TREATMENT PHASE (COURSES 1-8): Patients receive carfilzomib intravenously (IV) over 30 minutes on days 1, 2, 8, 9, 15, and 16. Treatment repeats every 28 days for 8 courses in the absence of disease progression or unacceptable toxicity. Patients achieving less than partial response (PR) also receive dexamethasone orally (PO) or IV weekly in courses 4-8.