Long-circulating liposomes radiolabeled with [18F]fluorodipalmitin ([18F]FDP)

Synthesis of a radiolabeled diglyceride, 3-[(18)F]fluoro-1,2-dipalmitoylglycerol [[(18)F]fluorodipalmitin ([(18)F]FDP)], and its potential as a reagent for radiolabeling long-circulating liposomes were investigated. The incorporation of (18)F into the lipid molecule was accomplished by nucleophilic substitution of the p-toluenesulfonyl moiety with a decay-corrected yield of 43+/-10% (n=12). Radiolabeled, long-circulating polyethylene-glycol-coated liposomes were prepared using a mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, cholesterol, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)-2000] ammonium salt (61:30:9) and [(18)F]FDP with a decay-corrected yield of 70+/-8% (n=4). PET imaging and biodistribution studies were performed with free [(18)F]FDP and liposome-incorporated [(18)F]FDP. Freely injected [(18)F]FDP had the highest uptake in the liver, spleen and lungs. Liposomal [(18)F]FDP remained in blood circulation at near-constant levels for at least 90 min, with a peak concentration near 2.5%ID/cc. Since [(18)F]FDP was incorporated into the phospholipid bilayer, it could potentially be used for radiolabeling a variety of lipid-based drug carriers.     

Synthesis of 2'-deoxy-2'-[18F]fluoro-5-bromo-1-beta-D-arabinofuranosyluracil ([18F]-FBAU) and 2'-deoxy-2'-[18F]fluoro-5-chloro-1-beta-D-arabinofuranosyl-uracil ([18F]-FCAU), and their biological evaluation as markers for gene expression

[(18)F]-FBAU and [(18)F]-FCAU have been synthesized and evaluated in vivo as markers for HSV1-tk gene expression. At 2 hours, uptake of [(18)F]-FBAU and [(18)F]-FCAU in HSV1-tk-positive tumors was 7.9-fold and 6.0-fold higher than the control tumors, respectively. Micro-PET images also showed very high uptake in HSV-tk tumors. Compared to [(14)C]-FMAU, total uptake of [(18)F]-FBAU and [(18)F]-FCAU was similar in tk-positive cells, but the uptake ratio (tk+/wild) was higher. [(18)F]-FBAU and [(18)F]-FCAU appear to be potential PET imaging agents for gene expression.     

A robotic synthesis of [18F]fluoromisonidazole ([18F]FMISO).

This study aimed to develop an automated synthesis of [18F]fluoromisonidazole ([18F]FMISO) using a Scanditronix Anatech RB III robotic system. [18F]HF was produced via the 18O(p,n)18F reaction using a Scanditronix MC17F cyclotron. On average, a typical run produced [18F]FMISO with an uncorrected radiochemical yield of 30+/-5% at end of synthesis (EOS) from the irradiation of 95% enriched [18O]water. The total synthesis time was 65 min. The retention time of [18F]FMISO (the radio-peak) was 4.9 min, which was consistent with the authentic FMISO (the ultraviolet peak). The radiochemical purity was greater than 97%. Preparation of [18F]FMISO using the automated robotic system is highly reliable and reproducible, and the radiation burden for the operator can be largely reduced. Sufficient radioactivities of [18F]FMISO could be obtained for non-invasive tumor hypoxia imaging in vivo with positron emission tomography (PET).     

Synthesis and evaluation of O-(3-[18F]fluoropropyl)-L-tyrosine as an oncologic PET tracer

O-(3-[(18)F]fluoropropyl)-L-tyrosine (FPT), an analogue of O-(2-[(18)F]fluoroethyl)-L-tyrosine (FET) as an amino acid tracer for tumor imaging with positron emission tomography (PET), was synthesized and evaluated. FPT was prepared by [(18)F]fluoropropylation of L-tyrosine in a two-step procedure. Biodistribution of FPT was determined in normal mice. FPT, FET and [(18)F]fluorine-2-deoxy-D-glucose (FDG) uptake studies were performed in mice bearing S18 fibrosarcoma and S. aureus-inoculated mice. Also, carcinoma-bearing mice and S. aureus-inoculated mice were imaged using FPT PET imaging compared with FET and FDG PET imaging. Synthesis of FPT was accomplished in about 60 min with an overall radiochemical yield of 25-30% (without decay correction) by manual operation. High uptake and long retention time of FPT and FET in kidney, liver, lung, blood, etc., and low uptake in brain were found. Furthermore, high FPT, FET and FDG uptake in tumor, and almost no FPT and FET uptake in inflammatory tissue, in contrast, high FDG uptake in inflammatory tissue, were observed. In conclusion, FPT is easy to prepare and superior to FDG in the differentiation of tumor and inflammation, and seems to be a potential amino acid tracer like FET for tumors imaging with PET.     

Synthesis of no-carrier-added N-([18F]fluoroalkyl)spiperone derivatives

3-N-([¹⁸F]fluoroalkyl)spiperone derivatives (2, 3) can be prepared by N-alkylation of spiperone (1) with fluoroalkyl halides. The fluoroalkylating species 2-[¹⁸F]fluoroethyl bromide (7), 3-[¹⁸F]fluoropropyl bromide (8) and 4-[¹⁸F]fluorobutyl bromide (9) were prepared by [¹⁸F]fluoride ion displacement of the corresponding trifluoromethanesulfonates (triflates 4, 5, 6). By this method, the 2-[¹⁸F]fluoroethyl-, 3-[¹⁸F]fluoropropyl-, and 4-[¹⁸F]fluorobutyl spiperone derivatives (2a–c) can be prepared and purified rapidly and conveniently, within 40 min, in yields of 30–40% (end of synthesis, EOS). An alternative approach, suitable for the preparation of 2-[¹⁸F]fluoroalkyl (ethyl, propyl, butyl, pentyl and hexyl) spiperone derivatives (3a–d), involves iodo[¹⁸F]fluorination of terminal olefins, followed by N-alkylation of spiperone. This sequence is less convenient and proceeds in lower overall yields (<5%).     

18F]fluoromethyl triflate, a novel and reactive [18F]fluoromethylating agent: preparation and application to the on-column preparation of [18F]fluorocholine.

The production and use of [18F]fluoromethyl triflate ([18F]CH2FOTf), a more reactive [18F]fluoromethylating agent than [18F]fluoromethyl bromide ([18F]CH2BrF), is described. [18F]CH2FOTf was prepared from [18F]CH2BrF. The latter was synthesized by nucleophilic substitution of CH2Br2 with no-carrier-added [18F]fluoride and purified by four Sep-Pak Plus silica cartridges connected in series. It was then quantitatively converted on-line to [18F]CH2FOTf by passing through a heated AgOTf column. Decay-corrected radiochemical yields of [18F]CH2FOTf based on [18F]fluoride were 47 +/- 8% (n = 20). Both [18F]CH2BrF and [18F]CH2FOTf were applied to solid-supported [18F]fluoromethylation of N,N-dimethylaminoethanol on a Sep-Pak Plus C18 cartridge to produce the 18F-labeled choline analogue, (beta-hydroxyethyl)dimethyl-[18F]fluoromethylammonium ([18F]fluorocholine). Depending on flow rate and amount of precursor used, decay corrected radiochemical yields of [18F]fluorocholine from [18F]CH2BrF ranged from 6% to 63%, while [18F]CH2FOTf afforded yields of more than 80%. Thus, by using the latter reagent and a subsequent purification on a Sep-Pak Accell CM cartridge, [18F]fluorocholine was produced from [18F]fluoride in overall radiochemical yields of 40% (decay corrected) in less than 30 min.     

Increased [18F]2-fluoro-2-deoxy-D-glucose ([18F]FDG) yield with recycled target [18O]water: factors affecting the [18F]FDG yield.

The reuse of [18O] water after being purified by distillation has been reported to give lower [18F]2-fluoro-2-deoxy-D-glucose ([18F]FDG) yields, probably due to the presence of organic impurities. In our routine production of [18F]FDG, however, we observed increased [18F]FDG yields with recycled [18O]water. Thus, factors affecting [18F]FDG yield were examined using as-purchased (virgin) and recycled (by photochemical combustion and distillation) [18O]water. [18F]FDG was synthesized by nucleophilic 18F-fluorination on a quaternary 4-aminopyridinium resin. The recycled [18O]water gave an [18F]FDG yield significantly higher than did the virgin water, without any significant difference in the [18F]fluoride yield. Levels of several ionic impurities including Cl- and Ca2+ were significantly higher in the virgin [18O]water than in the recycled water, while significantly larger amounts of organic impurities were detected in the former. Hence, trace amounts of organic impurities were not responsible for the lower [18F]FDG yield. Chloride anion in the [18O]water may compete with [18F]fluoride to lower the [18F]FDG yield.     

Fully automated [18F]fluorocholine synthesis in the TracerLab MX FDG Coincidence synthesizer

INTRODUCTION: We developed a new fully automated method for the radiosynthesis of [18F]fluorocholine by modifying the commercial 2-[18F]fluoro-2-d-deoxy-glucose ([18F]FDG) synthesizer module (GE TracerLab MX, formerly Coincidence). METHODS: [18F]Flurocholine was synthesized by (18)F-fluoroalkylation of N,N-dimethylaminoethanol using [18F]fluorobromomethane as fluoromethylating agent. [18F]Fluorobromomethane was produced by reaction of dibromomethane with [18F]fluoride, assisted by Kryptofix 2.2.2. RESULTS: After purification on solid-phase extraction cartridges, the [18F]fluorocholine was obtained in 15-25% radiochemical yields (decay not corrected), with more than 99% radiochemical purity. Specific activity was more than 37 GBq/micromol. Synthesis time was less than 35 min. CONCLUSION: This new automated synthesis technique provides high and reproducible yields that could be dedicated for routine use with the same [18F]FDG disposable cassette system.     

Simple,column purification technique for the fully automated radiosynthesis of [18F]fluoroazomycinarabinoside ([18F]FAZA)

A novel fully automated radiosynthesis procedure for the fluorine-18 analog of 1-α-D-(5′-deoxy-5′-fluoro-(1S,2R,3S,4S) arabinofuranosyl)-2-nitroimidazole ([¹⁸F]FAZA) using a commercially available combination column – Chromabond^® Set V (FDG-base-hydrolysis) – for purification was developed. [¹⁸F]FAZA was prepared via a one-pot, two-step synthesis using a nuclear interface nucleophilic synthesis module. Nucleophilic fluorination of the precursor molecule, 1-(2,3-di-O-acetyl-5-O-tosyl-α-D-arabinofuranosyl)-2-nitroimidazole, with no-carrier added [¹⁸F]fluoride followed by hydrolysis of the protecting groups with 0.3 M NaOH was performed. Purification was carried out using the Chromabond^® Set V column without any modifications. The overall radiochemical yield obtained was 21.98±1.40% (n=5, without decay correction) within a total synthesis period of 51±1 min. The radiochemical purity was greater than 95% and devoid of any other chemical impurities. The reported method can easily be adapted in any commercial FDG synthesis module.     

Biological evaluation of 2'-[18F]fluoroflumazenil ([18F]FFMZ), a potential GABA receptor ligand for PET

[(11)C]Flumazenil, a highly selective benzodiazepine antagonist is the most extensively used GABA(A) ligand for PET so far. To overcome half life disadvantages of (11)C a [(18)F]-labeled flumazenil derivative, 2'-[(18)F]fluoroflumazenil (FFMZ) was developed and biologically evaluated with respect to the GABA(A) receptor. Organ with the highest uptake was the pituitary gland. Brain uptake was high and followed the order cortex>thalamus>cerebellum>rest brain. Fluoroflumazenil displaced [(3)H]flumazenil binding from membrane GABA(A) receptors with an IC(50)value (3.5 nM) comparable to that of Flumazenil (2.8 nM). The presented data confirm the potential of [(18)F]FFMZ for PET imaging of the GABA-ergic system.     

A simplified one-pot synthesis of 9-[(3-[18F]fluoro-1-hydroxy-2-propoxy)methyl]guanine([18F]FHPG) and 9-(4-[18F]fluoro-3-hydroxymethylbutyl)guanine ([18F]FHBG) for gene therapy

9-[(3-[18F]Fluoro-1-hydroxy-2-propoxy)methyl]guanine ([18F]FHPG, 2) has been synthesized by nucleophilic substitution of N(2)-(p-anisyldiphenylmethyl)-9-[[1-(p-anisyldiphenylmethoxy)-3-toluenesulfonyloxy-2-propoxy]methyl]guanine (1) with potassium [18F]fluoride/Kryptofix 2.2.2 followed by deprotection with 1 N HCl and purification with different methods in variable yields. When both the nucleophilic substitution and deprotection were carried out at 90 degrees C and the product was purified by HPLC (method A), the yield of compound 2 was 5-10% and the synthesis time was 90 min from EOB. However, if both the nucleophilic substitution and deprotection were carried out at 120 degrees C and the product was purified by HPLC, the yield of compound 2 decreased to 2%. When compound 2 was synthesized at 90 degrees C and purified by Silica Sep-Pak (method B), the yield increased to 10-15% and the synthesis time was 60 min from EOB. Similarly, 9-(4-[18F]fluoro-3-hydroxymethylbutyl)guanine ([18F]FHBG, 4) was synthesized with method A and method B in 9% and 10-15% yield, respectively, in a synthesis time of 90 and 60 min, respectively, from EOB. Compound 2 was relatively unstable in acidic medium at 120 degrees C while compound 4 was stable under the same condition. Both compound 2 and compound 4 had low lipid/water partition coefficient (0.126 +/- 0.022, n=5 and 0.165 +/- 0.023, n=5, respectively). Although it contains non-radioactive ganciclovir ( approximately 5-30 microg) as a chemical by-product, compound 2 synthesized by method B has a similar uptake in 9L glioma cells as that synthesized by method A, and is a potential tracer for imaging herpes simplex virus thymidine kinase gene expression in tumors using PET. Similarly, compound 4 synthesized by method B contains approximately 10-25 microg of penciclovir as a chemical by-product. Thus, the simplified one pot synthesis (method B) is a useful method for synthesizing both compound 2 and compound 4 in good yield for routine clinical use, and the method is readily amenable for automation.     

Synthesis and evaluation of l-5-(2-[18F]fluoroethoxy)tryptophan as a new PET tracer

A convenient remote controlled synthesis of a new tryptophan analog, l-5-(2-[¹⁸F] fluoroethoxy)-tryptophan (5-¹⁸FEHTP) was described. The radiochemical yield within 65 min was about 12–16% without decay correction, the radiochemical purity was over 98%, and 5-¹⁸FEHTP dissolved in saline was stable over 6 h at room temperature. The biodistribution of 5-¹⁸FEHTP in mice and the high uptake of 5-¹⁸FEHTP in tumor demonstrated that it is very likely a new PET tracer for tumor imaging.     

A high yield robotic synthesis of 9-(4-[18F]-fluoro-3-hydroxymethylbutyl)guanine ([18F]FHBG) and 9-[3-[18F]fluoro-1-hydroxy-2-propoxy)methyl]guanine([18F] FHPG) for gene expression imaging.

The aim of this study was to develop an automated synthesis of 9-(4-[(18)F]-fluoro-3-hydroxymethylbutyl)guanine ([(18)F]FHBG) and 9-[(3-[(18)F]fluoro-1-hydroxy-2-propoxy)methyl]guanine ([(18)F]FHPG) using a Scanditronix Anatech RB III robotic system. [(18)F]HF was produced via (18)O(p, n)(18)F using a Scanditronix MC17F cyclotron. On average, a typical run produced [(18)F]FHBG and [(18)F]FHPG with an uncorrected radiochemical yield of 19% and 16%, respectively, at end of synthesis (EOS) from irradiation of 95% enriched [(18)O]water. The total synthesis time was 80 min. The retention time of [(18)F]FHBG and [(18)F]FHPG (the radio-peak) was 3.9 and 4.0 min, respectively, which was consistent with the [(19)F]FHBG and [(19)F]FHPG ultraviolet peak. The radiochemical purity was greater than 97%. A robotic, automated method for [(18)F]FHBG and [(18)F]FHPG radiosynthesis is therefore feasible. The radiation burden for the operator can be reduced as much as possible. Sufficient radioactivities of [(18)F]FHBG and [(18)F]FHPG could be obtained for non-invasive monitoring the expression of transfected gene in vivo with positron emission tomography (PET).     

Radiolysis of 2-[18F]fluoro-2-deoxy-d-glucose ([18F]FDG) and the role of ethanol and radioactive concentration

Radiolysis is the process by which radioactively labeled compounds degrade. Many positron emission tomography (PET) radiopharmaceuticals produced with high radioactive concentrations and specific activities exhibit low radiochemical purity because of radiolysis. Little data exist that describe the radiolytic decomposition of 2-[¹⁸F]fluoro-2-deoxy-d-glucose ([¹⁸F]FDG). The objective of our study was to profile the degradation of [¹⁸F]FDG at various radioactive concentrations by measuring radiochemical purity at different time intervals and to study the effects of ethanol, a well-known reductant stabilizer of [¹⁸F]FDG preparations.     

Synthesis of [18F]flunarizine

Flunarizine, a calcium channel antagonist of the piperazine class, has been labeled with the positron-emitter ¹⁸F. 4-[¹⁸F]Fluoro-4′-fluorobenzhydryl chloride was prepared in three steps from no-carrier-added [¹⁸F]fluoride ion, and used in the alkylation of N-cinnamylpiperazine to give [¹⁸F]flunarizine in 13% radiochemical yield (from [¹⁸F]fluoride).     

Synthesis of 2-(1,1-dicyanopropen-2-yl)-6-(2-[18F]-fluoroethyl)-methylamino-naphthalene ([18F]FDDNP).

2-(1,1-dicyanopropen-2-yl)-6-(2-[18F]-fluoroethyl)-methylamino-naphthalene ([18F]FDDNP) was synthesized in a single step labeling procedure. The precursor, 2-(1,1-dicyanopropen-2-yl)-6-(2-tosyloxyoethyl)-methylamino-naphthalene, was fluorinated with 18F in acetonitrile. After 15 min the reaction mixture was subjected to preparative HPLC purification. The product was isolated from the HPLC eluent with solid-phase extraction, and formulated in an ascorbic acid solution to prevent formation of side products during formulation. Quantitative sticking to tubing and filters was overcome by the addition of polysorbatum-80. This formulation yielded an isotonic, pyrogen-free and sterile solution of [18F]FDDNP. The overall decay-corrected radiochemical yield was 41+/-11% (n=22). Radiochemical purity was >98% and the specific activity was 102+/-56 GBq/micromol at the end of synthesis.     

Synthesis and evaluation of 2-amino-5-(4-[18F]fluorophenyl)pent-4-ynoic acid ([18F]FPhPA): A novel 18F-labeled amino acid for oncologic PET imaging

Introduction¹⁸ F-labeled amino acids are important PET radiotracers for molecular imaging of cancer. This study describes synthesis and radiopharmacological evaluation of 2-amino-5-(4-[¹⁸ F]fluorophenyl)pent-4-ynoic acid ([¹⁸ F]FPhPA) as a novel amino acid radiotracer for oncologic imaging.Methods¹⁸ F]FPhPA was prepared using Pd-mediated Sonogashira cross-coupling reaction between 4-[¹⁸ F]fluoroiodobenzene ([¹⁸ F]FIB) and propargylglycine. The radiopharmacological profile of [¹⁸ F]FPhPA was evaluated in comparison with O-(2-[¹⁸ F]fluoroethyl)-L-tyrosine ([¹⁸ F]FET) using the murine breast cancer cell line EMT6 involving cellular uptake studies, radiotracer uptake competitive inhibition experiments and small animal PET imaging.Results¹⁸ F]FPhPA was prepared in 42 ± 10% decay-corrected radiochemical yield with high radiochemical purity >95% after semi-preparative HPLC purification. Cellular uptake of L-[¹⁸ F]FPhPA reached a maximum of 58 ± 14 % radioactivity/mg protein at 90 min. Lower uptake was observed for racemic and D-[¹⁸ F]FPhPA.Radiotracer uptake inhibition studies by synthetic and naturally occurring amino acids suggested that Na⁺-dependent system ASC, especially ASCT2, and Na⁺-independent system L are important amino acid transporters for [¹⁸ F]FPhPA uptake into EMT6 cells. Small animal PET studies demonstrated similar high tumor uptake of [¹⁸ F]FPhPA in EMT6 tumor-bearing mice compared to [¹⁸ F]FET reaching a maximum standardized uptake value (SUV) of 1.35 after 60 min p.i.. Muscle uptake of [¹⁸ F]FPhPA was higher (SUV_30min = 0.65) compared to [¹⁸ F]FET (SUV_30min = 0.40), whereas [¹⁸ F]FPhPA showed a more rapid uptake and clearance from the brain compared to [¹⁸ F]FET.ConclusionL-[¹⁸ F]FPhPA is the first ¹⁸ F-labeled amino acid prepared through Pd-mediated cross-coupling reaction.Advances in Knowledge and Implications for patient CareL-[¹⁸ F]FPhPA displayed promising properties as a novel amino acid radiotracer for molecular imaging of system ASC and system L amino acid transporters in cancer.     

(+)-p-([18F]fluorobenzyl)spirotrozamicol [(+)-[18F]spiro-FBT]: synthesis and biological evaluation of a high-affinity ligand for the vesicular acetylcholine transporter (VAChT)

(+)-1'-[4-Hydroxy-1-(4-fluorobenzyl)piperidin-3-yl]spiro[1H- indene-1,4'- piperidine] {(+)-Spiro-FBT}, a high-affinity vesicular acetylcholine transporter ligand, was labeled with fluorine-18, and evaluated in the rat and monkey. In the rat brain, (+)-[18F]Spiro-FBT accumulated preferentially in the striatum, hippocampus, and cortex, brains regions containing high-to-moderate densities of cholinergic terminals. However, due to rapid metabolism, no preferential accumulation of the radiotracer was observed in corresponding regions of the monkey brain. Consequently, rapid metabolism renders (+)-[18F]Spiro-FBT unsuitable for studying cholinergic function with positron emission tomography.     

Assessment of radionuclidic impurities in 2-[18F]fluoro-2-deoxy-d-glucose ([18F]FDG) routine production.

In this paper, radionuclidic impurities generated during the bombardment of [18 O]water in the routine production of 2-[18F]fluoro-2-deoxy-d-glucose ([18F]FDG) were studied. In order to assess such impurities and the efficacy of purification methods through the different steps of the synthesis, samples of the target filters, purification columns, [18 O]water recovered after the synthesis, and the final solution was collected and their activities measured and analyzed by means of a gamma-ray spectrometry system. The data demonstrated that purification methods adopted for the synthesis provide the [18F]FDG radionuclidically pure, as requested by the EU Pharmacopeia.