Abstract A78: Simvastatin synergistically potentiates tipifarnib cytotoxic and apoptotic effects and disrupts Ras membrane localization in human leukemia cells.

Abstract

Abstract Background: Tipifarnib, a farnesyl transferase inhibitor, was initially designed to prevent Ras farnesylation and its subsequent membrane localization, which is necessary for Ras mediated signaling. Although, the clinical effectiveness of tipifarnib as a single agent in solid tumors was limited, it was modest in hematologic malignancies. Alternative prenylation by geranylgeranyl transferase has been postulated as an escape mechanism by which Ras evades the effect of tipifarnib. Simvastatin, an anti-hyperlipidemic drug that inhibits 3-hydroxy 3-methylglutaryl coenzyme A (HMG-CoA) reductase, has been shown in several studies to induce apoptosis in cancer cells through blockade of the geranylgeranylation pathway of small GTPases. Thus, we hypothesized that simvastatin can augment the cytotoxic effect of tipifarnib by blocking the alternative Ras prenylation pathway. Our studies were carried out in a panel of leukemia cell lines. Methods: Studies were carried out using five cell lines of varied leukemic origin including HL60, K562, Molt4, Jurkat and HSB2. Cells were exposed to increasing drug concentrations for 72 hours either as a single or combination treatment with simvastatin and tipifarnib. Cell viability was measured using AlamarBlue (Invitrogen) and the combination index values were calculated based on the Chou-Talalay equations. Apoptosis was evaluated using Annexin V assay. Biochemical assays assessing the expression of caspases, c-PARP and Ras, in both membrane and cytosolic fractions, were also used. Results: The combination of tipifarnib with simvastatin was shown to induce a synergistic cytotoxic effect in all cell lines tested. Notably, our results showed that coadministarion of simvastatin and tipifarnib resulted in the induction of apoptosis through the activation of several apoptotic markers including Caspase 3, 7 and 9 as well as the upregulation of cleaved PARP. These effects were accompanied by disruption of Ras membrane localization and its sequestration into the cytosol. Conclusions: These findings demonstrate that combining simvastatin with tipifarnib effectively disrupts Ras prenylation and induces apoptosis in human leukemia cell lines. These effects were observed with simvastatin and tipifarnib concentrations that can be achieved in the clinic. Thus, the effectiveness of this combination should be explored further in future clinical studies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A78.