US regulators are considering whether two anemia-fighting drugs, often given to cancer patients, may in some cases stimulate tumor growth, a question raised by studies of similar products sold in Europe.

The US FDA has scheduled a meeting of outside experts for May 4 to gather input on safety issues surrounding the medicines, Amgen Inc.'s Aranesp and Johnson & Johnson's Procrit.

The agency still believes the drugs are safe, and a valuable alternative to blood transfusions, when used as directed for treating anemia in patients receiving chemotherapy.

The treatments are bioengineered versions of a human protein, erythropoietin, that stimulates production of red blood cells. Combined sales of the anemia drugs, which are given by injection, amount to several billion dollars a year.

The medicines are prescribed for patients with cancer, kidney disease and other ailments to treat anemia, a deficiency of red blood cells that causes fatigue and other symptoms.

In the studies of the European products, the drugs were used differently to see if they might help fight cancer by boosting effectiveness of chemotherapy or radiation. Some patients received higher than normal doses, and some were not anemic.

The FDA wants advice from its outside advisory committee on whether those studies are relevant to the U.S. products, and whether more research on possible tumor effects is needed, an agency official said.

One study last year found patients undergoing radiation treatment for head and neck cancer had shorter survival times if they took Roche AG's NeoRecormon, the company's anemia treatment sold in Europe.

Another trial of breast cancer patients undergoing chemotherapy was halted early because of higher mortality among those given Johnson & Johnson's Eprex, also sold in Europe. Survival at 12 months was 76 percent in patients given a placebo, and 70 per cent among patients treated with Eprex.

Some deaths resulted from progression of cancer, and others from blood clots, a known risk, the researchers said.

Researchers said the findings were not definitive but did raise concerns.

Amgen has "fully examined all of our data, and we find no evidence of (tumor stimulation) with Aranesp," Amgen spokeswoman Kelly Stoddard said.

She said the studies of the European products were not pertinent to Aranesp because they involved different drugs.

Johnson & Johnson spokesman Mark Wolfe said the company was committed to providing patients and health-care providers "with the most complete and up-to-date information regarding the safe use of this important class of medications."

He declined to comment further.

Researchers do not know why the anemia drugs might prompt tumors to grow, but some theorize they might activate some receptors on tumors that help spur growth.