Fluorine‐18 labelling of PNAs functionalized at their pseudo‐peptidic backbone for imaging studies with PET

Abstract

AbstractPeptide nucleic acids (PNAs) form a unique class of synthetic macromolecules, originally designed as ligands for the recognition of double‐stranded DNA, where the deoxyribose phosphate backbone of original DNA is replaced by a pseudo‐peptide N‐(2‐aminoethyl)glycyl backbone, while retaining the nucleobases of DNA. We have previously developed an original method to label oligonucleotide‐based macromolecules with the short‐lived positron‐emitter fluorine‐18 (t1/2: 109.8 min) using the N‐(4‐[18F]fluorobenzyl)‐2‐bromoacetamide reagent. Using this method, we herein report the fluorine‐18‐labelling of 13 decameric PNAs (OLP_1‐13), of the same sequence (CTCATACTCT), but presenting selected modification of the pseudo‐peptidic backbone at two or three of the thymine residues (positions 2, 5 and 8). Structural characteristics of these backbone modifications include either an amino acid side chain (L‐Lys, L‐Glu, L‐Leu and L‐Arg) or a glycosyl moiety (mannose, galactose, fucose, N‐Ac‐galactosamine and N‐Ac‐glucosamine) attached via an appropriate spacer. N‐(4‐[18F]fluorobenzyl)‐2‐bromoacetamide was synthesized in three radiochemical steps from 4‐cyano‐N,N,N‐trimethylanilinium trifluoromethanesulfonate and HPLC‐purified in 85–90 min (typical production: 3.7–4.8 GBq starting from a batch of 29.6–31.4 GBq of [18F]fluoride). Conjugation of the fluorine‐18‐labelled bromoacetamide reagent with the PNAs was performed in a mixture of acetonitrile and HEPES buffer (0.1 M, pH 7.9) for 10 min at 60°C and gave the corresponding pure labelled conjugated PNAs ([18F]c‐OLP_1‐13) after RP‐HPLC purification. The whole synthetic procedure, including the preparation of the fluorine‐18‐labelled reagent, provides up to 0.9 GBq (25 mCi) of HPLC‐purified [18F]c‐OLP_1‐13 in 160 min with a specific radioactivity of 45–65 GBq/µmol (1.2–1.7 Ci/µmol) at the end of synthesis starting from 29.6 to 31.4 GBq (800–850 mCi) of [18F]fluoride. Copyright © 2004 John Wiley & Sons, Ltd.