UCB, the Brussels-based multinational biopharmaceutical company, has received approval from the US Food and Drug Administration (FDA) for KYGEVVI (doxecitine and doxribtimine), marking the first and only approved treatment for thymidine kinase 2 deficiency (TK2d) in adults and pediatric patients whose symptoms began at or before 12 years of age.

TK2d is an ultra-rare, life-threatening, genetic mitochondrial disease characterised by progressive (worsening over time) and severe muscle weakness (myopathy). Until now, patients had no treatment options beyond supportive care. The condition often proves fatal, with those experiencing initial symptoms on or before the age of 12 years facing a high risk of premature death (often occurring within three years after symptom onset). Worldwide prevalence is estimated at 1.64 cases per 1,000,000 people.

"The approval of doxecitine and doxribtimine represents a pivotal moment for the TK2d community, who previously had no FDA-approved treatment options for this rare genetic mitochondrial disease beyond supportive [palliative] care," said Donatello Crocetta, Chief Medical Officer at UCB.  

Kristen Clifford, United Mitochondrial Disease Foundation President and CEO, welcomed the milestone. "This is an ultra-rare disease community in dire need of treatment options. For too long, caregivers and their families have had to endure the burden of this disease. Having the first-ever FDA-approved therapy for TK2d in this patient population not only meets a critical medical need - it represents something greater - hope for the future,” she said.

The FDA approval was supported by data from one Phase 2 clinical study, two retrospective chart review studies, and an expanded access programme. Findings showed that KYGEVVI significantly improved survival time and reduced the overall risk of death by approximately 86 percent from treatment initiation.

Regulatory review of doxecitine and doxribtimine is currently underway at the European Medicines Agency (EMA), with additional global submissions planned. UCB expects KYGEVVI to be commercially available in the US in the first quarter of 2026. The company also plans to launch a personalised patient support programme to ensure equitable access and care.

In the US, KYGEVVI received Orphan Drug, Breakthrough Therapy, Priority Review and Rare Pediatric Disease designations from the FDA. As part of the approval, UCB was also granted a Rare Pediatric Disease Priority Review Voucher, which can be used for a future marketing application.