Opioid analgesics remain the drug of choice for management of acute and chronic pain. However, for treatment of pain in the presence of inflammation, their use is limited because inflammation reduces the efficacy of opioids. This is due primarily to chemokines whose release in the inflammatory state is responsible for promoting increased levels of glutamate that leads to a lowering of the pain threshold. In this regard, the use of morphine, either with a chemokine receptor antagonist or a metabotropic glutamate-5 receptor (mGluR5) antagonist, are known to increase the efficacy of morphine and prevent the establishment of side effects such as tolerance, dependence, constipation, etc., under inflammatory conditions. Given the coexistence of opioid, chemokine, and mGluR5 receptors in glia and neurons, together with reports that indicate coexpressed opioid/chemokine and opioid/mGluR5 receptors in cultured cells associate as heteromers, there is a reasonable possibility that similar association of these receptors exists in vivo. We therefore propose to synthesize and biologically evaluate ligands that selectively target putative heteromers of opioid-chemokine and opioid-mGlu5 receptors for use as pharmacological tools in an effort to bridge the divide between cultured cells and in vivo pharmacology. Our approach involves the attachment of a mu opioid agonist pharmacophore to either a chemokine (CCR5, or CXCR4) antagonist (series 1, 2) or mGluR5 antagonist (series 3) pharmacophore via a spacer. Each member of the series will contain homologous spacers in order to assess the optimal bridging to each of the heteromers. Monovalent ligands (series 4-7) containing the above pharmacophores also will be synthesized as controls for this study. Functional studies of these series will be conducted in cultured cells that contain heteromers (MOPR-CCR5, MOPR-CXCR4, and MOPR-mGluR5) or the corresponding homomers. Immunofluorescence studies on these cells will be conducted in order to study trafficking of receptors that are activated by heteromer-selective bivalent ligands. Bivalent ligands that are potent, selective, and whose characteristics are consistent with bridging of opioid-chemokine and opioid-mGluR5 heteromers devoid of tolerance will be tested in mice. In the long term, the approach of activating opioid receptors while simultaneously antagonizing chemokine or mGluR5 receptors could lead to the development of improved analgesics devoid of the classical side effects of morphine.
Opioid analgesics such as morphine have the drawback of tolerance, physical dependence, and abuse potential. This project proposes to prepare novel, potent analgesics that will produce no tolerance or physical dependence upon chronic administration.
