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Phase I study on a new vinca alkaloid derivative, KW-2307(vinorelbine), was conducted by multiple institutions in 40 patients with a variety of malignant tumors. KW-2307 was given intravenously by single administration or by weekly repeated for 4 weeks (hereinafter as the repeated administration). The initial dose, 10 mg/m2(ln), was escalated to 35 mg/m2(3.5n) in single administration and to 30 mg/m2(3n) in the repeated administration. A total of 35 cases were eligible for evaluation, consisting of 18 cases in single administration group and 17 in the repeated group. Both of single and repeated administration caused leukopenia dose-dependently, and the dose limiting toxicity (DLT) was judged as leukopenia. The maximum tolerated dose (MTD) was 30 mg/m2 in single administration and 25 mg/m2 in the repeated administration provided that administration is made weekly for 4 consecutive weeks. As subjective and objective adverse effects, general fatigue, anorexia, constipation, phlebitis etc. were observed, but all of these symptoms were relatively mild and recoverable. Peripheral neuropathy were found in two cases and judged as Grade 1. In this phase I study, a tendency of the decrease in tumor size was seen in 3 cases; 2 of non-small cell lung cancer and 1 of breast cancer. The pharmacokinetics of KW-2307 in blood after single administration showed a triphasic disappearance pattern with half-life of about 20 to 60 hrs. The data of pharmacokinetics after repeated administration revealed no accumulation of this agent. The recommended dose for early phase II study was supposed to be 20 to 25 mg/m2/week.