Siebert S, et al. - In serum samples obtained, interleukin-17A (IL-17A), IL-17F, IL-23, and CRP concentrations were measured as part of the 2 PSUMMIT phase III studies of ustekinumab in psoriatic arthritis (PsA) (n = 927) in order to assess the relations of CRP and circulating Th17-associated cytokine levels with PsA disease activity and therapeutic response to ustekinumab. Baseline serum IL-23/IL-17 levels associated with skin, although not with joint, disease activity, implying a tissue-specific difference. Nonetheless, neither baseline Th17-associated cytokine levels nor CRP level were portentous of therapeutic response to ustekinumab in the skin or joints, irrespective of accelerated declines in their levels after ustekinumab therapy.