Article

Assessment of Thiopurine Methyltransferase Activity in Patients Prescribed Azathioprine or Other Thiopurine-based Drugs.

ABSTRACT To examine whether pretreatment determination of thiopurine methyltransferase (TPMT) enzymatic activity (phenotyping) or TPMT genotype, to guide thiopurine therapy in chronic autoimmune disease patients, reduces treatment harms. Other objectives included assessing: preanalytic, analytic, and postanalytic requirements for TPMT testing; diagnostic accuracy of TPMT genotyping versus phenotyping; association of thiopurine toxicity with TPMT genotypic or phenotypic status; and costs of testing, care, and treating drug-associated complications.
MEDLINE®, EMBASE®, and Healthstar were searched from inception to May 2010; the Cochrane Library® to October 2009; and BIOSIS®, Genetics Abstracts, and EconLit™ to May 2009, for English language records.
A reviewer screened records, and a second reviewer verified exclusions and subsequent selection of relevant studies. Studies in patients with leukemia and organ transplant were excluded. Additionally, laboratories that provide TPMT analytical services were surveyed to assess means of TPMT testing in practice. Where possible, risk of bias was assessed using standard criteria. Meta-analyses estimated diagnostic sensitivity, and specificity; and odds ratios of associations.
1790 titles or abstracts, and 538 full text records were screened. 114 observational studies and one RCT were included. Majority of studies were rated fair quality, except for diagnostic studies with 37 percent of studies rated poor. In general, there were few patients who were homozygous (or compound heterozygous) for TPMT variant alleles in the included studies limiting applicability. There is insufficient evidence examining effectiveness of pretesting in terms of reduction in clinical adverse events. Sufficient preanalytical data were available regarding preferred specimen collection, stability and storage conditions for TPMT testing. There was no clinically significant effect of age, gender, various coadministered drugs, or most morbidities (with the exception of renal failure and dialysis). TPMT phenotyping methods had coefficients of variation generally below 10 percent. TPMT genotyping reproducibility is generally between 95-100 percent. The sensitivity of genotyping to identify patients with low or intermediate TPMT enzymatic activity is imprecise, ranging from 70.70 to 82.10 percent (95 percent CI, lower bound range 37.90 to 54.00 percent; upper bound range 84.60 to 96.90 percent). Sensitivity of homozygous TPMT genotype to correctly identify patients with low to absent enzymatic activity was 87.10 percent (95 percent CI 44.30 to 98.30 percent). Genotyping specificity approached 100 percent. Leukopenia was significantly associated with low and intermediate enzymatic activity (low activity OR 80.00, 95 percent CI 11.5 to 559; and intermediate activity OR 2.96, 95 percent CI 1.18 to 7.42), and homozygous and heterozygous TPMT variant allele genotype (OR 18.60, 95 percent CI 4.12 to 83.60; and 4.62, 95 percent CI 2.34 to 9.16, respectively). In general, TPMT phenotyping costs less than genotyping, although estimates across studies are quite heterogeneous.
There is insufficient direct evidence regarding the effectiveness of pretesting of TPMT status in patients with chronic autoimmune diseases. Indirect evidence confirms strong association of leukopenia with lower levels of TPMT activity and carrier genotype already established in the literature.

Download full-text

Full-text

Available from: Andrea Tricco, Sep 09, 2014
0 Followers
 · 
95 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Azathioprine (AZA), 6-mercaptopurine (6-MP), and thioguanine (TG) are thiopurine drugs. These agents are indicated for the treatment of various diseases including hematologic malignancies, inflammatory bowel disease (IBD), rheumatoid arthritis, and as immunosuppressants in solid organ transplants. Thiopurine drugs are metabolized, in part, by thiopurine methyltransferase (TPMT). TPMT displays genetic polymorphism resulting in null or decreased enzyme activity. At least 20 polymorphisms have been identified, of which, TPMT *2, *3A, *3B, *3C, and *4 are the most commonly studied. These polymorphisms have been associated with increased myelosuppression risk. TPMT genotyping may be useful to predict this risk.
    05/2011; 3:RRN1236. DOI:10.1371/currents.RRN1236
  • [Show abstract] [Hide abstract]
    ABSTRACT: To determine the incidence and predictors of thiopurine-related adverse events. Subjects with Crohn's disease who were followed in the Alberta Inflammatory Bowel Disease Consortium patient database registry were identified. Retrospective chart review was conducted between August 5(th), 2010 and June 1(st), 2012. We collected data on: age at diagnosis; sex; disease location and behaviour at time of prescribing thiopurine; perianal fistulising disease at or prior to thiopurine prescription; smoking status at time of thiopurine prescription, use of corticosteroid within 6 mo of diagnosis; dosage, age at onset, and cessation of 5-aminosalicyclic acid (5-ASA); anti-tumour necrosis factor medication exposure and intestinal resection before thiopurine prescription. The primary outcome of interest was the first adverse event that led to discontinuation of the first thiopurine medication used. Logistic regression models were used to associate clinical characteristics with outcomes after adjusting for potential confounders. Risk estimates were presented as odds ratios (OR) with 95% CI. Effect modification by age and sex were explored. Our cohort had a median follow-up duration of 5.8 years [interquartile range (IQR 25(th)-75(th)) 2.7-9.1]. Thiopurine therapy was discontinued in 31.3% of patients because of: hypersensitivity reactions (7.1%), acute pancreatitis (6.2%), gastrointestinal intolerance (5.4%), leucopenia (3.7%), hepatotoxicity (3.4%), infection (1.1%) and other reasons (4.3%). A higher incidence of thiopurine withdrawal was observed in patients over the age of 40 (39.4%, P = 0.007). A sex-by-age interaction (P = 0.04) was observed. Females older than 40 years of age had an increased risk of thiopurine discontinuation due to an adverse event (age above 40 vs age below 40, adjusted OR = 2.8; 95%CI: 1.4-5.6). In contrast, age did not influence thiopurine withdrawal in males (age above 40 vs below 40, adjusted OR = 0.9; 95%CI: 0.4-2.1). Other clinical variables (disease location and phenotype, perianal disease, smoking history, history of intestinal resection and prior 5-ASA or corticosteroid use) were not associated with an increased risk an adverse event leading to therapy cessation. Thiopurine withdrawal due to adverse events is commoner in women over the age of 40 at prescription. These findings need to be replicated in other cohorts.
    07/2015; 21(25):7795-804. DOI:10.3748/wjg.v21.i25.7795