* Rituxan approved to treat two rare, orphan disorders
* Rituxan manufactured by Roche unit Genentech (Adds details on disorder, company decline to give details)
WASHINGTON, April 19 (Reuters) - The U.S. health regulators approved Roche Holding AG's (ROG.VX) big selling arthritis and cancer drug, Rituxan, for treating two rare disorders that cause blood vessel inflammation.
The U.S. Food and Drug Administration approved Rituxan, in combination with certain steroids, to treat patients with Wegener's granulomatosis (WG) and microscopic polyangiitis (MPA).
Both disorders are considered orphan diseases as they affect less than 200,000 people in the United States. The causes of these disorders are unknown, the FDA said on its website.
Rituxan -- the first FDA-approved treatment for the two disorders -- is manufactured by San Francisco-based Genentech, which was acquired by Switzerland-based Roche in 2009.
WG is estimated to affect about three people per 100,000, while MPA affects one to three people per 100,000, a Genentech spokesman said.
The company denied to give any details on the expected sales from these two indications.
The drug, also known as rituximab, is already approved to treat rheumatoid arthritis, non-Hodgkin's lymphoma and chronic lymphocytic leukemia.
It is an antibody that works by binding itself to a specific protein found on the surface of malignant and normal B-cell lymphocytes, enlisting the body's own immune system to to kill marked cells.
The drug is set to lose its patent protection in 2012 and replaced Avastin as Roche's top-selling drug during the latest first quarter. [ID:nLDE73D063] (Reporting by Esha Dey; editing by Tim Dobbyn and Andre Grenon)