Spontaneous remission in patients with minimal change disease (MCD) occurs in 40% of cases. Approximately 80 to 90% of cases respond to initial therapy with corticosteroids. It is important to continue therapy for another 2 weeks after remission and change to a maintenance regimen to lower the possibility of relapse of MCD. For those patients who do not respond to corticosteroid therapy, which of the following is the most appropriate next step in treatment?