Delayed administration of memantine prevents N-methyl-D-aspartate receptor-mediated neurotoxicity

Potential and current use of N-methyl-D-aspartate (NMDA) receptor antagonists in diseases of aging

The N-methyl-D-aspartate (NMDA) receptor complex is a subtype of glutamate receptor and its dysfunction is involved in many neurological disorders associated with aging, including chronic pain, depression, stroke and Parkinson's disease. Multiple clinical trials using NMDA receptor antagonists have been aborted mainly due to the severe psychomimetic adverse effects of these drugs that occur before concentrations can reach an adequate level in the brain. In this review, we present the evidence that clinically safer NMDA antagonists such as memantine and nitroglycerin, and the combination drug nitro-memantine, are promising as drugs in treating neurodegenerative diseases.

Evaluation of memantine for neuroprotection in dementia

Memantine, a non-competitive NMDA antagonist, has been approved for use in the treatment of dementia in Germany for over ten years. The rationale for use is excitotoxicity as a pathomechanism of neurodegenerative disorders. Memantine acts as a neuroprotective agent against this pathomechanism, which is also implicated in vascular dementia. HIV-1 proteins Tat and gp120 have been implicated in the pathogenesis of dementia associated with HIV infection and the neurotoxicity caused by HIV-1 proteins can be blocked completely by memantine. Memantine has been investigated extensively in animal studies and following this, its efficacy and safety has been established and confirmed by clinical experience in humans. It exhibits none of the undesirable effects associated with competitive NMDA antagonists such as dizocilpine. The efficacy of memantine in a variety of dementias has been shown in clinical trials. Memantine is considered to be a promising neuroprotective drug for the treatment of dementias, particularly Alzheimer's disease for which there is no neuroprotective therapy available currently. It can be combined with acetylcholinesterase inhibitors which are the mainstay of current symptomatic treatment of Alzheimer's disease. Memantine has a therapeutic potential in numerous CNS disorders besides dementias which include stroke, CNS trauma, Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), epilepsy, drug dependence and chronic pain. If memantine is approved by the FDA for some of these indications by the year 2005, it can become a blockbuster drug by crossing the US$1 billion mark in annual sales.

Memantine is a clinically well tolerated N-methyl-D-aspartate (NMDA) receptor antagonist--a review of preclinical data

N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential in numerous CNS disorders ranging from acute neurodegeneration (e.g. stroke and trauma), chronic neurodegeneration (e.g. Parkinson's disease, Alzheimer's disease, Huntington's disease, ALS) to symptomatic treatment (e.g. epilepsy, Parkinson's disease, drug dependence, depression, anxiety and chronic pain). However, many NMDA receptor antagonists also produce highly undesirable side effects at doses within their putative therapeutic range. This has unfortunately led to the conclusion that NMDA receptor antagonism is not a valid therapeutic approach. However, memantine is clearly an uncompetitive NMDA receptor antagonist at therapeutic concentrations achieved in the treatment of dementia and is essentially devoid of such side effects at doses within the therapeutic range. This has been attributed to memantine's moderate potency and associated rapid, strongly voltage-dependent blocking kinetics. The aim of this review is to summarise preclinical data on memantine supporting its mechanism of action and promising profile in animal models of chronic neurodegenerative diseases. The ultimate purpose is to provide evidence that it is indeed possible to develop clinically well tolerated NMDA receptor antagonists, a fact reflected in the recent interest of several pharmaceutical companies in developing compounds with similar properties to memantine.

Neuroprotective concentrations of the N-methyl-D-aspartate open-channel blocker memantine are effective without cytoplasmic vacuolation following post-ischemic administration and do not block maze learning or long-term potentiation

The potential of most N-methyl-D-aspartate antagonists as neuroprotectants is limited by side effects. We previously reported that memantine is an open-channel N-methyl-D-aspartate blocker with a faster off-rate than many uncompetitive N-methyl-D-aspartate antagonists such as dizocilpine maleate. This parameter correlated with memantine's known clinical tolerability in humans with Parkinson's disease. Memantine is the only N-methyl-D-aspartate antagonist that has been used clinically for excitotoxic disorders at neuroprotective doses. Therefore, we wanted to investigate further the basis of its clinical efficacy, safety, and tolerability. Here we show for the first time for any clinically-tolerated N-methyl-D-aspartate antagonist that memantine significantly reduces infarct size when administered up to 2 h after induction of hypoxia/ischemia in immature and adult rats. We found that at neuroprotective concentrations memantine results in few adverse side effects. Compared to dizocilpine maleate, memantine displayed virtually no effects on Morris water maze performance or on neuronal vacuolation. At concentrations similar to those in brain following clinical administration, memantine (6-10 microM) did not attenuate long-term potentiation in hippocampal slices and substantially spared the N-methyl-D-aspartate component of excitatory postsynaptic currents, while dizocilpine maleate (6-10 microM) or D-2-amino-5-phosphovalerate (50 microM) completely blocked these phenomena. We suggest that the favorable kinetics of memantine interaction with N-methyl-D-aspartate channels may be partly responsible for its high index of therapeutic safety, and make memantine a candidate drug for use in many N-methyl-D-aspartate receptor-mediated human CNS disorders.

Neuronal injury associated with HIV-1: approaches to treatment

Mounting evidence suggests that cognitive dysfunction developing as a result of HIV-1 infection is mediated at least in part by generation of excitotoxins and free radicals in the brain. This syndrome is currently designated HIV-1-associated cognitive/motor complex, was originally termed the AIDS Dementia Complex, and for simplicity, is called AIDS dementia in this review. Recently, brains of patients with AIDS have been shown to manifest neuronal injury and apoptotic-like cell death. How can HIV-1 result in neuronal damage if neurons themselves are only rarely, if ever, infected by the virus? Experiments from several different laboratories have lent support to the existence of HIV- and immune-related toxins in a variety of in vitro and in vivo paradigms. In one recently defined pathway to neuronal injury, HIV-infected macrophages and microglia, or immune-activated macrophages and astrocytes (activated by the shed HIV-1 envelope protein, gp120, or other viral proteins and cytokines), appear to secrete excitants and neurotoxins. These substances may include arachidonic acid, platelet-activating factor, free radicals (NO. and O2.-), glutamate, quinolinate, cysteine, amines, and as yet unidentified factors emanating from stimulated macrophages and reactive astrocytes. A final common pathway for neuronal susceptibility is operative, similar to that observed in stroke and several neurodegenerative diseases. This mechanism involves excessive activation of N-methyl-D-aspartate (NMDA) receptor-operated channels, with resultant excessive influx of Ca2+ and the generation of free radicals, leading to neuronal damage. With the very recent development of clinically tolerated NMDA antagonists, there is hope for future pharmacological intervention.

A preliminary investigation of lamotrigine for cocaine abuse in HIV-seropositive patients

Theoretical considerations as well as pre-clinical data suggest a potential role for glutamate-inhibiting agents in the treatment of cocaine addiction. At present, however, there is little clinical data to inform the use of these agents for this application. In this preliminary study eighteen HIV-seropositive cocaine dependent, opiate-agonist maintained patients received lamotrigine (300 mg/day), an indirect glutamate release inhibitor, on either a standard (n = 8) or accelerated (n = 10) induction schedule for 12 weeks. Results showed a significant decrease in percentage of cocaine-positive urine screens in the standard induction lamotrigine group but not in the accelerated induction group. There were fewer reports of side-effects and fewer dropouts in the standard-induction lamotrigine group compared to the accelerated induction group. Neuropsychological assessments suggested a decrement in the Trail Making Tests, but no other decreases in cognitive functioning. We conclude that standard-induction lamotrigine warrants further investigation for the treatment of cocaine abuse in this patient population.

Molecular basis of glutamate toxicity in retinal ganglion cells

Loss of retinal ganglion cells (RGCs) is a hallmark of many ophthalmic diseases including glaucoma, retinal ischemia due to central artery occlusion, anterior ischemic optic neuropathy and may be significant in optic neuritis, optic nerve trauma, and AIDS. Recent research indicates that neurotoxicity is caused by excessive stimulation of receptors for excitatory amino acids (EAAs). In particular, the amino acid glutamate has been shown to act as a neurotoxin which exerts its toxic effect on RGCs predominantly through the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. NMDA-receptor-mediated toxicity in RGCs is dependent on the influx of extracellular Ca2+. The increase in [Ca2+]i acts as a second messenger that sets in motion the cascade leading to eventual cell death. Glutamate stimulates its own release in a positive feedback loop by its interaction with the non-NMDA receptor subtypes. Ca(2+)-induced Ca2+ release and further influx of Ca2+ through voltage-gated Ca2+ channels after glutamate-induced depolarization contribute to glutamate toxicity. In vitro and in vivo studies suggest that the use of selective NMDA receptor antagonists or Ca2+ channel blockers should be useful in preventing or at least abating neuronal loss in the retina. Of particular importance for future clinical use of NMDA receptor antagonists in the treatment of acute vascular insults is the finding that some drugs can prevent glutamate-induced neurotoxicity, even when administered a few hours after the onset of retinal ischemia.

Memantine reduces functional and morphological consequences induced by global ischemia in rats

In this study the effect of memantine, an antagonist at the N-methyl-D-aspartate receptor, on spatial learning deficit and on neuronal damage following transient cerebral ischemia was evaluated. Global ischemia was induced by four-vessel-occlusion (4VO) for 20 min in rats. Memantine was administered 20 min before induction of ischemia at a dose of 10 or 20 mg/kg. One week after surgery spatial learning was tested in the Morris water maze. Treatment with the higher dose of memantine reduced the increase in escape latency and in swim distance induced by 4VO. Neuronal damage in the CA1 sector of the hippocampus and in the striatum produced by 4VO was significantly attenuated by 20 mg/kg memantine. Treatment with the lower dose of memantine had no influence on the deficit in spatial learning and the neuronal damage resulting from ischemia. The present data demonstrate that treatment with a neuroprotective agent like memantine can reduce functional as well as morphological sequelae induced by ischemia.

Different binding affinities of NMDA receptor channel blockers in various brain regions--indication of NMDA receptor heterogeneity

The N-methyl-D-aspartate (NMDA) receptor-channel complex exists in multiple forms which probably have different physiological and pharmacological properties. To further evaluate this concept of different NMDA receptor subtypes, receptor binding and autoradiographic techniques were used to study the phencyclidine (PCP) binding site of the NMDA receptor ion-channel complex. [3H]MK-801 was employed to characterize binding properties of (+)-MK-801, (-)-MK-801, phencyclidine (PCP), (+/-)-ketamine, amantadine (1-amino-adamantane) and memantine (3,5-dimethyl-1-amino-adamantane) in different brain regions. Saturation experiments on homogenized membranes revealed the existence of single classes of binding sites in cortex and cerebellum but with significant different affinities between these regions (KD/Cortex = 4.59 nM, Bmax/Cortex = 0.836 pmol/mg protein; KD/Cereb. = 25.99 nM, Bmax/Cereb. = 0.573 pmol/mg protein) suggesting that the lower affinity in cerebellum indicates another population of NMDA receptor channels. In contrast, in striatum there was clear evidence for two binding sites (KD/high = 1.43 nM, Bmax/high = 0.272 pmol/mg protein; KD/low = 12.15 nM, Bmax/low = 1.76 pmol/mg protein). Displacement studies (autoradiography and binding) revealed a lower affinity for unlabeled (+)-MK-801 in striatum which was clearly not the case for memantine. In cerebellar membranes there was a significant decrease in the affinity for both MK-801 enantiomers and PCP but not for the 1-amino-adamantanes. In contrast, all compounds showed lowered affinity in the dentate gyrus. These findings support NMDA receptor heterogeneity which may be of particular relevance for the development of subtype-selective drugs.

Influence of ketamine on the neuronal death caused by NMDA in the rat hippocampus

The protection provided by ketamine against the neuronal cytotoxicity of NMDA was investigated and compared with that provided by dizocilpine (MK 801). A massive anaesthetic dose of ketamine (180 mg/kg) was required for substantial protection (about 70%) of rat dorsal hippocampal neurons. Protection was markedly decreased if the ketamine was given in three divided doses of 60 mg/kg over a period of 2 hr, rather than as a bolus injection of 180 mg/kg. A lower dose (60 mg/kg i.p.) gave no protection when given 10 min prior to NMDA, but some protection (up to 30%) was found when administration was delayed for 1-2 hr. After 3 hr, ketamine at this dose did not protect. In comparison, the toxicity of NMDA was reduced by about 70% by prior treatment with dizocilpine at 1 mg/kg, and completely eliminated at 10 mg/kg. The lack of protection when ketamine at 60 mg/kg was administered prior to NMDA may be due to a proconvulsant action of ketamine, as diazepam in the presence but not in the absence of ketamine significantly reduced the toxicity of NMDA. However, there was no behavioural or histological evidence of increased seizure activity in the presence of ketamine. Neuroprotectant effects may prevail with massive anaesthetic doses of ketamine or when diffusion has reduced the concentration of NMDA. The heroic doses of ketamine required for protection diminish its attractiveness as a potential anti-ischaemic agent.

Chronic treatment with the uncompetitive NMDA receptor antagonist memantine influences the polyamine and glycine binding sites of the NMDA receptor complex in aged rats

Receptor binding studies on rat cortical membranes were used to characterize the NMDA receptor in aged rats (22 months) treated for 20 months with a memantine containing diet delivering 30 mg/kg/day in comparison to aged and young/adult rats treated with control-diet. Spatial memory impairing effects of (+)-MK-801 (0.16 mg/kg) in the radial maze was not altered within the course of memantine-treatment (up to 16 months). However, chronic memantine-treatment significantly increased the number of [3H]MK-801 binding sites and the affinity of [3H]glycine. A non-significant trend to such changes was also seen in aged-control rats. Glycine-dependent [3H]MK-801 binding (functional binding under non-equilibrium conditions at a fixed L-glutamate concentration) revealed that a decreased ability of glycine to stimulate channel opening in aged rats was partially attenuated by the long-term memantine treatment. Furthermore, an increased ability of spermidine to enhance [3H]MK-801 binding in aged-control rats was even more pronounced in the aged memantine-treated group. Together these findings may indicate that changes in functional receptor-channel properties during the process of aging occur prior to a detectable loss of binding sites and that memantine enhances an endogenous compensatory mechanism triggered by glutamatergic hypofunction which is suggested to take place in aging.