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Abstract
Objective: A bioassay is developed for the measurement of methotrexate (MTX) in serum.
Methods: The assay is based on MTX inhibition of the proliferation of hypoxanthine‐guanosine phosphoribosyl transferase (HGPRT) negative mouse B‐cells (B9.H). HGPRT negative cells cannot use the salvage pathway of nucleotide synthesis to overcome inhibition by MTX.
Results: When B9.H cells are cultured with serial dilutions of serum, inhibition of proliferation is a measure of the amount of MTX in the serum. Circulating folates do not interfere with the assay.
Conclusion: This simple assay can detect low concentrations of MTX in serum: it is therefore useful for following the pharmacodynamics of functional MTX after low‐dose MTX treatment.