Jun Toyohara

α7 Nicotinic Receptor Agonists: Potential Therapeutic Drugs for Treatment of Cognitive Impairments in Schizophrenia and Alzheimer’s Disease

Accumulating evidence suggests that α7 nicotinic receptors (α7 nAChRs), a subtype of nAChRs, play a role in the pathophysiology of neuropsychiatric diseases, including schizophrenia and Alzheimer’s disease (AD). A number of psychopharmacological and genetic studies shown that α7 nAChRs play an important role in the deficits of P50 auditory evoked potential in patients with schizophrenia, and that (α nAChR agonists would be potential therapeutic drugs for cognitive impairments associated with P50 deficits in schizophrenia. Furthermore, some studies have demonstrated that α7 nAChRs might play a key role in the amyloid-β (Aβ)-mediated pathology of AD, and that α7 nAChR agonists would be potential therapeutic drugs for Aβ deposition in the brains of patients with AD. Interestingly, the altered expression of α7 nAChRs in the postmortem brain tissues from patients with schizophrenia and AD has been reported. Based on all these findings, selective α7 nAChR agonists can be considered potential therapeutic drugs for cognitive impairments in both schizophrenia and AD. In this article, we review the recent research into the role of α7 nAChRs in the pathophysiology of these diseases and into the potential use of novel α7 nAChR agonists as therapeutic drugs.

Preclinical and the first clinical studies on [11C]CHIBA-1001 for mapping α7 nicotinic receptors by positron emission tomography

Objective4-[11C]methylphenyl 2,5-diazabicyclo[3.2.2]nonane-2-carboxylate ([11C]CHIBA-1001), a 4-methyl-substituted derivative of the selective α7 nicotinic acetylcholine receptor (α7 nAChR) partial agonist 4-bromophenyl 1,4 diazabicyclo[3.2.2]nonane-4-carboxylate (SSR180711), is a potential radioligand for mapping α7 nAChRs in the brain by positron emission tomography (PET). In this study, we performed preclinical and first clinical PET studies using [11C]CHIBA-1001 for imaging α7 nAChRs in the human brain.Methods[11C]CHIBA-1001 was synthesized by methylation of the tributylstannyl precursor with [11C]CH3I in a palladium-promoted Stille cross-coupling reaction. The radiation absorbed-dose of [11C]CHIBA-1001 in humans was calculated from distribution data in mice. The acute toxicity of CHIBA-1001 at a dose of 3.20xa0mg/kg body weight, which is more than 41,000-fold the clinical equivalent dose of [11C]CHIBA-1001, was evaluated. The mutagenicity of CHIBA-1001 was studied by a reverse mutation test in Salmonella typhimurium (Ames test). Metabolite analysis in the mouse brain was carried out by high-performance liquid chromatography. The first clinical PET imaging of α7 nAChRs with [11C]CHIBA-1001 in a normal volunteer was also performed.ResultsA suitable preparation method for [11C]CHIBA-1001 injection was established. The radiation absorbed-dose by [11C]CHIBA-1001 in humans was low enough for clinical use, and no acute toxicity or mutagenicity of CHIBA-1001 was found. Most radioactivity in the mouse brain was detected as an unchanged form, although peripherally [11C]CHIBA-1001 was degraded. We successfully performed brain imaging by PET with [11C]CHIBA-1001 in a normal volunteer. A 90-min dynamic scan showed a rapid accumulation and gradual washout of radioactivity in the brain. The highest distribution volume of [11C]CHIBA-1001 was found in the thalamus; however, regional differences in brain radioactivity were small. Peripherally, [11C]CHIBA-1001 was stable in humans: >80% of the radioactivity in plasma was detected as the unchanged form for 60xa0min.ConclusionsThese results demonstrate that [11C]CHIBA-1001 is a suitable radioligand to use in clinical trials for imaging α7 nAChRs in the human brain, providing acceptable dosimetry and pharmacological safety at the dose required for adequate PET imaging.

High occupancy of σ1 receptors in the human brain after single oral administration of donepezil: a positron emission tomography study using [11C]SA4503

The acetylcholinesterase (AChE) inhibitor donepezil is also a sigma1 receptor agonist. We examined whether donepezil binds to sigma1 receptors in the living human brain after a single oral administration. Dynamic positron emission tomography (PET) data acquisition using the selective sigma1 receptor ligand [11C]SA4503 was performed to evaluate quantitatively the binding of [11C]SA4503 to sigma1 receptors in eight healthy male volunteers. Each subject had a PET scan before and after receiving a single dose of donepezil (5 or 10 mg). The binding potential of [11C]SA4503 was calculated. Doses of 5 mg and 10 mg donepezil bound to sigma1 receptors in the human brain with occupancies of approximately 60% and approximately 75%, respectively, in a dose-dependent manner. This study demonstrated that donepezil binds to sigma1 receptors in the living human brain at therapeutic doses. Therefore, sigma1 receptors may be implicated in the pharmacological mechanism of donepezil in the human brain.

[11C]Gefitinib ([11C]Iressa): Radiosynthesis, In Vitro Uptake, and In Vivo Imaging of Intact Murine Fibrosarcoma

ObjectiveGefitinib (N-(3-chloro-4-fluorophenyl)-7-methoxy-6-[3-(morpholin-4-yl)propoxy]quinazolin-4-amine, Iressa) is an approved anticancer drug. In this study, we labeled gefitinib with carbon-11 and evaluated [11C]gefitinib to explore its specific binding in intact fibrosarcoma (NFSa)-bearing mice.Methods[11C]Gefitinib was synthesized by the reaction of desmethyl precursor (1) with [11C]CH3I. In vitro uptake of [11C]gefitinib into NFSa, human-A431 epidermoid carcinoma, and Jurkat T cells was determined. Positron emission tomography (PET) imaging using [11C]gefitinib was performed for NFSa-bearing mice.Results[11C]Gefitinib accumulated into NFSa cells with 2.1 uptake ratio (UR)/mg protein in cells. Addition of nonradioactive gefitinib decreased uptake in a concentration-dependent manner. [11C]Gefitinib also had high uptake (2.6 UR/mg protein) into epidermal growth factor receptor/tyrosine kinase (EGFR/TK)-rich A431 cells but low uptake (0.2 UR/mg protein) into EGFR/TK-poor Jurkat cells. In vivo distribution study on NFSa-bearing mice by the dissection method revealed that [11C]gefitinib specifically accumulated into the tumor. The ratio of radioactivity in tumors to that in blood and muscle as two comparative regions increased from 0.4 to 6.0 and from 0.6 to 5.0 during this experiment (0–60xa0min), respectively. PET for NFSa-bearing mice produced a clear tumor image, although high radioactivity was distributed throughout the body. Treatment with nonradioactive gefitinib (100xa0mg/kg) decreased uptake in the tumor. In vivo metabolite analysis demonstrated that [11C]gefitinib was stable in the tumor, liver, kidney, and blood.ConclusionThese results demonstrated the promising potential of [11C]gefitinib to serve as a PET ligand for in vivo imaging of NFSa-bearing mice.

Biodistribution and radiation dosimetry of the α7 nicotinic acetylcholine receptor ligand [11C]CHIBA-1001 in humans

INTRODUCTIONn4-[(11)C]Methylphenyl 2,4-diazabicyclo[3.2.2]nonane-2-carboxylate ([(11)C]CHIBA-1001) is a newly developed positron emission tomography (PET) ligand for mapping α(7) nicotinic acetylcholine receptors. We investigated whole-body biodistribution and radiation dosimetry of [(11)C]CHIBA-1001 in humans and compared the results with those obtained in mice.nnnMETHODSnDynamic whole-body PET was carried out for three human subjects after administering a bolus injection of [(11)C]CHIBA-1001. Emission scans were collected in two-dimensional mode over five bed positions. Regions of interest were placed over 12 organs. Radiation dosimetry was estimated from the residence times of these source organs using the OLINDA program. Biodistribution data from mice were also used for the prediction of radiation dosimetry in humans, and results with and those without accommodation of different proportions of organ-to-total-body mass were compared with the results from the human PET study.nnnRESULTSnIn humans, the highest accumulation was observed in the liver, whereas in mice, the highest accumulation was observed in the urinary bladder. The estimated effective dose from the human PET study was 6.9 μSv/MBq, and that from mice was much underestimated.nnnCONCLUSIONnEffective dose estimates for [(11)C]CHIBA-1001 were compatible with those associated with other common nuclear medicine tests. Absorption doses among several organs were considerably different between the human and mouse studies. Human dosimetry studies for the investigation of radiation safety are desirable as one of the first clinical trials of new PET probes before their application in subsequent clinical investigations.

In Vivo Evaluation of α7 Nicotinic Acetylcholine Receptor Agonists [11C]A-582941 and [11C]A-844606 in Mice and Conscious Monkeys

Background The α7 nicotinic acetylcholine receptors (nAChRs) play an important role in the pathophysiology of neuropsychiatric diseases such as schizophrenia and Alzheimers disease. The goal of this study was to evaluate the two carbon-11-labeled α7 nAChR agonists [11C]A-582941 and [11C]A-844606 for their potential as novel positron emission tomography (PET) tracers. Methodology/Principal Findings The two tracers were synthesized by methylation of the corresponding desmethyl precursors using [11C]methyl triflate. Effects of receptor blockade in mice were determined by coinjection of either tracer along with a carrier or an excess amount of a selective α7 nAChR agonist (SSR180711). Metabolic stability was investigated using radio-HPLC. Dynamic PET scans were performed in conscious monkeys with/without SSR180711-treatment. [11C]A-582941 and [11C]A-844606 showed high uptake in the mouse brain. Most radioactive compounds in the brain were detected as an unchanged form. However, regional selectivity and selective receptor blockade were not clearly observed for either compound in the mouse brain. On the other hand, the total distribution volume of [11C]A-582941 and [11C]A-844606 was high in the hippocampus and thalamus but low in the cerebellum in the conscious monkey brain, and reduced by pretreatment with SSR180711. Conclusions/Significance A nonhuman primate study suggests that [11C]A-582941 and [11C]A-844606 would be potential PET ligands for imaging α7 nAChRs in the human brain.

Characterization of [3H]CHIBA-1001 binding to α7 nicotinic acetylcholine receptors in the brain from rat, monkey, and human

Accumulating evidence suggests that the alpha7 subtype of nicotinic acetylcholine receptors (nAChRs) plays a role in the pathophysiology of neuropsychiatric diseases, including schizophrenia and Alzheimers disease. Currently, there are no suitable small molecule radioligands for alpha7 nAChRs in the brain, although [(125)I]alpha-bungarotoxin has been widely used as a radioligand for alpha7 nAChRs. In the present study, we characterized a new radioligand, 4-[(3)H]methylphenyl 2,5-diazabicyclo[3.2.2]nonane-2-carboxylate ([(3)H]CHIBA-1001), a derivative of the selective alpha7 nAChR agonist SSR180711, in brain membranes from rat, monkey, and human. Scatchard analysis revealed an apparent equilibrium dissociation constant (Kd) of 193.4nM in rat brain membranes at 4 degrees C, and the maximal number of binding sites (Bmax) was 346.2fmol/mg protein. The order of drugs for the inhibition of [(3)H]CHIBA-1001 binding to rat brain membranes is SSR180711>A-844606>MG624>epibatidine>DMAB>A-582941, suggesting a similarity of alpha7 nAChR pharmacological profiles. In contrast, alpha-bungarotoxin, MLA, and nicotine were found to be very weak. The distribution of [(3)H]CHIBA-1001 binding to crude membranes from dissected regions of rat, monkey, and human brain was different from that of [(125)I]alpha-bungarotoxin binding, suggesting that [(3)H]CHIBA-1001 binding sites may not be identical to [(125)I]alpha-bungarotoxin binding in the brain. In summary, [(3)H]CHIBA-1001 would be a useful radioligand for alpha7 nAChRs in the brains of rodents, non-human primates, and humans.

Comparison of conventional and novel PET tracers for imaging mesothelioma in nude mice with subcutaneous and intrapleural xenografts.

INTRODUCTIONnMalignant mesothelioma is a highly aggressive tumor originating in the pleura, peritoneum and pericardium, and the prognosis of patients undergoing current treatment remains poor. To develop new therapies, it is important to have a noninvasive imaging system for evaluating the efficacy of such prospective treatments. We have established clinically relevant mouse models and evaluated conventional and novel positron emission tomography (PET) tracers.nnnMETHODSnEpithelioid and sarcomatoid mesothelioma cells were inoculated subcutaneously and intrapleurally into nude mice. Biodistribution and PET imaging studies were conducted by injecting [(18)F]fluoro-2-deoxy-D-glucose (FDG), 3-[(18)F]fluoro-3-doxythymidine (FLT) or 4-methyl-[(11)C]thiothymidine (S-dThd) into the mouse models. In vitro cellular uptake of [(14)C]FDG and [(3)H]FLT and thymidine kinase 1 (TK(1)) activity in both cell lines were measured. Expression of glucose transporter 1 (GLUT-1) and Ki-67 in xenografted tumors was evaluated by immunohistochemical staining.nnnRESULTSnIn epithelioid mesothelioma models, biodistribution experiments showed that tumor uptake of [(11)C]S-dThd was significantly higher than that of [(18)F]FDG. On the other hand, in sarcomatoid models, [(18)F]FDG showed significantly higher accumulation than the other two tracers. These differential uptakes of the three tracers were confirmed by PET imaging. The cellular uptake of [(14)C]FDG and [(3)H]FLT and TK(1) activity in sarcomatoid cells were higher than those of epithelioid cells. GLUT-1 protein was strongly expressed in sarcomatoid but not in epithelioid tumor. We observed a high percentage of Ki-67-positive cells in both epithelioid and sarcomatoid tumors.nnnCONCLUSIONSnWe established nude mouse models of epithelioid and sarcomatoid subtypes of mesothelioma. PET tracers applicable for the evaluation of epithelioid and sarcomatoid mesothelioma would vary: [(18)F]FLT and [(11)C]S-dThd seemed suitable for the epithelioid subtype and [(18)F]FDG seemed suitable for the sarcomatoid subtype in our mouse models. Our results indicated that cellular uptake and TK(1) activity in vitro are not always consistent with tracer uptake of [(18)F]FLT and [(11)C]S-dThd in vivo. These mouse models and PET imaging might be useful tools for evaluating new and effective treatments in mesothelioma.

In vivo evaluation of carbon-11-labelled non-sarcosine-based glycine transporter 1 inhibitors in mice and conscious monkeys.

INTRODUCTIONnGlycine transporter 1 (GlyT-1) is an attractive target in positron emission tomography (PET) studies. Here, we report the in vivo evaluation of three carbon-11-labelled non-sarcosine-type GlyT-1 inhibitors--[(11)C]SA1, [(11)C]SA2 and [(11)C]SA3--as novel PET tracers for GlyT-1.nnnMETHODSnThe regional brain distributions of the three compounds in mice were studied at baseline and under receptor-blockade conditions with co-injection of carrier loading or pretreatment with an excess of selective GlyT-1 inhibitors (ALX-5407 and SSR504734). Metabolic stability was investigated by radio high-performance liquid chromatography. Dynamic PET scans in conscious monkeys were performed with/without selective GlyT-1 inhibitors.nnnRESULTSnThe IC(50) values of SA1, SA2 and SA3 were 9.0, 6400 and 39.7 nM, respectively. The regional brain uptakes of [(11)C]SA1 and [(11)C]SA3 in mice were heterogeneous and consistent with the known distribution of GlyT-1. [(11)C]SA2 showed low and homogeneous uptake in the brain. Most radioactivity in the brain was detected in unchanged form, although peripherally these compounds were degraded. Carrier loading decreased the uptake of [(11)C]SA1 in GlyT-1-rich regions. However, similar reductions were not observed with [(11)C]SA3. Pretreatment with ALX-5407 decreased the uptake of [(11)C]SA1 in GlyT-1-rich regions. In the monkey at baseline, regional brain uptake of [(11)C]SA1 was heterogeneous and consistent with the known GlyT-1 distribution. Pretreatment with selective GlyT-1 inhibitors significantly decreased the distribution volume ratio of [(11)C] SA1 in GlyT-1-rich regions.nnnCONCLUSIONSn[(11)C]SA1 has the most suitable profile among the three carbon-11-labelled GlyT-1 inhibitors. Lead optimization of [(11)C]SA1 structure will be required to achieve in vivo selective GlyT-1 imaging.

Development of PET radiopharmaceuticals and their clinical applications at the Positron Medical Center.

The Positron Medical Center has developed a large number of radiopharmaceuticals and 36 radiopharmaceuticals have been approved for clinical use for studying aging and geriatric diseases, especially brain functions. Positron emission tomography (PET) has been used to provide a highly advanced PET‐based diagnosis. The current status of the development of radiopharmaceuticals, and representative clinical and methodological results are reviewed. Geriatr Gerontol Int 2010; 10 (Suppl. 1): S180–S196.