PM00104 (Zalypsis®) is a new synthetic alkaloid with potent cytotoxic activity against tumor cell lines. This phase I clinical trial determined the maximal tolerated dose (MTD) and recommended dose (RD) for phase II trials of PM00104 administered as a 1-hour intravenous (i.v.) infusion weekly for three consecutive weeks resting every fourth week (d1,8,15 q4wk). Forty-nine patients with advanced solid malignancies received PM00104 following a toxicity-guided, accelerated, dose-escalation design. Doses evaluated ranged from 0.07 to 3.0 mg/m(2). Dose-limiting toxicities (DLTs) appeared at the highest doses tested and comprised grade 3 diarrhea and grade 4 lipase increase at 2.0 mg/m(2); grade 1 thrombocytopenia and grade 2 neutropenia with two infusion omissions, grade 3 fatigue and grade 4 febrile neutropenia at 2.5 mg/m(2); and grade 3/4 fatigue, grade 4 neutropenia lasting >5 days and grade 4 thrombocytopenia at 3.0 mg/m(2). RD was established at 2.0 mg/m(2). PM00104-related adverse events at the RD were mostly grade 1/2, with fatigue, nausea and vomiting as the most common. Transient and manageable myelosuppression and transaminase increases were also reported. Main pharmacokinetic parameters increased linearly with dose. Disease stabilization lasting ≥ 3 months was found in 4 patients with cervical carcinoma, colorectal adenocarcinoma, lachrymal adenoid carcinoma, and bladder carcinoma (n=1 each). In conclusion, PM00104 2.0 mg/m(2) 1-hour, d1,8,15 q4wk showed a positive risk-benefit ratio, which has supported its further evaluation in three ongoing phase II clinical trials.