Farnesyltransferase inhibitor
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The farnesyltransferase inhibitors are a class of experimental cancer drugs that target 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 controversial, as will be explained below. After translation RAS goes through four steps of modification: isoprenylation, proteolysis, methylation and palmitoylation. Isoprenylation involves the enzyme farnesyltransferase (FFTase) transferring a farnesyl group from 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 (Reuter et al., 2000).
After a programme of high-throughput screening a class of drugs targeting the first step, the farnesyltransferase inhibitors (FTIs), were developed (Reuter et al., 2000). 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 (Ionafarnib) was the first to do so, followed by R115777 (Zarnestra, Tipifarnib) (Caponigro et al., 2003).
However, the predicted “early potential [of FTIs] has not been realised” (Downward J, 2003). 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 becomes blocked by FFTase inhibitors this pathway comes in to 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 surive (Downward J, 2003). So how to explain the preclinical successes – many N- or K-RAS transformed cell lines (and even tumor cell lines that do not harbor RAS mutations) are sensitive to FTase inhibitors? It has been suggested that this is due to inhibition of farnesylation of a number of other proteins (Reuter et al., 2000). Therefore it is hoped that FTIs, whilst not RAS specific, still have potential for cancer therapy.
References
- Caponigro F, Casale M, Bryce J. (2003). Farnesyl transferase inhibitors in clinical development. Expert Opin Investig Drugs. 12:943-54
- Downward J. (2003). Targeting the RAS Signalling Pathway in Cancer Therapy. Nat Rev Cancer, 3:11-22
- Reuter WM, Morgan MA, Bergmann L. (2000). Targeting the Ras signaling pathway: a rational, mechanism-based