The hunt for new therapeutics is daunting. Beyond the initial matchmaking of drug candidate and cellular target comes a string of hurdles—some sequential and many parallel—that include preclinical testing, formulation, manufacturing, stability, toxicology and clinical confirmation. As our knowledge of the myriad physiological pathways regulating the ocular surface has expanded, the number of potential targets for therapeutic intervention has grown. But the goal is not simply to accumulate prospects, but rather to identify and develop new treatments that address unmet needs.
In ocular allergy, we are well-served by the currently available cadre of antihistamines, anti-inflammatories and mast cell stabilizers, yet there remains a significant need for therapies that can alleviate chronic allergy and ocular inflammation. Most anti-allergic drugs target the mast cell and its chief minion histamine, but these cells are only one step in the ocular allergic cascade, and their activation frees a kaleidoscope of allergic mediators in addition to histamine. Many of these are molecules that we’ve examined over the years, while others, including those derived from neighboring tissues, have come to our attention only recently. This month we survey findings on new potential targets for therapeutic treatment of ocular allergy.
Mast Cells: An Allergic Nexus
Mast cells have been the target for most allergic therapies because of their role in the response to allergen exposure.Moon TC, Befus AD, Kulka M. Mast cell mediators: Their differential release and the secretory pathways involved. Front Immunol 2014;5:569. In an atopic response, the exposure to antigen involves processing of the offending agent—pollen, dander or dust mite—by antigen-presenting dendritic cells. These cells signal a stepwise activation of T and B lymphocytes, and eventual production of an allergen-specific IgE antibody. These steps are part of the adaptive immune process that’s hijacked in the development of allergic conjunctivitis and other allergic conditions.
When mast cells are activated by binding of the complex of allergen, specific IgE and IgE receptors (FcεRs) expressed on the mast cell surface, a cascade of cellular events is initiated that includes the release of pre-formed allergic mediators and the synthesis of additional lipid-derived signaling compounds. At this point it may seem self-evident that since mast cells elicit allergic responses, and mast cells release histamine, histamine must be responsible for the allergic response. But this hadn’t been established when we began looking at histamine levels in tears and the association between those levels and ocular allergic disease. Abelson MB, Baird RS, Allansmith MR. Tear histamine levels in vernal conjunctivitis and other ocular inflammations. Ophthalmology 1980;87:812-4. While we’ve established that antihistamines have a high degree of efficacy in relieving signs and symptoms of AC, there are still many patients who are not well-served by these compounds. Gomes PJ, Ousler GW, Welch DL, Smith LM, Coderre J, Abelson MB. Exacerbation of signs and symptoms of allergic conjunctivitis by a controlled adverse environment challenge in subjects with a history of dry eye and ocular allergy. Clin Ophthalmol 2013;7:157-165.
In addition to histamine, mast cells package and secrete proteoglycans, various hydrolases and signaling molecules, including interleukins, tumor necrosis factor and platelet activating factor. Lipid metabolism triggered by phospholipase A2 activation generates prostaglandins, leukotrienes and other lipid-based signaling molecules. In theory, all of these compounds represent potential targets for allergy therapy, and many have been investigated.
Mast Cell Targets
In previous installments of Therapeutic Topics, such as the May 2013 column, we discussed the importance of a number of protein kinases as potential targets for allergic therapy. Allergen cross-linking of the FcεRIs leads to activation of a series of kinases that provides the link between allergen and mast cell degranulation and activation. Gilfillan AM, Peavy RD, Metcalfe DD. Amplification mechanisms for the enhancement of antigen-mediated mast cell activation. Immunol Res 2009;43:15-24. For example, one of the earliest responses to surface antigen-antibody binding is phosphorylation of Lyn kinase, an enzyme that responds to this phosphorylation by physically associating with the antibody-receptor complex on the intracellular side of the cell membrane, initiating subsequent phosphorylation events. Spleen tyrosine kinase (Syk), phosphoinositide 3-kinase, and protein kinase C all participate in the activation chain, and so all are potential targets for intervention. Based upon issues of pharmacokinetics and tissue specificity, it turns out that Syk appears to be the best of these potential choices. There are small molecule inhibitors of Syk in development for a number of disorders, and their future testing in models of ocular allergy may not be far off. Coffey G, DeGuzman F, Inagaki M, et al. Specific inhibition of spleen tyrosine kinase suppresses leukocyte immune function and inflammation in animal models of rheumatoid arthritis. J Pharmacol Exp Ther 2012;340:2:350-9. Interestingly, a Japanese laboratory studying therapeutic effects of plant glycosides has identified Syk kinase inhibition as a potential mechanism in the treatment efficacy of Camellia japonica extracts in models of both allergic rhinitis and conjunctivitis. Kuba M, Tsuha K, Tsuha K, Matsuzaki G, Yasumoto T. In vivo analysis of the anti-allergic activities of Camellia japonica extract and okicamelliaside, a degranulation inhibitor. Journal of Health Science 2008;54:5:584-588.
Lipid-derived signaling molecules include prostaglandins, leukotrienes and PAF. Compounds that block cyclooxygenase, such as ketorolac or other NSAIDs, have been used in chronic allergy as steroid-sparing compounds, and they have demonstrated some efficacy, especially in VKC. Abelson MB, Butrus SI, Weston JH. Aspirin therapy in vernal conjunctivitis. Am J Ophthalmol 1983;95:502-5. Several recent clinical studies showed that another NSAID, pranoprofen, compared favorably with a topical steroid in patients with AC. Li Z, Mu G, Chen W, Gao L, Jhanji V, Wang L. Comparative evaluation of topical pranoprofen and fluorometholone in cases with chronic allergic conjunctivitis. Cornea 2013;32:579-82. In contrast to these results, our pilot studies testing the responses to topical leukotrienes showed little or no allergic responses. More recent trials of leukotriene inhibitors such as montelukast have confirmed that leukotrienes have little or no role in the etiology of AC. Gane J, Buckley R. Leukotriene receptor antagonists in allergic eye disease: A systematic review and meta-analysis. J Allergy Clin Immunol Pract 2013;1:65-74.
Studies dating back a decade or more demonstrated that PAF is chemotactic for eosinophils, and that this PAF-mediated chemotaxis has been shown to contribute to the chronic phase of allergic rhinitis and conjunctivitis. Zinchuk O, Fukushima A, Zinchuk V, Fukata K, Ueno H. Direct action of platelet activating factor (PAF) induces eosinophil accumulation and enhances expression of PAF receptors in conjunctivitis. Mol Vis 2005;11:114-23. , Kato M, Imoto K, Miyake H, Oda T, Miyaji S, Nakamura M. Apafant, a potent platelet-activating factor antagonist, blocks eosinophil activation and is effective in the chronic phase of experimental allergic conjunctivitis in guinea pigs. J Pharmacol Sci 2004;95:435-42. In addition, PAF induces degranulation of eosinophils and increases vascular permeability, two effects associated with chronic-phase allergy. The permeability response is separate from that elicited by histamine, as it isn’t blocked by antihistamines such as olopatadine. Yanni JM, Stephens DJ, Miller ST, et al. The in vitro and in vivo ocular pharmacology of olopatadine (AL-4943A), an effective anti-allergic/antihistaminic agent. J Ocul Pharmacol Ther 1996;12:389-400. Despite this, most research efforts on PAF have focused on its role in neuropathic and cancer-related pain. Morita K, Shiraishi S, Motoyama N, et al. Palliation of bone cancer pain by antagonists of platelet-activating factor receptors. PLoS One 2014;9:e91746. With renewed interest in finding therapies for more chronic allergic conditions, it may be time to give PAF a second look.
Studies suggest that the leaves of various species of Camellia are a rich source of compounds with potential as therapeutics for ocular allergy.
Tackling Allergic Inflammation
A significant part of perennial and chronic allergy is ocular inflammation and the associated infiltration of inflammatory cell types into the ocular surface environment. Established mast cell pre-formed mediators such as TNF-α are thought to be involved in this process as either direct chemo-attractants or as instigators of inflammatory cell recruitment. Recent studies of TNF-α in pre-clinical models of ocular inflammation suggest that topical use of inhibitors can reduce both inflammatory cell recruitment and production of inflammatory cytokines such as IL-6. Ji YW, Byun YJ, Choi W, et al. Neutralization of ocular surface TNF-α reduces ocular surface and lacrimal gland inflammation induced by in vivo dry eye. Invest Ophthalmol Vis Sci 2013;54:7557–7566. Perhaps the one-two punch of an antihistamine, which blocks the acute effects of degranulation, and an anti-allergic, which blocks TNF-α-mediated inflammation, could provide a more comprehensive anti-allergic response than the presently available therapeutic compounds, whose efficacy against severe ocular allergies is lacking.
Yet another approach to inflammation involves intervention beyond the mast cell. A key cytokine in inflammatory signaling is thymic stromal lymphopoietin, an epithelial cell-derived molecule that acts to shift adaptive responses toward a sensitized, allergic phenotype. Ziegler SF. Thymic stromal lymphopoietin and allergic disease. J Allergy Clin Immunol 2012;130:845-52. In the eye, TSLP appears to enhance allergic responses of antigen-presenting dendritic cells and mast cells, and it appears to have a role in the underlying etiology of AKC. Matsuda A, Ebihara N, Yokoi N, et al. Functional role of thymic stromal lymphopoietin in chronic allergic keratoconjunctivitis. Invest Ophthalmol Vis Sci 2010;51:1:151-5. In a recent clinical trial, treatment of asthmatic patients with a monoclonal antibody to TSLP reduced allergen-induced bronchoconstriction and indices of airway inflammation both before and after allergen challenge. Gauvreau GM, O’Byrne PM, Boulet LP, et al. Effects of an anti-TSLP antibody on allergen-induced asthmatic responses. N Engl J Med 2014;370:22:2102-10.
Monoclonals as Topicals?
Among current therapies, mast cell stabilizers such as pemirolast act at one of the earliest points in the allergic cascade, disrupting the linkage between FcεRI activation and mast cell degranulation. Abelson MB, Berdy GJ, Mundorf T, Amdahl LD, Graves AL; Pemirolast study group. Pemirolast potassium 0.1% ophthalmic solution is an effective treatment for allergic conjunctivitis: A pooled analysis of two prospective, randomized, double-masked, placebo-controlled, phase III studies. J Ocul Pharmacol Ther 2002; 18:5:475-88. While this is an attractive strategy, these drugs are limited by lower efficacy and a requirement for pretreatment, both of which reduce their overall utility. An alternative that would also halt the activation process before it starts would be an inhibitor like omalizumab, a humanized monoclonal antibody that binds to the CH3 domain near the binding site for the high-affinity type-I IgE Fc receptors of human IgE. Omalizumab can neutralize free IgE and inhibit the IgE allergic pathway without sensitizing mast cells or other cell types with surface FcεRI receptors such as those found on basophils. Yalcin AD. An overview of the effects of anti-IgE therapies. Med Sci Monit 2014;20:1691-9. Although this mechanism requires the mAb to be used in an injectable form, a recent report demonstrated its efficacy as a treatment for a case of severe VKC. de Klerk TA, Sharma V, Arkwright PD, Biswas S. Severe vernal keratoconjunctivitis successfully treated with subcutaneous omalizumab. J AAPOS 2013;17:305-6. Another potential target for mAb therapy, particularly in severe conditions such as AKC or VKC, is the IL-4 mAb Dupilumab (Regeneron), currently in development for atopic dermatitis. Malajian D, Guttman-Yassky E. New pathogenic and therapeutic paradigms in atopic dermatitis. Cytokine. 2014;in press Dec 23 [Epub ahead of print]
As in other disorders, use of mAbs targeting other candidates for allergic intervention—interleukins or interleukin receptors, for example—will sink or swim based upon issues of pharmacokinetics. Even monovalent antibody fragments are extremely large molecules by pharmaceutical standards, and wouldn’t be expected to appreciably penetrate ocular tissues when applied topically. Despite this, a number of published studies have provided encouraging evidence that topically applied mAbs can have a therapeutic impact on the ocular surface. In several recent trials employing the topical VEGF inhibitors ranibizumab or bevacizumab as a treatment for corneal neovascularization, both treatments were able to reduce vascular proliferation. Ozdemir O, Altintas O, Altintas L, Ozkan B, Akdag C, Yüksel N. Comparison of the effects of subconjunctival and topical anti-VEGF therapy (bevacizumab) on experimental corneal neovascularization. Arq Bras Oftalmol 2014;77:209-13. , Bucher F, Parthasarathy A, Bergua A, et al. Topical Ranibizumab inhibits inflammatory corneal hemangiogenesis and lymphangiogenesis. Acta Ophthalmol 2014;92:143-148. This finding establishes a proof of principle that even molecules as large as mAbs can be of benefit when delivered topically. While their therapeutic utility may be limited to the most severe cases of allergy with epithelial damage, they also can help to establish suitable targets for small molecule discovery.
Like the Japanese Camellia leaves that are used to make tea (and potentially, Syk inhibitors), another potential anti-allergic comes from an unlikely place: the kitchen. It turns out that turmeric roots, members of the ginger family that are commonly used as spices (especially in Indian foods) are also the source for curcumin, a polyphenol compound with multiple therapeutic applications. Among these is an ability to suppress responses to allergen challenge in a mouse model of allergic conjunctivitis. Chung SH, Choi SH, Choi JA, Chuck RS, Joo CK. Curcumin suppresses ovalbumin-induced allergic conjunctivitis. Mol Vis 2012;18:1966-72. Our own mouse model has been designed to test compounds that target both the acute, early phase (antihistamine) response, as well as later stage chronic (anti-inflammatory, steroid-like) responses. We know from our own studies that this pre-clinical confirmation of efficacy is an important step in the overall process of discovery. (McLaughlin JT, et al. IOVS 2013; 54: ARVO E-abstract 2553) In fact, the endpoints evaluated in early animal efficacy work mirror those that will ultimately be evaluated in the clinic, thus increasing the translatability of preclinical efficacy into success in the final stages of development.
It seems that we don’t have to look too far to find many potential targets for new therapies to treat ocular allergies, but as always, the real effort comes in sorting the true contenders from the false pretenders. Still, it’s encouraging to see that many of the newest treatment candidates have shown the promise of addressing our biggest current unmet need: chronic allergic conjunctivitis.
Dr. Abelson is a clinical professor of ophthalmology at Harvard Medical School. Dr. Gelfman is senior director of Pre-Clinical and Translational Services at Ora, Inc. Dr. McLaughlin is a medical writer at Ora Inc.