Abstract 5434: Combined BCL-2/XL and mTor inhibition promotes apoptosis in small cell lung cancer
Authors: Gao Li , Barnwell Brittany , Kalvala Arjun , Otterson Gregory A. , Duan Wenrui , Villalona-Calero Miguel A. 
Published: May 2015

The Bcl-2 family genes have emerged as potential targets for the treatment of small cell lung cancer (SCLC). Bcl-2 is a central apoptotic inhibitor associated with tumor progression and treatment resistance. Overexpression occurs in up to 80% of SCLC. ABT-263 (navitoclax) is an orally bioavailable small molecule, which inhibits Bcl-2 and Bcl-xL, disrupting their interactions with pro-death proteins, and leading to the initiation of apoptosis. However a recent clinical trial of single-agent ABT-263 (navitoclax) showed a low rate of response in advanced and recurrent SCLC, discouraging development.

Our laboratory had previously shown that SCLC cells rendered deficient in the Fanconi Anemia (FA) repair pathway by inhibition of FANCD2 were more sensitive to navitoclax compared to FA competent cells, and that 15% of small cell lung cancer patients' tumor samples, evaluated by immunofluorescence for FANCD2 foci formation are FA functionally deficient. In the search for pharmacologic inhibition of FANCD2, which could potentially expand the pool of susceptible patients to navitoclax, we learned that the mTOR inhibitor AZD8055 induced downregulation of the FANCD2 protein. Thus, we hypothesized that mTOR inhibition would potentiate the activity of navitoclax.

First we treated the human SCLC cancer cell H179 with navitoclax and AZD8055 at a dose of 5µM, either as single agents or in combination. MTT assay showed that the cell survival rate was 39% for AZD8055 and 52% for navitoclax when the cell was treated for 72 hours. Western analysis showed that AZD8055 inhibited FANCD2 protein expression completely, and when the cell was treated with the combination, the survival rate decreased to 24%. Increased apoptosis by Western immunoblot PARP cleavage assay was observed for the combination, as compared to navitoclax or AZD8055 alone.

Despite evidence showing that the clinically available mTOR inhibitor temsirolimus only inhibits TORC1 at clinically meaningful concentrations, we decided to evaluate if this agent could also inhibit FANCD2. Western immunoblot analysis with anti-FANCD2 antibody showed that temsirolimus caused marked FANCD2 protein reduction. Subsequent clonogenic assays using the human SCLC cells H719 and H792 showed that either temsirolimus or navitoclax treatment alone was cytotoxic to both SCLC cell lines. The cell survival rate for H719 was 48% for temsirolimus and 62% for navitoclax10 days following treatment. For H792 the survival rates were 59% for temsirolimus and 32% for navitoclax. Navitoclax and temsirolimus in combination led to further reduction in cell colony survival (5% for H719 and 14% H792). Western analysis showed that the apoptotic rate was increased for the combination as compared to single agent treatment.

The encouraging in vitro results described above suggest that evaluation of activity for the combination of navitoclax and temsirolimus in small cell lung cancer patients should be considered.

Citation Format: Li Gao, Brittany Barnwell, Arjun Kalvala, Gregory A. Otterson, Wenrui Duan, Miguel A. Villalona-Calero. Combined BCL-2/XL and mTor inhibition promotes apoptosis in small cell lung cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5434. doi:10.1158/1538-7445.AM2014-5434