A phase 1b/2, open label, dose-escalation study of margetuximab (M) in combination with pembrolizumab (P) in patients with relapsed/refractory advanced HER2+ gastroesophageal (GEJ) junction or gastric (G) cancer

TPS219 Background: Prognosis for advanced HER2+ GEJ and G cancers remains poor, with median survival just beyond one year. Trastuzumab (T) in combination with chemotherapy is the initial treatment of choice, but therapeutic options targeting HER2 beyond T are poorly defined. M is an Fc-enhanced monoclonal antibody (Mab) to HER2 that recognizes with similar affinity the same epitope as T and whose Fc domain, compared to T, binds with increased affinity to the activating CD16A Fc-receptor (FcR) and decreased affinity to the inhibitory CD32B FcR. Preliminary data shows that M monotherapy has clinical activity against HER2+ tumors in GEJ and G cancer patients previously treated with T or other anti-HER2 agents. P is a Mab that blocks the interaction of the immune checkpoint molecule, PD-1, with its ligands, facilitating tumor cell elimination by releasing tumor-associated T cells from exhaustion. Monotherapy P has demonstrated remarkable and durable clinical activity in a Phase I study. Safety profiles of M and P are acceptable and non-overlapping. Methods: This study advances a chemotherapy free combination of M + P treatment for advanced HER2+ GEJ and G cancer patients. Enrolled patients will have relapsed/refractory HER2+ GEJ or G adenocarcinoma with measurable disease that has progressed on T plus first line chemotherapy. HER2+ (IHC 3+ or ISH+) will be confirmed by central review. Two dose levels of M (10mg/kg and 15mg/kg) and a fixed dose of P (200mg) will be evaluated for safety and tolerability. Patients will receive combination treatment once every 21 days for up to 24 months, until confirmed disease progression or intolerable toxicity. Dose expansion will enroll up to 60 patients, with 20 undergoing pre- and on-treatment biopsy. Response will be assessed every 6 weeks for the first 6 months and every 12 weeks thereafter per RECIST v1.1 and immune RECIST to account for response patterns observed with immunotherapies. Primary endpoint is ORR and duration of response, and secondary endpoints include PFS and OS. The study was initiated on January 2016 and is ongoing in North America and Asia. Clinical trial information: NCT02689284.