[In vivo effect of etoposide on the pharmacokinetics of methotrexate]

High dose (5 g/m2/24 h) methotrexate therapy was combined two times with etoposide (100 mg/m2/1h) infusions as a part of the Medulloblastoma protocol developed in our Department Vepesid therapy was administered in two different schedules. The first group of the patients have received etoposide immediately before and at the end (24th h) of methotrexate treatment. The second group was treated with etoposide at 24 h and at 48 hour after starting methotrexate infusion. In this latter group treatment related grade 3-4 toxicity developed more frequently than in the first group (58.6% vs 33.3%). The authors observed that after the second dose of etoposide given at 48 h (second group) both total and unbound serum methotrexate levels (determined by high performance liquid chromatography) were elevated by 53.14-109.19%, and 25.86-64.95%, respectively by the third hour after completion of Vepesid infusion. This effect was detectable for 6 hours. All the liver and kidney functions of the patients were in the normal range. These results suggest the possibility of partial recirculation of extra/intracellular methotrexate into the blood after etoposide administration. Based on these results the therapeutic protocol has been modified and Vepesid is given prior to and at the end (24 h) of high dose methotrexate treatment. Under these conditions only a slight decrease of methotrexate elimination has been detected between the 25-28th h. These results emphasize the role of possible schedule dependent interactions of cytostatic drugs.