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).

Pharmacokinetics of controlled-release carbimazole tablets support once daily dosing in cats.

Carbimazole, a prodrug of methimazole, is used in the treatment of hyperthyroidism in cats. The pharmacokinetics of methimazole was investigated in healthy cats following oral administration of 15 mg of carbimazole as a controlled-release tablet (Vidalta), Intervet). The controlled-release tablet did not produce a pronounced concentration peak and methimazole was present in the circulation for a sustained period, compared with a conventional tablet formulation. The time to reach peak concentrations after carbimazole administration was quite long (t(max) 6 h). The absolute bioavailability of carbimazole was around 88 +/- 11%. Repeated oral administration daily for 13 consecutive days did not lead to accumulation of methimazole in plasma. The extent of absorption of carbimazole was about 40% higher when administered to cats that had been fed compared to fasted cats. The relative oral bioavailability of methimazole following administration of the controlled-release tablets was similar to that of a conventional release formulation (83 +/- 21%). The pharmacokinetics of this controlled-release formulation of carbimazole supports its use as a once daily treatment (both as a starting dose and for maintenance therapy) for cats with hyperthyroidism.

Radioiodine uptake and thyroid hormone levels on or off simultaneous carbimazole medication: a prospective paired comparison.

AIM: To allow radioiodine (RAI) treatment in patients with need for anti-thyroid drug medication and low RAI uptakes we investigated the feasibility of discontinuing carbimazole for 3 days to enhance the RAI uptake without concurrent exacerbation of hyperthyroidism. METHODS: We prospectively investigated RAI dynamics and thyroid hormone concentration in 12 patients with low RAI uptake (<30%) under simultaneous carbimazole medication and 3 days after discontinuation. At both time points fT(4), T(3) and TSH were monitored. RESULTS: Discontinuation of carbimazole for 3 days led to a significant increase of RAI uptake in all patients. We found an enhancement up to 4.9-fold compared to the measurement on carbimazole. The mean RAI uptake increased from 15.2 +/- 4.4% to 50.1 +/- 15.5% (p<0.001). The intrapersonal radioiodine half-life increased from 4.2 +/- 1.6 days to 5.4 +/- 0.7 days (p = 0.13). Mean thyroid hormone concentration was not affected by the three day withdrawal of anti-thyroid drugs and no patient suffered from an aggravation of biochemical hyperthyroidism. CONCLUSION: A withdrawal of carbimazole for 3 days is long enough to provide sufficiently high RAI uptakes for RAI treatment in patients with low RAI uptakes and short enough to avoid the risk of exacerbation of hyperthyroidism.

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.

The pharmacokinetics of methimazole in a novel lipophilic formulation administered transdermally to healthy cats.

AIM: To determine the pharmacokinetics of a novel lipophilic formulation of transdermal methimazole compared to oral carbimazole. METHODS: Healthy cats received 5 mg carbimazole orally every 12 hours for 13 treatments (n=6), then received transdermal methimazole (n=5) at a dose of 5 mg, then 10 mg, once daily on the pinna for 7 days, with 21 days between treatments. Concentrations of methimazole in serum over 24 hours and at 148 hours were determined by high performance liquid chromatography. RESULTS: Concentrations of methimazole in serum for the first 24 hours were not reliably detected in all cats treated with 5 mg methimazole transdermally, while for those receiving 5 mg carbimazole orally and 10 mg methimazole transdermally all cats had detectable concentrations of methimazole in serum. The maximum concentration and area under the curve were lower in cats receiving 10 mg methimazole transdermally (108 (SD 25) ng/mL and 2544 (SD 216) mg-hour/mL, respectively) than those receiving 5 mg oral carbimazole (355 (SD 113) ng/mL and 31,866 (SD 439) ng-hour/mL, respectively) (p<0.05). The time at maximal concentration and elimination half-life were longer for 10 mg transdermal methimazole (5.2 (SD 1.1) hours and 13 (SD 3) hours, respectively) compared to 5 mg oral carbimazole (2.1 (SD 1.6) hours and 5.1 (SD 1.2) hours, respectively). At 148 hours, mean concentrations of methimazole in serum were higher in cats receiving 10 mg methimazole transdermally (506 (SD 165) ng/mL) than for 5 mg oral carbimazole (255 (SD 28) ng/mL) or 5 mg transdermally (204 (SD 76) ng/mL). The mean relative bioavailability of 10 mg transdermal methimazole compared to oral carbimazole was 48 (min 43, max 55)%. CONCLUSION: Transdermal methimazole at a dose of 10 mg administered to the pinnae of healthy cats once daily in a novel lipophilic formulation has half the relative bioavailablity compared to 5 mg oral carbimazole. CLINICAL RELEVANCE: Transdermal methimazole can be absorbed from the skin of healthy cats.

Pharmacokinetics and pharmacotherapy of thionamides in pregnancy.

Hyperthyroidism occurs in approximately 1 in every 1000 to 2000 pregnancies. Although the signs and symptoms of the disease are similar in the pregnant and nonpregnant patient, the complications of hyperthyroidism can have even more profound consequences for the mother and fetus during gestation. These include maternal heart failure, preeclampsia, miscarriage, and preterm labor; as well as fetal loss and low birth weight. Furthermore, thyroid function and laboratory testing for hyperthyroidism are altered in pregnancy. The gestational increase in thyroid size, increased thyroid-binding globulin levels, increased serum total T4 and total T3 levels, and decreased thyroid stimulating hormone levels often confuses the evaluation of the thyroid status in pregnancy. Worldwide, the thionamides-propylthiouracil, methimazole, and carbimazole-have been used in pregnancy for the treatment of hyperthyroidism. However, propylthiouracil has been the drug of choice in the United States because it is believed to have less potential to induce fetal/neonatal hypothyrodism, to cross the placenta and into breast milk to a lesser degree, and to be less teratogenic than methimazole or carbimazole. None of the above have been substantiated in more recent studies. The pharmacokinetics of the thionamides in the pregnant and nonpregnant states, as well as the pharmacotherapeutic recommendation for hyperthyroidism will be reviewed.