A comparative study of N.C.A. Fluorine-18 labeling of proteins via acylation and photochemical conjugation

Abstract

Three methods for 18F-labeling of proteins were evaluated with respect to conjugation yields, suitability for remote-controlled routine synthesis, and in vivo stability of the conjugates— i.e., photochemical conjugation (PCC) using 4-azidophenacyl-[ 18F]fluoride ([ 18F]APF) as well as classical conjugation using 4-nitrophenyl 2-[ 18F]fluoropropionate ([ 18F]NPFP) and N-succinimidyl 4-[18F]fluorobenzoate ([ 18F]SFB). For this purpose, [ 18F]APF was synthesized in one step with a radiochemical yield (RCY) of up to 70% within about 15 min. The 18F-labeling was performed by photogeneration of the corresponding [ 18F]arylnitrene by irradiating [ 18F]APF with UV light in presence of the protein in aqueous buffered solution. Using this procedure, human serum albumin (HSA), transferrin, IgG, and avidin were labeled. The [ 18F]NPFP was synthesized according to a recently published method. Preparation of [ 18F]SFB was achieved within 35 min with radiochemical yields of 55 ± 10% by an improved method using O-( N-succinimidyl)- N-N,N′,N′-tetramethyluronium tetrafluoroborate (TSTU) as activating reagent. Compared to [ 18F]APF, protein labeling with [ 18F]NPFP and [ 18F]SFB gave rise to considerably higher RCY, of up to 90%. Labeling studies showed that conjugation yields using [ 18F]NPFP depend on the lysine, tyrosine, and histidine content of the proteins used, whereas conjugation with [ 18F]APF and [ 18F]SFB predominantly depends on the Lys content. Owing to competing O-acylation of Tyr residues, [ 18F]fluoropropionylated HSA was partially unstable under slightly basic conditions. Biodistribution studies with 18F-labeled HSA in NMRI mice revealed the highest in vivo stability for the [ 18F]SFB conjugate. Based on these results, [ 18F]SFB seems to be the most suitable 18F-labeling agent for proteins, particularly for the labeling of antibodies.