A COMPARATIVE-STUDY OF NCA F-18 LABELING OF PROTEINS VIA ACYLATION AND PHOTOCHEMICAL CONJUGATION

Three methods for F-18-labeling of proteins were evaluated with respec t to conjugation yields, suitability for remote-controlled routine syn thesis, and in vivo stability of the conjugates-i.e., photochemical co njugation (PCC) using 4-azidophenacyl-[F-18]fluoride ([F-18]APF) as we ll as classical conjugation using LC-nitrophenyl 2-[F-18]fluoropropion ate ([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 ph otogeneration 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 accordi ng to a recently published method. Preparation of [F-18]SFB was achiev ed within 35 min with radiochemical yields of 55 +/- 10% by an improve d method using O-(N-succinimidyl)-N-N, N',N'-tetramethyluronium tetraf luoroborate (TSTU) as activating reagent. Compared to [F-18]ApF, prote in labeling with [F-18]NPFP and [F-18]SFB gave rise to considerably hi gher RCY, of up to 90%. Labeling studies showed that conjugation yield s using [F-18]NPFP depend on the lysine, tyrosine, and histidine conte nt of the proteins used, whereas conjugation with [F-18]APF and [F-18] SFB predominantly depends on the Lys content. Owing to competing O-acy lation of Tyr residues, [F-18]fluoropropionylated HSA was partially un stable under slightly basic conditions. Biodistribution studies with F -18-labeled HSA in NMRI mice revealed the highest in vivo stability fo r 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 fo r the labeling of antibodies.