Synthesis of glutamate mimics as neuropathic pain modulating agents.

Abstract

As part of the vital search towards improved therapeutic agents for the treatment of neuropathic pain, the central nervous system ubiquitous glutamate receptors have become a major focus of research. As such, the discovery of glutamate receptor ligands with improved potency and selectivity has been an important area of study for many decades, though there is still much knowledge to be gained. Outlined herein are the syntheses towards a series of potentially biologically active 3’-cycloalkyl-substituted carboxycyclopropylglycine analogues. These syntheses utilize novel synthetic chemistry to construct the cyclopropane core with all required stereochemistry. As a consequence of this work, two new cycloalkylcarboxycycloproplyglycine analogues were successfully synthesized, utilizing the reaction of 1,2-dioxines with protected phosphonates in a 20% overall yield for one diastereoisomer. Secondly, the syntheses of a series of 1,4- and 1,5-substituted 1,2,3-triazole amino acids as a new class of potential glutamate receptor ligands. Briefly, a series of six 1,4- and 1,5-triazole amino acids were successfully synthesized utilizing both copper (I) and ruthenium-catalysed cycloaddition of functionalized azides and alkynes. Furthermore, contained within Chapter 4 are the details and results of in vitro binding assays used in screening for possible active compounds. As an example, in vitro drug screening at NMDA, kainate and AMPA ionotropic glutamate receptor subtypes revealed activity of triazole amino acid 48 with an EC₅₀ value of 49 μ M at AMPA receptors. Also, drug screening at metabotropic glutamate receptor subtypes 1, 2 and 4 revealed potent agonist activity of cyclopropane amino acid 44a at mGluR2 with an EC₅₀ value of 0.05 μ M. Cyclopropane amino acid 44a was thus selected for further testing in vivo in a rodent model of neuropathic pain. The results indicated that cyclopropane amino acid 44a significantly and dose-dependently decreased mechanical allodynia, one of the symptoms of neuropathic pain. It was suggested that this effect was due to activation of mGlu2 and 3 receptors located on both neuronal and glial cells within the dorsal horn of the spinal cord. Lastly, in an effort to rationalize the in vitro binding data, the newly synthesized cyclopropane and triazole amino acids were docked in silico into the NMDA, AMPA, mGluR1 and mGluR3 receptors available as x-ray crystal structures. Only limited data was obtained regarding the mGluR1 and mGluR3 dockings. However, AMPA receptor docking of the new in vitro active triazole amino acids 45 and 48 revealed positive docking interactions in agreement with those seen for the endogenous ligand, glutamate and the selective agonist AMPA. The docking of these new compounds was also computed to be highly energetically favourable, thus suggesting plausible binding modes.