Ocular Redness – II: Progress in Development of Therapeutics for the Management of Conjunctival Hyperemia

Abstract

Conjunctival hyperemia is one of the most common causes for visits to primary care physicians, optometrists, ophthalmologists, and emergency rooms. Despite its high incidence, the treatment options for patients with conjunctival hyperemia are restricted to over-the-counter drugs that provide symptomatic relief due to short duration of action, tachyphylaxis and rebound redness. As our understanding of the immunopathological pathways causing conjunctival hyperemia expands, newer therapeutic targets are being discovered. These insights have also contributed to the development of animal models for mimicking the pathogenic changes in microvasculature causing hyperemia. Furthermore, this progress has catalyzed the development of novel therapeutics that provide efficacious, long-term relief from conjunctival hyperemia with minimal adverse effects.

1. INTRODUCTION

Conjunctival hyperemia is the pathological vasodilatory response of the microvasculature in conjunctival tissue, which has the highest intensity at the fornices and fades toward the limbus [1,2]. It is caused by a wide range of etiologies with distinct immunopathological mechanisms and is the most common non-refractive ocular complaint requiring medical care.[3] According to one study, ocular complaints are the reason for 2-3% of patient visits to primary care physicians and emergency facilities, of which the majority are for treatment of conjunctival hyperemia. This incidence is expected to rise in the coming years due to changing environment and evolving human behavior. [4-6] Due to its ubiquity, ocular hyperemia not only impacts patients with direct costs in the form of consultations and pharmaceutical therapies but also indirectly due to its impact on social and work routines.

Currently, ophthalmic decongestants are the most commonly prescribed drugs for the treatment of conjunctival hyperemia. In the United States, the over-the-counter (OTC) eye-care market represents approximately $500-$700 million of sales annually. Redness relief products comprise 37% of unit sales, whereas redness and allergy relief therapies represent close to 60%.[4] However, these eye drops offer no long-term benefits due to their short duration of action, tachyphylaxis, and rebound redness. [5] Adverse effects from continuous use of ocular decongestants present a challenge in treating patients with chronic or recurrent disease, rendering their use ineffective for long-term management. The lack of treatment options for efficacious long-term management of conjunctival hyperemia highlights the necessity for a more thorough understanding of relevant pathological mechanisms underpinning this clinical presentation and the need to develop therapeutic options with fewer adverse effects.

Herein, we evaluate the available animal models that replicate the human disease and are essential tools for translating the research from bench to clinics. We evaluate the currently available therapeutic modalities for efficacious treatment of conjunctival hyperemia. Lastly, we provide insights into the development of novel therapies to manage conjunctival hyperemia and targets that may be of interest.

2. ANIMAL MODELS FOR CONJUNCTIVAL HYPEREMIA

As our understanding of complex immunological mechanisms that cause these changes at the ocular surface evolves, these processes are modeled in laboratory animals to further deepen our understanding of the pathophysiology of conjunctival hyperemia and develop novel therapies.

2.1. Allergic conjunctivitis models

Multiple animal models have been developed as the immunological mechanisms underpinning allergen-triggered conjunctivitis have been made clear. Allergic conjunctivitis is further categorized into seasonal and perennial allergic conjunctivitis, vernal and atopic keratoconjunctivitis, and giant papillary conjunctivitis depending on the underlying pathophysiological and immunological mechanisms. Despite this diagnostic granularity, the current peer-reviewed animal models are exclusively based on the sensitization against allergens such as ovalbumin ragweed pollen or major cat allergens and consecutive challenge. [6]

3.1.1. IgE mediated animal models

In 1971, Dwyer and Darougar were the first to study type 1 hypersensitivity in guinea pigs by intradermal injection of rabbit serum into the conjunctival sac causing localized edema and hyperemia due to vasodilation and permeability mediated by IgE. [7] Since then, a majority of guinea-pig studies have been performed using ovalbumin as the antigen. Two other allergens, ragweed (to mimic human hay fever conjunctivitis) and Japanese cedar pollen, have also been used as sensitizing allergens in guinea-pig models of allergic conjunctivitis [8-10]. Kato et al. used guinea-pig models to establish the role of platelet-activating factor and PAF-acetyl hydrolase in regulating conjunctival vascular permeability in allergic conjunctivitis [11]. Helleboid and colleagues established an association between the degranulation of mast cells to release histamine and increased localized production of the vasodilatory prostaglandin I₂. [12]. The ovalbumin-induced guinea-pig models also established the role of neuromediators such as substance P as a mediator of allergic conjunctivitis, causing conjunctival vasodilation and hyperpermeability through NK-1R receptors on blood vessels. [13] Sohn et al. demonstrated the role of uteroglobin and lipocortin-1, endogenously proteins produced in response to steroidal anti-inflammatory drugs that inhibit phospholipase A₂, thereby inhibiting the inflammatory changes associated with allergic conjunctivitis triggered by ragweed exposure in guinea pigs [14]. To study the kinetics of conjunctival hyperemia, Fukushima et al. established an allergic conjunctivitis model by directly applying histamine (20 μL of 1000 μg/mL in PBS) in guinea-pigs. [15] The authors reported a significant increase in conjunctival hyperemia one minute following application and a progressive increase up to 5 minutes post-histamine challenge, which then gradually reduced before reaching a post-induction baseline that was significantly higher than before the histamine challenge. The ovalbumin-based guinea-pig model has helped establish the therapeutic efficacy of a wide range of drugs under development or currently being prescribed for the management of allergic conjunctivitis, such as mast cell stabilizers, chlorpheniramine, ketotifen, leukotriene D4 receptor antagonists, 5-lipoxygenase inhibitors, salbutamol, theophylline, olopatadine, and levocabastine. [9,16-22].

The rat model of ovalbumin-based allergic conjunctivitis was developed using either direct or indirect sensitization. Brown-Norway rats have been used to study the role of IFN-γ in the pathogenesis of allergic conjunctivitis and were used to demonstrate the suppressive effect of IFN-γ during the induction phase of the disease.[23] Rat models of allergic conjunctivitis have also been established using ragweed pollen to generate an immune response. [24,25] These models have been used to test the efficacy of mast cell stabilizers such as nedocromil and sodium cromoglycate, cimetidine, levocabastine, and pemirolast.[26-28] Moreover, the Brown-Norway and Lewis rats with allergic conjunctivitis have been used to study the effect of FK506, an immunosuppressive compound, which suppresses the infiltration of both lymphocytes and eosinophils in the conjunctiva after active and passive immunization. [29,30]. These rat models have also been used to demonstrate the immunopathogenesis of late-phase reaction in allergic conjunctivitis mediated by T-cell and eosinophilic infiltration [31,32].

Yoshida and colleagues demonstrated that the genetic disparity in Brown-Norway and Lewis rats generated a variable immunopathogenic response. Conjunctivitis induced in Lewis rats on the application of ovalbumin in aluminum hydroxide did not generate IgE, and the pathogenesis was attributed to the cellular immune response. In contrast, Brown–Norway rats generated IgE following ovalbumin treatment. Additionally, IL-4 was only detected in Brown-Norway rats. The group also reported the development of milder allergic conjunctivitis in Fischer rats in comparison to Lewis rats. [33]

Merayo-Lloves et al. were the first to use SWR/J mice to model allergic conjunctivitis using ragweed by topical contact with the conjunctiva exhibiting specific anti-ragweed IgE and clinical signs specific to the disease.[34] Magone et al. established an SWR/J using short ragweed pollen in aluminum hydroxide. [35] Allergic conjunctivitis was induced after ten days of immunization, with one topical application of ragweed pollen to the eye, with characteristic chemosis, conjunctival hyperemia, and lid edema. They found dense conjunctival infiltration with polymorphonuclear leukocytes, macrophages, and CD4+ T lymphocytes, along with high levels of ragweed-specific IgG1 and IgE serum levels. In this study, the role of DNA immunostimulatory sequences (CpG motifs) in modulating allergic responses was also elucidated.[36,37] The authors reported that systemic and mucosal administration of immunostimulatory sequence oligonucleotide post-sensitization suppressed IgE-mediated hypersensitivity as well as late-phase cellular infiltration and instead only induced a ragweed-specific Th1 response. Miyazaki et al. used a murine model to show that IgE-mediated hypersensitivity response only occurred on cognate signals via eotaxin-1 to activate mast cells. [38] The authors reported an absence of hypersensitivity reactions in conjunctiva of mice deficient in eotaxin-1, despite normal levels of IgE and a mast cell population that did not undergo degranulation. Ozaki et al. reported the deleterious role of highly expressed T-cell specific suppressor of cytokine signaling (SOCS3) in disease pathogenesis [39]. Reduction in the expression levels of SOCS3 in heterozygous mice or transgenic mice reduced the severity of clinical features of the disease, including conjunctival hyperemia. The direct effects of CCR-2 were evaluated by Tominaga et al. in SWR/J mice with allergic conjunctivitis post-sensitization to ragweed pollen. [40] They observed that blocking CCR2 significantly suppressed the mast cell degranulation and clinical signs of the disease without altering the allergen-specific IgE or the release of Th2 cytokine from the isolated draining lymph node cells. CCL7 induces chemotaxis in mast cells in allergic conjunctivitis and is upregulated in conjunctival tissue during an ovalbumin-induced allergic response.[41] The authors reported a significantly lower conjunctival mast cell population in the conjunctiva of CCL7-deficient mice, thereby demonstrating the role of CCL-7 in ocular anaphylaxis, mast cell recruitment, and maximal FcεRI-mediated mast cell activation. Established murine models of allergic conjunctivitis have been used to study the therapeutic efficacy of novel drugs such as olopatadine, naphazoline, curcumin, levocetirizine, heparin, and thymoquinone.[42-46]

2.1.2. Non-IgE-mediated models

Guinea pigs have also been used to establish non-IgE-mediated models of allergic conjunctivitis, primarily using haptens for sensitization. Saiga and colleagues established a model by sensitizing conjunctivae of guinea pigs with IgG1 antisera to dinitrophenol carrier and subsequently topically challenged the ocular surface with haptens. [47] The authors observed biphasic conjunctival edema and hyperemia, and inflammatory tear cytology, due to an early-phase reaction at 30 minutes and a delayed late-phase response reaction after five to seven hours. This biphasic response U a in guinea-pig allergic conjunctivitis models by Leonardi and colleagues in a series of experiments. [48,49] Tiligada et al. used mast cell degranulating polymer-compound 48/80 for the induction of allergic conjunctivitis. [50]

As with established guinea-pig models, haptens such as 2,4-dinitrophenyl group (DNP) have been used for induction of conjunctivitis in rats through immunization with DNP-Ascaris and topical challenges with varying amounts of di-DNP-lysine. [51] The animals showed extensive conjunctival edema and hyperemia, and the histological evaluation showed increased neutrophil, eosinophil, lymphocyte, and atypical epithelial cell levels. This model was subsequently used to study alterations in conjunctival response by repeated hapten exposure and late-phase reaction. [52,53] Allansmith et al. reported that 250 μg of mast cell degranulating polymer-compound 48/80 induced severe conjunctival edema and hyperemia, replicating the ocular anaphylaxis seen in human patients. [54]

2.2. Non-allergic conjunctivitis animal models

2.2.1. Bimatoprost instillation models

Bimatoprost and latanoprost are ethyl amide derivative non-selective prostaglandin F (F.P.) receptor agonist - 17-phenyl-trinor PGF_2α. [55] The clinical trials conducted to test the efficacy of these drugs have reported ocular hyperemia to be the most common adverse event. [56,57] Vasorelaxation of the conjunctival microvasculature is caused by the stimulation of endothelial muscarinic receptors and mediated by endothelium-derived NO. [58] In endothelial cells, an increase in intracellular calcium activates endothelial nitric oxide synthase (eNOS) and leads to the generation of nitric oxide, a primary regulator of ocular blood flow. The authors observed significant ocular surface hyperemia in beagle dogs dosed topically with bimatoprost 0.1% and latanoprost 0.005%. The hyperemia was reversed on treatment with a nitric oxide synthase inhibitor L–NAME over six hours. The authors further reported that despite blocking NO synthesis, the hyperemia was not restored to baseline. This finding suggests the involvement of calcitonin gene-related peptide, substance P, or another tachykinin released from perivascular sensory nerves. Data from an in-vivo study in guinea pigs suggest that the application of bimatoprost causes significant vasodilation of conjunctival vessels for an average duration of more than six hours. [59]

2.2.2. Adrenomedullin

Adrenomedullin is a vasorelaxant peptide consisting of fifty-two amino acid residues and was first isolated from extracts of human pheochromocytoma. [60] The peptide is structurally homologous to CGRP and generates a comparable potent vasodilator effect to this peptide. [61]. Clementi and colleagues studied the vasodilatory effects of adrenomedullin in rabbits. [62] Topical administration of adrenomedullin to the rabbit eye at different doses (1.25, 2.5, and 5 μg/kg) caused acute and severe conjunctival hyperemia in a dose-dependent fashion. It also yielded an increase in inflammatory cells within the conjunctival tissue, elevated levels of prostaglandin E2 concentration in the aqueous humor, and increased uveal vascular response and myeloperoxidase activity. These changes were inhibited by pretreating the ocular surface with L-NAME, thereby showing the process to be NO-mediated or with the adrenomedullin receptor antagonist, adrenomedullin-(22-52) fragment.

2.2.3. Dapiprazole hydrochloride

The majority of established animal models discussed previously rely on inflammatory ocular surface disease, primarily allergic conjunctivitis, to study the associative changes in the vasculature. However, the underlying ocular surface disorders are not present in a proportion of patients who present with conjunctival hyperemia. Therefore, we established a novel, single drug-based animal model to investigate the localized vascular changes without any underlying inflammatory pathology.

Dapiprazole is a topical adrenergic antagonist that has vasodilating action and causes pupillary miosis. [63] Peterson and colleagues used 0.5% dapiprazole to induce vasodilation in the conjunctival microvasculature in patients. [64]

We have utilized the vasodilatory properties of topically administered dapiprazole to establish a rabbit model of conjunctival hyperemia. (Unpublished data) We applied topically four different concentration of dapiprazole (i.e., 0.5%, 1%, 2%, and 5%) on the ocular surface of the rabbits, and slit-lamp images were taken every thirty seconds for the first two minutes, every four minutes between two to eight minutes and every ten minutes till one-hour post-induction. Our data analysis using the ocular redness index showed a significant increase in conjunctival hyperemia for forty minutes post-induction in rabbits treated with 5% dapiprazole. (Figure 1)

Figure 1:

An animal (rabbit) model established in our laboratory using dapiprazole chloride. The animal model shows a significant increase in the ocular redness index score after forty minutes of induction on treatment with 5% dapiprazole.

3. CURRENT THERAPEUTIC MODALITIES FOR CONJUNCTIVAL HYPEREMIA AND UNDERLYING ETIOLOGIES: WHERE ARE WE AND WHERE ARE WE HEADING?

Successful management of conjunctival hyperemia, especially the chronic type, relies on understanding underlying mechanisms associated with the relevant etiology. As discussed in a previous section, given the vast array of different pathophysiological processes that may trigger conjunctival hyperemia, various treatment modalities are available (Table 1). The therapeutic strategies deployed for the management of conjunctival hyperemia must be tailored according to the underlying etiology deduced from the patient’s history of presentation and the observations derived from the eye examination. For instance, although both allergic conjunctivitis and dry eye disease may present with chronic conjunctival hyperemia, the former can usually be successfully treated with a combination of antihistamines and mast cell stabilizers; these agents are not ideal for dry eye disease. Conversely, treatment with ocular lubricants may aid both dry eyes and allergy (through dilution of the offending allergens on the ocular surface). In cases of conjunctival hyperemia where the etiology is not discernable despite workup, the management is less clear; many rely on decongestants and broad-spectrum anti-inflammatory ophthalmic medications such as glucocorticoids given that inflammation is a common proximal factor in conjunctival hyperemia pathophysiology. [65].

Table 1:

Commonly Used Pharmaceutical Categories in Management Ocular Redness^*

Category	Mechanism	Indications	Adverse effect
Ocular Decongestants	Induce vasoconstriction via α1/α2-adrenergic receptor stimulation, which reduce hyperaemia, chemosis and ocular redness through the constriction of blood vessels supplying the eye	Allergic conjunctivitis , Scleritis and Episcleritis , Pterygium	Blurred vision, burning/stinging/irritation of the eye; take longer to relieve symptoms and effect last only a few hours; sedation, excitability, dizziness or disturbed co-ordination
LUMIFY Eye Drops (brimonidine tartrate ophthalmic solution 0.025%)	A third-generation selective α2-adrenergic receptor agonist to induce vasoconstriction in the eye	Ocular redness with stable ocular health	May cause burning/stinging/irritation of the eye; may induce dry eye and conjunctival hemorrhage
Antihistamines & Mast Cell Stabilizers	Selective H1 receptor antagonist & inhibit the degranulation of sensitized mast cells Prevent release of inflammatory mediators Decrease redness, hyperaemia, itching and irritation	Allergic conjunctivitis, Contact-lens related problems	May cause burning/stinging/irritation after using the eye drops; may cause headache
Corticosteroids	Increase synthesis of lipocortin A, inhibiting local leukocyte adhesion and chemotaxis, and systemic inflammatory responses such as vasodilation and vascular permeability.	Dry eye, Allergic conjunctivitis , Viral conjunctivitis , Blepharitis Contact-lens related problems, Conjunctivitis medicamentosa, Immunogenic uveitis	Cannot be used for a long term Risk for inducing IOP increase and cataract formation
Nonsteroidal Anti-Inflammatory Drugs (NSAIDs)	Inhibit prostaglandin synthesis, reducing conjunctival redness and itching mediated by prostaglandins D2 and E2	Allergic conjunctivitis, Viral conjunctivitis	May temporarily sting or burn eyes when applied; eye redness and headache may also occur
Cyclosporine	Selectively inhibits interleukin-2 (IL-2), which is required for the transcription of T cells, thereby suppressing a cell-mediated immune response and interrupting inflammatory cytokine production	Dry eye, Allergic conjunctivitis, Graves’ Disease	A large group of nonresponders; may cause ocular surface pain and irritation
Ocular Lubricants	Replace and/or supplement the natural tear film Maintain corneal thickness, promote recovery of epithelial barrier function	Dry eye, Allergic conjunctivitis, Acute viral conjunctivitis, Contact-lens related problems	Blurred vision, variable levels of ocular discomfort and foreign body sensation Lont-term use is not effective for patients with severe symptoms.

^*Many agents & pharmaceutical used clinically for treat ocular redness are not approved for that indication.

Critical issues that are relevant for the treatment of ocular redness are the duration of action and whether the treatment is simply masking the signs of hyperemia or reversing the pathophysiologic aspects of the condition.

3.1. Pharmacological strategies

3.1.1. Ophthalmic Decongestants

Current over-the-counter ophthalmic decongestants consist of α-adrenergic receptor (α-AR) agonists with various receptor binding profiles. [66] Phenylephrine and tetrahydrozoline are selective α1-AR agonists, whereas naphazoline and oxymetazoline are considered mixed α1/α2-AR agonists. [67] These agents have been widely used for the treatment of conjunctival hyperemia since the 1960s and have well-established short-term efficacy. [68] However, their long-term use is limited by tachyphylaxis, rebound redness, and systemic side effects. [69-71] Because tachyphylaxis may occur due to α1-adrenoceptor-mediated downregulation and desensitization, brimonidine, a highly selective α2- receptor agonist, was recently approved by the U.S. Food and Drug Administration (FDA) to relieve conjunctival hyperemia. [71-73] Numerous clinical trials have endeavored to establish the duration of action and associated adverse effects for brimonidine. [74-76] Although it is safe and effective for month-long use with no evidence of tachyphylaxis and minimal rebound redness upon treatment discontinuation, there are still concerns about the induction of ocular allergy, dry eye disease, and eye irritation. [75] Long-term studies of brimonidine are warranted to confirm whether these adverse effects limit its continued use.

3.1.2. Antihistamines and Mast Cell Stabilizers

Due to the high prevalence of allergic conjunctivitis, topical histamine H1 receptor antagonists and mast cell stabilizers are often effective in treating acute conjunctival hyperemia due to atopic disease.[77,78] Histamine antagonists inhibit the histamine signaling pathway by blocking histamine H1 receptors, thereby suppressing conjunctival hyperemia. [79,80] Topical antihistamines have been shown to significantly reduce conjunctival hyperemia in conjunctivitis induced by allergen challenges in numerous clinical trials.[81-84] However, first-generation antihistamines can only provide short-term redness relief for less than four hours and pose the risk of causing significant systemic side effects such as sedation. [68,80,85] Recently, alcaftadine was shown to have a unique pharmacological profile with activity against H1, H2, and H4 receptors, in addition to reducing conjunctival eosinophil infiltration and the late-phase immune response of allergic conjunctivitis. [86-88] Moreover, alcaftadine 0.25% was proven to be superior to vehicle control in preventing conjunctival hyperemia when dosed eight hours before the allergen challenge. [81] Like all antihistamine agents, topical antihistamines are contraindicated in patients at risk for angle-closure glaucoma.

Mast cell stabilizers prevent mast cells from degranulating and consequently inhibit histamine release, reducing the influx of monocytes, eosinophils, and neutrophils at the ocular surface. [89,90] In a randomized controlled trial, Collum et al. compared disodium cromoglycate with placebo in the treatment of chronic blepharitis. The authors reported that topical mast cell stabilizers were significantly efficacious for conjunctival hyperemia after four weeks of treatment. [91] Nedocromil sodium, the second-generation mast cell stabilizer, effectively ameliorates the conjunctival hyperemia of vernal keratoconjunctivitis. [92] Dual-acting antihistamine/mast cell stabilizers are the newest class of agents for the treatment of allergy-associated symptoms. These dual-acting agents reduce ocular inflammation by i) preventing mast cell release of inflammatory mediators and ii) selectively blocking the H1-receptor, rendering them a potent therapeutic with potential use as a prophylactic agent. [93,94] It also renders the drug superior in terms of clinical effectiveness due to rapid onset and duration of action. [95-97] In different clinical trials for seasonal allergic conjunctivitis, topical olopatadine [98-101], epinastine [102,103], and ketotifen [104] were also shown to be significantly associated with a reduction in conjunctival hyperemia. However, olopatadine was more effective in treating ocular itching, while ketotifen had a better resolution of conjunctival hyperemia. [104] Additionally, olopatadine was also proven effective in alleviating conjunctival hyperemia of contact lens-induced mild to moderate papillary conjunctivitis. [105]

3.1.3. Anti-Inflammatory Drugs

Although ophthalmic decongestants and antihistamines are widely used for rapid resolution of conjunctival hyperemia, their long-term administration is limited by their short duration of action and potential adverse effects. For chronic and/or severe cases of conjunctival hyperemia, there is a need for targeted, long-term therapeutic strategies that are both efficacious and well-tolerated. Since chronic conjunctival hyperemia is considered a sign of immune dysregulation leading to a cycle of persistent inflammation, certain immunomodulatory and anti-inflammatory therapies in the form of corticosteroids, NSAIDs, and cyclosporine may be prescribed as well.

3.1.3.1. Corticosteroids

Corticosteroids are among the most potent medications available for the treatment of non-infectious ocular inflammatory disease, as they reduce inflammatory cellular infiltration, inhibit chemotaxis of leukocytes, and block the production of phospholipase A2 and restore normal vascular permeability. [106] Numerous studies have demonstrated the efficacy of corticosteroids in the treatment of conjunctival hyperemia. [92,107-109]

Pflugfelder et al. reported that loteprednol etabonate 0.5% significantly improved conjunctival hyperemia in dry eye disease patients compared to vehicle control. [110] In another DED study, fluorometholone was shown to markedly decrease conjunctival hyperemia compared to artificial tears. [108] Additionally, corticosteroids are considered the most effective drugs in the late inflammatory stage of seasonal allergic conjunctivitis (SAC). [111] Desonide was proven to be efficacious in reducing allergic conjunctival hyperemia[112] and, fluorometholone was shown to also produce a reduction in ocular surface temperature, decreasing conjunctival hyperemia. [92] For contact lens-induced papillary conjunctivitis, fluorometholone was reported to effectively alleviate mild to moderate conjunctival hyperemia and is comparable with olopatadine in efficacy. [105] However, long-term use of corticosteroids is limited by their extensive side effect profile as they increase the risk of infection, IOP elevation, cataract formation, and in some cases, corneal thinning. As a result, these agents are always used as adjunctive therapy with other targeting therapies and require close monitoring to avoid potential adverse effects. [108,113]

3.1.3.2. Topical Nonsteroidal Anti-Inflammatory Drugs

Nonsteroidal anti-inflammatory drugs (NSAID) are potent inhibitors of cyclooxygenase (COX) enzymes and inhibit the formation of prostaglandins, including PGE₂, PGD₂, PGF_2α, and PGI₂. [114] These prostaglandins can induce dilatation of conjunctival blood vessels through the local release of nitric oxide and sensory neuropeptides. [115] Topical NSAIDs may therefore be an effective therapeutic option for the treatment of conjunctival hyperemia. Oxyphenbutazone was also reported to reduce conjunctival hyperemia in cases of infectious conjunctivitis in the 1970s but was subsequently withdrawn from the market due to its significant side effect profile. [116] There are several studies suggesting that NSAIDs are effective in the treatment of conjunctival hyperemia secondary to allergic conjunctivitis. [98] Ballas et al. reported that ketorolac significantly decreased hyperemia secondary to allergic conjunctivitis compared to the placebo control using a double-masked, placebo randomized controlled study. [117] Numerous studies have also reported that ketorolac 0.5%[118,119], diclofenac 0.1%[118,120,121], and bromfenac 0.09%[122] are effective in reducing conjunctival hyperemia in vernal keratoconjunctivitis. As with corticosteroids, long-term use of topical NSAIDs is limited by potential significant corneal toxicities, such as corneal thinning disorders and melting. [123]

3.1.3.3. Cyclosporine A

Cyclosporine A (CsA) is a well-established immunomodulator and anti-inflammatory agent that has been available as a topical ophthalmic solution for nearly two decades. Its mechanism of action involves inhibition of both calcineurin and the synthesis of IL-2, a cytokine involved in the activation of lymphocytes.[124-126] In animal models of allergic conjunctivitis, CsA treatment was shown to reduce pro-inflammatory cytokine production and inhibit eosinophil infiltration.[127] In a cohort of vernal keratoconjunctivitis patients, Leonardi et al. demonstrated that CsA significantly reduced conjunctival hyperemia.[128] There are a few additional studies showing that CsA suppresses conjunctival hyperemia in late-phase reactions of allergic conjunctivitis.[129,130] Moreover, topical CsA has also been reported to significantly improve conjunctival hyperemia in patients with dry eye disease.[131] Notwithstanding these proven therapeutic benefits, the topical formulation of CsA may be locally irritating, and patients often report a burning sensation following treatment, which can lead to their cessation of therapy.[132]

3.2. Non-Pharmacologic Treatments

3.2.1. Ocular Lubricants

Ocular lubricants have been shown to i) reduce tear osmolarity, ii) restore physiologic ocular temperature, iii) dilute pro-inflammatory mediators and allergens, and iv) partially restore homeostasis of the ocular surface. [133,134] For these many reasons, lubricating eye drops are helpful in the management of ocular surface diseases and improve conjunctival hyperemia. Indeed, several clinical studies have shown that artificial tears can effectively improve conjunctival injections in patients with dry eye disease. [133,135,136] Other studies investigating the use of ocular lubricants in patients with seasonal allergic conjunctivitis report a significant decrease in bulbar hyperemia following treatment. [137,138] Although artificial tears were less effective in reducing conjunctival hyperemia than other more potent medications, they are widely used in the clinical setting, given its safety profile and minimal side effects.

3.2.2. Cold Compression

Numerous reports have suggested that cold compresses (CC) can reduce conjunctival hyperemia and eyelid redness. They do so, presumably, by lowering the temperature of the ocular surface, thereby causing transient vasoconstriction of conjunctival blood vessels. [137] Although the effectiveness of CC was supported mainly by anecdotal evidence, Bilkhu et al. demonstrated using a randomized, masked clinical trial the therapeutic benefits of CC in conjunction with artificial tears for the treatment of conjunctival hyperemia secondary to seasonal allergic conjunctivitis. [137] Their study also reported that CC could enhance the treatment effect of dual-action antihistamine-mast cell stabilizers in these patients. [137] These findings suggest that cold compresses may not only be used as supportive therapy in reducing conjunctival hyperemia of ocular allergy but may also effectively serve as a stand-alone treatment. However, the therapeutic effects of cold compresses in reducing conjunctival hyperemia are limited by their inconvenience for many and their very time-limited effects.

3.2.3. Dietary Supplementation

Recent clinical guidelines for the management of dry eye disease recommend the use of dietary supplements as a form of anti-inflammatory therapy. [126] This recommendation is based on clinical trial findings reported by LDECSG. [139] They noted that dietary supplementation with ω-3 essential fatty acids, antioxidants, vitamins, and minerals improved symptoms of dry eye disease, including a decrease in the use of artificial tears and a reduction of conjunctival hyperemia. [139] Moreover, ω-3 fatty acids have been shown to exhibit anti-inflammatory activity by blocking pro-inflammatory eicosanoids and reducing cytokine production, thereby potentially indirectly suppressing conjunctival hyperemia. [140] The therapeutic effects of nutraceutical supplementation were also demonstrated in patients with glaucoma. Indeed, dietary supplementation was found to reduce medication-induced chronic inflammation of the ocular surface and significantly decreased conjunctival hyperemia. [141] However, there remains a lack of consensus regarding the optimal supplementation protocol, particularly regarding dosage, composition, or duration of treatment. Additionally, the current literature on dietary supplementation for the treatment of ocular hyperemia has not investigated the potential adverse effects relating to toxicity. [139,141,142] That said, dietary supplementation of ω-3 essential fatty acids is still an adjunctive therapy to be considered by the treating physician.

4. NOVEL THERAPEUTIC STRATEGIES IN DEVELOPMENT

Recent progress in our understanding of the immunopathogenic mechanisms has catalyzed the development of novel therapies acting through the modulation of cytokine production, transcription factors, and immunophilins. Researchers have also identified several limitations of traditional drug delivery techniques and have developed several novel drug delivery techniques, such as nanoparticles, that increase bioavailability by prolonging the residence time of the drug at the ocular surface.

4.1. Therapeutic modalities

4.1.1. Topical anti-histaminic drugs

Bilastine is a second-generation selective H₁ receptor antagonist used for the management of urticaria and rhino-conjunctivitis in Europe [143]. The preclinical studies in mice models showed bilastine blocked the effects of histamine in a dose-dependent manner, at a potency comparable to cetirizine and up to ten times higher than fexofenadine.

The efficacy of bilastine for the management of ocular symptoms in patients with allergic rhinoconjunctivitis has been reported in three clinical trials. [144] In a randomized, double-blind, multi-center study in 721 patients with seasonal allergic rhinoconjunctivitis treated with bilastine 20 mg, desloratadine 5 mg, or placebo, Bachert et al. reported significant improvement in non-nasal symptoms including itching, redness, foreign body sensation, and tearing. [145] Kuna et al. conducted a randomized, double-blind, multi-center study conducted in 683 patients with seasonal allergic rhinoconjunctivitis, treated with bilastine 20 mg, cetirizine 10 mg, or placebo, and reported the drug to be more efficacious than placebo in reducing itching, conjunctival redness, and tearing by day seven and moderately reduced these symptoms by day 14 [146]. Horak et al. reported that bilastine was more efficacious in improving conjunctival redness one hour after drug administration, post-conjunctiva provocation test. [147] The efficacy of the reformulated topical Bilastine is now being studied for the management of conjunctival hyperemia associated with allergic conjunctivitis. The multi-center, double-masked, randomized phase 3 study comparing the efficacy of bilastine ophthalmic solution 0.6% to vehicle and ketotifen for the management of allergic conjunctivitis in August 2018, and the results are still pending. [148]

4.1.2. Selective Glucocorticoid Receptor Modulators and Agonists

Selective glucocorticoid receptor modulators and agonists (SEGRM/SEGRA) act through either transactivation or transrepression on binding to the glucocorticoid receptor. [149] These experimental drugs have anti-inflammatory activity comparable to glucocorticoids without the associated side effects.

Kato et al. evaluated the efficacy of ZK209614 in the carrageenan-induced conjunctivitis model and an allergic conjunctivitis model. [150] In the allergic conjunctivitis model, ZK209614 exhibited an inhibitory effect on the increased vascular permeability and had significant anti-inflammatory and antiallergic effects. However, the authors reported that the drug was comparatively less efficacious than dexamethasone in the allergic conjunctivitis model. Mapracorat, a SEGRA drug, has been evaluated for the topical treatment of inflammatory ocular surface disorders as it binds selectively to the human glucocorticoid receptor with high affinity but transactivates fewer several genes, resulting in fewer adverse events. [151] In a preclinical study, mapracorat injected in the conjunctival sac of ovalbumin (OVA)-sensitized guinea pigs two hours after the induction of allergic conjunctivitis efficaciously reduced the clinical signs, eosinophil infiltration, and eosinophil peroxidase activity in the conjunctiva tissue and also reduced conjunctival mRNA levels and protein expression of both CCL5 and CCL11. These sub-cellular changes result in efficacious antiallergic properties in ocular allergic conjunctivitis. A phase 2 clinical trial was initiated to evaluate the efficacy of the drug in patients with allergic conjunctivitis; conjunctival hyperemia, and pruritis are the primary outcomes. [152]

4.1.3. Anti-IgE monoclonal antibody

Omalizumab is a humanized, recombinant monoclonal antibody that binds to the FCεR3 portion of IgE and has been evaluated as a potential treatment modality for allergic disorders of the ocular surface. [153] The trials conducted to report the efficacy of the drug in allergic rhinitis also reported a significant reduction in ocular symptoms, including conjunctival hyperemia, in the treatment group compared with placebo after 12 and 16 weeks. [154,155] Although few cases have reported the efficacy of omalizumab for the management of AKC and VKC, no further studies have been conducted to study its efficacy on ocular symptoms. [156-158]

4.1.4. Urocanic Acid

Urocanic acid (UCA) is a chromophore that actively absorbs the ultraviolet (U.V.) wavelength of the light in the epidermis and acts as an initiator of UV-induced immunosuppression. [159] The trans-isomer of UCA is generated by histidine deamination, upon exposure to UV-B, forms a polymer with its cis isoform in a dose-dependent manner. [160] Jauhonen et al. demonstrated the productive immunosuppressive activity of Cis-UCA by reducing mast cell degranulation in both IgE mediated as well as IgE independent models. [161] In the same study, the authors also reported the efficacy of 2.5% cis -UCA had a comparable anti-inflammatory effect as dexamethasone, ketotifen, and olopatadine. The outcomes of phase 1, double-blinded, placebo-controlled trial reported that 0.5% and 2.5% cis -UCA eye drops were safe for topical application and were well-tolerated by the subjects [162].

4.1.5. DS-70 (α4 integrin antagonist)

DS-70 is a novel α/β-peptidomimetic α₄ integrin antagonist, which binds to α4β1 integrin with nanomolar affinity, thereby preventing the adhesion of α4 integrin–expressing cells, and consequently inhibiting the VCAM-1-mediated degranulation of mast cells. [163] to prevent the conjunctival infiltration of immune cells and clinical symptoms in a model of allergic conjunctivitis. In a preclinical study using the ovalbumin-induced guinea pig model of allergic conjunctivitis, treatment of topical 0.1% DS-70 before disease induction prevented the conjunctival hyperemia, epiphora, and chemosis. [163]

4.1.6. NK-1 Receptor antagonist

As our understanding of the pathophysiology of various ocular infectious and inflammatory diseases increases at the sub-cellular level, newer therapeutic targets are also being discovered, allowing us to tailor novel targeted therapies. For example, recent insights into the mechanisms of Substance P-induced neurogenic vasodilation have provided a new therapeutic target for the treatment of conjunctival hyperemia.

Substance P binds to NK-1R on various cell types, including mast cells, leading to degranulation of mast cells and release of biogenic amines, including histamine. Our group has recently exploited this pathway for the novel application of an NK-1R receptor antagonist in both allergic and non-allergic conjunctivitis guinea pig models. Preliminary data from our studies demonstrate the drug's efficacy in significantly reducing conjunctival hyperemia in both allergic and non-allergic models. Specifically, we induced conjunctival hyperemia in guinea pigs by applying topical dapiprazole (5%). Simultaneous single topical treatment with the drug led to significantly lower conjunctival redness over fifty minutes post-induction. (Figure 2) Similarly, we also studied the drug's efficacy in reducing conjunctival hyperemia in an allergen model by inducing conjunctival hyperemia in guinea by histamine (1.5mg/ml), topical application of the drug (1mg/ml) results in a significant reduction of the conjunctival hyperemia.

Figure 2:

Novel application of NK-1R antagonist for the treatment of conjunctival hyperemia. The drug blocks the neurogenic vasodilation by competitively inhibiting Substance P to NK-1R on the mast cells, leading to degranulation and release of histamine. These unpublished data are presented as mean ± SEM of three independent experiments, each consisting of 3 rabbits per group. (*p<0.05, **p<0.01)

4.2. Novel drug delivery systems

Drug delivery to ocular tissues is challenging despite the easy external access to the tissues and evasion of the first-pass metabolism. The conventional delivery formulations in the form of eye drops have relied upon as a conduit for delivering drugs for the treatment of myriad ocular surface diseases. However, multiple dynamic barriers such as choroidal and conjunctival blood flow, lymphatic clearance, and tear dilution reduce the bioavailability of topically applied drugs to less than 5%. [164,165]

4.2.1. Polymeric nanoparticles

The mucins in tear film help in maintaining the hydration of the ocular surface, provide lubrication and anti-adhesive properties between the conjunctival cells during blinking and also act as an epithelial barrier preventing debris and pathogens from binding to the ocular surface. [166] It has been demonstrated that charged polymers gain mucoadhesive properties through electrostatic interactions and the formation of hydrogen bonds. [167] These mucoadhesive forces have been used to develop vehicles for drug delivery with a prolonged residence time at the ocular surface.

Solani et al. demonstrated the application of cationic polymer nanoparticle, Eudragit RL-100, for delivering selective H1 antagonist Ketotifen fumarate. [168,169] The vehicle loaded with the drug showed a steady release of the different formulations of the drug at a rate of 65 to 88% over 24 hours. These experiments demonstrated that polymer nanoparticle vehicles with a higher polymer concentration resulted in a higher rate of drug release and penetration than formulations, which had a lower polymer concentration, with the latter releasing the drug over a longer duration and had a slower rate of tissue permeation.

4.2.2. Drug loaded microfilms

Poly (D, L-lactide-co-ε-caprolactone) microfilm is a polymeric implant manufactured by a matrix containing a biodegradable polymer, typically constructed from a synthetic aliphatic polyester of the poly-α-hydroxy acid family. [170] The physical properties of these drug delivery vehicles are easily are customized to modulate the drug release at therapeutic dosage over weeks to months [171,172]. Liu and colleagues have conducted preclinical studies reporting the in vitro and in vivo release profiles of tacrolimus in Balb/c mice immunized with short ragweed (SRW) injection and re-challenged with topical SRW containing topical solution. [173]. This novel delivery system delivered the therapeutic dosage of the drug at a rate of 0.212 to 0.243 μg/day in animals. The outcomes of these in-vivo experiments showed significantly reduced ocular allergy clinical scores throughout the four-week study period post-treatment. Furthermore, the immunohistochemical and histological analysis of the conjunctival tissue revealed suppressed eosinophils and reduced CD11c, CD4, and IL-4 expression in the treatment groups.

5. WHERE ARE WE HEADING: CURRENT CHALLENGES AND FUTURE DIRECTIONS IN MANAGEMENT OF OCULAR REDNESS

As detailed above, conjunctival hyperemia is known as a recognizable symptom of numerous ocular inflammatory conditions. General therapeutic strategies are specific to the underlying causes that evoke the inflammatory response of conjunctival vessels that can be reserved for allergic or infectious sources. However, clinical ophthalmology recognizes that conjunctival hyperemia may also occur in individuals without any evidence of ocular or systemic diseases. It is still a significant challenge to treat recurrent and/or chronic ocular redness. In recent decades, clinicians and scientists have tried to give an optimal resolution to conjunctival hyperemia by establishing proper animal models [6,174-176], developing automated clinical scales [177-180], and investigating the mechanisms of underlying causes [181-183] but few improvements have been made in this field. Since the problem brings a significant burden to both economics and society, immense efforts need to be devoted to the study of ocular redness in the future.

5.1. Modeling pathophysiology in animals

Animal models are essential tools in understanding the pathophysiology of a disease but also in the development of therapeutics [184]. The lack of predictive animal models restricts the development of novel therapeutics at the preclinical stage before proceeding the clinical trials in human subjects.

Since conjunctival hyperemia is associated with several etiologic factors, most established small animal models are typically used to study mechanisms of specific underlying disease and provide limited insight into the conjunctival microvascular dynamics during and post-treatment [15,44,185,186]. Large animal models established to assess the reliability of new quantification methods of conjunctival injections do not replicate ocular surface changes over a long duration, as seen in patients [62,187]. Moreover, the majority of available animal models assessing conjunctival hyperemia are primarily limited to allergen-induced conjunctivitis. Over the years, multiple groups have studied the effect of microbial agents at the ocular surface. They have focused their work on corneal tissue, with limited focus on immunopathological changes in surrounding conjunctival tissue [188-191]. Additionally, the immunopathological changes in animal models should discern between early and late-phase responses Accurate predictive animal models of conjunctival hyperemia resembling that seen in patients are necessary to develop efficacious therapeutic modalities.

5.2. Development of novel efficacious therapeutic modalities

For acute or recurrent conjunctival hyperemia with no apparent underlying pathology, topical decongestants may provide relief in the short term the short term. However, significant drawbacks are the induction of medicamentosa, development of rebound redness, and tachyphylaxis [69,73]. Amongst the newest generation of agents for the treatment of conjunctival hyperemia, low-dose brimonidine has been demonstrated to significantly improve redness with minimum rebounding, however, continuous use of these eye drops (for more than four weeks) may lead to allergic reactions [75]. Conjunctival hyperemia associated with prolonged application of ocular drugs in the management of chronic ocular diseases, such as topical prostaglandins prescribed for treating glaucoma, can affect patients’ decision on long-term continuation due to cosmetic concerns of redness [199,200]. Social implications concerning cosmetic appearances of the eye are demonstrated in redness relief products comprising 37% of unit sales of OTC medications [4].

Due to the main drawbacks of topical decongestants mostly occurring during the long-term use, topical decongestants are not appropriate for treating chronic conjunctival hyperemia that is associated with chronic underlying diseases, such as allergic conjunctivitis and dry eye disease, as they only mask relevant signs without addressing the underlying pathophysiologic mechanisms. More targeted therapeutic modalities are needed for the effective management of these conditions.

Recent advances in our understanding of the pathogenesis of the two most common conditions associated with the red eye (dry eye and allergy), and the contribution of neurogenic inflammation and nerve-derived factors in regulating vascular tone, have informed R&D efforts for the development of relevant new ophthalmic drugs. [201-203]. Studies focused on the mechanisms of action of neuropeptides and immune mediators in ocular inflammation will continue to provide a strong framework for the future development of new therapeutic strategies to address this common ocular morbidity.

6. CONCLUSIONS

Conjunctival hyperemia is one of the most common causes for visits to primary care physicians, optometrists, ophthalmologists, and emergency rooms. The treatment options for patients diagnosed with conjunctival hyperemia continue to be largely restricted to drugs that provide short-term relief. Although our understanding of the neurogenic and immune-mediated pathways that regulate the microvasculature of the ocular surface has significantly increased in recent years, large gaps in knowledge continue to persist, as detailed in this paper. As the prevalence of conjunctival hyperemia increases due to changes in human behavior and the environment, there is an ever-increasing need for the development of diagnostic techniques and rapid-acting therapeutics with minimal adverse effects.