Thiopurine Drug Toxicity Testing
Indications for Ordering
• Detect risk for severe myelosuppression with standard
dosing of thiopurine drugs
• Individualize dosing of thiopurine drugs
• Optimize therapy for thiopurine drugs
• Identify thiopurine metabolite concentrations that may
lead to toxicity
Test Description
Thiopurine methyltransferase, RBC
• Enzymatic/quantitative liquid chromatography-tandem
mass spectrometry
Thiopurine metabolites
• Quantitative liquid chromatography/tandem mass
spectrometry
TPMT Genotype
• Qualitative polymerase chain reaction
Tests to Consider
Primary tests
Thiopurine Methyltransferase, RBC 0092066
• Preferred screening test for pretherapeutic evaluation
• Detects risk for severe myelosuppression with standard
dosing of thiopurine drugs
• Assesses risk prior to treatment with thiopurine drugs
Thiopurine Metabolites 2011134
• Identifies thiopurine metabolite concentrations that may
lead to toxicity
• Optimizes therapy for thiopurine drugs
TPMT Genotype 2002573
• Consider if RBC thiopurine methyltransferase (TPMT) level
is abnormal
Disease Overview
Prevalence
• Very low/absent thiopurine methyltransferase (TPMT)
activity – ~30/100 individuals
• Intermediate TPMT activity – ~11% of Caucasian
individuals
• Normal TPMT activity – ~90% of individuals
• High TPMT activity – unknown
Pathophysiology
• Thiopurine drugs are purine antimetabolites and include
o Azathioprine (Imuran)
o 6-mercaptopurine (6-MP, purinethol)
o 6-thioguanine (6-TG, Tabloid)
• Thiopurines must be metabolized to 6-thioguanine
nucleotides (6-TGN) for activity
o Proportion of active 6-TGN is regulated by the balance
between activation and inactivation mechanisms
• Primary metabolic route for inactivation of thiopurine
drugs is catalyzed by TPMT
o Low TPMT activity – more 6-MP may be converted into
the active (cytotoxic) 6-TGN
o Active/cytotoxic metabolites will accumulate when
TPMT activity is low
 Excess 6-TG in the bone marrow inhibits purine
synthesis
• Inhibits cell proliferation
• Contributes to excessive myelosuppression
• TPMT enzyme can be inhibited by common drugs
o NSAIDs
 Ibuprofen
 Ketoprofen
 Naproxen
 Mefenamic acid
o Diuretics
 Furosemide
 Thiazides
o Ulcerative colitis drugs
 Mesalamine
 Olsalazine
 Sulfasalazine
Treatment Issues
• AZA, 6-MP, and 6-TG are inactive prodrugs
o Used to treat
 Acute lymphoblastic leukemia
 Autoimmune diseases
 Inflammatory bowel disease
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o Used to prevent rejection after solid organ transplant
 For patients with low to intermediate TPMT activity,
myelosuppression is a major issue
 Individuals with very low/no TPMT enzyme activity
universally experience severe myelosuppression when
receiving conventional thiopurine doses
 Reduced drug dosing in above patients (with very low
to intermediate TPMT activity) may reduce risk for
myelosuppression
 ~30-60% of individuals with intermediate TPMT
activity who receive conventional thiopurine doses
experience moderate to severe myelosuppression
• Thiopurine dosing should rely on disease-specific
guidelines and degree of myelosuppression
• Guidelines for thiopurine dosing by the Clinical
Pharmacogenetics Implementation Consortium (CPIC) can
be found at: https://www.pharmgkb.org/gene/PA356
Test Interpretation
Components
230-400 pmol/8 x 10 RBC
6-MMPN RBC
<5700 pmol/8 x 10 RBC
Units/milliliter
(U/mL)
High TPMT
activity
<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
th
th
20 -99
percentiles
Not well
characterized
8
8
• 6-TGN concentrations <230 pmol/8 x 10 RBC may
indicate a reduced response to therapy
8
• 6-TGN concentrations >400 pmol/8 x 10 RBC may
indicate a higher risk for leukopenia
8
• 6-MMPN concentrations >5700 pmol/8 x 10 RBC may
indicate a higher risk for hepatotoxicity
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
• TPMT testing – blood transfusion within 30 days will
reflect donor status
Thiopurine methyltransferase
Normal
TPMT
activity
8
6-TGN RBC
Results
Low TPMT
activity
Therapeutic Range
Thiopurine metabolites – limit of quantification (LOQ)
8
• LOQ – 12.5 pmol/8 x 10 RBC (6-TGN)
8
• LOQ – 325 pmol/8 x 10 RBC (6-methylmercaptopurine
nucleotide [6-MMPN])
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