Phase I clinical and pharmacokinetic study of S9788, a new multidrug-resistance reversal agent given alone and in combination with doxorubicin to patients with advanced solid tumors

Purpose: The objectives of this phase I study were to evaluate the toxic effects and the maximum tolerated dose (MTD) of S9788, a new modifier of multidrug resistance (MDR), when given alone and in combination with doxorubicin to patients with advanced solid tumors; to achieve a potentially active plasma concentration of S9788; and to study the pharmacokinetics of both drugs. Methods: A total of 26 patients (median age 58 years) entered the study. S9788 was given alone as a 30-min infusion at day 1 and in combination with a 50-mg/m² bolus of doxorubicin at days 8 and 29. Dose levels of S9788 were escalated from 8 to 96 mg/m² according to the modified Fibonacci scheme. Plasma samples were taken predose as well as during and up to 48 h after the beginning of infusion for S9788 and doxorubicin quantitation. Fractionated urine samples were also collected for up to 24 h for S9788 determination. Results: The dose-limiting side effects of S9788 consisted of bradycardia, sometimes associated with faintness or dizziness. The MTD of S9788 was 96 mg/m². No enhancement of doxorubicin toxicity was observed. One partial response (duration 140 days) was observed at 96 mg/m² in a patient with multiple lung metastases from a refractory urothelial carcinoma. Pharmacokinetic studies were performed in 24 patients. Since the mean apparent elimination half-life of S9788 was 46 ± 23 h and the last plasma sampling time was 48 h, only model-independent parameters were considered. Plasma levels of S9788 were below the limit of quantitation (4 × 10^–3μM ) before each drug administration. S9788 plasma levels of up to 3.7 μM could be obtained with this administration schedule. The urinary elimination of the unchanged drug was negligible, whatever the collection period. In spite of the large inter- and intraindividual variability, plasma pharmacokinetics of S9788 given as a 30-min i.v. infusion were linear up to 96 mg/m² and were not modified by doxorubicin administration. Doxorubicin pharmacokinetic parameters did not seem to be influenced by S9788 coadministration. Conclusion: The dose-limiting toxicity of S9788 consisted of bradycardia or clinical symptoms suggesting a vasovagal impact such as faintness or dizziness. The MTD of S9788 was 96 mg/m². The pharmacokinetic parameters of doxorubicin in this study were close to those usually described and were not influenced by escalation of the S9788 dose. No pharmacokinetic interaction was observed between S9788 and doxorubicin. The clinical tolerability of the combined treatment is in good agreement with the pharmacokinetic findings, since no enhancement of doxorubicin toxicity was observed. Received: 13 July 1996 / Accepted: 4 July 1997