Background FK866 is a potent inhibitor or NAD synthesis. This first-in-human study was performed to determine the maximum-tolerated dose, toxicity profile, and pharmacokinetics on a 96-h continuous infusion schedule. Materials and methods Twenty four patients with advanced solid tumor malignancies refractory to standard therapies were treated with escalating doses of FK866 as a continuous, 96-h infusion given every 28 days. Serial plasma samples were collected to characterize the pharmacokinetics of FK866. Further blood samples were collected for the measurement of plasma VEGF levels. Results There were 12 women and 12 men with a median age of 61 (range 34-78) and a median KPS of 80%, received a 4-day of infusion of FK866 at dose levels of 0.018 mg/m²/h (n = 3), 0.036 mg/m²/h (n = 3), 0.072 mg/m²/h (n = 3), 0.108 mg/m²/h (n = 4), 0.126 mg/m²/h (n = 6), and 0.144 mg/m²/h (n = 5). Thrombocytopenia was the dose limiting toxicity, observed in two patients at the highest dose level and one patient at the recommended phase II dose of 0.126 mg/m2/h No other hematologic toxicities were noted other than mild lymphopenia and anemia. There was mild fatigue and grade 3 nausea; the latter was controlled with antiemetics and was not a DLT. C_ss (the mean of the 72 and 96 h plasma concentrations) increased in relation to the dose escalation. The study drug did not significantly affect plasma concentrations of VEGF. There were no objective responses, although four patients had stable disease (on treatment for 3 months or greater). Conclusions The recommended phase II dose is 0.126 mg/m²/h given as a continuous 96-h infusion every 28 days. The dose limiting toxicity of FK866 is thrombocytopenia. Pharmacokinetic data suggest an increase in the plasma C_ss in relation to the escalation of FK866.