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Importance: High-cost biologic therapies have transformed the management of immune-mediated inflammatory diseases. To optimize outcomes and reduce costs, dose adjustment informed by measurement of circulating drug levels has been shown to be effective in various settings. However, limited evidence exists for this approach with the interleukin 12 and interleukin 23 inhibitor ustekinumab.\ud \ud Objective: To evaluate clinical utility of therapeutic drug monitoring for ustekinumab in patients with psoriasis.\ud \ud Design, Setting, and Participants: A prospective observational cohort of 491 adults with psoriasis was recruited to the multicenter Biomarkers of Systemic Treatment Outcomes in Psoriasis study within the British Association of Dermatologists Biologic and Immunomodulators Register from June 2009 to December 2017; samples from some patients were taken between 2009 and 2011 as part of a pilot study with the same inclusion criteria.\ud \ud Exposure: Serum ustekinumab level measured at any point during the dosing cycle using an enzyme-linked immunosorbent assay.\ud \ud Main Outcomes and Measures: Disease activity measured using the Psoriasis Area and Severity Index (PASI) score. Treatment response outcomes were PASI75 (75% reduction in PASI score from baseline [primary outcome]), PASI90 (90% reduction of PASI score from baseline), and absolute PASI score of 1.5 or less.\ud \ud Results: A total of 491 patients (171 women and 320 men; mean [SD] age, 45.7 [12.8] years) had 1 or more serum samples (total, 853 samples obtained 0-56 weeks from start of treatment) and 1 or more PASI scores within the first year of treatment. Antidrug antibodies were detected in only 17 of 490 patients (3.5%). Early measured drug levels (1-12 weeks after starting treatment) were associated with PASI75 response 6 months after starting treatment (odds ratio, 1.38; 95% CI, 1.11-1.71) when adjusted for baseline PASI score, age, and ustekinumab dose. However, this finding was not consistent across the other PASI outcomes (PASI90 and PASI score of ≤1.5).\ud \ud Conclusions and Relevance: This real-world study provides evidence that measurement of early serum ustekinumab levels could be useful to direct the treatment strategy for psoriasis. Adequate drug exposure early in the treatment cycle may be particularly important in determining clinical outcome.

The identification of robust endotypes—disease subgroups of clinical relevance—is fundamental to stratified medicine. We hypothesized that HLA-C∗06:02 status, the major genetic determinant of psoriasis, defines a psoriasis endotype of clinical relevance. Using two United Kingdom–based cross-sectional datasets—an observational severe-psoriasis study (Biomarkers of Systemic Treatment Outcomes in Psoriasis; n = 3,767) and a large population-based bioresource (UK Biobank, including n = 5,519 individuals with psoriasis)—we compared demographic, environmental, and clinical variables of interest in HLA-C∗06:02–positive (one or two copies of the HLA-C∗06:02 allele) with those in HLA-C∗06:02‒negative (no copies) individuals of European ancestry. We used multivariable regression analyses to account for mediation effects established a priori. We confirm previous observations that HLA-C∗06:02–positive status is associated with earlier age of psoriasis onset and extend findings to reveal an association with disease expressivity in females (Biomarkers of Systemic Treatment Outcomes in Psoriasis: P = 2.7 × 10 –14, UK Biobank: P = 1.0 × 10 –8). We also show HLA-C∗06:02–negative status to be associated with characteristic clinical features (large plaque disease, OR for HLA-C∗06:02 = 0.73, P = 7.4 × 10 –4; nail involvement, OR = 0.70, P = 2.4 × 10 –6); higher central adiposity (Biomarkers of Systemic Treatment Outcomes in Psoriasis: waist circumference difference of 2.0 cm, P = 8.4 × 10 –4; UK Biobank: waist circumference difference of 1.4 cm, P = 1.5 × 10 –4), especially in women; and a higher prevalence of other cardiometabolic comorbidities. These findings extend the clinical phenotype delineated by HLA-C∗06:02 and highlight its potential as an important biomarker to consider in future multimarker stratified medicine approaches.