Maternal transfer of methimazole and effects on thyroid hormone availability in embryonic tissues

Methimazole (MMI) is an anti-thyroid drug used in the treatment of chronic hyperthyroidism. There is, however, some debate about its use during pregnancy as MMI is known to cross the mammalian placenta and reach the developing foetus. A similar problem occurs in birds, where MMI is deposited in the egg and taken up by the developing embryo. To investigate whether maternally derived MMI can have detrimental effects on embryonic development, we treated laying hens with MMI (0.03% in drinking water) and measured total and reduced MMI contents in the tissues of hens and embryos at different stages of development. In hens, MMI was selectively increased in the thyroid gland, while its levels in the liver and especially brain remained relatively low. Long-term MMI treatment induced a pronounced goitre with a decrease in thyroxine (T₄) content but an increase in thyroidal 3,5,3'-triiodothyronine (T₃) content. This resulted in normal T₃ levels in tissues except in the brain. In chicken embryos, MMI levels were similar in the liver and brain. They gradually decreased during development but always remained above those in the corresponding maternal tissues. Contrary to the situation in hens, T₄ availability was only moderately affected in embryos. Peripheral T₃ levels were reduced in 14-day-old embryos but normal in 18-day-old embryos, while brain T₃ content was decreased at all embryonic stages tested. We conclude that all embryonic tissues are exposed to relatively high doses of MMI and its oxidised metabolites. The effect of maternal MMI treatment on embryonic thyroid hormone availability is most pronounced for brain T₃ content, which is reduced throughout the embryonic development period.

Management of hyperthyroidism during pregnancy and lactation

INTRODUCTION

Poorly treated or untreated maternal overt hyperthyroidism may affect pregnancy outcome. Fetal and neonatal hypo- or hyper-thyroidism and neonatal central hypothyroidism may complicate health issues during intrauterine and neonatal periods.

AIM

To review articles related to appropriate management of hyperthyroidism during pregnancy and lactation.

METHODS

A literature review was performed using MEDLINE with the terms 'hyperthyroidism and pregnancy', 'antithyroid drugs and pregnancy', 'radioiodine and pregnancy', 'hyperthyroidism and lactation', and 'antithyroid drugs and lactation', both separately and in conjunction with the terms 'fetus' and 'maternal.'

RESULTS

Antithyroid drugs are the main therapy for maternal hyperthyroidism. Both methimazole (MMI) and propylthiouracil (PTU) may be used during pregnancy; however, PTU is preferred in the first trimester and should be replaced by MMI after this trimester. Choanal and esophageal atresia of fetus in MMI-treated and maternal hepatotoxicity in PTU-treated pregnancies are of utmost concern. Maintaining free thyroxine concentration in the upper one-third of each trimester-specific reference interval denotes success of therapy. MMI is the mainstay of the treatment of post partum hyperthyroidism, in particular during lactation.

CONCLUSION

Management of hyperthyroidism during pregnancy and lactation requires special considerations and should be carefully implemented to avoid any adverse effects on the mother, fetus, and neonate.

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).

[Current problems in the treatment of Graves' disease in pregnancy and in lactation]

In Graves' patients complicated by pregnancy, both maternal and fetal problems related to the disease can be reduced or avoided by controlling hyperthyroidism. However, optimal treatment for mothers may exert detrimental effects on fetuses. Methimazole may cause "methimazole embryopathy". Antithyroid drug doses that maintain mothers in euthyroid status are sometimes excessive fetuses. Furthermore, successful treatment with surgery or radioiodine occasionally may result in fetal hyperthyroidism due to TSH receptor antibody(TRAb). There are approaches to manage these problems. Propylthiouracil is chosen in treating Graves' disease in early pregnancy. In later pregnancy, maternal free thyroxine is maintained near or somewhat above normal. Ablative therapy is not recommended in women whose TRAb levels are extremely high from the standpoint of fetal thyroid state.

Thermoresponsive drug delivery using liquid crystal-embedded cellulose nitrate membranes

The aim of this study was to investigate the use of thermotropic liquid crystalline (TLC) blends of 4-pentyl-4'-cyanobiphenyl (K15) and 4-heptyl-4'-cyanobiphenyl (K21) with appropriate nematic to isotropic phase temperature (Tn - i) just above body temperature as a temperature-modulated drug permeation system. Using differential scanning calorimetry (DSC) we showed that the phase transition temperature (Tn - i) of K15 and K21 were 34.2 degrees C and 41.5 degrees C respectively. However, the thermogram of K15 and K21 blends with different ratios was shown to be a single endothermic peak similar to that of pure TLCs. K15 and K21 blends did not behave as a physical blend of two thermotropic liquid crystals with different Tn - i. However, they are rather mixed together in such ways that behave like a single unit TLC. The Tn - i of these TLC mixtures was linearly proportionate to the ratio of K15:K21. Using appropriate ratio of K15:K21 TLC, a mixture with desirable phase transition temperature was obtained. A triple layer of cellulose nitrate membranes containing a 50:50 mixture of K15 and K21 was used for drug permeation studies. This composite membrane showed good pulsatile permeation of drug molecules in response to temperature changes below and above the Tn - i of the K15 and K21 blends in a reproducible and reversible manner. Paracetamol and methimazole were chosen as hydrophobic and hydrophilic drug models, respectively. Methimazole permeability through the TLC membrane was much higher (36.0 x 10(-5) cm/s) at temperatures above the phase transition temperature of liquid crystal blends than that (7.2 x 10(-5) cm/s) at temperatures below the phase transition temperature of liquid crystal blends (38.1 degrees C).

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.

Therapeutic drug monitoring of antithyroid drugs in pregnancy: the knowledge gaps

Despite being a common condition in pregnancy, and despite propylthiouracil (PTU) being perceived as safer than methimazole, there are virtually no epidemiological controlled studies on malformation rate an neurobehavioral outcomes with the former. This knowledge gap must be filled to ensure fetal safety.

Two new polymorphisms of the FMO3 gene in Caucasian and African-American populations: comparative genetic and functional studies

To characterize the contribution of the human flavin-containing monooxygenase form 3 (FMO3) in the metabolism and disposition of drugs and xenobiotics, we determined the single nucleotide polymorphisms in the coding region and adjacent splice junctions of FMO3 in 134 African Americans and 120 Caucasians from the United States. In the regions examined, DNA resequencing or high throughput MassEXTEND studies coupled with mass spectrometric genotyping showed that 12 sites of variation were present. Three variants encoding synonymous mutations and four polymorphisms were observed in the noncoding region. Another three variants, Lys158-FMO3, Met257-FMO3 and Gly308-FMO3, previously reported in similar populations, were prominent polymorphisms. Two new polymorphisms, His132-FMO3 and Pro360-FMO3, were identified in this study. Both variants were found only in African Americans. To evaluate the effect of the amino acid substitutions on the function of FMO3, each amino acid substitution was introduced by site-directed mutagenesis into a wild-type FMO3 cDNA. Selective functional activity was studied with methimazole, trimethylamine, and 10-(N,N-dimethylaminopentyl)-2-(trifluoromethyl) phenothiazine. Both His132-FMO3 and Pro360-FMO3 variants were able to metabolize the substrates examined. Compared with wild-type FMO3, the His132-FMO3 was less catalytically efficient. The His132-FMO3 variant moderately altered the catalytic efficiency of FMO3 (decrease of 30%, 60% and 6% with methimazole, trimethylamine and 10-(N,N-dimethylaminopentyl)-2-(trifluoromethyl)phenothiazine, respectively). The Pro360-FMO3 variant was more catalytically efficient than wild-type FMO3. Pro360-FMO3 oxygenated methimazole, trimethylamine and 10-(N,N-dimethylaminopentyl)-2-(trifluoromethyl)phenothiazine, respectively, 3-, 5- and 2-fold more efficiently than wild-type FMO3. Based on the functional activity of the variant FMO3 enzymes, it is likely that population differences exist for compounds primarily metabolized by FMO3.

Diagnosis and management of Graves' disease

Graves' disease is characterized by hyperthyroidism, diffuse goitre, ophthalmopathy and, rarely, dermopathy. Although diagnostic testing is straightforward once Graves' disease is suspected, physicians need to be aware of heterogeneous and even atypical presentations of the disease, particularly in elderly patients. Because morbidity may be associated with even subtle forms of hyperthyroidism, treatment promoting long-term euthyroidism is necessary. Although all of the available treatments are effective, compliance is best assured by a full discussion of the risks and benefits of each approach. This review focuses on issues of diagnosis and management that will allow the primary care physician to identify patients with Graves' disease and guide them to recovery.

Bioavailability of transdermal methimazole in a pluronic lecithin organogel (PLO) in healthy cats

The antithyroid drug methimazole is widely used for the medical management of feline hyperthyroidism. Recently, custom veterinary pharmacies have offered methimazole in a transdermal gel containing pluronic and lecithin (PLO), with anecdotal evidence of efficacy. The purpose of this study was to determine the bioavailability, relative to i.v. and oral routes of administration, of transdermal methimazole in a PLO gel in cats. Six healthy adult cats were assigned to receive 5 mg of methimazole by the i.v., oral, or transdermal routes, in a randomized triple crossover protocol with 1 week washout between doses. Blood samples were taken for high performance liquid chromatography (HPLC) determination of serum methimazole, at 0, 5, 15, 30, 60 min, and 2, 4, 6, 12 and 24 h after dosing. Methimazole absorption following transdermal administration was poor and variable, with only two of six cats achieving detectable serum methimazole concentrations at any time point following transdermal administration. Area under the concentration-time curve (AUC), maximum concentration (Cmax), and absolute bioavailability were all significantly lower for the transdermal route (0.39 +/- 0.63 microg h/mL, 0.05 +/- 0.09 microg/mL, and 11.4 +/- 18.7%, respectively) than for either i.v. (7.96 +/- 4.38 microg h/mL, 3.34 +/- 2.00 microg/mL, 100%) or oral routes (2.94 +/- 1.24 microg h/mL, 0.51 +/- 0.15 microg/mL, 40.4 +/- 8.1%). The results of this study indicate generally low to undetectable bioavailability of methimazole in a lecithin/pluronic gel given as a single transdermal dose to healthy cats, although one individual cat did achieve nearly 100% transdermal bioavailability relative to the oral route.

Thyroid function and intellectual development of infants nursed by mothers taking methimazole

For many years, breast-feeding was forbidden if antithyroid drugs were being used. Recently, limited studies have shown the relative safety of propylthiouracil and methimazole (MMI). It is not known whether MMI therapy of lactating mothers for 1 yr is safe for breast-fed infants and does not cause alterations in thyroid function and intellectual development. Between 1988 and 1998, 139 thyrotoxic lactating mothers and their infants were studied. Fifty-one thyrotoxic lactating mothers were treated with MMI during pregnancy, and MMI was continued during breast-feeding. Eighty-eight mothers were given 10 mg MMI (n 46) or 20 mg MMI (n = 42) daily for 1 month, 10 mg daily for the second month, and 5-10 mg daily thereafter. Serum T4, T3, and TSH concentrations were measured in thyrotoxic lactating mothers and their infants, before and at 1, 2, 4, 8, and 12 months. Serum MMI was measured in the infants of thyrotoxic lactating mothers taking 20 mg MMI. Thyroid function, urinary iodine, thyroid antibodies, intelligence quotient (IQ), verbal and functional components (Wechsler and Goodenough tests) were performed on 14 children of thyrotoxic lactating mothers between 48 and 74 months of age and on 17 controls. Mean +/- SD of FT4I in thyrotoxic lactating mothers treated with 10 mg MMI for 1 month decreased from 19.4 +/- 4.1 to 11.6 +/- 4.4 and from 20.5 +/- 4.7 to 9.8 +/- 1.5 when treated with 20 mg MMI. Values for FT3I decreased from 462 +/- 52 to 194 +/- 52 with 10 mg MMI and from 481 +/- 92 to 171 +/- 38 with 20 mg MMI. FT4I and FT3I were normal from the third to the twelfth months. In all infants FT4I, FT3I, and TSH concentrations were normal before and up to 12 months of MMI therapy in their lactating mothers. The lowest T4 and T3 values were 108 and 1.87 nmol/L, and the highest TSH value was 4.0 mU/L. Serum MMI levels in infants were less than 0.03 microg/mL. Six mothers receiving 20 mg MMI had increased serum TSH concentrations ranging from 26-135 mU/L after 1 month of treatment. Their infants were euthyroid with serum TSH values less than 2.6 mU/L. At 48-74 months of age, height, weight, FT4I, FT3I, TSH, and antithyroid antibody titers were not different than controls. The mean IQ was 107 +/- 14 vs. 106 +/- 16 (Goodenough test) and 103 +/- 10 vs. 103 +/- 16 (Wechsler test) for infants of thyrotoxic lactating mothers and control infants, respectively. Similarly, there was no difference in verbal and performance IQ and their components between infants of thyrotoxic lactating mothers and control children. No deleterious effects occur in thyroid function and physical and intellectual development of breast-fed infants whose lactating mothers were treated with doses of MMI up to 20 mg daily.

Methimazole toxicity in rodents: covalent binding in the olfactory mucosa and detection of glial fibrillary acidic protein in the olfactory bulb

Methimazole is an antithyroid drug reported to affect the sense of smell and taste in humans. The aim of the present study was to examine the distribution and effects of methimazole on the olfactory system in rodents. Autoradiography showed a selective covalent binding of 3H-labeled methimazole in the Bowman's glands in the olfactory mucosa, bronchial epithelium in the lungs, and centrilobular parts of the liver following an iv injection in mice. Histopathology showed an extensive lesion in the olfactory mucosa that was efficiently repaired 3 months after two consecutive ip doses of methimazole. The effect of methimazole on various brain regions was studied by determining levels and location of glial fibrillary acidic protein (GFAP). The results showed a threefold increase of GFAP in the olfactory bulb 2 weeks after treatment with methimazole whereas no change was observed 4 days after treatment. Pretreatment of mice with thyroxine did not protect against the methimazole-induced toxicity in the olfactory mucosa and bulb. In contrast, pretreatment with the cytochrome P450 inhibitor metyrapone completely prevented the covalent binding and toxicity of methimazole in the olfactory mucosa and bulb. The present results suggest that the methimazole-induced toxicity in the olfactory mucosa is mediated by a cytochrome P450-dependent metabolic activation of the compound into reactive metabolites that are bound to various tissues including the olfactory mucosa. The increase of GFAP in the olfactory bulb of methimazole-treated mice is suggested to be a secondary phenomenon due to the primary damage in the olfactory mucosa.

Methimazole and propylthiouracil equally cross the perfused human term placental lobule

Propylthiouracil (PTU) is widely believed to cross the placenta less freely than methimazole (MMI) and is therefore regarded as the preferred drug for treatment of hyperthyroidism in pregnancy. Clinical studies comparing the two drugs show, however, no differences in maternal or fetal thyroid function. We investigated transfer from the maternal to the fetal circuit in the isolated perfused term human placental lobule of low and high doses of PTU (4 micrograms/mL and 40 micrograms/mL) and MMI (1.5 micrograms/mL and 15 micrograms/mL) in protein-free perfusate and low doses of both drugs with addition of 40 g/L of bovine albumin. Both drugs readily crossed the placenta, reaching equilibrium in all experiments in about 2 h. Drug concentrations in the two circuits fitted a two compartmental model. Transfer kinetics for the two drugs were similar, nonsaturable, and unaffected by addition of albumin. Clearances (mL.min-1.g-1, means +/- SD) of PTU from maternal to fetal circuits were: 0.229 +/- 0.110, 0.216 +/- 0.065, and 0.170 +/- 0.032; and for transfer of MMI: 0.165 +/- 0.025, 0.232 +/- 0.153, and 0.174 +/- 0.009 (for low doses without, low doses with, and high doses without albumin, respectively). Clearances of PTU from fetal to maternal circuits were: 0.147 +/- 0.072, 0.109 +/- 0.014, and 0.116 +/- 0.028; and for transfer of MMI: 0.095 +/- 0.029, 0.122 +/- 0.088, and 0.12 +/- 0.005 (in the same experiments). There was no significant difference between drugs or drug doses and no effect of addition of albumin. We conclude that PTU and MMI have similar placental transfer kinetics.

A hyperthyroid patient with Graves' disease who was strongly resistant to methimazole: investigation on possible mechanisms of the resistance

We treated a patient who was hyperthyroid due to Graves' disease and strongly resistant to methimazole (MMI): in spite of good compliance, she needed 150 mg of MMI daily to control her hyperthyroidism. To elucidate the reasons of resistance to MMI, her serum and intrathyroidal MMI concentrations were determined by high pressure liquid chromatography (HPLC). After taking a 30 mg dose of MMI, she had a similar serum MMI concentration-time curve to that of a normal subject: drug malabsorption and rapid drug metabolism were not evident when studied after surgical treatment. After her serum containing MMI was incubated with Protein G, the MMI concentration of the fraction not bound to Protein G did not change significantly from that of untreated serum: the possibility of anti-MMI IgG antibody production was considered unlikely. Furthermore, the intrathyroidal concentration of MMI in a surgically obtained tissue specimen was 3 micrograms/g wet tissue and appeared to be comparable with those of other Graves' tissues reported. Considering that the patient had been taking 150 mg per day of MMI by the time of thyroidectomy, her intrathyroidal MMI concentration was relatively low, suggesting possible impairment of intrathyroidal MMI accumulation. The possibilities of impaired intrathyroidal actions and the severity of hyperthyroidism, especially high T3 levels, also remained as possible causes. In conclusion, here was a severely hyperthyroid patient who was poorly responsive to conventional doses of MMI, and impairment of thyroid uptake of MMI or of pathways after uptake were considered as possible mechanisms.

Methimazole has no dose-related effect on the intensity of the intrathyroidal autoimmune process in relapsing Graves' disease

Declining thyroid autoantibodies during treatment and decreased lymphocytic infiltration after treatment of patients with Graves' disease suggest immunosuppressive actions of antithyroid drugs. However, the recent report of similar relapse rates after low and high dose carbimazole treatment of Graves' disease seems to contradict the immunosuppression thesis. We therefore determined the intrathyroidal methimazole concentrations with a high performance liquid chromatography method in 17 patients undergoing subtotal thyroid resection for relapsing Graves' disease. The intensity of the intrathyroidal infiltration by immunoglobulin G-producing plasma cells, activated T cells, and antigen presenting cells, and the total number of lymphocytes were identified immunohistologically with monoclonal antibodies for kappa- and lambda-immunoglobulin light chains, UCHL1, and the S100 antibody, respectively, followed by morphometry. The intrathyroidal methimazole concentration and the cumulative preoperative methimazole doses did not correlate with the intensity of the intrathyroidal infiltration by any of these immunocompetent cells. Comparison of groups with significantly different intrathyroidal methimazole concentrations (134 ng/g, n = 8 vs. 993 ng/g, n = 7) showed no significant differences for any of the intrathyroidal immunocompetent cells. These findings suggest that there is no dose-related effect of methimazole on the intensity of the intrathyroidal autoimmune process of patients with relapsing Graves' disease. They provide an explanation for why it does not seem justifiable to recommend higher methimazole doses than those required for the control of hyperthyroidism with the goal of immunosuppression.

Pharmacokinetics and short-term clinicopathologic changes after intravenous administration of a high dose of methimazole in dogs

A bolus dose of methimazole (MMI) was administered IV over 1 minute to 5 healthy adult dogs at a dosage (40 mg/kg of body weight) known to impart protection against cisplatin-induced renal disease. Blood and urine samples for pharmacokinetic analysis were collected over a 24-hour period. Physical examination, CBC, determination of serum thyroid hormone concentrations, and serum biochemistry analysis were performed over a 10-day period to evaluate short-term toxicoses. At this dosage, MMI appears to be safe and well tolerated in dogs; only 1 of the 5 dogs had mild and transient increases in serum activity of hepatic enzymes. In addition, MMI did not alter serum thyroid hormone concentrations. Half-life of 8.82 hours and mean residence time of 12.18 hours were determined for MMI. Renal clearance of native MMI, along with sulfate and glucuronide conjugates, represented only 20% of total systemic clearance. Results of this study provide further information concerning clinical use of MMI in dogs and may contribute to better understanding of the mechanism of MMI protection against chemically induced nephrotoxicosis.

[In vivo nitrosation of methimazole in humans]

Monitoring the excretion of the non-carcinogenic nitroso-methylaminoantipyrine (NMAA) after peroral applications of the analgesic drug metamizole is a useful procedure for evaluating endogenous nitrosation, also in man. The outstanding nitrosatibility of this drug led to high amounts of excreted NMAA, easily detected without derivatisation by HPLC. By nitrate ingestion considered as normal dietary intake the nitrosation is scarcely modified by ascorbic acid, sodium-bicarbonate or sodium-bicarbonate plus metronidazole. These results are an other indication of a third, cell-mediated path of endogeneous nitrosation in man.

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.

Pharmacokinetics of intravenous and oral methimazole following single- and multiple-dose administration in normal cats

The pharmacokinetics of methimazole (MMI) administered intravenously and orally were determined in six adult domestic shorthaired cats. There was no significant difference between mean serum MMI concentrations after oral and i.v. administration by 30 min post-MMI administration, indicating relatively rapid and complete absorption of the drug. The bioavailability of MMI ranged from 27% to 100% (mean = 81.1 +/- 11.4%). The mean serum elimination half-life was 6.6 +/- 2.0 h, with a wide range of values (1.9 h to 15.1 h). After repeat i.v. administration of MMI following 2 weeks of oral administration of the drug, no significant difference was found between mean serum concentrations after single-dose and multiple-dose administration. No significant change in serum elimination half-life or total body clearance was found after multiple-dose administration of MMI. Two cats with the longest half-lives (9.9 h and 15.1 h), however, did exhibit markedly shorter t1/2 values (3.5 h and 3.3 h, respectively) after multiple-dose administration. Values for central and steady state volumes of distribution also decreased after multiple-dose administration, possibly indicating saturation of thyroid uptake of MMI with chronic administration. These results indicate that MMI has good oral bioavailability and has a longer mean serum elimination half-life than propylthiouracil, the other anti-thyroid drug that has been evaluated in cats. Although no significant change in mean values occurred after multiple-dose administration of MMI, drug-induced acceleration of metabolism may occur in some cats after long-term MMI administration.

[Comparison of thiamazole pharmacokinetics in healthy individuals and patients with hyperthyroidism]

Pharmacokinetic parameters of thiamazole in hyperthyroid patients (40 subjects with Graves-Basedow disease--32 female and 8 male patients) and in healthy individuals (8 subjects--5 women and 3 men) were compared. A one-compartment model was used for the analysis of examined pharmacokinetic parameters. Area under thiamazole concentration curve (AUC) and thiamazole peak plasma concentration (cmax) were significantly decreased in hyperthyroid patients in comparison with healthy individuals. An analysis of other pharmacokinetic parameters suggests that observed differences seem to depend upon lowered and retarded thiamazole absorption from the gut and acceleration of drug metabolism in hepatic microsomal system in hyperthyroid patients.

[Relation between the dosage of methimazole and its intrathyroidal concentrations]

Intrathyroidal concentrations of methimazole (MMI) were measured by means of high-performance liquid chromatography in 46 patients with Graves' disease who had undergone thyroidectomy. In this experiment, the dose of the drug varied from 5 mg/day to 40 mg/day, and the interval between the last dose and surgery ranged from 2 to 26 hours, and the mode of drug administration varied from a single dose to four divided doses. With dose of MMI from 5 to 15 mg/day, intrathyroid levels of MMI increased with increasing dose, but without significant increase when the dose was above 15 mg/day. There was no difference between patients taking the drug in divided doses and those taking it in one single dose of the same amount, or between patients taking the final dose 2 hours before thyroid excision aud those taking it 26 hours before excision. Our results indicate a saturable uptake mechanism for MMI by the thyroid, and MMI 15 mg/day is the most satisfactory dose for treating hyperthyroidism. The intrathyroidal concentrations are maintained for 26 hours at least, and the single daily dose of MMI is effective and reasonable for treatment of Graves' disease.

Pharmacokinetics of methimazole in normal cats and cats with hyperthyroidism

The intravenous and oral disposition of the antithyroid drug methimazole was determined in 10 clinically normal cats and nine cats with naturally occurring hyperthyroidism. After intravenous administration of 5 mg methimazole, the mean residence time was significantly (P less than 0.05) shorter in the cats with hyperthyroidism than in the normal cats, but there was no significant difference between the mean values for total body clearance (CL), steady state volume of distribution (Vdss), terminal elimination rate constant (ke), or serum terminal half-life (t1/2) in the two groups of cats. After oral administration, the mean bioavailability of methimazole was high in both the normal cats (77.6 per cent) and cats with hyperthyroidism (79.5 per cent). The values for mean residence time, ke and serum terminal t1/2 after oral dosing were significantly shorter in the cats with hyperthyroidism than in the normal cats. However, after oral administration of methimazole there were no significant differences between the mean values for CL, Vdss, bioavailability and maximum serum concentrations or the time for maximal concentrations to be reached in the two groups of cats. Overall, most pharmacokinetic parameters for methimazole were not altered by the hyperthyroid state. However, the cats with hyperthyroidism did show a trend toward faster elimination of the drug compared with the normal cats, similar to what has been previously described for the antithyroid drug propylthiouracil in cats. These results also indicate that methimazole is well absorbed when administered orally and has a higher bioavailability than that of propylthiouracil in cats with hyperthyroidism.(ABSTRACT TRUNCATED AT 250 WORDS)

[Acetylation phenotype in Graves' disease]

Acetylation phenotype was determined with sulfamidine technique in 25 patients with Graves-Basedow disease including 19 female and 6 male patients aged between 21 and 64 years. The control group consisted of 23 healthy volunteers (8 female and 15 male individuals aged between 28 and 57 years) from the Szczecin province . Acetylation phenotype was determined before and after therapy with thiamazole++. Prevalence of fast acetylators was noted in the patients with Graves-Basedow disease in comparison with healthy population. The administered treatment decreased the percentage of fast acetylators. Fast acetylators responded to the treatment earlier achieving clinical euthyreosis.

Metabolism of 35S- and 14C-labeled 1-methyl-2-mercaptoimidazole in vitro and in vivo

We previously described an in vitro incubation system for studying the mechanism of inhibition of thyroid peroxidase (TPO)-catalyzed iodination by the antithyroid drug 1-methyl-2-mercaptoimidazole (MMI). Inhibition of iodination in this system may be reversible or irreversible, depending on the relative concentrations of iodide and MMI and on the TPO concentration. Metabolism of the drug occurs under both conditions, and in the present investigation we used 35S- and 14C-labeled MMI together with reverse phase HPLC to examine the metabolic products associated with reversible and irreversible inhibition of iodination by MMI. Under conditions of reversible inhibition, MMI was rapidly metabolized and disappeared completely from the incubation mixture. With [35S]MMI, the earliest detectable 35S-labeled product was MMI disulfide, which reached a peak after a few minutes and then declined to undetectable levels. Coincident with the decrease in disulfide was the appearance of two 35S peaks, the major one corresponding to sulfate/sulfite, and the other to a component eluting at 7.5 min. Similar results were obtained for the disulfide and for the 7.5 min metabolite with [14C]MMI. The major 14C-labeled metabolite containing no S appeared to be 1-methylimidazole. Under conditions of irreversible inhibition, MMI disulfide was also the earliest detectable 35S-labeled metabolite. However, MMI decreased more slowly, and after reaching a nadir at about 6 min returned gradually to a level about halfway between the initial and the minimum value. The reformation of MMI appeared to involve the nonenzymatic disproportionation of MMI disulfide. Formation of the 7.5 min peak was also observed, but there was no formation of sulfate/sulfite. The difference in metabolic pattern between the reversible and irreversible conditions is primarily related to the rapid inactivation of TPO that occurs under irreversible conditions. The metabolism of [35S]MMI in thyroids of rats injected with the labeled drug resembles more closely conditions of reversible inhibition, since sulfate/sulfite is the only 35S-labeled metabolite. Neither [35S]MMI disulfide nor the 7.5 min component was detected in rat thyroids in vivo. However, it was demonstrated that these components do not survive homogenization with thyroid tissue, and failure to detect them in vivo does not exclude them as likely intermediates in intrathyroidal MMI metabolism. Based on the observations reported in this study, we present a revised scheme for the mechanism of inhibition of TPO-catalyzed iodination by MMI.