The major concern that has inhibited the use of thioguanine has been veno-occlusive disease and its histological precursor nodular regenerative hyperplasia. The incidence of NRH with thioguanine was reported as between 33–76%. 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. 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. 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.
Contraindications
Pregnancy
Lactation: The safety warning against breastfeeding may have been a conservative assessment, but research evidence suggests that thiopurines do not enter breastmilk.
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 may be responsive to thioguanine.
Pharmacogenetics
The enzyme thiopurine S-methyltransferase is responsible for the direct inactivation of thioguanine to its methylthioguanine base – this methylation prevents thioguanine from further conversion into active, cytotoxic thioguanine nucleotide metabolites. 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 when receiving thioguanine. 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. 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. 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. Indeed, testing for TPMT activity is currently one of the few examples of pharmacogenetics being translated into routine clinical care.
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%. 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 have longer half-lives and can therefore be measured after thioguanine is eliminated from the plasma. Thioguanine is catabolized via two pathways. 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 to be converted to 6-thioguanosine monophosphate. High concentrations of TGMP may accumulate intracellularly and hamper the synthesis of guanine nucleotides via the enzyme Inosine monophosphate dehydrogenase. TGMP is converted by phosphorylation to thioguanosine diphosphate and thioguanosine triphosphate. Simultaneously deoxyribosyl analogs are formed, via the enzyme ribonucleotide reductase. The TGMP, TGDP and TGTP are collectively named 6-thioguanine nucleotides. 6-TGN are cytotoxic to cells by: incorporation into DNA during the synthesis phase of the cell; and through inhibition of the GTP-binding protein Rac1, which regulates the Rac/Vav pathway.
Chemistry
It is a pale yellow, odorless, crystalline powder.
Names
Tioguanine, or thioguanine. Thioguanine is administered by mouth.
