New Bifunctional Chelators Incorporating Dibromomaleimide Groups for Radiolabeling of Antibodies with Positron Emission Tomography Imaging Radioisotopes

Abstract

Positron Emission Tomography (PET) imaging with antibody-based contrast agents frequently uses the radioisotopes [⁶⁴Cu]Cu²⁺ and [⁸⁹Zr]Zr⁴⁺. The macrobicyclic chelator commonly known as sarcophagine (sar) is ideal for labeling receptor-targeted biomolecules with [⁶⁴Cu]Cu²⁺. The siderophore chelator, desferrioxamine-B (dfo), has been widely used to incorporate [⁸⁹Zr]Zr⁴⁺ into antibodies. Here, we describe new bifunctional chelators of sar and dfo: these chelators have been functionalized with dibromomaleimides (dbm), that enable site-specific and highly stable attachment of molecular cargoes to reduced, solvent-accessible, interstrand native disulfide groups. The new sar-dbm and dfo-dbm derivatives can be easily conjugated with the IgG antibody trastuzumab via reaction with reduced interstrand disulfide groups to give site-specifically modified dithiomaleamic acid (dtm) conjugates, sar-dtm-trastuzumab and dfo-dtm-trastuzumab, in which interstrand disulfides are rebridged covalently with a small molecule linker. Both sar- and dfo-dtm-trastuzumab conjugates have been radiolabeled with [⁶⁴Cu]Cu²⁺ and [⁸⁹Zr]Zr⁴⁺, respectively, in near quantitative radiochemical yield (>99%). Serum stability studies, <i>in vivo</i> PET imaging, and biodistribution analyses using these radiolabeled immunoconjugates demonstrate that both [⁶⁴Cu]Cu-sar-dtm-trastuzumab and [⁸⁹Zr]Zr-dfo-dtm-trastuzumab possess high stability in biological milieu. Dibromomaleimide technology can be easily applied to enable stable, site-specific attachment of radiolabeled chelators, such as sar and dfo, to native interstrand disulfide regions of antibodies, enabling tracking of antibodies with PET imaging.