First treatment for rare thymidine kinase 2 deficiency

30 January 2026

Kygevvi improves motor function of patients with disease onset at or before 12 years of age

EMA has recommended granting a marketing authorisation in the European Union for Kygevvi (doxecitine and doxribtimine) in patients with genetically confirmed thymidine kinase 2 deficiency (TK2d) whose disease started at or before 12 years of age.

TK2d is a rare, life-threatening genetic disease that affects fewer than 1 in 1,000,000 people and for which there is no authorised treatment. Current care is limited to supportive measures such as providing nutrition through a feeding tube, supporting movement through physiotherapy and helping with breathing using a ventilator.

TK2d is caused by mutations in the thymidine kinase 2 (TK2) gene. These mutations prevent the TK2 enzyme, which helps produce and maintain mitochondrial DNA, from working properly. As a result, mitochondria cannot function correctly and muscles cannot produce enough energy, leading to progressive myopathy (muscle weakness), loss of motor function and the ability to walk, breathing difficulties and a shortened life expectancy.

The way Kygevvi works in patients has not been confirmed, but studies in animal models suggest that its active substances, the pyrimidine nucleosides doxecitine and doxribtimine, are modified and incorporated into mitochondrial DNA in muscle cells, improving the production and maintenance of mitochondrial DNA. By doing so, Kygevvi is expected to compensate for reduced TK2 activity and help slow the progression of the disease.

EMA’s recommendation is based on data from a retrospective chart review and a phase 2 single arm clinical study in 39 patients with TK2d whose disease onset occurred at or before 12 years of age. The effect of Kygevvi on motor function was assessed by comparing motor milestones before and after treatment in each patient. After treatment with Kygevvi, 84% of patients regained one or more motor milestones, suggesting that the medicine improves motor function.

The most common side effects reported with Kygevvi are diarrhoea, vomiting and abdominal pain.

The marketing authorisation for Kygevvi was recommended under exceptional circumstances. In this situation, authorisation can be granted with specific obligations that are reviewed each year. This approach was used because the applicant cannot provide full efficacy and safety data because of the rarity of TK2d. EMA requested the company to conduct a new study to confirm the safety and efficacy of Kygevvi.

Kygevvi was supported through EMA's PRIority MEdicines (PRIME) scheme, which provides early and enhanced scientific and regulatory support to medicines that have a potential to address patients' unmet medical needs.

The opinion adopted by EMA’s human medicines committee (CHMP) is an intermediary step on Kygevvi’s path to patient access. The opinion will now be sent to the European Commission for the adoption of a decision on an EU-wide marketing authorisation. Once a marketing authorisation has been granted, decisions about price and reimbursement will take place at the level of each Member State, taking into account the potential role or use of this medicine in the context of the national health system of that country.