Liquid chromatography/electrospray tandem mass spectrometry method for the quantitation of fosinoprilat in human serum using automated 96-well solid-phase extraction for sample preparation.

A sensitive, specific, accurate and reproducible liquid chromatography/electrospray tandem mass spectrometry method was developed and validated for the quantitation of fosinoprilat in 0.2 mL of human serum. The method employed acidification (with pH 4.0 sodium acetate buffer) of the serum samples to minimize the hydrolysis of the prodrug fosinopril to fosinoprilat prior to purification by automated 96-well solid-phase extraction. The required chromatographic separation of fosinoprilat and fosinopril was achieved isocratically on a Luna C8 analytical column (2 x 50 mm, 3 microm). The total run time was 2 min. The mobile phase contained methanol and water with 10 mM ammonium acetate. Detection was by positive ion electrospray tandem mass spectrometry. The standard curve, which ranged from 2.00 to 500 ng/mL, was fitted to a 1/x(2) weighted linear regression model. Fosinoprilat quality control (QC) samples used to determine the accuracy and precision of the method were prepared in human serum at concentrations of 5.00, 200, 400 and 1000 ng/mL. The assay accuracy was within 8% (dev). The intra- and inter-assay precisions were within 6 and 3% (RSD), respectively. Fosinopril QC samples used to gauge the rate of hydrolysis of fosinopril to fosinoprilat during the assay procedure were prepared in human serum at 500 ng/mL. The hydrolysis of fosinopril to fosinoprilat was </=1%. This degree of conversion would cause little error in the analysis of post-dose serum samples since such samples are known to contain low levels of the prodrug compared with the drug.