Laser differential light-scattering bioassay for methotrexate (NSC-740).

A new bioassay method utilizing laser light scattering from suspensions of drug-sensitive bacteria has been developed for the estimation of antitumor drugs in biologic samples. Changes in the light-scattering patterns of antibiotic-treated bacteria have recently been shown to provide a rapid and accurate indication of antibiotic sensitivity. Similar considerations for several antitumor drugs have shown the method capable of assaying 0.1 ml with drug concentrations as low as a few nanograms of drug per milliliter of sample. The first successful application of the methodology is described for the antitumor agent methotrexate. Studies of both drug-treated human serum specimens and dog serum levels and urinary excretion as a function of time indicate that assay results are available within 3 hours of preparing the serum. Time variations of drug serum levels and urinary excretion rates are compared via laser differential light-scattering assay, standard disc-diffusion assay, and previously published radioisotopic assays.