Farnesyltransferase inhibitors. Preclinical development

Farnesyltransferase Inhibitors and Their Role in the Treatment of Multiple Myeloma

BACKGROUND

Ras mutations are among the most common oncogene mutations found in multiple myeloma (MM). Patients with mutated Ras are less likely to respond to chemotherapy and have a shortened median survival. Therefore, targeting Ras farnesylation may be a valuable approach to treatment of MM. R115777 (tipifarnib) is a potent farnesyltransferase inhibitor (FTI) presently undergoing phase II/III clinical trials.

METHODS

We reviewed the preclinical and clinical experience of FTIs as antineoplastic agents and describe their potential role in the treatment of MM.

RESULTS

FTIs are a novel group of agents that selectively inhibit farnesyltransferase, an enzyme responsible for the posttranslational modification of several proteins including Ras. Since Ras is among the most commonly mutated oncogenes associated with cancer, this class of drugs has been evaluated in clinical trials in a diversity of tumors. R115777 has been evaluated in a phase II clinical trial in patients with advanced myeloma and found to be well tolerated. It induced disease stabilization in more than 60% of patients with advanced myeloma.

CONCLUSIONS

The drug selectively targets farnesyltransferase, but this effect did not correlate with disease stabilization, suggesting that these drugs may be targeting a survival pathway independent of Ras processing. Further studies will evaluate the use of FTI in maintenance therapy as well as in combination with other agents in advanced myeloma.

Farnesyltransferase inhibitors and their potential role in therapy for myelodysplastic syndromes and acute myeloid leukaemia

Novel strategies are required for treatment of acute myeloid leukaemia (AML) and higher risk myelodysplastic syndrome (MDS) patients who are not eligible for intensive chemotherapy and/or allogenic stem cell transplantation. As activating RAS mutations are frequent in these diseases, one novel approach, consisting of interfering with isoprenylation of RAS proteins by farnesyltransferase inhibitors (FTIs), has been proposed. Clinical phase II studies with the oral FTIs tipifarnib and lonafarnib in previously untreated AML, MDS and chronic myelomonocytic leukaemia yielded rather encouraging results while results in relapsed and/or refractory AML were disappointing. Results of a phase III trial in untreated AML in the elderly with tipifarnib were also disappointing. Clinical responses were not related to RAS mutations, suggesting additional actions of FTIs on other molecular targets. The combination of existing FTIs with other treatments, such as chemotherapy (in AML) and hypomethylating agents (in MDS and AML), is under investigation. Ongoing studies will also determine if gene profiling analysis may help to identify patients that will respond to FTIs.

Surfing the piperazine core of tricyclic farnesyltransferase inhibitors

In order to fully explore structure-activity relationships at the 1- and 2-positions of the piperazine core of tricyclic farnesyltransferase inhibitors, an 11,718-member ECLiPS library was synthesized and screened in a farnesyltransferase scintillation proximity assay. A detailed description of the library and analyses of the screening data will be provided.

Guiding farnesyltransferase inhibitors from an ECLiPS library to the catalytic zinc

Farnesyltransferase inhibitors identified from an ECLiPS library were optimized using solution-phase synthesis. X-ray crystallography of inhibited complexes was used to identify substructures that coordinate to the active site zinc. The X-ray structures were ultimately used to guide the design of second-generation analogs with FTase IC(50)s of less than 1.0 nM.

Investigation of the effect of the farnesyl protein transferase inhibitor R115777 on isoprenylation and intracellular signalling by the prostacyclin receptor

The human (h) and mouse (m) prostacyclin receptors (IPs) undergo isoprenylation through attachment of a C-15 farnesyl moiety within their conserved carboxyl terminal -CSLC sequences. Herein, the effects of a novel farnesyl transferase inhibitor R115777 on signalling by the hIP and mIP, overexpressed in human embryonic kidney 293 cells, and by the hIP endogenously expressed in human erythroleukaemia cells were investigated. R115777 significantly impaired IP-mediated cyclic AMP generation (IC(50) 0.37-0.60 nm) and intracellular calcium ([Ca(2+)](i)) mobilization (IC(50) 37-65 nm), but had no effect on signalling by the control nonisoprenylated beta(2) adrenergic receptor or the alpha or beta isoforms of the human thromboxane A(2) receptor (TP). Additionally, R115777 significantly reduced IP-mediated cross-desensitization of signalling by the TP alpha, but not by the TP beta, isoform of the human TP and impaired the farnesylation-dependent processing of the chaperone HDJ-2 protein (IC(50) 4.5 nm). Furthermore, R115777 fully impaired isoprenylation of both the Ha-Ras(WT) and Ha-Ras(CSLC) in vitro and in whole cells confirming that, unlike N-Ras and Ki-Ras, the -CSLC motif associated with the IP cannot support alternative geranylgeranylation in the presence of R115777 and does not act as a substrate for geranylgeranyl transferase 1 in vitro or in whole cells. In conclusion, these data confirm that R115777 potently impairs IP isoprenylation and signalling, and suggest that clinically it may not only target Ras proteins but may also disrupt IP isoprenylation, events which could impact on physiologic processes in which prostacyclin and its receptor are implicated.

Short-term local delivery of an inhibitor of Ras farnesyltransferase prevents neointima formation in vivo after porcine coronary balloon angioplasty

BACKGROUND

Mitogenic stimuli present at the site of coronary arterial balloon injury contribute to the progression and development of a restenotic lesion, many signaling through a common pathway involving the small G protein p21(ras). Our aim was to demonstrate in biochemical studies that farnesyl protein transferase inhibitor III (FPTIII) is an inhibitor of p21(ras) processing and that when it is given locally in vivo at the site of coronary balloon injury in a porcine model, it can inhibit neointima formation.

METHODS AND RESULTS

FPTIII (1 to 25 micromol/L) concentration-dependently reduced p21(ras) levels in porcine coronary artery smooth muscle cell membranes. FPTIII also prevented p42/p44 MAPK activation and DNA synthesis in response to platelet-derived growth factor in these cells at a concentration of 25 micromol/L. Application of 25 micromol/L FPTIII locally for 15 minutes to balloon-injured porcine coronary arteries in vivo prevented neointima formation assessed at 4 weeks, reduced proteoglycan deposition, and inhibited adventitial hypertrophy. Coronary arteries from FPTIII-treated pigs had no deterioration in contraction or in endothelium-dependent relaxation.

CONCLUSIONS

The study demonstrates in the pig that short-term local delivery of inhibitors of p21(ras)-dependent mitogenic signal transduction prevents restenosis after balloon angioplasty.

Novel compounds in the therapy of breast cancer: opportunities for integration with docetaxel

Increasingly, novel agents are being developed specifically at inhibition of growth factor receptors and events within the signal transduction pathway. These agents include the epidermal growth factor tyrosine kinase inhibitors, the farnesyl transferase inhibitors, and bcl-2 antisense oligonucleotides. Along with these new approaches to molecular targeting, it will be necessary to develop new study designs for drug evaluation. Target validation in both normal surrogate tissues and tumor tissue becomes increasingly relevant in early clinical trials. Furthermore, antitumor efficacy may no longer correlate with normal hematological or nonhematological toxicity, and it may be more appropriate in phase I trials to identify the maximum target inhibition dose rather than the maximum tolerated dose. Moreover, measures of cytoreduction, such as complete and partial response, may be less relevant than disease stabilization for some of these novel agents which have limited cytotoxic effects and would be considered cytostatic agents. Assessment of single-agent activity and the future role in conjunction with cytostatic agents represents the single most important challenge facing the clinical development of these molecular targeted therapies.

Inhibition of neointimal formation in porcine coronary artery by a Ras mutant

BACKGROUND

Therapeutic approaches to reduce the neointimal formation caused by balloon injury have been focused mainly on experimental models of restenosis in the rat carotid artery. However, restenosis in rat carotid artery may not replicate the coronary arterial responses to injury in larger animals and humans.

METHODS

In this study, we used pig coronary arteries as an animal model to evaluate the preventive effects of a virus-mediated dominant negative mutant RasN17 on balloon injury-induced restenosis. The viral particles were delivered to the balloon-injured coronary arteries via a dispatch catheter to keep the virus in a confined arterial segment for 10 min to reach optimal transfection. Six weeks after balloon injury, the pigs were sacrificed and the left anterior descending arteries were isolated for histological analysis.

RESULTS

Neointima formation was prominent in the group receiving balloon injury as compared with the uninjured controls. A remodeling process with migration of collagen was also found in the injured coronary arteries. The application of AdRasN17 led to a 56% decrease in neointima formation and a 75% increase in lumen size, as compared with the balloon-injured vessels treated with AdLacZ control.

CONCLUSIONS

These results suggest that AdRasN17 is an effective therapeutic gene in preventing balloon injury-induced neointimal formation in pig coronary arteries.

The microenvironment of the tumour-host interface

Throughout the entire process of cancer aetiology, progression and metastasis, the microenvironment of the local host tissue can be an active participant. Invasion occurs within a tumour-host microecology, where stroma and tumour cells exchange enzymes and cytokines that modify the local extracellular matrix, stimulate migration, and promote proliferation and survival. A new class of cancer therapies that targets this pathological communication interface between tumour cells and host cells is currently under development.

Inhibition of farnesyltransferase increases TGFbeta type II receptor expression and enhances the responsiveness of human cancer cells to TGFbeta

Several small GTPases of the Ras superfamily have been shown to antagonize TGFbeta signaling in human tumor cell lines. Some of these GTPases are post-translationally modified by farnesylation, a lipid modification catalyzed by farnesyltransferase and required for the proteins to attach to membranes and to function. In this study, we investigated the effect of the farnesyltransferase inhibitor FTI-277 on TGFbeta-regulated cell growth and transcription. Treatment of the human pancreatic tumor cell line, Panc-1, with FTI-277 enhanced the ability of TGFbeta to inhibit both anchorage-dependent and -independent tumor cell growth. FTI-277 also enhanced the ability of TGFbeta to induce transcription, as measured by p3TP-lux reporter activity and collagen synthesis. The enhancement of TGFbeta responses by FTI-277 correlated with the stimulation of transcription and protein expression of type II TGFbeta receptor (TbetaRII). Consequently, FTI-277-treated cells exhibited a higher level of TGFbeta binding to its receptor. Thus, inhibition of protein farnesylation stimulates TbetaRII expression, which leads to increased TGFbeta receptor binding and signaling as well as inhibition of tumor cell growth and transformation.