Impact of STI571 (Imatinib) on the Pharmacokinetics and In Vivo Efficacy of Etoposide and/or Ifosfamide.

Abstract

Using a human small cell lung cancer (SCLC) xenografted in nude mice, we previously reported an increase of tumor growth inhibition following chemotherapy in combination with STI571 (imatinib)(Int J Cancer. 2005; 113:849–856). This led us to study the in vivo impact of STI571 on the pharmacokinetics of chemotherapy. Plasma, urine, and fecal concentrations of etoposide were determined by a validated high performance liquid chromatography method, and plasma concentrations of ifosfamide were determined by a validated GAS chromatography assay with nitrogen-phosphorus detection. C-kit and MDR1/ABCB1 transcripts were quantified using real-time quantitative reverse transcription-PCR assays. We first observed an increase of the Cmax of both etoposide and ifosfamide on days 1 and 3 when mice were treated by chemotherapy combined with STI571 compared to chemotherapy alone. We then observed a high significant increase of the AUC 0–3h (Area Under the concentration-time Curve) of etoposide when mice were treated with etoposide + STI571, due to a decrease of its fecal excretion, without impact of concomitant administration of the CYT3A4 inhibitor fluconazole. As previously shown, this effect was observed despite the absence of c-kit receptor mRNA expression. Finally, we showed that STI571 enhanced the etoposide-induced tumor growth inhibition of a human xenografted lymphoma, but not that of an in vivo human SCLC tumor, and that this antitumor effect was correlated with the mRNA level of the MDR1/ABCB1 gene. This last observation suggests that imatinib-induced pharmacokinetic changes could be due to inhibition mediated via the MDR modulator of the P-glycoprotein transporter resulting in decreased elimination of etoposide. In conclusion, STI571 can reverse MDR1 drug resistance and potentiate the effects of etoposide in MDR1-expressing cancer cells such as lymphoma cell lines. These results show that further prospective phase I and II clinical trials with combinations of etoposide-based chemotherapy and imatinib are therefore warranted in some cancer patients, such as malignant lymphoma patients, with careful toxicologic monitoring.