Biotechnology firm Geron said last night that it would discontinue its stem-cell research program and halt a pioneering clinical study in people with spinal-cord injury.

The decision brings to a halt the world’s largest and longest-running program to develop medical treatments from embryonic stem cells, versatile cells able to form many other types of human tissue.

Geron’s project was one of the most aggressive, controversial, and far-reaching research efforts ever undertaken by a biotechnology company. It began more than 13 years ago, when Geron financed the initial discovery of human embryonic stem cells by university researchers in Wisconsin.

Those efforts, which cost several hundred million dollars, culminated last year in a first-of-a-kind clinical study to see if lab-grown nerve cells could return movement or sensation to people with spinal-cord injury. That study, profiled in TR’s July issue, was the first using embryonic stem cells ever approved by the U.S. Food and Drug Administration. 

After being paralyzed at the chest in a car crash in September 2010, a nursing student named T. J. Atchison became the first person to be treated with Geron’s cells. The company says it will no longer accept patients into the study, though Atchison and others who have already been treated will continue to be monitored.

Geron plans to discontinue its work on stem cells in order to focus on cancer drugs. In a statement, John A. Scarlett, who joined Geron in September as CEO, blamed the decision on an “environment of capital scarcity and uncertain economic conditions.”

The company denied it had given up on stem cells for scientific reasons. “We’re not doing this because we were souring on the field, or as a result of any problems—we have not had any safety issues at all,” Scarlett told Bloomberg news.

In recent months, Geron had seen the departure of key leaders, including the director of the stem-cell study, neuroscientist Ed Wirth, and its former CEO, Tom Okarma. “The writing was on the wall, but I am disappointed, because this could mean other companies will be a little more reluctant to follow their footsteps,” says Wise Young, an expert on spinal-cord injury at Rutgers University. Young estimated that there are still dozens of companies, mostly startups, working on potential stem-cell treatments.

The attempt to study stem cells in humans had proved stupendously expensive and slow-moving for Geron. The company estimated that it spent $45 million just to win FDA approval for the initial safety trial of its treatment, known as GRNOPC1. As of October, however, only four patients had been treated, and the company would have had to spend tens of millions more in order to finish the study.

GRNOPC1 consisted of a preparation of immature oligodendrocyte cells grown in the lab from embryonic precursors. While injections of such cells had been reported to help injured rats, Geron’s decision to test it in people had attracted scientific skeptics who believed GRNOPC1 was unlikely to work, and might even harm patients.

“A lot of experts in the field were surprised that they selected spinal-cord injury as their first application. There were not very good end points, and we knew it was going to be very difficult to show a biological effect,” says Robert Lanza, chief medical officer of Advanced Cell Technology. Advanced Cell, based in Marlborough, Massachusetts, is carrying out the only other FDA-approved trials using embryonic stem cells, which aim to treat macular degeneration, an eye disease that causes blindness. Lanza says Geron’s decision to quit the field “definitely puts a lot of pressure on us to deliver a success.”

In ending its stem-cell research, Geron will lay off 66 people, or 38 percent of its workforce.