Determination of a new angiotensin-converting enzyme inhibitor (CS-622) and its active metabolite in plasma and urine by gas chromatography-mass spectrometry using negative ion chemical ionization

A sensitive and specific gas chromatographic-mass spectrometric method for the simultaneous determination of angiotensin-converting enzyme inhibitor (I, CS-622) and its active desethyl metabolite (II, RS-5139) in plasma and urine was developed. Compound D5-RS-5139 was used as an internal standard and measurements were made by electron-capture negative ion chemical ionization. Extraction from plasma and urine was carried out using Sep-Pak C18 and silica cartridges. The extract of plasma or urine was treated with diazomethane followed by trifluoroacetic anhydride to convert I and II into their methyl ester trifluoroacetyl derivatives. The detection limit of I and II was 0.5 ng/ml in plasma and 5 ng/ml in urine. The proposed method was satisfactory for the determination of I and II in plasma and urine with respect to accuracy and precision. Thus it is suitable for measurement of bioavailability and pharmacokinetics of I and II in body fluids.