MELAS is a condition affecting the brain and muscles, caused by changes in mitochondrial DNA. This study is testing a medicine called zagociguat to see if it helps people with MELAS feel less tired and think better. It’s called a Phase 2b study, which means it’s checking if the medicine works and if it's safe. The study is randomized, double-blind, and placebo-controlled, meaning neither the participants nor the doctors know who is getting the real medicine or a placebo (a fake pill). Participants will take the medicine or placebo for 12 weeks, then switch after a break. Visits happen at the clinic or possibly at home. You must be 18 to 75 years old and meet specific conditions to join. If you finish the study, you might get to try the real medicine in a later study.

• Participate for about 28 weeks, including two 12-week treatment periods.
• Clinic visits at start, middle, and end of each period, with some possible home visits.
• Weekly assessments via phone app, plus extra check-ups during visits.