The U.S. Food and Drug Administration (FDA) has granted approval for KYGEVVI® (doxecitine and doxribtimine) powder for oral solution (2g/2g), a key announcement made by the global biopharmaceutical company, UCB. This approval sanctions KYGEVVI for the treatment of thymidine kinase 2 deficiency (TK2d) in adult and pediatric patients who experienced the onset of symptoms at or before 12 years of age.

KYGEVVI is the first and only FDA-approved therapy for this specific patient population suffering from this ultra-rare, life-threatening, genetic mitochondrial disease, which is characterized by progressive and severe muscle weakness (myopathy).

TK2d, which has an estimated worldwide prevalence of 1.64 cases per 1,000,000 people, is often fatal, particularly for those with early symptom onset who face a high risk of premature death, often within three years of symptoms beginning. The approval provides the first treatment option beyond supportive care, a development hailed by both UCB's Chief Medical Officer and leaders of the United Mitochondrial Disease Foundation as one that meets a critical medical need and provides hope for the future.

The approval is supported by safety and efficacy data drawn from a Phase 2 clinical study, two retrospective chart review studies, and an expanded access program, which collectively included 82 unique patients with TK2d symptom onset at age 12 or younger. Efficacy was primarily assessed by comparing overall survival in treated patients to an external control group of 78 matched, untreated patients. The results demonstrated a substantial survival benefit, showing that treatment with KYGEVVI reduced the overall risk of death from the start of treatment by approximately 86%. KYGEVVI is a combination of pyrimidine nucleosides intended to restore mitochondrial DNA copy number in skeletal muscle. The most common adverse reactions reported (incidence >5%) are diarrhea, abdominal pain, vomiting, and increases in liver transaminases (ALT and AST). Importantly, the drug carries warnings regarding the potential for increased liver transaminases and gastrointestinal adverse reactions like diarrhea and vomiting, which may necessitate dose adjustments or treatment interruption, and patients should be monitored for these effects. UCB, which received Orphan Drug, Breakthrough, Priority Review, and Rare Pediatric Disease designations from the FDA, and was also awarded a Rare Pediatric Disease Priority Review Voucher, expects KYGEVVI to be commercially available in the U.S. in the first quarter of 2026.