Pharmacokinetics of methotrexate and 7-hydroxymethotrexate following infusions of high-dose methotrexate

The pharmacokinetics of methotrexate (MXT) and 7-hydroxymethotrexate (7-OH-MTX) has been studied in seven patients treated for osteosarcoma with up to 27 cycles of MTX at doses of 140-350 mg/kg of body weight. The distribution volume of MTX was 0.186 +/- 0.062 liter/kg. Peak plasma levels ranged from 540 to 1700 microM for MTX and from 12 to 560 mu M for 7-OH-MTX. The MTX metabolite 2,4-diamino-N10-methylpteroic acid was occasionally detectable in plasma and urine at a level of 10(-7)M. Plasma disappearance of MTX was biphasic, with a terminal half-life of 2.1 +/- 0.6 hours (mean +/- SE). Plasma decay of 7-OH-MTX was mainly monoexponential, with a half-life of 23.8 +/- 15.2 hours. The renal clearance of MTX (0.0623 +/- 0.0232 liter/kg/hour) accounted for about 84% of the total clearance of MTX (0.0742 +/- 0.0288 liter/kg/hour). In urine, 70%-94% of the dose was recovered as MTX and 0.4%-11% as 7-OH-MTX. The renal clearance of 7-OH-MTX was in the range of 0.0173 +/- 0.0149 liter/kg/hour. The pharmacokinetics of MTX and 7-OH-MTX was influenced by orally coadministered activated charcoal, presumably by inhibition of enteral reabsorption of MTX and 7-OH-MTX.