Background: In the phase 3 KEYNOTE-604 study of extensive-stage small-cell lung cancer (ES-SCLC; NCT03066778), first-line pembrolizumab (pembro) plus etoposide and platinum (EP) significantly improved PFS vs placebo (pbo) plus EP (HR, 0.75; P = 0.0023), with favorable OS (significance threshold not met; HR, 0.80; P = 0.0164). PFS/OS were similar regardless of PD-L1 CPS. In this exploratory analysis, tumor mutational burden (TMB), 18-gene T cell–inflamed gene expression profile (Tcell_infGEP) and SCLC transcriptional subtypes were assessed as correlates of survival. Methods: Patients (pts) eligible for this analysis of KEYNOTE-604 had previously untreated ES-SCLC with evaluable pretreatment tumor samples. TMB was assessed by whole-exome sequencing (WES) of tumor and matched normal DNA. RNA-seq was used to determine Tcell_infGEP and SCLC transcriptional subtypes (ASCL1, POU2F3, NEUROD1, YAP1, or inflamed). Associations of TMB, Tcell_infGEP, and SCLC subtype with OS were analyzed using an adjusted Cox proportional hazards model. 1-sided (pembro + EP) and 2-sided (pbo + EP) P values were calculated for TMB and Tcell_infGEP (prespecified α = 0.05); 2-sided P values were calculated for SCLC subtype (multiplicity-adjusted, α = 0.10). Clinical utility was assessed using prespecified cutoffs of ≥175 mut/exome for TMB and the first tertile for Tcell_infGEP. Clinical data cutoff date was Dec 2, 2019. Results: Of 453 pts randomized in KEYNOTE-604 (ITT), 318 had WES data (pembro + EP, n = 167; pbo + EP, n = 151), and 316 had RNA-seq data (pembro + EP, n = 159; pbo + EP, n = 157). High TMB was positively associated with OS in the pbo + EP group (P = 0.005) but not the pembro + EP group (P = 0.450). There was a positive association between higher Tcell_infGEP and OS in the pembro + EP (P = 0.003) and pbo + EP (P < 0.005) groups. SCLC subtypes were not associated with OS in either group (pembro + EP, P = 0.960; pbo + EP, P = 0.999). Clinical benefit of pembro + EP over pbo + EP was demonstrated for TMB <175 mut/exome, but not for TMB ≥175 mut/exome. Pembro + EP benefit over pbo + EP was consistent across Tcell_infGEP subgroups. Conclusions: In this exploratory analysis of biomarker subgroups of KEYNOTE-604, TMB and SCLC subtypes were not associated with OS in the pembro + EP group in pts with ES-SCLC. While Tcell_infGEP was positively associated with OS in both treatment groups, no additional OS benefit was observed with pembro + EP. Further research is warranted to better identify predictive biomarkers to immunotherapy. Clinical trial information: NCT03066778.