Thiopurine Methyltransferase Testing - TPMT

  • Diagnosis
  • Monitoring
  • Background
  • Lab Tests
  • References
  • Related Content

Indications for Testing

  • Patients being considered for thiopurine therapy in order to
    • Detect risk for severe myelosuppression with standard full dosing of thiopurine drugs
    • Individualize dosing of thiopurine drugs
  • Patients who have experienced an adverse reaction to thiopurine therapy

Laboratory Testing

  • TPMT genotyping
    • Detects *2, *3A, *3B, and *3C alleles
    • Variant alleles are correlated with risk of severe myelosuppression with standard dosing of thiopurine drugs
    • Does not replace need for clinical monitoring
  • Pretherapeutic or posttherapeutic use
    • Detect common genotypes
    • Detect enzyme phenotype
  • Should not be used in patients with history of allogeneic bone marrow transplant
 
  • Therapeutic drug monitoring (metabolic phenotype)
    • Thiopurine drug metabolites
      • Thiopurine metabolite concentrations may be appropriate for individuals with deficient or high TPMT activity to monitor and optimize dose

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 posttransplant organ rejection. Patients with abnormal thiopurine methyltransferase (TPMT) enzyme activity due to genetics and/or drug-drug interactions have an increased risk of toxicity when given thiopurines.

Epidemiology

  • Prevalence of phenotype
    • Low thiopurine methyltransferase (TPMT) activity – ~0.3%
    • Intermediate TPMT activity – ~10%
    • Normal TPMT activity – ~90%
    • High TPMT activity – unknown

Genetics

  • TPMT gene – autosomal co-dominant inheritance
  • >20 TPMT deficiency alleles identified to date
    • TPMT deficiency alleles account for 95% of low-to-intermediate activity states
      • *2 (c.238G>C; p.Ala80Pro)
      • *3A (c.[460G>A;719A>G]; p.[Ala154Thr;Tyr240Cys])
      • *3B (c.460G>A; p.Ala154Thr)
      • *3C (c.719A>G; p.Tyr240Cys)
    • Homozygous or compound heterozygous
      • Associated with very low/no TPMT enzyme activity and high risk for drug-related toxicity with conventional thiopurine doses
    • Heterozygous
      • Associated with intermediate TPMT enzyme activity and increased risk for drug-related toxicity with conventional thiopurine doses
    • No variants detected
      • Predictive of *1 functional alleles
      • Predicts normal TPMT enzyme activity and normal risk for thiopurine drug-related toxicity

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 risk for toxicity
    • Accumulation of excessive nucleotides inhibit purine synthesis, most dramatically noted in the bone marrow, inhibiting cell proliferation and contributing to myelosuppression
    • Reduced drug dosing may prevent myelosuppression in patients with intermediate and low TPMT activity
      • Individuals with very low/no TPMT enzyme activity typically experience severe myelosuppression when receiving conventional thiopurine doses
      • An estimated 30-60% of individuals with intermediate TPMT activity who receive conventional thiopurine doses experience moderate to severe myelosuppression
    • Thiopurine dosing guidelines (Clinical Pharmacogenetics Implementation Consortium [CPIC])
    • TPMT can be inhibited by common drugs
      • NSAIDs
        • Ibuprofen
        • Ketoprofen
        • Naproxen
        • Mefenamic acid
      • Diuretics
        • Furosemide
        • Thiazides
      • Ulcerative colitis drugs
        • Sulfasalazine
        • Mesalamine
        • Olsalazine
  • Factors to consider if deciding when TPMT should be ordered
    • Disease state being tested
    • Starting dose
    • Need for immediate full dose
    • Previous documented tolerance of thiopurine medication at steady state doses

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

Thiopurine Methyltransferase, RBC 0092066
Method: Enzymatic/Quantitative Liquid Chromatography-Tandem Mass Spectrometry

Limitations

Does not replace clinical monitoring

TPMT inhibitors may contribute to false-low test results

TPMT activity should be assessed prior to treatment with thiopurine drugs

Thiopurine Methyltransferase (TPMT) Genotyping, 4 Variants 2012233
Method: Polymerase Chain Reaction/Fluorescence Monitoring

Limitations

Only targeted TPMT allele variants  will be detected by this panel

Diagnostic errors can occur due to rare sequence variations

Genotyping cannot distinguish between the *1/*3A and *3B/*3C genotypes

Genotyping does not replace the need for therapeutic drug monitoring or clinical observation

Genotyping in patients who have received allogenic stem cell/bone marrow transplant  will reflect donor status

Thiopurine drug metabolism and risk for toxicity may be affected by genetic and nongenetic factors that are not evaluated by this test

Test does not assess for TPMT allele variants associated with ultra-high enzyme activity

Thiopurine Drug Metabolites 2011134
Method: Quantitative Liquid Chromatography/Tandem Mass Spectrometry

Limitations

Limit of quantification (LOQ)

  • LOQ – 12.5 pmol/8 x 108 RBC (6-TGN)
  • LOQ – 325 pmol/ 8 x 108 RBC (6-methyl mercaptopurine nucleotide [6-MMPN])

General References

Booth R, Ansari M, Loit E, Tricco A, Weeks L, Doucette S, Skidmore B, Sears M, Sy R, Karsh J. Assessment of thiopurine S-methyltransferase activity in patients prescribed thiopurines: a systematic review. Ann Intern Med. 2011; 154(12): 814-23, W-295-8. PubMed

Chouchana L, Narjoz C, Beaune P, Loriot M, Roblin X. Review article: the benefits of pharmacogenetics for improving thiopurine therapy in inflammatory bowel disease. Aliment Pharmacol Ther. 2012; 35(1): 15-36. PubMed

DiPiero J, Teng K, Hicks K. Should thiopurine methyltransferase (TPMT) activity be determined before prescribing azathioprine, mercaptopurine, or thioguanine? Cleve Clin J Med. 2015; 82(7): 409-13. PubMed

Relling M, Gardner E, Sandborn W, Schmiegelow K, Pui C, Yee S, Stein C, Carrillo M, Evans W, Hicks J, Schwab M, Klein T, Clinical Pharmacogenetics Implementation Consortium. Clinical pharmacogenetics implementation consortium guidelines for thiopurine methyltransferase genotype and thiopurine dosing: 2013 update. Clin Pharmacol Ther. 2013; 93(4): 324-5. PubMed

Relling M, Gardner E, Sandborn W, Schmiegelow K, Pui C, Yee S, Stein C, Carrillo M, Evans W, Klein T, Clinical Pharmacogenetics Implementation Consortium. Clinical Pharmacogenetics Implementation Consortium guidelines for thiopurine methyltransferase genotype and thiopurine dosing. Clin Pharmacol Ther. 2011; 89(3): 387-91. PubMed

Smith M, Marinaki A, Sanderson J. Pharmacogenomics in the treatment of inflammatory bowel disease. Pharmacogenomics. 2010; 11(3): 421-37. PubMed

Medical Reviewers

Last Update: January 2016