ORAL MEMANTINE FOR THE TREATMENT OF GLAUCOMA

Summary of the article by RN Weinreb et al 

https://www.aaojournal.org/article/S0161-6420(18)30029-0/fulltext 

Memantine is an uncompetitive NMDA receptor antagonist that binds preferentially to NMDA receptor-operated cation channels; it is an open-channel blocking NMDA antagonist and thus binds only to channels that have been activated by glutamate binding to the receptor. Because memantine has a low to moderate affinity for the receptor and its inhibitory effect exhibits strong dependence on membrane voltage, it effectively blocks excessive glutamatergic activity at concentrations that do not affect normal neurotransmission.

Memantine can inhibit overstimulation of the NMDA receptor and potentially provide neuroprotection by preventing excessive calcium influx. In support of this mechanism, results from preclinical studies have shown that memantine reduces excitotoxicity in animal models of glaucoma.

Oral memantine was first offered for marketing by Merz & Co in 1982 and is currently approved in several countries for the treatment of moderate to severe Alzheimer’s and Parkinson’s diseases, 2 other age-related neurodegenerative pathologic conditions. 

Two phase 3 studies were conducted by Weinreb et al to evaluate the effectiveness and safety of oral memantine administered daily in doses of either 10 mg or 20 mg, compared with placebo, in patients with chronic open angle glaucoma (OAG) who were at risk of disease progression. 

Although patients on 30 mg dose did tolerate the medication, except for few side effects such as dizziness, it was thought that 20 mg would be a safer option for chronic use.

In the first study (NCT00141882), the SAP-based analysis showed that by month 48, the cumulative probability of glaucomatous visual field progression with memantine 10 mg was statistically significantly higher than with placebo. However, in the analyses based on FDT and stereoscopic optic disc photographs, there were no statistically significant differences in the cumulative probability of glaucomatous visual field progression and percentage of patients with progression of glaucomatous optic nerve damage, respectively, between memantine 10 mg or 20 mg and placebo groups. Interestingly, post hoc subgroup analysis of non-LTG patients during treatment showed a statistically significantly lower probability of glaucomatous visual field progression with memantine 20 mg compared with placebo.

The cumulative probability of glaucomatous visual field progression with memantine 10 mg was statistically significantly higher than with placebo when analyzed by FDT, not SAP. After amendment of the primary efficacy outcome, the FDT-based analysis in non-LTG patients showed that by month 48, there was a statistically significantly higher probability of glaucomatous visual field progression during treatment with memantine 10 mg, whereas memantine 20 mg had no effect, compared with placebo. Analyses based on SAP and optic disc assessments showed no statistically significant differences in the cumulative probability of glaucomatous visual field progression and percentage of patients with progression of glaucomatous optic nerve damage, respectively, among the memantine (10 or 20 mg) and placebo groups.

Stereo optic disc photographs did not reveal a consistent protective effect of memantine, and the primary endpoint was not met in either study or in the pooled analysis. 

Dizziness was the most frequently reported adverse events (AE).

Headache, fatigue, and insomnia were mild to moderate in severity. The most commonly reported serious AEs (regardless of causality) included prostate cancer, coronary artery disease, pneumonia, chest pain, atrial fibrillation, osteoarthritis, increased IOP, and congestive cardiac failure, of which only prostate cancer was reported in statistically significantly more patients in the memantine 20-mg per day group, compared with placebo group.

Although there were differences between the 20-mg and 10-mg dose groups versus placebo using different endpoints in each study, overall, there were no consistent or clinically meaningful outcomes. In addition, pooled results of visual field and optic disc analyses revealed no consistent or beneficial effects of memantine.

The cause of prostate cancer was attributed to spurious results.

it is possible that initiating memantine treatment earlier in the course of disease or in a population defined by more restrictive progression risk factors (e.g., age) may have produced different effects.

It is also worth considering the possibility that the 20-mg daily dose was too low to provide adequate neuroprotective activity and that a 30-mg dose might have been preferable, especially if administered as 15-mg tablets twice daily to minimize AEs.