This invention concerns bicarbocyclic and tricarbocyclic ethynyl derivatives which act as allosteric modulators of the metabotropic glutamate receptor 5 (mGlu5 receptors or mGluR5), as well as pharmaceutical compositions and methods of treatment utilizing these compounds.
Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. One means of modulating glutamate neurotransmission is through metabotropic glutamate receptors (mGluRs); another means being ionotropic receptors. Presently, eight mGluRs have been cloned and classified into three groups based on sequence homology, preferred signal transduction pathway and pharmacology. Group I of mGluRs includes mGluR1 and mGluR5, while Group II comprises mGluR2 and mGluR3 and Group III comprises mGlu4, 6, 7 and 8 receptors.
mGlu receptors have an essential role in normal brain functions, as well as in neurological, psychiatric, and neuromuscular disorders. mGlu5 receptors are located primarily postsynaptically and highly expressed in the limbic brain regions. mGlu5 receptors also are expressed in the thalamus, spinal cord, and vagal nerve systems, as well as peripherally in the skin on nerve endings and C fibers.
Ligands to the mGlu5 receptors have been shown to have promise for peripheral and central nervous system disorders. See e.g., G. Jaeschke et al., “mGlu5 receptor antagonists and their therapeutic potential,” Expert Opin. Ther. Patents, 2008, 18, 2: 123-142. Yet some proffer that glutamate analogs targeting the orthosteric binding site may be limited by low brain penetration and insufficient selectivity with respect to the different mGluRs subtypes. Synthetic agonists may lead to continuous stimulation of the receptor since they are often designed to be metabolically stable. This continuous stimulation is not necessarily desirable, due to potential receptor desensitization issues. Also, with respect to receptor occupancy, synthetic antagonists may lead to prolonged blockade of receptor function, which may not be compatible with the kinetics of the pathology of a central nervous system disorder.
However, a more selective and controlled “fine-tuning” action on the mGlu5 receptor is feasible through allosteric modulation. See e.g., P. Bach et al., “Metabotropic glutamate receptor 5 modulators and their potential therapeutic applications,” Expert Opin. Ther. Patents, 2007, 17, 4: 371-381. Allosteric modulation refers to binding by a modulator ligand to a site on a receptor that is different from the orthosteric primary substrate or ligand binding site. This ligand binding process results in conformational changes, which may profoundly influence the function of the protein (e.g., G protein-coupled receptors such as mGluRs, including mGluR5). Novel mGluR5 ligands that allosterically modulate the mGlu5 receptor may improve the therapeutic window of traditional central nervous system agents and/or the treatment of central nervous system disorders. The present invention is directed to these, and other important, ends.