(Adds analyst comment, updates shares)
By Natalie Grover
June 12 (Reuters) - Geron Corp said the U.S. Food and Drug Administration had lifted a partial clinical hold on a study testing its sole drug as a treatment for myelofibrosis, a rare form of blood cancer, sending the company's shares soaring in morning trading.
Enrollment in the early-stage trial, sponsored by the Mayo Clinic, was halted in March over concerns about liver toxicity pending followup data from Mayo's investigator on the possible reversibility of the liver damage.
Enrollment for the study ceased in January, and about 20 of the 79 patients dropped out.
Geron, whose shares rose as much as 33 percent on Thursday, did not then disclose the reason behind the dropouts, but said the remaining enrolled patients would continue to receive the drug, imetelstat.
However, in March the FDA also imposed a halt on separate company-sponsored trials evaluating the use of the drug in thrombocythemia and multiple myeloma, citing similar concerns.
Geron said on Thursday the company-sponsored trials remain suspended.
Stifel Nicolaus & Co analyst Brian Klein said that while the FDA's lift on the partial hold was positive, it was unlikely that the Mayo Clinic would enroll new patients.
As the partial hold had not prevented Mayo from dosing patients already enrolled, Thursday's announcement will make no difference to the final results of the study, he said.
Klein said he did not expect the FDA to lift its hold on company-sponsored trials in the next few months.
Geron, once one of the leading firms in the race to develop the first commercial stem-cell therapy, divested its stem-cell assets last year to focus on cancer treatments.
Imetelstat was touted as the company's savior after curing 22 percent of myelofibrosis patients in a trial last year.
Geron is likely to ask the FDA to lift the hold on the company-sponsored trials before providing all toxicity data from prior trials in order to initiate its own study in myelofibrosis patients, George Zavoico, an analyst at brokerage MLV & Co said.
"We believe the FDA will allow Geron to proceed, and if so the next trial could start in early 2015," he told Reuters.
If approved, Geron's drug would compete with Incyte Corp's myelofibrosis drug, Jakafi, which generated sales of $235.4 million last year.
Some analysts have indicated that imetelstat's ability to evoke a disease-modifying effect - defined as partial or complete remission - suggests it would be superior to Jakafi.
Incyte's shares were slightly higher at $53.90, while Geron's shares were up about 27 percent at $3.31 in early afternoon trading on the Nasdaq.
Imetelstat is designed to inhibit telomerase, an enzyme that enables the rapid multiplication of tumor cells.
The company had earlier discontinued testing the drug for treatement of breast and lung cancer after trial failures. (Reporting by Natalie Grover in Bangalore; Editing by Don Sebastian and Ted Kerr)