A proprietary humanized monoclonal antibody against human sodium-dependent phosphate transport protein 2B (SLC34A2; NaPi2b), with potential antineoplastic activity. Upon administration of upifitamab, the antibody targets and binds to NaPi2b expressed on tumor cells. Although the tumor cell killing effects of XMT-1535 are not established, this binding may induce an antibody-dependent cellular cytotoxicity (ADCC)-mediated immune response against NaPi2b-expressing tumor cells, and/or may inhibit NaPi2b-mediated sodium and phosphate ion cotransport activity and ion-dependent tumor cell signaling. NaPi2b, a tumor-associated antigen (TAA), is overexpressed on a variety of tumor cells. It plays a key role in the transport of inorganic phosphate (Pi) and the maintenance of phosphate homeostasis.