Radioimmunoassay for methotrexate using hydroxyethylmethacrylate hydrogel.

A sensitive, rapid method for the measurement of methotrexate in biologic fluids has been developed using a solid-phase radioimmunoassay (RIA). Hydroxyethylmethacrylate monomer was polymerized in the presence of rabbit antimethotrexate antisera and the resultant gels lyophilized, ground to a fine powder, and aliquoted into 3-ml syringes fitted with a fritted filter disc. Control gels without incorporated antibodies did not bind radiolabeled methotrexate, whereas a dose-response curve expressing percent bound methotrexate against antiserum concentration could be readily constructed for appropriate gels. Drug concentrations of less than 1 ng/ml can be measured. The fate of a bolus IV injection of methotrexate (100 mg) into an adult female baboon was evaluated by the HEMA-RIA system and compared with a previously published enzyme assay for the drug. These studies describe a stable and efficient system for rapid separation of bound antigen by simple filtration.