This is an open-label Phase 1b dose-escalation study to assess the safety, tolerability, and PK of vantictumab when combined with paclitaxel.

Once the maximum tolerated dose (MTD) or maximum administered dose (MAD) has been determined, up to 10 patients may be enrolled in the cohort-expansion phase to better characterize the safety, tolerability and PK of vantictumab combined with paclitaxel. Up to approximately 34 patients may be enrolled into the study.

Inclusion Criteria:

• Signed Informed Consent Form

• Age ≥18 years

• Histologically documented adenocarcinoma of the breast with locally recurrent or metastatic disease

  o Patients with breast cancer overexpressing HER2 are not eligible.

• Availability of tumor tissue, either archival FFPE or obtained at study entry through fresh biopsy

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

• All acute treatment-related toxicity from prior therapy must have resolved to Grade ≤ 1 prior to study entry

• Adequate hematologic and end-organ function

• Evaluable or measurable disease per RECIST v1.1

• For women of childbearing potential, agreement to use two effective forms of contraception

Exclusion Criteria:

• Known significant dose delays during prior treatment with a taxane due to drug-related toxicities
• Prior treatment with more than two regimens of systemic cytotoxic chemotherapy for locally recurrent or metastatic disease
• Treatment with any anti-cancer therapy within 3 weeks prior to initiation of study treatment