Influence of dosing schedule on toxicity and antitumor effects of a combination of adriamycin and docetaxel in mice.

PURPOSE Although the combination of Adriamycin (ADR) and docetaxel (DOC) showed a better cure rate against metastatic breast cancer in a clinical study, severe myelosuppression and cardiotoxicity were dose-limiting factors. The purpose of this study was to establish the most suitable dosing schedule to relieve severe adverse effects and improve the antitumor effects. EXPERIMENTAL DESIGN Both ADR and DOC were administered simultaneously in the simultaneous-dosing group (ADR/DOC), whereas in the intermittent-dosing groups (ADR-DOC and DOC-ADR), the second drug was administered 12 h after the first drug. Leukocyte counts and survival were measured to estimate adverse effects. After administration, ADR and DOC concentrations in blood, myelocyte cells, and heart were determined. To clarify the antitumor effect, tumor growth was measured in Ehrlich-cell-bearing mice after the initiation of drug injections. RESULTS The simultaneous-dosing group showed severe leukopenia compared with the saline-treated group. However, the toxicity was reduced in the intermittent-dosing groups. The DOC-ADR group showed the best survival rate in the dosing groups. In the pharmacokinetic study, ADR and DOC concentrations in plasma, myelocyte cells, and the heart were markedly higher in the simultaneous-dosing group than the intermittent-dosing groups. These results indicate that pharmacokinetic interactions may contribute to the change in leukopenia induced by concurrent administration of ADR and DOC. The antitumor effect in the DOC-ADR group was the highest in the dosing groups. CONCLUSIONS In the present study, the findings suggest that ADR administered 12 h after DOC injection (DOC-ADR group) not only inhibits tumor growth more strongly but also significantly reduces leukopenia compared with results for the simultaneous-dosing (ADR/DOC) group and significantly reduced the number of toxic deaths compared with the other groups.