Background: Small cell lung cancer (SCLC) is the most aggressive form of lung cancer, accounting for 14% of lung cancers. It is associated with poor outcomes and few effective treatments. Identification of novel therapeutic targets is imperative for improving treatment outcomes. In our previous work we identified several DNA repair proteins including check point kinase 1 (Chk1) and poly (ADP-ribose) polymerase 1 (PARP1) that are overexpressed in SCLC and are high priority candidate targets. TP53-mutant cells (a hallmark of SCLC) rely on Chk1 to arrest the S and G2 phases of the cell cycle. Hence, inhibition of Chk1 in p53 mutant cells abrogates the S and G2 checkpoints driving such cells to mitotic catastrophe and apoptosis. In this study we tested the in vitro efficacy of Chk1 inhibitors as single agents and in combination with a PARP1 inhibitor and cisplatin.

Experimental design: SCLC cell lines [human cell lines plus those from a patient derived xenograft (PDX) and genetically engineered mouse models (GEMMs)] were treated with Chk1 inhibitors (SCH900776, LY2603618, LY2606368) alone and in combination with olaparib (PARP inhibitor) or cisplatin. Cell proliferation was measured by Cell Titer Glo assay. Drug sensitivity (IC50) was correlated with baseline expression levels of >190 total or phosphorylated proteins measured by reverse phase protein array (RPPA) to identify potential predictive markers.

Result: Single-agent Chk1 suppression decreased proliferation of SCLC cells in a dose-dependent manner. SCLC lines exhibited varied levels of sensitivity to the Chk1 inhibitors, with greatest single agent activity observed with LY2606368 (IC50 2-30nM). LY2606368 also showed appreciable single agent activity in PDX and GEMM-derived cell lines (IC50 2-7nM). Single agent activity was also observed with the other Chk1 inhibitors (SCH900776 IC50 0.3-10uM; LY2603618 IC50 0.2-10uM). Combination of LY2606368 with olaparib and cisplatin revealed an additive effect. Proteomic analysis revealed an association between Chk1 inhibitor sensitivity and elevated basal expression of pro-apoptotic proteins (cleaved PARP, Bax, and Caspase 3; p <0.05). In contrast, Chk1 resistance was associated with higher levels of PI3K pathway proteins (pAkt, pGSK, p <0.05).

Conclusion: Identification and characterization of novel therapeutic targets is essential to improve efficacy of SCLC therapy. In this preliminary study we tested Chk1 inhibitors as potential therapies in SCLC and provide evidence that targeting Chk1 could be a promising therapeutic strategy for SCLC. Further preclinical and clinical investigation is needed to confirm the molecular mechanism and in vivo outcome of Chk1 targeting.

Acknowledgement: 1. 2013 NCI Cancer Clinical Investigator Team Leadership Award, P30 CA016672 (Byers) 2. UT MD Anderson Small Cell Lung Cancer Working Group and Abell Hangar Foundation Distinguished Professor Endowment (Glisson).

Citation Format: Aly A. Valliani, Triparna Sen, Fatehmeh Masrorpour, Lixia Diao, Robert J. Cardnell, Jing Wang, Bonnie S. Glisson, Helen Piwnica-Worms, Don L. Gibbons, Lauren A. Byers. Check point kinase 1 (Chk1) targeting as a novel therapeutic strategy in small cell lung cancer (SCLC). [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5316. doi:10.1158/1538-7445.AM2015-5316