The U.S. Food and Drug Administration (FDA) said on Thursday it approved Pfizer Inc's treatment for a rare type of blood cancer.

The treatment, Besponsa, carries a boxed warning, the agency's severest form of warning reserved to caution against the most serious adverse events.

Besponsa was approved to treat adults with relapsed or refractory B-cell precursor acute lymphoblastic leukemia (ALL), a disease that will affect about 5,970 people in the United States this year and kill about 1,440, according to the National Cancer Institute.

B-cell precursor ALL is a rapidly progressing type of cancer in which the bone marrow makes too many B-cell lymphocytes, an immature type of white blood cell.