Competitive protein binding assay for methotrexate

C. E. Myers, M. E. Lippman, H. M. Eliot, B. A. Chabner

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107 Scopus citations


A competitive protein binding assay has been developed for methotrexate based on the tight binding of this drug to Lactobacillus casei dihydrofolate reductase (= tetrahydrofolate dehydrogenase; 5,6,7,8 tetrahydrofolate: NADP+ oxidoreductase; EC Free drug may be separated from that bound to reductase by adsorption with dextran albumin coated charcoal. Scatchard plot analysis of the enzyme drug interaction confirmed the presence of a single homogeneous class of binding sites with an association constant K(a) of 2.1 x 108 M-1. This high affinity binding permits detection of methotrexate in the range of 0.3-30 pmol with a coefficient of variation of 15% or less. The predominant circulating folate, 5 methyl tetrahydrofolate, and the clinically useful rescue agent leucovorin (5 formyl tetrahydropteroyl glutamic acid) do not interfere with the assay, nor does the methotrexate metabolite 4 amino 4 deoxy 10 methylpteroic acid. Assay of clinical samples, including plasma and cerebrospinal fluid, showed close agreement between the previously described enzyme inhibition assay and the more rapid competitive binding method.

Original languageEnglish (US)
Pages (from-to)3683-3686
Number of pages4
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number9
StatePublished - 1975
Externally publishedYes

ASJC Scopus subject areas

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