ENZYME IMMUNOASSAYS FOR A NEW ANGIOTENSIN-CONVERTING ENZYME-INHIBITOR, ZABICIPRIL, AND ITS ACTIVE METABOLITE IN HUMAN PLASMA - APPLICATION TO PHARMACOKINETIC STUDIES

Zabicipril (S 9650) is a new angiotensin-converting enzyme inhibitor w hose hydrolysis in vivo produces the pharmacologically active metaboli te zabiciprilat (S 10211). Two competitive enzyme immunoassays specifi c for either zabicipril or zabiciprilat have been developed using acet ylcholinesterase (E.C. 3.1.1.7) as label. Antibodies were raised in ra bbits after immunization with lysil derivatives of zabicipril or zabic iprilat coupled with bovine serum albumin. Assays were performed in 96 -well microtiter plates coated with a monoclonal antibody raised again st rabbit immunoglobulin G, thus ensuring rapid separation of free and bound fractions of the tracer. The analysis does not require any extr action step. In the case of the assay of zabiciprilat, interference ge nerated by endogenous angiotensin-converting enzyme (ACE) was eliminat ed by the addition of perindoprilat, another ACE inhibitor. Perindopri lat was not recognized by the antibodies (cross-reactivity <0.01%) and did not affect assay efficiency. The specificity of the assays was ch ecked by high-performance liquid chromatography of human plasma sample s obtained after oral administration of 2 mg of zabicipril. No metabol ites or endogenous substances were detected. The mean reproducibility was 15% for the assay of zabicipril and 19% for the assay of zabicipri lat. The quantification limits were 1.2 ng/ml for the zabicipril assay and 0.8 ng/ml for the zabiciprilat assay. These assays are therefore suitable for pharmacokinetic studies and drug monitoring in clinical s tudies.