1. Introduction
The inflammatory process is defined as a natural response of the immune system to noxious stimuli affecting cells or tissues, triggered by chemical, physical or biological agents [
1,
2]. When activated excessively and persistently, this process can lead to organ and system damage associated with the pathogenesis of various diseases, including autoimmune diseases, cancers, musculoskeletal, gastrointestinal, and psychoemotional disorders, as well as worsen the prognosis of viral infections, thus contributing to healthcare costs [
3,
4,
5].
The pathophysiology of inflammation involves vasodilation resulting from the release of mediators by leukocytes that migrate from the blood to the site of injury, leading to characteristic symptoms such as edema, pain, and redness. If left unchecked, this process can lead to loss of organ or tissue function [
6]. Effector mechanisms of the immune system include the expression of cytokines, eicosanoids, vasoactive amines, and nitric oxide, which play a role in signaling and activating vascular endothelial cells and higher regulatory adhesion molecules that mediate leukocyte rolling, cellular adhesion, and extravasation to the site of inflammation, actively contributing to the cellular response [
2].
As mentioned above, an important mediator involved in the genesis of inflammation is nitric oxide (NO), a small signaling molecule whose functions are primarily regulated by the expression and activity of nitric oxide synthase (NOS) enzymes: neuronal (nNOS), endothelial (eNOS), or inducible (iNOS), which synthesize NO by converting L-arginine to L-citrulline [
7,
8]. Due to its anti-inflammatory and pro-inflammatory properties, the role of NO remains unclear; however, iNOS expression is known to result from inflammatory responses [
8].
In localized infections and sepsis, iNOS expression is associated with the inhibition of pathogen growth and platelet aggregation and the NO pathway is involved in peripheral analgesia in primary sensory neurons [
7,
8]. In addition, iNOS can be transcriptionally stimulated by the activation of TLRs in response to PAMPs, where biosynthesized NO exerts its vasoactive function in acute inflammatory responses accompanied by leukocyte migration throughout the process [
9].
Another critical event in the pathophysiology of inflammation is the biosynthesis and release of prostaglandins (PGs). The pharmacological action of classical nonsteroidal anti-inflammatory drugs (NSAIDs) is to suppress the biosynthesis of these mediators from arachidonic acid by selective or non-selective inhibition of cyclooxygenase (COX) enzymes [
2,
10]. Compared with steroidal anti-inflammatory drugs (SAIDs), long-term use of NSAIDs is considered safer, and they are among the most commonly used drugs, with a place in the World Health Organization Model List of Essential Medicines [
11].
Non-selective NSAIDs in conventional therapy, although effective, have undesirable adverse effects on the gastrointestinal (GI) tract, renal and hepatic systems due to the constitutive expression of cyclooxygenase 1 (COX-1) in the body, which serves as a source of cytoprotective PGs for the GI tract [
8,
11]. Therefore, the development of drugs with selective inhibition of cyclooxygenase 2 (COX-2) has been proposed, assuming greater therapeutic advantages since it is an isoform induced in response to the onset of an inflammatory process and associated with various pathological dysfunctions [
12]. However, these drugs may have serious cardiovascular side effects due to the suppression of cardioprotective PGs derived from COX-2 [
13].
Thus, the critical role of iNOS and COX-2 enzymes and cytokines in the initiation and progression of inflammation is evident. Therefore, drugs capable of modulating these targets and associated with a low toxicity profile are essential as potential pharmacological agents for new anti-inflammatory therapies [
14,
15]. In this context, acylhydrazone derivatives stand out as a class of synthetic chemical structures with diverse bioactive activities, among which the N-acylhydrazone function has shown relevant anti-inflammatory and analgesic responses [
14]. Therefore, this study aims to evaluate the in vivo anti-inflammatory potential of the compound N'-(3-(1H-indol-3-yl)benzylidene)-2-cyanoacetohydrazide (JR19) and to perform in silico and in vivo investigations of possible mechanisms of action involved in the anti-inflammatory response regarding cellular behavior and inflammatory cytokine expression.
4. Discussion
Initially, the peritonitis model was used to study cellular behavior in response to the inflammatory stimulus, using carrageenan as the inflammatory agent. This polypeptide promotes capillary vasodilation in the peritoneal membrane, leading to increased blood flow, structural changes in the microcirculation, allowing extravasation of plasma proteins into the interstitium as inflammatory exudate, and migration of leukocytes from the microcirculation that accumulate at the site of initial injury [
36,
37]. Substances with anti-inflammatory activity can reduce leukocyte migration into the peritoneal cavity by two mechanisms: by inhibiting the synthesis and/or release of chemotactic mediators or by inhibiting the expression of adhesion molecules, since the presence of chemotactic substances is necessary to facilitate their migration to the site of injury and to trigger their effects in an attempt to eliminate the aggressive agent [
38,
39].
Leukocytes play a critical role in the defense and repair provided by the inflammatory response, although platelets and erythrocytes also participate. Through the release of chemical mediators of inflammation, leukocytes migrate across interendothelial junctions (diapedesis) and move toward sites of inflammation [
40]. In addition to their chemotactic effect, these mediators can trigger a cascade capable of amplifying and releasing other stimulatory factors. Leukocyte activation results in the production of arachidonic acid (AA) metabolites, degranulation and secretion of lysosomal enzymes, cytokine secretion, and increased adhesion molecule expression and integrin exposure [
40,
41].
Thus, animal models of acute inflammation that allow the quantitative assessment of leukocyte migration have been widely used, making it possible to quantitatively measure cellular migration, inflammatory mediators, and plasma extravasation after an acute inflammatory process induced by various irritants applied in the cavity [
42,
43,
44]. The results obtained in this screening suggest that JR19 has anti-inflammatory activity, possibly related to its ability to suppress the action and/or release of vasoactive amines, NO, and PGs. These results are consistent with those described by other authors, such as Silva (2022), who tested a ferrocenyl-N-acylhydrazone derivative (100 μmol/kg) and found it to be active in this model.
The second model used to investigate the cellular migration profile of JR19 was the subcutaneous air pouch model. The main difference between the air pouch and carrageenan-induced peritonitis models is the predominant cell type in the exudate. In the subcutaneous air pouch model, neutrophils are the most abundant cells responsible for chemotaxis. In contrast, carrageenan-induced peritonitis shows a high amount of macrophages and mast cells attracted to the site of inflammation by pro-inflammatory cytokines, particularly IL-1β and TNF-α [
45,
46]. Based on the results obtained in this study, it can be concluded that the tested compounds specifically inhibit the production of mediators by neutrophils and macrophages, considering the significant inhibition observed in the air pouch and peritonitis assays, respectively.
Salomé et al. [
47] tested a series of naftyl-N-acylhydrazone derivatives and also obtained satisfactory results in the subcutaneous air pouch model. At the doses tested (1, 10, and 30 μmol/kg), most of the compounds showed statistically similar effects to the standard drug dexamethasone (6.5 μmol/kg). Similarly, Cordeiro [
48] tested doses of 30 and 100 μmol/kg of a new N-acylhydrazone derivative and its corresponding hydrochloride, using dexamethasone (65 μmol/kg) as a standard.
Considering the anti-inflammatory potential of JR19, it has been suggested that this derivative may also have an antinociceptive effect induced by formalin through a biphasic response. The two phases of the formalin test have different characteristics, making it useful for evaluating analgesic substances and elucidating the mechanism of analgesia. The first phase is characterized by the immediate transmission of impulses to the central nervous system and occurs during the first 5 minutes after the application of the inflammatory agent. It is sensitive to drugs that interact with the opioid system, such as morphine. This pain is caused by the direct action of formalin, especially on the C-afferent sensory fibers and partially on the Aδ afferent fibers [
49,
50]. The second phase is prolonged, occurring 15-30 minutes after formalin injection, and is associated with the development of an inflammatory response. It can be inhibited by non-steroidal anti-inflammatory drugs (NSAIDs) such as indomethacin. Thus, centrally-acting drugs (opioids) can inhibit both phases, whereas peripherally-acting drugs inhibit only the second phase [
28,
29,
51].
Compound JR19 was active only in the second phase of formalin administration, confirming its lack of central analgesic effect and possibly its retention of anti-inflammatory activity. This effect may be attributed to the inhibition of the synthesis of inflammatory mediators such as PGs, TXs, and LTs [
46]. From these results, it can be concluded that the observed antinociceptive effect involves peripheral mechanisms acting on inflammatory pain. These results support the hypothesis that the N-acylhydrazone derivative JR19 may have peripheral antinociceptive activity, possibly by inhibiting PG synthesis or by inhibiting the release and/or production of inflammatory mediators, such as cytokines, histamine, and serotonin, among others. In a study by Silva (2015) [
52] the antinociceptive activity of a series of new cyclohexyl-N-acylhydrazone derivatives was evaluated. Most of the compounds tested showed a relevant antinociceptive profile in the formalin-induced inflammatory response (second phase). The compounds active in the neurogenic phase were evaluated in the hot plate test, where the hypothesis of central antinociceptive activity was ruled out.
As demonstrated by molecular docking, the involvement of the enzymes “soluble guanylate cyclase” and “inducible nitric oxide synthase” was investigated using the carrageenan-induced peritonitis model. As mentioned above, this model induces acute inflammation in mice, and in response to the injury, the body mobilizes immune system cells, mainly neutrophils, to the site of injury. A fundamental step in this response is the adhesion of circulating leukocytes to the endothelial cells of blood vessels, allowing their subsequent migration through the barrier of these cells to the site of inflammation. Leukocyte migration from the circulation to the injured tissue is a critical event in the development of the inflammatory process [
40,
46]. As leukocytes migrate to the tissue, they release a number of chemical mediators that can amplify and prolong the inflammatory process. Therefore, compounds capable of inhibiting the activation of these cells can reduce the inflammatory process [
53].
The presence of NO in biological systems is often determined based on physiological effects such as vasodilation, activation of sGC, increased cGMP concentration, citrulline production, or inhibition of platelet aggregation [
54]. It can also be assessed using NO synthesis inhibitors such as L-arginine analogs or hemoglobin and by measuring nitrite and nitrate concentrations. All of these methods have varying degrees of specificity and provide indirect information on NO production [
55].
Our results suggest that JR19 may exert anti-inflammatory effects by modulating NO levels. NO plays a crucial role in various aspects of the inflammatory response, but its role in leukocyte migration is controversial in the literature, with evidence supporting both anti-inflammatory and pro-inflammatory effects [
56]. Studies suggest that NO exerts anti-inflammatory effects by interfering with leukocyte migration. An important piece of evidence for the anti-inflammatory effects of NO is related to the activation of the second messenger cGMP [
57,
58].
NO promotes the modulation of P-selectin (adhesion protein) expression in vitro induced by IL-1β produced in the endothelium through activation of the GCs enzyme, thereby reducing leukocyte adhesion to the vascular wall and neutrophil aggregation and secretion [
57]. Elevated cGMP levels are also associated with a reduction in the expression of glycoprotein IIb/IIIa, another important leukocyte adhesion molecule [
59,
60]. To support the experimental findings, we also investigated whether the anti-inflammatory activity represented by the reduction of leukocytes exerted by the compound JR19 would involve the GC-cGMP pathway through blockade of the GCs enzyme by methylene blue, a nonspecific inhibitor of this enzyme.
Iwata et al. [
61] and Florentino et al. [
62] demonstrated, in carrageenan-induced pleurisy models, that administration of a NO donor (L-arginine) reduces inflammatory cell migration and edema formation. This model, similar to ours, mimics an acute inflammatory process. The anti-inflammatory effect of NO has been attributed to the blockade of transcription factor binding to nuclear DNA (NFκB), thereby inhibiting the synthesis of chemokine IL-8 (a chemotactic factor secreted by activated monocytes and macrophages that promotes the coordinated and directional migration of immune cells such as neutrophils, basophils, and T lymphocytes), thereby compromising the chemotaxis process [
57,
63].
There is also an important relationship between NO and O
2- levels associated with leukocyte migration, which concerns the enzymatic system responsible for O
2- synthesis, which possesses a heme prosthetic group to which NO can bind, inhibiting the generation of this reactive oxygen species [
57,
64]. The reduction in O
2- availability prevents its dismutation to hydrogen peroxide (H
2O
2). This chemical species is responsible for inducing leukocyte adhesion by generating PAF and increasing the expression of other molecules involved in this process [
65,
66].
In addition, elevated levels of O
2- can activate NFκB, leading to increased production of cytokines and chemokines and stimulating mast cells to release pro-adhesive agents such as cytokines s [
67,
68,
69]. Therefore, we conclude that the anti-inflammatory effect of compound JR19, as observed by the reduction of peritoneal leukocytes, is due in part to the acute maintenance of NO levels.
Finally, peritonitis was used to study cytokine expression by JR19 through an sGC/NOS-dependent mechanism using the blockers L-NAME or methylene blue. JR19 (at a dose of 10 mg/kg) was effective in significantly reducing the levels of the pro-inflammatory cytokines IL-6, TNF-α, IL-17, and IFN-γ. IL-6 is a cytokine secreted by various cell types such as macrophages, monocytes, eosinophils, and hepatocytes, and TNF-α is a potent inducer of IL-6 [
43]. It is a pro-inflammatory cytokine that promotes the maturation and activation of neutrophils and macrophages as well as the differentiation/maintenance of cytotoxic T lymphocytes and NK cells. IL-6 is one of the earliest and most important mediators in the induction and control of acute phase protein synthesis and release by hepatocytes during painful stimuli such as infection [
65,
70] It is also involved in the metabolic regulation of C-reactive protein (CRP), an acute-phase protein that increases rapidly during inflammatory processes [
71,
72]. Elevation of serum IL-6 and CRP levels may cause downregulation of NO production by inhibiting the enzyme nitric oxide synthase [
73,
74].
TNF-α is mainly produced by macrophages. It acts on endothelial cells to promote vasodilation and stimulates chemokines to facilitate the chemotaxis of leukocytes, mainly neutrophils and monocytes [
75]. In addition to stimulating their activation, TNF-α also stimulates the production of acute-phase proteins and fibrinogen during the inflammatory process [
76]. Even at low concentrations, TNF-α induces the expression of adhesion molecules in endothelial cells and stimulates macrophages and other cells to secrete chemokines [
77,
78].
IFN-γ is primarily produced by NK cells and T lymphocytes and acts synergistically with TNF-α to stimulate chemokine secretion [
79]. In addition, IFN-γ induces the expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), promoting better leukocyte adhesion to the endothelium and facilitating their diapedesis to the site of inflammation [
79,
80]. It has been reported that this cytokine is involved in neutrophil infiltration in malignant tumors, which typically contain various types of immune cells in addition to cancer cells [
81].
IL-17 is a pro-inflammatory cytokine produced by T cells with the ability to activate and mature neutrophils [
82]. It also stimulates the production of IL-6 and IL-8 (responsible for the migratory stimulus of immune cells, mainly neutrophils, and increases the expression of adhesion molecules on endothelial cells) [
82,
83]. These pro-inflammatory cytokines, among others, stimulate the production of other cytokines and other inflammatory mediators such as inflammatory prostanoids via COX-2 [
84].
Freitas [
85] has classified N-acylhydrazone derivatives as inhibitors of p38 MAPK. Additionally, Cordeiro [
48] demonstrated that these derivatives inhibit the expression of mediators involved in this pathway, such as IL-1β, TNF-α, and NO (at doses of 10, 30, and 100 μmol/kg). Furthermore, Cordeiro [
48] found that at the same doses, there was a significant reduction in pain induced by formalin during the second phase, compared to the control group using morphine.
Since cytokines play a critical role in the inflammatory response and are produced by various cell types at the site of injury and by cells of the immune system during the inflammatory process, compounds that reduce the levels of pro-inflammatory cytokines may help to control inflammation. Thus, JR19 shows great promise in controlling inflammation by acting on key inflammatory pathways, making it a potential candidate for a new anti-inflammatory drug.