Abstract LB-22: A subset of small cell lung cancer (SCLC) cell lines is Mcl-1-dependent and responds to cyclin-dependent kinase (cdk)9 inhibition in vitro and in vivo
Authors: Qi Li , Xu Chunxiao , Sarosiek Kristopher A. , Ligon Azra H. , Rodig Scott J. , Wong Kwok-Kin , Letai Anthony G. , Shapiro Geoffrey I. 
Published: June 2012

Abstract
Small cell lung cancer (SCLC) accounts for approximately 13% of lung cancer cases, and current treatment options are limited to radiation and/or chemotherapy, which frequently lead to drug resistance. Recently, multiple SCLC cell lines have been found to be responsive to the Bcl-2/Bcl-xL inhibitor, ABT-737, both in vitro and in vivo. Clinical trials of the related orally-bioavailable compound ABT-263 have shown promising activity in SCLC patients. However, a subset of SCLC cell lines that express Mcl-1, another anti-apoptotic protein, are resistant to ABT-737. Therefore, drugs targeting Mcl-1 may be particularly useful in this disease. Here, we have investigated dependence of a panel of SCLC cell lines on Mcl-1 for survival. A subset of Mcl-1-expressing SCLC cell lines, including NCI-H82 and SHP-77, demonstrate apoptosis upon Mcl-1 knockdown. However, other SCLC cell lines, including NCI-H69 and SW1271, are not responsive to Mcl-1 knockdown. In addition, the dependence of SCLC cell lines on Mcl-1 for survival can be predicted by BH3 profiling, a methodology that measures dependence of a cell on a variety of anti-apoptotic proteins. We have also investigated the efficacy of Flavopiridol, a potent inhibitor of the transcriptional cyclin-dependent kinase (cdk)9, against the panel of SCLC cell lines. Flavopiridol acutely down-regulates Mcl-1 expression in all cell lines tested; however, only Mcl-1-dependent cells undergo abrupt apoptosis. In contrast, cells that were not found to be Mcl-1 dependent, including H69 and SW1271 cells, are not effectively killed by Flavopiridol. We have further investigated the activity of Flavopiridol against Mcl-1-dependent SCLC in vivo utilizing nude mice bearing H82 xenografts. Flavopiridol treatment results in substantial tumor growth inhibition in this aggressive and chemotherapy-refractory model. To potentially stratify patients who would be predicted to be responsive to Flavopiridol treatment, we have performed MCL-1 fluorescent in situ hybridization in SCLC cell lines and deidentified patient specimens. MCL-1 gene copy number gains exist in both patient and cell line samples, and copy number gain in cell lines correlates with Mcl-1 dependence. Finally, we have demonstrated that the combination of Flavopiridol and ABT-737 can synergistically kill both Mcl-1-dependent and – independent SCLC cells, with significantly reduced viability in SHP-77 and SW1271 cells compared to treatment with either drug alone. Taken together, our studies suggest that Flavopiridol-mediated cdk9 inhibition may effectively target Mcl-1-dependent SCLCs, including those resistant to Bcl-2/Bcl-xL inhibition.
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 LB-22. doi:10.1158/1538-7445.AM2011-LB-22