Doxecitin and Doxribthymine in Adult Subjects With Thymidine Kinase 2 (TK2) Deficiency

Inclusion Criteria:

1. Signed informed consent by the subject.

2. Subject must be greater than 18 years of age at time of consent.

3. Genetic diagnosis of TK2 deficiency

4. Subject should have evidence of a moderate to severe disease, with motor and orrespiratory involvement, shown by one of the following:

5. North Star Ambulatory Assessment Scale (NSAA) less than 30

6. 6-minute walking test less than 450 meters

7. Force Vital Capacity in the sitting position less than 70 percent or a drop inthe decubitus position greater than 10 percent or need for mechanicalventilation.

8. Female subjects must have no intention to become pregnant during the study. Femalesubjects who are of childbearing potential (that is, following menarche until atleast 1 year post-menopausal if not anatomically and physiologically incapable ofbecoming pregnant) must agree and commit to the use of highly effective methods ofbirth control for the duration of the study and for 30 days after the end of thestudy, and be willing to have additional pregnancy tests conducted during the study.Acceptable methods are defined as those that result, alone or in combination, in alow failure rate (that is, less than 1 percent per year) when used consistently andcorrectly, such as surgical sterilization, an intrauterine device, or hormonalcontraception in combination with a barrier method.

9. Male subjects with partners of childbearing potential must agree to use effectivecontraception methods during the study and for at least 90 days after the last doseof the study medication. Acceptable methods include the use of condoms combined withspermicidal foam/gel/film/cream/suppository.

10. Willingness to comply with the study protocol, including but not limited to, allstudy procedures, study visits, and study drug compliance.

Exclusion Criteria:

1. History of liver disease, or liver function test results (alanine aminotransferase [ALT], aspartate transaminase [AST], or total bilirubin) greater than or equal2times (2X) the upper limit of normal. Patients with transaminases greater than 2times (2X) can participate with the approval and monitoring of a doctor specializingin liver toxicity.

2. Participation in a previous trial of any investigational agent for primarymitochondrial disease within 1 year prior to informed consent, or use of any otherinvestigational therapy within 30 days (or 3 half-lives, whichever is longer) priorto informed consent, or participation in other clinical studies, within 30 daysprior to informed consent, which in the opinion of the study Sponsor, maypotentially confound results from this study.

3. Pregnant (females 10.0 years old or older will have a pregnancy test at screening),or breastfeeding