Toshio Yamasaki

Radioactive N,N‐Dimethylphenylethylamine: A Selective Radiotracer for In Vivo Measurement of Monoamine Oxidase‐B Activity in the Brain

N‐[methyl‐14C]N,N‐dimethylphenylethylamine (DMPEA) was synthesized and its availability as a selective radiotracer for in vivo measurement of mouse brain monoamine oxidase (MAO) activity was examined. Relatively high incorporation of labelled DMPEA into brain (about 10% of the injected dose/per gram of brain) was observed just after its injection; however, radioactive dimethylamine, a metabolite produced from labelled DMPEA in the brain 1 h after DMPEA injection, was reduced in a dose‐dependent manner by pretreatment with various doses of a specific MAO‐B inhibitor, 1‐deprenyl, but was not reduced appreciably by pretreatment with a specific MAO‐A inhibitor, clorgyline. Pretreatment with 1‐deprenyl did not affect significantly the rate of incorporation of the radiotracer DMPEA into the brain, suggesting that reduction of the radioactivity in brain by this compound might be due to a decrease in the rate of production of the radioactive metabolite dimethylamine by brain MAO‐B. The amount of the radioactive metabolite trapped in the brain was found to be proportional to the brain MAO‐B activity remaining after pretreatment with 1‐deprenyl. In vitro deamination of DMPEA by mouse brain MAO showed a higher sensitivity to inhibition by 1‐deprenyl than that by clorgyline. These results indicate that DMPEA is a selective substrate for mouse brain MAO‐B both in vivo and in vitro and that the positron emitter [11C]DMPEA might be used instead of [14C]DMPEA as a radiotracer for in vivo measurement of MAO‐B activity in human brain.

Synthesis of 13N-labelled amines by reduction of 13N-labelled amides

Abstract 13 N-β-phenethylamine and 13 N-n-octylamine were synthesized by LialH 4 reduction of 13 N-phenylacetamide and 13 N-octamide prepared by 13 N-ammonolysis in ether. The reaction produced a fairly good yield even in a non-carrier-added state. Both 13 N-amines were isolated by extraction, and organ distribution in mice was studied preliminarily. After i.v. administration of both 13 N-amines, high accumulation was observed in the brain, lung, and heart. Long-term retention of the radioactivity was observed in the brain and the heart.

Preparation of 13N-β-phenethylamine

Abstract Nitrogen-13-labelled β-phenethylamine([ 13 N]PEA) was synthesized by Hofmann rearrangement of [ 13 N]phenylpropionamide prepared from phenylpropionyl chloride and aqueous [ 13 N]ammonia solution. The reaction proceeded rapidly with a fairly good yield. [ 13 N]PEA was isolated using preparative thin-layer chromatography, and organ distribution in mice was studied preliminarily. After i.v. administration of [ 13 N]PEA, high accumulation and long-term retention of the radioactivity were observed in the brain and the heart.

[13N]ammonia in organic solvents; a potent synthetic precursor for 13N-labeling

Abstract 13 NH 3 in an organic solvent was prepared and its utility as a labeling precursor was studied. [ 13 N]adenine ([ 13 N]ADN), [ 13 N]nicotinamide ([ 13 N]NAM), [ 13 N] p -nitrophenyl carbamate ([ 13 N]NPC), and [ 13 N] l -glutamine ([ 13 N]Gln) were labeled utilizing this precursor. [ 13 N]ADN and [ 13 N]NAM were labeled in much better yields than from an aqueous solution of 13 NH 3 . [ 13 N]NPC and [ 13 N]Gln, which could not be labeled in an aqueous solution, were labeled in high radiochemical yields. Thus, the advantages of this precursor are the improvement of the labeling yield and the feasibility of labeling compounds unstable in aqueous conditions.

Evaluation of 13N-amines as tracers.

The organ distribution and the metabolic fate of the 13N-amines 13N-beta-phenethylamine, 13N-n-octylamine, and 13N-3,4-dimethoxyphenethylamine, were studied in detail. After administration 13N-amines were rapidly transferred to tissues and oxidized by MAO. 13N-ammonia formed thereby was converted into amino acids (mainly glutamine by glutamine synthetase) and trapped. 13N-amines were found to be potential metabolic trapping tracers for the study of disposition and metabolism of amines.

Organ distribution and metabolism of [13N]nicotinamide in mice

The organ distribution and the metabolic fate of 13N[amide-13N]nicotinamide ([13N]NAM) were studied in order to evaluate its potential as a radiotracer. After administration, [13N]NAM is transported, mainly by simple diffusion, into the brain and heart, where part of the tracer is metabolized into hydrophilic compounds ([13N]NAD etc.) and trapped. A very high radioactivity is accumulated in the small intestine, probably due to bile duct excretion of the tracer and its metabolites. [13N]NAM was found to be a useful tracer for the study of the utilization of the vitamin, nicotinamide.

A new method for in vivo measurement of brain monoamine oxidase activity

The radiotracers, C-14-N-methylphenylethylamine (MPEA) and N-methylphenylethanolamine (MPEOA) both rapidly entered mouse brain after their intravenous injection and were metabolized by brain monoamine oxidase (MAO) to C-14-methylamine and corresponding aldehydes. The labelled metabolite was trapped in the brain. Measurement of radioactivity showed that the amount of the metabolite produced in the brain from C-14-MPEA was proportional to the MAO activity remaining after combined treatment with a specific MAO-A inhibitor, clorgyline and a MAO-B inhibitor, 1-deprenyl, but not by treatment with either inhibitor alone. The rate of production of the labelled metabolite produced from C-14-MPEOA was highly sensitive to the extent of inhibition of MAO-B activity (with phenylethylamine as substrate) by pretreatment with 1-deprenyl, but was relatively insensitive to inhibitor clorgyline. This selectivity suggests that MPEOA is a specific substrate of MAO-B in mouse brain in vivo. The above results indicate that C-14-labelled N-methylphenylethylamine and N-methylphenylethanolamine derivatives can be used for measurement of brain MAO activity and that C-14-MPEOA is a specific substrate for mouse brain MAO-B. The value and possible applications of this method for measurement of MAO-B in brain under different physiological conditions are discussed.

Positron CT imaging using a high resolution PCT device (Positologica-I),11CO,13NH3, and18FDG in clinical evaluation of cerebrovascular diseases

Positron computed tomography (PCT) was performed in 3 normal volunteers and 21 patients with cerebrovascular diseases using a high resolution PCT device ‘Positologica-I’ and three tracers11CO,13NH3, and18FDG. Relatively early lesions showed various accumulation patterns, and metabolism and perfusion mismatches were clearly shown by this measurement. One type of mismatch is luxury perfusion which had a slight increase of blood volume. Another type of uncoupling is misery perfusion. Remote effects of ischemic lesions also appeared on PCT with18FDG and13NH3. From our clinical results, the PCT method with a high resolution device and radiopharmaceuticals such as11CO,13NH3, and18FDG is very useful in the assessment of cerebrovascular diseases and in defining circulatory dysfunction in man.

Production system for 18F-2-deoxy-2-fluoro-D-glucose--a trial for automatic production.

We have developed a production system for 18F-2-deoxy-2-fluoro-D-glucose (18F-2FDG), which assures reliable production with easy handling and reduces radiation exposures to the operator. Chemical procedures in this system are the same as manual method developed in NIRS. This system has 2 operation modes; one is remote controlled manual operation mode and the other is microcomputer controlled automatic operation mode. In remote controlled mode, we tested this system 5 times and 18F-2FDG synthesized was supplied for clinical use once. The mean radiochemical yield of 18F-2FDG from the target gas recovery with decay time correction was 8%, that is the same as in the manual synthesis. It took about 2 hours from end of bombardment (EOB) to end of synthesis (EOS). Since this time is shorter than in manual synthesis, the available activity at EOS is increased.