Clinical pharmacokinetics of methyldopa

Absorption of methyldopa from the gastrointestinal tract is incomplete and variable; bioavailability after oral administration is about 25% (range 8 to 62%). The average time to reach maximum plasma concentration (tmax) [chemically determined] is 2 hours, when the maximum plasma concentration of active drug accounts for 50% of the radioactivity, the remainder representing various metabolites. Physicochemical determination of methyldopa shows that bi-phasic elimination occurs after both intravenous and oral administration, the half-life of the alpha-phase being 0.21 hours (range 0.16 to 0.26 hours) and of the beta-phase 1.28 hours (range 1.02 to 1.69 hours) in normal subjects. Methyldopa is less than 15% protein bound, whereas the primary metabolite, which most probably is the O-sulphate, is about 50% protein bound. The apparent volume of distribution in the central compartment is about 0.23L/kg (range 0.19 to 0.32L/kg), and the total volume of distribution (calculated as Vdarea) is about 0.60L/kg (range 0.41 to 0.72L/kg) in healthy volunteers. Acid-labile conjugates are formed after oral administration. These acid-labile conjugates, in particular the O-sulphate, are probably formed in the intestinal cells, since they are detected in very small amounts after intravenous administration. Additionally, there is a rapid formation of partly unidentified metabolites after both intravenous and oral administration. After intravenous administration the quantitatively most prominent metabolites are methyldopamine and the glucuronide of dihydroxyphenylacetone, but traces of 5 or 6 other metabolites have also been found and identified. These metabolites are probably formed in the liver, but the complete metabolic pattern is still unknown. The renal clearance of methyldopa (95 ml/min/m2) is more than 50% higher than the endogenous creatinine clearance. Renal excretion of some metabolites is slower. Extrarenal elimination accounts for about 50% of the total body clearance of the drug. Renal excretion is very low in patients with renal failure, resulting in accumulation of both active drug and, in particular, its metabolites. There is a marked accumulation of unidentified metabolites in renal failure patients, which possibly explains the strong and prolonged hypotensive action of methyldopa in these patients.