Skysona Blood Cancer Side Effects Lead to New FDA Restrictions

Seven additional patients have developed blood cancer on Skysona, since the gene therapy drug first received accelerated approval in 2022.

Federal health regulators are requiring the gene therapy drug Skysona to carry new warnings and information about potential blood cancer side effects, indicating patients who take the drug will require life-long medical monitoring.

An update to the Skysona black box warning label was announced by the U.S. Food and Drug Administration (FDA) on August 7, after a review of updated clinical trial data found additional cases of life-threatening myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) in treated patients.

Skysona (elivaldogene autotemcel) is a stem cell-based gene therapy used to treat cerebral adrenoleukodystrophy (CALD), a severe neurological disorder that breaks down the protective barrier around nerve fibers in the brain and spinal cord.

CALD can cause rapid decline, leading to learning deficits, poor school performance, difficulty reading and understanding speech, visual disturbances, loss of hearing, seizures, and eventually the loss of all body functions and death.

The FDA granted Skysona accelerated approval in 2022, based on clinical trial results that showed blood cancer in 4% of patients, with three cases among 67 participants. However, by November 2024, an investigation into additional Skysona blood cancer cases led the agency required a label warning, which added new information about the potential side effects of the therapy.

Since then, the Skysona blood cancer risk rate has more than tripled. As of July 2025, 10 out of 67 patients in the trials, about 15%, have been diagnosed with blood cancer, including at least one death. Seven of those cases were reported after the drug’s initial approval. 

In some patients, blood cancers developed before Skysona could provide any therapeutic benefit for CALD, with diagnoses reported anywhere from 14 months to 10 years after treatment.

The Skysona boxed warning states that it should only be used in CALD patients who have no other suitable treatment options. It also requires lifelong monitoring for blood cancer, with complete blood counts at least every three months, and mandates that any cases be reported to the FDA’s MedWatch program.

The agency warns that patients treated with Skysona will need to be monitored for blood cancer for the rest of their lives, and should receive complete blood count tests at least every three months. If blood cancer is detected it should be reported to the MedWatch Adverse Event Reporting program.