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 screeningvisit.

• Participant has a diagnosis of isolated MMA due to MUT deficiency confirmed bymolecular genetic testing.

• Participant has a blood vitamin B12 level equal to or above the lower limit ofnormal (based on laboratory reference range) confirmed in the screening period.

• Participant or their legally authorized representative is willing and able toprovide informed consent and/or assent as mandated by local regulations and iswilling and able to comply with study-related assessments.

• Sexually active participants of childbearing or reproductive potential agree to usea 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 informedconsent/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 combinedMMA with homocystinuria.

• Participant has previously received gene therapy for the treatment of MMA.

• Participant has a history of organ transplantation or planned organ transplantationduring the period of study participation.

• Participant has an active, unstable, or clinically significant medical condition notrelated to MMA or history of noncompliance that, in the investigator's opinion,could potentiate the risk while participating in this study, interfere with theinterpretation of study results, or limit the participant's participation in thestudy. This may include, but is not limited to, history of relevant food or drugallergies; history of cardiovascular, central nervous, gastrointestinal, orinfectious disease; history of clinically significant pathology; and/or history ofcancer.

• (Part 2 only) Participant has the partial MUT deficiency disease phenotype, asassessed by genotyping, clinical phenotype/presentation, or vitamin B12-responsiveMMA.

Note: Additional inclusion/exclusion criteria may apply, per protocol.