Purpose: Azidothymidine (AZT, zidovudine) has been shown to reverse cisplatin resistance in cell culture. This phase I study was performed to determine the maximally tolerated dose (MTD) and dose-limiting toxicities of AZT when administered by continuous intravenous infusion in combination with cisplatin (CDDP), and to evaluate the pharmacokinetics of AZT in this setting. Patients and methods: Entered in the study were 61 patients with advanced, histologically confirmed malignancies which were unresponsive to or for which no "standard" chemotherapeutic regimen existed. AZT was administered as a 72-h infusion on days 1-3 and 14-16 of a 28-day cycle at dose levels from 400 through 14,364 mg/m2 per day. CDDP at dose levels of 30, 45, or 60 mg/m2 was administered at hour 36 of each AZT infusion. The plasma pharmacokinetics of AZT were determined in patients treated at representative dose levels. Results: Of the 61 patients who completed 125 courses of therapy, 21 had stable disease for a median of four cycles (range two to eight), 33 progressed on therapy, and 7 were not assessable for response. The major observed toxicity was myelosuppression. The MTD of AZT was 8135 mg/m2 per day when administered on this schedule. Escalation of CDDP did not result in additive toxicity. The mean steady-state level of AZT at the MTD was 44 μM (range 35-51 μM). Conclusions: Steady-state concentrations of AZT increased with dose. The plasma levels achieved at the MTD exceeded those required for drug resistance reversal in vitro. The administration of CDDP had no effect on AZT steady-state levels. The dose-limiting toxicity of this drug combination is myelosuppression. AZT may be useful in further studies utilizing combination therapy to achieve increased chemotherapy effectiveness.
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