Phase 1 trial of zolbetuximab in Japanese patients with CLDN18.2+ gastric or gastroesophageal junction adenocarcinoma

Zolbetuximab is a chimeric monoclonal antibody that targets claudin-18.2, a candidate biomarker in patients with advanced gastric/gastroesophageal cancer. This nonrandomized phase 1 study (NCT03528629) enrolled previously treated Japanese patients with claudin-18.2-positive locally advanced/metastatic gastric/gastroesophageal cancer in two parts: Safety (Arms A and B, n = 3 each) and Expansion (n = 12). Patients received intravenous zolbetuximab 800 mg/m² on cycle 1, day 1 followed by 600 mg/m² every 3 weeks (Q3W; Safety Part Arm A and Expansion) or 1000 mg/m² Q3W (Safety Part Arm B). For the Safety Part, the primary endpoint was safety (i.e., dose-limiting toxicities [DLTs]) and a secondary endpoint was objective response rate (ORR) by investigator. For the Expansion Part, the primary endpoint was ORR by investigator and secondary endpoints included ORR by central review and safety. Additional secondary endpoints for both the Safety and Expansion Parts were disease control rate (DCR), overall survival (OS), progression-free survival (PFS), duration of response, pharmacokinetics, and immunogenicity. In 18 patients, no DLTs (Safety Part) or drug-related treatment-emergent adverse events (TEAEs) grade ≥3 were observed. Most TEAEs were gastrointestinal. In 17 patients with measurable lesions, best overall response was stable disease (64.7%) or progressive disease (35.3%). The DCR was 64.7% (95% confidence interval 38.3-85.8). In Arm A and Expansion combined (n = 15), median OS was 4.4 months (2.6-11.4) and median PFS was 2.6 months (0.9-2.8). In Arm B (n = 3), median OS was 6.4 months (2.9-6.8) and median PFS was 1.7 months (1.2-2.1). Zolbetuximab exhibited no new safety signals with limited single-agent activity in Japanese patients.