By David N. Leff
EPO - recombinant human erythropoietin - has two main claims to fame - one licit, one illicit.
The hormone's legal use began in 1989 as a rebuilder of red blood cells to treat the anemia side effect of kidney dialysis. Its clinical applications have since spread to the anemias of HIV infection, cancer therapy and surgery.
In parallel, EPO has virtually cornered the market, illegally, as a blood booster in competitive sports. Unlike doping by traditional anabolic steroids, recombinant EPO - biochemically undistinguishable from the native hormone secreted by the kidneys - could not be detected in urine testing. But that deep cover, enjoyed till now by athletes taking what it takes to win, is about to become history, thanks to an ingenious technique for spotting the difference. (See BioWorld Today, June 8, 2000, p. 1.)
Now recombinant human erythropoietin (r-Hu-EPO) may be in for a new and expanded role in clinical medicine. The title of a paper in the latest Proceedings of the National Academy of Sciences (PNAS), dated Sept. 12, 2000, spells it out: "Erythropoietin crosses the blood-brain barrier to protect against experimental brain injury." Its senior author is biochemist Anthony Cerami, director of the Warren Laboratories, in Tarrytown, N.Y.
"Our basic discovery," Cerami told BioWorld Today, "is that erythropoietin is transported across the blood-brain barrier to allow peripherally administered material to get into, and affect nervous cells. EPO can not only promote erythropoiesis, but it can function as a cytokine that has been transported across the blood-brain barrier to protect nervous tissue - neuronal cells, glial cells, etcetera - from a number of insults.
"Previously," he went on, "people had found the erythropoietin receptor on neuronal cells, and that astrocytes could make the hormone. But the surprising thing that we discovered about 2.5 years ago - and we've been working on ever since - is the idea that we could peripherally administer EPO, and have it protect nervous tissue before, at the time of, and even after an insult."
Hypoxia Summons Erythropoietin To Action
As its anemia-alleviating role implies, EPO mans the body's key defense against hypoxia - a drop in oxygen. And besides its main source - the kidneys - other tissues, including those in the central nervous system, also produce EPO, though not enough, or quickly enough, to avert such major insults as ischemic stroke, concussion of the brain, epilepsy or the inflammation of autoimmune multiple sclerosis. The PNAS paper reported in vivo testing of r-Hu-EPO therapy against these four cerebral pathologies.
Just as step one in grilling a trout is "First, catch your fish," the initial goal of chemotherapy is to bring the medication into physical contact with its target tissue. In EPO's anti-anemia gig, that target was the bloodstream itself - no problem. But how to reach oxygen-starved cells inside the skull? As the paper points out, "direct delivery of r-Hu-EPO into the brain is not a practical approach in most clinical contexts." However, it adds, "this concept is based on the untested assumption that the blood-brain barrier (BBB) effectively excludes large glycosylated molecules such as EPO."
But only up to a point: "Recent study," Cerami pointed out, "clearly establishes that some large molecules can be specifically transported into the brain across the capillary endothelium to affect brain function."
Putting this concept to the test, they mobilized phalanxes of laboratory rats, subjected them, under anesthetic, to the model brain lesions, then dosed them with noninvasive, systemic EPO injection into the peritoneum. Blow by blow, these were their four in vivo experiments:
Ischemic stroke: To simulate this sudden drop in cerebral oxygen, the team cut off blood to the brain by clamping a carotid artery. The brain infarction was markedly reduced in animals dosed systemically up to 24 hours after carotid occlusion - with protective effect partially lost six hours later.
Brain concussion: To create this blunt trauma, they struck a short, moderately sharp blow to the front part of the animals' skulls, using a nitrogen-driven pneumatic piston, three millimeters in diameter, then administered systemic EPO at intervals after impact, for five days. Result: "Control animals not receiving r-Hu-EPO 24 hours before the blow exhibited extensive cavitary injury 10 days after it. Animals pretreated with EPO had significantly reduced concussive injury."
Multiple sclerosis: To make MS models of their rodents, the co-authors created the classical surrogate disease, EAE - experimental allergic encephalopathy - by injecting them with guinea pig basic myelin protein. Daily inspection revealed hallmark symptoms graduated from none to flaccid tail to ataxia (staggering gait) to hind-limb paralysis and urinary incontinence. EPO treatment "significantly delayed the onset and reduced the severity of symptoms, with no 'rebound'."
Epilepsy: In this case, mice, not rats, became animal models by receiving injections of kainate, a neurotoxin, which produces epileptic seizures. The experimenters measured these episodes on a scale of 5: (1) no seizure; (2) mild limbic activity; (3) brief, sustained seizures; (4) status epilepticus - non-stop - with loss of balance; (5) status epilpticus leading to death.
"Recombinant EPO acts like an anti-epileptic drug," Cerami pointed out. "But the thing that's different about it, compared to other anti-epileptic drugs, is that we can treat an animal and then three to five to seven days later, when the drug is gone, still see its therapeutic effects."
Human Studies Will Test Rodent Compatibility
"It really points to the potential of being able to use EPO in new clinical indications," Cerami suggested, "which means to administer large enough amounts to get it across the BBB, enough to protect the brain. And since the recombinant hormone is such a safe compound, it should be fairly easy to see whether or not what we've observed in animals will hold up in people, when we do clinical trials.
"We're setting these studies up now," he continued. Because r-Hu-EPO is an FDA-approved drug, the safety issue is pretty minimal. I guess it would be a Phase II study," he observed, "a new application for an existing compound. It's a lot different than if we would be taking a new chemical into man."