April 10, 2008 -- While politicians battle over where to set limits for human embryonic stem cell studies, regulators are mulling where it should set scientific limits on the promising but controversial research.
Despite widespread media coverage, embryonic stem cells and related cells have only been implanted in a handful of human patients. Researchers say some of the experiments have shown early signs of success. Others have been failures because they were ineffective or led to tumors.
Most research is still conducted in Petri dishes and animals like rats and pigs. But with the field on the brink of producing new therapies for humans, how to test those treatments -- and how much risk to tolerate -- remain open questions.
Stem cells' scientific promise lies in their ability to form dozens of different tissues in the body. As these cells divide and grow, they can be coaxed to form heart, lung, brain, or pancreatic cells. That makes them good candidates to engineer new tissues to repair diseases or injuries.
But their potential may also be a curse. Because embryonic stem cells are genetically programmed to easily divide and grow, research shows they also have the propensity to form tumors.
Cancer and Stem Cells
Scientists and regulators now see a narrow path for the research: designing stem cell studies bold enough to find successful treatments without overreaching and causing cancer.
"We are really in uncharted water," said Stanton L. Gerson, MD, a professor of medicine at Case Western Reserve University in Cleveland and a member of an FDA advisory panel on stem cell research that met outside Washington, D.C. on Thursday.
Research in animals has already shown that high doses of stem cells are ideal for guaranteeing enough cells will survive, reproduce, and grow into new tissue once they're in the body. The cells also grow much more readily if they are implanted at a very early stage, before they differentiate.
But many studies also show that higher doses of more primitive cells are the very ones likely to produce tumors.
So experts and regulators are now wrestling with what kinds of safety cushions researchers should show in animals before the FDA lets the experiments proceed in humans in years to come.
Human Testing: Risks vs. Benefits
All medical research is supposed to balance potential risks with benefits. But embryonic stem cell researchers feel their path is especially narrow, given the politically charged atmosphere around their field.
"It would take very few patients suffering [cancer] to really put a significant damper on the field for years to come," says Steven A. Goldman, MD, a stem cell researcher at the University of Rochester and a panel member.
"All it takes is one," says John W. McDonald, MD, PhD, another panel member and director of International Center for Spinal Cord Injury at the Kennedy Krieger Institute. "Something going awry would cost the field a tremendous amount."
The narrow political path is strikingly similar to the narrow scientific path laid out before the field, scientists suggest. Stem cell research remains controversial because it may require the destruction of human embryos. In 2001, President Bush barred the federal government from funding stem cell research outside a limited number of cell lines already created at the time.
Despite the limits, the resulting political confrontation has helped researchers generate funding from private investors and from taxpayers in several states.
Amy Rick, president of the Coalition for the Advancement of Medical Research, a pro-stem-cell science group, says the FDA should not set a different safety bar for the research than it sets for studies on drugs or medical devices.
"I would plead that you do not, as scientists allow external controversy to interfere in any way with your analysis," she says.
Rick, like many advocates, expects that any of the three remaining presidential candidates would lift the current federal funding restrictions. The move would send millions in public dollars flowing into the field.
"We happen to have on record that all three of them voted twice in favor" of repealing limits, she says, referring to Sen. Hillary Clinton, D-N.Y., Sen. John McCain, R-Ariz., and Sen. Barack Obama, D-Ill.
The deliberations were also being watched by embryonic stem cell research opponents. In a statement Thursday, Sen. Sam Brownback, R-Kan., said he was "astonished" that the FDA was considering sanctioning human trials testing embryonic stem cells.
"Human beings are not guinea pigs," the statement said.