Exploratory biomarker analysis of the phase 3 KEYNOTE-604 study of pembrolizumab plus etoposide for extensive-stage SCLC.

Abstract

8503 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 inf GEP) 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 inf GEP and SCLC transcriptional subtypes (ASCL1, POU2F3, NEUROD1, YAP1, or inflamed). Associations of TMB, Tcell inf GEP, 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 inf GEP (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 inf GEP. 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 inf GEP 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 inf GEP 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 inf GEP 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 . [Table: see text]