Browsing by Author "Riad, Aladdin"
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Item In vitro characterization of [(125)I]HY-3-24, a selective ligand for the dopamine D3 receptor(Frontiers Media S.A., 2024-04-24) Lee, Ji Youn; Kim, Ho Young; Martorano, Paul; Riad, Aladdin; Taylor, Michelle; Luedtke, Robert R.; Mach, Robert H.INTRODUCTION: Dopamine D3 receptor (D3R) ligands have been studied for the possible treatment of neurological and neuropsychiatric disorders. However, selective D3R radioligands for in vitro binding studies have been challenging to identify due to the high structural similarity between the D2R and D3R. In a prior study, we reported a new conformationally-flexible benzamide scaffold having a high affinity for D3R and excellent selectivity vs. D2R. In the current study, we characterized the in vitro binding properties of a new radioiodinated ligand, [(125)I]HY-3-24. METHODS: In vitro binding studies were conducted in cell lines expressing D3 receptors, rat striatal homogenates, and rat and non-human primate (NHP) brain tissues to measure regional brain distribution of this radioligand. RESULTS: HY-3-24 showed high potency at D3R (K(i) = 0.67 +/- 0.11 nM, IC(50) = 1.5 +/- 0.58 nM) compared to other D2-like dopamine receptor subtypes (D2R K(i) = 86.7 +/- 11.9 nM and D4R K(i) > 1,000). The K(d) (0.34 +/- 0.22 nM) and B(max) (38.91 +/- 2.39 fmol/mg) values of [(125)I]HY-3-24 were determined. In vitro binding studies in rat striatal homogenates using selective D2R and D3R antagonists confirmed the D3R selectivity of [(125)I]HY-3-24. Autoradiography results demonstrated that [(125)I]HY-3-24 specifically binds to D3Rs in the nucleus accumbens, islands of Calleja, and caudate putamen in rat and NHP brain sections. CONCLUSION: These results suggest that [(125)I]HY-3-24 appears to be a novel radioligand that exhibits high affinity binding at D3R, with low binding to other D2-like dopamine receptors. It is anticipated that [(125)I]HY-3-24 can be used as the specific D3R radioligand.Item Interaction of Ligands for PET with the Dopamine D3 Receptor: In Silico and In Vitro Methods(MDPI, 2021-04-02) Hsieh, Chia-JI; Riad, Aladdin; Lee, Ji Youn; Sahlholm, Kristoffer; Xu, Kuiying; Luedtke, Robert R.; Mach, Robert H.[(18)F]Fallypride and [(18)F]Fluortriopride (FTP) are two different PET radiotracers that bind with sub-nanomolar affinity to the dopamine D3 receptor (D3R). In spite of their similar D3 affinities, the two PET ligands display very different properties for labeling the D3R in vivo: [(18)F]Fallypride is capable of binding to D3R under "baseline" conditions, whereas [(18)F]FTP requires the depletion of synaptic dopamine in order to image the receptor in vivo. These data suggest that [(18)F]Fallypride is able to compete with synaptic dopamine for binding to the D3R, whereas [(18)F]FTP is not. The goal of this study was to conduct a series of docking and molecular dynamic simulation studies to identify differences in the ability of each molecule to interact with the D3R that could explain these differences with respect to competition with synaptic dopamine. Competition studies measuring the ability of each ligand to compete with dopamine in the beta-arrestin assay were also conducted. The results of the in silico studies indicate that FTP has a weaker interaction with the orthosteric binding site of the D3R versus that of Fallypride. The results of the in silico studies were also consistent with the IC50 values of each compound in the dopamine beta-arrestin competition assays. The results of this study indicate that in silico methods may be able to predict the ability of a small molecule to compete with synaptic dopamine for binding to the D3R.