Abstract B15: The oral Chk1 inhibitor, SRA737, synergizes with immune checkpoint blockade in small-cell lung cancer (SCLC)
Authors: Sen Triparna , Milutinovic Snezana , Cardnell Robert J. , Diao Lixia , Fan Youhong , Hansen Ryan J. , Strouse Bryan , Hedrick Michael P. , Hassig Christian , Wang Jing , Byers Lauren A. 
Published: June 2021

Abstract
Background: Small-cell lung cancer (SCLC) is the most aggressive form of lung cancer. Despite the recent success of immunotherapy in other indications, only a minority of SCLC patients respond to immune checkpoint blockade (ICB) targeting programmed cell death protein 1 (PD-1) or programmed death ligand 1 (PD-L1) either as a monotherapy or combination. Therefore, there is a strong need to develop strategies to enhance the efficacy of immunotherapy in SCLC. Our group previously discovered that SCLC exhibits high expression of checkpoint kinase 1 (Chk1) and showed that preclinical in vivo models of SCLC respond to Chk1 inhibition. Based on data from others and our group, we hypothesized that targeting Chk1 can enhance antitumor immunity and synergize with ICB.
Results: SRA737 treatment decreased cell viability with a range of potencies in a panel of SCLC cell lines in vitro. This was accompanied by an induction of double-strand breaks in sensitive cell lines as demonstrated by increased γ-H2AX. Intriguingly, SRA737 also led to an increase in micronuclei formation and STING activation in cells in vitro. Cell surface and total PD-L1 protein were increased following SRA737 treatment in vitro, further supporting a potential benefit of combining the drug with immune checkpoint blockade therapy in vivo. As hypothesized, SRA737 showed strong synergy with anti-PD-L1 antibody in an immunocompetent xenograft SCLC model. Triple-knockout SCLC cells generated from a GEMM mouse model with conditional deletion of Trp53, Rb1 and p130 were implanted into the flank of B6129F1 mice. The mice were treated for three weeks with either IgG (control), SRA737 (100mg/kg, either 3/7 or 5/7 days), anti-PD-L1 (300ug, 1/7 days) or the combination. While anti-PD1 antibody treatment was largely ineffective, SRA737 significantly delayed tumor growth (at Day 21: T/C=0.30 for 3/7 days and T/C=0.28 for 5/7 days). Combination treatment with SRA737 and anti-PD-L1 demonstrated remarkable antitumor efficacy, resulting in stable disease following SRA737 schedule of 3/7 days (T/C=0.12) and tumor regressions following SRA737 schedule of 5/7 days (T/C=0.1). These effects were sustained after treatment cessation and the long-term survival benefit is being assessed.
Discussion: The intrinsic antitumor activity of the Chk1 inhibitor, SRA737, was significantly enhanced by addition of an anti-PD-L1 antibody, leading to tumor regressions in an immunocompetent SCLC model. Preliminary evidence suggests SRA737 induces micronuclei formation, STING activation and PD-L1 expression in tumor cells. Further studies to elucidate the mechanism of Chk1 inhibition-induced antitumor immunity in SCLC are ongoing. SRA737 is currently being tested in clinical trials both as a monotherapy and in combination with other agents. Given that the anti-PD-L1 antibody opdivo is now approved for SCLC, our data suggest intriguing possibilities for therapeutic synergy between the oral Chk1 inhibitor, SRA737, and ICB therapy that warrant further clinical investigation.
Citation Format: Triparna Sen, Snezana Milutinovic, Robert J. Cardnell, Lixia Diao, Youhong Fan, Ryan J. Hansen, Bryan Strouse, Michael P. Hedrick, Christian Hassig, Jing Wang, Lauren A. Byers. The oral Chk1 inhibitor, SRA737, synergizes with immune checkpoint blockade in small-cell lung cancer (SCLC) [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B15.