Sequence-dependent alteration of doxorubicin pharmacokinetics by paclitaxel in a phase I study of paclitaxel and doxorubicin in patients with metastatic breast cancer.

Purpose: To determine whether a schedule-dependent interaction occurs when paclitaxel and doxorubicin are administered sequentially.
Patients and Methods: Ten patients with metastatic breast cancer received paclitaxel 125 mg/m2 over 24 hours either immediately before or after doxorubicin 48 mg/m2 over 48 hours as the initial chemotherapy treatment. Two such courses were given, and the sequence of administration was reversed after course 1. In cohort 1, paclitaxel preceded doxorubicin for course 1. In cohort 2, doxorubicin preceded paclitaxel for course 1. Doxorubicin levels were measured serially during the infusion and for 24 hours following it. Patients were assessed clinically for the occurrence of stomatitis and infection and granulocyte counts were measured twice weekly.
Results: Eight patients had complete pharmacokinetic sampling for both courses. The mean end-of-infusion plasma doxorubicin concentrations (Cmax) were 70% higher in the paclitaxel-doxorubicin sequence compared with the reverse sequence (45 +/- 8 ng/mL v 26 +/- 5 ng/ mL). The mean doxorubicin clearance was 32% lower in the paclitaxel-doxorubicin sequence (34.3 +/- 10.3 L/h v 51.6 +/- 16.1 L/h, P < .01). Clinically, hematologic and mucosal toxic effects were worse in the paclitaxel-doxorubicin sequence. The median absolute granulocyte count was 0.2/microL in the paclitaxel-doxorubicin sequence and 1.3/microL in the doxorubicin-paclitaxel sequence. Seven of 10 patients who received the paclitaxel-doxorubicin sequence had grade 2 (n = 4) or 3 (n = 3) stomatitis, while only one of 10 patients who received the doxorubicin-paclitaxel sequence had grade 2 stomatitis and none had grade 3.
Conclusion: When paclitaxel by 24-hour infusion precedes doxorubicin by 48-hour infusion, doxorubicin clearance is reduced by nearly one third, which results in grade 2 and 3 stomatitis. To prevent this effect when paclitaxel (by 24-hour infusion) and doxorubicin are administered sequentially, doxorubicin should be given first. The mechanisms for this effect are under investigation.