Just over a week after the Geron Corporation announced that the US Food and Drug Administration (FDA) had placed a hold on its stem cell trial aimed at treating patients with spinal cord injury with human embryonic stem cells (hESCs), the company revealed a reason: microscopic cysts.

Human embryonic stem cells Image: Wikimedia commons, Nissim Benvenisty

The cysts had appeared with a "very low frequency" in previous animal experiments of the drug GRNOPC1, but were "non-proliferative, confined to the injury site, and had no adverse effects on the animals," Geron's statement said. In a more recent animal trial, however, the cysts showed up more often, although again, they were small and did not appear to be harmful in any way. In fact, the statement pointed out, cysts are not uncommon in victims of spinal cord injury, developing in the spinal cord scar tissue in up to 50% of patients.

"I think it provides people with a reasonable explanation," Stephen Brozak, an analyst with WBB Securities LLC in Westfield, New Jersey, told Bloomberg.com, adding that it was good news that none of the cysts had developed into teratomas.

Geron is currently working with the FDA to resolve any issues that would prevent the continuation of the trial. "We are committed to the optimization of all our hESC-based products," they said.

The FDA had no comment, according to spokesperson Karen Riley, saying in an email to The Scientist that they "neither confirm nor comment on clinical holds."


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