Serum concentrations of methimazole in cats after a single oral dose of controlled-release carbimazole or sugar-coated methimazole (thiamazole).

Methimazole (thiamazole) is an antithyroid drug commonly used to treat feline hyperthyroidism. It is routinely given twice daily. Carbimazole is a methimazole derivative that is rapidly metabolized to methimazole in vivo. A controlled-release tablet for once-daily carbimazole therapy has recently been developed in an attempt to improve compliance during medical management of feline hyperthyroidism. The results of a crossover study in six cats suggest that the pharmacokinetics of methimazole with a single dose of this controlled-release tablet may be similar to those with a single dose of a sugar-coated methimazole tablet when the two drugs are given at an equimolar dose. The mean half-lives were nearly identical (3.12 hours, sugar-coated methimazole tablets; 3.28 hours, controlled-release carbimazole tablets). The serum concentrations of methimazole at 24 hours were 21.7 ± 28.9 ng/mL in the cats treated with 5-mg sugar-coated methimazole tablets and 28.7 ± 37 ng/mL in the cats treated with 10-mg carbimazole tablets (which provide approximately 25% more methimazole after conversion to the active metabolite).

Comparison of the disposition of carbimazole and methimazole in clinically normal cats.

The oral disposition of the antithyroid drugs methimazole and carbimazole were compared in nine clinically normal cats. After the administration of 5 mg of methimazole, serum concentrations of methimazole increased in all the cats, with mean drug concentrations reaching peak values (1.37 micrograms ml-1) at 30 minutes. After administration of 5 mg carbimazole, serum concentrations of carbimazole remained low, but serum methimazole became readily measurable, with mean drug concentrations reaching peak values (0.79 microgram ml-1) at 120 minutes. When serum concentrations of methimazole attained after administration of the two antithyroid drugs were compared, the mean maximum serum methimazole concentration achieved after administration of methimazole was approximately twofold higher than peak concentrations measured after administration of carbimazole. In addition, the mean area under the serum concentration curve (AUC) after administration of methimazole was approximately twofold higher than the mean AUC determined after administration of carbimazole. When the differences in molecular weight between the two drugs was taken into consideration, however, these methimazole:carbimazole ratios of 2:1 were nearly equivalent to the molar ratio of the 5 mg doses of the drugs given (1.63). Results of this study indicate that carbimazole is nearly totally converted to methimazole after oral administration to cats, similarly to the findings in man. The finding of less available serum methimazole after administration of a 5 mg tablet of carbimazole than after methimazole is also consistent with published antithyroid drug dosages needed to control hyperthyroidism in cats.

Rapid conversion of carbimazole to methimazole in serum; evidence for an enzymatic mechanism.

Carbimazole (CBZ) is one of the major drugs currently used for the treatment of Graves' disease. It is a carbethoxy derivative of methimazole (MMI), originally developed in the hope of obtaining a longer acting drug than methimazole. In the present study we have demonstrated that carbimazole is rapidly converted to methimazole in vitro by serum from rats and humans, and we have obtained evidence that this conversion is enzymatic. Experiments with [35S] CBZ in rats showed that the drug is so rapidly transformed to MMI after i.v. injection (within 3 min) that very little of the unchanged drug would be expected to reach the thyroid gland. The antithyroid action of CBZ in rats, therefore, can be ascribed entirely to the MMI to which it is rapidly converted. Although no experiments were performed with human subjects in vivo, the very rapid conversion of CBZ to MMI by human serum in vitro suggests that the antithyroid action of CBZ in humans can also be attributed to MMI. The original expectation of a longer acting drug has, therefore, not been met by CBZ. On the basis of the studies reported here there appears to be no advantage in using CBZ in preference to MMI for the treatment of Graves' disease. Although the in vivo action of CBZ must be attributed to its rapid conversion to MMI, the drug does possess inherent antithyroid activity. This was shown in the present study by the finding that CBZ is as potent as MMI in blocking thyroid peroxidase-catalysed iodination of thyroglobulin.

35-S-antithyroid drug concentration and organic binding of iodine in the human thyroid.

35S-methimazole (MMI), 35S-carbimazole or 35S-propylthiouracil (PTU) were given orally to fifty-five patients at various times up to 12 h before surgical thyroidectomy. The amount of 35S radioactivity and labelled drug in thyroid and plasma samples was measured. Intrathyroidal inhibition or organic binding of iodine by MMI, carbimazole and PTU was measured after intravenous administration of 131I, 132I or 125I-iodide. After administration of 35S-carbimazole or 35S-MMI the thyroid to serum (T/S) ratio of 35S radioactivity was greater in thyrotoxic glands than in non-toxic adenoma tissue. 35S-MMI was found in thyroid and plasma samples after administration of 35S-carbimazole. The T/S 35S-MMI was greater than 1 in most but not all patients. 35S radioactivity was also concentrated in the thyroid after administration of 35S-PTU. In thyrotoxic glands there was an 80% inhibition of iodine organification in patients receiving MMI and 60% for those receiving PTU. It is suggested that carbimazole and MMI can be given once or twice daily in some patients but PTU would be less suitable for this dose schedule.