Pfizer this week announced that Bosulif, a medication it developed for the treatment of a rare type of Leukemia, has been approved by the U.S. Food and Drug Administration.

The medication is approved for the treatment of adults with chronic, accelerated, or blast phase Philadelphia chromosome-positive chronic myelogenous leukemia (CML) that has resisted prior therapy. The American Cancer Society estimates that almost 5,500 new CML cases will be diagnosed in 2012, and that just over 600 people will die of the disease. Just over 10% of new leukemia cases are CML, and over half of the people who get it are over the age of 65. According to Pfizer, Bosulif blocks CML cell growth by inhibiting the Abl and Src signaling pathways.

“Bosulif is an important new addition to the CML treatment landscape,” said Dr. Jorge Cortes, professor of medicine in the Department of Leukemia at the University of Texas, and a lead investigator of the Pfizer’s registrational study for Bosulif. “Despite recent advances, an unmet need remains for many CML patients who are refractory to one or more tyrosine kinase inhibitors.”

Bosulif was quickly approved because of its status as an orphan drug. Orphan drugs are drugs that have been developed to treat rare diseases and conditions. In the U.S. and E.U., orphan drugs are fast-tracked for approval, and companies can often charge a premium for them. The wildly expensive scorpion antivenom Anascorp is another example of a recently approved orphan drug.