The pharmacokinetics of captopril and captopril disulfide conjugates in uraemic patients on maintenance dialysis: comparison with patients with normal renal function.

We have measured the plasma concentrations of captopril and total disulfide conjugates of captopril after a 50 mg oral dose in 6 uraemic patients on maintenance dialysis and in 8 hypertensive subjects with normal renal function. The mean peak plasma concentration of captopril was 2.5 times higher (0.447 micrograms X ml-1 vs 0.181 micrograms X ml-1) and the concentrations of the disulfides 4 times higher (3.62 micrograms X ml-1 vs 0.924 micrograms X ml-1) in the uraemic patients. Moreover captopril disulfide conjugates in the uraemic subjects reached peak concentrations at 8 h after the dose and subsequently felt. The apparent plasma half-time was 46 +/- 19 h. Only 15% of these conjugates were removed by dialysis. This marked accumulation of captopril conjugates was associated with a sustained fall in both systolic and diastolic blood pressures. In uraemic patients the mean maximum reduction in systolic and diastolic blood pressures were 37 +/- 7 mmHg and 24 +/- 9 mmHg respectively, occurring 6 h after the dose, compared with 8 +/- 7 and 8 +/- 1 mmHg respectively at 30 min in normal renal function patients. These results are consistent with the results of animal experiments, which show that captopril disulfides can be converted back to free captopril and can contribute to the antihypertensive effect of the drug. They provide a reationale for reducing the dose and frequency of administration of captopril in patients with significant renal impairment.