Development of Peripherally-acting Cannabinoid 1 Receptor Ligands

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DESCRIPTION (provided by applicant): Synthetic and naturally occurring cannabinoids are a focus of strong social, legal and medical controversy concerning their therapeutic utility, yet studies show that cannabinoids reduce the hyperalgesia and allodynia associated with persistent pain of neuropathic origin in humans and animals. Moreover, cannabinoids are effective in alleviating neuropathic pain symptoms after repeated treatment, unlike opioids, which have only limited effectiveness. However, the usefulness of existing cannabinoid-based analgesics is limited by their profile of psychotropic side-effects. The targets of cannabinoids may be defined by the distribution of two cloned subtypes of cannabinoid receptors, CB1R and CB2R. Both are members of the superfamily of G protein- coupled receptors of which CB1Ris expressed at high levels in the hippocampus, cortex, cerebellum and basal ganglia, whereas CB2R is primarily expressed in immunocompetent cells outside the central nervous system (CNS). CB1Rs are also found outside the CNS, especially in primary sensory neurons which are common sources of hyperexcitability thought responsible for the painful symptoms of many neuropathies. In this Exploratory and Developmental Research Project we seek to (Aim 1) develop peripherally-acting ligands at CB1Rs by designing analogs that are unable to cross the blood-brain barrier by virtue of their being charged compounds. Following their synthesis, these compounds will be compared with their parent brain-permeable CB1R ligands for their ability to activate CB1Rs using well established in vitro CB1R screening assays (Aim 2). Promising ligands will be screened for blood-brain barrier permeability in another in vitro assay to estimate the likely extent of the compounds' CNS penetration. These will be followed by (Aim 3) studying the effectiveness of peripherally-acting ligands in reducing the pain symptoms in a rat model of peripheral neuropathy. The most promising compounds will also be examined for potential CNS side effects in a modified tetrad assay. The data obtained from this pilot project are expected to prove useful to the development of new analgesics for the treatment of chronic pain of neuropathic origin. Our strategy may also contribute to the understanding of cannabinoid receptor pharmacology and to introduce a novel set of tools that can help study the physiological and pathophysiological importance of peripheral cannabinoid receptors. PUBLIC HEALTH RELEVANCE Neuropathic pain is extremely difficult to treat, in part because available drugs carry a high burden of central nervous system side-effects. Here we propose to design and test analogs of cannabinoid receptor ligands that are unable to cross the blood-brain barrier by virtue of their being charged compounds. As a result, we expect these drugs to exhibit analgesic properties with minimal psychotropic side-effects.