Abstract 4592: Anti-tumorigenic effects of silibinin in combination with histone dacetylase inhibitors (TSA and SAHA): Involvement of G2/M arrest and apoptotic cell death

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Deregulated histone alterations, (particularly those involving HDAC [histone deacetylase] enzyme) are hallmark epigenetic characteristics of a transformed/malignant cell phenotype. Due to their several histone and non-histone substrates and multi-faceted regulatory role in biological events such as cellular proliferation, migration, angiogenesis as well as control of gene expression, targeting HDACs using HDAC inhibitors (HDACi) alone or in combination with other therapies has emerged as an attractive therapeutic option in a broad spectrum of diseases including Non-small cell lung cancer (NSCLC). NSCLC comprises of ∼80% of newly diagnosed cases of lung cancer and current approaches including chemotherapy and HDACi are moderately efficacious. A natural agent silibinin (Sb) has shown strong efficacy against various cancers including NSCLC. Accordingly, here we assessed the efficacy and associated mechanisms of two pan-HDACi inhibitors (TSA and SAHA) alone and in combination with Sb both in vitro and in vivo in H1299 NSCLC cells. A combined treatment of cells with 75µM Sb and HDACi (0.5µM TSA or 5µM SAHA) for 24-72 hrs resulted in strong cell growth inhibition and cell death compared to each agent alone. Similar treatments showed a strong G2/M arrest and a linear increase in apoptotic cell population by these combinations compared to single agent. In mechanistic studies, addition of Sb to either TSA or SAHA decreased the levels of anti-apoptotic proteins including Bcl-xl, Bcl-2, Mcl-1, XIAP and survivin suggesting their involvement in the apoptosis induction by combined drugs exposure. Concomitant cleavage of caspases (9 and 3) and PARP and a marked induction of p21 and elevated phospho-cdc2 levels (48h) by similar combination treatments in H1299 cells further supported above observations. Next we assessed the in vivo anti-tumorigenic potential of these combinations in athymic nude mice in terms of H1299 xenograft growth. TSA (0.8mg/kg body weight) or SAHA (100 mg/kg body weight) was given intraperitoneally with or without oral silibinin (100 mg/kg body weight) combination every day. At the end of the treatment regimen at four weeks, compared to control (956 mm3 tumor volume/mouse), and single agent treatment groups i.e. silibinin alone (408 mm3 tumor volume/mouse), TSA alone (495 mm3 tumor volume/mouse) and SAHA alone (377 mm3 tumor volume/mouse), a significant reduction in tumor volume was observed in the combination treatment group of SAHA + Sb (208 mm3 tumor volume/mouse; p<0.05); TSA + Sb group showed comparable effect (486 mm3 tumor volume/mouse) to each agent alone. Overall, both in vitro and in vivo results implicate the promising use of silibinin along with HDACi SAHA in the treatment of NSCLC. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4592. doi:10.1158/1538-7445.AM2011-4592