Farnesyltransferase inhibitors target multiple endothelial cell functions in angiogenesis

Potential of Farnesyl Transferase Inhibitors in Combination Regimens in Squamous Cell Carcinomas

Current therapies for recurrent and metastatic SCC are associated with poor outcomes, and options for later lines of treatment are limited. Insights into potential therapeutic targets, as well as mechanisms of resistance to available therapies, have begun to be elucidated, creating the basis for exploration of combination approaches to drive better patient outcomes. Tipifarnib, a farnesyl transferase inhibitor (FTI), is a small molecule drug that has demonstrated encouraging clinical activity in a genetically-defined subset of head and neck squamous cell carcinoma (HNSCC)-specifically, tumors that express a mutation in the HRAS protooncogene. More recently, bioinformatic analyses and results from patient-derived xenograft modeling indicate that HRAS pathway dependency may extend to a broader subpopulation of SCCs beyond HRAS mutants in the context of combination with agents such as cisplatin, cetuximab, or alpelisib. In addition, tipifarnib can also inactivate additional farnesylated proteins implicated in resistance to approved therapies, including immunotherapies, through a variety of distinct mechanisms, suggesting that tipifarnib could serve as an anchor for combination regimens in SCCs and other tumor types.

A drug-repositioning screen using splicing-sensitive fluorescent reporters identifies novel modulators of VEGF-A splicing with anti-angiogenic properties

Alternative splicing of the vascular endothelial growth factor A (VEGF-A) terminal exon generates two protein families with differing functions. Pro-angiogenic VEGF-A_xxxa isoforms are produced via selection of the proximal 3' splice site of the terminal exon. Use of an alternative distal splice site generates the anti-angiogenic VEGF-A_xxxb proteins. A bichromatic splicing-sensitive reporter was designed to mimic VEGF-A alternative splicing and was used as a molecular tool to further investigate this alternative splicing event. Part of VEGF-A's terminal exon and preceding intron were inserted into a minigene construct followed by the coding sequences for two fluorescent proteins. A different fluorescent protein is expressed depending on which 3' splice site of the exon is used during splicing (dsRED denotes VEGF-A_xxxa and EGFP denotes VEGF-A_xxxb). The fluorescent output can be used to follow splicing decisions in vitro and in vivo. Following successful reporter validation in different cell lines and altering splicing using known modulators, a screen was performed using the LOPAC library of small molecules. Alterations to reporter splicing were measured using a fluorescent plate reader to detect dsRED and EGFP expression. Compounds of interest were further validated using flow cytometry and assessed for effects on endogenous VEGF-A alternative splicing at the mRNA and protein level. Ex vivo and in vitro angiogenesis assays were used to demonstrate the anti-angiogenic effect of the compounds. Furthermore, anti-angiogenic activity was investigated in a Matrigel in vivo model. To conclude, we have identified a set of compounds that have anti-angiogenic activity through modulation of VEGF-A terminal exon splicing.

Tipifarnib as a Precision Therapy for HRAS-Mutant Head and Neck Squamous Cell Carcinomas

Tipifarnib is a potent and highly selective inhibitor of farnesyltransferase (FTase). FTase catalyzes the posttranslational attachment of farnesyl groups to signaling proteins that are required for localization to cell membranes. Although all RAS isoforms are FTase substrates, only HRAS is exclusively dependent upon farnesylation, raising the possibility that HRAS-mutant tumors might be susceptible to tipifarnib-mediated inhibition of FTase. Here, we report the characterization of tipifarnib activity in a wide panel of HRAS-mutant and wild-type head and neck squamous cell carcinoma (HNSCC) xenograft models. Tipifarnib treatment displaced both mutant and wild-type HRAS from membranes but only inhibited proliferation, survival, and spheroid formation of HRAS-mutant cells. In vivo, tipifarnib treatment induced tumor stasis or regression in all six HRAS-mutant xenografts tested but displayed no activity in six HRAS wild-type patient-derived xenograft (PDX) models. Mechanistically, drug treatment resulted in the reduction of MAPK pathway signaling, inhibition of proliferation, induction of apoptosis, and robust abrogation of neovascularization, apparently via effects on both tumor cells and endothelial cells. Bioinformatics and quantitative image analysis further revealed that FTase inhibition induces progressive squamous cell differentiation in tipifarnib-treated HNSCC PDXs. These preclinical findings support that HRAS represents a druggable oncogene in HNSCC through FTase inhibition by tipifarnib, thereby identifying a precision therapeutic option for HNSCCs harboring HRAS mutations.

Lonafarnib is a potential inhibitor for neovascularization

Atherosclerosis is a common cardiovascular disease that involves the build-up of plaque on the inner walls of the arteries. Intraplaque neovacularization has been shown to be essential in the pathogenesis of atherosclerosis. Previous studies showed that small-molecule compounds targeting farnesyl transferase have the ability to prevent atherosclerosis in apolipoprotein E-deficient mice, but the underlying mechanism remains to be elucidated. In this study, we found that lonafarnib, a specific inhibitor of farnesyl transferase, elicits inhibitory effect on vascular endothelial capillary assembly in vitro in a dose-dependent manner. In addition, we showed that lonafarnib treatment led to a dose-dependent decrease in scratch wound closure in vitro, whereas it had little effect on endothelial cell proliferation. These data indicate that lonafarnib inhibits neovascularization via directly targeting endothelial cells and disturbing their motility. Moreover, we demonstrated that pharmacological inhibition of farnesyl transferase by lonafarnib significantly impaired centrosome reorientation toward the leading edge of endothelial cells. Mechanistically, we found that the catalytic β subunit of farnesyl transferase associated with a cytoskeletal protein important for the establishment and maintenance of cell polarity. Additionally, we showed that lonafarnib remarkably inhibited the expression of the cytoskeletal protein and interrupted its interaction with farnesyl transferase. Our findings thus offer novel mechanistic insight into the protective effect of farnesyl transferase inhibitors on atherosclerosis and provide encouraging evidence for the potential use of this group of agents in inhibiting plaque neovascularization.

Uses of the in vitro endothelial-fibroblast organotypic co-culture assay in angiogenesis research

Angiogenesis is a complex process that involves multiple cellular events. In addition to receiving inputs from a range of stimulatory and inhibitory factors, endothelial cells undergoing angiogenesis make multiple interactions with the extracellular matrix and with other cell types in the stroma. Recreating angiogenesis in vitro is probably an impossible goal; however, a number of assays have been developed that recapitulate many of the key events of the process. These assays are indispensible tools for investigating the signalling pathways that control the formation of new blood vessels. In the present paper, we review the organotypic co-culture assay of angiogenesis - until recently, a comparatively underemployed assay, but one with a number of powerful advantages for angiogenesis research. We give a set of optimized protocols for its use, including protocols for siRNA (small interfering RNA)-based screens, and we discuss appropriate methods for obtaining quantitative data from the assay.

Inhibition of farnesyltransferase reduces angiogenesis by interrupting endothelial cell migration

Inhibitors of farnesyltransferase (FTI) have been developed for cancer treatment for more than a decade. Aside from being a therapeutic target in tumor cells, little is known about the role of farnesyltransferase (FTase) in other physiological processes. In this study, we revealed the involvement of FTase in angiogenesis and showed that FTI inhibited angiogenesis by directly acting on endothelial cells. Inhibition of FTase interrupted cell migration in vitro and in vivo. In addition, we found that FTase was important for cell polarization, cell spreading and pseudopodia formation. We also found that FTase interacted with microtubule end binding protein 1 (EB1) and that this interaction was critical for the localization of EB1 to microtubule tips. Our findings thus offer novel insight into the functions of FTase in endothelial cells and provide valuable information for the use of FTI in cancer therapy.

Use of the mouse aortic ring assay to study angiogenesis

Here we provide a protocol for quantitative three-dimensional ex vivo mouse aortic ring angiogenesis assays, in which developing microvessels undergo many key features of angiogenesis over a timescale similar to that observed in vivo. The aortic ring assay allows analysis of cellular proliferation, migration, tube formation, microvessel branching, perivascular recruitment and remodeling-all without the need for cellular dissociation-thus providing a more complete picture of angiogenic processes compared with traditional cell-based assays. Our protocol can be applied to aortic rings from embryonic stage E18 through to adulthood and can incorporate genetic manipulation, treatment with growth factors, drugs or siRNA. This robust assay allows assessment of the salient steps in angiogenesis and quantification of the developing microvessels, and it can be used to identify new modulators of angiogenesis. The assay takes 6-14 d to complete, depending on the age of the mice, treatments applied and whether immunostaining is performed.

Lonafarnib (SCH66336) improves the activity of temozolomide and radiation for orthotopic malignant gliomas

Malignant gliomas are highly lethal tumors resistant to current therapies. The standard treatment modality for these tumors, surgical resection followed by radiation therapy and concurrent temozolomide, has demonstrated activity, but development of resistance and disease progression is common. Although oncogenic Ras mutations are uncommon in gliomas, Ras has been found to be constitutively activated through the action of upstream signaling pathways, suggesting that farnesyltransferase inhibitors may show activity against these tumors. We now report the in vitro and orthotopic in vivo results of combination therapy using radiation, temozolomide and lonafarnib (SCH66336), an oral farnesyl transferase inhibitor, in a murine model of glioblastoma. We examined the viability, proliferation, farnesylation of H-Ras, and activation of downstream signaling of combination-treated U87 cells in vitro. Lonafarnib alone or in combination with radiation and temozolomide had limited tumor cell cytotoxicity in vitro although it did demonstrate significant inhibition in tumor cell proliferation. In vivo, lonafarnib alone had a modest ability to inhibit orthotopic U87 tumors, radiation and temozolomide demonstrated better inhibition, while significant anti-tumor activity was found with concurrent lonafarnib, radiation, and temozolomide, with the majority of animals demonstrating a decrease in tumor volume. The use of tumor neurospheres derived from freshly resected adult human glioblastoma tissue was relatively resistant to both temozolomide and radiation therapy. Lonafarnib had a significant inhibitory activity against these neurospheres and could potentate the activity of temozolomide and radiation. These data support the continued research of high grade glioma treatment combinations of farnesyl transferase inhibitors, temozolomide, and radiation therapy.

The farnesyltransferase inhibitor LB42708 suppresses vascular endothelial growth factor-induced angiogenesis by inhibiting ras-dependent mitogen-activated protein kinase and phosphatidylinositol 3-kinase/Akt signal pathways

Farnesyltransferase (FTase) inhibitors induce growth arrest and apoptosis in various human cancer cells by inhibiting the post-translational activation of Ras. FTase inhibitors also function to suppress the release of vascular endothelial growth factor (VEGF) from tumor cells by inhibiting Ras activation; however, the effects of FTase inhibitors on VEGF-induced angiogenesis in endothelial cells have not been studied. We have investigated the antiangiogenic effect and molecular mechanism of 4-((1-((1-((4-bromophenyl)methyl)-1H-imidazol-5-yl)methyl)-4-(1-napthalenyl)-1H-pyrrol-3-yl)carbonyl)-(9C1)-morpholine (LB42708), a selective nonpeptidic FTase inhibitor, using in vitro and in vivo assay systems. LB42708 inhibited VEGF-induced Ras activation and subsequently suppressed angiogenesis in vitro and in vivo by blocking the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase/p38 mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K)/Akt/endothelial nitric-oxide synthase pathways in endothelial cells without altering FAK/Src activation. In addition, this inhibitor suppressed VEGF-induced endothelial cell cycle progression at the G(1) phase by suppressing cyclin D1 expression and retinoblastoma phosphorylation as well as up-regulating the cyclin-dependent kinase inhibitors p21 and p27. Knockdown of Ras by short interfering RNA revealed similar inhibitory effects on VEGF-induced angiogenic signal events compared with LB42708. Moreover, the inhibitory effects of LB42708 were significantly higher than those of 4-(2-(4-(8-chloro-3,10-dibromo-6,11-dihydro-5H-benzo-(5,6)-cyclohepta(1,2-b)-pyridin-11(R)-yl)-1-piperidinyl)-2-oxo-ethyl)-1-piperidinecarboxamide (SCH66336), a well known FTase inhibitor. LB42708 suppressed tumor growth and tumor angiogenesis in both xenograft tumor models of Ras-mutated HCT116 cells and its wild-type Caco-2 cells, indicating its potential application in the treatment of both Ras-mutated and wild type tumors. These data indicate that the antitumor effect of LB42708 can be associated with direct inhibition of VEGF-induced tumor angiogenesis by blocking Ras-dependent MAPK and PI3K/Akt signal pathways in tumor-associated endothelial cells.