Until recently, the therapeutic treatment of
breast cancer has been dominated by endocrine-based drugs (oestrogen receptor antagonists,
aromatase inhibitors etc.) and conventional cytotoxics (
doxorubicin,
cyclophosphamide,
5-fluorouracil etc.). However, the advent of new generation signal transduction inhibitor drugs targeted against the molecular abnormalities of
breast cancer (e.g., the antibody
trastuzumab, directed against the cERBB2 receptor) has the promise of providing a new era of more tumour selective
therapy. Inhibitors of the
enzyme farnesyl
transferase (FTIs) are now undergoing early-stage clinical trials, including in patients with advanced
breast cancer. Although originally developed as inhibitors of RAS signal transduction pathways, it is now apparent that these drugs are better described as prenylation inhibitors; the addition of a 15-carbon
prenyl or farnesyl moiety by farnesyl
transferase being critical to the function of a number of
proteins, including RAS. At least three FTIs are currently undergoing clinical evaluation;
R115777 (
tipifarnib,
Zarnestra),
SCH66336 (
lonafarnib,
Sarasar) and
BMS-214662. In terms of their potential use in the chemotherapeutic treatment of advanced
breast cancer, a Phase II trial of
R115777 (using either continuous or intermittent twice-daily oral dosing) has demonstrated promising activity (approximately 10% partial response rate). Overall, however, the single agent activity of FTIs in various Phase II trials has been rather modest (as well as the above mentioned
breast cancer trial, some responses have been seen in patients with acute and chronic myeloid leukaemias). The main dose-limiting toxicities that have been reported are myelosuppression and
fatigue and neurotoxicity (with
R115777). Two Phase III trials of
R115777 in colorectal (versus placebo) and pancreatic (with
gemcitabine versus placebo)
cancer have failed to show a survival benefit. It is likely that the future clinical direction of FTIs will be as combination
therapy, especially with the
taxanes, where synergy has been seen in a variety of preclinical studies.