Direct ¹⁸F-Labeling Of Biomolecules Via Spontaneous Site-Specific Nucleophilic Substitution By F^- On Phosphonate Prostheses

Biomolecules such as amino acids,vitamins,and peptides play an indispensable role in almost all life processes,and are increasingly becoming important lead compounds for preparing positron emission tomography(PET)tracers.Traditional 18F-labeling methods are mostly based on 18F-C bond formation,which usually demands multiple steps including 18F-labeling under harsh reaction conditions.Current direct methods include Al-18F bond formation based on complex chelation,B/Si/P-18F bond formation based on 18F/19F isotopic exchange,however,separation of the desired 18F-labeled products from the precursors is not implemented in these methods.Herein,direct 18F-labeling on phosphonate prostheses via spontaneous F-nucleophilic substitution is comprehensively investigated.The reaction rate constants and corresponding activation energies of this nucleophilic addition-elimination reaction are determined both theoretically and experimentally.The influence of different reaction temperature,reaction time,reaction solvent,precursor load on the 18F-labeling reaction was explored.Rome temperature labeling for 5 min with 0.32 μmol precursor in anhydrous CH3CN was found to be the optimal labeling condition for attaining>75%RCYs.Under the optimal labeling conditions,various substrates bearing functional groups such as carboxyl,hydroxyl,amino and heterocyclic scaffolds,including quinolines,benzofurans,and benzothiazoles were labeled.Further more,medicinally important biomolecules,such as 18F-labled amino acid mimics,[18F]PFA-pyridoxamine,[18F]PFA-folic acid,and[18F]PFA-E[c(RGDyK)]2,were radio-synthesized in high radiochemical yields(21-68%)and high molar activity(35.9-55.1 GBq μmol-1).In summary,we have introduced a spontaneous F-site-specific nucleophilic substitution on phosphonates to rapidly label biomolecules with positron-emitting 18F in a single step.This feasible method was successfully automated on a commercial multifunction radiosynthesis module to allow an efficient synthesis of 18F-tracers with high Am.We believe that this methodology will provide a valuable strategy for the direct nucleophilic 18F-labeling of some of the most challenging biomolecules.