Molecular mechanisms of lidocaine

∗ Corresponding author. Faculty of Medicine, Muhammadiyah University of Jakarta, Jl. Kh.Achmad Dahlan, RW.2, Cireundeu, Kec. Ciputat Tim, Kota Tangerang, Selatan, Banten, 15419, Indonesia. moc.oohay@rd_emilbus

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Abstract

Lidocaine is an amide-class local anesthetic used clinically to inhibit pain sensations. Systemic administration of lidocaine has antinociceptive, antiarrhythmic, anti-inflammatory, and antithrombotic effects. Lidocaine exerts these effects under both acute and chronic pain conditions and acute respiratory distress syndrome through mechanisms that can be independent of its primary mechanism of action, sodium channel inhibition. Here we review the pathophysiological underpinnings of lidocaine's role as an anti-nociceptive, anti-inflammatory mediated by toll-like receptor (TLR) and nuclear factor kappa-β (NF-kβ) signalling pathways and downstream cytokine effectors high mobility group box 1 (HMGB1) and tumour necrosis factor-α (TNF-α).

Highlights

Lidocaine has been safely used in humans for almost a century with a remarkable diversity of applications.

Lidocaines therapeutic effects include antinociceptive, antiarrhythmic, anti-inflammatory, and antithrombotic.

The mechanisms underlying lidocaine's many clinical effects include decreasing levels of inflammatory mediators.

New clinical applications are still being discovered, including a potential therapeutic to treat COVID-19 (lung injury).

1. Introduction

Post-surgery inflammation is characterized by increased blood flow and vascular permeability, the upregulation of inflammatory mediators, and leukocyte accumulation [1]. Cytokines are key modulators of inflammation and can play both pro- and anti-inflammatory roles. A dynamic balance exists between pro- and anti-inflammatory cytokines that affects organ dysfunction, immunity, infection, wound healing, and pain – all of which are associated with surgery [1,2]. Surgical injury induces endogenous mediators and activates hemodynamic, metabolic, and immune responses [1]. This immune response initiates immediately after a surgical injury. Polymorphonuclear leukocytes (PMNs), endothelial cells, macrophages, and lymphocytes are activated by the secretion of pro-inflammatory mediators including cytokines, chemokines, and other molecules including but not limited to reactive oxygen species, nitric oxide, and platelet-activating factor [3]. While essential, when unchecked, excessive inflammation can disrupt the body's immune system, potentially leading to certain inflammation-related conditions and even organ failure [4,5].

Lidocaine was first synthesized by Nils Lofgren in 1935 in the in the Stockholm laboratory of Professor Hans von Euler where Lofgren began tasting the compounds he and his colleagues had synthesized [6]. In 1943, Lofgren found that the 57th compound he tested rapidly numbed his tongue. The patent for Xylocaine® was approved in Sweden on May 11, 1948, based on Goldberg's (toxicology) and Gordh's (clinical results) papers suggesting that lidocaine had a strong and unexpected anesthetic effect. The Food and Drug Administration approved Xylocaine® for usage in the United States in November 1948 [6,7]. Torsten Gordh's clinical testing revealed that lidocaine represented a significant improvement over procaine, the gold standard for managing surgical pain at the time [8,9]. This amide-class anesthetic is still used widely to ease the pain associated with surgery, provide neuropathic pain relief, and treat ventricular arrhythmias [10,11].

In addition to its use as a local anesthetic and anti-arrhythmic agent, lidocaine has analgesic properties for various pain conditions. The nociceptive antagonist effects of intravenous lidocaine have been well established in a variety of acute and chronic pain conditions [7,12]. Moreover, preclinical and clinical data evidence the antihyperalgesic effects of parenteral lidocaine [7]. The recommended initial dose is 1–2 mg/kg administered intravenously followed by a continuous infusion of 2–4 mg/kg/h, resulting in steady plasma concentrations of 1–3 mg/ml [7,13].

The mechanism of action of lidocaine as a local anesthetic is through a blockade of voltage-gated sodium channels (VGSCs) leading to a reversible block of action potential propagation [7]. Lidocaine affects inflammatory cells in vitro, for example by inhibiting the priming of human peripheral PMNCs and neutrophils [7,14]. Lidocaine can further reduce the release of pro-inflammatory mediators such as IL-4, IL-6, and tumour necrosis factor-alpha (TNF-α) [7].

It is well established in the literature that lidocaine exerts its anti-inflammatory effects by inhibiting the expression of pro-inflammatory cytokines, the metabolic activity of leukocytes, and the release of histamine [10,15]. The effects of lidocaine are achieved by preventing NF-kβ activation and its downstream cytokine storm [16]. Specifically, lidocaine significantly reduces TNF-α levels relative to vehicle-treated controls [17]. Therefore, lidocaine's anti-inflammatory mechanism of action at the level of receptor engagement and pro-inflammatory cytokine release in the context of post-surgical injury is the entry point for the present review.

2. Systemic effect of lidocaine in molecular biology

Lidocaine is an amide class of local anesthetics used in medicine to inhibit pain sensations [17,18]. It consists of lipophilic and hydrophilic subunits connected by hydrocarbon chains. The hydrophilic portion is composed of tertiary amines (e.g., diethylamine) while the lipophilic portion is composed of unsaturated aromatic rings [e.g., para-aminobenzoic acid (PABA)] [19,20]. Based on this structure, these local anesthetics can be classified into amino-esters and amino-amides. The lipophilic portion determines the anesthetic activity of the local anesthetic drug [9]. Local anesthetics act on sodium ion channels to reduce the permeability of cell membranes, thereby blocking depolarization and preventing the conduction of the electrical impulse through which pain occurs [21].

Chemically, lidocaine [2- (diethylamino) -N- (2,6-dimethylphenyl) acetamide] contains three basic components: hydrophilic amine groups, aromatic residues, and intermediary groups that connect these two ( Fig. 1 ) [22,23]. The amine group is a tertiary or secondary amine, between an aromatic residue group and an intermediate group connected by an amide bond. Lidocaine is a weak alkaline with pKa of 8, protein binding of 64%, and fat solubility of 1%. Lidocaine remains the drug of choice for a variety of medical procedures due to its strong anesthetic potential, fast onset of action, and wide safety limits [24,25]. Moreover, lidocaine can be administered via many routes, including topical (i.e., skin and airway), subcutaneous, intravenous, perineural, epidural, and intrathecal [22,26,27]. After intravenous administration, peak plasma levels are achieved within 3–5 min with a half-life of 30–120 min [28].

Chemical structure of lidocaine [21].

In the liver, lidocaine is dealkylated by dual-function oxidizing enzymes to the pharmacologically active metabolite, monoethylglycinexylidide (MEGX) and then metabolized by the P450 3A4 isoenzyme into N-ethylglycine (NEG) and glycinexylidide (GX). MEGX is 80% as potent as the parent drug, whilst GX is nearly ineffective [29].

In clinical practice, lidocaine is also used as a class IB antiarrhythmic drug (sodium channel blocker). Sodium channels have three basic states: (1) resting (phase 1), while they await the arrival of an action potential; (2) open/active (phase 0), during which the channel is activated and conducts a sodium current; and (3) inactivated/refractory (phase 2), after the channel has conducted a sodium current but has not yet returned to its resting state. During this refractory period, the sodium channel cannot yet be activated again. Lidocaine occupies receptors on the sodium channel in its open/active (phase 0) and inactivated/refractory (phase 2) states, which lidocaine has a high affinity for its receptors during both phases [30].

Lidocaine's effects on the central nervous system include inhibiting nicotinic and acetylcholine receptors, inhibiting presynaptic calcium channels in the dorsal root ganglion, inhibiting opioid receptors, inhibiting of neurite growth, inhibiting muscarinic cholinergic receptors, and preventing substance P from binding to natural killer (NK) cell receptors [7,31,32].

The anti-inflammatory effect of lidocaine occurs at lower concentrations than required to block sodium channels [33,34]. The effect of lidocaine on inflammation, particularly against inflammatory polymorphonuclear granulocytes (PMNs), macrophages, and monocytes is not due to blocked sodium channels [35,36].

Priming can be described as a process that gives a resting neutrophil a functional response that can be greatly amplified upon exposure to another stimulus [37,38]. The second stimulus is usually considered an activating agent or agonist. The enhanced functional response keeps the neutrophils in an active state. Thus, full neutrophil activation is a two-step process, starting with initial exposure to primary agents such as cytokines (e.g., IL-1α, GM-CSF, and TNF-α) and antigens (e.g., pathogenic endotoxin). Priming and activation change neutrophils from a resting state to an active state, thus enabling them to perform antibacterial, pro- and anti-inflammatory functions [39]. PMN priming regulates the function of PMNs and is implicated in cases of excessive inflammatory responses that cause tissue damage [39,40]. Some potential mechanisms include local anesthetics inhibiting G-protein-coupled receptors (GPCRs) signals that mediate inflammatory responses such as lysophospathic acid and thromboxane A2 as well as the M1 muscarinic acetylcholine receptor. GPCRs consist of muscarinic acetylcholine receptors M1-M5, which regulate several functions of the nervous system ( Fig. 2 ) [23]. Furthermore, M1 and M4 receptors are the treatment sites for several central nervous system disorders such as Alzheimer's disease, schizophrenia, and drug addiction [23,41].

The mechanism of action of local anesthetics on inflammation [21].

3. Lidocaine as an anti inflammatory

Lidocaine is a potent anti-inflammatory agent, whose properties are often compared with steroids and non-steroidal anti-inflammatory drugs (NSAIDs) [42]. The definite anti-inflammatory mechanism of lidocaine remains vague; however, it is presumed that the drug affects a multitude of inflammatory processes such as phagocytosis, migration, exocytosis, and cellular metabolism. In vitro experiments on human polymorphonuclear granulocytes suggest that lidocaine the membrane-ion transporters, thereby, dysregulating cellular pH levels and eventually depressing cytokine release [43].

There is still no universal reference dose for lidocaine administration as an anti-inflammatory agent. Ortiz et al. conducted a double-blind, randomized trial studying the effect of endovenous lidocaine on serum inflammatory cytokine levels using bolus lidocaine of 1.5 mg/kg at the start of the procedure with a maintenance dose of 3 mg/kg/h until 1 h after the end of the surgery [44]. A significant reduction in serum levels of pro-inflammatory markers (IL-1, IL-6, TNF-α, and IFN-γ) was observed in the IV lidocaine group in comparison with a control group [44,45]. In this same study, there were no statistically significant differences regarding the postoperative pain intensity, morphine consumption, ileus, and hospital stay compared with the control group, meaning there were no proven secondary effects at this dose [44].

Multiple, complex mechanisms likely underlie lidocaine's anti-inflammatory effects in a synergy that involves numerous pathways, receptors, cells, and mediators. More specifically, the modulation of high mobility group box 1 (HMGB1), toll-like receptor (TLR), nuclear factor kappa-β (NF-kβ), and TNF-α have been implicated in previous studies.

3.1. High Mobility Group Box 1 (HMGB1) cytokine activity

HMGB1 functions as a pathogenetic cytokine regulator under these conditions. HMGBl is actively secreted from various cell types including macrophages, NK cells, dendritic cells (DCs), blood vessel endothelial, and platelets [46,47]. HMGB1 is also passively released from necrotic cells, damaged cells, and after extracellular injury [48]. Cells undergoing apoptosis release less HMGB1 than necrotic cells, yet macrophages covered with apoptotic cells can still stimulate the discharge of HMGB1 from WEHI-231, Jurkat, and HL-60 cells. Pyropticosis and caspase-1-associated necrosis are responsible for the continuous discharge of HMGB1 controlled by dsRNA-dependent protein kinase (PKR) and the inflammasome. Pyroptosis follows the activation of inflammasomes, leading to the expression of caspase-1 and its downstream effects, including generation of the cytokines IL-1β and IL-18 by the cleavage of their precursors [49].

Release mechanisms such as necrosis, macrophage activation, pyroptosis, and apoptosis discharge HMGB1 in various redox forms. Necrotic and pyroptotic cells produce thiol in its reduced form, which then binds to the chemokine CXCL12 and the CXCR4 receptor to stimulate the process of chemotaxis. Pyroptosis and TLR4 stimulation also release reduced HMGB1, disulfide bond HMGB1, C23, C45, and C106, which are all in thiol form. This form of HMGB1 then stimulates cytokine production via TLR4 signaling. Activated macrophages also release the cytokine-inducing form of HMGB1 upon TLT4 activation. Apoptotic cells release HMGB1 that is partially oxidized or completely oxidized at the critical cysteine residues. Completely oxidized HMGB1, with cysteines in the form of sulfonates, is unable to stimulate cytokines or induce chemotaxis; apoptotic cells expressing this form of oxidized HMGB1 can induce tolerance [49].

3.2. Cellular inflammatory response and HMGB1 receptors

HMGB1 is a classic pro-inflammatory mediator because [4]:

It can be specifically targeted to therapeutic advantage in sterile and infectious disease syndromes associated with elevated HMGB1 levels.

One difference between HMGB1 and conventional PCs (e.g., TNF-α, IL-1) is it stimulates systemic inflammatory responses through receptors that report and interact with foreign substances [50]. Unlike TNF-α and IL-1, the allied plasma membrane receptor family interacts with HMGB1 and initiates signal transduction through endogenous (RAGE) and exogenous ligands (TLR2, TLR4, and TLR9) [51,52]. These processes show that HMGB1 elicits various inflammatory responses to a diversity of infections and injuries. HMGB1's ability to modulate the magnitude of the inflammatory response in clinical syndromes associated with injury is discussed below. These mechanisms have been explored in loss-of-function-type studies based on HMGB1 antagonists and/or the deletion of receptors via genetic clustering techniques [53].

HMGB1 binding to TLR4-MD2 acts as a measure of surface plasmin resonance and signal transducers that stimulate macrophages to release TNF [4]. These processes require redox-sensitive cysteine 106, which prevents HMGB1 from adhering to TLR4, the endogenous HMGB1 receptor responsible for regulating macrophage activation, cytokine release, and tissue injury repair by activating IKB kinase (IKK)-β, IKK-α (active endotoxin only IKK-β), and active nuclear translocation NF-kβ [[54], [55], [56]] [[54], [55], [56]] [[54], [55], [56]]. One difference between HMGB1- and endotoxin-mediated signaling, is that the former does not bind to TLR4 as readily as LPS. Another difference is the pattern of gene expression induced upon activation. While both HMGB1 and LPS significantly increase NF-kβ nuclear translocation and Akt/p38 MAPK phosphorylation, LPS increases the production of NF-kβ and TNF more than HMGB1. Furthermore, HMGB1-induced secretion of TNF exhibits a biphasic kinetic profile, while the endotoxin LPS stimulates monophasic TNF release [56].

Animal studies have shown that HMGB1 levels increase during injury due to a lack of oxygen [4]. HMGB1 protein levels rise within 1 h of reperfusion and remain elevated for up to 24 h. Treating wild-type (C3H/HeOuj) mice with anti-HMGB1 antibodies helps to protect them from liver injury, while the TLR4 deficient (C3H/Hej) mice garner no benefit from these antibodies [4,16,57]. HMGB1 signaling via TLR4 is an effective target for solid tumors treatment strategies such as antigen cross-presentation or chemotherapy. Renal tubular TLR4 expression in donor kidneys is indicated by HMGB1 immunoreactivity, this reveals its importance in developing kidney graft inflammation and sterile injury [4].

The impact of HMGB1 protein binding on the release of cytokines (e.g., CXCL12, TLR9, thrombospondin, syndecan, TLR2, MAC1, TREM1) in the pathogenesis of sterile infection and injury remains unknown [4]. Furthermore, HMGB1 regulates the body's inflammatory response to sterile threats and infections through TLR4 receptor-mediated signaling [58,59]. HMGB1 then reacts with CD24, a plasma protein working with Siglec-10 to suppress nuclear translocation. This process is controlled by HMGB1 and mediated TLR4 activation, not pathogen-mediated TLR activation, indicating that the outcome of HMGB1 via TLR4 signaling is different from CD24-siglec-10 when it comes to sterile damage [56].

3.3. HMGB1 pathway to NF-kβ

Receptors involved in HMGB1 binding include the receptor for advanced glycation end products (RAGE), a transmembrane, cell surface, multi-ligand member of the large immunoglobulin family ( Fig. 3 ) [4]. Consequently, RAGE-mediate HMGB1 signaling stimulates chemotaxis, cell growth, differentiation, and the migration of immune/smooth muscle cells by engaging with cell-surface moieties such as RAGE/TLR4. Although HMGB1 and RAGE bind to each other, TLR4 still controls the secretion of HMGB1 from macrophages due to the inhibition of RAGE macrophages and the inactivation of TNF-producing macrophages by TLR2. However, TLR4 does not mediate macrophage inactivation [53].

The relationship between HMGB1, TLR4, and RAGE. TLR4 binding induces cytokine secretion from macrophages and monocytes (left), meanwhile, RAGE modulates endothelial and tumor cell function (right) [3].