(Reuters) - The U.S. Food and Drug Administration (FDA) on Tuesday approved Celgene Corp and Agios Pharmaceuticals Inc’s oral treatment for acute myeloid leukemia (AML) patients with a rare genetic mutation.

A view shows the U.S. Food and Drug Administration (FDA) headquarters in Silver Spring, Maryland August 14, 2012. REUTERS/Jason Reed/File Photo

The drug, Idhifa, will have a monthly list price of $24,872, Celgene said in an email, noting that the median time on therapy for patients was 4.3 months in the trial to secure the FDA’s approval.

The list price of a drug is not necessarily what patients actually pay. Their ‘out-of-pocket’ cost is based on their individual healthcare insurance plans and duration of treatment.

AML is a cancer that originates in the bone marrow and progresses rapidly, resulting in an abnormal increase in white blood cells. It is generally diagnosed in older people, and is uncommon before the age of 45.

The drug secured approval for relapsed or refractory AML patients with an IDH2 mutation. It is to be used along with a diagnostic test, developed by Abbott Laboratories, designed to detect the mutation.

“While the product isn’t expected to be a big needle mover for Celgene, it does represent the first approval for a partnered product,” J.P. Morgan analyst Cory Kasimov said, pointing out that the drug targeted “an area of clear high unmet need”.

Novartis AG’s recently approved AML drug, Rydapt, has a list price of $7,495 for a 14-day treatment duration and $14,990 for a 28-day duration.

However, Novartis’ treatment is approved for newly diagnosed patients with AML carrying a specific genetic mutation called FLT3.

Idhifa’s approval comes with a boxed warning — the strongest mandated by the FDA — designed to call attention to the risk of differentiation syndrome, an adverse reaction that could be fatal if untreated.

Differentiation syndrome is characterized by fever, respiratory distress and multi-organ dysfunction.

Idhifa is the first and only FDA-approved therapy for patients with an IDH2 mutation, a group that accounts for 8 to 19 percent of all AML patients. In the United States, that translates to about 1,200 to 1,500 patients, Celgene said.

Oppenheimer’s Leah Rush Cann said in a client note that the drug could generate sales of $1.4 billion in 2021.

About 21,380 new cases of AML will be diagnosed in 2017 and some 10,590 patients will succumb to the disease this year, according to estimates by the American Cancer Society.

Shares of Agios were up 5 percent at $58.77 in afternoon trading. Celgene was little changed at $135.09.