Introduction: Despite advances in treatment strategy, multiple myeloma (MM) remains a challenging, incurable disease as patients (pts) often relapse. Here we describe preclinical data of a novel cereblon (CRBN) E3 ligase modulatory compound (CELMoD), iberdomide (IBER; CC-220), as well as clinical and translational data from a current phase 1/2 clinical trial (NCT02773030). These data support a differentiated profile for IBER from immunomodulatory agents (IMiD agents), and development of IBER as a foundation of combination therapy for the treatment of relapsed/refractory MM (RRMM). Methods: Preclinical analyses were performed on pomalidomide (POM)-sensitive and acquired-resistant MM cell lines and peripheral blood mononuclear cells from healthy volunteers. Eligible pts enrolled on the clinical trial had RRMM and had received ≥ 2 prior regimens, containing at least an IMiD agent and proteasome inhibitor (PI), and had experienced disease progression within 60 days of last MM therapy. Escalating doses of IBER were given on Days 1-21, in combination with dexamethasone (DEX; 40 mg; 20 mg in pts aged > 75 years) on Days 1, 8, 15, and 22, of each 28-day cycle. Immunohistochemistry (IHC) data on CRBN, Ikaros, Aiolos, and ZFP91 expression was obtained from pt bone marrow specimens at screening and at Cycle 2, Day 15. Immune profiling was evaluated by flow cytometry of peripheral blood at Cycle 1, Day 1 and Cycle 2, Day 15. Primary objectives were to evaluate maximum tolerated dose (MTD), recommended phase 2 dose (RP2D), safety, and preliminary efficacy. Results: Preclinically, IBER exhibited potent tumoricidal anti-MM and immunostimulatory activity. IBER was more potent than lenalidomide (LEN) or POM in overcoming the immunosuppressive activity of bortezomib (BORT), inducing a more than twofold increase in T cell proliferation in the presence of BORT. IBER also enhanced the antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity of daratumumab (DARA), leading to deeper responses than combinations with LEN or POM. Further differentiating from both LEN and POM, IBER showed activity alone and in combination with BORT or DARA in POM-resistant MM cell lines. As of June 28, 2019, 69 pts with RRMM had received IBER + DEX. Median age was 65 years (range 33-80), and median number of prior regimens was 5 (range 2-12). Prior therapies included autologous stem cell transplantation (80%), LEN (100%), POM (70%), PIs (100%), DARA (71%) and anti-BCMA (6%), including CAR T cells. Clinical activity was observed across all dose levels with an overall response rate (ORR) of 29%, clinical benefit rate of 45%, and disease control rate of 80%. ORR was 30%, 28% and 29% in IMiD agent-refractory, DARA-refractory and Quad-class (IMiD agents/PIs/steroids/CD-38 antibodies)-refractory pts, respectively. IBER + DEX showed a favorable safety profile, with grade 3-4 neutropenia, infections, and thrombocytopenia, occurring in 29%, 25%, and 12% of pts, respectively. To date, IBER doses range from 0.3 to 1.3 mg and the MTD/RP2D has not yet been reached. Pharmacodynamic immunophenotyping changes were dose-dependent. Treatment with IBER (at doses > 0.75mg) + DEX doubled the percentage of proliferating T cells and NK cells (P < 0.001 for each) and increased activated and effector memory CD8+ and CD4+ T cells by > 50% (P < 0.01 for each subset). Pharmacodynamic changes in pt bone marrow by quantitative IHC scoring showed significant degradation of substrates (Ikaros, P = 0.006; Aiolos, P < 0.001; ZFP91, P < 0.001) and downregulation of c-myc (P = 0.027), including in pts refractory to POM and with low CRBN expression. Conclusions: IBER + DEX has shown notable clinical activity and favorable tolerability in heavily pretreated pts with RRMM, including pts refractory to prior IMiD therapy. Preclinical and translational data suggest a differentiated profile of IBER from IMiD agents in regard to activity in POM-resistant models, potency in combination with BORT and DARA, and by inducing changes in CD4+ T cells that were not observed in pts treated with POM from prior studies. Taken together, these data support a differentiated profile for IBER from IMiD agents, and the clinical development of IBER as a foundation of combination therapy for the treatment of RRMM. This study is ongoing, and includes cohorts evaluating the combinations of IBER + DEX with DARA, BORT, and carfilzomib. Updated results will be presented at the meeting.