogens.

[0025] The term "oxo" as used herein means an oxygen that is double bonded to a carbon atom.

[0026] Each of above terms (e.g. , "alkyl," "heteroalkyl," "cycloalkyl," "heterocycloalkyl," "aryl," "heteroaryl," and "arylalkyl," as well as their divalent radical derivatives) are meant to include, unless otherwise stated, both substituted and unsubstituted forms of the indicated radical. Preferred substituents for each type of radical are provided hereinafter.

[0027] Substituents for alkyl, heteroalkyl, cycloalkyl, heterocycloalkyl, and monovalent and divalent derivative radicals (including those groups often referred to as alkylene, alkenyl, heteroalkylene, heteroalkenyl, alkynyl, cycloalkenyl, and heterocycloalkenyl) can be one or more of a variety of groups selected from, but not limited to: -OR', =0, =NR', =N-OR', - NR'R", -SR', -halogen, -OC(O)R', -C(O)R', -CO₂R' ,-C(O)NR⁵R", -OC(O)NR⁵R", - NR"C(0)R', -NR'-C(0)NR"R"', -NR"C(0)0R\ -NR-C(NR'R")=NR"', -S(O)R', -S(O)₂R', -S(O)₂NR⁵R", -NRSO₂R', -CN and -NO₂ in a number ranging from zero to (2m'+l), where m' is the total number of carbon atoms in such radical. R', R" and R⁵" each preferably independently refer to hydrogen, or Ci-C₆ alkyl, cycloalkyl, or haloalkyl. Unless otherwise stated, when a compound of the invention includes more than one R group, for example, each of the R groups is independently selected as are each R', R" and R'" groups when more than one of these groups is present.

[0028] Similar to the substituents described for alkyl radicals above, exemplary substituents for aryl and heteroaryl groups ( as well as their divalent derivatives) are varied and are selected from, for example: -OR', -NR'R", -SR', -halogen, -OC(O)R', -C(O)R', -CO₂R', - C(O)NR⁵R", -OC(O)NR⁵R", -NR⁵⁵C(O)R⁵, -NR'-C(0)NR"R"\ -NR⁵⁵C(O)OR⁵, -NR- C(NR⁵R"R'")=NR"", -NR-C(NR'R")=NR"⁵, -S(O)R⁵, -S(O)₂R', -S(O)₂NR⁵R", -NRSO₂R', - CN and -NO₂, -R⁵, -N₃, -CH(Ph)₂, fluoro(Ci-C₄)alkoxo, and fluoro(Ci-C₄)alkyl, in a number ranging from zero to the total number of open valences on the aromatic ring system; and where R⁵, R⁵⁵, R⁵⁵⁵ and R"" are preferably independently selected from hydrogen, or Ci-Cβ alkyl, cycloalkyl, or haloalkyl. Unless otherwise stated, when a compound of the invention includes more than one R group, for example, each of the R groups is independently selected as are each R⁵, R⁵⁵, R⁵" and R"" groups when more than one of these groups is present.

[0029] As used herein, the term "heteroatom" or "ring heteroatom" is meant to include oxygen (O), nitrogen (N), and sulfur (S).

[0030] The compounds of the present invention may exist as salts. The present invention includes such salts. These salts may be prepared by methods known to those skilled in art. The term "pharmaceutically acceptable salts⁵⁵ is meant to include salts of active compounds which are prepared with relatively nontoxic acids or bases, depending on the particular substituent moieties found on the compounds described herein. When compounds of the present invention contain relatively acidic functionalities, base addition salts can be obtained by contacting the neutral form of such compounds with a sufficient amount of the desired base, either neat or in a suitable inert solvent. Examples of pharmaceutically acceptable base addition salts include sodium, potassium, calcium, ammonium, organic amino, or magnesium salt, or a similar salt. When compounds of the present invention contain relatively basic functionalities, acid addition salts can be obtained by contacting the neutral form of such compounds with a sufficient amount of the desired acid, either neat or in a suitable inert solvent. Examples of pharmaceutically acceptable acid addition salts include those derived from inorganic acids like hydrochloric, hydrobromic, nitric, carbonic, monohydrogencarbonic, phosphoric, monohydrogenphosphoric, dihydrogenphosphoric, sulfuric, monohydrogensulfuric, hydriodic, or phosphorous acids and the like, as well as the salts derived from relatively nontoxic organic acids like acetic, propionic, isobutyric, maleic, rnalonic, benzoic, succinic, suberic, fumaric, lactic, mandelic, phthalic, benzenesulfonic, p- tolylsulfonic, citric, tartaric, methanesulfonic, and the like. Also included are salts of amino acids such as arginate and the like, and salts of organic acids like glucuronic or galactunoric acids and the like (see, for example, Berge et al., "Pharmaceutical Salts", Journal of Pharmaceutical Science 66: 1-19 (1977)).

[0031] In addition to salt forms, the present invention provides compounds that are in a prodrug form. Prodrugs of the compounds described herein are those compounds that readily undergo chemical changes under physiological conditions to provide the compounds of the present invention. Additionally, prodrugs can be converted to the compounds of the present invention by chemical or biochemical methods in an ex vivo environment. For example, prodrugs can be slowly converted to the compounds of the present invention when placed in a transdermal patch reservoir with a suitable enzyme or chemical reagent.

[0032] The terms "a," "an," or "a(n)", when used in reference to a group of substituents herein, mean at least one. For example, where a compound is substituted with "an" alkyl or aryl, the compound is optionally substituted with at least one alkyl and/or at least one aryl. Moreover, where a moiety is substituted with an R substituent, the group maybe referred to as "R-substituted." Where a moiety is R-substituted, the moiety is substituted with at least one R substituent and each R substituent is optionally different.

II. Iminopyrrolidone Aziridine Compositions of the Present Invention

[0033] Embodiments of the present invention include enantiomerically enriched or enantiomerically pure compounds and compositions useful for the treatment of cancer, including carcinomas, sarcomas, melanomas, leukemias, and lymphomas. Isolated enantiomerically enriched or enantiomerically pure compounds of the present invention exhibit unexpectedly advantageous characteristics, including a markedly superior solubility profile in aqueous solutions, as compared to racemic mixtures of the compounds that are not enantiomerically enriched or enantiomerically pure.

[0034] In one aspect, the present invention provides a composition including an enantiomeric excess of an enantiomer of a compound of the formula:

[0035] In Formula (I), R¹ and R² are independently selected from hydrogen, or substituted or unsubstituted (Ci-Cio) alkyl or cycloalkyl. In one embodiment, the composition has an aqueous solubility in excess of 40 mg/ml. In another embodiment, the composition has an aqueous solubility in excess of 50 mg/ml. In still another embodiment, the composition has an aqueous solubility in excess of 60 mg/ml. In a preferred embodiment, R¹ and R² are hydrogen. It will be appreciated that certain compounds of the present invention may exist in tautomeric forms in which two or more structural isomers exist in equilibrium and are readily converted from one isomeric form to another. All such tautomeric forms of the compounds are intended to come within the scope of the present invention.

[0036] Enantiomerically enriched compositions of the present invention include enantiomers of a compound of Formula (I), one of which comprises, to varying degrees in different embodiments, a greater than 50% fraction (e.g., by weight) of the composition (i.e., one enantiomer is in enantiomeric excess). In other words, in some embodiments the (R)- stereoisomer of the compound is substantially free from the (^-stereoisomer of the compound and is, thus, in enantiomeric excess of the (5)-stereoisomer. In other embodiments, the (5)-stereoisomer of the compound is substantially free from the (R)- stereoisomer of the compound and is, thus, in enantiomeric excess of the (i?)-stereoisomer. In one embodiment of the invention, one enantiomer (either the (R)- or the (^-stereoisomer) is in at least about 60% enantiomeric excess. In other embodiments, one enantiomer (either the (R)- or the (5)-stereoisomer) is in at least about 80% enantiomeric excess. In other embodiments, one enantiomer (either the (R)- or the (^-stereoisomer) is in at least about 90% enantiomeric excess. In other embodiments, one enantiomer (either the (R)- or the (S)- stereoisomer) is in at least about 95% enantiomeric excess. In other embodiments, one enantiomer (either the (R)- or the (^-stereoisomer) is in at least about 99% enantiomeric excess. In still other embodiments, compositions of the present invention comprise one enantiomer (either the (R)- or the (5)-stereoisomer) in enantiomeric purity.

[0037] In one embodiment, the enantiomer present in enantiomeric excess in a composition in accordance with the present invention is an (i?)-iminopyrrolidone aziridine compound having the formula:

[0038] In Formula (II), R¹ and R² are independently selected from hydrogen, or substituted or unsubstituted (Ci-C_1O) alkyl or cycloalkyl. In one embodiment, the substituents R¹ and R² have no chiral centers within them. In another embodiment, the (i?)-irninopyrrolidone aziridine is in at least from about 60% to about 99% enantiomeric excess. In still another embodiment, the (i?)-iminopyrrolidone aziridine is substantially enantiomerically pure. In a preferred embodiment, R¹ and R² are hydrogen.

[0039] In another embodiment, the enantiomer present in enantiomeric excess in a composition in accordance with the present invention is an (5)-iminopyrrolidone aziridine compound having the formula:

[0040] In Formula (III), R¹ and R² are independently selected from hydrogen, or substituted or unsubstituted (C₁-Ci_O) alkyl or cycloalkyl. In one embodiment, the substituents R¹ and R² have no chiral centers within them. In another embodiment, the (5)-iminopyrrolidone aziridine is in at least from about 60% to about 99% enantiomeric excess, hi still another embodiment, the (ιS)-iminopyrrolidone aziridine is substantially enantiomerically pure. In a preferred embodiment, R¹ and R² are hydrogen.

A. Measuring Enantiomeric Enrichment or Enantiomeric Purity

[0041] Enantiomeric enrichment or enantiomeric purity in accordance with the present invention can be measured via resolution and detection of the enantiomeric components of a composition using chiral HPLC separation techniques. See, e.g., Okamoto et al., J Chromatography A 666: 403-491 (1994); and Kunath et al., J Chromatography A 728: 249- 257 (1996). In a particularly useful chiral HPLC resolution method for measuring enantiomeric enrichment or enantiomeric purity, iminopyrrolidone aziridine enantiomers in accordance with embodiments of the present invention can be resolved using an Astec Chirobiotic T, teicoplanin column (4.6 x 250 mm; 5μ pore size) with 60% methanol:40% 2- propanol (isocratic, but from separate reservoirs) as solvent. The resolved enantiomers can be detected using a photodiode array to generate a spectrum index plot {e.g., at 230 nm) to evaluate the degree of enantiomeric enrichment or to confirm enantiomeric purity. In addition, enantiomeric enrichment or enantiomeric purity can be measured using chiral shift NMR techniques that will be familiar to those skilled in the art (see, e.g., Meddour et al., J Am Chem Soc. 119: 4502-4508 (1997); and Merlet et al., JAm Chem Soc. 121: 5249-5258 (1999)), and optical rotation may be used to quantitatively evaluate enantiomeric excess if a linear relationship can be established between enantiomer proportion and optical rotation.

B. Assays for Testing the Anticancer Activity of Iminopyrrolidone Aziridine

Enantiomers

[0042] In many pharmaceutically active chiral compounds, one enantiomer of a racemic mixture is more physiologically or biologically potent than the other enantiomer. A number of biological assays for testing the antineoplastic activity of enantiomerically enriched or enantiomerically pure compounds of the present invention are available. These assays can roughly be split into two groups: Those involving in vitro exposure of tumor cells to a selected agent; and in vivo antitumor assays in rodent models, and rarely, in larger animals. Both types of assays are equally applicable to determining whether an enantiomerically enriched or enantiomerically pure compound of the present invention exhibits antineoplastic activity. As used herein, the term "antineoplastic" means inhibiting or preventing the growth of cancer, including reducing the growth of cancer relative to the absence of a given therapy or treatment.

[0043] In vitro cytotoxic assays generally involve the use of established tumor cell lines both of animal, and, especially, of human origin. These cell lines can be obtained from commercial sources such as the American Type Tissue Culture laboratory in Bethesda, Maryland, and from tumor banks at research institutions. Some assays (e.g., a colony forming assay) may use either established cell lines, or fresh tumor biopsies surgically removed from patients with cancer. Exposures of the cells to compounds of the present invention may be carried out under simulated physiological conditions of temperature, oxygen, and nutrient availability in the laboratory. The endpoints for these in vitro assays can involve colony formation, the reduction of a mitochondrial enzyme substrate (e.g., reduction of MTT to a blue formazan) by viable cells, the binding of a dye to cellular proteins (e.g., sulforhodamine B), the uptake of "vital" dyes that are excluded from cells with an intact cytoplasmic membrane, or the incorporation of radiolabeled nutrients into a proliferating (viable) cell. A more detailed description of in vitro assays for testing the antineoplastic activity of enantiomerically enriched or enantiomerically pure compounds of the present invention can be found in Hersh et al., J Natl Cancer Inst. 84: 1238-1244 (1992); Salmon et al, J Natl Cancer Inst. 86: 228-230 (1994); Dvorakova et al., Blood 97: 3544-3551 (2001); and Dorr et al., Int J Gastrointest Cancer 36: 15-28.

[0044] In vivo antitumor assays are generally conducted after antineoplastic activity is identified through in vitro cytotoxic assays, and most commonly use rodents for assessing the antineoplastic characteristics of selected compounds. In vivo tumor studies are typically carried out using human tumor cell lines that are implanted subcutaneously in the animals. The tumors are generally allowed to grow to a predetermined size (e.g., 100-200 mg), and the growth, or lack thereof, is then assessed periodically during a period of treatment with the investigative compound. The in vivo antineoplastic activity of a selected compound can be evaluated as a function of tumor growth inhibition calculated from the tumor weights in animals treated with the investigative compound, and in an untreated control group. Tumor weights are typically estimated from measurements of the dimensions of the tumor, which are often implanted in the front flank of the animals. The tumor growth inhibition is calculated as follows: T/C (%) = (median tumor weight of the treated group (T)/median tumor weight of the control group (C)) x 100. hi other assay methods, particularly for non-localized tumors, survival can be used as an endpoint and a comparison made between treated animals and an untreated control group. A more detailed description of in vivo assays for testing the antineoplastic activity of enantiomerically enriched or enantiomerically pure compounds of the present invention can be found in Hersh et al., J Immunother 13: 77-83 (1993); Dorr et al., Invest New Drugs 13: 113-116 (1995); and Pourpak et al., Anticancer Drugs, 17(10): 1179-1184 (2006).

[0045] Enantiomers of iminopyrrolidone aziridine compounds of the present invention exhibit substantially equipotent cytotoxic effects when compared to one another, and when compared to the racemie mixture. The antineoplastic activity of the enantiomers of imexon, as illustrated in Formulae (I) - (III) in which R¹ and R² are hydrogen, was evaluated in cytotoxic assays in comparison to the racemic mixture of imexon. The antineoplastic effects of the enantiomers of imexon and the imexon racemate were determined in RPMI 8226 multiple myeloma and Panc-1 pancreatic adenocarcinoma cell lines. The results of these antitumor assays are shown in Table 1.

Table 1. Antineoplastic Activity of Imexon Racemate and Individual Imexon Enantiomers

[0046] Growth inhibition was measured 48 hours after exposure at 37° C in a humidified incubator. The medium was RPMI 1640 with 10% calf serum to which penicillin and streptomycin were added. Cell growth was measured using the MTT assay, a well known measure of cell viability. Each drug concentration was tested in 8 wells, and each plating was repeated once. As illustrated by the data presented in Table 1, each of the enantiomers of imexon is substantially equipotent when compared to the other enantiomer and to the racemic mixture of imexon in these cell culture assays.

[0047] The imexon enantiomers A and B used in these antitumor assays, the results of which are illustrated in Table 1, were prepared from substantially pure enantiomers of phenyl 2-cyanoaziridine-l-carboxylate following resolution of these enantiomers by semi- preparative HPLC. The resolution of the phenyl 2-cyanoaziridine-l-carboxylate enantiomers was accomplished by dissolving a racemic mixture of phenyl 2-cyanoaziridine-l-carboxylate in a ratio of 1 g to 1 ml of acetonitrile, 9 ml of 2-propanol, and 36 ml of hexane, with vortexing and sonification. Aliquots (2-3 ml) of this solution were then injected into a Waters Prep LC 4000 system with a 10 ml sample loop at a flow rate of 8.0 ml/min using a Chiral Pak AD-H semi-prep column (2 x 25 cm) with an isocratic phase containing 90% hexane and 10% of (2:1 isopropanol:acetonitrile) acidified with 0.1 v/v% trifluoroacetic acid and at ambient conditions. The enantiomers were resolved at 21.9 and 24.7 minutes and collected with a Waters Fraction Collector II in the window collection mode. The UV spectral peaks were monitored in real time using a photodiode array detector to ensure that the peaks did not contain impurities. Substantially pure, unmixed, enantiomer fractions were collected and utilized in the synthesis of the substantially pure imexon enantiomers.

[0048] The synthesis of the imexon enantiomers was accomplished using Schemes VIIA and VIII, as described and illustrated hereinafter, wherein the enantiomerically pure phenyl 2- cyanoaziridine-1-carboxylate is treated with ammonia to yield 2-cyanoaziridine-l- carboxamide and ultimately the substantially enantiomerically pure imexon upon treatment of the 2-cyanoaziridine-l-carboxamide with a catalytic amount of Triton B^φ in ethanol. In these Schemes, R¹ and R² are hydrogen, and R⁴ is phenyl.

[0049] The analytical purity of the imexon enantiomers was determined on a Phenomenex Luna Cyano (100 A 250 x 4.6 mm) column with imexon samples dissolved in DMSO and eluted with 98% 0.01 M dibasic potassium phosphate adjusted to pH 2 and 2% acetonitrile as solvent. While the absolute configuration of the imexon enantiomers has not yet been determined, the first eluting imexon enantiomer (19.9 minutes) was designated imexon enantiomer A, and had a specific rotation of -276.2 deg., while the second eluting imexon enantiomer (21.7 minutes) was designated imexon enantiomer B, and had a specific rotation of +275.8 deg. Both enantiomers and racemic imexon showed identical ¹H-NMR spectra in DMSO-d6.

[0050] The chiral purity of the imexon enantiomers was evaluated using an Astec Chirobiotic T, teicoplanin column (4.6 x 250 mm; 5μ pore size) with 60% methanol:40% 2- propanol (isocratic but from separate reservoirs) as solvent. The detection method comprised a photodiode array with a spectrum index plot at 230 nm. The results of the chiral purity analysis indicated that the imexon enantiomers were individually in at least about 99% enantiomeric excess.

C. Methods of Synthesizing Iminopyrrolidone Aziridine Enantiomers

[0051] The enantiomers of compounds of the present invention can be synthesized from amino acids having the desired stereochemistry. This synthesis route may employ laboratory techniques generally apparent and accessible to those of skill in the relevant art. hi Schemes I- VIII, R¹ and R² are as defined previously with reference to Formulae (I), (II), and (III). While no specific stereochemistry is illustrated in the following synthesis schemes, it is equally applicable to either the (R)- or the (.^-stereoisomer of the starting amino acid, and will yield the R-substituted iminopyrrolidone aziridine compound having the corresponding stereochemistry.

Scheme I

1 2

[0052] In Scheme (I), the amino group of amino acid 1 is protected with the amino protecting group Y². The term "protecting group" as used herein, refers to a group designed to block one reactive site in a molecule while a chemical reaction is carried out at another reactive site. Useful amino protecting groups are described in detail in Greene et al., Protective Groups hi Organic Chemistry, 2nd Ed., John Wiley & Sons, New York, NY, 1991; and Stewart et al., Solid Phase Peptide Synthesis, 2nd Ed., 1984. The amino acid protecting group is selected such that removal in subsequent steps may be achieved under mild conditions (such as mild hydrogenation, mildly acidic, or mildly basic conditions). Exemplary amino protecting groups (Y²) include benzyloxy groups (e.g., carbobenzyloxy, tert-butyloxycarbonyl (BOC), and 9-fluorenylmethyloxycarbonyl (FMOC)).

[0053] In Scheme (I), the carboxylic acid is esterified with appropriate esterifying reagents, including alcohols and acidic catalysts such as HCl, H₂SO₄, BF₃, CF₃CO₂H, p- toluenesulfonic acid, and acid ion exchange resins. The esterifying reagent is selected such that the resulting ester may be converted to the corresponding aldehyde under mild conditions, such as mild reducing conditions (see Scheme II hereinbelow). Thus, in some embodiments, Y¹ is an unsubstituted C₁-C₆ alkyl (e.g., methyl, ethyl, etc.).

Scheme II

2 [0054] In Scheme (II), the protected amino acid 2 is reduced to the corresponding aldehyde 3 using any appropriate reducing agent, such as diisobutyl aluminum hydride.

Scheme III

[0055] In Scheme (III), the aldehyde of 3 is converted to the corresponding cyano group of 4 through an oxime intermediate using any appropriate hydroxylamine, such as O₅N- (bistrifluoroacetyl)hydroxylarnine. Methods of oxime formation with hydroxylamine hydrochloride followed by dehydration to the nitrile are presented in detail, for example, in Hunt, Chem lnd., 1873 (1961); Browne et al., J Org Chem. 22, 1320 (1957); Doyle, J Chem Soc, 2853 (1956); Mukuyama et al., Bull Chem Soc Japan 34: 99 (1961); Conley et al., J Org Chem. 26: 782 (1961), and Pomeroy et al., JAm Chem Soc. 81: 6340 (1954).

Scheme IV

[0056] In Scheme (IV), the hydroxyl group of 4 is activated as a leaving group using any appropriate reagent. In some embodiments, the reagent is a sulfonate derivative (such as mesylsulfonylchloride or para-toluenesulfonylchloride), or a substituted or unsubstituted benzene derivative (such as benzylchloride). Thus, Y³ may be methanesulfonyl, toluenesulfonyl, benzyl, or bromobenzyl. Scheme V

5 6

[0057] In Scheme (V), cyclization to form the aziridine compound 6 is accomplished using a hydrogenation agent. Hydrogenation agents are selected to eliminate the amino protecting group Y² while avoiding breakdown of 5 or 6 and allowing nucleophilic attack of the free amine to the -C-O-Y³ carbon center. Useful hydrogenation reagents include, for example, hydrogen/metal reagents such as hydrogen/palladium.

Scheme VI

[0058] In Scheme (VI), amidation of the aziridine nitrogen of 6 is accomplished using the appropriate isocyanate reagent having the formula R³=N=C=O. The substituent R³ may comprise -C(O)R^3A where R^3A is -CCl₃, -CF₃, or -OR^3A1, and R^3A1 is a (C₁-C₆) alkyl substituted with phenyl (e.g. R³ is a carbonylbenzyloxy).

Scheme VII

8

[0059] In Scheme (VII), R³ may be eliminated using an appropriate elimination reagent. One of skill in the art will immediately recognize that the elimination reagent will depend upon the specific identity of R³. For example, where R³ is BOC, hydrogenation/palladium may be employed. The reagent is selected to avoid breakdown of 7 or 8 while allowing removal of R³. Alternatively, R³ is displaced using a mild base such as ammonia.

Scheme VIA

R¹ R² R¹ R²

R' \ / CN ► R' \ / CN

N N

H I

O^o-R^< 6 7A

[0060] Alternatively, as shown in Scheme (VIA), 6 is derivatized with a chloroformate having the formula R⁴-O-C(O)-C1 to arrive at 7A. R⁴ can be hydrogen, alkyl, heteroalkyl, aryl, or heteroaryl, and in a preferred scheme R⁴ is unsubstituted phenyl or unsubstituted (Cr C₁₀) alkyl.

Scheme VIIA

7A 8

[0061] In Scheme (VIIA), -OR⁴ is substituted using an appropriate amination reagent such as ammonia to provide 8.

Scheme VHI

8 9

[0062] In Scheme (VIII), cyclization of 8 to form the (R)- or the (^-stereoisomer of the iminopyrrolidone aziridine 9 is achieved using a polar alcohol and a basic reagent. Polar alcohols are compounds having the formula X¹ -OH having a dielectric constant at 25°C greater than 20, where X¹ is selected from substituted or unsubstituted alkyl, substituted or unsubstituted heteroalkyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted heterocycloalkyl, substituted or unsubstituted aryl, and substituted or unsubstituted heteroaryl.

[0063] In some cases, the polar alcohol may be X¹ -OH having a dielectric constant at 25⁰C greater than 25, wherein X¹ is substituted or unsubstituted Ci-C₆ alkyl. The polar alcohol may also be selected from ethanol and methanol, hi some cases, the polar alcohol may be selected to allow in situ crystallization of 9, thereby minimizing or avoiding racemization.

[0064] The basic reagent is selected to avoid degradation of 8 and 9, and may be selected from a metal hydroxide or benzyl trimethylammonium (Triton B^®). The basic reagent is typically present in a catalytic amount, for example, Triton B^® may be present from 0.01 to 1.0 grams of 40% Triton B^® per gram of the 2-cyanoaziridine-l-carboxamide.

[0065] hi a preferred embodiment, the foregoing synthesis route (Schemes I- VIII) begins with either an i?-serine or a 5-serine as the ammo acid 1 of Scheme I to produce the corresponding (5i?)-4-amino-l,3-diazabicyclo[3.1.0]hex-3-en-2-one or (55)-4-amino-l,3- diazabicyclo[3.1.0]hex-3-en-2-one (iminopyrrolidone aziridine enantiomer 9 of Scheme VIII), respectively. Starting amino acids such as i?-serine or iS-serine are commercially available from several sources, including Spectrum Chemical Manufacturing, Inc. (Gardena, CA), and Sigma-Aldrich (St. Louis, MO).

III. Aqueous Solutions of Iminopyrrolidone Aziridine Compounds

[0066] Embodiments of the present invention include aqueous solutions of enantiomerically enriched or enantiomerically pure iminopyrrolidone aziridine compounds and compositions, and methods of increasing the aqueous solubility of iminopyrrolidone aziridine compounds of the present invention. As described hereinbefore, it has been discovered that, surprisingly, enantiomerically enriched or enantiomerically pure iminopyrrolidone aziridine compounds in accordance with the present invention exhibit dramatically enhanced aqueous solubility characteristics, as compared to racemic mixtures of the compounds that are not enantiomerically enriched or enantiomerically pure.

[0067] In one aspect, the present invention provides an aqueous solution comprising an iminopyrrolidone aziridine composition having an enantiomeric excess of an enantiomer of a compound of Formula (I), in which R¹ and R² comprise substituents as described hereinbefore, hi one embodiment, the composition is soluble in an aqueous solvent to a concentration in excess of 40 mg/ml. In another embodiment, the composition is soluble in an aqueous solvent to a concentration in excess of 50 mg/ml. In yet another embodiment, the composition is soluble in an aqueous solvent to a concentration in excess of 60 mg/ml.

[0068] Aqueous solutions in accordance with embodiments of the present invention can be prepared by contacting an enantiomerically enriched compound, as described in detail hereinbefore, with an aqueous solvent to yield a solution. The dissolution of the enantiomercially enriched compound in the aqueous solvent may be facilitated or expedited by mechanical stirring, mixing, vortexing, or the like. Methods for measuring the concentration of a solute in a solution are well known. Particularly useful methods include spectrophotometric absorbance measurement techniques, with or without associated chromatography. For example, the concentration of an enantiomerically enriched compound of the present invention in an aqueous solution can be determined by measuring absorbance with a UV- Vis spectrophotometer (e.g., at 230 nm) and comparing the result against a standard or standards of known concentration. In a preferred method, a spectophotometric detector is coupled to an HPLC apparatus to resolve the enantiomerically enriched compound from any potential impurities prior to measuring absorbance (e.g., at 230 nm), and the concentration of the enantiomerically enriched iminopyrrolidone aziridine composition is then determined by comparing the result (e.g., area under the curve ("AUC")) against a standard or standards of known concentration.

[0069] As will be appreciated, in those aqueous solutions of the present invention comprising iminopyrrolidone aziridine compounds in less than enantiomeric purity, the AUCs of resolved enantiomers will be combined to determine the total concentration of the iminopyrrolidone aziridine compound in the solution. It is also understood by those skilled in the art that the relationship between absorbance and concentration may be linear over only a finite concentration range, and as such, measurements of the concentration of enantiomerically enriched compounds and compositions of the present invention may require serial dilution of a sample to achieve a concentration range bracketed by a standard curve having a linear relationship between absorbance and concentration. Such dilutions will be accounted for in calculating the actual concentration of iminopyrrolidone aziridine compounds in aqueous solutions of the present invention. In addition, indications of iminopyrrolidone aziridine concentrations used herein are intended to encompass a narrow range corresponding to variability resulting from experimental uncertainty. [0070] Aqueous solutions of iminopyrrolidone aziridine compounds in accordance with the present invention may comprise an enantiomeric excess of either an (i?)-stereoisomer or an (^-stereoisomer of a compound of Formula (I) to achieve a concentration of the compound in excess of from 40 mg/ml to in excess of 60 mg/ml, and may comprise an enantiomeric excess of either enantiomer of from at least about 60% enantiomeric excess to at least about 99% enantiomeric excess in the aqueous solution. In other embodiments, aqueous solutions of the present invention comprise one enantiomer (either the (R)- or the (^-stereoisomer) in enantiomeric purity.

[0071] Samples of substantially pure enantiomers of the iminopyrrolidone aziridine compound imexon, prepared according to the methods described hereinbefore, were evaluated in solubility experiments to assess the aqueous solubility of the imexon enantiomers as compared to racemic imexon. Unexpectedly, the individual aqueous solubility of the enantiomers of imexon was dramatically improved as compared to the imexon racemate. At 23° C, imexon enantiomer A ((-)-imexon) had an aqueous solubility in sterile water of 69.3 mg/ml ± 4.6 mg/ml. The aqueous solubility characteristics (as well as other physical properties, except the rotation of plane polarized light) of imexon enantiomer B ((+)-imexon) are expected to be identical, within the bounds of experimental uncertainty, to those identified for imexon enantiomer A. This identity of physical characteristics between enantiomers (stereoisomers having opposite configurations at all chiral centers in the molecule) is well known in the chemical arts. Racemic imexon has a literature reported aqueous solubility of 23.6 mg/ml ± 0.5 mg/ml in sterile water (Den Brok et al., J Pharm ScL 94: 1101-1114 (2005), and a similar (25 mg/ml) aqueous solubility was observed in experiments conducted on racemic imexon under the same conditions used to evaluate the imexon enantiomers. These results are summarized in Table 2.

Table 2. Aqueous Solubility of Imexon Racemate and Individual Imexon Enantiomers

[0072] The solubility experiments were conducted by combining an excess of racemic or enantiomerically pure imexon with a volume of sterile water. The mixture was then agitated for 30-90 minutes and centrifuged to yield a supernatent free of undissolved material. The supernatent was then serially diluted into a concentration range of from about 100 μg/ml to about 1000 μg/ml and assayed using HPLC to measure the concentration of material present and determine the corresponding solubility.

[0073] While not intending to be bound by any particular theory, this unexpected and significant difference in the solubility of the imexon enantiomers, as compared to the racemic imexon, maybe a function of a difference in stability of the crystal structures of the compounds. Thus, in the racemic mixture, the fundamental hydrogen bonding interaction may be between one molecule of (-)-imexon and one molecule of (+)-imexon, while the corresponding interaction in the enantiomerically pure compositions clearly cannot be the same. Evidence in support of this supposition may be found in the differential thermal analyses of (-)-imexon, as compared to the racemate. The exothermic decomposition of (-)-imexon occurs at 135.6° C₅ whereas racemic imexon decomposes exothermically at 191.4° C.

[0074] In another aspect, the present invention provides a method of increasing the aqueous — solubilityOfiminopyrrolidone^~aziridine compounds, the method comprising contacting an iminopyrrolidone aziridine composition having at least about a 60% enantiomeric excess of an enantiomer of a compound of Formula (I), in which R¹ and R² comprise sύbstituents as described hereinbefore, with an aqueous solvent to yield a solution having an iminopyrrolidone aziridine concentration in excess of 40 mg/ml. In another embodiment, the solution has an iminopyrrolidone aziridine concentration in excess of 50 mg/ml. In yet another embodiment, the solution has an iminopyrrolidone aziridine concentration in excess of 60 mg/ml.

[0075] As will be appreciated, the iminopyrrolidone aziridine compositions in accordance with the methods of the present invention may comprise an enantiomeric excess of either an (i?)-stereoisomer or an (^-stereoisomer of a compound of Formula (I) to yield a solution having an iminopyrrolidone aziridine concentration in excess of from 40 mg/ml to in excess of 60 mg/ml. In one embodiment, the (R)- or the (5)-stereoisomer may be enantiomerically pure. In other embodiments, the method may include contacting an iminopyrrolidone aziridine compound having from at least about an 80% enantiomeric excess to at least about a 99% enantiomeric excess.

[0076] In one embodiment, the iminopyrrolidone aziridine compound is imexon, and the method comprises contacting an imexon composition having at least about a 60% enantiomeric excess of (5i?)-4-amino-l,3-diazabicyclo[3.1.0]hex-3-en-2-one with an aqueous solvent to yield a solution having an imexon concentration in excess of 40 mg/ml. In other embodiments, the imexon concentration is in excess of from 50 mg/ml to in excess of 60 mg/ml. In another embodiment, the iminopyrrolidone aziridine compound is imexon, and the method comprises contacting an imexon composition having at least about a 60% enantiomeric excess of (55)-4-amino-l,3-diazabicyclo[3.1.0]hex-3-en-2-one with an aqueous solvent to yield a solution having an imexon concentration in excess of 40 mg/ml. In other embodiments, the imexon concentration is in excess of from 50 mg/ml to in excess of 60 mg/ml.

IV. Methods of Formulating Iminopyrrolidone Aziridine Drug Products

[0077] Embodiments of the present invention include methods of formulating iminopyrrolidone aziridine drug products for use in the treatment of cancer. As evidenced by the cell viability assays described hereinbefore and summarized in Table 1, individual

-enantiomers of iminopyπ-olidone^zmdmexompounds ofihe present invention exhibit equipotent cytotoxicity against tumor cells, as compared to one another and to a racemic mixture. This observation, in conjunction with the unexpected discovery of the dramatically enhanced aqueous solubility characteristics of enantiomerically enriched compositions of the present invention, provide an opportunity for significant advantages in the preparation and formulation of drug product dosage forms useful in the treatment of cancer.

[0078] hi one aspect, the present invention provides a method of formulating an iminopyrrolidone aziridine drug product from enantiomerically enriched or enatiomerically pure iminopyrrolidone aziridine compounds of the present invention. In one embodiment, the method comprises:- dissolving a drug having at least about a 60% enantiomeric excess in an aqueous solvent to form a solution having a drug concentration in excess of 40 mg/ml, the drug being a compound of Formula (I) in which R¹ and R² are as described hereinbefore; sterilizing the solution by passing it through a bacteria-retentive filter; filling the solution into one or more individual containers; freezing the solution in a freeze-drying chamber; and applying a vacuum to the chamber to remove substantially all of the aqueous solvent from the one or more individual containers. In one embodiment, an i?-stereoisomer of the drug is in enantiomeric excess. In another embodiment, an ■S-stereoisomer of the drug is in enantiomeric excess. In a preferred embodiment, the drug is imexon.

[0079] As will be appreciated, the concentration of the drug in the aqueous solution can be measured by the spectrophotometric techniques described hereinbefore (e.g., by HPLC assay). As is frequently done in the preparation of a lyophilized dosage form, the drug concentration may be measured in the bulk solution prior to being filled into individual containers, which can then be made to contain a predefined dose of the drag by filling each individual container with a volume containing that dose as determined by the measured concentration of the bulk solution (e.g., 50 mg/ml concentration x 5 ml fill = 250 mg dose).

[0080] In other embodiments in accordance with the teachings of the present invention, one enantiomer of the drug is in at least about 80% enantiomeric excess. In other embodiments, one enantiomer of the drug is in at least about 90% enantiomeric excess. In other embodiments, one enantiomer of the drug is in at least about 95% enantiomeric excess. In still other embodiments, one enantiomer of the drug is in at least about 99% enantiomeric excess, and in some embodiments the drug is enantiomerically pure.

[0081] In still other embodiments of methods in accordance with the present invention, the drug may be dissolved in an aqueous solvent to form a solution having a drug concentration in excess of from 50 mg/ml to in excess of 60 mg/ml. As will be appreciated, one particular advantage associated with the use of a solution having a drug concentration in excess of from 40 mg/ml to in excess of 60 mg/ml in the preparation of a drug product is the opportunity to handle smaller volumes containing an equivalent dose of enatiomerically enriched or enantiomerically pure material, as compared to racemic material or non-enantiomerieally enriched material. In the preparation of lyophilized dosage forms useful for parenteral administration, smaller volumes can significantly reduce lyophilization cycle time, resulting in more efficient drug product manufacturing and substantial cost advantages.

[0082] In addition, the significant reduction in solution volume made possible by the enhanced aqueous solubility of enantiomerically enriched compounds of the present invention is of particular advantage in overcoming the limited aqueous stability of the iminopyrrolidone aziridine compounds in commercial scale production. The reduced volume facilitates an expedited production process, which limits the duration of exposure of these compounds to an aqueous environment in which they are prone to acid/base catalytic degradation. These challenges can be further overcome by optimizing the aqueous stability of the iminopyrrolidone aziridine compounds through a combination of solution pH and temperature. For example, preferred parameters include a pH of from about 7.2 to about 9.0, maintained, e.g., with a 0.1 M phosphate buffer, and a temperature of from about 5° C to about 20° C. Kuehl et al., Drug Development and Industrial Pharmacy 32: 687-697 (2006).

V. Methods of Treating Cancer

[0083] Drug products produced according to the methods of the present invention or comprising enantiomerically enriched or enantiomerically pure iminopyrrolidone aziridine compounds of Formula (I), in which R^! and R² are as hereinbefore described, are useful in the treatment of a variety of cancers, including without limitation, solid tumors such as breast cancer, colon cancer, kidney cancer, lung cancer, ovarian cancer, pancreas cancer, prostate cancer, fibrosarcoma, and melanoma, as well as hematological malignancies such as multiple myeloma, lymphomas, and leukemias.

[0084] As will be appreciated, enantiomerically enriched or enantiomerically pure compounds and compositions in accordance with the teachings of the present invention may be used in the treatment of cancer either alone, or in combination with a second antineoplastic agent. As used herein, the terms "combination therapy" and "adjunct therapy" mean that a patient in need of the drug is treated with or given another drug for the disease in conjunction with an enantiomerically enriched or enantiomerically pure iminopyrrolidone aziridine compound. In various embodiments, the combination therapy can be sequential therapy where the patient is treated first with one drug and then the other or the two drugs can be administered simultaneously. In some embodiments, the combination of an enantiomerically enriched or enantiomerically pure iminopyrrolidone aziridine compound and the second antineoplastic agent may exhibit a synergistic therapeutic cytotoxic effect, as assessed using the median-effect principle (Chou et al., Adv Enzyme Regul 22: 27-55 (1984)).

[0085] Synergistic combination therapies include the combination of an enantiomerically enriched or enantiomerically pure iminopyrrolidone aziridine compound in combination with an antineoplastic nucleic acid binding agent {e.g., dacarbazine, cisplatin, melphalan, carmustine, mechlorethamine, thiotepa, chlorambucil, lomustine, ifosfamide, mitomycin C, carboplatin, oxaliplatin, or cyclophosphamide), an antineoplastic antimetabolite base analog {e.g., gemcitabine, 5-flurouracil, cytarabine, mercaptopurine, thioguanine, azathioprine, fludarabine, cladribine, pentostatin, capecitabine, or floxuridine), an antineoplastic proteasome inhibitor (e.g., bortezomib, epoxomicin, eponemycin, lactacystin, clasto- lactacystin β-lactone, PSI, TMC-95A, MG-115, or MG-132), an antineoplastic corticosteroid (e.g., dexamethasone, prednisone, betamethasone, prednisolone, methylprednisolone, cortisone, hydrocortisone, or triamcinolone), or docetaxel.

[0086] Methods of treating cancer with enantiomerically enriched or enantiomerically pure compounds and compositions in accordance with the teachings of the present invention may comprise any suitable method that is effective in the treatment of the particular cancer or tumor type being treated. Treatment may be facilitated by oral, rectal, topical, parenteral or intravenous administration, or by injection into the tumor or cancer. It is believed that parenteral treatment by intravenous, subcutaneous, or intramuscular application of an enantiomerically enriched or enantiomerically pure iminopyrrolidone aziridine compound, formulated with an appropriate pharmaceutically acceptable carrier to facilitate application, will be the preferred method of administering compounds and compositions of the present invention. The compounds and compositions of the present invention can be formulated in any suitable manner applicable to the chosen route of administration, as will be familiar to those skilled in the pharmaceutical art, and may be administered in a therapeutically effective dose, which may vary according to the particular disease being treated, the severity of the disease, and the response to treatment. By way of general guidance, a dose of from about 10 mg/m² of body surface area to about 2500 mg/m² of body surface area will comprise a therapeutically effective dose that may be administered periodically from several times per day to several times per week as part of a cycle of therapy.

[0087] One skilled in the art will recognize that the efficacy of the compounds can be ascertained through routine screening using known cancer cell lines both in vitro and in vivo. Cell lines are available from American Tissue Type Culture or other laboratories.

A. Measuring Response to Pharmaceutical Formulations

[0088] Tumor load is generally assessed prior to therapy by means of objective scans of the tumor such as with x-ray radiographs, computerized tomography (CAT scans), nuclear magnetic resonance (NMR) scans or direct physical palpation of the tumor mass. Alternatively, the tumor may secrete a marker substance such as alphafetoprotein from colon cancer, CAl 25 antigen from ovarian cancer, or serum myeloma "M" protein from multiple myeloma. The levels of these secreted products then allow for an estimate of tumor burden to be calculated. These direct and indirect measures of the tumor load are done pretherapy, and are then repeated at intervals following the administration of the drug in order to gauge whether or not an objective response has been obtained. An objective response in cancer therapy generally indicates >50% shrinkage of the measurable tumor disease (a partial response), or complete disappearance of all measurable disease (a complete response). Typically these responses must be maintained for a certain time period, usually one month, to be classified as a true partial or complete response. In addition, there may be stabilization of the rapid growth of a tumor or there may be tumor shrinkage that is <50%, termed a minor response or stable disease. In general, increased survival is associated with obtaining a complete response to therapy, and in some cases, a partial response, if maintained for prolonged periods can also contribute to enhanced survival in the patient.

[0089] Patients receiving chemotherapy are also typically "staged" as to the extent of their disease before beginning chemotherapy, and are then restaged following chemotherapy to see if this disease extent has changed. In some situations the tumor may shrink sufficiently, and if no metastases are present, to make surgical excision possible after chemotherapy treatment where it was not possible beforehand due to the widespread disease. In this case the chemotherapy treatment with the novel pharmaceutical compositions is being used as an adjuvant to potentially curative surgery. In addition, patients may have individual lesions in the spine or elsewhere that produce symptomatic problems such as pain and these may need to have local radiotherapy applied. This may be done in addition to the continued use of the systemic pharmaceutical compositions of the present invention.

B. Assessing Toxicity and Setting Dosing Regimens

[0090] Patients are assessed for toxicity with each course of chemotherapy, typically looking at effects on liver function enzymes and renal function enzymes such as creatinine clearance or BUN as well as effects on the bone marrow, typically a suppression of granulocytes important for fighting infection and/or a suppression of platelets important for hemostasis or stopping blood flow. For such myelosuppressive drugs, the nadir in these normal blood counts is reached between 1-3 weeks after therapy and recovery then ensues over the next 1-2 weeks. Based on the recovery of normal white blood counts, treatments may then be resumed.

[0091] In general, complete and partial responses are associated with at least a 1-2 log reduction in the number of tumor cells (a 90-99% effective therapy). Patients with advanced cancer will typically have >10⁹ tumor cells at diagnosis, and multiple treatments will often be required in order to reduce tumor burden to a very low state and potentially obtain a cure of the disease.

[0092] Treatment schedules or dosing regimens for the administration of compounds or pharmaceutical compositions in accordance with the present invention conventionally comprise cycles of treatment wherein a specified dose of the compound, or each composition of a combination therapy, is administered to a patient at defined intervals over the period of a cycle, and then repeated in each subsequent cycle. The period of a cycle may be defined in any suitable manner, and may comprise, for example, a twenty-one day cycle, a twenty-eight day cycle, or the like. Within the period of a cycle of treatment, the specified dose of a compound in accordance with the present invention can be administered to the patient at defined intervals, such as for example, for five consecutive days every other week (e.g., days 1-5 and 15-19 of a 28-day cycle), for five consecutive days every three weeks (e.g., days 1-5 of a 21 -day cycle), once per week (e.g., days 1, 8 and 15 of a 21 -day cycle), or the like.

C. Clinical Management of Patients

[0093] At the end of a treatment cycle with a pharmaceutical formulation of the present invention, which could comprise several weeks of continuous drug dosing, patients will be evaluated for response to therapy (complete and partial remissions), toxicity measured by blood work and general well-being classified performance status or quality of life analysis. The latter includes the general activity level of the patient and their ability to do normal daily functions. It has been found to be a strong predictor of response and some anticancer drugs may actually improve performance status and a general sense of well-being without causing a significant tumor shrinkage. The antimetabolite gemcitabine is an example of such a drug that was approved in pancreatic cancer for benefiting quality of life without changing overall survival or producing a high objective response rate. Thus, for some cancers that are not curable, the pharmaceutical formulations may similarly provide a significant benefit, well- being performance status, etc. without affecting true complete or partial remission of the disease.

[0094] In hematologic disorders such as multiple myeloma, lymphoma and leukemia, responses are not assessed via the measurement of tumor diameter since these diseases are widely metastatic throughout the lymphatic and hematogenous areas of the body. Thus, responses to these diffusely disseminated diseases are usually measured in terms of bone marrow biopsy results wherein the number of abnormal tumor cell blasts are quantitated and complete responses are indicated by the lack of detection (e.g., microscopic detection) of any tumor cells in a bone marrow biopsy specimen. With the B-cell neoplasm multiple myeloma, a serum marker, the M protein, can be measured by electrophoresis and, if substantially decreased, this is evidence of the response of the primary tumor. Again, in multiple myeloma, bone marrow biopsies can be used to quantitate the number of abnormal tumor plasma cells present in the specimen. For these diseases, higher dose therapy is typically used to affect responses in the bone marrow and/or lymphatic compartments.

[0095] While the invention is described here in the context of a limited number of embodiments, and with reference to specific details and examples, the invention may be embodied in many forms without departing from the spirit of the essential characteristics of the invention. The exemplary and described embodiments, including what is described in the summary of the invention and the abstract of the disclosure, are therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Claims

WHAT IS CLAIMED IS:

1. A composition comprising an enantiomeric excess of an enantiomer of a compound of the formula:

wherein R¹ and R² are independently selected from hydrogen, or substituted or unsubstituted (Ci-Ci₀) alkyl or cycloalkyl, and wherein the composition has an aqueous solubility in excess of 40 mg/ml.

2. The composition of claim 1 , wherein the enantiomer is an R- stereoisomer of the compound.

3. The composition of claim 1 , wherein the enantiomer is an S- stereoisomer of the compound.

4. The composition of claim 1 , wherein the enantiomer is in at least about 60% enantiomeric excess.

5. The composition of claim 1 , wherein the enantiomer is in at least about 80%, at least about 90%, or at least about 95% enantiomeric excess.

6. The composition of claim 1 , wherein the enantiomer is in at least about 99% enantiomeric excess.

7. The composition of claim 1, wherein the enantiomer is enantiomerically pure within a range corresponding to experimental uncertainty.

8. The composition of claim 1 , wherein R¹ and R² are hydrogen.

9. The composition of claim 1, wherein the aqueous solubility is in excess of 50 mg/ml.

10. The composition of claim 1, wherein the aqueous solubility is in excess of 60 mg/ml.

11. An aqueous solution comprising a composition having an enantiomeric excess of an enantiomer of a compound of the formula:

wherein R¹ and R² are independently selected from hydrogen, or substituted or unsubstituted (Ci-C_1O) alkyl or cycloalkyl, and wherein the composition is soluble to a concentration in excess of 40 mg/ml.

12. The aqueous solution of claim 11 , wherein the enantiomer is an R- stereoisomer of the compound.

13. The aqueous solution of claim 11 , wherein the enantiomer is an S- stereoisomer of the compound.

14. The aqueous solution of claim 11 , wherein the enantiomer is in at least about 60% enantiomeric excess.

15. The aqueous solution of claim 11 , wherein the enantiomer is in at least about 80% enantiomeric excess.

16. The aqueous solution of claim 11 , wherein the enantiomer is in at least about 90% enantiomeric excess.

17. The aqueous solution of claim 11 , wherein the enantiomer is in at least about 95% enantiomeric excess.

18. The aqueous solution of claim 11, wherein the enantiomer is in at least about 99% enantiomeric excess.

19. The aqueous solution of claim 11 , wherein the enantiomer is enantiomerically pure within a range corresponding to experimental uncertainty.

20. The aqueous solution of claim 11, wherein R¹ and R² are hydrogen.

21. The aqueous solution of claim 11, wherein the composition is solubleo a concentration in excess of 50 mg/ml.

22. The aqueous solution of claim 11 , wherein the composition is soluble to a concentration in excess of 60 mg/ml.

23. A method of formulating a drug product, the method comprising:

(a) dissolving a drug having at least about a 60% enantiomeric excess in an aqueous solvent to form a solution having a drug concentration in excess of 40 mg/ml, the drug having the formula:

wherein R¹ and R² are independently selected from hydrogen, or substituted or unsubstituted (C₁-C_1O) alkyl or cycloalkyl;

(b) sterilizing the solution;

(c) filling the solution into one or more individual containers;

(d) freezing the solution in a freeze-drying chamber; and

(e) applying a vacuum to the chamber to remove substantially all of the aqueous solvent from the one or more individual containers.

24. The method of claim 23, wherein the drug is imexon.

25. The method of claim 23, wherein an ^-stereoisomer of the drug is in enantiomeric excess.

26. The method of claim 23, wherein an S-stereoisomer of the drug is in enantiomeric excess.

27. The method of claim 23, wherein the drug has at least about an 80%, at least about a 90%, or at least about a 95% enantiomeric excess.

28. The method of claim 23, wherein the drag has at least about a 99% enantiomeric excess.

29. The method of claim 23, wherein the drug is enantiomerically pure within a range corresponding to experimental uncertainty.

30. The method of claim 23, wherein the solution has a drug concentration in excess of 50 mg/ml.

31. The method of claim 23 , wherein the solution has a drug concentration in excess of 60 mg/ml.

32. The method of claim 23, wherein the solution is sterilized by passing it through a bacteria-retentive filter.

33. A method of increasing solubility of imexon, the method comprising contacting a composition containing at least about a 60% enantiomeric excess of one imexon enantiomer with an aqueous solvent to yield a solution having an imexon concentration in excess of 40 mg/ml.

34. The method of claim 33, wherein the imexon enantiomer is (5i?)-4- amino-l,3-diazabicyclo[3.1.0]hex-3-en-2-one.

35. The method of claim 33 , wherein the imexon enantiomer is (55)-4- amino-l,3-diazabicyclo[3.1.0]hex-3-en-2-one.

36. The method of claim 33, wherein the imexon concentration is in excess of 50 mg/ml.

37. The method of claim 33, wherein the imexon concentration is in excess of 60 mg/ml.

38. The method of claim 33, wherein the composition contains one imexon enantiomer in at least about 80%, at least about 90%, or at least about 95% enantiomeric excess.

39. The method of claim 33, wherein the composition contains one imexon enantiomer in at least about 99% enantiomeric excess.

40. The method of claim 33, wherein the composition contains one imexon enantiomer in enantiomeric purity within a range corresponding to experimental uncertainty.

41. An aqueous solution comprising (5i?)-4-amino-l ,3- diazabicyclo[3.1.0]hex-3-en-2-one in at least about 95% enantiomeric excess at a concentration in excess of 50 mg/ml.

42. An aqueous solution comprising (55)-4-amino-l,3- diazabicyclo[3.1.0]hex-3-en-2-one in at least about 95% enantiomeric excess at a concentration in excess of 50 mg/ml.