Modular Medical Imaging Agents Based on Azide-Alkyne Huisgen Cycloadditions: Synthesis and Pre-Clinical Evaluation of ¹⁸ F-Labeled PSMA-Tracers for Prostate Cancer Imaging

Since the seminal contribution of Rolf Huisgen to develop the [3+2] cycloaddition of 1,3-dipolar compounds, its azide-alkyne variant has established itself as the key step in numerous organic syntheses and bioorthogonal processes in materials science and chemical biology. In the present study, the copper(I)-catalyzed azide-alkyne cycloaddition was applied for the development of a modular molecular platform for medical imaging of the prostate-specific membrane antigen (PSMA), using positron emission tomography. This process is shown from molecular design, through synthesis automation and in vitro studies, all the way to pre-clinical in vivo evaluation of fluorine-18- labeled PSMA-targeting 'F-PSMA-MIC' radiotracers (t_1/2 =109.7 min). Pre-clinical data indicate that the modular PSMA-scaffold has similar binding affinity and imaging properties to the clinically used [⁶⁸ Ga]PSMA-11. Furthermore, we demonstrated that targeting the arene-binding in PSMA, facilitated through the [3+2]cycloaddition, can improve binding affinity, which was rationalized by molecular modeling. The here presented PSMA-binding scaffold potentially facilitates easy coupling to other medical imaging moieties, enabling future developments of new modular imaging agents.