Definition

A radioimmunoconjugate containing YS5, a human immunoglobulin G1 (IgG1) monoclonal antibody directed against the cell surface antigen cluster of differentiation 46 (CD46; membrane cofactor protein; MCP), and linked, via the chelator desferrioxamine (DFO), to the radioisotope zirconium Zr 89 (Zr89), with potential diagnostic properties upon positron emission tomography (PET) imaging. Upon administration, the monoclonal antibody moiety of zirconium Zr 89-DFO-YS5 specifically targets and binds to a conformational epitope formed within Sushi domains (complement control protein repeats; CCPs) 1 and 2 on the immune modulatory receptor CD46 expressed on certain tumor cells. Upon binding, internalization and PET, CD46-expressing tumor cells can be imaged and assessed. The conformational epitope on CD46 is highly expressed in multiple tumor cell types while minimally expressed or absent in normal, healthy tissues. CD46 plays a key role in the negative regulation of innate immunity.