Tadashi Nozaki

Design and evaluation of radioactive acetylcholine analogs for mapping brain acetylcholinesterase (AchE) in vivo

For mapping brain acetylcholinesterase (AchE) in vivo, seven radioactive acetylcholine analogs, N-[14C]methylpiperidyl-3- and 4-acetates, propionates, isobutyrates, and 3-butyrate were newly synthesized and evaluated in mice. The esters readily entered the brain and were hydrolyzed into the hydrophilic metabolite, which was trapped. In brain homogenates, the esters showed a wide range of enzymatic reactivity (about 40-fold), and high specificity for AchE (more than 82%) except the butyrate. Intra-brain distribution of the esters reflected a pattern of AchE activity.

Synthesis and in vivo evaluation of [11C]semotiadil, a benzothiazine calcium antagonist.

A carbon-11 labeled benzothiazine calcium antagonist, (+)-(R)-2-[5-methoxy-2-[3-[methyl[2-[(3,4- methylenedioxy)phenoxy]ethyl]amino]propoxy]phenyl]-4-methyl-2H-1,4- benzothiazin-3(4H)-one (semotiadil), and its enantiomer were prepared by N-methylation of the corresponding norderivatives with 11CH3I: decay-corrected radiochemical yields of 16-27% based on 11CH3I, radiochemical, chemical and optical purity of > 99%, sp. act. of 11-50 GBq/mumol and preparation time of 35-40 min. In mice, saturable and stereo-selective uptake in the hippocampus, striatum and hypothalamus was observed. The potential of the compound to visualize the regional brain calcium channels in vivo by positron emission tomography was indicated; however, no promising sign was found in the myocardium.

A distillation method of preparing C-11 labeled acetate for routine clinical use

Abstract A semi-automated synthesis of [1- 11 C]acetate for routine clinical studies by positron emission tomography is described. Carboxylation of methylmagnesium chloride in THF with [ 11 C]CO 2 followed by the HCl hydrolysis at low temperature gave [1- 11 C]acetic acid, which was distilled with a N 2 flow into a NaHCO 3 solution. After radioactive and nonradioactive undesirable products were removed by evaporation of the solution to dryness, a sterile and apyrogenic [1- 11 C]acetate injection was obtained within 20 min.

Synthesis and preliminary evaluation of [1-11C]hexanoate as a PET tracer of fatty acid metabolism

The potential of [1-11C]hexanoate (11C-HA) as a radiopharmaceutical assessing fatty acid metabolism of the myocardium and brain tissues by PET studies was evaluated.11C-HA was synthesized by the Grignard reaction of pentylmagnesium bromide and11CO2.11C-HA, [1-14C]acetate and [3H]deoxyglucose were simultaneously injected i.v. into mice, and the tissue distribution of the three radionuclides was measured. In the heart, high uptake and rapid clearance of11C and14C was found. The brain uptake of11C was twice as high as that of14C, and both11C and14C decreased slowly compared to the heart. The level of3H increased with time in both the heart and brain. In fasting conditions, the uptake of11C by the heart was enhanced and the level of3H decreased with time. The brain uptake of11C and3H was also enhanced. The fasting conditions did not affect the distribution of14C. The radiation absorbed dose of11C-HA was also estimated.

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.

Malonic ester and acetoacetic ester synthesis of 2-[11,14C]methyl-fatty acids

Abstract Suitable conditions were sought for the synthesis of various 2-[ 11,14 C]methyl-fatty acids by malonic ester synthesis and acetoacetic ester synthesis, both involving radio-methylation with [ 11,14 C]Ch 3 I. The malonic ester synthesis gave 2-[ 11 C]methyl-fatty acids with over 60% decay-corrected yields in about 40 min and the [ 14 C]products with somewhat higher yields at a longer time. In the acetoacetic ester synthesis, several 2-[ 14 C]methyl-fatty acids were synthesized in 50–70% yields by the hydrolysis of the radio-methylated acetoacetates with a concentrated potassium hydroxide solution, together with by-prodcut ketones. This hydrolysis was completed in 5 min at 70°C, whereas rather drastic conditions or a longer time were needed for the thermal decarboxylation in the malonic ester synthesis.

A brain uptake study of [1-11C]hexanoate in the mouse: The effect of hypoxia, starvation and substrate competition

We evaluated the potential of sodium [1-11C]hexanoate (11C-HA) as a radiopharmaceutical with which to assess oxidative metabolism of the brain by PET.11C-HA, sodium [1-14C]acetate and [3H]deoxyglucose were simultaneously injected into mice under control, hypoxic and starving conditions. In the control, the brain uptake of11C was maximal at 3 min (% ID/g = 2.2–2.5), being twice as high as that of14C, followed by a gradual clearance. The time-radioactivity curve of11C was similar to that of14C. Hypoxia enhanced the brain uptake of3H, but not of either11C or14C. Starvation enhanced the brain uptake of3H and11C. The clearance rate of11C was not significantly affected by either condition. In the control brain at 3 min postinjection of HA, 65% of the total radioactivity was detected as labeled glutamate and glutamine, which was gradually decreased by 47% at 30 min. The brain to blood ratios of11C-HA at 3 min were significantly reduced by butyrate, hexanoate and octanoate loading but not by that with other monocarboxylic acids or ketone bodies.

Production of fine aerosols labelled with various radionuclides by sublimation from a graphite boat, and their properties and tracer use

Abstract Fine aerosols labelled with 18 F, 42 K, 64 Cu, 68 Ga and 99m Tc were produced by a commercial apparatus called Technegas Generator. Their properties were studied by various methods, before and after the particulates were introduced in water. The nuclides in the water were found to exist as: 18 F, entirely anion, 42 K, entirely cation; 68 Ga, mainly cation; 64 Cu, mainly in carbon particulates; and 99m Tc, almost entirely in carbon particulates. Examples are shown for the utilization of these aerosols in various radiotracer studies.

Isotopic double tracers for the measurement of the biological half-life of an element or compound, exemplified by 125I131I taken up in a seaweed

Abstract As an example for the measurement of biological half-life of an element using isotopic double tracers, a seaweed was cultured in sea water containing constant concentrations of 125 I and 131 I and the activities taken up were measured for various parts and compositions of the seaweed. Mathematical relationships between the biological half-life of iodine and the 125 I/ 131 I ratio were obtained and compared with the experimental data. The half-life in each composition was shown to increase with the resistivity to extraction or decomposition. Also, iodide in the sea water was found to be oxidized in the presence of seaweed in the light.

Characterization of [3H]nemonapride binding to mouse brain dopamine D2 receptors assessed in vivo and ex vivo for metabolic modeling in PET studies.

We characterized [3H]nemonapride ([3H]NEM, [3H]YM-09151-2) binding to dopamine D2 receptors. In mice given [3H]NEM with and without sulpiride, thein vivo specific binding of the [3H]NEM to the D2 receptors in the striatum was assessed: SBin vivo-1, striatal uptake minus cerebellar uptake; and SBin vivo-2, uptake in the control mice minus uptake in the sulpiride-treated mice. Tissue homogenates were divided into cytosol and the membrane binding fraction (MB). When the MB was incubatedin vitro with sulpiride, the dissociated and nondissociated fractions were defined as theex vivospecific binding (SBex vivo) and theex vivo nonspecific binding (NBex vovo), respectively. HPLC revealed that most of the radioactivity in the MB was [3H]NEM, whereas metabolites were found in the cytosol. In the striatum, the SBex vivo increased with time (50% of the total tissue uptake at 60 min), and was equivalent to the SBin vivo-2. The SBin vivo-1 was comparable to the MB. In the cerebral cortex and cerebellum, the SBex vivo decreased with time and the SBex vivo/free [3H]NEM ratios were smaller than those in the striatum.