981 ATR inhibition activates cancer cell cGAS/STING-interferon signaling and promotes anti-tumor immunity in small cell lung cancer

Background

Small cell lung cancer (SCLC) is the most aggressive subtype of lung cancer. Recent clinical trials of immune checkpoint blockade (ICB) combined with chemotherapy delivered only very modest benefits. In this study, we investigated the effect of targeting ataxia telangiectasia and rad3-related (ATR), the primary activator of the replication stress response, in SCLC.

Methods

In this study, we performed genetic and pharmacological inhibition of ATR in a panel of human and murine SCLC cell lines. Furthermore, we investigated the effect of ATR inhibition either alone or in combination with PD-L1 blockade in multiple immunocompetent mouse models of SCLC. The downstream effects of ATR inhibition was assessed by bulk RNA sequencing, multicolor flow cytometry, western blot analysis and real-time qRT-PCR. The SCLC clinical samples from treatment naïve and patients treated with an ATR inhibitor we analysis by single cell and bulk RNA sequencing to ascertain the effect of ATR on immune subsets.

Results

In multiple immunocompetent SCLC mouse models, ATR inhibition (ATRi) remarkably enhanced the anti-tumor effect of PD-L1 blockade. We next tested the ATR inhibition either alone or in combination with PD-L1 in the second-line regimen for SCLC. We observed that ATR inhibition in combination with PD-L1 blockade significantly reduced tumor volume and prolonged survival of aggressive mice models when compared to PD-L1 alone. Targeting ATR enhanced the expression of PD-L1, activated the cGAS/STING pathway, induced the expression of Type I and II interferon pathways, and caused significant infiltration of cytotoxic and memory/effector T-cells into tumors. Interestingly, ATRi also led to significant induction of MHC class I in SCLC in vitro and in vivo models. Single-cell analysis of patient samples confirms immunosuppressed phenotype in SCLC. Analysis of pre-and post-treatment clinical samples from a proof-of-concept study of a first-in-class ATR inhibitor, M6620 (VX970, berzosertib), and TOP1 inhibitor topotecan, in patients with relapsed SCLCs, validated the induction of MHC class I and interferon pathway genes, for the first time in this disease.

Conclusions

Our findings highlight ATRi as a potentially transformative vulnerability of SCLC, paving the way for combination clinical trials with anti-PD-L1. Given the increasing importance of immunotherapy for the management of SCLC and that ATR inhibitors are already in clinical trials, combining an ATR inhibitor with PD-L1 blockade may offer a particularly attractive strategy for the treatment of SCLC and contribute to the rapid translation of this combination into the clinic.