Development and optimisation of a radioimmunoassay for plasma captopril.

A specific and sensitive radioimmunoassay has been developed to monitor plasma levels of captopril, the first orally active angiotensin-converting enzyme inhibitor. Because of the reactive nature of the captopril thiol group, captopril was measured as the captopril-N-ethylmaleimide complex (captopril-NEM). Accuracy studies, using samples with known added concentrations of captopril, were satisfactory (r = 0.95, p less than 0.001, y = 0.97x - 2.02), and the radioimmunoassay results compared well with those determined by a gas chromatography/mass spectrometric method (r = 0.98, p less than 0.0005, y = 1.2x - 19). The minimum detection limit of the assay was 2 micrograms captopril/litre plasma.