A drug used to treat a rare condition called Castleman disease in patients who do not have HIV or human herpesvirus 8. It is also being studied in the treatment of multiple myeloma. Sylvant binds to a protein called interleukin-6 (IL-6), which is made by some white blood cells and other cells in the body. Sylvant may help reduce inflammation and stop the growth of cancer cells or abnormal blood cells. It is a type of monoclonal antibody. Also called anti-IL-6 chimeric monoclonal antibody, cCLB8, CNTO 328, and siltuximab.