The low-affinity, use-dependent NMDA receptor antagonist AR-R 15896AR. An update of progress in stroke

Ion channels involved in stroke

Ion channels are membrane proteins that flicker open and shut to regulate the flow of ions down their electrochemical gradient across the membrane and consequently regulate cellular excitability. Every living cell expresses ion channels, as they are critical life-sustaining proteins. Ion channels are generally either activated by voltage or by ligand interaction. For each group of ion channels the channels' molecular biology and biophysics will be introduced and the pharmacology of that group of channels will be reviewed. The in vitro and in vivo literature will be reviewed and, for ion channel groups in which clinical trials have been conducted, the efficacy and therapeutic potential of the neuroprotective compounds will be reviewed. A large part of this article will deal with glutamate receptors, focusing specifically on N-methyl-D-aspartate (NMDA) receptors. Although the outcome of clinical trials for NMDA receptor antagonists as therapeutics for acute stroke is disappointing, the culmination of these failed trials was preceded by a decade of efforts to develop these agents. Sodium and calcium channel antagonists will be reviewed and the newly emerging efforts to develop therapeutics targeting potassium channels will be discussed. The future development of stroke therapeutics targeting ion channels will be discussed in the context of the failures of the last decade in hopes that this decade will yield successful stroke therapeutics.

A randomized placebo-controlled trial of an NMDA receptor antagonist in sleep-disordered breathing

Hypoxemia is a powerful stimulus of glutamate release in the central nervous system (CNS) and a hallmark phenomenon in sleep disordered breathing (SDB). Glutamate effects that include neuronal damage and apoptosis following hypoxemia and apnea following microinjections in animal models are in part mediated via postjunctional N-methyl-D-aspartate (NMDA) receptors. This was a double blind, randomized, placebo-controlled single dose cross-over study of the NMDA receptor antagonist AR-R15896AR in 15 male patients with moderate to severe SDB. Seven patients received 120 mg and eight patients received 350 mg AR-R15896AR or corresponding placebo (given by 2 h infusion) starting half an hour before estimated sleep onset. AR-R15896AR concentrations were in line with the predicting kinetic model. A standard polysomnographic montage was applied. Repeated plasma samples were obtained in nine patients for analysis of plasma glutamate. Glutamate concentration in plasma did not change overnight and was unrelated to severity of SDB. Overall AHI (apnea-hypopnea index; primary efficacy variable) or investigated oxygen saturation variables were not significantly changed after AR-R15896AR at either dosage level. Side effects were mostly confined to the higher dose level and included vivid dreams, nightmares as well as in two cases mild hallucinations. The previously postulated role of glutamate in SDB could not be confirmed after AR-R15896AR induced NMDA-receptor blockade.

Neuroimaging: New Approaches for Neurotoxicology

Over the last 20 years, the impact of imaging on the clinical sciences is unquestionable. It has revolutionized the diagnosis and treatment of disease. Interestingly, the use of imaging in preclinical neurotoxicology has been relatively negligible. This has been in part due to the lack of knowledge or understanding of the capabilities of these powerful technologies. However, some of the more immediately applicable imaging approaches could impact the present approach to neurotoxicology. In addition, the recent advent of the development of imagers specifically for application to small animals will provide the opportunity of obtaining information for neurotoxicological risk assessment in a more timely and relevant manner. The ability to visualize changes in structure and function due to neurotoxic insult in a noninvasive manner is a promising direction. Changes in anatomy of soft and hard tissue, metabolism, function and gene expression can now be done in both a preclinical and a clinical setting using such technologies as magnetic resonance imaging (MRI), magnetic resonance imaging microscopy (MRM), and positron emission tomography (PET). This type of information is not readily accessible using conventional preclinical neurotoxicological procedures and usually requires total destruction of the intrinsic structure of the sample of interest. Imaging provides an opportunity to produce much of these data in a nondestructive manner and presents the data in a three-dimensional format. This permits longitudinal studies of the same subject subsequently reducing the number of animals required for studies while providing more information. In addition, as these technologies have been primarily developed for clinical purposes, they provide an outstanding opportunity for cross-species and animal-to-human extrapolation and testing.

Apoptotic death of beta cells after optic nerve transection in adult cats

We have revealed previously that the survival rate of beta cells of cat retinal ganglion cells (RGCs) rapidly decreased to 29% on day 7 after optic nerve transection, whereas that of alpha cells slowly decreased to 64% on day 14 (Watanabe et al., 2001). The reason that beta cells die more rapidly than alpha cells was not clear. In the present study, we tested the possibility that the rapid death of beta cells is attributable to apoptosis, as shown for some axotomized RGCs in rats. The following results were obtained. First, the proportion of pyknotic cells in Nissl-stained cat retinas started to increase sharply starting on day 4 and reached a peak on day 6 after optic nerve transection. The time course of occurrence of pyknotic cells corresponded well with that of the rapid death of axotomized beta cells. Secondly, the proportion of pyknotic cells was the highest in the area centralis (AC), in which beta cells are densely distributed. The preferential death of axotomized RGCs in the AC was also confirmed by terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling staining in cross sections. Thirdly, after the intravitreal injection of caspase 3 inhibitor (z-DEVD-cmk) the survival of axotomized beta cells on day 7 was significantly enhanced, whereas no such survival-promoting effect was obtained in axotomized alpha cells. Taken together, these results suggest that the rapid death of axotomized beta cells is attributable mainly to apoptosis, which is mediated by caspase 3.

Immune-related mechanisms participating in resistance and susceptibility to glutamate toxicity

Glutamate is an essential neurotransmitter in the CNS. However, at abnormally high concentrations it becomes cytotoxic. Recent studies in our laboratory showed that glutamate evokes T cell-mediated protective mechanisms. The aim of the present study was to examine the nature of the glutamate receptors and signalling pathways that participate in immune protection against glutamate toxicity. We show, using the mouse visual system, that glutamate-induced toxicity is strain dependent, not only with respect to the amount of neuronal loss it causes, but also in the pathways it activates. In strains that are genetically endowed with the ability to manifest a T cell-dependent neuroprotective response to glutamate insult, neuronal losses due to glutamate toxicity were relatively small, and treatment with NMDA-receptor antagonist worsened the outcome of exposure to glutamate. In contrast, in mice devoid of T cell-dependent endogenous protection, NMDA receptor antagonist reduced the glutamate-induced neuronal loss. In all strains, blockage of the AMPA/KA receptor was beneficial. Pharmacological (with alpha2-adrenoceptor agonist) or molecular intervention (using either mice overexpressing Bcl-2, or DAP-kinase knockout mice) protected retinal ganglion cells from glutamate toxicity but not from the toxicity of NMDA. The results suggest that glutamate-induced neuronal toxicity involves multiple glutamate receptors, the types and relative contributions of which, vary among strains. We suggest that a multifactorial protection, based on an immune mechanism independent of the specific pathway through which glutamate exerts its toxicity, is likely to be a safer, more comprehensive, and hence more effective strategy for neuroprotection. It might suggest that, because of individual differences, the pharmacological use of NMDA-antagonist for neuroprotective purposes might have an adverse effect, even if the affinity is low.

Glutamate release promotes growth of malignant gliomas

Glutamate neurotoxicity has been implicated in stroke, head trauma, multiple sclerosis and neurodegenerative diseases. Although recent data show that cultured glioma cells secrete glutamate, the growth potential of brain tumors has not yet been linked to an excitotoxic mechanism. Using bioluminescence detection of glutamate release from freshly prepared brain slices, we show that implanted glioma cells continue to secrete glutamate. Moreover, gliomas with high glutamate release have a distinct growth advantage in host brain that is not present in vitro. Treatment with the NMDA receptor antagonists MK801 or memantine slowed the growth of glutamate-secreting tumors in situ, suggesting that activation of NMDA receptors facilitates tumor expansion. These findings support a new approach for therapy of brain tumors, based upon antagonizing glutamate secretion or its target receptors.

On the relationship between plasma concentrations of drugs and outcome of stroke studies in laboratory animals

In assessing plasma concentrations of drugs in relation to neuroprotective effect, emphasis should be placed on measured or calculated concentrations during the window of opportunity for effect, rather than at the end of the experiment. Unbound (plasma free) concentrations should be especially considered as should brain penetration to the stroked area. Problem-solving exercises should include post hoc assessment of dosing residues and proof of exposure. The shape of the graph of response versus concentration in plasma is very steep, giving the impression of an all-or-none effect. Although higher doses lead to greater effects, attempts to statistically correlate plasma level and infarct size are likely to be unsuccessful. There is strong evidence that the pharmacokinetic properties of drugs are affected by the physiological consequences of ischemia.

A comparative assessment of the efficacy and side-effect liability of neuroprotective compounds in experimental stroke

There are many examples of compounds showing neuroprotective efficacy in animal models of stroke but not in clinical trials. It is possible that some or all of these compounds possess poor therapeutic ratios, which results in the administration of sub-efficacious doses in order to avoid the emergence of side-effects. In order to explore this possibility, this study compared the therapeutic ratios of a number of neuroprotective agents that have undergone clinical trials. Neuroprotective efficacy was established using the mouse permanent (24 h) middle cerebral artery occlusion model. Side-effect liability was determined by assessment of motor coordination using the rotarod test. The therapeutic ratio was calculated as the ratio between the minimum effective dose (MED) for significant impairment in rotarod performance and the MED for significant neuroprotection. Compounds were administered i.p. 30 min prior to rotarod testing or onset of ischemia. Drugs such as Ifenprodil, Cerestat and Selfotel, that have failed in clinical trials, were found to have very low therapeutic ratios of < or = 1, whereas compounds with more tolerable clinical side-effect profiles were found to have higher therapeutic ratios (2, 10 and 10 for Sipatrigine, Remacemide and sPBN, respectively). It is concluded that the lack of efficacy of a number of neuroprotectants in clinical trials may well be a consequence of their poor therapeutic ratios.