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Introduction Ustekinumab is an effective treatment for Crohn’s disease (CD) and ulcerative colitis (UC). The advent of biosimilar ustekinumab in 2024 and potential healthcare savings drove a national agenda to use ustekinumab biosimilars for licensed indications. Pharmacists predominantly oversaw the switch. We aimed to assess the effectiveness, safety and tolerability of an ustekinumab originator-to-biosimilar switching program in inflammatory bowel disease (IBD).Methods Patients with IBD established on originator ustekinumab across 13 UK sites and undergoing a biosimilar switch were included. Each site determined the specific biosimilar to be used according to local medicines governance protocols. Retrospective review of prospectively collected records was performed 12-24 weeks after the switch. Baseline data included C-reactive protein (CRP), faecal calprotectin (FCP), clinical disease activity indices (Harvey-Bradshaw Index (HBI) for CD and Simple Clinical Colitis Activity Index (SCCAI) for UC) and a baseline clinician’s global assessment (CGA). Demographic data, treatment history and adverse drug reactions (ADR) were recorded. Wilcoxon signed rank test was used to compare pre- and post- switch variables via GraphPad Prism 10.6.1.Results We included 545 patients (293, 54% female) of whom 461 (85%) had CD and 84 (15%) had UC. Three biosimilars were used; Wezenla 423 (77%) patients, Pyzchiva 80 (15%) patients and Uzpruvo 42 (8%) patients. Prior treatment exposure was anti-tumour necrosis factor agents (anti-TNF) in 442 (81%) patients, vedolizumab in 28 (5%) patients and both anti-TNF and vedolizumab in 59 (11%) patients. 72 (13%) patients were advanced treatment naïve. Between baseline and week 12-24 there were no statistically significant differences in biochemical or clinical markers ( table 1), except CGA scores increased significantly post switch (p = 0.0001). However, the absolute change in mean CGA values was clinically insignificant (0.31 to 0.51). Biosimilar treatment persistence was observed in 495 (91%) patients. Of the remaining 50 (9%), 39 (7%) were switched to another treatment and 11 (2%) switched back to originator. 32 (6%) patients experienced an ADR; 5 (0.9%) patients reported an injection site reaction.Conclusion We observed high treatment persistence rates and stable clinical disease activity and biomarkers with ustekinumab biosimilars. Biosimilar ustekinumab was well tolerated and appears safe. This cohort will be evaluated again for 12-month outcomes.Abstract P213 Table 1Disease activity indices before switch and early after switchVariableMedian (IQR)Number of patientsPre-switchEarly post-switchP valueHBI2161 (0 – 3.2)2 (0 – 4)0.0594SCCAI431 (0 – 2)0 (0 – 2)0.2898CGA4300 (0 - 0)0 (0 - 1)0.0001CRP (mg/L)3634 (1 - 5)3 (1 - 5)0.075FCP (micrograms/g)149101 (30 - 266)84 (33 - 278)0.5383