Angiotensin-converting enzyme inhibitors: measurement of relative inhibitory potency and serum drug levels by radioinhibitor binding displacement assay.

Radioinhibitor binding displacement, a new method for the measurement of angiotensin-converting enzyme (ACE) competitive inhibitors, has been used to assess the relative potency of nine synthetic ACE inhibitors. MK351A, tyrosyl derivative of enalaprilic acid was iodinated with 125I and used as the radioligand. [125I]MK351A bound to human serum ACE in a concentration-dependent manner. It was displaced in a concentration-dependent manner by all ACE inhibitors tested. Fifty percent displacement of bound [125I]MK351A (DD50) for each ACE inhibitor correlated well with inhibitor potency ID50, estimated using an ACE enzymatic activity assay using Hip-His-Leu as substrate (r = 0.96, p less than 0.001; n = 9). The radioinhibitor binding displacement assay was used to measure serum concentration of enalaprilic acid (MK422) in human serum samples. Drug concentration estimated by radioinhibitor binding displacement assay correlated closely (r = 0.96, p less than 0.001; n = 22) with the drug concentration measured by a specific radioimmunoassay. The radioinhibitor binding displacement technique using [125I]MK351A as the ligand for human serum ACE has general application to all competitive ACE inhibitors, allowing comparison of in vitro ACE inhibitor potencies and estimation of serum ACE inhibitor concentrations.