This document outlines a presentation on thiopurine drugs including azathioprine, 6-mercaptopurine, and 6-thioguanine. It discusses their metabolism, mechanisms of action, adverse drug reactions, and importance of testing for thiopurine S-methyltransferase (TPMT) polymorphisms. TPMT activity affects levels of active drug metabolites and risk of toxicity. Genotype correlates with but does not perfectly predict phenotype. The presentation emphasizes dose adjustment based on TPMT levels and close monitoring to improve outcomes with thiopurine treatment.

1. Nin Prapongsena, MPh.
Faculty of pharmacy
Huachiew chalermprakiet University

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

3. Thiopurine drugs

4. Azathioprine (AZA)
Pharmacological category: Immunosuppressant
Indication:
- Rheumatoid arthitis (RA)
- Inflammatory bowel disease (IBD)
- Psoriasis
- Prevent graft rejection
ADR:
- N/V
- Hepatotoxicity
- Leukopenia & thrombocytopenia
- Cutaneous ADR

5. 6-Mercaptopurine (6-MP): 50 mg/m2
Pharmacologic category:
Antineoplastic agent & immunosuppressant
Indication:
- Acute myelogenous leukemia (AML)
in induction & maintenance phase
(remission ≈ 80%)
- Inflammatory bowel disease (IBD)
ADR:
- Myelosuppression
(onset 7 d, nadir 14-16 d, recovery 21-28 d)
- Hepatotoxicity
- Cutaneous ADR

6. 6-Thioguanine (6-TG): 40 mg/m2
Pharmacologic category:
Antineoplastic agent
Indication:
- Acute lymphoblastic leukemia (ALL)
- Acute myelogenous leukemia (AML)
ADR:
- Myelosuppression
(onset 7-10 d, nadir 14 d, recovery 21 d)
- Hepatotoxicity
- Cutaneous ADR

7. Metabolism of thiopurines
Azathioprine (AZA)
6-Mercaptopurine (6-MP)
Glutathione S- transferase (GST)
Xanthine oxidase
(XO)
Thiouric acid
(inactive)
*Thiopurine
methyltransferase
(TPMT)
6-Methyl mercaptopurine
(antiangiogenesis)
Hypoxanthine
phosphoribosyl
transferase
(HPRT)
Thioinosine monophosphate
(TIMP)

8. Metabolism of thiopurines (continue)
TIMP
*TPMT
6-Methyl thioinosine
monophosphate
(MeTIMP)
Hepatotoxicity
Inosine monophosphate
Dehydrogenase (IMPDH)
6-Thioguanine
monophosphate
(TGMP)
Guanosine monophosphate
Synthetase
Purine
biosynthesis

9. Metabolism of thiopurines (continue)
TGMP
6-MP 6-Thioguanine (6-TG)
HPRT
*TPMT
6-Methylthioguanine
Monophosphate (MeTGMP)
Phosphokinase
Deoxythioguanine
triphosphate (TGN)
Purine
biosynthesis

10. Deoxythioguanine triphosphate effect
Deoxythioguanine
triphosphate (TGN)
Caused cell death
& myelosuppression

11. ADR of thiopurines
• Found ADR 25.9% in patients who taking thiopurines
ADR of thiopurine drugs Liver abnormalities
Allergy
Pancreatitis
Bonemarrow
suppression
N/V
Other
34 %
6%
9 %
***7 %
25 %
7 %

12. Related to dose and duration
of 6-MP & 6-TG treatment in acute leukemia

13. How thiopurine treatment failure ? ? ?
• Drug interactions
• Food interactions
• Pharmacogenomics
Changed
active metabolites level
Treatment failure
• ADR (especially neutropenia)
• Too low dose

14. Thiopurine methyltransferase (TPMT) gene
• Located at chromosome 6 (6p22)
• 3.4 kb (composed of 10 exon)
• TPMT found in liver, renal, RBC, etc.
• TPMT expression α TPMT activity α 1/TGN

15. Weinshilboum RM., and Sladek SL
Am J Hum Genet. 1980; 32: 651-652.
Mercaptopurine pharmacogenetics:
monogenic inheritance of erythrocyte
thiopurine methyltransferase activity

16. Undetected
(0.3%)
Intermediate or Low
Activity (11%)
Normal activity
(89%)
X-axis: RBC TPMT activity (U/mL)
6-mp 4mM
Y-axis:%ofsubjects RBC TPMT activity

17. Polymorphisms of thiopurine
methyltransferase (TPMT) gene

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

19. mRNA of TPMT

20. RBC TPMT activity

21. Stability of TPMT

22. Polymorphisms of thiopurine
methyltransferase (TPMT) gene
Normal actvity
T1/2: 18 h
Low TPMT activity
T1/2: 15 min
Low TPMT activity

23. Polymorphism of TPMT gene in Thailand
200 unrelated samples

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

27. Correlation of TPMT genotype and phenotype

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

29. Have False positive
(Specificity is not 100%):
cause low dose of 6-MP/6-TG usage

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

32. Cost effective for screening TPMT mutation

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.

34. Case report

38. Cutaneous ADR occured

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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