 In Phase 1 Dose Escalation, patients will be randomized to either once-weekly Arm A or twice- weekly Arm B dosing with selinexor. Dose escalation will be performed within each arm for both lenalidomide and selinexor to determine the selinexor MTD for that arm. Each arm will be expanded until approximately 17 patients have been treated at the MTD in each arm. The Sponsor and Investigator will review the MTD, efficacy, and safety data from Phase 1 to determine which dose schedule Arm A or B to be used as the RP2D dose in the Expansion Phase. 