The purpose of this study is to investigate the safety, tolerability, and pharmacokinetics when MT-3921 or a placebo is intravenously given to Japanese healthy adult male subjects

Inclusion Criteria: Additional screening criteria check may apply for qualification:

• Japanese healthy adult males
• Subjects aged between 18 and 55 years inclusive at the time of informed consent
• Subjects with a full understanding of the nature of this study who have consented in writing to participate in the study

Exclusion Criteria: Additional screening criteria check may apply for qualification:

• Subjects with or having a history or treatment history of disorders, including cardiac, hepatic, renal, gastrointestinal, respiratory, neuropsychiatric, hematopoietic, or endocrine system disorder, who are judged by the investigator (or subinvestigator) to be unfit for study participation
• Subjects with a history of drug or food allergies, including anaphylaxis or significant allergic reactions
• Subjects with a body mass index (BMI) of less than 18.5 kg/m^2 or greater than 25 kg/m^2 at the time of screening or Day -1 or those with a body weight of less than 50 kg or greater than 80 kg
• Subjects who have donated blood or from whom blood samples have been collected in a total amount of 400 mL or more within 12 weeks prior to informed consent, 200 mL or more within 4 weeks prior to informed consent, or 800 mL or more within 1 year prior to informed consent
• Subjects who have donated blood component or platelet apheresis within 2 weeks prior to informed consent
• Subjects with or having a history of drug dependence or alcohol dependence
• Subjects with clinically significant abnormalities on a 12-lead ECG, with a Fridericia-corrected QT (QTcF) interval of ≥450 msec, or with clinically significant abnormalities on an auscultation
• Subjects with a positive result for hepatitis B virus surface (HBs) antigen, serological test for syphilis, hepatitis C virus (HCV) antibody, or human immunodeficiency virus (HIV) antigen/antibody at screening
• Subjects with a positive polymerase chain reaction (PCR) test for Coronavirus disease 2019 (COVID-19)
• Subjects with a family history of sudden death
• Subjects who answered "yes" to any of the items of suicide ideation or suicide attempts in the Columbia Suicide Rating Scale (C-SSRS) assessment in the screening period
• Subjects who do not agree to practice contraception during the study period
• Subjects who have previously received anti-RGMa antibodies, including this investigational product
• Subjects who have participated in other clinical studies and received other investigational products within 12 weeks before informed consent
• Subjects who have used any drug other than the investigational product in the period within 7 days before the start of investigational product administration or 5 times the half-life of the drug, whichever is longer