Sotatercept (ACE-011) With Lenalidomide or Pomalidomide and Dexamethasone in Relapsed/Refractory Multiple Myeloma

After the screening procedures confirm that you are eligible to participate in the research study: Assignment to dose level of study treatment Since we are looking for the highest dose of the ACE 011 combined with lenalidomide that can be administered safely without severe or unmanageable side effects in participants that have multiple myeloma, not everyone who participates in this research study will receive the same dose of these drugs. The dose you get will depend on the number of participants who have been enrolled in the study before you and how well they have tolerated their doses. Study treatment Study treatment will be given in 28 day cycles. * ACE-011 will be given as an injection under the skin, Day 1 of each cycle (every 28 days). You will receive each injection at the clinic. * Lenalidomide will be taken by mouth: once daily days 1 21, followed by a 7 day rest period during which no lenalidomide will be taken. * Dexamethasone will be taken by mouth once per week (days 1, 8, 15 and 22) of each cycle. You will be given drug diaries to record the doses of lenalidomide and dexamethasone taken. The study staff will tell you how to complete the diaries