A Study to Assess the Safety, Pharmacokinetics, and Pharmacodynamics of mRNA-3705 in Participants With Isolated Methylmalonic Acidemia

Key Inclusion Criteria: * (Part 1 only) Participant has a body weight of ≥11.0 kilograms at the screening visit. * Participant has a diagnosis of isolated MMA due to MUT deficiency confirmed by molecular genetic testing. * Participant has a blood vitamin B12 level equal to or above the lower limit of normal (based on laboratory reference range) confirmed in the screening period. * Participant or their legally authorized representative is willing and able to provide informed consent and/or assent as mandated by local regulations and is willing and able to comply with study-related assessments. * Sexually active participants of childbearing or reproductive potential agree to use a highly effective method of contraception, consistent with local regulations, during the study and for 3 months after the last administration of study drug. * (Part 2 only) Participants with 2 screening MMA levels ≥400 micromolar. * (Parts 2 and 3 only) Participant is ≥5 years of age at the time of informed consent/assent. Key Exclusion Criteria: * Participant has a diagnosis of isolated MMA cofactor adenosyl-cobalamin (cb1A, cb1B, or cb1D) enzymatic subtypes or methylmalonyl-CoA epimerase deficiency or combined MMA with homocystinuria. * Participant has previously received gene therapy for the treatment of MMA. * Participant has a history of organ transplantation or planned organ transplantation during the period of study participation. * Participant has an active, unstable, or clinically significant …