Synthesis of 6-[18F] and 4-[18F]fluoro-L-m-tyrosines via regioselective radiofluorodestannylation

Production of 6-l-[18F]Fluoro-m-tyrosine in an Automated Synthesis Module for 11C-Labeling

6-l-[¹⁸F]Fluoro-m-tyrosine (6-l-[¹⁸F]FMT) represents a valuable alternative to 6-l-[¹⁸F]FDOPA which is conventionally used for the diagnosis and staging of Parkinson’s disease. However, clinical applications of 6-l-[¹⁸F]FMT have been limited by the paucity of practical production methods for its automated production. Herein we describe the practical preparation of 6-l-[¹⁸F]FMT using alcohol-enhanced Cu-mediated radiofluorination of Bpin-substituted chiral Ni(II) complex in the presence of non-basic Bu₄ONTf using a volatile iPrOH/MeCN mixture as reaction solvent. A simple and fast radiolabeling procedure afforded the tracer in 20.0 ± 3.0% activity yield within 70 min. The developed method was directly implemented onto a modified TracerLab FX C Pro platform originally designed for ¹¹C-labeling. This method enables an uncomplicated switch between ¹¹C- and ¹⁸F-labeling. The simplicity of the developed procedure enables its easy adaptation to other commercially available remote-controlled synthesis units and paves the way for a widespread application of 6-l-[¹⁸F]FMT in the clinic.

Mechanistic Studies and Radiofluorination of Structurally Diverse Pharmaceuticals with Spirocyclic Iodonium(III) Ylides

Theoretical studies provide insight into radiofluorination of non-activated electron-rich and sterically hindered ¹⁸F-arenes using a new class of adamantyl-based spirocyclic iodonium(iii) ylide precursors.

Synthesis of non-activated electron-rich and sterically hindered ¹⁸F-arenes remains a major challenge due to limitations of existing radiofluorination methodologies. Herein, we report on our mechanistic investigations of spirocyclic iodonium(iii) ylide precursors for arene radiofluorination, including their reactivity, selectivity, and stability with no-carrier-added [¹⁸F]fluoride. Benchmark calculations at the G2[ECP] level indicate that pseudorotation and reductive elimination at iodine(iii) can be modeled well by appropriately selected dispersion-corrected density functional methods. Modeling of the reaction pathways show that fluoride–iodonium(iii) adduct intermediates are strongly activated and highly regioselective for reductive elimination of the desired [¹⁸F]fluoroarenes (difference in barriers, ΔΔG^‡ > 25 kcal mol^–1). The advantage of spirocyclic auxiliaries is further supported by NMR spectroscopy studies, which bolster evidence for underlying decomposition processes which can be overcome for radiofluorination of iodonium(iii) precursors. Using a novel adamantyl auxiliary, sterically hindered iodonium ylides have been developed to enable highly efficient radiofluorination of electron-rich arenes, including fragments of pharmaceutically relevant nitrogen-containing heterocycles and tertiary amines. Furthermore, this methodology has been applied for the syntheses of the radiopharmaceuticals 6-[¹⁸F]fluoro-meta-tyrosine ([¹⁸F]FMT, 11 ± 1% isolated radiochemical yield, non-decay-corrected (RCY, n.d.c.), n = 3), and meta-[¹⁸F]fluorobenzylguanidine ([¹⁸F]mFBG, 14 ± 1% isolated RCY, n.d.c., n = 3) which cannot be directly radiolabeled using conventional nucleophilic aromatic substitution with [¹⁸F]fluoride.

A three-step radiosynthesis of 6-[(18) F]fluoro-L-meta-tyrosine starting with [(18) F]fluoride

The radiosynthesis of 6-[(18)  F]fluoro-L-m-tyrosine has generally been performed by electrophilic radiofluorination, which exhibits several drawbacks. In the present work, a three-step radiochemical synthesis is described starting from [(18)  F]fluoride. The synthetic sequence, including isotopic exchange, Baeyer-Villiger oxidation, and hydrolysis, were examined comparing four fluorobenzophenone derivatives as labeling precursors. Of those, (2S,5S)-tert-butyl 5-(5-acetyl-2-fluorobenzyl)-2-tert-butyl-3-methyl-4-oxoimidazolidine-1-carboxylate (1a) and (2S,5S)-tert-butyl 2-tert-butyl-5-(2-fluoro-5-(2,2,2-trifluoroacetyl)benzyl)-3-methyl-4-oxoimidazolidine-1-carboxylate (1d) proved to be the most suitable ones. 6-[(18)  F]Fluoro-L-m-tyrosine was obtained with overall radiochemical yields of 8-13% and an enantiomeric excess of up to 98%.

Spirocyclic hypervalent iodine(III)-mediated radiofluorination of non-activated and hindered aromatics

Fluorine-18 (t½=109.7 min) is the most commonly used isotope to prepare radiopharmaceuticals for molecular imaging by positron emission tomography (PET). Nucleophilic aromatic substitution reactions of suitably activated (electron-deficient) aromatic substrates with no-carrier-added [(18)F]fluoride ion are routinely carried out in the synthesis of radiotracers in high specific activities. Despite extensive efforts to develop a general (18)F-labelling technique for non-activated arenes there is an urgent and unmet need to achieve this goal. Here we describe an effective solution that relies on the chemistry of spirocyclic hypervalent iodine(III) complexes, which serve as precursors for rapid, one-step regioselective radiofluorination with [(18)F]fluoride. This methodology proves to be efficient for radiolabelling a diverse range of non-activated functionalized arenes and heteroarenes, including arene substrates bearing electron-donating groups, bulky ortho functionalities, benzylic substituents and meta-substituted electron-withdrawing groups. Polyfunctional molecules and a range of previously elusive (18)F-labelled building blocks, compounds and radiopharmaceuticals are synthesized.

A dual-tracer study of extrastriatal 6-[18F]fluoro-m-tyrosine and 6-[18F]-fluoro-L-dopa uptake in Parkinson's disease

6-[(18)F]-Fluoro-L-dopa (FDOPA) has been widely used as a biomarker for catecholamine synthesis, storage, and metabolism--its intense uptake in the striatum, and fainter uptake in other brain regions, is correlated with the symptoms and pathophysiology of Parkinson's disease (PD). 6-[(18)F]fluoro-m-tyrosine (FMT), which also targets L-amino acid decarboxylase, has potential advantages over FDOPA as a radiotracer because it does not form catechol-O-methyltransferase (COMT) metabolites. The purpose of the present study was to compare the regional distribution of these radiotracers in the brains of PD patients. Fifteen Parkinson's patients were studied with FMT and FDOPA positron emission tomography (PET) as well as high-resolution structural magnetic resonance imaging (MRI). MRI's were automatically parcellated into neuroanatomical regions of interest (ROIs) in Freesurfer (http://surfer.nmr.mgh.harvard.edu); region-specific uptake rate constants (Kocc) were generated from coregistered PET using a Patlak graphical approach. The essential findings were as follows: (1) regional Kocc were highly correlated between the radiotracers and in agreement with a previous FDOPA studies that used different ROI selection techniques; (2) FMT Kocc were higher in extrastriatal regions of relatively large uptake such as amygdala, pallidum, brainstem, hippocampus, entorhinal cortex, and thalamus, whereas cortical Kocc were similar between radiotracers; (3) while subcortical uptake of both radiotracers was related to disease duration and severity, cortical uptake was not. These results suggest that FMT may have advantages for examining pathologic changes within allocortical loop structures, which may contribute to cognitive and emotional symptoms of PD.

A within-subject comparison of 6-[18F]fluoro-m-tyrosine and 6-[18F]fluoro-L-dopa in Parkinson's disease

Progression of Parkinson's disease symptoms is imperfectly correlated with positron emission tomography biomarkers for dopamine biosynthetic pathways. The radiopharmaceutical 6-[(18) F]fluoro-m-tyrosine is not a substrate for catechol-O-methyltransferase and therefore has a more favorable uptake-to-background ratio than 6-[(18) F]fluoro-L-dopa. The objective of this study was to evaluate 6-[(18) F]fluoro-m-tyrosine relative to 6-[(18) F]fluoro-L-dopa with partial catechol-O-methyltransferase inhibition as a biomarker for clinical status in Parkinson's disease. Twelve patients with early-stage Parkinson's disease, off medication, underwent Unified Parkinson Disease Rating Scale scoring, brain magnetic resonance imaging, and 3-dimensional dynamic positron emission tomography using equivalent doses of 6-[(18) F]fluoro-m-tyrosine and 6-[(18) F]fluoro-L-dopa with tolcapone, a catechol-O-methyltransferase inhibitor. Images were realigned within subject, after which the tissue-derived uptake rate constant was generated for volumes of interest encompassing the caudate nucleus, putamen, and subregions of the putamen. We computed both bivariate (Pearson) and partial (covariate of age) correlations between clinical subscores and tissue-derived uptake rate constant. Tissue-derived uptake rate constant values were correlated between the radiopharmaceuticals (r = 0.8). Motor subscores were inversely correlated with the contralateral putamen 6-[(18) F]fluoro-m-tyrosine tissue-derived uptake rate constant (|r| > 0.72, P < .005) but not significantly with the 6-[(18) F]fluoro-L-dopa tissue-derived uptake rate constant. The uptake rate constants for both radiopharmaceuticals were also inversely correlated with activities of daily living subscores, but the magnitude of correlation coefficients was greater for 6-[(18) F]fluoro-m-tyrosine. In this design, 6-[(18) F]fluoro-m-tyrosine uptake better reflected clinical status than did 6-[(18) F]fluoro-L-dopa uptake. We attribute this finding to 6-[(18) F]fluoro-m-tyrosine's higher affinity for the target, L-aromatic amino acid decarboxylase, and the absence of other major determinants of the uptake rate constant. These results also imply that L-aromatic amino acid decarboxylase activity is a major determinant of clinical status.

Functional effects of AAV2-GDNF on the dopaminergic nigrostriatal pathway in parkinsonian rhesus monkeys

We investigated the safety and neuroregenerative potential of an adeno-associated virus (AAV2) containing human glial cell line-derived neurotrophic factor (GDNF) in an MPTP primate model of Parkinson's disease. Dopaminergic function was evaluated by positron emission tomography with 6-[(18)F]fluoro-l-m-tyrosine (FMT) before and after AAV2-GDNF or phosphate-buffered saline infusion bilaterally into the putamen. FMT uptake was significantly increased bilaterally in the putamen of AAV2-GDNF but not phosphate-buffered saline-treated animals 6 months after infusion, indicating increased dopaminergic activity in the nigrostriatal pathways. AAV2-GDNF-treated animals also showed clinical improvement without adverse effects. These findings are consistent with our previous report in aged nonhuman primates that showed evidence of enhanced use of striatal dopamine and dopaminergic nigrostriatal innervation. Clinical improvement and evidence of functional recovery in the nigrostriatal pathway, and the absence of adverse effects, support the safety of this approach for the delivery of GDNF over a 6-month period.

Striatal dopamine predicts outcome-specific reversal learning and its sensitivity to dopaminergic drug administration

Individual variability in reward-based learning has been ascribed to quantitative variation in baseline levels of striatal dopamine. However, direct evidence for this pervasive hypothesis has hitherto been unavailable. We demonstrate that individual differences in reward-based reversal learning reflect variation in baseline striatal dopamine synthesis capacity, as measured with neurochemical positron emission tomography. Subjects with high baseline dopamine synthesis in the striatum showed relatively better reversal learning from unexpected rewards than from unexpected punishments, whereas subjects with low baseline dopamine synthesis in the striatum showed the reverse pattern. In addition, baseline dopamine synthesis predicted the direction of dopaminergic drug effects. The D(2) receptor agonist bromocriptine improved reward-based relative to punishment-based reversal learning in subjects with low baseline dopamine synthesis capacity, while impairing it in subjects with high baseline dopamine synthesis capacity in the striatum. Finally, this pattern of drug effects was outcome-specific, and driven primarily by drug effects on punishment-, but not reward-based reversal learning. These data demonstrate that the effects of D(2) receptor stimulation on reversal learning in humans depend on task demands and baseline striatal dopamine synthesis capacity.

Real-time MR imaging of adeno-associated viral vector delivery to the primate brain

We are developing a method for real-time magnetic resonance imaging (MRI) visualization of convection-enhanced delivery (CED) of adeno-associated viral vectors (AAV) to the primate brain. By including gadolinium-loaded liposomes (GDL) with AAV, we can track the convective movement of viral particles by continuous monitoring of distribution of surrogate GDL. In order to validate this approach, we infused two AAV (AAV1-GFP and AAV2-hAADC) into three different regions of non-human primate brain (corona radiata, putamen, and thalamus). The procedure was tolerated well by all three animals in the study. The distribution of GFP determined by immunohistochemistry in both brain regions correlated closely with distribution of GDL determined by MRI. Co-distribution was weaker with AAV2-hAADC, although in vivo PET scanning with FMT for AADC activity correlated well with immunohistochemistry of AADC. Although this is a relatively small study, it appears that AAV1 correlates better with MRI-monitored delivery than does AAV2. It seems likely that the difference in distribution may be due to differences in tissue specificity of the two serotypes.

Striatal dopamine and working memory

Recent studies have emphasized the importance of dopamine projections to the prefrontal cortex (PFC) for working memory (WM) function, although this system has rarely been studied in humans in vivo. However, dopamine and PFC activity can be directly measured with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), respectively. In this study, we examined WM capacity, dopamine, and PFC function in healthy older participants in order to test the hypothesis that there is a relationship between these 3 factors. We used the PET tracer 6-[18F]fluoro-L-m-tyrosine to measure dopamine synthesis capacity in the striatum (caudate, putamen), and event-related fMRI to measure brain activation during different epochs (cue, delay, probe) of a WM task. Caudate (but not putamen) dopamine correlated positively with WM capacity, whereas putamen (but not caudate) dopamine correlated positively with motor speed. In addition, delay-related fMRI activation in a left inferior prefrontal region was related to both caudate dopamine and task accuracy, suggesting that this may be a critical site for the integration of WM maintenance processes. These results provide new evidence that striatal dopaminergic function is related to PFC-dependent functions, particularly brain activation and behavioral performance during WM tasks.

PET 6-[F]fluoro-L-m-tyrosine Studies of Dopaminergic Function in Human and Nonhuman Primates

Although positron emission tomography (PET) and the aromatic L-amino acid decarboxylase (AADC) tracer 6-[¹⁸F]fluoro-L-m-tyrosine (FMT) has been used to assess the integrity of the presynaptic dopamine system in the brain, relatively little has been published in terms of brain FMT uptake values especially for normal human subjects. Twelve normal volunteer subjects were scanned using PET and FMT to determine the range of normal striatal uptake values using Patlak graphical analysis. For comparison, seven adult rhesus monkeys were studied and the data analyzed in the same way. A subset of monkeys that were treated with a unilateral intracarotid artery infusion of the dopamine neurotoxin MPTP showed an 87% decrease in striatal FMT uptake. These findings support the use of PET and FMT to image AADC distribution in both normal and diseased brains using Patlak graphical analysis and tissue input functions.

Working memory capacity predicts dopamine synthesis capacity in the human striatum

Evidence from psychopharmacological research has revealed that dopamine receptor agents have opposite effects on cognitive function depending on baseline levels of working memory capacity. These contrasting effects have been interpreted to reflect differential baseline levels of dopamine. Here we demonstrate for the first time that working memory capacity as measured by listening span predicts dopamine synthesis capacity in the striatum, indicating that subjects with low working memory capacity have low DA synthesis capacity in the striatum, whereas subjects with high working memory capacity have high DA synthesis capacity in the striatum.

Aromatic l-amino acid decarboxylase turnover in vivo in rhesus macaque striatum: A microPET study

The aromatic L-amino acid decarboxylase (AAAD) is involved in the de novo synthesis of dopamine, a neurotransmitter crucial in cognitive, neurobehavioral and motor functions. The goal of this study was to assess the in vivo turnover rate of AAAD enzyme protein in the rhesus macaque striatum by monitoring, using microPET imaging with the tracer [(18)F]fluoro-m-tyrosine (FMT), the recovery of enzyme activity after suicide inhibition. Results showed the AAAD turnover half-life to be about 86 h while total recovery was estimated to be 16 days after complete inhibition. Despite this relatively slow AAAD recovery, the animals displayed normal movement and behavior within 24 h. Based on the PET results, at 24 h, the animals have recovered about 20% of normal AAAD function. These findings show that normal movement and behavior do not depend on complete recovery of AAAD function but likely on pre-synaptic and post-synaptic compensatory mechanisms.

Experiment assessment of mass effects in the rat: implications for small animal PET imaging

In vivo imaging using positron emission tomography (PET) is important in the development of new radiopharmaceuticals in rodent animal models for use as biochemical probes, diagnostic agents, or in drug development. We have shown mathematically that, if small animal imaging studies in rodents are to have the same "quality" as human PET studies, the same number of coincidence events must be detected from a typical rodent imaging "voxel" as from the human imaging voxel. To achieve this using the same specific activity preparation, we show that roughly the same total amount of radiopharmaceutical must be given to a rodent as to a human subject. At high specific activities, the mass associated with human doses, when administered to a rodent, may not decrease the uptake of radioactivity at non saturable sites or sites where an enzyme has a high capacity for a substrate. However, in the case of binding sites of low density such as receptors, the increased mass injected could saturate the receptor and lead to physiologic effects and non-linear kinetics. Because of the importance of the mass injected for small animal PET imaging, we experimentally compared high and low mass preparations using ex vivo biodistribution and phosphorimaging of three compounds: 2-fluoro-2-deoxyglucose (FDG), 6-fluoro-L-metatyrosine (FMT) and one receptor-directed compound, the serotonin 5HT1A receptor ligand, trans-4-fluoro-N-[2-[4-(2-methoxylphenyl) piperazino]ethyl]-N-(2-pyridyl) cyclohexane- carboxamide (FCWAY). Changes in the mass injected per rat did not affect the distribution of FDG, FMT, and FCWAY in the range of 0.6-1.9 nmol per rat. Changes in the target to nontarget ratio were observed for injected masses of FCWAY in the range of approximately 5-50 nmol per rat. If the specific activity of such compounds and/or the sensitivity of small animal scanners are not increased relative to human studies, small animal PET imaging will not correctly portray the "true" tracer distribution. These difficulties will only be exacerbated in animals smaller than the rat, e.g., mice.

A new precursor for the preparation of 6-[18F]Fluoro-L-m-tyrosine ([18F]FMT): efficient synthesis and comparison of radiolabeling

For the electrophilic preparation of 6-[18F]fluoro-L-m-tyrosine ([18F]FMT), a PET tracer for measuring changes in dopaminergic function in movement disorders, a novel precursor, N-(tert-butoxycarbonyl)-3-(tert-butoxycarbonyloxy)-6-trimethylstannnyl-L-phenylalanine ethyl ester, was synthesized in four steps and 26% yield starting from L-m-tyrosine. [18F]FMT produced by two methods at two institutions was comparable in both radiochemical yield, 25-26%, and quality (chemical, enantiomeric, and radiochemical purity and specific activity) as that obtained with the original N-trifluoroacetyl-3-acetyl-6-trimethylstannyl-L-m-tyrosine ethyl ester [18F]FMT precursor.

Blood-brain barrier and neuronal membrane transport of 6-[18F]fluoro-L-DOPA

The transport of 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine ([18F]FDOPA) across the blood-brain barrier (BBB) and neuronal membranes was compared with that of L-3,4-dihydroxyphenylalanine (L-DOPA) in rats. The carotid injection method was used as a direct measurement of [18F]FDOPA, 1-[14C]-L-DOPA, and 3-[14C]-L-DOPA transport across the BBB, while isolated nerve terminals were used to examine neuronal membrane transport of [3H]-L-DOPA. [18F]FDOPA appeared to use the same large neutral amino acid carrier for BBB transport as L-DOPA and L-phenylalanine. In addition, carbidopa [L-alpha-hydrazino-alpha-methyl-beta-(3,4-dihydroxyphenyl)propionic acid] was found not to have direct interference with the transport carrier on the BBB, but indirectly inhibited aromatic L-amino acid decarboxylase (AAAD) activity in brain endothelium by depletion of pyridoxal phosphate, a necessary cofactor of the enzyme. In striatal and cortical synaptosomes, [3H]-L-DOPA uptake was inhibited by non-radioactive L-DOPA, FDOPA, and 6-fluoro-L-meta-tyrosine (6-FMT). The inhibition was significantly greater in terminals isolated from the striatum than in those from the cerebral cortex. FDOPA, 6-FMT, and L-DOPA equally inhibited the neuronal transport of [3H]-L-DOPA. This suggests that FDOPA and 6-FMT compete with L-DOPA at similar transport sites at the neuronal membrane.

Synthesis of stereo (R and S) and geometric (E and Z) isomers of [18F]fluoro-beta-fluoromethylene-m-tyrosine derivatives: in vivo probes of central dopaminergic function

Fluorination of pure R and S enantiomers of (E)-beta-fluoromethylene-m-tyrosine [(E)-FMMT] and its racemic geometric isomer, (Z)-beta-fluoromethylene-m-tyrosine [(Z)-FMMT] with [18F]acetyl hypofluorite ([18F]AcOF) gave a mixture of aromatic ring fluorinated products and a pair of diastereomeric products of addition across the exocyclic double bond. Semipreparative high performance liquid chromatography (HPLC) enabled a complete separation and isolation of these products, namely, 6-[18F]fluoro, 2-[18F]fluoro, and 2,6-[18F]difluoro (E)-FMMT and (Z)-FMMT derivatives. No attempt was made to isolate the individual components of the addition product. Pure racemic 4-[18F]fluoro-(E)-beta-fluoromethylene-m-tyrosine was also synthesized from a substituted (E)-FMMT precursor involving a fluorodestannylation reaction with [18F]F2. The availability of stereo (R and S) isomers of 6-[18F]fluoro and 2-[18F]fluoro (E)-FMMT and those of the racemic (Z)-FMMT along with 4-[18F]fluoro-(E)-beta-fluoromethylene-m-tyrosine would now enable a systematic investigation of the central monoamine oxidase/aromatic amino acid decarboxylase enzyme system with positron emission tomography.

Evaluation of dopaminergic presynaptic integrity: 6-[18F]fluoro-L-dopa versus 6-[18F]fluoro-L-m-tyrosine

The effectiveness of 6-[18F]fluoro-L-m-tyrosine (6FMT) to evaluate dopamine presynaptic integrity was compared to that of 6-[18F]fluoro-L-dopa (6FDOPA) in vivo by positron emission tomography (PET). Six normal and six 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys received 6FDOPA and 6FMT PET scans on separate occasions with identical scanning protocols. Four measures, the rate of uptake of tracer into striatum using either the arterial input function (Ki) or the activity in the occipital cortex as the input function (Kc), the rate of loss of striatal radioactivity (k(loss)), and an index of "effective turnover" of dopamine (k(loss)/Ki), were obtained for both tracers during extended PET studies. 6-[18F]Fluoro-L-m-tyrosine was as effective as 6FDOPA in separating normals from MPTP-lesioned subjects on the basis of the uptake rate constants Ki and Kc. However, in contrast to 6FDOPA, it was not possible to differentiate the normal from the lesioned animal using k(loss) or k(loss)/Ki for 6FMT. Thus, FMT appears to be a reasonable, highly specific tracer for studying the activity of aromatic dopa decarboxylase enzyme as an index of presynaptic integrity. However, if one is interested in investigating further the metabolic pathway and obtaining an in vivo estimate of the effective turnover of dopamine (after pharmacologic manipulation, for example), 6FDOPA remains the tracer of choice.

6-[18F]fluoro-L-m-tyrosine: metabolism, positron emission tomography kinetics, and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine lesions in primates

The tracer 6-[18F]fluoro-L-m-tyrosine (FMT) was studied with regard to its biochemistry and kinetics, as well as its utility in evaluating brain dopaminergic function in primates before and after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment using positron emission tomography (PET). Plasma analysis of FMT and its F18-labeled metabolites 6-fluoro-3-hydroxyphenylacetic acid (FPAC) and 6-fluoro-3-hydroxyphenylethylamine (FMA) during PET scanning enabled kinetic analysis of FMT uptake. A separate study examined brain FMT metabolism in MPTP-naive monkeys euthanized 60 or 120 min after FMT injection. Almost 60% of total plasma F-18 activity was associated with FPAC and FMA 120 min after FMT injection. The FMT signal accumulated preferentially in dopaminergic areas such as caudate and putamen. This bilateral FMT signal was disrupted after unilateral intracarotid artery (ICA) MPTP infusion which reduced ipsilateral striatal activity. A three compartment three kinetic rate constant model for FMT uptake revealed reduced FMT decarboxylation (k3) in ipsilateral caudate and putamen after unilateral MPTP although a further decrease was not evident after intravenous MPTP. FPAC was the major F-18 species in all brain regions except in cerebellum where FMT was predominant 60 min post-mortem. FPAC was most concentrated in dopaminergic areas whereas lower levels occurred in areas containing few dopamine terminals. These data demonstrate preferential FMT metabolism and F-18 retention in dopaminergic tissue and support the use of FMT to evaluate normal and abnormal dopaminergic function.

Radiofluorinated L-m-tyrosines: new in-vivo probes for central dopamine biochemistry

In this work, we introduce 6-[18F]fluoro-L-m-tyrosine (6-FMT) and compare its in-vivo kinetic and bio-chemical behaviors in monkeys and rodents with those of 4-FMT and 6-[18F]fluoro-L-3, 4-dihydroxyphenylalanine (DOPA) (FDOPA). These radiofluorinated m-tyrosine presynaptic dopaminergic probes, resistant to peripheral 3-O-methylation, offer a nonpharmacological alternative to the use of catechol-O-methyltransferase inhibitors. Like FDOPA, 4-FMT and 6-FMT are analogs that essentially follow the L-DOPA pathway of central metabolism. After i.v. administration in nonhuman primates and rodents, these new radiofluorinated m-tyrosine analogs accumulate selectively in striatal structures and allow for the detection of additional innervation sites (e.g., brain stem) rich in aromatic amino acid decarboxylase. Bio-chemical analyses in rodents and monkeys revealed the specificity of their central and peripheral metabolism. Molecular and enzymatic mechanisms involved in their retention in central brain structures are consistent with involvement of dopaminergic neurons. The high signal-to-noise ratios observed make these radiofluorinated m-tyrosine analogs outstanding candidates for probing the integrity of central dopaminergic mechanisms in humans.

Identification and quantitation of gaseous compounds of fluorine generated in [18F]F2 target systems

The first direct evidence for the chemical identity of the electrophilic fluorinating agents generated in 20Ne(d, alpha)18F (single-step), 18O(p,n)18F (single-step and two-step) and 16O(3He2+, p)18F (single-step) gas target systems, utilizing aluminum, silver, copper, nickel and gold plated copper target bodies, has been established with multinuclear NMR and mass spectral techniques. The major components of the reactive fraction from these targets were also quantitated using 19F NMR. Fluorine-19 NMR data of the reactive fraction of all proton and 3He2+ irradiated oxygen gas target systems showed the presence of oxygen difluoride in various proportions. Samples from the single-step method contained up to 20% OF2 while those from the two-step process had 0-5%. Fluorine nitrate (FONO2) was observed only as a minor component (0-3%) in the reactive fraction. The presence of OF2 and FONO2 was further confirmed by 17O and 15N NMR, respectively, using [17O]O2 and [15N]N2 spiked oxygen gas targets. The NMR results were supported by mass spectral data collected with a residual gas analyzer (RGA). Both 19F NMR and mass spectroscopy showed CF4 as the only major inert component in the single-step oxygen target products. As expected, the 19F NMR and mass spectral data showed that the reactive fraction of the neon gas target constituted only F2 and the inert fraction comprised of CF4 and NF3.

Synthesis and evaluation of an 18F-labeled dopa prodrug as a PET tracer for studying brain dopamine metabolism

In the quantitative studies of presynaptic dopamine metabolism by PET with 6-[18F]fluoro-L-dopa (6-[18F]FDOPA), metabolic analysis in the plasma is required to determine the precise input function because of susceptibility of the compound to peripheral metabolism. In this study, we prepared 6-[18F]-fluoro-O-pivaloyl-L-dopa (6-[18F]FPDOPA) as a prodrug of 6-[18F]FDOPA, and evaluated its potential as a PET tracer in mice. If the 6-[18F]FPDOPA is stable peripherally and is hydrolyzed to 6-[18F]FDOPA in the brain tissues, disadvantage of the 6-[18F]FDOPA will be overcome. Compared with the 6-[18F]FDOPA, the initial brain uptake of the 6-[18F]FPDOPA was lower; however, the uptake in the latter become comparable, and the uptake ratios of striatum to other reference regions were larger. Medication of mice with inhibitors of aromatic amino acid decarboxylase and catechol-O-methyl transferase greatly enhanced the striatal uptake of the two compounds. The reduced brain uptake of the compounds by L-phenylalanine-loading suggested transport through the blood-brain barrier by the neutral amino acid transporter. HPLC analysis showed the presence of 6-[18F]FPDOPA, 6-[18F]FDOPA and 6-[18F]fluorodopamine in the striatum; however, 6-[18F]fluoro-3-O-methyl-L-dopa was a predominate metabolite in the brain and plasma as in the case of [18F]FDOPA. Results suggested that 6-[18F]FPDOPA had characteristics as a prodrug of 6-[18F]FDOPA; however, the compound was also labile to metabolic alteration in vivo.

Metals suitable for fluorine gas target bodies: first use of aluminum for the production of [18F]F2

A comprehensive evaluation of different metals (aluminum, silver, copper, nickel, and gold-plated copper) was undertaken for the fabrication of target bodies with straight and conical bore shapes for the production of [18F]F2 via the 20Ne(d,alpha)18F nuclear reaction. Of these metals, aluminum, silver and copper have never been used for the production of [18F]F2. All these target bodies were easily passivated using a mild beam-induced plasma technique in the presence of 1% F2 in neon or argon. The recovery of 18F activity was higher with electroformed nickel and silver bodies, probably due to favorable thermal conductivities. Aluminum proved to be a useful material for fluorine gas targets. The consistent recovery of 18F activities, ease and low cost of manufacturing and low nuclear activation properties all make aluminum an ideal choice for fluorine gas targetry. To our knowledge, this investigation is the first to highlight the use of aluminum as a target body material for the routine production of [18F]F2. A reasonable mechanism based on the Langmuir-Rideal surface atom recombination is also proposed for the behavior of [18F]F2 recovery from a nickel target body.

Proton irradiation of [18O]O2: production of [18F]F2 and [18F]F2 + [18F] OF2

The production of 18F electrophilic reagents via the 18O(p,n)18F reaction has been investigated in small-volume target bodies made of aluminum, copper, gold-plated copper and nickel, having straight or conical bore shapes. Three irradiation protocols-single-step, two-step and modified two-step-were used for the recovery of the 18F activity. The single-step irradiation protocol was tested in all the target bodies. Based on the single-step performance, aluminum targets were utilized extensively in the investigation of the two-step and modified two-step irradiation protocols. With an 11-MeV cyclotron and using the two-step irradiation protocol, > 1Ci [18F]F2 was recovered reproducibly from an aluminum target body. Probable radical mechanisms for the formation of OF2 and FONO2 (fluorine nitrate) in the single-step and modified two-step targets are proposed based on the amount of ozone generated and the nitrogen impurity present in the target gases, respectively.

Sporadic ALS and chromosome 22: evidence for a possible neurofilament gene defect

ALS is associated with the P2 blood group phenotype. Molecular evidence now shows the gene encoding this antigen to be on the long arm of human chromosome 22 near the newly discovered gene for heavy neurofilament (NF-H). Since an ALS-type condition can be generated in transgenic mice expressing the human NF-H gene, and since the gene for the CNTF-related cytokine leukemia inhibitory factor (LIF) is located adjacent to this gene, it is hypothesized that a defect on the chromosome 22 band region q12 is involved in the pathogenesis of sporadic ALS.