Washington

Muscling in: stem cells are injected into a patient's heart during bypass surgery. Credit: BENETTI FOUNDATION

The prospects for using adult stem cells in medical treatments improved this week, when cardiac surgeons reported that injecting bone-marrow cells into the heart can boost its function.

The finding, released on 25 April at the annual meeting of the American Association for Thoracic Surgery in Toronto, Canada, is set to add heat to an ongoing debate about whether such therapies have benefits that outweigh their risks.

The study looked at 20 people with heart failure and is the largest clinical trial so far to include stem-cell treatment. The hearts of ten patients were injected with stem cells purified from their bone marrow during bypass surgery. The other ten underwent bypass surgery but did not receive any stem cells, says Amit Patel of the University of Pittsburgh School of Medicine in Pennsylvania, the principal investigator.

Six months after surgery, the patients' heart function was assessed by checking the amount of blood pumped with each beat. The score on this scale for those who received stem cells was almost ten points higher than for those who only had the bypass.

Stem cells from bone marrow are classified as ‘adult’ and their use is less politically fraught than that of embryonic stem cells. Adult stem cells have long been viewed as less flexible than embryonic stem cells, which can divide to produce any cell type in the body. But recent studies of human cells suggest that adult stem cells can also turn into many cell types, including heart, brain and liver cells. Supporters of adult stem-cell therapies say that this is what probably happened in Patel's trial — but not everyone agrees.

“This clinical trial basically says that bone-marrow cells do differentiate” into heart muscle, says cardiologist Piero Anversa of the New York Medical College in Valhalla, New York, who has reported the same finding in mice1.

But critics say that when scientists use genetic markers to track bone-marrow cells injected into mouse hearts, the cells do not change type2,3. They speculate that other mechanisms, such as the effects of chemicals used in the bone-marrow preparation, could explain the improvement in heart function.

“I'm a little sceptical about the result because it is a very limited study, and so far the evidence for the regenerating effect of bone marrow is very controversial,” says cardiac surgeon Philippe Menasche of the Georges Pompidou European Hospital in Paris.

Patel admits that it is not clear whether the bone-marrow cells actually changed type. He adds that his group will soon begin a follow-up study to examine this question in humans. Patients who are to undergo a heart transplant will have their hearts injected with bone-marrow cells months before their operation. Then, when they undergo surgery, the team will dissect and examine the old hearts to see whether the bone-marrow cells did indeed change type.

Many scientists say that they are reluctant to undertake further large clinical trials of the technique until they hear more definitive answers about the risks and benefits of adult stem-cell therapies.

“I'm not trying to downplay the fact that they've seen a benefit,” says developmental biologist Jonathan Epstein of the University of Pennsylvania in Philadelphia. “But there's a risk to implanting cells into the heart, and if the benefit is unrelated to the regeneration of cells then we might be able to develop a therapy that has fewer risks.”