2. Outline
• Introduction of thiopurines
• Thiopurine metabolism and their mechanism of actions
• ADR of thiopurines
• How thiopurine treatment failure ???
• Polymorphisms of thiopurine methyltransferase
(TPMT) gene and their clinical effects of TPMT activity
• Correlation of TPMT genotype and phenotype
• Limitation of TPMT genotype and phenotype
assessment
• Conclusion
• Pharmacist’s role
18. Proc. Natl. Acad. Sci. USA
Vol. 94, pp. 6444–6449, June 1997
Medical Sciences
Enhanced proteolysis of thiopurine S-transferase
(TPMT) encoded by mutant alleles in human
TPMT*3A, TPMT*2): mechanisms for the genetic
polymorphism of TPMT activity
Tai HL., Krynetski EY., Yanishevski Y., and Evans WE
24. Clinical effects of TPMT activity
• High TPMT activity (If TPMT level > 65 U/mL):
- High meTIMP level
- Low TGN level
Inhibited purine synthesis
Hepatotoxicity
Risk to failure in ALL treatment
• How to solve this problems ?
Recommend increasing dose of 6-MP or 6-TG than
usual dose and closely hepatic enzymes monitoring
25. Clinical effects of TPMT activity
• Intermediate TPMT activity
(25 U/mL > TPMT level < 45 U/mL):
- High TGN level Myelosuppression
Increased risk of 2nd malignancy
• How to solve this problems ?
Recommend reducing dose to 50-80 %
of standard dose
26. Clinical effects of TPMT activity
• Low TPMT activity or TPMT deficiency
(If TPMT level < 25 U/mL):
- Very high TGN level Severe myelosuppression
High risk of 2nd malignancy
• How to solve this problems ?
Recommend reducing dose to 10-20 %
of standard dose
28. Assessment of Thiopurine S-Methyltransferase
Activity in Patients Prescribed Thiopurines: A
Systematic Review
Ann Intern Med. 2011;154:814-823.
Ronald A. Booth, PhD; Mohammed T. Ansari, MBBS, MMedSc, MPhil; Evelin Loit,
PhD; Andrea C. Tricco, PhD; Laura Weeks, PhD; Steve Doucette, MSc; Becky
Skidmore, MLS; Margaret Sears, PhD; Richmond Sy, MD; and Jacob Karsh, MDCM
30. Limitation of TPMT phenotype and
genotype testing
Genotype testing
Missed target:
pseudogene on
chromosome 18
Phenotype testing
• Factors affected to
TPMT activity:
- Age
- Blood transfusion
• Loss sample between
testing
Investigated both genotype and phenotype
of TPMT are more utility
31. Cost effective for screening TPMT mutation
J Am Acad Dermatol 2000;42: 628-632
33. Conclusion
• TPMT gene mutation affected to TPMT level, activity
and stability
• TPMT expression α TPMT activity α 1/TGN level,
TGN level α myelosuppression & 2nd malignancy
• Only TPMT*3C found in Thais (≈ 9%)
• Dose adjustment:
- High TPMT (> 65 U/mL): dose
- Intermediate TPMT (25-45 U/mL): dose 20-50%
- Low or absence TPMT (< 25 U/mL): dose 80-90%
•TPMT genotype correlated with phenotype but not
100% (Many factors affected to TPMT activity)
• There are limitation of genotype and phenotype testing,
then investigated both are more utility.
40. Pharmacist’s role
Prevented
Screening TPMT genotype Testing TPMT activity
Considered appropriate initial dose
Monitor CBC
and hepatic enzymes
Toxicity or ADR occurred:
Adjusted dose or delayed dose
± antibiotic (NCCN guideline)
± myelosuppressive treatment (NCCN guideline)
and closely liver enzymes monitoring
Corrected
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