Farnesyltransferase and geranylgeranyltransferase I inhibitors in cancer therapy: Important mechanistic and bench to bedside issues

The fact that proteins such as Ras, Rac and RhoA require farnesylation or geranylgeranylation to induce malignant transformation prompted many investigators to develop farnesyltransferase (FTase) and geranylgeranyltransferase I (GGTase I) inhibitors (FTIs and GGTIs, respectively) as novel anticancer drugs. Although FTIs have been shown to antagonise oncogenic signalling, reverse malignant transformation, inhibit human tumour growth in nude mice and induce tumour regression in transgenic mice without any signs of toxicity, their mechanism of action is not known. This review will focus on important mechanistic issues as well as bench to bedside translational issues. These will include the relevance to cancer therapy of the alternative geranylgeranylation of K-Ras when FTase is inhibited; a thorough discussion about evidence for and against the involvement of inhibition of prenylation of Ras and RhoB in the mechanism of FTIs' antitumour activity as well as effects of FTIs and GGTIs on the cell cycle machinery and the dynamics of bipolar spindle formation and chromosome alignment during mitosis. Bench to bedside issues relating to the design of hypothesis-driven clinical trials with biochemical correlates for proof-of-concept in man will also be discussed. This will include Phase I issues such as determining maximally tolerated dose (MTD) versus effective biological dose (EBD), as well as whether Phase II trials are still needed for clinical evaluations of anti-signalling agents. Other questions that will be addressed include: what levels of inhibition of FTase activity are required for tumour response in Phase II clinical evaluations? What FTase substrates are most relevant as biochemical correlates? Are signalling pathways such as H-Ras/P13K/Akt and K-Ras/Raf/MEK/Erk significant biological readouts? Does Ras mutation status predict response? What are appropriate clinical end-points for FTI Phase II trials? For this latter important question, time to tumour progression, median survival, percentage of patients that progress, clinical benefits and improvement in quality of life will all be discussed.