Topical TRPM8 Agonist for Relieving Neuropathic Ocular Pain in Patients with Dry Eye: A Pilot Study

Efficacy and safety of transient receptor potential channel modulators for dry eye: A systematic review and meta-analysis

PURPOSE

To investigate the efficacy and safety of transient receptor potential (TRP) channel modulators for dry eye.

METHODS

A thorough search for randomized clinical trials was conducted in seven databases up to February 16, 2024. Suitable studies were identified according to inclusion and exclusion criteria, extracted data were synthesized and analyzed using Review Manager 5.4, and risk of bias and quality of evidence were assessed using recommended tools.

RESULTS

Six trials with 884 patients using two kinds of TRP channel modulators (TRPV1 antagonist and TRPM8 agonist) were included. TRP channel modulators were effective in reducing subjective symptom scores (standardized mean differences [SMD], -0.41; 95 % confidence interval [CI], -0.75 to -0.07), corneal fluorescence staining (mean differences [MD], -0.19; 95 % CI, -0.37 to -0.02), impact on quality of life and daily function, and increasing the non-anesthetic Schirmer's test score (MD, 11.71; 95 % CI, 9.59 to 13.83), instant tear meniscus height (MD, 245.52; 95 % CI, 14.02 to 477.02), improvement in conjunctival hyperemia (risk ratio [RR], 2.08; 95 % CI, 1.07 to 4.06). There was no increased incidence of adverse events with TRP modulators compared to vehicle solution controls.

CONCLUSIONS

TRP channel modulators have shown positive therapeutic effects in reducing the symptoms and signs of dry eye and improving the quality of life associated with the disease without raising the risk of adverse events. These modulators are expected to become a new therapy for dry eye.

CORONIS symposium 2023: Scientific and clinical frontiers in ocular surface innervation

The 5th CORONIS Symposium, held during the 2023 Congress of the European Association for Vision and Eye Research (EVER), highlighted the growing importance of ocular surface innervation in eye surface disorders. This article summarises the insights and perspectives shared during the symposium, which focused on the clinical relevance of ocular surface innervation, as well as on the development of innovative diagnostic and therapeutic approaches for ocular surface pathologies linked to disturbed sensory innervation. Through robust interdisciplinary collaborations, these developments hold great potential to improve patient outcomes and quality of life.

Lysophosphatidic acid receptor 3 (LPAR3) regulates ocular surface chloride transport via calcium signaling

Dry eye is a multifactorial disease associated with impaired tear film homeostasis, damaging the ocular surface epithelium. Lysophosphatidic acid receptors (LPARs) are G-protein coupled receptors involved in Ca2+ and cAMP signaling via PLC and adenylyl cyclase activation. LPAR activation is involved in cell proliferation and wound healing in human corneal epithelial cells (HCECs) and in neuropathic pain. This study investigates the expression and functions of LPARs in ocular surface epithelial cells. Functional measurements of ocular surface potential difference (OSPD) were done in mice with topically applied, selective LPAR modulators. LPAR3 immunostaining was performed in mouse and human cornea and conjunctiva, and mouse lacrimal gland. LPAR-induced Ca2+ signaling was studied in primary and immortalized HCECs. The general LPAR agonist, linoleoyl LPA, and the LPAR3 selective agonist, 2S-OMPT, stimulated ocular surface Cl- secretion via Ca2+-activated Cl- channels (CaCCs). LPAR3 was expressed in the corneal and conjunctival epithelia of mice and humans, as well as in mouse lacrimal gland. Activation of LPAR and LPAR3 in HCECs transiently elevated intracellular Ca2+ through the Gq/PLC signaling pathway. LPAR3 agonists may potentially have therapeutic efficacy in ocular surface diseases, including dry eye disease.

Corneal Sensory Receptors and Pharmacological Therapies to Modulate Ocular Pain

Nociceptors respond to noxious stimuli and transmit pain signals to the central nervous system. In the cornea, the nociceptors located in the most external layer provide a myriad of sensation modalities. Damage to these corneal nerve fibers can induce neuropathic pain. In response, corneal nerves become sensitized to previously non-noxious stimuli. Assessing corneal pain origin is a complex ophthalmic challenge due to variations in its causes and manifestations. Current FDA-approved therapies for corneal nociceptive pain, such as acetaminophen and NSAIDs, provide only broad-acting relief with unwanted side effects, highlighting the need for precision medicine for corneal nociceptive pain. A few targeted treatments, including perfluorohexyloctane (F6H8) eye drops and Optive Plus (TRPV1 antagonist), are FDA-approved, while others are in preclinical development. Treatments that target signaling pathways related to neurotrophic factors, such as nerve growth factors and ion channels, such as the transient receptor potential (TRP) family or tropomyosin receptor kinase A, may provide a potential combinatory therapeutic approach. This review describes the roles of nociceptors in corneal pain. In addition, it evaluates molecules within nociceptor signaling pathways for their potential to serve as targets for efficient therapeutic strategies for corneal nociceptive pain aimed at modulating neurotrophic factors and nociceptive channel sensitivity.

Refrigeration reduces instillation discomfort of a 0.09% cyclosporine A solution

SIGNIFICANCE

Topical cyclosporine A (CsA) for the treatment of dry eye disease is often associated with instillation discomfort, which may negatively influence patient adherence to therapy. This study found that refrigerating topical CsA reduced instillation discomfort compared with instillation of warm CsA. Thus, refrigerating CsA prior to instillation may improve patient experience when using CsA to manage dry eye disease.

PURPOSE

This study aimed to quantify instillation discomfort associated with cold or warm instillation of a 0.09% CsA.

METHODS

Forty participants with symptomatic aqueous deficient dry eye were enrolled. A drop of cold (4°C) CsA was instilled in one eye, and a drop of warm (23°C) CsA was instilled in the other eye. The order and eye receiving the cold drop were randomized. Participants rated the discomfort of each eye (0, no discomfort; 10, maximal discomfort) prior to drop instillation, immediately post-instillation, and at each subsequent minute for 10 minutes. Area under the curve was used to quantify cumulative discomfort.

RESULTS

Forty participants (39.6 ± 18.9 years old, 82% female) completed the study. A majority of participants (n = 24, 60%) experienced reduced cumulative discomfort with cold CsA, whereas the remainder experienced minimal difference (n = 10, 25%) or increased cumulative discomfort (n = 6, 15%). For those with reduced discomfort (n = 24), cumulative discomfort associated with cold instillation (median, 11.5 [2.2, 20.0]) was significantly lower (p<0.01) than cumulative discomfort associated with warm instillation (median, 17.5 [11.2, 32.2]). Cold instillation was associated with a median reduction of 1 discomfort point immediately post-instillation and at all subsequent time points (all p≤0.04, but not significant at t = 10), compared with warm instillation.

CONCLUSIONS

Up to 60% of participants found that cold instillation of CsA solution induced less discomfort than warm instillation, lasting up to 9 minutes post-instillation. In contrast, although 15% of participants found reduced discomfort with warm instillation, the magnitude of discomfort associated with warm instillation was not significantly different than cold instillation.

Thermosensitive TRP Channels Are Functionally Expressed and Influence the Lipogenesis in Human Meibomian Gland Cells

While the involvement of thermosensitive transient receptor potential channels (TRPs) in dry eye disease (DED) has been known for years, their expression in the meibomian gland (MG) has never been investigated. This study aims to show their expression and involvement in the lipogenesis of the MG, providing a possible new drug target in the treatment of DED. Our RT-PCR, Western blot and immunofluorescence analysis showed the expression of TRPV1, TRPV3, TRPV4 and TRPM8 in the MG at the gene and the protein level. RT-PCR also showed gene expression of TRPV2 but not TRPA1. Calcium imaging and planar patch-clamping performed on an immortalized human meibomian gland epithelial cell line (hMGECs) demonstrated increasing whole-cell currents after the application of capsaicin (TRPV1) or icilin (TRPM8). Decreasing whole-cell currents could be registered after the application of AMG9810 (TRPV1) or AMTB (TRPM8). Oil red O staining on hMGECs showed an increase in lipid expression after TRPV1 activation and a decrease after TRPM8 activation. We conclude that thermo-TRPs are expressed at the gene and the protein level in MGs. Moreover, TRPV1 and TRPM8's functional expression and their contribution to their lipid expression could be demonstrated. Therefore, TRPs are potential drug targets and their clinical relevance in the therapy of meibomian gland dysfunction requires further investigation.

A Comprehensive Review on the Regulatory Action of TRP Channels: A Potential Therapeutic Target for Nociceptive Pain

The transient receptor potential (TRP) superfamily of ion channels in humans comprises voltage-gated, non-selective cation channels expressed both in excitable as well as non-excitable cells. Four TRP channel subunits associate to create functional homo- or heterotetramers that allow the influx of calcium, sodium, and/or potassium. These channels are highly abundant in the brain and kidney and are important mediators of diverse biological functions including thermosensation, vascular tone, flow sensing in the kidney and irritant stimuli sensing. Inherited or acquired dysfunction of TRP channels influences cellular functions and signaling pathways resulting in multifaceted disorders affecting skeletal, renal, cardiovascular, and nervous systems. Studies have demonstrated the involvement of these channels in the generation and transduction of pain. Based on the multifaceted role orchestrated by these TRP channels, modulation of the activity of these channels presents an important strategy to influence cellular function by regulating intracellular calcium levels as well as membrane excitability. Therefore, there has been a remarkable pharmaceutical inclination toward TRP channels as therapeutic interventions. Several candidate drugs influencing the activity of these channels are already in the clinical trials pipeline. The present review encompasses the current understanding of TRP channels and TRP modulators in pain and pain management.

Loss of TRPM8 Exacerbate Herpes Simplex Keratitis Infection in Mice by Promoting the Infiltration of CD11b+ Ly6G+ Cells and Increasing the Viral Load in the Cornea

Purpose

To reveal the role of transient receptor potential cation subfamily M member 8 (TRPM8) channels in herpes simplex keratitis (HSK).

Methods

HSK models were established using TRPM8 knockout (TRPM8-/-) mice and their wild-type (WT) littermates. The infected corneas were graded and harvested to evaluate the mRNA levels of inflammatory factors through quantitative real-time polymerase chain reaction (RT-PCR), as well as the infiltration of inflammatory cells through immunofluorescence staining and flow cytometry. Viral titers were determined by plaque assay and absolute quantitative method. RNA-sequencing was conducted to elucidate the transcriptome of corneal epithelium in response to TRPM8 knockout after infection. The anti-inflammatory effect of TRPM8 agonist menthol was documented via subconjunctival administration.

Results

Compared to their wild-type counterparts, TRPM8-deficient mice exhibited exacerbated infection symptoms and thicker corneas in HSK models. Infection in TRPM8-deficient mice resulted in significant lymphocyte infiltration, primarily consisting of Ly6G+ CD11b+ cells. Additionally, TRPM8-deficient mice displayed increased levels of corneal viral titers after infection, along with decreased expression of interferon-stimulated genes (ISGs). Subconjunctival administration of menthol effectively alleviated infection-induced symptoms and Ly6G+ CD11b+ cell infiltration in herpes simplex virus type 1 (HSV-1)-treated mice.

Conclusions

TRPM8 promoted host resistance to HSV-1 infection by suppressing the accumulation of Ly6G+ CD11b+ cells and virus replication. These findings suggest that targeting TRPM8 could be valuable for therapeutic interventions against HSV-1 infections.

The Molecular Mechanisms Responsible for Tear Hyperosmolarity-Induced Pathological Changes in the Eyes of Dry Eye Disease Patients

Dry eye disease (DED) is a multifactorial disorder of the lacrimal system and ocular surface, characterized by a deficiency in the quality and/or quantity of the tear fluid. The multifactorial nature of DED encompasses a number of interconnected underlying pathologies, including loss of homeostasis, instability and hyperosmolarity of the tears, and the induction and propagation of detrimental inflammatory responses in the eyes, which finally results in the development of neurosensory dysfunction and visual disruption. Dryness, grittiness, scratchiness, discomfort, inflammation, burning, watering, ocular fatigue, pain, and decreased functional visual acuity are common symptoms of DED. Eye dysfunction drastically attenuates patients' quality of life. Accordingly, a better understanding of the pathogenic processes that regulate the development and progression of DED is crucially important for the establishment of new and more effective DED-related treatment approaches, which would significantly improve the quality of life of DED patients. Since the process of osmoregulation, which guards the ocular surface epithelia and maintains normal vision, is affected when the osmolarity of the tears is greater than that of the epithelial cells, tear hyperosmolarity (THO) is considered an initial, important step in the development, progression, and aggravation of DED. In order to delineate the role of THO in the pathogenesis of DED, in this review article, we summarize current knowledge related to the molecular mechanisms responsible for the development of THO-induced pathological changes in the eyes of DED patients, and we briefly discuss the therapeutic potential of hypo-osmotic eye drops in DED treatment.

Novel Treatments for Chronic Ocular Surface Pain

ABSTRACT

Several etiologies can contribute to ocular surface pain including nociceptive, peripheral neuropathic, and central neuropathic mechanisms. Clinical clues can help identify contributors to ocular surface pain in a patient. In individuals whose pain persists despite targeting nociceptive contributors, neuropathic mechanisms should be considered and addressed using oral, topical, and/or adjuvant agents.

Ocular Surface Ion Transport and Dry Eye Disease

Purpose of Review

To review the role of ocular surface epithelial (corneal and conjunctival) ion transporters in the pathogenesis and treatment of dry eye disease (DED).

Recent Findings

Currently, anti-inflammatory agents are the mainstay of DED treatment, though there are several agents in development that target ion transport proteins on the ocular surface, acting by pro-secretory or anti-absorptive mechanisms to increase the tear fluid Film volume. Activation or inhibition of selected ion transporters can alter tear fluid osmolality, driving water transport onto the ocular surface via osmosis. Several ion transporters have been proposed as potential therapeutic targets for DED, including the cystic fibrosis transmembrane conductance regulator (CFTR), calcium-activated chloride channels (CaCCs), and the epithelial sodium channel (ENaC).

Summary

Ocular surface epithelial cell ion transporters are promising targets for pro-secretory and anti-absorptive therapies of DED.

A randomized, vehicle-controlled, Phase 2b study of two concentrations of the TRPM8 receptor agonist AR-15512 in the treatment of dry eye disease (COMET-1)

PURPOSE

Dry eye disease (DED) symptoms can negatively impact quality of life (QoL). AR-15512, a transient receptor potential melastatin 8 (TRPM8) agonist, was evaluated as a potential therapy for DED.

METHODS

In a Phase 2b study, patients with DED were randomized 1:1:1 to 0.0014% AR-15512, 0.003% AR-15512, or vehicle twice daily for 12 weeks. Eligibility criteria included DED signs and symptoms of prespecified severity levels. Outcomes assessed were DED signs (Schirmer score ± anesthetic, ocular surface staining, hyperemia), symptoms (Ocular Discomfort [ODS-VAS], Symptoms Assessment iN Dry Eye [SANDE], Eye Dryness-VAS, Ocular Pain-VAS), QoL-VAS, and adverse events. Co-primary endpoints were changes from baseline in ODS-VAS and anesthetized Schirmer score at Day 28.

RESULTS

0.003% AR-15512 (n = 122) was associated with early and sustained improvements in unanesthetized Schirmer score (Days 1 and 14, p < 0.0001), as well as improvements in ocular surface staining (Days 14 and 84, p ≤ 0.0365) and hyperemia (Day 84, p < 0.0215). Statistically significant improvements in symptoms were observed for the 0.003% concentration on SANDE (Days 14, 28, and 84, p ≤ 0.0254), ODS-VAS (Day 84, p = 0.0281), Eye Dryness-VAS (Day 84, p = 0.0302), and multiple QoL measures (Days 14, 28, and 84, p < 0.05). There were no significant differences between active and vehicle groups for the co-primary endpoints. The most common adverse events were burning and stinging upon instillation.

CONCLUSIONS

Although predefined co-primary study endpoints were not met, AR-15512 demonstrated statistically significant improvements in DED signs, symptoms, and disease-related QoL.

Role of ocular surface neurobiology in neuronal-mediated inflammation in dry eye disease

Inflammation is the consequence of dry eye disease regardless of its etiology. Several injurious or harmless processes to the ocular surface neurons promote ocular surface neurogenic inflammation, leading to the vicious cycle of dry eye disease. These processes include the regular release of neuromediators during the conduction of ocular surface sensations, hyperosmolarity-induced ocular surface neuronal damage, neuro-regenerative activities, and neuronal-mediated dendritic cell activities. Neurogenic inflammation appears to be the main culprit, instigating the self-perpetuating inflammation observed in patients with dry eye disease.

Transient Receptor Potential Channels: Important Players in Ocular Pain and Dry Eye Disease

Dry eye disease (DED) is a multifactorial disorder in which the eyes respond to minor stimuli with abnormal sensations, such as dryness, blurring, foreign body sensation, discomfort, irritation, and pain. Corneal pain, as one of DED’s main symptoms, has gained recognition due to its increasing prevalence, morbidity, and the resulting social burden. The cornea is the most innervated tissue in the body, and the maintenance of corneal integrity relies on a rich density of nociceptors, such as polymodal nociceptor neurons, cold thermoreceptor neurons, and mechano-nociceptor neurons. Their sensory responses to different stimulating forces are linked to the specific expression of transient receptor potential (TRP) channels. TRP channels are a group of unique ion channels that play important roles as cellular sensors for various stimuli. These channels are nonselective cation channels with variable Ca²⁺ selectivity. TRP homologs are a superfamily of 28 different members that are subdivided into 7 different subfamilies based on differences in sequence homology. Many of these subtypes are expressed in the eye on both neuronal and non-neuronal cells, where they affect various stress-induced regulatory responses essential for normal vision maintenance. This article reviews the current knowledge about the expression, function, and regulation of TRPs in ocular surface tissues. We also describe their implication in DED and ocular pain. These findings contribute to evidence suggesting that drug-targeting TRP channels may be of therapeutic benefit in the clinical setting of ocular pain.

Neuropathic ocular surface pain: Emerging drug targets and therapeutic implications

INTRODUCTION

Dysfunction at various levels of the somatosensory system can lead to ocular surface pain with a neuropathic component. Compared to nociceptive pain (due to noxious stimuli at the ocular surface), neuropathic pain tends to be chronic and refractory to therapies, making it an important source of morbidity in the population. An understanding of the options available for neuropathic ocular surface pain, including new and emerging therapies, is thus an important topic.

AREAS COVERED

This review will examine studies focusing on ocular surface pain, emphasizing those examining patients with a neuropathic component. Attention will be placed toward recent (after 2017) studies that have examined new and emerging therapies for neuropathic ocular surface pain.

EXPERT OPINION

Several therapies have been studied thus far, and continued research is needed to identify which individuals would benefit from specific therapies. Gaps in our understanding exist, especially with availability of in-clinic diagnostics for neuropathic pain. A focus on improving diagnostic capabilities and researching gene-modulating therapies could help us to provide more specific mechanism-based therapies for patients. In the meantime, continuing to uncover new modalities and examining which are likely to work depending on pain phenotype remains an important short-term goal.

Ocular Surface Ion-Channels Are Closely Related to Dry Eye: Key Research Focus on Innovative Drugs for Dry Eye

Abundant ion-channels, including various perceptual receptors, chloride channels, purinergic receptor channels, and water channels that exist on the ocular surface, play an important role in the pathogenesis of dry eye. Channel-targeting activators or inhibitor compounds, which have shown positive effects in in vivo and in vitro experiments, have become the focus of the dry eye drug research and development, and individual compounds have been applied in clinical experimental treatment. This review summarized various types of ion-channels on the ocular surface related to dry eye, their basic functions, and spatial distribution, and discussed basic and clinical research results of various channel receptor regulatory compounds. Therefore, further elucidating the relationship between ion-channels and dry eye will warrant research of dry eye targeted drug therapy.

Mucosal immunology of the ocular surface

The eye is a sensory organ exposed to the environment and protected by a mucosal tissue barrier. While it shares a number of features with other mucosal tissues, the ocular mucosal system, composed of the conjunctiva, Meibomian glands, and lacrimal glands, is specialized to address the unique needs of (a) lubrication and (b) host defense of the ocular surface. Not surprisingly, most challenges, physical and immunological, to the homeostasis of the eye fall into those two categories. Dry eye, a dysfunction of the lacrimal glands and/or Meibomian glands, which can both cause, or arise from, sensory defects, including those caused by corneal herpes virus infection, serve as examples of these perturbations and will be discussed ahead. To preserve vision, dense neuronal and immune networks sense various stimuli and orchestrate responses, which must be tightly controlled to provide protection, while simultaneously minimizing collateral damage. All this happens against the backdrop of, and can be modified by, the microorganisms that colonize the ocular mucosa long term, or that are simply transient passengers introduced from the environment. This review will attempt to synthesize the existing knowledge and develop trends in the study of the unique mucosal and immune elements of the ocular surface.

TRPM8 Channels: Advances in Structural Studies and Pharmacological Modulation

The transient receptor potential melastatin subtype 8 (TRPM8) is a cold sensor in humans, activated by low temperatures (>10, <28 °C), but also a polymodal ion channel, stimulated by voltage, pressure, cooling compounds (menthol, icilin), and hyperosmolarity. An increased number of experimental results indicate the implication of TRPM8 channels in cold thermal transduction and pain detection, transmission, and maintenance in different tissues and organs. These channels also have a repercussion on different kinds of life-threatening tumors and other pathologies, which include urinary and respiratory tract dysfunctions, dry eye disease, and obesity. This compendium firstly covers newly described papers on the expression of TRPM8 channels and their correlation with pathological states. An overview on the structural knowledge, after cryo-electron microscopy success in solving different TRPM8 structures, as well as some insights obtained from mutagenesis studies, will follow. Most recently described families of TRPM8 modulators are also covered, along with a section of molecules that have reached clinical trials. To finalize, authors provide an outline of the potential prospects in the TRPM8 field.

Experimental Pharmacotherapy for Dry Eye Disease: A Review

Dry eye disease (DED) is a complex multifactorial disease showing heterogenous symptoms, including dryness, photophobia, ocular discomfort, irritation and burning but also pain. These symptoms can affect visual function leading to restrictions in daily life activities and reduction in work productivity with a consequently high impact on quality of life. Several pathological mechanisms contribute to the disease: evaporative water loss leads to impairment and loss of tear homeostasis inducing either directly or indirectly to inflammation, in a self-perpetuating vicious cycle. Dysregulated ocular immune responses result in ocular surface damage, which further contributes to DED pathogenesis. Currently, DED treatment is based on a flexible stepwise approach to identify the most beneficial intervention. Although most of the available treatments may control to a certain extent some signs and symptoms of DED, they show significant limitations and do not completely address the needs of patients suffering from DED. This review provides an overview of the emerging experimental therapies for DED. Several promising therapeutic strategies are under development with the aim of dampening inflammation and restoring the homeostasis of the ocular surface microenvironment. Results from early phase clinical trials, testing the effects of EnaC blockers, TRPM8 agonist or mesenchymal stem cells in DED patients, are especially awaited to demonstrate their therapeutic value for the treatment of DED. Moreover, the most advanced experimental strategies in the pipeline for DED, tivanisiran, IL-1R antagonist EBI-005 and SkQ1, are being tested in Phase III clinical trials, still ongoing. Nevertheless, although promising results, further studies are still needed to confirm efficacy and safety of the new emerging therapies for DED.