Small cell lung cancer (SCLC) is the most aggressive lung cancer subtype. Recent trials of immune checkpoint blockade (ICB) combined with chemotherapy have observed modest benefits. This study explores the therapeutic potential of targeting ATR, the key activator of the replication stress response, in SCLC.

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 were assessed by bulk RNA sequencing, multicolor flow cytometry, western blot analysis, and real-time qRT-PCR. SCLC clinical samples from treatment-naïve patients and patients treated with an ATR inhibitor were analyzed by single-cell and bulk RNA sequencing to ascertain the effect of ATR on immune subsets.

In multiple immunocompetent SCLC mouse models, ATR inhibition (ATRi) remarkably enhanced the anti-tumor effect of PD-L1 blockade. We next tested 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 mouse models 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. 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. Single-cell analysis of patient samples revealed how ATR inhibition modulated antigen presentation and confirmed the immunosuppressed phenotype in SCLC.

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 the fact that ATR inhibitors are already in clinical trials, combining an ATR inhibitor with PD-L1 blockade may offer a particularly attractive strategy for treating SCLC and contribute to the rapid translation of this combination into the clinic.