Neuroprotection in Ophthalmology: A Review 17

Regardless of the cause and nature of neuronal injury, the lesion spreads not only to other parts of the affected cells but also beyond the directly injured neurons to affect neighboring neurons that escaped or were only partially affected by the primary insult. The primary insult, whether mechanical, ischemic, degenerative, or radia- tional, leads to changes in the extracellular envi- ronment, which in turn engender degeneration of adjacent neurons that escaped the primary injury.

The purpose of neuroprotective ther- apy is to minimize this process of secondary degeneration – to reduce the ensuing morpho- logic and functional damage as well as maximize the recovery of a neural system after acute or chronic insult. This feat can be brought about by lessening the noxiousness of the extracellular milieu caused by substances secreted from the primarily injured axons, by preventing the adverse effect of those substances on healthy adjacent neurons, by assisting the latter to resist these effects, by minimizing the spread of dam- age in the injured neurons and encouraging them to regenerate.

The term neuroprotection should only be used to designate interference with processes that occur after the primary neural insult. That is, neuroprotection interferes with processes associated with secondary degeneration, not with the direct effects of the primary insult. For instance, in the case of glaucoma, neuroprotec- tion should not be used to describe the use of ocular hypotensive drugs. Rather, in this con- text, neuroprotection means the use of modali-

ties that, by ameliorating secondary degenera- tion processes in the optic nerve and retinal gan- glion cell (RGC) layer, will arrest or diminish the progress of glaucomatous neuropathy.

Many compounds are involved in the process of secondary neuronal degeneration. Those neu- rotoxic substances include, among others, exci- tatory amino acids (e.g., glutamate), free radicals, nitric oxide, lipid peroxidation products (e.g., polyunsaturated fatty acid), eicosanoids, cations, monoamines (e.g., 5-hydroxytryptamine), and opioids. Thus, modalities that can inhibit the synthesis, release, spread, or interaction of these compounds with neighboring neurons have been investigated as potential neuropro- tective drugs. Hundreds of agents have been tested in animals, but only a few have reached phase III clinical trials and only two have achieved Food and Drug Administration (FDA) approval for clinical use: Riluzole for amyotrophic lateral sclerosis

and memantine for moderate to severe Alzheimer’s disease.

Although many agents have been associated with secondary degeneration, they cause neu- ronal death by only a few common final path- ways. The most prevalent and best-known one is apoptosis. Apoptotic death of RGCs was shown, for example, in animal models of pressure- induced glaucoma and after optic nerve axo- tomy.

When in excessive concentration, the most common retinal neurotransmitter, gluta- mate, induces apoptosis in retinal cells as a mediator of an ischemic insult through hyperac- tivation of the N-methyl-

-aspartate (NMDA)

17

Neuroprotection in Ophthalmology: A Review

Yaniv Barkana and Michael Belkin

237

receptor,

and NMDA has shown a similar effect directly.

Recent studies have provided evi- dence of apoptosis associated with RGC death in human eyes with glaucoma

and ischemic optic neuropathy.

In theory, many ophthalmic diseases can be treated to an extent hitherto impossible if effica- cious clinical neuroprotection becomes avail- able. Diseases of the retina that could be a target for neuroprotective therapy include age-related and other forms of degeneration, diabetic retinopathy, arterial and venous occlusions, retinal trauma including that caused by surgery, retinal ischemia, retinal edema, and damage induced by photocoagulation or accidental laser injuries. Optic nerve diseases that could poten- tially be treated include glaucomatous neuropa- thy, ischemic optic neuropathy, optic nerve trauma and surgery, optic neuritis, and damage from increased intracranial pressure. The neural lesions of these diseases are currently untreat- able and as a consequence they account for a major proportion of the cases of blindness and visual disability in the world. Three diseases, age-related macular degeneration, glaucoma, and diabetic retinopathy cause the vast majority of yet untreatable blindness and visual impair- ment in the industrial world.

In the less developed parts of the globe, although cataract is the main cause of visual function decrement, the retinal diseases listed above account for a very considerable proportion of visual disabil- ity. The worldwide toll on vision by those dis- eases is expected to increase with the aging of the population and the increase in prevalence of diabetes.

This review will briefly summarize the rele- vance and potential applications of neuro- protection in ophthalmology, the preclinical research performed on animal models of eye diseases, and the beginning of research in clin- ical ophthalmology. An attempt will be made to explain the fact that, despite the enormous promise, extensive laboratory and clinical research, and the expenditure of enormous financial resources associated with neuropro- tection as a therapeutic modality, there is almost a complete lack of clinical success of neuroprotective therapies in neurologic and ophthalmic diseases. Finally, a new, immune- based approach to the subject, which holds promise of solving some of the problems that plague the clinical development of neuropro- tection, will be described.

Animal Models for Testing Neuroprotection

Retinal Ischemia

Acute ischemia and reperfusion is the most commonly used model of retinal neuronal injury because it is technically relatively easy (Table 17.1). It is performed by increasing the intraocular pressure (IOP) above systolic blood pressure for a limited, predefined duration, usu- ally 30 to 60 minutes. During the weeks after the insult, neurons, mainly in the inner retina, die, a process which is morphologically quantifiable by histologic assays or functionally by elec- troretinography (ERG).

Ischemia can be also modeled by inducing vascular occlusion, such as ligating ophthalmic or carotid vessels, or producing vascular throm- bosis.

These procedures lead to a prolonged but less precisely controllable retinal ischemia.

Exposure to Endogenous Excitotoxins

Exposing the retina to a measurable high con- centration of a noxious substance is facilitated by the ease of in vitro retinal exposure or injections into the vitreous body. The endogenous excito- toxin most widely investigated is the neurotrans- mitter glutamic acid which, when present in excessive concentrations, is a major cause of cytotoxicity leading to secondary degeneration after brain injury and in brain disorders.

The direct detrimental effect of glutamate on RGCs and other cell types is observed by exposing the retina to high glutamate concentrations in vitro

or in animal models where glutamate is injected intravitreally.

Because the neuro-

Table 17.1.Models of neurotoxicity

Model References

Retinal ischemia 9, 15–20, 27, 54

Retinal exposure to glutamate 24–26, 30 Retinal exposure to NMDA 1, 27–29, 63, 70 Retinal laser injury 38, 42, 43 Acute partial crush lesion of the optic nerve 52, 53

Primate glaucoma model 44–47

Rat glaucoma model 7, 48–50

toxic effect of glutamate is mediated primarily through its NMDA receptors, similar models involve injection of NMDA.

Whereas most of those models attempt to mimic acute pathologi- cal conditions, “chronic” exposure to relatively low-dose intravitreal glutamate for 3 months was described.

In this connection, it is interesting to note that an increase in intraocular glutamate has been reported in the eyes of humans and monkeys with glaucoma

and in the crush- injured optic nerve of the rat.

Retinal Laser Injury

Laser photocoagulation of the retina is used extensively in the treatment of many vision- impairing retinal disorders, such as diabetic retinopathy and age-related macular degenera- tion.

Clinical experience has shown that this treatment is associated with some loss of vision, especially when treatment is applied to sub- foveal or juxtafoveal lesions. Spreading of the destructive effect to adjacent retinal tissue not directly destroyed by the laser beam has been demonstrated.

The latest method of laser treatment of retinal diseases, photodynamic therapy, although pur- portedly limiting the damage to blood vessels, is associated with a considerable number of visual side effects, probably resulting from neuronal damage.

Neuroprotective therapy that preserves the neurosensory retina from the injurious effect would protect the patient’s sight and allow the irradiation to be applied closer to the fovea, thereby enhancing the beneficial effects of laser treatment. Thus, models of laser-induced injury of the rat retina are useful animal models of acute retinal damage, which are accurately quantifiable both with respect to lesion diameter and to the number of cells injured and res- cued.

Furthermore, they mimic precisely the prevalent clinical problem on which they are based.

Animal Models of Glaucoma

There are many glaucoma models in which the optic neuropathy is induced by increasing IOP, although none of them emulate precisely the

pathogenesis and chronic development of pri- mary open-angle glaucoma. The most com- monly used glaucoma model in primates is laser photocoagulation of the trabecular meshwork, resulting in its blockage, failure of aqueous humor outflow, and consequently an increase in IOP and neuropathy.

This model was used in numerous studies of pressure-induced changes in the optic nerve, such as histologic changes in RGC populations, as well as changes in pattern- evoked retinal potentials, blood flow in the optic nerve head, and in the extracellular matrix. This model is also in common use for pharmacologic studies.

Rat models offer clear advantages of avail- ability, cost, and ease of handling without the need for general anesthesia of primates. An increase in IOP in rats’ eyes can be induced by various techniques of blood flow obstruction in the episcleral veins

or laser photocoag- ulation of the trabecular meshwork.

The rat glaucoma model has become even more use- ful since the development of the TonoPen tonometer with its small applanation tip, which greatly facilitates the measurement of IOP in this animal.

Optic Nerve Injury

One of the models of the primary, initial glau- comatous neuropathy lesion

consists of a reproducible, well-calibrated, and partial crush injury of the optic nerve of the adult rat.

Because the initial mechanical neuronal loss is controlled, the secondary loss can easily be quantified. The measurement is performed by retrograde labeling of RGCs after applying a dye distal to the site of the lesion and counting the labeled ganglion cell bodies in the correspon- ding retina. Another method to quantify the neuronal loss is to measure the change in action potential in response to electrical stimulation of the excised nerve or the visual evoked potential response to light. These amplitudes are propor- tional to the numbers of excitable fibers left in the optic nerve.

Levkovitch-Verbin et al.

described a mon-

key model in which the upper third of the optic

nerve was partially transected. Because there is

vertical topographic separation of RGCs in the

primate retina and their corresponding axons in

the optic nerve, the observed loss of RGCs in the

inferior retina was attributed to secondary degeneration.

Compounds Investigated in Neuroprotection Research

NMDA Receptor Antagonists

MK-801 is one of the first neuroprotective com- pounds to be developed. It is a potent noncom- petitive NMDA receptor blocker, shown to be effective in protecting retinal cells in various animal models (Table 17.2).

Solberg et al.,

using a rat model of argon laser irradiation-induced retinal lesion, found significant beneficial effects of MK-801 on the diameter of the retinal lesions and the percent- age of surviving photoreceptor cells in the outer nuclear layer of the retina. The treated rats also showed a pronounced, dose-dependent inhibi- tion of proliferation of the retinal pigment epithelium (RPE), fibrosis, and neovasculariza- tion. Similarly, MK-801 significantly inhibits the death of RGCs in models of ischemia in the rat and cat.

MK-801 is also effective in treat- ing partial crush injury of the rat optic nerve where it triples the survival rate of neurons.

MK-801 also prevents cellular loss

and apopto-

sis

after intravitreal injection of NMDA in rats.

The compound, although quite effective as a neuroprotectant, cannot be used in humans because it was found to have psychotropic side effects in clinical trials.

NMDA receptors are very common on RGC bodies, suggesting that they might be the site for MK-801 and other NMDA antagonist-induced protection of optic neurons from secondary degeneration.

There are other NMDA receptor antagonists that were tested for retinal neuroprotective activity. Dextromethorphan was found to pro- tect the rabbit retina from ischemia

and laser- induced injury.

Flupirtine protected rabbit retina from ischemia,

and rat RGCs from both ischemia and NMDA-induced toxicity.

Riluzole antagonizes glutamate excitotoxicity by inhibiting its release rather than binding to the glutamate receptor. In a rat model of retinal ischemia/reperfusion, riluzole attenuated dam- age, measured by indices of retinal cellular necro- sis and apoptosis and by reduction of ERG a and b waves.

This drug was the first FDA-approved neuroprotective drug. It is used for the treat- ment of amyotrophic lateral sclerosis but is not very effective, prolonging life of the patients by an average of 2 months.

The most currently clinically relevant neuro- protective drug to ophthalmology is memantine, which is in an advanced phase III clinical trial for glaucoma. It is a noncompetitive and low- to moderate-affinity NMDA antagonist, but seems to lack the central nervous system (CNS) side effects associated with other NMDA antago- nists.

Memantine is the only neuroprotective compound approved by the FDA for clinical use in Alzheimer’s disease, as its clinical benefit is clearly similar to those observed with acetyl- cholinesterase inhibitors.

In ophthalmology, memantine was shown to protect rat and rabbit retinal cells after ischemia/reperfusion injury.

It also protected ganglion cells in a model of chronic (3-month) elevation in vitreal glutamate.

Of more clinical relevance is the investigation showing that it was safe and effective in reducing damage in a monkey model of glaucoma as evinced by histo- logic measurements of RGC survival as well as tomographic measurements of nerve head topography without affecting IOP.

Its short- term effectiveness was demonstrated in the same model by conventional and multifocal recordings of the electroretinogram (ERG).

Table 17.2. Neuroprotective agents

Agent References

MK-801 9, 10, 19, 27, 29, 43, 54–56 Dextromethorphan 57, 58

Flupirtine 59, 60

Memantine 6, 30, 49, 62, 64–66, 78, 101

Riluzole 61

Corticosteroids 42, 74–81 Brimonidine 82–85 Betaxolol 15, 16, 86, 87 Flunarizine 54, 89, 90 Brain-derived 104–107, 110

neurotrophic factor Pigment epithelium- 50

derived factor Aminoguanidine 98 Minocycline 99–103

Autoimmune T cells 15, 21, 39, 41, 106, 111, 112, 117–120, 128 Copaxone 121, 123–126

There was no effect on the visually evoked cor- tical potential.

Memantine was effective in clinical trials of a variety of dementias, mainly Alzheimer’s disease, without causing the toxic side effects of MK-801 and earlier NMDA antag- onists.

It is also somewhat effective in Parkinson’s disease.

However, it is not effective in the treatment of other neural diseases such as in ameliorating the symptoms of patients with phantom pains after upper limb amputation.

Prednisolone

Prednisolone is a potent corticosteroid that exerts a variety of physiological and cellular reactions, such as inhibition of arachidonic acid metabolism, stabilization of lysosomal mem- branes, modification of edema formation, and antioxidant activity. High-dose methylpred- nisolone therapy is effective in the treatment of CNS injury in different animal models

and in controlled clinical trials for spinal cord trauma for which high doses are the standard treatment in the acute stage.

Treatment with corticosteroids has been inves- tigated in animal models of laser injury to the eye, with mixed results. Lam et al.

reported a benefi- cial effect of treatment with methylprednisolone administered as an intravenous bolus of 30 mg/kg and a maintenance dose of 5.4 mg/kg per hour in laser injuries of nonhuman primate retina.

Morphologically, the treated retina showed rapid reestablishment of retinal and choroidal vascula- ture, proliferation, and organization of the RPE and reformation of the outer limiting membrane, less macrophage activity, and reduced photore- ceptor damage at the periphery of the lesion.

After a single retinal argon laser lesion, Naveh and Weissman

found that vitreal accumulation of protein and prostaglandin E

was reduced in rab- bits treated daily with intramuscular dexametha- sone (0.5 mg/kg weight). Wilson et al.

showed that breakdown of the blood–retinal barrier after panretinal photocoagulation was reduced by intravitreal injection of triamcinolone. Ishibashi et al.

reported that continuous intravitreal infu- sion of steroids inhibits subretinal neovascular- ization after laser induction of retinal lesions in monkeys.

However, there are compelling doubts regard- ing the efficacy of corticosteroids as neuropro- tective drugs for minimizing visual loss in

laser-induced retinal injuries. Rosner et al.

reported that systemic corticosteroids had only a short-term effect on argon-laser-induced lesions in the retinas of pigmented rats. Corticosteroids were shown to promote sparing of neurons after mechanical injury but to interfere with regen- eration of neural tissue, for which inflamma- tion was necessary.

Marshall

reported that steroids slowed the regeneration of the outer blood–retinal barrier by RPE cells and suggested that steroids not be used to treat retinal laser burns until more data become available.

Schuschereba et al.

reported a deleterious effect of mega doses of intravenous steroids on the prognosis of argon-laser-induced retinal injuries in rabbits. On the whole, it seems that corticosteroids are not an effective neuroprotec- tant in ophthalmic diseases.

Adrenergic Agonists and Antagonists

The α

-adrenergic agonist brimonidine, a com- mercially available topical ocular hypotensive glaucoma drug, has been associated with a neu- roprotective effect on RGCs. Yoles et al.

found that a single injection of brimonidine (and other α

-adrenergic agonists) resulted in a signifi- cantly smaller loss of RGCs and lesser reduction in action potential amplitude after partial crush injury of the rat optic nerve. Timolol, a β-recep- tor blocker that similarly lowers IOP, had no such protective effect in that model.

Donello et al.

showed in a rat model of acute retinal ischemia that pretreatment with brimonidine has a marked protective role on RGC loss, as evaluated by histology, cell labeling, and ERG.

Furthermore, brimonidine treatment prevented an increase in the vitreous body of glutamate and aspartate, indicating interference with the excitotoxic mechanism of secondary degenera- tion. However, it seems likely that it exerts its neuroprotective effect by another mechanism, not demonstrated for other ophthalmic neuro- protective compounds. Gao et al.

showed that brimonidine promotes the production of the growth factor brain-derived neurotrophic factor (BDNF) in RGCs, a finding that may account for its significant neuroprotective efficacy.

The neuroprotective capabilities of this drug

have been further demonstrated by improv-

ing, in an IOP independent manner, the contrast

sensitivity of glaucoma patients.

The most

compelling evidence for the effects of topical brimonidine in reducing the rate of ganglion cell loss in severe glaucoma was reported by Gandolfi et al. in the 2004 meeting of the Association for Research in Vision and Ophthalmology. He studied 52 high-risk eyes on two antihypertensive drugs in a masked man- ner. All eyes exhibited progressive visual field loss during the 18 months preceding the trial.

The subjects were randomly assigned to treat- ment with topical brimonidine or to undergo laser trabeculoplasty without changing the hypertensive treatment. Eighteen further months later, the brimonidine-treated eyes showed significantly less visual field loss than the laser-treated eyes although the extent of IOP reduction was lower in the former eyes.

Adrenergic antagonists have also shown neuroprotective capability in animal experi- ments. Osborne et al.

reported that betaxolol, a β

-adrenergic receptor blocker, protects rat RGCs from ischemia/reperfusion insult when injected intraperitoneally or intravitreally and that topical use of betaxolol significantly reduces the death of RGCs after ischemia/

reperfusion in rabbit eyes and after intravitreal injection of NMDA into the rat eye.

The neu- roprotective effect of betaxolol was attributed to the drug’s action as a calcium channel blocker, reducing excessive influx of calcium into stressed cells, and was thought to be unre- lated to its β

receptor-mediated reduction of IOP.

Betaxolol has also been shown by others to have a neuroprotective effect when applied top- ically before or after an ischemic insult.

Calcium Channel Blockers

Calcium ions, when entering a cell through either voltage-sensitive or receptor-operated channels have a cardinal role in the pathophysi- ology of apoptosis leading to tissue destruction after CNS insult of all types. The increase in intracellular calcium exerts a neurotoxic effect through the activation of Ca

-dependent cata- bolic enzymes, leading to lethal alteration of the cell’s metabolism.

It was therefore thought that blocking cal- cium ions egress into cells would prevent their death, providing neuroprotection. Indeed, flu- narizine, a potent antagonist of the neuronal

voltage-dependent calcium channel, when administered daily to rats after unilateral axo- tomy of the optic nerve, was found to somewhat enhance RGC survival which was measured 14 days after the injury.

Both flunarizine and another calcium blocking drug, lomerizine, have a protective effect after induction of retinal ischemia in rats by temporary increase of the IOP.

The beneficial effect of treatment with Ca

channel blockers was shown in one moderate- term clinical trial of patients with normal-ten- sion glaucoma.

The relevance of the results of this trial is still moot. Even if further clini- cal evidence will accumulate for calcium chan- nel blockers having beneficial effects on glaucoma, it is still uncertain whether the observed effects are neuroprotective in nature or operate via a different mechanism such as vasodilation.

Nitric Oxide Synthase Inhibitors

Nitric oxide has multiple physiological roles in the normal metabolism of various tissues, including neural ones. As is the case with excito- toxic amino acids, its presence in high concen- trations is noxious to cells. Consequently, many research projects were devoted to understand- ing the cellular origins and synthetic mecha- nisms of the compound and its function as a neurotoxin in animal models of human neural degenerative diseases, including glaucoma.

Shareef et al.

demonstrated that, in the rat model of gradual chronic IOP elevation, iso- forms of the enzyme nitric oxide synthase are induced in optic nerve astrocytes, such as nitric oxide synthase 2. The induction of this enzyme is in turn predicated on activation of epidermal growth factor receptor-induced nitric oxide syn- thase-2 in the astrocytes.

These authors subsequently showed that aminoguanidine, a relatively specific inhibitor of this isoform, reduces the loss of RGCs in this model.

Minocycline

This second-generation tetracycline antibiotic is

an inhibitor of caspase-3, an essential enzyme in

the apoptosis pathway, and thus is anti-apoptotic

in neural and non-neural cells. Quite a few prom- ising preliminary studies showed this drug to be effective as a neuroprotectant in neurode- generative diseases animal models. The oph- thalmic models included light-induced retinal degeneration,

retinal cell culture,

and reti- nal degeneration.

However, some negative results using minocycline were obtained in animal testing as well as serious side effects,

casting doubt on the future of minocycline as a neuroprotectant drug.

Neurotrophic Factors

BDNF, a member of the nerve growth factor family of proteins, is highly effective in reducing the rate of RGC death after optic nerve axotomy in rats

and optic nerve crush in cats.

These findings may be particularly pertinent to the therapy of glaucoma, where death of RGCs may be partly attributed to a decrease in axonal transport of trophic factors.

Experiments with other growth factors also indicate that the growth factors are potent agents for providing neuroprotection and inducing regeneration in neural tissues after insults. Intravitreal injection of adenovirus vec- tors containing pigment epithelium-derived growth factor 4 days before inducing retinal ischemia was found to preserve inner retinal thickness and cell density in the ganglion cell layer, and to decrease cell apoptosis.

The problem associated with the clinical use of BDNF and any other protein with neuropro- tective and regeneration enhancement property in neurologic and ophthalmic diseases is the inability of large molecules to penetrate the blood–brain barrier or the blood–retina barrier in sufficient quantities to exert significant clini- cal impact. Those compounds, therefore, have to be delivered intravitreally or intrathecally.

Indeed, in the clinical trials in using BDNF, e.g., for amyotrophic lateral sclerosis, the compound is injected intrathecally.

Apoptosis Prevention

Another possible approach to neuroprotection is interference with the final common pathway of apoptosis, or programmed cell death, shown

to occur in RGCs under a variety of conditions, both naturally occurring and experimental.

As the genetic and molecular mechanisms of apoptosis are gradually elucidated, new targets for neuroprotection appear. Current research efforts are aimed at up-regulating anti-apoptotic proteins such as BCLX, and inhibiting the pro- moters of apoptosis such as BAX and the execu- tors of apoptosis, the caspase proteases.

Immune-Based Neuroprotection

A very promising, experimentally advanced technique for providing neuroprotection to injured neural tissues utilizes the body’s immune system to supply the necessary neuro- protective compounds, at the proper regimen, to the site of the primary insult. This potential therapy is based on the novel insights gained and promulgated by Schwartz

on the nature of autoimmunity. Autoimmunity in general, and in the CNS in particular, has long been considered to be harmful and this was considered the rea- son for the CNS “immune free” status. Schwartz has shown that the adaptive immune response displayed by regulatory T cells directed against self-antigens (“autoimmune” T cells) is essential for body homeostasis in the eye and other parts of the CNS.

The autoimmune system is acti- vated after CNS trauma

and can be therapeu- tically manipulated to provide more effective neuroprotection than it affords without such medical maneuvers.

This immune mechanism was demonstrated

to be effective in preventing secondary degener-

ation and promoting significant recovery from

axonal injuries more than other neuroprotective

modalities.

It was first demonstrated in

rodents who underwent optic nerve crush injury

and were injected systemically with T cells that

recognize CNS self-antigens such as myelin

basic protein. The resulting protection of RGCs

was demonstrated by morphologic and electro-

physiological means.

The neuroprotection

afforded by the T cells was shown not only by

passive T cell transfer, but also by active vacci-

nation with the relevant antigens.

Of even

greater immediate clinical relevance is the pro-

nounced neuroprotection achieved using

Copaxone, a drug that prevents the development

of experimental autoimmune encephalomyelitis

in rodents and is clinically used to ameliorate

multiple sclerosis in humans

as a vaccine that

preferentially directed T cells unto the lesion site.

Protection of RGCs by vaccination with Copaxone was also observed in rats into whose vitreous a toxic dose of glutamate was injected.

From the clinical point of view, an even more directly pertinent finding was that in a rat model of glaucoma the RGCs were saved from destruc- tion by Copaxone vaccination, even if the IOP was not reduced,

giving a prospect of develop- ing vaccination against glaucoma,

a treat- ment for other optic nerve diseases and injuries,

and other retinal diseases.

The possible mechanism whereby the system functions when activated at the lesion site, is by local secretion of neurotrophic factors and other agents which lessen the secondary degeneration and promote healing and regeneration. This secretion is effected either directly or through interaction with microglia.

The advantage of the neuroprotective immunity is in its ability to circumvent the blood–brain barrier and provide large molecules such as growth factors in the lesion site.

Neuroprotective vaccination has another major advantage over other neuropro- tective modalities, each of which addresses one secondary degeneration agent of the multiple compounds involved in the process of second- ary degeneration. In contradistinction to those, the T cells at the lesion site presumably secrete the right mix of factors according to their proper timing, dosage, and duration.

Neuroprotective Therapy – A Thumbnail History and State of Art

A therapeutic idea that would have been called today neuroprotection was put forward by Dr.

Bernard Becker in the early 1970s who suggested using the antiepileptic drug phenytoin for glau- coma control (personal communication by Paul Palmberg, 2004). The concept underlying neuro- protection, secondary degeneration, was described by Astrup in 1979 who described an area which he later termed the ischemic penum- bra around the primary infarct in stroke.

In the same year, barbiturates were tried to amelio- rate the functional effects of stroke, with the same idea in mind.

The first ophthalmic experiments were performed by Tso who tried the use of ascorbate to treat a model of a photic injury of the retina.

The term neuroprotection

was first used in 1987 to describe the effect of MK-801 on brain ischemia, and the first rea- soned suggestion for the use of the technique to moderate glaucomatous neuropathy was made by Schwartz in 1996.

During all this period, to the present day, there has been a continuous effort to translate the vast scientific knowledge collected over recent decades on the subject of neuroprotec- tion into clinical therapeutic reality. As a part of the research aimed at developing neuroprotec- tive drugs for neural diseases, hundreds of com- pounds have been subjected to preclinical testing, scores have reached clinical trials, and many have made it to phase III testing. Yet, only two drugs of this class have been approved by the FDA, namely, riluzole for amyotrophic lat- eral sclerosis, and memantine for moderate to severe Alzheimer’s disease. Both drugs, espe- cially the former, are of marginal clinical value and have not manifested the great hopes associ- ated with neuroprotection.

This meager clinical return for enormous

decades-long research efforts is attributable to

many factors. The main one is probably the fact

that the secondary degeneration of neurons is

mediated by many noxious agents. Thus, miti-

gating the harmful effects of one of them is

insufficient to produce meaningful clinical

effects. Furthermore, all of the agents that

induce secondary degeneration are compounds

intimately involved with the normal functioning

of the nervous system. Their degeneration-

inducing effects are attributable to their pres-

ence at abnormally high concentrations in the

injury site. The primary example of this problem

is glutamate, which is a major neurotransmitter

in the retina and elsewhere. It becomes toxic as

a result of its release at abnormally high concen-

trations from the primarily injured neurons into

the extracellular environment. Blocking the

effects of glutamate and the other secondary

degeneration agents too intensively is liable to

produce severe side effects, as was the case with

MK-801. Furthermore, to achieve clinically rele-

vant neuroprotection, the activities of some of

the agents of secondary degeneration would

have to be blocked to the right extent and with

the appropriate timing. Finding the proper com-

bination of drugs, and ensuring that they all act

according to the proper sequence, concentra-

tion, and timing is impossible by present-day

medical practice. Other reasons for the almost

uniform failure of neuroprotective therapy as a

medical modality are associated with the differ- ences between preclinical testing and medical practice. The animal models used are mostly acute and do not represent the human disease accurately. The animals tested do not ade- quately represent the patient population, the former being of uniform strain, age, sex, and extent of lesion. For acute diseases, there is the problem of the “window of opportunity,” the time interval after the onset of the disease dur- ing which the process of secondary degenera- tion can be arrested or decelerated. The animals are always treated within this window, whereas most patients do not seek treatment nor are diagnosed within this time period. The clinical trial population differs considerably from the animal population in being young and healthy in phase I, too few in phase II, and old, heterogeneous and with comorbidities in phase III.

Despite all the difficulties and disappointing track record, the search for new means of pro- viding neuroprotection to neural diseases and injuries is continuing, because the concept holds a promise to reduce morbidity and mor- tality from some of humanity’s most devastat- ing diseases. It seems, however, that new approaches to the problem are required and interfering with a single secondary degenera- tion agent will not be useful as a clinical modal- ity. Methods of providing generalized, timely amelioration of the secondary degeneration and of promoting regeneration, as described above, will possibly provide effective neuro- protection in the not too distant future.

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