The purpose of this study was to determine the safety and maximum tolerated dose (MTD) of BZL101 (FDA IND# 59,521), an orally delivered aqueous extract from the herb Scutellaria barbata, in women with metastatic breast cancer (MBC). The trial was an open-label, phase 1B, multicenter, dose escalation study. Eligible patients had histologically confirmed breast cancer and measurable stage IV disease. The standard phase 1 "3 + 3" study design was used to determine the MTD. Primary endpoints were toxicity and MTD of BZL101. Secondary outcomes included efficacy based on RECIST criteria. A total of 27 women with a median of 2 prior chemotherapy treatments for metastatic disease were treated in four different dose cohorts. Grade 3 and 4 adverse events (AEs) were uncommon. Dose-limiting toxicities included the following: grade 4 AST elevation, grade 3 diarrhea, grade 3 fatigue, and grade 3 rib pain. Fourteen patients were evaluable according to Response Evaluation Criteria in Solid Tumors. Investigator assessment classified three patients with stable disease for >120 days (21%). One patient was on BZL101 for 449 days and remains stable for 700 + days. Independent radiology review identified three patients with objective tumor regression (>0% and <30%). The MTD was not reached, thus per protocol, the MTD was defined as the maximum administered dose of BZL101 40 g/day. In conclusion, oral administration of BZL101 was safe, well tolerated, and showed promising clinical evidence of anticancer activity in this heavily pretreated population of women with MBC.