(Reuters) - The U.S. Food and Drug Administration on Friday approved Stemline Therapeutics Inc’s Elzonris for the treatment of a rare blood disease in adults and children aged two years and above.

This is the first approved treatment for the condition, blastic plasmacytoid dendritic cell neoplasm (BPDCN), Richard Pazdur, director of the FDA’s Oncology Center of Excellence, said.

Elzonris will be commercially available in early 2019, the company said.

The labeling for Elzonris contains a boxed warning, FDA’s harshest, flagging increased risk of capillary leak syndrome, which may be life-threatening or fatal to patients in treatment.

Last year, the drug developer said a patient death had occurred in the clinical studies of Elzonris, after developing capillary leak syndrome, a “well-documented side effect” of the drug.

“Boxed warning should have been expected given the history of capillary leak syndrome, but we don’t think it will affect uptake,” Wedbush Securities analyst David Nierengarten said.

Stemline Therapeutics shares dropped as much as 9.9 percent to $7.82, before partially recovering to $8.5.

BPDCN is an aggressive and rare disease of the bone marrow and blood that can affect multiple organs, including the lymph nodes and the skin, the FDA said.

Roth Capital analyst Jotin Marango estimated 350 BPDCN patients in the U.S., while the annual treatment cost of a patient is estimated at $285,000.

The approval unlocks lateral indications for the drug, Marango said in a note.

Elzonris is also being tested for other indications, including bone marrow disorders chronic myelomonocytic leukemia and myelofibrosis.