Competitive inhibitor binding assay (CIBA) of ACE inhibitors

We describe a new principle for measuring concentrations of pharmacologically active captopril or other angiotensin-converting enzyme (ACE) inhibitors in blood. Serum is incubated with 125-I-labeled ACE inhibitor (substance 351A, a lisinopril analogue, Merck Sharp & Dohme) in a nonequilibrated system, in which label and ACE inhibitor compete for binding to added serum ACE. Free label is separated by adsorption to coated charcoal. Concentration of captopril or other ACE inhibitor is calculated from a standard curve. Results in healthy volunteers showed rapid absorption of captopril with maximal concentration of active drug within 1 h, and fast disappearance within 2.5 h. Stability of captopril was improved by immediate 1:100 dilution of blood samples with assay buffer. In spite of this precaution, analysis should be performed within two days to avoid loss of active drug due to polymerization and protein binding. Samples of other tested ACE inhibitors can be frozen and later analyzed at convenience. The new principle is simple, sensitive, and specific.