58.Stewart WC, Kruft B, Nelson LA, et al. Ophthalmologist attitudes regarding fixed combination treatment for glaucoma in the European Union. Eur J Ophthalmol 2009; 19: 588-593.

59.Baiza-Duran L, Varma R. Clinical Study of A Fixed Combination of Timolol-Brimonidine- Dorzolamide. Presented at: Association for Research in Vision and Ophthalmology 2009.

60.Dunker S, Schmucker A, Maier H. Tolerability, quality of life, and persistency of use in patients with glaucoma who are switched to the fixed combination of latanoprost and timolol. Adv Ther 2007; 24: 376-386.

VII Investigational and Future Drugs

Carol Toris, Malik Kahook, Paul Kaufman, Hidenobu Tanihara

Currently, there are six classes of drugs on the market for the treatment of elevated IOP. Despite the variety of choices, there remain large numbers of patients who do not respond well to these drugs and blindness results. A variety of investigational drugs from a growing number of classes are being developed and tested for their IOP lowering efficacy and side effect profile. These classes are summarized below.

Prostaglandin analogs

Prostaglandin EP2/EP4 analogs

The prostanoid EP2 receptor agonist, butaprost,1 an EP4 agonist (ARVO 2010 abstract #151) and a mixed EP2/4 agonist (ARVO 2010 abstract #2007) have recently been reported to lower intraocular pressure in monkeys to a level similar to FP agonists such as latanoprost. This IOP effect is accomplished in monkeys by increasing uveoscleral outflow. Similar studies2 of another selective EP4 receptor agonist, 3,7-dithia PGE1 showed a significant increase in outflow facility without an effect on uveoscleral outflow. These IOP-reducing effects are similar to FP agonists that have been reported sometimes to have no effect on outflow facility but other times to increase outflow facility.3 EP2/EP4 analogs can be classified as outflow drugs with IOP efficacy at least as good as FP agonists,4 although hyperemia and stinging and burning seem to be greater.

Nitric oxide donating prostaglandin analogs

PF-3187207, a nitric oxide donating prostaglandin analog lowers IOP in glaucomatous beagles, in rabbits with saline induced transient ocular hypertension, and in monkeys with laser induced glaucoma. The IOP reduction with this compound was more than with latanoprost alone presumably as a consequence of a contribution by NO in addition to its prostaglandin activity (ARVO 2009 abstract #1471). Its IOP-lowering mechanism of action has not been elucidated to date. The compound is now in clinical development for the treatment of ocular hypertension.

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Serotonin agonists

Serotonin, (5-hydroxy tryptamine, 5-HT) is an important endogenous neurotransmitter in the mammalian central nervous system and it is found throughout the eye, which has led some investigators to consider serotonin agonists as a potential class of IOP-lowering drugs. 5HT2A agonists effectively lower IOP in normotensive and hypertensive eyes of monkeys.5 R-DOI decreased IOP in ocular normotensive monkeys by increasing uveoscleral outflow.6 Of the many 5HT receptors, 5HT2 appears to be the one most involved in the maintenance of IOP.5

Dopaminergic agonists and antagonists

Cabergoline is an interesting dopaminergic agonist at D2/3 receptors with some serotonergic activity at 5HT2 and 5HT1A receptors. Cabergoline and other ergot derivatives (bromocriptine, lergotrile, lisuride and pergolide) reduce IOP in research animals.7 The 5HT2 and dopaminergic agonist activities of cabergoline probably mediate the IOP reduction in monkeys by increasing uveoscleral outflow.7

Angiotensin AT1 receptor antagonists

Recent evidence suggests that components of the renin-angiotensin hormone system are involved in the regulation of IOP. The angiotensin II AT1 receptor is one of two receptor subtypes able to bind angiotensin II. AT1 receptors are localized in ocular tissues of rabbits and humans. These receptors mediate vasoconstriction and extracellular matrix formation, two factors that can affect aqueous humor dynamics. An early study in monkeys found an IOP lowering effect of the angiotensin converting enzyme inhibitor, enalaprilat. The drug appeared to promote the formation of endogenous prostaglandins which in turn modified the outflow pathway and caused an increase in outflow facility, thus explaining the IOP effect.8 Topical AT1 receptor antagonists (sartans) reduce IOP in ocular hypertensive monkeys. One of these antagonists, olmesartan lowered IOP in ocular hypertensive rabbits by increasing uveoscleral outflow with no effects on aqueous flow and outflow facility.9,10 Olmesartan also appeared to decrease IOP and increase uveoscleral outflow in monkeys but, as in rabbits, the effect was small.11 The currently available AT1 receptor antagonists do not appear to be efficacious enough to be developed for clinical use.

Cytoskeletal drugs

The actin cytoskeleton and associated cellular-adhesion proteins are attractive targets for novel therapeutic approaches for glaucoma.12,13 No commonly employed therapies directly target and enhance outflow facility through the conventional outflow pathway (through the trabecular meshwork (TM) and Schlemm’s canal (SC)). This pathway can account for 50% to 75% of aqueous humor outflow.13

New therapies are in early clinical trials that target the structures and enzymes involved in maintaining actin associated cell-cell, cell shape and cell-ECM interactions.12,13 These compounds are intended to reduce the resistance to outflow by affecting cellular and tissue contractility/relaxation in the outflow pathways.

Direct perturbation of the actin microfilament system (by cytochalasins, latrunculins, etc.)14,15 or acto-myosin contractility of trabecular meshwork cells (by myosin light chain kinase or rho kinase inhibitors or by over-expression of caldesmon)16-19 dramatically reduces outflow resistance in live monkeys and/ or in human/monkey organ cultured perfused anterior segments. Morphological studies show that the common effects of these agents is the relaxation of TM, JCT and IW cells, as well as the TM overall. Cellular relaxation leading to a ‘relaxed’ tissue configuration may be the geometrically/biomechanically critical event and may be the fundamental endogenous control mechanism for outflow resistance, providing some validation of this as a therapeutic target for resistance reduction in glaucomatous eyes.12,13

Potential issues with some agents in this class are adverse corneal epithelial or endothelial effects due to altered permeability, and conjunctival hyperemia or hemorrhage.20,21 Dose and delivery refinements could help overcome these issues. Novel gene transfer approaches that would over-express cytoskeletonrelaxing proteins such as caldesmon in the TM are attractive delivery options for producing long-term therapeutic effects.13

ROCK inhibitors

Rho GTPase and its effector, ROCK (Rho-associated coiled coil-forming kinase) participate in signaling pathways that regulate actin stress fiber formation, focal adhesion, cell shape, cell motility and smooth muscle contraction. Recent investigations showed significant intraocular pressure (IOP)-lowering effects of ROCK inhibitors.17 The IOP-lowering effects are attributed to improved outflow facility, possibly caused by rearrangement of the actin cytoskeleton and resultant relaxation of cells in the conventional outflow pathway.18,21 Clinical trials revealed IOP-lowering effects of ophthalmic solution of a selective ROCK inhibitor with no systemic adverse events in humans.22 The most frequent adverse event after instillation of ophthalmic solution of a selective ROCK inhibitor was transient bulbar conjunctival hyperemia. Also, the potential risk of conjunctival hemorrhages has been suggested21 ROCK inhibitors have been shown to inhibit scar formation in animal models of glaucoma surgery, suggesting utility as effective anti-scarring agents after filtration surgeries.23 Taken together, the findings suggest that ROCK inhibitors may be effective treatments for open-angle glaucoma and ocular hypertension.

Endothelin

Endothelin, produced by endothelial cells, is a potent vasoconstrictor that plays a major role in ocular physiology and pathology, including glaucoma. Two recep-

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tors for ET-1, ETA and ETB, mediate vasoconstriction through the regulation (increase) of intracellular calcium levels.24 The ETB receptor acts as a vasoconstrictor when found on smooth muscle vasculature as well as a vasodilator, through the production of NO, when found on the surface of endothelial cells.25 The biological effect of endothelin on the vasculature results from the balance of ETA and ETB effects.26

Endothelin is associated with many different pathologic conditions, including glaucoma. Vascular dysregulation and altered ocular blood flow may lead to ischemic damage to the optic nerve head and retinal ganglion cells. The role of endothelin in these processes remains under intense study. ETA and ETB receptors on both the ciliary body and trabecular meshwork contribute to contractility and outflow resistance.27,28 Changes in outflow resistance, directly affects IOP. ET-1 has been implicated in the loss of RGCs and linked to astrogliosis, extracellular matrix remodeling and nitric oxide induced damage. ET-1 contributes to the disruption of anterograde axonal transport. ET-1 may also mediate ECM remodeling at the level of the ONH, possibly contributing to increased collagen deposition, reduced aqueous humor outflow facility and progressive damage to the optic nerve head.29

Several ET-1 receptor antagonists have been tested in animals and humans. In glaucomatous monkeys, avosentan (SPP 301), an ETA receptor antagonist, significantly reduced IOP.30 In a clinical trial, a dual ETA/ETB receptor blocker, bosentan, significantly increased blood flow to the retina, choroid, and optic nerve head but had no effect on IOP.31

Sulfisoxazole a non-selective ET-1 receptor antagonist reduced ET-1 induced elevation of NO. Additionally, sulfisoxazole reduced the number of GABA positive neurons, used as a measure of toxicity, by 41%. This evidence shows that ET-1 blockage can have a protective effect on the retinal ganglion cells of the optic nerve head.32

ET-1 is present and involved in a vast array of processes within the eye, and many may be directly related to the pathophysiology of glaucoma. The ET-1 pathway appears to show promise as a target for glaucoma therapies other than IOP reduction.

References

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2.Woodward DF, Nilsson SF, Toris CB, Kharlamb AB, Nieves AL, Krauss AH. Prostanoid EP4 receptor stimulation produces ocular hypotension by a mechanism that does not appear to involve uveoscleral outflow. Invest Ophthalmol Vis Sci 2009; 50: 3320-3328.

3.Toris CB, Gabelt BT, Kaufman PL. Update on the mechanism of action of topical prostaglandins for intraocular pressure reduction. Surv Ophthalmol 2008; 53 (Suppl1): S107-120.

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8.Lotti V.J and Pawlowski N. Prostaglandins mediate the ocular hypotensive action of the angiotensin converting enzyme inhibitor MK-422 (enalaprilat) in African green monkeys. J Ocul Pharmacol 1990; 6: 1-7.

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11.Wang RF, Podos SM, Mittag TW, Yokoyoma T. Effect of CS-088, an angiotensin AT1 receptor antagonist, on intraocular pressure in glaucomatous monkey eyes. Exp Eye Res 2005; 80: 629-632.

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13.Kaufman PL. Enhancing trabecular outflow by disrupting the actin cytoskeleton, increasing uveoscleral outflow with prostaglandins, and understanding the pathophysiology of presbyopia interrogating Mother Nature: asking why, asking how, recognizing the signs, following the trail. Exp Eye Res 2008; 86: 3-17.

14.Tian B, Gabelt BT, Geiger B, Kaufman PL. Combined effects of H-7 and cytochalasin B on outflow facility in monkeys. Exp Eye Res 1999; 68: 649-655.

15.Peterson JA, Tian B, McLaren JW, Hubbard WC, Geiger B, Kaufman PL. Latrunculins’ effects on intraocular pressure, aqueous humor flow, and corneal endothelium. Invest Ophthalmol Vis Sci 2000; 41: 1749-1758.

16.Honjo M, Inatani M, Kido N, Sawamura T, Yue BY, Honda Y, Tanihara H. A myosin light chain kinase inhibitor, ML-9, lowers the intraocular pressure in rabbit eyes. Exp Eye Res 2002; 75: 135-142.

17.Honjo M, Tanihara H, Inatani M, Kido N, Sawamura T, Yue BY, Narumiya S, Honda Y. Effects of rho-associated protein kinase inhibitor Y-27632 on intraocular pressure and outflow facility. Invest Ophthalmol Vis Sci 2001; 42: 137-144.

18.Rao PV, Deng PF, Kumar J, Epstein DL. Modulation of aqueous humor outflow facility by the Rho kinase-specific inhibitor Y-27632. Invest Ophthalmol Vis Sci 2001; 42: 1029-1037. (Erratum in: Invest Ophthalmol Vis Sci 2001; 42: 1690.)

19.Gabelt BT, Hu Y, Vittitow JL, Rasmussen CR, Grosheva I, Bershadsky AD, Geiger B, Borrás T, Kaufman PL. Caldesmon transgene expression disrupts focal adhesions in HTM cells and increases outflow facility in organ-cultured human and monkey anterior segments. Exp Eye Res 2006; 82: 935-944.

20.Sabanay I, Tian B, Gabelt BT, Geiger B, Kaufman PL. Latrunculin B effects on trabecular meshwork and corneal endothelial morphology in monkeys. Exp Eye Res 2006; 82: 236-246.

21.Tokushige H, Inatani M, Nemoto S, Sakaki H, Katayama K, Uehata M, Tanihara H. Effects of topical administration of y-39983, a selective rho-associated protein kinase inhibitor, on ocular tissues in rabbits and monkeys. Invest Ophthalmol Vis Sci 2007; 48: 3216-3222.

22.Tanihara H, Inatani M, Honjo M, Tokushige H, Azuma J, Araie M. Intraocular pressurelowering effects and safety of topical administration of a selective ROCK inhibitor, SNJ-1656, in healthy volunteers. Arch Ophthalmol 2008; 126: 309-315.