[Show abstract][Hide abstract] ABSTRACT: Studies have demonstrated the presence of allosteric binding sites on each of the muscarinic acetylcholine receptor (mAChR) subtypes. Since most drugs targeting muscarinic receptors bind to the highly conserved orthosteric binding site, they fail to achieve appreciable subtype selectivity. Targeting non-conserved allosteric sites may provide a new way of enhancing selectivity for individual subtypes of muscarinic receptor. Tetra(ethyleneglycol)(3-methoxy- 1,2,5-thiadiazol-4-yl)[3-(1-methyl-1,2,5,6-tetrahydropyrid-3-yl)-1,2, 5-thiadiazol-4-yl] ether, CDD-0304 (10), was found to be a M1/2/4 selective muscarinic agonist and might prove useful in treating the symptoms associated with schizophrenia (J. Med. Chem. 2003, 46, 4273). It was hypothesized that the observed subtype selectivity demonstrated by 10 may be due to its ability to function as a bitopic ligand (J. Med. Chem. 2006, 49, 7518). To further investigate this possibility, a novel series of compounds was synthesized using a 1,2,5-thiadiazole moiety along with varying lengths of a polyethylene glycol linker and terminal groups, for evaluation as potential allosteric modulators of muscarinic receptors. Preliminary biological studies were performed using carbachol to stimulate M1 and M5 receptors. No significant agonist activity was observed at either M1 or M5 receptors for any of the compounds. Compound 18, 2-(4-methoxy-1,2,5-thiadiazol-3-yloxy)-N,N-dimethylethanamine fumarate (CDD-0361F) was found to block the effects of carbachol at M5 muscarinic receptors.
[Show abstract][Hide abstract] ABSTRACT: Theoretical calculations were carried out for studying the tautomeric protonation of N-methylpiperazine as a prototype six-member aliphatic ring containing a secondary and a tertiary nitrogen atom. The protonation was investigated in three solvents, water, acetonitrile, and dichloromethane. Calculations were performed up to the B3LYP/aug-cc-pvtz and QCISD(T)/CBS levels by applying the IEF-PCM polarizable continuum dielectric solvent model. Relative solvation free energies also were calculated upon explicit solvent models by utilizing the free-energy perturbation theory as implemented in Monte Carlo simulations. The relative free energy for the N-methylpiperazine tautomer protonated at the secondary (NMps) rather than at the tertiary (NMpt) nitrogen was calculated at a ratio of 47/53 in infinitely dilute aqueous solution. The ratio further decreased in lower-polarity solvents. In contrast, NMR experiments suggested that the protonation takes place primarily at the secondary nitrogen in 0.37 molar aqueous solution with NMps/NMpt = 80/20. The NMps tautomer was exclusive in dichloromethane at the same concentration. The discrepancy between theory and experiment was resolved by considering association equilibria in parallel with the protonation for the solute. As a result, the theoretically predicted tautomer ratios were obtained in close agreement with the experimental values. The NMps tautomer could form a preferable dimeric structure, where one or two chloride anion(s) was/were in hydrogen bonds with protons of the associating monomers. The calculations suggest that the proton relocation may take place by solvent assistance in water or along an intramolecular proton jump in the twist-boat conformation. The predicted activation free energy was about 10 kcal/mol on the basis of variable-temperature NMR experiments in DCM.
Full-text · Article · Dec 2010 · The Journal of Physical Chemistry B