DNA damage response signaling as a predictive biomarker and synergistic therapeutic target for anti‐PD‐1/PD‐L1 immunotherapy in non‐small cell lung cancer

Lung cancer is the most common cancer and leading cause of cancer-related mortality worldwide. In 2015, more than 610,000 people died from lung cancer in China. Nonsmall cell lung cancer (NSCLC) accounts for approximately 85% of lung cancer cases and the five-year survival rate of advanced NSCLC remains < 15%. PD-1 is an important immune checkpoint protein that mediates immunosuppression by binding to PD-L1. Compared to conventional chemotherapy, antibody-directed therapies against the PD1/PD-L1 checkpoint have shown remarkable and persistent anti-cancer efficacy for advanced NSCLC. The United States Food and Drug Administration has approved several monoclonal antibodies directed to the PD-1/PD-L1 pathway (nivolumab, pembrolizumab, atezolizumab, durvalumab) for first-line or second-line treatment of advanced NSCLC. Presently, PD-1/PD-L1 inhibitors play an important role in therapeutic strategies for advanced NSCLC. In China, domestic pharmaceutical companies have made enormous efforts to develop counterpart antibodies. The China Food and Drug Administration (CFDA) has approved clinical trial applications for 16 agents: PD-1 inhibitors including JS001, SHR-1210, IBI308, BGB-A317, GLS-010, GB226, BAT1306, LZM009, HLX10, and HX008; and PD-L1 inhibitors including KN035, SHR-1316, TQB2450, KL-A167, ZKAB001, and CS1001. Moreover, at least six agents (CS1003, AK104, AK105, HLX20, MSB2311, BGB-A333) are under review by the CFDA for clinical trial approval. Numerous randomized clinical trials of PD-1/PD-L1 inhibitors both domestically and abroad are being performed in patients with advanced NSCLC to explore the efficacy, safety, and optimal regimen of these agents. Early clinical trials with PD-1/PD-L1 inhibitors have shown durable responses and good tolerance in patients with advanced NSCLC. However, the response rate in the overall population remains only 14–20%. Many studies have attempted to identify effective biomarkers for predicting the clinical efficacy of PD-1/PD-L1 inhibitors. PD-L1 expression was initially examined as a predictive biomarker. The results of clinical trials (CheckMate 017, CheckMate 057, POPLAR phase 2, durvalumab phase 1b, avelumab phase 1b) reported that patients with PDL1-positive NSCLC benefitted from these agents. However, the predictive ability of PD-L1 expression remains controversial. Not all patients with PD-L1-positive NSCLC benefit from PD-1/PD-L1 inhibitors. Additionally, some patients with PD-L1-low-expressing or PD-L1-negative NSCLC benefit from these agents. Pembrolizumab has only been approved for patients with PD-L1-positive tumors, while nivolumab has been approved regardless of PD-L1 status. These inconsistencies have created confusion in clinical practice. Technical challenges related to the antibodies used, assay methods, scoring cutoffs, heterogeneity, and dynamic changes in PD-L1 expression contribute to this dilemma, indicating that any single assay for PD-L1 expression is not sufficient to distinguish PD-L1-lowexpressing or PD-L1-negative patients who may benefit from the inhibitors. Given the limitations of using PD-L1 expression as a definitive biomarker, other potential biomarkers for PD-1/ PD-L1 inhibitors have been identified in diverse clinical trials, including oncogenic driver mutations, mismatch repair deficiency (dMMR), and tumor mutation burden (TMB). The results of the KEYNOTE-001 trial of pembrolizumab demonstrated that progression-free survival was shorter in EGFR-mutant patients compared to EGFR wildtype patients. Similarly, the CheckMate 057 trial showed that EGFR-mutant patients were less likely to benefit from nivolumab. This difference was also observed between KRAS-mutant and KRAS wild-type patients. However, these findings were only observed in the PD-L1-negative subgroup, while those with high PD-L1 expression were predicted to have a higher response to pembrolizumab regardless of their mutation status. In recent years, researchers have observed that tumors generally contain large numbers of somatic mutations that induce the production of neoantigenes. The recognition of mutation-associated neoantigenes by tumor-infiltrating T cells is essential to achieve the effects of PD-1/PD-L1 inhibitors. High TMB has been shown to have a greater association with the benefits of PD-1/PD-L1 inhibitors compared to PD-L1 expression. However, TMB reflects the mutational landscape requiring additional technical support, and the TMB scoring cutoffs remain disputable. dMMR results in the accumulation of somatic mutations and the appearance of multiple alleles at microsatellite loci, known as microsatellite instability (MSI). Previous studies have suggested that dMMR/MSI has a notable positive correlation with high TMB. In 2017, pembrolizumab was approved for dMMR/MSI-high patients regardless of their