Comparative bioavailability of carbimazole and methimazole

Area of Science:

• Pharmacology
• Endocrinology
• Drug Metabolism

Background:

• Antithyroid drugs are crucial for managing hyperthyroidism.
• Methimazole and carbimazole are commonly prescribed, but their comparative bioavailability requires clarification.

Purpose of the Study:

• To investigate and compare the oral bioavailability and pharmacokinetics of therapeutic doses of methimazole and carbimazole.
• To determine if carbimazole is equipotent to methimazole on a molar basis.

Main Methods:

• Seven euthyroid subjects received oral doses of methimazole and carbimazole.
• Deuterium-labeled methimazole served as an internal standard for enhanced accuracy.
• Plasma concentrations were measured using a sensitive gas chromatographic-mass spectrometric assay.

Main Results:

• Carbimazole (15 mg) yielded methimazole plasma concentrations and pharmacokinetic profiles comparable to equimolar methimazole (9.2 mg).
• Peak plasma concentrations (163 ng/ml for carbimazole, 149 ng/ml for methimazole) were achieved at 0.9 hours.
• Plasma half-lives were similar (5.7 h for carbimazole, 5.4 h for methimazole) with minimal interindividual variation.

Conclusions:

• Carbimazole is rapidly and completely bioactivated to methimazole.
• Methimazole and carbimazole should be considered equipotent antithyroid drugs when compared on a molar basis.