Ligand-binding radioassay for the antifolate compounds: application in patients receiving methotrexate.

The general principles of ligand-binding radioassays are reviewed and a specific noncompetitive system is described for the measurement of methotrexate (MTX) using dihydrofolate reductase as the binding determinant. This type of radioassay can also be exploited to measure any antifolate compound which binds to this enzyme even if [3H]MTX is used as the tracer. The radioassay has now been used to measure MTX in all types of body fluids, tissue extracts, and rbc lysates from patients receiving this drug.