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A Direct Ligand-binding Radioassay for the Measurement of Methotrexate in Tissues and Biological Fluids¹

Division of Hematology-Oncology, Department of Medicine, New York Medical College, New York, New York 10029

A direct ligand-binding radioassay for methotrexate (MTX) has been developed using dihydrofolate reductase, contained in the lysate of L1210 leukemia cells, as the binding determinant. The procedure is a two-phase reaction system where standard MTX concentrations or the sample being assayed is incubated with the reagent lysate in the first phase, and [³H]MTX is then added in the second phase to titrate the remaining unoccupied binding sites on the enzyme. This method eliminates the need for measuring the residual catalytic activity of the enzyme.

The sensitivity of the radioassay is limited only by the specific activity of the [³H]MTX and now approximates 10 pg of the drug. Folic acid, methyltetrahydrofolate, formyltetrahydrofolate, and dihydrofolate in concentrations that are physiological do not interfere in the radioassay. Both mercaptoethanol and reduced nicotinamide adenine dinucleotide phosphate increase the binding capacity of the lysate for MTX; but the reduced nucleotide also increases the affinity of the enzyme for the inhibitor.

MTX added to serum can be assayed without extraction if the concentration is greater than 500 pg/ml and recovery of the drug added to serum is about 92%. MTX has been assayed in serum, spinal fluid, and urine of patients who were treated with this drug. It has also been assayed in the lysates of L1210 cells from C57BL x DBA/2 F₁ mice treated with MTX. The procedure is simple, rapid, and accurate and should permit better correlation of the therapeutic and toxic effects of MTX with blood concentrations over long-term treatment periods.

¹ This work was supported by Research Grants CA 08976 and AM 16220 from the NIH and by a grant from the National Leukemia Association, Inc.

² Medical student at New York Medical College.

³ Career Investigator of The New York City Health Research Council, I-423. To whom request for reprints should be sent, at Division of Hematology-Oncology, Department of Medicine, New York Medical College, 1249 Fifth Avenue, New York, N. Y. 10029.

⁴ Special fellow of the Leukemia Society of America.

⁵ Kresevich Foundation Cancer Research Scholar.

Received 1/27/75. Accepted 4/22/75.