Assay of metyrapone, metyrapol and the isomeric mono-N-oxides of metyrapone in biological fluids by high-performance liquid chromatography

A sensitive high-performance liquid chromatographic (HPLC) method for the analysis of metyrapone 2-methyl-1,2-di-(3-pyridyl)-1-propanone], its reduced metabolite metyrapol and metyrapone mono-N-oxide metabolites in biological fluids is reported. These components were extracted into dichloromethane (2 x 5 ml) from alkalinised microsomal incubates, urine and blood (final pH about 12.5), or from cytosolic incubates at pH 7.4 (final aqueous volume 2-4 ml). Recoveries were in the range 70-100% under these conditions. The intact drug and metabolites were separated by reversed-phase HPLC with ultraviolet detection at 261 nm. All calibration curves were linear (correlation coefficient greater than 0.997). For the analysis of hepatic microsomal or cytosolic incubates, the coefficient of variation was less than 10% for samples over the range 2.5-250 nmol/ml N-oxides and 10-250 nmol/ml metyrapol. Measurement of metyrapone and metyrapol in rat blood (0.25-ml sample volume) was linear in the ranges 4.4-265 and 26-263 nmol/ml, respectively, the lower concentration being the limit of detection. The coefficient of variation was less than 20% for samples over the ranges tested for both these compounds. The N-oxide metabolites were not detectable in blood using this assay, their concentrations being below the limit of detection.