A specific HPLC assay to determine the pharmacokinetics of methotrexate in patients

The pharmacokinetics of methotrexate and its metabolite, 7-hydroxymethotrexate (7-OH-MTX), were evaluated by a specific HPLC assay in patients receiving low dose (40 mg/m2) or high dose (750-1500 mg/m2) MTX. A wide range of total body clearance (1.03-5.13 ml/min/kg) was obtained in four subjects. The apparent renal clearances of MTX and 7-OH-MTX were found to decline with increasing serum levels, indicating the existence of saturable tubular secretion for the drug. The steady-state volume of distribution of MTX was 0.41-0.95 l/kg. Urinary excretion of MTX was almost complete at 36-48 h after dosing. The nonrenal excretion accounted for 17-39% of the dose. Extensive formation of 7-OH-MTX occurred at all dose levels, with their serum levels being 2.4- to 10-fold higher than MTX levels 24 h post infusion. The mean apparent renal clearance of 7-OH-MTX (0.08-0.13 ml/min/kg) was much smaller compared with that of MTX (0.63-2.62 ml/min/kg). The average saliva/serum level ratios were 0.009 and 0.078 in two patients studied.