Mechanism of Action of Thioureylene Antithyroid Drugs in the Rat: Possible Inactivation of Thyroid Peroxidase by Propylthiouracil*

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Abstract
The thioureylene antithyroid drugs 6-propyl-2-thiouracil (PTU) and 1-methyl-2-mercaptoimidazole (MMI) can inactivate thyroid peroxidase (TPO) in a model iodination system containing relatively high concentrations of I. It was determined whether these drugs may also inactivate TPO in vivo in rats. Assays for total TPO activity after injection of PTU or MMI did not prove to be a valid approach. As TPO inactivation might be expected to result in a relatively prolonged inhibition of enzyme activity, most of our experiments involved measurement of the duration of the inhibitory effect of a single injection of drug. Young rats were injected with low doses of PTU or MMI, and the effect on thyroidal organic I formation was determined at intervals after injection, either by 1-h pulse labeling with 131I- in vivo or by incubation of excised thyroid lobes in a medium containing 131I-. Results of both types of experiment demonstrated that the inhibitory effect of a small dose of PTU (1 .mu.mol/100 g body wt) was still very marked 17-18 h after injection. Moreover, an inhibitory effect of this small dose of PTU on the metabolism of [35S]MMI could also be demonstrated. Administration of MMI to rats did not show the prolonged inhibitory effect observed with PTU. This is most likely attributable to the much lower thyroidal uptake of MMI than of PTU in rats. Intrathyroidal metabolism of [35S]PTU and [35S]MMI was also investigated. In contrast to the rapid disappearance of 35S from plasma, both drugs showed accumulation and retention of 35S in the thyroid. No evidence was obtained that thyroidal accumulation of PTU or one of its metabolites could explain the prolonged inhibitory effect of this drug. It seemed more likely that this was attributable to TPO inactivation. The clinical implications of the findings are discussed with relation to the dosage schedule commonly employed in the treatment of Graves'' disease with antithyroid drugs.