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Increases in methyldopa absorption and renal excretion after multiple doses
1Discipline of Medicine, Faculty of Medicine, Memorial University of Newfoundland, St John's.
Abstract:
A retrospective analysis of previous studies examining methyldopa absorption suggested the possibility that the absorption of methyldopa might increase on repeat methyldopa ingestion. A prospective study was undertaken to determine the effect of repeated oral doses of methyldopa on methyldopa absorption. Thirteen healthy subjects ingested single 250 mg methyldopa doses on days 0, 7, 14, 28, 56, and 112; 24 urine samples were collected and analyzed for methyldopa and its major metabolites on each study day and methyldopa plasma levels were measured over 8 hours at days 0 and 56. There were significant increases in the absorption of methyldopa (as estimated by the urinary excretion of methyldopa and the measured metabolites over 24 hours) at day 56 (33.4 +/- 8.9%, P less than .025) compared with day 0 (26.0 +/- 10.8%). There was also a significant increase in renal clearance of unmetabolized methyldopa (62.7 +/- 13.6 vs. 99.3 +/- 29.1 mL/min, P less than .01) and a decrease in the plasma half-life of methyldopa at day 56 (2.22 +/- 0.91 vs. 1.56 +/- 0.68 hr, P less than .05). There was a tendency toward increases in methyldopa absorption at day 7, 14, 28, and 112. Several possible explanations for the changes in methyldopa disposition are discussed.
Insights
Repeated oral doses of methyldopa increase its absorption and alter its disposition in healthy subjects. This study investigated the impact of repeated methyldopa ingestion on drug absorption and clearance.
Area of Science:
- Pharmacokinetics
- Drug Metabolism and Disposition
Background:
- Previous studies suggested methyldopa absorption may increase with repeated ingestion.
- Understanding methyldopa absorption is crucial for optimizing hypertension treatment.
Purpose of the Study:
- To prospectively evaluate the effect of repeated oral methyldopa doses on its absorption.
- To assess changes in methyldopa plasma levels and renal clearance over time.
Main Methods:
- Prospective study involving 13 healthy subjects.
- Single 250 mg methyldopa doses administered on days 0, 7, 14, 28, 56, and 112.
- 24-hour urine collection for methyldopa and metabolite analysis; 8-hour plasma level measurements on days 0 and 56.
Main Results:
- Significant increase in methyldopa absorption by day 56 (33.4% vs. 26.0%, P < .025).
- Increased renal clearance of unmetabolized methyldopa (99.3 vs. 62.7 mL/min, P < .01).
- Decreased methyldopa plasma half-life by day 56 (1.56 vs. 2.22 hr, P < .05).
Conclusions:
- Repeated methyldopa administration enhances its absorption and alters its pharmacokinetic profile.
- Findings suggest potential for dose adjustment or modified dosing regimens.
- Further research is needed to elucidate the mechanisms behind these observed disposition changes.