Cysteinyl Leukotriene Receptor Antagonism by Montelukast to Treat Visual Deficits

Introduction

Montelukast, an orally available drug approved by the Food and Drug Administration (FDA) to treat asthma and allergic rhinitis, is a critical anti-inflammatory agent that modulates leukotriene (LT) receptors. ¹ These receptors, particularly the cysteinyl leukotriene receptor (CysLTR1), have a significant role in various physiological processes, including bronchoconstriction, mucus secretion, plasma leakage, and vascular permeability. ² By antagonizing CysLTR1, montelukast mitigates these effects, relieving asthma and allergic rhinitis symptoms.

The type 1 CysLTR1, identified in 1999, serves as a primary receptor for LTs ³ Primarily synthesized by leukocytes during inflammatory responses, LTs are the major lipid mediators derived from arachidonic acid. LTs play a vital role in various biological processes, including chemotaxis, activation of leukocytes, and production of chemokines. Cysteinyl leukotrienes (CysLTs) are involved in immune and inflammatory responses and are generated from arachidonic acid through a series of enzymatic conversions. The leukotrienes LTC4, LTD4, and LTE4 are produced within cells by a single synthetic process. This process begins with the oxidation of arachidonic acid by an enzyme called 5-lipoxygenase (5-LO), which results in the leukotriene A4 (LTA4) formation, but as LTA4 is unstable, it is converted to LTC4 by an enzyme known as leukotriene C4 synthase (LTC4S). LTC4 is then exported from the cell, where it undergoes further conversions to generate LTD4 and, finally, LTE4, the most stable form of CysLT. ⁴

Cysteinyl leukotrienes mediate their biological actions by interactions with two major receptors, namely CysLT receptor 1 (CysLTR1) and CysLT receptor 2 (CysLTR2). Among these, CysLTR1 signaling is particularly important. It is a major target for pharmacological intervention in asthma and allergic rhinitis. ⁵ Understanding the synthesis and actions of CysLTs and their interactions with their receptors provides insight into the pathophysiology of inflammatory diseases. It informs the development of therapeutic strategies to modulate these pathways to alleviate symptoms and improve patient outcomes. ⁴ Activation of CysLTR1 by leukotriene D4 (LTD4) triggers a cascade of intracellular events, including G protein activation and the release of numerous second messengers such as calcium ions, inositol phosphates, and diacylglycerol. This cascade ultimately leads to protein kinase C (PKC) activation. ^3,5 PKC phosphorylates various target proteins, leading to diverse cellular responses, including changes in gene and protein expression related to inflammation. The activation of CysLTR1 can also trigger a signaling pathway that activates PI3K, which in turn stimulates Akt and Rho/Rac, leads to NF-kB activation, and contributes to the inflammatory response. ⁶ Additionally, Akt activation results in the accumulation of beta-catenin and the activation of proteins such as COX-2, cyclin D1, and c-Myc, which are essential for cell proliferation and migration. ⁷ Activation of the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (Erk) pathway with activation of CysLTR1 causes p-ERK to move from the cytosol to the nucleus, where it interacts with p90RSK, leading to the activation of transcription factors that regulate cell growth, migration, and survival. ^7,8 CysLTR1/ERK signaling can also increase COX-2 levels and subsequently elevate the expression of the antiapoptotic protein Bcl-2. ⁹ Recent research has also uncovered a previously under recognized role of the unfolded protein response (UPR) in this process, emphasizing the interaction between the LT system and UPR. ^10,11 These findings highlight the importance of targeting CysLTR to develop effective treatments and address disease pathology (Fig. 1).

Beyond its well-established role in respiratory conditions, recent studies suggest that the LT signaling pathway targeted by montelukast has broader implications in various other diseases like fibrosis, cardiovascular diseases, malignant tumors, cerebrovascular disease, and immune host defense. ¹² A literature review suggests several ocular disorders where LTs and their cognate receptors were implicated; however, the specific expression of CysLTRs and their cell signaling pathways in various ocular cell types is emerging. ¹² A predominant literature reports LTB4, a lipid mediator produced from arachidonic acid by an LTA4 hydrolase, as a significant player in allergic conjunctivitis, uveitis, age-related macular degeneration, and diabetic retinopathy. For a more comprehensive literature review on LTB4 leukotrienes and their receptor’s role in ophthalmical diseases, readers should refer to eminent reviews. ^13,14 While it is reported that CysLTR1 and CysLTR2 are expressed by several ocular tissues, including the cornea, conjunctiva, iris, lens, ciliary body, retina, and choroid, inhibition of such receptor regulation is linked to RGC survival, ¹⁵ protection against VEGF-induced vascular permeability, ¹⁶ reduction in microglial activation, ¹⁵ and increased autophagic activity in RPE, ¹⁷ suggesting a causal connection between LTs and CysLTRs in visual deficits.

This expanded understanding indicates that montelukast could potentially be repurposed to treat conditions where aberrant stress mechanisms play a significant role. In particular, the potential for repurposing montelukast for ocular conditions is intriguing, as many of these conditions share similar pathophysiological mechanisms involving inflammation and oxidative stress. Understanding the mechanisms by which montelukast’s therapeutic effects in various diseases will pave the way for exploring its efficacy in treating ocular conditions and other ailments beyond its current indications.

Inflammation

Inflammation is a coordinated response involving signaling pathways that regulate inflammatory mediators in tissue and recruit immune cells from the blood. ¹⁸ It is common in chronic diseases such as cardiovascular and bowel diseases, diabetes, arthritis, and cancer. ¹⁹ The inflammatory process generally involves the following sequence of events: 1) Harmful stimuli detection by cell surface receptors, 2) activation of inflammatory pathways, 3) release of inflammatory markers, and 4) recruitment of immune cells to the inflammation site. ²⁰

Ocular inflammatory diseases can be categorized into two main groups: those primarily fueled by inflammation and those where inflammation plays a significant role in pathological or degenerative processes. Conditions such as keratitis, conjunctivitis, and uveitis fall into the first category, where inflammation is the main culprit. In the second category, diseases such as diabetic retinopathy (DR), glaucoma, dry eye syndrome, and age-related macular degeneration (AMD) progress significantly due to inflammation. ²¹ These inflammatory diseases can impact various parts of the eye, including the retinal vessels, cornea, orbit, optic nerve, uvea, ocular adnexa, sclera, and conjunctiva. They present a range of diagnostic and therapeutic challenges, from minor, short-lived issues to serious, vision-threatening disorders. Inflammation in the eye can also occur postsurgery, potentially leading to complications such as macular cystoid edema. Treatment typically involves pharmacological therapy or minimally invasive procedures. ¹⁹

LT receptor antagonists have demonstrated the ability to block the proinflammatory effects of cysteinyl LTs. ²² In the kidneys, montelukast treatment has been observed to lower levels of transforming growth factor-β1 (TGF-β1) and tumor necrosis factor-α (TNF-α). ²³ The capacity of montelukast to diminish retinal inflammation and decrease superoxide accumulation in early DR supports its use as a therapy to preserve retinal health and prevent further damage to capillaries and neurons. These findings suggest promising prospects for novel treatment approaches to mitigate diabetes-related complications. ¹⁶ Montelukast has also notably reduced the diabetes-induced expression of the cytokine receptor TNFR1 and the adhesion molecule ICAM1 in the retina. This treatment suppresses inflammatory reactions and improves retinal function, leading to enhanced ERG measurements. ²⁴ Studies by Bapputty et al. indicate that montelukast appears to alleviate early inflammatory changes in leukocytes in diabetes and sustain endothelial cell viability by preserving tight junction proteins. ²⁵ In mouse retinal endothelial cells, montelukast treatment inhibited the NF-kB inflammatory cascade, reduced ICAM1 levels, with a dose-dependent decrease in the phosphorylation levels of NF-kB and IkBα (inhibitor of NF-kB alpha) at a standard dose of 2 µM. ²⁶ Furthermore, a decrease in inflammation and orbital congestion has been observed following combined therapy of oral montelukast and cetirizine in thyroid eye disease. ²⁷ Additionally, montelukast has been shown to diminish the activation of microglial cells in the retina and optic nerve ¹⁵ and alleviate vasculitic neuropathic pain resulting from ischemia-reperfusion (I/R), likely due to its anti-inflammatory and neuroprotective properties. ²⁸ These findings underscore the anti-inflammatory potential of montelukast, which could be explored further in treating inflammation-mediated ocular pathologies.

Oxidative Stress

In eukaryotic cells, oxidative metabolism predominantly occurs in the mitochondria and endoplasmic reticulum, where metabolic byproducts such as free radicals get produced during normal oxidation-reduction reactions. ²⁹ Reactive oxygen species (ROS) and reactive nitrogen species (RNS) ascribe free radicals with unpaired electrons combined with peroxides or other electrophilic compounds. ^29,30 ROS production is a physiological process countered by the antioxidant defense system before oxidative damage. Indeed the build-up of ROS results in oxidative damage, linked to the onset of multiple illnesses such as diabetes, cancer, heart disease, neurological conditions, and several ophthalmical ailments. ³¹

There are two main mechanisms by which oxidative stress results in a disease. The first mechanism is where oxidative stress mediated production of reactive species such as HOCl, ONOO−, •OH, and H₂O₂ cause direct damage to nucleic acids, structural proteins, membrane lipids, enzymes culminating in cell death, or abnormal cell functioning. The second mechanism involves aberrant redox signaling resulting in oxidative stress. ^32,33 In some diseases such as diabetes, both mechanisms of oxidative stress are observed where buildup of advanced glycation products as well as abnormal stress signal pathways cause compilcations. ³⁴

Oxidative stress-related pathologies can be divided into two categories based on their etiological role. The first category includes conditions where oxidative stress is the primary pathology, such as in atherosclerosis and toxicities caused by radiation and paraquat. The second category comprises diseases where oxidative stress is a secondary factor contributing to disease progression, such as hypertension, Alzheimer’s disease, and chronic obstructive pulmonary disease. ³² The visual system is especially susceptible to oxidative stress because it is constantly exposed to light and comprises tissues with high metabolic activity making it prone to damage. As a result, both primary and secondary factors contribute to oxidative stress in ocular pathologies. Examples of oxidative stress-driven ocular disorders include corneal diseases such as keratoconus, diabetic keratopathy, dry eye disease, pterygium, and conditions such as cataract and Fuchs endothelial corneal dystrophy. ³⁵

Given the involvement of oxidative stress in many diseases, there is a pressing need to develop effective therapies to mitigate its pathological effects. Montelukast has shown promise in several conditions, either as a standalone treatment or in combination with other therapies to reduce oxidative stress. For instance, montelukast has been identified as a potential treatment for osteoarthritis, where it alleviates oxidative stress by inhibiting CYSLTR1 and activating Kruppel Like Factor 2 (KLF2) in IL-1β-induced cells. ⁵ It also reduced oxidative stress markers such as TBARS (thiobarbituric acid reactive substances) and MPO (myeloperoxidase) activity in a rat model of ischemia—reperfusion injury. ²⁸ Additionally, montelukast mitigated oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) by enhancing antioxidant enzyme levels and reducing lipid peroxidation in an in vivo rat model. ³⁶ Furthermore, in cases of prednisolone-induced oxidative stress in rat livers, montelukast improved mitochondrial function and reduced lipid and protein damage. ³⁷ In the treatment of diabetic nephropathy, montelukast was compared with Losartan and demonstrated significant reductions in oxidative stress, as indicated by decreased malondialdehyde levels and increased levels of reduced glutathione and superoxide dismutase in renal and aortic tissues. ²³ The protective effects of montelukast against oxidative stress, marked by decreased malondialdehyde levels and increased glutathione levels, have also been reported in various other studies. ^{38 –40} Moreover, in a diabetic mouse model, montelukast curbed the rise in retinal and leukocyte-derived superoxide. ¹⁶ Due to its ability to mitigate oxidative stress across various pathological conditions, montelukast shows promise as a therapeutic agent in the prevention or treatment of oxidative stress-related ocular pathogenesis.

Endoplasmic Reticulum Stress

The endoplasmic reticulum (ER) functions as a quality-control organelle essential for maintaining protein homeostasis. ⁴¹ Various components, including chaperones, ATPases, glucose-regulated protein 94 (Grp94), BiP (a member of the Hsp70 family), and two proteolytic systems—the ubiquitin-proteasome and lysosome-autophagy systems—are crucial for preserving proteostasis. ⁴² Under normal physiological conditions, these enzymes and molecular chaperones assist in assembling newly synthesized proteins and prevent the misfolding and aggregation of existing proteins. Misfolded proteins are promptly removed by the ER-associated degradation (ERAD) pathway. ⁴³ Cells constantly monitor the levels of misfolded proteins in the ER lumen to maintain a balance between protein folding capacity and demand. ⁴⁴ When the accumulation of unfolded and misfolded proteins exceeds a critical threshold, it causes ER stress, triggering the UPR. The UPR is a regulatory mechanism aimed at restoring ER homeostasis and preventing further cellular damage. ⁴⁵ This response is initiated by three sets of ER transmembrane proteins: IRE1α (inositol-requiring enzyme 1α), PERK (protein kinase RNA-like ER kinase), and ATF6 (activating transcription factor 6). ^44,46 Activation of the UPR leads to the inhibition of protein synthesis, regulation of gene expression, ^44,47 and decisions about cell fate, including apoptosis, ⁴⁸ to meet the cell’s needs.

ER stress is implicated in numerous pathways and is crucial in many pathogenic processes. For example, ER stress and persistent UPR signaling have been observed in tissues affected by inflammatory diseases, neurodegeneration, diabetes, pulmonary fibrosis, stroke, cancer, viral infection, and heart disease. ⁴⁹ Additionall y, ER stress contributes to various ocular diseases, including DR, ^41,50 glaucoma, cataract, AMD, corneal diseases, achromatopsia, myopia, uveal melanoma, and uveitis. In ocular autoimmune disease, ER stress activation is linked to inflammation-driven proteolytic activity. Research indicates that the inflammatory cytokine TNFα induces ER stress on the ocular surface, triggering the expression of matrix metalloproteinase 9 (MMP9) via the c-Fos protein. ⁴³ Chronic exposure to harmful factors such as TNFα and high glucose induces ER stress, disrupting retinal cell metabolism and contributing to DR through pathways involving ATF6, PERK, and IRE1. ⁵¹ The accumulation of misfolded proteins due to impaired folding caused by external or internal factors appears to be a common feature across these disorders. ⁴⁹ For example, ER stress results in the translocation of glucose-regulated protein (GRP78) to the plasma membrane, leading to O-GlcNAcylation of junction proteins, increased transmigration of leukocytes, and increased endothelial permeability under these stress conditions, creating a perpetual motion of inflammation and ER stress. ^{52 –54}

Montelukast has shown potential in alleviating ER stress caused by various insults. In human hepatocytes, montelukast treatment was reported to inhibit PEM-induced ER stress by suppressing the eIF-2α/ATF4 signaling pathway, thereby preventing the expression of ER stress markers GADD34 and CHOP. ⁵⁵ Another study in a type 2 diabetic rat model found that montelukast, alone or combined with dapagliflozin (a diabetes drug), positively affected ER stress response elements and improved insulin sensitivity in skeletal muscle. This suggests that montelukast may be beneficial in reducing ER stress associated with diabetes. ⁵⁶ Given that the mechanisms contributing to ER stress-related pathologies are similar, montelukast could potentially be a therapeutic option for visual disorders as well.

Apoptosis

Apoptosis is largely defined as a genetically regulated process of programmed cell death and conventionally coincides with normal physiological processes such as embryogenesis, homeostasis, and aging. ⁵⁷ In contrast to the other major mechanism of cell death, necrosis, apoptosis is a process of cell shrinkage, nuclear fragmentation, and membrane blebbing to form apoptotic bodies that are rapidly engulfed by body phagocytes, ultimately preventing spillage of cellular contents into neighboring areas as well as the incitation of inflammatory responses. ^58,59 When cellular damage or other proapoptotic signals are sensed, a subset of cysteine proteases called caspases is activated and facilitates this process of cell dismantling. ⁵⁹

Although apoptosis occurs as a process of natural biological functioning, dysregulated or inappropriate apoptosis is a defining property of several pathological conditions. Some disorders arise due to an inability to prevent aberrant apoptosis, while others are identified by the ability to evade cell death altogether through alterations in the apoptotic signaling pathway. ⁵⁹ In the case of neurogenerative disorders, excessive apoptosis drives neuronal deterioration, which ultimately defines these conditions. ^{58 –60} Contrastingly, many cancer cells can elude apoptosis by several mechanisms, including increasing expression of antiapoptotic proteins such as Bcl-2 while simultaneously downregulating proapoptotic proteins such as Bax/Bak. ^{58 –60} These examples demonstrate that abnormal apoptosis is attributed to detrimental diseases, and regulating this process through various therapeutic strategies could mitigate these consequences.

Montelukast has previously been used to manage allergies and symptoms of asthma; however, several reports have detailed the potential of utilizing this therapeutic as a means of treating ailments characterized by abnormal apoptosis. Two studies describing montelukast’s potential to treat or prevent Alzheimer’s disease in rat models demonstrated that montelukast not only improved cognitive ability but also reversed proapoptotic changes seen in the treatment groups by reducing activated caspase-3 and increased Bcl-2 levels. ^61,62 Treatment with montelukast also seemingly prevents apoptosis following spinal cord injury by decreasing FAS ligand, another regulator of apoptosis, and Bax expression, further validating the potential benefit of montelukast in treating neurodegenerative disorders. ⁶³ Furthermore, montelukast shows potential in mitigating inflammation and resulting apoptosis associated with osteoarthritis, as demonstrated via in vitro chondrocyte models. ⁵

The dysregulation of apoptosis is thought to be a major constituent of several ophthalmical pathologies, including glaucoma, retinoblastoma, DR, retinitis pigmentosa, cataract formation, and several other ocular diseases. ⁶⁴ In DR, for example, the progression of the disease is largely defined by the activation of the apoptotic pathway, affecting several cell types. ⁶⁵ Studies have proven that in hyperglycemic conditions, caspase activity in capillary endothelial cells, as well as in rat retinal pericytes, increases, resulting in apoptosis and a reduction in vascular integrity. ⁶⁵ It has also been reported that increased expression of Bcl-2 promotes endothelial cell survival and stability in diabetic mouse eyes. ⁶⁵ Additionally, glaucoma is thought to be associated with the FAS signaling pathway, ultimately resulting in apoptosis of retinal ganglion cells. ⁶⁶ Because montelukast has been proven to reduce cell death through the mediation of apoptotic markers such as caspase, Bcl-2, and FAS ligand levels, it is reasonable to conclude that formulation of montelukast for ocular use might be beneficial in the treatment of ocular disorders characterized by apoptosis.

Mitophagy

Mitochondria are vitally important organelles responsible for energy production and cellular metabolism. When these organelles are damaged, they undergo a process termed mitophagy. Mitophagy is defined as the selective autophagy of injured, damaged, or unnecessary mitochondria and is largely responsible for the mechanism of mitochondrial quality control. ⁶⁷ If these regulatory mechanisms fail, mitochondria that cannot perform proper functions accumulate, ultimately leading to tissue and organ damage. ⁶⁷

Mitochondrial dysregulation can be detrimental to cellular functioning and is thought to be the hallmark of several diseases, including cancer, neurodegenerative, ageing, skeletal muscle disorders, cardiovascular disease, and metabolic disease. ⁶⁸ In diseases such as Alzheimer’s, Parkinson’s, diabetes, and others, defective mitochondrial performance, as well as reduced uptake of damaged mitochondria, have been identified. ⁶⁷ Mitochondrial dysfunction can generally be recognized as an increase in ROS formation, mtDNA damage, decreased synthesis of ATP, and inflammation. ⁶⁹ Although the mechanism by which this influences disease pathology is still largely unknown, it is hypothesized that an accumulation of injured mitochondria, as assessed by these factors, results from dysregulated mitophagy. ⁶⁷

Evidence suggests that montelukast is a prime therapeutic candidate in regulating dysfunctional mitophagy and mitochondrial dysfunction, as seen in several experimental models. In models of lung disease attributable to epithelial dysfunction, treatment with montelukast improved mitochondrial biogenesis markers such as mitochondrial mass, mtDNA, and cytochrome B expression. ⁷⁰ Interestingly, montelukast was also found to increase respiratory rate and ATP generation, suggesting a gain of function effect in treated human bronchiole epithelial cells. ⁷⁰ Montelukast was also found to ameliorate neurodegeneration and mitochondrial dysfunction in rat models exposed to quinolinic and malonic acid, as well as in kainic acid-exposed rats. ^71,72 When rats exposed to either quinolinic or malonic acid were treated with montelukast, mitochondrial function improved, as demonstrated through increased mitochondrial redox activity and activity of mitochondrial enzyme complexes. ⁷¹ In kainic acid models, treatment with montelukast showed restoration of oxidative stress, superoxide dismutase, and glutathione levels, implying potential antioxidant effects. ⁷² Furthermore, damage to mitochondrial enzyme complexes was reduced while activity was increased. ⁷² Montelukast has also been proven to promote hepatoprotection in prednisone-treated rats by similarly improving mitochondrial complex activity. ³⁷ These findings demonstrate montelukast’s potential to enhance and regulate mitochondrial functioning.

When assessing ocular diseases, it is evident that several major ophthalmical disorders are characterized by mitophagy dysfunction and mitochondrial malfunctioning. Fuchs Endothelial Corneal Dystrophy, for example, is characterized by damage to mtDNA and oxidative stress, leading to a decrease in ATP production. ⁷³ This decrease in ATP production, however, has been shown to be ameliorated with the addition of the antioxidant N-acetylcysteine. ⁷⁴ Additionally, in age-related macular degeneration, mitophagy, and mitochondrial redox dysregulation have also been documented, as well as alterations in mitochondrial structure, reduction in mitochondrial size, and increased levels of oxidative stress. ⁷³ Montelukast’s hypothesized antioxidant potential and ability to regulate mitochondrial mass, oxidative stress, and other properties of mitochondrial dysfunction suggest that this therapeutic might be beneficial in mitigating mitochondrial and mitophagy dysfunction seen in these disorders.

Endothelial Dysfunction

The endothelium comprises a single layer of endothelial cells and is the cellular lining of blood and lymphatic vessels. This inner cell layer is a major regulator in the passage of substances between the blood vessels and tissues and, therefore, is important in facilitating immunological and inflammatory responses. ⁷⁵ The endothelium plays an essential part in maintaining vascular tone through the production and release of vasoactive factors such as nitric oxide and angiotensin II. ⁷⁵ When an imbalance of these factors results in enhanced vasoconstriction, platelet aggregation, and inflammation of vessel walls, it is termed endothelial dysfunction. ^75,76 Endothelial dysfunction can lead to apoptosis of endothelial cells and disruption of vessel wall integrity, promoting vascular disease. ^53,76

As one of the largest bodily organs, dysfunctions in the endothelium can profoundly impact many body systems. ⁷⁷ In cardiovascular diseases, diminished nitric oxide production and impaired vasodilation of vessels can lead to hypertension, atherosclerosis, thrombosis, and angina, further exasperating this condition. ^77,78 Insulin resistance and hyperglycemia seen in diabetes can similarly cause endothelial dysfunction by reducing nitric oxide production, impairing barrier functions and regulators, causing accumulation of oxidized fatty acids, and increasing oxidative stress and inflammatory markers. ⁷⁹ Because the involvement and dysregulation of the endothelial barrier are so prevalent in the progression of many diseases, the endothelium is an attractive target for the treatment and prevention of such disorders.

Montelukast has been shown to mitigate endothelial dysfunction and could serve as a novel therapeutic in diseases characterized by this pathology. One study investigating the impact of montelukast on endothelial dysfunction in a streptozotocin-induced diabetic mouse model found that montelukast improved the viability and integrity of retinal endothelial cells by preserving levels of a tight junction membrane protein. ⁷⁷ Montelukast dose-dependently attenuated endothelial dysfunction in a renovascular hypertension-induced vascular dementia rat model. ⁸⁰ In another study, brain endothelial cells subjected to oxygen glucose-deprivation/reoxygenation (OGD/R) resulting in altered expression of inhibitors of matrix metalloproteinases (TIMPs) and TNFα could be alleviated with montelukast. ⁸¹ Montelukast significantly suppressed ox-LDL-induced monocyte adhesion to human umbilical vein endothelial cells via its inhibitory effects on adhesion molecule expression, including VCAM-1 and E-selectin. ⁸² These studies prove that montelukast provides endothelial protection that could potentially be beneficial in treating several diseases.

Endothelial dysfunction has been observed in many ophthalmical disorders, such as DR, Fuchs endothelial corneal dystrophy, glaucoma, and others. ^{83 –85} In the case of DR, increased inflammatory and advanced glycosylation end products result in a rise in ROS, decreased nitric oxide levels and enhanced permeability of retinal endothelium through the reduced expression of tight junction proteins. ⁸³ As mentioned previously, montelukast has proven to be effective in treating such pathologies and, therefore, might provide another avenue in treating DR, as well as other ocular disorders characterized by endothelial dysfunction. Future studies should investigate further the effect of montelukast on ocular endothelial dysfunction, as well as the potentiality of montelukast to treat DR.

Autophagy

Autophagy is a highly conserved homeostatic mechanism responsible for cellular degradation and recycling. ⁸⁶ Three main categories of autophagy exist, including microautophagy, macroautophagy, and chaperone-mediated autophagy, each of which results in lysosomal degradation of cellular contents and recycling of metabolites. ⁸⁶ The process of autophagy plays an essential role in cellular maintenance and survival, as toxins and waste products can be removed from the cell and degraded. ^86,87 Once broken down, the byproducts of substances such as damaged organelles or misfolded proteins can be used to construct new cellular constituents. ^86,87 Additionally, excessive autophagy can lead to cellular demise, further contributing to cell regulation and maintenance. ⁸⁶

Due to the essential role autophagy plays in cellular maintenance and survival, it is unsurprising that dysregulation of this process is characteristic of several human diseases. In pathologies such as cancer, the promotion of autophagic mechanisms can be beneficial to tumorigenesis, as cell survival is aided by the removal of damaged or detrimental substances. ⁸⁷ In other disorders, such as neurodegenerative diseases or cystic fibrosis, autophagy dysfunction has been identified. ⁸⁷ Diseases such as these are characterized by misfolded or nonfunctional proteins, and impairing autophagy can lead to an accumulation of these protein aggregates, further exasperating disease pathology. ⁸⁷ Because of the involvement of autophagy in human disease, targeting this process through therapeutic strategies might be a beneficial treatment plan for such disorders.

Recent studies investigating the therapeutic potential of montelukast as a means to regulate autophagy have documented the potentiality of this drug to improve autophagic biomarkers and promote the modulation of autophagic activity. One such study demonstrated that treatment with montelukast increased expression of Beclin-1, a known autophagy regulator, in dementia with Lewy bodies mouse model, suggesting the potential of montelukast to improve and restore autophagy in neurodegenerative disorders. ⁸⁸ Montelukast was also found to produce similar regulatory effects in diabetic nephropathy models, as determined by an increase in autophagy markers such as LC3-II. ⁸⁹ Interestingly, in a rat model of hemorrhagic cystitis, montelukast reversed autophagic dysfunction induced by cyclophosphamide treatment through an increased expression of Beclin-1 and LC3-II and a downregulation of the negative P13K/Akt/mTOR regulatory pathway. ⁹⁰ The same results were not produced by the standard treatment of 2-mercaptoethane sulfonate sodium (MESNA) treatment group, indicating that montelukast may have more beneficial effects than MESNA in treating cyclophosphamide-induced hemorrhagic cystitis. ⁹⁰ In contrast to the aforementioned revelations, montelukast has also been shown to mitigate overexpression of autophagy, as treatment with montelukast reduced autophagy markers such as Beclin-1, LC3, and CD4 in mice models of autoimmune hepatitis. ⁹¹ These reports suggest that montelukast treatment may have a substantial influence on the regulation of autophagy and could potentially serve as a reasonable therapeutic option for diseases characterized by autophagic dysfunction.

In addition to previously mentioned disorders, the dysregulation of autophagy is characteristic of several ocular pathologies. In the case of glaucoma, an increase in intraocular pressure is associated with increased autophagy markers such as Beclin-1 and LC3 early on in the disease progression. ⁹² Over time, however, chronic increases in intraocular pressure lead to a decrease in the expression of these markers and in autophagic response, resulting in the axonal degeneration of retinal ganglion cells, potentially contributing to disease progression. ⁹² Similarly, autophagic dysfunction is thought to instigate cataract formation as ROS and damaged cellular components can accumulate, decreasing lens transparency. ⁹² Contrastingly, in DR models, excessive autophagic activity caused by chronic exposure to oxidative stress escalated disease progression and pathology, as inappropriate autophagy led to pericyte necrosis and photoreceptor death. ⁹² Because of the strong association of autophagic dysregulation in ocular disease, therapies that target this process are attractive methods to treat many ophthalmical disorders. Montelukast, having documented the ability to restore autophagic markers, might potentially provide another avenue to treating ophthalmical diseases characterized by dysfunctional autophagy. The effects of montelukast in ocular models should be further investigated to determine montelukast’s capability to ameliorate such dyscrasias.

Clinical Trials and Adverse Effects

Various case reports and clinical trials have been conducted to explore the role of montelukast in treating inflammatory and neurodegenerative diseases (Table 1). LTs are known to cause inflammation and vasoconstriction, leading to endothelial dysfunction and vascular damage. Therefore, inhibiting these effects with montelukast could benefit vascular disorders and is currently under investigation in multiple studies. One study with 200 participants aims to assess the impact of montelukast on the reocclusion rates of lower limb arteries in patients with peripheral artery disease (NCT04277702). Another trial investigated the effects of montelukast on arterial vasculature in heart disease patients, hypothesizing that hospitalized patients with coronary artery disease might show endothelial dysfunction due to prolonged LT-induced vasoconstriction. This study planned to measure changes in brachial artery diameter and PAT responses to temporary forearm ischemia but was terminated due to recruitment challenges (NCT00351364). Air pollution, which increases pulmonary inflammation and oxidative stress, leading to atherosclerosis, is another area where montelukast’s potential is being explored (NCT04762472). Montelukast has shown effectiveness in reducing relapse rates of steroid-sensitive nephrotic syndrome in children. Additionally, two clinical trials are evaluating montelukast’s ability to decrease proteinuria and vascular dysfunction caused by Diabetic Kidney Disease, although one was terminated due to funding issues (NCT05362474, NCT05498116). In rheumatoid arthritis, a study found that montelukast, combined with antirheumatic drugs, enhanced the anti-rheumatic therapy’s effect and reduced disease activity by lowering serum VCAM-1 levels. ⁹³ Montelukast also proved to be effective in treating nephrotic syndrome and other renal disorders. ⁹⁴ Regarding ocular conditions, a case-control study reported a reduced risk of DR associated with prior oral montelukast use. ⁹⁵ Montelukast’s neuroprotective effects are being investigated for potential treatment of neurodegenerative diseases such as Parkinson’s and Alzheimer’s (NCT03991988, NCT06113640). While most trials use oral montelukast, a study with mild to moderate Alzheimer’s patients utilized a buccal film of the drug (NCT03402503). These studies suggest montelukast could improve outcomes in a variety of diseases. Although several trials examine montelukast in asthma and rhinitis (not discussed here), the studies described here highlight the montelukast’s potential for repurposing to treat other inflammation and neurodegeneration-driven disorders, including ocular conditions.

Overall, montelukast is considered to be a safe and easily tolerated drug. ⁹⁶ The most common adverse effects associated with montelukast use include gastrointestinal disturbances, ^{97 –99} headache, ^{97 –99} cough, ^98,99 pharyngitis, ^98,99 laryngitis, ^98,99 sinusitis, ^98,99 and viral infection. ^98,99 Neuropsychiatric events such as anxiety, depression, suicidal thoughts and behavior, aggression, sleep disturbances, hallucinations, and others have been reported with the use of montelukast ⁹⁹ ; however, the FDA acknowledges that a lack of information surrounding these events and an insufficient understanding of the mechanism montelukast may use to cause such events makes it difficult to ascertain risk. ¹⁰⁰ In addition to the more common adverse effects associated with montelukast, case reports of hepatitis, ^{101 –103} ecchymosis, ¹⁰⁴ pemphigus, ¹⁰⁵ urticaria, ^106,107 and angio-edema ¹⁰⁸ have been documented. Side effects such as otitis, conjunctivitis, pruritus, eczema, arthralgia, and myalgia are also associated with montelukast. ⁹⁹ Montelukast is not known to have any carcinogenic, mutagenic, or teratogenic effects; however, patients with a history of montelukast hypersensitivity were advised not to take this drug. ⁹⁹ Furthermore, patients with phenylketonuria must exercise caution when taking montelukast, as some formulations of the drug contain phenylalanine. ⁹⁹ Although montelukast is an effective means of treating asthma, bronchospasms, and potentially many other disorders, prescribers should be aware of the adverse effects associated with this drug to reduce such events and improve patient outcomes. No studies to date have shown any ocular adverse effects.

Conclusions and Future Directions

While Montelukast shows promise, further research is needed to fully understand its effectiveness in humans. Considering the neuropsychiatric events reported with its use, ¹⁰⁹ the FDA has warned against its use in mild-symptom subjects. ¹¹⁰ Whether such side effects are possible in ocular conditions needs to be tested. One advantage for ocular conditions is the availability of a drug that can be delivered locally, either in the form of an ophthalmical topical application or intravitreally, thus avoiding the complications of crossing the blood-brain barrier. Future studies should also aim to test a direct intravitreal injection or topical application to see if such complications could be avoided. Several structurally different CysLT1 antagonists other than montelukast are already available for clinical use (zafirlukast and pranlukast), which have been shown with varied rank order of potency for LTD₄ ¹¹¹ should be tested in ocular conditions. Collaborating with ophthalmologists, researchers, and clinicians will be essential to designing robust studies and validating CysLTR1 antagonism for its potential benefits in ocular diseases.

Authors’ Contributions

A.T.M.S. and C.E.O.: Conceived and designed the research, conducted experiments resulting in the data, analyzed and interpreted the data, drafted the article, and made critical revisions of the article. R.G.: Conceived and designed the research, analyzed and interpreted the data, obtained the funding, supervised research personnel, drafted the article, and made critical revisions of the article.

Data Availability Statement

The data supporting this study’s findings were from literature. The original data supporting this study’s findings are available on request from the corresponding author.

Author Disclosure Statement

None of the other authors declare any financial conflicts.

Funding Information

Department of Defense, W81XWH-22–1-0697 (R.G.); National Eye Institute, EY034716 (R.G.), and unrestricted funds from Research to Prevent Blindness (Hamilton Eye Institute).