A Study of XmAb®18087 in Subjects With NET and GIST

Inclusion Criteria: * Histologically or cytologically confirmed well differentiated low or intermediate grade (World Health Organization \[WHO\] Grade 1 or 2) NET of pancreatic, gastrointestinal, lung, or undetermined origin that is locally advanced or metastatic and has progressed within the past 12 months * Histologically confirmed GIST that is locally advanced or metastatic * NET and GIST tumors must be unresectable * NET subjects must have progressed on or been ineligible for treatment with somatostatin analogues (SSA) and at least one other FDA-approved targeted therapy (everolimus or sunitinib). * GIST subjects must have previously received all FDA-approved therapies (imatinib mesylate, sunitinib malate, and regorafenib) for which they are eligible * Must have disease measurable by RECIST 1.1 criteria using either computed tomography (CT) or magnetic resonance imaging (MRI) scan * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 Exclusion Criteria: * Diagnosis of high-grade (WHO Grade 3) or poorly differentiated NET; high-grade neuroendocrine carcinoma; large cell neuroendocrine carcinoma, small cell carcinoma, or mixed small and large cell carcinoma. * Subjects currently receiving anti-cancer therapies (other than SSAs, which may continue). * Subjects who have received anti-cancer therapies within 2 weeks of the start of study drug (including chemotherapy, radiation therapy, immunotherapy, etc.). * Must not be experiencing a Grade 3 or 4 toxicity from previous anti-cancer treatment * Must not be receiving other anti-cancer therapies (except somatostatin analogues, which may be allowed) * Must not have poorly controlled diabetes mellitus, known central nervous system involvement by malignant disease or insufficient bone marrow, renal, or hepatic function