human phase III clinical trial used the same dose of memantine as approved for the treatment of Alzheimer’s disease, which for the reasons alluded to above, may represent too low a dose to be effective in glaucoma.

NitroMemantines

NMDA receptors have several cysteine residues whose S-nitrosylation results in downregulation of NMDA receptor activity (Sullivan et al., 1994; Lipton et al., 1998; Lipton, 1999; Choi et al., 2000). Nitroglycerin, which generates NO-related species and is widely used for management of angina pectoris, can decrease NMDA receptormediated channel activity (Lipton et al., 1998). Interestingly, nitroglycerin appears to retard progression of glaucomatous changes in primary open-angle glaucoma patients (Zurakowski et al., 1998). Although improved circulation provided by the vasodilating action of nitroglycerin could be the mechanism for this effect, downregulation of excessive NMDA receptor activity by S-nitrosyla- tion of the receptors might have also contributed by attenuating excitotoxicity of RGCs. However, it is not practical to use nitroglycerin for glaucoma treatment because of its significant cardiovascular effects, including hypotension. Therefore, we designed a novel set of dual-acting memantine derivatives, called NitroMemantines, which contain an NO group tethered to memantine in order to target NO to the critical cysteine residues of the NMDA receptor via memantine’s specific interaction with overly active NMDA receptor-associated channels (Lipton, 2004, 2006, 2007; Chen and Lipton, 2006). Our preliminary studies have revealed that NitroMemantines display increased neuroprotective properties over memantine, both in culture and in vivo.

Drugs targeting downstream signaling molecules in NMDA-induced cell death pathways

p38 MAPK inhibitors

The p38 MAPK inhibitor, SB203580, can offer neuroprotection. Structural biology studies revealed

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that SB203580 binds to the ATP-binding site of p38, thus blocking kinase activity without affecting phosphorylation of p38 (Wilson et al., 1997). The neuroprotective potential of SB203580 was first observed in cerebellar granule cells (Kawasaki et al., 1997). We previously showed that SB203580 inhibited NMDA-induced RGC death in culture (Kikuchi et al., 2000) (Fig. 5A). This p38 MAPK inhibitor also protected rat RGCs in vivo from NMDAinduced excitotoxicity (Manabe and Lipton, 2003) (Fig. 5B) and from optic nerve axotomy (Kikuchi et al., 2000) (Fig. 5C). Recent development of improved, clinically tolerated p38 MAPK antagonists for other diseases, including rheumatoid arthritis, suggests the possibility of future clinical application of these drugs to optic neuropathies such as glaucoma.

Averting caspase-mediated neurodegeneration

Caspases are a group of enzymes that cleave their substrates in the execution of apoptotic cell death. These enzymes contribute to apoptotic neuronal cell death observed during mild NMDA-induced excitotoxicity. Thus, a candidate therapeutic strategy in glaucoma is to protect RGCs from excessive caspase activity. Conventional irreversible caspase inhibitors such as N-benzyloxycarbonyl-Val-Ala- Asp-fluoromethyl ketone (z-VAD-fmk) work as pseudosubstrates; they bind to caspases like decoys in place of the actual substrate and thus block interaction between caspases and substrates. Because this type of inhibition is irreversible and therefore possibly toxic, these chemicals to date have not proven useful in the clinic for neuroprotection. We and our collaborators have found that a hexapeptide, IQACRG, protects a variety of neuronal cells from apoptotic-like cell death, including rat pheochromocytoma-derived PC12 cell death induced by trophic factor withdrawal and downregulation of superoxide dismutase 1 (Troy et al., 1996), and cultured cerebrocortical neurons exposed to NMDA (Tenneti et al., 1998). The peptide is homologous to the active site of caspases-1, -2, -3, -6, -7, and -14 but is incapable of substrate cleavage. IQACRG is thought to bind to caspase substrates by mimicking the enzyme, thus acting as a pseudoenzyme to protect substrates

from caspase cleavage (Troy et al., 1996). We have recently found that the IQACRG peptide protects RGCs from NMDA-induced cell death both in culture and in vivo after intravitreal injection. Future studies investigating the protective

properties of IQACRG in more relevant animal models of glaucoma and also exploring clinically applicable modes of drug delivery could make this peptide a viable therapeutic to prevent RGC death in diseases such as glaucoma.