Studies Of Cannabinoid Analgesic Drugs With Limited Side-effects: Characterization Of The Endogenous Peptidic Agonist And The Novel Strategy Of Combined Treatment

Pain is one of the most common and difficult syndromes of human. The classical analgesic drugs (eg. opioids) in pain management are always accompanied by undesirable side-effects.Recently, a number of biological studies suggested that cannabinoids were the attractive candidates in developing potent and low side-effects drugs. However, high doses of cannabinoid drugs, which are required to treat severe pain, are accompanied by CNS (central nervous system)side effects, such as hypothermia, hypolocomoter, gastrointestinal transit inhibition, and antinociception tolerance. Thus, this study present two strategies for potent and low side-effects drugs development: 1) the characterization of cannabinoid novel peptide ligand (m)VD-Hp?,and expected to provide a new template molecule for cannabinoid analgesic drugs development, 2)the combination of NPFF (neuropeptide FF) receptor ligand and cannabinoids to produce limited side-effects in pain management.Recently, researchers have found some new cannabinoid peptide ligands, their chemical structures are totally different from endogenous lipid ligands of cannabinoid receptors, and they have high affinities for cannabinoid receptors and exert cannabinoid-like activities in in vitro functional assays. However, the chemical structures and pharmacokinetics of these peptide ligands are very different from that of other cannabinoid agonists. Thus, the identification of the antinociceptive effects of these peptides may provide a new candidate molecule for the development of potent cannabinoid analgesic drugs.In the present study, the effects of (m)VD-Hpa, an mouse a-hemoglobin-derived peptide on pain management and its CNS effects were fully characterized. In the mouse tail-flick test,(m)VD-Hpa induced antinociception after supraspinal (EC50 = 6.69 nmol) and spinal (EC50 = 2.88 nmol). The central antinociception of (m)VD-Hpa (i.c.v. and i.t.) were mediated by CB1 receptor.In addition, only at the highly antinociceptive dose, (m)VD-Hpa exerted hypothermia and hypoactivity after supraspinal administration, and it has no effect on gastrointestinal transit even at 3 × EC50 dose. Repeated i.c.v. injection of (m)VD-Hpa did not induce rewarding property. At the highly antinociceptive dose (2 × EC50 and 3 × EC50),(m)VD-Hpa also induce tolerance to antinociception, but the effect is less than that of WIN55,212-2. In addition, central injection of(m)VD-Hpa stimulated food consumption. Therefore, compared with traditional CB1 receptor agonists, (m)VD-Hpa induced antinociception accompanied by limited CNS effects.In combination treatment, the previous studies indicated that combination of low doses of cannabinoids with other analgesics, provide an attractive strategy to overcome the undesired effects of cannabinoid drugs in pain management. In the present work, we systematically studied the interaction between NPFF and cannabinoid systems, and then promoted a new drug combined treatment strategy.In the present study, NPFF and related peptides elicit different modulating effects on the central (CB1 receptor mediated) and peripheral (CB2 receptor mediated) antinociception of WIN55,212-2 via its own receptors. Central (i.c.v.) administered NPFF receptor non-selective agonist NPFF can significantly decrease the central and peripheral antinociception of WIN55,212-2, while NPFF1 and NPFF2 receptor selective agonists NPVF and dNPA induced opposite modulating effects. NPVF produced more potent enhanced effects on the central antinociception of WIN55,212-2 than dNPA.Considering the enhanced antinociception of cannabinoids by NPVF, the combination treatment was further studied in its CNS effects. In mouse tail-flick test, WIN55,212-2 induced antinociception preceded by NPVF treatment was more potent than cannabinoids alone, with an EC50 shift from 3.51 to 0.69 nmol. 9 nmol WIN55,212-2 alone, and 3 nmol WIN55,212-2 combined with NPVF induced equivalent antinociception after i.c.v. injection. The enhancement effect of NPVF was mediated by its own receptor. In addition, the combination of WIN55,212-2 and NPVF also exerted significant hypothermia and hypoactivity as high dose of WIN55,212-2 did. However,the combined treatment produced less inhibition of gastrointestinal transit and nontolerance-forming antinociception. These data suggest that cannabinoids combined with NPVF produce nontolerance-forming antinociception with limited CNS effects.Collectively, the present study provides two strategies for reduce cannabinoids CNS effects: 1)The CB, receptor agonist (m)VD-Hpa induce potent antinociception with limited side effects, thus,it is expected that it may serve as an peptide candidate in development of new cannabinoid drugs.2) The combination of NPVF and WIN55,212-2 to produce antinociception in the absence of tolerance provides evidence that enhance the analgesic properties of cannabinoids and minimize the side-effects associated with higher doses alone. Therefore, this combined treatment represents a unique prototypic, anti-tolerance-forming way for pain relief with future therapeutic potential.