Kygevvi (doxecitine and doxribtimine)

The first FDA-approved therapy for thymidine kinase 2 deficiency (TK2d), an ultra-rare mitochondrial muscle disease. Kygevvi is an oral powder containing two pyrimidine nucleosides (doxecitine and doxribtimine) that act as substitute building blocks for mitochondrial DNA. Approved on November 3, 2025 for adults and pediatric patients whose TK2d symptoms began on or before age 12.

The most common side effects (occurring in 5% or more of patients) are diarrhea, abdominal pain, vomiting, and elevated liver enzymes (alanine aminotransferase / ALT and aspartate aminotransferase / AST). Liver function blood tests are required before starting and periodically during treatment. Some side effects are more pronounced when treatment is first started or when the dose is increased. Diarrhea and vomiting can be severe: some patients needed a hospital stay, a lower dose, or to stop treatment, so tell the care team if these happen or do not go away.

Kygevvi is supplied as 2g/2g powder packets (2 grams of each ingredient per packet) for oral solution. The powder is mixed with water once a day to make a 1-day supply, then taken by mouth (or by feeding tube) in 3 equal doses about 6 hours apart, with food. Dosing is based on body weight for all patients, adults and children, and is increased in steps up to 800mg/kg/day as tolerated. UCB dispenses Kygevvi in the U.S. through PANTHERx Rare, its only specialty pharmacy for the drug, which announced the arrangement on March 23, 2026.

Yes, Doxecitine and doxribtimine (Kygevvi) is FDA approved (2025) for the treatment of Thymidine Kinase 2 Deficiency.

Yes. On November 3, 2025, the FDA approved Kygevvi (doxecitine and doxribtimine) as the first and only treatment specifically for thymidine kinase 2 deficiency in adults and pediatric patients whose symptoms began on or before age 12. On March 23, 2026, PANTHERx Rare announced that UCB had chosen it as the only specialty pharmacy that dispenses Kygevvi in the U.S.^[7].

TK2d patients have a broken enzyme called thymidine kinase 2 that normally helps mitochondria make and repair their own DNA. Kygevvi is an oral powder containing two pyrimidine nucleosides (deoxycytidine and deoxythymidine analogs) that bypass the broken enzyme and provide the building blocks mitochondria need directly. It does not correct the underlying TK2 gene mutation, so it must be taken long-term.

Across UCB's combined clinical program, treatment reduced the overall risk of death by approximately 86% compared with matched untreated patients in a natural-history comparison. In pooled data UCB presented in March 2025, 30 of 40 treated patients whose symptoms began by age 12 (75%) regained at least one motor milestone they had lost (such as sitting, standing, or walking), and of 31 patients on ventilatory support, 5 cut their daily use and 5 stopped it altogether^[6]. Earlier treatment was associated with greater benefit.

The most common side effects (occurring in 5 percent or more of patients) are diarrhea, abdominal pain, vomiting, and elevated liver enzymes (ALT and AST, the standard liver-function blood tests). Liver enzyme monitoring is required before starting Kygevvi and periodically during treatment. Side effects tend to be more pronounced when starting therapy or increasing the dose.

Yes, discuss with a genetic counselor or metabolic specialist. TK2d is autosomal recessive, so each sibling has a 25 percent chance of being affected. Because earlier treatment with Kygevvi is associated with greater benefit, identifying an affected sibling before significant muscle damage occurs can change their lifetime trajectory.