On April 15, 2026, UCB officially announced that the European Commission had approved KYGEVVI® ( deoxycytidine and deoxythymidine ) 2g/2g oral solution powder, making it the first and currently only approved drug in the EU for the treatment of genetically confirmed thymidine kinase 2 deficiency (TK2d), suitable for children and adults with symptom onset at age 12 years and under . This is the first and currently only approved treatment for TK2d.Publicly available information indicates that the main mechanism of action of KYGEVVI (deoxycytidine and deoxythymidine) is the integration of the pyrimidine nucleosides deoxycytidine (dC) and deoxythymidine (dT) into the mitochondrial DNA of skeletal muscle , thereby restoring mitochondrial DNA copy number and improving skeletal muscle function in TK2d patients. Deoxycytidine and deoxythymidine may utilize residual TK2 activity and cytoplasmic phosphorylation pathways (such as thymidine kinase 1 and deoxycytidine kinase) to increase the mitochondrial DNA precursors deoxycytidine triphosphate and deoxythymidine triphosphate in mitochondria.
The supporting data upon which the European Commission's approval was based came from a pooled analysis of two studies* on deoxycytidine and deoxythymidine treatment in genetically diagnosed patients with TK2d who were ≤12 years of age at symptom onset. These studies assessed the impact of treatment on functional outcome measures (including motor developmental milestones, respiratory support, and feeding support) and survival. During the studies, deoxycytidine and deoxythymidine were well tolerated, with the most commonly reported adverse reactions being diarrhea (86%), vomiting (28%), and abdominal pain (including epigastric pain) (26%).

• Motor development milestones: After the start of treatment, a reduction in the loss of motor milestones was observed; 26/31 (84%) of patients recovered one or more motor milestones (e.g., sitting upright without assistance, lifting head without assistance).
• Respiratory support: Before treatment began, 18/39 (46%) of the participants started respiratory support, and none stopped. After treatment began, 5/21 (24%) of the participants started respiratory support, while 5/23 (22%) stopped.
• Feeding support: Before treatment began, 12/39 (31%) of the participants used feeding tubes. After treatment began, 4/28 (14%) of the participants started feeding support, of whom 2 stopped feeding support after treatment began.
TK2d is an extremely rare, life-threatening inherited mitochondrial disease characterized by progressive (worsening over time) and severe muscle weakness (myopathy). TK2d can affect all aspects of a patient's daily life and overall health, and can worsen over time, impacting the ability to walk, eat, and breathe independently, typically requiring 24/7 care support. The disease is often fatal, with patients experiencing symptom onset at age 12 or younger facing a higher risk of premature death (usually within 3 years of symptom onset). The global prevalence of TK2d is estimated at 1.64 per million people. For a long time, apart from supportive care, no effective treatments for TK2d have been approved, leaving patients and their families struggling in despair.
On November 3, 2025, the U.S. FDA approved KYGEVVI (deoxycytidine and deoxythymidine) for the treatment of adult patients with thymidine kinase 2 deficiency (TK2d) and pediatric patients with onset at age 12 years or younger. The approval of KYGEVVI brings hope to TK2d patients worldwide and provides valuable insights for the development of the rare disease ecosystem in China.

https://mp.weixin.qq.com/s/q3VKEPqcjygFqzQqbcILMg