UCB publishes long-term KYGEVVI data in TK2 deficiency, showing major survival gains and functional recovery. FDA- and EMA-approved therapy remains the only treatment for pediatric and adult patients with early-onset TK2d.
Written By: Kirti Kumbhar, M. Pharm (QA)
Reviewed By: Pharmacally Editorial Team
UCB has published new clinical evidence supporting KYGEVVI (doxecitine and doxribtimine) in patients with thymidine kinase 2 deficiency (TK2d), a rare inherited mitochondrial disease. Two manuscripts published in Brain Communications showed improved survival and functional outcomes in treated patients and provided one of the largest assessments to date of disease progression in untreated individuals.
The findings add to the evidence supporting KYGEVVI, the first approved treatment for early-onset TK2d, which received U.S. FDA approval in November 2025 and European Commission authorization under exceptional circumstances in March 2026.
Understanding TK2 Deficiency and KYGEVVI
TK2 deficiency is an ultra-rare mitochondrial disorder caused by mutations in the TK2 gene that impair mitochondrial DNA maintenance. The disease can lead to progressive muscle weakness, loss of motor function, respiratory failure, feeding difficulties, and premature death, particularly in children with early symptom onset.
KYGEVVI combines the nucleosides doxecitine and doxribtimine to support mitochondrial DNA replication and cellular energy production.
Integrated Analysis Demonstrates Survival and Functional Gains
The integrated efficacy and safety analysis included 104 treated patients and 114 untreated controls.
Among patients whose symptoms began at or before 12 years of age, treatment reduced the risk of death by 92% to 94% from symptom onset. Over a modeled 30-year period, treated patients achieved an estimated average survival of 29.2 years compared with 14.4 years in untreated patients.
Functional improvements were also substantial. Seventy-five percent of treated patients regained at least one lost motor milestone after treatment initiation, while 22.5% regained four or more milestones.
“These results represent an important step forward for this rare genetic mitochondrial disease, offering new hope for patients, particularly those with early-onset TK2d,” said Dr. Michio Hirano, Professor of Neurology and Chief of the Division of Neuromuscular Medicine at Columbia University Irving Medical Center.
Treatment was generally well tolerated. Diarrhea was the most common treatment-emergent adverse event, reported in 86% of patients, and was typically mild to moderate and manageable with dose adjustments. Overall, 13.4% of patients discontinued treatment because of adverse events, while 23.9% required dose reductions.
Natural History Study Highlights Disease Burden
A separate natural history study evaluated 257 untreated patients, making it one of the largest published datasets in TK2 deficiency.
The analysis confirmed the severe and progressive nature of the disease. Among patients whose symptoms began at or before 12 years of age, 56.4% died, with a median age at death of 1.9 years. Mortality reached 66.7% among patients whose symptoms appeared by two years of age.
Motor decline was common. More than 81% of patients lost at least one previously acquired motor milestone, and over one-third lost four or more. Recovery of lost function without treatment was rare. Many patients also required ventilatory and feeding support, highlighting the substantial burden of disease.
Clinical Significance
The findings underscore the importance of early genetic diagnosis and timely treatment. With approvals now in place in the United States and Europe, the latest analyses provide further evidence that early intervention with KYGEVVI can significantly improve outcomes in this life-threatening mitochondrial disorder.
What This Means for Patients
For patients and families affected by early-onset TK2 deficiency, these findings provide evidence that treatment with KYGEVVI can significantly extend survival and help restore lost motor abilities. The data also highlight the importance of early genetic testing and diagnosis, as untreated disease often progresses rapidly and can lead to severe disability, respiratory complications, and premature death. Early intervention may offer the best chance to improve long-term outcomes and quality of life.
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About the Writer
Kirti Kumbhar (LinkedIn) is an M.Pharm graduate with experience in Quality Assurance at Lupin Limited and a strong interest in clinical research, regulatory affairs, and Trial Master File (TMF) management. She has developed knowledge of regulatory documentation, quality systems, compliance, and healthcare research through her professional experience. Passionate about clinical development and continuous learning, Kirti is committed to supporting high-quality healthcare documentation, regulatory excellence, and research-driven healthcare advancements.