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Background TK2 deficiency (TK2d) is an autosomal recessive mitochondrial disease, characterised by progressive myopathy with early respiratory involvement and external ophthalmoplegia. Deoxynucleoside replacement therapy has emerged as a promising therapeutic intervention for patients with TK2d.Aims To report clinical characteristics and treatment response in patients with TK2d under an expanded access programme (EAP)Methods/Materials Combination deoxynucleoside therapy was administered to two paediatric and one adult patients with TK2d as per protocol. Outcome measures including Revised Hammersmith Scale (RHS) and 6 minute-walk test (6MWT) were performed at follow up to evaluate treatment response.Results Proximal and axial myopathy, and hyperCKaemia were present in three patients. One child was non-ambulatory and required nocturnal non-invasive ventilation. Two transient adverse reactions (loose stool) were reported to date. The most noticeable improvement in RHS was observed in the non-ambulatory child (3 to 19) compared to the other paediatric patient (64 to 69) at 9-month follow up, and the adult patient at 3-month follow up (51 to 58). Clinically-meaningful change of 6MWT was evident in both ambulatory patients (53.5m and 86.4m, respectively).Conclusion Early recognition and diagnosis of TK2 deficiency will enable patients to be treated with deoxynucleoside therapy which is generally well-tolerated, safe and efficacious.Teddymoe29@gmail.com