Stroke Researchers Aim to Stem the “Ischemic Cascade”

by Carl Sherman

May 14, 2012

A stroke may be sudden, but much stroke damage is not. While brain cells completely deprived of blood at the core of an ischemic stroke (the most common kind)  die within minutes, in the broader “penumbra” where circulation is down but not out, the process is gradual—and reversible.

Neuroscientists speak of the “ischemic cascade:” Without the energy from oxygen and glucose required to maintain neurotransmitter storage, neurons release massive amounts of glutamate. The resulting excitotoxicity allows a flood of calcium, sodium, and water into the cell, producing excessive nitric oxide and leading to inflammation, free radical formation, and, ultimately, the death of the cell.

“Cells in the penumbra stay for hours, maybe days in a meta-stable state,” says Michael A. Moskowitz, professor of neurology at Harvard. “They don’t function normally and don’t carry impulses, but they are alive and rescuable.” 

The one approved treatment for ischemic stroke, tissue plasminogen activator (tPA), saves brain in the penumbra in the most direct way:  by dissolving the clot and restoring circulation. The drug must  be given within 3-4 hours of symptoms to do any good, though, and restoring circulation can bring problems of its own, including hemorrhage. In practice only 5 percent of patients benefit.

In recent years, researchers have sought to widen the window of therapeutic opportunity by targeting the ischemic cascade itself—halting the destructive process through neuroprotective strategies. Their activity has been intense: More than a thousand compounds have been considered, and more than one hundred tested in clinical trials—but none has yet succeeded.

 “An enormous amount of energy has been put into this, but no one has hit pay dirt,” says Moskowitz. “It’s been a very bad time for people interested in neuroprotection.” More than the innate difficulty of the problem, he suggests, the failure reflects serious flaws in the science behind screening: “Proof of concept has been lacking for most drugs chosen for clinical trials. They were tested without any demonstration that they could actually get into the brain, bind to their receptors, and do what they were intended to do.”

 On the other hand, poor experimental design may have meant promising possibilities were overlooked. “We may have thrown the baby out with the bathwater in some previous studies,” Moskowitz says.

He emphasizes that this dismal history by no means discredits the concept of neuroprotection. “There shouldn’t be so much doom and gloom. This isn’t an easy business, but there’s no theoretical reason why we can’t do a better job of rescuing cells. We need to reinvent the field.”

A most promising study

Asked about current research, Moskowitz mentioned Michael Tymianski. “He’s a very good, thoughtful investigator. I’d say, from the excitement point of view, [his work] may be the most encouraging thing we’ve seen.”

Tymianski’s research involves the excessive release of glutamate that occurs early in the ischemic cascade, the first step toward catastrophic excitotoxicity. Attempts to abort this process by blocking the NMDA glutamate receptor itself haven’t worked because glutamate neurotransmission is essential to normal neuron function. Tymianski’s approach is more selective: to inhibit a protein, PSD-95, that links the receptor to molecular events within the cell that promote overproduction of nitric oxide and the influx of calcium.

Tymianski, a senior scientist at Toronto Western Research Institute, and his colleagues have been developing a PSD-95 inhibitor for 15 years, testing it in cell cultures and rodent models of stroke. In his most recent study, reported in the March 8 issue of Nature, they administered the compound, Tat-NR2B9, to macaques, non-human primates whose brain closely resemble ours.

Findings were encouraging: the drug reduced the area of brain loss, compared with placebo, when given 1-3 hours after a large cerebral artery was blocked to simulate a stroke. The animals also fared significantly better in tests of neurological function up to two weeks later, confirming that the simulated stroke had done less damage.

“Our results show that neuroprotection is unequivocably feasible in the complex brain,” Tymianski says. “The challenge now is to design a human trial to show clinical benefit.”

The compound has already been shown to be safe in a recent clinical trial in which it was given to patients just after surgery to repair a brain aneurysm, a procedure that carries a high risk of stroke. Although he could not discuss further results in detail, Tymianski called them favorable, suggesting that ischemic damage had been reduced in patients who had strokes after the procedure.

Next, he hopes to test the compound in patients with acute ischemic stroke. Because the drug is apparently safe even in the face of hemorrhagic stroke, it might be given by emergency medical personnel en route to the hospital without the expert screening needed for tPA, dramatically shortening the time to potential neuroprotection. [Tymianski heads a company established to develop the drug in question.]

Tymianski’s success has led researchers to seek other ways to block the PSD-95 pathway. “His work is really impressive, but we like to think we’ve made a better compound,” said Anders Bach, a postdoctoral fellow at University of Copenhagen. The molecule developed by his group has a much higher affinity for PSD-95, and results of a study in mice, published in Proceedings of the National Academy of Sciences in February 2012, suggests that this enhanced its ability to protect the brain.   

Other routes to neuroprotection

Researchers elsewhere are addressing other parts of the ischemic cascade.

“In my lab we’ve used two approaches to promote survival in the penumbra,” says Nicolas G. Bazan, director of the Neuroscience Center of Excellence at Louisiana State University and a member of the Dana Alliance for Brain Initiatives. “We’ve devised new molecules that can cross the blood-brain barrier and block bad things happening. And we’ve looked inside the brain to piece out the intrinsic mechanisms that the brain sets in motion to protect itself.”

Much of his research over the past several decades has involved the release of free fatty acids in stroke, with a particular eye toward an endogenous molecule derived from the fatty acid DHA, neuroprotectin-D1, which appears to reduce the impact of ischemia.

A recent focus of his attention has been platelet activating factor (PAF), a compound that normally aids in blood clotting but when released in large amounts by ischemia apparently participates, along with glutamate, in the cascade of excitotoxicity and its consequences.

In a study reported in the March 2012 issue of Translational Stroke Research, Bazan and his colleagues showed that timely administration of a PAF antagonist, LAU-0901, to rats reduced the area of brain damage after experimental stroke, limited inflammation, and improved neuron survival. Animals treated with LAU-0901 showed significantly less behavioral and neurological impairment up to a week later, compared to those given placebo. [Louisiana State University holds the patent on LAU-0901]

Another conspicuously active area of neuroprotection research is hypothermia. Lowering body temperature by just a few degrees appears to slow multiple destructive processes unleashed by ischemia—excitotoxicity, inflammation, free radical release— simultaneously, according to Midori A. Yenari, professor of neurology at University of California, San Francisco.

Hypothermia has been shown to protect the brain against disrupted circulation in conditions other than stroke—it is recommended for resuscitation of cardiac arrest survivors, for example—and animal experiments have been encouraging.

 What’s more, its benefits may persist long after the immediate post-stroke period. “There are a few studies suggesting a positive downstream effect on recovery— when cooling [is initiated] the first day, restorative processes like neurogenesis are improved months later,” Yenari says.

Like other stroke interventions, hypothermia would probably be used along with thrombolytic therapy, but how the two interact remains an open question. “[Some] studies suggest that thrombolysis doesn’t work as well when the brain is cooled, but other research indicates that if  tPA is given, the risk of hemorrhage is reduced,” she says.

The most imposing barriers to hypothermia for acute stroke are practical: Lowering body temperature can induce uncomfortable shivering, disturb electrolyte balance, and raise the risk of pneumonia or cardiac complications, particularly in older patients with other illnesses.

Researchers have used surface cooling, circulating ice water to cool blood vessels internally, and measures like helmets to cool the brain selectively. “People are now trying to identify drugs to cool the body instead of mechanical measures” in hopes of avoiding complications, says Yenari.