A Phase 1/2 Study of MEDI4276 in Adults Subjects With Select HER2-expressing Advanced Solid Tumors.

Inclusion Criteria: 1. Age ≥ 18 years. 2. Histologically or cytologically documented unresectable, locally advanced or metastatic breast cancer or gastric cancer refractory to standard therapy. 1. For subjects with breast cancer: * Prior treatment with trastuzumab, pertuzumab, and T-DM1, either alone or in combination, is required. * Subjects with a primary tumor that is hormone (estrogen, progesterone, or both) receptor-positive or receptor-negative are eligible. * Prior hormone therapy is allowed, but last dose must be at least 14 days prior to first dose of MEDI4276. 2. For subjects with gastric cancer: * Prior treatment with a trastuzumab containing chemotherapy regimen is required. 3. HER2 Positive disease documented as FISH-positive and/or 3+ by IHC on previously collected tumor tissue. 4. At least one lesion measurable by RECIST Version 1.1. Exclusion Criteria: 1. Receipt of any conventional or investigational anticancer treatment within 28 days prior to the first dose of MEDI4276. 2. History of exposure to the following cumulative doses of anthracyclines: 1. Doxorubicin or liposomal doxorubicin \>350 mg/m². 2. Epirubicin \>530 mg/m². 3. Mitoxantrone \>90 mg/m² and idarubicin \> 70 mg/m². 4. If another anthracycline or more than 1 anthracycline has been used, then the cumulative dose must not exceed the equivalent of 350 mg/m² of doxorubicin. 3. Known brain metastases that are untreated, symptomatic, or require therapy to control symptoms; or any radiation, surgery or other therapy to control symptoms from brain metastases within 2 months prior to first dose of MEDI4276.