Mechanisms of in vitro acquired resistance to vorinostat (suberoylanilide hydroxamic acid, SAHA)

AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA 694 Vorinostat is a histone deacetylase (HDAC) inhibitor that induces differentiation, growth arrest and/or apoptosis of malignant cells both in vitro and in vivo . Vorinostat has demonstrated antitumor activity in both hematological malignancies and solid tumors, including an overall response rate of approximately 30% in cutaneous manifestations of cutaneous T-cell lymphoma (CTCL) in patients with progressive, persistent, or recurrent disease on or following 2 systemic therapies. As not all clinical responses are permanent, we developed model systems to investigate the mechanisms of acquired resistance to vorinostat and used these results to identify rational combination strategies to overcome this clinical issue. Using a dose escalation protocol vorinostat resistant clones were derived from sensitive HCT116 colon carcinoma cells and A549 non-small cell lung cancer (NSCLC) cells. This method established cell lines that can be maintained in media containing vorinostat at 2 μM (A549-VR2), 3μM (A549-VR3), 5 μM (A549-VR5) and 8 μM (HCT116-VR8) for a prolonged period of time (3 weeks) without signs of apoptosis. Gene expression profiling was performed on the parental HCT116 cells and vorinostat-resistant cells. In addition to an unbiased analysis of the data, the status of several pathways possibly related to vorinostat's mechanism of action were examined. Preliminary analyses revealed alterations in the expression of genes that participate in the apoptotic cascade (e.g. Bcl-2, survivin), protein acetylation (CBP/p300), redox homeostasis (SOD2, metallothionein) and Wnt/ β-catenin signaling pathway (frizzle, β-catenin and cyclin D1). The expression of candidate genes from this analysis was assessed across the panel of vorinostat-resistant A549 clones. The results indicate that acquired resistance to vorinostat appears to comprise mechanisms that inhibit the induction of apoptosis, including an increase in antioxidants and in anti-apoptotic Bcl-2 levels. In particular, upregulation of antioxidant enzymes and thiol modulating proteins including SOD2 and metallothioneins have been associated with clinical resistance to cytotoxic drugs and radiation therapy as well as poor prognosis. Future studies will help to address the clinical relevance of these findings. One prediction from this analysis is that compounds that inhibit the glutathione system like buthionine sulfoximine or agents that downregulate or block Bcl-2 could be used to revert these mechanisms of resistance. Combination strategies with emerging treatment modalities may help to increase vorinostat efficacy in specific genetic contexts.