The development of very fast, clean, and selective methods for indirect labeling in PET tracer synthesis is an ongoing challenge. Here we present the development of an ultrafast photoclick method for the synthesis of short-lived 18 F-PET tracers based on the photocycloaddition reaction of 9,10-phenanthrenequinones with electron-rich alkenes. The respective precursors are synthetically easily accessible and can be functionalized with various target groups. Using a flow photo-microreactor, the photoclick reaction can be performed in 60 s, and clinically relevant tracers for prostate cancer and bacterial infection imaging were prepared to demonstrate practicality of the method.
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 (t1/2=109.7 min). Pre‐clinical data indicate that the modular PSMA‐scaffold has similar binding affinity and imaging properties to the clinically used [68Ga]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.
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