First-line pembrolizumab (P), trastuzumab (T), capecitabine (C) and oxaliplatin (O) in HER2-positive metastatic esophagogastric adenocarcinoma.

Abstract

4011 Background: Trastuzumab stimulates HER2-specific T cell responses and increases tumor PD-L1 expression, and anti-PD-1 antibody can help enhance T cell-specific immunity of trastuzumab. We conducted a phase II trial of pembrolizumab with chemotherapy/trastuzumab. Methods: Patients (pts) with previously untreated HER2 IHC 3+ or FISH+ tumors irrespective of PD-L1 status received intravenous P 200 mg flat dose, T 6 mg/kg (after 8 mg/kg load), O 130 mg/m2 every 3 weeks and oral C 850 mg/m2 2 weeks on/1 week off. 22 pts received 1 cycle of induction P/T prior to initiation of chemotherapy. The primary endpoint was 6-months PFS; with target accrual of 37 pts. Secondary endpoints included safety, OS, ORR, and biomarker analysis. Results: Accrual completed and 100% of the 32 evaluable pts had tumor regression (ranging from -20% to -100%). The RECIST 1.1 ORR was 87% (25 PR, 3 CRs), and 12 (52%) of pts that received induction P/T x 1 cycle showed reduction in target lesions. Median PFS was 11.3 months (mo), with 67% 6 mo PFS. Median follow up was only 6.6 mo. In pts with available material, 14/36 (40%) had PD-L1 CPS >1 and median TMB was 4.4 mut/Mb (0-10.6). There was no correlation between PD-L1 status and PFS or OS. ERBB2 amplification was evident by tissue-NGS in 17/29 (61%) and ctDNA-NGS in 17/30 (58%) pre-treatment, while the remaining pts were ERBB2- by NGS likely due to tumor heterogeneity or low tumor content. CtDNA maxVAF decreased in 16/24 tested pts after 1 cycle of induction T/P alone. irAEs included interstitial nephritis Gr4 (3%), transaminitis Gr3 (11%), Gr4 (3%), colitis Gr3 (3%). Conclusions: Most pts (51%) remain on therapy, and so the primary endpoint should be reached by 6/19. Updated survival, correlative studies and will be presented. These promising preliminary safety and efficacy results led to initiation of a definitive phase III Keynote 811 trial. Clinical trial information: NCT02954536.