Development and pharmacological characterization of a rat model of osteoarthritis pain.

Osteoarthritis (OA) is an age-related joint disease characterized by degeneration of articular cartilage and is associated with chronic pain. Although several experimental models of OA have been employed to investigate the underlying etiologies of the disease, there has been relatively little investigation into development of animal models of OA to study the pain associated with the condition. In the present study, we investigated OA induced by injection of either iodoacetate or papain into the knee joint of rats, and assessed the joint degeneration with radiographic analyses and measured pain behavior using hind limb weight bearing. We found that injection of iodoacetate, but not papain, resulted in a chronic joint degeneration as measured by decreased bone mineral content and bone mineral density, necrosis of articular cartilage and osteophyte formation. These pathological changes were associated with pain that manifested as time- and concentration-dependent alterations in hind limb weight bearing. These alterations in hind limb weight bearing were reversed with morphine, but were not significantly affected by acute administration of either indomethacin or celecoxib. However, administration of 30 mg/kg celecoxib twice daily for 10 days resulted in a significant restoration of hind limb weight bearing. We conclude that the iodoacetate model of OA is a relevant animal model to study pain associated with OA, and can be used to test potential therapeutic agents.

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.

Synthesis and evaluation of novel peripherally restricted kappa-opioid receptor agonists.

A series of 3-substituted analogs (3) of the parent kappa agonist, 1, were prepared to limit access to the central nervous system. With the exception of compound 3j, all other compounds bound to the human kappa opioid receptor with high affinity (K(i)=0.31-9.5 nM) and were selective for kappa over mu and delta opioid receptors. Compounds 3c, d, and 3g-i produced potent antinociceptive activity in the rat formalin assay (i.paw) and the mouse acetic acid-induced writhing assay (s.c.), with weak activity in the mouse platform sedation test. The peripheral restriction indices of 3c, d, 3g, and 3i were improved 2- to 7-fold compared to the parent compound 1, and these compounds were approximately 2- to 5-fold more potent than the peripheral kappa agonist ICI 204448.

Azepinone as a conformational constraint in the design of kappa-opioid receptor agonists.

A new class of kappa-opioid receptor agonists is described. The design of these agents was based upon energy minimization and structural overlay studies of the generic azepin-2-one structure 3 with the crystal structure of arylacetamide kappa agonist 1, ICI 199441. The most active compound identified was ligand 4a (K(i)=0.34 nM), which demonstrated potent antinociceptive activity after oral administration in rodents.