can be extremely misleading. The treatment effect could be 40% higher with the new treatment, but if the number of patients is small and the variability of response high, this difference may not be statistically significant, even though the effect could clearly be clinically meaningful. In analyzing study results, look at the actual data, and do not be misled by an insignificant P value, especially if the study is inadequately powered to detect a clinically meaningful difference. Remember that clinical studies can never definitively prove that two treatments have the same effect.

Clinical trials of neuroprotection

Clinical trials in neuroprotection pose unique challenges. First, the neurological diseases studied to date have been difficult to treat. This occurs either because the disorders encompass an acute and often devastating neurological insult, such as stroke or acute traumatic brain injury, where therapeutic interventions are administered too late, or because the disorders progress at such a slow rate that a therapy-induced delay in progression is difficult to demonstrate. In the case of a sudden, devastating neurologic insult, it is important to have a study where the disease is quickly diagnosed and the study allows for prompt institution of therapy. This is often difficult since enrollment in a

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clinical trial requires full explanation of the study prior to obtaining informed consent from the patient. Oftentimes, these patients have impaired cognitive function, and obtaining informed consent from the patient is not possible. Slowly progressing neurological diseases such as ALS or Parkinson’s disease offer another challenge. To show a delay in the progression of these diseases, following large numbers of patients for a long period of time is required. Furthermore, the neurologic deterioration caused by the diseases is usually highly variable; therefore, clinical trial results are affected by the time of day that endpoints are assessed. In contrast to therapies that prevent or slow progression, it is much easier to demonstrate the efficacy of therapies that improve function or cure a disease. This is illustrated in Fig. 1.

A second challenge in clinical trials of neuroprotection is the choice of valid and clinically relevant endpoints. To date, it has not been possible to measure the exact number of functioning neurons in patients with neurodegenerative diseases. The use of biomarkers can be important for the initial assessment of therapies for neurologic diseases. These biomarkers include diagnostic assessments, such as laboratory tests or imaging studies, which can predict a clinically relevant outcome. Not only are there few predictive biomarkers for neurodegenerative diseases, there is a dearth of validated and

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objective clinical endpoints. Many trials of neuroprotection have used the assessment of cognitive function as the primary efficacy measure. Until recently, well-validated measures of neurological function were not widely available. Nevertheless, these validated measures of cognitive function remain subjective and variable. Objective endpoints, such as death or the requirement for mechanical ventilation, have been used as the primary measure of some of the severe neurodegenerative diseases.

Just because a therapy has a positive impact on a neurodegenerative disease does not mean that it is neuroprotective. Drugs or surgical interventions can provide symptomatic relief and lead to improved clinical outcomes. L-Dopa for Parkinson’s disease provides symptomatic improvement but is not considered neuroprotective.

Importantly, many neurodegenerative diseases are slowly progressive. Clinically relevant declines in function may take years for many neurodegenerative diseases like ALS or glaucoma. As a result, drugs that lead to symptomatic improvement often take less time and fewer patients to be adequately powered to show statistically significant change in a clinically relevant primary endpoint. Trials of drugs that lead to symptomatic improvement can show effects in weeks to months (Fig. 1). Neuroprotective drugs often need to be administered for months to years before clinical benefit is definitively demonstrated. This is especially true if the natural history of the disease is characterized by exacerbations and remissions as part of a slow overall diminution of function (Fig. 1).

To address this issue of progression, clinical proof of neuroprotection requires exploratory studies showing an impact on biomarkers of neuronal function. In addition, clinical study designs where the slope of deterioration is assessed can help prove that an intervention is truly neuroprotective (Bodick et al., 1997; Clarke, 2004). However, study designs that better distinguish actual neuroprotection from a symptomatic improvement often require thousands of patients followed for a number of years. These trial designs are expensive and time consuming and partially explain why few therapies have been proven to be neuroprotective.

As a result, it is not surprising that there are very few randomized clinical trials of neuroprotective medications in medicine. One such study, the Glycine Antagonist in Neuroprotection (GAIN) trial, was a randomized, double-masked, placebocontrolled trial conducted to examine the efficacy of gavestinel, an antagonist of the glycine site of the N-methyl-D-aspartate (NMDA) receptor, as a neuroprotective therapy for acute ischemic stroke (Sacco et al., 2001). The main outcome measure was functional capability at 3 months, measured by the Barthel Index. This study concluded that gavestinel administered up to 6 h after an acute ischemic stroke did not improve functional outcome at 3 months. This study illustrated the difficulties of treating acute conditions soon after a neurological insult. Even treatment within 6 h after an acute ischemic stroke may not be soon enough to impact the disease.

Two neuroprotective therapies have been approved by the US FDA: riluzole for ALS and memantine for Alzheimer’s disease. Both drugs are thought to prevent glutamate-mediated excitotoxicity. Glutamate is the major excitatory neurotransmitter in the brain (Fonnum, 1984). Excess glutamate at the synapse can lead to excitotoxicity and subsequent neuronal death.

ALS is a progressive and fatal neurodegenerative disease (Charcot, 1877) caused by degeneration of motor system neurons (Mitchell and Borasio, 2007). Although the exact cause of the selective degeneration is unknown, glutamate toxicity at the synapse leads to neuronal death (Plaitakis and Caroscio, 1987; Rothstein et al., 1992). Riluzole [2-amino-6-(trifluoromethoxy)ben- zothiazole] is thought to block glutamate-mediated neurotoxicity (Mizoule et al., 1985). In a randomized controlled trial in patients with ALS, the antiglutamate agent riluzole appeared to slow the progression of disease. After 12 months, survival was 74% in patients receiving riluzole compared to 58% in patients receiving placebo (P ¼ 0.014) (Fig. 2) (Bensimon et al., 1994).

Alzheimer’s disease is also a progressive neurologic disease leading to impaired cognition and function. Glutamate stimulates a number of receptors including the NMDA receptor. The NMDA receptor is thought to be involved in the

pathogenesis of Alzheimer’s disease (Chohan and Iqbal, 2006; Palla`s and Camins, 2006). Recent data suggest that blocking glutamate excitotoxicity at the NMDA receptor can be useful in the treatment of Alzheimer’s disease (Cosman et al., 2007).

Memantine, an uncompetitive NMDA-receptor antagonist, has been approved by the US FDA for moderate-to-severe Alzheimer’s disease. In one of the pivotal phase 3 clinical trials, the primary efficacy variables were the Clinician’s InterviewBased Impression of Change Plus Caregiver Input (CIBIC-Plus) and the Alzheimer’s Disease Cooperative Study Activities of Daily Living Inventory modified for severe dementia (ADCS-ADLsev). In this trial, patients receiving memantine had a significantly better outcome for both endpoints than those receiving placebo (Fig. 3) (Reisberg

et al., 2003). In a second phase 3 trial, memantine was compared with placebo in patients already receiving a cholinesterase inhibitor. The primary outcome measures in this trial were change from baseline on the Severe Impairment Battery (SIB), a measure of cognition, and on a modified 19-item Alzheimer’s Disease Cooperative Study Activities of Daily Living Inventory (ADCS-ADL19). Patients receiving memantine in this trial had significantly better outcomes at 24 weeks than patients receiving placebo (Tariot et al., 2004).

Clinical trials of neuroprotection in ophthalmology

Proof of neuroprotection in ophthalmology will require a well-designed clinical trial employing

many of the strategies detailed above. The most definitive evidence will require a randomized clinical trial comparing the potential neuroprotective treatment to a control treatment or placebo. Unfortunately, there are few controlled clinical trials of neuroprotection in ophthalmology. One example is the Ischemic Optic Neuropathy Decompression Trial (IONDT) (The Ischemic Optic Neuropathy Decompression Trial Research Group, 1997). This was an National Eye Institute–spon- sored multicenter clinical trial designed to assess the safety and efficacy of optic nerve decompression surgery compared with careful follow-up alone in patients with NAION. The IONDT illustrates the need for a well-controlled clinical study to adequately assess neuroprotective therapy, since the natural history of many degenerative diseases has not been well documented. Prior to this trial, improvement in visual acuity in patients with NAION was thought to be rare (less than 10%) (Boghen and Glaser, 1975; Repka et al., 1983), and several nonrandomized trials showed benefits of optic nerve decompression; however, none of these

were randomized studies (Sergott et al., 1989; Kelman and Elman, 1991; Spoor et al., 1991, 1993). Interestingly, results from the IONDT showed that patients assigned to surgery did no better than patients assigned to careful follow-up (The Ischemic Optic Neuropathy Decompression Trial Research Group, 1997). Improved visual acuity of three or more lines was achieved by 23.6% of the surgery group compared with 42.7% of the careful follow-up group. In fact, patients receiving surgery had a significantly greater risk of losing three or more lines of vision at 6 months: 23.9% in the surgery group worsened compared with 12.4% in the careful follow-up group. Optic nerve decompression surgery was found to be ineffective and potentially harmful to patients with this disease.

Endpoints

A critical factor in designing clinical trials for neuroprotection in ophthalmology is endpoint

selection. The primary outcome measures in the IONDT were gain or loss of three or more lines of visual acuity on the New York Lighthouse chart at 6 months after randomization (The Ischemic Optic Neuropathy Decompression Trial Research Group, 1997). This is a functional endpoint that has been used in a number of ophthalmology trials and is felt to be clinically meaningful. Visual acuity has also been used as a standard clinical endpoint for clinical trials in macular degeneration, where central acuity can be affected early in the course of the disease. Unfortunately, changes in central acuity may be an insensitive endpoint for many ophthalmic diseases. For example, central visual acuity loss occurs relatively late in the course of some diseases, including glaucoma and retinitis pigmentosa. Although visual field loss can be used as a functional endpoint, it occurs slowly and may require many years before meaningful changes occur.

Identification and validation of surrogate endpoints will improve our ability to assess neuroprotective therapies. In a sense, IOP has been used as a surrogate endpoint for glaucoma treatments. Without well-controlled data, lowering IOP was felt to reduce the risk of vision loss in patients with glaucoma. Data from randomized clinical trials that support the benefits of lowering IOP are now becoming available. In a recent report from the Advanced Glaucoma Intervention Study (AGIS), investigators showed that eyes with 100% of visits with IOP less than 18 mmHg over 6 years had mean changes from baseline in visual field defect scores close to zero during follow-up, whereas eyes with less than 50% of visits with IOP less than 18 mmHg had an estimated worsening over followup of 0.63 units of visual field defect score (The AGIS Investigators, 2000).

Studies have documented that rates of change differ among measures of visual function. Depending on the disease, endpoints may change at differing rates and have a large impact on length of clinical trials and the required sample size. In a trial of patients with retinitis pigmentosa, investigators assessed changes in measures of visual function in patients over time (Holopigian et al., 1996). The smallest amount of change occurred for visual acuity and hue discrimination, and the

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greatest amount of change occurred for visual field area.

New methods for assessing visual function may be useful for measuring endpoints in clinical trials for neuroprotection. Electrophysiological testing, newer methods of visual field testing including Frequency Doubling Technology (FDT), Short Wavelength Automated Perimetry (SWAP), and contrast sensitivity are just a few examples. The electroretinogram was used as the primary outcome measure in a randomized clinical trial of vitamin A and vitamin E supplementation for retinitis pigmentosa (Berson et al., 1993). This was a National Eye Institute–sponsored, randomized, double-masked trial to determine whether supplements of vitamin A or vitamin E alone or in combination affect the course of retinitis pigmentosa. In this study, the main outcome measure was the cone electroretinogram amplitude. Patients receiving 15,000 IU/day of vitamin A were 32% less likely to have a decline in amplitude of 50% or more from baseline than those not receiving this dosage (P ¼ 0.03). Although not statistically significant, similar trends were observed for rates of decline of visual field area. These data support the potential benefit of identifying endpoints that may be more sensitive or less variable in clinical trials.

More accurate and sensitive measures of visual function will improve our ability to test neuroprotective therapies in ophthalmology. The scanning laser ophthalmoscope to assess perimetry (Sunness et al., 1995) or the multifocal electroretinogram (Hood et al., 1997) may be useful to evaluate endpoints in trials of retinal disease. Similarly, new methods of assessing glaucomatous damage such as the Heidelberg retinal tomograph (HRT), the GDx nerve fiber analyzer, and the optical coherence tomograph (OCT) may be important measures of visual function in neuroprotection trials in patients with glaucoma (Wollstein et al., 2000; Bowd et al., 2001; Miglior et al., 2001; Zangwill et al., 2001).

Measurements of visual field have become standard ways to assess the functional progression of glaucoma. There have been a number of recent developments in automated perimetry that have led to improved diagnosis and assessment of disease progression in glaucoma (Johnson, 2002).