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

Concomitant Agranulocytosis and Hepatotoxicity After Treatment with Carbimazole

Objective:

To describe a case of agranulocytosis and severe hepatotoxicity associated with Carbimazole treatment.

Case Summary:

A 37-year-old woman was diagnosed with severe hyperthyroidism resulting from Graves' disease. Treatment with Carbimazole 30 mg/day was initiated. Within 15 days following the start of therapy, both minor (eg, pruritus, rash, urticaria, fever, arthralgias) and potentially life-threatening (eg, agranulocytosis, severe mixed hepatotoxicity with severe cholestatic jaundice) adverse effects developed. The patient's symptoms and laboratory abnormalities resolved following withdrawal of Carbimazole. Treatment with other antithyroid drugs was not attempted, and 131I ablation of the thyroid was successfully performed. Thyroid function was maintained with standard follow-up care. Agranulocytosis, identified following bone marrow biopsy, was treated with granulocyte colony-stimulating factor.

Discussion:

Agranulocytosis and hepatotoxicity are rare adverse effects associated with Carbimazole treatment and are usually dose- and age-related. The likelihood that Carbimazole induced these undesirable events in our patient is rated as probable based on the Naranjo probability scale. We believe this case to be the first to describe minor and major adverse effects related to Carbimazole therapy in a patient with Graves' disease.

Conclusions:

Major adverse effects associated with Carbimazole are infrequent. However, clinicians need to be aware that the effects described here, including severe liver failure and bone marrow toxicity, may occur in patients receiving this drug.

Antithyroid drugs and their analogues: synthesis, structure, and mechanism of action

Thyroid hormones are essential for the development and differentiation of all cells of the human body. They regulate protein, fat, and carbohydrate metabolism. In this Account, we discuss the synthesis, structure, and mechanism of action of thyroid hormones and their analogues. The prohormone thyroxine (T4) is synthesized on thyroglobulin by thyroid peroxidase (TPO), a heme enzyme that uses iodide and hydrogen peroxide to perform iodination and phenolic coupling reactions. The monodeiodination of T4 to 3,3',5-triiodothyronine (T3) by selenium-containing deiodinases (ID-1, ID-2) is a key step in the activation of thyroid hormones. The type 3 deiodinase (ID-3) catalyzes the deactivation of thyroid hormone in a process that removes iodine selectively from the tyrosyl ring of T4 to produce 3,3',5'-triiodothyronine (rT3). Several physiological and pathological stimuli influence thyroid hormone synthesis. The overproduction of thyroid hormones leads to hyperthyroidism, which is treated by antithyroid drugs that either inhibit the thyroid hormone biosynthesis and/or decrease the conversion of T4 to T3. Antithyroid drugs are thiourea-based compounds, which include propylthiouracil (PTU), methimazole (MMI), and carbimazole (CBZ). The thyroid gland actively concentrates these heterocyclic compounds against a concentration gradient. Recently, the selenium analogues of PTU, MMI, and CBZ attracted significant attention because the selenium moiety in these compounds has a higher nucleophilicity than that of the sulfur moiety. Researchers have developed new methods for the synthesis of the selenium compounds. Several experimental and theoretical investigations revealed that the selone (C═Se) in the selenium analogues is more polarized than the thione (C═S) in the sulfur compounds, and the selones exist predominantly in their zwitterionic forms. Although the thionamide-based antithyroid drugs have been used for almost 70 years, the mechanism of their action is not completely understood. Most investigations have revealed that MMI and PTU irreversibly inhibit TPO. PTU, MTU, and their selenium analogues also inhibit ID-1, most likely by reacting with the selenenyl iodide intermediate. The good ID-1 inhibitory activity of PTU and its analogues can be ascribed to the presence of the -N(H)-C(═O)- functionality that can form hydrogen bonds with nearby amino acid residues in the selenenyl sulfide state. In addition to the TPO and ID-1 inhibition, the selenium analogues are very good antioxidants. In the presence of cellular reducing agents such as GSH, these compounds catalytically reduce hydrogen peroxide. They can also efficiently scavenge peroxynitrite, a potent biological oxidant and nitrating agent.

Antithyroid medications and psychosis

INTRODUCTION

Antithyroid drugs (ATDs) are used in the treatment of hyperthyroidism. Very rarely ATDs were reported to trigger acute psychosis in patients with no history of psychiatric disturbances. Our aim is to review the literature on psychosis as a side effect of ATD and to give a personal opinion on this issue.

AREAS COVERED

The cases of acute psychosis elicited by ATD are few and most were reported many years ago, before radioimmunoassay for thyroid-stimulating hormone (TSH) and thyroid hormones was introduced. Most of those cases lack a description of serum thyroid hormone profile before, during and after the appearance of the psychiatric disorder; hence, an abrupt shift from hyperthyroidism to euthyroidism or hypothyroidism cannot be excluded. In addition, patients underwent specific psychiatric therapy, so that it is difficult to attribute the disappearance of the mental disorders to the withdrawal of ATD per se.

EXPERT OPINION

Patients who develop mental disorders while under ATD should be followed by an accurate evaluation of TSH, free triiodothyronine (FT3), and free thyroxine (FT4) levels throughout the course of the psychiatric disease. The use of new imaging techniques could be helpful in ruling out the encephalopathy associated with autoimmune thyroid diseases and other cerebral pathologies that might be possible causes of these mental disorders.

Antithyroid drug carbimazole and its analogues: synthesis and inhibition of peroxidase-catalyzed iodination of L-tyrosine

Synthesis and biological activity of the antithyroid drug carbimazole (CBZ) and its analogues are described. The introduction of an ethoxycarbonyl group in methimazole and its selenium analogue not only prevents the oxidation to the corresponding disulfide and diselenide but also reduces the zwitterionic character. A structure-activity correlation in a series of CBZ analogues suggests that the presence of a methyl substituent in CBZ and related compounds is important for their antithyroid activity.

Change in the intrathyroidal kinetics of radioiodine under continued and discontinued antithyroid medication in Graves’ disease

PURPOSE

This study evaluated the thyroidal kinetics of radioiodine in Graves' disease under continued thiamazole medication and after discontinuation of thiamazole for 1-2 days, with a view to keeping the period of discontinuation as short as possible and to exploring the underlying mechanism of a postulated radioprotective effect of antithyroid drugs.

METHODS

In 316 patients, diagnostic and therapeutic radioiodine kinetics were followed up for 2 days by ten uptake measurements each and were defined mathematically by a two-compartment model.

RESULTS

Without thiamazole or when thiamazole was discontinued for at least 2 days, all uptake curves could be fitted perfectly by a simple in- and output function; the mean square error (mse) was 0.38 (test) and 0.28 (therapy). Under continued thiamazole medication (11.0+/-7.0 mg/day), the energy dose delivered to the thyroid was lowered by factor of 2.5. Uptake curves were deformed (mse: 1.06, test and 0.86, therapy) and appeared two peaked, suggesting coexistence of follicles with blocked and follicles with intact hormone synthesis and hence heterogeneous radioiodine uptake in the thyroid. In patients with maximally altered uptake curves, the success rate was as low as 31%. One day after discontinuation of thiamazole, mse was still increased (0.78, test), while 2 days afterwards it had normalised (0.36, test) and 3 days afterwards (mse: 0.24, therapy) the success rate was 87%.

CONCLUSION

Efficacy of radioiodine therapy under continued thiamazole medication is reduced not only by a lower uptake and shorter half-life of radioiodine, but also by a heterogeneous energy dose distribution in the thyroid. Discontinuation of thiamazole (but probably not of propylthiouracil) for at least 2 days is required to restore the efficacy of radioiodine.

Feline hyperthyroidism: spectrum of clinical presentions and response to carbimazole therapy

OBJECTIVE

To determine the spectrum of clinical presentations of hyperthyroidism in cats and response to carbimazole therapy by analysis of historical, clinical and laboratory data.

DESIGN

A prospective clinical study involving client-owned cats presenting to a private veterinary practice in Australia.

PROCEDURE

Twenty-five cats diagnosed as hyperthyroid during a 23-month period participated in the study with owner consent. Therapy with carbimazole was instituted and revisits were scheduled 2, 6 and 13 weeks after diagnosis. The cats were physically examined and underwent haematological and serum biochemical testing at each revisit. Owners were also asked to assess clinical signs in their cats in the periods between veterinary examinations. Cats with underlying renal disease were managed by alterations or cessation of carbimazole therapy.

RESULTS

A high prevalence of lethargic or inappetent cats without detectable underlying nonthyroidal illness was found. There was also a high prevalence of cats less than 10-years-old and cats in good body condition. Fourteen cats treated with carbimazole and monitored for 13 weeks responded favourably to therapy. Side-effects were minor and uncommon. Cats with underlying renal disease that became apparent during the study, responded well to alterations or cessation of carbimazole therapy.

CONCLUSION

The trend towards more subtle clinical presentations of hyperthyroid cats reported previously continued in this study. The findings of the current study do not appear to support the traditional view of hyperthyroid cats as being old, hyperactive, hungry and thin. Carbimazole therapy was found to decrease the prevalence of almost all clinical abnormalities in 14 cats and side-effects were minor and uncommon. This study demonstrates the usefulness of medical management of hyperthyroidism in the cat when radioiodine therapy is not possible due to renal compromise or other factors.

The selenium analog of methimazole. Measurement of its inhibitory effect on type I 5'-deiodinase and of its antithyroid activity

Methimazole (MMI), unlike propylthiouracil (PTU) is a poor inhibitor of type I iodothyronine deiodinase (ID-1). Inhibition of the enzyme by PTU was attributed initially to formation of a mixed disulfide between PTU and a cysteine residue at the active site. Presumably, MMI was unable to form a stable mixed disulfide and thus did not inhibit the enzyme. However, it has been demonstrated recently that ID-1 is a selenium-containing enzyme, with selenocysteine, rather than cysteine, at the active site. This observation raised the possibility that the selenium analog of MMI, methyl selenoimidazole (MSeI), might be a better inhibitor of ID-1 than MMI itself, as formation of the Se-Se bond with the enzyme would be expected to occur more readily than formation of the S-SE bond. To test this possibility, we developed a procedure for the synthesis of MSeI and compared MSeI with MMI and PTU for inhibition of ID-1 and for antithyroid activity. For inhibition of ID-1, MMI and MSeI were tested at concentrations of 10-300 microM. No significant inhibition was observed with MMI. MSeI showed slight but significant inhibition only in the 100-300 microM range. PTU, on the other hand, showed marked inhibition at 1 microM. Thus, replacement of the sulfur in MMI with selenium only marginally increases its inhibitory effect on ID-1. As an inhibitor of ID-1, MSeI is much less than 1% as potent as PTU. MMI and MSeI were also compared for antithyroid activity, both in vivo and in vitro. As an inhibitor of the catalytic activity of thyroid peroxidase, MMI was 4-5 times more potent than MSeI in a guaiacol assay, but only twice as potent in an iodination assay. In in vivo experiments with rats, MMI was at least 50 times more potent than MSeI in inhibiting thyroidal organic iodine formation. The relatively low potency of MSeI in vivo suggests that it is much less well concentrated by the thyroid than in MMI.

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.

Initial treatment of thyrotoxic Graves' disease with methimazole: a randomized trial comparing different dosages

We evaluated the efficacy of different doses of methimazole (MMI) as the initial therapy for Graves' disease. Fourteen patients were treated with 15 mg/die of the drug (group A) and 14 with 30 mg/die (group B). Blood samples for T3, T4, FT3 and FT4 were obtained before beginning therapy, every 48 h during the first 12 days and on the 45th day of treatment. All these hormonal parameters fell significantly from the 2nd day of therapy in both groups. All the patients, except for one in group B, had normal or subnormal levels of thyroidal hormones on the 45th day of treatment. The comparison between the two groups of regression coefficients over the first 12 days showed no significant differences. The absolute decrease of each examined parameter on day 12 was positively correlated with the relevant pretreatment value. These results demonstrate that doses of MMI (15 mg/die) much lower than those commonly recommended are able to rapidly control thyroidal overproduction as effectively as 30 mg/die.

Clinical significance of esterases in man

Esterases, hydrolases which split ester bonds, hydrolyse a number of compounds used as drugs in humans. The enzymes involved are classified broadly as cholinesterases (including acetylcholinesterase), carboxylesterases, and arylesterases, but apart from acetylcholinesterase, their biological function is unknown. The acetylcholinesterase present in nerve endings involved in neurotransmission is inhibited by anticholinesterase drugs, e.g. neostigmine, and by organophosphorous compounds (mainly insecticides). Cholinesterases are primarily involved in drug hydrolysis in the plasma, arylesterases in the plasma and red blood cells, and carboxylesterases in the liver, gut and other tissues. The esterases exhibit specificities for certain substrates and inhibitors but a drug is often hydrolysed by more than one esterase at different sites. Aspirin (acetylsalicylic acid), for example, is hydrolysed to salicylate by carboxylesterases in the liver during the first-pass. Only 60% of an oral dose reaches the systemic circulation where it is hydrolysed by plasma cholinesterases and albumin and red blood cell arylesterases. Thus, the concentration of aspirin relative to salicylate in the circulation may be affected by individual variation in esterase levels and the relative roles of the different esterases, and this may influence the overall pharmacological effect. Other drugs have been less extensively investigated than aspirin and these include heroin (diacetylmorphine), suxamethonium (succinylcholine), clofibrate, carbimazole, procaine and other local anaesthetics. Ester prodrugs are widely used to improve absorption of drugs and in depot preparations. The active drug is released by hydrolysis by tissue carboxylesterases. Individual differences in esterase activity may be genetically determined, as is the case with atypical cholinesterases and the polymorphic distribution of serum paraoxonase and red blood cell esterase D. Disease states may also alter esterase activity.

Preliminary evaluation of mesoionic 6-substituted 1-methylimidazo[2,1-b][1,3]thiazine-5,7-diones as potential novel prodrugs of methimazole

A series of five 6-alkyl- and 6-aryl-mesoionic 1-methylimidazo[2,1-b][1,3]thiazine-5,7-diones was synthesized and found to produce 1-methyl-3H-imidazole-2-thione (methimazole) upon alkaline hydrolysis or treatment with amine or thiol reagents. The alkaline hydrolysis followed a second-order rate expression, being dependent on both substrate and hydroxide-ion concentrations. The rate constants for the five derivatives fell within 6-15 x 10(-2) liter/mole min at 40 degrees. These compounds were stable in aqueous acidic solutions and in human serum or rat liver homogenate under conditions producing rapid hydrolysis of the methimazole prodrug 1-carbethoxy-2-methylimidazole-2-thione (carbimazole).

Clinical pharmacokinetics of antithyroid drugs

Organic antithyroid drugs used today include propylthiouracil and the mercaptoimidazolines, carbimazole and methimazole. They can be measured with accuracy and in small quantities in serum by gas-liquid chromatography, high performance liquid chromatography and radio-immunoassay. Bioavailability of these drugs varies from 80 to 95%. During absorption carbimazole, which itself is inactive, is completely converted to methimazole. The total volume of distribution is about 40L for methimazole and around 30L for propylthiouracil, which is about 80% protein-bound, while methimazole is virtually non-protein-bound. Drug transfer across the placenta and into breast milk is also higher for the more lipid-soluble methimazole than for propylthiouracil, which is excreted into breast milk only in small quantities so that no harmful effect to the suckling infant is to be expected. Both drugs are concentrated in the thyroid gland, exerting an effect on intrathyroidal iodine metabolism for periods exceeding those in which serum concentrations can be measured. Less than 10% of both drugs is excreted unchanged in the urine, but detailed metabolic pathways are unknown. The half-life of methimazole is 3 to 5 hours with a total clearance of about 200ml/minute. Propylthiouracil has a half-life of 1 to 2 hours with a clearance of around 120ml/min/m2. Some studies have shown an increased rate of metabolism of anti-thyroid drugs in hyperthyroidism, in particular for methimazole. No reliable information exists regarding pharmacokinetics of these agents in renal and hepatic failure or in children. The clearance of propylthiouracil is unchanged in the elderly. Several mechanisms for the inhibiting effect of these agents on intrathyroidal hormone metabolism have been suggested. In contrast to methimazole, propylthiouracil inhibits the peripheral conversion of thyroxine to triiodothyronine. Preliminary dose-response studies with propylthiouracil suggest a peak therapeutic serum concentration of above 4 micrograms/ml in the treatment of thyrotoxicosis. The choice between the antithyroid drugs is based more upon personal preference and experience than on strict pharmacological principles, as no important differences exist between these drugs with regard to the rate of remission or frequency of occurrence of serious adverse reactions.

Studies of thyroid hormone and methimazole levels in patients with Graves' disease on a standardized anti-thyroid drug regimen

The outcome of a standardized carbimazole (CMI) regimen was evaluated in fifty-four patients with Graves' disease. Triiodothyronine (T3) and thyroxine (T4) serum values were determined before and throughout the first 6-8 months of therapy. Before therapy, decreasing T4/T3 ratios were found with increasing T3 levels, reflecting a relative increase in T3 production compared with T4 in thyrotoxicosis. High pretreatment T3 values were comparatively common among patients in whom a fixed CMI dosage had a insufficient effect. T4 levels were less informative in this respect. Serum levels of methimazole (MMI), the active metabolite of CMI, were dose-dependent. A 50% reduction in dosage resulted in a similar decrease in the serum values. For a give dose, the MMI levels differed considerably among individuals. The variation was not related to thyroid hormone values. It appears that the response to the anti-thyroid drug is primarily dependent upon the severity of the disease, Which is clearly reflected in serum T3 levels, rather than individual differences in drug handling. A routine CMI regimen for the treatment of thyrotoxicosis is suggested.

Thionamides and analogues: a reapraisal of antithyroid and thyroid carcinogenic effects

As commonly depicted, in the thione form, thionamides (including thiourea and its derivatives) bear little resemblance to thyroid hormone. However, if placed in the thiol resonance form, these molecules are noted to structurally mimic the end of the tyrosine molecule (the precursor of thyroid hormone). Alternatively, tyrosine written in the -one configuration resembles the thionamides. This permits a better appreciation of why the thionamides may themselves be iodinated by thyroid peroxidase in some cases, and under other circumstances induce changes in the enzyme. The thiol resonant form can be written for naturally occurring goitrin, and for thiobarbituric acids with antithyroid activity. An hydroxyl resonant configuration can be drawn for the antithyroid compound 3-hydroxy-4-pyridone. Tetramethylthiourea is a thyroid carcinogen in rats. The compound can not be readily placed in the thiol resonant configuration; it also contains 2 methyl groups at both ends of the molecule. The hair dye 2,4-diaminoanisole sulfate induces thyroid neoplasms in rats. A similarity is pointed out both to tyrosine an to lower potency antithyroid aminobenzenes (described by Astwood and coworkers). Pyrazole, known to produce thyroid necrosis, is seen to have a distinct resemblance to the opposite end of several of these compounds.