Thiopurine Methyltransferase Activity - TPMT

Diagnosis

Indications for Testing

  • Detect risk for severe myelosuppresssion with standard dosing of thiopurine drugs
  • Individualized dosing of thiopurine drugs

Laboratory Testing

TPMT activity in red blood cells (RBC)
 

Low TPMT activity

Normal TPMT activity

High TPMT activity

Units/milliliter (U/mL)

<25

25-65

>65

Associated with

High risk of bone marrow toxicity

Low risk of bone marrow toxicity

Therapeutic failure

Dose adjustment suggested

Dramatic dose reduction (80-90%) may be required

None

Higher than standard dosing may be required

Comments

Therapeutic drug monitoring may help optimize dose

Represents 20th-99th percentiles

Not well-characterized

  • Limitations for testing
    • Does not replace clinical monitoring
    • Genotype cannot be inferred from TPMT activity (phenotype)
    • TPMT inhibitors may contribute to false-low test results
    • TPMT activity should be assessed prior to treatment with thiopurine drugs
    • Blood transfusion within 30 days will reflect donor status

Monitoring

  • Therapeutic monitoring (thiopurine metabolite concentrations) may be appropriate for individuals with deficient or high TPMT activity

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
    • Low thiopurine methyltransferase (TPMT) activity – ~1/300 Caucasian individuals
    • Intermediate TPMT activity – ~11% of Caucasian population
    • High TPMT activity – unknown

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
    • TPMT can be inhibited by common drugs (eg, NSAIDs, diuretics)

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
Test Name and Number Recommended Use Limitations Follow Up
Thiopurine Methyltransferase, RBC 0092066
Method: Enzymatic/Quantitative Liquid Chromatography-Tandem Mass Spectrometry

Detect risk for severe myelosuppression with standard dosing of thiopurine drugs

Individualize dosing of thiopurine drugs

Does not replace clinical monitoring

Genotype cannot be inferred from TPMT activity (phenotype)

TPMT inhibitors may contribute to false-low test results

TPMT activity should be assessed prior to treatment with thiopurine drugs 

Blood transfusion within 30 days will reflect donor status

 
TPMT Genotype 2002573
Method: Qualitative Polymerase Chain Reaction

Consider genotyping if RBC level is abnormal

   
Thiopurine Metabolites 2002575
Method: Qualitative High Performance Liquid Chromatography

Use to monitor patients with deficient or high TPMT activity