Importance Ustekinumab is a human monoclonal antibody that binds to the shared p40 subunit of interleukin (IL) 12 and IL-23. It is approved in the United States for adults (18 years) with moderate to severe plaque psoriasis who are candidates for phototherapy or systemic therapy. In 1 phase 2 trial of ustekinumab for treatment of psoriatic arthritis, joint disease improved. Observation We report 4 cases of ustekinumab monotherapy for plaque psoriasis that resulted in disabling flares of known psoriatic arthritis or unmasked previously occult joint disease. In all of our cases, psoriasis improved dramatically with ustekinumab therapy while psoriatic arthritis flared. Conclusions And Relevance Despite early results of a phase 2 ustekinumab trial suggesting efficacy for both plaque psoriasis and psoriatic arthritis, our case series raises concern that ustekinumab may unmask or aggravate joint disease in selected patients. These data underscore the need for further investigation of ustekinumab's effects on psoriatic arthritis.
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