Inclusion Criteria:

Part A

• Patients must have a known diagnosis of multiple myeloma (MM) with evidence of measurable disease, as defined below, and have evidence of disease progression based on International Myeloma Working Group (IMWG) criteria:
• Serum M-protein ≥1g/dL, or urine M-protein ≥200 mg/24 hours, OR
• In the absence of measurable M-protein, serum immunoglobulin free light chain ≥10 mg/dL, and abnormal serum immunoglobulin kappa lambda free light chain ratio.
• Patients must have received at least 3 prior lines of therapy for MM and must include treatment with an immunomodulatory drug (IMiD) (for ≥2 cycles or ≥2 months of treatment) and a proteasome inhibitor (for ≥2 cycles or ≥2 months of treatment). Induction therapy and stem cell transplant (± maintenance) will be considered as one regimen within a line, OR
• Patients whose disease is double refractory to an IMiD and a proteasome inhibitor. For patients who have received more than one type of IMiD and proteasome inhibitor, their disease must be refractory to the most recent one.
• Patients must have achieved a minimal response (MR) or better to at least one prior line of therapy.
• Patients must have received an alkylating agent (for ≥2 cycles or ≥2 months of treatment) either alone or in combination with other MM treatments (history of stem cell transplant is acceptable). Treatment with high-dose Melphalan for stem cell transplantation meets this requirement.
• Signed written informed consent and be willing and able to complete all study-related procedures.

Part B

• Patients must have a known diagnosis of multiple myeloma (MM) with evidence of measurable disease, as defined below, and have evidence of disease progression based on International Myeloma Working Group (IMWG) criteria:
• Serum M-protein ≥1g/dL, or urine M-protein ≥200 mg/24 hours, OR
• In the absence of measurable M-protein, serum immunoglobulin free light chain ≥10 mg/dL, and abnormal serum immunoglobulin kappa lambda free light chain ratio.
• Patients must have received at least 3 cycles of daratumumab treatment with at least 6 weeks from the last treatment with daratumumab to the first study treatment OR at least 2 cycles of daratumumab treatment in case another therapy is given between daratumumab and isatuximab with at least 12 weeks from the last treatment with daratumumab to the first study treatment.
• Patients must have achieved MR or better to at least 1 prior line of therapy.
• Signed written informed consent and be willing and able to complete all study-related procedures.

Exclusion Criteria:

• Patients <18 years old.
• Eastern Cooperative Oncology Group (ECOG) performance status >2.
• Poor bone marrow reserve.
• Poor organ function.
• Known intolerance/hypersensitivity to IMiDs, dexamethasone, boron or mannitol, sucrose, histidine, or polysorbate 80.
• Any serious active disease (including clinically significant infection that is chronic, recurrent, or active) or comorbid condition, which, in the opinion of the Investigator, could interfere with the safety, the compliance with the study, or with the interpretation of the results.
• Any severe underlying medical conditions including presence of laboratory abnormalities, which could impair the ability to participate in the study or the interpretation of its results.

The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.