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Therapeutic approaches to prevent RGC death by targeting the pathways involved in NMDA excitotoxicity

In this section, on the basis of laboratory research and clinical trials, we will discuss if NMDA receptors and downstream signaling molecules contributing to excitotoxic pathways can be targets of therapeutic intervention to prevent RGC death in glaucoma. Importantly, glutamate mediates synaptic transmission, which is essential for the normal function of the nervous system. Hence, complete blockade of NMDA receptor activity can be deleterious because physiological activity is impaired. To be clinically acceptable and well tolerated, anti-excitotoxic therapies must block only excessive activation of the NMDA receptors, while leaving normal function relatively intact to avoid side effects. Although there are a number of potential targets for therapeutic intervention, in this chapter we will focus on molecular pathways that we have been studying in detail.

Drugs targeting NMDA receptors

Kinetics of NMDA receptor antagonists

Various NMDA antagonists have been tested for their therapeutic potential in neurodegenerative disorders. Nevertheless, many of them failed chiefly because of lack of safety and tolerability. If an antagonist binds too tightly (has too high an affinity) and totally blocks activity of the target, it will inhibit the normal activity necessary to maintain physiological function and thus prove to be clinically unacceptable. However, it is important not to confuse affinity with selectivity. If an antagonist acts selectively and specifically on a target, and the therapeutic concentration is achieved, high affinity is not necessary and can even become detrimental (Lipton, 2006). For example, a ‘‘neuroprotective’’ dose of MK-801 (a noncompetitive NMDA receptor antagonist, which blocks the associated ion channel with high affinity) causes drowsiness, hallucinations, and even coma (Lipton, 2004). Indeed, antagonists with high affinity for the NMDA receptors cause psychiatric and other side effects (Domino and

Luby, 1981; Leppik, 1988; Javitt and Zukin, 1991; Muir and Lees, 1995).

In addition to affinity, the mechanism of antagonism (competitive, noncompetitive, or uncompetitive) is also important. An uncompetitive antagonist acts at an allosteric site that is different from the agonist-binding site (Lipton, 2004), and its inhibitory action is contingent on prior activation of the receptor by the agonist (Lipton, 2007). Accordingly, an uncompetitive antagonist provides stronger antagonism when an agonist exists at higher concentration, whereas a competitive antagonist exhibits weaker antagonism at higher agonist concentration because it competes with the antagonist for the binding site (Lipton, 2006, 2007) (Fig. 3A). For instance, we have shown that the uncompetitive NMDA receptor antagonist, memantine, inhibits NMDA receptor channel activity only when the receptors are activated by NMDA (Chen et al., 1992). Moreover, an uncompetitive antagonist will block receptor activity more effectively when the receptor is excessively (pathologically) activated, while relatively preserving lower (normal) activity (Lipton, 2004). As a matter of fact, memantine is relatively ineffective at blocking normal synaptic NMDA receptor activity involved in physiological function, but is exceptionally effective when the receptors (usually representing extrasynaptic receptors under pathological conditions) are activated by higher concentration of glutamate (Chen et al., 1992).

Importantly, the off-rate from the NMDA receptor is also a critical factor for developing clinically tolerated drugs. For example, let us contrast MK-801 and memantine. Because MK801 has a much slower off-rate from NMDA receptor-operated channels (in part accounting for its high affinity), MK-801 resides in the channels for prolonged periods of time and accumulates there. This fact produces prolonged antagonism of NMDA receptors even after attempted washout of MK-801 (Fig. 3B, gray trace), which in turn results in prolonged blockade of not only pathological activity but also normal physiological activity. Conversely, NMDA receptor activity recovers quickly after the washout of memantine owing to its relatively fast off-rate from NMDA-operated channels (Chen et al., 1992; Lipton, 2006) (Fig. 3B,

black trace). This fast off-rate also allows memantine to block the channels when they are excessively open but leave the channels unblocked when activity returns to normal (Lipton, 2003). Therefore, during prolonged activation of NMDA receptor, as hypothesized to occur in retinal ischemia, glaucoma, and other chronic neurodegenerative disorders, memantine will be a very efficacious blocker (Lipton, 2004). In conclusion, uncompetitive antagonists of NMDA receptors with high selectivity, low affinity, and relatively rapid off-rate can be clinically tolerated (Lipton, 1993, 2001, 2003, 2004, 2006, 2007).

Memantine

Memantine (1-amino-3,4-dimethyladamantane hydrochloride) is an adamantane derivative (an analog of the antiviral drug amantadine) that

works as an open-channel blocker of NMDA receptors at or near the Mg2+ site within the ion channel (Chen et al., 1992; Chen and Lipton, 1997). What characterizes memantine as unique and safe are the four properties described above (uncompetitive antagonist with high selectivity, low affinity, and fast off-rate) (Chen et al., 1992, 1998). Memantine enters NMDA receptor channels preferentially when the channels are excessively (pathologically) active, while relatively preserving normal channel activity, contributing to the drug’s safety and tolerability (Lipton, 2004, 2006). Indeed, clinical trials of memantine for Alzheimer’s disease (Reisberg et al., 2003, 2006; Ott et al., 2007) and vascular dementia (Orgogozo et al., 2002; Wilcock et al., 2002) reported no serious adverse events. Similarly, memantine seems less likely to adversely affect visual function because memantine treatment did not alter the electroretinogram (ERG) or visual-evoked

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potentials (VEPs) of monkeys (Hare et al., 2004a). In addition to proving safe, memantine was also effective in the clinical trials mentioned above and, as a result, has been approved by the European Union and the U.S. Food and Drug Administration (FDA) for treatment of moderate-to-severe Alzheimer’s disease. Clinical studies of memantine in the treatment of HIV-associated dementia have also been encouraging (Lipton, 2004; Schifitto et al., 2007). Interestingly, however, the milder the disease, the higher the concentration of memantine needed to combat the damage, since fewer NMDA receptor-operated channels are open in milder disease. Hence, the effective treatment of glaucoma, which often continues for decades in the typical human patient, may necessitate a higher dose of memantine than Alzheimer’s disease in which the average patient lives for approximately 7 years (Lipton, 2006, 2007).

At the basic research level, there have been many studies demonstrating therapeutic efficacy of memantine for RGC insults. Our earlier studies have shown that memantine prevents excitotoxic

cell death in RGC cultures (Fig. 4A). Additionally, in vivo treatment with memantine inhibited RGC death caused by mild chronic glutamate elevation in rats (Vorwerk et al., 1996) (Fig. 4B) and in EAAT-1 (a glial glutamate transporter) knockout mice, which manifest glaucoma-like pathology (Harada et al., 2007). Memantine also protected the retina, both histologically and functionally, in a rodent model of retinal ischemia caused by raising IOP, which mimics, in part, acute angle closure glaucoma (Osborne, 1999). Additionally, in a monkey model of glaucoma induced by laser coagulation of the trabecular meshwork, memantine successfully inhibited RGC death (Hare et al., 2004b) and shrinkage of neurons in the lateral geniculate nucleus (the major projection site of RGC axons in primates) (Yucel et al., 2006). Encouragingly, memantine treatment improved functional outcomes measured by ERG and VEP in this monkey glaucoma model (Hare et al., 2004a). These results from basic laboratory research imply that memantine may be useful in the treatment of glaucoma. However, a recent