Patients receive selinexor orally (PO) once daily (QD) on days 1, 3, 8, 10, 15, and 17, carfilzomib intravenously (IV) over 30 minutes on days 1, 2, 8, 9, 15, and 16, and dexamethasone PO QD or IV on days 1, 2, 8, 9, 15, 16, 22, and 23. Courses repeat every 28 days in the absence of disease progression or unacceptable toxicity.