Thiopurine Methyltransferase Activity - TPMT

Diagnosis

Indications for Testing

• Consider TPMT testing in patients who will receive thiopurine drugs

Laboratory Testing

• TPMT activity in red blood cells (RBC)
  • If enzymatic level is reported as
    • Normal (25-65 U/mL) – low risk of bone marrow toxicity as a consequence of standard thiopurine therapy; no dose adjustment necessary
    • Abnormal/low (<25 U/mL) – high risk of bone marrow toxicity as a consequence of standard thiopurine dosing; reduction of drug dose by 80-90% recommended
    • Intermediate – reduction of drug dose by 20-50% may be necessary
    • High (>65 U/mL) – risk of therapeutic failure due to excessive inactivation of thiopurine drugs; may require higher drug dosing
• Patient should not be taking certain drugs when tested (xanthine oxidase inhibitors) because levels may appear falsely low
  • If RBC level is abnormal, consider TPMT genotyping

Monitoring

Clinical Background

Thiopurine drugs such as azathioprine (AZA), 6-mercaptopurine (6-MP), and 6-thioguanine (6-TG) are widely used in the treatment of acute lymphoblastic leukemia (ALL), autoimmune diseases, inflammatory bowel disease, and post-transplant organ rejection. Patients with abnormal thiopurine methyltransferase (TPMT) enzyme activity have an increased risk of toxicity when given thiopurines.

Epidemiology

• Prevalence – 1/300 Caucasians are TPMT deficient
  • 11% of Caucasians are heterozygous for TPMT mutations and may have intermediate TPMT activity

Genetics

• Autosomal recessive inheritance
  • Homozygous state – associated with deficient enzymatic activity
  • Heterozygous state – may be associated with intermediate enzymatic activity
• >20 mutational TPMT alleles identified to date; *2, *3A and *3C alleles account for 80-95% of low-to-intermediate activity states

Pathophysiology

• AZA, 6-MP, and 6-TG are inactive prodrugs used to treat a variety of different disease states and are metabolized by three different enzymes into three different 6-thioguanine nucleotides for activity
  • AZA is metabolized to 6-MP (one of most commonly prescribed)
    • 6-MP is converted into two pharmacologically inactive metabolites
      • 6-thiouric acid by the enzyme xanthine oxidase (XO)
      • 6-methylmercaptopurine (6-MMP) by thiopurine S-methyltransferase (TPMT)
        • Primary metabolic route for inactivation of nucleotides is catalyzed by TPMT
    • 6-MP is also converted into active thioguanine nucleotides by the enzyme hypoxanthine guanine phosphoribosyltransferase (HGPRT) to exert therapeutic cytotoxic effects
    • When 6-MP is converted to inactive metabolites, the amount of 6-TG nucleotides reduces, which balances the amount of 6-TG nucleotides needed to achieve required cytotoxicity for therapeutic treatment
    • When TPMT enzyme activity is low, proportionately more 6-MP is converted into cytotoxic 6-TG nucleotides, which increases toxicity
  • Accumulation of excessive nucleotides inhibit purine synthesis, most dramatically noted in the bone marrow, causing toxicity with myelosuppression
  • Reduced drug dosing may prevent myelosuppression in patients with intermediate and low TPMT activity

Indications for Laboratory Testing

• Tests generally appear in the order most useful for common clinical situations
• Click on number for test-specific information in the ARUP Laboratory Test Directory