FIGURE 3 Direct and indirect mechanisms
Many studies and reports highlighted that acetaminophen has broad mechanism of action not limited to COX pathway (Ohashi & Kohno, 2020; Kanchanasurakit et al., 2020).Therefore, this review revises the potential mechanistic pathways of acetaminophen in relation to its clinical applications.
ACETAMINOPHEN AND COX-3
COX-3 is a novel COX variant proposed to mediate the action of acetaminophen in both animals and humans. COX-3 encoded gen is differed from that of COX-1 and COX-2 (Ogemdi, 2019). Though, it was suggested that COX-3 is a variant of COX-1 but with different molecular effects (Li et al., 2008). Recently, COX variants are sequenced, and found that COX-3 is a variant of COX-1 (Esh et al., 2021). Acetaminophen can induce analgesia and hypothermia by reducing PGE2 via suppression of COX-3 in mice (Ogemdi, 2019; Li et al., 2008). Acetaminophen can selectively inhibits COX-2 but with very low potency compared to selective COX-2 inhibitors by about 433 fold (Esh et al., 2021). Acetaminophen blocks the peroxidase activity rather than inhibition of COX enzymes (Aminoshariae & Khan, 2015). It has been reported that COX-3 expression is higher in the brain only (Chandrasekharan et al., 2002) that mediate the antipyretic and analgesic effect of acetaminophen. Supporting to this notion, deletion of COX-1 prevents the antipyretic and analgesic effect of acetaminophen in mice (Ayoub & Flower, 2019) suggesting that COX-3 is a variant of COX-1. Likewise, selective COX-3 inhibitors such as antipyrine and aminopyrine produce similar antipyretic and analgesic effects of acetaminophen (Ayoub et al., 2004). An updated experimental study confirmed the expression of COX-3 in knee joint and implicated in the development of arthritis (Biswas et al., 2023). Therefore, COX-3 like COX-1 is peripherally and centrally expressed.
Despite of these preclinical findings, there is strong argument regarding the effect of acetaminophen in relation to COX-3 (Davies et al., 2004; Snipes et al., 2005). The potential conflicting for the functional activity of COX-3 is related to many points. COX-3 protein is detected in human tissues, though the functional activity of COX-3 enzyme is not sequenced. In addition, many variants of COX-1 are identified in animals and humans, and none of these variants are targeted by acetaminophen (Perrone et al., 2010). As well, COX-1 gene can produce mRNA products that not involved in the degradation process or synthesis of PGs, thus, COX-3 might be inert mRNA products (Mahesh et al., 2010). Furthermore, preclinical studies concerning effect of acetaminophen on COX-3 were inappropriate to be translated in clinical practice due to species difference regarding the expression and activity of COX-1 variants (Kotowska-Rodziewicz et al., 2023). Moreover, acetaminophen inhibits COX-1 when AA concentration is low, and COX-1 knockout mice but not COX-2 knockout mice prevent acetaminophen effect (Graham et al., 2013). Thus, COX-3 is not the potential target of acetaminophen effect.
ACETAMINOPHEN AND SEROTONIN PATHWAY
It has been shown that acetaminophen analgesic effect is mediated by modulating of brain serotonergic neurotransmission (Hamurtekin et al., 2020) and interfering with spinal serotonergic pathway by serotonin antagonist reduces the analgesic effect of acetaminophen in mice (Karandikar et al., 2016). Furthermore, acetaminophen augments serotonin level in the pons and cerebral cortex signifying supra-spinal analgesic effect of acetaminophen (Fukushima et al., 2017). An experimental study demonstrated that intraperitoneal administration of acetaminophen increases serotonin level in the pons by 40% and in the cerebral cortex by 75% through modulation of 5HT2R (Ruggieri et al., 2008). In addition, chronic acetaminophen administration in rats increases serotonin level in the prefrontal cortex, hypothalamus, thalamus and striatum (Blecharz-Klin et al., 2013). Acetaminophen-induced increase of brain serotonin may be through reducing of serotonin metabolism, increasing its release or through inhibition of serotonin reuptake. However, the exact mechanism of acetaminophen effect on brain serotonin is not fully elucidated.
Of interest, acetaminophen metabolite AM404 is generated by FAAH enzyme expressed in brain, dorsal root ganglion and spinal cord (Nilsson et al., 2021). AM404 has 50% analgesic effect of acetaminophen, as AM404 activates 5HT3 but not 5HT1A or 5HT2 which mediate the analgesic effect of acetaminophen (Mallet et al., 2023). Acetaminophen but not AM404 augment brain serotonin (Mallet et al., 2023). In addition, AM404 activates descending serotonergic pathway which has an antinociceptive effect (Kaur, 2020). Presynaptic autoreceptors 5HT1A and 5HT1B inhibit serotonin release from presynaptic neurons. Administration of 5HT1A agonist buspirone blocks the analgesic effect of acetaminophen in mice (Sandrini et al., 2003). Therefore, acetaminophen may act as an antagonist for presynaptic 5HT1A leading to increase serotonin release.
Furthermore, postsynaptic 5HT2 which mediate the excitatory effect of serotonin mediates the analgesic effect of acetaminophen. Systemic administration of 5HT2 antagonist ketanserin attenuates the analgesic effect of acetaminophen but not AM404 (Kose et al., 2019). However, intrathecal administration of ketanserin did not affect the analgesic effect of acetaminophen (Ledebuhr et al., 2022). These findings suggest that 5HT2 mediates the supraspinal but not spinal analgesic effect of acetaminophen. Likewise, spinal 5HT3 mediates the analgesic effect of acetaminophen, and administration of 5HT3 antagonist tropisetron blocks systemic and intrathecal acetaminophen (Irinmwinuwa et al., 2022). However, 5HT3 antagonist ondansetron which act centrally did not affect the analgesic effect of acetaminophen (Libert et al., 2004). Inhibition of spinal 5HT3 by oligodeoxynucleotide did no reduce the effect of acetaminophen (Libert et al., 2004). These findings were confirmed clinically (Pickering et al., 2006; Bandschapp et al., 2011). These findings indicated that 5HT3 mediates supraspinal analgesic effect of acetaminophen.
Moreover, 5HT7 is highly expressed in the brain, involved in antinociception effect, is also mediate the analgesic effect of acetaminophen at spinal level (Kose et al., 2019). A preclinical study found that 5HT7 did not mediate the antipyretic effect of acetaminophen (Hamurtekin et al., 2020). However, preclinical studies demonstrated that the analgesic effect of acetaminophen at spinal level is mediated by activating 5HT7 in the descending antinociceptive serotonergic pathway (Liu et al., 2013; Dogrul et al., 2012).
Therefore, the analgesic effect of acetaminophen at spinal and supraspinal levels is mediated by activation serotonin release or direct activation of serotonin receptors.
ACETAMINOPHEN AND NITRIC OXIDE PATHWAY
Nitric oxide (NO) is a small molecule widely expressed in the CNS and act as neurotransmitter; it modulates pain transmission negatively or positively (Lundberg & Weitzberg, 2002). Of note, NO and NO synthase are involved in the analgesic effect of low-dose but not high-dose acetaminophen (Angelis et al., 2021). Acetaminophen has ability to inhibit NO synthase in the spinal cord (Godfrey et al., 2007). Therefore, the central analgesic effect acetaminophen could be mediated by suppressing neuronal NO synthase. However, NO-acetaminophen combination was used to reduce acetaminophen-induced hepatotoxicity. NO-acetaminophen has profound analgesic and can reduce neuropathic pain compared to acetaminophen alone (Cooper et al., 2022). Moreover, NMDA receptor produces excitotoxicity by activating the release of NO (Negri et al., 2021). A previous experimental study showed that acetaminophen attenuates substance P and NMDA-mediated spinal hyperalgesia (Björkman et al., 1994). Choi et al (2001) study demonstrated that acetaminophen inhibits spinal nociceptive effect mediated by glutamate and substance P. Therefore; acetaminophen inhibits NO-induced pain transmission. In addition, AM404 inhibits neuronal NO, and release of pro-inflammatory cytokines through inhibition of microglial activation (Costa et al., 2006). Therefore, acetaminophen and its metabolite attenuate NO-induced nociception and pain transmission.
ACETAMINOPHEN AND TRANSIENT RECEPTOR POTENTIAL VANILLOID 1
Transient receptor potential vanilloid 1 (TRPV1) is a non-selective channel receptor triggered by vanilloids and temperature, and mediates the central hyperalgesia (Garami et al., 2020). TRPV1 is highly expressed in nociceptive and sensory neurons, such as sensory C fiber which mediate inflammatory and neuropathic pain (Chang et al., 2021). TRPV1 is mainly present in trigeminal ganglion and dorsal root ganglion (Cha et al., 2020). In the CNS, TRPV1 is expressed in specific brain regions may involve in pain transmission and thermoregulation, such as thalamus, hypothalamus, cerebral cortex, cerebellum, striatum and substantia nigari (Meza et al., 2022). TRPV1 expression is augmented by neuronal injury and inflammation (Meza et al., 2022). Notoriously, TRPV1 agonists inhibit inflammation by reducing the expression of pro-inflammatory TNF-α (Abdel-Salam et al., 2023). Activation of TRPV1 induces the release of different neuropeptides intricate in pain transmission such as somatostatin, substance P (SP) and calcitonin gene related peptide (CGRP) (Messlinger et al., 2020). Therefore, activation of TRPV1 leads to anti-inflammatory and immunomodulatory effects by reducing the release of pro-inflammatory cytokines and induction of neuropeptides. However, TRPV1 antagonists were suggested to be effective for painful conditions, though these agents were withdrawn because of risk of hyperthermia and cognitive impairment (Garami et al., 2020; Caballero, 2022). Thus, TRPV1 agonists were proposed to be effective against pyrexia, diabetic neuropathy, post-herpetic neuralgia and osteoarthritis (Iftinca et al., 2021; Liao et al., 2023). TRPV1 agonist capsaicin is effective in patients with diabetic neuropathy and osteoarthritis (Liao et al., 2023). Therefore, TRPV1 agonists have important anti-inflammatory effects either directly or indirectly by inducing the release neuropeptide such as SP, CGRP and somatostatin (Figure 4).