A comparative study of NCA fluorine-18 labeling of proteins via acylation and photochemical conjugation

Three methods for F-18-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-[F-18]fluoride ([F-18]APF) as well as classical conjugation using LC-nitrophenyl 2-[F-18]fluoropropionate ([F-18]NPFP) and N succinimidyl 4-[F-18]fluorobenzoate ([F-18]SFB). is to 10% within about 15 min. The F-18-labeling was performed by photogeneration of the corresponding [F-18]arylnitrene by irradiating [(18)F]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 [(18)]NPFP was synthesized according to a recently published method. Preparation of [F-18]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 [F-18]ApF, protein labeling with [F-18]NPFP and [F-18]SFB gave rise to considerably higher RCY, of up to 90%. Labeling studies showed that conjugation yields using [F-18]NPFP depend on the lysine, tyrosine, and histidine content of the proteins used, whereas conjugation with [F-18]APF and [F-18]SFB predominantly depends on the Lys content. Owing to competing O-acylation of Tyr residues, [F-18]fluoropropionylated HSA was partially unstable under slightly basic conditions. Biodistribution studies with F-18-labeled HSA in NMRI mice revealed the highest in vivo stability for the [F-18]SFB conjugate. Based on these results, [F-18]SFB seems to be the most suitable F-18-labeling agent for proteins, particularly for the labeling of antibodies.