A late stage trial of a Gilead Sciences drug in previously treated patients with a particular type of leukemia was unblinded early after independent monitors determined the medicine provided significant benefit in delaying worsening of the disease, the company said on Monday.

The study was testing Gilead's Zydelig in combination with the standard treatment, Rituxan and Treanda, compared with the standard treatment alone in patients with chronic lymphocytic leukemia (CLL) whose disease had progressed after prior therapy.

Generally, when it becomes clear that a new drug will succeed in a trial, the study is unblinded so that patients receiving other drugs or a placebo can be offered the study drug.

A planned interim analysis of data from the 415-patient study by the data monitoring committee found a statistically significant benefit in progression-free survival and overall survival and ordered the trial to be unblinded, Gilead said.

Details of the trial and the magnitude of benefit will be presented at a major medical meeting next month.

Independent safety monitors oversee blinded trials so that they can be halted or unblinded early in case it is determined that the drug is causing harm or if the benefit becomes clear so that the medicine can be offered to placebo patients. A trial can also be stopped for futility if monitors determine the study is doomed to fail.

Gilead said it plans to submit supplemental regulatory filings in the United States and Europe early next year with the hope of expanding the Zydelig approval for use with Rituxan and Treanda in previously treated CLL patients.

Zydelig, known chemically as idelalisib, is already approved to treat CLL in combination with Rituxan (rituximab), and for a type of non-Hodgkin lymphoma in patients who have received at least two prior therapies.

(Reporting by Bill Berkrot; Editing by Dan Grebler)