A retrospective, observational study of medical records showed patients with Thymidine Kinase 2 deficiency (TK2d) treated with Kygevvi (doxecitine and doxribtimine) powder showed a 95% reduction in risk of death compared to those who were not, The results were recently published in the journal Neurology.
Kygevvi became the first and only drug to be approved to treat TK2d by the U.S. Food and Drug Administration in November 2025. It is approved for patients whose symptoms began by or before age 12. These results, along with results from a Phase 2 clinical trial and an expanded access use program, were submitted to the agency for approval.
Kygevvi combines two substances, doxecitine and doxribtimine, which help the body make healthy mitochondrial DNA in muscle cells.
The study looked at medical records from patients treated at multiple centers around the world. Researchers reviewed charts from people with TK2d who received KYGEVVI and compared them to patients who did not receive the treatment. This type of study is called a retrospective chart review.
The results were very encouraging. None of the treated patients died, while 58% of untreated patients did. Overall, the treatment reduced the risk of death by as much as 95%. It’s important to note that these are results from one study, and the results of a clinical trial published later showed a reduced risk of death of approximately 86%.
Learn more about TK2d signs and symptoms
The study also showed improvements in daily function. Before treatment, about 71% of patients had lost at least one motor skill, such as the ability to sit, stand or walk. Almost 30% had lost four or more skills. During treatment, no patients lost any motor skills. Even more promising, 65% of patients who had lost skills regained at least one motor milestone after starting treatment.
Breathing also improved for some patients. Half of the patients needed breathing support before treatment. After treatment, nearly one-third of those patients were able to reduce their need for ventilatory support.
Most side effects were mild, such as diarrhea, fever or temporary changes in blood tests, and very few patients stopped treatment because of side effects.
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