Publications -- Robert Luedtke

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This collection is limited to articles published under the terms of a creative commons license or other open access publishing agreement since 2016. It is not intended as a complete list of the author's works.


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    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.
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    Design and Synthesis of D(3)R Bitopic Ligands with Flexible Secondary Binding Fragments: Radioligand Binding and Computational Chemistry Studies
    (MDPI, 2024-01-11) Tian, Gui-Long; Hsieh, Chia-Ju; Taylor, Michelle; Lee, Ji Youn; Luedtke, Robert R.; Mach, Robert H.
    A series of bitopic ligands based on Fallypride with a flexible secondary binding fragment (SBF) were prepared with the goal of preparing a D(3)R-selective compound. The effect of the flexible linker ((R,S)-trans-2a-d), SBFs ((R,S)-trans-2h-j), and the chirality of orthosteric binding fragments (OBFs) ((S,R)-trans-d, (S,R)-trans-i, (S,S)-trans-d, (S,S)-trans-i, (R,R)-trans-d, and (R,R)-trans-i) were evaluated in in vitro binding assays. Computational chemistry studies revealed that the interaction of the fragment binding to the SBF increased the distance between the pyrrolidine nitrogen and ASP110(3.32) of the D(3)R, thereby reducing the D(3)R affinity to a suboptimal level.
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    Selective Activation of D3 Dopamine Receptors Ameliorates DOI-Induced Head Twitching Accompanied by Changes in Corticostriatal Processing
    (MDPI, 2023-06-10) Estrada-Sanchez, Ana M.; Rangel-Barajas, Claudia; Howe, Andrew G.; Barton, Scott J.; Mach, Robert H.; Luedtke, Robert R.; Rebec, George V.
    D3 receptors, a key component of the dopamine system, have emerged as a potential target of therapies to improve motor symptoms across neurodegenerative and neuropsychiatric conditions. In the present work, we evaluated the effect of D3 receptor activation on the involuntary head twitches induced by 2,5-dimethoxy-4-iodoamphetamine (DOI) at behavioral and electrophysiological levels. Mice received an intraperitoneal injection of either a full D3 agonist, WC 44 [4-(2-fluoroethyl)-N-[4-[4-(2-methoxyphenyl)piperazin 1-yl]butyl]benzamide] or a partial D3 agonist, WW-III-55 [N-(4-(4-(4-methoxyphenyl)piperazin-1-yl)butyl)-4-(thiophen-3-yl)benzamide] five minutes before the intraperitoneal administration of DOI. Compared to the control group, both D3 agonists delayed the onset of the DOI-induced head-twitch response and reduced the total number and frequency of the head twitches. Moreover, the simultaneous recording of neuronal activity in the motor cortex (M1) and dorsal striatum (DS) indicated that D3 activation led to slight changes in a single unit activity, mainly in DS, and increased its correlated firing in DS or between presumed cortical pyramidal neurons (CPNs) and striatal medium spiny neurons (MSNs). Our results confirm the role of D3 receptor activation in controlling DOI-induced involuntary movements and suggest that this effect involves, at least in part, an increase in correlated corticostriatal activity. A further understanding of the underlying mechanisms may provide a suitable target for treating neuropathologies in which involuntary movements occur.
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    Design and Synthesis of Conformationally Flexible Scaffold as Bitopic Ligands for Potent D(3)-Selective Antagonists
    (MDPI, 2023-01-09) Kim, Ho Young; Lee, Ji Youn; Hsieh, Chia-Ju; Taylor, Michelle; Luedtke, Robert R.; Mach, Robert H.
    Previous studies have confirmed that the binding of D(3) receptor antagonists is competitively inhibited by endogenous dopamine despite excellent binding affinity for D(3) receptors. This result urges the development of an alternative scaffold that is capable of competing with dopamine for binding to the D(3) receptor. Herein, an SAR study was conducted on metoclopramide that incorporated a flexible scaffold for interaction with the secondary binding site of the D(3) receptor. The alteration of benzamide substituents and secondary binding fragments with aryl carboxamides resulted in excellent D(3) receptor affinities (Ki = 0.8-13.2 nM) with subtype selectivity to the D(2) receptor ranging from 22- to 180-fold. The beta-arrestin recruitment assay revealed that 21c with 4-(pyridine-4-yl)benzamide can compete well against dopamine with the highest potency (IC(50) = 1.3 nM). Computational studies demonstrated that the high potency of 21c and its analogs was the result of interactions with the secondary binding site of the D(3) receptor. These compounds also displayed minimal effects for other GPCRs except moderate affinity for 5-HT(3) receptors and TSPO. The results of this study revealed that a new class of selective D(3) receptor antagonists should be useful in behavioral pharmacology studies and as lead compounds for PET radiotracer development.
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    Ligand with Two Modes of Interaction with the Dopamine D2 Receptor-An Induced-Fit Mechanism of Insurmountable Antagonism
    (ACS Publications, 2020-09-15) Agren, Richard; Zeberg, Hugo; Stepniewski, Tomasz Maciej; Free, R. Benjamin; Reilly, Sean W.; Luedtke, Robert R.; Arhem, Peter; Ciruela, Francisco; Sibley, David R.; Mach, Robert H.; Selent, Jana; Nilsson, Johanna; Sahlholm, Kristoffer
    A solid understanding of the mechanisms governing ligand binding is crucial for rational design of therapeutics targeting the dopamine D2 receptor (D2R). Here, we use G protein-coupled inward rectifier potassium (GIRK) channel activation in Xenopus oocytes to measure the kinetics of D2R antagonism by a series of aripiprazole analogues, as well as the recovery of dopamine (DA) responsivity upon washout. The aripiprazole analogues comprise an orthosteric and a secondary pharmacophore and differ by the length of the saturated carbon linker joining these two pharmacophores. Two compounds containing 3- and 5-carbon linkers allowed for a similar extent of recovery from antagonism in the presence of 1 or 100 muM DA (>25 and >90% of control, respectively), whereas recovery was less prominent ( approximately 20%) upon washout of the 4-carbon linker compound, SV-III-130, both with 1 and 100 muM DA. Prolonging the coincubation time with SV-III-130 further diminished recovery. Curve-shift experiments were consistent with competition between SV-III-130 and DA. Two mutations in the secondary binding pocket (V91A and E95A) of D2R decreased antagonistic potency and increased recovery from SV-III-130 antagonism, whereas a third mutation (L94A) only increased recovery. Our results suggest that the secondary binding pocket influences recovery from inhibition by the studied aripiprazole analogues. We propose a mechanism, supported by in silico modeling, whereby SV-III-130 initially binds reversibly to the D2R, after which the drug-receptor complex undergoes a slow transition to a second ligand-bound state, which is dependent on secondary binding pocket integrity and irreversible during the time frame of our experiments.
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    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.
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    Evaluation of Substituted N-Phenylpiperazine Analogs as D3 vs. D2 Dopamine Receptor Subtype Selective Ligands
    (MDPI, 2021-05-26) Lee, Boeun; Taylor, Michelle; Griffin, Suzy A.; McInnis, Tamara; Sumien, Nathalie; Mach, Robert H.; Luedtke, Robert R.
    N-phenylpiperazine analogs can bind selectively to the D3 versus the D2 dopamine receptor subtype despite the fact that these two D2-like dopamine receptor subtypes exhibit substantial amino acid sequence homology. The binding for a number of these receptor subtype selective compounds was found to be consistent with their ability to bind at the D3 dopamine receptor subtype in a bitopic manner. In this study, a series of the 3-thiophenephenyl and 4-thiazolylphenyl fluoride substituted N-phenylpiperazine analogs were evaluated. Compound 6a was found to bind at the human D3 receptor with nanomolar affinity with substantial D3 vs. D2 binding selectivity (approximately 500-fold). Compound 6a was also tested for activity in two in-vivo assays: (1) a hallucinogenic-dependent head twitch response inhibition assay using DBA/2J mice and (2) an L-dopa-dependent abnormal involuntary movement (AIM) inhibition assay using unilateral 6-hydroxydopamine lesioned (hemiparkinsonian) rats. Compound 6a was found to be active in both assays. This compound could lead to a better understanding of how a bitopic D3 dopamine receptor selective ligand might lead to the development of pharmacotherapeutics for the treatment of levodopa-induced dyskinesia (LID) in patients with Parkinson's disease.