Background: Five-year survival with advanced G/GEJ is poor, and limited biomarkers exist to inform optimal treatment selection. Pembrolizumab, an anti–programmed death-1 receptor (PD-1) antibody, is approved for advanced/metastatic PD-ligand 1–positive (PD-L1⁺) G/GEJ that progressed after ≥2 lines of therapy. The transmembrane tight junction protein claudin 18.2 (CLDN18.2) is normally confined to gastric mucosa but is often overexpressed in G/GEJ with roughly one-third of patients (pts) having high expression (≥75%). Zolbetuximab, a chimeric IgG1 monoclonal antibody, binds to CLDN18.2 and mediates cancer cell death through antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity. Phase 2 (NCT01630083) results showed prolonged survival with zolbetuximab + epirubicin, oxaliplatin, and capecitabine (EOX) vs EOX alone in G/GEJ. Results of nonclinical studies showed enhanced antitumor activity with zolbetuximab + anti-murine PD-1 antibody, and it was hypothesized that a combination with pembrolizumab (new Cohort 3) might augment ADCC and antitumor immune response in CLDN18.2 overexpressing G/GEJ. Methods: This phase 2 open-label study (NCT03505320) will enroll ~112 adult pts from 22 sites in 5 countries into 3 cohorts; this abstract describes Cohort 3 (~62 pts). Key eligibility criteria are advanced/metastatic G/GEJ, measurable disease (RECIST v1.1), adequate organ function and performance status, and high/intermediate (Cohort 3A) or high (Cohort 3B) expression of CLDN18.2. Central testing of tumor tissue will determine CLDN18.2 expression; pts are considered CLDN18.2 positive (CLDN18.2⁺) if ≥75% (high) or ≥50% to < 75% (intermediate) of tumor cells demonstrate moderate-to-strong membranous IHC staining. Patients in Cohort 3B are required to be PD-L1⁺, defined as a combined positive score ≥1 (IHC staining per the Dako 22C3 PD-L1 assay). Patients will receive zolbetuximab + pembrolizumab in the third/later line in Cohort 3A and third line in Cohort 3B. In Cohort 3A (safety cohort), zolbetuximab will be administered at a loading dose of 800 mg/m² IV on Day 1 Cycle 1 followed by 600 mg/m² IV every 3 weeks; a reduction from 600 mg/m² every 3 weeks is permitted. Pembrolizumab 200 mg IV will be administered on Day 1 of each 21-day cycle. Cohort 3B (expansion cohort) zolbetuximab dose is determined from results of Cohort 3A. Imaging will occur every 6 weeks for 24 weeks and every 12 weeks thereafter. The primary endpoint is objective response rate; additional endpoints include duration of response, disease control rate, and progression-free survival by independent review committee and investigator assessment. Pharmacokinetics, safety/tolerability, quality of life, and immunogenicity will be assessed. The study is currently recruiting pts. Clinical trial information: NCT03505320