BRUSSELS, Belgium, June 17, 2026
UCB has announced the publication of two significant peer-reviewed studies in Brain Communications demonstrating the positive impact of KYGEVVI® (doxecitine and doxribtimine) in patients living with Thymidine Kinase 2 Deficiency (TK2d), a rare and life-threatening genetic mitochondrial disease. The newly published data provide compelling evidence that treatment with KYGEVVI significantly improves survival rates and functional outcomes while highlighting the devastating burden of disease progression among untreated patients. The publications represent one of the most comprehensive analyses ever conducted in TK2d and reinforce the critical importance of early diagnosis, genetic testing, and timely therapeutic intervention. KYGEVVI became the first and only approved treatment for early-onset TK2d, receiving approval from the U.S. Food and Drug Administration in November 2025 and European approval in March 2026. The latest findings further strengthen the clinical evidence supporting the therapy and provide new hope for patients and families affected by this ultra-rare neuromuscular disorder.
Clinical Data Demonstrate Significant Survival and Functional Benefits
The integrated efficacy and safety analysis included data from 104 treated patients and 114 untreated patients, making it one of the largest assessments of TK2d outcomes reported to date. Researchers found that KYGEVVI treatment significantly improved survival and functional abilities, particularly among patients who developed symptoms at or before 12 years of age. The analysis showed that treatment reduced the risk of death by 92% to 94% from symptom onset in early-onset patients. Over a 30-year period, treated individuals were estimated to live an average of 29.2 years compared with 14.4 years for untreated patients, representing a substantial improvement in life expectancy.
Furthermore, approximately 75% of treated patients regained at least one motor milestone, while nearly one-quarter regained four or more milestones, highlighting meaningful improvements in mobility and quality of life. The treatment also demonstrated an acceptable safety profile, with most adverse events being manageable and not leading to treatment discontinuation.
Largest Untreated TK2d Dataset Highlights Urgent Need for Diagnosis
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A second publication examined the natural history of TK2d using data from 257 untreated patients, providing one of the most comprehensive evaluations of disease burden ever reported. The findings revealed the severe consequences of untreated disease, particularly among children with early symptom onset. Researchers observed that 56.4% of patients who developed symptoms at or before 12 years of age died prematurely, with a median age at death of just 1.9 years. Loss of motor function was widespread, with more than 81% of patients losing previously acquired motor milestones and over one-third losing four or more milestones during disease progression.
Many patients required ventilatory support and nutritional assistance, emphasizing the profound impact of the disorder on patients and caregivers. These findings underscore the urgent need for earlier recognition of symptoms, improved genetic testing strategies, and broader access to disease-modifying therapies capable of altering the course of this devastating condition.
First Approved Therapy Marks Major Advance for Rare Mitochondrial Disease
TK2d is an ultra-rare mitochondrial disorder characterized by progressive muscle weakness, respiratory complications, and significant reductions in life expectancy. Prior to the availability of KYGEVVI, treatment options were limited largely to supportive care. The newly published clinical evidence demonstrates how targeted nucleoside therapy can meaningfully alter disease progression and improve long-term outcomes.
UCB emphasized its commitment to addressing unmet needs within the rare disease community and advancing awareness of TK2d among healthcare professionals. Experts involved in the studies noted that early intervention appears to be a critical factor in maximizing patient benefit, reinforcing the importance of timely diagnosis and treatment initiation. As additional real-world data emerge, KYGEVVI may continue to reshape the treatment landscape for patients living with this rare and previously untreatable mitochondrial disease.
Source: UCB press release
