Carborane-NHS esters as versatile reagents for amine-selective small-molecule modification, bioconjugation, and peptide-polymer assembly

Carboranes - hydrophobic, boron-rich clusters with unique structural properties - offer broad potential in materials science and biomedicine. They can mimic carbocyclic frameworks for designing advanced materials such as polymers and MOFs, and when incorporated into biomolecules, reduce local polarity while enriching boron content: a feature attractive for boron neutron capture therapy (BNCT). Here, we report the development of organic carborane reagents based on N-hydroxysuccinimide (NHS) esters serving as versatile platform for selective conjugation to amine residues, enabling functionalization of small molecules, peptides, and antibodies. We further introduce bifunctional carborane reagents bearing two NHS ester sites, which mediate inter- and intramolecular coupling as well as peptide-driven polymerization. The resulting carborane-peptide polymers, composed of fragments with distinct physicochemical features, represent a new class of hybrid materials. To demonstrate biomedical utility, therapeutic antibodies (trastuzumab, cetuximab, and daratumumab) were conjugated with these reagents, attaching up to 13 carboranes per antibody. Linker length was found to be critical for maximizing loading efficiency. nLC-MS/MS mapping of trastuzumab conjugates identified 15 preferential lysine modification sites, located outside CDR regions. Importantly, trastuzumab-carborane conjugates retained native HER2-targeting activity in BT-474 cell assays. These structures, integrating therapeutic antibody efficacy with boron delivery capability, hold promise as multimodal agents for BNCT.