New Debate on Human Test of Stem Cells
Scientists Wrestle With Risks and Benefits of Implanting Stem Cells in People
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
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
"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