Long-Term Safety and Efficacy of KL1333 in Adults with Primary Mitochondrial Disease

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What is this study about?

This clinical trial is studying primary mitochondrial disease, a group of rare conditions in which the body’s cells do not make energy normally. The treatment being used is KL1333 (napazimone), an oral tablet taken by mouth. The purpose of the study is to learn more about the long-term safety and tolerability of this medicine and to see how it may affect the disease over time.

This is an extension study, which means it follows people who have already taken KL1333 or placebo in a previous study. During the study, the medicine is taken for about 48 weeks, and health checks are done throughout the study to watch for side effects and other safety issues. The study also looks at how people feel and function in daily life, including tiredness, movement, and overall health.

Primary mitochondrial disease can cause muscle weakness, severe tiredness, exercise problems, and other body-wide symptoms because the cells do not produce enough energy. The study is focused on adults with this condition, and the medicine is given as a tablet taken orally, meaning by mouth.

1 study treatment period

after joining the study, you receive KL1333 (napazimone) as an oral tablet.

the dose is 100 mg by mouth, once daily.

treatment continues for 48 weeks.

2 safety monitoring during the study

throughout the study, your safety is checked with adverse events (any unwanted medical problems), physical examination, vital signs (such as blood pressure and pulse), ecg (a test that records the heart’s electrical activity), c-ssrs (a questionnaire about thoughts of self-harm), and safety laboratory tests, including blood counts, blood chemistry, and urine tests.

during the study, the trial also monitors for metabolic decompensation (a serious worsening of body chemistry), lactic acidosis (too much acid in the blood from lactic acid), and image-verified stroke-like episodes that happen after gastrointestinal side effects, meaning side effects involving the stomach or intestines.

3 patient-reported and clinical assessments

during the study, you complete questionnaires and tests that measure tiredness, daily function, and overall health.

these include PROMIS fatigue pmd short form (a short questionnaire about tiredness in primary mitochondrial disease), the 30-second sit-to-stand test (how many times you can stand up from a chair in 30 seconds), neuro-qol lower extremity function short form (a questionnaire about leg and mobility function), patient global impression questionnaires about severity and change, eq-5d-5l (a questionnaire about health and quality of life), clinician global impression of pmd questionnaires about severity and change, and nmdas subscales i-iii (a set of measures of disease progression).

if you have diabetes, hba1c is also measured. hba1c is a blood test that shows average blood sugar over time.

Who Can Join the Study?

Be an adult patient, meaning 18 years of age or older.
Have completed the FALCON study.
Have followed the study requirements in the FALCON study, according to the study doctor and sponsor.
Be willing and able to give informed consent, which means agreeing to take part after the study has been explained.
Be willing and able to attend the study visits within the required time windows.
Be willing and able to complete electronic patient-reported outcomes (ePROs), which are questionnaires answered on an electronic device.
Have other medicines that are expected to stay stable during the study, if this is medically possible.
Be willing to keep idebenone stopped during the study.
If female, not be pregnant at the start of the study.
If female and able to become pregnant, agree not to try to become pregnant and use a highly effective birth control method from consent until at least 36 days after the last dose of the study medicine.
If female and able to become pregnant, agree not to donate ova (eggs) during the study and for 36 days after the last dose of the study medicine.
If female, agree not to breastfeed during the study and for 36 days after the last dose of the study medicine.
If male and having a female partner who can become pregnant, agree to use a male condom plus a highly effective birth control method during treatment and for 96 days after the last dose of the study medicine.
If male and having a partner who is pregnant or breastfeeding, agree to use a male condom during treatment and for 96 days after the last dose of the study medicine.
If male, agree not to donate sperm during the study and for 96 days after the last dose of the study medicine.

Who Cannot Join the Study?

The person is considered unlikely to follow the study rules or is otherwise not suitable for the study, including if there is cognitive impairment (problems with thinking, memory, or understanding).
Any medical, mental health, laboratory, or other condition that could make study participation unsafe, affect the results, or make the person inappropriate for the study.
Use of idebenone within 14 days before the first study dose. Idebenone is a medicine that may affect the study results.
A history of unstable or severe illness involving the lungs, immune system, cancer, liver, kidneys, or any other serious medical problem that could interfere with the study, or use of a medicine that could affect safety checks, tolerability, or study results.
A positive pregnancy test at screening for female participants.
Use of an investigational drug within 30 days or 5 half-lives before screening, whichever is longer, or a plan to use another investigational drug during the study. An investigational drug is a medicine still being tested and not yet approved for general use.
General fatigue or muscle weakness caused by something other than mitochondrial disease, if the investigator thinks this is the cause.
Significant heart disease, such as a serious or ongoing abnormal heart rhythm, enlarged heart chambers, reduced heart function, or certain heart conduction problems.
An abnormal ECG (electrocardiogram, a test that records the heart’s electrical activity) that is considered clinically important by the investigator.
A QTcF value above the allowed limit on the ECG: more than 450 milliseconds for males or more than 470 milliseconds for females. QTcF is a heart timing measurement corrected for heart rate.
A recent history of unstable disease or not fully recovered from stroke-like episodes, including:
- one or more stroke-like episodes within the last 6 months
- more than 1 seizure per month within the last 6 months
- hospitalization for status epilepticus within the last 6 months (a dangerous seizure that lasts a long time or happens repeatedly without full recovery)
- more than 4 days of migraine episodes per month within the last 6 months
A history of inflammatory bowel disease (long-term swelling of the digestive tract), stomach erosions (small sores in the stomach lining), peptic ulcer disease (ulcers in the stomach or upper intestine), or gastrointestinal bleeding (bleeding in the stomach or intestines).
Gastroesophageal reflux disease confirmed by tests and causing symptoms at any time in the last 6 months. Gastroesophageal reflux disease means stomach acid moving back into the food pipe.
One or more blood or urine test results at screening that are outside the normal range and could be risky for the person’s safety.
Kidney function that is too low, shown by an estimated glomerular filtration rate below 30 mL/min/1.73 m². This is a measure of how well the kidneys filter blood.
Bilirubin in the blood above 1.5 times the upper normal limit, unless the increase is due to Gilbert’s syndrome and liver disease has been ruled out.
ALT or AST liver enzyme levels above 2 times the upper normal limit, unless the study team allows a limited exception. ALT and AST are blood tests that can show liver injury.
In the investigator’s opinion, severe ataxia (poor coordination), neuropathy (nerve damage), balance problems, or any other condition that would make the 30-second sit-to-stand test hard to evaluate.
Untreated or poorly treated sleep apnea, which means repeated breathing pauses during sleep.

Where you can join this trial?

Verified and Recommended Sites

No sites found in this category

Verified Sites

Site Name	City	Country
Katholieke Universiteit te Leuven	Leuven	Belgium
Fondazione Policlinico Universitario Agostino Gemelli IRCCS	Rome	Italy
Centre Hospitalier Universitaire De Lille	Lille	France
Université Libre de Bruxelles – Hôpital Erasme	Brussels	Belgium

Other Sites

Site Name	City	Country
Universitair Ziekenhuis Gent	Gent	Belgium
Vseobecna Fakultni Nemocnice V Praze	Prague	Czechia
Roskilde University	Roskilde	Denmark
A.O.U. Policlinico G. Martino Di Messina	Messina	Italy
Azienda Unita Sanitaria Locale Di Bologna	Bologna	Italy
Fondazione I.R.C.C.S. Istituto Neurologico Besta	Milan	Italy
Centre Hospitalier Universitaire De Nice	Nice	France
Centre Hospitalier Universitaire De Nantes	Nantes	France
Centre Hospitalier Universitaire De Bordeaux	Bordeaux	France
Hopital Beaujon	Clichy	France
Hospital Universitari General De Catalunya	Sant Cugat Del Valles	Spain
Hospital Universitario 12 De Octubre	Madrid	Spain
Charite Universitaetsmedizin Berlin KöR	Berlin	Germany
Azienda Ospedaliero Universitaria Pisana	Pisa	Italy
Coywvh Hwqbyctkchj Rvcibxnp Delpkgqtmpciwy	Angers	France
Mcekidldiwjlqorsqbxwnalyiy Htzmdjorgkrmycjp	Halle (Saale)	Germany
Hwedktgr Uzshydkujocjkv Svwbvmcpwt &iczopv Hqrysdm do Hzjplgtjxwx	STRASBOURG, Alsace	France

Trial status

Country	Status	Recruitment Start
Belgium	Not yet recruiting	29.06.2026
Czechia	Not yet recruiting	29.06.2026
Denmark	Not yet recruiting	29.06.2026
France	Not yet recruiting	29.06.2026
Germany	Not yet recruiting	29.06.2026
Italy	Not yet recruiting	29.06.2026
Spain	Not yet recruiting	29.06.2026

Trial locations

KL1333 is the study medicine being tested in this trial. It is taken by mouth as a tablet and is being given to people with primary mitochondrial disease to see how safe it is, how well it is tolerated, and whether it may help over the long term. In this extension study, participants continue treatment with KL1333 so researchers can learn more about its effects over time.

Primary Mitochondrial Disease – Primary mitochondrial disease is a group of inherited disorders that affect how cells make energy, most often harming muscles, the brain, and other organs. It usually begins in childhood or adulthood and can slowly worsen over time. Symptoms may include tiredness, muscle weakness, exercise intolerance, and problems with movement or balance. Some people also develop stomach, heart, eye, hearing, or diabetes-related problems. The course can vary widely from person to person, even within the same family.

Trial ID:

2025-524367-20-00

Protocol code:

KL1333 2025-104B

Trial Phase:

Therapeutic exploratory (Phase II)

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Long-Term Safety and Efficacy of KL1333 in Adults with Primary Mitochondrial Disease

What is this study about?

Who Can Join the Study?

Who Cannot Join the Study?