The use of stem cells is one of the most promising, yet ethically controversial issues in modern medicine. The primordial cells with the potential to become any tissue in the body—including the almost impossible to regenerate nerve, vascular, and ligamentous tissues—have sparked policy debates around the world because one of the most promising human sources for stem cells is blastocysts and embryos—specifically, those obtained after in vitro fertilization and abortions. Many governments, including the U.S. government, highly restrict the use of embryonic stem cells in research. Some stem-cell treatments, particularly those from embryonic stem cells, also carry high risks to patients, who may experience side effects including anaphylactic shock or tumors. At the same time, many scientists have considered adult stem cells—those sourced from bone marrow and fat—to hold far less potential for medical purposes and to be extremely difficult to harvest and cultivate.
However, an American-owned company headquartered in Haifa, Israel, may be on the verge of a breakthrough in preventing amputations due to peripheral artery disease (PAD), using a safe, effective stem-cell treatment. Pluristem Therapeutics received U.S. Food and Drug Administration (FDA) permission on June 16 to conduct the first in-human safety trials of a non-controversial form of stem cells that shows potential to treat vascular disease without risk of tumors or rejection.
The trials, which will be conducted at sites in the United States and Europe, will treat patients who have end-stage critical limb ischemia associated with PAD. The treatment substance is PLX-PAD, Pluristem's injectable form of stem cells that are harvested from human placentas and grown in a unique bioreactor system. The bioreactor system, called the PluriX™ 3D Bioreactor system, imparts stem cells with unusual immunoregulatory properties without altering the cells' genetics in any way. The FDA considers the treatment to be so potentially safe and valuable that it approved PLX-PAD for Stage 1 human safety trials just 30 days after Pluristem applied for permission.
Peripheral Artery Disease
The potential benefits of this treatment are great. According to the American Heart Association, approximately eight million people have critical limb ischemia associated with PAD. The disease narrows and hardens the arteries in a sufferer's limb, oftentimes leading to nerve damage, tissue degeneration, gangrene, and finally, amputation. It is caused and/or aggravated by diabetes and smoking, among other risk factors.
Two Kinds of Cells
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According to William Prather, RPh, MD, Pluristem's vice president of corporate development, stem cells from a variety of sources have been successfully isolated from and cultured into different types of tissue. The most well-known type, embryonic stem cells, are typically isolated from a blastocyst, a fertilized human egg that has gone through five to seven days of cell division but isn't ready to implant in the uterus. Blastocysts typically contain just 250 stem cells. Those cells are pluripotent, meaning that if the embryo develops, each of those 250 cells has the potential to divide into members of any of the three germ layers, or basic types of embryonic tissue. Once developed, these germ layers, called the mesoderm, ectoderm, and endoderm, each give rise to particular types of adult tissues. For example, if a pluripotent embryonic stem cell became part of the ectoderm, its daughter cells could become organ cells, including pancreatic cells, thyroid cells, and alveolar cells of the lungs. If a stem cell joined the mesoderm, it could generate cells including red blood cells and skeletal muscle cells, but not ectodermal cells. Once an embryonic stem cell's daughter cells differentiate into one type of tissue, they will continue producing only that type of tissue.
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In contrast, adult stem cells are only partially pluripotent. They have the potential to develop into tissues derived from only one of the three germ layers. Adult stem cells derived from placental tissue, for example, will produce only tissues that originally derive from the mesoderm.
Before Pluristem's work, adult stem cells were harvested surgically through procedures such as liposuction and bone-marrow extraction. Pluristem instead extracts a type of adult stem cells called mesenchymal stromal cells from the placenta.
"Everybody loves us because we're taking adult stem cells from the placenta, an organ that's traditionally been thrown away," Prather says.
The 'Jungle Gym'
According to Prather, Pluristem places these cells inside the bioreactor, whose interior is filled with what Prather calls a "jungle gym" of plastic scaffolding that mimics the environment of bone marrow. "Because of the spatial configuration, they don't pile up on top of each other the way they do in a typical petri dish, so the cells grow like a weed," Prather says. "They tell me in Haifa that once we get up to speed, will be able to grow something like one trillion cells from one placenta."
After the mature stem cells are removed from the bioreactor, they are rinsed with an antibody that blocks immune receptors on the stem cells' surface that could cause an immune flare-up in patients. In animal testing, the cells have so far caused no immune reactions, no matter what the blood type of the recipient. This means that PLX-PAD can potentially serve as a one-size-fits-all allogenic treatment that does not requiring donor matching. It also means that the treatments can be stored ready to use, unlike autologous stem cell treatments in which cells are surgically extracted from one patient, cultured, and then reinjected back into the same patient.
Positive Effects
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| In mice with induced PAD symptoms, tissue injected with PLX-PAD cells showed improved capillary growth (blue arrows, left) and muscle architecture compared to tissue injected with saline (right). Image magnified 400x. Image courtesy of Pluristem. |
PLX-PAD's effects on animals have been impressive. In a study published in the July 2009 Cytotherapy, Pluristem's team induced a condition similar to PAD in one back leg of a group of rodents, let the animals' wounds heal, then injected a dose of human-derived PLX-PAD into the damaged leg of half of the animals. In the other animals, a control saline solution was injected into the injured leg. The researchers measured blood flow in all of the legs with non-contact laser Doppler. Blood flow in the legs treated with PLX-PAD average 45 to 61 percent of that in the normal legs, while circulation in the control-treated legs dropped to zero as the limbs died. Three weeks later, the rodents were killed, and dissection revealed statistically significant angiogenesis—growth of new blood vessels—in the treated leg.
However, Prather says the PLX-PAD stem cells are not turning into blood vessels with the stem cells' DNA. Instead, he says, the PLX-PAD stem cells act through immune modulation. The inflamed or injured cells communicate with PLX-PAD stem cells, which then regulate the production of cytokines (immune-modulating molecules), determining where the cytokines are secreted and which ones are produced.
"I tell people that we basically inject the cells and get out of the way," Prather says. "We let the cell interaction do the work. If you look at the new blood vessels...they're made of the animal's own cells. We're sticking human placental cells into an animal, they grow new blood vessels, and the blood vessels are not of human origin."
Angiogenesis is not without risk—it is an integral part of the development of cancerous tumors. However, Prather contends that in Pluristem's trials, "We've done literally thousands of animals…and we've never seen a tumor…. We've [also] never seen an anaphylactic reaction and for that matter neither have any of the other companies that work with allogenic stem cells. That's why the FDA in our clinical trials will allow us to inject the patients and then observe them as outpatients for just six hours before we send them home."
On June 21, the first of 24 human patients was injected with a low dose of PLX-PAD to test it for safety. Each patient in the study has end-stage limb ischemia and is facing amputation. The study is testing for safety, Prather says, "but we'll also be looking at things like blood-flow measurements, transcutaneous oxygen levels, wound healing, pain assessment, and quality-of-life scores."
If the treatment proves safe after three months, Prather expects the first dosage tests to begin in 2010.
Looking to the Future
Pluristem is already looking at other applications for its stem cells—wound-healing, orthopedic-injury, and peripheral neuropathy applications have already received federal approval for safety testing in humans. The future of such treatments looks extremely bright, Prather concludes. "The three-dimensional environment allows us to expand and grow the cells in commercial quantities very cheaply. We've even had talks with companies that want to use them in companion animals."
Morgan Stanfield can be reached at