During the treatment period, eligible patients will be included in the study and receive maintenance treatment with lenalidomide/dexamethasone versus lenalidomide/dexamethasone/MLN9708. Each cycle will last 28 days. Treatment arm A will consist of oral administration of 15 mg/day of oral lenalidomide on days 1-21, and 20 mg/day of dexamethasone administered orally on days 1-4 and 9-12 for a period of two years. Arm B of the maintenance treatment will be the same as arm A, with the addition of MLN9708 during the two year maintenance period, at a dose of 4 mg/day on days 1, 8 and 15 of the cycle.