Abstract
A competitive protein binding assay for piritrexim (PTX, 1) that makes use of a commercially available radioassay kit for methotrexate has been developed. After it is selectively extracted from plasma, PTX competes with [125l]methotrexate for binding to dihydrofolate reductase isolated from Lactobacillus casei. Free drug is separated from bound drug by adsorption to dextran-coated charcoal. Piritrexim is measurable over a range of 0.01 to 10.0, μg/mL in plasma with a coefficient of variation <15%. The limit of sensitivity of the assay is ~2 ng/mL. An excellent correlation between this assay and a previously published HPLC method was found.
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