Investigation of PARP1 as a therapeutic target in small cell lung cancer.

Abstract

7096 Background: Small cell lung cancer (SCLC) is an aggressive malignancy that differs from non-small cell lung cancer (NSCLC) in its metastatic potential and response to treatment. To date, no molecularly-targeted agent has prolonged survival of SCLC patients. Using a proteomic approach, we previously identified high expression of the DNA repair protein poly (ADP-ribose) polymerase 1 (PARP1) in SCLC cell lines and tumors. Here we test in vitro sensitivity of SCLC to PARP inhibition or knockdown. Methods: Cell lines were treated with PARP inhibitor olaparib or AG014699 for 14d +/- chemotherapy. Relative cell viability was assessed by cell count. siRNA against PARP1 was compared with scrambled siRNA and mock transfected cells. To assess DNA repair, RAD51 foci were counted after 1µM or 5µM olaparib and after 8Gy irradiation (RT). Results: SCLC cell lines were highly sensitive to PARP inhibition by olaparib (IC50s <0.5 µM for H69; ≤2µM in H524, H82, and H526) and AG014699 (IC50s <0.5 µM for H82, H69, and H524; 2.2 µM for H526 and H841). In contrast, A549 (NSCLC) was resistant to both drugs (IC50s >8µM). Because BRCA1/2 and PTEN mutations are associated with greater sensitivity to PARP inhibitors, we compared SCLC sensitivity with that of BRCA1-mutated (HCC1395) and PTEN-mutated (MDA-MB-468) breast lines. As expected, HCC1395 and MDA-MB-468 were sensitive to both PARP inhibitors. Remarkably, however, SCLC cell lines were as sensitive or more so. Combination of olaparib with topotecan or irinotecan (commonly used in SCLC) decreased tumor cell viability more than either agent alone (p <0.03). Consistent with the drug studies, knockdown of PARP1 by siRNA decreased growth of SCLC compared with that of controls. RAD51 foci increased in SCLC after olaparib treatment (>4-fold) and RT (>18-fold). Conclusions: SCLC lines were as sensitive to PARP inhibition as BRCA1- or PTEN-mutated breast cancer lines. Moreover, PARP inhibition enhanced the effect of chemotherapy on SCLC lines. Increased formation of RAD51 foci in SCLC cells after olaparib or RT suggests a deficiency in homologous recombination that may account for the sensitivity to PARP inhibitors. These results support the investigation of PARP inhibition as a novel therapeutic approach in SCLC lung cancer.