Muscarinic agonists as analgesics. Antinociceptive activity versus M1 activity: SAR of alkylthio-TZTP's and related 1,2,5-thiadiazole analogs

1,2,5-Thiadiazole Scaffold: A review on recent progress in biological activities

BACKGROUND

Thiadiazoles can be considered as the privileged scaffold having diverse pharmacological potentials such as antihypertensive, anti-HIV, antimicrobials, antileishmanial agents, etc. In particular, 1,2,5-thiadiazoles and their fused analogues are subjects of fast-growing interest due to their higher significance in the fields of biomedicine and material sciences.

OBJECTIVE

This study aims to collect detailed medicinal information about aspects of 1,2,5-thiadiazole.

METHODS

A systemic search has been carried out using PubMed, Google Scholar, CNKI, etc., for relevant studies having the keyword, '1,2,5-thiadiazole'.

RESULTS AND CONCLUSION

In this mini-review, we have covered known procedures of the synthesis and explored in detail all known advancements of this scaffold concerning to its biological activities.

Targeting the cholinergic system as a therapeutic strategy for the treatment of pain

Acetylcholine mediates its effects through both the nicotinic acetylcholine receptors (ligand-gated ion channels) and the G protein-coupled muscarinic receptors. It plays pivotal roles in a diverse array of physiological processes and its activity is controlled through enzymatic degradation by acetylcholinesterase. The effects of receptor agonists and enzyme inhibitors, collectively termed cholinomimetics, in antinociception/analgesia are well established. These compounds successfully inhibit pain signaling in both humans and animals and are efficacious in a number of different preclinical and clinical pain models, suggesting a broad therapeutic potential. In this review we examine and discuss the evidence for the therapeutic exploitation of the cholinergic system as an approach to treat pain.

Loss of muscarinic M4 receptors in spinal cord of arthritic rats: implications for a role of M4 receptors in pain response

Changes in the levels of muscarinic M4 receptors in spinal cord of acute and chronic arthritic rats (animal models of pain) were studied by receptor autoradiography using muscarinic M4 receptor subtype selective ligand. Arthritis was induced in female Lewis rats by single intradermal injection of heat-killed Mycobacterium butyricum and sacrificed 12 days (acute group) and 30 days (chronic and control groups) after induction of arthritis. Our results demonstrate significant reduction in the level of M4 receptors in the spinal cord (Rexed laminae I-X) of acute and chronic arthritic rats compared to controls. These findings suggest that the muscarinic M4 receptor subtype may be involved in cholinergic mechanisms of analgesia.

Intrathecal clonidine reduces hypersensitivity after nerve injury by a mechanism involving spinal m4 muscarinic receptors

alpha2-Adrenergic agonists reduce mechanical and thermal hypersensitivity in animals with nerve injury and effectively treat neuropathic pain in humans. Previous studies indicate a reliance of alpha2-adrenergic agonists in this setting on spinal cholinergic activation and stimulation of muscarinic receptors. The subtype(s) of muscarinic receptors in the spinal cord that produces antinociception in normal animals is controversial, and those involved in reducing hypersensitivity and interacting with alpha2-adrenergic systems after nerve injury are unstudied. To examine this, the left L5 and L6 spinal nerves were tightly ligated in rats, resulting in reduction in withdrawal threshold to punctate mechanical stimuli. Intrathecal clonidine, 15 micro g, returned the withdrawal threshold to normal. Using highly specific m1 and m4 antagonists, we observed no reduction in the effect of clonidine by the m1 antagonist, but inhibition of clonidine's effect by the m4 antagonist. Western analysis revealed no difference in quantitative expression of m1 and m4 receptor protein in the dorsal spinal cord of spinal nerve-injured animals compared with sham-operated controls, suggesting this interaction with m4 receptors does not reflect an increase in receptor expression.

IMPLICATIONS

Neuraxial clonidine is an effective adjunct in the treatment of neuropathic pain and increases acetylcholine concentrations in cerebrospinal fluid in humans. These data in animals suggest that spinal m4 type muscarinic receptors are important to the effect of clonidine in treating hypersensitivity to touch after nerve injury.

Central muscarinic inhibition of lower urinary tract nociception

Previous studies indicate cholinergic systems suppress somatic nociception. The present studies determined if cholinergic muscarinic systems suppress visceral nociception, specifically, chemical irritation of the lower urinary tract. Bladders of urethane-anesthetized rats were cannulated through the dome for continuous-infusion cystometrogram recordings. EMG electrodes recorded anal sphincter activity. Infusion of 0.5% acetic acid into the bladder to produce irritation increased bladder activity and anal sphincter activity (i.e. activation of a nociceptive vesicoanal reflex). Oxotremorine (a muscarinic agonist) and (-)butylthio[2.2.2] (a mixed muscarinic agonist/antagonist) dose-dependently inhibited vesicoanal reflex activity. This inhibition was antagonized by atropine (a centrally active muscarinic antagonist) but not by scopolamine methylbromide (a peripherally restricted muscarinic antagonist). Physostigmine (a centrally active cholinesterase inhibitor) also dose-dependently inhibited vesicoanal reflex activity in an atropine-sensitive manner, while neostigmine (a peripherally restricted cholinesterase inhibitor) did not. Atropine alone (i.e. administered without prior administration of muscarinic agonist or cholinesterase inhibitor) produced robust but transient (15 min) increases in vesicoanal activity and bladder activity under conditions of acetic acid infusion into the bladder. Under conditions of saline infusion into the bladder, atropine had the opposite effect on bladder activity (i.e. inhibition). These studies indicate that an endogenous cholinergic muscarinic system can be activated by lower urinary tract irritation to suppress visceral nociception through central nervous system mechanisms.

Synthesis, analgesic activity, and binding properties of some epibatidine analogs with a tropine skeleton

A series of epibatidine analogs and their positional isomers bearing an 8-azabicyclo[3.2.1]octane moiety is described. Also some of their simplified analogs bearing a 3-piperidine moiety are reported. Their receptor binding profiles (5-HT1A, 5-HT1B, M1, M2, neuronal nicotinic receptor) and analgesic activity (hot plate, acetic acid induced writhing) have been studied. Some of the compounds, especially those containing an 8-azabicyclo[3.2.1]oct-2-ene moiety possess high afinity for the nicotinic cholinergic receptor. The most analgesically active compounds are also highly toxic. Optimized structures (PM3-MOPAC, Alchemy 2000, Tripos Inc.) of compounds 1-9 were compared with that of epibatidine.

Role of spinal muscarinic and nicotinic receptors in clonidine-induced nitric oxide release in a rat model of neuropathic pain

Intrathecal administration of alpha(2) adrenergic agonists, such as clonidine, is capable of alleviating neuropathic pain. Recent studies suggest that spinal nitric oxide (NO) mediates the analgesic effect of intrathecal clonidine. Furthermore, compared to nicotinic receptors, spinal muscarinic receptors play a greater role in the analgesic effect of intrathecal clonidine. In the present study, we tested a hypothesis that clonidine-evoked NO release is dependent primarily on muscarinic receptors in the spinal cord after nerve injury. A rat model of neuropathic pain was induced by ligation of the left L(5)/L(6) spinal nerves. Using an in vitro spinal cord perfusion preparation, the effect of muscarinic and nicotinic receptor antagonists on clonidine-evoked nitrite (a stable product of NO) release was determined. Both muscarinic and nicotinic antagonists dose-dependently attenuated clonidine-elicited nitrite release. In spinal cords from the neuropathic rats, the inhibitory effect of muscarinic receptor antagonists (atropine and scopolamine) on clonidine-elicited nitrite release was more potent than that of nicotinic receptor antagonists (mecamylamine and hexamethonium). However, in spinal cords obtained from sham animals, the inhibitory effect of muscarinic and nicotinic antagonists did not differ significantly. These results indicate that muscarinic, as well as nicotinic, receptors mediate clonidine-induced NO release in the spinal cord. These data also suggest that after nerve injury, the cascade of activation of alpha(2) adrenergic receptors-muscarinic receptors-NO in the spinal cord likely plays a predominant role in the analgesic effect of intrathecal clonidine on neuropathic pain.

Synthesis and binding studies of some epibatidine analogues

A series of epibatidine analogues and their positional isomers bearing an 8-azabicyclo[3.2.1]octane moiety is described. Some of the compounds, especially those containing 8-azabicyclo[3.2.1]oct-2-ene moiety show high affinity for the nicotinic cholinergic receptor.

Identification of side chains on 1,2,5-thiadiazole-azacycles optimal for muscarinic m1 receptor activation

Series of analogs to the functional m1 selective agonist, xanomeline (hexyloxy-TZTP), were evaluated for their in vitro m1 efficacy in cell lines transfected with the human m1 receptor. Systematic variation of the side chain and the azacyclic ring led to the discovery of potent muscarinic agonists with robust m1 efficacy, all having the phenylpropargyloxy/thio as the side chain. The most selective compound was the phenylpropargylthio-[3.2.1] endo analog 28, which is a potent and efficacious m1 agonist with no m2 activity.

Conformationally constrained analogues of the muscarinic agonist 3-(4-(methylthio)-1,2,5-thiadiazol-3-yl)-1,2,5,6-tetrahydro-1-methylpyr idine. Synthesis, receptor affinity, and antinociceptive activity

Conformationally constrained analogues of the potent muscarinic agonist 3-(4-methylthio)-1,2,5-thiadiazol-3-yl)-1,2,5,6-tetrahydro-1-methy lpyridine (methylthio-TZTP, 17) were designed and synthesized with the aim of (a) improving the antinociceptive selectivity over salivation and tremor and (b) predicting the active conformation of 17 with respect to the dihedral angle C4-C3-C3'-N2'. Using MOPAC 6.0 tricyclic analogues (7, 15, 16) with C4-C3-C3'-N2' dihedral angles close to 180 degrees and a rotation hindered analogue (9) with a C4-C3-C3'-N2' dihedral angle close to 274 degrees were designed, as these conformations had previously been suggested as being the active conformations. The analogues were tested for central muscarinic receptor binding affinity, for their antinociceptive activity in the mouse grid shock test, and, in the same assay, for their ability to induce tremor and salivation. The data showed that the tricyclic analogues (7, 15, 16) were equipotent with 17 as analgesics, but with no improved side effect profiles. The rotation-hindered analogue 9 had neither muscarinic receptor binding affinity nor antinociceptive activity. These results suggest that the active conformation of 17 has a C3-C4-C3'-N2' dihedral angle close to 180 degrees.

Transduction is a major factor influencing receptor characterization

Two state (agonist-antagonist) receptor systems may explain many discrepancies in receptor classification, but the role of transduction (G protein coupling) may be critical. We propose that in some instances synthetic agonists and antagonists may interact with the receptor in such a way as to modify coupling compared with endogenous agonists, and that the transduction system together with the local environment, may contribute more to the rank order of potency of agonists and antagonists than the receptor subtype as defined by structure. Allosteric interactions at ion channels and receptors require a modification of concepts of coupling. Imidazoline ligands have different efficacy in coupling alpha 2-adrenoceptors to G proteins, compared with adrenaline and noradrenaline, and do not show a marked sodium shift, implying that the sodium site, and by implication the arginine switch, is implicated in the differential coupling. The alpha 2-adrenoceptor labeled with a natural agonist does not show subtype selectivity whereas antagonist-labeled alpha 2-adrenoceptors show subtype selectivity. In the 5-HT1A receptor, palmitoylation (of receptor or G proteins) allows the expression of different agonist states. Thus transduction and G protein coupling must be taken into account in receptor classification, even if the primary classification may be structural.

Muscarinic analgesics with potent and selective effects on the gastrointestinal tract: potential application for the treatment of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a pathopysiolocal condition characterized by abnormal bowel habits that are frequently accompanied by abdominal pain. Current therapy based on reducing high-amplitude GI contractions with nonselective muscarinic antagonists is limited in efficacy due to typical muscarinic side effects and provides no pain relief. We have previously found potent antinociceptive agents acting through muscarinic receptors. In the present work, new 1,2,5-thiadiazole-based structures with muscarinic activity have been evaluated both for activity as analgesics in the mouse withing assay and for activity in normalizing spontaneous cluster contractions in ferret jejunum as a model of IBS in humans. (5R,6R)-exo-6-[4-[(4,4,4-Trifluorobutyl)thio]-1,2,5-thiadiazol+ ++-3-yl] -1-azabicyclo[3.2.1]octane (35, LY316108/NNC11-2192) was found to offer an exceptional profile combining analgesic potency in mouse writhing (ED50 = 0.1 mg/kg) along with potency for normalization of GI motility (ED50 = 0.17 mg/kg). This combination of GI and analgesic potency suggests 35 as an excellent candidate for evaluation as a potential treatment of IBS.