Kuntarat Arunrungvichian

α₃β₄ Nicotinic acetylcholine receptor (nAChR) has been recognized as an emerging biomarker for the early detection of drug addiction. Herein, α₃β₄ nAChR ligands were designed and synthesized to improve the binding affinity and selectivity of two lead compounds, (S)-QND8 and (S)-T2, for the development of an α₃β₄ nAChR tracer. The structural modification was achieved by retaining the key features and expanding the molecular structure with a benzyloxy group to increase the lipophilicity for blood-brain barrier penetration and to extend the ligand-receptor interaction. The preserved key features are a fluorine atom for radiotracer development and a p-hydroxyl motif for ligand-receptor binding affinity. Four (R)- and (S)-quinuclidine-triazole (AK1-AK4) were synthesized and the binding affinity, together with selectivity to α₃β₄ nAChR subtype, were determined by competitive radioligand binding assay using [³H]epibatidine as a radioligand. Among all modified compounds, AK3 showed the highest binding affinity and selectivity to α₃β₄ nAChR with a K_i value of 3.18 nM, comparable to (S)-QND8 and (S)-T2 and 3069-fold higher affinity to α₃β₄ nAChR in comparison to α₇ nAChR. The α₃β₄ nAChR selectivity of AK3 was considerably higher than those of (S)-QND8 (11.8-fold) and (S)-T2 (294-fold). AK3 was shown to be a promising α₃β₄ nAChR tracer for further development as a radiotracer for drug addiction.

The selective binding of six (S)-quinuclidine-triazoles and their (R)-enantiomers to nicotinic acetylcholine receptor (nAChR) subtypes α3β4 and α7, respectively, were analyzed by in silico docking to provide the insight into the molecular basis for the observed stereospecific subtype discrimination. Homology modeling followed by molecular docking and molecular dynamics (MD) simulations revealed that unique amino acid residues in the complementary subunits of the nAChR subtypes are involved in subtype-specific selectivity profiles. In the complementary β4-subunit of the α3β4 nAChR binding pocket, non-conserved AspB173 through a salt bridge was found to be the key determinant for the α3β4 selectivity of the quinuclidine-triazole chemotype, explaining the 47–327-fold affinity of the (S)-enantiomers as compared to their (R)-enantiomer counterparts. Regarding the α7 nAChR subtype, the amino acids promoting a however significantly lower preference for the (R)-enantiomers were the conserved TyrA93, TrpA149 and TrpB55 residues. The non-conserved amino acid residue in the complementary subunit of nAChR subtypes appeared to play a significant role for the nAChR subtype-selective binding, particularly at the heteropentameric subtype, whereas the conserved amino acid residues in both principal and complementary subunits are essential for ligand potency and efficacy.

Objective The current study was conducted to improve the understanding of relationships between regional cortical amyloid load, glucose metabolism, cortical morphology (volume), and severity of clinical symptoms in patients with AD, MCI, and age-matched controls. Methods To objectivize the radiological evaluation of patients with suspected AD, head-to-head multi-modality imaging studies were conducted using MRI and PET/CT with [¹⁸F]FDG and [¹⁸F]AV45 for visualization and quantitation of brain morphology, glucose metabolism, and amyloid levels, respectively. A total of 84 subjects was studied, including 33 patients with AD, 31 patients with MCI, and 20 age-matched healthy controls (HC). A new quantitative index was calculated as a ratio of regional SUV of [¹⁸F]AV45 (normalized to cerebellar cortex) over the corresponding regional SUV of [¹⁸F]FDG, divided by the corresponding regional volume, measured from the co-registered MRI and normalized to the normal age-matched control group (AV45/FDG/NVol index). Relationships between clinical scores (TMSE, ADAS) and AV45/FDG/NVol indices for different structures of the brain in study groups were determined using linear regression analyses. Results A significant direct linear correlation was observed between the AV45/FDG/NVol index and ADAS-Cog test score and an inverse correlation with TMSE score at baseline and with the degree of changes in ADAS and TMSE scores assessed one year later (disease progression). The observed correlations between AV45/FDG/NVol index and clinical scores were higher than those with MRI-based cortical volumes, FDG SUV, or cerebellum-normalized AV45 SUV alone. Conclusions Current study demonstrated that AV45/FDG/NVol index mapping of the brain is a novel quantitative molecular imaging biomarker that correlates with clinical neurocognitive status and may facilitate more accurate diagnosis, staging, and prognosis of AD. Additional larger scale clinical studies are required to further evaluate the efficacy of this new quantitative index as a diagnostic and prognostic biomarker of AD as well as for the evaluation of safety and efficacy of novel agents undergoing clinical trials for therapy of AD.

An investigation on the selective binding of six quinuclidine-triazole enantiomeric pairs to nicotinic acetylcholine receptor (nAChR) subtypes, (S)-enantiomers for a3b4-nAChR and its (R)-counterpart for a7-nAChR, was performed in silico to provide the insight into the molecular basis for subtype discrimination of the quinuclidine-triazole enantiomers. The homology modeling and molecular docking analyses revealed that unique amino acid residues in the complementary subunit of nAChR subtypes are related to a high subtype selectivity profile. One non-conserved residue AspB173 in a complementary b4-subunit of the a3b4-nAChR binding pocket was found to be a primary determinant for the a3b4 selectivity of the quinuclidine-triazole chemotype as evidenced by the more pronounced enantioselectivity of (S)-enantiomers for the a3b4 nAChR, 47-326 times greater than its corresponding (R)-enantiomers. For (R)-enantiomers toward the a7 subtype, the interacting amino acid residues were the conserved TyrA93 and TrpA149 and TrpB55, leading to a lesser degree of stereoselectivity. The interaction with non-conserved amino acid residues in the complementary subunit of nAChR subtypes appeared to be the determinant for the nAChR subtype-selective binding, particularly at the heteropentameric subtype.