**1. Introduction**

Multiple myeloma (MM), a malignancy of plasma cells, ranks second among all blood cancers in the U.S., representing about 1% of all diagnosed malignancies. In 2020, an estimated 32,270 individuals (54.3% male), the majority over age 65, will be diagnosed with the disease and approximately 12,830 will succumb to it [1]. Classical symptoms of active MM include hypercalcemia, renal insufficiency, anemia, and bone lesions (CRAB) and often are preceded by an asymptomatic stage referred to as monoclonal gammopathy of undetermined significance (MGUS). The risk of progression from MGUS to MM is about 1% per year [2] and may include another asymptomatic state known as smoldering myeloma [3]. Current guidelines for the diagnosis and treatment of MM have been published by the National Comprehensive Cancer Network (NCCN) [4]. Treatment modalities for MM have seen enormous advances since the beginning of the current century with the introduction of drugs working by different mechanisms, e.g., proteasome inhibition (bortezomib, carfilzomib, and ixazomib) and immunomodulation (lenalidomide

and pomalidomide), which were added to the long-established treatments based on alkylating agents (melphalan and cyclophosphamide) and corticosteroids (dexamethasone). These measures, together with autologous stem cell transplantation (ASCT), first introduced for MM in the 1990's, have increased the five-year survival rate for the disease from 24% in the mid-1970s to 55% in the 2010–2016 period [5]. Furthermore, the relatively recent arrival on the scene of monoclonal antibodies (mAbs), beginning with the U.S. Food and Drug Administration (FDA) approval of daratumumab in 2015, has greatly expanded the therapeutic options available to treat MM. However, in spite of these advances, MM remains incurable as patient relapse and refractoriness to treatment continue as major issues. This review focuses on the contributions made by those mAbs currently approved for MM, as well as on those under investigation as potential future therapies for this disease.
