The Renal Excretion of Methyldopa

The renal excretion of unconjugated methyldopa was studied in 17 patients on long-term therapy and in 7 patients following intravenous injection of 200 mg of the drug. The renal clearance of the conjugated drug was also determined in 9 cases. The renal function of the patients ranged from normal to severely impaired; even a patient with total anuria was studied. The clearance rate of unconjugated methyldopa was, on an average, 90% of the endogenous creatinine clearance. This strongly indicates that the renal elimination of unconjugated methyldopa depends on glomerular filtration. Ultra-filtration in vitro of plasma from normal subjects given methyldopa showed that there was no protein-binding of the unconjugated drug, whereas about 50% of the conjugated drug was protein-bound. The renal clearance of conjugated methyldopa was about 50% of the clearance of the unconjugated drug, possibly owing to protein-binding of the conjugated drug. Clearance studies, as well as findings in one patient with anuria, suggest that there is some extra-renal elimination both of unconjugated and conjugated methyldopa.