P703 Addition of azathioprine to switch of anti-TNF in patients with IBD in clinical relapse with pharacokinetic failure: A post hoc analysis of a prospective randomised trial using drug-tolerant assay

Abstract Background In a recent prospective, randomised trial, we showed that the rates of clinical failure and occurrence of unfavourable pharmacokinetics using a drug-sensitive assay after a switch of anti-TNF were significantly higher in monotherapy compared with combination therapy (Log-rank test p < 0.0001), whatever the anti-TNF used in IBD patients. We aimed in a post hoc analysis to assess the time-course of antidrug antibodies in the two groups of patients using a drug-tolerant assay previously described (1). Methods After switching of anti-TNF under mono or combotherapy, blood samples were uptake at various time-points (6, 12, 18 and 24 months). Using a drug-tolerant assay, we defined pharmacokinetic failure when antidrug antibodies were detected (> 2.5 μg/ml Eq). Transient anti-drug antibodies were defined as antibodies that appeared during the course of anti-TNF therapy with no clinical worsening, and finally that disappeared after 2 consecutive measurements. Results Ninety patients were analysed. According to a drug-tolerant assay, the occurrence of antibodies against anti-TNF was significantly higher in patients under monotherapy (log-rank test p < 0.0001 (HR = 3.36 [1.94–5.86]) than under combination therapy. The rates of anti-drug antibody were significantly lower under IFX-AZA combotherapy compared with ADA-AZA combotherapy or under monotherapy. Monotherapy ADA (HR = 5.55[2.46–12.34]) or IFX (HR = 6.05[2.67–13.70]) and combination therapy ADA-AZA (HR = 3.34 [1.407.98]) increased significantly the rates of pharmacokinetic failure using a drug-tolerant assay compared with IFX-AZA combotherapy. The time-course without pharmacokinetic failure was significantly different between the 4 groups of patients from 15 months (p = 0.04) but was not significantly different at 6 months (p = 0.12). Overall, 9% of patients developed transient antibodies. Detection of antibodies against anti-TNF at 6 months using a drug-tolerant assay predicted significantly an immunogenic failure assessed previously by a drug-sensitive assay with a sensitivity of 57.1%, a specificity of 92.7%, a positive predictive value of 90.3%. In contrary to the association between clinical failure and immunogenic failure using drug-sensitive assay, the association of detectable drug and positive antibodies using a drug-tolerant assay was not significantly associated with clinical failure (HR = 1.3[0.3–6.8], p = 0.51). Conclusion Pharmacokinetic failure using a drug-tolerant assay was significantly lower under combotherapy, especially for IFX-AZA. Detection of anti-drug antibodies could predict pharmacokinetic failure. Reference