© 2015 AACR. Purpose: This phase I study examined the toxicity and tolerability of pegylated arginine deiminase (ADI-PEG 20) in combination with docetaxel in patients with advanced solid malignancies. Experimental Design: Eligible patients had histologically proven advanced solid malignancies, with any number of prior therapies, Zubrod performance status 0-2, and adequate organ function. Patients received ADI-PEG 20 weekly intramuscular injection ranging from 4.5 to 36 mg/m2 and up to 10 doses of docetaxel (75 mg/m2) every 3 weeks. Primary endpoints were safety, toxicity, and a recommended phase II dose. Circulating arginine levels were measured before each cycle. Tumor response was measured as a secondary endpoint every 6 weeks on study. Results: Eighteen patients received a total of 116 cycles of therapy through four dose levels of ADI-PEG 20. A single doselimiting toxicity (grade 3 urticarial rash) was observed at the 1st dose level, with no additional dose-limiting toxicities observed. Hematologic toxicities were common with 14 patients experiencing at least one grade 3 to 4 leukopenia. Fatigue was the most prevalent toxicity reported by 16 patients. Arginine was variably suppressed with 10 patients achieving at least a 50% reduction in baseline values. In 14 patients with evaluable disease, four partial responses (including 2 patients with PSA response) were documented, and 7 patients had stable disease. Conclusions: ADI-PEG 20 demonstrated reasonable toxicity in combination with docetaxel. Promising clinical activity was noted, and expansion cohorts are now accruing for both castrate-resistant prostate cancer and non-small cell lung cancer at a recommended phase II dose of 36 mg/m2.