**5. Difficulties in designing a clinical trial**

**Figure 4.** Neurotrophin Signaling Summary. (courtesy of Dr A. Gravanis). Neurotrophins include the first to be discov‐ ered: Nerve Growth Factor (NGF) as well as Brain Derived Neurotrophic Factor (BDNF) and Neurotrophin 3 and 4 (NT-3 and NT-4). There are two classes of receptors for neurotrophins: p75 and the "Trk" family of Tyrosine kinase receptors. P75 can bind all factors, whereas Trk receptors are specific for their ligands. Binding of neurotrophins to their recep‐

Tumor necrosis factor-alpha (TNF-α) can lead to death of retinal ganglion cells by inducing mitochondrial damage and activation of caspases. [114] Inhibition of TNF-α via the use of etanercept [115] or an anti-TNF-α neutralizing antibody [116] can protect retinal ganglion

Providing free radical scavengers, such as coenzyme Q10 [117, 118] or thioredoxin [119], or in‐ hibiting the formation of free radicals by blocking the action of nitric oxide synthase [120] has shown promise as an alternative neuroprotective strategy. Inflammatory and immune mecha‐ nisms also play a role in glaucomatous damage of retinal ganglion cells. [121] Immunomodula‐ tion [122], inhibition of calcineurin by tacrolimus [123], and subcutaneous injection of granulocyte-colony stimulating factor [124] also have neuroprotective potential in glaucoma.

Other mechanisms that have been investigated in the lab with encouraging results include the use of mesenchymal stem cells, which are thought to exert their effect through produc‐ tion of neurotrophins or stimulation of inflammation [125, 126]. A recent study has shown that amyloid β (Aβ) is found to be elevated in an animal model of ocular hypertension in‐ duced glaucoma and that inhibition of the generation of amyloid β led to preservation of

tors leads to activation of many pro-survival signals including PI3K, Akt, and MAPK among others.

cells in mouse models of glaucoma.

186 Glaucoma - Basic and Clinical Aspects

Over the last 30 years, numerous pharmacologic agents and gene therapeutic approaches have been shown to be neuroprotective in animal models of retinal and optic nerve injury. To date, none of the trials on neuroprotection of the visual system have shown efficacy and none of the agents developed in the laboratory have translated into a definitive clinical treat‐ ment. [131] There are several reasons for the universal failure of clinical trials to confirm preclinical results; they stem from the nature of glaucoma itself and from the poor design of previous neuroprotective trials. [51, 131] First, the long, slow course of glaucomatous optic neuropathy hinders our efforts to measure whether an improvement in progressive worsen‐ ing has been achieved and asks for a design of therapeutic trials that last several years. Sec‐ ond, the rate of worsening varies among patients and thus a larger sample size is required to account for this inherent variability in disease progression. Third, the current standard of outcome measurement remains visual field testing and this carries a significant test-retest variance even among patients who are adept in taking the test. Fourth, any neuroprotection clinical trial would have to include patients that are already on topical medications that low‐ er IOP. IOP-lowering agents have proven effective in slowing glaucoma progression in sev‐ eral controlled clinical trials [8-10] and it would be unethical to preclude neuroprotection study patients from using IOP-lowering medications.

#### **6. Failed and ongoing trials**

Success in the lab has not paralleled success in neuroprotection clinical trials. A Cochrane review in 2010 failed to identify any neuroprotection studies with significant results. [132] The largest neuroprotection study to date consisted of two industry-supported, parallel, randomized, phase III clinical trials on oral memantine in patients with chronic progressive open angle glaucoma. A total of 2,200 patients were enrolled into the trials at 89 sites and they were followed for at least 4 years. Despite the success of memantine in animal models of glaucoma, both clinical trials failed to show efficacy with respect to their primary out‐ come measures. The results of these two trials are as of yet unpublished and only two press releases hinted on their results. [133] The first release stated, "Two measures of visual func‐ tion were selected in the statistical analysis plan to assess the efficacy of memantine in glau‐ coma. The functional measure chosen as the endpoint (glaucomatous field progression) did not show a benefit of memantine in preserving visual function. In a number of analyses us‐ ing the secondary functional measure, memantine demonstrated a statistically significant benefit of the high-dose compared to placebo." The second release stated, "Allergan un‐ masked the second Phase III clinical trial examining the safety and efficacy of oral meman‐ tine as a treatment for glaucoma. Although the study showed that the progression of disease was significantly lower in patients receiving the higher dose of memantine compared to pa‐ tients receiving the low dose of memantine, there was no significant benefit compared to pa‐ tients receiving placebo. Therefore, the study failed to meet its primary endpoint and to sufficiently replicate the results of the first Phase III trial." In going forward, knowing the specifics of these trials and the reasons for failure would facilitate a more well-thought de‐ sign of future trials; yet, the results remain unpublished.

phic factor (CNTF) implant in patients with chronic progressive open angle glaucoma (trial

Neuroprotection in Glaucoma http://dx.doi.org/10.5772/54294 189

Designing neuroprotection trials in glaucoma is challenging. Modifications in study de‐ sign, patient selection, and outcome measures can aid in the clinical testing of neuropro‐ tective agents with positive pre-clinical results. [51] Instead of the standard randomized controlled clinical trial prototype, neuroprotection trials in glaucoma may be served better by using a so-called futility design strategy. Detection of beneficial agents with robust treatment effects in a short period of time in a single treatment group (i.e. there is no need for a control group) are advantages of futility design. Clearly, a major disadvantage of this approach is the inability to adequately assess for side effects and time-dependent treatment effects in a trial that has fewer patients and a shorter testing time frame. [138, 139] In addition, selection of patients whose disease is rapidly can maximize the opportu‐ nity to detect differences after the use of neuroprotective agents. Older age, higher base‐ line intraocular pressure, bilateral disease, low perfusion pressure, presence of exfoliation, disc hemorrhages, and thinner central corneal thickness are all risk factors for rapid pro‐ gression and should be used in the selection of the study population in neuroprotection trials. [51, 140] The agent in question also should reach its target tissue(s), the retina and optic nerve head. Steps should be taken to ensure patient adherence with medication ad‐ ministration or the results of any neuroprotective trial that is performed on a background of co-administered IOP-lowering therapy are deemed to be confounded. [141]. If the IOP is reduced to an identical degree, while one agent (like brimonidine or other neuroporo‐ tectant) shows fewer injuries to the visual fields, this would indirectly support the addi‐

tional neuroprotective effect of the agent on top of its IOP lowering effects.

In terms of endpoints for such trials, visual field testing is likely more suitable than struc‐ tural measures since it has been employed extensively to measure progression of disease. Its disadvantages of high variability in some test point areas, the patient effort it requires, and the insensitivity to show the earliest stages of damage are well known. Nevertheless, it has been well established as a method to assess glaucoma progression. Given the lack of reliable software to measure progression using structural tests, such as the Heidelberg Retinal Tomograph or Optical Coherence Tomography, visual field testing remains the most reliable endpoint to use. If structural measures are to be used in the future, one should keep in mind that the more optic nerve damage present at the outset, the less sen‐

Novel neuroprotective agents and mechanisms show promise in pre-clinical studies and ani‐ mal models. However, translating these findings into effective treatments still remains a

identifier: NCT01408472).

**7. Hope for success**

sitive structural change will be. [51]

**8. Conclusion**

The second most studied agent in neuroprotection trials is the highly selective alpha-2 adrenergic agonist, brimonidine, which had also shown great promise in animal studies. The first trial included 9 patients with Leber's hereditary optic neuropathy in whom use of brimonidine after loss of vision in one eye failed to prevent loss of vision in the second eye, which naturally occurs within weeks to months after first eye involvement. [134] The sec‐ ond trial also failed to show efficacy of brimonidine in aiding recovery of vision loss in pa‐ tients with anterior ischemic optic neuropathy, though the trial itself may have been underpowered. [135]

The last trial assessing the neuroprotective effects of brimonidine was the Low-Pressure Glaucoma Treatment Study, which recruited patients with normal-tension glaucoma and randomized them to either treatment with brimonidine or with timolol. [136] Timolol has no neuroprotective properties and it thus served as a control for the pressure-lowering effects of brimonidine. However, the IOP was only minimally lowered in both groups, which raises concerns about patient adherence to the treatment regimen. [51] Results of the trial were published in 2011 and showed that low-pressure glaucoma patients treated with brimoni‐ dine were less likely to have visual field progression than patients treated with timolol. [137]

A review of ongoing trials at clinicaltrials.gov revealed one phase I trial that is still recruit‐ ing patients and aims to investigate the safety and efficacy of the NT-501 clinical neurotro‐ phic factor (CNTF) implant in patients with chronic progressive open angle glaucoma (trial identifier: NCT01408472).
