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 Tmax 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 Cmax and AUC0-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 Tmax to a median of 2 to 4 hours and increased plasma exposure: baseline-corrected plasma dC Cmax and AUC0-t increased by ∼79% to 96% and 137% to 250%, respectively, and dT Cmax and AUC0-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 (Fe<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.

The impact of TK2 deficiency syndrome and its treatment by nucleoside therapy on quality of life.

TK2d is an ultrarare autosomal recessive mitochondrial DNA depletion syndrome. Nucleoside therapy improves or stabilizes disease across key outcomes including survival, ambulation, and requirement for mechanical ventilation. However, little is known about the effects of nucleoside therapy treatment of TK2d from the patient's perspective. This study sought to address this knowledge gap. Participants with TK2d and/or their parents/caregivers completed online surveys with standardized health measures and interviews. During interviews, participants rated and described TK2d's impact on 13 quality of life domains, changes since starting nucleoside therapy, and if they would recommend nucleoside therapy. Twenty-five individuals participated (17 adults with TK2d, 4 parent-participant pairs, 4 parents of children with TK2d). Adult participants with TK2d had clinically meaningfully worse scores than the general population on global physical and mental health, physical function, pain interference, fatigue, anxiety, and social function. Children's mobility and pain interference were significantly worse than the general pediatric population. Physical domains most affected by TK2d were: mobility (84%), fatigue (60%), respiratory function (56%), and hospitalizations (55%). Psychosocial domains most affected were: impact on family members (39%), mood (36%), and social life (28%). Most (77%) treated patients reported improvement; whereas, 67% in the untreated group reported worsening. All participants would recommend nucleoside therapy. In summary, TK2d has significant negative impacts on most areas of life and function. Measures of fatigue, sleep, swallowing/eating, speaking, and mood, should be considered as outcomes in clinical trials and research studies. Nucleoside therapy appears to provide meaningful improvements across many health domains affected by TK2d. SYNOPSIS: The consequences of having TK2d are devastating for both those with the disorder and their families; however, nucleoside therapy appears to provide meaningful improvements across many health domains affected by TK2d.

Metrics of progression and prognosis in untreated adults with thymidine kinase 2 deficiency: An observational study.

This historical cohort study evaluated clinical characteristics of progression and prognosis in adults with thymidine kinase 2 deficiency (TK2d). Records were available for 17 untreated adults with TK2d (mean age of onset, 32 years), including longitudinal data from 6 patients (mean follow-up duration, 26.5 months). Pearson's correlation assessed associations between standard motor and respiratory assessments, clinical characteristics, and laboratory values. Longitudinal data were assessed by linear regression mixed models. Respiratory involvement progressed at an annual rate of 8.16% decrement in forced vital capacity (FVC). Most patients under noninvasive ventilation (NIV) remained ambulant (12/14, 86%), reduced FVC was not associated with concomitant decline in 6-minute walk test (6MWT), and 6MWT results were not correlated with FVC. Disease severity, assessed by age at NIV onset, correlated most strongly at diagnosis with: creatinine levels (r = 0.8036; P = 0.0009), followed by FVC (r = 0.7265; P = 0.0033), mtDNA levels in muscle (r = 0.7933; P = 0.0188), and age at disease onset (r = 0.7128; P = 0.0042). This population of adults with TK2d demonstrates rapid deterioration of respiratory muscles, which progresses independently of motor impairment. The results support FVC at diagnosis, mtDNA levels in muscle, and age at disease onset as prognostic indicators. Creatinine levels may also be potentially prognostic, as previously reported in other neuromuscular disorders.

Collaborative model for diagnosis and treatment of very rare diseases: experience in Spain with thymidine kinase 2 deficiency.

BACKGROUND: Mitochondrial diseases are difficult to diagnose and treat. Recent advances in genetic diagnostics and more effective treatment options can improve patient diagnosis and prognosis, but patients with mitochondrial disease typically experience delays in diagnosis and treatment. Here, we describe a unique collaborative practice model among physicians and scientists in Spain focused on identifying TK2 deficiency (TK2d), an ultra-rare mitochondrial DNA depletion and deletions syndrome. MAIN BODY: This collaboration spans research and clinical care, including laboratory scientists, adult and pediatric neuromuscular clinicians, geneticists, and pathologists, and has resulted in diagnosis and consolidation of care for patients with TK2d. The incidence of TK2d is not known; however, the first clinical cases of TK2d were reported in 2001, and only ~ 107 unique cases had been reported as of 2018. This unique collaboration in Spain has led to the diagnosis of more than 30 patients with genetically confirmed TK2d across different regions of the country. Research affiliate centers have led investigative treatment with nucleosides based on understanding of TK2d clinical manifestations and disease mechanisms, which resulted in successful treatment of a TK2d mouse model with nucleotide therapy in 2010. Only 1 year later, this collaboration enabled rapid adoption of treatment with pyrimidine nucleotides (and later, nucleosides) under compassionate use. Success in TK2d diagnosis and treatment in Spain is attributable to two important factors: Spain's fully public national healthcare system, and the designation in 2015 of major National Reference Centers for Neuromuscular Disorders (CSURs). CSUR networking and dissemination facilitated development of a collaborative care network for TK2d disease, wherein participants share information and protocols to request approval from the Ministry of Health to initiate nucleoside therapy. Data have recently been collected in a retrospective study conducted under a Good Clinical Practice-compliant protocol to support development of a new therapeutic approach for TK2d, a progressive disease with no approved therapies. CONCLUSIONS: The Spanish experience in diagnosis and treatment of TK2d is a model for the diagnosis and development of new treatments for very rare diseases within an existing healthcare system.