Clinical relevance of innovative immunoassays for serum ustekinumab and anti‐ustekinumab antibody levels in Crohn's disease

Background and Aim Ustekinumab is a human monoclonal antibody to the p40 subunit of human IL‐12/IL‐23. The purpose of this report is to verify the newly developed immunoassays for serum ustekinumab and anti‐ustekinumab antibody (AUA) concentrations and assess their clinical utility. Methods Serum ustekinumab trough levels and AUA levels were measured using new immunoassays in 38 patients with Crohn's disease under ustekinumab maintenance injection. Results Mean ustekinumab trough levels were 2.54 ± 2.1 μg/mL, and 3 of 38 patients (7.9%) were positive for AUAs. There was no association between ustekinumab trough levels and AUA levels. The optimal trough level of ustekinumab to maintain negative C‐reactive protein levels (≤ 0.3 mg/dL) was 1.67 μg/mL determined by receiver operating characteristic curve analysis. Ustekinumab trough level negatively but significantly correlated with C‐reactive protein, erythrocyte sedimentation rate, and Crohn's disease activity index and positively and significantly correlated with serum albumin levels. Ustekinumab trough levels were significantly higher in biologics‐naïve patients than in biologics‐experienced patients, although there was no difference in AUA levels. Conclusions We developed new assays for serum ustekinumab trough and AUA levels. These assays might provide new insights into therapeutic drug monitoring‐based management of Crohn's disease patients under ustekinumab therapy.