Excitotoxicity: Still Hammering the Ischemic Brain in 2020

Interest in excitotoxicity expanded following its implication in the pathogenesis of ischemic brain injury in the 1980s, but waned subsequent to the failure of N-methyl-D-aspartate (NMDA) antagonists in high profile clinical stroke trials. Nonetheless there has been steady progress in elucidating underlying mechanisms. This review will outline the historical path to current understandings of excitotoxicity in the ischemic brain, and suggest that this knowledge should be leveraged now to develop neuroprotective treatments for stroke.

Predicting the Pharmacokinetic Characteristics of Edaravone Intravenous Injection and Sublingual Tablet Through Modeling and Simulation

PURPOSE

Edaravone is a free-radical scavenger with relatively favorable properties of brain penetration. It has been approved for the indications of acute ischemic stroke and amyotrophic lateral sclerosis (ALS). This study aimed to establish a pharmacokinetic (PK) model to fit the PK profile of edaravone after a single sublingual (SL) dose of a novel edaravone tablet and single IV infusion of injectable edaravone in healthy Chinese volunteers participating in a bioavailability study. The model is expected to be useful for predicting the concentration-time profiles of edaravone following different dosing regimens in a healthy Chinese population. The purposes were to identify an optimal dose and dosing regimen for the new SL formulation and to support future clinical exploration of this tablet product in its approved indications and other therapeutic fields being developed.

METHODS

The PK profiles after a single SL dose or IV infusion of edaravone 30 mg can be well described by a 3-compartment linear disposition model, on which a first-order absorption model with a lag time and a parameter for bioavailability was incorporated to fit the absorption phase of the SL dose. Performance of these PK models was evaluated for goodness of fit, residual trends, visual predictive checks, as well as precision of model predictions against external data. The validated models were employed for simulating the PK profiles of edaravone after a single SL dose of 60 mg and IV infusion of 60 mg for 60 min.

FINDINGS

The resultant estimates support the possibility and feasibility of demonstrating bioequivalence between an SL administration of edaravone 60 mg and the currently approved dosing regimen for ALS (ie, 60 mg IV over 60 min) once per day. The calculation of sample size suggested that the requirement for subject number was acceptable considering the general capacity of a Phase I study center, and so were the procedures defined in the protocol.

IMPLICATION

The models can be further applied to simulate favorable concentration-time profiles in diseases with different underlying courses of oxidative stress, and hence facilitate the optimization of current dosing regimens.

How is edaravone effective against acute ischemic stroke and amyotrophic lateral sclerosis?

Edaravone is a low-molecular-weight antioxidant drug targeting peroxyl radicals among many types of reactive oxygen species. Because of its amphiphilicity, it scavenges both lipid- and water-soluble peroxyl radicals by donating an electron to the radical. Thus, it inhibits the oxidation of lipids by scavenging chain-initiating water-soluble peroxyl radicals and chain-carrying lipid peroxyl radicals. In 2001, it was approved in Japan as a drug to treat acute-phase cerebral infarction, and then in 2015 it was approved for amyotrophic lateral sclerosis (ALS). In 2017, the U.S. Food and Drug Administration also approved edaravone for treatment of patients with ALS. Its mechanism of action was inferred to be scavenging of peroxynitrite. In this review, we focus on the radical-scavenging characteristics of edaravone in comparison with some other antioxidants that have been studied in clinical trials, and we summarize its pharmacological action and clinical efficacy in patients with acute cerebral infarction and ALS.

Antioxidant therapies for acute spinal cord injury

One of the most investigated molecular mechanisms involved in the secondary pathophysiology of acute spinal cord injury (SCI) is free radical-induced, iron-catalyzed lipid peroxidation (LP) and protein oxidative/nitrative damage to spinal neurons, glia, and microvascular cells. The reactive nitrogen species peroxynitrite and its highly reactive free radicals are key initiators of LP and protein nitration in the injured spinal cord, the biochemistry, and pathophysiology of which are first of all reviewed in this article. This is followed by a presentation of the antioxidant mechanistic approaches and pharmacological compounds that have been shown to have neuroprotective properties in preclinical SCI models. Two of these, which act by inhibition of LP, are high-dose treatment with the glucocorticoid steroid methylprednisolone (MP) and the nonglucocorticoid 21-aminosteroid tirilazad, have been demonstrated in the multicenter NASCIS clinical trials to produce at least a modest improvement in neurological recovery when administered within the first 8 hours after SCI. Although these results have provided considerable validation of oxidative damage as a clinically practical neuroprotective target, there is a need for the discovery of safer and more effective antioxidant compounds for acute SCI.

Neuroprotection and acute spinal cord injury: a reappraisal

It has long been recognized that much of the post-traumatic degeneration of the spinal cord following injury is caused by a multi-factorial secondary injury process that occurs during the first minutes, hours, and days after spinal cord injury (SCI). A key biochemical event in that process is reactive oxygen-induced lipid peroxidation (LP). In 1990 the results of the Second National Acute Spinal Cord Injury Study (NASCIS II) were published, which showed that the administration of a high-dose regimen of the glucocorticoid steroid methylprednisolone (MP), which had been previously shown to inhibit post-traumatic LP in animal models of SCI, could improve neurological recovery in spinal-cord-injured humans. This resulted in the registration of high-dose MP for acute SCI in several countries, although not in the U.S. Nevertheless, this treatment quickly became the standard of care for acute SCI since the drug was already on the U.S. market for many other indications. Subsequently, it was demonstrated that the non-glucocorticoid 21-aminosteroid tirilazad could duplicate the antioxidant neuroprotective efficacy of MP in SCI models, and evidence of human efficacy was obtained in a third NASCIS trial (NASCIS III). In recent years, the use of high-dose MP in acute SCI has become controversial largely on the basis of the risk of serious adverse effects versus what is perceived to be on average a modest neurological benefit. The opiate receptor antagonist naloxone was also tested in NASCIS II based upon the demonstration of its beneficial effects in SCI models. Although it did not a significant overall effect, some evidence of efficacy was seen in incomplete (i.e., paretic) patients. The monosialoganglioside GM1 has also been examined in a recently completed clinical trial in which the patients first received high-dose MP treatment. However, GM1 failed to show any evidence of a significant enhancement in the extent of neurological recovery over the level afforded by MP therapy alone. The present paper reviews the past development of MP, naloxone, tirilazad, and GM1 for acute SCI, the ongoing MP-SCI controversy, identifies the regulatory complications involved in future SCI drug development, and suggests some promising neuroprotective approaches that could either replace or be used in combination with high-dose MP.

Effect of glutathione augmentation on lipid peroxidation after spinal cord injury

Lipid peroxidation (LPO) is considered a major factor in damage spread after spinal cord injury (SCI). Therapies that limit LPO after SCI have demonstrated some utility in clinical trials, but more effective treatments are needed. In the present study the effects of augmenting SC levels of the endogenous antioxidant glutathione (GSH) on LPO after SCI were studied in a rat contusion injury model. A significant decrease in GSH occurred 1h after SCI which was paralleled by increases of 123% in malondialdehyde (MDA) and >500% in the 4-hydroxyalkenals (4-HA's), two LPO products. SC irrigation with gamma-glutamylcysteine (GC) preserved GSH and reduced 4-HA's below naive levels but had no effect on MDA. By 24 h after SCI, MDA returned to naive levels but 4-HA's were still elevated. Once again, GC treatment reduced 4-HA's. 4-HA's are much more reactive than MDA and are considered among the most toxic LPO products. These results suggest that (1) conditions after SCI may favor particular branches of the LPO pathway leading to differential LPO product levels, (2) MDA measurement is not by itself an adequate test for the presence or magnitude of LPO after SCI, (3) binding of GSH to 4-HA's may be an important mechanism by which the GSH system confers protection against LPO after SCI, and (4) SC GSH can be augmented after trauma by local irrigation with GC. These results also suggest that GSH augmentation may be an effective strategy for curtailment of LPO-mediated damage in acute phase SCI.

Peroxynitrite scavengers for the acute treatment of traumatic brain injury

Recent evidence has suggested that the superoxide and nitric oxide-derived reactive oxygen species peroxynitrite (ONOO-) may play a significant role in the acute pathophysiology of brain injury. One pharmacological mechanism by which ONOO(-)-mediated damage might be interrupted is by the administration of scavenging compounds such as the thiol-containing compound penicillamine. In the present study, we examined the ability of either penicillamine (Pen) or the more brain penetrable penicillamine methyl ester (PenME) (0.01, 0.1, 1.0 or 10.0 mg/kg i.v. 5 min post-injury) to improve the early (1 hr) neurological recovery (grip score) of male CF-1 mice after a severe (900 g-cm; 50 g x 18 cm) injury. Pen produced a dose-related improvement in grip score. At 1.0 mg/kg, a +112% improvement was observed compared to vehicle-treated mice, and at 10.0 mg/kg, the increase was +168% (both, p < 0.05). PenME more potently improved the 1-hr grip score, but the magnitude of the optimal effect (+96% at 0.1 mg/kg; p < 0.02) was no greater than that observed with Pen, which largely remains in the cerebral microvasculature. These results are consistent with a role of ONOO- in acute head injury, but suggest that microvascular scavenging may be of primary therapeutic importance during the early post-traumatic period.

Novel pharmacologic strategies in the treatment of experimental traumatic brain injury: 1998

The mechanisms underlying secondary or delayed cell death following traumatic brain injury are poorly understood. Recent evidence from experimental models suggests that widespread neuronal loss is progressive and continues in selectively vulnerable brain regions for months to years after the initial insult. The mechanisms underlying delayed cell death are believed to result, in part, from the release or activation of endogenous "autodestructive" pathways induced by the traumatic injury. The development of sophisticated neurochemical, histopathological and molecular techniques to study animal models of TBI have enabled researchers to begin to explore the cellular and genomic pathways that mediate cell damage and death. This new knowledge has stimulated the development of novel therapeutic agents designed to modify gene expression, synthesis, release, receptor or functional activity of these pathological factors with subsequent attenuation of cellular damage and improvement in behavioral function. This article represents a compendium of recent studies suggesting that modification of post-traumatic neurochemical and cellular events with targeted pharmacotherapy can promote functional recovery following traumatic injury to the central nervous system.

Attenuation of motor nerve terminal repetitive discharge by the 21-aminosteroid tirilazad: evidence of a neural calcium antagonist action

Pretreatment with the 21-aminosteroid antioxidant compound tirilazad mesylate has been previously shown to retard the axotomy-induced anterograde degeneration of soleus motor nerve terminals in the cat. In the present study, we examined tirilazad's effects (7.7, 13.0 or 30.0 mg/kg twice daily P.O. for 6 days) on the excitability of normal cat soleus motor nerve terminals. Low frequency (0.4 Hz) neuromuscular transmission was measured as well as the occurrence of muscle contractile potentiation in response to either a 400 Hz/10 s episode of tetanic conditioning stimulation of the soleus nerve or the administration of a 200 microg/kg i.v. dose of the neuromuscular facilitatory drug edrophonium. The mechanism of the post-tetanic potentiation (PTP) or edrophonium-induced facilitatory response involves the occurrence of a stimulus-dependent repetitive discharge of the soleus motor nerve terminals due to an exaggeration of the nerve terminal Ca2+-mediated after-depolarization. Tirilazad pretreatment caused a dose-related suppression of PTP and the edrophonium response indicative of a suppression of motor nerve terminal repetitive discharge. These effects were not shared by 6 days of oral pretreatment of cats with a high dose combination of the antioxidants vitamin E (200 I.U./day) and selenium (50 microg/day). Thus, it is unlikely that the antioxidant properties of tirilazad are involved in the suppression of motor nerve terminal excitability. Rather, it is proposed that tirilazad suppresses delayed motor nerve terminal Ca2+ conductances secondary to its ability to decrease membrane phospholipid fluidity, and that this action might in some circumstances contribute to its neuroprotective activity.