The U.S. Food and Drug Administration (FDA) has approved KYGEVVI® (doxecitine and doxribtimine) powder for oral solution for the treatment of thymidine kinase 2 deficiency (TK2d) in adults and pediatric patients with an age of symptom onset on or before 12 years, UCB (Euronext Brussels: UCB) announced. KYGEVVI is the first and only FDA-approved therapy for this ultra-rare mitochondrial disease.
TK2d is a life-threatening, genetic mitochondrial disorder marked by progressive and severe muscle weakness (myopathy). The disease often leads to premature death, typically within three years of symptom onset in early childhood. Global prevalence is estimated at roughly 1.64 cases per million people.
“The approval of doxecitine and doxribtimine represents a pivotal moment for the TK2d community, who previously had no FDA-approved treatment options beyond supportive care,” said Dr. Donatello Crocetta, Chief Medical Officer at UCB.
Advocacy leaders also welcomed the approval, calling it a “critical milestone” for families affected by the condition.
FDA approval was supported by data from one Phase 2 trial, two retrospective chart reviews, and an expanded access program involving 82 patients with early-onset TK2d.
Across these studies, treatment with KYGEVVI or pyrimidine nucleosides was associated with an 86% reduction in overall risk of death (95% CI: 61%, 96%) compared to matched untreated patients.
The median duration of treatment was four years. Common side effects (≥5%) included diarrhea, abdominal pain, vomiting, and elevated liver enzymes (ALT and AST).
KYGEVVI combines doxecitine and doxribtimine, pyrimidine nucleosides designed to restore mitochondrial DNA levels in skeletal muscle. Elevated liver enzymes and gastrointestinal effects were among the key safety concerns identified; routine monitoring of liver transaminases and bilirubin is recommended.
KYGEVVI received multiple FDA designations, including Orphan Drug, Breakthrough Therapy, Priority Review, and Rare Pediatric Disease status. With this approval, UCB has been awarded a Rare Pediatric Disease Priority Review Voucher, which may be used for a future marketing application.
A regulatory review is ongoing with the European Medicines Agency (EMA), and additional submissions are planned globally. UCB expects KYGEVVI to be commercially available in the U.S. in Q1 2026 and plans to launch a patient-support program to facilitate access.
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