WEDNESDAY, Sept. 20 (HealthDay News) -- Two German trials that used injected stem cells to strengthen the heart muscle after a heart attack got good results, while a small Norwegian trial showed no benefit.

All three trials used stem cells derived from bone marrow. Perhaps the best results came from the largest trial, done at the University of Frankfurt, which enlisted 204 patients, half of whom had stem cells injected three to seven days after a heart attack. Four months later, the injection fraction -- a measure of the heart's ability to pump blood -- was significantly better in patients who got the stem cells.

The injection fraction of the stem-cell recipients improved by 5.5 percent, compared to 3 percent for those who got conventional treatment, according to the report in the Sept. 21 New England Journal of Medicine.

After a year, the stem-cell recipients had a significantly lower incidence of second heart attacks. Their death rate was lower and fewer of them needed treatment to reopen blocked blood vessels.

A smaller trial, including 75 patients, by German researchers had some receiving either stem cells or injections of a molecule that stimulates stem-cell growth. The treatment produced moderate but significant improvement in the ejection fraction after three months, the researchers reported.

In sharp contrast, the Norwegian study of 100 heart attack patients, half of whom got stem cells, showed no beneficial effect. There was no improvement in ejection fraction, the amount of heart muscle damage, and the incidence of adverse effects was the same for patients who did or did not get stem-cell therapy.

"I think this therapy has no place in medical practice at this moment," said Dr. Ketil Lunde, a research fellow at the Rikshospitalet University, and lead author of the Norwegian report. "It should be applied only for research applications."

That's the same opinion of Dr. Andreas Zeiher, professor of medicine at the University of Frankfurt and leader of the two German studies.

"For the time being, there is no recommendation for routine use in clinical practice," Zeiher said. "We have to show in larger trials reduced endpoints such as second heart attacks, deaths and rehospitalizations."

The Frankfurt group plans such an expanded study, which will include 200 or more heart-attack patients, Zeiher said. "We hope to start it by the end of this year or early next year and have it complete in two years," he said.

And Lunde said he hasn't given up hope that stem-cell therapy might prove effective. It's possible that the Norwegian trial was too small to detect a positive effect, he said, so "larger trials are needed."

Dr. Anthony Rosenzweig, director of cardiovascular research at Beth Israel Deaconess Medical Center in Boston, who wrote an accompanying editorial in the journal, said the German studies provide "the best evidence yet for beneficial effects" of stem-cell therapy after a heart attack.

"The bottom line is that two of the three studies showed benefit," Rosenzweig said. "But the benefit was relatively modest. This approach remains experimental. Neither the long-term effect nor the long-term risk have been established."

The best strategy for future trials would be to enroll the sickest patients, Rozenzweig said.

"We should guard against both premature declarations of victory and premature abandonment of a promising therapeutic strategy," he wrote.

More information

Basic facts about stem cell therapy are provided by the U.S. National Institutes of Health.