What Is Kygevvi, and Why Does It Matter?
Kygevvi is a new oral medicine for people with thymidine kinase 2 deficiency (TK2d), an extremely rare genetic condition affecting the muscles. The FDA has approved it for the treatment of adults and children with TK2d whose symptoms began at age 12 or younger.
TK2d affects the mitochondria – tiny structures inside your cells that produce energy to carry out vital cellular functions. TK2d reduces your body's ability to maintain mitochondrial DNA, causing serious muscle weakness and breathing problems that often get worse with time. It can be life-threatening, especially when symptoms begin early in life. Until now, there were no approved treatments for TK2d, and care was limited to managing symptoms.
Kygevvi combines two ingredients – doxecitine and doxribtimine – that are natural building blocks of DNA called pyrimidine nucleosides. Kygevvi delivers these DNA building blocks to the mitochondria in muscle cells, helping restore the number of mitochondrial DNA copies and improve muscle strength and function.
This approval is important because Kygevvi is the first and only medicine in the U.S. approved to treat TK2d, offering families a new treatment option for a rare condition that affects about one to three people per million worldwide.
Why Was It Approved?
The FDA approved Kygevvi after clinical studies showed that it helped people with TK2d live longer and significantly lowered the overall risk of death after the start of treatment.
The studies included both adults and children who started having symptoms before age 12. In one analysis involving 78 matched pairs, only three patients (4%) treated with Kygevvi, versus 28 patients (36%) in the untreated group, died. People who took Kygevvi had about an 86% lower risk of death, compared with those who didn't get the medicine. Most people took the medicine for several years, and it was generally well tolerated.
The most common side effects were diarrhea, pain in the belly area, vomiting, and higher levels of liver enzymes.
What Do I Need to Know?
Kygevvi comes as a powder that needs to be dissolved in water before use. This liquid is taken by mouth three times a day with food. You'll use a special kit called the ZX2000 to mix and measure each dose. Measuring cups and spoons from home aren't accurate enough for proper dosing. The amount you take depends on your weight and may be increased slowly over time, based on how your body handles it. Once mixed, the solution must be used within 16 hours.
If a dose is spit out or not fully taken, don't give another one. Just wait for the next scheduled dose. If you miss a dose, take it as soon as you can, but avoid taking it if the next one is due in less than two hours. In that case, skip the missed dose and stick to your regular schedule. Never take two doses at once.
Before starting Kygevvi, your health care provider will check your liver function. Let your health care provider know if you have any liver, stomach, or kidney problems and about other details of your medical history. If you have signs of liver problems or ongoing stomach issues like vomiting or diarrhea, your provider may pause or stop the medicine.
It is not known if the medicine affects a fetus or passes into breast milk. Talk to your health care provider if you are pregnant, planning pregnancy, or breastfeeding while on Kygevvi.
Contact your health care provider right away if you lose your appetite or have belly pain, notice dark urine or yellowing of your skin or eyes, or have diarrhea and vomiting that lasts longer than a few days. These could be signs of serious side effects.
Kygevvi is expected to be available in the U.S. in early 2026.