Lotte Brehm

Resolution, absolute stereochemistry, and enantiopharmacology of the GluR1–4 and GluR5 antagonist 2-amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl]propionic acid

The phosphono amino acid, (RS)-2-amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl+ ++]propio nic acid (ATPO), is a structural hybrid between the NMDA antagonist (RS)-2-amino-7-phosphonoheptanoic acid (AP7) and the AMPA and GluR5 agonist, (RS)-2-amino-3-(5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid (ATPA). ATPO has been resolved into (S)-ATPO and (R)-ATPO using chiral HPLC, and the absolute stereochemistry of the two enantiomers was established by an X-ray crystallographic analysis of (R)-ATPO. (S)-ATPO and (R)-ATPO were characterized pharmacologically using rat brain membrane binding and electrophysiologically using the cortical wedge preparation as well as homo- or heteromeric GluR1-4, GluR5-6, and KA2 receptors expressed in Xenopus oocytes. (R)-ATPO was essentially inactive as an agonist or antagonist in all test systems. (S)-ATPO was an inhibitor of the binding of [(3)H]AMPA (IC(50) = 16 +/- 1 microM) and of [(3)H]-6-cyano-7-nitroquinoxaline-2,3-dione ([(3)H]CNQX) (IC(50) = 1.8 +/- 0.2 microM), but was inactive in the [(3)H]kainic acid and the [(3)H]-(RS)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid ([(3)H]CPP) binding assays. (S)-ATPO did not show detectable agonist effects at any of the receptors under study, but antagonized AMPA-induced depolarization in the cortical wedge preparation (IC(50) = 15 +/- 1 microM). (S)-ATPO also blocked kainic acid agonist effects at GluR1 (K(i) = 2.0 microM), GluR1+2 (K(i) = 3.6 microM), GluR3 (K(i) = 3.6 microM), GluR4 (K(i) = 6.7 microM), and GluR5 (K(i) = 23 microM), but was inactive at GluR6 and GluR6+KA2. Thus, although ATPO is a structural analog of AP7 neither (S)-ATPO nor (R)-ATPO are recognized by NMDA receptor sites.

Exploring the GluR2 ligand‐binding core in complex with the bicyclical AMPA analogue (S)‐4‐AHCP

The X‐ray structure of the ionotropic GluR2 ligand‐binding core (GluR2‐S1S2J) in complex with the bicyclical AMPA analogue (S)‐2‐amino‐3‐(3‐hydroxy‐7,8‐dihydro‐6H‐cyclohepta[d]‐4‐isoxazolyl)propionic acid [(S)‐4‐AHCP] has been determined, as well as the binding pharmacology of this construct and of the full‐length GluR2 receptor. (S)‐4‐AHCP binds with a glutamate‐like binding mode and the ligand adopts two different conformations. The Ki of (S)‐4‐AHCP at GluR2‐S1S2J was determined to be 185 ± 29 nm and at full‐length GluR2(R)o it was 175 ± 8 nm. (S)‐4‐AHCP appears to elicit partial agonism at GluR2 by inducing an intermediate degree of domain closure (17°). Also, functionally (S)‐4‐AHCP has an efficacy of 0.38 at GluR2(Q)i, relative to (S)‐glutamate. The proximity of bound (S)‐4‐AHCP to domain D2 prevents full D1–D2 domain closure, which is limited by steric repulsion, especially between Leu704 and the ligand.

The Glutamate Receptor Glur5 Agonist (S)-2-Amino-3-(3-Hydroxy-7,8-Dihydro-6H-Cyclohepta[D]Isoxazol-4-Yl)Propionic Acid and the 8-Methyl Analogue: Synthesis, Molecular Pharmacology, and Biostructural Characterization

The design, synthesis, and pharmacological characterization of a highly potent and selective glutamate GluR5 agonist is reported. (S)-2-Amino-3-((RS)-3-hydroxy-8-methyl-7,8-dihydro-6H-cyclohepta[d]isoxazol-4-yl)propionic acid (5) is the 8-methyl analogue of (S)-2-amino-3-(3-hydroxy-7,8-dihydro-6H-cyclohepta[d]isoxazol-4-yl)propionic acid ((S)-4-AHCP, 4). Compound 5 displays an improved selectivity profile compared to 4. A versatile stereoselective synthetic route for this class of compounds is presented along with the characterization of the binding affinity of 5 to ionotropic glutamate receptors (iGluRs). Functional characterization of 5 at cloned iGluRs using a calcium imaging assay and voltage-clamp recordings show a different activation of GluR5 compared to (S)-glutamic acid (Glu), kainic acid (KA, 1), and (S)-2-amino-3-(3-hydroxy-5-tert-butyl-4-isoxazolyl)propionic acid ((S)-ATPA, 3) as previously demonstrated for 4. An X-ray crystallographic analysis of 4 and computational analyses of 4 and 5 bound to the GluR5 agonist binding domain (ABD) are presented, including a watermap analysis, which suggests that water molecules in the agonist binding site are important selectivity determinants.

Structure and pharmacology of 4,5,6,7-tetrahydroisothiazolo[5,4-c]pyridin-3-ol (Thio-THIP), an agonist/antagonist at GABAA receptors

Summary 4,5,6,7-Tetrahydroisothiazolo[5,4-c]pyridin-3-ol (Thio-THIP), an analogue of the potent and efficacious partial GABAA agonist, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), shows rather potent agonist effects at spinal GABAA receptors in vivo, but remarkably low affinity for brain GABAA receptors in vitro. 2-Methyl-4,5,6,7-tetrahydropyrazolo[5,4-c]pyridin-3-ol (2-Me-Aza-THIP) does not bind detectably to GABAA receptors. The conformation of the molecule of Thio-THIP, which has now been determined by an X-ray crystallographic analysis, is very similar to those previously described for THIP and 2-Me-Aza-THIP. At human GABAA receptors of α3β2γ2 or α5β3γ2 subunit configurations, expressed in Xenopus oocytes, at which THIP shows low- (44%) or high-efficacy (99%) GABAA agonism, respectively, Thio-THIP was shown to be a competitive antagonist. At GABAA receptors in cultured cerebellar granule cells, Thio-THIP turned out to be a weak low-efficacy (2–9%) partial GABAA agonist.

3-Pyrazolone analogues of the 3-isoxazolol metabotropic excitatory amino acid receptor agonist homo-AMPA. Synthesis and pharmacological testing

We have previously shown that the higher homologue of (S)-glutamic acid [(S)-Glu], (S)-alpha-aminoadipic acid [(S)-alpha-AA] is selectively recognized by the mGlu(2) and mGlu(6) subtypes of the family of metabotropic glutamic acid (mGlu) receptors. Furthermore, a number of analogues of (S)-alpha-AA, in which the terminal carboxyl group has been replaced by various bioisosteric groups, such as phosphonic acid or 3-isoxazolol groups, have been shown to interact selectively with different subtypes of mGlu receptors. In this paper we report the synthesis of the 3-pyrazolone bioisosteres of alpha-AA, compounds (RS)-2-amino-4-(1,2-dihydro-5-methyl-3-oxo-3H-pyrazol-4-yl)butyric acid (1) and (RS)-2-amino-4-(1,2-dihydro-1,5-dimethyl-3-oxo-3H-pyrazol-4-yl)butyric acid (2). At a number of steps in the reaction sequences used, the reactions took unexpected courses and provided products which could not be transformed into the target compounds, and attempts to synthesize the 2,5-dimethyl isomer of 2, compound 3, failed. An X-ray crystallographic analysis of the intermediate 1,2-dihydro-4-(2-hydroxyethyl)-2,5-dimethyl-3H-pyrazol-3-one (5b) confirmed the expected regioselectivity of the reaction between methylhydrazine and alpha-acetylbutyrolactone (4). Neither 1 nor 2 showed significant effects at the different types of ionotropic glutamic acid receptors or at mGlu(1a) (group I), mGlu(2) (group II), and mGlu(4a) and mGlu(6) (group III) receptors, representing the three indicated groups of mGlu receptors.

Biostructural and pharmacological studies of bicyclic analogues of the 3-isoxazolol glutamate receptor agonist ibotenic acid.

We describe an improved synthesis and detailed pharmacological characterization of the conformationally restricted analogue of the naturally occurring nonselective glutamate receptor agonist ibotenic acid (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-7-carboxylic acid (7-HPCA, 5) at AMPA receptor subtypes. Compound 5 was shown to be a subtype-discriminating agonist at AMPA receptors with higher binding affinity and functional potency at GluA1/2 compared to GluA3/4, unlike the isomeric analogue (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-5-carboxylic acid (5-HPCA, 4) that binds to all AMPA receptor subtypes with comparable potency. Biostructural X-ray crystallographic studies of 4 and 5 reveal different binding modes of (R)-4 and (S)-5 in the GluA2 agonist binding domain. WaterMap analysis of the GluA2 and GluA4 binding pockets with (R)-4 and (S)-5 suggests that the energy of hydration sites is ligand dependent, which may explain the observed selectivity.

Ibotenic acid analogues. Synthesis and biological testing of two bicyclic 3-isoxazolol amino acids

The bicyclic 3-isoxazolol amino acids (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-4-carboxylic acid (5, 4-HPCA) and (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-6-carboxylic acid (11, 6-HPCA) were synthesized as model compounds for studies of the structural requirements of central excitatory amino acid neurotransmitter receptors. 4-HPCA was synthesized via introduction of a methoxycarbonyl group into the 4-position of the lithiated N-nitroso intermediate 1. The key reaction in the synthesis of 6-HPCA is an intramolecular N-alkylation of the appropriately substituted acetamidomalonate derivative 7 using sodium hydride as a base. On the basis of the pKA values for 4-HPCA the existence of an intramolecular hydrogen bond in the zwitterionic form of this amino acid is proposed. 6-HPCA was shown by 1H NMR spectroscopy to adopt preferentially a conformation with the carboxylate group in an equatorial position. 4- and 6-HPCA were tested as agonists and antagonists at excitatory amino acid receptors on neurones in the cat spinal cord using microelectrophoretic techniques. Neither compound showed significant effects at these receptors.

Aryl and cycloalkyl analogues of AMPA: synthetic, pharmacological and stereochemical aspects

We have previously shown that (RS)-2-amino-3-(3-hydroxy-5-phenylisoxazol-4-yl)propionic acid (APPA, 2) is a functional partial agonist at the (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) subtype of excitatory amino acid receptors, reflecting that (S)-APPA is a full agonist and (R)-APPA a competitive antagonist at AMPA receptors. We have now synthesized and pharmacologically characterized (RS)-2-amino-3-[3-hydroxy-5-(2-fluorophenyl)isoxazol-4-yl]propioni c acid (2-F-APPA, 5a), 3-F-APPA (5b), 4-F-APPA (5c), (S)-4-F-APPA (6), (R)-4-F-APPA (7), and the fully and partially, respectively, saturated APPA (2) analogues, (RS)-2-amino-3-(3-hydroxy-5-cyclohexylisoxazol-4-yl)propionic acid (5d) and compound 5e containing a 1-cyclohexenyl ring. The absolute stereochemistry of 6 and 7 was established on the basis of comparative circular dichroism studies on 6, 7, and (S)- and (R)-APPA. 4-F-APPA (5c), (S)-4-F-APPA (6), 5d, and 5e were shown to selectively inhibit [3H]AMPA binding and to activate AMPA receptors. Whereas (S)-4-F-APPA (6) showed full AMPA receptor agonism, (R)-4-F-APPA (7) was an AMPA receptor antagonist. Co-administration of (S)- and (R)-4-F-APPA to the rat cortical wedge preparation produced functional partial AMPA receptor agonism. Semi empirical calculations showed that the magnitude of the torsional angle of the bond connecting the two rings in the series of nonannulated bicyclic AMPA analogues appears to be of importance for the potency and efficacy of these compounds.

Structural features of muscimol, a potent GABAA receptor agonist, crystal structure and quantum chemicalab initio calculations

Muscimol, a constituent of the mushroomAmanita muscaria, is a semirigid analogue of the inhibitory neurotransmitter 4-aminobutyric acid (GABA). X-ray structure determinations and quantum chemicalab initio calculations (HF/6-31G*) have been carried out on the muscimol zwitterion. The solid-state conformations of the muscimol zwitterion are calculated to be 1.6–2.2 kcal/mol higher in energy than that of the calculated minimum energy structurein vacuo. A comparison of the calculated and experimental structures indicates that the hydrogen bonding network in the solid state significantly affects the geometry of the molecular structure. This conclusion is supported by results ofab initio calculations on binary complexes between muscimol and an ammonium ion and between muscimol and a methoxide anion, simulating observed hydrogen bonding in the crystal structures.

Heterocyclic GABA agonists. Synthesis and crystal structure of (RS)-5-(N-t-butyloxycarbonylaminomethyl)-3-oxoisoxazolidine-2-carboxamide, a derivative of dihydromuscimol

Reaction of the O- and N-protected (RS)-3-hydroxy-4-aminobutyric acid ester (4) with hydroxyurea under basic conditions unexpectedly yielded (RS)-5-(N-t-butyloxycarbonylaminomethyl)-3-oxoisoxazolidine-2-carboxamide (8), the structure of which has been confirmed by an X-ray analysis. Elimination of the cyanate ion, which could be trapped with dimethylamine, converted (8) into the corresponding 2-isoxazolin-3-ol derivative (7). Attempts to prepare the individual enantiomers of (8) were unsuccessful. The (R)-(–)-3-hydroxy-4-aminobutyric acid derivative (5) yielded racemic (8). The cyclization reactions of (5) and (4) are believed to involve elimination of toluene-4-sulphonate giving the corresponding α,β-unsaturated ester as an intermediate.