Pharmacological and pharmacokinetic characterization of the cannabinoid receptor 2 agonist, GW405833, utilizing rodent models of acute and chronic pain, anxiety, ataxia and catalepsy.

To date, two cannabinoid receptors have been identified, CB1 and CB2. Activation of these receptors with non-selective cannabinoid receptor agonists reduces pain sensitivity in animals and humans. However, activation of CB1 receptors is also associated with central side effects, including ataxia and catalepsy. More recently, a role for selective CB2 agonists in pain modification has been demonstrated. GW405833, a selective CB2 agonist, was recently reported to partially reverse the inflammation and hyperalgesia in a rat model of acute inflammation. In the current report, we extend the characterization and therapeutic potential of this compound. For the first time, we show that GW405833 selectively binds both rat and human CB2 receptors with high affinity, where it acts as a partial agonist (approximately 50% reduction of forskolin-mediated cAMP production compared to the full cannabinoid agonist, CP55,940). We also report for the first time that intraperitoneal administration of GW405833 (0.3-100 mg/kg) to rats shows linear, dose-dependent increases in plasma levels and substantial penetration into the central nervous system. In addition, GW405833 (up to 30 mg/kg) elicits potent and efficacious antihyperalgesic effects in rodent models of neuropathic, incisional and chronic inflammatory pain, the first description of this compound in these models. In contrast, analgesia, sedation and catalepsy were not observed in this dose range, but were apparent at 100 mg/kg. Additionally, GW405833 was not antihyperalgesic against chronic inflammatory pain in CB2 knockout mice. These data support the tenet that selective CB2 receptor agonists have the potential to treat pain without eliciting the centrally-mediated side effects associated with non-selective cannabinoid agonists, and highlight the utility of GW405833 for the investigation of CB2 physiology.

A role for cannabinoid receptors, but not endogenous opioids, in the antinociceptive activity of the CB2-selective agonist, GW405833.

Several recent reports have demonstrated a role for selective cannabinoid CB2 receptor agonists in pain modulation, showing both analgesic and antihyperalgesic activities. While the mechanism of action is poorly understood, it has been postulated that these effects may be indirect, involving release of endogenous opioids. We have previously reported that administration of the selective cannabinoid CB2 receptor agonist GW405833 (2,3-dichloro-phenyl)-[5-methoxy-2-methyl-3-(2-morpholin-4-yl-ethyl)-indol-1-yl]-methanone) to rats elicits potent and efficacious antihyperalgesic effects against neuropathic and inflammatory pain and, at high dose (100 mg/kg), is analgesic and ataxic [Valenzano, K.J., Tafesse, L., Lee, G., Harrison, J.E., Boulet, J., Gottshall, S.L., Mark, L., Pearson, M.S., Miller, W., Shan, S., Rabadi, L., Rotstheyn, Y., Chaffer, S.M., Turchin, P.I., Elsemore, D.A., Toth, M., Koetzner, L., Whiteside, G.T., 2005. Pharmacological and pharmacokinetic characterization of the cannabinoid receptor 2 agonist, GW405833, utilizing rodent models of acute and chronic pain, anxiety, ataxia and catalepsy. Neuropharmacology 48, 658-672]. In the current study, we confirm these properties using mouse models and investigate the role of cannabinoid CB2 receptors using knockout animals. Furthermore, we provide evidence that the antinociceptive properties of GW405833 are opioid independent. GW405833 elicited robust antihyperalgesic effects in mouse models of inflammatory (Freund's complete adjuvant) and neuropathic (Seltzer) pain. In contrast, GW405833 showed no antihyperalgesic activity against Freund's complete adjuvant-mediated inflammatory pain in cannabinoid CB2 receptor knockout mice. As in rats, high-dose GW405833 (100 mg/kg) showed both analgesic and sedative activities in wild-type mice, activities that were also apparent in cannabinoid CB2 receptor knockout mice. In rats, neither the antihyperalgesic effect in the Freund's complete adjuvant model nor the analgesic effects in tail flick and hot plate assays were inhibited by pre-treatment with the non-selective opioid receptor antagonist, naltrexone. These data demonstrate that the antihyperalgesic effects of GW405833 are mediated via the cannabinoid CB2 receptor, whereas the analgesic and sedative effects are not. Furthermore, these data suggest that the mechanism of action for GW405833 does not depend on the release of endogenous opioids.

Pharmacology of 2-[4-(4-chloro-2-fluorophenoxy)phenyl]-pyrimidine-4-carboxamide: a potent, broad-spectrum state-dependent sodium channel blocker for treating pain states.

Voltage-gated Na(+) channels may play important roles in establishing pathological neuronal hyperexcitability associated with chronic pain in humans. Na(+) channel blockers, such as carbamazepine (CBZ) and lamotrigine (LTG), are efficacious in treating neuropathic pain; however, their therapeutic utility is compromised by central nervous system side effects. We reasoned that it may be possible to gain superior control over pain states and, in particular, a better therapeutic index, by designing broad-spectrum Na(+) channel blockers with higher potency, faster onset kinetics, and greater levels of state dependence than existing drugs. 2-[4-(4-Chloro-2-fluorophenoxy)phenyl]-pyrimidine-4-carboxamide (PPPA) is a novel structural analog of the state-dependent Na(+) channel blocker V102862 [4-(4-fluorophenoxy)benzaldehyde semicarbazone]. Tested on recombinant rat Na(v)1.2 channels and native Na(+) currents in cultured rat dorsal root ganglion neurons, PPPA was approximately 1000 times more potent, had 2000-fold faster binding kinetics, and > or =10-fold higher levels of state dependence than CBZ and LTG. Tested in rat pain models against mechanical endpoints, PPPA had minimal effective doses of 1 to 3 mg/kg p.o. in partial sciatic nerve ligation, Freund's complete adjuvant, and postincisional pain. In all cases, efficacy was similar to clinically relevant comparators. Importantly, PPPA did not produce motor deficits in the accelerating Rotarod assay of ataxia at doses up to 30 mg/kg p.o., indicating a therapeutic index >10, which was superior to CBZ and LTG. Our experiments suggest that high-potency, broad-spectrum, state-dependent Na(+) channel blockers will have clinical utility for treating neuropathic, inflammatory, and postsurgical pain. Optimizing the biophysical parameters of broad-spectrum voltage-gated Na(+) channel blockers may lead to improved pain therapeutics.

DiPOA ([8-(3,3-diphenyl-propyl)-4-oxo-1-phenyl-1,3,8-triazaspiro[4.5]dec-3-yl]-acetic acid), a novel, systemically available, and peripherally restricted Mu opioid agonist with antihyperalgesic activity: II. In vivo pharmacological characterization in the rat.

Mu opioid receptors are expressed throughout the central and peripheral nervous systems. Peripheral inflammation leads to an increase in mu receptor present on the peripheral terminals of primary sensory neurons. Activation of peripheral mu receptors produces potent antihyperalgesic effects in both humans and animals. Here, we describe the in vivo pharmacological properties of the structurally novel, highly potent, systemically available yet peripherally restricted mu opioid agonist, [8-(3,3-diphenyl-propyl)-4-oxo-1-phenyl-1,3,8-triaza-spiro[4.5]dec-3-yl]-acetic acid (DiPOA). DiPOA administered i.p. produced naltrexone-sensitive, dose-dependent reversal of Freund's complete adjuvant-induced inflammatory mechanical hyperalgesia (1-10 mg/kg). Maximum percent reversal (67%) was seen 1 h postadministration at 10 mg/kg (the highest dose studied). DiPOA also proved antihyperalgesic in a model of postsurgical pain with a maximum percent reversal of 85% 1 h postadministration at 30 mg/kg i.p. (the highest dose studied). DiPOA administered i.p. had no effect in the tail flick assay of acute pain (0.1-10 mg/kg), produced no ataxia as measured by latency to fall from an accelerating rotarod (3-30 mg/kg), and was not antihyperalgesic in the Seltzer model of neuropathic pain (1-10 mg/kg). This is the first report of a peripherally restricted, small-molecule mu opioid agonist that is nonsedating, antihyperalgesic, and effective against inflammatory and postsurgical pain when administered systemically.