Thioguanine, Azathioprine, Mercaptopurine (Dosage)

The thiopurine family consists of purine metabolism antagonist drugs, widely used in the treatment of autoimmune diseases and some types of cancer. Thiopurine methyltransferase, encoded by the TPMT gene, is involved in their metabolism, so knowing the metabolizer profile can be useful for correct dosage adjustment and thus avoid the appearance of undesirable adverse effects.

Azathioprine

Azathioprine is an immunosuppressant derived from mercaptopurine that antagonizes purine metabolism. Its preferred use is in autoimmune diseases with insufficient response to corticosteroids, as well as in the prevention of rejection in transplant patients (associated with other immunosuppressants).

The most important side effects of azathioprine are bone marrow depression, gastrointestinal intolerance and hepatotoxicity. It should be used carefully in patients with gout or bacterial infections, and periodic hematological and biochemical controls should be performed.

Thioguanine

Thioguanine is an antineoplastic that also antagonizes purine metabolism. Upon activation it inhibits de novo purine synthesis as well as purine nucleotide interconversions. This drug is used in the treatment of acute leukemia.

The main side effects are related to gastrointestinal (nausea, vomiting, diarrhea), hepatobiliary (jaundice), metabolic (hyperuricemia) and hematological (myelosuppression) disorders.

Clinical implications

When there is some failure affecting these genes it can lead to TPMT deficiency, and with it, an abnormal metabolism of thiopurines. In most people this deficiency does not cause any health problems, since it is not very common to receive treatment with azathioprine or 6-mercaptopurine. However, those who receive these drugs and have TPMT deficiency metabolize these drugs more slowly and are exposed to more erratic responses and a higher rate of adverse effects. Some of these include nausea and increased transaminases, as well as others that can be serious, such as marrow suppression (inability to form new blood cells).

In addition to the TPMT gene, variations in the NUDT15 gene are associated with impaired metabolization of thiopurines and therefore present an increased risk of adverse reactions similar to those of the TPMT gene.

Genes analyzed

NUDT15 TPMT

Bibliography

Dickson AL, Daniel LL, Zanussi J, et al. TPMT and NUDT15 Variants Predict Discontinuation of Azathioprine for Myelotoxicity in Patients with Inflammatory Disease: Real-World Clinical Results. Clin Pharmacol Ther. 2022 Jan;111(1):263-271.

Maillard M, Nishii R, Yang W, et al. Additive effects of TPMT and NUDT15 on thiopurine toxicity in children with acute lymphoblastic leukemia across multiethnic populations. J Natl Cancer Inst. 2024 May 8;116(5):702-710.

Relling MV, Schwab M, Whirl-Carrillo M, et al. Clinical Pharmacogenetics Implementation Consortium Guideline for Thiopurine Dosing Based on TPMT and NUDT15 Genotypes: 2018 Update. Clin Pharmacol Ther. 2019 May;105(5):1095-1105.

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