Health regulators on Tuesday approved a Pfizer Inc pill for a rare type of leukemia, another step in the company's effort to expand its oncology business.

The medicine, called Bosulif, treats chronic myelogenous leukemia (CML), a blood and bone marrow disease that usually affects older adults. About 26,000 Americans live with the cancer, and 5,430 people in the United States expected to be diagnosed with it annually, the U.S. Food and Drug Administration said.

Most people with CML have a specific type of genetic mutation called the Philadelphia chromosome. This mutation causes bone marrow to make an enzyme that triggers the abnormal growth of white blood cells. Bosulif, known generically as bosutinib, blocks the enzyme's signal that causes the white blood cells to grow.

"We are seeing improvements in the treatment of CML based on a better understanding of the molecular basis of the disease," Dr. Richard Pazdur, head of the FDA's cancer drugs center, said in a statement.

Pfizer's drug is meant for people who have CML with the Philadelophia mutation who cannot tolerate other medicines, such as Novartis AG's Gleevec, or whose cancer has stopped responding to the older treatments.

The FDA gave Bosulif orphan status, which means it treats a condition or disease that affects fewer than 200,000 people in the United States. The designation comes with a seven-year period of marketing exclusivity.

The medicine is expected to reach peak global sales of $341 million in 2016, according to the average forecast of analysts polled by Thomson Reuters.

Bosulif is the second Pfizer cancer drug to get the FDA's nod this year, after its kidney cancer drug Inlyta. Investors are looking for signs of the company's research productivity, to help replace lost revenue from its cholesterol fighter Lipitor. The world's top-selling drug began facing generic competition last year.

Shares of Pfizer were largely flat at $23.85 in after-market trading on Tuesday.