Valemetostat and trastuzumab deruxtecan (T-DXd) in previously treated advanced or metastatic human epidermal growth factor receptor 2 (HER2)-positive gastric cancer or gastro-esophageal junction (GEJ) adenocarcinoma.

TPS508 Background: Valemetostat tosylate (valemetostat), a novel and selective dual inhibitor of enhancer of zeste homolog (EZH)2 and EZH1, has demonstrated clinical efficacy and tolerability in multiple hematologic cancers (200 mg orally [PO] once daily [QD]). EZH2 inhibition by valemetostat is expected to upregulate DNA/RNA helicase Schlafen 11 expression and sensitize tumor cells to DNA damaging agents such as topoisomerase I inhibitor antibody–drug conjugates (ADCs). Topoisomerase inhibitors cause DNA strand breaks, which may synergize with valemetostat. HER2 gene amplification or protein expression has been associated with gastric cancer (GC) and GEJ adenocarcinoma. Patients with HER2+ locally advanced or metastatic gastric adenocarcinoma who progress after first line treatment have poor prognosis and limited therapeutic options. T-DXd is approved globally for HER2+ locally advanced or metastatic GC previously treated with a trastuzumab-based regimen. This study evaluates the clinical activity and safety of valemetostat in combination with T-DXd in patients with GC or GEJ adenocarcinoma as part of a Master Protocol trial evaluating valemetostat in combination with ADCs in solid tumors. Methods: This global, phase 1b, multicenter, open-label Master Protocol study (NCT06244485) will enroll ~ 70 patients per sub-protocol in multiple regions, including the US and Japan. The gastric cohort enrolls patients with previously treated advanced or metastatic HER2+ GC or GEJ adenocarcinoma. The dose-escalation (Part 1) will determine the recommended dose for expansion (RDE), combining valemetostat 50–200 mg PO QD with T-DXd 5.4 or 6.4 mg/kg intravenously every 3 weeks. The dose-expansion (Part 2) will assess the efficacy of valemetostat + T-DXd at the RDE. Primary endpoints include safety and tolerability in Part 1 and the objective response rate in Part 2. Secondary endpoints include response duration, progression-free and overall survival, pharmacokinetics, and HER2 expression by immunohistochemistry and in situ hybridization with clinical response. An interim futility analysis will be performed when 20 patients are enrolled with ≥ 6 months of follow-up. Clinical trial information: NCT06244485 .