R. Cantineau

Serotonin 5HT2 receptor imaging in the human brain using positron emission tomography and a new radioligand, [18F]Altanserin : results in young normal controls

Changes in serotonin-2 receptors have been demonstrated in brain autopsy material from patients with various neurodegenerative and affective disorders. It would be desirable to locate a ligand for the study of these receptors in vivo with positron emission tomography (PET). Altanserin is a 4-benzoylpiperidine derivative with a high affinity and selectivity for S2 receptors in vitro. Dynamic PET studies were carried out in nine normal volunteers with high-specific activity (376–1,680 mCi/μmol) [18F]altanserin. Arterial blood samples were obtained and the plasma time–activity curves were corrected for the presence of labeled metabolites. Thirty minutes after injection, selective retention of the radioligand was observed in cortical areas, while the cerebellum, caudate, and thalamus had low radioactivity levels. Specific binding reached a plateau between 30 and 65 min postinjection at 1.8% of the injected dose/L of brain and then decreased, indicating the reversibility of the binding. The total/nonspecific binding ratio reached 2.6 for times between 50 and 70 min postinjection. The graphical analysis proposed by Logan et al. allowed us to estimate the binding potential (Bmax/KD). Pretreatment with ketanserin was given to three volunteers and brain activity remained uniformly low. An additional study in one volunteer showed that [18F]altanserin can be displaced from the receptors by large doses of ketanserin. At the end of the study, unchanged altanserin was 57% of the total plasma activity. These results suggest that [18F]altanserin is selective for S2 receptors in vivo as it is in vitro. They indicate that [18F]altanserin is suitable for imaging and quantifying S2 receptors with PET in humans.

A New Route for the Synthesis of [18f]Fluoroaromatic Substituted Amino Acids: No Carrier Added L-P-[18f]Fluorophenylalanine

L-p-[18F]fluorophenylalanine was designed as a potential marker for probing protein synthesis in the human brain by positron emission tomography. This radiotracer has been synthesized using nucleophilic displacement of the activated nitro group of p-nitrobenzaldehyde by NCA 18F- obtained from the 18O (p, n) reaction on enriched water. The L-form of the [18F]fluoroamino acid can be separated on an analytical scale chiral column. A typical production run of 22.2 GBq (600 mCi) of 18F obtained after a 10 microA.h bombardment of 18 MeV protons on 99.8% 18O-enriched water leads to a batch of 1.11 GBq (30 mCi) of NCA L-p-[18F]fluorophenylalanine after a total synthesis time of 120 min.

Synthesis of fluorine-18 substituted aromatic aldehydes and benzyl bromides, new intermediates for n.c.a. [18F]fluorination☆

The synthesis of various [18F]fluoroaromatic aldehydes using activated nitro precursors and amino-polyether supported nucleophilic substitution with 18F− is reported. These radiolabelled fluorinated aldehydes (radiochemical yields: 50–75%) are powerful key intermediates leading after treatment with NaBH4 and SOBr2 (SOCL2) to further active intermediates for example substituted [18F]fluorobenzyl bromides (yields 30–50% EOB). These benzaldehydes and bromides are particularly useful for the preparation of new radiopharmaceuticals (e.g. fluorotroprapride, fluorodexetimide) either by reductive amination or by aromatic N-alkylation. The preparation of various amino acids in D, L (50:50) or enriched L form by asymmetric synthesis is also possible (e.g. L-6-[18F]fluorodopa, L-4-[18F]fluoro-m-tyrosine). It can be anticipated that the 18F-labelled fluoroaldehydes will find widespread application in radiopharmaceutical chemistry.

Nucleophilic enantioselective synthesis of 6-[18F]Fluoro-l-dopa via two chiral auxiliaries

Abstract Asymmetric nucleophilic synthesis of 6-[18F]fluoro- l -dopa was investigated in order to reach an enantiomeric excess of close to 100% of the l form of this amino acid. The radiochemical synthesis required [18F]fluoride as fluorinating agent and regioselective nucleophilic substitution of commercially available 6-nitroveratraldehyde. The [18F]fluorobenzaldehyde thus obtained was easily converted to the corresponding 2-[18F]fluoro-4,5-dimethoxybenzyl bromide. This alkylating agent was added to the lithium enolates of 1-(S)-(−)camphor imine of t-butyl glycinate (1) and (S)-(−)-1-Boc-2-t-butyl-3-methyl-4-imidazolidinone [(S)- Boc-BMI] (2) in order to compare the enantiomeric excess of the l form obtained in each case with these two chiral inductors. The l -isomer of fluorodopa was isolated after H1 hydrolysis and HPLC purification in 5–10% radiochemical yield (decay corrected). The overall synthesis time was of 110 min. Through this synthetic pathway, the l -isomer of fluorodopa was obtained in 83% e.e with 1 and 96% e.e with 2 respectively, as determined by chiral HPLC. A practical three step preparative scale synthesis of 6-[19F]fluoro- d,l -dopa is also presented.

An approach to the asymmetric synthesis of l-6-[18F]fluorodopa via NCA nucleophilic fluorination

Abstract The NCA asymmetric synthesis of l -6-[18F]fluorodopa starting from (1R,2R,5R)-[(+)-2-hydroxypinanyl- 3-idene]glycine t-butyl ester as chiral agent has been developed. After 18F-fluorination of the two commercially available aldehydes either 6-nitroveratraldehyde or 6-nitropiperonal, the required alkylating [18F]fluorobenzyl bromide derivative can be easily prepared by treatment with NaBH4 followed by SOBr2. Alkylation of the Schiff base was carried out with the lithium salt of 2,2,6,6-tetramethylpiperidine as base in anhydrous THF at −78°C. Following hydrolysis of the protecting groups with hydroxylamine and Hl, the l -amino acid was obtained in 75% l form (ee 50%) with a 10% decay corrected (120 min) radiochemical yield.

PET Radiopharmaceutical Metabolism — Plasma Metabolite Analysis

PET radiopharmaceutical metabolism is discussed in general. Methodology for determining the proportion of unchanged radiopharmaceutical in plasma is discussed with regard to preparation of sample, current analytical techniques, sources of errors and pitfalls. The literature on metabolite analysis in plasma is discussed for several popular radiopharmaceuticals, including labelled amino acids, flow tracers and radioligands for acetylcholine, adrenergic, benzodiazepine, dopamine and serotonin receptors.

Chemical processing for production of no-carrier-added selenium-73 from germanium and arsenic targets and synthesis of l-2-amino-4-([73Se]methylseleno) butyric acid (l-[73Se]selenomethionine)

The Ge(4He, xn) and 75As(p, 3n) reactions were compared as the best potential routes for routine production of selenium-73 (73Se) for medical applications. With 26 MeV alpha particles, available with compact cyclotrons, the first reaction required an enriched 70Ge target of sodium metagermanate to give a production yield of 1 mCi/microAh (0.037 GBq/microAh) in a 105 mg/cm2 target. With 55 MeV protons the As(p, 3n) reaction on natural arsenic yielded 20 mCi/microAh (0.74 GBq/microAh) in a 685 mg/cm2 target. A simple method was developed and optimized for both targets in order to isolate and purify the no-carrier-added selenium in the elemental form with a radiochemical yield greater than 75% in less than 90 min. An automated radiochemical processing unit has been designed for the routine production of 100-150 mCi (3.7-5.5 GBq) batches of carrier-free 73Se ready for radiopharmaceutical labeling. 30 mCi (1.11 GBq) (EOS) of L-2-amino-4-([73Se]methylseleno) butyric acid (L-[73Se]selenomethionine) ready for injection with a specific activity of 5 Ci/mmol (185 GBq/mmol) (EOS) were obtained through a fast chemical synthesis. Radiation absorbed dose estimates for L-[73Se]selenomethionine have been determined. A value of 0.70 rem/mCi (0.19 mSv/MBq) administered was calculated for the risk from irradiation in man. The first human PET investigation with [73Se]selenomethionine showed a very good delineation between liver and pancreas.

Fast chemical synthesis of [75Se] L-selenomethionine

A fast chemical synthesis of high specific activity [75Se]L-selenomethionine (10 Ci/mmol--370 GBq/mmol) is described with a view to 73Se labeling and PET studies. The overall radiochemical yield of the preparation is better than 80%. The purification method uses commercially available reverse phase HPLC columns and 9% NaCl as mobile phase. The final labeled compound is obtained in less than 3 h and the chemical, radiochemical and optical purities of the L-isomer are higher than 99.0%.

2- and 4-[18F]fluorotropapride, two specific D2 receptor ligands for positron emission tomography: N.C.A. syntheses and animal studies.

Tropapride, (exo)-2,3-dimethoxy-N-[8-(phenylmethyl)-8- azabicyclo[3.2.1]oct-3-yl]benzamide hydrochloride, has been labeled with fluorine-18 at the 2- and 4-positions of its benzylic group. Two synthetic pathways were investigated: the first one required the alkylation of the norbenzyl precursor with 2- or 4-[18F]fluorobenzyl bromide (radiochemical yield of 5% EOB, 180 min); the second method consisted of a reductive amination of norbenzyl tropapride with 2- or 4-[18F]fluorobenzaldehyde (20% EOB, 110 min). In both cases, the specific activity was found to be greater than 1 Ci/mumol (EOS). Animal studies in rats showed the percentage of the injected dose localizing in the whole brain to be 0.6 +/- 0.09 and 0.2 +/- 0.03 at 2 h post injection for the para- and the ortho-[18F]fluoro analogs of tropapride respectively. Cerebral biodistribution studies showed at 4 h a striatum uptake of 5 +/- 0.7% of the injected dose per gram of striatum for the para derivative with a low fixation into the frontal cortex and the cerebellum (% ID/g FC < 0.4 and % ID/g Cb < 0.3). The selectivity of 4-[18F]fluorotropapride for D2 dopaminergic sites was demonstrated through blocking experiments with ketanserin, spiperone and halopemide. The saturability was confirmed by the use of variable specific activities. These preliminary results showed that 4-[18F]fluorotropapride can be considered as a potent radiopharmaceutical for the study of the dopaminergic system with PET.

High-yield synthesis of a [3H]ethylenediamine ditetrodotoxin derivative

[3H]Tetrodotoxin [( 3H]TTX) and a [3H]ethylenediamine derivative of TTX are the most widely used ligands for the study of the Na channel. The former ligand presents a low specific radioactivity (1 Ci/mmol) while the latter is highly labeled (30 Ci/mmol). However, its two-step synthesis, i.e., mild oxidation followed by coupling of [3H]ethylenediamine, has been described with a low overall yield of 1.7%. In this work, more favorable experimental conditions are defined for the limiting reaction, i.e., the oxidation step, using [14C]testosterone as a model molecule. Applied to the oxidation of tetrodotoxin, this procedure produces yield values of 30-50%, as determined by high-performance liquid chromatography. Moreover, two oxidized TTX molecules appear to be covalently linked to [3H]ethylenediamine, yielding a new labeled tetrodotoxin derivative with a specific radioactivity of 45 Ci/mmol and a dissociation constant of 0.6 nM for electroplax membranes.