Development and evaluation of an enzyme inhibition assay for methotrexate.

A method for the measurement of methotrexate based on its inhibition of dihydrofolate reductase (EC 1.5.1.3) is described. The performance of the method is evaluated and a brief assessment of its clinical usefulness made. Reaction time, pH, and the relative concentration of enzyme and inhibitor all have a significant effect on the shape of the standard curve. Pre-incubation of enzyme with methotrexate resulted in increased inhibition, but did not improve the sensitivity or linearity of the assay. Data are presented on the imprecision, specificity and accuracy of the method. The method is simple to perform, rapid and inexpensive. The sensitivity and specificity of the method are such as to allow for effective monitoring of patients on methotrexate therapy.