* FDA approves Rituxan for most common adult leukemia

* Approved as first-line CLL treatment

* Can also be used after other drugs fail

* Had 2009 sales of $5.6 bln

NEW YORK, Feb 18 - U.S. health regulators approved Roche's (ROG.VX) blockbuster cancer drug Rituxan to treat the most common form of leukemia in adults.

The Food and Drug administration said on Thursday that Roche can now promote Rituxan to treat chronic lymphocytic leukemia (CLL) in patients beginning chemotherapy for the first time, as well as for those who have not responded to treatment with other cancer drugs.

Rituxan has been widely used as a treatment for non-Hodgkin's lymphoma and rheumatoid arthritis.

The drug, which is sold outside the United States under the brand name Mabthera, had 2009 sales of 6.1 billion Swiss Francs, or about $5.6 billion.

The new approval should expand sales of the drug.

In 2009, more than 15,000 new CLL cases were diagnosed in the United States and about 90,000 people are living with the disease, according to the American Cancer Society.

CLL is a slowly progressing cancer of the blood and bone marrow that typically affects people older than 50 years of age.

In a late-stage clinical trial of first time CLL patients, those who received Rituxan and chemotherapy went eight months longer before their disease worsened than those who just got chemotherapy.

Rituxan is a biotechnology drug that was developed and is manufactured by Genentech, which is now a unit of the Swiss drugmaker Roche Holding AG. (Reporting by Bill Berkrot; Editing by Tim Dobbyn)