Multivalent ²¹¹At-Radiolabeled Fibroblast Activation Protein Inhibitors for Targeted Alpha-Therapy of Glioma

Multivalent ligand strategy prevails recently in the design of radioligands for small-molecules due to its successes in enhancing the therapeutic efficacy of radiometallic ligands. In this work, ²¹¹At-FAPI-Dimer and ²¹¹At-APBA-FAPI-Dimer, two multivalent ²¹¹At-conjugated fibroblast activation protein inhibitors (FAPIs), were successfully synthesized with the expectation of narrowing the significant gap between extant radioastatinated FAPIs and clinical application. Cellular pharmacokinetic assessments show that the multivalent strategy could improve the cellular internalization and retention of the radioligand but compromise cellular uptake efficiency relative to monovalent FAPI ligand. Molecular simulations reveal that FAPI-Dimer and IPBA-FAPI-Dimer were able to bind to the fibroblast activation protein-α (FAPα) via hydrogen bonding and hydrophobic forces, but the total binding capacity was obviously weaker than that of monovalent IPBA-FAPI. In addition, despite longer tumor retention, ²¹¹At-FAPI-Dimer and ²¹¹At-APBA-FAPI-Dimer did not present expected advantages in ²¹¹At endoradiotherapy of murine xenograft models over corresponding monovalent ²¹¹At-APBA-FAPI. All these findings suggest that the application of multivalent ligand strategies in therapeutic radiopharmaceuticals should be approached with caution. In particular, more rational molecular design is warranted for ²¹¹At-labeled FAPIs.