ORLANDO, Fla. --

Protein Design Labs

(PDLI) - Get Report

said Sunday that its experimental leukemia drug, Zamyl, doesn't work well enough to pass muster with U.S. drug regulators -- another setback for the troubled biotech company.

The Fremont, Calif.-based company said 36% of patients suffering from advanced acute myeloid leukemia responded to Zamyl when given in combination with chemotherapy. But 28% of patients given just chemotherapy also responded -- a statistically insignificant difference. The disappointing data was released Sunday at the annual meeting of the American Society of Clinical Oncology.

"Based on this analysis, we do not plan to seek meetings with regulatory authorities or to consider filing a Biologic License Application for Zamyl at this time," said PDL's acting CEO Doug Ebersole, in a statement. Ebersole took over the company's reins after former CEO Larry Korn

resigned unexpectedly May 1.

Korn's resignation sent PDL investors running for the exits because of fears that his departure signaled bad news at ASCO -- something denied at the time by company executives. So much for those assurances.

Last December, PDL released more positive, albeit preliminary, data on Zamyl's efficacy. Then, 43% of leukemia patients responded to Zamyl treatment, compared with 26% of patients receiving chemotherapy alone.

But in the past six months, an independent panel of doctors reviewed the patient data and discovered problems, forcing PDL to reclassify 14, or 7% of patients, in the study, according to the company.

"Zamyl has continued to demonstrate positive trends and possible therapeutic activity, and we expect to explore partnering opportunities that would allow continued development of this novel, humanized antibody," added Ebersole.

PDL has been beset by a series of clinical disappointments over the past six months. The Zamly bad news on Sunday follows poor results from its other cancer drug candidate, Remitogen. An experimental psoriasis drug, Zenapax, has also failed to live up to expectations.

PDL closed Friday at $12.27 a share.