NEOMERCAZOLE Tablet Ref.[49851] Active ingredients: Carbimazole

Source: Health Products Regulatory Authority (IE)  Revision Year: 2021  Publisher: Amdipharm Limited, Temple Chambers, 3 Burlington Road, Dublin 4, Ireland

5.1. Pharmacodynamic properties

Pharmacotherapeutic group: Sulfur-containing imidazole derivatives
ATC Code: H03BB01

Mechanism of action

Carbimazole, a thionamide, is a pro-drug which undergoes rapid and virtually complete metabolism to the active metabolite, thiamazole, also known as methimazole. The method of action is believed to be inhibition of the organification of iodide and the coupling of iodothyronine residues which in turn suppress the synthesis of thyroid hormones.

5.2. Pharmacokinetic properties

Absorption

Carbimazole is rapidly metabolised to thaimazole. After oral ingestion, peak plasma concentrations of thiamazole, the active moiety, occur at 1 to 2 hours.

Distribution

The total volume of distribution of thiamazole is 0.5 1/kg. Thiamazole is concentrated in the thyroid gland. This intrathyroidal concentration of thiamazole has the effect of prolonging its activity. However, thiamazole has a shorter half-life in hyperthyroid patients than in normal controls and so more frequent initial doses are required while the hyperthyroidism is active.

Biotransformation

Thiamazole is moderately bound to plasma proteins.

Carbimazole has a half-life of 5.3 to 5.4 hours. It is possible that the plasma half-life may also be prolonged by renal or hepatic disease. See section 4.2.

Thiamazole crosses the placenta and appears in breast milk. The plasma:milk ratio approaches unity.

Elimination

Over 90% of orally administered carbimazole is excreted in the urine as thiamazole or its metabolites. The remainder appears in faeces. There is 10% enterohepatic circulation.

5.3. Preclinical safety data

There are no preclinical data of relevance to the prescriber which are additional to that already included in other sections of the Summary of Product Characteristics.

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