Mercaptopurine is metabolized primarily in the liver via the enzyme thiopurine S-methyltransferase (TPMT). The activity of TPMT varies among individuals due to genetic polymorphisms, which can influence the drug's efficacy and toxicity. Patients with low or absent TPMT activity are at increased risk for severe myelosuppression. Genetic testing for TPMT activity is often recommended before initiating therapy to tailor the dosage appropriately and mitigate toxicity risks.
