Supramolecular Chemistry Enables Radiolabeling of Proteins under Mild Conditions

Monoclonal antibodies have proven to be highly effective for in vivo targeting in nuclear medicine applications. However, typical radiolabeling methods, for PET/SPECT imaging and targeted radionuclide therapy, often require conditions that may affect the structural integrity of these fragile biomolecules. In this study, we introduce a novel methodology, based on supramolecular chemistry, for efficient radiolabeling of antibodies under mild conditions. Specifically, we report the synthesis of DOTA-cucurbit[7]uril (DOTA-CB[7]), a precursor that can be readily radiolabeled with a variety of radionuclides (¹¹¹In, ⁶⁸Ga, ⁸⁹Zr, ¹⁷⁷Lu, ²²⁵Ac). [¹¹¹In]In-DOTA-CB[7] was conjugated to an anti-HER2 antibody (trastuzumab) site─specifically grafted with adamantan-1-amine groups. The supramolecular conjugation, driven by the exceptionally strong affinity between CB[7] and adamantan-1-amine enabled rapid and efficient formation of a stable antibody-radiometal complex under mild conditions. The resulting radioimmunoconjugate demonstrated high in vitro and in vivo stability and effective targeting of HER2+ tumors, as evidenced by SPECT/CT imaging in mice xenografted with HER2+ SKOV-3 cells. These findings highlight the potential of DOTA-CB[7] for supramolecular radiolabeling, offering a simple and versatile alternative to traditional covalent approaches, particularly for applications requiring compatibility with heat-sensitive biomolecules.