Sequence-dependent hematological toxicity associated with the 3-hour paclitaxel/cyclophosphamide doublet

Paclitaxel is active in metastatic breast cancer. Combination studies have demonstrated complex interactions between paclitaxel and other cytotoxic agents, including sequence-dependent cytotoxic, toxicological, and pharmacological effects. The principal objectives of this study were to determine the maximum tolerated doses of paclitaxel (3-h infusion) and cyclophosphamide (1-h infusion) administered every 3 weeks with granulocyte colony-stimulating factor (Filgrastim) and to determine if the sequence-dependent toxicological effects that have previously been observed with this combination when paclitaxel was administered over 24 h were evident when paclitaxel was administered over 3 h. Fifteen women with metastatic breast cancer were treated. Starting doses were 200 mg/m2 paclitaxel and 1600 mg/m2 cyclophosphamide, with granulocyte colony-stimulating factor (5 micrograms/kg/day) given s.c. beginning 24 h after chemotherapy. Doses of both drugs were escalated in cohorts of at least four patients. The sequence of drug administration was alternated with each consecutive patient and with each subsequent course of therapy in each individual patient, enabling the evaluation of sequence-dependent toxicological and pharmacological effects. Severe myelosuppression was the principal dose-limiting toxicity for this regimen, precluding dose escalation above 200 mg/m2 paclitaxel and 1600 mg/m2 cyclophosphamide, the maximum tolerated dose for this combination on this schedule. As has been previously demonstrated with this combination, when paclitaxel is administered over 24 h, the hematopoietic toxicity was sequence dependent. Paired analysis of toxicity data using each patient as her own control indicated more severe hematological toxicity in courses in which paclitaxel was administered first. There was no evidence of sequence-dependent effects on the pharmacokinetics of these drugs that might account for this phenomenon. The impact of drug sequencing on toxicity should be considered in the further development of combination therapy containing alkylating agents and paclitaxel, when the latter is administered over 3 h.