Farnesyltransferase inhibitor

The farnesyltransferase inhibitors (FTIs) are a class of experimental cancer drugs that target protein farnesyltransferase with the downstream effect of preventing the proper functioning of the Ras (protein), which is commonly abnormally active in cancer. The farnesyltransferase inhibitors (FTIs) are a class of experimental cancer drugs that target protein farnesyltransferase with the downstream effect of preventing the proper functioning of the Ras (protein), which is commonly abnormally active in cancer. Studies have suggested that interference with certain post-translational modification processes seem to have quite a high selectivity for targeting cells displaying tumour phenotypes, although the reason for this is a matter of controversy. After translation, Ras goes through four steps of modification: isoprenylation, proteolysis, methylation and palmitoylation. Isoprenylation involves the enzyme farnesyltransferase (FTase) transferring a farnesyl group from farnesyl pyrophosphate (FPP) to the pre-Ras protein. Also, a related enzyme geranylgeranyltransferase I (GGTase I) has the ability to transfer a geranylgeranyl group to K and N-Ras (the implications of this are discussed below). Farnesyl is necessary to attach Ras to the cell membrane. Without attachment to the cell membrane, Ras is not able to transfer signals from membrane receptors. After a program of high-throughput screening of a class of drugs targeting the first step, the farnesyltransferase inhibitors (FTIs) were developed. One FTI found in the screening was clavaric acid, a mushroom isolate. A number of molecules were found to have FTI activity. Some earlier compounds were found to have major side effects, and their development was discontinued. The others have entered clinical trials for different cancers. SCH66336 (Lonafarnib) was the first to do so, followed by R115777 (Zarnestra, Tipifarnib). Unfortunately, the predicted 'early potential has not been realised'. The anti-tumour properties of FTIs were attributed to their action on Ras processing; however this assumption has now been questioned. Of the three members (H, N and K) of the Ras family, K-Ras is the form found most often mutated in cancer. As noted above, as well as modification by FFTase an alternative route to creation of biologically active Ras is through GGTase modification. When FFTase is blocked by FFTase inhibitors this pathway comes into operation – both K and N-Ras are able to be activated through this mechanism. In recognition of this a joint administration of FTIs and GTIs was tried, however this resulted in high toxicity. It is in fact thought that the lack of FTI toxicity may be due to a failure to fully inhibit Ras: FTIs actually target normal cells but alternative pathway allow these cells to survive (Downward J, 2003). It has been suggested that the preclinical successes showing that many N- or K-Ras transformed cell lines (and even tumor cell lines that do not harbor Ras mutations) are sensitive to FTase inhibitors due to inhibition of farnesylation of a number of other proteins. Therefore, it is hoped that FTIs, whilst not Ras specific, still have potential for cancer therapy.