N-Debenzoyl-N-tert-butoxycarbonyl-10-deacytyl taxol (Taxotere, RP 56976) is a semisynthetic analogue of taxol, prepared from a noncytotoxic precursor extracted from the needles of the European yew tree (Taxus baccata L.). It has a broad spectrum of antitumor activity against a variety of transplantable tumors in mice. In vitro cytotoxicity assays suggest that it is 2-5-fold more potent than taxol. In this phase I study Taxotere was administered by 24 h i.v. infusion at 3-week intervals. Thirty patients with solid tumors refractory to conventional therapy were treated; 70 courses of Taxotere were administered at doses ranging from 10 to 90 mg/m2. Grade 4 neutropenia and grade 3 mucositis were dose limiting but reversible at 90 mg/m2. The pattern and grade of toxicity at this dose were similar in 3 heavily pretreated patients compared with 7 patients who had received a maximum of one previous chemotherapy regimen. Alopecia occurred at 55 mg/m2 and above. Other mild toxicities included phlebitis, diarrhea, emesis, and sensory peripheral neuropathy, but these were neither dose-limiting nor clearly dose-related. One patient treated at 70 mg/m2 had an anaphylactoid reaction following the second dose of Taxotere. No cardiovascular toxicity was observed. No partial or complete responses were documented. Plasma concentrations of Taxotere were determined by high-performance liquid chromatography, and end-of-fusion levels at the maximum tolerated dose exceeded drug concentrations which are cytotoxic in vitro. The maximum tolerated dose for Taxotere administered as a 24-h infusion is 90 mg/m2.