The association of tissue tumor mutational burden (tTMB) using the Foundation Medicine genomic platform with efficacy of pembrolizumab versus paclitaxel in patients (pts) with gastric cancer (GC) from KEYNOTE-061.
4537 Background: KEYNOTE-061 (NCT02370498) was a randomized, open-label, phase 3 study of pembrolizumab vs paclitaxel in pts with advanced gastric or gastroesophageal junction (GEJ) adenocarcinoma with tumor progression after first-line therapy (N = 592). In this analysis, we evaluated tTMB using FoundationOne CDx (F1CDx; Foundation Medicine) in pts with gastric or GEJ cancer in KEYNOTE-061. Methods: In pts with evaluable F1CDx tTMB data (n = 204), we analyzed the association of tTMB with confirmed objective response rate (ORR), progression-free survival (PFS), and overall survival (OS) within each treatment arm using one-sided (pembrolizumab) and two-sided (paclitaxel) Wald test nominal P for logistic regression (ORR) and Cox proportional hazards regression (PFS; OS) adjusted for ECOG performance status; significance was prespecified at 0.05. The clinical utility of tTMB was assessed using the prespecified cutoff of 10 mut/Mb for F1CDx. Clinical data cutoff: Oct 26, 2017. Results: tTMB was positively associated with ORR ( P < 0.001; AUROC, 0.68), PFS ( P < 0.001), and OS ( P = 0.003) with pembrolizumab but not paclitaxel (ORR, P = 0.047; AUROC, 0.30; PFS, P = 0.605; OS, P = 0.084). Pt outcomes by tTMB cutoff are reported in the Table; prevalence of TMB ≥10 mut/Mb was 17%. In pts with microsatellite stable disease-only, HRs (95% CI) by treatment arm for OS by F1CDx cutoff were 0.40 (0.14-1.17) for tTMB ≥10 mut/Mb (n = 21) vs 0.97 (0.70-1.34) for tTMB <10 mut/Mb (n = 168). Conclusions: In this exploratory analysis from KEYNOTE-061, tTMB as determined by F1CDx demonstrated a positive association with clinical outcomes with pembrolizumab, but not paclitaxel, in pts with GC; these findings are consistent with those reported with whole exome sequencing. Pembrolizumab demonstrated an OS benefit vs paclitaxel in the subgroup with tTMB ≥10 mut/Mb, which remained when pts with microsatellite instability-high disease were excluded. Clinical trial information: NCT02370498 . [Table: see text]