This clinical trial is focused on studying the long-term safety and effects of a treatment called mRNA-3705 in individuals with a rare genetic condition known as methylmalonic acidemia (MMA). This condition is caused by a deficiency in an enzyme called methylmalonyl-coenzyme A mutase, which is important for breaking down certain proteins and fats in the body. The study involves participants who have previously been part of other clinical trials involving mRNA-3705.

The purpose of this study is to evaluate how safe mRNA-3705 is when used over a longer period. Participants will receive the treatment through an intravenous infusion, which means it is delivered directly into the bloodstream. The study will monitor participants for any side effects and changes in their condition over time. This includes looking at levels of certain substances in the blood that are related to MMA, as well as any changes in the frequency of hospital visits or the impact of the disease on daily activities like school or work.

In addition to mRNA-3705, the study may involve other medications such as dexamethasone, paracetamol, cetirizine dihydrochloride, ibuprofen, and famotidine, which are used to manage symptoms or support the treatment process. Some participants may receive a placebo, which is a substance with no active medication, to help compare the effects of the treatment. The study is expected to continue until January 2029, allowing researchers to gather comprehensive data on the long-term safety and effectiveness of mRNA-3705 in managing MMA.