First in Human Dose Escalation of M3258 as a Single Agent and Expansion Study of M3258 in Combination With Dexamethasone

Inclusion Criteria:

• Participants having Eastern Co-operative Oncology Group (ECOG) Performance Status less than or equals to (<=) 1
• Adequate hematological, hepatic and renal function as defined in the protocol
• Participant must have measurable disease of Multiple Myeloma (MM) and received greater than (>) 3 prior lines of therapy for MM including a Proteasome Inhibitors (PI), an Immunomodulatory Imide Drug (IMiD) and an anti-CD38 mAb or who are refractory to at least PI agent (carfilzomib or bortezomib) and IMiD according to the International Myeloma Working Group (IMWG) criteria
• Participant must have documented evidence progressive disease as defined by the IMWG criteria either on or after their last regimen
• Other protocol defined inclusion criteria could apply

Exclusion Criteria:

• Any condition, including any uncontrolled disease state that in the Investigator's opinion constitutes an inappropriate risk or a contraindication for participation in the study or that could interfere with the study objectives, conduct, or evaluation.
• An active second malignancy or evidence of disease of cancer (other than MM) before the date of enrollment (exceptions are squamous and basal cell carcinomas of the skin and carcinoma in situ of the cervix, or malignancy that in the opinion of the Investigator, with concurrence with the Sponsor's medical monitor, is considered cured with minimal risk of recurrence within 3 years).
• Cerebrovascular accident/stroke (< 6 months prior enrollment) or neurologic instability per clinical evaluation due to tumor involvement of the Central Nervous System
• Diagnosis of fever within 1 week prior to study intervention administration
• Part B: Participants planning to undergo a stem cell transplant should not be enrolled to reduce disease burden prior to transplant.
• Other protocol defined exclusion criteria could apply