This is a Phase 1 clinical study to investigate the safety, pharmacokinetics, and pharmacodynamics of U3-1287 (AMG 888), a fully human monoclonal antibody targeting the HER3 receptor, in patients with advanced solid tumors. Eligible patients will have disease that is refractory or resistant to standard treatments or for which no standard therapy exists. The study will be conducted in two parts; a dose escalation (Part 1) and a dose expansion (Part 2). The hypothesis for this study is that U3-1287 (AMG888) will be safe and well tolerated in patients with advanced solid tumors and will show initial evidence of anti-tumor activity.

Key Inclusion Criteria:

• Pathologically or cytologically documented advanced solid tumor that is refractory to standard treatment, for which no standard therapy is available, or the patient refuses standard therapy.
• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2.
• Men or women at least 18 years of age
• Adequate hematologic, cardiac, renal, hepatic, metabolic, and coagulation function

Key Exclusion Criteria:

• Presence of untreated or symptomatic brain metastasis.
• Presence of ascites or pleural effusion requiring chronic medical intervention.
• Uncontrolled hypertension
• Concurrent or previous (within 1 week of study day 1) anticoagulation therapy, except low-dose warfarin (≤ 2 mg/day) or low dose,low molecular weight heparin for prophylaxis against central venous catheter thrombosis.
• Recent major surgical procedure or not yet recovered from major surgery
• Recent participation in clinical drug trials.
• Participation in other investigational procedures.
• Unresolved toxicities from prior anti-cancer therapy
• Patient who is pregnant (e.g. positive human choriogonadotropin [HCG] test) or nursing.