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Thiopurine Methyltransferase Activity

Azathioprine (Imuran) and 6-mercaptopurine (Purinethol, 6-MP) are thiopurine drugs that are used to treat neoplasms such as acute lymphoblastic leukemia and a variety of rheumatologic, dermatologic, and neurologic diseases which are believed to have an immune etiology.Both drugs are metabolized to purine nucleotides, which are subsequently incorporated into DNA. This step is necessary for their antimetabolite activity and therapeutic efficacy.

The enzyme, thiopurine methyltransferase (TPMT), provides a competitive pathway for inactivation of these drugs by thiol methylation. A balance must be established between these 2 competing metabolic pathways such that sufficient drug is converted to 6-thiguanine to act as an antimetabolite, but the level does not become so high as to cause lethal bone marrow suppression.

Thiopurine Methyltransferase Activity

Distinct inheritable differences in levels of red blood cell TPMT have been detected among patients.Patients with low or absent TPMT activity are at an increased risk of developing severe, life-threatening myelosupression if conventional doses are given.

Approximately 10% of the population is heterozygous and 0.3% is homozygous for low TPMT activity. A reduced dose of azathioprine or 6-MP is recommended for heterozygous individuals, while alternative therapies should be considered for homozygous individuals.

Reference values are:

TPMT Level

Interpretation

15.1-26.4 U/mL RBC

Normal

6.3-15.0 U/mL RBC

Heterozygous low TPMT

<6.3 U/mL RBC

Homozygous low TPMT

Numerous patients have values which are near the cutoff between normal and the heterozygous state due to both assay variability and biological variation.

Specimen requirement is one 5 mL green-top (heparin) tube of whole blood. Specimen must be refrigerated and tested within 72 hours of draw.

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