Doxecitine/Doxribitine Approval Redefines Care in Thymidine Kinase 2 Deficiency: Fabian Somers, PhD

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"This approval is more than just a clinical milestone—it gives the gift of hope and the gift of time. For the first time, physicians can move beyond palliation and truly change the trajectory of TK2d."

Thymidine kinase 2 deficiency (TK2d) is an ultra-rare autosomal recessive mitochondrial myopathy caused by pathogenic variants in the TK2 gene, which encodes thymidine kinase 2, a mitochondrial matrix enzyme important for the mitochondrial nucleotide salvage pathway. Because TK2 is responsible for phosphorylation of deoxythymidine (dT) and deoxycytidine (dC) within mitochondria, its deficiency leads to mitochondrial DNA (mtDNA) depletion and/or multiple mtDNA deletions.

Recently, the FDA approved UCB’s dC and dT, a fixed-dose combination therapy, as the first treatment for patients with Tk2d, marking a groundbreaking decision for patients and families affected by the disorder. Marketed as Kygevvi, the treatment’s clinical program was comprised of a phase 2 study (NCT03845712), as well as 2 retrospective chart review studies, and an expanded access program. Overall, treatment with the combination therapeutic led to an 86% (95% CI, 61%-96%) reduced risk of death over a median duration of 4 years (range, 1 day to 12 years).

Following the approval, NeurologyLive^® sat down with Fabian Somers, PhD, vice president and Asset Head of Rare and Ultra-Rare Diseases at UCB, to get thoughts and perspectives on the significance of the first approved treatment for Tk2d. In the interview, Somers spoke on the clinical data that led to the agent’s approval, as well as the impact the treatment could have beyond clinical benefits. Overall, he stressed that the approval delivers what the community values most: hope and time, giving physicians a therapeutic tool to truly alter the course of Tk2d.