Tioguanine

Last updated
Tioguanine
Tioguanine.svg
Tioguanine 3D spacefill.png
Clinical data
Trade names Lanvis, Tabloid, others
AHFS/Drugs.com International Drug Names
MedlinePlus a682099
License data
Routes of
administration 		By mouth
ATC code
Legal status
Legal status
Pharmacokinetic data
Bioavailability 30% (range 14% to 46%)
Metabolism Intracellular
Elimination half-life 80 minutes (range 25–240 minutes)
Identifiers
  • 2-amino-1H-purine-6(7H)-thione
CAS Number
PubChem CID
IUPHAR/BPS
DrugBank
ChemSpider
UNII
KEGG
ChEBI
ChEMBL
CompTox Dashboard (EPA)
ECHA InfoCard 100.005.299 OOjs UI icon edit-ltr-progressive.svg
Chemical and physical data
Formula C5H5N5S
Molar mass 167.19 g·mol−1
3D model (JSmol)
  • Nc2nc(=S)c1[nH]cnc1[nH]2
  • InChI=1S/C5H5N5S/c6-5-9-3-2(4(11)10-5)7-1-8-3/h1H,(H4,6,7,8,9,10,11) Yes check.svgY
  • Key:WYWHKKSPHMUBEB-UHFFFAOYSA-N Yes check.svgY
 X mark.svgNYes check.svgY  (what is this?)    (verify)

Tioguanine, also known as thioguanine or 6-thioguanine (6-TG) or tabloid is a medication used to treat acute myeloid leukemia (AML), acute lymphocytic leukemia (ALL), and chronic myeloid leukemia (CML). [2] Long-term use is not recommended. [2] It is given by mouth. [2]

Contents

Common side effects include bone marrow suppression, liver problems and inflammation of the mouth. [2] [3] It is recommended that liver enzymes be checked weekly when on the medication. [2] People with a genetic deficiency in thiopurine S-methyltransferase are at higher risk of side effects. [3] Avoiding pregnancy when on the medication is recommended. [2] Tioguanine is in the antimetabolite family of medications. [3] It is a purine analogue of guanine and works by disrupting DNA and RNA. [4]

Tioguanine was developed between 1949 and 1951. [5] [6] It is on the World Health Organization's List of Essential Medicines. [7]

Medical uses

Side effects

Hepatic veno-occlusive disease

The major concern that has inhibited the use of thioguanine has been veno-occlusive disease (VOD) and its histological precursor nodular regenerative hyperplasia (NRH). The incidence of NRH with thioguanine was reported as between 33 and 76%. [10] The risk of ensuing VOD is serious and frequently irreversible so this side effect has been a major concern. However, recent evidence using an animal model for thioguanine-induced NRH/VOD has shown that, contrary to previous assumptions, NRH/VOD is dose dependent and the mechanism for this has been demonstrated. [11] This has been confirmed in human trials, where thioguanine has proven to be safe but efficacious for coeliac disease when used at doses below those commonly prescribed. [12] This has led to a revival of interest in thioguanine because of its higher efficacy and faster action compared to other thiopurines and immunosuppressants such as mycophenylate. [13]

Contraindications

Interactions

Cancers that do not respond to treatment with mercaptopurine do not respond to thioguanine. On the other hand, some cases of IBD that are resistant to mercaptopurine (or its pro-drug azathioprine) may be responsive to thioguanine.

Pharmacogenetics

The enzyme thiopurine S-methyltransferase (TPMT) is responsible for the direct inactivation of thioguanine to its methylthioguanine base – this methylation prevents thioguanine from further conversion into active, cytotoxic thioguanine nucleotide (TGN) metabolites. [15] [16] [17] Certain genetic variations within the TPMT gene can lead to decreased or absent TPMT enzyme activity, and individuals who are homozygous or heterozygous for these types of genetic variations may have increased levels of TGN metabolites and an increased risk of severe bone marrow suppression (myelosuppression) when receiving thioguanine. [15] In many ethnicities, TPMT polymorphisms that result in decreased or absent TPMT activity occur with a frequency of approximately 5%, meaning that about 0.25% of patients are homozygous for these variants. [15] [18] However, an assay of TPMT activity in red blood cells or a TPMT genetic test can identify patients with reduced TPMT activity, allowing for the adjustment of thiopurine dose or avoidance of the drug entirely. [15] [19] The FDA-approved drug label for thioguanine notes that patients who are TPMT-deficient may be prone to developing myelosuppression and that laboratories offer testing for TPMT deficiency. [20] Indeed, testing for TPMT activity is currently one of the few examples of pharmacogenetics being translated into routine clinical care. [21]

Metabolism and pharmacokinetics

A single oral dose of thioguanine has incomplete metabolism, absorption and high interindividual variability. The bioavailability of thioguanine has an average of 30% (range 14-46%). The maximum concentration in plasma after a single oral dose is attained after 8 hours.

Thioguanine, like other thiopurines, is cytotoxic to white cells; as a result it is immunosuppressive at lower doses and anti-leukemic/anti-neoplastic at higher doses. Thioguanine is incorporated into human bone marrow cells, but like other thiopurines, it is not known to cross the blood-brain barrier. Thioguanine cannot be demonstrated in cerebrospinal fluid, similar to the closely related compound 6-mercaptopurine which also cannot penetrate to the brain.

The plasma half-life of thioguanine is short, due to the rapid uptake into liver and blood cells and conversion to 6-TGN. The median plasma half-life of 80-minutes with a range of 25–240 minutes. Thioguanine is excreted primarily through the kidneys in urine, but mainly as a metabolite, 2-amino-6-methylthiopurine. However, the intra-cellular thio-nucleotide metabolites of thioguanine (6-TGN) have longer half-lives and can therefore be measured after thioguanine is eliminated from the plasma.

Thioguanine is catabolized (broken down) via two pathways. [22] One route is through the deamination by the enzyme guanine deaminase to 6-thioxanthine, which has minimal anti-neoplastic activity, then by oxidation by xanthine oxidase of the thioxanthine to thiouric acid. This metabolic pathway is not dependent on the efficacy of xanthine oxidase, so that the inhibitor of xanthine oxidase, the drug allopurinol, does not block the breakdown of thioguanine, in contrast to its inhibition of the breakdown of the related thiopurine 6-mercaptopurine. The second pathway is the methylation of thioguanine to 2-amino-6-methylthiopurine, which is minimally effective as an anti-neoplastic and significantly less toxic than thioguanine. This pathway also is independent of the enzyme activity of xanthine oxidase.

Mechanism of action

6-Thioguanine is a thio analogue of the naturally occurring purine base guanine. 6-thioguanine utilises the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) to be converted to 6-thioguanosine monophosphate (TGMP). High concentrations of TGMP may accumulate intracellularly and hamper the synthesis of guanine nucleotides via the enzyme Inosine monophosphate dehydrogenase (IMP dehydrogenase), leading to DNA mutations. [23]

TGMP is converted by phosphorylation to thioguanosine diphosphate (TGDP) and thioguanosine triphosphate (TGTP). Simultaneously deoxyribosyl analogs are formed, via the enzyme ribonucleotide reductase. The TGMP, TGDP and TGTP are collectively named 6-thioguanine nucleotides (6-TGN). 6-TGN are cytotoxic to cells by: (1) incorporation into DNA during the synthesis phase (S-phase) of the cell; and (2) through inhibition of the GTP-binding protein (G protein) Rac1, which regulates the Rac/Vav pathway. [24]

Chemistry

It is a pale yellow, odorless, crystalline powder.

Names

Tioguanine (INN, BAN, AAN), or thioguanine (USAN).

Thioguanine is administered by mouth (as a tablet – 'Lanvis').

Related Research Articles

<span class="mw-page-title-main">Xanthine</span> Chemical compound

Xanthine is a purine base found in most human body tissues and fluids, as well as in other organisms. Several stimulants are derived from xanthine, including caffeine, theophylline, and theobromine.

<span class="mw-page-title-main">Xanthine oxidase</span> Class of enzymes

Xanthine oxidase is a form of xanthine oxidoreductase, a type of enzyme that generates reactive oxygen species. These enzymes catalyze the oxidation of hypoxanthine to xanthine and can further catalyze the oxidation of xanthine to uric acid. These enzymes play an important role in the catabolism of purines in some species, including humans.

<span class="mw-page-title-main">Phosphodiesterase</span> Class of enzymes

A phosphodiesterase (PDE) is an enzyme that breaks a phosphodiester bond. Usually, phosphodiesterase refers to cyclic nucleotide phosphodiesterases, which have great clinical significance and are described below. However, there are many other families of phosphodiesterases, including phospholipases C and D, autotaxin, sphingomyelin phosphodiesterase, DNases, RNases, and restriction endonucleases, as well as numerous less-well-characterized small-molecule phosphodiesterases.

<span class="mw-page-title-main">Allopurinol</span> Medication

Allopurinol is a medication used to decrease high blood uric acid levels. It is specifically used to prevent gout, prevent specific types of kidney stones and for the high uric acid levels that can occur with chemotherapy. It is taken orally or intravenously.

<span class="mw-page-title-main">Azathioprine</span> Immunosuppressive medication

Azathioprine, sold under the brand name Imuran, among others, is an immunosuppressive medication. It is used for the treatment of rheumatoid arthritis, granulomatosis with polyangiitis, Crohn's disease, ulcerative colitis, and systemic lupus erythematosus; and in kidney transplants to prevent rejection. It is listed by the International Agency for Research on Cancer as a group 1 human carcinogen. It is taken by mouth or injected into a vein.

Drug metabolism is the metabolic breakdown of drugs by living organisms, usually through specialized enzymatic systems. More generally, xenobiotic metabolism is the set of metabolic pathways that modify the chemical structure of xenobiotics, which are compounds foreign to an organism's normal biochemistry, such as any drug or poison. These pathways are a form of biotransformation present in all major groups of organisms and are considered to be of ancient origin. These reactions often act to detoxify poisonous compounds. The study of drug metabolism is the object of pharmacokinetics. Metabolism is one of the stages of the drug's transit through the body that involves the breakdown of the drug so that it can be excreted by the body.

<span class="mw-page-title-main">Mercaptopurine</span> Chemical compound

Mercaptopurine (6-MP), sold under the brand name Purinethol among others, is a medication used for cancer and autoimmune diseases. Specifically it is used to treat acute lymphocytic leukemia (ALL), acute promyelocytic leukemia (APL), Crohn's disease, and ulcerative colitis. For acute lymphocytic leukemia it is generally used with methotrexate. It is taken orally.

<span class="mw-page-title-main">Antimetabolite</span> Chemical that inhibits the use of a metabolite

An antimetabolite is a chemical that inhibits the use of a metabolite, which is another chemical that is part of normal metabolism. Such substances are often similar in structure to the metabolite that they interfere with, such as the antifolates that interfere with the use of folic acid; thus, competitive inhibition can occur, and the presence of antimetabolites can have toxic effects on cells, such as halting cell growth and cell division, so these compounds are used in chemotherapy for cancer.

<span class="mw-page-title-main">History of cancer chemotherapy</span>

The era of cancer chemotherapy began in the 1940s with the first use of nitrogen mustards and folic acid antagonist drugs. The targeted therapy revolution has arrived, but many of the principles and limitations of chemotherapy discovered by the early researchers still apply.

<span class="mw-page-title-main">Thiopurine methyltransferase</span>

Thiopurine methyltransferase or thiopurine S-methyltransferase (TPMT) is an enzyme that in humans is encoded by the TPMT gene. A pseudogene for this locus is located on chromosome 18q.

<span class="mw-page-title-main">CYP2C9</span> Enzyme protein

Cytochrome P450 family 2 subfamily C member 9 is an enzyme protein. The enzyme is involved in the metabolism, by oxidation, of both xenobiotics, including drugs, and endogenous compounds, including fatty acids. In humans, the protein is encoded by the CYP2C9 gene. The gene is highly polymorphic, which affects the efficiency of the metabolism by the enzyme.

Purine metabolism refers to the metabolic pathways to synthesize and break down purines that are present in many organisms.

<span class="mw-page-title-main">Febuxostat</span> Chemical compound

Febuxostat, sold under the brand name Uloric among others, is a medication used long-term to treat gout due to high uric acid levels. It is generally recommended only for people who cannot take allopurinol. It is taken by mouth.

Histamine <i>N</i>-methyltransferase Mammalian enzyme involved in the metabolism of histamine

Histamine N-methyltransferase (HNMT) is a protein encoded by the HNMT gene in humans. It belongs to the methyltransferases superfamily of enzymes and plays a role in the inactivation of histamine, a biomolecule that is involved in various physiological processes. Methyltransferases are present in every life form including archaeans, with 230 families of methyltransferases found across species.

<span class="mw-page-title-main">Thiopurine</span> Class of chemical compounds

The thiopurine drugs are purine antimetabolites widely used in the treatment of acute lymphoblastic leukemia, autoimmune disorders, and organ transplant recipients.

Purine analogues are antimetabolites that mimic the structure of metabolic purines.

<span class="mw-page-title-main">UGT1A9</span> Protein-coding gene in the species Homo sapiens

UDP-glucuronosyltransferase 1-9 is an enzyme that in humans is encoded by the UGT1A9 gene.

<span class="mw-page-title-main">Thiouric acid</span> Chemical compound

Thiouric acid, more accurately called 6-thiouric acid, is a main inactive metabolite of the immunosuppressive drugs azathioprine, mercaptopurine and tioguanine.

<span class="mw-page-title-main">Cancer pharmacogenomics</span>

Cancer pharmacogenomics is the study of how variances in the genome influences an individual’s response to different cancer drug treatments. It is a subset of the broader field of pharmacogenomics, which is the area of study aimed at understanding how genetic variants influence drug efficacy and toxicity.

Howard L. McLeod is an American pharmacogeneticist and implementation scientist specialized in precision medicine.

References

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  2. 1 2 3 4 5 6 British National Formulary: BNF 69 (69th ed.). British Medical Association. 2015. pp. 588, 592. ISBN   978-0-85711-156-2.
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  12. ↑ Tack GJ, van Asseldonk DP, van Wanrooij RL, van Bodegraven AA, Mulder CJ (August 2012). "Tioguanine in the treatment of refractory coeliac disease--a single centre experience". Alimentary Pharmacology & Therapeutics. 36 (3): 274–281. doi: 10.1111/j.1365-2036.2012.05154.x . PMID   22646133. S2CID   24811114.
  13. ↑ Van Asseldonk DP, Oancea I, Jharap B, et al. (March 2012). "Is thioguanine-associated sinusoidal obstruction syndrome avoidable? Lessons learned from 6-thioguanine treatment of inflammatory bowel disease and a mouse model" (PDF). Journal of the Brazilian Medical Association. 58 (Suppl.1): S8–13.
  14. ↑ Gardiner SJ, Gearry RB, Roberts RL, Zhang M, Barclay ML, Begg EJ (October 2006). "Exposure to thiopurine drugs through breast milk is low based on metabolite concentrations in mother-infant pairs". British Journal of Clinical Pharmacology. 62 (4): 453–456. doi:10.1111/j.1365-2125.2006.02639.x. PMC   1885151 . PMID   16995866.
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  17. ↑ Fujita K, Sasaki Y (August 2007). "Pharmacogenomics in drug-metabolizing enzymes catalyzing anticancer drugs for personalized cancer chemotherapy". Current Drug Metabolism. 8 (6): 554–562. doi:10.2174/138920007781368890. PMID   17691917. Archived from the original on 2013-01-12.{{cite journal}}: CS1 maint: unfit URL (link)
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  24. ↑ de Boer NK, van Bodegraven AA, Jharap B, de Graaf P, Mulder CJ (December 2007). "Drug Insight: pharmacology and toxicity of thiopurine therapy in patients with IBD". Nature Clinical Practice. Gastroenterology & Hepatology. 4 (12): 686–694. doi:10.1038/ncpgasthep1000. PMID   18043678. S2CID   23686284.

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