Ustekinumab is a monoclonal antibody used in Crohn’s disease (CD). Dose optimization in case of non-response and the role of pharmacokinetic–pharmacodynamic (PK-PD) monitoring remain unresolved dilemmas in clinical practice. We aimed to develop a population PK-PD model for ustekinumab in CD and simulate efficacy of alternative dosing regimens. We included 57 patients and recorded their characteristics during 32 weeks after starting with ustekinumab therapy. Serum ustekinumab concentration was prospectively measured and fecal calprotectin (FC) concentration was used to monitor the disease activity. Ustekinumab PK-PD was described by a two-compartment target-mediated drug disposition model linked to an indirect response model. Lower fat-free mass, higher serum albumin, previous non-exposure to biologics, FCGR3A-158 V/V variant and lower C-reactive protein were associated with higher ustekinumab exposure. Model-based simulation suggested that 41.9% of patients receiving standard dosing achieve biochemical remission at week 32. In patients not achieving remission with standard dosing at week 16, transition to 4-weekly subcutaneous maintenance dosing with or without intravenous reinduction resulted in comparably higher remission rates at week 32 (51.1% vs. 49.2%, respectively). Our findings could be used to guide stratified ustekinumab treatment in CD, particularly in patients with unfavorable characteristics, who might benefit from early transition to 4-weekly maintenance dosing.

Background: Data on the incidence of IgG4-related disease (IgG4-RD) are scarce. Our aim was to determine the incidence of IgG4-RD in a well-defined region. Methods: This retrospective study covered the Ljubljana region over the period from January 2012 to December 2024. A review of cases diagnosed with IgG4-RD was performed at several departments of the University Medical Centre Ljubljana—an integrated secondary/tertiary university teaching hospital (rheumatology, nephrology, angiology, gastroenterology, abdominal surgery, ENT surgery, ophthalmology). While IgG4-RD cases at the Department of Rheumatology were collected prospectively, potential cases at other departments were retrieved by searching electronic medical database for the keyword “IgG4”. In addition, the Institute of Pathology, Faculty of Medicine, University of Ljubljana, provided a list of patients with histological features consistent with IgG4-RD. Year-specific incidence rates and an average incidence rate over the 13-year period were determined. Clinical features of patients were analysed. Results: During the observation period, 58 cases of IgG4-RD were diagnosed. Of these, 35 patients were residents of the Ljubljana region, which had an average adult population of 541,600. The estimated average annual incidence rate of IgG4-RD was 5.0 per million (95% confidence interval: 3.5; 6.9), with year-specific incidence rates fluctuating between 1.8 and 9.3 per million adults. The cases were stratified into four phenotypic categories: pancreato-hepato-biliary (17%), retroperitoneal fibrosis-aortitis (43%), head and neck-limited (14%), and Mikulicz syndrome with systemic involvement (26%). Conclusions: The average annual incidence rate of IgG4-RD was 5 per million adults, with the retroperitoneal fibrosis-aortitis phenotype predominating in our cohort.