PURPOSE: To define the maximum tolerated dose (MTD), recommended phase II dose (RPTD), and assess safety and tolerability for the combination of pazopanib plus TH-302, an investigational hypoxia-activated prodrug (HAP), in adult patients with advanced solid tumors. METHODS: This was an open-label, non-randomized, single-center, phase I trial consisting 2 stages. Stage 1 was a standard "3 + 3" dose escalation design to determine safety and the RPTD for TH-302 plus pazopanib combination. Stage 2 was an expanded cohort to better describe the tolerability and toxicity profile at the MTD. Pazopanib was orally dosed at 800 mg daily on days 1-28 for all cohorts. TH-302 was administered intravenously on days 1, 8 and 15 of a 28-day cycle at doses of 340 mg/m2 (cohort 1) or 480 mg/m2 (cohort 2). Dose limiting toxicity (DLT) was assessed in the first 28-day cycle. Efficacy was assessed every 2 cycles. RESULTS: Thirty patients were enrolled between December 2011 and September 2013. In the dose escalation stage, 7 patients were enrolled in the 340 mg/m2 TH-302 cohort and 6 patients in the 480 mg/m2 TH-302 cohort. Ten patients were evaluable for DLT. DLTs included grade 2 intolerable esophagitis (n = 1) in the 340 mg/m2 TH-302 cohort, and grade 3 vaginal inflammation (n = 1) and grade 3 neutropenia with grade 3 thrombocytopenia (n = 1, same patient) in the 480 mg/m2 TH-302 cohort. The 340 mg/m2 TH-302 cohort was determined to be MTD and RPTD. The most common treatment-related adverse events were hematologic (anemia, neutropenia, and thrombocytopenia), nausea/vomiting, palmar-plantar erythrodysesthesia syndrome, constipation, fatigue, mucositis, anorexia, pain, and hypertension. Partial response (PR) was observed in 10% (n = 3) of patients, stable disease (SD) in 57% (n = 17), and progressive disease (PD) in 23% (n = 7). Due to toxicity, 3 patients were discontinued from study drug prior to first radiographic assessment but were included in these calculations. Disease control ≥6 months was observed in 37% of patients (n = 11). CONCLUSIONS: The RPTD for this novel combination is pazopanib 800 mg daily on days 1-28 plus TH-302 340 mg/m2 on days 1, 8 and 15 of each 28-day cycle. Preliminary activity was seen in treatment-refractory cancers and supports potential value of co-targeting tumor angiogenesis and tumor hypoxia.