A new, convenient method for the preparation of 4-[18F]fluorobenzyl halides

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.

An Automated Multidose Synthesis of the Potentiometric PET Probe 4-[18F]Fluorobenzyl-Triphenylphosphonium ([18F]FBnTP)

PURPOSE

The aim of this study was the automated synthesis of the mitochondrial membrane potential sensor 4-[¹⁸F]fluorobenzyl-triphenylphosphonium ([¹⁸F]FBnTP) on a commercially available synthesizer in activity yields (AY) that allow for imaging of multiple patients.

PROCEDURES

A three-pot, four-step synthesis was implemented on the ELIXYS FLEX/CHEM radiosynthesizer (Sofie Biosciences) and optimized for radiochemical yield (RCY), radiochemical purity (RCP) as well as chemical purity during several production runs (n = 24). The compound was purified by solid-phase extraction (SPE) with a Sep-Pak Plus Accell CM cartridge, thereby avoiding HPLC purification.

RESULTS

Under optimized conditions, AY of 1.4-2.2 GBq of [¹⁸F]FBnTP were obtained from 9.4 to 12.0 GBq [¹⁸F]fluoride in 90-92 min (RCY = 28.6 ± 5.1 % with n = 3). Molar activities ranged from 80 to 99 GBq/μmol at the end of synthesis. RCP of final formulations was > 99 % at the end of synthesis and > 95 % after 8 h. With starting activities of 23.2-33.0 GBq, RCY decreased to 16.1 ± 0.4 % (n = 3). The main cause of the decline in RCY when high amounts of [¹⁸F]fluoride are used is radiolytic decomposition of [¹⁸F]FBnTP during SPE purification.

CONCLUSIONS

In initial attempts, the probe was synthesized with RCY < 0.6 % when starting activities up to 44.6 GBq were used. Rapid radiolysis of the intermediate 4-[¹⁸F]fluorobenzaldehyde and the final product [¹⁸F]FBnTP during purification was identified as the main cause for low yields in high-activity runs. Radiolytic decomposition was hindered by the addition of radical scavengers during synthesis, purification, and formulation, thereby improving AY and RCP. The formulated probe in injectable form was synthesized without the use of HPLC and passed all applicable quality control tests.

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).

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.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

PET imaging of bacterial infections with fluorine-18-labeled maltohexaose

A positron emission tomography (PET) tracer composed of (18)F-labeled maltohexaose (MH(18)F) can image bacteria in vivo with a sensitivity and specificity that are orders of magnitude higher than those of fluorodeoxyglucose ((18)FDG). MH(18)F can detect early-stage infections composed of as few as 10(5) E. coli colony-forming units (CFUs), and can identify drug resistance in bacteria in vivo. MH(18)F has the potential to improve the diagnosis of bacterial infections given its unique combination of high specificity and sensitivity for bacteria.

Single-step syntheses of no-carrier-added functionalized [18F]fluoroarenes as labeling synthons from diaryliodonium salts

Radiotracers labelled with short-lived fluorine-18 (t(1/2) = 109.7 min) are keenly sought for biomedical imaging with positron emission tomography (PET). The radiotracers are mostly required at high specific radioactivities, necessitating their radiosyntheses from cyclotron-produced no-carrier-added [(18)F]fluoride ion. PET radiotracers encompass wide structural diversity and molecular weight. Hence, diverse (18)F-labeling methodology is needed to accomplish the required radiosyntheses in a simple and rapid manner. A useful strategy is to introduce nucleophilic [(18)F]fluoride ion first into a labeling synthon that may then be applied to label the target radiotracer. Here, we show that various functionalized [(18)F]fluoroarenes may be rapidly synthesized as labeling synthons through single-step reactions of appropriate diaryliodonium salts with [(18)F]fluoride ion. Decay-corrected radiochemical yields (RCYs) varied with position of functional group, choice of electron-rich aryl ring in the diaryliodonium salt, and choice of anion. Under best conditions, (18)F-labeled fluorobenzaldehydes, fluorobenzyl halides, fluorobenzoic acid esters and fluorophenyl ketones were obtained selectively in 40-73%, 20-55%, 46-89% and 81-98% RCYs, respectively. This versatile straightforward methodology will enhance the scope for producing structurally complex, yet useful, PET radiotracers.

Positron emission tomography molecular imaging for drug development

Human in vivo molecular imaging with positron emission tomography (PET) enables a new kind of 'precision pharmacology', able to address questions central to drug development. Biodistribution studies with drug molecules carrying positron-emitting radioisotopes can test whether a new chemical entity reaches a target tissue compartment (such as the brain) in sufficient amounts to be pharmacologically active. Competition studies, using a radioligand that binds to the target of therapeutic interest with adequate specificity, enable direct assessment of the relationship between drug plasma concentration and target occupancy. Tailored radiotracers can be used to measure relative rates of biological processes, while radioligands specific for tissue markers expected to change with treatment can provide specific pharmacodynamic information. Integrated application of PET and magnetic resonance imaging (MRI) methods allows molecular interactions to be related directly to anatomical or physiological changes in a tissue. Applications of imaging in early drug development can suggest approaches to patient stratification for a personalized medicine able to deliver higher value from a drug after approval. Although imaging experimental medicine adds complexity to early drug development and costs per patient are high, appropriate use can increase returns on R and D investment by improving early decision making to reduce new drug attrition in later stages. We urge that the potential value of a translational molecular imaging strategy be considered routinely and at the earliest stages of new drug development.

Automated radiosynthesis of the thiol-reactive labeling agent N-[6-(4-[18F]fluorobenzylidene)aminooxyhexyl]maleimide ([18F]FBAM)

The two-step radiosynthesis of N-[6-(4-[(18)F]fluorobenzylidene)aminooxyhexyl]maleimide ([(18)F]FBAM) was adapted to a remotely controlled synthesizer module. After optimization of reaction conditions as well as solid phase extraction based purification steps, the final [(18)F]FBAM was obtained in a decay-corrected radiochemical yield of 29±4% (related to [(18)F]fluoride, n=12) within a total synthesis time of 40 min. The radiochemical purity of [(18)F]FBAM was in the range 94-98%, the specific activity was determined with 13.4-17.2 GBq/μmol.

Synthesis of hypoxia imaging agent 1-(5-deoxy-5-fluoro-α-D-arabinofuranosyl)-2-nitroimidazole using microfluidic technology

INTRODUCTION

Microfluidic technology allows fast reactions in a simple experimental setup, while using very low volumes and amounts of starting material. Consequently, microfluidic technology is an ideal tool for radiolabeling reactions involving short-lived positron emitters. Optimization of the complex array of different reaction conditions requires knowledge of the different reaction parameters linked to the microfluidic system as well as their influence on the radiochemical yields. 1-(5-Deoxy-5-fluoro-α-d-arabinofuranosyl)-2-nitroimidazole ([(18)F]FAZA) is a frequently used radiotracer for PET imaging of tumor hypoxia. The present study describes the radiosynthesis of [(18)F]FAZA by means of microfluidic technology and subsequent small animal PET imaging in EMT-6 tumor-bearing mice.

METHODS

Radiosyntheses were performed using the NanoTek Microfluidic Synthesis System (Advion BioSciences, Inc.). Optimal reaction conditions were studied through screening different reaction parameters like temperature, flow rate, residency time, concentration of the labeling precursor (1-(2,3-di-O-acetyl-5-O-tosyl-α-d-arabinofuranosyl)-2-nitroimidazole) and the applied volume ratio between the labeling precursor and [(18)F]fluoride.

RESULTS

Optimized reaction conditions at low radioactivity levels (1 to 50 MBq) afforded 63% (decay-corrected) of HPLC-purified [(18)F]FAZA within 25 min. Higher radioactivity levels (0.4 to 2.1 GBq) gave HPLC-purified [(18)F]FAZA in radiochemical yields of 40% (decay-corrected) within 60 min at a specific activity in the range of 70 to 150 GBq/μmol. Small animal PET studies in EMT-6 tumor-bearing mice showed radioactivity accumulation in the tumor (SUV(20min) 0.74 ± 0.08) resulting in an increasing tumor-to-muscle ratio over time.

CONCLUSIONS

Microfluidic technology is an ideal method for the rapid and efficient radiosynthesis of [(18)F]FAZA for preclinical radiopharmacological studies. Careful analysis of various reaction parameters is an important requirement for the understanding of the influence of different reaction parameters on the radiochemical yield using microfluidic technology. Exploration of microfluidic technology for the radiosynthesis of other PET radiotracers in clinically relevant radioactivity levels is currently in progress.

Synthesis and radiopharmacological investigation of 3-[4'-[(18)F]fluorobenzylidene]indolin-2-one as possible tyrosine kinase inhibitor

The radiosynthesis and radiopharmacological evaluation of 3-[4'-[(18)F]fluorobenzylidene]indolin-2-one, a derivative of tyrosine kinase inhibitor SU5416, is described. The radiosynthesis was accomplished by Knoevenagel condensation of 4-[(18)F]fluorobenzaldehyde with oxindole in a remotely controlled synthesis module. The reaction conditions were optimized through screening the influence of different bases on the radiochemical yield. The radiotracer was obtained after a two-step labelling procedure in 4% decay-corrected radiochemical yield at a specific activity of 48-61GBq/micromol within 90min. The radiochemical purity after semi-preparative HPLC purification exceeded 98%. The biodistribution was studied in Wistar rats. After distribution the radiotracer was rapidly accumulated in the adrenals, liver and kidneys, however, it was cleared from these and the most other organs. Only the adipose tissue remained the activity over 60min. Unexpected high transient uptake was observed in the brain, pancreas, heart and lung. The fast clearance of 3-[4'-[(18)F]fluorobenzylidene]indolin-2-one was caused by excretion, approximately one half each was renal and biliary excreted and the other part cleared by metabolic processes. In arterial blood plasma two more polar metabolites were found by radio-HPLC. After 20min post-injection, only 12% of intact radiotracer has been detected. Consequently, in small animal PET studies with FaDu tumour bearing mice no specific uptake in the tumours could be observed.

New strategy for the preparation of clickable peptides and labeling with 1-(azidomethyl)-4-[(18)F]-fluorobenzene for PET

The alkyne-azide Cu(I)-catalyzed Huisgen cycloaddition, a click type reaction was used to label a peptide with fluorine-18. A novel solid phase synthesis approach for the preparation of clickable peptides has been developed and has also permitted the straightforward preparation of reference compounds. A complementary azide labeling agent (1-(azidomethyl)-4-[(18)F]-fluorobenzene) has been produced in a four step procedure in 75 min with a 34% radiochemical yield (decay corrected). Conjugation of [(18)F]fluoroazide with a model alkyne-neuropeptide produced the desired (18)F-radiolabeled peptide in less than 15 min with a yield of 90% and excellent radiochemical purity.

Fully automated synthesis procedure of 4-[18F]fluorobenzaldehyde by commercial synthesizer: amino-oxi peptide labelling prosthetic group

Automatic synthesis of 4-[18F]fluorobenzaldehyde has been developed by a commercially available TRACERlab FX(F-N) synthesis module to be used as prosthetic group for amino-oxy functionalized peptide labelling in clinical routine application. In addition a handmade purification device (HPD) has been setup to perform automatic cartridge purification as well as to back-up the reactor where one-pot synthesis is not applicable. Cartridges for solid phase extraction such as C18, C8, phenyl has been tested to best perform purification as well as activity recovery. Radiochemical yield (RCY) at end of synthesis (EOS) was in average 67% after about 45 min (90% decay corrected at EOB). The RCY of the entire procedure was 54% with a radiochemical purity above 99%.

Fluorine-18 labeling of small molecules: the use of 18F-labeled aryl fluorides derived from no-carrier-added [18F]fluoride as labeling precursors

The favourable long-half life, the ease of production and the low energy of the emitted positron make 18F an ideal radionuclide for PET imaging. Radiochemistry of 18F basically relies on two distinctive types of reactions: nucleophilic and electrophilic reactions. All syntheses of 18F-labeled radiotracers are based on either [18F]fluoride ion or [18F]fluorine gas as simple primary labeling precursors which are obtained directly from the cyclotron. They can be applied either directly to the radiosynthesis or they can be transformed into more complex labeling precursors enabling the multi-step build-up of organic tracer molecules. The topic of this review is a survey on the application of several 18F-labeled aryl fluorides as building blocks derived from no-carrier-added (n.c.a.) [18F] fluoride to build up small monomeric PET radiotracers at high specific radioactivity by multi-step synthesis procedures.

Synthesis modules and automation in F-18 labeling

Fast implementation of PET into clinical studies and research has resulted in high demands in the automated modules for the preparation of PET radiopharmaceuticals in a safe and reproducible manner. 18F-labeled radiotracers are of considerable interest due to longer half-life of fluorine-18 allowing remote site application, as demonstrated by [18F]FDG. In this chapter, the state of the art of commercially available modules for [18F]FDG is reviewed with the emphasis on multibatch production of this important radiotracer. Examples are given on the syntheses of other clinically relevant 18F-labeled radiotracers by using existing [18F]FDG synthesizers or with the help of general-purpose [18F]nucleophilic fluorination modules. On-going research and progress in the automation of complex radio labeling procedures followed by development of flexible multipurpose automated apparatus are discussed. The contribution of radiochemists in facilitating automation via introduction of new 18F-labeling techniques and labeling synthons, on-line reactions and purifications etc. is outlined.

An improved method of 18F peptide labeling: hydrazone formation with HYNIC-conjugated c(RGDyK)

Radiolabeled alpha(v)beta(3)-integrin antagonists are increasingly investigated as a means of imaging angiogenesis. Several methods of labeling alpha(v)beta(3)-integrin binding peptide with (18)F have been reported recently. In the present study, we devised a straightforward means for labeling Arg-Gly-Asp (RGD) peptide with (18)F via hydrazone formation between c(RGDyK)-hydrazinonicotinic acid (HYNIC) (3) and 4-[(18)F]-fluorobenzaldehyde ([(18)F]4). The resulting reaction mixture was purified by HPLC to give 4'-[(18)F]-fluorobenzylidenehydrazone-6-nicotinamide-c(RGDyK) ([(18)F]5). The conjugation efficiency of 3 and 4 to form [(18)F]5 was 95.2%, and the radiochemical purity of [(18)F]5 after purification was >99%. The specific activity of [(18)F]5 estimated by radio-HPLC was 20.5 GBq/mumol (end of synthesis). Competitive binding assay of c(RGDyK) (1) and 5 was performed using [(125)I]iodo-c(RGDyK) as a radioligand, and K(i) values were found to be 2.8 and 21.7 nM, respectively. For the biodistribution study, the angiogenic mouse model was established by inducing unilateral ischemia on the left hindlimbs of ICR mice after femoral artery ablation. Seven days after inducing ischemia, [(18)F]5 was administered to the mice through the tail vein. Ischemic muscle uptake of [(18)F]5 was significantly higher than that of normal muscle (P<.01). Specific uptake was confirmed by coinjection of 1 with [(18)F]5. Here, we successfully labeled RGD peptide with (18)F via hydrazone formation between 3 and 4, resulting to [(18)F]5. [(18)F]5 was found to have high affinity for alpha(v)beta(3)-integrin and to accumulate specifically in ischemic hindlimb muscle of mice. We suggest that (18)F labeling via formation of hydrazone between HYNIC peptide and [(18)F]4 is a useful method for labeling c(RGDyK), which can be applied for imaging angiogenesis.

Preparation of 18F-human serum albumin: a simple and efficient protein labeling method with 18F using a hydrazone-formation method

18F-labeling of proteins and peptides is important for positron emission tomography (PET). Although there are many methods for the labeling of proteins with (18)F, most of these are characterized by complicated procedures or low yields. Here, we report a novel and simple method which includes the preparation of [18F]fluorobenzaldehyde ([18F]FBA) and successive conjugation with hydrazinonicotinic acid-human serum albumin conjugate (HYNIC-HSA) via hydrazone formation. HYNIC-HSA, which can also be used for labeling with (99m)Tc, was prepared via reaction with N-hydroxysuccinimide (NHS) or tetrafluorophenyl (TFP) esters of HYNIC with HSA. No-carrier-added [18F]FBA was prepared by the nucleophilic substitution of [18F]fluoride to 4-trimethylammonium benzaldehyde triflate in the presence of tetrabutylammonium bicarbonate. [18F]FBA was purified by passing ion exchange cartridges (IC-H and QMA) and was adsorbed to a C18 Sep-Pak cartridge. The adsorbed [18F]FBA was then eluted with 50% ethanol. HYNIC-HSA was added to the solution and conjugated with [18F]FBA via hydrazone formation. 18F-HSA was purified with a PD10 column. Biodistribution of 18F-HSA, (99m)Tc-HSA, and [18F]FBA in mice were investigated at 10, 20, and 60 min after intravenous injection. The number of conjugated HYNIC molecules per HSA ranged from 5.2 to 23.2 depending on the reaction conditions. The labeling efficiency of 18F-FBA was 67 +/- 15.7%. The radiochemical purity after purification was over 99%. The conjugation efficiency of HYNIC-HSA with [18F]FBA was between 25% and 90%. The conjugation efficiency was observed to increase with increases in the number of conjugated HYNIC, the HYNIC-HSA concentration, or temperature. 18F-HSA exhibited a biodistribution pattern similar to that of (99m)Tc-HSA while [18F]FBA showed much lower blood activity than that of 18F-HSA and (99m)Tc-HSA. We concluded that 18F-HSA was successfully labeled using a novel method which involves hydrazone formation between [18F]FBA and HYNIC-HSA. This method can be applied to the 18F-labeling of other proteins or peptides.

First (18)F-labeled tracer suitable for routine clinical imaging of sst receptor-expressing tumors using positron emission tomography

PURPOSE

Despite excellent radionuclide characteristics, no (18)F-labeled peptides are available for quantitative peptide receptor mapping using positron emission tomography (PET) so far, mainly due to time-consuming multistep radiosyntheses with limited overall yields. A newly developed two-step chemoselective conjugation method allows rapid and high-yield [(18)F]fluorination of peptides via oxime formation and was applied for the synthesis of new (18)F-labeled carbohydrated Tyr(3)-octreotate (TOCA) analogs with optimized pharmacokinetics suitable for clinical routine somatostatin-receptor (sst) imaging.

EXPERIMENTAL DESIGN

(18)F-labeled glucose (Gluc-S-) and cellobiose (Cel-S-) derivatives of aminooxy-functionalized TOCA were synthesized via oxime formation with 4-[(18)F]fluorobenzaldehyde ([(18)F]FBOA-peptides). Both the in vitro internalization profile of Gluc-S-Dpr([(18)F]FBOA)TOCA and Cel-S-Dpr([(18)F]FBOA)TOCA in hsst(2)-expressing Chinese hamster ovary cells (dual tracer protocol) and their biodistribution in AR42J tumor-bearing mice were investigated and compared with two [(18)F]fluoropropionylated ([(18)F]FP) analogs, Gluc-Lys([(18)F]FP)TOCA and Gluc-S-Dpr([(18)F]FP)TOCA.

RESULTS

In contrast to [(18)F]FP-labeling (3 h), chemo-selective [(18)F]FBOA-formation (50 min) afforded the respective radiopeptides in high yields (65-85%). In vitro, Gluc-S-Dpr([(18)F]FBOA)TOCA and Cel-S-Dpr([(18)F]FBOA)-TOCA showed high internalization (139 +/- 2 and 163 +/- 8 of the reference [(125)I]Tyr(3)-octreotide, respectively), which was reflected by high tumor accumulation in vivo [21.8 +/- 1.4 and 24.0 +/- 2.5% of injected dose/g (1 h), respectively]. How-ever, only Cel-S-Dpr([(18)F]FBOA)TOCA and Gluc-S-Dpr([(18)F]FP)TOCA (tumor: 15.1 +/- 1.5% of injected dose/g) with its very low accumulation in all of the nontarget organs showed improved tumor:organ ratios compared with Gluc-Lys([(18)F]FP)TOCA. For Cel-S-Dpr([(18)F]FBOA)TOCA,tumor:organ ratios (1 h) were 42:1, 27:1, 15:1, 3:1, and 208:1 for blood, liver, intestine, kidney, and muscle, respectively.

CONCLUSION

Due to the fast and high-yield chemoselective radiofluorination strategy and to its excellent pharmacokinetics, Cel-S-Dpr([(18)F]FBOA)TOCA represents the first tracer suitable for routine clinical application in PET somatostatin receptor imaging.

Synthesis of a new heterobifunctional linker, N-[4-(aminooxy)butyl]maleimide, for facile access to a thiol-reactive 18F-labeling agent

A new heterobifunctional linker containing an aldehyde-reactive aminooxy group and a thiol-reactive maleimide group, namely N-[4-(aminooxy)butyl]maleimide, was synthesized as a stable HCl salt by O-alkylation of either N-hydroxyphthalimide or N-(4-monomethoxytrityl)hydroxylamine, followed by N-alkylation of maleimide, in an overall yield of 18% (seven steps) or 29% (five steps), respectively. This heterobifunctional linker allowed a simple and efficient synthesis of a maleimide-containing thiol-reactive (18)F-labeling agent. Thus, N-[4-[(4-[(18)F]fluorobenzylidene)aminooxy]butyl]maleimide (specific activity: approximately 3000 Ci/mmol at end of synthesis) was synthesized in two steps involving the preparation of 4-[(18)F]fluorobenzaldehyde, followed by its aminooxy-aldehyde coupling reaction to the heterobifunctional linker, with an overall radiochemical yield of approximately 35% (decay corrected) within approximately 60 min from end of bombardment. Initial (18)F-labeling experiments were carried out using a thiol-containing tripeptide glutathione (GSH) and a 5'-thiol-functionalized oligodeoxynucleotide (5'-S-ODN) in phosphate-buffered saline (PBS, pH 7.5). After standing at room temperature for 10 min, the (18)F-labeled GSH and 5'-S-ODN were obtained in (18)F-labeling yields of approximately 70% and approximately 5% (decay-corrected), respectively. The heterobifunctional linker is easy to synthesize and provides a facile access to the maleimide-containing thiol-reactive (18)F-labeling agent, which could be advantageously employed in the development of (18)F-labeled biomomolecules for use with positron emission tomography.

Catalytic enantioselective synthesis of 18F-fluorinated α-amino acids under phase-transfer conditions using (s)-NOBIN

We describe a new method for the asymmetric synthesis of [(18)F]fluorinated aromatic alpha-amino acids (FAA) under phase transfer conditions using achiral glycine derivative NiPBPGly and (S)-NOBIN as a novel substrate/catalyst pair. The key alkylation step proceeds under mild conditions. Substituted [(18)F]fluorobenzylbromides were prepared using nucleophilic [(18)F]fluoride and were used as alkylation agents. Two important FAA, 2-[(18)F]fluoro-L-tyrosine (2-FTYR) and 6-[(18)F]fluoro-L-3,4-dihydroxyphenylalanine (6-FDOPA), were synthesized with an ee of 92 and 96%, respectively. The total synthesis time was 110-120 min and radiochemical yields (d.c.) were 25+/-6% for 2-FTYR and 16+/-5% for 6-FDOPA.

Radiolabeled guanine derivatives for the in vivo mapping of O(6)-alkylguanine-DNA alkyltransferase: 6-(4-[(18)F]Fluoro-benzyloxy)-9H-purin-2-ylamine and 6-(3-[(131)I]Iodo-benzyloxy)-9H-purin-2-ylamine

Two radiolabeled analogues of 6-benzyloxy-9H-purin-2-ylamine (O(6)-benzylguanine; BG) potentially useful in the in vivo mapping of O(6)-alkylguanine-DNA alkyltransferase (AGT) were synthesized. Fluorine-18 labeling of the known 6-(4-fluoro-benzyloxy)-9H-purin-2-ylamine (FBG; 6) was accomplished by the condensation of 4-[(18)F]fluorobenzyl alcohol with 2-aminopurin-6-yltrimethylammonium chloride (4) or 2-amino-6-chloropurine in average decay-corrected radiochemical yields of 40 and 25%, respectively. Unlabeled 6-(3-iodo-benzyloxy)-9H-purin-2-ylamine (IBG; 7) was prepared from 4 and 3-iodobenzyl alcohol. Radioiodination of 9, prepared from 7 in two steps, and subsequent deprotection gave [(131)I]7 in about 70% overall radiochemical yield. The IC(50) values for the inactivation of AGT from CHO cells transfected with pCMV-AGT were 15 nM for IBG and 50 nM for FBG. The binding of [(18)F]6 and [(131)I]7 to purified AGT was specific and saturable with both exhibiting similar IC(50) values (5-6 microM).