The use of pentafluorophenyl derivatives for the 18F labelling of proteins

Isotope Exchange-Based 18F-Labeling Methods

¹⁸F-Labeling methods for the preparation of ¹⁸F-labeled molecular probes can be classified into electrophilic fluorination, nucleophilic fluorination, metal-F coordination, and ¹⁸F/¹⁹F isotope exchange. Isotope exchange-based ¹⁸F-labeling methods demonstrate mild conditions featuring water resistance and facile high-performance liquid chromatography-free purification in direct ¹⁸F-labeling of substrates. This paper systematically reviews isotope exchange-based ¹⁸F-labeling methods sorted by the adjacent atom bonding with F, i.e., carbon and noncarbon atoms (Si, B, P, S, Ga, Fe, etc.). The respective isotope exchange mechanism, radiolabeling condition, radiochemical yield, molar activity, and stability of the ¹⁸F-product are mainly discussed for each isotope exchange-based ¹⁸F-labeling method as well as the cutting-edge application of the corresponding ¹⁸F-labeled molecular probes.

SNAr Radiofluorination with In Situ Generated [18F]Tetramethylammonium Fluoride

This report describes a method for the nucleophilic radiofluorination of (hetero)aryl chlorides, (hetero)aryl triflates, and nitroarenes using a combination of [¹⁸F]KF·K_2.2.2 and Me₄NHCO₃ for the in situ formation of a strongly nucleophilic fluorinating reagent (proposed to be [¹⁸F]Me₄NF). This method is applied to 24 substrates bearing diverse functional groups, and it generates [¹⁸F](hetero)aryl fluoride products in good to excellent radiochemical yields in the presence of ambient air/moisture. The reaction is applied to the preparation of ¹⁸F-labeled HQ-415 for potential (pre)clinical use.

One-pot synthesis and biodistribution of fluorine-18 labeled serum albumin for vascular imaging

INTRODUCTION

Equilibrium single-photon radionuclide imaging methods for assessing cardiac function and the integrity of the vascular system have long been in use for both clinical and research purposes. However, positron-emitting blood pool agents that could provide PET equivalents to these (and other) clinical procedures have not yet been adopted despite technical imaging advantages offered by PET. Our goal was to develop a PET blood pool tracer that not only meets necessary in vivo biological requirements but can be produced with an uncomplicated and rapid synthesis method which would facilitate clinical translation. Herein, albumin labeled with fluorine-18 was synthesized using a one-pot method and evaluated in vitro and in vivo in rats.

METHODS

A ligand (NODA-Bz-TFPE), containing NODA attached to a tetrafluorophenylester (TFPE) via a phenyl linker (Bz), was labeled with aluminum fluoride (Al[18F]F). Conjugation of the serum albumin with the ligand (Al[18F]F-NODA-Bz-TFPE), followed by purification (size exclusion chromatography), yielded the final product (Al[18F]F-NODA-Bz-RSA/HSA). In vitro stability was evaluated in human serum albumin by HPLC. Rat biodistributions and whole-body PET imaging over a 4 h time course were used for the in vivo evaluation.

RESULTS

This synthesis exhibited an overall radiochemical yield of 45 ± 10% (n = 30), a 50-min radiolabeling time, a radiochemical purity >99% and apparent stability up to 4 h in human serum. Blood had the highest retention of Al[18F]F-NODA-Bz-RSA at all times with a blood half-life of 5.2 h in rats. Al[18F]F-NODA-Bz-RSA distribution in most rat tissues remained relatively constant for up to 1 h, indicating that the tissue radioactivity content represents the respective tissue plasma volume. Dynamic whole-body PET images were in agreement with these findings.

CONCLUSIONS

A new ligand has been developed and radiolabeled with Al[18F]F that allows rapid (50-min) preparation of fluorine-18 serum albumin in one-pot. In addition to increased synthetic efficiency, the construct appears to be metabolically stable in rats. This method could encourage wider use of PET to quantify cardiac function and tissue vascular integrity in both research and clinical settings.

Small Prosthetic Groups in 18F-Radiochemistry: Useful Auxiliaries for the Design of 18F-PET Tracers

Prosthetic group (PG) applications in ¹⁸F-radiochemistry play a pivotal role among current ¹⁸F^-labeling techniques for the development and availability of ¹⁸F-labeled imaging probes for PET (Wahl, 2002) (¹). The introduction and popularization of PGs in the mid-80s by pioneers in ¹⁸F-radiochemistry has profoundly changed the landscape of available tracers for PET and has led to a multitude of new imaging agents based on simple and efficiently synthesized PGs. Because of the chemical nature of anionic ¹⁸F^- (apart from electrophilic low specific activity ¹⁸F-fluorine), radiochemistry before the introduction of PGs was limited to simple nucleophilic substitutions of leaving group containing precursor molecules. These precursors were not always available, and some target compounds were either hard to synthesize or not obtainable at all. Even with the advent of recently introduced "late-stage fluorination" techniques for the ¹⁸F-fluorination of deactivated aromatic systems, PGs will continue to play a central role in ¹⁸F-radiochemistry because of their robust and almost universal usability. The importance of PGs in radiochemistry is shown by its current significance in tracer development and exemplified by an overview of selected methodologies for PG attachment to PET tracer molecules. Especially, click-chemistry approaches to PG conjugation, while furthering the historical evolution of PGs in PET tracer design, play a most influential role in modern PG utilization. All earlier and recent multifaceted approaches in PG development have significantly enriched the contingent of modern ¹⁸F-radiochemistry procedures and will continue to do so.

Synthesis of fluorine-18 radio-labeled serum albumins for PET blood pool imaging

We sought to develop a practical, reproducible and clinically translatable method of radiolabeling serum albumins with fluorine-18 for use as a PET blood pool imaging agent in animals and man. Fluorine-18 radiolabeled fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester, [(18)F]F-Py-TFP was prepared first by the reaction of its quaternary ammonium triflate precursor with [(18)F]tetrabutylammonium fluoride ([(18)F]TBAF) according to a previously published method for peptides, with minor modifications. The incubation of [(18)F]F-Py-TFP with rat serum albumin (RSA) in phosphate buffer (pH9) for 15 min at 37-40 °C produced fluorine-18-radiolabeled RSA and the product was purified using a mini-PD MiniTrap G-25 column. The overall radiochemical yield of the reaction was 18-35% (n=30, uncorrected) in a 90-min synthesis. This procedure, repeated with human serum albumin (HSA), yielded similar results. Fluorine-18-radiolabeled RSA demonstrated prolonged blood retention (biological half-life of 4.8 hours) in healthy awake rats. The distribution of major organ radioactivity remained relatively unchanged during the 4 hour observation periods either by direct tissue counting or by dynamic PET whole-body imaging except for a gradual accumulation of labeled metabolic products in the bladder. This manual method for synthesizing radiolabeled serum albumins uses fluorine-18, a widely available PET radionuclide, and natural protein available in both pure and recombinant forms which could be scaled up for widespread clinical applications. These preclinical biodistribution and PET imaging results indicate that [(18)F]RSA is an effective blood pool imaging agent in rats and might, as [(18)F]HSA, prove similarly useful as a clinical imaging agent.

Radiopharmaceutical chemistry for positron emission tomography

Molecular imaging is an emerging technology that allows the visualization of interactions between molecular probes and biological targets. Molecules that either direct or are subject to homeostatic controls in biological systems could be labeled with the appropriate radioisotopes for the quantitative measurement of selected molecular interactions during normal tissue homeostasis and again after perturbations of the normal state. In particular, positron emission tomography (PET) offers picomolar sensitivity and is a fully translational technique that requires specific probes radiolabeled with a usually short-lived positron-emitting radionuclide. PET has provided the capability of measuring biological processes at the molecular and metabolic levels in vivo by the detection of the gamma rays formed as a result of the annihilation of the positrons emitted. Despite the great wealth of information that such probes can provide, the potential of PET strongly depends on the availability of suitable PET radiotracers. However, the development of new imaging probes for PET is far from trivial and radiochemistry is a major limiting factor for the field of PET. In this review, we provided an overview of the most common chemical approaches for the synthesis of PET-labeled molecules and highlighted the most recent developments and trends. The discussed PET radionuclides include ¹¹C (t₁(/)₂=20.4min), ¹³N (t₁(/)₂=9.9min), ¹⁵O (t₁(/)₂=2min), ⁶⁸Ga (t₁(/)₂=68min), ¹⁸F (t₁(/)₂=109.8min), ⁶⁴Cu (t₁(/)₂=12.7h), and ¹²⁴I (t₁(/)₂=4.12d).

Direct one-step 18F-labeling of peptides via nucleophilic aromatic substitution

Methods for the radiolabeling molecules of interest with [18F]-fluoride need to be rapid, convenient, and efficient. Numerous [18F]-labeled prosthetic groups, e.g., N-succinimidyl 4 [18F]-fluorobenzoate ([18F]-SFB), 4-azidophenacyl-[18F]-fluoride ([18F]-APF), and 1-(3-(2-[18F]fluoropyridin-3-yloxy)propyl)pyrrole-2,5-dione ([18F]-FpyMe), for conjugating to biomolecules have been developed. As the synthesis of these prosthetic groups usually requires multistep procedures, there is still a need for direct methods for the nucleophilic [18F]-fluorination of biomolecules. We report here on the development of a procedure based on the trimethylammonium (TMA) leaving group attached to an aromatic ring and activated with different electron-withdrawing groups (EWGs). A series of model compounds containing different electron-withdrawing substituents, a trimethylammonium leaving group, and carboxylic functionality for subsequent coupling to peptides were designed and synthesized. The optimal model compound, 2-cyano-4-(methoxycarbonyl)-N,N,N-trimethylbenzenaminium trifluoromethanesulfonate, was converted to carboxylic acid and coupled to peptides. The results of the one-step [18F]-fluorination of tetrapeptides and bombesin peptides show that the direct 18F-labeling of peptides is feasible under mild conditions and in good radiochemical yields.

One step radiosynthesis of 6-[(18)F]fluoronicotinic acid 2,3,5,6-tetrafluorophenyl ester ([(18)F]F-Py-TFP): a new prosthetic group for efficient labeling of biomolecules with fluorine-18

The labeling of biomolecules for positron emission tomography (PET) with no-carrier-added fluorine-18 is almost exclusively accomplished using prosthetic groups in a two step procedure. The inherent complexity of the process renders full automation a challenge and leads to protracted synthesis times. Here we describe a new (18)F-labeled prosthetic group based on nicotinic acid tetrafluorophenyl ester. Reaction of [(18)F]fluoride at 40 degrees C with the trimethylammonium precursor afforded 6-[(18)F]fluoronicotinic acid tetrafluorophenyl ester ([(18)F]F-Py-TFP) directly in 60-70% yield. [(18)F]F-Py-TFP was conveniently purified by Sep-Pak cartridge prior to incubation with a peptide containing the RGD sequence. The desired conjugate was formed rapidly and in good yields. An in vitro receptor-binding assay for the integrin alpha(v)beta(3) was established to explore competition with peptide and peptidomimetic prepared from F-Py-TFP with (125)I-echistatin. The nonradioactive conjugates were found to possess high binding affinities with calculated K(i) values in the low nanomolar range.

PET designated flouride-18 production and chemistry

Positron emission tomography (PET) is a nuclear medicine imaging technology which allows for four-dimensional, quantitative determination of the distribution of labeled biological compounds within the human body. PET is becoming an increasingly important tool for the measurement of physiological, biochemical and pharmacological functions at the molecular level in healthy and pathological conditions. This review will focus on Flouride-18, one of the common isotopes used for PET imaging, which has a half life of 109.8 minutes. This isotope can be produced with an efficient yield in a cyclotron as a nucleophile or as an electrophile. Flouride-18 can be thereafter introduced into small molecules or biomolecules using various chemical synthetic routes, to give the desired imaging agent.

Synthesis of 4-([18F]fluoromethyl)-2-chlorophenylisothiocyanate: A novel bifunctional 18F-labelling agent

The one-step radiosynthesis of 4-([18F]fluoromethyl)-2-chlorophenylisothiocyanate 18F-7 as a novel bifunctional 18F-labelling agent is described. Optimised reaction conditions in a remotely controlled synthesis module gave isothiocyanate 18F-7 in radiochemical yields of 45% (decay-corrected) within 40 min and high radiochemical purity of >95% after solid-phase-extraction. Coupling of compound 18F-7 with the primary amine benzylamine as a model reaction afforded the corresponding ((4-[18F]fluoromethyl)-2-chloro-phenyl)-benzyl thiourea 18F-8 in a high radiochemical yield of >90%. Stability studies of thiourea 18F-8 in terms of radiodefluorination showed appreciable buffer stability at pH 7.4, whereas significant radiodefluorination was observed when 18F-8 was incubated in buffers at pH 3.6 and pH 9.4. Preliminary dynamic PET studies with thiourea 18F-8 in male Wistar rats showed high bone accumulation, indicative of high in vivo radiodefluorination.

Kit-like 18F-labeling of proteins: synthesis of 4-(di-tert-butyl[18F]fluorosilyl)benzenethiol (Si[18F]FA-SH) labeled rat serum albumin for blood pool imaging with PET

Radiosyntheses of 18F-radiopharmaceuticals for positron emission tomography (PET) normally require an extraordinarily high effort of technical equipment and specially trained personnel. We recently reported a novel method for the introduction of fluorine-18 into peptides for PET-imaging based on silicon-18F-chemistry (SiFA technique). We herewith introduce the first SiFA-based Kit-like radio-fluorination of a protein (rat serum albumin,RSA) and demonstrate its usefulness for in vivo imaging with microPET in normal rats as well as in a rat heterotropic transplanted heart model. As a labeling agent, we prepared 4-(di-tert-butyl[18F]fluorosilyl)benzenethiol (Si[18F]FASH)by simple isotopic exchange in 40-60% radiochemical yield (RCY) and coupled it directly to a Sulfo-SMCC derivatized RSA in an overall RCY of 12% within 20-30 min. The technically simple labeling procedure does not require any elaborated purification procedures and is a straightforward example of a successful application of Si-18F chemistry for in vivo imaging with PET.

para-Functionalized aryl-di-tert-butylfluorosilanes as potential labeling synthons for (18)F radiopharmaceuticals

Broad spectrum: Novel para-functionalized aryl-di-tert-butylfluorosilanes, p-(tBu(2)FSi)C(6)H(4)X (X=functional group), have been made available and broaden the spectrum of silicon-based (18)F acceptors (SiFAs) for potential PET applications. For example, the [(18)F]maleimido derivative 1 has been employed for the synthesis of [(18)F]1- labeled rat serum albumin (RSA), the applicability of which for PET has been verified by in vivo experiments.The syntheses of the functionalized triorganofluorosilanes tBu(2)(p-XC(6)H(4))SiF (3 a, X=SH; 4 a, X=NCS; 4 b, X=NCO; 5, X=NC(4)H(2)O(2); 7, X=COOH; 8 a, X=COONC(4)H(4)O(2); 8 b, X=COOC(6)F(5)) are reported. These compounds display potential as silicon-based fluoride acceptors (SiFAs). The molecular structures of compounds 5, 7, and 8 a have been determined by single-crystal X-ray diffraction studies. With the exception of compounds 8 a and 8 b, all of the compounds could be (18)F-labeled by isotopic exchange in good to high radiochemical yields (RCY) with good to excellent specific activities. As proof of applicability, the maleimido-functionalized SiFA derivative 5, which is specific for thiol groups, has been used for the labeling of rat serum albumin (RSA) that had been derivatized with 2-iminothiolane. The incorporation of [(18)F]5 into the derivatized RSA reached a maximum yield after 30 min at ambient temperature. After purification, the [(18)F]RSA was evaluated in a healthy rat by means of muPET and displayed an expedient in vivo stability over 180 min.

Site-specific (18)F-labeling of the protein hormone leptin using a general two-step ligation procedure

The protein hormone leptin acts to regulate body fat and energy expenditure. Resistance to this hormone is implicated in human obesity and its pathophysiological consequences. In order to gain insight into the mechanism of leptin resistance, an (18)F-labeled derivative was developed to study the biodistribution of the hormone using positron emission tomography (PET). A two-step, site specific ligation approach was developed for this purpose, in which an aminooxy-reactive group was incorporated at the C-terminus of leptin using expressed protein ligation (EPL), which was subsequently derivatized with [ (18)F]fluorobenzaldehyde using an aniline-accelerated radiochemical oximation reaction. The modified hormone was shown to be biologically active in vitro and in vivo, and it was applied to PET imaging in ob/ ob mice. These protocols will allow for the routine production of site-specifically (18)F radiolabeled leptin, as well as other proteins, for use in PET imaging in systems from mouse to man.

Synthesis and application of [18F]FDG-maleimidehexyloxime ([18F]FDG-MHO): a [18F]FDG-based prosthetic group for the chemoselective 18F-labeling of peptides and proteins

2-[(18)F]Fluoro-2-deoxy-D-glucose ([(18)F]FDG) as the most important PET radiotracer is available in almost every PET center. However, there are only very few examples using [(18)F]FDG as a building block for the synthesis of (18)F-labeled compounds. The present study describes the use of [(18)F]FDG as a building block for the synthesis of (18)F-labeled peptides and proteins. [(18)F]FDG was converted into [(18)F]FDG-maleimidehexyloxime ([(18)F]FDG-MHO), a novel [(18)F]FDG-based prosthetic group for the mild and thiol group-specific (18)F labeling of peptides and proteins. The reaction was performed at 100 degrees C for 15 min in a sealed vial containing [(18)F]FDG and N-(6-aminoxy-hexyl)maleimide in 80% ethanol. [(18)F]FDG-MHO was obtained in 45-69% radiochemical yield (based upon [(18)F]FDG) after HPLC purification in a total synthesis time of 45 min. Chemoselecetive conjugation of [(18)F]FDG-MHO to thiol groups was investigated by the reaction with the tripeptide glutathione (GSH) and the single cysteine containing protein annexin A5 (anxA5). Radiolabeled annexin A5 ([(18)F]FDG-MHO-anxA5) was obtained in 43-58% radiochemical yield (based upon [(18)F]FDG-MHO, n = 6), and [(18)F]FDG-MHO-anxA5 was used for a pilot small animal PET study to assess in vivo biodistribution and kinetics in a HT-29 murine xenograft model.

Aspects of positron emission tomography radiochemistry as relevant for food chemistry

Positron emission tomography (PET) is a medical imaging technique using compounds labelled with short-lived positron emitting radioisotopes to obtain functional information of physiological, biochemical and pharmacological processes in vivo. The need to understand the potential link between the ingestion of individual dietary agents and the effect of health promotion or health risk requires the exact metabolic characterization of food ingredients in vivo. This exciting but rather new research field of PET would provide new insights and perspectives on food chemistry by assessing quantitative information on pharmocokinetics and pharmacodynamics of food ingredients and dietary agents. To fully exploit PET technology in food chemistry appropriately radiolabelled compounds as relevant for food sciences are needed. The most widely used short-lived positron emitters are (11)C (t(1/2) = 20.4 min) and (18)F (t(1/2) = 109.8 min). Longer-lived radioisotopes are available by using (76)Br (t(1/2) = 16.2 h) and (124)I (t(1/2) = 4.12 d). The present review article tries to discuss some aspects for the radiolabelling of food ingredients and dietary agents either by means of isotopic labelling with (11)C or via prosthetic group labelling approaches using the positron emitting halogens (18)F, (76)Br and (124)I.

18F-labelled annexin V: a PET tracer for apoptosis imaging

Annexin V can be used to detect apoptotic cells in vitro and in vivo, based on its ability to identify extracellular phosphatidylserine, which arises during apoptosis. In the present study, we examined the synthesis of fluorine-18 labelled annexin V as a positron emission tomography tracer for apoptosis imaging. The distribution of [18F]annexin V and technetium-99m labelled annexin V, a well-characterised SPET tracer for apoptosis imaging, was compared. [18F]annexin V was synthesised using N-succinimidyl 4-[18F]fluorobenzoate as an 18F labelling reagent. Synthesised and purified [18F]annexin V was confirmed by SDS-PAGE. In an ex vivo imaging experiment, [18F]annexin V was intravenously injected into rats 24 h after the induction of myocardial ischaemia, and accumulation in the left ventricle was examined. [18F]annexin V accumulated in the infarct area of the left ventricle, where apoptotic cells were observed. In separate experiments, [18F]annexin V or [(99m)Tc]annexin V was intravenously injected into ischaemic or normal animals, and the distribution of the tracers was compared. In ischaemic animals, accumulation of [18F]annexin V and [(99m)Tc]annexin V in the infarct area was about threefold higher than in the non-infarct area. Furthermore, the ratio of accumulation in the normal heart to the blood radioactivity was not significantly different between the tracers. In normal animals, however, the uptake of [18F]annexin V in the liver, spleen and kidney was much lower than that of [(99m)Tc]annexin V. The low uptake of [18F]annexin V in these organs might represent an advantage over [(99m)Tc]annexin V.

Radiolabelling of isopeptide N epsilon-(gamma-glutamyl)-L-lysine by conjugation with N-succinimidyl-4-[18F]fluorobenzoate

The isopeptide N(epsilon)-(gamma-glutamyl)-L-lysine 4 was labelled with 18F via N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB). A modified approach for the convenient synthesis of [18F]SFB was used, and [18F]SFB could be obtained in decay-corrected radiochemical yields of 44-53% (n = 20) and radiochemical purity >95% within 40 min after EOB. For labelling N(epsilon)-(gamma-glutamyl)-L-lysine with [18F]SFB the effects of isopeptide concentration, temperature, and pH were studied to determine the optimum reaction conditions. The coupling reaction was shown to be temperature and pH independent while being strongly affected by the isopeptide concentration. Using the optimized labelling conditions, in a typical experiment 1.3GBq of [18F]SFB could be converted into 447MBq (46%, decay-corrected) of [18F]fluorobenzoylated isopeptide within 45 min, including HPLC purification.

Characterizing multicomponent adsorbed protein films using electron spectroscopy for chemical analysis, time-of-flight secondary ion mass spectrometry, and radiolabeling: capabilities and limitations

Characterization of complex adsorbed protein films is a critical aspect of biomaterials science, particularly in understanding the in vivo response to biomaterials. The surface analysis techniques electron spectroscopy for chemical analysis (ESCA) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are particularly suited to the analysis of complex adsorbed protein films due to their wide applicability to a variety of materials. We have investigated the applicability of ESCA for studying the structure of adsorbed serum and plasma protein layers. ESCA was able to monitor the thickness of the adsorbed protein film. Due to its chemical specificity, ToF-SIMS was used to estimate the composition of the plasma and serum protein layers by comparison of their spectra with the spectra of single protein films. The limit of detection of ToF-SIMS for the plasma protein fibrinogen was determined by comparison with independent radiolabeled fibrinogen adsorption measurements. While ToF-SIMS was able to determine some qualitative trends in the composition of the plasma protein films as a function of adsorption time, the detection limit of the minor components in multicomponent adsorbed protein films ultimately limits the ability of ToF-SIMS to quantify the composition of these films. However, both ESCA and ToF-SIMS can provide useful information on adsorbed plasma protein films without further sample treatment. This study outlines the strengths and weaknesses of ESCA and ToF-SIMS for studying multicomponent adsorbed plasma protein films.

18F-labeled radiopharmaceuticals for PET in oncology, excluding FDG

This article reviews possible use of (18)F-labelled radiopharmaceuticals in oncology with positron emission tomography. The characteristics of various (18)F-labelled compounds are proteins and peptides, those that bind to. receptors, agents to assess hypoxia, and agents to evaluate gene therapy are highlighted. Furthermore, different (18)F-labelled tissue specific agents are indicated for the detection and monitoring of various malignancies: melanoma, brain tumours, breast cancer, prostate cancer and colorectal cancer. (18)F-fluorodeoxyglucose has been excluded from this summary.

Novel halogenated sulfonamides inhibit the growth of multidrug resistant MCF-7/ADR cancer cells

In this report, we describe the synthesis of halogenated benzenesulfonamide compounds and their ability to inhibit the growth of HeLa, MCF-7 and MCF-7/ADR tumor cells in vitro. The multidrug resistance (MDR) phenotype of certain cells does not affect their sensitivity to these compounds. These agents belong to a family of compounds previously shown to bind irreversibly to cysteine-239 of beta-tubulin. Consistent with this mechanism of action, the cytotoxicities of these compounds appear to correlate with their ability to undergo nucleophilic aromatic substitution.

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

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.