Inhibition of RAS-targeted prenylation: protein farnesyl transferase inhibitors revisited

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Abstract

The ras oncogene and its 21 kD protein product, Ras, has emerged during the last decade as a potentially exploitable target for anticancer drug development. The knowledge that Ras was readily prenylated by protein farnesyl transferase (PFTase) and that inhibition of this prenylation had functional consequences for the transformed phenotype that expressed oncogenic Ras provided the rational for the development of PFTase inhibitors. The initial enthusiasm for this approach seemed justified by the early identification of PFTase inhibitors that were able potently and specifically to block Ras processing, signalling and transformation in transformed and tumour cell lines in vitro and in certain selected animal models. More recently the recognition that geranylgeranyl transferase (GGTase) I might also be a therapeutic target is being actively researched. The last couple of years though have proved remarkable with the disclosure of a series of structurally-diverse molecules, whose major in vivo preclinical activites have been well documented against experimental animal tumours, and culminating this year in preliminary reporting of their Phase I clinical evaluations. Nevertheless, during the research and development phases of PFTase inhibitors as pharmaceutical agents for clinical use, there have been several unexpected findings which have raised intriguing and potentially crucial questions about their activities. This review aims to highlight and offer new insights into many of these issues and to bring into perspective concerns arising from basic research, as well as from clinical studies. There seems little doubt that these inhibitors of RAS-targeted prenylation represent a new generation of anticancer drugs for the preclinical researcher, whether they can be successfully exploited in clinical practice should be resolved early in the next millenium.

Section snippets

Background and introduction

The 1990s were to be the era for exploiting the earlier advances in molecular oncology which had greatly increased our understanding of certain genetic events that were associated with the propensity for uncontrolled proliferation leading to tumourigenesis. Such knowledge had opened up potentially exploitable targets for anti-cancer drug development-one of which was the ras oncogene and its 21 kD protein product, Ras. The knowledge that Ras was readily prenylated by protein farnesyl transferase

Increasing complexity of the ras signalling pathway

Ras comprises a family of GTP-dependent proteins (G-proteins) that are localised at the inner surface of the cell membrane and participate in transmitting signals for growth and many other processes from the outside to the inside of the cell [11], [12], [13].

These G-proteins cycle between a GTP-binding active ‘ON’ state and a GDP-binding inactive ‘OFF’ state. The biological activity of Ras is controlled by a regulated GDP/GTP cycle. Guanine nucleotide exchange factors (GEFs; RasGRF 1 / 2 and

Initial characterisation of PFTase inhibitors

Inevitably the first screen for PFTase inhibitors involves their evaluation in direct enzyme assays, initially H-Ras was commonly used, and literature reports abound now with examples citing IC50 values in nanomolar or even subnanomolar ranges. The next stage requires an evaluation of their ability to inhibit Ras processing in whole cells and again experimenters generally favoured H-Ras overexpressing cells. Due to poor cell permeability and instability, compounds frequently have lost 2–3 logs

Perspectives and conclusions

The question as to how these novel prenyltransferase inhibitors might best be used clinically is crucial to their subsequent development, not only in the clinical setting itself, but also in their preclinical development study design. Realistically, on the present evidence, with the current generation of PFTase and GGTase I inhibitors, their successful use in cancer therapy as single agents is highly unlikely. Thus we are left to consider the question of their usage in combinations and here it

Reviewers

Dr François F. Lavelle, Rhône-Poulenc Rorer, Centre de Recherche de Vitry-Alfortville, 13, quai Jules Guesde, F-94403 Vitry-sur-Seine, France.
Dr Serge Halazy, Department of Chemistry Serono Pharmaceutical Research Institute, CH-1228 Plan-les-Ouates, Switzerland.
Professor Gilles Favre, Scientific Director, Centre de Lutte contre le Cancer Claudius Regaud, Université Paul Sabatier, 20–24, rue du pont Saint Pierre, F-31052 Toulouse, France.

Dr Bridget T Hill PhD, EurChem, FRSC, FIBiol, graduated from the University of London obtaining her doctorate in Biochemistry before gaining post-doctoral experience in London, Philadelphia (USA) and Toronto (Canada). She returned to the ICRF Laboratories, London (UK) and as Head of Cellular Chemotherapy researched fundamental drug resistance mechanisms and collaborated in clinical studies devising new therapies for cancers of the head and neck and breast. In 1993 she moved to direct cancer

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    Dr Bridget T Hill PhD, EurChem, FRSC, FIBiol, graduated from the University of London obtaining her doctorate in Biochemistry before gaining post-doctoral experience in London, Philadelphia (USA) and Toronto (Canada). She returned to the ICRF Laboratories, London (UK) and as Head of Cellular Chemotherapy researched fundamental drug resistance mechanisms and collaborated in clinical studies devising new therapies for cancers of the head and neck and breast. In 1993 she moved to direct cancer research at CRPF, Castres, France, concentrating on new drug discovery, with a current interest in PFTase inhibitors.

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