Abstract 4686: Synergistic effect of olaparib (AZD2281) with combination of cisplatin on PTEN-deficient lung cancer cell lines

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Abstract Poly(ADP-ribose) polymerase (PARP) enzyme plays a key role in the cellular machinery responsible for DNA damage repair. Although inhibition of this enzyme using a pharmacological inhibitor is postulated to enhance the cytotoxicity of DNA-damaging cytotoxic chemotherapy, this has not been profoundly studied in lung cancer. PTEN (phosphatase and tensin homologue deleted on chromosome ten) is a tumor-suppressor gene deactivating PI3K downstream of EGFR (epidermal growth factor receptor) signaling. Approximately 2% to 9% of non-small cell lung cancers (NSCLC) are considered to have PTEN loss, and a recurrent gross-mutation of the PTEN gene is identified in lung cancer with deficient DNA double-strand break repair. We hypothesize that there is a cellular threshold for error-free DNA-repair in lung cancer cells and that the absence of PTEN can sensitize these cells to a concurrent treatment of a DNA-damaging agent (cisplatin) and a PARP inhibitor (olaparib). To investigate the effect of olaparib and cisplatin on PTEN deficient lung tumors, two EGFR mutant (deletion in exon19) NSCLC cell lines, PC-9 (PTEN wild type) and NCI-H1650 (PTEN loss) were used. Two additional cell lines with PTEN loss, prostate cancer cell line (PC-3) and glioblastoma cell line (U-87MG) were also examined. We transfected intact PTEN gene into H1650 cells and knocked down PTEN expression in the PC-9 cells using shRNA. Antiproliferative activity was determined by MTT assay in terms of 50% inhibitory concentration (IC50) values. Olaparib alone was not effective for H1650 cells and other PTEN-deficient cells. IC50 values ranged from 6 to 40 μM for olaparib and from 0.2 to 2 μM for cisplatin. Combination of cisplatin with olaparib showed a synergistic effect in vitro according to the combination index (CI). CIs were 0.23, 0.20, 0.57 and 0.29 when concentration ratios of cisplatin and olaparib were designed to be molar ratios of 1:1, 1:2, 1:3 and 1:5, respectively. Restoration of PTEN in the H1650 cells decreased sensitivity to olaparib and cisplatin (CI >1). Ablation of PTEN in PC-9 cells increased sensitivity to olaparib and cisplatin (CI <1). We also examined the effectiveness of the cisplatin (5 mg/kg/week) and olaparib (50 mg/kg/day) in a xenograft model using H1650 cells. The combination of cisplatin with olaparib was more effective than each agent individually. This effect was not observed in a xenograft model using PTEN transfected H1650 cells. Mechanistic investigations revealed that PTEN-deficiency caused a reduction in radiation-induced nuclear RAD51 focus formation and activation of Chk1 S345. Thus, genetic inactivation of PTEN led to suppression of DNA repair. The combination of cisplatin with olaparib in PTEN deficient lung tumors should be further pursued. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4686. doi:1538-7445.AM2012-4686