Development and application of a radioimmunoassay for methotrexate

Summary.-An antiserum to methotrexate has been produced in a sheep against a conjugate of ovalbumin and methotrexate (MTX) prepared using a water-soluble carbodiimide. The antibodies produced were specific for substances containing the 2,4-diamino pteridine structure. Naturally occurring folates did not interfere with the assay. A radioimmunoassay has been developed using this antiserum, which can be used to measure MTX concentrations of less than 1 ng/ml in biological samples without prior extraction. The concentrations of MTX in the blood and urine of patients following a single i.v. bolus injection and following oral administration of the drug have been measured. The published radioimmunoassays for MTX have been compared. THE antifolate drug methotrexate (MTX) is extensively used in the treat-ment of various forms of neoplastic disease. The great improvement in anti-tumour therapy achieved with it in recent years has resulted not only from increased clinical experience, but also from a knowledge of the pharmaco-kinetics of the drug (Henderson et al., 1965a; Henderson, Adamson and Oliverio