Doxecitine

The progressive, often fatal myopathy seen in Thymidine Kinase 2 deficiency (TK2d) is caused by mutations in the nuclear TK2 gene, leading to a dysfunctional or deficient mitochondrial TK2 enzyme. This enzyme is essential for phosphorylating the pyrimidine nucleosides doxecitine (dC) and thymidine (dT) into dCMP and dTMP, which are precursors for the deoxyribonucleotide triphosphates (dNTPs) required for mitochondrial DNA (mtDNA) maintenance and replication. The resulting metabolic block causes severe mtDNA depletion and dysfunction of the mitochondrial respiratory chain.[] Doxecitine, in combination with doxribtimine, functions as a nucleoside substrate enhancement therapy that successfully bypasses the defective mitochondrial pathway. The high concentrations of exogenous dC and dT are primarily phosphorylated by intact cytosolic salvage enzymes, specifically Deoxycytidine Kinase (dCK) and Thymidine Kinase 1 (TK1). The nucleosides are actively transported into cells and across the blood-brain barrier via equilibrative nucleoside transporters. The resulting phosphorylated dNTP precursors are then made available to the mitochondria. This replenishes the critically depleted dNTP pools, which facilitates the replication of mtDNA by the mitochondrial DNA Polymerase gamma.