Still, a handful of terminally ill patients have undergone stem-cell therapy, even knowing that the experimental technique might not work.
"You have to move quickly when you have three years to live," said Stephen Heywood, diagnosed with amyotrophic lateral sclerosis, also known as Lou Gehrig's disease, in 1998. Last year, doctors at Thomas Jefferson University's CNS Gene Therapy Center harvested stem cells from his marrow and injected them directly into his spinal cavity in an attempt to grow functional motor neurons damaged by the disease. Two other ALS patients followed Heywood, who lives in Massachusetts with his wife and child. The results were less than promising; according to a recent study published in the Journal of Hematotherapy and Stem Cell Research, the technique didn't change the course of the fatal disease.
Some researchers developing cells isolated from the human body say that it is too early in the scientific investigation to even think about moving into humans.
"There is a widespread view in the field that any such introduction of immature cells whether they are embryonic, tumor-derived or other kinds of stem cells into a clinical situation is premature," said Dr. Jeffrey Macklis, associate professor of neurology and neuroscience at Harvard Medical School. "We need to learn more about how to control such cells and to understand what may be their great future potential."
The hope is that the cells ultimately can be used either to replace dying or damaged cells or serve as pumps to deliver growth factors to help damaged cells survive. The fear is that the cells might make things worse.
Researchers who work with human embryonic stem cells have come to learn about the potential pitfalls. Some of these cells can transform into tumors. "We can enhance and select for different cell types, but the question of purity is always on our minds," said Dr. John Gearhart, a scientist at Johns Hopkins Medical Institutions who developed the first human embryonic germ-cell line.
Experimental tumor-derived cells had been infused into at least a dozen stroke patients before the Food and Drug Administration convened a panel of stem-cell experts to determine whether such cells were ready for use in humans. The year was 2000, and the consensus, according to Dr. Steven Goldman, a researcher at Cornell University Medical Center in Manhattan, was, no, science was not ready to place any experimental cell lines into humans.
Goldman, Macklis and other stem-cell researchers agree that it may be naive to think that one could dump cells in a diseased area and magically fix what's broken. "Going slowly and carefully is very important," Macklis said.
The first patient received transplanted tumor-derived cells in July 1998, in an FDA-approved study conducted by Layton Bioscience, a small biotech company in California. Layton had bought rights to a line of cells engineered at the University of Pennsylvania. But the cell line from the start was risky: The cells were originally culled from a patient with testicular cancer that had metastasized into germ cells in the patient's body. It was an interesting tumor line to study because germ cells can become any cell type in the body. Stem cells possess a similar ability but are derived from early embryos.
Amid controversy, Layton manufactured the cell line to produce neuronal cells. Scientists in the field worried openly about using cancer-derived cells, but the cells seem to work without causing tumors.
Layton president Gary Snable and University of Pittsburgh's Dr. Douglas Kondziolka, the surgeon who has performed the transplant in 12 patients since the summer of 1998, have said there were no side effects from the transplanted cells and there may have even been some improvement.
Experts outside of the Layton study still wonder about the risk for unforeseen problems when putting these cells into humans.
But hopes have been raised in animal studies with stem cells. Cells infused into the damaged spinal cords of mice and rats seemed to grow, and the animals were able to move their hind legs.
Hopkins' Gearhart says he worries that laboratory-grown cells behave differently depending on where in the body they are placed. "Our concern is that we sometimes find tumors in the areas where we put these cells," he said.
Other scientists are turning to different cell sources. Dr. Seth Finklestein, a stroke researcher with the biotech company Viacell in Massachusetts, said that umbilical-cord blood is proving to be a good source for stem cells.