Cannabinoid receptor ligand bias: implications in the central nervous system

The human cannabinoid subtype 1 receptor (hCB1R) is highly expressed in the CNS and serves as a therapeutic _target for endogenous ligands as well as plant-derived and synthetic cannabinoids. Unfortunately, acute use of hCB1R agonists produces unwanted psychotropic effects and chronic administration results in development of tolerance and dependence, limiting the potential clinical use of these ligands. Studies in β-arrestin knockout mice suggest that interaction of certain GPCRs, including μ-, δ-, κ-opioid and hCB1Rs, with β-arrestins might be responsible for several adverse effects produced by agonists acting at these receptors. Indeed, agonists that bias opioid receptor activation toward G-protein, relative to β-arrestin signaling, produce less severe adverse effects. These observations indicate that therapeutic utility of agonists acting at hCB1Rs might be improved by development of G-protein biased hCB1R agonists. Our laboratory recently reported a novel class of indole quinulidinone (IQD) compounds that bind cannabinoid receptors with relatively high affinity and act with varying efficacy. The purpose of this study was to determine whether agonists in this novel cannabinoid class exhibit ligand bias at hCB1 receptors. Our studies found that a novel IQD-derived hCB1 receptor agonist PNR-4-20 elicits robust G protein-dependent signaling, with transduction ratios similar to the non-biased hCB1R agonist CP-55,940. In marked contrast to CP-55,940, PNR-4-20 produces little to no β-arrestin 2 recruitment. Quantitative calculation of bias factors indicates that PNR-4-20 exhibits from 5.4-fold to 29.5-fold bias for G protein, relative to β-arrestin 2 signaling (when compared to G protein activation or inhibition of forskolin-stimulated cAMP accumulation, respectively). Importantly, as expected due to reduced β-arrestin 2 recruitment, chronic exposure of cells to PNR-4-20 results in significantly less desensitization and down-regulation of hCB1Rs compared to similar treatment with CP-55,940. PNR-4-20 (i.p.) is active in the cannabinoid tetrad in mice and chronic treatment results in development of less persistent tolerance and no significant withdrawal signs when compared to animals repeatedly exposed to the non-biased full agoinst JWH-018 or Δ9-THC. Finally, studies of a structurally similar analog PNR-4-02 show that it is also a G protein biased hCB1R agonist. It is predicted that cannabinoid agonists that bias hCB1R activation toward G protein, relative to β-arrestin 2 signaling, will produce fewer and less severe adverse effects both acutely and chronically.

The CB1 and CB2 cannabinoid receptors (CB1R, CB2R) are members of the G protein coupled receptor (GPCR) family that were identified over 20 years ago. CB1Rs and CB2Rs mediate the effects of Δ9-tetrahydrocannabinol (Δ9-THC), the principal psychoactive constituent of marijuana and subsequently identified endogenous cannabinoids (endocannabinoids) anandamide and 2-arachidonoyl glycerol. CB1Rs and CB2Rs have both similarities and differences in their pharmacology. Both receptors recognize multiple classes of agonist and antagonist compounds and produce an array of distinct downstream effects. Natural polymorphisms and alternative splice variants may also contribute to their pharmacological diversity. As our knowledge of the distinct differences grows, we may be able to _target select receptor conformations and their corresponding pharmacological responses. This chapter will discuss their pharmacological characterization, distribution, phylogeny and signaling pathways. In addition, the effects of extended agonist exposure and how that affects signaling and expression patterns of the receptors is considered.

Background Activation of cannabinoid CB1 receptors suppresses pathological pain but also produces unwanted central side effects. We hypothesized that a positive allosteric modulator (PAM) of CB1 signaling would suppress inflammatory and neuropathic pain without producing cannabimimetic effects or physical dependence. We also asked whether a CB1-PAM would synergize with inhibitors of endocannabinoid deactivation and/or an orthosteric cannabinoid agonist. Methods GAT211, a novel CB1-PAM, was evaluated for antinociceptive efficacy and tolerance in models of neuropathic and/or inflammatory pain. Cardinal signs of direct CB1-receptor activation were evaluated together with propensity to induce reward/aversion and physical dependence. Comparisons were made with inhibitors of endocannabinoid deactivation (JZL184, URB597) or an orthosteric cannabinoid agonist (WIN55,212-2). All studies used 5-11 subjects per group. Results GAT211 suppressed allodynia induced by complete Freund’s adjuvant (CFA) and the chemotherapeutic agent paclitaxel in wildtype but not CB1 knockout mice. GAT211 did not impede paclitaxel-induced tumor cell line toxicity. GAT211 did not produce cardinal signs of direct CB1-receptor activation in the presence or absence of pathological pain. GAT211 produced synergistic anti-allodynic effects with fatty-acid amide hydrolase (FAAH) and monoacylglycerol lipase (MGL) inhibitors in paclitaxel-treated mice. Therapeutic efficacy was preserved over 19 days of chronic dosing with GAT211 but not the MGL inhibitor JZL184. The CB1 antagonist rimonabant precipitated withdrawal in mice treated chronically with WIN55,212-2 but not GAT211. GAT211 did not induce condition place preference or aversion. Conclusions Positive allosteric modulation of CB1 receptor signaling shows promise as a safe and effective analgesic strategy that lacks tolerance, dependence and abuse liability.