Background & Aims: In patients with inflammatory bowel diseases, the combination of infliximab and thiopurines (such as 6-thioguanine) is more effective treatment than monotherapy. We assessed the correlation between serum levels of 6-thioguanine (6-TGN) and infliximab levels or antibodies to infliximab (ATI). Methods: We performed a cross-sectional study of 72 patients receiving maintenance therapy with infliximab and a thiopurine for inflammatory bowel disease at the Crohn's and Colitis Center of the University of Miami, FL. We collected clinical, endoscopic, and biochemical data, and levels of thiopurine metabolites. The primary outcomes were trough level of infliximab and the presence of ATI. Results: Levels of 6-TGN correlated with those of infliximab (ρ, 0.53; P < .0001). The cut-off point of 6-TGN that best predicted a higher level of infliximab was 125 pmol/8× 108 red blood cells (RBCs) (area under receiver operating characteristic, 0.86; P < .001). Patients in the lowest quartile of 6-TGN had infliximab levels that were similar to patients on no thiopurines (4.3 vs 4.8 mcg/mL, respectively; P= .8). An infliximab level of 8.3 mcg/mL or greater was associated with mucosal healing. Only 8 patients (11%) had detectable ATI. Patients with 6-TGN levels less than 125 pmol/8× 108 RBCs were significantly more likely to have ATI (odds ratio, 1.3; 95% confidence interval, 2.3-72.5; P < .01). Conclusions: Although 6-TGN levels of greater than 230 pmol/8× 108 RBCs have been associated with improved outcomes in patients on monotherapy, a level of 6-thioguanine of 125 pmol/8× 108 RBCs or greater may be adequate to achieve therapeutic levels of infliximab. In the long term, this may minimize the toxicity for patients on combination therapy.
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