Effect of etoposide on the pharmacokinetics of methotrexate in vivo

The effect of etoposide on the pharmakokinetics of methotrexate (MTX) was examined in vivo. High-dose (5g/m2/24 h) MTX therapy was combined with two etoposide (100mg/m2/ 1 h) infusions as a part of the medulloblastoma protocol developed in our department. Vepesid therapy was administered in two different schedules. The first group of patients received etoposide immediately before and at the end (24 h) of MTX treatment. The second group was treated with etoposide at 24 and at 48 h after starting MTX infusion. In this latter group both treatment-related grade III and grade IV toxicity developed more frequently than in the first group (58.6 versus 29.2%, for grade 3 toxicity p=0.019, for grade 4 toxic signs p=0.040, respectively). We observed that after the second dose of etoposide given at 48 h (second group) both total and unbound serum MTX levels (determined by high-performance liquid chromatography) were elevated by 53-109 and 26-65%, respectively, by the third hour after completion of Vepesid infusion. This effect was detectable for 6 h. All the liver and kidney functions of the patients were within the normal range. These results suggest the possibility of partial recirculation of extra/intracellular MTX 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 MTX treatment. Under these conditions only a slight decrease of MTX elimination has been detected between 25 and 28 h. These results emphasize the role of possible schedule-dependent interactions of cytostatic drugs.