Nucleoside therapy for thymidine kinase 2 deficiency: Long-term outcomes from a Brazilian cohort

Background Thymidine Kinase 2 deficiency (TK2d) is a rare, mitochondrial DNA (mtDNA) depletion/deletions syndrome leading to a severe and progressive myopathic disorder. Nucleoside supplementation (deoxythymidine and deoxycytidine) has been shown to favorably alter the disease's course, particularly in severe infantile-onset cases. Long-term data on efficacy and safety, especially in the adult patient population, remain limited. Methods This is a retrospective, long-term follow-up study of 14 TK2d patients (five children and nine adults with childhood-onset disease) treated with nucleosides. Patients were systematically evaluated over a period ranging from 9 to 36 months, with assessments conducted every 3 months during the first year of treatment, and every 6 months thereafter. Comprehensive functional assessments of motor, respiratory, and bulbar function were performed. Periodic measurements of liver and pancreatic function monitored safety and tolerability. Results All 14 TK2d patients showed beneficial effects across motor, respiratory, and bulbar function domains. Among pediatric patients, a rapid treatment response was observed early on, with functional gains sustained and continuing beyond 12 months of therapy. Adults experienced substantial improvements in motor and respiratory capacity but most of them reported severe gastrointestinal symptoms. Liver and pancreatic enzymes abnormalities were noticed mainly in adults. Conclusions Deoxythymidine and deoxycytidine were found to be safe and beneficial in this long-term cohort of TK2d patients, but elevation in liver and pancreatic enzymes were present and required regular monitorization. This study provided valuable evidence supporting this therapy as an effective and safe, long-term disease-modifying treatment option for both pediatric and adult patients.