This is a Phase 1, multiple dose, ascending dose escalation study; to define a MTD/RD and regimen consisting of a first "priming" dose and escalated subsequent doses of XmAb18087; to describe safety and tolerability; to assess PK and immunogenicity; and to preliminarily assess anti-tumor activity of XmAb18087 in subjects with advanced NET or GIST.

The study will enroll dosing cohorts to establish a MTD/RD and regimen in subjects with advanced NET or GIST, then enroll additional subjects into separate NET and GIST expansion cohorts to collect additional data on safety and potential efficacy of XmAb18087.

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