BEYOND IOP

While there is a consensus that reducing intraocular pressure (iOP) in glaucoma patients can reduce the progression of the disease, research also suggests that elevated iOP is not glaucoma's only risk factor and may not even be the most important one in all cases. The evidence that other factors than iOP are important in the pathophysiology of glaucomatous disease includes the observation that some glaucoma patients continue to progress despite good reductions in iOP and that, in patients with normal tension glaucoma, the disease progresses despite perfectly normal iOP. in addition, in the five-year OhTs study more than 90 per cent of patients did not develop glaucoma. Therefore, intensive research is now under way throughout the world devoted to finding potentially modifiable risk factors other than iOP.

Neuroprotective strategies IOP reduction can prevent or slow the loss of retinal ganglion cells in eyes with glaucoma, including those with normal tension glaucoma. it therefore follows that the retinal ganglion cells in eyes with glaucomatous disease may have an increased sensitivity to iOP, said Jeffrey L Goldberg MD, PhD, associate professor of ophthalmology, Bascom Palmer Eye institute, University of Miami, Miller school of Medicine, Miami, Florida, Us. 'Formerly, glaucoma was thought of primarily as a disease of high intraocular pressure, but in recent years there has been a subtle but important shift in our understanding of the disease. We now think of glaucoma as a disease of susceptibility to insults, including intraocular pressure. so the disease isn't the outflow problem or the problem with pressure regulation up at the front of the eye. The disease fundamentally is that some people for unknown reasons are very sensitive to some insults like pressure. At present, we don't understand the reasons for that susceptibility but we're making progress,' Dr Goldberg told EuroTimes in an interview.

There are numerous theories as to why retinal ganglion cells die in eyes with glaucoma. They range from such obvious possibilities as simple mechanical damage to the cells and hypoxia to more complex biomolecular and pathophysiological mechanisms. But all of the theories remain unproven and it is not clear if the cause of retinal ganglion cell death is the same in all cases, he said. 'We don't really understand yet how the pressure in the eye at any level translates into the insults to retinal ganglion cells that ultimately lead to their death. it could be any or all of a number of reasons, including pathways such as excitotoxicity, vascular dysregulation, hypoxia, or the cells being cut off from their target-derived trophic factors. it could be some transduction of that pressure that messes up the neuron/ glial interaction in the optic nerve head or interferes with synapse formation between the retinal ganglion cells and their partners in the retina or their partners in the brain. it could be that one patient is sensitive for one reason and another patient is sensitive for another reason,' Dr Goldberg said.

Failures and successes in clinical trials The best known clinical trial involving a proposed neuroprotective agent in glaucoma is the Phase iii memantine trial carried out by Allergan. Memantine is an NMDA glutamate receptor antagonist that has been FDA approved for the treatment of Alzheimer's disease. The theory behind its use in glaucoma is that it could reduce glutamate excitoxicity in the optic nerve head, which is a proposed mechanism of retinal ganglion cell loss. The international Phase iii trial involving 1,136 patients began in 1999 and concluded in 2006. Although the study showed that the progression of glaucoma was significantly lower in patients receiving the higher dose of memantine than in patients receiving the low dose of the drug, there was no significant difference between the results in active treatment groups and the placebo group. Another agent, brimonidine (Alphagan®, Allergan), is showing more promise as a neuroprotectant. it also has the advantage of already being approved for use in glaucoma patients because of its iOPlowering properties. in a randomised controlled trial involving patients with low-pressure glaucoma, 91 patients receiving topical brimonidine had significantly less visual field progression than 79 patients receiving timolol over a mean of 31 weeks of follow-up, despite similar reductions of iOP in both groups (Krupin et al. Am J Ophthalmol 2011 ;151:671– 681). There are numerous theories regarding how brimonidine achieves its apparent neuroprotective effect. For example, it may enhance the function of brain-derived neurotrophic factors in the retinal ganglion cells, it may inhibit ischemia-induced release of glutamate, or it may activate the production of anti-apoptotic compounds, Dr Goldberg noted. The prospects of brimonidine as a neuroprotectant may improve still further if it proves effective in trials where it is used in a biodegradable intravitreal implant formulation similar to that used in the Ozurdex implant, he added. 'it is unlikely that brimonidine will supplant prostaglandins as a firstline glaucoma eye drop but the Low Tension Glaucoma study Group's data is probably strong enough to warrant using brimonidine, at a minimum, as a second agent for patients who are progressing or having difficulty despite good pressure control while receiving prostaglandins,' Dr Goldberg said.

Salvaging dysfunctional retinal ganglion cells A frequent finding of studies that have charted glaucoma progression over several years is that, in the earlier stages of the disease, visual field loss will sometimes precede detectable changes in the optic nerve head. Although this can be a result of measurement artefacts that in turn result from media opacity, patient inattention, and other neuroretinal tissues masquerading as retinal ganglion cells, the finding may also represent retinal ganglion cells that are still alive but are dysfunctional. 'There is a lot of evidence from animal models of glaucoma and from the human disease that there is an interval between dysfunction and the actual death of the retinal ganglion cells. That suggests that, if we could intervene and give these dysfunctional cells a sort of booster shot, then we might acutely enhance function, in addition to preventing their eventual demise,' Dr Goldberg said. One such cell-salvaging approach is the use of neurotrophic factors that could both enhance axon growth and enhance retinal ganglion cell survival. That is the rationale behind a phase i clinical trial that is currently under way in the Us involving the treatment of primary open-angle glaucoma (POAG) patients with intravitreal implants containing encapsulated cell lines expressing ciliary neurotrophic factor. Another approach might involve the use of agents that block the inhibition of axon re-growth that normally occurs in the mature eye. such agents include antibodies to the oligodendrocyte-derived protein Nogo, and the Rho-kinase inhibitors, which also have iOP-lowering properties. Both agents are undergoing clinical evaluation for the treatment of spinal cord injury.

A third approach is to improve the function of retinal ganglion cells through electrical stimulation. One of the main hypotheses for how retinal ganglion cells die in glaucoma is that the loss of electrical activity and synaptic function sets in motion the biomolecular processes that ultimately lead to their death. Therefore, enhancing the retinal ganglion cells' function could, in theory, yield the added benefit of improving their survival, Dr Goldberg said. 'The appreciation of glaucoma as a neurodegenerative disease has been increasing. We think that many of the neurodegenerative diseases throughout the brain and spinal cord also involve a loss of activity or synaptic function on the way to the cell's death. if that's true in glaucoma as well, that would link glaucoma even more strongly to the pathology of other neurodegenerative diseases,' he added.

Improving ocular blood flow Among the many risk factors that research has identified for glaucoma onset and progression, one of the most significant is a low ocular perfusion pressure. Ocular perfusion pressure is estimated as blood pressure minus intraocular pressure, or better, as arterial pressure minus venous pressure. Both intraocular pressure and systemic blood pressure fluctuate over the course of 24 hours, with blood pressure generally rising in daytime hours and falling during the evening and intraocular pressure doing the reverse.

One school of thought suggests that an impaired autoregulation of ocular blood flow can play a role in the progression of glaucoma and that this impairment plays an especially important role in patients with normal tension glaucoma and in glaucoma patients who progress despite optimum iOP control. Patients with a condition called vascular dysregulation syndrome are especially prone to an impaired autoregulation, said Josef Flammer MD, professor and head, Department of Ophthalmology, University of Basel, Basel, switzerland. 'if you have primary vascular dysregulation you are more likely to have a disturbed autoregulation and that is why these people are at a higher risk for glaucoma. They are not automatically glaucomatous, the vast majority won't get glaucoma, just as the majority with iOP greater than 25 mmhg will never get glaucoma, but the risk is greater,' he told EuroTimes in an interview. Unstable ocular blood flow can in theory contribute significantly to the retinal ganglion cell death and excavation of the optic nerve head that characterise glaucoma, he said. it can result from extreme fluctuations of iOP and from extreme fluctuations in blood pressure. The result in either case is a fluctuation in the oxygen supply to the cells in the optic nerve head. 'We are pretty sure that in the end one major factor is oxidative stress. That is because we know that hypoxia, for example, might kill axons but does not normally kill the astrocytes in the optic nerve head. Whereas, we know that oxidative stress can damage the mitochondria, causing the optic nerve head to suffer, including the axons and therefore also the neurons they are attached to. At the same time, the oxidative stress in the first stage causes the astrocytes to produce more nitric oxide, but in the long run they also disappear because they cannot survive this oxidative stress in the long run,' Dr Flammer explained. Weakened autoregulation may intensify the injurious effects of iOP and blood pressure fluctuations. Where a patient's autoregulation is disturbed, even normal fluctuations of iOP and blood pressure can lead to the same kind of oxygen fluctuation as occurs with high fluctuations of iOP and blood pressure, Dr Flammer noted. 'in other words, high iOP, low blood pressure, disturbed auto regulation, they can all individually and combined lead to the same picture, namely oxidative stress,' he added.

The characteristics of patients with vascular dysregulation syndrome commonly include low blood pressure, low body mass index, cold hands and a tendency not to feel thirsty. They also have a tendency to take longer to fall asleep, are unlikely to work outdoors and are more sensitive to some systemic medication, Dr Flammer said. he added at present he treats glaucoma patients who have very low blood pressure with salt, patients with dysregulation syndrome with magnesium and very low doses of calcium channel blockers and with ginkgo. The sign he looks for as indications that the treatment is working is improved visual fields and improved regulation of ocular blood flow. if such techniques are not available the reduction of the condition's symptoms is a good surrogate, he said. he noted that he frequently finds that patients with vascular dysregulation syndrome who progress despite an iOP of 12 mmhg will stabilise once their dysregulation has been successfully treated, even if their iOP rises back up to 18 mmhg. 'Unfortunately, from a scientific point of view, we still lack good, big controlled studies although in my experience spanning over three decades – and i've used it in thousands of patients – i am very satisfied with the treatment. But the pharmaceutical companies will not sponsor that absolutely meaningful type of study because the agents i use cannot be patented,' Dr Flammer said. he added that glaucoma in such patients is often only the tip of the iceberg and that their dysregulation syndrome can also result in other pathologies including migraine headaches, hearing loss and silent myocardial ischaemia and others. 'if a patient has a dysregulation he doesn't only have a problem in his eye. i have had a number of patients with untreated dysregulation that had myocardial infarctions very early in their life or else sudden hearing loss. so while you treat the dysregulation you don't only benefit the patient's eyes. That is why patients often report an improved quality of life after they start their treatment,' Dr Flammer said.

New clues to glaucoma genetics Since a family history of glaucoma is a risk factor for the disease it is likely that genetic factors play a role in its aetiology. however, the genetic factors involved appear to be much more complex than those that lead to colour blindness or eye colour, traits which can be linked to a single gene. instead, the genetic basis of glaucoma appears to resemble more closely diseases such as AMD and diabetes, which appear to involve the interaction of a number of different genes. isolating the genes involved would therefore be almost impossible using classical family history analysis. however, in recent years geneticists have developed a cohort based approach called genome-wide association. studies using this approach compare the entire genome of large cohorts of patients to reveal the locations of subtle variations in nucleotide sequences on the chromosomes, called single nucleotide polymorphisms (sNPs), associated with specific traits. For example, an Australian group has found associations between variations in the ATOh7 gene and variations in optic disc size (Macgregor et al, Hum. Mol. Genet. 2010; 19 (13): 2716-2724). The findings were replicated in a UK twin cohort and the Wellcome Trust Case-Control Consortium (WTCCC) cohort. Animal studies have shown that the gene plays a key role in retinal ganglion cell formation. Another group has identified an association between glaucoma and an sNP in close proximity to the CAV1 and CAV2 genes (Thorleifsson et al, Nature Genetics 2010; 42:906–909). Both of the genes are expressed in the trabecular meshwork and retinal ganglion cells. More recently, in the largest genomewide association study of POAG conducted to date, a group of investigators funded by the Us National Eye institute identified two sNPs in two different chromosomes significantly associated with normal tension glaucoma and exfoliative glaucoma. Both sNPs were in the vicinity of genes associated with transforming growth factor beta (TGFbeta), a molecule that regulates cell growth and survival throughout the body and which plays an important role in the health and function of the optic nerve. 'This is the first indication that there is significant association between single nucleotide polymorphism markers particularly on chromosome 9 and chromosome 8 with the normal tension glaucoma patients that is highly significant. But we don't yet know how much of the genetic risk that represents,' hemin R Chin PhD, associate director of ophthalmic genetics and program director, ocular genetics, National Eye institute, National institutes of health, Bethesda, Maryland, Us. The study's investigators analysed the complete genome of 6,633 participants, half of whom had POAG. The participants were part of two Nih-funded studies: GLAUGEN (Glaucoma Genes and Environment) and NEiGhBOR (NEi Glaucoma human genetics collaBORation), conducted at 12 sites in the Us. The findings were published in the journal PLoS Genetics, (April 26, 2012). 'in this study we used 3,000 cases and 3,000 managing controls and we were able to identify the most significant highly probable alleles. There may be many other genetic risk factors that will contribute to the disease to a smaller degree and we are now trying to get at those factors by combining forces with groups in Europe, Australia, singapore and all over the world,' Dr Chin added.

IOP reduction still essential While researchers continue to search for clues into glaucoma's cause and ways to treat the disease, the best evidence-based approach to glaucoma therapy remains iOP reduction, said Carlo Traverso MD, University of Genoa, Genoa, italy. 'Agents that preserve visual function independently of iOP could be of great advantage so long as iOP is very low, but i don't think there will ever be a neuroprotectant or something that works on the blood flow that will work if iOP is not lowered to an optimal level,' he told EuroTimes. he noted for example that in the brimonidine trial, which showed an advantage of the alpha 2 adrenergic antagonist over timolol, the patients had achieved low iOP and that was an essential component of brimonidine's neuroprotectant effect. 'in that study, the patients in the two treatment arms had well-controlled iOP – otherwise they would have been dropped from the study – and those receiving brimonidine had better optic nerve function. That doesn't mean that iOP is irrelevant, brimonidine would not have achieved the neuroprotective effect unless it also controlled iOP,' he added.