Methylthiouracil targets the thyroid peroxidase enzyme system within the thyroid gland. By inhibiting thyroid peroxidase, it blocks the iodination of tyrosine residues in thyroglobulin, thereby reducing the production of thyroxine (T4) and triiodothyronine (T3). This decreases the formation of stored thyroid hormone. Methylthiouracil also suppresses the production of pro-inflammatory cytokines TNF-α and IL-6, and inhibits the activation of NF-κB and ERK1/2 signaling pathways.

Following a 4-hour addition of LPS (100 ng/mL), HUVEC were exposed to varying MTU doses (0–20 μM) for six hours. MTU suppresses endothelial cells' LPS-mediated hyperpermeability; its actions are most effective at concentrations greater than 5 μM. Using F-actin-labeled fluorescein phalloidin for immunofluorescence labeling of HUVEC monolayers, the impact of MTU on the actin cytoskeletal structure of the cells was investigated. F-actin was dispersed randomly throughout the control HUVEC, with actin filament bundles localized at cell borders. In HUVECs, barrier breakdown caused by LPS (100 ng/mL) showed up as the creation of paracellular gaps. Moreover, thick F-actin rings were generated and LPS-induced paracellular spaces were prevented by post-treatment with MTU (10 or 20 μM). A cell survival assay was conducted in HUVEC treated with MTU for 24 hours in order to evaluate the cytotoxicity of the drug. At doses as high as 20 μM, MTU has no effect on cell viability [1].
In vitro, Methylthiouracil suppresses LPS-mediated hyperpermeability in HUVEC endothelial cells, with effects most effective at concentrations greater than 5 μM. It prevents LPS-induced paracellular gap formation and the generation of thick F-actin rings. At doses as high as 20 μM, MTU has no effect on cell viability. It suppresses the production of TNF-α and IL-6, and inhibits the activation of NF-κB and ERK1/2. Methylthiouracil decreases the formation of stored thyroid hormone as thyroglobulin in the thyroid gland.

Methylthiouracil can be used to create models of brain and cardiovascular diseases in animals.
In vivo, Methylthiouracil can be used to create models of brain and cardiovascular diseases in animals. Following intravenous injection of 5 mg/rat MTU, 84-90% of the dose can be recovered from the carcass within 1 minute. After 3 hours, the concentration of MTU in the thyroid gland is approximately 1 mg/g tissue. Antithyroid drugs can cross the placental barrier and are detectable in breast milk. The usual clinical dose for hyperthyroidism is 200 mg/day in two to four equally spaced doses.

In vitro enzyme/receptor binding studies for Methylthiouracil typically involve thyroid peroxidase inhibition assays. Thyroid peroxidase enzyme is incubated with iodide and tyrosine or thyroglobulin in the presence of increasing concentrations of MTU, and enzyme activity is measured by monitoring iodide oxidation or iodothyronine formation. Standard protocols use spectrophotometric or radiometric detection methods. The compound's mechanism involves blocking the iodination of tyrosine residues in thyroglobulin. Similar to other thionamides, it inhibits the thyroid peroxidase enzymes involved in thyroid hormone synthesis.

In vitro cellular assays for Methylthiouracil involve culturing HUVEC endothelial cells or other cell types in the presence of serial dilutions of the compound. Cells are stimulated with LPS (100 ng/mL) and treated with MTU (0-20 μM) for 6 hours. Endothelial barrier function is assessed by measuring paracellular permeability and F-actin cytoskeletal organization using fluorescent phalloidin labeling. Cell viability is assessed using standard MTT or cell survival assays. Cytokine production (TNF-α, IL-6) is measured by ELISA, and NF-κB and ERK1/2 activation is assessed by Western blotting.

In vivo animal studies for Methylthiouracil involve administration to rats or other animals to evaluate antithyroid effects and create disease models. Following intravenous injection of 5 mg/rat MTU, tissue distribution and pharmacokinetics are assessed. Thyroid hormone levels are measured in serum, and thyroid gland histology is examined to evaluate goitrogenic effects. The compound can be used to create models of brain and cardiovascular diseases. Standard protocols include dose-response assessment, comparison to vehicle or reference antithyroid drugs, and evaluation of safety parameters.

Absorption, Distribution and Excretion
Following intravenous injection of 5 mg/rat MTU, 84-90% of the dose can be recovered from the carcass within 1 minute, and 55-60% can be recovered from the carcass of animals euthanized after 3 hours. After 3 hours, the concentration of MTU in the thyroid gland is approximately 1 mg/g tissue. Antithyroid drugs can cross the placental barrier and are also detectable in breast milk. Metabolism/Metabolites 48 hours after oral administration of radioactive methylthiouracil, the metabolites excreted in rat urine are 6-methyluracil, 6-methyl-2-methylthiouracil, 6-methyl-4-oxopyrimidine, 2-amino-6-methyl-4-oxopyrimidine, and urea.
Pharmacokinetic properties of Methylthiouracil include rapid distribution following intravenous administration. After injection of 5 mg/rat MTU, 84-90% of the dose can be recovered from the carcass within 1 minute. After 3 hours, the concentration in the thyroid gland is approximately 1 mg/g tissue. Antithyroid drugs can cross the placental barrier and are detectable in breast milk. Following oral administration of radioactive MTU, metabolites excreted in rat urine include 6-methyluracil, 6-methyl-2-methylthiouracil, 6-methyl-4-oxopyrimidine, 2-amino-6-methyl-4-oxopyrimidine, and urea. The compound has a molecular weight of 142.18 g/mol and a formula of C₅H₆N₂OS.

Interactions
Dietary iodine content was negatively correlated with the incidence of MTU-induced thyroid tumors. Topical application of 9,10-dimethyl-1,2-benzanthracene promoted the induction of cervical and vaginal sarcomas and epithelial tumors by methylthiouracil. Simultaneous administration of methylthiouracil and carbon tetrachloride prevented carbon tetrachloride-induced chronic hepatotoxicity in rats. Theophylline enhanced the effect of thyroid-stimulating hormone at the level of the thyroid adenylate cyclase system. Concomitant administration of theophylline enhanced the goitrogenic effect of 0.05% methylthiouracil.
Methylthiouracil has significant toxicity concerns. According to California labor law, it may be carcinogenic. Dietary iodine content was negatively correlated with the incidence of MTU-induced thyroid tumors. Topical application of 9,10-dimethyl-1,2-benzanthracene promoted the induction of cervical and vaginal sarcomas and epithelial tumors by methylthiouracil. Simultaneous administration of MTU and carbon tetrachloride prevented carbon tetrachloride-induced chronic hepatotoxicity in rats. Theophylline enhanced the goitrogenic effect of 0.05% MTU. It is a white crystalline powder with an onion-like odor and a bitter taste.

[1]. Anti-inflammatory effects of methylthiouracil in vitro and in vivo. Toxicol Appl Pharmacol. 2015 Nov 1;288(3):374-86.

According to California labor law, methylthiouracil may be carcinogenic. Methylthiouracil is a white crystalline powder with an onion-like odor and a bitter taste. Its saturated aqueous solution is neutral or slightly acidic. (NTP, 1992) Methylthiouracil is a pyrimidinone drug. It is a thionamide antithyroid drug that inhibits the synthesis of thyroid hormones. It is used to treat hyperthyroidism. Mechanism of Action: Thiamide derivatives (including methylthiouracil)...inhibit the synthesis and secretion of hormones until spontaneous remission occurs during the course of the disease. Therapeutic Uses: Antithyroid Drugs: Antithyroid drugs...are used to treat thyrotoxic crisis, or before or after radioactive iodine therapy. /Antithyroid Drugs/ After discontinuation, approximately 20% of patients experience a rapid relapse of hyperthyroidism within 2 months. ...Drug treatment appears to reduce the incidence of progressive eye disease. /Antithyroid Drugs/
For more complete data on the therapeutic uses of methylthiouracil (6 types), please visit the HSDB record page.

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Drug Warnings
...The incidence of side effects with methylthiouracil is approximately six times that of propylthiouracil.
The side effects of this drug...are similar to those of propylthiouracil...The most serious side effects...include agranulocytosis, drug fever, and dermatitis. Joint pain and urticaria may occur.
Cross-sensitivity reactions with other thionamide drugs may occur.
...Less common complications include...paresthesia, headache, nausea, and hair loss or depigmentation. ...Hepatitis and nephritis are very rare. /Thionamide Drugs/
...When methylthiouracil is used to treat hyperthyroidism, nystagmus and visual illusions of motion of the surrounding environment have been reported, accompanied by severe vertigo...
Methylthiouracil has been reported to affect taste and smell. /Excerpt from Table/
Methylthiouracil is a thionamide antithyroid drug introduced in the mid-1940s for the treatment of hyperthyroidism. Its CAS number is 56-04-2. It is also known as 6-methyl-2-thiouracil and MTU. It is no longer marketed in many countries and has been largely replaced by other antithyroid drugs such as propylthiouracil and methimazole. The usual dose is 200 mg/day in two to four equally spaced doses. It remains a research compound for studying thyroid function, inflammation, and disease models. It is not approved for clinical use in many jurisdictions but is available for research purposes.

C5H6N2OS

142.1789

142.02

56-04-2

667493

White to off-white solid powder

1.4±0.1 g/cm3

342.3ºC at 760 mmHg

~330 °C (dec.)(lit.)

160.8ºC

1.638

0.31

2

2

0

9

197

0

HWGBHCRJGXAGEU-UHFFFAOYSA-N

InChI=1S/C5H6N2OS/c1-3-2-4(8)7-5(9)6-3/h2H,1H3,(H2,6,7,8,9)

6-methyl-2-sulfanylidene-1H-pyrimidin-4-one

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : ≥ 50 mg/mL (~351.67 mM)

Solubility in Formulation 1: ≥ 2.5 mg/mL (17.58 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (17.58 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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 (Please use freshly prepared in vivo formulations for optimal results.)