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).