Supra-spinal FAAH is required for the analgesic action of paracetamol in an inflammatory context

Temperature-dependent solid-state phase transition with twinning in the crystal structure of 4-meth-oxy-anilinium chloride

At room temperature, the title salt, C7H10NO+·Cl-, is ortho-rhom-bic, space group Pbca with Z' = 1, as previously reported [Zhao (2009 ▸). Acta Cryst. E65, o2378]. Between 250 and 200 K, there is a solid-state phase transition to a twinned monoclinic P21/c structure with Z' = 2. We report the high temperature structure at 250 K and the low-temperature structure at 100 K. In the low-temperature structure, the -NH3 hydrogen atoms are ordered and this group has a different orientation in each independent mol-ecule, in keeping with optimizing N-H⋯Cl hydrogen bonding, some of which are bifurcated: these hydrogen bonds have N⋯Cl distances in the range 3.1201 (8)-3.4047 (8) Å. In the single cation of the high-temperature structure, the NH hydrogen atoms are disordered into the average of the two low-temperature positions and the N⋯Cl hydrogen bond distances are in the range 3.1570 (15)-3.3323 (18) Å. At both temperatures, the meth-oxy group is nearly coplanar with the rest of the mol-ecule, with the C-C-O-C torsion angles being -7.0 (2)° at 250 K and -6.94 (12) and -9.35 (12)° at 100 K. In the extended ortho-rhom-bic structure, (001) hydrogen-bonded sheets occur; in the monoclinic structure, the sheets propagate in the (010) plane.

N-(4-Meth-oxy-3-nitro-phen-yl)acetamide

The title compound, C₉H₁₀N₂O₄, crystallizes with a disordered nitro group in twinned crystals. Both the meth-oxy group and the acetamide groups are nearly coplanar with the phenyl ring, and the C-N-C-O torsion angle [0.2 (4)°] is also insignificantly different from zero. Overall, the 12-atom meth-oxy-phenyl-acetamide group is nearly planar, with an r.m.s. deviation of 0.042 Å. The nitro group is twisted out of this plane by about 30°, disordered into two orientations with opposite senses of twist. In the crystal, the N-H group donates a hydrogen bond to a nitro oxygen atom, generating chains propagating in the [101] direction. The amide carbonyl oxygen atom is not involved in the hydrogen bonding.

An Updated Review on the Metabolite (AM404)-Mediated Central Mechanism of Action of Paracetamol (Acetaminophen): Experimental Evidence and Potential Clinical Impact

Paracetamol remains the recommended first-line option for mild-to-moderate acute pain in general population and particularly in vulnerable populations. Despite its wide use, debate exists regarding the analgesic mechanism of action (MoA) of paracetamol. A growing body of evidence challenged the notion that paracetamol exerts its analgesic effect through cyclooxygenase (COX)-dependent inhibitory effect. It is now more evident that paracetamol analgesia has multiple pathways and is mediated by the formation of the bioactive AM404 metabolite in the central nervous system (CNS). AM404 is a potent activator of TRPV₁, a major contributor to neuronal response to pain in the brain and dorsal horn. In the periaqueductal grey, the bioactive metabolite AM404 activated the TRPV₁ channel-mGlu5 receptor-PLC-DAGL-CB1 receptor signaling cascade. The present article provides a comprehensive literature review of the centrally located, COX-independent, analgesic MoA of paracetamol and relates how the current experimental evidence can be translated into clinical practice. The evidence discussed in this review established paracetamol as a central, COX-independent, antinociceptive medication that has a distinct MoA from non-steroidal anti-inflammatory drugs (NSAIDs) and a more tolerable safety profile. With the establishment of the central MoA of paracetamol, we believe that paracetamol remains the preferred first-line option for mild-to-moderate acute pain for healthy adults, children, and patients with health concerns. However, safety concerns remain with the high dose of paracetamol due to the NAPQI-mediated liver necrosis. Centrally acting paracetamol/p-aminophenol derivatives could potentiate the analgesic effect of paracetamol without increasing the risk of hepatoxicity. Moreover, the specific central MoA of paracetamol allows its combination with other analgesics, including NSAIDs, with a different MoA. Future experiments to better explain the central actions of paracetamol could pave the way for discovering new central analgesics with a better benefit-to-risk ratio.

Role of T CD4+ cells, macrophages, C-low threshold mechanoreceptors and spinal Cav 3.2 channels in inflammation and related pain-like symptoms in murine inflammatory models

BACKGROUND AND PURPOSE

T-type calcium channels, mainly the Ca_v 3.2 subtype, are important contributors to the nociceptive signalling pathway. We investigated their involvement in inflammation and related pain-like symptoms.

EXPERIMENTAL APPROACH

The involvement of Ca_v 3.2 and T-type channels was investigated using genetic and pharmacological inhibition to assess mechanical allodynia/hyperalgesia and oedema development in two murine inflammatory pain models. The location of Ca_v 3.2 channels involved in pain-like symptoms was studied in mice with Ca_v 3.2 knocked out in C-low threshold mechanoreceptors (C-LTMR) and the use of ABT-639, a peripherally restricted T-type channel inhibitor. The anti-oedema effect of Ca_v 3.2 channel inhibition was investigated in chimeric mice with immune cells deleted for Ca_v 3.2. Lymphocytes and macrophages from either green fluorescent protein-targeted Ca_v 3.2 or KO mice were used to determine the expression of Ca_v 3.2 protein and the functional status of the cells.

KEY RESULTS

Ca_v 3.2 channels contributed to the development of pain-like symptoms and oedema in the two murine inflammatory pain models. Our results provided evidence of the involvement of Ca_v 3.2 channels located on C-LTMRs and spinal cord in inflammatory pain. Ca_v 3.2 channels located in T cells and macrophages contribute to the inflammatory process.

CONCLUSION AND IMPLICATIONS

Ca_v 3.2 channels play crucial roles in inflammation and related pain, implying that targeting of Ca_v 3.2 channels with pharmacological agents could be an attractive and readily evaluable strategy in clinical trials, to relieve chronic inflammatory pain in patients.

Systematic Review of Systemic and Neuraxial Effects of Acetaminophen in Preclinical Models of Nociceptive Processing

Acetaminophen (APAP) in humans has robust effects with a high therapeutic index in altering postoperative and inflammatory pain states in clinical and experimental pain paradigms with no known abuse potential. This review considers the literature reflecting the preclinical actions of acetaminophen in a variety of pain models. Significant observations arising from this review are as follows: 1) acetaminophen has little effect upon acute nociceptive thresholds; 2) acetaminophen robustly reduces facilitated states as generated by mechanical and thermal hyperalgesic end points in mouse and rat models of carrageenan and complete Freund’s adjuvant evoked inflammation; 3) an antihyperalgesic effect is observed in models of facilitated processing with minimal inflammation (eg, phase II intraplantar formalin); and 4) potent anti-hyperpathic effects on the thermal hyperalgesia, mechanical and cold allodynia, allodynic thresholds in rat and mouse models of polyneuropathy and mononeuropathies and bone cancer pain. These results reflect a surprisingly robust drug effect upon a variety of facilitated states that clearly translate into a wide range of efficacy in preclinical models and to important end points in human therapy. The specific systems upon which acetaminophen may act based on targeted delivery suggest both a spinal and a supraspinal action. Review of current targets for this molecule excludes a role of cyclooxygenase inhibitor but includes effects that may be mediated through metabolites acting on the TRPV1 channel, or by effect upon cannabinoid and serotonin signaling. These findings suggest that the mode of action of acetaminophen, a drug with a long therapeutic history of utilization, has surprisingly robust effects on a variety of pain states in clinical patients and in preclinical models with a good therapeutic index, but in spite of its extensive use, its mechanisms of action are yet poorly understood.

Behavioral Battery for Testing Candidate Analgesics in Mice. II. Effects of Endocannabinoid Catabolic Enzyme Inhibitors and ∆9-Tetrahydrocannabinol

Enhanced signaling of the endocannabinoid (eCB) system through inhibition of the catabolic enzymes monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) has received increasing interest for development of candidate analgesics. This study compared effects of MAGL and FAAH inhibitors with effects of ∆9-tetrahydrocannabinol (THC) using a battery of pain-stimulated, pain-depressed, and pain-independent behaviors in male and female mice. Intraperitoneal injection of dilute lactic acid (IP acid) served as an acute visceral noxious stimulus to stimulate two behaviors (stretching, facial grimace) and depress two behaviors (rearing, nesting). Nesting and locomotion were also assessed in the absence of IP acid as pain-independent behaviors. THC and a spectrum of six eCB catabolic enzyme inhibitors ranging from MAGL- to FAAH-selective were assessed for effectiveness to alleviate pain-related behaviors at doses that did not alter pain-independent behaviors. The MAGL-selective inhibitor MJN110 produced the most effective antinociceptive profile, with 1.0 mg/kg alleviating IP acid effects on stretching, grimace, and nesting without altering pain-independent behaviors. MJN110 effects on IP acid-depressed nesting had a slow onset and long duration (40 minutes to 6 hours), were blocked by rimonabant, and tended to be greater in females. As inhibitors increased in FAAH selectivity, antinociceptive effectiveness decreased. PF3845, the most FAAH-selective inhibitor, produced no antinociception up to doses that disrupted locomotion. THC decreased IP acid-stimulated stretching and grimace at doses that did not alter pain-independent behaviors; however, it did not alleviate IP acid-induced depression of rearing or nesting. These results support further consideration of MAGL-selective inhibitors as candidate analgesics for acute inflammatory pain. SIGNIFICANCE STATEMENT: This study characterized a spectrum of endocannabinoid catabolic enzyme inhibitors ranging in selectivity from monoacylglycerol lipase-selective to fatty acid amide hydrolase-selective in a battery of pain-stimulated, pain-depressed, and pain-independent behaviors previously pharmacologically characterized in a companion paper. This battery provides a method for prioritizing candidate analgesics by effectiveness to alleviate pain-related behaviors at doses that do not alter pain-independent behaviors, with inclusion of pain-depressed behaviors increasing translational validity and decreasing susceptibility to motor-depressant false positives.

Paracetamol (acetaminophen): A familiar drug with an unexplained mechanism of action

Paracetamol (acetaminophen) is undoubtedly one of the most widely used drugs worldwide. As an over-the-counter medication, paracetamol is the standard and first-line treatment for fever and acute pain and is believed to remain so for many years to come. Despite being in clinical use for over a century, the precise mechanism of action of this familiar drug remains a mystery. The oldest and most prevailing theory on the mechanism of analgesic and antipyretic actions of paracetamol relates to the inhibition of CNS cyclooxygenase (COX) enzyme activities, with conflicting views on the COX isoenzyme/variant targeted by paracetamol and on the nature of the molecular interactions with these enzymes. Paracetamol has been proposed to selectively inhibit COX-2 by working as a reducing agent, despite the fact that in vitro screens demonstrate low potency on the inhibition of COX-1 and COX-2. In vivo data from COX-1 transgenic mice suggest that paracetamol works through inhibition of a COX-1 variant enzyme to mediate its analgesic and particularly thermoregulatory actions (antipyresis and hypothermia). A separate line of research provides evidence on potentiation of the descending inhibitory serotonergic pathway to mediate the analgesic action of paracetamol, but with no evidence of binding to serotonergic molecules. AM404 as a metabolite for paracetamol has been proposed to activate the endocannabinoid and the transient receptor potential vanilloid-1 (TRPV1) systems. The current review gives an update and in some cases challenges the different theories on the pharmacology of paracetamol and raises questions on some of the inadequately explored actions of paracetamol. List of Abbreviations: AM404, N-(4-hydroxyphenyl)-arachidonamide; CB1R, Cannabinoid receptor-1; Cmax, Maximum concentration; CNS, Central nervous system; COX, Cyclooxygenase; CSF, Cerebrospinal fluid; ED50, 50% of maximal effective dose; FAAH, Fatty acid amidohydrolase; IC50, 50% of the maximal inhibitor concentration; LPS, Lipopolysaccharide; NSAIDs, Non-steroidal anti-inflammatory drugs; PGE2, Prostaglandin E2; TRPV1, Transient receptor potential vanilloid-1.

Paracetamol analogues conjugated by FAAH induce TRPV1-mediated antinociception without causing acute liver toxicity

Paracetamol, one of the most widely used pain-relieving drugs, is deacetylated to 4-aminophenol (4-AP) that undergoes fatty acid amide hydrolase (FAAH)-dependent biotransformation into N-arachidonoylphenolamine (AM404), which mediates TRPV1-dependent antinociception in the brain of rodents. However, paracetamol is also converted to the liver-toxic metabolite N-acetyl-p-benzoquinone imine already at therapeutic doses, urging for safer paracetamol analogues. Primary amine analogues with chemical structures similar to paracetamol were evaluated for their propensity to undergo FAAH-dependent N-arachidonoyl conjugation into TRPV1 activators both in vitro and in vivo in rodents. The antinociceptive and antipyretic activity of paracetamol and primary amine analogues was examined with regard to FAAH and TRPV1 as well as if these analogues produced acute liver toxicity. 5-Amino-2-methoxyphenol (2) and 5-aminoindazole (3) displayed efficient target protein interactions with a dose-dependent antinociceptive effect in the mice formalin test, which in the second phase was dependent on FAAH and TRPV1. No hepatotoxicity of the FAAH substrates transformed into TRPV1 activators was observed. While paracetamol attenuates pyrexia via inhibition of brain cyclooxygenase, its antinociceptive FAAH substrate 4-AP was not antipyretic, suggesting separate mechanisms for the antipyretic and antinociceptive effect of paracetamol. Furthermore, compound 3 reduced fever without a brain cyclooxygenase inhibitory action. The data support our view that analgesics and antipyretics without liver toxicity can be derived from paracetamol. Thus, research into the molecular actions of paracetamol could pave the way for the discovery of analgesics and antipyretics with a better benefit-to-risk ratio.

Paracetamol - An old drug with new mechanisms of action

Paracetamol (acetaminophen) is the most commonly used over-the-counter (OTC) drug in the world. Despite its popularity and use for many years, the safety of its application and its mechanism of action are still unclear. Currently, it is believed that paracetamol is a multidirectional drug and at least several metabolic pathways are involved in its analgesic and antipyretic action. The mechanism of paracetamol action consists in inhibition of cyclooxygenases (COX-1, COX-2, and COX-3) and involvement in the endocannabinoid system and serotonergic pathways. Additionally, paracetamol influences transient receptor potential (TRP) channels and voltage-gated Kv7 potassium channels and inhibits T-type Cav3.2 calcium channels. It also exerts an impact on L-arginine in the nitric oxide (NO) synthesis pathway. However, not all of these effects have been clearly confirmed. Therefore, the aim of our paper was to summarize the current state of knowledge of the mechanism of paracetamol action with special attention to its safety concerns.

The Antitumor Peptide ERα17p Exerts Anti-Hyperalgesic and Anti-Inflammatory Actions Through GPER in Mice

Persistent inflammation and persistent pain are major medical, social and economic burdens. As such, related pharmacotherapy needs to be continuously improved. The peptide ERα17p, which originates from a part of the hinge region/AF2 domain of the human estrogen receptor α (ERα), exerts anti-proliferative effects in breast cancer cells through a mechanism involving the hepta-transmembrane G protein-coupled estrogen receptor (GPER). It is able to decrease the size of xenografted human breast tumors, in mice. As GPER has been reported to participate in pain and inflammation, we were interested in exploring the potential of ERα17p in this respect. We observed that the peptide promoted anti-hyperalgesic effects from 2.5 mg/kg in a chronic mice model of paw inflammation induced by the pro-inflammatory complete Freund's adjuvant (CFA). This action was abrogated by the specific GPER antagonist G-15, leading to the conclusion that a GPER-dependent mechanism was involved. A systemic administration of a Cy5-labeled version of the peptide allowed its detection in both, the spinal cord and brain. However, ERα17p-induced anti-hyperalgesia was detected at the supraspinal level, exclusively. In the second part of the study, we have assessed the anti-inflammatory action of ERα17p in mice using a carrageenan-evoked hind-paw inflammation model. A systemic administration of ERα17p at a dose of 2.5 mg/kg was responsible for reduced paw swelling. Overall, our work strongly suggests that GPER inverse agonists, including ERα17p, could be used to control hyperalgesia and inflammation.

Paracetamol is a centrally acting analgesic using mechanisms located in the periaqueductal grey

BACKGROUND AND PURPOSE

We previously demonstrated that paracetamol has to be metabolised in the brain by fatty acid amide hydrolase enzyme into AM404 (N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide) to activate CB₁ receptors and TRPV1 channels, which mediate its analgesic effect. However, the brain mechanisms supporting paracetamol-induced analgesia remain unknown.

EXPERIMENTAL APPROACH

The effects of paracetamol on brain function in Sprague-Dawley rats were determined by functional MRI. Levels of neurotransmitters in the periaqueductal grey (PAG) were measured using in vivo ¹ H-NMR and microdialysis. Analgesic effects of paracetamol were assessed by behavioural tests and challenged with different inhibitors, administered systemically or microinjected in the PAG.

KEY RESULTS

Paracetamol decreased the connectivity of major brain structures involved in pain processing (insula, somatosensory cortex, amygdala, hypothalamus, and the PAG). This effect was particularly prominent in the PAG, where paracetamol, after conversion to AM404, (a) modulated neuronal activity and functional connectivity, (b) promoted GABA and glutamate release, and (c) activated a TRPV1 channel-mGlu₅ receptor-PLC-DAGL-CB₁ receptor signalling cascade to exert its analgesic effects.

CONCLUSIONS AND IMPLICATIONS

The elucidation of the mechanism of action of paracetamol as an analgesic paves the way for pharmacological innovations to improve the pharmacopoeia of analgesic agents.

Pharmacogenetics in Pain Treatment

Pain is an unpleasant feeling usually resulting from tissue damage that can persist along weeks, months, or even years after the injury, turning into pathological chronic pain, the leading cause of disability. Currently, pharmacology interventions are usually the first-line therapy but there is a highly variable analgesic drug response. Pharmacogenetics (PGx) offers a means to identify genetic biomarkers that can predict individual analgesic response opening doors to precision medicine. PGx analyze the way in which the presence of variations in the DNA sequence (single-nucleotide polymorphisms, SNPs) could be responsible for portions of the population reaching different levels of pain relief (phenotype) due to gene interference in the drug mechanism of action (pharmacodynamics) and/or its concentration at the place of action (pharmacokinetics). SNPs in the cytochrome P450 enzymes genes (CYP2D6) influence metabolism of codeine, tramadol, hydrocodone, oxycodone, and tricyclic antidepressants. Blood concentrations of some NSAIDs depend on CYP2C9 and/or CYP2C8 activity. Additional candidate genes encode for opioid receptors, transporters, and other molecules important for pharmacotherapy in pain management. However, PGx studies are often contradictory, slowing the uptake of this information. This is likely due, in large part, to a lack of robust evidence demonstrating clinical utility and to its polygenic response modulated by other exogenous or epigenetics factors. Novel therapies, including targeting of epigenetic changes and gene therapy-based approaches, broaden future options to improve understanding of pain and the treatment of people who suffer it.

Acetaminophen Relieves Inflammatory Pain through CB1 Cannabinoid Receptors in the Rostral Ventromedial Medulla

Acetaminophen (paracetamol) is a widely used analgesic and antipyretic drug with only incompletely understood mechanisms of action. Previous work, using models of acute nociceptive pain, indicated that analgesia by acetaminophen involves an indirect activation of CB₁ receptors by the acetaminophen metabolite and endocannabinoid reuptake inhibitor AM 404. However, the contribution of the cannabinoid system to antihyperalgesia against inflammatory pain, the main indication of acetaminophen, and the precise site of the relevant CB₁ receptors have remained elusive. Here, we analyzed acetaminophen analgesia in mice of either sex with inflammatory pain and found that acetaminophen exerted a dose-dependent antihyperalgesic action, which was mimicked by intrathecally injected AM 404. Both compounds lost their antihyperalgesic activity in CB₁^-/- mice, confirming the involvement of the cannabinoid system. Consistent with a mechanism downstream of proinflammatory prostaglandin formation, acetaminophen also reversed hyperalgesia induced by intrathecal prostaglandin E₂ To distinguish between a peripheral/spinal and a supraspinal action, we administered acetaminophen and AM 404 to hoxB8-CB₁^-/- mice, which lack CB₁ receptors from the peripheral nervous system and the spinal cord. These mice exhibited unchanged antihyperalgesia indicating a supraspinal site of action. Accordingly, local injection of the CB₁ receptor antagonist rimonabant into the rostral ventromedial medulla blocked acetaminophen-induced antihyperalgesia, while local rostral ventromedial medulla injection of AM 404 reduced hyperalgesia in wild-type mice but not in CB₁^-/- mice. Our results indicate that the cannabinoid system contributes not only to acetaminophen analgesia against acute pain but also against inflammatory pain, and suggest that the relevant CB₁ receptors reside in the rostral ventromedial medulla.SIGNIFICANCE STATEMENT Acetaminophen is a widely used analgesic drug with multiple but only incompletely understood mechanisms of action, including a facilitation of endogenous cannabinoid signaling via one of its metabolites. Our present data indicate that enhanced cannabinoid signaling is also responsible for the analgesic effects of acetaminophen against inflammatory pain. Local injections of the acetaminophen metabolite AM 404 and of cannabinoid receptor antagonists as well as data from tissue-specific CB₁ receptor-deficient mice suggest the rostral ventromedial medulla as an important site of the cannabinoid-mediated analgesia by acetaminophen.

Pharmacogenomics in pain treatment

The experience of chronic pain is one of the commonest reasons for seeking medical attention, being a major issue in clinical practice. While pain is a universal experience, only a small proportion of people who felt pain develop pain syndromes. In addition, painkillers are associated with wide inter-individual variability in the analgesic response. This may be partly explained by the presence of single nucleotide polymorphisms in genes encoding molecular entities involved in pharmacodynamics and pharmacokinetics. However, uptake of this information has been slow due in large part to the lack of robust evidences demonstrating clinical utility. Furthermore, novel therapies, including targeting of epigenetic changes and gene therapy-based approaches are further broadening future options for the treatment of chronic pain. The aim of this article is to review the evidences behind pharmacogenetics (PGx) to individualize therapy (boosting the efficacy and minimizing potential toxicity) and genes implicated in pain medicine, in two parts: (i) genetic variability with pain sensitivity and analgesic response; and (ii) pharmacological concepts applied on PGx.

TRPA1 mediates the hypothermic action of acetaminophen

Acetaminophen (APAP) is an effective antipyretic and one of the most commonly used analgesic drugs. Unlike antipyretic non-steroidal anti-inflammatory drugs, APAP elicits hypothermia in addition to its antipyretic effect. Here we have examined the mechanisms responsible for the hypothermic activity of APAP. Subcutaneous, but not intrathecal, administration of APAP elicited a dose dependent decrease in body temperature in wildtype mice. Hypothermia was abolished in mice pre-treated with resiniferatoxin to destroy or defunctionalize peripheral TRPV1-expressing terminals, but resistant to inhibition of cyclo-oxygenases. The hypothermic activity was independent of TRPV1 since APAP evoked hypothermia was identical in wildtype and Trpv1^−/− mice, and not reduced by administration of a maximally effective dose of a TRPV1 antagonist. In contrast, a TRPA1 antagonist inhibited APAP induced hypothermia and APAP was without effect on body temperature in Trpa1^−/− mice. In a model of yeast induced pyrexia, administration of APAP evoked a marked hypothermia in wildtype and Trpv1^−/− mice, but only restored normal body temperature in Trpa1^−/− and Trpa1^−/−/Trpv1^−/− mice. We conclude that TRPA1 mediates APAP evoked hypothermia.