Purpose: To determine the bioavailability (F) and the pharmacokinetic profile of both etoposide and its prodrug, etoposide phosphate, after oral and intravenous administration of etoposide phosphate, and to determine the maximum-tolerable dose (MTD) of oral etoposide phosphate administered daily for 5 consecutive days every 3 weeks. In addition, we sought to develop and validate two limited-sampling models (LSMs) to predict the etoposide area under the curve (AUC) 24 hours after administration of oral and intravenous etoposide phosphate.
Patients and methods: In the F part of the study, patients were assessed for pharmacokinetic studies after one oral and one intravenous administration of the same dose of etoposide phosphate. Etoposide phosphate and etoposide plasma concentrations were assayed by high-performance liquid chromatography (HPLC). To develop LSMs after oral and intravenous administration, patients were randomized between the training and validation data sets. In the phase I part of the study, which followed the F part, the dose of etoposide phosphate was escalated from 50 mg/m2/d for etoposide equivalents for 5 days to 220 mg/m2/d for 5 days.
Results: Forty adult patients with solid tumors or lymphoma entered the study and 35 were assessable for toxicity. The MTDs were defined as 175 mg/m2 and 220 mg/m2 in previously treated and untreated patients, respectively. Neutropenia was dose-limiting, with high interpatient variability. Within 15 minutes after intravenous administration, etoposide phosphate was no longer detectable in plasma, and it was never detectable after oral administration. Plasma concentrations and pharmacokinetic parameters of etoposide following etoposide phosphate were comparable to those reported for etoposide. The relative F (mean +/- SD) of etoposide after oral etoposide phosphate was 76 +/- 27%, with a range of 37% to 144%.
Conclusion: The clinical and pharmacokinetic results of this study confirm the prodrug hypothesis of etoposide phosphate. Although firm conclusions cannot be drawn, the F of oral etoposide phosphate seems to be comparable to or only slightly better than that of oral etoposide.