Methotrexate concentrations in biological fluids: comparison of results obtained by radioimmunoassay and direct ligand binding radioassay.

A sensitive (sensitivity 2.2 X 10(-9) mol/l) and specific (practically no cross-reaction with circulating folates) radioimmunoassay for the determination of methotrexate concentrations in biological fluids in described and compared with a commercial competitive protein binding assay. Antiserum with high titer was produced in rabbits immunized with MTX-human serum albumin conjugate. Fitness for use in pharmacokinetic drug level determinations was shown in three patients, who received both low doses and high dose therapy combined with citrovorum factor rescue. An excellent correlation was found between plasma and urine MTX concentrations obtained by RIA and competitive protein binding assay. A two-compartment pharmacokinetic model was found adequately describing the serum decay curves, but there was a great interindividual variability in the calculated pharmacokinetic parameters.