This study was designed to determine the safety and optimal dosing of TAS-102, a novel oral combination of alphaalphaalpha-trifluorothymidine (FTD) and an inhibitor of thymidine phoshorylase, in patients with solid tumors. Patients who met the eligibility criteria were treated with one of two different TAS-102 regimens: once per day on either days 1-5 and 8-12 every 4 weeks (schedule A) or days 1-5 every 3 weeks (schedule B). The primary objectives were the determination of the maximum tolerated dose, dose-limiting toxicities (DLTs), and recommended phase II dose. Pharmacokinetic analysis was conducted during courses 1 and 2. Sixty-three patients received a total of 172 courses of therapy with the median number of courses delivered on both schedules being 2. DLTs were observed in three patients on schedule A, 70 mg/m(2)/day (1) and 110 mg/m(2)/day (2); and in five patients on schedule B, 120 mg/m(2)/day (1), 170 mg/m(2)/day (2), 180 mg/m(2)/day (2). Granulocytopenia was the DLT in seven of the eight cases. The most frequent toxicities were nausea, fatigue, granulocytopenia, anemia, diarrhea, and abdominal pain. Twelve patients, 6 on schedule A and 6 on schedule B, were treated at the recommended phase II dose, with good tolerance. No objective responses were seen in this heavily pretreated, 5-FU-refractory population. The pharmacokinetic parameters of FTD are a T (max) of 0.53 to 3.15 h, t (1/2) of 1.46 to 4.20 h, volume of distribution of 0.0526 to 0.483 l/kg, and clearance of 0.0194 to 0.197 1/h/kg. The recommended phase II doses for TAS-102 are 100 mg/m(2)/day on schedule A and 160 mg/m(2)/day on schedule B. Future development of TAS-102 should focus upon multiple daily dosing schedules.