Body Mass Index and Smoking Affect Thioguanine Nucleotide Levels in Inflammatory Bowel Disease.

Introduction: Optimal levels of the thiopurine metabolite, 6-thioguanine nucleotides [6-TGN] correlate with remission of inflammatory bowel disease [IBD]. Apart from variations in the thiopurine methyl transferase [TPMT] gene, little is known about other predictors of 6-TGN levels. Obesity adversely affects response to infliximab and adalimumab and clinical course in IBD, but little is known about the interaction of thiopurines and obesity. We investigated the relationship between body mass index [BMI] and 6-TGN levels and sought to examine other predictors of 6-TGN levels.

Methods: This retrospective cohort study included patients with concurrent measurements of 6-TGN and BMI. The association between 6-TGN and clinical variables including BMI was estimated using a multivariable linear regression model.

Results: Of 132 observations, 77 [58%] had Crohn's disease and 55 [42%] ulcerative colitis. BMI, smoking, and TPMT levels were associated with 6-TGN levels in multivariable analysis. Every 5kg/m(2) increase in BMI was associated with an 8% decrease in 6-TGN (0.92; 95% confidence interval [CI] 0.87-0.98; p = 0.009). Smokers had higher 6-TGN levels in comparison with non-/ex-smokers [1.43; 95% CI 1.02-2.02; p = 0.041]. Patients with intermediate TPMT had higher 6-TGN compared to those with normal levels [2.13; 95% CI 1.62-2.80; p < 0.001]. Obese patients were more likely to have sub-therapeutic 6-TGN levels and a higher methyl mercaptopurine nucleotide [MMPN/TGN] ratio despite a similar dose of thiopurines.

Conclusions: Active smoking and intermediate TPMT values were associated with higher 6-TGN levels but increasing BMI resulted in lower 6-TGN and higher MMPN levels. This may explain the worse outcome that has been reported previously in obese IBD subjects.