MacroGenics, Inc. announced presentation of data from its clinical trial of margetuximab plus pembrolizumab for patients with advanced gastric and gastroesophageal junction (GEJ) cancers in a poster session at the 2018 ASCO Gastrointestinal Cancers Symposium in San Francisco, California. The poster was titled Phase 1b/2 Study of Margetuximab Plus Pembrolizumab in Advanced HER2+ Gastroesophageal Junction or Gastric Adenocarcinoma. This Phase 1b/2 open-label, dose escalation study evaluates margetuximab, an Fc-optimized anti-HER2 monoclonal antibody, in combination with pembrolizumab, an anti-PD-1 antibody. The trial seeks to characterize the safety, tolerability, maximum tolerated dose, and preliminary anti-tumor activity of this combination. Enrolled patients had relapsed or refractory advanced HER2+ gastric or GEJ cancer with disease progression after or resistance to treatment with trastuzumab plus chemotherapy. Study patients were enrolled irrespective of PD-L1 expression status. Phase 1b dose escalation segment of the study tested dose levels of 10 and 15 mg/kg margetuximab in combination with a flat dose of 200 mg pembrolizumab every three weeks. After completion of dose escalation, Phase 2 dose expansion cohorts were enrolled, including a 30 patient cohort in North America and a 30 patient cohort in Asia. Sixty dose expansion patients received margetuximab at 15 mg/kg and 200 mg of pembrolizumab every three weeks. Acceptable tolerability was observed in the safety population of 67 patients. Grade 3 or higher treatment-related adverse events (TRAE) occurred in 11.9% of patients. The most common TRAE of any grade was fatigue (14.9%). As of the December 4, 2017 data cut-off date, responses were evaluable from 51 patients, including 25 with gastric and 26 with GEJ cancer. The Overall Response Rate (ORR) was higher in patients with gastric vs. GEJ cancer (32% vs. 4%). ORR across all patients in the study was 18% (six confirmed and three unconfirmed patients). Similarly, Disease Control Rate (including partial responses and stable disease) was higher in patients with gastric vs. GEJ cancer (72% vs. 38%). Median progression-free survival was also higher in patients with gastric vs. GEJ cancer (5.5 vs. 1.4 months).