Abstract C153: Investigation of Chk1 as a novel therapeutic target for small cell lung cancer (SCLC)
Authors: Sen Triparna , Tong Pan , Stewart C. Allison , Hong-Fan You , Feng Ying , Wang Jing , Byers Lauren A. 
Published: May 2016

Background: Small cell lung cancer (SCLC) is the most lethal form of lung cancer, accounting for around 15% of lung cancers in the United States, currently having no targeted therapies with established efficacy. Using a high-throughput proteomic screen, we demonstrated overexpression of checkpoint kinase 1 (Chk1) in SCLC. Chk1 being the master regulator of the cell cycle in absence of p53, we hypothesize that Chk1 targeting may be effective SCLC because of near ubiquitous loss of p53. In the present study, we tested the effect of Chk1 targeting by shRNA-mediated knockdown and pharmacologic inhibition in an extensive panel of SCLC cell lines and in animal models.

Experimental procedures: shRNA-mediated knockdown of Chk1 was done in select human SCLC lines and confirmed at the mRNA and protein level. The effect of Chk1 inhibition by LY2606368 alone and in combination with cisplatin or the PARP1/2 inhibitor talazoparib was tested in 39 SCLC cell lines. Proliferation was assessed by cell titer glo analysis and effect on cell cycle was tested by flow cytometry. Drug sensitivity (IC50) was then correlated with baseline expression level of &gt;190 total or phosphorylated proteins measured by reverse phase protein array (RPPA) to identify potential predictive biomarkers. In vivo effect of the Chk1 inhibitor with or without cisplatin or talazoparib was tested in syngeneic and xenograft model of SCLC.

Results: shRNA-mediated targeted Chk1 knockdown significantly decreased the proliferation of SCLC lines confirming Chk1 as a viable target (p&lt;0.01). Targeted Chk1 knockdown also increased the expression of pγH2Ax indicating double-stranded DNA damage upon Chk1 inhibition. Chk1 inhibition by LY2606368 had striking single-agent activity in a majority of SCLC lines (median IC50 2-50nM) and the combination of LY2606368 with either cisplatin or talazoparib was additive in a majority of cell lines. Proteomic analysis revealed association between Chk1 inhibitor sensitivity and elevated basal expression of total and phospho cMyc (p = 0.01), 14-3-3 protein (p = 0.003), pro-apoptotic protein Bax (p&lt;0.05); in contrast, resistance to LY2606368 at multiple doses was correlated to with higher levels of PARP, SGK and p90RSK (p&lt;0.05). In syngeneic in vivo model, single agent LY2606368 was well tolerated and resulted in near-total regression of tumor (&gt;90% reduction in volume) which was durable (ongoing response &gt;50 days in 7/9 animals). The combination with cisplatin or talazoparib also resulted in tumor regression and was significantly more effective than either cisplatin or talazoparib alone. Tumor regression following LY2606368 treatment was also observed in cis-resistant H69 xenograft model of SCLC.

Conclusion: The study established checkpoint kinase 1 (Chk1) targeting as a novel and active therapeutic strategy for SCLC, a disease for which the standard of care has remained unchanged for &gt;30 years. Chk1 knockdown was sufficient to inhibit proliferation in SCLC cell lines and LY2606368, a Chk1 inhibitor in current clinical trials, showed activity as a single agent and in combination with cisplatin and PARP inhibition in vitro and in vivo. TP53-mutant cells (hallmark of SCLC) rely on Chk1 for cell cycle arrest; hence, inhibition of Chk1 in SCLC likely works by driving cells to mitotic catastrophe and apoptosis. These findings support the further investigation of Chk1 targeting and Chk1 predictive biomarkers in SCLC.

Citation Format: Triparna Sen, Pan Tong, C. Allison Stewart, You Hong-Fan, Ying Feng, Jing Wang, Lauren A. Byers. Investigation of Chk1 as a novel therapeutic target for small cell lung cancer (SCLC). [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C153.