| 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. |