ABSTRACT
Title
Analgesic activity of new hybrid muscarinic agonists in mice
Authors
L. Flammini1, S. Bertoni1, F. Saccani1, A. Rapalli1, V. Arcaro1, G. Domenichini1, E. Barocelli1
1Dept. of Pharmacological, Biological and Applied Chemical Sciences, University of Parma
1Dept. of Pharmacological, Biological and Applied Chemical Sciences, University of Parma
Abstract
Background and Aim. It has long been known that cholinergic system is involved in the modulation of nociceptive transmission. In particular, muscarinic receptors, primarily represented by M2 and M4 subtypes, tonically regulate pain transmission by modulating the response of spinal dorsal horn neurons to algesic stimuli (Cai et al., 2009). Muscarinic agonists could therefore represent alternative promising agents endowed with a good analgesic potency but devoid of the heavy side effects, represented by tolerance, physical dependence and respiratory depression, typical of the classic opioid drugs.
This work is aimed at preliminarily defining the pharmacological profile of two new series of bis-quaternary hybrid compounds obtained by incorporating the acetylenic fragment of a potent yet non-selective oxotremorine analogue Iperoxo (Barocelli et al., 2001; Antony et al., 2009) into the structure of the M2-selective allosteric modulators W84 (Compounds 1 and 2) and naphmethonium (Compounds 3 and 4).
Materials and Methods. We evaluated the in vitroagonistic potency and efficacy of the new molecules towards M2 (guinea pig electrically-stimulated left atrium) and M3 (guinea pig ileum) receptors and their ability to inhibit rat cerebral cholinesterase enzyme using Ellman colorimetric assay. In vivo, the novel compounds were tested from 0.5 to 10 mg/kg s.c. for their analgesic activity in mouse writhing test, a model of visceral inflammatory pain, and for the occurrence in the same species of peripheral and central cholinergic side effects, such as salivation, intestinal hypermotility, tremor and hypothermia.
Results. All the compounds behaved as full and potent M2 agonists (-LogEC50 values ranging from 7.30 of Compound 1to 8.2 of Compound 4), displaying a preference towards M2 vs. M3 sites, maximal for Compound 1, and a moderate anticholinesterase activity (pIC50 ranging from 4.7 to 5.1), more pronounced in the naphmethonium series.
In the writhing test, Compound 4 emerged as the most potent analgesic agent, being able to exert at 0.5 mg/kg a significant antinociceptive effect, completely prevented by atropine (5 mg/kg s.c.) but unaffected by mecamylamine (1 mg/kg s.c.) or naloxone (1 mg/kg s.c.) pre-treatment. A marked hypothermia and peripheral unwanted side effects were detected in 50% of treated mice. Undesired effects were evoked by Compound 1only in 25% of mice when administered at a dose significantly blocking (80%) acetic acid-induced writhes.
Conclusions. In this preliminary investigation, among the new hybrid M2 and M3 muscarinic agonists, Compound 1 emerges since it is endowed with the highest M2/M3 selectivity and a marked analgesic activity combined with a reduced appearance of peripheral cholinergic undesired responses. These initial results suggest to get a deeper insight into the analgesic properties of these agents in other models of pain and could be a starting point to synthesize subtype-selective derivatives with improved safety profiles.
References:
Cai et al. (2009), J Neurochem. 111(4): 1000-10.
Barocelli et al., (2001) Life Sciences 68: 1775-85.
Antony et al., (2009) FASEB J. 23(2): 442-50
This work is aimed at preliminarily defining the pharmacological profile of two new series of bis-quaternary hybrid compounds obtained by incorporating the acetylenic fragment of a potent yet non-selective oxotremorine analogue Iperoxo (Barocelli et al., 2001; Antony et al., 2009) into the structure of the M2-selective allosteric modulators W84 (Compounds 1 and 2) and naphmethonium (Compounds 3 and 4).
Materials and Methods. We evaluated the in vitroagonistic potency and efficacy of the new molecules towards M2 (guinea pig electrically-stimulated left atrium) and M3 (guinea pig ileum) receptors and their ability to inhibit rat cerebral cholinesterase enzyme using Ellman colorimetric assay. In vivo, the novel compounds were tested from 0.5 to 10 mg/kg s.c. for their analgesic activity in mouse writhing test, a model of visceral inflammatory pain, and for the occurrence in the same species of peripheral and central cholinergic side effects, such as salivation, intestinal hypermotility, tremor and hypothermia.
Results. All the compounds behaved as full and potent M2 agonists (-LogEC50 values ranging from 7.30 of Compound 1to 8.2 of Compound 4), displaying a preference towards M2 vs. M3 sites, maximal for Compound 1, and a moderate anticholinesterase activity (pIC50 ranging from 4.7 to 5.1), more pronounced in the naphmethonium series.
In the writhing test, Compound 4 emerged as the most potent analgesic agent, being able to exert at 0.5 mg/kg a significant antinociceptive effect, completely prevented by atropine (5 mg/kg s.c.) but unaffected by mecamylamine (1 mg/kg s.c.) or naloxone (1 mg/kg s.c.) pre-treatment. A marked hypothermia and peripheral unwanted side effects were detected in 50% of treated mice. Undesired effects were evoked by Compound 1only in 25% of mice when administered at a dose significantly blocking (80%) acetic acid-induced writhes.
Conclusions. In this preliminary investigation, among the new hybrid M2 and M3 muscarinic agonists, Compound 1 emerges since it is endowed with the highest M2/M3 selectivity and a marked analgesic activity combined with a reduced appearance of peripheral cholinergic undesired responses. These initial results suggest to get a deeper insight into the analgesic properties of these agents in other models of pain and could be a starting point to synthesize subtype-selective derivatives with improved safety profiles.
References:
Cai et al. (2009), J Neurochem. 111(4): 1000-10.
Barocelli et al., (2001) Life Sciences 68: 1775-85.
Antony et al., (2009) FASEB J. 23(2): 442-50