Mapping and fitting the peripheral benzodiazepine receptor binding site by carboxamide derivatives. Comparison of different approaches to quantitative ligand-receptor interaction modeling

Engineering of Phenylpyridine- and Bipyridine-Based Covalent Organic Frameworks for Photocatalytic Tandem Aerobic Oxidation/Povarov Cyclization

Covalent organic frameworks (COFs) are emerging as a new class of photoactive organic semiconductors, which possess crystalline ordered structures and high surface areas. COFs can be tailor-made toward specific (photocatalytic) applications, and the size and position of their band gaps can be tuned by the choice of building blocks and linkages. However, many types of building blocks are still unexplored as photocatalytic moieties and the scope of reactions photocatalyzed by COFs remains quite limited. In this work, we report the synthesis and application of two bipyridine- or phenylpyridine-based COFs: TpBpyCOF and TpPpyCOF. Due to their good photocatalytic properties, both materials were applied as metal-free photocatalysts for the tandem aerobic oxidation/Povarov cyclization and α-oxidation of N-aryl glycine derivatives, with the bipyridine-based TpBpyCOF exhibiting the highest activity. By expanding the range of reactions that can be photocatalyzed by COFs, this work paves the way toward the more widespread application of COFs as metal-free heterogeneous photocatalysts as a convenient alternative for commonly used homogeneous (metal-based) photocatalysts.

Acetamidobenzoxazolone scaffold as a promising translocator protein (18 kDa, TSPO) marker for neuroinflammation imaging: Advancement in last decennial period

Inflammation has been linked to the onset and progression of a wide range of neuropathological disorders. The well-conserved outer mitochondrial membrane 18 kDa translocator protein (TSPO) is perceived as an in vivo neuroinflammation marker. A dearth of a reference region, genetic disparity influencing the ligand's affinity for TSPO, and a substantial signal in the endothelium of the brain veins contributes toward complications in quantifying TSPO positron emission tomography (PET) image. Up to the present time several radiotracers based on different pharmacophore such as (R)[¹¹ C]PK11195, [¹⁸ F]DPA714, [¹¹ C]PBR28, [¹¹ C]ER176, and many more have been recognized for envisaging the prominent TSPO level observed in neurological conditions. Recently acetamidobenzoxazolone (ABO) scaffold, a bicyclic ring system composed of a phenyl ring fused to a carbamate and its substituted radiolabelled analogues especially at C-5 position has evidenced encouraging outcomes as next generation of TSPO PET ligands. Diverse ABO framework-based TSPO ligands have been designed embracing imperative aspects such as lipophilicity, metabolic profile, and capability to penetrate the blood-brain barrier apart from least effect of polymorphism (rs6971). Over the years numerous systematic literature reviews compiling different structural class of TSPO ligands characterized on the grounds of their binding affinity and metabolite profile have been reported but none is especially focused toward a fascinating benzoxazolone scaffold. This review exclusively bestows an overview of the recent advancements on ABO derivatives with neuroinflammation imaging potential and emphases on the structural features accountable for visualizing TSPO in-vivo with collation of published reports during last 10 years.

Exploring Translocator Protein (TSPO) Medicinal Chemistry: An Approach for Targeting Radionuclides and Boron Atoms to Mitochondria

Translocator protein 18 kDa [TSPO or peripheral-type benzodiazepine receptor (PBR)] was identified in the search of binding sites for benzodiazepine anxiolytic drugs in peripheral regions. In these areas, binding sites for TSPO ligands were recognized in steroid-producing tissues. TSPO plays an important role in many cellular functions, and its coding sequence is highly conserved across species. TSPO is located predominantly on the membrane of mitochondria and is overexpressed in several solid cancers. TSPO basal expression in the CNS is low, but it becomes high in neurodegenerative conditions. Thus, TSPO constitutes not only as an outstanding drug target but also as a valuable marker for the diagnosis of a number of diseases. The aim of the present article is to show the lesson we have learned from our activity in TSPO medicinal chemistry and in approaching the targeted delivery to mitochondria by means of TSPO ligands.

The Ligands of Translocator Protein: Design and Biological Properties

In 2020, it is already 43 years since Braestrup and Squires discovered 18 kDa translocator protein (TSPO), known until 2006 as "peripheral benzodiazepine receptor". During this time, the functions of this receptor, which is located on the outer membrane of mitochondria, were studied in detail. One of the key functions of TSPO is the transfer of cholesterol from the outer to the inner mitochondrial membrane, which is the limiting stage in the synthesis of neurosteroids. TSPO is also involved in the transport of porphyrins, mitochondrial respiration, the opening of mitochondrial pores, apoptosis and cell proliferation. This review presents current information on the structure of TSPO, the mechanism of its participation in neurosteroidogenesis, as well as endogenous and synthetic TSPO ligands. Particular emphasis is placed on the analysis of approaches to the design of synthetic ligands and their neuropsychotropic activity in vitro and in vivo. The presented review demonstrates the promise of constructing new neuropsychotropic drugs in the series of TSPO ligands.

Design, Synthesis and Anxiolytic Activity Evaluation of N-Acyltryptophanyl- Containing Dipeptides, Potential TSPO Ligands#

Background:

The 18 kDa translocator protein (TSPO), previously known as the peripheral- type benzodiazepine receptor, plays a key role for the synthesis of neurosteroids by promoting transport of cholesterol from the outer to the inner mitochondrial membrane, which is the ratelimiting step in neurosteroid biosynthesis. Neurosteroids interact with nonbenzodiazepine site of GABAa receptor causing an anxiolytic effect without the side effects.

Methods:

Using the original peptide drug-based design strategy, the first putative dipeptide ligand of the TSPO N-carbobenzoxy-L-tryptophanyl-L-isoleucine amide (GD-23) was obtained. Molecular docking of GD-23 in the active pocket of the TSPO receptor using Glide software was carried out. The lead compounds GD-23 and its analogues were synthesized using activated succinimide esters coupling method. The anxiolytic activity of GD-23 and its analogues was investigated in vivo, using two validated behavioral tests, illuminated open field and elevated plus-maze.

Results:

he in vivo studies revealed that the following parameters are necessary for the manifestation of anxiolytic activity of new compounds: the L-configuration of tryptophan, the presence of an amide group at the C-terminus, the specific size of the N-acyl substituent at the Nterminus. Compound GD-23 (N-carbobenzoxy-L-tryptophanyl-L-isoleucine amide) demonstrated a high anxiolytic-like effect in the doses of 0.05–1.0 mg/kg i.p. comparable with that of diazepam. Compound GD-23 was also active in the open field test when was administered orally in the doses of 0.1-5.0 mg/kg. The involvement of TSPO receptor in the mechanism of anxiolytic-like activity of new compounds was proved by the antagonism of compound GD-23 with TSPO selective inhibitor PK11195 as well as with inhibitors of enzymes which are involved in the biosynthesis of neurosteroids, trilostane and finasteride.

Conclusion:

A series of N-acyl-tryptophanyl-containing dipeptides were designed and synthesized as 18 kDa translocator protein (TSPO) ligands. Using a drug-based peptide design method a series of the first dipeptide TSPO ligands have been designed and synthesized and their anxiolytic activity has been evaluated. In general, some of the compounds displayed a high level of anxiolytic efficacy comparable with that of diazepam. The involvement of TSPO receptor in the mechanism of anxiolytic activity of new compounds was proved using two methods. On this basis, the N-acyl-Ltryptophanyl- isoleucine amides could potentially be a novel class of TSPO ligands with anxiolytic activity.

Oxone promoted dehydrogenative Povarov cyclization of N-aryl glycine derivatives: an approach towards quinoline fused lactones and lactams

Oxone promoted intramolecular dehydrogenative imino Diels–Alder reaction (Povarov cyclization) of alkyne tethered N-aryl glycine esters and amides has been explored, thus affording biologically significant quinoline fused lactones and lactams.

Total Synthesis of Tetarimycin A, (±)-Naphthacemycin A9, and (±)-Fasamycin A: Structure-Activity Relationship Studies against Drug-Resistant Bacteria

Making use of a reductive olefin coupling reaction and Michael-Dieckmann condensation as two key operations, we have completed a concise total synthesis of tetarimycin A, (±)-naphthacemycin A₉, and (±)-fasamycin A in a highly convergent and practical protocol. Synthetic procedures thus developed have also been applied to provide related analogues for structure-activity relationship studies, thereby coming to the conclusion that the free hydroxyl group at C-10 is essential for exerting inhibitory activities against a panel of Gram-positive bacteria, including drug-resistant strains VRE and MRSA.

Metal-free photocatalyzed aerobic oxidative Csp3–H functionalization of glycine derivatives: one-step generation of quinoline-fused lactones

This reaction provides facile access to a series of valuable quinoline-fused lactones at room temperature under an air atmosphere.

Structure-based QSAR, molecule design and bioassays of protease-activated receptor 1 inhibitors

Quantitative structure-activity relationship (QSAR) studies were performed on a series of protease-activated receptor 1 (PAR1) inhibitors to identify the key structural features responsible for their biological activity. Induced-fit docking (IFD) was used to explore the active mechanisms of all PAR1 inhibitors at the active pocket of PAR1, and the best plausible conformation was determined by IFD for further QSAR studies. Based on the best plausible conformation, structure-based descriptors and ligand descriptors incorporating the ligand-receptor interaction were calculated. The random forest method was used to select important descriptors and build the 2D-QSAR model. The results of the 2D-QSAR model gave a squared correlation coefficient (R²) of 0.937, a prediction squared correlation coefficient (R²_pred) of 0.845 and a mean square error (MSE) of 0.056. Furthermore, a 3D-QSAR model was developed via topomer comparative molecular field analysis (Topomer CoMFA), resulting in an R² of 0.938, a cross-validated Q² of 0.503 and a R²_pred of 0.758. Based on the developed QSAR model, Topomer search was used for virtual screening of the R2 fragment in lead-like inhibitors from the National Cancer Institute (NCI) database, which contains 260,000 molecules. Eighty-two compounds were designed with different R2 fragments, and four of these compounds were selected for further biological testing. All four compounds showed inhibitory potency against PAR1.

TSPO ligand residence time: a new parameter to predict compound neurosteroidogenic efficacy

The pharmacological activation of the cholesterol-binding Translocator Protein (TSPO) leads to an increase of endogenous steroids and neurosteroids determining benefic pleiotropic effects in several pathological conditions, including anxiety disorders. The relatively poor relationship between TSPO ligand binding affinities and steroidogenic efficacies prompted us to investigate the time (Residence Time, RT) that a number of compounds with phenylindolylglyoxylamide structure (PIGAs) spends in contact with the target. Here, given the poor availability of TSPO ligand kinetic parameters, a kinetic radioligand binding assay was set up and validated for RT determination using a theoretical mathematical model successfully applied to other ligand-target systems. TSPO ligand RT was quantified and the obtained results showed a positive correlation between the period for which a drug interacts with TSPO and the compound ability to stimulate steroidogenesis. Specifically, the TSPO ligand RT significantly fitted both with steroidogenic efficacy (Emax) and with area under the dose-response curve, a parameter combining drug potency and efficacy. A positive relation between RT and anxiolytic activity of three compounds was evidenced. In conclusion, RT could be a relevant parameter to predict the steroidogenic efficacy and the in vivo anxiolytic action of new TSPO ligands.

Structure of the mammalian TSPO/PBR protein

The 3D structure of the 18-kDa transmembrane (TM) protein TSPO (translocator protein)/PBR (peripheral benzodiazepine receptor), which contains a binding site for benzodiazepines, is important to better understand its function and regulation by endogenous and synthetic ligands. We have recently determined the structure of mammalian TSPO/PBR in complex with the diagnostic ligand PK11195 [1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide; Jaremko et al. (2014) Science 343: , 1363-1366], providing for the first time atomic-level insight into the conformation of this protein, which is up-regulated in various pathological conditions including Alzheimer's disease and Parkinson's disease. Here, we review the studies which have probed the structural properties of mammalian TSPO/PBR as well as the homologues bacterial tryptophan-rich sensory proteins (TspOs) over the years and provide detailed insight into the 3D structure of mouse TSPO (mTSPO)/PBR in complex with PK11195.

A novel 18F-labelled high affinity agent for PET imaging of the translocator protein

The translocator protein (TSPO) is an important target for imaging focal neuroinflammation in diseases such as brain cancer, stroke and neurodegeneration, but current tracers for non-invasive imaging of TSPO have important limitations. We present the synthesis and evaluation of a novel 3-fluoromethylquinoline-2-carboxamide, AB5186, which was prepared in eight steps using a one-pot two component indium(iii)-catalysed reaction for the rapid and efficient assembly of the 4-phenylquinoline core. Biological assessment and the implementation of a physicochemical study showed AB5186 to have low nanomolar affinity for TSPO, as well as optimal plasma protein binding and membrane permeability properties. Generation of [18F]-AB5186 through 18F incorporation was achieved in good radiochemical yield and subsequent in vitro and ex vivo autoradiography revealed the ability of this compound to bind with specificity to TSPO in mouse glioblastoma xenografts. Initial positron emission tomography imaging of a glioma bearing mouse and a healthy baboon support the potential for [18F]-AB5186 use as a radiotracer for non-invasive TSPO imaging in vivo.

Deepening the Topology of the Translocator Protein Binding Site by Novel N,N-Dialkyl-2-arylindol-3-ylglyoxylamides

As a continuation of our studies on 2-phenylindol-3-ylglyoxylamides as potent and selective translocator protein (TSPO) ligands, two subsets of novel derivatives, featuring hydrophilic group (OH, NH2, COOH) at the para-position of the pendent 2-phenyl ring (8-16) or different 2-aryl moieties, namely, 3-thienyl, p-biphenyl, 2-naphthyl (23-35), were synthesized and biologically evaluated, some of them showing Ki values in the subnanomolar range and the 2-naphthyl group performance being the best. The resulting SARs confirmed the key role played by interactions taking place between ligands and the lipophilic L1 pocket of the TSPO binding site. Docking simulations were performed on the most potent compound of the present series (29) exploiting the recently available 3D structures of TSPO bound to its standard ligand (PK11195). Our theoretical model was fully consistent with SARs of the newly investigated as well of the previously reported 2-phenylindol-3-ylglyoxylamide derivatives.

2-Phenylimidazo[1,2-a]pyridine-containing ligands of the 18-kDa translocator protein (TSPO) behave as agonists and antagonists of steroidogenesis in a mouse leydig tumor cell line

Ligands of 18-kDa translocator protein (TSPO) are known for their ability to potently and dose-dependently stimulate steroid biosynthesis in steroidogenic cells. In this study, we investigated a number of 2-phenyl-imidazo[1,2-a]pyridine acetamide derivatives, analogs of alpidem, for their ability to bind TSPO and to affect steroidogenesis in a mouse Leydig tumor cell line. We observed that not only some compounds behaved as agonists, stimulating steroidogenesis (e.g., 3 and 4) with EC50 values (15.9 and 6.99μM) close to that determined for FGIN-1-27 used as positive control (7.24μM), but two compounds, namely 5 and 6, which on the other hand are the most lipophilic ones in the investigated series, behaved as antagonists, by significantly inhibiting steroid production at concentrations at least twenty times lower than the cytotoxic ones. To our surprise, the newly synthesized compound 3, which is a strict analog of alpidem bearing at the para position of the 2-phenyl group a methoxy group instead of chlorine, achieved a ten-fold stimulation of the steroid production (for comparison FGIN-1-27 achieved 1.6-fold stimulation). Within the limits of the examined property space, some unprecedented SARs were unveiled, which can help in understanding the key molecular factors underlying the transition from agonism to antagonism in the steroidogenesis process. Besides the substitution pattern and the physicochemical features (mainly hydrogen bonding potential) of the substituents at the positions C(6) and C(8) of the imidazo[1,2-a]pyridine nucleus, and at the para position of the 2-phenyl group, the structure-activity relationship analysis suggested lipophilicity, whose increase seems to be generally related to steroidogenesis inhibition, and steric hindrance, which appeared as a stimulation-limiting factor, as two main properties to control in the design or optimization of novel imidazo[1,2-a]pyridine-based TSPO ligands endowed with potential in modulating the steroidogenesis process.

The 18 kDa translocator protein, microglia and neuroinflammation

The 18 kDa translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, is expressed in the injured brain. It has become known as an imaging marker of "neuroinflammation" indicating active disease, and is best interpreted as a nondiagnostic biomarker and disease staging tool that refers to histopathology rather than disease etiology. The therapeutic potential of TSPO as a drug target is mostly based on the understanding that it is an outer mitochondrial membrane protein required for the translocation of cholesterol, which thus regulates the rate of steroid synthesis. This pivotal role together with the evolutionary conservation of TSPO has underpinned the belief that any loss or mutation of TSPO should be associated with significant physiological deficits or be outright incompatible with life. However, against prediction, full Tspo knockout mice are viable and across their lifespan do not show the phenotype expected if cholesterol transport and steroid synthesis were significantly impaired. Thus, the "translocation" function of TSPO remains to be better substantiated. Here, we discuss the literature before and after the introduction of the new nomenclature for TSPO and review some of the newer findings. In light of the controversy surrounding the function of TSPO, we emphasize the continued importance of identifying compounds with confirmed selectivity and suggest that TSPO expression is analyzed within specific disease contexts rather than merely equated with the reified concept of "neuroinflammation."

Construction and validation of an atomic model for bacterial TSPO from electron microscopy density, evolutionary constraints, and biochemical and biophysical data

The 18 kDa protein TSPO is a highly conserved transmembrane protein found in bacteria, yeast, animals and plants. TSPO is involved in a wide range of physiological functions, among which the transport of several molecules. The atomic structure of monomeric ligand-bound mouse TSPO in detergent has been published recently. A previously published low-resolution structure of Rhodobacter sphaeroides TSPO, obtained from tubular crystals with lipids and observed in cryo-electron microscopy, revealed an oligomeric structure without any ligand. We analyze this electron microscopy density in view of available biochemical and biophysical data, building a matching atomic model for the monomer and then the entire crystal. We compare its intra- and inter-molecular contacts with those predicted by amino acid covariation in TSPO proteins from evolutionary sequence analysis. The arrangement of the five transmembrane helices in a monomer of our model is different from that observed for the mouse TSPO. We analyze possible ligand binding sites for protoporphyrin, for the high-affinity ligand PK 11195, and for cholesterol in TSPO monomers and/or oligomers, and we discuss possible functional implications.

Translocator protein (18 kDa) (TSPO) as a therapeutic target for anxiety and neurologic disorders

The translocator protein (18 kD) (TSPO) plays a crucial role for the synthesis of neurosteroids by promoting the transport of cholesterol to the inner mitochondrial membrane, which is the rate-limiting step in neurosteroidogenesis. Neurosteroids are allosteric modulators of GABA(A) receptor function, which plays an important role in the pathophysiology of anxiety disorders. The TSPO ligand XBD173 enhances GABAergic neurotransmission by promoting neurosteroidogenesis without direct effects at the GABA(A) receptor. In humans, XBD173 shows potent antipanic efficacy without sedation and withdrawal after 7 days of treatment. XBD173 therefore appears to be a promising compound for rapid anxiolytic efficacy with a favorable side-effect profile. Furthermore, TSPO ligands show neuroprotective and antiinflammatory effects in experimental models of peripheral neuropathies and traumatic brain injury. These compounds might therefore also be valuable for the treatment of neurologic diseases with inflammation-related pathophysiology.

Solution structures of the prototypical 18 kDa translocator protein ligand, PK 11195, elucidated with 1H/13C NMR spectroscopy and quantum chemistry

Eighteen kilodalton translocator protein (TSPO) is an important target for drug discovery and for clinical molecular imaging of brain and peripheral inflammatory processes. PK 11195 [1a; 1-(2-chlorophenyl)-N-methyl-(1-methylpropyl)-3-isoquinoline carboxamide] is the major prototypical high-affinity ligand for TSPO. Elucidation of the solution structure of 1a is of interest for understanding small-molecule ligand interactions with the lipophilic binding site of TSPO. Dynamic (1)H/(13)C NMR spectroscopy of 1a revealed four quite stable but interconverting rotamers, due to amide bond and 2-chlorophenyl group rotation. These rotamers have been neglected in previous descriptions of the structure of 1a and of the binding of 1a to TSPO. Here, we used quantum chemistry at the level of B3LYP/6-311+G(2d,p) to calculate (13)C and (1)H chemical shifts for the rotamers of 1a and for the very weak TSPO ligand, N-desmethyl-PK 11195 (1b). These data, plus experimental NMR data, were then used to characterize the structures of rotamers of 1a and 1b in organic solution. Energy barriers for both the amide bond and 2'-chlorophenyl group rotation of 1a were determined from dynamic (1)H NMR to be similar (ca.17 to 18 kcal/mol), and they compared well with those calculated at the level of B3LYP/6-31G*. Furthermore, the computed barrier for Z to E rotation is considerably lower in 1a(18.7 kcal/mol) than in 1b (25.4 kcal/mol). NMR (NOE) unequivocally demonstrated that the E rotamer of 1a is the more stable in solution by about 0.4 kcal/mol. These detailed structural findings will aid future TSPO ligand design and support the notion that TSPO prefers to bind ligands as amide E-rotamers.

Translocator protein (18 kDa) as a target for novel anxiolytics with a favourable side-effect profile

Anxiety disorders are frequent and highly disabling diseases with considerable socio-economic impact. In the treatment of anxiety disorders, benzodiazepines (BZDs) as direct modulators of the GABA(A) receptor are used as emergency medication because of their rapid onset of action. However, BZDs act also as sedatives and rather quickly induce tolerance and abuse liability associated with withdrawal symptoms. Antidepressants with anxiolytic properties are also applied as first line long-term treatment of anxiety disorders. However, the onset of action of antidepressants takes several weeks. Obviously, novel pharmacological approaches are needed that combine a rapid anxiolytic efficacy with the lack of tolerance induction, abuse liability and withdrawal symptoms. Neurosteroids are potent allosteric modulators of GABA(A) receptor function. The translocator protein (18 kDa) (TSPO) plays an important role for the synthesis of neurosteroids by promoting the transport of cholesterol from the outer to the inner mitochondrial membrane, which is the rate-limiting step in neurosteroidogenesis. Etifoxine not only exerts anxiolytic effects as a TSPO ligand by enhancing neurosteroidogenesis, but also acts as a weak direct GABA(A) receptor enhancer. The TSPO ligand XBD173 enhances GABAergic neurotransmission via the promotion of neurosteroidogenesis without direct effects at the GABA(A) receptor. XBD173 counteracts pharmacologically-induced panic in rodents in the absence of sedation and tolerance development. Also in humans, XBD173 displays antipanic activity and does not cause sedation and withdrawal symptoms after 7 days of treatment. XBD173 therefore appears to be a promising candidate for fast-acting anxiolytic drugs with less severe side-effects than BZDs. In this review, we focus on the pathophysiology of anxiety disorders and TSPO ligands as a novel pharmacological approach in the treatment of these disorders.

Translocator protein (18 kDa) (TSPO) as a therapeutic target for neurological and psychiatric disorders

The translocator protein (18 kDa) (TSPO) is localized primarily in the outer mitochondrial membrane of steroid-synthesizing cells, including those in the central and peripheral nervous system. One of its main functions is the transport of the substrate cholesterol into mitochondria, a prerequisite for steroid synthesis. TSPO expression may constitute a biomarker of brain inflammation and reactive gliosis that could be monitored by using TSPO ligands as neuroimaging agents. Moreover, initial clinical trials have indicated that TSPO ligands might be valuable in the treatment of neurological and psychiatric disorders. This Review focuses on the biology and pathophysiology of TSPO and the potential of currently available TSPO ligands for the diagnosis and treatment of neurological and psychiatric disorders.

Three-dimensional structure of TspO by electron cryomicroscopy of helical crystals

The 18 kDa TSPO protein is a polytopic mitochondrial outer membrane protein involved in a wide range of physiological functions and pathologies, including neurodegeneration and cancer. The pharmacology of TSPO has been extensively studied, but little is known about its biochemistry, oligomeric state, and structure. We have expressed, purified, and characterized a homologous protein, TspO from Rhodobacter sphaeroides, and reconstituted it as helical crystals. Using electron cryomicroscopy and single-particle helical reconstruction, we have determined a three-dimensional structure of TspO at 10 Å resolution. The structure suggests that monomeric TspO comprises five transmembrane α helices that form a homodimer, which is consistent with the dimeric state observed in detergent solution. Furthermore, the arrangement of transmembrane domains of individual TspO subunits indicates a possibility of two substrate translocation pathways per dimer. The structure provides the first insight into the molecular architecture of TSPO/PBR protein family that will serve as a framework for future studies.

New iodinated quinoline-2-carboxamides for SPECT imaging of the translocator protein

With the aim of developing new SPECT imaging agents for the translocator protein (TSPO), a small library of iodinated quinoline-2-carboxamides have been prepared and tested for binding affinity with TSPO. N,N-Diethyl-3-iodomethyl-4-phenylquinoline-2-carboxamide was found to have excellent affinity (K(i) 12.0 nM), comparable to that of the widely used TSPO imaging agent PK11195.

Ethyl 2,6-dichloro-4-phenyl-quinoline-3-carboxyl-ate

In the title compound, C₁₈H₁₃Cl₂NO₂, the quinoline ring system is almost planar (r.m.s. deviation 0.009 Å), and the phenyl and carboxyl­ate planes are twisted away from it by 59.2 (1) and 65.9 (2)°, respectively.

Synthesis and biological characterization of novel 2-quinolinecarboxamide ligands of the peripheral benzodiazepine receptors bearing technetium-99m or rhenium

Potential receptor imaging agents based on Tc-99m for the in vivo visualization of the peripheral benzodiazepine receptor (PBR) have been designed on the basis of the information provided by the previously published structure-affinity relationship studies, which suggested the existence of tolerance to voluminous substituents in the receptor area interacting with 3-position of the quinoline nucleus of 2-quinolinecarboxamides 5. In the first step of the investigation, the stereoelectronic features of the above-indicated receptor area were also probed by means of 4-phenyl-3-[(1-piperazinyl)methyl]-2-quinolinecarboxamide derivatives bearing different substituents on the terminal piperazine nitrogen atom (compounds 6a-f). The structure-affinity relationship data confirmed the existence of a tolerance to bulky lipophilic substituents and stimulated the design of bifunctional ligands based on the 4-phenyl-3-[(1-piperazinyl)methyl]-2-quinolinecarboxamide moiety (compounds 6h,j,k,m). The submicromolar PBR affinity of rhenium complexes 6j,m suggests that the presence of their metal-ligand moieties with encaged rhenium is fairly compatible with the interaction with the PBR binding site. Thus, in order to obtain information on the in vivo behavior of these bifunctional ligands, (99m)Tc-labeled compounds 6h,k were synthesized and evaluated in preliminary biodistribution and single photon emission tomography (SPET) studies. The results suggest that both tracers do not present a clear preferential distribution in tissues rich in PBR, probably because of their molecular dimensions, which may hamper both the intracellular diffusion toward PBR and the interaction with the binding site.

Synthesis, labeling, and biological evaluation of halogenated 2-quinolinecarboxamides as potential radioligands for the visualization of peripheral benzodiazepine receptors

The previous exploration of the structure-affinity relationships concerning 4-phenyl-2-quinolinecarboxamide peripheral benzodiazepine receptor (PBR) ligands 6 showed as an interesting result the importance of the presence of a chlorine atom in the methylene carbon at position 3 of the quinoline nucleus. The subnanomolar PBR affinity shown by N-benzyl-3-chloromethyl-N-methyl-4-phenyl-2-quinolinecarboxamide (6b) suggested its chlorine atom to be replaced with other halogens in order to optimize the interaction of the quinolinecarboxamide derivatives with PBR and to develop suitable candidates for positron emission tomography (PET) or single photon emission computed tomography (SPECT) studies. The binding studies led to the discovery of fluoromethyl derivative 6a, which showed an IC50 value of 0.11 nM and is, therefore, one of the most potent PBR ligands so far described. Fluoromethyl derivative 6a has been labeled with 11C (t1/2=20.4 min, beta+=99.8%) starting from the corresponding des-methyl precursor (14) using [11C]CH3I in the presence of tetrabutylammonium hydroxide in DMF with a 35-40% radiochemical yield (corrected for decay) and 1.5 Ci/micromol of specific radioactivity. Ex vivo rat biodistribution and inhibition (following intravenous pre-administration of PK11195) studies showed that [11C]6a rapidly and specifically accumulated in PBR-rich tissues such as heart, lung, kidney, spleen, and adrenal, and at a lower level in other peripheral organs and in the brain. The images obtained in mouse with small animal YAP-(S)PET essentially confirmed the result of the ex vivo biodistribution experiments. The biological data suggest that [11C]6a is a promising radioligand for peripheral benzodiazepine receptor PET imaging in vivo.

Insight into 2-phenylpyrazolo[1,5-a]pyrimidin-3-yl acetamides as peripheral benzodiazepine receptor ligands: Synthesis, biological evaluation and 3D-QSAR investigation

The present paper reports the synthesis and binding studies of new 2-phenylpyrazolo[1,5-a]pyrimidin-3-yl acetamides as selective Peripheral Benzodiazepine Receptor (PBR) ligands. The variability of substituents at the 3-position was investigated and a 3D-QSAR model was proposed to evaluate the effect of different substitutions on the acetamide moiety. In addition, a subset of the novel compounds showing high affinity for PBR was tested for their ability to modulate the steroid biosynthesis in C6 glioma cells.

Evaluating molecular similarity using reduced representations of the electron density

A model system of four benzodiazepine-like ligands for the central benzodiazepine receptors (CBRs) and peripheral benzodiazepine receptors (PBRs)is examined using a genetic algorithm procedure (GAGS) designed for evaluating molecular similarity. The method is based on the alignment of reduced representations generated from the critical points of the electron density computed at medium crystallographic resolution. The results are further characterized by a comparison with alignments produced by MIMIC, a field-based superimposition method that matches both steric and electrostatic molecular fields. The alignments produced by the two methods are generally seen to be consistent. The relationships of the compounds' binding affinities for both CBRs and PBRs to the alignments determined by GAGS yield a set of structural features required for significant binding to benzodiazepine receptors. Benefits of using reduced representations for evaluating molecular similarities and for constructing pharmacophore models are discussed.

Structure−Activity Relationships and Effects on Neuroactive Steroid Synthesis in a Series of 2-Phenylimidazo[1,2-a]pyridineacetamide Peripheral Benzodiazepine Receptors Ligands

A series of 36 imidazopyridineacetamides (2-37) were designed and synthesized to evaluate the effects of structural changes on the amide nitrogen at both central (CBRs) and peripheral benzodiazepine receptors (PBRs). These changes include variations in the length and number of the alkyl groups as well as introduction of different aromatic, heteroaromatic, and conformationally constrained groups. The affinities of these compounds for CBRs and PBRs were determined, and the results indicate that bulkiness of the substituents, their branching, and length beyond an optimal value may cause hindrance to the ligand in its interaction with the receptor. The presence of aromatic or conformationally constrained substituents on the carboxamide nitrogen can be conducive to high affinity and selectivity. Furthermore, the ability of a subset of the most active ligands to stimulate synthesis of neuroactive steroids in plasma and brain was evaluated in vivo and in vitro. Compound 3 exhibited very marked effects on the peripheral and central synthesis of neuroactive steroids, while 36 (potent at subnanomolar level) showed a slight ability to affect neuroactive steroid content in the cerebral cortex.

Synthesis, central and peripheral benzodiazepine receptor affinity of pyrazole and pyrazole-containing polycyclic derivatives

A series of new pyrazole-condensed 6,5,5 tricyclic compounds were synthesized and tested to evaluate their binding affinities at both central (CBR) and peripheral (PBR) benzodiazepine receptors. Some 1-aryl-5-phenylpyrazole derivatives were also prepared and tested for comparison with their corresponding rigid tricyclic analogs. Among the newly synthesized 1-aryl-1,4-dihydro-indeno[1,2-c]pyrazoles bearing both an ethoxycarbonyl group at position 3 and a carbonyl function at the position 4, compound 4b emerged as a new potent (IC(50) = 26.4 nM) and selective CBR ligand. The 4-oxo-1-aryl-1,4-dihydro-indeno[1,2-c]pyrazole diethylamide derivative 14a was instead identified as a relatively potent (IC(50) = 124 nM) but highly selective PBR ligand.