In vivo determinations of Ki values for angiotensin converting enzyme inhibitors

We present two methods for calculating Ki values of angiotensin converting enzyme (ACE) inhibitors, such as captopril, in anesthetized or conscious rabbits. Both methods are based on indicator-dilution type determinations of single pass transpulmonary metabolism of the ACE substrate benzoyl-phe-ala-pro (BPAP). The first method involves two determinations of Michaelis-Menten constants Km and Amax (product of Vmax and lung capillary plasma volume) of endothelial-bound ACE for BPAP. Thirty seconds before the second determination of kinetic constants, the inhibitor is administered iv (e.g. captopril, 12 nmol/kg). Comparisons of the apparent Km and Amax values, obtained after the inhibitor to the control values obtained from the first determination, provide Ki values. With the second method, the ratio Amax/Km is obtained, under first-order reaction conditions, before and 30 sec after administration of inhibitor. These apparent and control ratios are used to calculate Ki values. In both methods, plasma levels of the inhibitor at the time of the determination of apparent kinetic constants are estimated by injecting radio-labelled inhibitor (e.g. 3H-captopril), analyzing radioactivity in arterial samples and correcting for plasma protein binding. These methods are potentially applicable to the clinical evaluation of new ACE inhibitors, in vivo, under normal or pathologic conditions.