Ras oncogenes are the most frequently identified activated oncogenes in human tumors (1-3). It is known that the ras oncogenes encode 21,000 dalton G-proteins (p21ras) which play an essential role in growth factor signal transduction, proliferation and malignant transformation (1-7). Association of p21ras with the plasma membrane is required for its transforming activity (8,9). Post-translational events leading to membrane association of p21ras have previously been disclosed (10-14). The p21ras proteins are first made as pro-p21ras in the cytosol where they are modified on cysteine 186 of their carboxyl terminal sequence CA.sub.1 A.sub.2 X (C=cysteine, A.sub.1 and A.sub.2 =isoleucine, leucine or valine and X=methionine or serine) by the cholesterol biosynthesis intermediate farnesyl pyrophosphate (FPP). This farnesylation reaction is then followed by peptidase removal of the A.sub.1 A.sub.2 X tripeptide and carboxymethylation of the remaining cysteine. The processed p21ras proteins associate with the inner surface of the plasma membrane and are further modified on cysteines 181-184 by another lipid, palmitic acid (10-14).
p21Ras farnesyltransferase, the enzyme responsible for catalyzing the transfer of farnesyl, a 15-carbon isoprenoid, from FPP to the cysteine of the CA.sub.1 A.sub.2 X carboxyl terminus of p21ras, has been purified to homogeneity from rat brain (15,16). The enzyme is a heterodimer composed of .alpha. and .beta. subunits of molecular weights 49 and 46 kDa, respectively (17). The B subunit has been shown (17) to bind p21ras and the .alpha. subunit is believed to bind FPP. Because p21ras farnesylation and subsequent membrane association are required for p21ras transforming activity, it has been proposed that p21ras farnesyltransferase would be a useful anticancer therapy target. Accordingly, an intensive search for inhibitors of the enzyme is underway (18-24). Potential inhibitor candidates are CA.sub.1 A.sub.2 X tetrapeptides which have been shown to be farnesylated by p21ras farnesyltransferase and appear to be potent inhibitors of this enzyme in vitro (15,18,21-24). Competition studies have demonstrated that CA.sub.1 A.sub.2 X peptides with the greatest inhibitory activity are those where A.sub.1 and A.sub.2 are hydrophobic peptides with charged or hydrophilic residues in the central positions demonstrating very little inhibitory activity (18,21,23).
The research efforts directed towards farnesyltransferase and the inhibition of its activity are further illustrated by the following patents or published patent applications:
U.S. Pat. No. 5,141,851 PA1 WO 91/16340 PA1 WO 92/18465 PA1 EPA 0456180 A1 PA1 EPA 0461869 A2 PA1 EPA 0512865 A2 PA1 EPA 0520823 A2 PA1 EPA 0523873 A1 PA1 Cys is a cysteine amino acid; PA1 Xaa.sup.1 is an amino acid in natural L-isomer form; PA1 dXaa.sup.2 is an amino acid in unnatural D-isomer form; and PA1 Xaa.sup.3 is an amino acid in natural L-isomer form. PA1 Cys-Val-Leu-Ser PA1 Cys-Val-Ile-Met PA1 Cys-Val-Val-Met PA1 C stands for the cysteine radical; PA1 AMBA is the radical of an aminomethylbenzoic acid; and PA1 X is the radical of an amino acid, preferably Met or Phe, although for certain purposes, other amino acids, e.g. Lys, Glu, Cys or Leu, may also be used. Any other natural or unnatural amino acid can also be used at this position.
Of the above disclosures, EPA 0520823 A2 discloses compounds which are useful in the inhibition of farnesyl-protein transferase and the farnesylation of the oncogene protein ras. The compounds of EPA 0520823 A2 are illustrated by the formula: EQU Cys-Xaa.sup.1 -dXaa.sup.2 -Xaa.sup.3
or pharmaceutically acceptable salts thereof,
wherein
The preferred compounds are said to be CV(D)lS and CV(Df)M, the amino acids being identified by conventional 3 letter and single letter abbreviations as follows:
______________________________________ Cysteine Cys C Glycine Gly G Isoleucine Ile I Leucine Leu L Methionine Met M Phenylalanine Phe F Serine Ser S Threonine Thr T Valine Val V ______________________________________
EPA 0523873 A1 discloses a modification of the compounds of EPA 0520823 A2 wherein Xaa.sup.3 is phenylalanine or p-fluorophenylalanine.
EPA 0461869 describes compounds which inhibit farnesylation of Ras protein of the formula:(SEQ ID NO:1) EQU Cys-Aaa.sup.1 -Aaa.sup.2 -Xaa
where Aaa.sup.1 and Aaa.sup.2 are aliphatic amino acids and Xaa is an amino acid. The aliphatic amino acids which are disclosed are Ala, Val, Leu and Ile. Preferred compounds are those where Aaa.sup.1 is Val, Aaa.sup.2 is Leu, Ile or Val and Xaa is Ser or Met. Preferred specific compounds are:
U.S. Pat. No. 5,141,851 and WO 91/16340 disclose the purified farnesyl protein transferase and certain peptide inhibitors therefor, including, for example, CVIM, TKCVIM and KKSKTKCVIM. (SEQ ID NO;2 through SEQ ID NO:4)
WO 92/18465 discloses certain farnesyl compounds which inhibit the enzymatic methylation of proteins including ras proteins.
EPA 0456180 A1 is directed to a farnesylprotein transferase assay which can be used to identify substances that block farnesylation of ras oncogene gene products while EPA 0512865 A2 discloses certain cyclic compounds that are useful for lowering cholesterol and inhibiting farnesylprotein transferase.
As will be evident from the foregoing, there is a great deal of research effort directed towards the development of inhibitors of farnesyltransferase. However, there still remains a need for improvements in this critically important area.