Cyclohexanediamine Triazole (CHDT) Functionalization Enables Labeling of Target Molecules with Al 18 F/ 68 Ga/ 111 In.

The Al ¹⁸ F-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 (±)-H ₃ RESCA , which allows Al ¹⁸ F-labeling already at ambient temperature. While the suitability of (±)-H ₃ RESCA 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 Al ¹⁸ F-labeling, the cyclohexanediamine triazole (CHDT) moiety allows stable complexation of ⁶⁸ Ga and ¹¹¹ In. Three novel CHDT-functionalized PSMA inhibitors were synthesized and their Al ¹⁸ F-, ⁶⁸ Ga-, and ¹¹¹ In-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 Al ¹⁸ F-labeled PSMA ligands were characterized for their biodistribution in a LNCaP derived tumor xenograft mouse model by PET imaging. One radioligand, Al[ ¹⁸ F]F-CHDT-PSMA-1 , bearing a small azidoacetyl linker at the glutamate-urea-lysine motif, provided an in vivo performance comparable to that of [ ¹⁸ F]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 Al ¹⁸ F, ⁶⁸ Ga, and ¹¹¹ In and the triazole ring seems to entail favorable pharmacokinetic properties for molecular imaging purposes.