Infliximab Serum Level Assessment After Switching from Intravenous to Subcutaneous Administration: The ISLASIS Study.

Introduction: Subcutaneous infliximab (SC-IFX) has emerged as an alternative maintenance therapy for inflammatory bowel disease (IBD), offering absorption and sustained systemic exposure. Although higher and more stable serum concentrations have been reported with SC-IFX, prospective real-world data evaluating pharmacokinetic evolution after transition from intravenous infliximab (IV-IFX) remain limited, particularly in Latin America. We assessed infliximab trough concentrations following transition from IV-IFX to SC-IFX and explored associations with biomarkers and clinical outcomes. Methods: ISLASIS was a prospective, open-label study including adults with Crohn's disease (CD) or ulcerative colitis (UC) receiving IV-IFX. All transitioned to SC-IFX and were evaluated at baseline (V1), week 12 (V2), and week 24 (V3). Infliximab trough concentrations were measured at each visit. Fecal calprotectin and clinical indices were assessed at baseline and week 24. Subgroup analyses explored pharmacokinetic behavior according to disease phenotype, prior biologic exposure, and IV regimen. Results: Nineteen patients were included; 89.5% had CD, with 35.3% presenting perianal disease. Median trough concentrations increased from 8.7 μg/mL at baseline to 20.0 μg/mL at week 12 and remained at 20.0 μg/mL at week 24 (p=0.00029). Fecal calprotectin decreased from 239 to 142 μg/g (p=0.00097). Median Harvey-Bradshaw Index decreased from 8 to 3, while partial Mayo score declined from 2 to 0. Increases in serum infliximab concentrations were observed across the clinical and treatment subgroups evaluated, although interpretation of concentrations above 20 µg/mL was limited by the assay measurement range. No anti-infliximab antibodies were detected. One mild, self-limited injection-site reaction occurred. Conclusion: Transition from IV-IFX to SC-IFX was associated with rapid and sustained increases in trough concentrations, reductions in fecal calprotectin, and favorable changes in clinical activity scores. These findings should be interpreted in light of the small sample size and the upper measurement limit of the infliximab assay. Clinical Trial Registration: This investigator-initiated prospective observational study was not registered in a clinical trial registry because treatment allocation was not determined by the study protocol. All participants received standard-of-care treatment, and the decision to switch from intravenous to subcutaneous infliximab was made as part of routine clinical practice.