Abstract A108: DNA damage repair targeting upregulates PD-L1 level and potentiates the effect of PD-L1 blockade in small cell lung cancer
Authors: Sen Triparna , Chen Limo , Rodriguez B. Leticia , Gay Carl M. , Li Lerong , Li Yanli , Fan Youhong , Glisson Bonnie , Wang Jing , Piwnica-Worms Helen , Sage Julien , Heymach John V. , Gibbons Don L. , Byers Lauren A. 
Published: August 2018

Abstract
Background: Effective targeted therapies for small-cell lung cancer (SCLC), the most aggressive form of lung cancer, are urgently needed. SCLC has a relatively immunosuppressed phenotype with relatively low levels of infiltrating T-cells and reduced antigen presentation. Only a minority of SCLC patients responds to programmed cell death protein 1 (PD-1) or programmed death ligand 1 (PD-L1) inhibitors as monotherapy. Therefore, there is a critical need to develop strategies to enhance the efficacy of immunotherapy in SCLC. We previously discovered that DNA damage repair (DDR) protein, checkpoint kinase 1 (CHK1), is overexpressed in SCLC; CHK1 inhibitor, LY2606368, has activity in preclinical models of SCLC, a finding that supported the phase 2 clinical trial of LY2606368 for SCLC patients (NCT02735980). We hypothesize that targeting CHK1 by inducing DNA damage can enhance immunogenicity, antitumor immunity, and response to immune checkpoint targeting. Methods: SCLC cell lines and immune competent murine models were treated with single agent LY2606368, anti-PD-L1, or combination. End point analyses were done by Western blot, real-time RT-PCR, multicolor flow cytometry, and reverse phase protein array (RPPA). Result: SCLC tumors with higher DDR mutation burden have a higher expression of PD-L1 (p&lt;0.0001). LY2606368 treatment enhanced the protein (but not mRNA) and surface expression of PD-L1 in SCLC cell lines and tumor models. We further observed increased PD-L1 glycosylation post-LY2606368 treatment. LY2606368+/- anti-PD-L1 combination activates mTOR and inactivates GSK3β pathway, thus providing mechanistic insight into CHK1-targeting mediated PD-L1 glycosylation and stabilization. We treated tumor-bearing immune-competent B6129F1 mice with either IgG (control), LY2606368 (10mg/kg, 2 out of 7 days), anti-PD-L1 (300ug, 1 out of 7 days) or combination (n=10/group). At Day 21, anti-PD-L1 did not cause tumor regression and LY2606368 treatment delayed tumor growth [T/C=0.31(p&lt;0.001)]. However, most notably, 60% mice treated with anti-PD-L1+LY2606368 had a complete tumor regression within 15 days of treatment [T/C=0.05(p&lt;0.001)]. Cotargeting CHK1+PD-L1 significantly increases the level of CD8+ cytotoxic-T cell tumor infiltration, decreases exhausted T cell population, and enhances the expression of IFN gamma in SCLC models. CD8 depletion was able to reverse the antitumor effect of the combination, demonstrating the role of cytotoxic T-cell infiltration in CHK1+PD-L1 response. Significance: Our study demonstrates, for the first time, the potential for combining the two promising modalities, immune checkpoint targeting and CHK1 inhibition, in SCLC. This study further demonstrates the novel mechanism of DDR-mediated PD-L1 regulation in SCLC. 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. This study suggests intriguing possibilities for therapeutic synergy and scientific rationale for the evaluation of PD-L1/PD-1 blockade with CHK1i in clinical trials for SCLC, a disease with dismal 5-year survival rate of &lt;5%, which has remained unchanged for &gt;30 years.
Citation Format: Triparna Sen, Limo Chen, B. Leticia Rodriguez, Carl M. Gay, Lerong Li, Yanli Li, Youhong Fan, Bonnie Glisson, Jing Wang, Helen Piwnica-Worms, Julien Sage, John V. Heymach, Don L. Gibbons, Lauren A. Byers. DNA damage repair targeting upregulates PD-L1 level and potentiates the effect of PD-L1 blockade in small cell lung cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A108.