Kim Moran, Head of U.S. Rare Diseases at UCB, discusses the approval of Kygevvi (doxecitine and doxribtimine) for patients with thymidine kinase 2 deficiency (TK2d).
TK2d is a rare genetic disease characterized by progressive and severe muscle weakness. It is often fatal, with those experiencing initial symptoms on or before the age of 12 years facing a high risk of premature death (often occurring within 3 years after symptom onset). Other common symptoms include breathing difficulties, weakness in the eye muscles, and trouble chewing and swallowing. TK2d is caused by genetic mutations in the TK2 gene.
Kygevvi is a combination of two pyrimidine nucleosides, doxecitine and doxribtimine, that target and restore mitochondrial DNA in the skeletal muscle. It is the first and only treatment, approved by the U.S. Food and Drug Administration (FDA) in November 2025, for patients with TK2d. The approval is based on safety and efficacy data showing Kygevvi reduced the overall risk of death from the start of treatment by 86%.
As Dr. Moran explains, there are only 110 patients diagnosed with TK2d in the United States. To address this unique population, the clinical development program was designed by taking data from chart reviews and a retrospective study and creating a natural history. This natural history data was used as the control group and compared with a single-arm, open-label cohort.
Dr. Moran notes how this approval not only gives physicians a better treatment option for their patients, but also gives the TK2d community hope for better disease management and better treatment options to come.
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To learn more about TK2d and other rare genetic diseases, visit https://checkrare.com/diseases/congenital-and-genetic-conditions/
