[Radioimmunoassay of the antitumor drug--methotrexate (MTX)].

The specific efficient anti-MTX antibody was obtained from the rabbits immunized with an artificial antigen produced by condensing MTX and bovine serum albumin in water-soluble carbodi-imide. The affinity constant (Ka) of the antibody is 1.8 X 10(9) L/mol. The rapid, sensitive and specific radioimmunoassay was developed with the tritium-labeled MTX as the tracer and the dextran-coated charcoal as the separating reagent. The concentration of MTX was measured within 1.5 hours and the lowest detectable level was 0.1 ng using only 10 microliter of serum or other biological fluids. Folic acid and its derivatives, hormones and other chemotherapy drugs have no influence on this assay. The method has an acceptable accuracy and precision: the recovery is 96.5%; the coefficients of variation (CV) within and between assay are 8% and 14%. The method is successful for the monitor of the MTX concentration in patient's plasma and study of its metabolism and pharmacokinetics.