Thiopurine S-methyltransferase polymorphisms in acute lymphoblastic leukemia, inflammatory bowel disease and autoimmune disorders: influence on treatment response

Figures & data

Figure 1 Metabolic pathways involved in the mechanism of action of thiopurines.

Notes: This figure illustrated the pathways involved in the metabolism of Azathioprin, 6-mercaptopurine and thioguanine highlighting the genes/enzymes that can potentially affect the metabolism of these drugs.
Abbreviations: 6-MP, 6-mercaptopurine; 6-Me-MP, 6-methyl-mercaptopurine; 6-Me-TG, 6-methyl-thioguanine; 6-Me-tIMP, 6-methyl-thioinosine-monophosphate; 6-Me-tITP, 6-methyl-thioinosine-triphosphate; 6-TG, thioguanine; 6-TGN, 6-thioguanine nucleotides; 6-tIDP, 6-thio-inosine diphosphate; 6-tIMP, 6-thio-inosine monophosphate; 6-tITP, 6-thio-inosine triphosphate; AZA, azathioprine; GMPS, guanosine monophosphatase synthetase; HGPRT, hypoxanthine guanine phosphoribosyl transferase; IMPDH, inosine monophosphate dehydrogenase; ITPA, inosine triphosphate pyrophosphatase; SAH, S-adenosyl-L-homocysteine; SAM, S-adenosyl-L-methionine; TPMT, thiopurine S-methyltransferase; XO, xanthine oxidase.

Table 1 Summary of selected studies that investigated the influence of TPMT pharmacogenetics on thiopurine treatment response in childhood acute lymphoblastic leukemia

Trial name	Design relative to TPMT genotype	6-MP treatment strategy	6-MP dosing (maintenance phase)	Genotype included	Finding	Conclusion	Author (reference no.)
Total Therapy Study XII	Observational/retrospective	Individualized therapy (dose reduction in patients experiencing myelosuppression to the highest tolerable dose)	75 mg/m^2/day with selective dose reduction in patients based on clinical tolerance	TPMT*2, *3A, *3B and *3C	Highest cumulative incidence of toxicity-induced reductions among patients homozygous for mutant TPMT alleles (100%), intermediate among heterozygous (35%) and lowest among wild-type patients (7%) Reduced TPMT activity tends to be associated with improved EFS. 6-MP dose intensity is a significant predictor of EFS Reduced TPMT activity tends to associate with higher risk of secondary AML Significantly higher cumulative risk of brain tumors among patients with reduced activity TPMT polymorphisms, especially when combined with high doses of antimetabolites	TPMT genotype is an important determinant of 6-MP toxicity and tolerance Treatment should aim to administer maximal tolerable protocol dose of 6-MP, particularly to patients with wild-type TPMT activity TPMT-deficient genotype may be associated with an increased risk of secondary AML. TPMT-deficient genotype may be associated with an increased risk of radiation-associated brain tumors	Relling et al^Citation10 Relling et al^Citation23 Relling et al^Citation53 Relling et al^Citation52
Total Therapy Study XIIIB	Individualized/prospective	Individualized therapy (dose reduction in patients experiencing myelosuppression to the highest tolerable dose) with pharmacogenetics compartment	75 mg/m^2/day with selective dose reduction in patients with low or intermediate TPMT activity based on a strategy that involves up-front knowledge of TPMT status combined with clinical tolerance and measurement of thiopurine metabolites	TPMT*2, *3A, *3B and *3C	No association between hematologic relapse (or other long-term outcomes) and TPMT status	Considering pharmacogenetics of TPMT for dose adjustment of 6-MP can help reduce treatment toxicity while not compromising its efficacy	Relling et al^Citation55 Rocha et al^Citation56
NOPHO-ALL-92	Observational/retrospective	Individualized randomized maintenance therapy based on clinical response and levels of metabolites	75 mg/m^2/day with subsequent dose adjustment to a target WBC	TPMT*3A, *3B and *3C	Higher risk of relapse in patients with high TPMT activity. Increased leukemogenic risk and higher rate of SMN in patients with low activity	No difference in OS between low vs high TPMT activity groups as the improved EFS is offset by the higher risk of SMN	Thomsen et al^Citation60 Schmiegelow et al^Citation59 Schmiegelow et al^Citation36 Schmiegelow et al.^Citation38 Schmiegelow et al.^Citation58
NOPHO- ALL-2000	Individualized/prospective	Individualized randomized maintenance therapy based on clinical response, levels of metabolites and pharmacogenetics	75 mg/m^2/day for wild-type patients, 50 mg/m^2/day for heterozygous patients and 5–10 mg/m^2/day for TPMT deficient patients, with subsequent dose adjustment to a target WBC during the first year of maintenance therapy	TPMT*3A, *3B and *3C	Reduced risk of SMN in heterozygous patients compared to ALL-92. Similar relapse risk between low TPMT activity patients and wild-type patients. No difference in overall EFS (any event) or OS compared to previous protocol	Initial 6-MP dose adjustment based on TPMT genotype did reduce the risk of SMN in heterozygous patients but at the expense of an increased risk of relapse	Schmiegelow et al^Citation58 Levinsen et al^Citation61
UK-ALL97 and ALL97/99	Individualized	Individualized randomized maintenance therapy based on TPMT activity and clinical response	75 mg/m^2/day for both TPMT heterozygous and wild-type patients subsequently titrated to toxicity Patients with TPMT deficiency were titrated from a starting dose of 10% of protocol dose (75 mg/m^2/day)	TPMT*2, *3A, *3B and *3C	Patients with TPMT*1/*3A genotype had better EFS than both wild-type TPMT*1/*1 and TPMT*1/*3C patients. Heterozygosity was not associated with a higher rate of second cancers. Higher hematotoxicity in heterozygous patients compared to wild-type patients	While, heterozygous patients had significantly more cytopenias and required dose adjustments more often than wild-type patients, TPMT*1/*3A patients had better EFS.	Lennard et al^Citation62
UK-ALL-2003	Individualized/prospective	Risk stratification based on MRD and individualized randomized maintenance therapy based on clinical response, TPMT activity and pharmacogenetics	75 mg/m^2/day for both TPMT heterozygous and wild-type patients subsequently titrated to toxicity Patients with TPMT deficiency were titrated from a starting dose of 10% of protocol dose (75 mg/m^2/day)	TPMT*2, *3A, *3B, *3C and *9	Improved overall EFS (all TPMT genotypes confounded) compared to previous protocol. No difference in EFS, RFS or OS between TPMT genotypes. The only significant factor affecting EFS was MRD status	Refinements of the risk stratification process and treatment strategies have reduced the influence of TPMT genotype on treatment outcome	Lennard et al^Citation33
ALL-BFM-2000	Observational/prospective	Measurement of MRD load before and after 6-MP treatment in heterozygous vs wild-type patients	During consolidation phase, on treatment day 78, heterozygous and wild-type homozygous patients received a 4-week cycle of (60 mg/m^2/day of 6-MP). Patients homozygous for mutant TPMT alleles were excluded from the analyses. During maintenance phase, 50 mg/m^2/day of 6-MP. The doses of methotrexate and 6-MP were adjusted to a target WBC	TPMT*2, *3A,	Significant reduction in the risk of having detectable MRD in TPMT heterozygous patients after induction consolidation treatment compared with wild-type patients. No difference in the rate of hematopoietic toxicity between TPMT heterozygous carriers and homozygous wild-type carriers. No association between low TPMT activity and risk of SMN	TPMT genotype has a significant impact on MRD during induction consolidation treatment as heterozygous patients showed a better MRD response, indicating an increased clearance of disease likely due to higher intensity of 6-MP effect. TPMT polymorphisms do not influence the risk of developing SMN in ALL patients treated with BFM protocols	Stanulla et al^Citation63 Stanulla et al^Citation64

Abbreviations: 6-MP, 6-mercaptopurine; ALL, acute lymphoblastic leukemia; AML, acute myeloid leukemia; BFM, Berlin-Frankfurt-Münster; EFS, event-free survival; MRD, minimal residual disease; NOPHO, Nordic Society of Paediatric Haematology and Oncology; OS, overall survival; RFS, relapse-free survival; SMN, secondary malignant neoplasms; TPMT, thiopurine S-methyltransferase; WBC, white blood cells.

Table 2 Summary of selected studies which investigated the influence of TPMT pharmacogenetics on thiopurine treatment response in inflammatory bowel disease

Author (reference no.)	Design relative to TPMT genotype	Treatment strategy	Genotypes included	Findings and conclusion
Derijks et al^Citation69	Observational	6-MP as a single oral 50 mg evening dose	TPMT*2, *3A, *3B and *3C	TPMT genotype correlated with 6-TGN concentrations. Patients with mutant alleles have higher risk of developing leukopenia
Hindrof et al^Citation67	Observational	Dose escalation schedule to the target dose of (2.5 mg/kg) for azathioprine and (1.25 mg/kg) for 6-MP by week 3	TPMT*2, *3A, *3B, *3C, *3D, *4, *5, *6, *7, *8, *10, *14 and *15	Overall, thiopurine-related ADRs were significantly more common among patients with low-to-intermediate TPMT activity; particularly myelotoxicity in TPMT-deficient patients
Ansari et al^Citation68	Observational	AZA was started at 2 mg/kg daily and without dose alteration	TPMT*3A, *3B and *3C	Heterozygous TPMT genotype strongly predicts treatment withdrawal due to early-onset of ADRs. Gastric intolerance was the most frequent reason for withdrawal among heterozygous patients. Myelotoxicity and gastric intolerance occurred significantly more frequently among heterozygous than with TPMT wild-type patients
Newman et al^Citation16, TARGET trial	Individualized/prospective	Arm 1: Standard dosing without genotyping vs Arm 2: Pretreatment TPMT genotyping and AZA dosing. Wild-type (1.5–3 mg/kg/day); heterozygous (25–50 mg/day) and titrate to the maintenance dose; homozygous for TPMT variant alleles were given alternative drugs	TPMT*2, *3A, *3B and *3C	No differences between the two study arms or between heterozygous and wild-type homozygous patients with respect to the rate of stopping azathioprine due to ADRs. No difference in the rate of remission between the intervention and control groups
Coenen et al^Citation22, TOPIC trial	Individualized/prospective	Arm 1: Control group. No genotyping + standard dosing: 2–2.5 mg/kg/day AZA or 1–1.5 mg/kg/day 6-MP. Arm 2: Intervention group. Genotyping + individualized dosing. TPMT wild-type: same as control; heterozygous: 50% of control and homozygous variant: 0%–10% of control	TPMT*2, *3A and *3C	No significant overall impact of TPMT genotype-guided dosing of thiopurines on treatment efficacy or on the risk of hematologic ADRs (i.e., leukopenia and thrombocytopenia) between the genotyped and nongenotyped arms. Carriers of at least one genetic variant in the pharmacogenetics arm had a significant reduction in the risk of hematologic ADRs compared with same group in the conventional arm

Abbreviations: 6-MP, 6-mercaptopurine; 6-TGN, 6-thioguanine nucleotides; ADRs: adverse drug reactions; AZA, azathioprine; TPMT, thiopurine S-methyltransferase; TARGET, TPMT: Azathioprine Response to Genotyping and Enzyme Testing; TOPIC, Thiopurine response Optimization by Pharmacogenetic testing in Inflammatory bowel disease Clinics.

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