A clinical trial plans to assess the safety and efficacy of doxecitin and doxribtimine (dC + dT) in patients with thymidine kinase 2 deficiency (TK2d), according to details published on clinicaltrials.gov.
The single-center study will recruit 10 to 15 patients aged 18 years and older who received care at the 12 de Octubre Hospital in Madrid, Spain. All participants will take three doses of oral dC + dT daily, waiting six to eight hours between each dose.
In order to participate, individuals must have a genetically confirmed diagnosis of TK2d and have moderate to severe symptoms. In addition, patients must have motor involvement, respiratory involvement or both.
Initially, patients will receive 130 mg dC and 130 mg dT per kilogram of body weight per day. If patients experience no side effects, the dose will be increased to a maximum of 400 mg dC and 400 mg dT per kilogram of body weight daily. The investigators expect to treat patients for 24 months.
Read more about TK2d therapies
Throughout the study, participants must undergo evaluation to investigate the safety and efficacy of the treatment. The investigators will measure motor function via the Medical Research Council Scale for Muscle Strength, the 6-minute walk test, the 100-meter run test, the North Star Ambulatory Assessment and the FACIT-Fatigue score. The study will also perform spirometry and collect data on use of ventilatory support to assess respiratory function.
In addition, the trial will measure levels of mitochondrial DNA (mtDNA) at the start of treatment and six months later via a muscle biopsy. This will allow the researchers to determine the extent to which dC + dT restores mtDNA production.
The study, which is being conducted in collaboration with UCB Pharma, is expected to be completed by Dec. 31, 2027.
The U.S. Food and Drug Administration approved dC + dT for children and adults with TK2d in November 2025. The European Medicines Agency is currently reviewing the combination therapy.
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