Modulating radiochemical behaviour and biodistribution of ²¹¹ At-FAPI via host–guest complexation

Abstract

Radioastatinated fibroblast activation protein inhibitor (FAPI) ligands are promising candidates for targeted alpha therapy (TAT), yet their clinical translation is hampered by unfavourable physicochemical properties originating from conventional organotin-based astatination. The mandatory lipophilic aryl moiety introduced during radiolabelling causes poor aqueous solubility, and irregular in vivo radionuclide biodistribution, narrowing the effective application window of astatine radiotherapeutic agents. To address these radiochemical limitations, we developed a facile post-regulation supramolecular host–guest strategy using (2-hydroxypropyl)-α-cyclodextrin (HP-α-CD) for non-covalent encapsulation of radioastatinated FAPI, without covalent structural modification or auxiliary carrier scaffolds. Molecular docking and UV titration validated HP-α-CD as the optimal macrocycle for selective inclusion of the lipophilic halogenated aryl segment. NMR spectroscopic characterisation using stable iodine-127 surrogates confirmed specific host–guest complexation while preserving the integrity of the FAP-targeting pharmacophore. The resulting [ ¹³¹ I]I-IPBA- and [ ²¹¹ At]At-APBA-FAPI⊂CD complexes exhibited significantly improved aqueous solubility and favourable radiochemical stability. Cellular assays verified uncompromised FAP-specific binding ability after encapsulation. In vivo biodistribution measurements demonstrated that [ ²¹¹ At]At-APBA-FAPI⊂CD exhibits altered tumour uptake kinetics and substantially reduced non-specific pulmonary accumulation at 3 h post-injection (≈46 %), yielding improved tumour-to-nontarget ratios and mitigated radiation-induced organ damage. This work demonstrates that macrocyclic host–guest chemistry serves as an efficient radiochemical tool to modulate the physicochemical properties and in vivo behaviour of astatinated radiopharmaceuticals, providing a practical formulation optimisation strategy for ²¹¹ At-based targeted agents.