Kygevvi is indicated for patients with thymidine kinase 2 deficiency whose symptoms arise by 12 years of age. The disease manifests as muscle weakness and can become life-threatening in severe instances.
The FDA signed off on the use of UCB’s combination of doxecitine and doxribtimine for the treatment of thymidine kinase 2 deficiency, marking the first-ever approved therapy for the ultra-rare condition. The drugs are delivered together in an oral solution and will be marketed as Kygevvi.
Kygevvi is indicated for adults and children with thymidine kinase 2 deficiency (TK2d) in whom symptoms start at or before 12 years of age. UCB will make Kygevvi available on U.S. shelves in the first quarter of 2026.
Affecting fewer than 2 patients per 1,000,000 people, TK2d is caused by a mutation in the TK2 gene, which under healthy conditions produces a protein that helps maintain the proper function of the mitochondria. In patients with TK2d, this organelle weakens over time and becomes unable to provide enough energy for cells, in turn resulting in muscle weakness across the body—a key symptom.
In its more severe forms, TK2d also leads to developmental and cognitive delays, seizures, problems with moving, speaking and breathing, and, eventually, death.
Kgevvi’s approval was based on findings from a Phase II clinical trial and two retrospective chart studies, bolstered by data from an expanded access program. All told, data from 82 TK2d patients constituted the drug’s evidence base. Taken together, results showed that Kygevvi significantly reduced the overall risk of death by 86% compared to an external control group of similarly aged untreated patients.
Common safety concerns in Kygevvi-treated patients included diarrhea, vomiting and abdominal pain. There were also instances of elevated liver enzyme levels.
“It’s hard to overstate the importance of this FDA approval,” Kristen Clifford, president and CEO of the United Mitochondrial Disease Foundation, said in a statement alongside UCB’s press announcement. Patients with TK2d and their caregivers are “in dire need of treatment options,” she added, and Kygevvi will help address this “critical medical need.”
Kygevvi’s approval also marks the continuation of the regulatory winning streak in the rare disease space this year.
Last month, for instance, Glaukos won the FDA’s nod for its corneal therapy Epioxa, indicated for keratoconus. In September, the regulator also signed off on two other rare disease medicines: Crinetics’ SST2 agonist oral drug Palsonify and Stealth BioTherapeutics’ Forzinity—the first-ever treatment for Barth syndrome.
