An immunoconjugate consisting of a monoclonal antibody directed against syndecan-1 (CD138) covalently attached to the maytansinoid DM4, a derivative of the cytotoxic agent maytansine (DM1), with potential antineoplastic activity. Upon administration, indatuximab ravtansine binds to syndecan-1-expressing tumor cells; upon internalization the DM4 moiety is released, binding to tubulin and disrupting microtubule assembly/disassembly dynamics, which may result in the inhibition of cell division and cell growth of syndecan-1-expressing tumor cells. Syndecan-1, a type 1 transmembrane proteoglycan and tumor-associated antigen (TAA), is overexpressed in a variety of cancer cells and plays a key role in the regulation of cell growth, differentiation, and adhesion.