FDA

FDA OKs first drug for thymidine kinase 2 deficiency

November 5, 2025

Brand name: Kygevvi

Generic name: doxecitine and doxribtimine powder

Manufacturer: UCB Inc.

Approval date: November 3, 2025

FDA approved Kygevvi (doxecitine and doxribtimine) powder to treat thymidine kinase 2 deficiency (TK2d) in adults and pediatric patients who start to show symptoms when they’re 12 years old or younger. Kygevvi received Breakthrough Therapy Designation for this indication.

TK2d is a rare, inherited genetic disorder that affects the body's ability to produce and repair mitochondrial DNA (mtDNA). Conditions that cause low levels of mtDNA, including TK2d, are called mitochondrial depletion syndromes. Symptoms of TK2d can include muscle weakness and respiratory failure. While the exact frequency of TKd2 isn’t known, it is considered very rare. Approximately 120 patients have been described in the medical literature, although the condition may be underdiagnosed.

Efficacy

Efficacy of Kygevvi was established based on data from one phase 2 clinical study, two retrospective chart review studies, and an expanded access program. Survival in patients treated with Kygevvi was compared with that in an untreated external control group composed of untreated patients from published literature and one of the retrospective studies in this drug development program.

Survival analysis for 78 matched pairs of treated and untreated patients showed there were three deaths (4%) in the patients receiving Kygevvi compared with 28 deaths (36%) in the external control group. Mean survival time at 10 years was 9.6 years for Kygevvi recipients vs. 5.7 years in the control group.

Safety

The most common adverse reactions (≥5%) are diarrhea, vomiting, increased ALT and AST, and abdominal pain.

Sources:

FDA approves 1st drug for thymidine kinase 2 deficiency, a very rare mitochondrial disease. [News release]. 2025. https://www.fda.gov/drugs/news-events-human-drugs/fda-approves-1st-drug-thymidine-kinase-2-deficiency-very-rare-mitochondrial-disease

Kygevvi (doxecitine and doxribtimine) [package insert]. Food and Drug Administration. https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/219792s000lbl.pdf Revised November 2025. Accessed November 4, 2025.

US FDA approves Kygevvi® (doxecitine and doxribtimine), the first and only treatment for adults and children living with thymidine kinase 2 deficiency (TK2d). [News release]. 2025. https://www.ucb.com/newsroom/press-releases/article/us-fda-approves-kygevvir-doxecitine-and-doxribtimine-the-first-and-only-treatment-for-adults-and-children-living-with-thymidine-kinase-2-deficiency-tk2d