1189PD - Determinants of response of HER2+ gastric cancer (GC) vs gastroesophageal junction adenocarcinoma (GEJ) to margetuximab (M) plus pembrolizumab (P) post trastuzumab (T)

Abstract

BackgroundT + chemo is standard 1st line therapy for HER2+ gastroesophageal adenocarcinoma; however, patients (pts) tend to progress in 6-8 months. Up to 40% show loss of HER2 expression post T, likely underlying the lack of efficacy of anti-HER2 agents in 2nd line therapy. We report here a clinical update and biomarker analysis of an ongoing study in pts receiving M, an anti-HER2 Fc-optimized mAb, plus P in HER2+ GEA pts in post T, 2nd line, chemotherapy-free treatment. MethodsEndpoints described herein are safety, objective response rate (ORR), disease control rate (DCR), archival HER2 IHC level and/or ERBB2 amplification (amp) status pre-M+P in cell free DNA (cfDNA) by NGS (Guardant360), PD-L1 CPS by IHC (22C3 pharmDx), anti-HER2 T- cell immunity by ELISPOT on PBMCs, and NanoString PanCancer IO360™ assay on archival FFPE biopsies. Results92 pts (66% GC, 34% GEJ) received 15mg/kg M+200mg P Q3W. 99% were MSS and 43% were PD-L1+(>1%). Related adverse events ≥grade 3 were 18.5%. Confirmed ORR was 19% with 54% DCR; interestingly, higher response rates occurred in GC vs GEJ (25% vs 7% ORR, p=0.047 and 64% vs 33% DCR, p=0.008). A higher proportion of GC vs GEJ cancers was HER2 3+ on archival IHC (90% vs 53%, p=0.0075), furthermore, a 5.3-fold greater fraction of GC vs GEJ pts showed upregulated tumor ERBB2 RNA expression (p<0.001). Pts with upregulated tumor ERBB2 RNA expression showed a 3-fold greater frequency of DCR (p=0.003). While not statistically significant, more GC pts retained HER2+ status post T (62.5% vs 50%) or expressed PD-L1 by IHC (46.3% vs 31.8%); notably, however, the frequency of dual ERRB2 amplified plus PD-L1+ was >6-fold higher in GC vs GEJ (34.7% vs 5.5%, p=0.03) and associated with 53% confirmed ORR and 82% DCR in GC. GC pts with higher baseline T-cell immunity toward HER2 (p59 class II peptide) had greater probability of ORR (p=0.005). ConclusionsIn this study, M+P demonstrated tolerability and preliminary evidence of anti-tumor activity as a chemotherapy-free regimen in 2nd line HER2+ GC; biomarker analysis suggests an association with higher expression and retention of HER2 and PD-L1 together with pre-existing anti-HER2 immunity. Clinical trial identificationNCT02689284. Legal entity responsible for the studyMacroGenics, Inc. FundingMacroGenics, Inc. DisclosureH. Park: Research grant / Funding (institution): Ambrx; Research grant / Funding (institution): Amgen; Research grant / Funding (institution): Array BioPharma; Research grant / Funding (institution): AstraZeneca; Research grant / Funding (institution): Bayer; Research grant / Funding (institution): BeiGene; Research grant / Funding (institution): Bristol-Myers Squibb; Research grant / Funding (institution): Daiichi Sankyo; Research grant / Funding (institution): EMD Serono; Research grant / Funding (institution): Gilead Sciences; Research grant / Funding (institution): GlaxoSmithKline; Research grant / Funding (institution): Incyte; Research grant / Funding (institution): Lilly; Research grant / Funding (institution): MacroGenics; Research grant / Funding (institution): MedImmune; Research grant / Funding (institution): Medivation; Research grant / Funding (institution): Merck; Research grant / Funding (institution): Millennium; Research grant / Funding (institution): Novartis; Research grant / Funding (institution): Pfizer; Research grant / Funding (institution): Puma Biotechnology; Research grant / Funding (institution): Regeneron; Research grant / Funding (institution): Roche; Research grant / Funding (institution): Taiho Pharmaceutical; Research grant / Funding (institution): Vertex. H. Uronis: Shareholder / Stockholder / Stock options, Full / Part-time employment: GeneCentric; Advisory / Consultancy, Research grant / Funding (institution), Travel / Accommodation / Expenses: Bristol-Myers Squibb; Research grant / Funding (institution): Advaxis; Research grant / Funding (institution): Genentech/Roche; Research grant / Funding (institution): Lycera; Research grant / Funding (institution): MacroGenics; Research grant / Funding (institution): Merck. Y. Kang: Advisory / Consultancy: Bristol-Myers Squibb; Advisory / Consultancy, Research grant / Funding (institution): DAE HWA Pharmaceutical; Advisory / Consultancy: Lilly/Imclone; Advisory / Consultancy: Merck/Serano; Advisory / Consultancy: Ono Pharmaceutical; Advisory / Consultancy: Roche/Genentech; Research grant / Funding (institution): Taiho Pharmaceutical; Research grant / Funding (institution): LSK Biopharma. M.C.H. Ng: Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: MSD Oncology; Honoraria (self), Travel / Accommodation / Expenses: Taiho Pharmaceutical; Advisory / Consultancy: Bristol-Myers Squibb; Research grant / Funding (institution): ASLAN Pharmaceuticals. P. Enzinger: Advisory / Consultancy: Astellas Pharma; Advisory / Consultancy: Five Prime Therapeutics; Advisory / Consultancy: Merck; Advisory / Consultancy: Taiho Pharmaceutical. K.W. Lee: Research grant / Funding (institution): Array BioPharma; Research grant / Funding (institution): ASLAN Pharmaceuticals; Research grant / Funding (institution): AstraZeneca/MedImmune; Research grant / Funding (institution): Five Prime Therapeutics; Research grant / Funding (institution): Green Cross Corp; Research grant / Funding (institution): LSK BioPharma; Research grant / Funding (institution): MacroGenics; Research grant / Funding (institution): Merck KGaA; Research grant / Funding (institution): MSD; Research grant / Funding (institution): Ono Pharmaceutical; Research grant / Funding (institution): Pfizer; Research grant / Funding (institution): Pharmacyclics; Honoraria (self): Bristol-Myers Squibb; Honoraria (self): Lilly; Research grant / Funding (institution): ALX Oncology. S. Rutella: Leadership role: Clinical & Biomarkers Data Sharing Subcommittee, Society for Immunotherapy of Cancer; Research grant / Funding (institution): John and Lucille van Geest Foundation. S.E. Church: Shareholder / Stockholder / Stock options, Full / Part-time employment: NanoString Technologies. J. Nordstrom: Shareholder / Stockholder / Stock options, Full / Part-time employment: MacroGenics, Inc. T. Wu: Shareholder / Stockholder / Stock options, Full / Part-time employment: MacroGenics, Inc. J. Yen: Shareholder / Stockholder / Stock options, Full / Part-time employment: Guardant Health. A. Franovic: Shareholder / Stockholder / Stock options, Full / Part-time employment: Guardant Health. J. Muth: Shareholder / Stockholder / Stock options, Full / Part-time employment: MacroGenics, Inc. M. Rosales: Shareholder / Stockholder / Stock options, Full / Part-time employment: MacroGenics, Inc. J. Davidson-Moncada: Shareholder / Stockholder / Stock options, Full / Part-time employment: MacroGenics, Inc. Y. Bang: Advisory / Consultancy, Research grant / Funding (institution): AstraZeneca/MedImmune; Advisory / Consultancy, Research grant / Funding (institution): Bayer; Advisory / Consultancy, Research grant / Funding (institution): Beigene; Advisory / Consultancy, Research grant / Funding (institution): Green Cross; Advisory / Consultancy: Hanmi; Advisory / Consultancy, Research grant / Funding (institution): Merck Serono; Advisory / Consultancy, Research grant / Funding (institution): MSD Oncology; Advisory / Consultancy, Research grant / Funding (institution): Novartis; Advisory / Consultancy: Samyang; Advisory / Consultancy, Research grant / Funding (institution): Taiho Pharmaceutical; Research grant / Funding (institution): Astellas Pharma; Research grant / Funding (institution): Boehringer Ingelheim; Research grant / Funding (institution): Boston Biomedical; Research grant / Funding (institution): Bristol-Myers Squibb; Research grant / Funding (institution): CKD; Research grant / Funding (institution): Curis; Research grant / Funding (institution): Daiichi Sankyo; Research grant / Funding (institution): Five Prime Therapeutics; Research grant / Funding (institution): Genentech/Roche; Research grant / Funding (institution): GlaxoSmithKline; Research grant / Funding (institution): Lilly; Research grant / Funding (institution): MacroGenics; Research grant / Funding (institution): Ono Pharmaceutical; Research grant / Funding (institution): Pfizer; Research grant / Funding (institution): Takeda. D.V. Catenacci: Honoraria (self), Advisory / Consultancy: Amgen; Honoraria (self), Advisory / Consultancy: Bristol-Myers Squibb; Honoraria (self): Five Prime Therapeutics; Honoraria (self), Speaker Bureau / Expert testimony: Foundation Medicine; Honoraria (self), Advisory / Consultancy: Genentech/Roche; Honoraria (self): GenMab; Honoraria (self): Gritstone Oncology; Honoraria (self), Speaker Bureau / Expert testimony: Guardant Health; Honoraria (self), Advisory / Consultancy: Lilly; Honoraria (self): Merck; Honoraria (self): NantOmics; Honoraria (self): OncoPlex Diagnostics; Honoraria (self), Advisory / Consultancy: Taiho Pharmaceutical; Advisory / Consultancy: Astellas Pharma. All other authors have declared no conflicts of interest.