One-step synthesis of 18F labeled [18F]-N-succinimidyl 4-(fluoromethyl)benzoate for protein labeling

Radiolabelled Peptides for Positron Emission Tomography and Endoradiotherapy in Oncology

This review deals with the development of peptide-based radiopharmaceuticals for the use with positron emission tomography and peptide receptor radiotherapy. It discusses the pros and cons of this class of radiopharmaceuticals as well as the different labelling strategies, and summarises approaches to optimise metabolic stability. Additionally, it presents different target structures and addresses corresponding tracers, which are already used in clinical routine or are being investigated in clinical trials.

Development of [18F]AlF-NOTA-NT as PET Agents of Neurotensin Receptor-1 Positive Pancreatic Cancer

Several studies have suggested that neurotensin receptors (NTRs) and neurotensin (NT) greatly affect the growth and survival of pancreatic ductal adenocarcinoma (PDAC). Developing NTR-targeted PET probes could therefore be important for the management of a pancreatic cancer patient by providing key information on the NTR expression profile noninvasively. Despite the initial success on the synthesis of ¹⁸F-labeled NT PET probes, the labeling procedure generally requires lengthy steps including azeotropic drying of ¹⁸F. Using a straightforward chelation method, here we report the simple preparation of aluminum-¹⁸F-NOTA-NT starting from aqueous ¹⁸F. The cell binding test demonstrated that [¹⁹F]AlF-NOTA-NT maintained high receptor-binding affinity to NTR1. This probe was then further evaluated in NTR1 positive pancreatic tumor models (AsPC-1 and PANC-1). After the administration of [¹⁸F]AlF-NOTA-NT, small animal PET studies showed a high contrast between tumor and background in both models at 1 and 4 h time points. A blocking experiment was performed to demonstrate the receptor specificity: the tumor uptake in AsPC1 without and with blocking agent was 1.0 ± 0.2 and 0.1 ± 0.0%ID/g, respectively, at 4 h post injection. In summary, a NTR specific PET agent, [¹⁸F]AlF-NOTA-NT, was prepared through the simple chelation method. This NTR-targeted PET probe may not only be used to detect NTR1 positive pancreatic tumors (diagnosis), but also it may be fully integrated to NTR target therapy leading to personalized medicine (theranostic).

18F-AlF Labeled Peptide and Protein Conjugates as Positron Emission Tomography Imaging Pharmaceuticals

The clinical applications of positron emission tomography (PET) imaging pharmaceuticals have increased tremendously over the past several years since the approval of ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) by the Food and Drug Administration (FDA). Numerous ¹⁸F-labeled target-specific potential imaging pharmaceuticals, based on small and large molecules, have been evaluated in preclinical and clinical settings. ¹⁸F-labeling of organic moieties involves the introduction of the radioisotope by C-¹⁸F bond formation via a nucleophilic or an electrophilic substitution reaction. However, biomolecules, such as peptides, proteins, and oligonucleotides, cannot be radiolabeled via a C-¹⁸F bond formation as these reactions involve harsh conditions, including organic solvents, high temperature, and nonphysiological conditions. Several approaches, including ¹⁸F-labeled prosthetic groups, silicon, boron, and aluminum fluoride acceptor chemistry, and click chemistry have been developed, in the past, for ¹⁸F labeling of biomolecules. Linear and macrocyclic polyaminocarboxylates and their analogs and derivatives form thermodynamically stable and kinetically inert aluminum chelates. Hence, macrocyclic polyaminocarboxylates have been used for conjugation with biomolecules, such as folate, peptides, affibodies, and protein fragments, followed by ¹⁸F-AlF chelation, and evaluation of their targeting abilities in preclinical and clinical environments. The goal of this report is to provide an overview of the ¹⁸F radiochemistry and ¹⁸F-labeling methodologies for small molecules and target-specific biomolecules, a comprehensive review of coordination chemistry of Al³⁺, ¹⁸F-AlF labeling of peptide and protein conjugates, and evaluation of ¹⁸F-labeled biomolecule conjugates as potential imaging pharmaceuticals.

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.

A 18F-labeled dibenzocyclooctyne (DBCO) derivative for copper-free click labeling of biomolecules

The new prosthetic group ¹⁸F-TEG-DBCO (dibenzocyclooctyne) can be prepared within a total reaction time of 60 min including purification with an overall yield (n.d.c.) of 34 ± 5%. Copper-free click cycloadditions with various biomolecule-azides resulted in very high RCYs under mild conditions.

18F-labeling using click cycloadditions

Due to expanding applications of positron emission tomography (PET) there is a demand for developing new techniques to introduce fluorine-18 (t_1/2 = 109.8 min). Considering that most novel PET tracers are sensitive biomolecules and that direct introduction of fluorine-18 often needs harsh conditions, the insertion of ¹⁸F in those molecules poses an exceeding challenge. Two major challenges during ¹⁸F-labeling are a regioselective introduction and a fast and high yielding way under mild conditions. Furthermore, attention has to be paid to functionalities, which are usually present in complex structures of the target molecule. The Cu-catalyzed azide-alkyne cycloaddition (CuAAC) and several copper-free click reactions represent such methods for radiolabeling of sensitive molecules under the above-mentioned criteria. This minireview will provide a quick overview about the development of novel ¹⁸F-labeled prosthetic groups for click cycloadditions and will summarize recent trends in copper-catalyzed and copper-free click ¹⁸F-cycloadditions.

Anti-CEA antibody fragments labeled with [(18)F]AlF for PET imaging of CEA-expressing tumors

A facile and rapid method to label peptides with (18)F based on chelation of [(18)F]AlF has been developed recently. Since this method requires heating to 100 °C, it cannot be used to label heat-sensitive proteins. Here, we used a two-step procedure to prepare (18)F-labeled heat-labile proteins using the [(18)F]AlF method based on hot maleimide conjugation. 1,4,7-Triazacyclononae-1,4-diacetate (NODA) containing a methyl phenylacetic acid group (MPA) functionalized with N-(2-aminoethyl)maleimide (EM) was used as a ligand which was labeled with [(18)F]AlF and then conjugated to the humanized anti-CEA antibody derivatives hMN-14-Fab', hMN-14-(scFv)2 (diabody), and a Dock-and-Lock engineered dimeric fragment hMN-14 Fab-AD2 at room temperature. The in vivo tumor targeting characteristics of the (18)F-labeled antibody derivatives were determined by PET imaging of mice with s.c. xenografts. NODA-MPAEM was radiolabeled with [(18)F]AlF at a specific activity of 29-39 MBq/nmol and a labeling efficiency of 94 ± 2%. The labeling efficiencies of the maleimide conjugation ranged from 70% to 77%, resulting in [(18)F]AlF-labeled hMN14-Fab', hMN14-Fab-AD2, or hMN14-diabody with a specific activity of 15-17 MBq/nmol. The radiolabeled conjugates were purified by gel filtration. For biodistribution and microPET imaging, antibody fragments were injected intravenously into BALB/c nude mice with s.c. CEA-expressing LS174T xenografts (right flank) and CEA-negative SK-RC-52 xenografts (left flank). All [(18)F]AlF-labeled conjugates showed specific uptake in the LS174T xenografts with a maximal tumor uptake of 4.73% ID/g at 4 h after injection. Uptake in CEA-negative SK-RC-52 xenografts was significantly lower. Tumors were clearly visualized on microPET images. Using a [(18)F]AlF-labeled maleimide functionalized chelator, antibody fragments could be radiofluorinated within 4 h at high specific activity. Here, we translated this method to preclinical PET imaging studies and showed feasibility of [(18)F]AlF-fluorinated hMN-14-Fab', [(18)F]AlF-hMN-14-Fab-AD2, and [(18)F]AlF-hMN-14-diabody for microPET imaging of CEA-expressing colonic cancer.

One-step radiosynthesis of 4-nitrophenyl 2-[(18) F]fluoropropionate ([(18) F]NFP); improved preparation of radiolabeled peptides for PET imaging

The versatile (18) F-labeled prosthetic group, 4-nitrophenyl 2-[(18) F]fluoropropionate ([(18) F]NFP), was synthesized in a single step in 45 min from 4-nitrophenyl 2-bromopropionate, with a decay corrected radiochemical yield of 26.2% ± 2.2%. Employing this improved synthesis of [(18) F]NFP, [(18) F]GalactoRGD - the current 'gold standard' tracer for imaging the expression of αV β3 integrin - was prepared with high specific activity in 90 min and 20% decay corrected radiochemical yield from [(18) F]fluoride.

Enhanced aqueous Suzuki-Miyaura coupling allows site-specific polypeptide 18F-labeling

The excesses of reagents used in protein chemistry are often incompatible with the reduced or even inverse stoichiometries used for efficient radiolabeling. Analysis and screening of aqueous Pd(0) ligand systems has revealed the importance of a guanidine core and the discovery of 1,1-dimethylguanidine as an enhanced ligand for aqueous Suzuki–Miyaura cross-coupling. This novel Pd catalyst system has now allowed the labeling of small molecules, peptides, and proteins with the fluorine-18 prosthetic [¹⁸F]4-fluorophenylboronic acid. These findings now enable site-specific protein ¹⁸F-labeling under biologically compatible conditions using a metal-triggered reaction.

N-Succinimidyl 4-[(18)F]-fluoromethylbenzoate-labeled dimeric RGD peptide for imaging tumor integrin expression

RGD peptides, radiolabeled with (18)F, have been used in the clinic for PET imaging of tumor angiogenesis in cancer patients. RGD peptides are typically labeled using a prosthetic group such as N-succinimidyl 4-[(18)F]-fluorobenzoate ([(18)F]SFB) or 4-nitrophenyl 2-[(18)F]-fluoropropionate ([(18)F]NPFP). However, the complex radiosynthetic procedures have impeded their broad application in clinical studies. We previously radiolabeled proteins and peptides with the prosthetic group, N-succinimidyl 4-[(18)F]-fluoromethylbenzoate ([(18)F]SFMB), which was prepared in a simple one-step procedure. In this study, we labeled a PEGylated cyclic RGD peptide dimer, PEG(3)-E[c(RGDyK)](2) (PRGD2), using [(18)F]SFMB and evaluated for imaging tumor αvβ3 integrin expression with positron emission tomography (PET). [(18)F]SFMB was prepared in one step using [(18)F]fluoride displacement of a nitrobenzenesulfonate leaving group under mild reaction conditions followed by HPLC purification. The (18)F-labeled peptide, [(18)F]FMBPRGD2 was prepared by coupling PRGD2 with [(18)F]SFMB in pH 8.6 borate buffer and purified with HPLC. The direct labeling on BMBPRGD2 was also attempted. A Siemens Inveon PET was used to image the uptake of the [(18)F]FMBPRGD2 into a U87MG xenograft mouse model. [(18)F]FMBPRGD2, was prepared with a 15% overall radiochemical yield (uncorrected) in a total synthesis time of 90 min, which was considerably shorter than the preparation of [(18)F]SFB- and [(18)F]NPFP-labeled RGD peptides. The direct labeling, however, was not successful. High quality microPET images using [(18)F]FMBPRGD2 clearly visualized tumors by 15 min with good target to background ratio. Early tracer accumulation in the bladder suggests fast renal clearance. No obvious bone uptake can be detected even at 4-h time point indicating that fluorine attachment is stable in mice. In conclusion, N-succinimidyl 4-[(18)F]-fluoromethylbenzoate ([(18)F]SFMB) prosthetic group can be a good alternative for labeling RGD peptides to image αvβ3 integrin expression and for labeling other peptides.

A novel PET tracer for the imaging of αvβ3 and αvβ5 integrins in experimental breast cancer bone metastases

The aim of this study was the evaluation of (68)Ga-DOTA-E-[c(RGDfK)](2) as a novel PET tracer to image αvβ3 and αvβ5 integrins. For this purpose, DOTA-E-[c(RGDfK)](2) was labeled with (68)Ga, which was obtained from a (68)Ge/(68)Ga generator, purified by solid-phase extraction and the radiochemical purity analyzed by radio-RP-HPLC. (68) Ga-DOTA-E-[c(RGDfK)](2) was obtained reproducibly in radiochemical yields of 60 ± 6% and with an excellent radiochemical purity of >99%. In nude rats bearing bone metastases after injection of MDA-MB-231 human breast cancer cells, biodistribution studies were performed to evaluate the accumulation of the radiotracer in selected organs, blood and bone metastases 0.5, 1, 2 and 3 h post injection. A rapid uptake into the bone metastases and rapid blood clearance was observed, resulting in tumor-blood ratios of up to 26.6 (3 h post injection) and tumor-muscle ratios of up to 7.9 (3 h post injection). A blocking experiment with coinjected αvβ3/αvβ5 antagonist showed the tumor uptake to be receptor-specific. In an initial in vivo micro PET evaluation of the tracer using the same animal model, the bone metastasis was clearly visualized. These results suggest that (68)Ga-DOTA-E-[c(RGDfK)](2) is a promising PET tracer suitable for the imaging of αvβ3 and αvβ5 integrins in bone metastases. This novel PET tracer should be further evaluated concerning its usefulness for early detection of bone metastases and monitoring treatment response of these lesions.

Synthesis and in vitro evaluation of 68Ga-DOTA-4-FBn-TN14003, a novel tracer for the imaging of CXCR4 expression

The expression of the chemokine receptor CXCR4 in tumors is associated with tumor aggressiveness and poor prognosis for the patient and contributes to metastatic seeding. Therefore it is of high interest to find a specific PET tracer for the imaging of CXCR4 expression in tumors. The aim of this study was the synthesis, (68)Ga labeling and first evaluation of DOTA-4-FBn-TN14003 as a potential PET tracer for this purpose. DOTA-4-FBn-TN14003 was synthesized using solid phase peptide synthesis and radiolabeling of this versatile precursor was performed with (68)Ga, which was obtained from a (68)Ge/(68)Ga generator. (68)Ga-DOTA-4-FBn-TN14003 was reproducibly obtained in isolated radiochemical yields of 72.5±4.9% with an excellent radiochemical purity of >99.5%. Specific activities of up to 29.8±3.1 GBq/μmol were achieved. In competition binding assays with SDF-1α, human T cell lymphoma Jurkat cells expressed high levels of CXCR4 whereas human breast cancer MDA-MB-231 cells expressed significantly lower levels of this chemokine receptor. The inhibition constants (IC(50)) of Ga-DOTA-4-FBn-TN14003 and 4-FBn-TN14003 to CXCR4 were determined in a competition assay against (125)I-SDF-1α using Jurkat as well as MDA-MB-231 cells. The IC(50) values of Ga-DOTA-4-FBn-TN14003 (1.99±0.31 nM) and 4-FBn-TN14003 (4.07±1.00 nM) proved to be comparable, indicating negligible influence of the metal complex. These results suggest (68)Ga-DOTA-4-FBn-TN14003 as a promising agent for the imaging of CXCR4 expression in tumors and metastases.

Optimized labeling of NOTA-conjugated octreotide with F-18

We recently reported a facile method based on the chelation of [¹⁸F]aluminum fluoride (Al¹⁸F) by NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid). Here, we present a further optimization of the ¹⁸F labeling of NOTA-octreotide (IMP466). Octreotide was conjugated with the NOTA chelate and was labeled with ¹⁸F in a two-step, one-pot method. The labeling procedure was optimized with regard to the labeling buffer, ionic strength, peptide concentration, and temperature. Radiochemical yield, specific activity, in vitro stability, and receptor affinity were determined. Biodistribution of ¹⁸F-IMP466 was studied in AR42J tumor-bearing mice. In addition, microPET/CT images were acquired. IMP466 was labeled with Al¹⁸F in a single step with 97% yield in the presence of 80% (v/v) acetonitrile or ethanol. The labeled product was purified by HPLC to remove unlabeled peptide and unbound Al¹⁸F. The radiolabeling, including purification, was performed for 45 min. Specific activities of 48,000 GBq/mmol could be obtained. ¹⁸F-IMP466 showed a high tumor uptake and excellent tumor-to-blood ratios at 2 h post-injection. In addition, the low bone uptake indicated that the Al¹⁸F–NOTA complex was stable in vivo. PET/CT scans revealed excellent tumor delineation and specific accumulation in the tumor. Uptake in receptor-negative organs was low. NOTA-octreotide could be labeled with ¹⁸F in quantitative yields using a rapid two-step, one-pot, method. The compound was stable in vivo and showed rapid accretion in SSTR₂-receptor-expressing AR42J tumors in nude mice. This method can be used to label other NOTA-conjugated compounds such as RGD peptides, GRPR-binding peptides, and Affibody molecules with ¹⁸F.

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.

A novel facile method of labeling octreotide with (18)F-fluorine

Several methods have been developed to label peptides with (18)F. However, in general these are laborious and require a multistep synthesis. We present a facile method based on the chelation of (18)F-aluminum fluoride (Al(18)F) by 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA). The method is characterized by the labeling of NOTA-octreotide (NOTA-d-Phe-cyclo[Cys-Phe-d-Trp-Lys-Thr-Cys]-Throl (MH(+) 1305) [IMP466]) with (18)F.

METHODS

Octreotide was conjugated with the NOTA chelate and labeled with (18)F in a 2-step, 1-pot method. The labeling procedure was optimized with regard to the labeling buffer, peptide, and aluminum concentration. Radiochemical yield, specific activity, in vitro stability, and receptor affinity were determined. Biodistribution of (18)F-IMP466 was studied in AR42J tumor-bearing mice and compared with that of (68)Ga-labeled IMP466. In addition, small-animal PET/CT images were acquired.

RESULTS

IMP466 was labeled with Al(18)F in a single step with 50% yield. The labeled product was purified by high-performance liquid chromatography to remove unbound Al(18)F and unlabeled peptide. The radiolabeling, including purification, was performed in 45 min. The specific activity was 45,000 GBq/mmol, and the peptide was stable in serum for 4 h at 37 degrees C. Labeling was performed at pH 4.1 in sodium citrate, sodium acetate, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, and 2-(N-morpholino)ethanesulfonic acid buffer and was optimal in sodium acetate buffer. The apparent 50% inhibitory concentration of the (19)F-labeled IMP466 determined on AR42J cells was 3.6 nM. Biodistribution studies at 2 h after injection showed a high tumor uptake of (18)F-IMP466 (28.3 +/- 5.2 percentage injected dose per gram [%ID/g]; tumor-to-blood ratio, 300 +/- 90), which could be blocked by an excess of unlabeled peptide (8.6 +/- 0.7 %ID/g), indicating that the accumulation in the tumor was receptor-mediated. Biodistribution of (68)Ga-IMP466 was similar to that of (18)F-IMP466. (18)F-IMP466 was stable in vivo, because bone uptake was only 0.4 +/- 0.2 %ID/g, whereas free Al(18)F accumulated rapidly in the bone (36.9 +/- 5.0 %ID/g at 2 h after injection). Small-animal PET/CT scans showed excellent tumor delineation and high preferential accumulation in the tumor.

CONCLUSION

NOTA-octreotide could be labeled rapidly and efficiently with (18)F using a 2-step, 1-pot method. The compound was stable in vivo and showed rapid accretion in somatostatin receptor subtype 2-expressing AR42J tumors in nude mice. This method can be used to label other NOTA-conjugated compounds with (18)F.

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.

Imaging of alpha(v)beta(3) expression by a bifunctional chimeric RGD peptide not cross-reacting with alpha(v)beta(5)

PURPOSE

To test whether a novel bifunctional chimeric peptide comprising a cyclic Arg-Gly-Asp pentapeptide covalently bound to an echistatin domain can discriminate alpha(v)beta(3) from alpha(v)beta(5) integrin, thus allowing the in vivo selective visualization of alpha(v)beta(3) expression by single-photon and positron emission tomography (PET) imaging.

EXPERIMENTAL DESIGN

The chimeric peptide was preliminarily tested for inhibition of alpha(v)beta(3)-dependent cell adhesion and competition of 125I-echistatin binding to membrane of stably transfected K562 cells expressing alpha(v)beta(3) (Kalpha(v)beta(3)) or alpha(v)beta(5) (Kalpha(v)beta(5)) integrin. The chimeric peptide was then conjugated with diethylenetriaminepentaacetic acid and labeled with 111In for single-photon imaging, whereas a one-step procedure was used for labeling the full-length peptide and a truncated derivative, lacking the last five C-terminal amino acids, with 18F for PET imaging. Nude mice bearing tumors from Kalpha(v)beta(3), Kalpha(v)beta(5), U87MG human glioblastoma, and A431 human epidermoid cells were subjected to single-photon and PET imaging.

RESULTS

Adhesion and competitive binding assays showed that the novel chimeric peptide selectively binds to alpha(v)beta(3) integrin and does not cross-react with alpha(v)beta(5). In agreement with in vitro findings, single-photon and PET imaging studies showed that the radiolabeled chimeric peptide selectively localizes in tumor xenografts expressing alphavbeta3 and fails to accumulate in those expressing alpha(v)beta(5) integrin. When 18F-labeled truncated derivative was used for PET imaging, alphavbeta3- and alpha(v)beta(5)-expressing tumors were visualized, indicating that the five C-terminal amino acids are required to differentially bind the two integrins.

CONCLUSION

Our findings indicate that the novel chimeric Arg-Gly-Asp peptide, having no cross-reaction with alphavbeta5 integrin, allows highly selective alphavbeta3 expression imaging and monitoring.

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.

Radiosynthesis of 18F-(R8,15,21, L17)-vasoactive intestinal peptide and preliminary evaluation in mice bearing C26 colorectal tumours

BACKGROUND

Radiolabelled vasoactive intestinal peptide (VIP) and its analogues have shown their potential as imaging agents for diagnosing tumours expressing VIP receptor. However, the fast proteolytic degradation in vivo has limited their clinical use.

AIM

To prepare the 18F-labelled (R8,15,21, L17)-VIP analogue in a convenient way and to evaluate its potential as an imaging agent for VIP receptor-positive tumours.

METHODS

Radiolabelled (R8,15,21, L17)-VIP was obtained by conjugation with N-succinimidyl 4-([18F]fluoromethyl) benzoate and purified by HPLC. Radiochemical purity and specific radioactivity were measured by analytical HPLC. In-vitro stability of the product was carried out in HSA solution and analysed by HPLC. Biodistribution study was carried out in mice bearing C26 colorectal tumours.

RESULTS

18F-(R8,15,21, L17)-VIP was obtained in greater than 99% radiochemical purity within 60 min in decay-for-corrected radiochemical yields of 21.8+/-4.7% (n=5) and a specific activity of 17.76 GBq x mumol(-1) at the end of synthesis (EOS). Results of in-vitro studies demonstrated a high stability in human serum albumin (HSA) solution. Biodistribution data showed a rapid blood clearance and specific binding towards receptor-positive tumours.

CONCLUSION

18F-(R8,15,21, L17)-VIP was prepared by a convenient method. Preliminary biodistribution results showed its potential for imaging tumours over-expressing VIP receptors and encouraged further investigation.

Fluorine-18 labeling of peptides and proteins

The pool of promising peptides worthy of investigation and evaluation for clinical use is continuously filled from different sources. Driven by the promising results obtained with peptides addressing somatostatin-2 receptor positive (sst2+) neuroendocrine tumours, other peptides targeting further receptor systems are being studied and evaluated. Progress in profiling the density and incidence of peptide hormone receptors in human cancer has initiated and will further promote research on the corresponding peptidic binders. In addition, industrial pharmaceutical research will be another significant source of peptides in the future. A recent prognosis revealed that about 50% of the drugs entering clinical trials in the next years will be peptides. The extensive research activities in genomics and proteomics will point out and quantify new and already known target structures upregulated in specific diseases. Based on the knowledge of their endogenous ligands or via selection of suitable candidates by phage display, suitable peptide ligands for e.g. membrane associated receptors can be identified and thus allow targeting of such binding sites. Thus, bioactive peptides specifically addressing relevant molecular targets are expected to become an important class of tracers, also due to the possibility of bridging imaging with therapeutic approaches. In this brief overview a summary of methods and strategies for the 18F-labeling of peptides and proteins is given.

Targeting prostate cancer with radiolabelled bombesins

The fact that a number of common human tumours, including those of breast and prostate, express increased levels of the gastrin-releasing peptide receptor (GRP-R) means that this receptor is a potential target for peptide receptor mediated scintigraphy and targeted radionuclide therapy. Although clinical application is yet in its infancy, there is a considerable literature on preclinical studies aimed at developing suitable radioligands for potential clinical application. This brief review provides an overview of this research and also describes some of the limited clinical studies that have been published.

Chemoselective hydrazone formation between HYNIC-functionalized peptides and (18)F-fluorinated aldehydes

INTRODUCTION

Since the demand for (18)F-fluorinated peptides for quantitative in vivo receptor imaging using PET has increased, a new chemoselective two-step (18)F-labeling strategy based on hydrazone formation between an unprotected hydrazine-functionalized peptide and an (18)F-labeled aldehyde was developed.

METHODS

First, 4-[(18)F]fluorobenzaldehyde ([(18)F]FB-CHO) was prepared from 4-formyl-N,N,N-trimethylanilinium triflate via direct no-carrier-added (18)F-fluorination (dimethyl sulfoxide, 90 degrees C, 5 min) and purified by RP-HPLC. Hydrazone formation between [(18)F]FB-CHO and 6-hydrazinonicotinic acid (HYNIC) and the unprotected HYNIC-functionalized peptides (HYNIC-d-Phe(1))-Tyr(3)-Thr(8)-octreotide and (HYNIC-Arg(1))-substance P was evaluated with respect to the dependence of radiochemical yield on pH, precursor concentration and temperature. The stability of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)(NH(2))-octreotide in aqueous solution at various pH (4.0, 5.5 and 7.5) as well as the in vivo stability of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)-octreotide in mouse blood (30 min p.i.) was investigated.

RESULTS

Yields of the hydrazone formation were independent of pH between pH 0.5 and 5.5. Optimal labeling yields of 85% were obtained with a precursor concentration of 2.1 mM at 70 degrees C for 10 min. The labeling products were stable at pH 7.5 at 37 degrees C, while in more acidic media (pH 4.0) the product slowly decomposed to form up to 31+/-2% [(18)F]FB-CHO within 5 h. Metabolite studies showed no detectable degradation of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)-octreotide in mouse blood (30 min p.i.).

CONCLUSIONS

In conclusion, chemoselective hydrazone formation between unprotected HYNIC-functionalized peptides and [(18)F]FB-CHO is a fast and straightforward radiolabeling method leading to high yields under mild acidic conditions. In addition, it represents a powerful and versatile radiolabeling strategy that is applicable to a variety of radionuclides and peptide precursors already available for (99m)Tc labeling.

Fully automated preparation of n.c.a. 4-[18F]fluorobenzoic acid and N-succinimidyl 4-[18F]fluorobenzoate using a Siemens/CTI chemistry process control unit (CPCU)

The widely used bifunctional labeling reagent 4-[18F]fluorobenzoic acid ([18F]FBA) and its activated form N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB) were prepared using a modified Siemens/CTI chemistry process control unit (CPCU) double vessel [18F]FDG module. The products were obtained with a radiochemical yield greater than 99% with decay corrected yields of 77+/-5% (n=4) for [18F]FBA and 46+/-5% (n=4) for [18F]SFB. The presented synthetic procedure is highly reproducible and fully automated.

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.