Inhibition of farnesyltransferase with A-176120, a novel and potent farnesyl pyrophosphate analogue

Pharmacophore mapping of diverse classes of farnesyltransferase inhibitors

Protein farnesyltransferase (FTase) is a zinc-dependent enzyme that catalyzes the attachment of a farnesyl lipid group to the sulfur atom of a cysteine residue of numerous proteins involved in cell signaling including the oncogenic H-Ras protein. Pharmacophore models were developed by using Catalyst HypoGen program with a training set of 22 farnesyltransferase inhibitors (FTIs), which were carefully selected with great diversity in both molecular structure and bioactivity for discovering new potent FTIs. The best pharmacophore hypothesis (Hypo 1), consisting of four features, namely, one hydrogen-bond acceptor (HBA), one hydrophobic point (HY), and two ring aromatics (RA), has a correlation coefficient of 0.961, a root mean square deviation (RMSD) of 0.885, and a cost difference of 62.436, suggesting that a highly predictive pharmacophore model was successfully obtained. For the test series, a classification scheme was used to distinguish highly active from moderately active and inactive compounds on the basis of activity ranges. Hypo 1 was validated with 181 test set compounds, which has a correlation coefficient of 0.713 between estimated activity and experimentally measured activity. The model was further validated by screening a database spiked with 25 known inhibitors. The model picked up all 25 known inhibitors giving an enrichment factor of 10.892. The results demonstrate that the hypothesis derived in this study can be considered to be a useful and reliable tool in identifying structurally diverse compounds with desired biological activity.

Potent farnesyltransferase inhibitor ABT-100 abrogates acute allograft rejection

BACKGROUND

Farnesyltransferase inhibitors (FTIs) inhibit the function of Ras, a GTPase involved in carcinogenesis and T cell activation. We evaluated the in vitro and in vivo immunomodulatory properties of a rationally designed FTI, ABT-100.

METHODS

The effects of ABT-100 on human peripheral blood mononuclear cell (PBMC) proliferation and the expression of the T cell activation markers CD25 and CD69 were studied. In a Wistar to Lewis rat heterotopic cardiac transplant model, ABT-100 was orally dosed alone or with a subtherapeutic course of cyclosporine (CsA). The degree of graft immune cell infiltrate was determined.

RESULTS

ABT-100 potently inhibited PBMC proliferation, but did not decrease expression of CD25 and CD69 during activation. Treatment with 25, 50 and 100 mg/kg ABT-100 BID increased allograft mean survival time (MST) to 12.8+/-3 days, 13.5+/-5 days and 13.8+/-3 days, respectively (vs 6.5+/-3 days for controls, p<0.001 by log rank). A subtherapeutic course of CsA increased MST to 12.7+/-3 days (p<0.001 vs control). Combination with ABT-100 at 25 and 100 mg/kg BID improved MST to 18.7+/-5 days and 19.5+/-4 days (both p<0.001 vs control and respective monotherapy groups). ABT-100 treatment at 100 mg/kg BID significant decreased the amount of graft infiltrate (2.5+/-4 mononuclear cells/high power field (hpf) vs 29+/-11 cells/hpf, p<0.001).

CONCLUSIONS

This is the first report that a specific FTI delays the development of acute rejection and supports the strategy of inhibiting Ras to impart immunomodulation. The antirejection and anticarcinogenic effects make FTIs a potentially useful adjunct in the antirejection regimens of malignancy-prone organ transplant recipients.

3D-QSAR studies of farnesyltransferase inhibitors: A comparative molecular field analysis approach

3D-QSAR analysis has been performed on a series of previously synthesized benzonitrile derivatives, which were screened as farnesyltransferase inhibitors, using comparative molecular field analysis (CoMFA) with partial least-square fit to predict the steric and electrostatic molecular field interactions for the activity. The CoMFA study was carried out using a training set of 34 compounds. The predictive ability of the model developed was assessed using a test set of eight compounds (r(pred)(2) as high as 0.770). The analyzed 3D-QSAR CoMFA model has demonstrated a good fit, having r(2) value of 0.991 and cross-validated coefficient q(2) value as 0.619. The analysis of CoMFA contour maps provided insight into the possible modification of the molecules for better activity.

A highly potent and selective farnesyltransferase inhibitor ABT-100 in preclinical studies

Ras mutation has been detected in approximately 20-30% of all human carcinomas, primarily in pancreatic, colorectal, lung and bladder carcinomas. The indirect inhibition of Ras activity by inhibiting farnesyltransferase (FTase) function is one therapeutic intervention to control tumor growth. Here we report the preclinical anti-tumor activity of our most advanced FTase inhibitor (FTI), ABT-100, and a direct comparison with the current clinical candidates. ABT-100 is a highly selective, potent and orally bioavailable FTI. It broadly inhibits the growth of solid tumors in preclinical animal models. Thus, ABT-100 is an attractive candidate for further clinical evaluation. In addition, our results provide plausible insights to explain the impressive potency and selectivity of ABT-100. Finally, we have demonstrated that ABT-100 significantly suppresses the expression of vascular endothelial growth factor (VEGF) mRNA and secretion of VEGF protein, as well as inhibiting angiogenesis in the animal model.

A hypoxia-driven vascular endothelial growth factor/Flt1 autocrine loop interacts with hypoxia-inducible factor-1alpha through mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2 pathway in neuroblastoma

Flt1, an "fms-like tyrosine kinase" receptor, has been suggested to play an active role in vascular endothelial growth factor (VEGF)-mediated autocrine signaling of tumor growth and angiogenesis. Here, we used a neuroblastoma model to investigate the role of VEGF/Flt1 signaling in hypoxia-mediated tumor cell survival, drug resistance, and in vivo angiogenesis. SK-N-BE2, a highly malignant neuroblastoma cell line resistant to hypoxia-induced apoptosis expresses active Flt1 but lacks VEGFR2 expression. We found that 24-hour hypoxia (<0.1% O2) alone (no serum deprivation) showed sustained activation of extracellular signal-regulated kinase 1/2 (ERK1/2) associated with bcl-2 up-regulation and resistance to etoposide-induced (5 mumol/L) apoptosis. Treatment with anti-VEGF and anti-Flt1 antibodies inhibited ERK1/2 activation, down-regulated bcl-2, and reversed the hypoxia-mediated drug resistance to etoposide. Similar results were obtained with U0126 and ursolic acid, specific and nonspecific inhibitors of ERK1/2, respectively. We confirmed the protective role of Flt1 receptor by small interfering RNA knockout and Flt1 overexpression studies. Subsequently, we found that inhibition of VEGF/Flt1 autocrine signaling led to reduced hypoxia-inducible factor-1alpha (HIF-1alpha) phosphorylation. Furthermore, the reduced phosphorylation was associated with down-regulation of basic fibroblast growth factor, a downstream target of the HIF-1alpha and VEGF pathways. Our findings suggested an expanded autocrine loop between VEGF/Flt1 signaling and HIF-1alpha. We investigated the angiogenic activity of the loop in an in vivo Matrigel plug assay. The hypoxia-treated conditioned medium induced a strong angiogenic response, as well as the cooption of surrounding vessels into the plugs; ursolic acid inhibited the angiogenesis process. We also found that three other Flt1-expressing neuroblastoma cell lines show hypoxia-mediated drug resistance to etoposide, melphalan, doxorubicin, and cyclophosphamide. Taken together, we conclude that a hypoxia-driven VEGF/Flt1 autocrine loop interacts with HIF-1alpha through a mitogen-activated protein kinase/ERK1/2 pathway in neuroblastoma. The interaction, in the form of an autocrine loop, is required for the hypoxia-driven cell survival, drug resistance, and angiogenesis in neuroblastoma.

The effect of HMG-CoA reductase inhibitors on chronic allograft rejection

Hydroxy-methyl-glutaryl coenzyme A (HMG-CoA) reductase inhibitors have pleiotropic actions that affect many systems other than lowering blood cholesterol concentrations. Hypercholesterolaemia is an adverse effect of immunosuppressive drug therapy and hence it is a common finding after organ transplantation. HMG-CoA reductase inhibitors lower cholesterol concentrations in transplant recipients but they also offer additional benefits. Since they impair the production of mevalonate, they reduce the concentrations of downstream products including farnesyl and geranyl phosphate. These isoprenoid moieties are required for protein prenylation and HMG-CoA reductase inhibitors impair this function in some cells. This action affects the immune system, especially in patients taking cyclosporin, and has been proposed as the mechanism whereby these drugs increase the half-life of transplanted organs. Other mechanisms have also been proposed including an increase in the free fraction of cyclosporin and a reduction in the time that low density lipoprotein (LDL) spends in blood. The latter effect reduces the extent of oxidation of LDL and hence reduces the damage caused by oxidised LDL. Chronic rejection is poorly understood but appears to involve both immune and non-immune processes. HMG-CoA reductase inhibitors affect both processes. At present, the evidence of benefit from statin prescription is confined to heart and kidney transplant recipients but it is likely that recipients of other organ transplants would also benefit. Drug interactions between cyclosporin and HMG-CoA reductase inhibitors are a limiting factor to their use. Pravastatin appears to be the best HMG-CoA reductase inhibitor for organ transplant recipients because of its lesser potential to interact with cyclosporin and hence cause myositis, which may thus allow higher doses to be used. Other, non-immunosuppressive drugs (including diltiazem and ketoconazole) have been shown to reduce transplant organ damage by unknown mechanisms and are widely prescribed in some transplant centres. More specific inhibitors of protein prenylation may afford useful immunosuppression, thereby prolonging transplant organ half-lives and also reducing the risk of cancer.

Stimulation of angiogenesis by Ras proteins

Cells that have acquired a proliferative advantage form islets of hyperplasia during the initial stages of tumor development. Like normal cells, they require oxygen and nutrients to survive and proliferate. The centre of the islets is characterized by low oxygen pressure and low pH, conditions that stimulate the sprouting of new capillaries from nearby vascular beds. It is now well established that neovascularisation (angiogenesis) of the hyperplasias is essential for further development of the tumor. The family of ras oncogenes promotes the initiation of tumor growth by stimulating tumor cell proliferation, but also ensures tumor progression by stimulating tumor-associated angiogenesis. Oncogenic Ras proteins stimulate a number of effector pathways that culminate in the transcriptional activation of genes that control angiogenesis. Moreover, Ras signaling leads to stabilization of the produced mRNAs and, possibly, to enhanced initiation of their translation. In this review we describe the mechanisms that underlie Ras regulation of vascular endothelial growth factor (VEGF), cyclooxygenases (COX-1/-2), thrombospondins (TSP-1/-2), urokinase plasminogen activator (uPA) and matrix metalloproteases-2 and -9 (MMP-2/-9). As a result of these Ras-regulated changes in gene expression, the tumor cells cause stimulation of endothelial cells in nearby vascular beds (directly via VEGF, and indirectly via COX-produced prostaglandins) and promote remodeling of the extracellular matrix (by lowering TSP and increasing uPA/MMPs). The latter effect makes growth factors available for endothelial cell activation and migration. In addition, tumor cell-activated stromal cells also contribute to the stimulation of angiogenesis by further enhancing the production and secretion of pro-angiogenic factors into the tumor stroma.

Design, Synthesis, and Biological Activity of 4-[(4-Cyano-2-arylbenzyloxy)-(3-methyl-3H-imidazol-4-yl)methyl]benzonitriles as Potent and Selective Farnesyltransferase Inhibitors

A novel series of 4-[(4-cyano-2-arylbenzyloxy)-(3-methyl-3H-imidazol-4-yl)methyl]benzonitriles have been synthesized as selective farnesyltransferase inhibitors using structure-based design. X-ray cocrystal structures of compound 20-FTase-HFP and A313326-FTase-HFP confirmed our initial design. The decreased interaction between the aryl groups and Ser 48 in GGTase-I binding site could be one possible reason to explain the improved selectivity for this new series of FTase inhibitors. Medicinal chemistry efforts led to the discovery of compound 64 with potent cellular activity (EC(50) = 3.5 nM) and outstanding pharmacokinetic profiles in dog (96% bioavailable, 18.4 h oral t(1/2), and 0.19 L/(h x kg) plasma clearance).

Farnesyltransferase inhibition: a novel method of immunomodulation

Farnesyltransferase inhibitors (FTIs) are anticancer compounds that inhibit Ras GTPases. Since Ras GTPases play key roles in T cell activation and function, we hypothesized that FTIs have immunomodulatory properties and are potential antirejection agents. An investigation was performed on a potent FTI to evaluate this hypothesis in the in vitro setting. The in vitro effects of the FTI A-228839 were evaluated. Lectin- or antigen presenting cell (APC)-induced lymphocyte proliferation in the presence of A-228839 was measured. The effects of A-228839 on 1E5 T cell polarity were assessed by microscopy. Intracellular calcium ([Ca(2+)](i)) kinetics of lectin-activated lymphocytes was monitored by flow cytometry. The effects of A-228839 on peripheral blood mononuclear cell (PBMC) cytokine production was assessed by a cytometric bead array method. Activation-induced apoptosis was measured with an annexin V staining assay.A-228839 inhibited lectin-induced proliferation (IC(50)=0.24+/-0.11 microM). The inhibitory effects of A-228839 on lectin induced lymphocyte proliferation were additive to those of CsA. A-228839 was more effective in inhibiting APC-induced T cell proliferation (IC(50)=0.10+/-0.09 microM). A-228839 significantly disrupted the polarized shape of 1E5 T cells at physiologic concentrations. A-228839 altered PBMC baseline [Ca(2+)](i) but did not affect [Ca(2+)](i) kinetics during lectin-induced lymphocyte activation. A-228839 inhibited lymphocyte Th1 cytokine production at submicromolar levels and promoted apoptosis in lectin-activated lymphocytes.A-228839 potently inhibits lymphocyte activation and function. Our results suggest that FTIs may represent a new class of clinically useful immunomodulatory agents. A-228839 has potent in vitro immunomodulatory properties that warrant in vivo evaluation as an antirejection agent.

Efficacy of the farnesyl transferase inhibitor R115777 in chronic myeloid leukemia and other hematologic malignancies

We investigated the clinical activity of the farnesyl transferase inhibitor R115777 in 22 patients with chronic myelogenous leukemia (CML) in chronic, accelerated, or blastic phase and in 8 patients with myelofibrosis (MF) and 10 patients with multiple myeloma (MM). R115777 was administered at 600 mg orally twice daily for 4 weeks every 6 weeks. Seven patients with CML (6 in chronic phase, 1 in advanced phase) achieved complete or partial hematologic response. Four of them had a minor cytogenetic response. Responses were transient, with a median duration of 9 weeks (range, 3-23 weeks). Two patients discontinued therapy because of toxicity while in complete hematologic response. Two MF patients had a significant decrease in splenomegaly, one had normalization of white blood cell count and differential, and one became transfusion independent. One patient with MM had a reduction in monoclonal protein of 34%. Adverse events included nausea in 22 patients (55%; all grade 2 or lower) and fatigue in 19 (48%; grade 3 or higher in 1). Other grade 3 or 4 toxicities included skin rash (4 patients, 10%), peripheral neuropathy (2 patients, 5%), and liver toxicity (2 patients, 5%). Patients who responded to therapy had significantly higher plasma vascular endothelial growth factor (VEGF) concentrations prior to treatment than nonresponders. Plasma concentrations decreased significantly during therapy among responders. R115777 showed clinical activity in patients with CML and MF. The effect on VEGF needs to be further investigated to determine whether this might be a possible mechanism of action of R115777.

2-Arylindole-3-acetamides: FPP-competitive inhibitors of farnesyl protein transferase

A series of 2-arylindole-3-acetamide farnesyl protein transferase inhibitors has been identified. The compounds inhibit the enzyme in a farnesyl pyrophosphate-competitive manner and are selective for farnesyl protein transferase over the related enzyme geranylgeranyltransferase-I. A representative member of this series of inhibitors demonstrates equal effectiveness against HDJ-2 and K-Ras farnesylation in a cell-based assay when geranylgeranylation is suppressed.