Studies on an (S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor antagonist IKM-159: asymmetric synthesis, neuroactivity, and structural characterization

Oxa-Michael-based divergent synthesis of artificial glutamate analogs

Herein we report stereoselective generation of two skeletons, 1,3-dioxane and tetrahydropyranol, by oxa-Michael reaction as the key reaction from δ-hydroxyenone. The construction of the 1,3-dioxane skeleton, achieved through hemiacetal formation followed by oxa-Michael reaction from δ-hydroxyenone, was exploited to access structurally diverse heterotricyclic artificial glutamate analogs. On the other hand, formation of a novel tetrahydro-2H-pyranol skeleton was accomplished by the inverse reaction order: oxa-Michael reaction followed by hemiacetal formation. Thus, this study succeeded in showing that structural diversity in a compound collection can be acquired by interchanging the order of just two reactions. Among the skeletally diverse, heterotricyclic artificial glutamate analogs synthesized in this study, a neuronally active compound named TKM-50 was discovered in the mice in vivo assay.

By interchanging the order of reactions, two types of skeletons were created and a neuroactive artificial glutamate analog was developed.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Menthyl esterification allows chiral resolution for the synthesis of artificial glutamate analogs

Herein, we report the enantiospecific synthesis of two artificial glutamate analogs designed based on IKM-159, an antagonist selective to the AMPA-type ionotropic glutamate receptor. The synthesis features the chiral resolution of the carboxylic acid intermediate by the esterification with ʟ-menthol, followed by a configurational analysis by NMR, conformational calculation, and X-ray crystallography. A mice in vivo assay showed that (2R)-MC-27, with a six-membered oxacycle, is neuroactive, whereas the (2S)-counterpart is inactive. It was also found that TKM-38, with an eight-membered azacycle, is neuronally inactive, showing that the activity is controlled by the ring C.

Diverse Aromatic Metabolites in the Solitary Tunicate Cnemidocarpa irene

Fourteen aromatic metabolites (6-19) were isolated from an aqueous extract of the solitary tunicate Cnemidocarpa irene collected in Hokkaido, Japan. The structures of the metabolites were determined based on the spectroscopic interpretations, including one- and two-dimensional NMR, mass spectra, UV, and circular dichroism data. The biopterin analogue 10 modulated the behavior of mice after intracerebroventricular injection and showed a weak affinity to ionotropic glutamate receptor subtypes. Analyses of fluorescent coelomic fluid of the tunicate revealed that pterin 12 was responsible for the fluorescence of the blood cells, while β-carbolines 1 and 3 were fluorescent compounds in the serum. The metabolic profiles in adults, juveniles, larvae, and eggs of the animal differed substantially, suggesting that the metabolism of the animal, especially biosynthesis of aromatic secondary metabolites, changes over different life stages.

New efficient synthesis of benzofuro[2,3-b]pyrroles utilizing a reactive nitrilium trapping approach by an acid-promoted cascade addition/cyclization sequence

The first p-TSA·H₂O-catalyzed highly efficient one-pot cascade reaction for the synthesis of novel fused tricyclic benzofuro[2,3-b]pyrroles has been discovered.

Pharmacological characterization and binding modes of novel racemic and optically active phenylalanine-based antagonists of AMPA receptors

In order to map out molecular determinants for the competitive blockade of AMPA receptor subtypes, a series of racemic aryl-substituted phenylalanines was synthesized and pharmacologically characterized in vitro at native rat ionotropic glutamate receptors. Most of the compounds showed micromolar affinity and preference for AMPA receptors. Individual stereoisomers of selected compounds were further evaluated at recombinant homomeric rat GluA2 and GluA3 receptors. The most potent compound, (-)-2-amino-3-(6-chloro-2',5'-dihydroxy-5-nitro-[1,1'-biphenyl]-3-yl)propanoic acid, the expected R-isomer showing K_i of 1.71 μM at the GluA2 subtype, was found to competitively antagonize GluA2(Q)_i receptors in TEVC electrophysiological experiments (K_b = 2.13 μM). Molecular docking experiments allowed us to compare two alternative antagonist binding modes for the synthesized phenylalanines at the GluA2 binding core, showing the direction for further structural modifications.

Three-Component, Diastereoselective Prins-Ritter Reaction for cis-Fused 4-Amidotetrahydropyrans toward a Precursor for Possible Neuronal Receptor Ligands

Here, we report an unprecedented, highly diastereoselective Prins-Ritter reaction of aldehydes, homoallylic alcohols, and nitriles in a three-component coupling reaction for the synthesis of tetra-cis-substituted 4-amidotetrahydropyrans. In this study, the reaction was not only applied for carbohydrate-based heterobicycles but also for more complex heterotricycles, showing acceptable levels of conversion yield (42-97% BRSM) and exclusive diastereoselectivity. Furthermore, the latter heterotricycles were converted to nine analogues of our neuronal receptor ligands IKM-159 and MC-27. An in vivo assay by intracerebroventricular injection in mice suggested that the substituent at C9 of the novel analogues interferes with the molecular interactions with the AMPA receptor, which was originally observed in the complex of IKM-159 and the GluA2 ligand binding domain. Our research has thus shown the power of a multicomponent coupling reaction for the preparation of a structurally diverse compound collection to study structure-activity relationships of biologically active small molecules.

Tweaking Subtype Selectivity and Agonist Efficacy at (S)-2-Amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionic acid (AMPA) Receptors in a Small Series of BnTetAMPA Analogues

A series of analogues of the (S)-2-Amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionic acid (AMPA) receptor agonist BnTetAMPA (5b) were synthesized and characterized pharmacologically in radioligand binding assays at native and cloned AMPA receptors and functionally by two-electrode voltage clamp electrophysiology at the four homomeric AMPA receptors expressed in Xenopus laevis oocytes. The analogues 6 and 7 exhibit very different pharmacological profiles with binding affinity preference for the subtypes GluA1 and GluA3, respectively. X-ray crystal structures of three ligands (6, 7, and 8) in complex with the agonist binding domain (ABD) of GluA2 show that they induce full domain closure despite their low agonist efficacies. Trp767 in GluA2 ABD could be an important determinant for partial agonism of this compound series at AMPA receptors, since agonist efficacy also correlated with the location of the Trp767 side chain.

Studies on Aryl-Substituted Phenylalanines: Synthesis, Activity, and Different Binding Modes at AMPA Receptors

A series of racemic aryl-substituted phenylalanines was synthesized and evaluated in vitro at recombinant rat GluA1-3, at GluK1-3, and at native AMPA receptors. The individual enantiomers of two target compounds, (RS)-2-amino-3-(3,4-dichloro-5-(5-hydroxypyridin-3-yl)phenyl)propanoic acid 37 and (RS)-2-amino-3-(3'-hydroxybiphenyl-3-yl)propanoic acid 38, were characterized. (S)-37 and (R)-38 were identified as the only biologically active isomers, both being antagonists at GluA2 receptors with Kb of 1.80 and 3.90 μM, respectively. To address this difference in enantiopharmacology, not previously seen for amino acid-based AMPA receptor antagonists, X-ray crystal structures of both eutomers in complex with the GluA2 ligand binding domain were solved. The cocrystal structures of (S)-37 and (R)-38 showed similar interactions of the amino acid parts but unexpected and different orientations and interactions of the biaromatic parts of the ligands inside the binding site, with (R)-38 having a binding mode not previously identified for amino acid-based antagonists.

Studies on lipase-catalyzed asymmetric synthesis of (S)-(hydroxymethyl)glutamic acid (HMG)

(S)-(Hydroxymethyl)glutamic acid was successfully synthesized in total 12 % yield over eight steps from tris(hydroxymethyl)aminomethane hydrochloride (Tris·HCl), employing lipase TL-induced enantioselective acetylation of a prochiral 1,3-diol as the key step.

Synthesis, structure-activity relationships, and anticonvulsant activities of 2-amino-4H-pyrido[3,2-e][1,3]thiazin-4-one derivatives as orally active AMPA receptor antagonists

As part of a program aimed at discovering orally active 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor antagonists, we screened our compound library and identified 2-[allyl(4-methylphenyl)amino]-4H-pyrido[3,2-e][1,3]thiazin-4-one (7) as a lead compound that inhibited kainate-induced neurotoxicity mediated by AMPA receptors in rat hippocampal cultures. Structure-activity relationship studies of a series of 2-amino-4H-pyrido[3,2-e][1,3]thiazin-4-one derivatives revealed that substituents on the phenyl ring attached to the 2-amino group and the 4H-pyrido[3,2-e][1,3]thiazin-4-one ring system play an important role in inhibitory activity against kainate-induced neurotoxicity. Several analogs bearing a phenyl group with a 4-substituent or five- or six-membered ring fused at the 3,4-positions exhibited potent inhibitory activity against kainate-induced neurotoxicity. Further, some of these compounds exhibited significant suppression of maximal electroshock seizure in mice following oral administration. Of these compounds, 2-[(4-chlorophenyl)(methyl)amino]-4H-pyrido[3,2-e][1,3]thiazin-4-one (16i) (YM928) demonstrated the most potent inhibitory effect with an ED50 value of 7.4mg/kg.

Recent progress in neuroactive marine natural products

This review covers the isolation, chemical structure, biological activity, structure activity relationships including synthesis of chemical probes, and pharmacological characterization of neuroactive marine natural products; 302 references are cited.