The purposes of this study is to assess the safety, tolerability, and pharmacokinetics of MT-3921 in subjects with Human T-cell Leukemia Virus Type 1 (HTLV-1)-Associated Myelopathy(HAM).

Subjects meeting eligibility criteria will enter the 6-month double-blind period. Subjects will be randomized in a 2:1 ratio to receive MT-3921 or placebo in a double blind manner.

Inclusion Criteria:

Additional screening criteria check may apply for qualification:

• Subjects aged 20 years or older on the day of consent
• Subjects with a confirmed diagnosis of HAM according to the diagnostic algorithm of Practical Guideline for HAM 2019 on the day of consent
• Subjects with an Osame's motor disability score (OMDS) of ≥4 and ≤6 at Screening and on the first day of the Treatment period (predose)
• Subjects with no change in OMDS for at least 3 months before the day of consent
• Subjects with a CSF concentration neopterin of ≥6 pmol/mL at Screening
• Subjects on maintenance oral steroid therapy who have continued to receive the same dose equivalent to ≤10 mg/day of prednisolone for at least 3 months before the day of consent

Exclusion Criteria:

Additional screening criteria check may apply for qualification:

• Subjects who have a history of anaphylaxis or clinically significant allergic reactions due to administration of antibody products
• Subjects exhibiting or with a history of malignant tumor.
• Subjects with adult T-cell leukemia/lymphoma (ATL) or those with positive HTLV-1 clonality test (Southern blot) at screening and suspected of having ATL
• Subjects with spinal cord compression lesions, such as cervical spine disease, disc herniation, and ossification of the yellow ligament
• Subjects with psychiatric disorders, epileptic seizures, or dementia.
• Subjects with suicide attempts or suicidal ideation corresponding to item 4 or 5 on the Columbia Suicide Rating Scale (C-SSRS) at screening and/or on the first day of the Treatment period (predose)
• Subjects exhibiting or with a history of hepatitis B virus (HBV), hepatitis C virus (HCV), or human immunodeficiency virus (HIV) infection.
• Subjects with the novel Coronavirus disease 2019 (COVID-19)
• Subjects with severe illness
• Male or female subjects of childbearing potential who do not agree to use a contraceptive measure from the day of consent to 16 weeks after the last dose of the investigational medical product
• Female subjects who are pregnant, lactating, or may be pregnant
• Subjects who have received anti-repulsive guidance molecule (RGM) a antibody containing this investigational medical product
• Subjects who participated in other clinical studies (including clinical trials) and received drugs (including investigational medical products) or therapy within 12 weeks before the day of consent