Doxecitine and Doxribtimine: What the Evidence Behind the First TK2 Deficiency Approval Shows

Key Takeaways

• The FDA approved Kygevvi (doxecitine and doxribtimine), from UCB, on November 3, 2025 for TK2d in adults and children whose symptoms began at or before age 12. It is the first approved therapy for the disease, according to an AdisInsight report in Pediatric Drugs.
• Approval rested on pooled data from two retrospective chart reviews, one open-label single-arm trial, and an expanded-access program, compared with untreated patients, not on a randomized trial.
• In patients with onset at 12 or younger, 4% of treated and 57% of untreated patients died in the pooled analysis. The report finds no survival difference in those with later onset. Diarrhea was the most common adverse event.

Hannah A. Blair of Springer Nature summarizes the development of doxecitine and doxribtimine in an AdisInsight report published in Pediatric Drugs. TK2d is an ultra-rare, life-threatening mitochondrial DNA depletion syndrome caused by variants in the TK2 gene. It mainly affects the muscles, causing progressive weakness, respiratory failure, swallowing and feeding difficulties, and early death in severe forms. Until now, care has been supportive only, such as ventilation and physical, occupational, and speech therapy.

The two pyrimidine nucleosides, deoxycytidine and deoxythymidine, are meant to restore the building blocks that TK2-deficient mitochondria lack for copying their DNA. The drug is a powder for oral solution, mixed with water or given by feeding tube, taken with food in three equal doses about six hours apart. Dosing is weight-based and titrated in steps: 260 mg/kg/day to start, then 520, then a maintenance dose of 800 mg/kg/day, with at least two weeks at each step depending on tolerability. Warnings in the US label include elevated liver enzymes and gastrointestinal reactions.

Because the disease is so rare, the clinical program didn't follow a traditional path. The US label's data package was pooled from two retrospective chart reviews, a prospective open-label single-arm trial, and an expanded-access program. These were compared with untreated patients drawn from a literature search and from one chart review. In the first chart review, 38 treated patients were compared with 69 untreated patients. No treated patient died, versus 58% of untreated patients, and the report cites an estimated 85% to 93% reduction in the risk of death. In the pooled analysis of patients with symptom onset at age 12 or younger, death occurred in 4% of 82 treated patients and 57% of 93 untreated patients, and the estimated 30-year restricted mean survival time was 29.2 years for treated patients versus 14.4 years for untreated patients. Among these patients, 75% regained at least one previously lost motor milestone, 16% stopped using ventilatory support, and fewer had a feeding tube placed after treatment began (12% versus 24%). In patients whose symptoms began after age 12, deaths were 18% of 22 treated and 24% of 21 untreated patients, with no significant survival difference, though 33% regained a lost motor milestone.

Safety data came from pooled analyses. Among 67 patients (345 patient-years of exposure), 24% had adverse events leading to dose reduction and 13% had events leading to discontinuation. Serious adverse events led to discontinuation in 6%, and 5% had fatal serious events, none judged related to treatment. Diarrhea was the most common adverse event in the pooled data (86%), and in the open-label trial (47 patients) the most frequent reactions were diarrhea (72%), abdominal pain (23%), vomiting (21%), and raised liver enzymes. Children had more vomiting and raised liver enzymes than adults did. Ongoing studies include the open-label trial, a phase 2 trial in adults, a phase 1/2 trial, and two expanded-access programs. The report says a European marketing authorization application, accepted in February 2025, was under regulatory review.

The evidence comes from non-randomized, largely retrospective data, compared with external untreated groups, and the pooled analyses are cited from conference abstracts. The survival benefit was seen only in patients with onset at 12 or younger. The report is a summary of published and registry information, not a new analysis, and it notes the manufacturer was allowed to comment during peer review. Even so, it concludes that the approval gives patients their first approved option, and it describes the program as a response to the challenge of studying an ultra-rare disease.