Nin Prapongsena, MPh.Faculty of pharmacy
Huachiew chalermprakiet University
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
Thiopurine drugs
Azathioprine (AZA)
Pharmacological category: ImmunosuppressantIndication:
- Rheumatoid arthitis (RA)- Inflammatory bowel disease (IBD)- Psoriasis- Prevent graft rejection
ADR: - N/V- Hepatotoxicity- Leukopenia & thrombocytopenia- Cutaneous ADR
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-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
Metabolism of thiopurines
Azathioprine (AZA)
6-Mercaptopurine (6-MP)
Glutathione S- transferase (GST)
Xanthine oxidase(XO)
Thiouric acid(inactive)
*Thiopurinemethyltransferase
(TPMT)
6-Methyl mercaptopurine(antiangiogenesis)
Hypoxanthine phosphoribosyl
transferase(HPRT)
Thioinosine monophosphate(TIMP)
Metabolism of thiopurines (continue)
TIMP
*TPMT
6-Methyl thioinosinemonophosphate
(MeTIMP)
Hepatotoxicity
Inosine monophosphateDehydrogenase (IMPDH)
6-Thioguanine monophosphate
(TGMP)
Guanosine monophosphateSynthetase
Purinebiosynthesis
Metabolism of thiopurines (continue)
TGMP
6-MP 6-Thioguanine (6-TG)
HPRT
*TPMT
6-Methylthioguanine Monophosphate (MeTGMP)
Phosphokinase
Deoxythioguaninetriphosphate (TGN)
Purinebiosynthesis
Deoxythioguanine triphosphate effect
Deoxythioguaninetriphosphate (TGN)
Caused cell death& myelosuppression
ADR of thiopurines
• Found ADR 25.9% in patients who taking thiopurines
ADR of thiopurine drugs Liver abnormalities
Allergy
Pancreatitis
Bonemarrowsuppression
N/V
Other
34 %
6%9 %
***7 %
25 %
7 %
Related to dose and durationof 6-MP & 6-TG treatment in acute leukemia
How thiopurine treatment failure ? ? ?
• Drug interactions• Food interactions• Pharmacogenomics
Changed active metabolites level
Treatment failure• ADR (especially neutropenia)
• Too low dose
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
Weinshilboum RM., and Sladek SL
Am J Hum Genet. 1980; 32: 651-652.
Mercaptopurine pharmacogenetics: monogenic inheritance of erythrocyte thiopurine methyltransferase activity
Undetected(0.3%)
Intermediate or LowActivity (11%)
Normal activity (89%)
X-axis: RBC TPMT activity (U/mL)6-mp 4mM
Y-a
xis:
% o
f su
bje
cts
RBC TPMT activity
Polymorphisms of thiopurinemethyltransferase (TPMT) gene
Proc. Natl. Acad. Sci. USAVol. 94, pp. 6444–6449, June 1997Medical 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
mRNA of TPMT
RBC TPMT activity
Stability of TPMT
Polymorphisms of thiopurinemethyltransferase (TPMT) gene
Normal actvityT1/2: 18 h
Low TPMT activityT1/2: 15 min
Low TPMT activity
Polymorphism of TPMT gene in Thailand
200 unrelated samples
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 thanusual dose and closely hepatic enzymes monitoring
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
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
Correlation of TPMT genotype and phenotype
Assessment of Thiopurine S-MethyltransferaseActivity 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
Have False positive(Specificity is not 100%): cause low dose of 6-MP/6-TG usage
Limitation of TPMT phenotype and genotype testing
Genotype testing
Missed target:pseudogene onchromosome 18
Phenotype testing
• Factors affected to TPMT activity:
- Age- Blood transfusion
• Loss sample between testing
Investigated both genotype and phenotypeof TPMT are more utility
Cost effective for screening TPMT mutation
J Am Acad Dermatol 2000;42: 628-632
Cost effective for screening TPMT mutation
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 not100% (Many factors affected to TPMT activity)
• There are limitation of genotype and phenotype testing, then investigated both are more utility.
Case report
Cutaneous ADR occured
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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