Cyclohexanediamine Triazole (CHDT) Functionalization Enables Labeling of Target Molecules with Al¹⁸F/⁶⁸Ga/¹¹¹In

The Al¹⁸F-labeling approach offers a one-step access to radiofluorinated biomolecules by mimicking the labeling process for radiometals. Although these labeling conditions are considered to be mild compared to classic radiofluorinations, improvements of the chelating units have led to the discovery of (±)-H₃RESCA, which allows Al¹⁸F-labeling already at ambient temperature. While the suitability of (±)-H₃RESCA for functionalization and radiofluorination of proteins is well established, its use for small molecules or peptides is less explored. Herein, we advanced this acyclic pentadentate ligand by introducing an alkyne moiety for the late-stage functionalization of biomolecules via click chemistry. We show that in addition to Al¹⁸F-labeling, the cyclohexanediamine triazole (CHDT) moiety allows stable complexation of ⁶⁸Ga and ¹¹¹In. Three novel CHDT-functionalized PSMA inhibitors were synthesized and their Al¹⁸F-, ⁶⁸Ga-, and ¹¹¹In-labeled analogs were subjected to a detailed in vitro radiopharmacological characterization. Stability studies in vitro in human serum revealed among others a high kinetic inertness of all radiometal complexes. Furthermore, the Al¹⁸F-labeled PSMA ligands were characterized for their biodistribution in a LNCaP derived tumor xenograft mouse model by PET imaging. One radioligand, Al[¹⁸F]F-CHDT-PSMA-1, bearing a small azidoacetyl linker at the glutamate-urea-lysine motif, provided an in vivo performance comparable to that of [¹⁸F]PSMA-1007 but with even higher tumor-to-blood and tumor-to-muscle ratios at 120 min p.i. Overall, our results highlight the suitability of the novel CHDT moiety for functionalization and radiolabeling of small molecules or peptides with Al¹⁸F, ⁶⁸Ga, and ¹¹¹In and the triazole ring seems to entail favorable pharmacokinetic properties for molecular imaging purposes.