P886 Ustekinumab concentrations in induction are associated with mid-term endoscopic outcomes in patients with inflammatory bowel disease

Background It is unclear whether ustekinumab (UST) concentrations can predict the clinical course of inflammatory bowel disease (IBD) and guide treatment algorithms during the induction phase. The aim of our study was to assess the association between serum UST concentrations during the induction phase and clinical outcomes at week 24 and to determine the validity of a UST threshold for guiding intensification strategies. Methods We conducted a retrospective study including Crohn's disease (CD) and ulcerative colitis (UC) patients who started UST treatment between June 2022 and February 2023. Intensification strategies were determined according to standard clinical practice. UST concentrations were collected at weeks 8, 16, and 24. Quartile analysis and logistic regression were performed to evaluate the association between UST concentrations and treatment targets. Definitions are clinical steroid-free remission as a Harvey-Bradshaw index <5 and a partial Mayo score <2; endoscopic remission as a simple endoscopic score (SES-CD) ≤2 and Mayo endoscopic score (EMS) ≤1; and endoscopic response as a ≥50% reduction in SES-CD and ≥1 point in EMS. Results We included 42 patients (CD: 24). At week 24, clinical remission rates of 67% and endoscopic response and remission rates of 57% and 28%, respectively, were achieved. At week 24, the majority of patients continued intensified treatment: 90 mg subcutaneously every 4 weeks in 55% and 130 mg intravenously every 4 weeks in 36%. Patients who achieved an endoscopic response at week 24 had higher UST levels at week 8 (4.1 vs. 2.9 µg/ml, p=0.029). No significant differences between endoscopic remission rates and UST levels at any week were observed. The differences observed in the quartile analysis between the UST concentrations at week 8 and the endoscopic response were not statistically significant (p=0.451). The area under the ROC curve value for UST levels at week 8 to predict endoscopic response was 0.734 (p=0.012). Logistic regression analysis identified prior exposure to vedolizumab and absence of perianal disease as predictors of endoscopic response and remission at week 24 in univariate analysis, but not in multivariate analysis. No association was observed between UST levels and drug persistence rates. Conclusion In this real-world cohort, higher UST concentrations at week 8 were associated with higher rates of endoscopic response at week 24. A reliable concentration threshold for predicting endoscopic response and treatment intensification could not be determined, probably due to the insufficient number of patients included in the study and the relatively poor performance of the ROC curve. Prospective, randomized studies are needed to validate these results.