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

**Background:** Paracetamol (acetaminophen) is one of the most widely used over-the-counter analgesics globally and remains the recommended first-line option for mild-to-moderate acute pain by numerous scientific societies, particularly in vulnerable populations including geriatrics, children, patients with peptic ulcers, and pregnant women. Despite over a century of clinical use, the analgesic mechanism of action of paracetamol has been subject to ongoing debate. Historically attributed to cyclooxygenase (COX) inhibition and prostaglandin synthesis suppression—similar to NSAIDs—this view has been challenged by accumulating evidence demonstrating that paracetamol's analgesic effects are predominantly mediated through centrally located, COX-independent pathways involving the endocannabinoid, serotonergic, and nitric oxide systems.

**Methods:** A bibliographic literature search was performed on Medline via PubMed from database inception to May 2022 using the keywords: (paracetamol[MeSH Terms]) OR (acetaminophen[MeSH Terms])) AND (AM404). The online search was complemented by manual screening of relevant references. This review synthesizes experimental and clinical evidence on the central mechanism of action of paracetamol via its bioactive metabolite AM404.

**Key Results:** The seminal 2005 study by Högestätt et al demonstrated that paracetamol undergoes hepatic deacetylation to p-aminophenol, which crosses the blood–brain barrier and is converted in the brain to N-arachidonoylphenolamine (AM404) by fatty acid amide hydrolase (FAAH). The first human evidence of this central conversion was provided by Sharma et al in 2017, where 26 patients received 1 g of paracetamol, and AM404 was detected in the cerebrospinal fluid of 17 patients, confirming central conversion in humans. Centrally injected AM404 induces analgesia, while FAAH inhibition (genetically or pharmacologically) suppresses paracetamol's analgesic effect. AM404 activates multiple molecular targets: (1) TRPV1 channels—supraspinal TRPV1 activation induces anti-nociception, with diminished paracetamol action in TRPV1 knockout mice and in humans with specific TRPV1 rs224534 variants; (2) Cav3.2 T-type calcium channels, acting downstream of TRPV1—Cav3.2 knockout mice show impaired response to paracetamol; (3) CB1 cannabinoid receptors—CB1 knockout models show no analgesic response to paracetamol, with periaqueductal grey-located CB1 receptors identified as essential through an AM404-activated TRPV1 channel–mGlu5 receptor–PLC–DAGL–CB1 receptor signaling cascade; (4) descending serotonergic pathways, though the exact spinal receptor involved remains to be elucidated. Regarding COX-dependent mechanisms, the proposed COX-3 splice variant hypothesis has been invalidated in humans, as full-length catalytically active COX-3 does not exist. Importantly, analgesic doses of paracetamol in mice did not reduce brain PGE2 content, and the antinociceptive profile differed from that of ibuprofen, indicating that analgesic action cannot be attributed to central COX inhibition. Clinically, paracetamol is dosed at a maximum single dose of 1 g every 4–6 hours up to 4 g daily. A Cochrane review concluded that adverse event rates were similar between higher (1 g) and lower (500–650 mg) single doses. Paracetamol demonstrates a favorable safety profile compared to NSAIDs: it does not cause gastrointestinal ulcers or bleeding, is generally safe in renal impairment, and shows no robust evidence of increased cardiovascular adverse outcomes, though some observational studies suggest blood pressure elevation in chronic users. Combination therapy data include: paracetamol plus opioids provides superior pain relief and lower opioid dose requirements; paracetamol/NSAID combinations (particularly ibuprofen) demonstrate additive analgesic effects—the PANSAID trial showed higher analgesia and morphine-sparing effect with the combination versus paracetamol alone after total hip arthroplasty. However, a 13-week trial showed increased risk of hemoglobin decrease (≥1 g/dL) with the combination, and a population-based retrospective cohort study suggested increased upper GI events with traditional NSAID plus acetaminophen combinations versus either agent alone. Paracetamol analogs that target brain TRPV1 receptors without producing the hepatotoxic metabolite NAPQI have been developed and exhibit dose-dependent antinociceptive effects without liver cell necrosis or elevation in liver enzyme levels.

**Clinical Implications:** The evidence establishes paracetamol as a central, COX-independent analgesic with a distinct mechanism of action from NSAIDs and a more tolerable safety profile. Paracetamol remains the preferred first-line option for mild-to-moderate acute pain management in healthy adults and special populations, including the elderly and patients with gastrointestinal, renal, or cardiovascular disease. Healthcare practitioners should individualize dosing in special populations to avoid hepatotoxicity. The central mechanism supports multimodal analgesia approaches: paracetamol potentiates opioid effects without increasing adverse events, and paracetamol/NSAID combinations provide additive analgesia. Further high-quality evidence is needed on morphine-sparing effects and long-term safety of combinations. Paracetamol analogs targeting TRPV1 signaling without NAPQI production represent a promising avenue for developing safer analgesics with improved benefit-to-risk ratios.