Palladium-Mediated Incorporation of Carboranes into SmallMolecules, Peptides, and Proteins

Carboranes represent a class of compounds withincreasing therapeutic potential. However, few general approachesto readily embed carboranes into small molecules, peptides, andproteins are available. We report a strategy based on palladium-mediated C-X (X = C, S, and N) bond formation for theinstallation of carborane-containing moieties onto small moleculesand peptides. We demonstrate the ability of Pd-based reagents withappropriate ligands to overcome the high hydrophobicity of thecarborane group and enable chemoselective conjugation of cysteineresidues at room temperature in aqueous buffer. Accordingly,carboranes can be efficiently installed on proteins by employing acombination of a bis-sulfonated biarylphosphine-ligated Pd reagentin an aqueous histidine buffer. This method is successfully employed on nanobodies, a fully synthetic affibody, and the antibodytherapeutics trastuzumab and cetuximab. The conjugates of the affibody ZHER2and the trastuzumab antibody retained binding totheir target antigens. Conjugated proteins maintain their activity in cell-based functional assays in HER2-positive BT-474 cell lines.This approach enables the rapid incorporation of carborane moieties into small molecules, peptides, and proteins for furtherexploration in boron neutron capture therapy, which requires the targeted delivery of boron-dense groups.