Responsiveness of Serum C-Reactive Protein, Interleukin-17A, and Interleukin-17F Levels to Ustekinumab in Psoriatic Arthritis: Lessons From Two Phase III, Multicenter, Double-Blind, Placebo-Controlled Trials

Objective To evaluate the associations of C-reactive protein (CRP) and circulating Th17-associated cytokine levels with psoriatic arthritis (PsA) disease activity and therapeutic response to ustekinumab. Methods Interleukin-17A (IL-17A), IL-17F, IL-23, and CRP concentrations were measured in serum samples collected as part of the 2 PSUMMIT phase III studies of ustekinumab in PsA (n = 927). In post hoc analyses, relationships of IL-17A, IL-17F, and CRP levels at baseline, week 4, and week 24 with baseline skin and joint disease activity and response to therapy were evaluated using generalized linear models and Pearson's product-moment correlation tests. Results Baseline serum levels of IL-17A and IL-17F were positively correlated with baseline skin disease scores (r = 0.39-0.62). IL-23 levels were correlated with skin disease scores to a lesser extent (r = 0.26-0.31). No significant correlations were observed between these cytokine or CRP levels and baseline joint disease activity. There was no significant association of baseline levels of IL-17A, IL-17F, IL-23, or CRP with therapeutic response to ustekinumab in either the skin or joints. Significant reductions from baseline in levels of IL-17A, IL-17F, and CRP were seen in patients treated with ustekinumab compared to those treated with placebo. Ustekinumab-treated patients in whom 75% improvement in the Psoriasis Area and Severity Index score or 20% improvement according to the American College of Rheumatology criteria was achieved after 24 weeks of treatment had greater reductions in CRP level (geometric mean decreases of 51-58% versus 32-33%; P < 0.05), but not IL-17A or IL-17F levels, than nonresponders. Conclusion Baseline serum IL-23/IL-17 levels correlated with skin, but not joint, disease activity, suggesting tissue-specific variation. However, neither baseline Th17-associated cytokine levels nor CRP level were predictive of therapeutic response to ustekinumab in the skin or joints, despite rapid reductions in their levels following ustekinumab therapy.