In Vivo Evaluation of 11C-labeled Three Radioligands for Glycine Transporter 1 in the Mouse Brain

OBJECTIVE: Glycine transporter 1 (GlyT-1) is one of the most attractive therapeutic targets for schizophrenia. There is great interest in developing radioligands for in vivo imaging of GlyT-1 in the brain using positron emission tomography. Here, we report the properties of three novel non-sarcosine-based radioligands [11C]CHIBA-3007, [11C]CHIBA-3009, and [11C]CHIBA-3011, for GlyT-1 imaging in the mouse brain in vivo. METHODS: The three radioligands were synthesized by N-[11C] methylation of the corresponding desmethyl precursor. A pharmacological characterization of these radioligands for in vivo imaging of GlyT-1 in the brain was conducted using male ddY mice. RESULTS: [11C]CHIBA-3009 and [11C]CHIBA-3011 were scarcely incorporated into the brain, whereas [11C]CHIBA-3007 showed slight but considerable brain uptake. Regional brain uptake of [11C]CHIBA-3007 (medulla oblongata>cerebellum>cortex) was similar to the distribution of the GlyT-1 protein. However, pretreatment with CHIBA-3007 (1 mg/kg) or the GlyT-1 selective inhibitor ALX5407 (N-[(3R)-3-([1,1'-Biphenyl]-4-yloxy)-3-(4-fluorophenyl)propyl]-N-methylglycine) (30 mg/kg) did not significantly decrease brain uptake of [11C]CHIBA-3007, suggesting low specific binding to GlyT-1. Pretreatment with cyclosporin A significantly increased brain uptake of [11C]CHIBA-3009 and [11C]CHIBA-3011, suggesting a role for P-glycoprotein in the brain uptake of these ligands. All three radioligands were rapidly degraded intact forms were 3-18% in plasma and 15-74% in the brain at 15 min after injection. CONCLUSION: The results suggest that these three radioligands are not suitable for in vivo imaging of GlyT-1 in the brain because of low brain uptake and rapid metabolism. Further structural refinement is necessary to enhance brain uptake.

Occupancy of alpha7 Nicotinic Acetylcholine Receptors in the Brain by Tropisetron: A Positron Emission Tomography Study Using 11CCHIBA-1001 in Healthy Human Subjects

OBJECTIVE: Agonists of alpha7-nicotinic acetylcholine receptors (nAChRs) have been developed as potential therapeutic drugs for neuropsychiatric diseases such as schizophrenia and Alzheimer's disease. Positron emission tomography (PET) is a noninvasive brain imaging technique to measure receptor occupancy in the living human brain. Although much effort has been expended to create specific PET radioligands for alpha7-nAChRs in the brain, only 4-[11C]methylphenyl-1,4-diazabicyclo[3.2.2.]nonane-4-carboxylate ([11C]CHIBA-1001) is currently available for clinical studies. In contrast, two 5-hydroxytryptamine-3 (5-HT3) receptor antagonists, tropisetron and ondansetron, have been used to treat patients with chemotherapy-induced or postoperative nausea and vomiting. Furthermore, tropisetron, but not ondansetron, possesses high affinity for alpha7-nAChRs. In the present study, we evaluated the receptor occupancy in the human brain after a single oral administration of tropisetron and ondansetron using [11C]CHIBA-1001 and PET. METHODS: Two serial dynamic PET scans using [11C]CHIBA-1001 in healthy non-smoking male subjects were performed before and after receiving an oral administration of these medications. RESULTS: A single oral administration of tropisetron, but not ondansetron, decreased the total distribution volume of [11C]CHIBA-1001 in the human brain. CONCLUSION: This study shows that tropisetron, but not ondansetron, could bind to alpha7-nAChRs in the human brain after a single oral administration. Therefore, [11C]CHIBA-1001 may be a useful PET radioligand to measure the occupancy of alpha7-nAChRs in the human brain.