Pustular Rash in Crohn's Patient on Ustekinumab Raises Concern for Drug-Induced Paradoxical Psoriasis.

We report the case of a 51-year-old male with Crohn's disease (CD) who developed a reproducible pustular rash after ustekinumab (UST) administration. The patient first presented with a pustular rash on his hands, body, extremities, and scalp starting 5 weeks after his initial weight-based UST induction. The rash resolved spontaneously, then recurred 4 weeks after his first subcutaneous maintenance dose of UST 90 mg. Biopsy of the affected area demonstrated subcorneal pustular dermatosis (SPD). UST was discontinued and the rash resolved. Unfortunately, the patient experienced clinical recurrence of CD, and given prior failure of multiple CD medications, UST was restarted with premedication. Two weeks after UST re-induction, the rash recurred, though less severe. Given improvement in CD symptoms, UST was continued and the rash managed with topical corticosteroids. This is the first case of drug-induced SPD associated with UST. One case report has previously described de novo pustular psoriasis associated with UST in a patient with CD and enteropathic arthritis. Notably, SPD and pustular psoriasis can be histologically indistinguishable. The development of a paradoxical psoriasiform rash is thought to be one of the few dose and duration dependent side effects of TNF-antagonist therapy but has not previously been established as a side effect of UST. This case demonstrates a new potential side effect of UST.