A novel, sensitive and rapid liquid chromatographic-electrospray ionization mass spectrometric method was developed and validated for the determination of zofenopril and its active metabolite zofenoprilat in human plasma. The method was based on a single extraction step using methyl tert-butyl ether and did not require chemical derivatization. The chromatographic conditions were optimized; separation was performed on a phenyl-hexyl column (5μm, 250mm×4.6mm i.d.) with a mobile phase consisting of a solution of methanol and water (95:5, v/v) that also contained 0.1% of formic acid. A flow rate of 1.0mL/min was used. Zofenopril, zofenoprilat and the internal standard (IS) fosinopril sodium were measured using an electrospray ion source in a positive reaction monitoring mode. Linear calibration curves were generated for zofenopril concentrations between 0.1052 and 1052ng/mL and for zofenoprilat concentrations between 0.2508 and 2508ng/mL. In both cases, the coefficients of determination were greater than 0.995. The extraction recovery for zofenopril was 93.5% on average. It was 92.5% for zofenoprilat. The inter- and intra-batch precision and accuracy for both zofenopril and zofenoprilat were higher than 14%. The method was applied to measure the concentrations of zofenopril and zofenoprilat in plasma samples.
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