Abstract 208: Low-dose actinomycin D decreases Mcl-1 expression and acts synergistically with ABT-737 to induce apoptosis of SCLC cell lines

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC SCLC is initially a highly chemotherapy-responsive disease but relapse and progressive development of chemotherapy resistance is the rule. Overexpression of pro-survival members of the Bcl-2 family, including Bcl-2, Bcl-XL and Mcl-1 are thought to play a role in the pathogenesis and chemotherapy resistance of SCLC. ABT-737, a BH3-mimetic drug which blocks the function of Bcl-2 and Bcl-XL, has shown single agent activity in preclinical models of SCLC and enhances the activity of a variety of cytotoxic agents. Low levels of Mcl-1 expression and high levels of Noxa expression have been characterized as markers of sensitivity to ABT-737 in SCLC and other malignancies. We have also noted that Noxa-mediated downregulation of Mcl-1 expression is absent in 4/4 SCLC cell lines examined, suggesting roles for this defect in the pathogenesis of SCLC and in resistance to ABT-737. Because of the short half-life of Mcl-1 mRNA and protein, we studied the effects of combining ABT-737 and actinomycin D, a general transcriptional inhibitor used in the treatment of Wilms tumor, sarcomas, germ cell and trophoblastic tumors. Actinomycin D decreased Mcl-1 expression in a dose-dependent fashion in the low ng/ml range, 3 logs lower than that required for general transcriptional inhibition. While exposure to actinomycin resulted in a cell line-dependent increase in Noxa expression, Noxa was not required for the decrease in Mcl-1 expression, as it occurred in the H209 cell line, which does not express Noxa, as well as the H69, WBA and H526 cell lines which do. Concentrations of actinomycin D from 0.4-4 ng/ml showed single agent activity across a panel of SCLC cell lines in 72 hr MTT viability assays and specifically induced a high grade G2-M cell cycle blockade. When combined with low micromolar doses of ABT-737, near complete loss of viability was seen with combination indices in the 0.5 range, indicating synergy by Chou-Talalay analysis, without a significant effect on MRC-5 pulmonary fibroblasts. In addition to loss of Mcl-1 expression and enhanced Noxa expression, correlates of apoptosis induced by the combination included marked Bid cleavage. Exposure to 4 ng/ml actinomycin was only required for the first 24 hrs of the combined incubation but the presence of ABT-737 was required for the full 72 hrs to see maximum loss of viability and clonogenic potential. This actinomycin exposure mimics a clinically achievable AUC of 100 ng.hr/ml after bolus administration, suggesting the feasibility of combination studies utilizing bolus actinomycin D administration. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 208.