Other targets

If the Geron test gets the OK and proves effective, the company hopes to seek approval eventually for similar tests of human embryonic stem-cell therapies for congestive heart failure, Parkinson's disease, diabetes and Osteoporosis, among other things.

But other scientists are skittish, including Evan Snyder, director of the stem cell and Regeneration program at the Burnham Institute in La Jolla. He praised Geron for making great progress with its research. Nonetheless, he said the company should do more non-human tests, and make sure their tests precisely mimic the kinds of spinal cord injuries humans get.

``I'm not convinced they have done that yet,'' Snyder said.

Cancerous side effects are another worry. Because human embryonic stem cells can become any tissue, Arnold Kriegstein, director of the Institute for Stem Cell and Tissue Biology at UC-San Francisco, fears the injections might trigger tumors. As a result, he said, the FDA and Geron need to be especially cautious.

``The bar would have to be a little higher'' for human experiments, Kriegstein said. ``There is a great potential for harm.''

Still others are nervous about Geron using a federally approved line of stem cells, many of which have been nourished in so-called feeder layers made from mouse or calf tissue. Studies have found such cells can be contaminated with a molecule that could prompt an immune system attack if injected into humans.

But Geron's chief executive, Dr. Tom Okarma, said none of those concerns warrant banning human tests.

For one thing, he said, Geron has developed feeder layers that are free of contamination. And to prove it, Geron published data last year that found no Immune Response in mice injected with its human embryonic stem cells.

Okarma said no cancer has turned up in any animals that have received the company's cells, either.

And although Geron is still compiling data in preparation for filing a formal request to the FDA to do the test, he's confident from the work already done at UC-Irvine and at his company that the procedure will be safe.

``We're turning over every stone,'' to avoid harming anyone in the experiment, Okarma said. ``The last thing in the world a public company wants to do is fall on its nose in its first clinical trial.''
Contact Steve Johnson at sjohnson@mercurynews.com or (408) 920-5043.