Farnesyltransferase inhibitors

J. Gibbs Ph.D., C. Buser Ph.D., J. deSolms Ph.D., C. Dinsmore Ph.D., S. Graham Ph.D., G. Hartman Ph.D., D. Heimbrook Ph.D., H. Huber Ph.D., K. Koblan Ph.D., R. Lobell Ph.D., C. Omer Ph.D., S. Stirdivant Ph.D., T. Williams Ph.D., N.E. Kohl Ph.D.,

Departments of Cancer Research and Medicinal Chemistry, Merck Research Laboratories, West Point, PA

AIM: Mutant ras genes that encode constitutively active Ras proteins are frequently found in a wide variety of human tumors. The Ras oncoproteins acquire their transforming ability upon localization to the inner surface of the plasma membrane via a series of post-translational modifications. The most frequently mutated form of ras in human tumors is Ki-ras. Ki-Ras is a substrate for both farnesyl protein transferase (FPTase) and the related enzyme geranylgeranyl protein transferase type I (GGPTase-I). Thus, when FPTase is inhibited, Ki-Ras becomes geranylgeranylated and localizes to the plasma membrane where it is biologically active. Therefore, we sought to identify and develop an inhibitor of FPTase and GGPTase-I. METHODS: Specific compounds were tested in vitro for inhibition of FPTase and GGPTase-I. The most active compounds were tested for inhibition of protein substrates in cell based assays. Finally, FPTase inhibitors with the appropriate pharmacological properties were tested in animal tumor models including mouse tumor xenografts and transgenic models of spontaneous mammary tumors. RESULTS: L-778123 is a potent inhibitor of FPTase (IC50 = 2 nM) with modest inhibitory activity toward GGPTase-I (IC50 = 98 nM). In cells, L-778123 inhibited the prenylation of both FPTase and GGPTase-I substrates, including Ki-Ras. Furthermore, L-778123 blocked the signaling pathway driven by Ki-Ras. In both MMTV-Ha-ras and MMTV-Ki-rasB transgenic mouse models, L-778123 inhibited tumor growth. However, analysis of protein prenylation in tumors from the treated MMTV-Ki-rasB mice demonstrated that inhibition of only a FPTase substrate but not a GGPTase-I substrate or Ki-Ras. Titration of L-778123 in tumor-bearing mice revealed that very high doses of compound were required to minimally inhibit Ki-Ras prenylation. Further analyses demonstrated that significant inhibition of Ki-Ras processing could be detected in tumors from mice treated with other, more potent dual prenyltransferase inhibitors as well as combinations of FPTase and GGPTase-I inhibitors. However, administration of these compounds for more than one day to mice resulted in lethality. This lethality correlated with significant inhibition of GGPTase-I and inhibition of Ki-Ras prenylation. While one of the more potent dual prenyltransferase inhibitors induced tumor regression in MMTV-Ki-rasB mice following a single dose, a similar tumor regression was observed in mice treated with a selective FPTase inhibitor administered at a dose that yielded equipotent FPTase inhibitory activity (but no GGPTase-I inhibition). These data suggest that inhibition of the transforming activity of oncogenically mutated Ki-Ras by inhibition of prenylation may not be a tractable anti-tumor strategy. L-778123 has undergone phase I and phase II studies in humans. Analysis of prenylation of a FPTase substrate in blood cells from treated patients demonstrated a dose dependent inhibition of the enzyme in vivo. Several pharmaceutical companies are currently pursuing the development of FPTase inhibitors in the clinic.

For more information, contact

Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Paris, France; February 9 - 12, 2002; in the section on Novel Molecular Therapies.