A 29-year-old woman with a 5-year history of Crohn disease involving the ileum and right colon has been maintained on mesalamine and a course of prednisone for flares. Over the past 3 months, attempts to taper prednisone below 20 mg daily have been unsuccessful, and she remains steroid-dependent despite the addition of azathioprine at standard dosing. She continues to experience abdominal pain and intermittent diarrhea. Which of the following is the most appropriate next step in her management?