Preclinical and clinical evaluation of farnesyltransferase inhibitors

Protein Farnesylation on Nasopharyngeal Carcinoma, Molecular Background and Its Potential as a Therapeutic Target

Simple Summary

Nasopharyngeal carcinoma is distinguished from other head and neck carcinomas by the association of its carcinogenesis with the Epstein–Barr virus. It is highly metastatic, and a novel therapeutic modality for metastatic nasopharyngeal carcinoma is keenly awaited. Protein farnesylation is a C-terminal lipid modification of proteins and was initially investigated as a key process in activating the RAS oncoprotein through its association with the cellular membrane structure. Since then, more and more evidence has accumulated to indicate that proteins other than RAS are also farnesylated and have significant roles in carcinogenesis. This review delineates molecular pathogenesis through protein farnesylation in the context of nasopharyngeal carcinoma and discusses the potential of farnesylation as a therapeutic target.

Abstract

Nasopharyngeal carcinoma (NPC) is one of the Epstein–Barr virus (EBV)-associated malignancies. NPC is highly metastatic compared to other head and neck carcinomas, and evidence has shown that the metastatic features of NPC are involved in EBV infection. The prognosis of advanced cases, especially those with distant metastasis, is still poor despite advancements in molecular research and its application to clinical settings. Thus, further advancement in basic and clinical research that may lead to novel therapeutic modalities is needed. Farnesylation is a lipid modification in the C-terminus of proteins. It enables proteins to attach to the lipid bilayer structure of cellular membranes. Farnesylation was initially identified as a key process of membrane association and activation of the RAS oncoprotein. Farnesylation is thus expected to be an ideal therapeutic target in anti-RAS therapy. Additionally, more and more molecular evidence has been reported, showing that proteins other than RAS are also farnesylated and have significant roles in cancer progression. However, although several clinical trials have been conducted in cancers with high rates of ras gene mutation, such as pancreatic carcinomas, the results were less favorable than anticipated. In contrast, favorable outcomes were reported in the results of a phase II trial on head and neck carcinoma. In this review, we provide an overview of the molecular pathogenesis of NPC in terms of the process of farnesylation and discuss the potential of anti-farnesylation therapy in the treatment of NPC.

Ras Isoforms from Lab Benches to Lives-What Are We Missing and How Far Are We?

The central protein in the oncogenic circuitry is the Ras GTPase that has been under intense scrutiny for the last four decades. From its discovery as a viral oncogene and its non-oncogenic contribution to crucial cellular functioning, an elaborate genetic, structural, and functional map of Ras is being created for its therapeutic targeting. Despite decades of research, there still exist lacunae in our understanding of Ras. The complexity of the Ras functioning is further exemplified by the fact that the three canonical Ras genes encode for four protein isoforms (H-Ras, K-Ras4A, K-Ras4B, and N-Ras). Contrary to the initial assessment that the H-, K-, and N-Ras isoforms are functionally similar, emerging data are uncovering crucial differences between them. These Ras isoforms exhibit not only cell-type and context-dependent functions but also activator and effector specificities on activation by the same receptor. Preferential localization of H-, K-, and N-Ras in different microdomains of the plasma membrane and cellular organelles like Golgi, endoplasmic reticulum, mitochondria, and endosome adds a new dimension to isoform-specific signaling and diverse functions. Herein, we review isoform-specific properties of Ras GTPase and highlight the importance of considering these towards generating effective isoform-specific therapies in the future.

Phase I study of lonafarnib (SCH66336) in combination with trastuzumab plus paclitaxel in Her2/neu overexpressing breast cancer: EORTC study 16023

PURPOSE

This phase I study was performed to determine the maximum tolerated dose (MTD), dose-limiting toxicities (DLT), safety profile, recommended dose for phase II studies, the pharmacokinetics, and antitumor activity of the combination of lonafarnib (farnesyl transferase inhibitor), trastuzumab, and paclitaxel in Her2-positive advanced breast cancer.

METHODS

Twenty-three patients with Her2-overexpressing breast cancer received in the first cycle paclitaxel and trastuzumab and from cycle 2 onwards lonafarnib which was added to the combination. Dose-limiting toxicity (DLT) was determined during the second cycle.

RESULTS

The MTD and the recommended dose for phase II trials are lonafarnib: 250 mg/day [125 mg/bi-daily (BID)] continuously, paclitaxel: 175 mg/m² 3-h infusion every 3 weeks, and trastuzumab: 4 mg/kg loading dose and 2 mg/kg/week thereafter. The most frequently observed adverse events starting from cycle 1 onwards were alopecia, myalgia, sensory neuropathy, fatigue, arthralgia, leukocytopenia, and neutropenia. From cycle 2 onwards, additional adverse events appeared, such as diarrhea, nausea, dyspepsia, vomiting, and allergy. The mean systemic exposures of both lonafarnib and paclitaxel through all dose levels were higher in the regimen with all three study medications but with no statistically significant difference. Preliminary antitumor activity (CR + PR) was observed in 58% of all patients.

CONCLUSION

Lonafarnib can be safely combined and tolerated with full doses of paclitaxel and trastuzumab in Her2-positive advanced breast cancer patients. Promising preliminary antitumor activity warrants further evaluation of lonafarnib in combination with paclitaxel and trastuzumab in Her2-positive breast cancer.

Long-term treatment of farnesyltransferase inhibitor FTI-277 induces neurotoxicity of hippocampal neurons from rat embryo in a ROS-dependent manner

Despite the well established anti-cancer effect of farnesyltransferase inhibitor FTI-277, the neurotoxic effects of the agent are not yet clearly defined at the molecular and cellular levels. Here, we report the neurotoxic effects of FTI-277 and the involvement of reactive oxygen species (ROS) in FTI-induced neurotoxicity. Although there is no significant effect of FTI-277 for 2 days, long-term treatment of FTI-277 for 4 days induced dramatic reduction in outgrowth, maturation and branching of neuritis and considerable cytoxicity in a dose- and time-dependent manner in primary cultured rat embryo hippocampal neurons. Interestingly, FTI-277 for 4 days dramatically decreased expression of synapsin I, a crucial molecule involved in the neuronal growth and plasticity, and increased a cytotoxic G-protein RhoB of which ectopic expression induced the neurotoxicity in hippocampal neurons. Moreover, treatment with FTI-277 dramatically increased intracellular levels of ROS, which was sustained for 4 days; while blockage of ROS rescued FTI-277-induced neurotoxicity as well as both decrease of synapsin I and increase of RhoB. Taken together, these results provide the molecular insights for the mechanisms which might be of use aiming for avoiding neurotoxic side effects by FTI agent for a drug development for a clinical use.

SCH66336, inhibitor of protein farnesylation, blocks signal transducer and activators of transcription 3 signaling in lung cancer and interacts with a small molecule inhibitor of epidermal growth factor receptor/human epidermal growth factor receptor 2

Signal transducer and activators of transcription 3 (STAT3) is an important transcription factor that is essential for lung cancer cell survival. STAT3 is activated by diverse upstream receptor and nonreceptor tyrosine kinases, and blockade of STAT3 results in tumor growth inhibition. Therefore, a search for STAT3 inhibitors is under way. We demonstrate that SCH66336, at 4 mumol/l, completely blocks STAT3 phosphorlyation in a variety of nonsmall cell lung carcinoma (NSCLC) cell lines, whereas the effect on AKT and extracellular signal-regulated kinase activation is variable. Furthermore, SCH66336 has antiproliferative effects on NSCLC cells. When NSCLC cells are exposed sequentially to SCH66336 and a small molecule dual tyrosine kinase inhibitor of epidermal growth factor receptor and human epidermal growth factor receptor 2, synergistic activity is observed with an increase in the fraction of cells undergoing apoptosis. Concurrent exposure to both agents is, however, associated with antagonism and decreased apoptosis. We conclude that blockade of STAT3 phosphorylation might be one of the mechanisms by which SCH66336 exerts its antitumor activity, and that this can be synergistic in vitro when administered sequentially with epidermal growth factor receptor inhibitors.

Inhibition of the CaaX proteases Rce1p and Ste24p by peptidyl (acyloxy)methyl ketones

The CaaX proteases Rce1p and Ste24p can independently promote a proteolytic step required for the maturation of certain isoprenylated proteins. Although functionally related, Rce1p and Ste24p are unrelated in primary sequence. They have distinct enzymatic properties, which are reflected in part by their distinct inhibitor profiles. Moreover, Rce1p has an undefined catalytic mechanism, whereas Ste24p is an established zinc-dependent metalloprotease. This study demonstrates that both enzymes are inhibited by peptidyl (acyloxy)methyl ketones (AOMKs), making these compounds the first documented dual specificity inhibitors of the CaaX proteases. Further investigation of AOMK-mediated inhibition reveals that varying the peptidyl moiety can significantly alter the inhibitory properties of AOMKs toward Rce1p and Ste24p and that these enzymes display subtle differences in sensitivity to AOMKs. This observation suggests that this compound class could potentially be engineered to be selective for either of the CaaX proteases. We also demonstrate that the reported sensitivity of Rce1p to TPCK is substrate-dependent, which significantly alters the interpretation of certain reports having used TPCK sensitivity for mechanistic classification of Rce1p. Finally, we show that an AOMK inhibits the isoprenylcysteine carboxyl methyltransferase Ste14p. In sum, our observations raise important considerations regarding the specificity of agents targeting enzymes involved in the maturation of isoprenylated proteins, some of which are being developed as anti-cancer therapeutic agents.

Molecular targets in melanoma from angiogenesis to apoptosis

Angiogenesis is a hallmark of melanoma progression. Antiangiogenic agents have been infrequently tested in patients with advanced melanoma. Experience with most other cancers suggests that single-agent application of angiogenic inhibitors is unlikely to have substantial clinical antitumor activity in melanoma. It is more likely that combinations of antiangiogenic agents with either chemotherapy or other targeted therapy will be needed to produce significant clinical benefit. In melanoma, numerous cellular pathways important to cell proliferation, apoptosis, or metastases have recently been shown to be activated. Activation occurs through specific mutations (B-RAF, N-RAS, and PTEN) or changes in expression levels of various proteins (PTEN, BCL-2, NF-kappaB, CDK2, and cyclin D1). Agents that block these pathways are rapidly entering the clinical setting, including RAF inhibitors (sorafenib), mitogen-activated protein kinase inhibitors (PD0325901), mammalian target of rapamycin inhibitors (CCI-779), and farnesyl transferase inhibitors (R115777) that inhibit N-RAS and proteasome inhibitors (PS-341) that block activation of nuclear factor-kappaB (NF-kappaB). It will be a challenge to evaluate these agents alone, in combination with each other, or with chemotherapy in patients with melanoma. Trials with large populations of biologically ill-defined tumors run the risk of missing clinical antitumor activity that is important for a particular yet-to-be-defined subset of patients. To rationally and optimally develop these targeted agents, it will be critical to adequately test for the presence of the presumed cellular target in tumor specimens and the effect of therapy on the proposed target (biological response). Investigators in this field will need to carefully plan these trials so that at the end of the day, we learn from both the failures and successes of targeted therapy.

Solid-phase synthesis and pharmacological evaluation of a library of peptidomimetics as potential farnesyltransferase inhibitors: an approach to new lead compounds

Oncogenic Ras proteins whose activation is farnesylation by farnesyltransferase have been seen as important targets for novel anticancer drugs. Inhibitors of this enzyme have already been developed as potential anti-cancer drugs, particularly by rational design based on the structure of the CA(1)A(2)X carboxyl terminus of Ras. Synthesis of a peptidomimetics library via solid-phase synthesis using the Multipin method is described here. The most active hits on cellular assays were resynthesized and enzymatic activity was measured. Compounds A1, A5 and A7 present significant activity on the isolated enzyme (IC(50)=117, 57.3 and 28.5 nM) and their molecular docking in the active site of the enzyme provides details on key interactions with the protein.

Mutational analysis of the ras converting enzyme reveals a requirement for glutamate and histidine residues

The Ras converting enzyme (RCE) promotes a proteolytic activity that is required for the maturation of Ras, the yeast a-factor mating pheromone, and certain other proteins whose precursors bear a C-terminal CAAX tetrapeptide motif. Despite the physiological importance of RCE, the enzymatic mechanism of this protease remains undefined. In this study, we have evaluated the substrate specificity of RCE orthologs from yeast (Rce1p), worm, plant, and human and have determined the importance of conserved residues toward enzymatic activity. Our findings indicate that RCE orthologs have conserved substrate specificity, cleaving CVIA, CTLM, and certain other CAAX motifs, but not the CASQ motif, when these motifs are placed in the context of the yeast a-factor precursor. Our mutational studies of residues conserved between the orthologs indicate that an alanine substitution at His194 completely inactivates yeast Rce1p enzymatic activity, whereas a substitution at Glu156 or His248 results in marginal activity. We have also determined that residues Glu157, Tyr160, Phe190, and Asn252 impact the substrate selectivity of Rce1p. Computational methods predict that residues influencing Rce1p function are all near or within hydrophobic segments. Combined, our data indicate that yeast Rce1p function requires residues that are invariably conserved among an extended family of prokaryotic and eukaryotic enzymes and that these residues are likely to lie within or immediately adjacent to the transmembrane segments of this membrane-localized enzyme.

Farnesyl-transferase inhibitor R115,777 enhances tamoxifen inhibition of MCF-7 cell growth through estrogen receptor dependent and independent pathways

INTRODUCTION

We have previously shown that FTI-277, a farnesyl transferase inhibitor (FTI), enhances the efficacy of tamoxifen (Tam) in inhibiting the proliferation of the estrogen dependent MCF-7 cell line. As the cellular response to Tam is the result of an inhibition of both estrogen receptor-dependent and -independent pathways, we have used the estrogen receptor selective anti-estrogen ICI182,780 and N-pyrrolidine(-phenylmethyl-phenoxy)-ethanamine-HCl (PBPE), a selective ligand of anti-estrogen binding site (AEBS), to dissect out the mechanism(s) associated with the observed additivity resulting from combination treatment with FTI-277 and Tam. Moreover, for these studies, FTI-277 has been replaced by R115,777, a FTI currently in phase III clinical trials.

METHODS

The quantitative sulphorhodamine B (SRB) colorimetric assay was used to determine the growth inhibitory effect of agents on MCF-7 cells. Dose response interactions between R115,777-Tam, R115,777-ICI182,780 and R115,777-PBPE were evaluated, at the IC50 point, using the isobologram method. Apoptotic cell death (DNA fragmentation, nucleus condensation and cytokeratin 18 cleavage) and inhibition of the mevalonate pathway (western blot) were also determined.

RESULTS

Combinations of the specific FTI R115,777 with either ICI182,780 or PBPE exhibit a synergistic effect on MCF-7 cell growth inhibition, while its combination with Tam is additive, as previously reported for FTI-277. Apoptosis is detected after treatment with combinations of R115,777 with either Tam or PBPE but not with ICI182,780, suggesting that each combination inhibits cell proliferation by different mechanisms. Even though the ER pathway has not yet been deciphered, it is shown here that the AEBS pathway is able to interfere with the mevalonate pathway at the level of protein farnesylation.

CONCLUSION

Overall, this work reveals that combinations of R115,777 with either selective ER ligands or a selective AEBS ligand are able to induce large increases in their anti-proliferative activities on MCF-7 cells. Moreover, these results suggest that it may be of definite interest to evaluate combinations of R115,777 with different anti-estrogens in the treatment of ER positive breast tumours. Based on these experimental data, such combinations may prove beneficial in different clinical scenarios or when used in specific sequences; studying the combination of R115,777 with ICI182,780 for early treatment and reserving combinations with either Tam or a selective AEBS ligand, such as BMS-217380-01, for more resistant disease.

Targeted therapy: for kids, too

Innovative hypothesis-driven clinical trials have achieved major successes over the past several decades in treating children and adolescents with cancer. DNA-damaging cytotoxic agents have cured children with cancer. While the mission is not yet accomplished, chemotherapy has been validated. None of these drugs were designed specifically for a pediatric disease. Continued progress will require new strategies. Now being tested for adult cancers, these strategies include gene therapy, immunotherapy, cancer prevention, and signal transduction inhibitor (STI) therapy. Of these, the most promising is STI therapy, also known as molecular therapeutics or targeted therapy. For this therapy to succeed, components of signal transduction (i.e., candidate drug targets) must be identified, the targets relevant to cancers, and the drugs available for trial. Because STI therapy is biologically driven and because therapy will be tailored depending on the molecular profile of a specific patient's tumor, clinical pediatric oncologists will need to acquire greater understanding of signaling pathways and their therapeutic relevance. With examples drawn from pediatric oncology, the critical steps in the pre-clinical development of targeted therapy are reviewed here.

The farnesyltransferase inhibitor L744832 potentiates UCN-01-induced apoptosis in human multiple myeloma cells

PURPOSE

The purpose of this study was to characterize interactions between the farnesyltransferase inhibitor L744832 and the checkpoint abrogator UCN-01 in drug-sensitive and drug-resistant human myeloma cell lines and primary CD138+ multiple myeloma cells.

EXPERIMENTAL DESIGN

Wild-type and drug-resistant myeloma cell lines were exposed to UCN-01 +/- L744832 for 24 hours, after which mitochondrial injury, caspase activation, apoptosis, and various perturbations in signaling and survival pathways were monitored.

RESULTS

Simultaneous exposure of myeloma cells to marginally toxic concentrations of L744832 and UCN-01 resulted in a synergistic induction of mitochondrial damage, caspase activation, and apoptosis, associated with activation of p34cdc2 and c-Jun-NH2-kinase and inactivation of extracellular signal-regulated kinase, Akt, GSK-3, p70(S6K), and signal transducers and activators of transcription 3 (STAT3). Enhanced lethality for the combination was also observed in primary CD138+ myeloma cells, but not in their CD138- counterparts. L744832/UCN-01-mediated lethality was not attenuated by conventional resistance mechanisms to cytotoxic drugs (e.g., melphalan or dexamethasone), addition of exogenous interleukin-6 or insulin-like growth factor-I, or the presence of stromal cells. In contrast, enforced activation of STAT3 significantly protected myeloma cells from L744832/UCN-01-induced apoptosis.

CONCLUSIONS

Coadministration of the farnesyltransferase inhibitor L744832 promotes UCN-01-induced apoptosis in human multiple myeloma cells through a process that may involve perturbations in various survival signaling pathways, including extracellular signal-regulated kinase, Akt, and STAT3, and through a process capable of circumventing conventional modes of myeloma cell resistance, including growth factor- and stromal cell-related mechanisms. They also raise the possibility that combined treatment with farnesyltransferase inhibitors and UCN-01 could represent a novel therapeutic strategy in multiple myeloma.

Simultaneous determination of farnesyl and geranylgeranyl pyrophosphate levels in cultured cells

A sensitive, nonradioactive analytical method has been developed to simultaneously determine the concentrations of farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP) in cultured cells. Following extraction, enzyme assays involving recombinant farnesyl protein transferase or geranylgeranyl protein transferase I are performed to conjugate FPP or GGPP to dansylated peptides. The reaction products are then separated and quantified by high-performance liquid chromatography coupled to a fluorescence detector at the excitation wavelength 335 nm and the emission wavelength 528 nm. The retention times for farnesyl-peptide and geranylgeranyl-peptide are 8.4 and 16.9 min, respectively. The lower limit of detection is 5 pg of FPP or GGPP ( approximately 0.01 pmol). A linear response has been established over a range of 5-1000 pg ( approximately 0.01-2 pmol) with good reproducibility. The method has been used to determine the levels of FPP (0.125+/-0.010 pmol/10(6)cells) and GGPP (0.145+/-0.008 pmol/10(6)cells) in NIH3T3 cells. Furthermore, changes in FPP and GGPP levels following treatment of cells with isoprenoid biosynthetic pathway inhibitors were measured. This method is suitable for the determination of the concentrations of FPP and GGPP in any cell type or tissue.

Phase I Study of the Farnesyltransferase Inhibitor BMS-214662 Given Weekly in Patients with Solid Tumors

Purpose: A phase I trial of BMS-214662, a selective farnesyltransferase inhibitor with significant preclinical antitumor activity in which drug was given as a weekly 1-hour infusion for four of six weeks, was conducted to evaluate the tolerability, pharmacokinetics, and pharmacodynamic effect on farnesyltransferase activity in peripheral blood mononuclear cells.

Experimental Design: BMS-214662 was given to 27 patients with solid tumors at 10 escalating dose levels (28-220 mg/m2) allowing intrapatient dose escalation; pharmacokinetics and pharmacodynamics were done at the first seven dose levels.

Results: Grade 4 neutropenia (four patients) was the most common dose-limiting toxicity followed by aminotransferase elevation (grade 3 alanine aminotransferase and grade 4 aspartate aminotransferase) and grade 3 dehydration. Most frequent toxicities were neutropenia in 11 (14%), anemia in 15 (19%), fatigue in 9 (12%), and nausea and diarrhea in 6 (8%) of courses, respectively. One minor response lasting 18 weeks in a patient with non–small cell lung cancer, serum calcitonin level reduction accompanied by disease stabilization in two of four patients with medullary thyroid carcinoma, and stable disease in 16 of 25 evaluable patients was seen. No correlation was observed between dose and Cmax, total body clearance (mean, 26.15 ± 10.88 L per hour per m2), volume of distribution at steady state (mean, 39.51 ± 17.91 L/m2), or half-life (mean, 2.63 ± 1.81 hours); a moderate correlation existed between dose given and systemic drug exposure (AUC). Substantial inhibition of peripheral blood mononuclear cell farnesyltransferase activity but near complete recovery by 24 hours was seen.

Conclusion: BMS-214667 was well tolerated as a weekly 1-hour i.v. infusion for four of six weeks with evidence of pharmacodynamic effect. The study was terminated before maximum tolerated dose was reached. Alternative schedules of drug administration might result in improved pharmacodynamic profile.

Synthesis of acyloxymethyl ester prodrugs of the transferable protein farnesyl transferase substrate farnesyl methylenediphosphonate

Three isoprenoid diphosphate analogues of farnesyl diphosphate (FPP) where the diphosphate has been replaced by methylene diphosphonate and the negative charges masked by frangible pivaloyloxymethyl (POM) esters were prepared. Farnesyl methylenediphosphonate is a sub-micromolar substrate for protein farnesyl transferase. The tripivaloyloxymethyl esters of isoprenoid methylenediphosphonate have significantly increased lipophilicity and may act as important farnesyl diphosphate prodrugs.

High affinity for farnesyltransferase and alternative prenylation contribute individually to K-Ras4B resistance to farnesyltransferase inhibitors

Farnesyltransferase inhibitors (FTIs) block Ras farnesylation, subcellular localization and activity, and inhibit the growth of Ras-transformed cells. Although FTIs are ineffective against K-Ras4B, the Ras isoform most commonly mutated in human cancers, they can inhibit the growth of tumors containing oncogenic K-Ras4B, implicating other farnesylated proteins or suggesting distinct functions for farnesylated and for geranylgeranylated K-Ras, which is generated when farnesyltransferase is inhibited. In addition to bypassing FTI blockade through geranylgeranylation, K-Ras4B resistance to FTIs may also result from its higher affinity for farnesyltransferase. Using chimeric Ras proteins containing all combinations of Ras background, CAAX motif, and K-Ras polybasic domain, we show that either a polybasic domain or an alternatively prenylated CAAX renders Ras prenylation, Ras-induced Elk-1 activation, and anchorage-independent cell growth FTI-resistant. The polybasic domain alone increases the affinity of Ras for farnesyltransferase, implying independent roles for each K-Ras4B sequence element in FTI resistance. Using microarray analysis and colony formation assays, we confirm that K-Ras function is independent of the identity of the prenyl group and, therefore, that FTI inhibition of K-Ras transformed cells is likely to be independent of K-Ras inhibition. Our results imply that relevant FTI targets will lack both polybasic and potentially geranylgeranylated methionine-CAAX motifs.