• Patients with histologically confirmed MM with progressive disease according to the IMWG criteria 47 during or within 60 days of their last regimen who have received 1-3 lines of prior therapy (inclusive of a lenalidomide-containing regimen) and have measurable disease within 4 weeks of enrollment based on one of the following:

‣ Serum monoclonal protein ≥ 1.0 g/dL

⁃ Urine monoclonal protein ≥ 200 mg/24 hour

⁃ Involved serum immunoglobulin free light chains (FLC) ≥ 10 mg/dL AND abnormal kappa/lambda ratio.

• Note: Because the primary endpoint is MRD-negativity rate, per the discretion of the Principal Investigator (PI), patients without measurable disease (e.g., M-spike \< 1.0 g/dL) may also be enrolled in line with the IMWG MM response criteria 47.

• Prior treatment with cluster of differentiation 38 (CD38) -directed therapy is permitted only if all the following are fulfilled:

‣ Best response achieved during CD38-directed therapy was ≥ PR.

⁃ Patient did not progress while receiving CD38-directed therapy or within 60 days of last dose of therapy.

⁃ Patient did not discontinue CD38-directed therapy due to a related AE.

• Prior treatment with carfilzomib is permitted only if all the following are fulfilled:

‣ Best response achieved during carfilzomib-based therapy was ≥ PR.

⁃ Patient did not progress while receiving carfilzomib-based therapy or within 60 days of last dose of therapy.

⁃ Patient did not discontinue carfilzomib due to a related AE.

• Creatinine Clearance (CrCl) ≥60 ml/min measured within 4 weeks of enrollment. CrCl can be measured or estimated using Cockcroft-Gault method, Modification of Diet in Renal Disease (MDRD), or Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formulae.

• Age ≥ 18 years at the time of signing the informed consent documentation. Age limit of 75 years.

• Eastern Cooperative Oncology Group (ECOG) performance status 0-2 within 4 weeks of enrollment (Appendix A).

• Absolute neutrophil count (ANC) ≥ 1.0 K cells/µL, hemoglobin ≥ 8 g/dL, and platelet count ≥ 50 K platelets/µL measured within 4 weeks of enrollment unless cytopenias are deemed to be due to disease at the discretion of the clinical Investigator. Transfusions and administration of growth factors are permissible.

• Adequate hepatic function with bilirubin \< 1.5 x the upper limit of normal (ULN) and aspartate aminotransferase (AST) and alanine aminotransferase (ALT) \< 3.0 x ULN measured within 4 weeks of enrollment.

• All study participants must be able to tolerate one of the following thromboprophylactic strategies: oral factor Xa inhibitors or low molecular weight heparin or alternative anti-coagulant.

⁃ Females of childbearing potential (FCBP)† must have a negative serum or urine pregnancy test within 10-14 days and again within 24 hours prior to prescribing of iberdomide for Cycle 1 (prescriptions must be filled within 7 days) and must either commit to continued abstinence from heterosexual intercourse or begin two acceptable methods of birth control, one highly effective method and one additional effective (barrier) method, at the same time at least 28 days before she starts taking iberdomide without interruption. FCBP must also agree to ongoing pregnancy testing. Men must agree to use a latex condom during sexual contact with a FCBP even if they have had a successful vasectomy.

∙ A FCBP is a female who: 1) has achieved menarche at some point, 2) has not undergone a hysterectomy or bilateral oophorectomy, or 3) has not been naturally postmenopausal (amenorrhea following cancer therapy does not rule out childbearing potential) for at least 24 consecutive months (i.e., has had menses at any time in the preceding 24 consecutive months). Females who do not meet the above definition of FCBP should be classified as females not of childbearing potential (FNCBP).