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.

Paracetamol for multimodal analgesia

Pain and related disability remain a major social and therapeutic problem. Comorbidities and therapies increase drug interactions and side effects making pain management more compounded especially in the elderly who are the fastest-growing pain population. Multimodal analgesia consists of using two or more drugs and/or techniques that target different sites of pain, increasing the level of analgesia and decreasing adverse events from treatment. Paracetamol enhances multimodal analgesia in experimental and clinical pain states. Strong preclinical evidence supports that paracetamol has additive and synergistic interactions with anti-inflammatory, opioid and anti-neuropathic drugs in rodent models of nociceptive and neuropathic pain. Clinical studies in young and adult elderly patients confirm the utility of paracetamol in multimodal, non-opioid or opioid-sparing, therapies for the treatment of acute and chronic pain.

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.

Synergism between oral paracetamol and nefopam in a murine model of postoperative pain

BACKGROUND

The use of paracetamol or nefopam for postoperative pain control is limited by the need of high doses associated with unwanted effects. Previous works suggest positive interactions between both compounds that may be exploited to obtain potentiation of antinociception.

METHODS

Mechanical and heat antinociception induced by oral doses of paracetamol, nefopam or their combination was studied by isobolographic analysis in a murine model of postsurgical pain. The effective doses that produced 50% antinociception (ED₅₀ ) were calculated from the log dose-response curves for each compound. Subsequently, the effects of ED_8.7 s, ED_12.5 s, ED_17.5 s and ED₃₅ s of nefopam and paracetamol combined were assessed.

RESULTS

Oral paracetamol induced dose-dependent relief of postoperative sensitivity and showed higher efficacy reducing mechanical hypersensitivity (ED₅₀ 177.3 ± 15.4 mg/kg) than heat hyperalgesia (ED₅₀ 278.6 ± 43 mg/kg). Oral nefopam induced dose-dependent antinociception with similar efficacy for mechanical and heat hypersensitivity (ED₅₀ s 5.42 ± 0.81 vs. 5.83 ± 0.72). Combinations of increasing isoeffective doses revealed that combined ED_17.5 s (85.76 mg/kg paracetamol and 1.9 mg/kg nefopam) and ED₃₅ s (132.67 mg/kg and 3.73 mg/kg) showed synergistic effects leading to 75% and 90% mechanical antinociception, respectively. These mixtures were defined by interaction indexes of 0.43 and 0.41 and ratios 45:1 and 35:1 paracetamol:nefopam, respectively. The same combinations showed additive effects for the inhibition of incisional thermal hyperalgesia.

CONCLUSIONS AND LIMITATIONS

This work describes a synergistic antinociceptive interaction between low doses of nefopam and paracetamol for the treatment of postoperative hypersensitivity to peripheral stimuli. The promising results obtained on reflexive nociceptive responses of young male mice subjected to plantar surgery highlight the interest of further research evaluating the effects of this mixture on the affective-motivational component of pain and in females and additional age groups. Confirmation of pain-relieving efficacy and safety of this oral combination clinically available in European and Asian countries could provide a useful tool for postsurgical pain management.

SIGNIFICANCE

Early postoperative pain is currently undertreated and has been recognized as a relevant source of chronic postsurgical pain. Oral efficient treatments could facilitate fast-track surgeries and patient recovery at home. Here, we identify in a mouse model of postoperative pain a potent synergistic oral combination consisting of low paracetamol and nefopam doses that provides relief of postsurgical hypersensitivity to mechanical and thermal stimuli. Oral multimodal paracetamol-nefopam mixtures represent a potential clinically available pharmacological strategy for the relief of incisional sensitivity and the promotion of patient recovery.

Co-administration of Pregabalin and Curcumin Synergistically Decreases Pain-Like Behaviors in Acute Nociceptive Pain Murine Models

Analgesic drugs in a combination-form can achieve greater efficacy with lesser side effects compared to either drug alone. The combination of drugs acting at different targets or mechanisms of action has been recognized as an alternative approach for achieving optimal analgesia. In this study, the analgesic effects of pregabalin (30, 60, 100, 200 mg/kg), curcumin (15, 30, 60, 100, 120 mg/kg), and 1:1 fixed-dose ratio of the pregabalin-curcumin combination were assessed using two acute nociceptive pain models, the acetic acid-induced writhing and tail-flick tests in mice. The pregabalin-curcumin combination produced a dose-dependent decrease in mean of writhes and an increase in the percentage of antinociception by the acetic acid-induced writhing test. In the tail-flick test, the combination also showed an improvement in antinociception indicated by the tail-flick latency, % antinociception, and area under the curve (AUC). Isobolographic analysis of interactions demonstrated a significant synergistic interaction effect between pregabalin and curcumin in both acute nociceptive pain models with the experimental ED₅₀ below the predicted additive line and the combination index < 1. These findings demonstrate that the combination of pregabalin and curcumin exhibits a synergistic interaction in mouse models of acute nociceptive pain.

Antiepileptic drugs as analgesics/adjuvants in inflammatory pain: current preclinical evidence

Inflammatory pain is the most common type of pain that is treated clinically. The use of currently available treatments (classic analgesics - NSAIDs, paracetamol and opioids) is limited by insufficient efficacy and/or side effects/tolerance development. Antiepileptic drugs (AEDs) are widely used in neuropathic pain treatment, but there is substantial preclinical evidence on their efficacy against inflammatory pain, too. In this review we focus on gabapentinoids (gabapentin and pregabalin) and dibenzazepine AEDs (carbamazepine, oxcarbazepine, and recently introduced eslicarbazepine acetate) and their potential for relieving inflammatory pain. In models of somatic, visceral and trigeminal inflammatory pain, that have a translational value for inflammatory conditions in locomotor system, viscera and head/face, AEDs have demonstrated analgesic activity. This activity was mostly consistent, dependent on the dose and largely independent on the site of inflammation and method of its induction, nociceptive stimuli, species, specific drug used, its route of administration and dosing schedule. AEDs exerted comparable efficacy with classic analgesics. Effective doses of AEDs are lower than toxic doses in animals and, when expressed as equivalent human doses, they are largely overlapping with AEDs doses already used in humans for treating epilepsy/neuropathic pain. The main mechanism of antinociceptive/antihyperalgesic action of gabapentinoids in inflammatory pain models seems to be α₂δ-dependent suppression of voltage-gated calcium channels in primary sensory neurons that leads to reduced release of neurotransmitters in the spinal/medullar dorsal horn. The suppression of NMDA receptors via co-agonist binding site primarily at spinal sites, activation of various types of K⁺ channels at spinal and peripheral sites, and activation of noradrenergic and serotonergic descending pain modulatory pathways may also contribute. Inhibition of voltage-gated sodium channels along the pain pathway is probably the main mechanism of antinociceptive/antihyperalgesic effects of dibenzazepines. The recruitment of peripheral adrenergic and purinergic mechanisms and central GABAergic mechanisms may also contribute. When co-administered with classic/other alternative analgesics, AEDs exerted synergistic/additive interactions. Reviewed data could serve as a basis for clinical studies on the efficacy/safety of AEDs as analgesics/adjuvants in patients with inflammatory pain, and contribute to the improvement of the treatment of various inflammatory pain states.

The influence of excipients on physical and pharmaceutical properties of oral lyophilisates containing a pregabalin-acetaminophen combination

OBJECTIVES

The purpose of the study was to investigate and characterize the oral lyophilisates containing the pregabalin-acetaminophen drug combination and as xcipients mannitol with microcrystalline cellulose or hydroxypropyl methylcellulose, in order to conclude upon drug-excipient interactions and their stability implications, impact of excipients on drug release and on the physicochemical and mechanical properties of the pharmaceutical formulations.

METHODS

The oral tablets were made by using a Christ freeze-dryer alpha 2-4-LSC lyophilizer, and evaluated for stability, drug-excipient compatibility and homogeneity of the prepared pharmaceutical formulations. The formulations were evaluated for in vivo absorption in rabbits by histopathological exams.

RESULTS

FTIR and thermogravimetric analyses, DLS technique, SEM and NIR-CI studies confirmed the compatibility between compounds. From the determined physical and biochemical parameters of the formulations it was established that they are stable, homogeneous, and meet the conditions for orally disintegrating tablets.

CONCLUSION

In the case of the investigated pharmaceutical formulations the study evidenced the assembling through physical bonds between the excipients and the 'codrug' complex, which do not affect the release of the bioactive compounds.