If you choose to join this study, you will go through a screening period to find out if you are eligible. During this screening period, you will:

• Have a physical exam
• Have a bone survey (a series of x-rays of all the bones in the body)
• Have blood draws and give urine samples
• Have bone marrow samples taken
• Have images taken (X-rays, CT, MRI, or PET-CT scans)
• Have a tumor assessment

If you are eligible to continue after screening, you will begin the study drug period. Each drug cycle during this period is 28 days long. Drug Cycle 1: You will get an isatuximab infusion once weekly on days 1, 8, 15, and 22 of this cycle. Lenalidomide will be taken every day during this cycle for three weeks, followed by a one week break (you will not take lenalidomide on the 4th week). You will take a dexamethasone pill on days you are scheduled to receive an isatuximab infusion. You should take the dexamethasone pill before the isatuximab infusion. Cycles 2-8: You will get an isatuximab infusion once every other week, specifically on days 1 and 15 of each study drug cycle. Lenalidomide will be taken every day during each study drug cycle for three weeks followed by a one week break. On days you are scheduled to receive isatuximab infusion, you should take a dexamethasone pill before the infusion. Cycles 9-17: you will receive isatuximab infusion once every other week, specifically on days 1 and 15 of each study drug cycle. Lenalidomide will be taken every day during each study drug cycle for three weeks followed by a one week break. You will not take any dexamethasone during these study drug cycles, unless the study doctor decides that you should continue taking it. Cycles 18-24: you will receive isatuximab infusion once only on Day 1 of each study drug cycle. Lenalidomide will be taken every day during each study drug cycle for three weeks followed by a one week break. You will not take any dexamethasone during cycles 18-24, unless the study doctor decides that you should continue taking it. After completing 24 study drug cycles, you will move into the maintenance period where you will take lenalidomide only. You will continue taking lenalidomide for as long as it benefits you.