Advanced neuroprotection for brain ischemia: an alternative approach to minimize stroke damage

Abstract

Despite decades of research on neuroprotectants in the fight against ischemic stroke, no successful results have been obtained and new alternative approaches are urgently needed. Translation of effective candidate drugs in experimental studies to patients has systematically failed. However, some of those treatments or neuroprotectant diets which demonstrated only beneficial effects if given before (but not after) ischemia induction and discarded for conventional neuroprotection, could be rescued in order to apply an ‘advanced neuroprotection strategy’ (ADNES). Herein, the authors discuss how re-profiling those neuroprotective candidate drugs and diets with the best potential, some of which are mentioned in this article as an ADNES, may be a good approach for developing successful treatments that protect the brain against ischemic damage. This novel approach would try to protect the brain of patients who are at high risk of suffering a stroke, before damage occurs, in order to minimize brain injury by having the neuroprotectant drug or diet ‘on board’ if unfortunately stroke occurs.

Keywords:

• advanced neuroprotection
• brain ischemia
• diet
• ischemic stroke
• neuroprotection
• nutrition intervention
• ω-3
• pharmacological intervention
• prevention
• statin

1. Introduction

Stroke is one of the leading causes of death in developed countries and the main cause of permanent disability in adults. One in six people will suffer a stroke during their life. Thus, it is a true vascular epidemic and a medical emergency that must be effectively addressed. Therefore, all measures aimed at identifying new therapeutic strategies and implementation of prevention is crucial [1].

After several decades of research on neuroprotectants against ischemic stroke, only unsuccessful results have been obtained and no single neuroprotectant agent has been approved. Therefore, new alternative approaches are necessary to win this battle. Translation from candidate drugs effective in experimental studies to patients has systematically failed, however, some of those treatments which demonstrated only beneficial effects if given before (but not after) ischemia induction, discarded therefore for conventional neuroprotection, could be rescued in order to apply an ‘advanced neuroprotection strategy’ (ADNES). In this review, we will try to support and elaborate more on this alternative strategy and on the candidate treatments with the best profile to be used for ADNES.

Ischemic stroke is mainly due to a blockage in cerebral arteries decreasing blood flow and leading to brain infarcts. This process causes a severe metabolic stress and triggers several cellular mechanisms known as ischemic cascade, which includes excitotoxicity, calcium influx, oxidative stress, a secondary microcirculation damage, inflammation and apoptosis. This is a dynamic process that produces an irreversible loss of cell viability and finally drives to neuronal death [2].

To date, intravenous thrombolysis with tissue plasminogen activator is the only drug approved and available for the acute phase of ischemic stroke. The aim of this treatment is to restore blood flow, preventing the progress of the ischemic cascade and limiting the damage caused by ischemia. In recent months, a real revolution on reperfusion strategies using thrombectomy devices (stent retrievers) adds hope to improve efficacy and extend time-windows of conventional reperfusion therapies [3].

Therefore, it seems clear that ‘plumbers’ are beating ‘chemists’ in the field of stroke; nevertheless, strict inclusion criteria for reperfusion eligibility (including a therapeutic window limited to 4.5 h after symptoms onset) restricts thrombolytic opportunity to < 5% of stroke patients [4]. Thus, investigating new ways of implementing neuroprotective strategies is essential to limit brain damage in most of the stroke patients who remain untreated.

Two major strategies can be afforded in stroke prevention, one related to health policies (e.g., reducing salt content in foods) and another more medical strategy consisting of identifying target populations at highest vascular risk to actively prevent the occurrence of stroke. It is well known that there are several groups of patients with high risk of brain ischemia such as patients undergoing carotid or heart surgery or patients with huge risk of having a stroke in the coming days or weeks, such as patients with silent brain infarcts, intracranial stenosis, transient ischemic attacks and heart arrhythmias (Table 1).

Table 1. Potential groups of patients that might be target of ADNES and that would require short- or long-term ADNES.

Groups of patients with high risk of brain ischemia (candidates for short-term ADNES)

Patients with intracranial stenosis or carotid disease that receive angioplasty and stenting

Patients undergoing carotid endarterectomies Patients receiving endovascular or surgical treatment for cerebral aneurysms

Cardiac patients receiving heart surgery

Groups of patients with high risk of stroke (candidates for long-term ADNES)

Patients with accumulation of several vascular risk factors such as hypertension, diabetes and dyslipidemia

Old patients with unhealthy diets, physical inactivity and obesity

Patients with silent brain infarcts

Patients with intracranial and carotid stenosis

Transient ischemic attacks

Patients with arrhythmias (such as atrial fibrillation)

ADNES: Advanced neuroprotection strategy.

Our hypothesis is that in some of these groups the risk is so high that it would make sense to start neuroprotective treatments before an ischemic event occurs. Our goal would be to protect the brain at risk before the stroke takes place and to minimize brain damage when it occurs.

The aim of ‘classical’ neuroprotection is to prevent death of vulnerable cells in the ischemic penumbra which surrounds the infarct core. This tries to interfere in the early stages of the ischemic cascade with the purpose of limiting the extension of brain injury.

However, those classical approaches have systematically failed [5]. In fact, more than a thousand compounds have been tested and hundreds of clinical trials performed. Many of those neuroprotective agents have shown beneficial effects in animal models of cerebral ischemia, but none has proven to reduce tissue damage and improve the neurological outcome in human stroke patients when given at patient’s arrival to emergency departments. Translation from animal studies to human patients has failed due to several caveats both at the lab and at the clinic and mainly driven by differences between animal models and human studies and the time window between ischemia and treatment. Stroke therapy academic industry roundtable recommendations try to solve part of those discrepancies proposing that after initial evaluations in young, healthy male animals, further studies should be performed in females, aged animals and animals with comorbid conditions such as hypertension, diabetes and hypercholesterolemia [6]. Moreover, in many of the preclinical studies, the animals are pre-treated with the neuroprotective agent, and in fact several of these neuroprotective agents, administered before the induction of the experimental ischemia yielded good results, but were discarded due to absence of effectiveness when administered minutes or hours after occlusion of a cerebral artery as recommended by the stroke therapy academic industry roundtable criteria.

After systematic failures in the translation process, the scientific community is trying to address these methodological issues to find effective neuroprotectant for acute stroke and as for example in the MULTI-PART initiative multicenter experimental studies mimicking human clinical trials are planned [7].

However, we believe that not all those previous results should be abandoned forever and that there is still potential for the use of those neuroprotective agents in other scenarios that are not the urgent treatment of stroke at the emergency department.

The acquired knowledge from past failed studies might be useful if re-profiled into new therapeutic opportunities such as ADNES to protect the brain at risk in order to minimize part of the brain injury that will occur (Figure 1).

Figure 1. Neuroprotectant drugs or preventive diets which demonstrated beneficial effects if given before ischemia induction and discarded for conventional neuroprotection could be rescued in order to apply ‘advanced neuroprotection strategy’ (ADNES). ADNES is a novel approach that would try to protect the brain at risk before stroke occurs, to prevent and especially to minimize brain injury in order to be applied to patients at high risk for stroke. Images show the possible differences in brain damage after stroke in a patient at high risk of cerebral ischemia with or without ADNES.

Are there any data supporting this new strategy? Yes, in fact there are studies showing that the use of statins, hypothermia or PSD-95 inhibitors has shown promise as neuroprotectants for stroke, specifically when used in an advanced manner.

For example, patients using statins before a stroke had a better neurologic outcome [8] and are therefore beginning to be tested in acute stroke [9].

Hypothermia is currently used in patients to prevent secondary brain ischemia after cardiac arrest and resuscitation [10] and is being tested in acute stroke in clinical trials such as EUROHYP [11].

Also the neuroprotective compound NA-1, an inhibitor of PSD-95, has been tested in patients with iatrogenic stroke occurring during aneurysm repair (ENACT trial) anticipating ischemia limited its consequences [12].

These results will probably encourage additional trials in carotid surgery, open heart surgery and many other invasive treatments with a risk of brain injury with such type of strategies (stroke, hypothermia, NA-1) to be given for a short period of time before the intervention (what we named short-term ADNES).

In addition, the use of several compounds with an excellent safety profile which demonstrated its neuroprotective effect in different models of cerebral ischemia models would allow them to be administered for long periods of time before the stroke. The paradigm of a safe compound to be given for months or years would be a neuroprotective diet.

Some diets used also for prevention of cardiovascular events such as ω 3-enriched diets have shown to reduce infarct size (fish oil-enriched food for 6 weeks) as an example of ADNES [13].

The target population of that strategy would be people with accumulation of vascular risk factors, with high risk of suffering a stroke. Stroke might occur weeks, months or years after initiation of ADNES that would be what we have named a long-term ADNES.

In fact, the type of diet of our elders can contribute to neurovascular disease. Recently, this effect has been demonstrated in a clinical trial with the Mediterranean diet that supplemented with extra-virgin olive oil or nuts reduced the risk of myocardial infarction, stroke or cardiovascular death by 30% [14]. In addition, we hypothesize that the course of ischemic injury, if it occurs, would be different depending on the patient diet. The diet would not only act as a preventive strategy counteracting vascular risk factors, but it also would be a factor that could modulate the degree of injury when a cerebral artery occlusion occurs.

It is well known that diets rich in fiber, low fat and low sugar content are possibly positively associated with lower cardiovascular risk and that nutritional qualities of each diet are determined by their components. For example, seafood intake is inversely related to the cardiovascular and cerebrovascular mortality [15]. Moderate consumption of tuna or other fish, except fried fish, was associated with lower incidence of subclinical brain infarcts and white matter abnormalities on MRI examinations [16]. The intake of fish with high content in ω-3 fatty acids may have clinically important benefits to health.

In Table 2, we show that some of those compounds discarded for conventional neuroprotection could be rescued in order to be applied in advanced neuroprotection and also some natural products and natural product-derived compounds known for having neuroprotective effects.

Table 2. Potential treatments to apply the advanced neuroprotection strategy.

Neuroprotective agent	Bioactive compounds	Properties
Candidates for short-term ADNES
Glutamate blockers	Tat-NR2B9c (NA-1)	Anti-excitotoxicity, inhibitor of post-synaptic density-95 protein/nNOS complex
	Magnesium sulfate	Anti-excitotoxicity, NMDA ion channel blocker; physiological calcium channel blocker, neuroprotection
Scavenger of ROS	NXY-059	Antioxidant, free-radical-trapping, neuroprotective
Cytokine antagonists	IL-1ra	Inhibitor at the IL-1 receptor. Anti-inflammatory
Statins	Simvastatin	HMG-CoA reductase inhibitors; pleiotropic activities
Antibiotics	Minocycline	Anti-inflammatory, antioxidant, anti-apoptotic, inhibitor of microglia
Immunosuppressant	FK506 (tacrolimus)	Anti-inflammatory, anti-apoptosis, inhibitor of calcineurin activity
Hormone	Melatonin	Antioxidant, free radical scavenging, neuroprotective, anti-inflammatory
Hematopoietic growth factor	Erythropoietin	Anti-apoptosis, neuroprotective
Candidates for long-term ADNES
Fish and vegetables oil	Polyunsaturated fatty acids (ω-3)	Reduces cardiovascular risk, decreases blood pressure and blood triglyceride concentrations, decreases inflammation, improve vascular function, neuroprotective
Grapes, red wine	Resveratrol	Antioxidant, upregulates antioxidant enzymes, prevents lipid peroxidation, anti-inflammatory, anti-cancerous, reduces cardiovascular risk
Olive oil	Monounsaturated fatty acids, vitamin E, polyphenols	Decreases LDL levels, increases HDL levels, antioxidant, decreases oxidation of LDL, decreases proinflammatory and prothrombotic mediators, anticancerous, neuroprotective
Onion	Quercetin	Antioxidant, free radical scavenger, anti-inflammatory, neuroprotective, inhibition of MMP activity, BBB protection, reduces excitotoxicity, anti-aggregant, anti-hypertensive, anti-atherosclerotic
Tomato	Carotenoids, lycopene	Antioxidant, anti-inflammatory, antitumor activity, reduces cardiovascular risk, hypocholesterolemic, reduces blood pressure, neuroprotective
Green tea	Teanin, catechins	Reduces cardiovascular risk, hypocholesterolemic, anti-hypertensive effect, reduces atherosclerosis, regulates vascular tone, anti-aging, antioxidant, anticancerous, neuroprotective
Blueberry	Polyphenols, anthocyanins	Anti-inflammatory, reduces proinflammatory markers, antioxidant, scavenging free radical species, chelating metals, reduce oxidative DNA damage, antilipidemic, antiobesity
Algae	Spirulina	Neuroprotective, immunostimulatory, reduces cholesterol, antimicrobial, anticancer, antioxidant, metalloprotective
Sesame	Sesamin	Antioxidant, reduces blood pressure, antithrombotic, inhibition of cholesterol absorption, neuroprotective
Curcume	Curcumin	Antioxidant, anticancerous, anti-inflammatory, immunomodulatory neuroprotective, inhibits mitochondrial-mediated apoptotic signaling cascade, decreases lipid peroxidation and mitochondrial dysfunction, BBB protection
Ginseng	Ginseroside-Rg1	Calcium channel antagonist, neuroprotective, reduces BBB damage, antioxidant, anti-inflammatory, anti-apoptotic and immune-stimulatory activities

Drugs more suited for short-term ADNES and dietetic compounds more suited for long-term ADNES are shown.

ADNES: Advanced neuroprotection strategy; BBB: Blood–brain barrier; HDL: High-density lipoprotein; HMG-CoA: 3-Hydroxy-3-methylglutaryl-coenzyme A; LDL: Low-density lipoprotein; MMP: Matrix metalloprotease.

Among those bioactive compounds are the flavonoids. It is well known that flavonoids from fruits and vegetables have a positive impact on blood pressure, vascular function and serum lipid levels [17]. Similarly, a study of neuroprotective effects of NT-020 (nutritional supplement with blueberry polyphenols, green tea catechins, amino acids and vitamin D3) in cerebral ischemia model has shown anti-inflammatory and antioxidant properties related to functional improvement in animals and cell proliferation in the infarct [18]. In fact, a diet with 2% blueberry has been related to reduction of infarct volume and apoptotic cell death [19]. It is important to understand the mechanisms of action of these compounds; in this case, the nuclear factor erythroid 2-related factor 2 has been described as a critical regulator of flavonoids-mediated protection [20].

Although we tried to differentiate short-term ADNES drugs and long-term ADNES diet, this is an artificial distinction and those compounds might of course be useful if they finally work in both conditions. In fact, components such as melatonin with an excellent safety profile might be given for years if shown effective as ADNES [21].

Furthermore, although administration of isolated bioactive compound can be beneficial, synergies among the food components may occur producing more benefit than either treatment alone. Thus, more research is needed to determine the optimal quantity and food matrix to confer a significant clinical benefit. Moreover, due to the complexity of the ischemic cascade, ideal drugs or bioactive compounds should be targeting multiple mechanisms of damage in order to suppress them at the same time. Accordingly, the use of a combined therapy could be effective.

Finally and following with the idea of rescuing the acquired knowledge during decades of research for stroke neuroprotection, we believe that transgenic models could be a great tool to develop and demonstrate the effectiveness of ADNES. Transgenic animals are modified before induction of ischemia, that is, they are already conditioned before suffering experimental ischemic brain damage. As an example, when a KO mouse shows clear reduction of brain infarct following middle cerebral artery occlusion, that gene or pathway is a theoretical good candidate for ADNES [22]. Understanding how the genetic modification affects the development of the disease can aid directing efforts on design and synthesis of new drugs on these targets. This novel approach could be considered for future neuroprotective drug development for ADNES.

As an example, a recent study using mice expressing the human apolipoprotein E4 allele fed on a high-fat diet before permanent middle cerebral artery occlusion has shown that mice expressing this protein were more susceptible to sensorimotor deficits induced by brain ischemia. These deficits are accompanied by altered astroglial activation, neurogenesis, cyclooxygenase-2 immunoreactivity and increased plasma IL-6 [23].

Of course, well-designed studies are necessary to demonstrate efficacy and safety of this new approach. In our laboratory, we will focus in this novel strategy at the experimental and clinical levels in the coming years.

In conclusion, rescuing the acquired knowledge obtained over decades on drugs that are effective when given before experimental brain ischemia and re-profiling those best potential neuroprotective candidates, in order to apply an ADNES may be a good approach to develop a successful treatment to protect the human brain against the ischemic damage.

2. Expert opinion

ADNES is a novel approach that would try to protect the brain at risk before damage occurs, to prevent and especially to minimize brain injury in order to be applied to patients at high risk for stroke. Although translation from candidate neuroprotective drugs effective in experimental studies to stroke patients has systematically failed, some of those treatments which demonstrated only beneficial effects if given before ischemia induction, discarded for conventional neuroprotection could be rescued and reprofiled.

We have selected and reviewed a list of candidate treatments (drugs and diets) with the best profile to be used for ADNES. Strategically investing in nutritional agents (long-term ADNES) versus neuroprotective pharmacological interventions (short-term ADNES) has multiple implications. In this battle between drugs versus food, it is clear that adequate nutrition strategies offer one of the most effective and least costly ways to reduce many diseases and their associated risk factors, especially in long-term treatments. The pharmacological therapies could be useful in short-term treatments but agents with high safety profile could be beneficial also for long-term treatments. New efforts to design trials that allow testing ADNES will be required and since high sample sizes will be required, collaborative efforts will be mandatory. We believe that applying an advanced neuroprotection on a specific patient population with high risk of ischemic event, to prevent or reduce ischemic damage, may yield success to reduce costs of stroke in the coming decades.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending or royalties.

Notes