Secondary neuroprotective effects of hypotensive drugs and potential mechanisms of action

Abstract

Primary open-angle glaucoma, a long-term degenerative ocular neuropathy, remains a significant cause of vision impairment worldwide. While many risk factors have been correlated with increased risk for primary open-angle glaucoma, intraocular pressure (IOP) remains the only modifiable risk factor and primary therapeutic target. Pharmacologic therapies are administered topically; these include α₂-agonists, β-antagonists, prostaglandin analogs and carbonic anhydrase inhibitors. Some of these topical medications exhibit secondary neuroprotective effects independent of their effect on IOP. This review covers the possible mechanisms of neuroprotection stimulated by drugs currently marketed for the lowering of IOP, based on known literature. While the neuroprotective properties of many glaucoma pharmaceuticals are promising from an experimental standpoint, key challenges for the development of new clinical practices include unknown systemic side effects, limited methods of drug delivery to the retina and optic nerve, and development of extended-release formulations.

Keywords::

• α-agonists
• β-antagonists
• carbonic anhydrase inhibitors
• glaucoma
• neuroprotection
• optic nerve degeneration
• prostaglandins

Medscape: Continuing Medical Education Online

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All other clinicians completing this activity will be issued a certificate of participation. To participate in this journal CME activity: (1) review the learning objectives and author disclosures; (2) study the education content; (3) take the post-test with a 70% minimum passing score and complete the evaluation at http://www.medscape.org/journal/expertop; (4) view/print certificate.

Release date: 28 March 2012; Expiration date: 28 March 2013

Learning objectives

Upon completion of this activity, participants should be able to:

• • Assess the relationship between IOP and the progression of POAG

• • Distinguish topical treatments for POAG with neuroprotective properties

• • Analyze the mechanism of neuroprotection of different topical therapies for POAG

• • Evaluate the clinical use of topical therapies for neuroprotection

Financial & competing interests disclosure

EDITOR

Elisa Manzotti,Publisher, Future Science Group, London, UK

Disclosure:Elisa Manzotti has disclosed no relevant financial relationships.

CME AUTHOR

Charles P Vega, MD,Health Sciences Clinical Professor; Residency Director, Department of Family Medicine, University of California, Irvine, CA, USA.

Disclosure:Charles P Vega, MD, has disclosed no relevant financial relationships.

AUTHORS

Grace C Shih,The Vanderbilt Eye Institute, Department of Ophthalmology and Visual Sciences, Vanderbilt University School of Medicine, Nashville, TN, USA.

Disclosure:Grace C Shih would like to acknowledge the support of NIH EY017427, an Unrestricted Grant from Research to Prevent Blindness to the Vanderbilt University School of Medicine Department of Ophthalmology and Visual Science, the Melza M and Frank Theodore Barr Foundation through the Glaucoma Research Foundation, an American Health Assistance Foundation National Glaucoma Research Award, and the Vanderbilt Vision Research Center (P30EY008126).

David J Calkins, PhD,The Vanderbilt Eye Institute, Department of Ophthalmology and Visual Sciences, Vanderbilt University School of Medicine, Nashville, TN, USA.

Disclosure:David J Calkins, PhD, acts as a consultant for Allergan, QLT and Acucela, which also have provided funding for his laboratory. He would like to acknowledge the support of NIH EY017427, an unrestricted grant from Research to Prevent Blindness to the Vanderbilt University School of Medicine Department of Ophthalmology and Visual Science, the Melza M and Frank Theodore Barr Foundation through the Glaucoma Research Foundation, an American Health Assistance Foundation National Glaucoma Research Award, and the Vanderbilt Vision Research Center (P30EY008126).

Figure 1. Possible mechanisms of neuroprotection mediated by α₂-adrenergic agonists.

α₂-agonists such as brimonidine initiate multiple possible protective cascades. Direct binding of the α₂-receptor on RGCs could promote Bcl-mediated anti-apoptotic cascades, while binding to receptors expressed by glial or pigment epithelial cells could induce trophic signaling involving BDNF and bFGF, and their respective receptors.

bFGF: Basic FGF; NMDA-R: NMDA receptor; RGC: Retinal ganglion cell.

Figure 2. Putative neuroprotective actions of β-receptor antagonists.

Based on known expression patterns, β-receptor antagonists could bind channels in presynaptic circuits to reduce the release of glutamate and postsynaptic activation of NMDA receptors on RGCs. Antagonists could also exert influence on receptors expressed by vascular elements, leading to increased blood supply and reduced ischemia-related stress. Finally, certain β-receptor antagonists upregulate expression of BDNF in retinal glia.

NMDA-R: NMDA receptor; RGC: Retinal ganglion cell; TRKB: Tyrosine receptor kinase B.

Figure 3. Possible neuroprotective mechanisms of prostaglandin F₂ receptor agonists on retinal ganglion cells.

Evidence suggests a role for prostaglandin agonists in reducing excitotoxic stress. Direct binding of prostaglandin receptors on RGCs could reduce COX-2-mediated induction of nitric oxide synthase.

NMDA-R: NMDA receptor; RGC: Retinal ganglion cell.

Figure 4. Interaction of glutamate with NMDA receptors.

In many neurodegenerative conditions, excessive glutamate release from presynaptic terminals binds to postsynaptic NMDA-Rs, depolarizing the postsynaptic membrane through removal of the Mg²⁺ block. Subsequent influx and build-up of excess calcium leads to excitotoxicity and neuronal apoptosis.

NMDA-R: NMDA receptor.

Glaucoma, a leading cause of blindness worldwide, is a long-term degenerative ocular neuropathy resulting in retinal ganglion cell (RGC) dysfunction and corresponding loss to the visual field. It is estimated that over 60 million individuals were afflicted with open-angle and angle-closure glaucoma as of 2010, which will increase to almost 80 million by 2020 [1]. The disease affects all ethnicities, and 6.7 million people are bilaterally blind as a result [1]. The most common type of glaucoma is primary open-angle glaucoma (POAG), which presents clinically with increased cupping of the optic disk and associated field deficits, although the anterior chamber angle remains open and additional acute factors, such inflammation or trauma, are absent.

Various risk factors are associated with the development of glaucoma, including several comorbidities. The most prominent primary risk factors are of course age, intraocular pressure (IOP) and ethnicity. A meta-analysis of recent population-based studies in the USA, Australia and Europe found that subjects of African descent had almost three-times the prevalence of POAG compared with Caucasians [2]. The age-adjusted rate of blindness from glaucoma is 6.6-fold higher than that among Caucasians and blindness begins an average of 10 years earlier [3]. However, in older age groups, the prevalence of POAG in Latin American and Chinese populations approached that of African-descent individuals [1].

Despite the diversity of risk factors associated with glaucoma, IOP is currently the only modifiable risk factor and therefore the main target for therapeutic interventions. However, glaucomatous progression has been estimated to continue in as many as half of glaucoma patients undergoing an IOP-lowering regimen [4]. While controversial and highly variable across studies, such estimates support the growing consensus that pathophysiologic factors aside from IOP may play an important role in the progression of vision loss in glaucoma. For this reason, over the past decade in particular, medications currently on the market to lower IOP in glaucoma have been the subject of investigations to reveal secondary neuroprotective properties. The nature of these medications and the possible mechanisms underlying their putative neuroprotective effects will be the focus of this review.

Glaucoma & IOP

A long and detailed history embedded deeply in the literature supports a strong association between IOP and the development and progression of POAG [5–8]. Results from the Early Manifest Glaucoma Trial indicated that for patients diagnosed with POAG, mean IOP was a significant risk factor for glaucoma progression over an average of 8 years, even when IOP was within the ‘normal’ range of 8–22 mmHg [9]. Additionally, lowering IOP with topical drugs in patients with elevated IOP but without demonstrable visual field defects can delay or prevent disease onset [10]. From a structural standpoint, the use of IOP-lowering medication also reduces the risk of both optic disc deterioration and changes in visual field performance [11].

The relationship between IOP and glaucoma is complex. Despite popular simplifications, POAG is not synonymous with elevated IOP. Approximately 15–25% of individuals with open-angle glaucoma are classified as normotensive based on IOP measurements, and 33–50% of individuals with changes in optic disc appearance and visual field deficits demonstrate IOP within the normal range [12–14]. These patients have been placed in a subgroup commonly referred to as ‘low-tension’ or ‘normal-tension’ glaucoma. It is noteworthy, however, that the Baltimore Eye Study concluded that such a distinction is artificial and probably does not represent discrete etiological subgroups [15]. Across the board, glaucomatous pathology without elevated IOP has been estimated at representing roughly half of all diagnoses [16]. The majority of patients diagnosed with POAG in Asia have IOPs in the normal-tension range [8]. By contrast, many patients with consistently elevated IOP never develop characteristic glaucomatous optic disk appearance or deficits in visual fields [6,11].

For the reasons outlined above, although elevated IOP and glaucoma are clearly associated, elevations beyond what is considered the normal IOP range are neither necessary nor sufficient for diagnosis. Even so, IOP-lowering topical medications are still the standard of treatment for all patients, even those classified as normal tension. This is certainly justified. The Collaborative Normal-Tension Glaucoma Study found a slower rate of incident visual field loss in cases with a 30% or more decrease in IOP [17]. It is important to note that even in this study, 20% of eyes continued with progression to glaucomatous changes, even when IOP was reduced 30% or more from baseline. This highlights the need for newer therapies that directly target the neural substrates for vision loss in glaucoma. Based on the evidence, it would make sense to begin with common topical hypotensives (Box 1), especially should further investigation support secondary actions directly modulating pathogenic mechanisms at the neural level.

Progression of neurodegeneration in glaucoma

Glaucoma is most frequently diagnosed by assessment of the optic disc and retinal nerve fiber layer, with concurrent monitoring of IOP and assessment of the visual field. While the full extent of pathophysiology across the disease spectrum is not completely understood, the various forms of glaucoma are categorically optic neuropathies and therefore are shared as a substrate for vision loss degeneration of the RGC projection to the brain. Loss of tissue in the retina, especially RGC axons, results in the distinct appearance of the optic disk and has been linked to concomitant visual field loss [18]. The assessment of correlations between RGC loss in the retina, deficits in visual sensitivity and retinal nerve fiber layer thickness are an active area of research, with much quantitative progress arising from experimental models using nonhuman primates [19,20].

The RGC projection to the brain is extensive, stretching many centimeters along the optic nerve between the posterior globe and central targets. It makes sense that an early and persistent focus on dissecting pathogenic mechanisms has been damage to the RGC axons, both proximal to the globe and at distal sites in the brain. Deficits in functional axonal transport have been described since the mid-1970s [21–23], and more recent investigations have described possible pathogenic mechanisms at the optic nerve head (ONH) as damaging normal axoplasmic flow [24–26]. ONH injury is thought to reduce retrograde transport of prosurvival factors such as BDNF from the RGC synaptic terminal in the brain to the cell body, thus triggering downstream apoptotic cascades [27,28]. However, in animal models of glaucoma, impaired anterograde axonal transport first becomes apparent in the RGC projection in the brain, far distal to the ONH, with progression working backwards towards the retina [29]. Therefore, mechanisms both at the ONH or elsewhere in the projection may transduce stress signals within the axons and inhibit transport more globally.

More and more evidence indicates that axonal injury is early in glaucoma and probably manifests as deficits in axonal transport [30]. While the progression of neurodegenerative events ultimately results in mitochondrial-mediated, caspase-dependent RGC apoptosis [31–36], there is growing movement away from viewing apoptosis as the direct cause of clinical presentation. Increasing support for the compartmentalization of neuronal degeneration suggests that RGC neuronal processes are affected separately from the cell bodies, and may actually precede cell body loss [29,37–41]. For instance, deletion of the proapoptotic gene BAX in the DBA/2J mouse model of pigmentary glaucoma demonstrates a protective effect on the RGC body, but does not prevent optic nerve degeneration [42]. In addition, distal structures in the optic projection structure persist, even after the loss of axonal transport [29,40]. The persistence of the RGC soma following the loss of axonal transport and the axon itself may suggest a ‘dying back’ progression as part of glaucomatous neurodegeneration – a progressive distal-to-proximal cascade that begins at the synaptic terminals [43]. However, it seems likely that even this axonal injury may progress from critical pathogenic events at the ONH, which are transduced along the axon.

Targets for neuroprotection in glaucoma

Since the neuroretina develops as an evagination of the CNS, glaucoma shares a number of mechanistic elements with other neurodegenerative disorders of the CNS. Indeed, a diverse array of recent publications suggests numerous commonalities between glaucoma and CNS disorders. While disorders such as Alzheimer’s, Parkinson’s, amyotrophic lateral sclerosis and Huntington’s disease result from diverse etiologies, their progression involves many common elements that may serve as targets for potential therapeutic interventions. Neurodegeneration in glaucoma shares many such common pathway components, and indicates that they hold promise as therapeutic targets [40,44–46].

Regarding glaucoma as a neurodegenerative disease introduces the possibility that neuroprotective strategies might be an efficacious means to slow or even stop degenerative progression entirely. Two definitions of neuroprotective agents prevail in the existing literature. The first is obvious: agents that indirectly counteract RGC degeneration by reducing eye-related stress, for example by reducing IOP. The second is far more intriguing from the standpoint of understanding mechanisms of progression: substances that slow degeneration through direct effects on components of the optic projection. Practically speaking, neuroprotective agents that conform to the latter definition must directly reach the retina and/or optic nerve and act upon cellular elements therein, such as medications that stimulate glial cells to produce insult-nullifying or other trophic factors (Box 2).

Cascades that contribute to RGC degeneration in glaucoma are as complex as they are diverse [30,46]. Equally diverse, then, are potential targets for neuroprotective drug therapies, including mitochondrial dysfunction, protein misfolding, oxidative stress, inflammatory mediators and neurotrophin signaling. Additionally, the ONH is an early site for changes in glial reactivity [47], which generates an inflammatory milieu for neighboring RGCs and supporting cell types [48–51]. Other evidence implicates a particular form of ischemic insult that triggers glutamate-induced, NMDA receptor-mediated excitotoxicity [52–55]. Several studies have suggested the involvement of reduced ocular blood flow in the pathogenesis of POAG [56–59], with one demonstrating up to a 24% reduction in blood flow through the optic nerve [60]. Ischemic reperfusion injury due to compromised circulation is believed to be one of the primary initiators of RGC death, and may be among the earliest events associated with RGC loss [31,61]. For this reason, it is essential to understand the effects of current IOP-lowering medications not only on RGCs and their axons, but also on elements of the retinal and optic nerve vasculature. Unfortunately, validating changes in ONH blood flow is difficult, and there is no way to separate primary effects from secondary effects that could be due to tissue loss.

α₂-adrenergic agonists

The α₂-adrenergic agonists include well-known topical medications such as brimonidine and apraclonidine. These lower IOP by decreasing aqueous humor production through inhibition of adenylate cyclase inhibition, thus lowering cAMP levels [62]. The drugs also increase uveoscleral outflow [63]. Work in animal models has demonstrated α_2A-receptors in nonpigmented ciliary epithelium and in corneal conjunctival epithelia of the anterior segment and throughout cell bodies of the retina in the posterior segment [64]. Additionally, α_2B-receptors localize in neuronal dendrites and axons as well as glia, while α_2C-receptors localize in photoreceptor cell bodies and inner segments [64]. Similarly, in human cadaveric eyes, α₂-agonist sites have been identified primarily in iris epithelium and ciliary epithelium. Additional binding sites have also been localized to the ciliary muscle, retina, retinal pigment epithelium and choroid [65].

The α₂-agonists have been well marketed as glaucoma medications, and there has been long-held interest in their secondary neuroprotective effects. Many studies have documented enhancement of RGC cell body survival and of axonal function across a variety of acute models utilizing both ocular hypertension and other optic nerve injuries with systemic application of α-agonists [66–68]. These are reviewed in a recent study that found that systemic application of brimonidine prevented early axonopathy, including deficits in anterograde transport to the brain, and ensuing optic nerve and retinal degeneration with prolonged ocular hypertension [69]. A 2009 literature review of 48 articles addressing whether brimonidine met the criteria of neuroprotection found that it met all but the final neuroprotective criterion of success in humans [70].

The mechanisms of secondary neuroprotective effects afforded by the α-agonists have been more difficult to pinpoint and probably involve multiple pathways. Brimonidine appears to upregulate the expression of endogenous BDNF in rat RGCs [71]. BDNF has long been recognized for supporting the survival of existing neurons and encouraging the growth and differentiation of new neurites and synapses. Brimonidine also is linked to the upregulation in the retina of several additional prosurvival factors. These include the vascular basement membrane protein bFGF, the anti-apoptotic factors Bcl-2 and Bcl-xl, and the extracellular signal-regulated kinases (ERKs) and PI3K/Akt pathways [72,73]. Pretreatment of RGCs with brimonidine also resulted in significantly reduced NMDA-elicited whole-cell currents and cytosolic apoptotic calcium signals in RGCs [74], suggesting a mechanism of neuroprotection via RGC NMDA receptors (Figure 1).

Whatever the mechanisms that mediate neuroprotective properties for the α-agonists, they probably do not primarily involve increasing choroidal and optic nerve vascular flow. Topical α₂-agonists cause potent vasoconstriction and increased vascular resistance in choroidal vessels [75]. Brimonidine and other α₂-agonists have also been implicated as vasoconstrictors that can affect systemic blood pressure [76].

β-blockers

Topical β-adrenoreceptor blockers are one of the most commonly prescribed hypotensive medications for glaucoma. Their hypotensive effect is primarily mediated by the decrease of aqueous fluid with antagonism of β-adrenoreceptors in the anterior chamber of the eye [77,78]. Multiple studies have demonstrated evidence for a secondary neuroprotective effect of this class of drugs. Topical application of betaxolol, a selective β₁-receptor antagonist, attenuated thinning of the inner plexiform layer and loss of immunoreactivity for choline acetyltransferase following ischemic–reperfusion injury, the implication being rescue of synaptic connections [79]. Timolol, a more commonly prescribed nonselective β-blocker, exhibited protective effects on RGCs in a rat experimental glaucoma model [80]. The drug was found to reduce cell loss in the ganglion cell layer and to rescue a- and b-waves in the electroretinogram following both glutamate-induced excitotoxic insult and ischemic–reperfusion injury [81].

The β-blockers are likely to exert a secondary neuroprotective effect primarily via regulation of sodium and calcium channels, which are linked to the release of glutamate and subsequent activation of NMDA receptors (Figure 2). β-blockers were demonstrated to block calcium channels in the retina [82], and the neuroprotective effect of betaxolol and the nonselective β-blockers metipranolol and timolol, is thought to be elicited through reduction in sodium and calcium influx through voltage-sensitive channels [83]. Levobetaxolol is a more effective neuroprotectant than timolol, likely owing to greater capacity to block sodium and calcium influx [61]. Additionally, levobetaxolol may blunt ischemic injury by upregulation of BDNF mRNA in the retina [84]. The improvement in both neurological and histological outcomes in transient cerebral ischemia following administration of β-adrenoreceptor antagonists is partly attributed to attenuation of glutamate release [85]. Prosurvival pathways downstream of astrocyte activation may also play a role in β-receptor-mediated neuroprotection [86].

Aside from ion channel regulation, β-blockers have long been recognized to alter vascular dynamics, both systemically and in the eye. The β-adrenoreceptor receptors are localized to the ciliary epithelium and vascular smooth muscle, so β-blockers are intimately involved not only in the mediation of aqueous humor production, but also smooth muscle relaxation. While β-receptors have long been known to localize to both retinal arteries and veins [87], β-adrenergic binding sites also localize to vessel-free areas of the neural retina and optic nerve [77,88,89]. Blockers such as betaxolol have been demonstrated to increase blood velocity in the human ONH, thus supporting the hypothesis that mediation of vasculature effects may temper ischemia-induced RGC injury [90].

Of note, although β₂-agonists are not currently marketed as antiglaucoma medications, recent work has demonstrated potential neuroprotection via β₂-receptor activation and microglial inhibition, possibly by induction of β-arrestin 2 and modulation of glutamate homeostasis [91]. Additionally, β₂-adrenoceptor agonists promote anti-inflammatory and neurotrophic actions in nonglaucoma animal models of excitotoxicity [92].

Prostaglandin analogs

Prostaglandin analogs reduce intraocular pressure by enhancing uveoscleral outflow and are welltolerated with few systemic side effects. Additionally, they are considered advantageous from a compliance standpoint owing to their potency; the drugs are also useful in experimental models [93]. Pharmacological evidence suggests that bimatoprost acts by binding ‘prostamide’ receptors at the trabecular meshwork [94], the site of uveoscleral outflow. A secondary neuroprotective effect has also been recognized in prostaglandin analogs used to topically treat glaucoma. For example, topically applied latanoprost reduced the number of apoptotic RGCs following optic nerve crush [95], while the drug also exerts a neuroprotective effect on cells challenged by glutamate toxicity [96,97].

Multiple pathways for the anti-apoptotic effect of prostaglandin analogs have been proposed (Figure 3). It has been suggested that latanoprost may work by negative feedback on neuronal COX-2 activity, as it prevented lactate accumulation in the retinal tissue of animals subjected to acute ischemia [96]. Additionally, this same study found that COX-2 activity was reduced by both arachidonic acid and latanoprost in RGCs exposed to excess glutamate and that inhibition of inducible nitric oxide synthase occurred with the same drug concentrations. Latanoprost may also exhibit a direct anti-apoptotic effect via neurite outgrowth and caspase-3 inhibition, mediated by p44/p42 mitogen-activated protein kinase [98,99]. There are several other hypotheses regarding secondary neuroprotective mechanisms for prostaglandin analogs, including effects on ocular and ONH hemodynamics [100,101].

Carbonic anhydrase inhibitors

Carbonic anhydrase (CA) inhibitors are established as hypotensive agents, diuretics and antiepileptics, with additional use in the management of gastric and duodenal ulcers, neurological disorders and osteoporosis. CA-II is the isoenzyme that plays a role in aqueous humor production in the human anterior segment [102]. Dorazolamide and brinzolamide both potently inhibit CA-II and significantly decrease aqueous levels [102]. By acting upon CA-II, acetazolamide’s inhibition of sodium accession decreases bicarbonate formation in the ciliary epithelium [103]. While CA activity localizes histochemically in the retina to Müller cells, cones and the pigment epithelium [104], distribution of the CA inhibitor trifluormethazolamide in tissue showed high concentrations in the ciliary body [105]. In retinal tissue in culture, the CA inhibitor dorzolamide reduced apoptotic pathways with exposure to methylglyoxal and glyoxal (intermediates of advanced glycation end products) and H₂O₂[106].

This class of medications also demonstrates a vasodilatory effect [107], likely through a mechanism similar to CO₂-induced changes [108–110]. CA inhibitors increase cerebral blood flow following systemic administration [107,111], and ocular blood supply increases topical application [112]. Furthermore, membrane-associated CA activity within neuronal processes is also likely to modulate the pH of extracellular fluid, which can affect metabolic activity [106,113]. Additionally, ocular pulse amplitude increases following dorzolamide administration [114,115]. However, no changes were noted in ONH blood flow following dorzolamide administration in healthy human subjects [116].

A word on NMDA receptor antagonists

NMDA receptor antagonists (memantine in particular) have received wide attention as potential neuroprotective agents through their suppression of potentially excitotoxic pathways [117–119]. The putative neuroprotective action of NMDA receptor antagonists occurs through the reduction of potentially excitotoxic signaling due to excess glutamate [120], which is the primary mediator of excitatory neurotransmission in the mammalian CNS. It binds to three classes of ionotropic receptors, as well as metabotropic subunits, although its toxic effects are primarily mediated by binding of NMDA receptor subunits [121]. An excess of glutamate causes subsequent release of excess intracellular calcium (Figure 4), leading to neuronal death [121–123]. Excitotoxicity through excessive glutamate and stimulation of glutamate receptors (such as the NMDA receptor) has been implicated at various stages of neurodegenerative diseases [123]. NMDA receptor antagonists therefore probably exert their neuroprotective effects by directly inhibiting already metabolically stressed neuronal cell types from responding to excess glutamate.

Expert commentary

The concept of employing neuroprotectant medications to slow or even prevent irreversible glaucomatous damage to the optic projection is undoubtedly appealing. An extreme viewpoint might foresee the day when chronic IOP management is no longer relevant. The literature we have reviewed indicates that most of the common drugs used as part of a topical hypotensive regimen have direct neuroprotective properties independent of their action in the anterior chamber. These mechanisms include neuronal, glial and vascular pathways. However, most of the work described has been completed in animal models, and it is difficult to extrapolate both the mechanisms and the potential for direct neuroprotection of such medications to human patients. Careful clinical trials are necessary, as in the Low-Pressure Glaucoma Treatment Study, which recently demonstrated a protective effect of topical brimonidine in the absence of overtly elevated IOP [124]. This is especially important, since we often presume that secondary neuroprotective use needs to be optimized for direct retinal and/or optic nerve delivery. In the burgeoning age of personalized medicine, we must also emphasize the importance of interplay between genetic and environmental factors in influencing not only the onset and progression of glaucoma, but also the response to treatments.

Obviously, potential side effects of such potent drugs must be fully understood and adequately controlled prior to systemic or targeted delivery in human patients. Most of the medications with secondary neuroprotective effects we have described act upon receptors that are generally distributed across multiple organ systems. The prevention of undesired side effects is no small feat and one that must be taken seriously in at-risk populations with multiple comorbidities.

Finally, the methods by which one may directly deliver neuroprotective medications to the retina and optic nerve represent a growing industry and a matter of considerable debate. The attraction for direct delivery is to avoid the primary difficulty that physicians encounter with topical medications, which is patient compliance. Topical use of neuroprotective medications would require efficient diffusion through the aqueous humor to the posterior segment and adequate permeability through the inner limiting membrane to reach a sufficient concentration for therapeutic efficacy. Intravitreal injections are a solution to patient compliance and delivery to the posterior segment, but raise the risk of infection and patient discomfort, assuming that adequate penetration and dosing is possible.

Five-year view

Currently, IOP is the only modifiable risk factor for glaucoma and the primary target of most glaucoma therapeutics. Several currently marketed medications may confer secondary neuroprotective benefits to the retina and optic nerve. In particular, a substantial body of empirical evidence suggests the α₂-adrenergic agonists (e.g., brimonidine) hold particular promise in abating the earliest pathogenic events in glaucomatous RGC degeneration [69,70]. As the neuroprotective mechanisms of disparate drug classes appear to work via different pathways, combination therapies may be the ideal method of combating neurodegeneration in glaucoma for those who do not respond to hypotensive regimens. In fact, synergistic neuroprotective effects using β₂-agonists and NMDA-receptor antagonists have already been demonstrated following stroke [125]. Over the coming years, current glaucoma medications marketed in combination formulations may help overcome progression to vision loss for nonresponders. The fundamental problem for neuroprotection does not seem to be a lack of available drugs, since the major hypotensives discussed here also demonstrate at least some neuroprotective effect in experimental systems. The problem is getting the drug to where it is needed in sufficient concentration to exert a sustained effect.

As the neuroprotective signaling pathways effected by common hypotensive drugs used in glaucoma are elucidated in greater detail, future neuroprotective treatments will likely target intermediates to abrogate degenerative pathways as a way to avoid systemic or other unwanted side effects. The overlap between glaucomatous neurodegeneration and other degenerative diseases of the CNS, such as Alzheimer’s or amyotrophic lateral sclerosis, encourages cross-fertilization between fields. Mechanisms involving glial signaling or neurovascular interactions are of increasing relevance, not only in chronic disease, but also in trauma (e.g., stroke). These too will represent additional therapeutic targets for glaucoma in the coming years. Although barriers to the approval of use of experimental therapeutic compounds are daunting, the significant morbidity of glaucomatous disease warrants continued investigation into the mechanisms and delivery of neuroprotective agents, especially those already approved to lower IOP.

Box 1. Currently marketed topical hypotensive medications with putative neuroprotective properties.

α₂-agonists

• • Apraclonidine

• • Brimonidine

β-antagonists

• • Betaxolol

• • Levobetaxolol

• • Timolol

Prostaglandin analogs

• • Bimatoprost

• • Latanoprost

• • Tafluprost

• • Travoprost

Carbonic anhydrase inhibitors

• • Acetazolamide

• • Brinzolamide

• • Dorazolamide

Box 2. Targets of neuroprotective agents.

Direct neuroprotectants

• • Cationic excitotoxicity

• • Oxidative stress

• • Mitochondrial by-products

• • Calcium-induced injury

Indirect neuroprotectants

• • Reduction of intraocular pressure

• • Stimulation of glial cells

• • Regulation of inflammatory mediators

• • Augmentation of ocular blood flow

Key issues

• • Intraocular pressure (IOP) is the only modifiable risk factor for glaucoma, although many patients have IOP within the normal range.

• • The four most common classes of drugs used topically to lower IOP are α₂-agonists, β-antagonists/blockers, prostaglandin analogs and carbonic anhydrase inhibitors. Each of these classes has at least some experimental evidence of direct, secondary neuroprotective action on the retina and/or optic nerve.

• • The α₂-agonists could exert a neuroprotective effect by upregulating expression of BDNF, basic FGF, the anti-apoptotic factors Bcl-2 and Bcl-xl, and the extracellular signal-regulated kinases and PI3K/Akt pathways.

• • The β-antagonists could decrease glutamate-mediated NMDA receptor activation. Levobetaxolol also upregulates BDNF, while betaxolol increases tissue blood velocity in the optic nerve head, thus tempering ischemia-induced neuronal stress.

• • Prostaglandin F₂ analogs are likely to exhibit a negative feedback mechanism on COX-2 activity and increase blood flow to the retina and optic nerve.

• • Carbonic anhydrase inhibitors may increase retinal blood flow velocity and oxygenation, while additionally reducing the detrimental effects of advanced glycation end products and possibly oxidative stress. They can also exert an effect on extracellular pH by modulating membrane-bound carbonic anhydrase.

• • Since the neuroprotective mechanisms of disparate drug classes appear to work via different pathways, combination therapies may be the ideal method of combating glaucomatous neurodegeneration.

• • A key problem remains targeted delivery of known neuroprotectants in sufficient concentrations to exert a sustained effect.

• • The overlap between other CNS neurodegenerative diseases encourages careful attention to mechanistic and therapeutic research being conducted in such disease fields.

References

• Quigley HA, Broman AT. The number of people with glaucoma worldwide in 2010 and 2020. Br. J. Ophthalmol.90(3), 262–267 (2006).
   PubMed Web of Science ®Google Scholar
• Friedman DS, Wolfs RC, O’Colmain BJ et al. Prevalence of open-angle glaucoma among adults in the United States. Arch. Ophthalmol.122(4), 532–538 (2004).
   PubMed Web of Science ®Google Scholar
• Sommer A, Tielsch JM, Katz J et al. Racial differences in the cause-specific prevalence of blindness in east Baltimore. N. Engl. J. Med.325(20), 1412–1417 (1991).
   PubMed Web of Science ®Google Scholar
• Leske MC, Heijl A, Hussein M, Bengtsson B, Hyman L, Komaroff E. Factors for glaucoma progression and the effect of treatment: the early manifest glaucoma trial. Arch. Ophthalmol.121(1), 48–56 (2003).
   PubMed Web of Science ®Google Scholar
• Heijl A, Leske MC, Bengtsson B, Hyman L, Hussein M. Reduction of intraocular pressure and glaucoma progression: results from the Early Manifest Glaucoma Trial. Arch. Ophthalmol.120(10), 1268–1279 (2002).
   PubMed Web of Science ®Google Scholar
• Miglior S, Pfeiffer N, Torri V, Zeyen T, Cunha-Vaz J, Adamsons I. Predictive factors for open-angle glaucoma among patients with ocular hypertension in the European Glaucoma Prevention Study. Ophthalmology114(1), 3–9 (2007).
   PubMed Web of Science ®Google Scholar
• Ekstrom C. Risk factors for incident open-angle glaucoma: a population-based 20-year follow-up study. Acta Ophthalmol. doi:10.1111/j.1755-3768.2010.01943.x (2010) (Epub ahead of print).
   Google Scholar
• Cedrone C, Mancino R, Cerulli A, Cesareo M, Nucci C. Epidemiology of primary glaucoma: prevalence, incidence, and blinding effects. Prog. Brain Res.173, 3–14 (2008).
   PubMed Web of Science ®Google Scholar
• Bengtsson B, Leske MC, Hyman L, Heijl A. Fluctuation of intraocular pressure and glaucoma progression in the early manifest glaucoma trial. Ophthalmology114(2), 205–209 (2007).
   PubMed Web of Science ®Google Scholar
• Maier PC, Funk J, Schwarzer G, Antes G, Falck-Ytter YT. Treatment of ocular hypertension and open angle glaucoma: meta-analysis of randomised controlled trials. BMJ331(7509), 134 (2005).
   PubMed Web of Science ®Google Scholar
• Kass MA, Heuer DK, Higginbotham EJ et al. The Ocular Hypertension Treatment Study: a randomized trial determines that topical ocular hypotensive medication delays or prevents the onset of primary open-angle glaucoma. Arch. Ophthalmol.120(6), 701–713; discussion 829–830 (2002).
   PubMed Web of Science ®Google Scholar
• Tielsch JM, Katz J, Quigley HA, Javitt JC, Sommer A. Diabetes, intraocular pressure, and primary open-angle glaucoma in the Baltimore Eye Survey. Ophthalmology102(1), 48–53 (1995).
   PubMed Web of Science ®Google Scholar
• Klein BE, Klein R, Knudtson MD. Intraocular pressure and systemic blood pressure: longitudinal perspective: the Beaver Dam Eye Study. Br. J. Ophthalmol.89(3), 284–287 (2005).
   PubMed Web of Science ®Google Scholar
• Mitchell P, Smith W, Attebo K, Healey PR. Prevalence of open-angle glaucoma in Australia. The Blue Mountains Eye Study. Ophthalmology103(10), 1661–1669 (1996).
   PubMed Web of Science ®Google Scholar
• Sommer A, Tielsch JM, Katz J et al. Relationship between intraocular pressure and primary open angle glaucoma among white and black Americans. The Baltimore Eye Survey. Arch. Ophthalmol.109(8), 1090–1095 (1991).
   PubMed Web of Science ®Google Scholar
• Shields MB. Normal-tension glaucoma: is it different from primary open-angle glaucoma? Curr. Opin. Ophthalmol.19(2), 85–88 (2008).
   PubMed Web of Science ®Google Scholar
• Anderson D. Collaborative normal tension glaucoma study. Curr. Opin. Ophthalmol.14(2), 89–90 (2003).
   Google Scholar
• Wheat JL, Rangaswamy NV, Harwerth RS. Correlating RNFL thickness by OCT with perimetric sensitivity in glaucoma patients. J. Glaucoma21(2), 95–101 (2012).
   Google Scholar
• Harwerth RS, Wheat JL, Fredette MJ, Anderson DR. Linking structure and function in glaucoma. Prog. Retin. Eye Res.29(4), 249–271 (2010).
   PubMed Web of Science ®Google Scholar
• Harwerth RS, Vilupuru AS, Rangaswamy NV, Smith EL III. The relationship between nerve fiber layer and perimetry measurements. Invest. Ophthalmol. Vis. Sci.48(2), 763–773 (2007).
   PubMed Web of Science ®Google Scholar
• Anderson DR, Hendrickson A. Effect of intraocular pressure on rapid axoplasmic transport in monkey optic nerve. Invest. Ophthalmol.13(10), 771–783 (1974).
   PubMedGoogle Scholar
• Minckler DS, Tso MO, Zimmerman LE. A light microscopic, autoradiographic study of axoplasmic transport in the optic nerve head during ocular hypotony, increased intraocular pressure, and papilledema. Am. J. Ophthalmol.82(5), 741–757 (1976).
   PubMed Web of Science ®Google Scholar
• Minckler DS, Bunt AH, Johanson GW. Orthograde and retrograde axoplasmic transport during acute ocular hypertension in the monkey. Invest. Ophthalmol. Vis. Sci.16(5), 426–441 (1977).
   PubMed Web of Science ®Google Scholar
• Quigley HA, Addicks EM, Green WR, Maumenee AE. Optic nerve damage in human glaucoma. II. The site of injury and susceptibility to damage. Arch. Ophthalmol.99(4), 635–649 (1981).
   PubMed Web of Science ®Google Scholar
• Hollander H, Makarov F, Stefani FH, Stone J. Evidence of constriction of optic nerve axons at the lamina cribrosa in the normotensive eye in humans and other mammals. Ophthalmic Res.27(5), 296–309 (1995).
   PubMed Web of Science ®Google Scholar
• Burgoyne CF, Downs JC, Bellezza AJ, Suh JK, Hart RT. The optic nerve head as a biomechanical structure: a new paradigm for understanding the role of IOP-related stress and strain in the pathophysiology of glaucomatous optic nerve head damage. Prog. Retin. Eye Res.24(1), 39–73 (2005).
   PubMed Web of Science ®Google Scholar
• Pease ME, McKinnon SJ, Quigley HA, Kerrigan-Baumrind LA, Zack DJ. Obstructed axonal transport of BDNF and its receptor TRKB in experimental glaucoma. Invest. Ophthalmol. Vis. Sci.41(3), 764–774 (2000).
   PubMed Web of Science ®Google Scholar
• Quigley HA, McKinnon SJ, Zack DJ et al. Retrograde axonal transport of BDNF in retinal ganglion cells is blocked by acute IOP elevation in rats. Invest. Ophthalmol. Vis. Sci.41(11), 3460–3466 (2000).
   PubMed Web of Science ®Google Scholar
• Crish SD, Sappington RM, Inman DM, Horner PJ, Calkins DJ. Distal axonopathy with structural persistence in glaucomatous neurodegeneration. Proc. Natl Acad. Sci. USA107(11), 5196–5201 (2010).
   PubMed Web of Science ®Google Scholar
• Crish SD, Calkins DJ. Neurodegeneration in glaucoma: progression and calcium-dependent intracellular mechanisms. Neuroscience176, 1–11 (2011).
   PubMedGoogle Scholar
• Quigley HA. Neuronal death in glaucoma. Prog. Retin. Eye Res.18(1), 39–57 (1999).
   PubMed Web of Science ®Google Scholar
• McKinnon SJ, Lehman DM, Kerrigan-Baumrind LA et al. Caspase activation and amyloid precursor protein cleavage in rat ocular hypertension. Invest. Ophthalmol. Vis. Sci.43(4), 1077–1087 (2002).
   PubMed Web of Science ®Google Scholar
• Tahzib NG, Ransom NL, Reitsamer HA, McKinnon SJ. Alpha-fodrin is cleaved by caspase-3 in a chronic ocular hypertensive (COH) rat model of glaucoma. Brain Res. Bull.62(6), 491–495 (2004).
   PubMed Web of Science ®Google Scholar
• Waldmeier PC, Tatton WG. Interrupting apoptosis in neurodegenerative disease: potential for effective therapy? Drug Discov. Today9(5), 210–218 (2004).
   PubMedGoogle Scholar
• Huang W, Dobberfuhl A, Filippopoulos T et al. Transcriptional up-regulation and activation of initiating caspases in experimental glaucoma. Am. J. Pathol.167(3), 673–681 (2005).
   Google Scholar
• Qu J, Wang D, Grosskreutz CL. Mechanisms of retinal ganglion cell injury and defense in glaucoma. Exp. Eye Res.91(1), 48–53 (2010).
   PubMed Web of Science ®Google Scholar
• Schlamp CL, Li Y, Dietz JA, Janssen KT, Nickells RW. Progressive ganglion cell loss and optic nerve degeneration in DBA/2J mice is variable and asymmetric. BMC Neurosci.7, 66 (2006).
   PubMed Web of Science ®Google Scholar
• Stevens B, Allen NJ, Vazquez LE et al. The classical complement cascade mediates CNS synapse elimination. Cell131(6), 1164–1178 (2007).
   PubMed Web of Science ®Google Scholar
• Howell GR, Libby RT, Jakobs TC et al. Axons of retinal ganglion cells are insulted in the optic nerve early in DBA/2J glaucoma. J. Cell. Biol.179(7), 1523–1537 (2007).
   PubMed Web of Science ®Google Scholar
• Buckingham BP, Inman DM, Lambert W et al. Progressive ganglion cell degeneration precedes neuronal loss in a mouse model of glaucoma. J. Neurosci.28(11), 2735–2744 (2008).
   PubMed Web of Science ®Google Scholar
• Fu QL, Li X, Shi J et al. Synaptic degeneration of retinal ganglion cells in a rat ocular hypertension glaucoma model. Cell. Mol. Neurobiol.29(4), 575–581 (2009).
   Google Scholar
• Libby RT, Li Y, Savinova OV et al. Susceptibility to neurodegeneration in a glaucoma is modified by Bax gene dosage. PLoS Genet.1(1), 17–26 (2005).
   PubMed Web of Science ®Google Scholar
• Coleman M. Axon degeneration mechanisms: commonality amid diversity. Nat. Rev. Neurosci.6(11), 889–898 (2005).
   PubMed Web of Science ®Google Scholar
• McKinnon SJ. Glaucoma: ocular Alzheimer’s disease? Front. Biosci.8, S1140–S1156 (2003).
   PubMed Web of Science ®Google Scholar
• Steele MR, Inman DM, Calkins DJ, Horner PJ, Vetter ML. Microarray analysis of retinal gene expression in the DBA/2J model of glaucoma. Invest. Ophthalmol. Vis. Sci.47(3), 977–985 (2006).
   PubMed Web of Science ®Google Scholar
• Kong GY, Van Bergen NJ, Trounce IA, Crowston JG. Mitochondrial dysfunction and glaucoma. J. Glaucoma18(2), 93–100 (2009).
   PubMed Web of Science ®Google Scholar
• Son JL, Soto I, Oglesby E et al. Glaucomatous optic nerve injury involves early astrocyte reactivity and late oligodendrocyte loss. Glia58(7), 780–789 (2010).
   PubMedGoogle Scholar
• Guo L, Tsatourian V, Luong V et al. En face optical coherence tomography: a new method to analyse structural changes of the optic nerve head in rat glaucoma. Br. J. Ophthalmol.89(9), 1210–1216 (2005).
   PubMedGoogle Scholar
• Hernandez MR. The optic nerve head in glaucoma: role of astrocytes in tissue remodeling. Prog. Retin. Eye Res.19(3), 297–321 (2000).
   PubMed Web of Science ®Google Scholar
• Tezel G, Hernandez MR, Wax MB. In vitro evaluation of reactive astrocyte migration, a component of tissue remodeling in glaucomatous optic nerve head. Glia34(3), 178–189 (2001).
   PubMedGoogle Scholar
• Nakazawa T, Nakazawa C, Matsubara A et al. Tumor necrosis factor-alpha mediates oligodendrocyte death and delayed retinal ganglion cell loss in a mouse model of glaucoma. J. Neurosci.26(49), 12633–12641 (2006).
   PubMed Web of Science ®Google Scholar
• Hiraoka M, Inoue K, Ninomiya T, Takada M. Ischaemia in the Zinn-Haller circle and glaucomatous optic neuropathy in macaque monkeys. Trans. Am. Ophthalmol. Soc.101, 163–169; discussion 169–171 (2003).
   Google Scholar
• Kuehn MH, Fingert JH, Kwon YH. Retinal ganglion cell death in glaucoma: mechanisms and neuroprotective strategies. Ophthalmol. Clin. North Am.18(3), 383–395, vi (2005).
   PubMedGoogle Scholar
• Rokicki W, Dorecka M, Romaniuk W. [Retinal ganglion cells death in glaucoma – mechanism and potential treatment. Part II]. Klin. Oczna109(7–9), 353–355 (2007).
   Google Scholar
• Osborne NN, Ugarte M, Chao M et al. Neuroprotection in relation to retinal ischemia and relevance to glaucoma. Surv. Ophthalmol.43(Suppl. 1), S102–S128 (1999).
   PubMed Web of Science ®Google Scholar
• Yoshida Y, Sugiyama T, Utsunomiya K, Ogura Y, Ikeda T. A pilot study for the effects of donepezil therapy on cerebral and optic nerve head blood flow, visual field defect in normal-tension glaucoma. J. Ocul. Pharmacol. Ther.26(2), 187–192 (2010).
   PubMedGoogle Scholar
• Harris A, Chung HS, Ciulla TA, Kagemann L. Progress in measurement of ocular blood flow and relevance to our understanding of glaucoma and age-related macular degeneration. Prog. Retin. Eye Res.18(5), 669–687 (1999).
   PubMedGoogle Scholar
• Bathija R. Optic nerve blood flow in glaucoma. Clin. Exp. Optom.83(3), 180–184 (2000).
   Google Scholar
• Zhao DY, Cioffi GA. Anterior optic nerve microvascular changes in human glaucomatous optic neuropathy. Eye (Lond.)14(Pt 3B), 445–449 (2000).
   Web of Science ®Google Scholar
• Grunwald JE, Piltz J, Hariprasad SM, DuPont J. Optic nerve and choroidal circulation in glaucoma. Invest. Ophthalmol. Vis. Sci.39(12), 2329–2336 (1998).
   PubMed Web of Science ®Google Scholar
• Osborne NN, Wood JP, Chidlow G, Casson R, DeSantis L, Schmidt KG. Effectiveness of levobetaxolol and timolol at blunting retinal ischaemia is related to their calcium and sodium blocking activities: relevance to glaucoma. Brain Res. Bull.62(6), 525–528 (2004).
   PubMed Web of Science ®Google Scholar
• Ogidigben M, Chu TC, Potter DE. Alpha-2 adrenoceptor mediated changes in aqueous dynamics: effect of pertussis toxin. Exp. Eye Res.58(6), 729–736 (1994).
   PubMed Web of Science ®Google Scholar
• Toris CB, Camras CB, Yablonski ME. Acute versus chronic effects of brimonidine on aqueous humor dynamics in ocular hypertensive patients. Am. J. Ophthalmol.128(1), 8–14 (1999).
   PubMed Web of Science ®Google Scholar
• Woldemussie E, Wijono M, Pow D. Localization of alpha 2 receptors in ocular tissues. Vis. Neurosci.24(5), 745–756 (2007).
   Google Scholar
• Matsuo T, Cynader MS. Localization of alpha-2 adrenergic receptors in the human eye. Ophthalmic Res.24(4), 213–219 (1992).
   PubMed Web of Science ®Google Scholar
• Wheeler LA, Lai R, Woldemussie E. From the lab to the clinic: activation of an alpha-2 agonist pathway is neuroprotective in models of retinal and optic nerve injury. Eur. J. Ophthalmol.9(Suppl. 1), S17–S21 (1999).
   PubMed Web of Science ®Google Scholar
• Yoles E, Wheeler LA, Schwartz M. Alpha2-adrenoreceptor agonists are neuroprotective in a rat model of optic nerve degeneration. Invest. Ophthalmol. Vis. Sci.40(1), 65–73 (1999).
   PubMed Web of Science ®Google Scholar
• Pinar-Sueiro S, Urcola H, Rivas MA, Vecino E. Prevention of retinal ganglion cell swelling by systemic brimonidine in a rat experimental glaucoma model. Clin. Experiment. Ophthalmol.39(8), 799–807 (2011).
   PubMed Web of Science ®Google Scholar
• Lambert WS, Ruiz L, Crish SD, Wheeler LA, Calkins DJ. Brimonidine prevents axonal and somatic degeneration of retinal ganglion cell neurons. Mol. Neurodegener.6(1), 4 (2011).
   PubMed Web of Science ®Google Scholar
• Saylor M, McLoon LK, Harrison AR, Lee MS. Experimental and clinical evidence for brimonidine as an optic nerve and retinal neuroprotective agent: an evidence-based review. Arch. Ophthalmol.127(4), 402–406 (2009).
   PubMed Web of Science ®Google Scholar
• Gao H, Qiao X, Cantor LB, WuDunn D. Up-regulation of brain-derived neurotrophic factor expression by brimonidine in rat retinal ganglion cells. Arch. Ophthalmol.120(6), 797–803 (2002).
   PubMedGoogle Scholar
• Lai RK, Chun T, Hasson D, Lee S, Mehrbod F, Wheeler L. Alpha-2 adrenoceptor agonist protects retinal function after acute retinal ischemic injury in the rat. Vis. Neurosci.19(2), 175–185 (2002).
   PubMed Web of Science ®Google Scholar
• Tatton W, Chen D, Chalmers-Redman R, Wheeler L, Nixon R, Tatton N. Hypothesis for a common basis for neuroprotection in glaucoma and Alzheimer’s disease: anti-apoptosis by alpha-2-adrenergic receptor activation. Surv. Ophthalmol.48(Suppl. 1), S25–S37 (2003).
   PubMed Web of Science ®Google Scholar
• Dong CJ, Guo Y, Agey P, Wheeler L, Hare WA. Alpha2 adrenergic modulation of NMDA receptor function as a major mechanism of RGC protection in experimental glaucoma and retinal excitotoxicity. Invest. Ophthalmol. Vis. Sci.49(10), 4515–4522 (2008).
   PubMed Web of Science ®Google Scholar
• Polska E, Simader C, Weigert G et al. Regulation of choroidal blood flow during combined changes in intraocular pressure and arterial blood pressure. Invest. Ophthalmol. Vis. Sci.48(8), 3768–3774 (2007).
   PubMed Web of Science ®Google Scholar
• Quaranta L, Gandolfo F, Turano R et al. Effects of topical hypotensive drugs on circadian IOP, blood pressure, and calculated diastolic ocular perfusion pressure in patients with glaucoma. Invest. Ophthalmol. Vis. Sci.47(7), 2917–2923 (2006).
   PubMed Web of Science ®Google Scholar
• Dawidek GM, Robinson MI. Beta-adrenergic receptors in human anterior optic nerve: an autoradiographic study. Eye (Lond.)7(Pt 1), 122–126 (1993).
   Google Scholar
• Dailey RA, Brubaker RF, Bourne WM. The effects of timolol maleate and acetazolamide on the rate of aqueous formation in normal human subjects. Am. J. Ophthalmol.93(2), 232–237 (1982).
   PubMed Web of Science ®Google Scholar
• Cheon EW, Park CH, Kang SS et al. Betaxolol attenuates retinal ischemia/reperfusion damage in the rat. Neuroreport14(15), 1913–1917 (2003).
   Google Scholar
• Seki M, Tanaka T, Matsuda H et al. Topically administered timolol and dorzolamide reduce intraocular pressure and protect retinal ganglion cells in a rat experimental glaucoma model. Br. J. Ophthalmol.89(4), 504–507 (2005).
   PubMed Web of Science ®Google Scholar
• Goto W, Ota T, Morikawa N et al. Protective effects of timolol against the neuronal damage induced by glutamate and ischemia in the rat retina. Brain Res.958(1), 10–19 (2002).
   PubMed Web of Science ®Google Scholar
• Osborne NN, Cazevieille C, Carvalho AL, Larsen AK, DeSantis L. In vivo and in vitro experiments show that betaxolol is a retinal neuroprotective agent. Brain Res.751(1), 113–123 (1997).
   PubMed Web of Science ®Google Scholar
• Wood JP, Schmidt KG, Melena J, Chidlow G, Allmeier H, Osborne NN. The beta-adrenoceptor antagonists metipranolol and timolol are retinal neuroprotectants: comparison with betaxolol. Exp. Eye Res.76(4), 505–516 (2003).
   PubMed Web of Science ®Google Scholar
• Wood JP, DeSantis L, Chao HM, Osborne NN. Topically applied betaxolol attenuates ischaemia-induced effects to the rat retina and stimulates BDNF mRNA. Exp. Eye Res.72(1), 79–86 (2001).
   PubMed Web of Science ®Google Scholar
• Goyagi T, Nishikawa T, Tobe Y. Neuroprotective effects and suppression of ischemia-induced glutamate elevation by beta1-adrenoreceptor antagonists administered before transient focal ischemia in rats. J. Neurosurg. Anesthesiol.23(2), 131–137 (2011).
   Web of Science ®Google Scholar
• Junker V, Becker A, Huhne R et al. Stimulation of beta-adrenoceptors activates astrocytes and provides neuroprotection. Eur. J. Pharmacol.446(1–3), 25–36 (2002).
   PubMed Web of Science ®Google Scholar
• Denis P, Elena PP. [Retinal vascular beta-adrenergic receptors in man]. Ophtalmologie3(1), 62–64 (1989).
   PubMedGoogle Scholar
• Ferrari-Dileo G. Beta 1 and beta 2 adrenergic binding sites in bovine retina and retinal blood vessels. Invest. Ophthalmol. Vis. Sci.29(5), 695–699 (1988).
   PubMedGoogle Scholar
• Elena PP, Kosina-Boix M, Moulin G, Lapalus P. Autoradiographic localization of beta-adrenergic receptors in rabbit eye. Invest. Ophthalmol. Vis. Sci.28(8), 1436–1441 (1987).
   Google Scholar
• Tamaki Y, Araie M, Tomita K, Nagahara M. Effect of topical betaxolol on tissue circulation in the human optic nerve head. J. Ocul. Pharmacol. Ther.15(4), 313–321 (1999).
   PubMed Web of Science ®Google Scholar
• Qian L, Wu HM, Chen SH et al. Beta2-adrenergic receptor activation prevents rodent dopaminergic neurotoxicity by inhibiting microglia via a novel signaling pathway. J. Immunol.186(7), 4443–4454 (2011).
   Google Scholar
• Gleeson LC, Ryan KJ, Griffin EW, Connor TJ, Harkin A. The beta2-adrenoceptor agonist clenbuterol elicits neuroprotective, anti-inflammatory and neurotrophic actions in the kainic acid model of excitotoxicity. Brain Behav. Immun.24(8), 1354–1361 (2010).
   PubMed Web of Science ®Google Scholar
• Aihara M, Lindsey JD, Weinreb RN. Reduction of intraocular pressure in mouse eyes treated with latanoprost. Invest. Ophthalmol. Vis. Sci.43(1), 146–150 (2002).
   PubMed Web of Science ®Google Scholar
• Smid SD. Role of prostaglandins and specific place in therapy of bimatoprost in the treatment of elevated intraocular pressure and ocular hypertension: a closer look at the agonist properties of bimatoprost and the prostamides. Clin. Ophthalmol.3, 663–670 (2009).
   PubMedGoogle Scholar
• Kanamori A, Naka M, Fukuda M, Nakamura M, Negi A. Latanoprost protects rat retinal ganglion cells from apoptosis in vitro and in vivo. Exp. Eye Res.88(3), 535–541 (2009).
   PubMed Web of Science ®Google Scholar
• Drago F, Valzelli S, Emmi I, Marino A, Scalia CC, Marino V. Latanoprost exerts neuroprotective activity in vitro and in vivo. Exp. Eye Res.72(4), 479–486 (2001).
   PubMed Web of Science ®Google Scholar
• Yamagishi R, Aihara M, Araie M. Neuroprotective effects of prostaglandin analogues on retinal ganglion cell death independent of intraocular pressure reduction. Exp. Eye Res.93(3), 265–270 (2011).
   PubMed Web of Science ®Google Scholar
• Zheng J, Feng X, Hou L et al. Latanoprost promotes neurite outgrowth in differentiated RGC-5 cells via the PI3K-Akt-mTOR signaling pathway. Cell. Mol. Neurobiol.31(4), 597–604 (2011).
   PubMed Web of Science ®Google Scholar
• Nakanishi Y, Nakamura M, Mukuno H, Kanamori A, Seigel GM, Negi A. Latanoprost rescues retinal neuro-glial cells from apoptosis by inhibiting caspase-3, which is mediated by p44/p42 mitogen-activated protein kinase. Exp. Eye Res.83(5), 1108–1117 (2006).
   PubMed Web of Science ®Google Scholar
• Boltz A, Schmidl D, Weigert G et al. Effect of latanoprost on choroidal blood flow regulation in healthy subjects. Invest. Ophthalmol. Vis. Sci.52(7), 4410–4415 (2011).
   PubMed Web of Science ®Google Scholar
• Kurashima H, Watabe H, Sato N, Abe S, Ishida N, Yoshitomi T. Effects of prostaglandin F(2alpha) analogues on endothelin-1-induced impairment of rabbit ocular blood flow: comparison among tafluprost, travoprost, and latanoprost. Exp. Eye Res.91(6), 853–859 (2010).
   PubMed Web of Science ®Google Scholar
• Sugrue MF. Pharmacological and ocular hypotensive properties of topical carbonic anhydrase inhibitors. Prog. Retin. Eye Res.19(1), 87–112 (2000).
   PubMed Web of Science ®Google Scholar
• Maren TH. The rates of movement of Na+, Cl-, and HCO-3 from plasma to posterior chamber: effect of acetazolamide and relation to the treatment of glaucoma. Invest. Ophthalmol.15(5), 356–364 (1976).
   PubMedGoogle Scholar
• Musser GL, Rosen S. Localization of carbonic anhydrase activity in the vertebrate retina. Exp. Eye Res.15(1), 105–119 (1973).
   PubMedGoogle Scholar
• Maren TH, Jankowska L, Sanyal G, Edelhauser HF. The transcorneal permeability of sulfonamide carbonic anhydrase inhibitors and their effect on aqueous humor secretion. Exp. Eye Res.36(4), 457–479 (1983).
   PubMed Web of Science ®Google Scholar
• Kniep EM, Roehlecke C, Ozkucur N et al. Inhibition of apoptosis and reduction of intracellular pH decrease in retinal neural cell cultures by a blocker of carbonic anhydrase. Invest. Ophthalmol. Vis. Sci.47(3), 1185–1192 (2006).
   PubMed Web of Science ®Google Scholar
• Okazawa H, Yamauchi H, Sugimoto K, Toyoda H, Kishibe Y, Takahashi M. Effects of acetazolamide on cerebral blood flow, blood volume, and oxygen metabolism: a positron emission tomography study with healthy volunteers. J. Cereb. Blood Flow Metab.21(12), 1472–1479 (2001).
   PubMed Web of Science ®Google Scholar
• Reitsamer HA, Bogner B, Tockner B, Kiel JW. Effects of dorzolamide on choroidal blood flow, ciliary blood flow, and aqueous production in rabbits. Invest. Ophthalmol. Vis. Sci.50(5), 2301–2307 (2009).
   PubMed Web of Science ®Google Scholar
• Torring MS, Holmgaard K, Hessellund A, Aalkjaer C, Bek T. The vasodilating effect of acetazolamide and dorzolamide involves mechanisms other than carbonic anhydrase inhibition. Invest. Ophthalmol. Vis. Sci.50(1), 345–351 (2009).
   PubMed Web of Science ®Google Scholar
• Pedersen DB, Koch Jensen P, la Cour M et al. Carbonic anhydrase inhibition increases retinal oxygen tension and dilates retinal vessels. Graefes Arch. Clin. Exp. Ophthalmol.243(2), 163–168 (2005).
   PubMed Web of Science ®Google Scholar
• Laux BE, Raichle ME. The effect of acetazolamide on cerebral blood flow and oxygen utilization in the rhesus monkey. J. Clin. Invest.62(3), 585–592 (1978).
   Google Scholar
• Martinez A, Sanchez-Salorio M. A comparison of the long-term effects of dorzolamide 2% and brinzolamide 1%, each added to timolol 0.5%, on retrobulbar hemodynamics and intraocular pressure in open-angle glaucoma patients. J. Ocul. Pharmacol. Ther.25(3), 239–248 (2009).
   PubMed Web of Science ®Google Scholar
• Ridderstrale Y, Wistrand PJ. Membrane-associated carbonic anhydrase activity in the brain of CA II-deficient mice. J. Neurocytol.29(4), 263–269 (2000).
   Google Scholar
• Sanchez-Pulgarin M, Martinez-de-la-Casa JM, Garcia-Feijoo J et al. [Comparative study of the pressure lowering efficacy and variations in the ocular pulse amplitude between fixed combinations of dorzolamide/timolol and brinzolamide/timolol.] Arch. Soc. Esp. Oftalmol.86(5), 149–153 (2011).
   PubMedGoogle Scholar
• Harris A, Arend O, Kagemann L, Garrett M, Chung HS, Martin B. Dorzolamide, visual function and ocular hemodynamics in normal-tension glaucoma. J. Ocul. Pharmacol. Ther.15(3), 189–197 (1999).
   PubMed Web of Science ®Google Scholar
• Pillunat LE, Bohm AG, Koller AU, Schmidt KG, Klemm M, Richard G. Effect of topical dorzolamide on optic nerve head blood flow. Graefes Arch. Clin. Exp. Ophthalmol.237(6), 495–500 (1999).
   PubMed Web of Science ®Google Scholar
• Hare WA, WoldeMussie E, Weinreb RN et al. Efficacy and safety of memantine treatment for reduction of changes associated with experimental glaucoma in monkey, II: structural measures. Invest. Ophthalmol. Vis. Sci.45(8), 2640–2651 (2004).
   PubMed Web of Science ®Google Scholar
• Vorwerk CK, Gorla MS, Dreyer EB. An experimental basis for implicating excitotoxicity in glaucomatous optic neuropathy. Surv. Ophthalmol.43(Suppl. 1), S142–S150 (1999).
   PubMed Web of Science ®Google Scholar
• Kemp JA, McKernan RM. NMDA receptor pathways as drug targets. Nat. Neurosci.5(Suppl.), 1039–1042 (2002).
   PubMed Web of Science ®Google Scholar
• Choi DW. Glutamate neurotoxicity and diseases of the nervous system. Neuron1(8), 623–634 (1988).
   PubMed Web of Science ®Google Scholar
• Hahn JS, Aizenman E, Lipton SA. Central mammalian neurons normally resistant to glutamate toxicity are made sensitive by elevated extracellular Ca2+: toxicity is blocked by the N-methyl-D-aspartate antagonist MK-801. Proc. Natl Acad. Sci. USA85(17), 6556–6560 (1988).
   Google Scholar
• Sucher NJ, Aizenman E, Lipton SA. N-methyl-D-aspartate antagonists prevent kainate neurotoxicity in rat retinal ganglion cells in vitro. J. Neurosci.11(4), 966–971 (1991).
   Google Scholar
• Lipton SA. The molecular basis of memantine action in Alzheimer’s disease and other neurologic disorders: low-affinity, uncompetitive antagonism. Curr. Alzheimer Res.2(2), 155–165 (2005).
   PubMedGoogle Scholar
• Krupin T, Liebmann JM, Greenfield DS, Ritch R, Gardiner S. A randomized trial of brimonidine versus timolol in preserving visual function: results from the Low-Pressure Glaucoma Treatment Study. Am. J. Ophthalmol.151(4), 671–681 (2011).
   PubMed Web of Science ®Google Scholar
• Culmsee C, Junker V, Kremers W, Thal S, Plesnila N, Krieglstein J. Combination therapy in ischemic stroke: synergistic neuroprotective effects of memantine and clenbuterol. Stroke35(5), 1197–1202 (2004).
   PubMed Web of Science ®Google Scholar

Secondary neuroprotective effects of hypotensive drugs and potential mechanisms of action

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1. Your patient is a 60-year-old woman with a 3-year history of POAG. She receives a combination of topical medications for her condition.

What should you consider regarding the efficacy of controlling IOP among patients with POAG?

• □ A Progression of glaucoma is rare after initiation of treatment to lower IOP

• □ B Glaucoma progression halts when the IOP is lowered to normal levels

• □ C Lowering IOP can prevent visual field defects

• □ D Lowering IOP cannot reduce the risk for optic disc degeneration

2. Which of the following classes of topical treatments for glaucoma is least likely to be neuroprotective?

• □ A Alpha-2 agonists

• □ B Beta antagonists

• □ C Sympathomimetics

• □ D Carbonic anhydrase inhibitors

3. Which of the following potential neuroprotective mechanisms is correctly matched with its topical medication below?

• □ A Alpha-2 agonists: upregulating expression of brain-derived neurotrophic factor (BDNF), basic fibroblast growth factor (bFGF), and the anti-apoptotic factors Bcl-2 and Bcl-xl

• □ B Beta antagonists: increase retinal blood flow velocity and oxygenation while also reducing the detrimental effects of advanced glycation end products and possibly oxidative stress

• □ C Prostaglandin analogs: decrease glutamate-mediated NMDA receptor activation

• □ D Carbonic anhydrase inhibitors: exhibit a negative feedback mechanism on COX-2 activity

4. What should you consider regarding neuroprotective properties of topical agents for glaucoma as you treat the patient in question 1?

• □ A Beta antagonists have the strongest track record of neuroprotection among humans

• □ B Prostaglandin analogs have the strongest track record of neuroprotection among humans

• □ C There is insufficient human data regarding the neuroprotective qualities of glaucoma medications

• □ D Combination therapy for neuroprotection is best avoided to prevent overlapping mechanisms of action