Prospective study to characterize adalimumab exposure in pediatric patients with rheumatic diseases

Background: In pediatric rheumatic diseases (PRD), adalimumab is dosed using fixed weight-based bands irrespective of methotrexate co-treatment, disease activity (DA) or other factors that might influence adalimumab pharmacokinetics (PK). In rheumatoid arthritis (RA) adalimumab exposure between 2–8 mg/L is associated with clinical response. PRD data on adalimumab is scarce. Therefore, this study aimed to analyze adalimumab PK and its variability in PRD treated with/without methotrexate. Methods: A two-center prospective study in PRD patients aged 2–18 years treated with adalimumab and methotrexate (G_A-M) or adalimumab alone (G_A) for ≥ 12 weeks was performed. Adalimumab concentrations were collected 1–9 (maximum concentration; C_max), and 10–14 days (minimum concentration; C_min) during ≥ 12 weeks following adalimumab start. Concentrations were analyzed with enzyme-linked immunosorbent assay (lower limit of quantification: 0.5 mg/L). Log-normalized C_min were compared between G_A-M and G_A using a standard t-test. Results: Twenty-eight patients (14 per group), diagnosed with juvenile idiopathic arthritis (71.4%), non-infectious uveitis (25%) or chronic recurrent multifocal osteomyelitis (3.6%) completed the study. G_A-M included more females (71.4%; G_A 35.7%, p = 0.13). At first study visit, children in G_A-M had a slightly longer exposure to adalimumab (17.8 months [IQR 9.6, 21.6]) compared to G_A (15.8 months [IQR 8.5, 30.8], p = 0.8). Adalimumab dosing was similar between both groups (median dose 40 mg every 14 days) and observed DA was low. Children in G_A-M had a 27% higher median overall exposure compared to G_A, although median C_min adalimumab values were statistically not different (p = 0.3). C_min values ≥ 8 mg/L (upper limit RA) were more frequently observed in G_A-M versus G_A (79% versus 64%). Overall, a wide range of C_min values was observed in PRD (0.5 to 26 mg/L). Conclusion: This study revealed a high heterogeneity in adalimumab exposure in PRD. Adalimumab exposure tended to be higher with methotrexate co-treatment compared to adalimumab monotherapy although differences were not statistically significant. Most children showed adalimumab exposure exceeding those reported for RA with clinical response, particularly with methotrexate co-treatment. This highlights the need of further investigations to establish model-based personalized treatment strategies in PRD to avoid under- and overexposure. Trial registration: NCT04042792 , registered 02.08.2019.