Abstract A153: Targeting DNA damage response upregulates PD-L1 level and promotes antitumor immunity in small-cell lung cancer
Authors: Sen Triparna , Rodriguez Bertha Leticia , Chen Limo , Morikawa Naoto , Fujimoto Junya , Diao Lixia , Fan Youhong , Wang Jing , Glisson Bonnie S. , Wistuba Ignasio , Sage Julien , Heymach John V. , Gibbons Don L. , Byers Lauren A. 
Published: February 2019

Abstract
Purpose of the Study: Despite recent advances in the use of immunotherapy, only a minority of small cell lung cancer (SCLC) patients respond to immune checkpoint blockade (ICB) with programmed cell death protein 1 (PD-1) or programmed death ligand 1 (PD-L1) antibodies as monotherapy or combination. Therefore, there is a critical need to develop strategies to enhance the efficacy of ICB in SCLC, an otherwise immunosuppressed disease with dismal 5-year survival rate of &lt;5%, which has remained unchanged for &gt;30 years. We have previously shown that prexasertib and olaparib treatment enhanced the protein (but not mRNA) and surface expression of PD-L1 in SCLC cell lines and tumor models. Here we elucidate the previously unexplored mechanism of how DNA damage response targeting enhances antitumor immunity in SCLC. Methods: SCLC cell lines and immune competent murine models were treated with inhibitors targeting DNA damage repair (DDR) proteins, checkpoint kinase 1 (CHK1) (by prexasertib), poly (ADP-ribose) polymerase (PARP) (by olaparib) either as single agents or in combination with anti-PD-L1. End point analyses were done by Western blot, real-time RT-PCR, multicolor flow cytometry and reverse phase protein array (RPPA). Result: We observed increased PD-L1 glycosylation post-prexasertib and olaparib treatment. Prexasertib and/or olaparib +/- anti-PD-L1 combination activates mTOR and inactivates GSK3β pathway thus providing mechanistic insight into DDR-targeting mediated PD-L1 glycosylation and stabilization. We treated tumor-bearing immune-competent B6129F1 mice with either IgG (control), prexasertib (10mg/kg, 2/7), anti-PD-L1 (300ug, 1/7) or combination (n=10/group). At Day 21, anti-PD-L1 did not cause tumor regression and prexasertib treatment delayed tumor growth [T/C=0.31(p&lt;0.001)]. However, most notably, 60% mice treated with anti-PD-L1+ prexasertib had a complete tumor regression within 15 days of treatment and the other 40% had no further increase in tumor growth beyond baseline [T/C=0.05(p&lt;0.001)]. Co-targeting CHK1+PD-L1 significantly increases the level of CD8+ cytotoxic-T-cell tumor infiltration and decreases exhausted T-cell population in SCLC models. CD8 depletion was able to reverse the antitumor effect of the combination demonstrating the role of cytotoxic T-cell infiltration in CHK1i+PD-L1 response. Next, we treated the tumor-bearing B6129F1 mice with either IgG (control), olaparib (100mg/kg, 4/7), anti-PD-L1 (300ug, 1/7) or combination (n=10/group). At Day 21, anti-PD-L1 or olaparib did not cause tumor regression. However, there was a significant delay in tumor growth when the mice were treated with olaparib+anti-PD-L1 combination [T/C=0.26(p&lt;0.001)]. Immune profiling demonstrated increased CD8+ cytotoxic-T-cell tumor infiltration, decreases exhausted T-cell population in the olaparib+anti-PD-L1 combination group. Conclusions: Expression of PD-L1 is associated with tumors deficient in DNA damage repair and we, for the first time, provide rationale for investigating the role of immunotherapy in the context of DDR targeting in SCLC. This study further demonstrates the novel mechanism of DDR-mediated PD-L1 regulation in SCLC via GSK3β inactivation and stabilization. Moreover, our results demonstrating the remarkable efficacy of the combination of PD-L1 blockade with PARP and CHK1 inhibition provide a strong scientific rationale for combining these modalities in clinical trials for SCLC patients. Combining DDR inhibition with ICB increases CD8+ cytotoxic T-cell infiltration in SCLC models. The immune profiling and correlative biomarker data from our study further provide valuable mechanistic insight and indicate the subset of the patient population who are most likely to respond to these treatments. Because prexasertib, olaparib and other PARP inhibitors are already in clinical trials for SCLC, we expect that this hypothesis has the potential for rapid translation into the clinic.
Citation Format: Triparna Sen, Bertha Leticia Rodriguez, Limo Chen, Naoto Morikawa, Junya Fujimoto, Lixia Diao, Youhong Fan, Jing Wang, Bonnie S. Glisson, Ignasio Wistuba, Julien Sage, John V. Heymach, Don L. Gibbons, Lauren A. Byers. Targeting DNA damage response upregulates PD-L1 level and promotes antitumor immunity in small-cell lung cancer [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A153.