During phase 1a, patients will receive 1 cycle of ACY-241 monotherapy. Patients who complete the ACY-241 monotherapy cycle without a dose limiting toxicity (DLT) and are clinically stable may continue to phase 1b combination therapy, beginning with Cycle 2. During phase 1b, patients will receive ACY 241 in combination with pomalidomide and low dose dexamethasone at the currently approved pomalidomide dose and schedule. Each cohort of patients in phase 1a and phase 1b will consist of at least 3 patients. The first patient enrolled in each cohort of phase 1a will be observed for 1 week before enrollment of subsequent patients in the cohort. Patients who withdraw consent before entering phase 1b will be replaced. (Replacement patients must complete phase 1a prior to continuing to phase 1b.) Patients who experience a DLT or other unacceptable toxicity in Cycle 1 of phase 1a monotherapy or Cycle 2, the first cycle of phase 1b combination therapy, will be removed from study treatment. An assessment of the safety of treatment will be completed by the Safety Review Committee (SRC) prior to dose-escalation.