Pharmacokinetics and Safety of a 1:1 Mixture of Doxecitine and Doxribtimine: Open-label Phase 1 Single Ascending Dose and Food Effect Studies in Healthy Adults

Purpose Doxecitine (deoxycytidine [dC]) and doxribtimine (deoxythymidine [dT]) powder for oral solution is a 1:1 mixture consisting of equal weights 2'-deoxycytidine (dC) and 2'-deoxythymidine (dT). Doxecitine and doxribtimine (referred to as study drug) is being developed as treatment for people with thymidine kinase 2 deficiency (TK2d). TK2d is an ultra-rare mitochondrial DNA depletion and multiple deletion syndrome characterized by progressive muscle weakness and premature death. Here, we report the pharmacokinetics (PK), the effect of food, and the tolerability of 2 study drug formulations, evaluated in 2 studies (Study MT-1621-103 and Study MT-1621-105). Methods A sequential, ascending 1:1 dose ratio was used for both studies (n = 14 healthy volunteer adult participants/study). After a 28-day (Study MT-1621-103) or 35-day (Study MT-1621-105) screening period, participants fasted overnight and sequentially received 86.6, 173.4, and 266.6 mg/kg study drug with a 48-hour PK assessment period and 48-hour washout period between doses. After 48 additional hours, participants were fed a high-fat meal and received 266.6 mg/kg study drug. Plasma and urine were collected before dosing and throughout the 48-hour PK period. dC and dT concentrations were analyzed by validated liquid chromatography mass spectrometry methods. Safety was evaluated throughout the study and at 2-week follow-up. Findings Plasma levels of dC and dT increased rapidly and dose-dependently above endogenous levels for both formulations, with a median T max of 1 to 2 hours under fasting conditions. Post-dose plasma dC and dT concentrations declined to nearly pre-dose (baseline) concentrations after 8 to 12 hours, suggesting rapid elimination. Peak and extent of plasma exposure (baseline-corrected C max and AUC 0-t ) tended to increase less than dose-proportionally for plasma dC and greater than dose-proportionally for plasma dT. PK variability of dC and dT was moderate-to-high (>30%). Administration with food delayed T max to a median of 2 to 4 hours and increased plasma exposure: baseline-corrected plasma dC C max and AUC 0-t increased by ∼79% to 96% and 137% to 250%, respectively, and dT C max and AUC 0-t increased by 27% to 29% and 74% to 89%, respectively, indicating a significant food effect. Renal clearance played a minor role in the elimination of systemically available intact dC and dT (F e <0.3%). The study drug was generally well tolerated; most frequent study-drug–related adverse events (AEs) were diarrhea (n = 4/29, 14%) and dizziness (n = 3/29, 10%). Most AEs were mild-to-moderate in severity. Implications Doxecitine and doxribtimine are orally bioavailable in the intended clinical dose range. The PK profile supports a formulation consisting of equal doses of doxecitine and doxribtimine, a 3-times-daily dosing regimen, and administration with food.

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