11/3/25, 5pm EST — Earlier today, the U.S. Food and Drug Administration (FDA) approved UCB’s Kygevvi (doxecitine and doxribtimine) to treat the mitochondrial disease thymidine kinase 2 deficiency (TK2d).

“It’s hard to put into words what today’s decision means for patients living with TK2d,” said United Mitochondrial Disease Foundation President and CEO Kristen Clifford. “Too many families have had to endure the terrible burden of this disease. The approval of the first-ever therapy for TK2d is more than a medical milestone – it’s a moment of hope, validation, and possibility for every patient and caregiver who has never stopped believing.”

TK2d is an ultra-rare mitochondrial disease and enzyme deficiency defined by muscle weakness, breathing difficulty, limb weakness that impairs gait or causes loss of ability to walk, droopy or saggy eyelids, and trouble chewing and swallowing that is often treated with feeding tubes. While estimating TK2d population size is challenging, the most recent literature puts the disease’s prevalence at 1.64 patients per million people.

Kygevvi is the first FDA-approved therapy for TK2d and marks the second FDA-approved therapy to treat a form of mitochondrial disease this year. Forzinity was approved for Barth syndrome in September of 2025.

UMDF has long been a leader in the TK2d community, having hosted an FDA Listening Session focused on the disease in 2022, and just this past June, a Masterclass with a TK2d focus at Mitochondrial Medicine Conference 2025  to help clinicians identify and provide superior care to affected patients.

Dr. Philip Yeske, UMDF Science and Alliance Officer, said this decision should bring hope to all mitochondrial disease patients.

“We thank the many patients, caregivers, clinicians, researchers, and partners who helped make this possible. Every success story like this brings more attention and interest to the mitochondrial disease therapy space,” said Dr. Yeske. “While we still remain in the early stages of approvals, today’s announcement reaffirms our belief that every mitochondrial disease patient deserves access to an effective therapy – and we won’t rest until that holds true.”

Full prescribing information can be found on ucb.com.