Small constrained SP1-7 analogs bind to a unique site and promote anti-allodynic effects following systemic injection in mice.

Previous results have shown that the substance P (SP) N-terminal fragment SP1-7 may attenuate hyperalgesia and produce anti-allodynia in animals using various experimental models for neuropathic pain. The heptapeptide was found to induce its effects through binding to and activating specific sites apart from any known neurokinin or opioid receptor. Furthermore, we have applied a medicinal chemistry program to develop lead compounds mimicking the effect of SP1-7. The present study was designed to evaluate the pharmacological effect of these compounds using the mouse spared nerve injury (SNI) model of chronic neuropathic pain. Also, as no comprehensive screen with the aim to identify the SP1-7 target has yet been performed we screened our lead compound H-Phe-Phe-NH2 toward a panel of drug targets. The extensive target screen, including 111 targets, did not reveal any hit for the binding site among a number of known receptors or enzymes involved in pain modulation. Our animal studies confirmed that SP1-7, but also synthetic analogs thereof, possesses anti-allodynic effects in the mouse SNI model of neuropathic pain. One of the lead compounds, a constrained H-Phe-Phe-NH2 analog, was shown to exhibit a significant anti-allodynic effect.

Synthesis of novel 5-substituted 3-amino-3,4-dihydro-2H-1-benzopyran derivatives and their interactions with the 5-HT1A receptor.

A series of new enantiomerically pure 3-amino-3,4-dihydro-2H-1-benzopyrans (3-aminochromans) has been synthesized from (R)- and (S)-5-methoxy-3-amino-3,4-dihydro-2H-1-benzopyran. The absolute configuration of the respective (R)- and (S)-enantiomers was deduced from X-ray crystallography of (R)-3-(N-isopropylamino)-5-methoxy-3,4-dihydro-2H-1-benzopyran, (R)-9a. Various 5-substituents were introduced via palladium-catalyzed carbonylation of N-substituted 3-amino-5-trifluoromethanesulfonyloxy-3,4-dihydro-2H-1-benzopyran. The effect of N- and 5-substitution on affinity for the 5-HT1A receptor was evaluated in competition experiments using rat hippocampal membranes and [3H]8-OH-DPAT as radioligand. Selected compounds were also tested for their affinity to the D1 (rat striatum), D2 (rat striatum), D2A (human cloned), and 5-HT2A (rat cortex) receptors. The intrinsic activity of the compounds was evaluated by measuring their effect on VIP-stimulated cAMP production in GH4ZD10 cells stably transfected with the 5-HT1A receptor. High-affinity compounds with high selectivity for the 5-HT1A receptor were found among structures substituted with carboxylate esters, amides, and ketones in the 5-position. Primary and secondary amines bound with lower affinity than tertiary amines. Larger substituents were well-tolerated by the receptor, but the smaller N-ethyl-N-isopropyl bound with lower affinity. Generally, the (R)-enantiomers displayed higher affinity for the 5-HT1A receptor than the corresponding (S)-enantiomers. In the present series of compounds, both full and partial agonists were found.

Interactions of ligands with active and inactive conformations of the dopamine D2 receptor.

The affinities of 19 pharmacologically diverse dopamine D2 receptor ligands were determined for the active and inactive conformations of cloned human dopamine D2 receptors expressed in Ltk cells. The agonist [3H]quinpirole was used to selectively label the guanine nucleotide-binding protein-coupled, active receptor conformation. The antagonist [3H]raclopride, in the presence of the non-hydrolysable GTP-analogue Gpp(NH)p and sodium ions and in the absence of magnesium ions, was used to label the free inactive receptor conformation. The intrinsic activities of the ligands were determined in a forskolin-stimulated cyclic AMP assay using the same cells. An excellent correlation was shown between the affinity ratios (KR/KRG) of the ligands for the two receptor conformations and their intrinsic activity (r=0.96). The ligands included eight structurally related and enantiopure 2-aminotetralin derivatives; the enantiomers of 5-hydroxy-2-(dipropylamino)tetralin, 5-methoxy-2-(dipropylamino)tetralin, 5-fluoro-2-(dipropylamino)tetralin and 2-(dipropylamino)tetralin. The (S)-enantiomers behaved as full agonists in the cyclic AMP assay and displayed a large KR/KRG ratio. The (R)-enantiomers were classified as partial agonists and had lower ratios. The structure-affinity relationships of these compounds at the active and the inactive receptor conformations were analysed separately, and used in conjunction with a homology based receptor model of the dopamine D2 receptor. This led to proposed binding modes for agonists, antagonists and partial agonists in the 2-aminotetralin series. The concepts used in this study should be of value in the design of ligands with predetermined affinity and intrinsic activity.

Site-directed mutagenesis of the human 5-HT1B receptor.

Site-directed mutagenesis was used to investigate the molecular interactions involved in ligand binding to the human 5-HT1B receptor. Six mutants were constructed at four positions and expressed in Chinese hamster ovary cells. Substitution of the amino acid F185 in transmembrane region IV by an alanine increased the affinities of sumatriptan, methysergide and 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) 3-4-fold and substitution by a methionine increased the affinities of methysergide and methiothepin 2- and 3-fold, respectively. Substitution of amino acid S334 in transmembrane region VI by an alanine increased the affinity of 8-OH-DPAT 5-fold. In accordance with this, the EC50 value of 8-OH-DPAT was decreased 7-fold. This suggests that the serine at position 334 causes steric hindrance for 8-OH-DPAT binding that is lost in the S334A mutant. Mutation of F354 in transmembrane region VII, which differs between receptor subtypes, increased the affinity of methiothepin 2-3-fold but the affinities of the other compounds tested were essentially unchanged.

Mutagenesis of the human 5-HT1B receptor: differences from the closely related 5-HT1A receptor and the role of residue F331 in signal transduction.

We have used a combination of sequence comparisons, computer-based modeling and site-directed mutagenesis to investigate the molecular interactions involved in ligand binding and signal transduction of the human 5-HT1B receptor. Two amino acid residues, S212 in transmembrane region (TM) V and F331 in TM VI, were replaced by alanines. These amino acids are conserved in many G protein-coupled receptors and therefore likely to be important for receptor function. The mutant receptors were expressed in Chinese hamster ovary cells. The 5-HT-like agonist 5-carboxamido-tryptamine (5-CT) bound with 15-fold lower affinity to the S212A mutant as compared to wild-type receptor and the antagonist methiothepin bound with 17-fold lower affinity to the F331A mutant. No reduction in the affinity of 5-HT was seen for the S212A mutant, although an equivalent mutation in the 5-HT1A receptor resulted in a 100-fold reduction of 5-HT binding. The inhibition of forskolin-stimulated cyclic AMP production by 5-HT was significantly reduced in cells expressing the F331A mutant, even though the endogenous ligand 5-HT bound with somewhat increased affinity. Methiothepin acted as an inverse agonist and increased the forskolin-stimulated cyclic AMP production at both the wild-type receptor and the mutants, and the effect was stronger on the F331A mutant. These results suggest that F331 is involved in the conformational changes necessary for signal transduction.

Mutagenesis study on human galanin receptor GalR1 reveals domains involved in ligand binding.

Many receptor mutants were generated and several NH2-terminally modified galanin analogs synthesized to define the regions of hGalR1 involved in galanin binding. Ligand binding properties and functionality of mutant receptors were evaluated. The His264Ala and Phe282Ala receptor mutants, although deficient in binding in the concentration range of galanin used, remained functional albeit at least 20-fold less efficient than the wild-type receptor in the inhibition of stimulated cAMP production. Hence, His264 and Phe282 of hGalR1 are directly involved in galanin binding. NH2-terminal carboxylic acid analogs of galanin (1-16) have a very low affinity for the wild-type receptor, but substantially increased affinity for the Glu271Lys-hGalR1, suggesting that the NH2-terminus of galanin binds to the receptor near the transmembrane (TM) VI. Based on these findings and computer-aided molecular modeling, we propose a binding site model for the hGalR1 receptor (possibly also for other galanin receptor subtypes): galanin binds with its NH2-terminus to the pocket between TM III and TM VI, Trp2 of galanin interacts with His264 of the receptor, and Tyr9 is involved in an aromatic-aromatic type of interaction with Phe282 of ECIII of GalR1.

Antimuscarinic 3-(2-furanyl)quinuclidin-2-ene derivatives: synthesis and structure-activity relationships.

A series of 25 derivatives of the muscarinic antagonist 3-(2-furanyl)quinuclidin-2-ene (4) was synthesized and evaluated for muscarinic and antimuscarinic properties. Substitution at all three positions of the furan ring has been investigated. The affinities of the new compounds were determined by competition experiments in homogenates of cerebral cortex, heart, parotid gland, and urinary bladder from guinea pigs using (-)-[3H]-3-quinuclidinyl benzilate as the radioligand, and the antimuscarinic potency was determined in a functional assay on isolated guinea pig urinary bladder using carbachol as the agonist. Several of the novel derivatives displayed high muscarinic affinities. Whereas the affinity of lead compound 4 for cortical muscarinic receptors is moderate (Ki = 300 nM), it is much higher for the 5-methyl (48; Ki = 12 nM), 5-ethyl (52; Ki = 7.4 nM), 5-bromo (33; Ki = 6.4 nM), and 3-phenyl (49; Ki = 2.8 nM) substituted derivatives. The substituent-induced increases in affinity do not appear to be additive as a 5-bromo-3-phenyl (54), and a 5-methyl-3-phenyl (55) substitution pattern only slightly increases affinity (Ki = 1.55 and 2.39 nM, respectively). The conformational preferences of the 3-phenyl (49) and 5-phenyl (51) derivatives were studied by X-ray crystallography and molecular mechanics calculations. Because of the observed high affinity of 49, a series of 16 meta- and para-substituted analogues of 49 was synthesized and tested. The m-hydroxy derivative (68) exhibited more than 10-fold improvement in affinity as compared to 49. The structure-activity relationships of the new series are well described with QSAR and CoMFA models.

Mutagenesis and ligand modification studies on galanin binding to its GTP-binding-protein-coupled receptor GalR1.

In this study, a large number of receptor mutants were generated and several N-terminally modified galanin analogues synthesized to refine the previously proposed binding site model for galanin to its GTP-binding-protein-coupled receptor GalR1. In addition to ligand-binding studies, the functionality of mutant receptors was evaluated by assessing their ability to mediate galaninergic inhibition of isoproterenol-stimulated adenylyl cyclase activity. The His264Ala and Phe282Ala receptor mutants, although deficient in binding in the concentration range of galanin used, remain functional albeit 20-fold less efficient than the wild-type receptor in mediating inhibition of stimulated cAMP production by galanin. The His267Ala mutant is, apart from being deficient in galanin binding, also severely impaired in functional coupling. While His264 and Phe282 seem to be important in forming the binding pocket for galanin, His267 might play a role in forming or stabilizing the active conformation of the GalR1 receptor rather than directly participating in the formation of the binding pocket for galanin. N-terminal carboxylic acid analogues of galanin have low affinity to wild-type GalR1, but substantially increased affinity to the Glu271Lys receptor mutant. This, together with the finding that an alanine substitution of Phe115 in TM III results in a tenfold decrease in affinity for galanin, suggests that the N-terminus of galanin interacts with Phe115. In contrast to the Phe282Ala mutation in TM VII, a conservative mutation of Phe282 to tyrosine did not alter the affinity for galanin. Thus, the interaction between Tyr9 of galanin and Phe282 is likely to be of an aromatic-aromatic nature.

Molecular design using the minireceptor concept.

Explicit molecular binding pockets were constructed and optimized around sets of superimposed ligands using the minireceptor concept. The resulting binding sites incorporate the properties of the different ligands and were shown to be suitable for the design of molecules presenting novel interaction patterns. Two applications of minireceptor construction and/or optimization, followed by molecular design are described. In the pursuit of new ligands mimicking the action of paclitaxel, a minireceptor was constructed using the primary amino acid sequence of the target protein as a guide. The active site extracted from a homology-based model of the serotonin 5-HT1A receptor was optimized around a set of three ligands using the same approach.

10-substituted 11-oxygenated (R)-aporphines: synthesis, pharmacology, and modeling of 5-HT1A receptor interactions.

Derivatives of the selective serotonin 5-HT1A receptor agonist (R)-11-hydroxy-10-methylaporphine (2) having various substituents in the C10-position or at the nitrogen have been synthesized from natural morphine or 6-O-acetylcodeine, respectively. The C10-substituents were introduced using efficient Stille or Suzuki cross-coupling reactions. The compounds were evaluated for their affinities to 5-HT1A and dopamine (DA) D1 and D2A receptors in vitro. All compounds tested displayed low (micromolar) affinities to D1 and D2A receptors. In addition, changes in steric bulk and/or electronic properties of the C10-substituent as compared to a C10-methyl group, as well as substitution of the N-methyl group for a hydrogen or a larger N-alkyl group, produced a marked decrease in the affinities to 5-HT1A receptors. Selected compounds that displayed moderate to high affinities to 5-HT1A receptors were evaluated for their ability to stimulate 5-HT1A receptors in vivo. The evaluated compounds behaved as agonists at 5-HT1A receptors, except for the N-propyl analogue of 2, (R)-11-hydroxy-10-methyl-N-propylnoraporphine (23), which displayed weak DA receptor agonism at the doses tested. Hence, the substitution pattern of 2 (a C10-methyl, a C11-hydroxy, and an N-methyl group) appears to be optimal for potent interaction of 10,11-disubstituted (R)-aporphines with 5-HT1A receptors. Modeling of ligand-5-HT1A receptor interactions was performed in an attempt to rationalize the observed affinity data. The binding site model suggests the presence of a "methyl pocket" in the 5-HT1A receptor binding ste. The C11-methoxy-substituted aporphines appear to have a different binding mode compared to 2, implying a different accessibility of these compounds to the "methyl pocket".

11-substituted (R)-aporphines: synthesis, pharmacology, and modeling of D2A and 5-HT1A receptor interactions.

A series of C11-substituted (R)-aporphines and C11-oxygenated (R)-noraporphines has been synthesized and evaluated for central serotonergic and dopaminergic effects in vitro and in vivo. The various C11-substituents were introduced using efficient nickel- and palladium-catalyzed reactions of the corresponding triflate (R)-11-[[(trifluoromethyl)sulfonyl]oxy]aporphine (6). Several compounds display high affinity to serotonin 5-HT1A receptors in spite of major differences in steric bulk and electronic properties of the various C11-substituents. A change of the N-methyl group of the nonselective 3 to H [23, (R)-11-hydroxynoraporphine] or propyl [2, (R)-11-hydroxy-N-propylnoraporphine] increases the selectivity for 5-HT1A receptors (100-fold) and dopamine D2A receptors (3-fold), respectively. Compounds 3 and 23 have similar affinities to 5-HT1A receptors, whereas the propyl substituent of 2 not only enhances the selectivity for D2A receptors but also increases the D2A affinity. Modeling of ligand-receptor binding site interactions yielded an interaction site model for the 5-HT1A receptor that describes a gradual change in binding mode for C11-hydroxy, -methoxy-, and -phenyl-substituted derivatives. Hydrogen bonding is hereby gradually replaced by van der Waals interactions involving a relatively large lipophilic pocket. The derived D2A receptor model can accommodate both the N-propyl substituent of 2 and the C11-ethyl substituent of 11 [(R)-11-ethylaporphine].

3-(2-Benzofuranyl)quinuclidin-2-ene derivatives: novel muscarinic antagonists.

A series of 26 derivatives of the novel muscarinic antagonist 3-(2-benzofuranyl)quinuclidin-2-ene (1) has been synthesized and evaluated for muscarinic and antimuscarinic properties. The affinity of the compounds was determined by competition experiments in homogenates of cerebral cortex, heart, parotid gland, and urinary bladder from guinea pigs using (-)-[3H]-3-quinuclidinyl benzilate as the radioligand, and the antimuscarinic-potency was determined in a functional assay on isolated guinea pig urinary bladder using carbachol as the agonist. The 5-fluorobenzofuranyl derivative was slightly more potent than 1. The 7-bromo-substituted 8 displayed a 14-fold tissue selectivity ratio for muscarinic receptors in the cortex versus the parotid gland. Comparative molecular field analysis and quantitative structure-activity relationship models were developed for this series of substituted benzofuranyl derivatives.

Delineation of the peptide binding site of the human galanin receptor.

Galanin, a neuroendocrine peptide of 29 amino acids, binds to Gi/Go-coupled receptors to trigger cellular responses. To determine which amino acids of the recently cloned seven-transmembrane domain-type human galanin receptor are involved in the high-affinity binding of the endogenous peptide ligand, we performed a mutagenesis study. Mutation of the His264 or His267 of transmembrane domain VI to alanine, or of Phe282 of transmembrane domain VII to glycine, results in an apparent loss of galanin binding. The substitution of Glu271 to serine in the extracellular loop III of the receptor causes a 12-fold loss in affinity for galanin. We combined the mutagenesis results with data on the pharmacophores (Trp2, Tyr9) of galanin and with molecular modelling of the receptor using bacteriorhodopsin as a model. Based on these studies, we propose a binding site model for the endogenous peptide ligand in the galanin receptor where the N-terminus of galanin hydrogen bonds with Glu271 of the receptor, Trp2 of galanin interacts with the Zn2+ sensitive pair of His264 and His267 of transmembrane domain VI, and Tyr9 of galanin interacts with Phe282 of transmembrane domain VII, while the C-terminus of galanin is pointing towards the N-terminus of th

Synthesis and pharmacology of the enantiomers of UH301: opposing interactions with 5-HT1A receptors.

The (S)-enantiomer of 5-fluoro-8-hydroxy-2-(dipropylamino) tetralin [(S)-2a; (S)-UH301] was the first reported 5-HT1A receptor antagonist. We now give a full account on the synthetic effort leading to the preparation of the racemate and the enantiomers of 2a. The crystal and molecular structure of 2a. HBr has been determined by X-ray diffraction and the absolute configuration has been deduced using statistical tests of the crystallographic R values. The unit cell is tetragonal (P4(1)2(1)2) with a = b = 13.2235(2), c = 39.560(1) A and contains two crystallographically independent molecules in each asymmetric unit. The two solid state conformers differ in the conformation of the N-propyl groups. The pharmacological characterization of the enantiomers was done by use of in vivo biochemical and behavioural assays in rats. The (R)-enantiomer of 2a is a 5-HT1A receptor agonist of low potency while (S)-2a does not exhibit any agonist properties at 5-HT1A receptors. As a consequence of the opposing effects of the enantiomers, the racemate, rac-2a, does not produce any clear-cut effects in rats. The reduced efficacy of (S)-2a as compared to the well known 5-HT1A receptor agonist 8-hydroxy-2-(dipropylamino) tetralin (1;8-OH-DPAT) may be due to the fluoro-substituent induced negative potential of the aromatic ring.

3-Heteroaryl-substituted quinuclidin-3-ol and quinuclidin-2-ene derivatives as muscarinic antagonists. Synthesis and structure-activity relationships.

A number of 3-heteroaryl-substituted quinuclidin-3-ol and quinuclidin-2-ene derivatives have been prepared and evaluated for muscarinic and antimuscarinic properties. The affinities of the new compounds (13, 14, 16-32, and 36-52a,b) were tested in homogenates of cerebral cortex, heart, parotid gland, and urinary bladder from guinea pigs using (-)-[3H]-3-quinuclidinyl benzilate [(-)-[3H]QNB] as the radioligand and in a functional assay using isolated guinea pig urinary bladder. The present compounds behaved as competitive muscarinic antagonists in the urinary bladder. The highest receptor binding affinity, Ki (cortex) = 9.6 nM, was observed for 3-(2-benzofuranyl)quinuclidin-2-ene (31). The corresponding 3-benzofuranyl (36) and 3-benzothienyl (37) homologues had about 3.5-fold lower affinity for cortical muscarinic receptors. All quinuclidin-3-ol derivatives (14 and 16-25) had lower binding affinities for the different muscarinic receptor subtypes than the corresponding quinuclidin-2-ene analogues when examined in the various tissue homogenates. In general, the new compounds showed low subtype selectivity. The structure-affinity relationships are discussed in terms of differences in proton basicity of the azabicyclic nitrogen and differences in geometric, conformational, and/or electronic properties of the compounds. The cortical antimuscarinic potency is also related to the complementarity of the compounds to the putative binding site of the muscarinic m1 receptor.

(R)-11-hydroxy- and (R)-11-hydroxy-10-methylaporphine: synthesis, pharmacology, and modeling of D2A and 5-HT1A receptor interactions.

(R)-11-Hydroxyaporphine (2) and (R)-11-hydroxy-10-methylaporphine (3) were synthesized from natural morphine by using new, short, and efficient synthetic sequences. The dopaminergic and serotonergic effects of 2 and 3 were evaluated by use of in vitro and in vivo test systems. The results indicate that 3 is a potent, selective, and efficacious 5-HT1A receptor agonist. In contrast, 2 is a partial 5-HT1A receptor agonist of low potency which has affinity also for central D1 and D2A receptors. The differences in pharmacological profiles were rationalized by modeling of ligand-receptor interactions using homology-based receptor models of the 5-HT1A and D2A receptor binding site. The selective and pronounced serotonergic effects of 3 appear to be due to the C10-methyl group, which is accommodated by a lipophilic pocket in the 5-HT1A receptor. In contrast, the C10-methyl group of 3 is not accommodated by the binding site model of the D2A receptor.

Derivatives of cis-2-amino-8-hydroxy-1-methyltetralin: mixed 5-HT1A-receptor agonists and dopamine D2-receptor antagonists.

(1S,2R)-8-Hydroxy-1-methyl-2-(dipropylamino)tetralin [(1S,2R)-3] has been previously characterized as a selective and potent but partial 5-HT1A-receptor agonist. In the present study, we have prepared derivatives of (1S,2R)- and (1R,2S)-3 in which various C8-substituents have been introduced. In addition, the enantiomers of the N-isopropyl-N-n-propylamino derivative of 3 were prepared. The new derivatives were tested in vivo by use of behavioral and biochemical tests in rats. In addition, the affinity of the compounds was studied by competition experiments with [3H]-8-OH-DPAT in rat brain tissue. The only new derivative which behaved like a selective 5-HT1A-receptor agonist was the C8-carboxamide derivative (1S,2R)-13. The other active derivatives, including (1S,2R)-3, have more complicated pharmacological profiles and may be best characterized as mixed 5-HT1A-receptor agonists/dopamine D2-receptor antagonists.

Quinuclidin-2-ene-based muscarinic antagonists.

A series of achiral 3-heteroaryl substituted quinuclidin-2-ene derivatives and related compounds have been synthesized by facile methods. The compounds were evaluated for muscarinic and antimuscarinic properties in receptor binding studies using (-)-[3H]-QNB as the radioligand and in a functional assay using isolated guinea pig urinary bladder. 3-(2-Benzofuranyl)-quinuclidin-2-ene (15) displayed the highest M1-receptor affinity in the present series (Ki = 9.6 nM).