[¹⁸F]FBAM and [¹⁸F]FBOM: novel prosthetic groups for the mild labeling of thiol group-containing biomacromolecules

Introduction:

The incorporation of 18F into peptides and proteins usually takes advantage of prosthetic groups, also referred to as bifunctional labeling agents. This approach comprises 18F incorporation into a small organic molecule capable of being linked to peptides and proteins under mild conditions. This work deals with a comparative discussion on the synthesis and application of N-[6-(4-[18F]fluorobenzylidene)aminooxyhexyl]-maleimide ([18F]FBAM) and 4-[18F]fluorobenzaldehyde-O-(2-{2-[2-(pyrol-2,5-dion-1-yl)ethoxy]ethoxy}ethyl)oxim ([18F]FBOM) as novel prosthetic groups for the mild and selective conjugation to thiol group-containing biomacromolecules (Fig. 1).

Experimental:

The aminooxy-functionalized labeling precursor for radiosynthesis of [18F]FBAM was prepared in a convenient three-step synthesis sequence in a total yield of 59%. The corresponding labeling precursor for the radiosynthesis of [18F]FBOM succeeded in a four-step reaction sequence in 14% total yield. Formation of the prosthetic groups [18F]FBAM and [18F]FBOM was achieved through condensation reaction with [18F]fluorobenzaldehyde to form the desired oximes in radiochemical yields of 20-30% ([18F]FBAM) and of 14-19% ([18F]FBOM), respectively.

Results and Discussion:

The syntheses were carried out in a remotely-controlled radiofluorination module allowing the convenient and reliable performance of the radiolabeling reactions. The radiochemical purity exceeded 95% and the specific activity ranged from 50 to 80 GBq/µmol. The total synthesis time was 70 to 80 min. The lipophilicity was determined to be logP=2.71 for [18F]FBAM and logP=0.84 for [18F]FBOM. The usefulness of [18F]FBAM and [18F]FBOM as thiol-reactive group prosthetic group was demonstrated by the reaction with glutathion, low density lipoproteins (LDL) and modified neurotensin derivatives. Reaction of [18F]FBAM and [18F]FBOM with glutathione showed that the use of as little as 1 µg/ml glutathione after 30 min provides excellent radiochemical yields of 95% of the desired coupled products. Labeling of LDL showed superior results with [18F]FBAM, whereas comparable results for both prosthetic groups were obtained for the labeling of modified neurotensin derivatives.

Conclusion:

The ease of production and the excellent performance in radiolabeling reactions make compounds [18F]FBAM and [18F]FBOM very promising prosthetic groups for the mild and selective conjugation to thiol group-containing biomacromolecules.