123

MEASUREMENT OF THIOPURINE DRUG METABOLITE RATIOS PORTENDS THERAPEUTIC OUTCOMES AND REFLECTS THIOPURINE METHYLTRANSFERASE GENOTYPE IN IBD

M Girardin, Y Theoret, D Sinnett, D Amre, F Costea, EG Seidman
McGill University Health Center, Ste Justine Hospital, McGill University, University of Montreal, Montreal, Quebec

BACKGROUND: Efficient conversion of thiopurine drugs 6-mercaptopurine (6-MP) and its pro-drug azathioprine (AZA) to their active metabolite, 6-thioguanine nucleotides (6TGN), is the principal determinant to clinical efficacy. The principal catabolic metabolite, 6-methylmercaptopurine (6MMP), is formed via the competing enzymatic pathway by thiopurine methyltransferase (TPMT).
AIM: To assess the clinical utility of the calculated metabolite ratio [6-MMP/6-TGN] in predicting efficacy or myelotoxicity risk due to TPMT mutations.
PATIENTS AND METHODS: 6-MP metabolite levels were measured prospectively at 4-6 monthly intervals, at the time of a clinical relapse or toxicity in IBD patients (adult & pediatric) on 6-MP or AZA for > 3 mo. Therapeutic response was determined by modified Harvey Bradshaw Index and by physical exam ascertained at each clinical evaluation point and correlated to 6-MP metabolite measurements. Erythrocyte 6-TGN and 6-MMP concentrations (pmol/8x10⁸ RBC) were measured by HPLC assay at Ste Justine Hospital. Patients were genotyped for common polymorphisms in TPMT transitions using a PCR-allele specific oligonucleotide hybridization assay. Logistic regression analyses were carried out after controlling for correlation between repeated measures.
RESULTS: Data was analyzed for 1243 clinical evaluation points with metabolite values and ratios in 308 patients. Clinical remission correlated with higher erythrocyte 6-TG levels (< 0.001), but not with the dose of 6-MP/AZA. Higher 6-MMP values (> 5380) were associated with reduced likelihood of success (p<0.001). A significantly increased risk for relapse (p=0.002) was associated with high (> 30.7) ratios of 6MMP/6TGN. With each 15 unit increase in the ratio above 30, the probability of a successful outcome on thiopurine drug therapy decreased by 29% (p<0.001). Ratios above 65 indicate very low likelihood (< 10%) of therapeutic success. Heterozygous mutations in TPMT (n=33) were associated with very low ratios (mean 2.07), and a high likelihood (81.8%) of 6-TGN levels in the myelotoxicity range (450-1243).
CONCLUSIONS: Calculated 6-MMP/6-TGN ratios portend outcomes to thiopurine drug therapy: High ratios (> 30) due to excessive TPMT activity are associated with a higher likelihood of drug failure, while low ratios (< 3) are associated with TPMT mutations and higher risk of myelotoxicity.

PREVIOUS NEXT