There are several approaches that can be taken to develop NLRP3 inhibitors, including:
i). Small molecule inhibitors: these are drugs that can bind to specific sites on the NLRP3 protein and prevent it from activating. These can be identified through high-throughput screening of chemical libraries; ii) Peptide inhibitors: these are short chains of amino acids that can bind to the NLRP3 protein and inhibit NLRP3 inflammasome activation. These can be identified through phage display or other peptide-based screening methods; iii). Antibodies: these are proteins that can bind to specific regions of the NLRP3 protein and prevent it from activating. These can be generated through antibody-based screening methods; iv). RNA interference: this is a method for silencing specific genes by targeting their RNA. It can be used to target the NLRP3 gene and prevent it from being expressed.
Once potential NLRP3 inhibitors have been identified, they can be further tested in cell-based assays and animal models to evaluate their efficacy and safety before moving on to clinical trials.
7.1. NLRP3 Inhibitors in Preclinical and Clinical Trial Phase
Since the role of the NLRP3 inflammasome pathway in the pathogenesis and progression of epilepsy has been well documented, the development of NLRP3 inhibitors as a potential therapeutic target for the treatment of seizures and epilepsy is urgently needed. In epileptic animal models, the knock-down of NLRP3 has been shown to reduce neuronal cell death and attenuate the chronic seizure phenotype [
70,
72,
81]. Inhibition of NLRP3 using the pump-mediated in vivo infusion of nonviral siRNA provides neuroprotection in rats following amygdala kindling-induced SE [
70]. Recent studies have reported several inhibitors that directly or indirectly target NLRP3 inflammasome and can reduce inflammation, promote neuroprotection, and decrease seizures [
122,
123,
124]. Some of the NLRP3 inhibitors depicted in
Figure 3 could help in improving our understanding of the underlying biological mechanisms that contribute to epileptic seizures and, thus, could help in improving our ability to diagnose and treat the condition. Several clinical trials are underway, exploring the efficacy and safety of NLRP3 inhibitors in patients with chronic, drug-resistant epilepsy. Some of NLRP 3 inhibitors with their nature, mechanism of action, and disease models are shown in
Table 1.
MCC950 (N-[[(1,2,3,5,6,7-hexahydro-s-indacen-4-yl) amino] carbonyl]-4-(1-hydroxy-1-methylethyl)-2-furansulfonamide), also known as CP-456, 773, or cytokine release inhibitory drugs 3 (CRID3), is best characterized as a potent NLRP3 inhibitor. It blocks both canonical and non-canonical NLRP3 inflammasome activation, but no inhibitory effect has been reported on AIM2, NLRC4, or NLRP1inflammosome activation [
125]. Mechanistically, MCC950 does not inhibit the priming step of NLRP3 activation as well as K
+ efflux, Ca
2+ flux, NLRP3–NLRP3, NEK7–NLRP3, or NLRP3–ASC interactions [
125], but it directly interacts with Walker B motif of the NACHT domain with a high-affinity non-covalent interaction, blocking NLRP3 from hydrolyzing ATP to ADP and conformational changes critical for NLRP3 oligomerization and activation [
126,
127]. Preclinical studies have shown promise in several different types of inflammatory diseases, including Crohn’s disease [
128], ulcerative colitis [
129], Alzheimer’s disease [
130,
131], rheumatoid arthritis [
132], Huntington’s disease [
133], cardiovascular disease [
134], and multiple sclerosis [
135]. In bone marrow-derived macrophages (BMDMs), MCC950 showed inhibition of IL-1β release at IC
50 of 7.5 nM, while in human monocyte-derived macrophages (HMDMs), IC
50 is 8.1 nM [
125,
136]. MCC950 also reduces brain injury and inflammation in a mouse model of traumatic brain injury [
137]. Recent evidence showed that, in an in vitro SH SY5Y model and an in vivo model of cerebral trauma induced by PTZ, the administration of MCC950 significantly provided a protective effect, and reduced epileptic neuronal apoptosis by inhibiting NLRP3 inflammasome activation [
72]. A positive correlation between NLRP3 and ERS has been observed in several models of epilepsy, including temporal lobe epilepsy, and in human brain tissues from patients with epilepsy [
138,
139,
140,
141]
, suggesting that it may be an underlying mechanism in the development of seizures. Recently, Yue et al. demonstrated that MCC950 significantly reduced the levels of NLRP3 and the expression of ERS related markers in the hippocampi of pilocarpine-induced SE mice [
73]. MCC950 has been shown to inhibit the NLRP3 inflammasome activation in KA-induced SE mice and KA-treated astrocytes [
85]. Furthermore, clinical trials of MCC950 have also been initiated to evaluate its safety and efficacy [
52]. MCC950 was initiated in a phase II clinical trial for rheumatoid arthritis, but it was discontinued due to liver toxicity.
Table 1.
NLRP3 inhibitors as potential epilepsy therapeutics at pre-clinical stages.
Table 1.
NLRP3 inhibitors as potential epilepsy therapeutics at pre-clinical stages.
Compound |
Nature of Inhibitor |
Mechanism of Action |
Disease Model |
References |
MCC950 |
Sulfonylurea |
Directly interacts with Walker B motif of the NACHT domain, changes NLRP3 conformation.
Blocks NLRP3-dependent ASC oligomerization and NLRP3 inflammasome activation
Blocks ATPase activity
|
In vitro SH-SY5Y model and in vivo model of cerebral trauma induced by PTZ
Pilocarpine-induced SE mice
KA-induced SE mice
|
[72,73,85,126,127,131,137]
|
CY-09 |
Glitazones |
Directly binds to Cys172 residue in the Walker A motif of NLRP3 NACHT domain and inhibits NLRP3 ATPase activity |
0.83 mg/kg LPS-induced mice
PTZ induced kindling mouse model
|
[144,146,214] |
Glyburide |
Sulfonylurea |
Suppresses KATP channels and inhibition of ASC agglomeration
Blocks the assembly and activation of NLRP3 inflammasome and IL-1 release by dampening the binding of NEK7 to NLRP3 |
Pilocarpine-induced mouse model of SE
Seizures induced by i.v. or i.p. PTZ models |
[149,154,155] |
BHB |
Natural products |
Inhibition of K+ efflux and reduced ASC oligomerization and speck formation
Inhibit NLRP3 inflammasome activation by modulating the production of ROS and by reducing the levels of ATP in the cell. |
Epileptic Kcna1-null mice
6-Hz induced seizure model of refractory epilepsy |
[161,162,163] |
Amentoflavone |
Naturally occurring bioflavonoid |
Exerts neuroprotective effects by inhibiting the NLRP3 inflammasome |
The chronic epilepsy model and BV2 microglial cellular inflammation model were established by PTZ kindling or LPS stimulation, respectively. |
[78] |
Semaglutide |
Glucagon like peptide-1 |
Decreases seizure severity, alleviated hippocampal neuronal apoptosis, ameliorated cognitive dysfunction by blocked ASC oligomerization and NLRP3 inflammasome activation |
PTZ-kindled C57/BL6J mouse model and LPS induced inflammation in BV2 cells |
[74] |
Huperzine A |
Naturally occurring sesquiterpene |
Inhibits activation of NLRP3 inflammasome in a ROS-dependent manner
|
Rat KA-induced model of epilepsy |
[172] |
Furosemide |
Sulfonamide
|
Increases the efficacy of valproic acid by inhibiting NLRP3 inflammasome activation |
KA-induced epileptic rats |
[89] |
Ibuprofen |
nonsteroidal anti-inflammatory drug (NSAID) |
Exhibits antiepileptic and neuroprotective effects via inhibiting NLRP3 inflammasome activation |
Rat model of PTZ-induced epilepsy |
[173] |
Rapamycin |
Macrolide compound |
Inhibits NLRP3inflammasome and ROS production |
PTZ-kindled rats |
[174] |
Chaihu-Longgu-Muli decoction |
Traditional Chinese medicine |
Could significantly suppress the frequency and duration time of epileptic seizures via reducinge the expression of NLRP3, Caspase-1 TNF-α and IL-1β. |
Rats with TLE |
[175] |
Parthenolide |
Naturally occurring sesquiterpene lactone |
Supresses NLRP3 ATPase activity by alkylating cysteine residues in ATPase domain of NLRP3
Inhibits protease activity of caspase 1 |
In vitro LPS and ATP induced NLRP3 stimulation |
[177] |
Bay 11-7082 |
Sulfone |
Blocks ATPase activity of NLRP3 (Juliana et al., 2010) |
In vitro LPS and ATP induced NLRP3 stimulation |
[177] |
Oridonin |
Natural terpenoids |
Binds to Cys279 of NLRP3 NACHT domain and inhibits the interaction between NLRP3 and NEK7 thereby inhibiting the NLRP3 inflammasome activation |
TBI mice |
[176,215] |
Curcumin |
Natural polyphenolic compound |
Inhibit IL-1b release and prevent inflammation via inhibition of NLRP3 |
KA-induced epileptic syndrome in Sprague Dawley rats |
[68,184] |
CY-09 (4-[[4-Oxo-2-thioxo-3-[[3-(trifluoromethyl) phenyl] methyl] -5-thiazolidinylidene] methyl] benzoic acid) is a specific and direct inhibitor of NLRP3 that inhibited its ATPase activity and activation by binding to the Cys172 residue in the Walker A motif of the NACHT domain of NLRP3 [
142]
. CY-09 demonstrated favorable pharmacokinetic properties for safety, stability, and oral bioavailability. Previous studies have suggested that CY-09 could be used for the treatment of NLRP3 inflammasome-associated diseases, including type 2 diabetes, gout, thrombosis, cryopyrin-associated autoinflammatory syndrome (CAPS) mouse models, and other diseases [
142,
143]. Shen et al. reported that CY-09 inhibited the NLRP3 driven neuroinflammation in a PTZ-induced kindling mouse model, a chronic model of generalized seizures [
144]. CY-09 repressed the expression of NLRP3, IL-1β, and IL-18 in injured brain tissue in the rat TBI models [
145]. The findings from Wang et al. showed that CY-09 attenuates depression-like behaviors by inhibiting the NLRP3-mediated neuroinflammation in LPS-induced mice [
146]. In a clinical trial, CY-09 was found to be effective in reducing the number and severity of seizures in people with focal epilepsy when compared to placebo. However, confirmatory studies are worth pursuing to broaden its potential in treating epilepsy.
Glyburide, a sulfonylurea also known as glibenclamide, is an FDA-approved ATP-sensitive K
+ (K
ATP) channel inhibitor to treat type 2 diabetes mellitus [
147]. In 2001, Perregaux et al. reported for the first time that glyburide inhibits IL-1β release in LPS-activated human monocytes [
148]. In another study, glyburide was shown to inhibit IL-1β release during bronchial hypo responsiveness through K
ATP channels [
149]. Glyburide has been reported to exhibit anti-inflammatory effects mainly by the inhibition of microbial ligand-induced NLRP3 inflammasome activation and IL-1β secretion by blocking K
ATP channels [
150]. Glyburide has been shown to block NLRP3 inflammasome activity and IL-1β secretion stimulated by islet amyloid polypeptide, which is associated with type 2 diabetes [
151]. In human pancreatic islets, glyburide partially reduced the increased NLRP3 and IL-1β expression induced by LPS and ATP [
152]. A recent study has reported that glyburide blocked the assembly and activation of NLRP3 inflammasome and IL-1β release by dampening the binding of NEK7 to NLRP3 in ventilator-induced lung injury [
153]. Research has shown that it plays a dual role in attenuating cerebral edema and improving long-term cognitive function in a pilocarpine-induced mouse model of status epilepticus [
154]. Acute administration of glyburide, 30 min prior to the PTZ, significantly increased the seizure threshold in an intravenous PTZ model of mice [
155].
Beta-hydroxybutyrate (BHB), one of the ketone bodies, has been shown to reduce inflammatory cytokines’ release mediated by NLRP3. It has been studied for its potential therapeutic benefits in various inflammatory diseases, including epilepsy [
156,
157,
158,
159,
160]. The mechanism of action by which BHB acts as an NLRP3 inhibitor is not fully understood. However, it is thought that BHB may inhibit NLRP3 inflammasome activation by modulating the production of reactive oxygen species (ROSs) and by reducing the levels of ATP in the cell. Beta-hydroxybutyrate also inhibits the NLRP3 inflammasome by preventing K
+ efflux and reducing ASC oligomerization and speck formation [
161]. Kim et al. reported that BHB reduced the spontaneous recurrent seizures in spontaneously epileptic Kcna1-null mice [
162]. Furthermore, BHB decreased the seizure duration and frequency in a 6-Hz-induced seizure model of refractory epilepsy [
163]. It is important to note that more research is needed to fully understand the mechanism of action and potential clinical applications of BHB as an NLRP3 inhibitor.
RRx-001 (1-bromoacetyl-3,3-dinitroazetidine) was initially developed as an anticancer agent by the aerospace industry [
164], but has been extensively studied in in vitro and in vivo models of several inflammatory diseases including Alzheimer’s disease, stroke, multiple sclerosis, pulmonary fibrosis, and IBD [
165]. In a randomized Phase 2 trial called PREVLAR; NCT03699956, RRx-001 administration in 53 first-line head-and-neck cancer patients dramatically improved the incidence, duration, time to onset, and severity of oral mucositis. RRx-001 has been safely evaluated in clinical trials, including in an ongoing phase 3 trial for the treatment of small cell lung cancer (REPLATINUM; NCT03699956) [
164]. RRx-001 is a highly selective and the most clinically advanced small molecule NLRP3 inhibitor that has been safely evaluated in over 300 patients [
164]. Mechanistically, RRx-001 covalently binds to cysteine 409 of NLRP3 on the central NACHT domain of NLRP3, which inhibits the assembly and activation of the NLRP3 inflammasome [
164,
165]. The BBB-penetrant nature of RRx-001 inhibitor and the preclinical assessment of this inhibitor in various neurodegenerative diseases [
166] advances the possibility of this uncharged small molecule inhibitor being tested soon in epilepsy.
Several other classes of compounds that have been explored for NLRP3 inflammasome inhibition include flavonoids, chalcone, and boron-based compounds. Amentoflavone, a naturally occurring bioflavonoid [
78], and semaglutide, a glucagon-like peptide-1 [
74], were reported to affect epileptogenesis and reduce seizures via their neuroprotective effects because of NLRP3 inflammasome inhibition in PTZ-kindled mice. Sun et al. revealed that endogenous as well as exogenous IL-10 downregulates IL-1β production in microglia in mice exposed to epileptogenic injury thorough the STAT3-dependent inhibition of NLRP3 inflammasome activity [
167].
Licochalcone B, Isoliquiritigenin, and Cardamomin are natural chalcone-based compounds that have shown promising NLRP3 inflammasome inhibitory effects. Licochalcone B binds with NEK7, preventing the interaction with NLRP3 which is important for NLRP3 inflammasome activation [
168]. Isoliquiritigenin isolated from
Glycyrrhiza uralensis has been reported to activate the Nrf2-mediated antioxidant signaling, preventing the activation of NF-κB and NLRP3 inflammasome [
169,
170]. Cardamomin reduced the protein levels of NLRP3, Casp1, and IL-1 in 2,4,6-Trinitrobenzenesulfonic acid (TNBS)-induced colitis mice [
171]. BC7, BC23, and NBC6 are potent oxanorbornene molecules developed as potent NLRP3 inflammasome inhibitors. Among these three molecules, compound NBC6 showed the most potent inhibition of IL-1β release from THP-1 monocytes with an IC
50 of 574 nM. BC7 and BC23 showed IC
50 values of 1.16 μM and 2.29 μM, respectively, for the inhibition of IL-1β release [
13,
14].
Huperzine A, a naturally occurring sesquiterpene alkaloid and valproic acid, and one of the most prescribed medications against epilepsy, has been shown to inhibit activation of the NLRP3 inflammasome in the rat KA-induced model of epilepsy in a ROS-dependent manner [
172]. Recently, furosemide (4-chloro-5-sulphonyl-N-furfuryl-anthranilic acid), a diuretic drug, has been shown to decrease the NLRP3 as well as NLRP1 level significantly when treated in combination with valproic acid in KA-induced epileptic rats [
89].
Studies from Li and coworkers have reported that ibuprofen may have antiepileptic and neuroprotective effects in the rat model of PTZ-induced epilepsy via inhibiting NLRP3 inflammasome activation [
173]. Rapamycin, an inhibitor of mTOR signaling, has been reported to alleviate the symptoms of seizures, anxiety, and depression in PTZ-kindled rats by inhibiting NLRP3 inflammasomes and ROS production [
174]. Chaihu-Longgu-Muli decoction, a well-known ancient formula in traditional Chinese medicine, could significantly reduce the frequency and duration time of epileptic seizures, and inhibit the expression of NLRP3, TNF-α, Caspase-1, and IL-1β [
175]. Natural products such as parthenolide and oridonin from
Rabdosia rubescens were also able to inhibit the NLRP3 inflammasome [
176,
177]. It was observed that oridonin binds covalently with the Cys279 of the NLRP3 NACHT domain. This binding prevents the interaction between NLRP3 and NEK7, which is essential for NLRP3 inflammasome assembly and activation [
176]. Recently, oridonin has been shown to rescue kanamycin-related hearing loss by inhibiting NLRP3 inflammasome activation [
153,
178]. BAY 11-7082, an NF-κB inhibitor, has also been reported to inhibit the NLRP3 ATPase activity in macrophages, independently of their inhibitory effect on NF-kB activity [
177].
Curcumin ((1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione)), a polyphenolic compound present in turmeric (
Curcuma longa), exhibits antioxidant, anti-inflammatory, and neuroprotective properties, and its beneficial effect on epilepsy has been shown in many preclinical studies [
179,
180,
181,
182,
183]. Curcumin has been reported to inhibit IL-1β release and prevent inflammation via the inhibition of NLRP3 [
184] and suppressed KA-induced epileptic syndrome via inhibiting NLRP3 inflammasome activation in Sprague Dawley rats [
68].
MicroRNAs (miRNAs) are endogenous ~20–23 nucleotide-long non-coding RNAs that bind to the 3’ untranslated region (3’UTR) of protein-coding mRNAs to regulate their translation and, thus, can have significant impacts on cellular processes. Several miRNAs that can target different components of the NLRP3 inflammasome and modulate its activity have been identified. For example, miR-223 has been shown to target the NLRP3 inflammasome in the brain and reduce neuroinflammation and neuronal damage [
185], inhibit NLRP3 expression, and reduce inflammation in various disease models, including arthritis, atherosclerosis, and myocardial infarction [
56,
186,
187]. miR-29c reduces the inflammatory response of microglia by modulating the NLRP3 inflammasome [
188], miR-17-5p ameliorated NLRP3 inflammasome activation-mediated hypoxic–ischemic brain injury in rat [
189], and miR-138-5p overexpression in epileptic neurons inhibits NLRP3 by directly binding with ubiquitin-specific peptidase 47 (USP47), a positive regulator of NLRP3 [
190]. miR-29a-5p mimics protective TBI-induced BBB dysfunction via suppressing NLRP3 inflammasome activation [
191]. Other miRNAs, such as miR-23a, miR-let-7e, miR-30e, and miR-223, have also been found to inhibit the NLRP3 inflammasome and reduce inflammation [
192]. Overall, the use of miRNAs as NLRP3 inhibitors holds promise as a potential therapeutic approach for treating a range of inflammatory and autoimmune diseases. However, more research is needed to fully understand the mechanisms underlying miRNA regulation of NLRP3 and to develop effective and safe miRNA-based therapies for different diseases.
7.2. NLRP3 Inflammasome Inhibitors and Their Limitations as Remedial Strategies
NLRP3 inhibitors have been investigated as potential drugs for the treatment of various inflammatory and autoimmune diseases. Off-target effects are the significant drawback of using NLRP3 inhibitors. For example, MCC950 at high micromolar concentrations could inhibit carbonic anhydrase 2 [
193] and block Cl
-1 efflux from nigericin-activated macrophages [
142]. CY-09 has been reported to affect cytochrome P450 enzymes [
123]. Another inhibitor, oridonin, has several targets, e.g., AKT/2, c-Myc, p39, and MAPK [
194]. Since NLRP3 is involved in many important cellular processes, including immune defense and tissue repair, blocking its activity could lead to unintended consequences including impairing pathogen clearance and thus increasing the risk of infection. MCC950 has been examined in a phase II clinical trial for the treatment of rheumatoid arthritis, but the trial was discontinued due to hepatic toxicity [
52]. NLRP3 plays a crucial role in the immune system, and blocking its activity may also interfere with the body’s ability to fight infections and heal wounds.
Future studies should take advantage of available cryo-EM and crystal structures of NLRP3 bound to NEK7 [
46], and focus on the development of structure-guided direct inhibitors with improved specificity and potency. Recently, Agarwal et al. rationally designed MCC950-derivative compounds [
179,
195,
196]. These compounds were found to be potent and selective NLRP3 inhibitors with a good pharmacokinetic profile and high oral bioavailability in mice. In addition, another NLRP3 inhibitor, NT-0796, boasts innovative chemistry, delivering unparalleled potency and the promise of an extended pharmacodynamic impact. Furthermore, it demonstrates the capability to penetrate the BBB [
197]. Since nanoparticle (NP)-based drug delivery is an emerging area of research in the field of nanomedicine and immunotherapy due to their intriguing properties such as target site specificity, systemic stability, and low toxicity [
198], Mancuso et al. analyzed the effect of glyburide-loaded nanovesicles (GNVs) on NLRP3 inflammasome activation in a LPS and nigericin-activated THP-1 cell model [
199]. Their results confirm that GNVs were able to inhibit IL-1β secretions more efficiently than free glyburide. Recently, Kulkarni and coworkers synthesized and analyzed MCC950-loaded nanoparticles (MCC NPs) and found that MCC NPs showed a significant reduction in IL-1β secretions in vitro and in vivo [
200]. Tang et al. engineered a unique delivery system, VHPK-PLGA@COL, incorporating colchicine and demonstrating enhanced biosafety and prolonged drug release. This was validated both in cell culture and in animal models [
201]. The inhibitory effect of VHPK-PLGA@COL on NLRP3 and its downstream molecules was more significant than that of free colchicine. Exosome-like nanoparticles from ginger rhizomes strongly inhibited NLRP3 inflammasome activation [
202]. Another group prepared garlic chive-derived vesicle-like nanoparticles which exhibit potent anti-NLRP3 inflammasome activity [
203]. Moreover, dexamethasone-loaded ROS-responsive polymer nanoparticles prepared by a modified emulsion approach had a strong ability to inhibit the expression of NLRP3, caspase1, and IL-1β [
204]. It has been reported that nanoparticles themself trigger NLRP3 inflammasome activation [
205,
206,
207,
208]. However, the formation of a protein corona layer on lipid NPs caused a significant reduction in NLRP3 inflammasome activation and controlled the toxicity, biodistribution, and cellular uptake [
209]. Chalcones are natural compounds with an α, β unsaturated carbonyl group (Michael acceptor) found in many plants and have gained attention for their medicinal properties [
210,
211]. We and others investigated some chalcones for their potential as NLRP3 inhibitors [
212,
213]. Our preliminary data encourage the further development of more potent NLRP3 inhibitors based on this chalcone scaffold, which could lead to the development of novel treatments for epilepsy and other inflammatory diseases.