Copper-free click chemistry with the short-lived positron emitter fluorine-18.

The copper-free strain-promoted click chemistry between (18)F-labeled aza-dibenzocyclooctyne [(18)F]FB-DBCO and various azides is described. [(18)F]FB-DBCO was prepared in 85% isolated radiochemical yield (decay-corrected) through acylation of amino aza-dibenzocyclooctyne 1 with N-succinimidyl 4-[(18)F]fluorobenzoate ([(18)F]SFB). [(18)F]FB-DBCO showed promising radiopharmacological profil with fast blood clearance as assessed with dynamic small animal PET studies. Metabolic stability of [(18)F]FB-DBCO was 60% of intact compound after 60 min post injection in normal Balb/C mice and blood clearance half-life was determined to be 53 s based on the time-activity-curve (TAC). Copper-free click chemistry was performed with various azides at low concentrations (1-2 μM) which differed in their structural complexity in different solvents (methanol, water, phosphate buffer and in bovine serum albumin (BSA) solution). Reaction proceeded best in methanol (>95% yield after 15 min at room temperature), whereas reaction in BSA required longer reaction times of 60 min and 40 °C upon completion.