Investigation of synergistic effects in trials of combination therapies with immune-checkpoint inhibitors in advanced gastric or gastroesophageal junction cancer.

405 Background: Combinations of immune checkpoint inhibitors (ICI) and chemotherapy (CT) have been approved for gastric cancer. However, there is a hypothesis that this combination may blunt antitumor immune responses because most chemotherapeutic agents also target lymphocytes. The primary objective was to investigate that the ICI and CT does not interfere each other’s therapeutic effects in advanced gastric cancer (GC) or gastroesophageal junction cancer (GEJC) patients. Methods: The reconstructed individual patient data was electronically extracted from the Kaplan-Meier curve of phase III randomized controlled trials (RCTs). The observed PFS curve of each constituent monotherapies was used to estimate simulated PFS curves expected under a model of independent drug action. If the observed curve demonstrated significantly better PFS than simulated curve, the combination of ICI and CT may have a synergistic effect, implying a superior outcome compared to simply adding the component monotherapy. Results: The study included 2,538 unresectable advanced, recurrent, or metastatic GC or GEJC patients from three RCTs comparing pembrolizumab (KEYNOTE-061, KEYNOTE-062 and KEYNOTE-859). In patients with programmed cell death ligand 1 (PD-L1) combined positive score (CPS) of 1 or greater, the 1-year and median PFS of the observed and simulated curves were 28.0% vs. 27.9%, and 6.89 months vs. 6.88 months, respectively. One sample log-rank test showed no significant differences between the observed and simulated curves (p = 0.107). In the subgroups with PD-L1 CPS ≥10 or <1, the 1-year PFS of the observed and simulated curves was 34.7% vs 32.8%, and 28.5% vs 26.8%. Conclusions: The observed PFS of ICT involving pembrolizumab was comparable to the simulated PFS estimated from the data for each monotherapy regardless of the magnitude of PD-L1 CPS. Although it was not clear whether potential synergies existed for ICT, these findings at least suggest that the benefits of ICI and CT are not interfering each other, thereby providing theoretical support for the efficacy of ICT in patients with advanced GC or GEJC.