Background: Caprin-1 is strongly expressed on the cell membrane of many cancer types including G/GEJ and on cancer stem cells. High expression is associated with enhanced in vitro colony-forming activity and in vivo tumorigenicity. TRK-950 is a first in class humanized antibody targeting CAPRIN-1¹⁾. In the phase I study (NCT02990481), TRK-950 was well tolerated as monotherapy with no DLTs and MTD not reached at doses of 3-30mg/kg IV weekly. Subsequently the Phase 1b study demonstrated that TRK-950 was well tolerated with a high potential for synergistic anti-tumor activity when used in combination with standard of care treatment(s) in selected advanced solid tumors (NCT03872947). Specifically, Regimen D which evaluated TRK-950 in combination with Ramucirumab and Paclitaxel in G/GEJ showed a DCR of 100% in all nine patients receiving 10 mg/kg and partial responses were seen in 5/9 (55%) patients. Caprin-1 strong expression was observed in 4/9 patients and an ORR of 100% was seen in those patients²⁾. A Phase II clinical trial in gastric/GEJ cancer patients with strong Caprin-1 expressions is now enrolling patients. Methods: The phase II study is a multi-national, open-label, randomized, multicenter study evaluating the efficacy of TRK-950 in combination with ramucirumab (Ram) + paclitaxel (PTX ) as compared with Ram + PTX treatment alone in patients with previously treated metastatic G/GEJ adenocarcinoma (NCT06038578). Patients enrolled across 10 sites in Japan, South Korea and the United States are randomized 1:1:1 by the permuted block method to (arm A) TRK-950 5mg/kg plus Ram + PTX, (arm B) TRK-950 10mg/kg plus Ram + PTX or (arm C) Ram + PTX alone. Inclusion criteria include patients with Caprin-1 positive (cutoff; 30% at ≥ 2+ staining) advanced/metastatic G/GEJ adenocarcinoma who have received first line therapy. Key exclusion criteria include HER2 positive disease. The primary endpoint is progression free survival (PFS) based on Independent Central Review (ICR) assessment per RECIST v1.1. Key secondary and exploratory endpoints include overall survival (OS), ORR, DoR, and biomarker evaluation. An interim analysis will be performed after 105 patients are randomized in stage 1 to determine the optimal dose of TRK-950 (5mg/kg or 10mg/kg) that will be evaluated in arm stage 2. In total 146 patients will be enrolled (40 patients in one arm and each 53 patients in two arms). This study opened in September 2023. 1. Okano F, et al. Cancer Res Commun. 2023 Apr 18;3(4):640-658. 2. Cassier P, et al. J Clin Oncol. 2023 Vol. 41, No. 4, suppl. Clinical trial information: NCT06038578.