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

Key Inclusion Criteria:

• Participant has a body weight of ≥11.0 kilograms (kg) at the Screening Visit.

• 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. Forthose participants found to have an elevated blood vitamin B12 level, theparticipant may enter if, in the opinion of the Investigator, the cause of theelevation is secondary to B12 supplementation.

• 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 females of childbearing potential and sexually active males ofreproductive potential agree to use a highly-effective method of contraceptionduring the study and for 3 months after the last administration of study drug.

• (Part 2 only) At least 1 documented MDE in the 12-month period before consent.

Key Exclusion Criteria:

• Participant has a diagnosis of isolated MMA cb1A, cb1B, or cb1D enzymatic subtypesor 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 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) History of hepatitis B (known positive hepatitis B surface antigen [HbsAg]), hepatitis C virus (HCV), or HIV (positive HIV1/HIV-2 antibodies).Participants with a past or resolved hepatitis virus B (HBV) infection (defined asthe presence of hepatitis B core antibody and absence of HbsAg) are eligible.Participants with history of positive results for HCV antibody are eligible only ifpolymerase chain reaction is negative for HCV RNA.