The Tumor Immune Microenvironment and Frameshift Neoantigen Load Determine Response to PD-L1 Blockade in Extensive-Stage SCLC

INTRODUCTION: Despite a considerable benefit of adding immune checkpoint inhibitors (ICIs) to platinum-based chemotherapy for patients with extensive-stage SCLC (ES-SCLC), a durable response to ICIs occurs in only a small minority of such patients. METHODS: A total of 135 patients with ES-SCLC treated with chemotherapy either alone (chemo-cohort, n = 71) or together with an ICI (ICI combo-cohort, n = 64) was included in this retrospective study. Tumors were classified pathologically as inflamed or noninflamed on the basis of programmed death-ligand 1 expression and CD8(+) tumor-infiltrating lymphocyte density. Immune-related gene expression profiling was performed, and predicted neoantigen load was determined by whole-exome sequencing. RESULTS: Among patients in the ICI combo-cohort, median progression-free survival was 10.8 and 5.1 months for those with inflamed (n = 7) or noninflamed (n = 56) tumors, respectively (log-rank test p = 0.002; hazard ratio of 0.26). Among the 89 patients with immune-related gene expression profiling data available, inflamed tumors had a higher T cell-inflamed GEP score than did noninflamed tumors (−0.18 versus −0.58, p < 0.001). The 12-month progression-free survival rate was 16.1% and 0% for patients in the ICI combo-cohort harboring tumors with a high (n = 26) or low (n = 18) frameshift neoantigen load, respectively. A high-frameshift neoantigen load was associated with up-regulation of gene signatures related to antigen presentation and costimulatory signaling. A durable clinical benefit of ICI therapy was observed only in patients with inflamed tumors and a high-frameshift neoantigen load. CONCLUSIONS: Expression of programmed death-ligand 1, CD8(+) T cell infiltration, and a high-frameshift neoantigen load are associated with clinical benefit of ICI therapy in ES-SCLC. CLINICAL TRIAL REGISTRATION: UMIN000041056