Early studies of the cellular thrombin receptor indicated that more than one species exists in platelets (Greco, N. J. and G. A. Jamieson. 1991. PSEBM 198: 792-799; Harmon, J. T. and G. A. Jamieson. 1986. J. Biol. Chem.

261: 15928-15933). The first cellular thrombin receptor cloned and sequenced was PAR-1 (Vu, T-K. H. et al. 1991.

Cell 64: 1057-1068). Human platelets respond to the activation of PAR-1 and a second minor receptor PAR-4 (Kahn, M. L. et al. 1998. Nature 394: 690-694) while the recently cloned PAR-3 is either absent on human platelets or present in trace amounts (Ishihara, H. et al. 1997.

Nature 386: 502-506). Mouse platelets respond to a-thrombin primarily through PAR-3 and secondarily through PAR-4, with no involvement of PAR-1 (Vu, T-K. H. et al. 1991. Cell 64: 1057-1068). Another platelet membrane protein, GP Ib, may also function at least in part as a thrombin receptor (Greco, N. J. and G. A. Jamieson. 1991. PSEBM 198: 792-799; Harmon, J. T. and G. A. Jamieson. 1986. J. Biol. Chem.

261: 15928-15933; Clemetson, K. J. 1995. Thromb. Haemost.

74: 111-116; Greco, N. J. et al. 1996. Biochemistry 35: 915).

The physiologic functions of the three purported platelet thrombin receptors (PAR-1, PAR-4 and GP Ib) have not yet been clearly defined. A functional role for PAR-1 in a-thrombin-induced platelet aggregation has been shown in vitro (Vu, T-K. H. et al. 1991. Cell 64: 1057-1068).

Human platelets have been shown to respond to y-thrombin, an autoproteolytic product of a-thrombin, through activation of PAR-4. However, the interactions of anti- thrombotic and anti-platelet therapies with the GP Ib and/or PAR-4 receptor have not been described.

Histones are nuclear proteins involved in DNA packaging, transcription and replication (Isenberg, I.

1979. Ann. Rev. Biochem. 48: 159-191). They are naturally occurring integral components of functional cell membranes (Watson, K. et al. 1994. Biochem. Soc. Trans. 22: 199S; Watson, K. et al. 1995. Biochem. Pharmacol. 50: 299-309).

In the past few years, histones have been found to have many more functions outside of the nucleus and even outside the cell. For example, It has been shown that if a cell undergoes a malignant transformation to a cancer cell, the molecular makeup of the cell is disturbed resulting in a higher percentage of histone molecules in the cell membrane (Class, R. et al. 1996. Am. J. Clin. Oncol. 19: 522-531).

Histones share homology with classic membrane proteins in that they have a hydrophobic core and hydrophilic C-and N- terminal tails allowing them to insert into the lipid bilayer system. The core domain of histones are composed of cylinder-shaped sequences of a-helices and ß-sheets called the histone fold motif. This motif is used in protein oligomerization and channel formation (Arents, et al. 1995. Proc. Natl. Acad. Sci. USA 92: 11170-11174).

If normal orientations are changed, histone molecules have the potential to form higher-order complexes with

other histones, ultimately resulting in the formation of a potentially lethal channel in the membrane. Such channel formation has been demonstrated in Friend erythroleukemia and Jurkat and ascites tumor cells (Gamberrucci et al.

1998. Biochem. J. 331: 623-630). Histone H1 has been shown to bind strongly to artificial cell membranes containing anionic phospholipids (Koiv, A. et al. 1995. Biochemistry 34: 8018-8027; Goldberg, E. M. et al. 1998. Eur. J. Biochem.

258: 722-728; Subramanian, M. et al. 1998. Biochemistry 37: 1394-1402). Similar results have been reported in patients with systemic lupus erythematosus, an autoimmune disease involving histone H1 (Pereira et al. 1994. Clin.

Exp. Immunol. 97: 175-180). The sub-critical amount of histone molecules in the membrane of cancer cells can be induced to turn on oligomerization and lethal channel formation by the addition of extracellular histone H1.

Cell lysis, therefore, appears to be critically controlled by the amount of extracelluarly added free histone H1.

The family of histone proteins consists of five members including the core histones H2A, H2B, H3 and H4 and the linker histone H1. Histone H1 has a low immunogenicity and antigenicity, making generation of neutralizing antibodies in patients injected with histone H1 to be an unlikely event. Histone H1 can be used to cross the blood- brain barrier and has been used to treat brain metastases (Partridge et al. 1989. J. Pharmacol. Exp. Ther. 251: 821- 826).

It has now been found that histones such as H1 have the ability to specifically block y-thrombin-induced platelet aggregation.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for preventing platelet aggregation comprising

administering an effective amount of a histone compound.

Another object of the present invention is to provide a composition for preventing platelet aggregation comprising a histone compound and a pharmaceutically acceptable vehicle.

Another object of the present invention is to provide a method for treating cardiovascular disorders related to thrombotic events comprising administering an effective amount of a composition comprising a histone compound and a pharmaceutically acceptable vehicle.

Another object of the present invention is to provide an implantable device comprising a device to be inserted into the body of a patient said device being coated with a histone compound.

DETAILED DESCRIPTION OF THE INVENTION Clinical data has demonstrated that patients taking anti-thrombosis drugs still suffer from relapses, thus indicating that another pathway is active that can induce platelet aggregation in blood vessels. Experiments performed in vitro suggest that the y-thrombin pathway may still be active in patients undergoing anti-platelet or anti-thrombotic therapy, and may be responsible for at least part of the thrombotic response. It has now been found that a composition comprising a histone compound has the ability to specifically and effectively block y- thrombin-induced platelet aggregation without interfering with platelet aggregation mediated by alpha-thrombin.

The present invention relates to compositions and methods for prevention of platelet aggregation and for treatment of cardiovascular disease related to thrombotic events. Compositions of the present invention are comprised of at least one histone compound administered in a pharmaceutically acceptable vehicle directly to an animal, including humans, or used as a coating for medical devices

implanted in blood vessels or the heart or brain. The histone compound may comprise a H1, H2A, H2B, H3 or H4 histone. In a preferred embodiment the histone compound comprises a H1 histone. The H1 histone may be a H1 subtype such as, Hl. 0, Hl. l, H1. 2, H1. 3, H1. 4, or H1. 5. In a preferred embodiment the histone is a human histone. In another preferred embodiment, the histone compound is recombinantly produced. In a preferred embodiment, the pharmaceutically acceptable vehicle comprises isotonic saline. The present invention also provides methods for preventing platelet aggregation and for treating cardiovascular disease related to thrombotic events which comprise administration of a composition comprising at least one histone.

Studies were performed to examine the role of a composition comprising histone H1 as an anti-thrombotic agent. Platelet aggregation assays were performed in an aggregometer that monitors increasing light transmission as larger platelet aggregates form in solution. y-Thrombin rapidly induced platelet aggregation in the presence of cofactors. Addition of 5 Al of a composition comprising histone H1 (from a 9.5 AM stock solution) significantly reduced the aggregation of platelets within the observation time period. Administration of a larger dose, 25 Al histone H1 (from a 9.5 AM stock solution) completely abolished platelet aggregation.

Additional experiments were performed with a y- thrombin mimetic agent, TRAP-4 (Thrombin Receptor Agonist Peptide-4) instead of y-thrombin itself. TRAP-4 produced a lower level platelet aggregation response. However, addition of a composition comprising histone Hl significantly reduced the level of platelet aggregation induced by TRAP-4.

The effects of a composition comprising histone H1 to inhibit y-thrombin-induced platelet aggregation were dose-

dependent. When doses of 0.95,4.75 and 9.5 AM histone H1 were added to platelet cells in vitro, aggregation was inhibited. a dose of only 0.95 AM histone H1 caused a greater than 95% inhibition of platelet aggregation.

Experiments were also performed to examine the specificity of the interaction of histone H1. When histone H1 was added to a-thrombin-induced platelet preparations, there was no significant effect on a-thrombin-induced platelet aggregation. These results indicate that the interaction of histone H1 is specific for the y-thrombin pathway.

Studies with other histone compounds such as H2A, H2B, H3, and H4 are indicative of these histone compounds having similar activity to histone H1. Further the histone H1 and histone H1 subtypes such as, H1. 0, Hl. l, H1. 2, H1. 3, H1. 4 and H1. 5, were both successful in preventing platelet aggregation. Compositions of the present invention comprise a histone compound formulated in a pharmaceutically acceptable vehicle for ingestion or injection as a solution or as a pill form for oral administration. In the context of the present invention by histone compound, it is meant to be inclusive of, but is not limited to the H1, H2A, H2B, H3 and H4 histones, as well as analogs of histones including but not be limited to histone subtypes, histone fragments, recombinantly produced histones, truncated histones, histones with conservative amino acid substitutions, modifications to histones that do not alter the primary and/or secondary structure of the histone (e. g., glycosylation, phosphorylation, methylation, pegylation), D-amino acid-containing histones, and histones fused or linked to other agents. For example, for the H1 histone, useful subtypes include, but are not limited to H1. 0, Hl. l, H1. 2, H1. 3, H1. 4 and H1. 5. Compositions in solution can be injected intravenously, intramuscularly, subcutaneously, intraperitoneally, or instilled directly

into the eye or muscosal areas.

Cardiovascular disorders in patients relating to thrombotic events may be treated by administering an effective amount of a histone compound to the patient.

Further, compositions of the present invention comprising a histone compound can be given prophylactically to patients at high risk of thrombotic episodes. For example, a composition of the present invention can be administered to a patient following surgery in lieu of standard treatments such as heparin. One of skill can administer the compositions of the present invention in accordance with well known techniques in the art. Further, one of skill can routinely select an effective amount of a histone compound to be administered based upon the experimental data provided herein. Extrapolation from effective amounts in cells to doses to be administered in vivo is performed routine by those skilled in the art.

Histone compounds can also be used to coat implantable devices such as stents or valves, or to coat devices inserted in the body of a patient for either short or prolonged periods such as catheters. In the context of the present invention an implantable device would include devices that are placed inside the blood vessels or the heart for any amount of time, either short-term or long- term. To prepare the coating, the histone compound is formulated in a physiologically acceptable matrix. In the context of the present invention a physiologically acceptable matrix is one that is not recognized as foreign to the immune system and is biocompatible (a substance that does not produce any irritation, immunogenic, or inflammatory response).

The following non-limiting examples are provided to further illustrate the present invention.

EXAMPLES Example 1: Platelet Aggregation Assay Blood was drawn by venopuncture into plastic tubes that contained 1/10 volume 3.8% citrate and platelet-rich plasma (PRP) was prepared in accordance with known techniques (Soslau, G. and J. Giles. 1982. Thrombos. Res.

26: 443-455). Blood samples were obtained from healthy donors who were medication free. Washed platelets were prepared from the PRP by known methods (Basheer, A. R. et al. 1995. Biochim. Biophys. Acta 1250: 97-109). Briefly, PRP was diluted with 3 volumes of 100 nM citrate buffer (pH 6.0) plus 1 to 2 volumes of hepes Tyrode's buffer (pH 7.4), final volume of 50 ml, and resuspended in hepes Tyrode's buffer with 1 mg/ml dextrose plus 1 mg/ml bovine serum albumin at 2 x 1061au to 3 x 106/yl (a 10X normal concentration).

Platelet aggregations were performed on a dual- channel Chronolog lumi aggregometer (Chronolog Corp., Havertown, PA) by known methods (Soslau, G. et al. 1988.

Biochem. Biophys. Res. Commun. 15: 909-916). Aggregations were conducted with 480 Al washed platelets or a 50 Al sample of the concentrated platelets added to 430 Al hepes Tyrode's buffer with a final platelet count of 2 x 105/yl to 3 x 105/yl. Agonists and antagonists were added at various concentrations.