Synthesis of [¹¹C]PBR06 and [¹⁸F]PBR06 as agents for positron emission tomographic (PET) imaging of the translocator protein (TSPO)

The translocator protein 18 kDa (TSPO) is an attractive target for molecular imaging of neuroinflammation and tumor progression. [(18)F]PBR06, a fluorine-18 labeled form of PBR06, is a promising PET TSPO radioligand originally developed at NIMH. [(11)C]PBR06, a carbon-11 labeled form of PBR06, was designed and synthesized for the first time. The standard PBR06 was synthesized from 2,5-dimethoxybenzaldehyde in three steps with 71% overall chemical yield. The radiolabeling precursor desmethyl-PBR06 was synthesized from 2-hydroxy-5-methoxybenzaldehyde in five steps with 12% overall chemical yield. The target tracer [(11)C]PBR06 was prepared by O-[(11)C]methylation of desmethyl-PBR06 with [(11)C]CH(3)OTf in CH(3)CN at 80°C under basic condition and isolated by HPLC combined with SPE purification with 40-60% decay corrected radiochemical yield and 222-740 GBq/μmol specific activity at EOB. On the similar grounds, [(18)F]PBR06 was also designed and synthesized. The previously described Br-PBR06 precursor was synthesized from 2,5-dimethoxybenzaldehyde in two steps with 78% overall chemical yield. A new radiolabeling precursor tosyloxy-PBR06, previously undescribed tosylate congener of PBR06, was designed and synthesized from ethyl 2-hydroxyacetate, 4-methylbenzene-1-sulfonyl chloride, and N-(2,5-dimethoxybenzyl)-2-phenoxyaniline in four steps with 50% overall chemical yield. [(18)F]PBR06 was prepared by the nucleophilic substitution of either new tosyloxy-PBR06 precursor or known Br-PBR06 precursor in DMSO at 140°C with K[(18)F]F/Kryptofix 2.2.2 for 15 min and HPLC combined with SPE purification in 20-60% decay corrected radiochemical yield, >99% radiochemical purity, 87-95% chemical purity, and 37-222 GBq/μmol specific activity at EOB. Radiosynthesis of [(18)F]PBR06 using new tosylated precursor gave similar radiochemical purity, and higher specific activity, radiochemical yield and chemical purity in comparison with radiosynthesis using bromine precursor.

[Adaptation of the (18)FDG module for the preparation of a sodium fluoride [(18)F] injection solution in agreement with the United States (USP 32) and European Pharmacopeia (PhEur 6)]

OBJECTIVE

To establish an automated procedure for the preparation of sodium fluoride (18)F injection using the resources available in our laboratory for the preparation of (18)FDG and to analyze the repercussion of the conditioning column of the fluoride ion entrapment on the characteristics of the final product.

MATERIAL AND METHOD

The sequence of an (18)FDG synthesis module prepared so that it traps the fluoride ion from the cyclotron in ion-exchange resin diluted with 0.9% sodium chloride. The final solution was dosified and sterilized in a final vial in an automatized dispensing module. Three different column conditioning protocols within the process were tested. Quality controls were run according to USP 32 and EurPh 6, adding control of ethanol levels of residual solvent and quality controls of the solution at 8 h post-preparation.

RESULTS

Activation of the resin cartridges with ethanol and water was the chosen procedure, with fluoride ion trapping > 95% and pH around 7. Ethanol levels were < 5.000 ppm. Quality controls at 8 h indicated that the solution was in compliance with the USP 32 and EurPh 6 specifications.

CONCLUSION

This is an easy, low-cost, reliable automated method for sodium fluoride preparation in PET facilities with existing equipment for (18)FDG synthesis and quality control.

Syntheses of mGluR5 PET radioligands through the radiofluorination of diaryliodonium tosylates

3-Fluoro-1-((thiazol-4-yl)ethynyl)benzenes constitute an important class of high-affinity metabotropic glutamate subtype 5 receptor (mGluR5) ligands, some of which have been labeled with fluorine-18 (t(1/2) = 109.7 min), to provide radioligands for molecular imaging of brain mGluR5 in living animal and human subjects with positron emission tomography (PET). Labeling in the 3-fluoro position of such ligands can be achieved through aromatic nucleophilic substitution of a halide leaving group with [(18)F]fluoride ion when a weakly activating m-nitrile group is present, but is generally very low yielding (<8%). Here we used a microfluidic reaction platform to show that greatly enhanced (up to 6-fold) radiochemical yields can be achieved from suitably synthesized diaryliodonium tosylate precursors. The presence of a m-nitrile or other activating group is not required. Similar conditions were adopted in a more conventional automated radiochemistry platform having a single-pot reactor, to produce mGluR5 radioligands with useful radioactivities for PET imaging.

t-Bu2SiF-derivatized D2-receptor ligands: the first SiFA-containing small molecule radiotracers for target-specific PET-imaging

The synthesis, radiolabeling and in vitro evaluation of new silicon-fluoride acceptor (SiFA) derivatized D(2)-receptor ligands is reported. The SiFA-technology simplifies the introduction of fluorine-18 into target specific biomolecules for Positron-Emission-Tomography (PET). However, one of the remaining challenges, especially for small molecules such as receptor-ligands, is the bulkiness of the SiFA-moiety. We therefore synthesized four Fallypride SiFA-conjugates derivatized either directly at the benzoic acid ring system (SiFA-DMFP, SiFA-FP, SiFA-DDMFP) or at the butyl-side chain (SiFA-M-FP) and tested their receptor affinities. We found D(2)-receptor affinities for all compounds in the nanomolar range (K(i(SiFA-DMFP)) = 13.6 nM, K(i(SiFA-FP)) = 33.0 nM, K(i(SiFA-DDMFP)) = 62.7 nM and K(i(SiFA-M-FP)) = 4.21 nM). The radiofluorination showed highest yields when 10 nmol of the precursors were reacted with [(18)F]fluoride/TBAHCO(3) in acetonitrile. After a reversed phased cartridge purification the desired products could be isolated as an injectable solution after only 10 min synthesis time with radiochemical yields (RCY) of more than 40% in the case of SiFA-DMFP resulting in specific activities >41 GBq/µmol (>1,100 Ci/mmol). Furthermore, the radiolabeled products were shown to be stable in the injectable solutions, as well as in human plasma, for at least 90 min.

Radiolabeling of phosphatidylserine-binding peptides with prosthetic groups N-[6-(4-[18F]fluorobenzylidene)aminooxyhexyl]maleimide ([18F]FBAM) and N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB)

The widely used (18)F-prosthetic group N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB) and the recently developed N-[6-(4-[(18)F]fluorobenzylidene)aminooxyhexyl]maleimide ([(18)F]FBAM) were investigated for radiolabeling of two representative phosphatidylserine-binding peptides. The prosthetic groups were compared with respect to required reactions conditions for optimum labeling, radiolabeling yield and chemoselectivity. The N-terminus labeled product produced by reaction of [(18)F]SFB with binding peptide LIKKPF was produced in 18% radiochemical yield while no N-terminus labeled product could be isolated following [(18)F]SFB reaction with PDGLSR. When the peptides were modified by addition of a cysteine residue at the N-terminus they provided almost quantitative radiochemical yields with [(18)F]FBAM. Results indicate that for the peptides in this study, [(18)F]FBAM is a more useful prosthetic group compared to [(18)F]SFB due to its excellent chemoselectivity and high radiochemical yield.

Synthesis and in vivo imaging of a 18F-labeled PARP1 inhibitor using a chemically orthogonal scavenger-assisted high-performance method

Pedal to the metal: Using inverse Diels-Alder catalyst free TCO/Tz cycloadditions, we were able to quickly and selectively generate an 18F-labeled AZD2281-derivative from multiple different scaffolds (A–E). Excess cold material was removed within minutes using a TCO scavenger resin. This protocol allows the parallel synthesis of a library of potential PET imaging agents in a short time, increasing the efficiency of lead compound detection. The novel PET probe was successfully tested in biological assays and its potency and targeted accumulation was confirmed in vivo .

Cross-coupling reactions as valuable tool for the preparation of PET radiotracers

The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of new radiotracers and novel radiolabeling procedures with the most prominent short-lived positron emitters carbon-11 and fluorine-18. Radiolabeling with these radionuclides represents a remarkable challenge. Special attention has to be paid to synthesis time and specific labeling techniques due to the short physical half life of the respective radionuclides ¹¹C (t_1/2 = 20.4 min) and ¹⁸F (t_1/2 = 109.8 min). In the past, numerous transition metal-catalyzed reactions were employed in organic chemistry, even though only a handful of these coupling reactions were adopted in radiochemical practice. Thus, the implementation of modern synthesis methods like cross-coupling reactions offers the possibility to develop a wide variety of novel radiotracers. The introduction of catalysts based on transition metal complexes bears a high potential for rapid, efficient, highly selective and functional group-tolerating incorporation of carbon-11 and fluorine-18 into target molecules. This review deals with design, application and improvement of transition metal-mediated carbon-carbon as well as carbon-heteroatom cross-coupling reactions as a labeling feature with the focus on the preparation of radiolabeled compounds for molecular imaging.

Frequency-swept pulse sequences for 19F heteronuclear spin decoupling in solid-state NMR

Heteronuclear spin decoupling pulse sequences in solid-state NMR have mostly been designed and applied for irradiating 1H as the abundant nucleus. Here, a systematic comparison of different methods for decoupling 19F in rigid organic solids is presented, with a special emphasis on the recently introduced frequency-swept sequences. An extensive series of NMR experiments at different MAS frequencies was conducted on fluorinated model compounds, in combination with large sets of numerical simulations. From both experiments and simulations it can be concluded that the frequency-swept sequences SWf-TPPM and SWf-SPINAL deliver better and more robust spin decoupling than the original sequences SPINAL and TPPM. Whereas the existence of a large chemical shift anisotropy and isotropic shift dispersion for 19F does compromise the decoupling efficiency, the relative performance hierarchy of the sequences remains unaffected. Therefore, in the context of rigid organic solids under moderate MAS frequencies, the performance trends observed for 19F decoupling are very similar to those observed for 1H decoupling.

Synthesis and evaluation of 1,2,4-triazolo[1,5-c]pyrimidine derivatives as A2A receptor-selective antagonists

Movement disorders such as Parkinson's disease and Huntington's disease are serious life-limiting and debilitating movement disorders. Their onset typically occurs from mid-life to late in life, and effective diagnostic techniques for detecting and following the disease course are lacking. Our goal is to develop receptor imaging agents for positron emission tomography (PET) that selectively target the most relevant subtype of adenosine receptors (AR) that are highly expressed in the striatum, that is, the A(2A) AR. To further this goal, we have synthesized and characterized pharmacologically a family of high affinity A(2A) AR ligands, based on the known antagonist, SCH 442416 (R=-Me), which have structural variability on the terminus (R=-Et, -i-Pr, -allyl, and others). A O-fluoroethyl analogue suitable for use as a PET tracer had a K(i) value of 12.4 nM and was highly selective for the A(2A) AR in comparison to the A(1) and A(3) ARs.

Biodistribution and radiation dosimetry of a positron emission tomographic ligand, 18F-SP203, to image metabotropic glutamate subtype 5 receptors in humans

PURPOSE

A new PET ligand, 3-fluoro-5-(2-(2-(18)F-(fluoromethyl)-thiazol-4-yl)ethynyl)benzonitrile ((18)F-SP203), is a positron emission tomographic radioligand selective for metabotropic glutamate subtype 5 receptors. The purposes of this study were to estimate the radiation-absorbed doses of (18)F-SP203 in humans and to determine from the distribution of radioactivity in bone structures with various proportions of bone and red marrow whether (18)F-SP203 undergoes defluorination.

METHODS

Whole-body images were acquired for 5 h after injecting (18)F-SP203 in seven healthy humans. Urine was collected at various time points. Radiation-absorbed doses were estimated by the Medical Internal Radiation Dose scheme.

RESULTS

After injecting (18)F-SP203, the two organs with highest radiation exposure were urinary bladder wall and gallbladder wall, consistent with both urinary and fecal excretion. In the skeleton, most of the radioactivity was in bone structures that contain red marrow and not in those without red marrow. Although the dose to red marrow (30.9 microSv/MBq) was unusually high, the effective dose (17.8 microSv/MBq) of (18)F-SP203 was typical of that of other (18)F radiotracers.

CONCLUSION

(18)F-SP203 causes an effective dose in humans typical of several other (18)F radioligands and undergoes little defluorination.

Conformational structure and energetics of 2-methylphenyl(2'-methoxyphenyl)iodonium chloride: evidence for solution clusters

Diaryliodonium salts allow the efficient incorporation of cyclotron-produced [(18)F]fluoride ions into electron-rich and electron-deficient arenes to provide potential radiotracers for molecular imaging in vivo with positron emission tomography (PET). This process (ArI(+)Ar'+(18)F(-)→Ar(18)F+Ar'I) is still not well understood mechanistically. To better understand this and similar reactions, it would be valuable to understand the structures of diaryliodonium salts in organic media, where the reactions are typically conducted. In this endeavor, the X-ray structure of a representative iodonium salt, 2-methylphenyl(2'-methoxyphenyl)iodonium chloride (1), was determined. Our X-ray structure analysis showed 1 to have the conformational M-P dimer as the unit cell with hypervalent iodine as a stereogenic center in each conformer. With the ab initio replica path method we constructed the inversion path between the two enantiomers of 1, thereby revealing two additional pairs of enantiomers that are likely to undergo fast interconversion in solution. Also LC-MS of 1 showed the presence of dimeric and tetrameric anion-bridged clusters in weak organic solution. This observation is consistent with the energetics of 1, both as monomeric and dimeric forms in MeCN, calculated at the B3LYP/DGDZVP level. These evidences of the existence of dimeric and higher order clusters of 1 in solution are relevant to achieve a deeper general understanding of the mechanism and outcome of reactions of diaryliodonium salts in organic media with nucleophiles, such as the [(18)F]fluoride ion.

A novel 18F-labeled pyridyl benzofuran derivative for imaging of β-amyloid plaques in Alzheimer's brains

A potential probe for PET targeting β-amyloid plaques in Alzheimer's disease (AD) brain, FPYBF-1 (5-(5-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)benzofuran-2-yl)-N,N-dimethylpyridin-2-amine), was synthesized and evaluated. In experiments in vitro, FPYBF-1 displayed high affinity for Aβ(1-42) aggregates (K(i)=0.9 nM), and substantial labeling of β-amyloid plaques in sections of postmortem AD brains but not control brains. In experiments in vivo, [(18)F]FPYBF-1 displayed good initial uptake (5.16%ID/g at 2 min postinjection) and rapid washout from the brain (2.44%ID/g at 60 min postinjection) in normal mice, and excellent binding to β-amyloid plaques in a murine model of AD. Furthermore, the specific labeling of plaques labeling was observed in autoradiographs of autopsied AD brain sections. [(18)F]FPYBF-1 may be a useful probe for imaging β-amyloid plaques in living brain tissue.

Aerobic biodegradation of [14C] 6:2 fluorotelomer alcohol in a flow-through soil incubation system

The aerobic biodegradation of [1,2-(14)C] 6:2 FTOH [F(CF(2))(6)(14)CH(2)(14)CH(2)OH] in a flow-through soil incubation system is described. Soil samples dosed with [1,2-(14)C] 6:2 FTOH were analyzed by liquid scintillation counting, LC/ARC (liquid chromatography/accurate radioisotope counting), LC/MS/MS, and thermal combustion to account for 6:2 FTOH and its transformation products over 84 d. Half of the [1,2-(14)C] 6:2 FTOH disappeared from soil in 1.3 d, undergoing simultaneous microbial degradation and partitioning of volatile transformation product(s) and the 6:2 FTOH precursor into the air phase. The overall (14)C (radioactivity) mass balance in live and sterile treatments was 77-87% over 84-d incubation. In the live test system, 36% of total (14)C dosed was captured in the airflow (headspace), 25% as soil-bound residues recovered via thermal combustion, and 16% as soil extractable. After 84 d, [(14)C] 5:2 sFTOH [F(CF(2))(5)CH(OH)(14)CH(3)] was the dominant transformation product with 16% molar yield and primarily detected in the airflow. The airflow also contained [1,2-(14)C] 6:2 FTOH and (14)CO(2) at 14% and 6% of total (14)C dosed, respectively. The other significant stable transformation products, all detected in soil, were 5:3 acid [F(CF(2))(5)CH(2)CH(2)COOH, 12%], PFHxA [F(CF(2))(5)COOH, 4.5%] and PFPeA [F(CF(2))(4)COOH, 4.2%]. Soil-bound residues as well as conjugates between fluorinated transformation products and dissolved soil components were only observed in the live test system and absent in the sterile soil, suggesting that such binding and complexation are microbially or enzymatically driven processes. At day 84, 5:3 acid is postulated to be the major transformation product in soil-bound residues, which may not be available for further biodegradation in soil environment.

Halogenated benzimidazole carboxamides target integrin alpha4beta1 on T-cell and B-cell lymphomas

Integrin alpha(4)beta(1) is an attractive but poorly understood target for selective diagnosis and treatment of T-cell and B-cell lymphomas. This report focuses on the rapid microwave preparation, structure-activity relationships, and biological evaluation of medicinally pertinent benzimidazole heterocycles as integrin alpha(4)beta(1) antagonists. We documented tumor uptake of derivatives labeled with (125)I in xenograft murine models of B-cell lymphoma. Molecular homology models of integrin alpha(4)beta(1) predicted that docked halobenzimidazole carboxamides have the halogen atom in a suitable orientation for halogen-hydrogen bonding. The high-affinity halogenated ligands identified offer attractive tools for medicinal and biological use, including fluoro and iodo derivatives with potential radiodiagnostic ((18)F) or radiotherapeutic ((131)I) applications, or chloro and bromo analogues that could provide structural insights into integrin-ligand interactions through photoaffinity, cross-linking/mass spectroscopy, and X-ray crystallographic studies.

Fast and high-yield microreactor syntheses of ortho-substituted [(18)F]fluoroarenes from reactions of [(18)F]fluoride ion with diaryliodonium salts

A microreactor was applied to produce ortho-substituted [(18)F]fluoroarenes from the reactions of cyclotron-produced [(18)F]fluoride ion (t(1/2) = 109.7 min) with diaryliodonium salts. The microreactor provided a very convenient means for running sequential reactions rapidly with small amounts of reagents under well-controlled conditions, thereby allowing reaction kinetics to be followed and Arrhenius activation energies (E(a)) to be measured. Prepared symmetrical iodonium chlorides (Ar(2)I(+)Cl(-)) rapidly (<4 min) gave moderate (Ar = 2-MeOC(6)H(4), 51%) to high (Ar = Ph or 2-MeC(6)H(4), 85%) decay-corrected radiochemical yields (RCYs) of a single radioactive product (Ar(18)F). Reaction velocity with respect to Ar group was 2-MeOC(6)H(4) < Ph < 2-MeC(6)H(4). Activation energies were in the range 18-28 kcal/mol. Prepared unsymmetrical salts (e.g., 2-RC(6)H(4)I(+)2'-R'C(6)H(4)X(-); X = Cl or OTs) also rapidly gave two products (2-RC(6)H(4)(18)F and 2-R'C(6)H(4)(18)F) in generally high total RCYs (79-93%). Selectivity for product [(18)F]fluoroarene was controlled by the nature of the ortho substituents. The power of ortho substituents to impart an ortho-effect was in the following order:, 2,6-di-Me > 2,4,6-tri-Me > Br > Me > Et approximately (i)Pr >> H > OMe. For (2-methyphenyl)(phenyl)iodonium chloride, the time-course of reaction product selectivity was constant and consistent with the operation of the Curtin-Hammett principle. These results will aid in the design of diaryliodonium salt precursors to (18)F-labeled tracers for molecular imaging.

PET imaging of multidrug resistance in tumors using 18F-fluoropaclitaxel

The failure of solid tumors to respond to chemotherapy is a complicated and clinically frustrating issue. The ability to predict which tumors will respond to treatment could reduce the human and monetary costs of cancer therapy by allowing pro-active selection of a chemotherapeutic to which the tumor does not express resistance. PET/CT imaging with a radiolabeled form of paclitaxel, F-18 fluoropaclitaxel (FPAC), may be able to predict the uptake of paclitaxel in solid tumors, and as a substrate of P-glycoprotein, it may also predict which tumors exhibit multidrug resistance (MDR), a phenotype in which tumors fail to respond to a wide variety of chemically unrelated chemotherapeutic agents. This article reviews the synthetic, preclinical and early human data obtained during the development phase of this promising new radiopharmaceutical.

Radiolabelling of proteins with fluorine-18 via click chemistry

The study describes for the first time the application of Cu(I)-mediated 1,3-dipolar [3+2]cycloaddition for the labelling of proteins with the short-lived positron emitter fluorine-18 as exemplified with azide-functionalized human serum albumin (HSA).

1H-19F spin-lattice relaxation spectroscopy: proton tunnelling in the hydrogen bond studied by field-cycling NMR

Proton tunnelling in the hydrogen bonds of two fluorine substituted benzoic acid dimers has been investigated using field-cycling NMR relaxometry. The close proximity of the (19)F nuclei to the hydrogen bond protons introduces heteronuclear (19)F-(1)H dipolar interactions into the spin-lattice relaxation processes. This renders the (1)H magnetisation-recovery biexponential and introduces multiple spectral density components into the relaxation matrix characterised by frequencies that are sums and differences of the (19)F and (1)H Larmor frequencies. Using field-cycling NMR pulse sequences that measure the spin-lattice relaxation and cross-relaxation rates we demonstrate how some of these multiple spectral density components can be separately resolved. This leads to an accurate determination of the correlation times that characterise the proton tunnelling motion. A broad spectrum of relaxation behaviour is illustrated and explored in the chosen samples and the investigation is used to explore the theory and practise of field-cycling NMR relaxometry in cases where heteronuclear interactions are significant.

Fluorine-containing nanoemulsions for MRI cell tracking

In this article we review the chemistry and nanoemulsion formulation of perfluorocarbons used for in vivo(19)F MRI cell tracking. In this application, cells of interest are labeled in culture using a perfluorocarbon nanoemulsion. Labeled cells are introduced into a subject and tracked using (19)F MRI or NMR spectroscopy. In the same imaging session, a high-resolution, conventional ((1)H) image can be used to place the (19)F-labeled cells into anatomical context. Perfluorocarbon-based (19)F cell tracking is a useful technology because of the high specificity for labeled cells, ability to quantify cell accumulations, and biocompatibility. This technology can be widely applied to studies of inflammation, cellular regenerative medicine, and immunotherapy.

Fluorine-containing nanoemulsions for MRI cell tracking

In this article we review the chemistry and nanoemulsion formulation of perfluorocarbons used for in vivo(19)F MRI cell tracking. In this application, cells of interest are labeled in culture using a perfluorocarbon nanoemulsion. Labeled cells are introduced into a subject and tracked using (19)F MRI or NMR spectroscopy. In the same imaging session, a high-resolution, conventional ((1)H) image can be used to place the (19)F-labeled cells into anatomical context. Perfluorocarbon-based (19)F cell tracking is a useful technology because of the high specificity for labeled cells, ability to quantify cell accumulations, and biocompatibility. This technology can be widely applied to studies of inflammation, cellular regenerative medicine, and immunotherapy.

The use of the exploratory IND in the evaluation and development of 18F-PET radiopharmaceuticals for amyloid imaging in the brain: a review of one company's experience

AIM AND METHODS

The regulatory mechanism of exploratory INDs established in 2006 by the US Food and Drug Administration (FDA) is useful for the evaluation of tracer dose radiopharmaceutical agents, and especially valuable for development of amyloid imaging agents because of the absence of appropriate animal models. The authors employed exploratory INDs to study four related novel 18F-labeled positron emission tomography (PET) amyloid imaging agents, 18F-AV-19, 18F-AV-45, 18F-AV-138 and 18F-AV-144. These exploratory INDs contained preclinical data on the mechanism of action, secondary pharmacology, biodistribution, pharmacokinetics and dosimetry and results from a single dose, extended acute toxicology study. Each compound was then tested in a human PET study in up to 15 healthy elderly controls (HC) and 15 patients with AD. Compared to HC, patients with AD showed accumulation of tracer in cortical areas expected to be high in amyloid deposition with all four tracer compounds, and no serious adverse events were observed for any of the tracers.

RESULTS

.18F-AV-45 showed the best imaging characteristics and was chosen for further development under a traditional IND.

CONCLUSIONS

In summary the exploratory IND pathway was very useful for comparing four related agents with respect to efficacy (amyloid plaque binding), kinetics and dosimetry.

PET imaging of hepatocellular carcinoma with 2-deoxy-2[18F]fluoro-D-glucose, 6-deoxy-6[18F] fluoro-D-glucose, [1-11C]-acetate and [N-methyl-11C]-choline

AIM

This study was designed to investigate the performance of positron emission tomography (PET) imaging for hepatocellular carcinoma (HCC) on a hepatitis viral infection-induced woodchuck model using existing tracers such as 2-deoxy-2[(18)F]fluoro-D-glucose (2FDG), 6-deoxy-6[(18)F]fluoro-D-glucose (6FDG), [1(-11)C]acetate (acetate) and [N-methyl(-11)C]choline (choline).

METHODS

Fourteen woodchucks with HCC were imaged with different radiotracers: 13 (10 with HCC and 3 controls) with 2FDG; 4 (3 with HCC and 1 control) with 6FDG; 13 (10 with HCC and 3 controls) with acetate; 4 (2 with HCC and 2 controls) with choline. The woodchucks were euthanized after imaging experiments and liver tissues were harvested for histology, for enzymatic activities including hexokinase (HK), glucose-6-phosphatase, acetyl-CoA synthetase (ACAS) and choline kinase (CK), and for differential gene expressions between the HCCs and the surrounding hepatic tissues.

RESULTS

2FDG detected 7/13 tumors with a tumor-to-liver uptake ratio (T/L) of 1.36+/-0.13. Five of these HCCs were moderately- or poorly-differentiated. The HK/glucose-6-phosphatase ratio was significantly higher in HCCs compared to the surrounding liver tissues (P=0.05). None of the HCCs imaged with 6FDG were detected by PET (T/L=1.01+/-0.11). Acetate detected 16/17 HCCs (T/L=2.02+/-0.7). ACAS activity was significantly higher in HCCs (P=0.01) and lipids-related genes were found up-regulated. Choline imaging detected all HCCs (T/L=1.63+/-0.34). CK activity was significantly higher in HCCs (P=0.001).

CONCLUSIONS

Well-differentiated and some moderately-differentiated HCCs do not uptake 2FDG more than the surrounding liver tissues, but display increased acetate uptake. There is no contrast between HCCs and the surrounding liver tissues on the 6FDG PET images. Despite elevated background signal from the liver, choline uptake seems to be detectable in the HCCs scanned in this study.

Imaging nanoparticle stability and activation in vivo

While liposomes and nanoparticles have been the subject of intense research for more than 40 years, few particles have been translated into clinical practice. Advantages of these particles include the potential to overcome the cardiac, renal or neural toxicity of systemic chemotherapy, the opportunities for multivalent targeting, the gradual yet significant accumulation within tumors due to leaky blood vessels and the myriad of new approaches to locally alter the properties of the particle in the region of interest. Given the complexity of the design and co-optimization of the surface architecture, shell formulation and drug loading, methods to image the pharmacokinetics of nanoparticles in living systems are an essential part of an efficient research methodology. Here, we describe our efforts to label the shell and drug core of lipid-shelled particles with a goal of facilitating translation of activatable particles.

Synthesis, radiosynthesis, and biological evaluation of fluorine-18-labeled 2beta-carbo(fluoroalkoxy)-3beta-(3'-((Z)-2-haloethenyl)phenyl)nortropanes: candidate radioligands for in vivo imaging of the serotonin transporter with positron emission tomography

The meta-vinylhalide fluoroalkyl ester nortropanes 1-4 were synthesized as ligands of the serotonin transporter (SERT) for use as positron emission tomography (PET) imaging agents. In vitro competition binding assays demonstrated that 1-4 have a high affinity for the SERT (K(i) values = 0.3-0.4 nM) and are selective for the SERT over the dopamine and norepinephrine transporters (DAT and NET). MicroPET imaging in anesthetized cynomolgus monkeys with [(18)F]1-[(18)F]4 demonstrated that all four tracers behave similarly with peak uptake in the SERT-rich brain regions achieved after 45-55 min, followed by a steady washout. An awake monkey study was performed with [(18)F]1, which demonstrated that the uptake of [(18)F]1 was not influenced by anesthesia. Chase studies with the SERT ligand 15 displaced [(18)F]1-[(18)F]4, but chase studies with the DAT ligand 16 did not displace [(18)F]1-[(18)F]4 thus indicating that the tracers were binding specifically to the SERT.

Methods for 18F-labeling of RGD peptides: comparison of aminooxy [18F]fluorobenzaldehyde condensation with 'click labeling' using 2-[18F]fluoroethylazide, and S-alkylation with [18F]fluoropropanethiol

Three strategies for chemoselective labeling of RGD peptides with (18)F have been compared. Aminooxy [(18)F]fluorobenzaldehyde conjugation provided 40 +/- 12% decay-corrected radiochemical yield using a fully automated method. An one-pot protocol for 'click labeling' of the RGD scaffold with 2-[(18)F]fluoroethylazide afforded 47 +/- 8% decay-corrected radiochemical yield. Attempted conjugation with 3-[(18)F]fluoropropanethiol led to extensive decomposition and was therefore found unsuitable for labeling of the RGD peptide investigated. The results suggest that 'click labeling' of RGD peptides provides an attractive alternative to aminooxy aldehyde condensation, however, 2-[(18)F]-fluoroethylazide may be too small to allow separation of large (18)F-labeled RGD peptides from their precursors.

Microwave-induced nucleophilic [18F]fluorination on aromatic rings: synthesis and effect of halogen on [18F]fluoride substitution of meta-halo (F, Cl, Br, I)-benzonitrile derivatives

The meta-halo-3-methylbenzonitrile derivatives (-F, -Cl, -Br, -I) were synthesized as model compounds to study reactivity towards aromatic nucleophilic substitution. A single-mode microwave system was incorporated into a commercial radiochemical synthetic module for (18)F labeling. Labeling yields of 64% for fluoro-, 13% for bromo- and 9% for chloro-precursors were achieved in DMSO in <3 min. The observed order of reactivity of the leaving groups toward aromatic nucleophilic substitution was F>>Br>Cl>>>I.

Synthesis of N-succinimidyl 4-[18F]fluorobenzoate, an agent for labeling proteins and peptides with 18F

This protocol describes the step-by-step procedure for the synthesis of N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB), an agent widely used for labeling proteins and peptides with the positron-emitting radionuclide 18F. The protocols for the synthesis of unlabeled SFB and the quaternary salt precursor 4-formyl-N,N,N-trimethyl benzenaminium trifluoromethane sulfonate also are described. For the [18F]SFB synthesis, the quaternary salt is first converted to 4-[18F]fluorobenzaldehyde. Oxidation of the latter provides 4-[18F]fluorobenzoic acid, which is converted to [18F]SFB by treatment with N,N-disuccinimidyl carbonate. Using this method, [18F]SFB can be synthesized in decay-corrected radiochemical yields of 30%-35% and a specific radioactivity of 11-12 GBq micromol(-1). The total synthesis and purification time required is about 80 min, starting from delivery of the [18F]fluoride. [18F]SFB remains an optimal reagent for labeling proteins and peptides with 18F because of good conjugation yields and metabolic stability.

Synthesis and reactivity of [18F]-N-fluorobenzenesulfonimide

A novel [18F]NF reagent and two novel radiochemical transformations have been developed: [18F]NFSi has been prepared from sodium dibenzenesulfonimide and reacted in the presence of silyl enol ethers and allylsilanes to deliver labelled fluorinated ketones and allylic fluorides respectively; the radiosynthesis of the fluorinated A ring of vitamin D3 has also been completed with success.

One-step high-radiochemical-yield synthesis of [18F]FP-CIT using a protic solvent system

Although [18F] fluoropropylcarbomethoxyiodophenylnortropane (FP-CIT) is a promising radiopharmaceutical for dopamine transporter imaging, it has not been used for clinical studies because of low radiochemical yield. The purpose of our study was to develop a new radiochemistry method using a protic solvent system to obtain a high radiochemical yield of [18F]FP-CIT in single-step manual and automatic preparation procedures. [18F]F(-) was trapped on a QMA Sep-Pak cartridge or PS-HCO(3) cartridge and eluted with Cs2CO(3)/K222 buffer or TBAHCO3 respectively, or 8 microl of TBAOH was added directly to [18F]F(-)/H(2)(18)O solution in a reactor without using a cartridge. After drying, 18F] fluorination was performed with 2-6 mg of mesylate precursor, 100 microl of CH(3)CN and 500 microl of t-BuOH at 50-120 degrees C for 5-30 min, followed by high-performance liquid chromatography (HPLC) purification to obtain the final product. For comparison, the same procedure was performed with a tosylate precursor. Manual synthesis gave a decay-corrected radiochemical yield of 52.2+/-4.5%, and optimal synthesis conditions were as follows: TBAOH addition, 4 mg of precursor, 100 degrees C and 20 min of [18F] fluorination (n=3). We obtained low radiochemical yields of [18F]FP-CIT with carbonate elution systems such as Cs2CO(3) or TBAHCO3. We also developed an automatic synthesis method based on manual synthesis results. In automatic production, we obtained a decay-corrected radiochemical yield of 35.8+/-5.2% after HPLC purification, and we did not have any synthesis failures (n=14). Here, we describe our new method for the synthesis of [18F]FP-CIT using a protic solvent system. This method gave a high radiochemical yield with high reproducibility and might enable [18F]FP-CIT to be used clinically and commercially.

19F-NMR Reveals Metal and Operator-induced Allostery in MerR

Metalloregulators of the MerR family activate transcription upon metal binding by underwinding the operator-promoter DNA to permit open complex formation by pre-bound RNA polymerase. Historically, MerR's allostery has been monitored only indirectly via nuclease sensitivity or by fluorescent nucleotide probes and was very specific for Hg(II), although purified MerR binds several thiophilic metals. To observe directly MerR's ligand-induced behavior we made 2-fluorotyrosine-substituted MerR and found similar, minor changes in (19)F chemical shifts of tyrosine residues in the free protein exposed to Hg(II), Cd(II) or Zn(II). However, DNA binding elicits large chemical shift changes in MerR's tyrosine residues and in DNA-bound MerR Hg(II) provokes changes very distinct from those of Cd(II) or Zn(II). These chemical shift changes and other biophysical and phenotypic properties of wild-type MerR and relevant mutants reveal elements of an allosteric network that enables the coordination state of the metal binding site to direct metal-specific movements in the distant DNA binding site and the DNA-bound state also to affect the metal binding domain.

Characterization of optically resolved 9-fluoropropyl-dihydrotetrabenazine as a potential PET imaging agent targeting vesicular monoamine transporters

Labeling derivatives of dihydrotetrabenazine (DTBZ) with F-18 (T(1/2)=110 min) instead of C-11 (T(1/2)=20 min) would improve their utility and availability for imaging vesicular monoamine transporters (VMAT2) in clinical settings. The successful synthesis, reported previously, of two novel 9-fluoroalkyl(+/-)-DTBZ ligands prompted us to study the optically resolved active ligand 9-fluoropropyl-(+)-DTBZ (FP-(+)-DTBZ), which may have more promising characteristics. The inhibition constant (K(i)) estimated for FP-(+)-DTBZ (using [(3)H](+/-)-DTBZ as the labeled ligand in rat striatal homogenates) showed a lower value as compared to the racemic FP-(+/-)-DTBZ (0.10+/-0.01 vs 0.19+/-0.04 nM). The inactive isomer, FP-(-)-DTBZ, displayed a much lower binding affinity with a K(i) value >3000 nM. Biodistribution studies in mice after an iv injection of [(18)F]FP-(+)-DTBZ exhibited a ratio of striatum (ST, target) to cerebellum (CB, background) of 4.51 at 30 min postinjection, which is a higher value than previously obtained with the racemic ligand [(18)F]FP-(+/-)-DTBZ (ST/CB=2.95). Brain extraction at 30 min after the tracer injection in mice showed that >95% of the radioactivity corresponded to the parent, nonmetabolized, compound remaining in the ST, suggesting that the tracer has an excellent in vivo stability. Furthermore, localization of the tracer in the brain examined with ex vivo autoradiography displayed a typical distribution pattern consistent with VMAT2 sites. The highest labeling was observed in monoaminergic neuron regions (caudate putamen, olfactory tubercle, nucleus accumbens, substantia nigra, dorsal raphe and locus coerules). We also tested the selective labeling of this tracer at the dopamine neurons in unilateral-lesioned mice (treated with 6-hydroxydopamine). When [(18)F]FP-(+)-DTBZ and [(125)I]IPT ((N-(3'-iodopropen-2'-yl)-2-beta-carbomethoxy-3-beta-(4-chlorophenyl)tropane, a selective marker for dopamine transporters (DATs) in dopaminergic neurons) were simultaneously injected into lesioned mice, we observed an excellent correlation (r=0.95) for these tracers. From these findings, we conclude that [(18)F]FP-(+)-DTBZ is a sensitive and selective tracer for VMAT2 binding sites and it may be useful for in vivo evaluation of diseases relating to changes of monoamine neuronal integrity.

PET imaging of infection with a HYNIC-conjugated LTB4 antagonist labeled with F-18 via hydrazone formation

It was previously shown that the (99m)Tc-labeled hydrazinonicotinamide (HYNIC)-conjugated LTB4 antagonist MB81 visualized infectious foci in rabbits adequately and within a few hours after injection. Here, the bivalent HYNIC-conjugated LTB4 antagonist MB67 (analog of MB81) was fluorinated with (18)F via hydrazone formation and tested in vivo.

METHODS

MB67 was [(18)F]-fluorinated via reaction of the [(18)F]-fluorinated intermediate p-[(18)F]-fluorobenzaldehyde ([(18)F]FB) and the HYNIC moiety of MB67 via hydrazone formation. For comparison, MB67 was also labeled with (99m)Tc. The biodistribution of (18)F- and (99m)Tc-labeled MB67 was investigated in rabbits with intramuscular infection.

RESULTS

[(18)F]-MB67 was obtained at a maximum specific activity of 1200 GBq/mmol and proved to be stable in phosphate buffered saline (PBS) at 37 degrees C for at least 4 h. PET images obtained with [(18)F]-MB67 clearly delineated the abscess at 2 and 4 h pi. Counting of dissected tissues at 4 h pi revealed an abscess uptake of 0.073+/-0.005 %ID/g, as compared to 0.160+/-0.010 %ID/g for the (99m)Tc-labeled analog. Abscess-to-muscle ratios were 23+/-4 for [(18)F]-MB67 and 35+/-9 for [(99m)Tc]-MB67.

CONCLUSION

The present study showed the feasibility of a new [(18)F]-labeling methodology and its application in the production of a new PET tracer for imaging of infection, [(18)F]-MB67.

Determination of the 19F NMR chemical shielding tensor and crystal structure of 5-fluoro-dl-tryptophan

5-Fluoro-dl-tryptophan (5F-Trp) is a very sensitive probe used to investigate orientation and dynamics of biomacromolecules at the in situ level. In order to establish a (19)F NMR strategy, the crystal structure and (19)F chemical shielding tensor of 5F-Trp are reported. A novel approach was developed to use F-F homonuclear dipole-dipole coupling information to analyze single-crystal NMR data without determining crystal orientations. The measured values for the principal components of the shielding tensor are sigma(11)=0.9, sigma(22)=-63.3, and sigma(33)=-82.9 ppm relative to TFA in D(2)O. The principal axes of the shielding tensors coincide with the indole ring symmetry, which makes it a straightforward and powerful tool to monitor protein alignment in oriented environments. Hartree-Fock (HF) and density functional theory (DFT) calculations of the chemical shielding tensors are also reported.

Radiolabeling of multimeric neurotensin(8–13) analogs with the short-lived positron emitter fluorine-18

Three methods for (18)F-labeling of dimeric and tetrameric neurotensin(8-13) derivatives were evaluated with respect to the labeling yield and the required peptide amounts. Labeling using N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB) gave low radiochemical yield for the dimeric peptides. Coupling of the tetramer with [(18)F]SFB was not successful. High yields were obtained for labeling of the aminooxy-functionalized neurotensin(8-13) dimer using 4-[(18)F]fluorobenzaldehyde ([(18)F]FBA) whilst coupling of the corresponding tetramer gave only low yields. Labeling of sulfydryl-functionalized neurotensin(8-13) derivatives using the maleinimide 4-[(18)F]fluorobenzaldehyde-O-[6-(2,5-dioxo-2,5-dihydro-pyrrol-1-yl)-hexyl]-oxime ([(18)F]FBAM) resulted in high radiochemical yields for both, the dimer and the tetramer. Therefore, [(18)F]FBAM seems to be the most suitable (18)F-labeling agent for multivalent neurotensin(8-13) derivatives.

Synthesis and evaluation of fluorine-18-labeled SA4503 as a selective sigma1 receptor ligand for positron emission tomography

The [(18)F]fluoromethyl analog of the sigma(1) selective ligand 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503) ([(18)F]FM-SA4503) was prepared and its potential evaluated for the in vivo measurement of sigma(1) receptors with positron emission tomography (PET). FM-SA4503 had selective affinity for the sigma(1) receptor (K(i) for sigma(1) receptor, 6.4 nM; K(i) for sigma(2) receptor, 250 nM) that was compatible with the affinity of SA4503 (K(i) for sigma(1) receptor, 4.4 nM; K(i) for sigma(2) receptor, 242 nM). [(18)F]FM-SA4503 was synthesized by (18)F-fluoromethylation of O-demethyl SA4503 in the radiochemical yield of 2.9-16.6% at the end of bombardment with a specific activity of 37.8-283 TBq/mmol at the end of synthesis. In mice, the uptake of [(18)F]FM-SA4503 in the brain was gradually increased for 30 min after injection, and then decreased. In the blocking study, brain uptake was significantly decreased by co-injection of haloperidol to 32% of control, and FM-SA4503 to 52% of control. In PET study of the monkey brain, high uptake was found in the cerebral cortex, thalamus and striatum. The radioactivity level of [(18)F]FM-SA4503 in the brain regions gradually increased over a period of 120 min after injection, followed by a stable plateau phase until 180 min after injection. In pretreatment with haloperidol measurement of the monkey brain, the radioactivity level was 22-32% and 11-25% of the baseline at 60 and 180 min, respectively, after injection, suggesting high receptor-specific binding. [(18)F]FM-SA4503 showed specific binding to sigma(1) receptors in mice and monkeys; therefore, [(18)F]FM-SA4503 has the potential for mapping sigma(1) receptors in the brain.

Simultaneous Characterization of a Mixture of Fluorochemicals Using Three-Dimensional 19F−1H Heteronuclear TOCSY Filtered/Edited NMR Experiments

Heteronuclear 19F-1H cross-polarization can be used effectively as a tool for both spectral filtering and editing in the NMR analysis of the increasing number of fluorine-containing compounds encountered in drug discovery. Combined with LC-MS, three-dimensional 19F-1H heteronuclear TOCSY filtered experiments based on this approach have enabled the simultaneous identification of a mixture of closely related dexamethasone derivatives without the need for isolation.

Spin Relaxation Effects in Photochemically Induced Dynamic Nuclear Polarization Spectroscopy of Nuclei with Strongly Anisotropic Hyperfine Couplings

We describe experimental results and theoretical models for nuclear and electron spin relaxation processes occurring during the evolution of 19F-labeled geminate radical pairs on a nanosecond time scale. In magnetic fields of over 10 T, electron-nucleus dipolar cross-relaxation and longitudinal DeltaHFC-Deltag (hyperfine coupling anisotropy--g-tensor anisotropy) cross-correlation are shown to be negligibly slow. The dominant relaxation process is transverse DeltaHFC-Deltag cross-correlation, which is shown to lead to an inversion in the geminate 19F chemically induced dynamic nuclear polarization (CIDNP) phase for sufficiently large rotational correlation times. This inversion has recently been observed experimentally and used as a probe of local mobility in partially denatured proteins (Khan, F.; et al. J. Am. Chem. Soc. 2006, 128, 10729-10737). The essential feature of the spin dynamics model employed here is the use of the complete spin state space and the complete relaxation superoperator. On the basis of the results reported, we recommend this approach for reliable treatment of magnetokinetic systems in which relaxation effects are important.

Fluorine-18-labeled benzamide analogues for imaging the sigma2 receptor status of solid tumors with positron emission tomography

A series of fluorine-containing benzamide analogs was synthesized and evaluated as candidate ligands for positron emission tomography (PET) imaging of the sigma-2 (sigma2) receptor status of solid tumors. Four compounds having a moderate to high affinity for sigma2 receptors and a moderate to low affinity for sigma-1 (sigma1) receptors were radiolabeled with fluorine-18 via displacement of the corresponding mesylate precursor with [18F]fluoride. Biodistribution studies in female Balb/c mice bearing EMT-6 tumor allografts demonstrated that all four F-18-labeled compounds had a high tumor uptake (2.5-3.7% ID/g) and acceptable tumor/normal tissue ratios at 1 and 2 h post-i.v. injection. An analysis of the chemistry and biodistribution data suggested that N-(4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)butyl)-2-(2-[18F]-fluoroethoxy)-5-methylbenzamide ([18F]3c) and N-(4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)butyl)-2-(2-[18F]-fluoroethoxy)-5-iodo-3-methoxybenzamide ([18F]3f) are acceptable compounds for imaging the sigma2 receptor status of solid tumors.

Radiosynthesis and biological evaluation of an 18F-labeled derivative of the novel pyrazolopyrimidine sedative-hypnotic agent indiplon

INTRODUCTION

Gamma amino butyric acid type A (GABA(A)) receptors are involved in a variety of neurological and psychiatric diseases, which have promoted the development and use of radiotracers for positron emission tomography imaging. Radiolabeled benzodiazepine antagonists such as flumazenil have most extensively been used for this purpose so far. Recently, the non-benzodiazepine pyrazolopyrimidine derivative indiplon with higher specificity for the alpha(1) subtype of the GABA(A) receptor has been introduced for treatment of insomnia. The aim of this study was the development and biological evaluation of an (18)F-labeled derivative of indiplon.

METHODS

Both [(18)F]fluoro-indiplon and its labeling precursor were synthesized by two-step procedures starting from indiplon. The radiosynthesis of [(18)F]fluoro-indiplon was performed using the bromoacetyl precursor followed by multiple-stage purification using semipreparative HPLC and solid phase extraction. Stability, partition coefficients, binding affinities and regional brain binding were determined in vitro. Biodistribution and radiotracer metabolism were studied in vivo.

RESULTS

[(18)F]Fluoro-indiplon was readily accessible in good yields (38-43%), with high purity and high specific radioactivity (>150 GBq/micromol). It displays high in vitro stability and moderate lipophilicity. [(18)F]Fluoro-indiplon has an affinity to GABA(A) receptors comparable to indiplon (K(i)=8.0 nM vs. 3.4 nM). In vitro autoradiography indicates high [(18)F]fluoro-indiplon binding in regions with high densities of GABA(A) receptors. However, ex vivo autoradiography and organ distribution studies show no evidence of specific binding of [(18)F]fluoro-indiplon. Furthermore, the radiotracer is rapidly metabolized with high accumulation of labeled metabolites in the brain.

CONCLUSIONS

Although [(18)F]fluoro-indiplon shows good in vitro features, it is not suitable for in vivo imaging studies because of its metabolism. Structural modifications are needed to develop derivatives with higher in vivo stability.

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).

The automatic production of 16α-[18F]fluoroestradiol using a conventional [18F]FDG module with a disposable cassette system

We have developed a fully automatic method for the synthesis of 16alpha-[18F]fluoroestradiol ([18F]FES) using a disposable cassette system and conventional [18F]FDG module. [18F]FES was synthesized using a GE TracerLab MX module and a modified module control program. Following [18F]fluorination, we hydrolyzed the product three times with a mixture of 2N HCl and CH(3)CN. After HPLC purification, the decay corrected radiochemical yield of [18F]FES was 45.3+/-2.8%, which was stable to 98.2+/-0.2% at 6h after synthesis. This new automated synthesis method provides high and reproducible yields with the advantage of a disposable cassette system.

Determination of residual Kryptofix 2.2.2 levels in [18F]-labeled radiopharmaceuticals for human use

4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (Kryptofix 2.2.2) is used in the routine preparation of [18F]-labeled tracers employed in positron emission tomography (PET) imaging. Confirming the absence of Kryptofix in radiopharmaceuticals is a quality control criterion required before they can be released for human use. Analysis of Kryptofix levels using the iodoplatinate spot-test can be complicated by false-positive results due to nitrogen containing tracers and/or false-negative results caused by added stabilizers. To overcome this issue, we have developed a universal TLC method for the rapid and reliable determination of Kryptofix levels in the wide range of fluorine-18 radiopharmaceuticals we prepare, including complex multi-component formulations.

6-Chloro-3-(((1-[11C]methyl)-2-(S)-pyrrolidinyl)methoxy)-5-(2-fluoropyridin-4-yl)pyridine ([11C]JHU85270), a potent ligand for nicotinic acetylcholine receptor imaging by positron emission tomography

6-Chloro-3-((1-methyl)-2-(S)-pyrrolidinyl)methoxy)-5-(2-fluoropyridin-4-yl)pyridine (JHU85270), a novel high-affinity ligand for the alpha4beta2 nicotine acetylcholine receptor (nAChR) (K(i)=86, 115 pM; K(i)(JHU85270)/K(i)(epibatidine)=1.7) with a log D(7.4)=1.6 was synthesized in 56% overall yield. [(11)C]JHU85270 was synthesized from [(11)C]-methyl iodide and the corresponding normethyl precursor. The average time of radiosynthesis, purification, and formulation was 37 min from the end of bombardment. The average radiochemical yield of [(11)C]JHU85270 was 37%+/-3% (non-decay corrected). The average specific radioactivity was 398+/-165 GBq/micromol (10750+/-4468 mCi/micromol) and the radiochemical purity was greater than 99%.

18F-labeled mini-PEG spacered RGD dimer (18F-FPRGD2): synthesis and microPET imaging of alphavbeta3 integrin expression

PURPOSE

We have previously reported that (18)F-FB-E[c(RGDyK)](2) ((18)F-FRGD2) allows quantitative PET imaging of integrin alpha(v)beta(3) expression. However, the potential clinical translation was hampered by the relatively low radiochemical yield. The goal of this study was to improve the radiolabeling yield, without compromising the tumor targeting efficiency and in vivo kinetics, by incorporating a hydrophilic bifunctional mini-PEG spacer.

METHODS

(18)F-FB-mini-PEG-E[c(RGDyK)](2) ((18)F-FPRGD2) was synthesized by coupling N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB) with NH(2)-mini-PEG-E[c(RGDyK)](2) (denoted as PRGD2). In vitro receptor binding affinity, metabolic stability, and integrin alpha(v)beta(3) specificity of the new tracer (18)F-FPRGD2 were assessed. The diagnostic value of (18)F-FPRGD2 was evaluated in subcutaneous U87MG glioblastoma xenografted mice and in c-neu transgenic mice by quantitative microPET imaging studies.

RESULTS

The decay-corrected radiochemical yield based on (18)F-SFB was more than 60% with radiochemical purity of >99%. (18)F-FPRGD2 had high receptor binding affinity, metabolic stability, and integrin alpha(v)beta(3)-specific tumor uptake in the U87MG glioma xenograft model comparable to those of (18)F-FRGD2. The kidney uptake was appreciably lower for (18)F-FPRGD2 compared with (18)F-FRGD2 [2.0 +/- 0.2%ID/g for (18)F-FPRGD2 vs 3.0 +/- 0.2%ID/g for (18)F-FRGD2 at 1 h post injection (p.i.)]. The uptake in all the other organs except the urinary bladder was at background level. (18)F-FPRGD2 also exhibited excellent tumor uptake in c-neu oncomice (3.6 +/- 0.1%ID/g at 30 min p.i.).

CONCLUSION

Incorporation of a mini-PEG spacer significantly improved the overall radiolabeling yield of (18)F-FPRGD2. (18)F-FPRGD2 also had reduced renal uptake and similar tumor targeting efficacy as compared with (18)F-FRGD2. Further testing and clinical translation of (18)F-FPRGD2 are warranted.

Automated synthesis of 11β-methoxy-4,16α-[16α-18F]difluoroestradiol (4F-M[18F]FES) for estrogen receptor imaging by positron emission tomography

Addition of both a 4-fluoro and 11beta-methoxy group onto 16alpha-[(18)F]fluoroestradiol ([(18)F]FES) yields 11beta-methoxy-4,16alpha-[16alpha-(18)F]difluoroestradiol (4F-M[(18)F]FES) with potential improved properties for positron emission tomography (PET) imaging of estrogen receptor densities in breast cancer patients. In order to provide 4F-M[(18)F]FES as a radiopharmaceutical for clinical trials, we developed an automated synthesis procedure using 3-O-methoxymethyl-11beta-methoxy-4-fluoro-16,17-O-sulfuryl-16-epiestriol as precursor. The radio synthesis involves stereoselective opening of the protected cyclic sulfone precursor via nucleophilic fluorination with [(18)F]fluoride in acetonitrile. After removal of the protecting ether and 17beta-sulphate groups by rapid hydrolysis in acidic ethanol and subsequent reversed-phase HPLC purification, the pure 4F-M[(18)F]FES was obtained as a sterile physiological saline solution in 45-50% radiochemical yield (decay corrected). The radiochemical purity of the final product was >98% and the effective specific activity (ESA) of 4F-M[(18)F]FES prepared under optimized conditions was >15,000 Ci/mmol. The total preparation time was 110+/-5 min and the product was shown to be stable for at least 6 h.

Further Insights into the Spectroscopic Properties, Electronic Structure, and Kinetics of Formation of the Heme−Peroxo−Copper Complex [(F8TPP)FeIII−(O22-)−CuII(TMPA)]+

In the further development and understanding of heme-copper O2-reduction chemistry inspired by the active-site chemistry in cytochrome c oxidase, we describe a dioxygen adduct, [(F8TPP)FeIII-(O22-)-CuII(TMPA)](ClO4) (3), formed by addition of O2 to a 1:1 mixture of the porphyrinate-iron(II) complex (F8TPP)FeII (1a) {F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate dianion} and the copper(I) complex [(TMPA)CuI(MeCN)](ClO4) (1b) {TMPA = tris(2-pyridylmethyl)amine}. Complex 3 forms in preference to heme-only or copper-only binuclear products, is remarkably stable {t1/2 (RT; MeCN) approximately 20 min; lambda max = 412 (Soret), 558 nm; EPR silent}, and is formulated as a peroxo complex on the basis of manometry {1a/1b/O2 = 1:1:1}, MALDI-TOF mass spectrometry {16O2, m/z 1239 [(3 + MeCN)+]; 18O2, m/z 1243}, and resonance Raman spectroscopy {nu(O-O) = 808 cm-1; Delta16O2/18O2 = 46 cm-1; Delta16O2/16/18O2 = 23 cm-1}. Consistent with a mu-eta2:eta1 bridging peroxide ligand, two metal-O stretching frequencies are observed {nu(Fe-O) = 533 cm-1, nu(Fe-O-Cu) = 511 cm-1}, and supporting normal coordinate analysis is presented. 2H and 19F NMR spectroscopies reveal that 3 is high-spin {also muB = 5.1 +/- 0.2, Evans method} with downfield-shifted pyrrole and upfield-shifted TMPA resonances, similar to the pattern observed for the structurally characterized mu-oxo complex [(F8TPP)FeIII-O-CuII(TMPA)]+ (4) (known S = 2 system, antiferromagnetically coupled high-spin FeIII and CuII). Mössbauer spectroscopy exhibits a sharp quadrupole doublet (zero field; delta = 0.57 mm/s, |DeltaEQ| = 1.14 mm/s) for 3, with isomer shift and magnetic field dependence data indicative of a peroxide ligand and S = 2 formulation. Both UV-visible-monitored stopped-flow kinetics and Mössbauer spectroscopic studies reveal the formation of heme-only superoxide complex (S)(F8TPP)FeIII-(O2-) (2a) (S = solvent molecule) prior to 3. Thermal decomposition of mu-peroxo complex 3 yields mu-oxo complex 4 with concomitant release of approximately 0.5 mol O2 per mol 3. Characterization of the reaction 1a/1b + O2 --> 2 --> 3 --> 4, presented here, advances our understanding and provides new insights to heme/Cu dioxygen-binding and reduction.

Radiosynthesis of 2'-deoxy-2'-[18F]-fluoro-5-methyl-1-beta-L-arabinofuranosyluracil ([18F]-L-FMAU) for PET

Radiosynthesis of 2'-deoxy-2'-[(18)F]-fluoro-5-methyl-1-beta-L-arabinofuranosyluracil ([(18)F]-L-FMAU) is reported. Compound 1 was synthesized and converted to 2-triflate 2. Compound 3 was prepared from 2 using tetrabutylammonium[(18)F]fluoride, converted to 4, and then coupled with 5. The crude product was hydrolyzed, and purified by HPLC to obtain 7a. The radiochemical yield of [(18)F]-L-FMAU was 26% decay corrected (d.c.) in four runs with radiochemical purity >99% and specific activity 2200 mCi/micromol. The synthesis time was 3.3-3.5h from the end of bombardment (EOB).

Polyfluorinated Amino Acids for Sensitive 19F NMR-Based Screening and Kinetic Measurements

Two novel series of polyfluorinated amino acids (PFAs) were designed and synthesized according to a very short and scalable synthetic sequence. The advantages and limitations of these moieties for screening purposes are presented and discussed. The potential applications of these PFAs were tested with their incorporation into small arginine-containing peptides that represent suitable substrates for the enzyme trypsin. The enzymatic reactions were monitored by 19F NMR spectroscopy, using the 3-FABS (three fluorine atoms for biochemical screening) technique. The high sensitivity achieved with these PFAs permits a reduction in substrate concentration required for 3-FABS. This is relevant in the utilization of 3-FABS in fragment-based screening for identification of small scaffolds that bind weakly to the receptor of interest. The large dispersion of 19F isotropic chemical shifts allows the simultaneous measurement of the efficiency of the different substrates, thus identifying the best substrate for screening purposes. Furthermore, the knowledge of KM and Kcat for the different substrates allows the identification of the structural motifs responsible for the binding affinity to the receptor and those affecting the chemical steps in enzymatic catalysis. This enables the construction of suitable pharmacophores that can be used for designing nonpeptidic inhibitors with high affinity for the enzyme or molecules that mimic the transition state. The novel PFAs can also find useful application in the FAXS (fluorine chemical shift anisotropy and exchange for screening) experiment, a 19F-based competition binding assay for the detection of molecules that inhibit the interaction between two proteins.