Specifically, 28 patients (48%) achieved biochemical complete remission (CR), 4 (7%) very good partial remissions (VGPR), and 8 (14%) partial remissions (PR). months. == Conclusions == WY-135 Despite the risk of infectious complications, the prognosis of MM patients can be significantly improved with vigilant monitoring and proactive management of infections. This realworld data highlight the potential of BCMA Tcell engagers in treating MM, emphasizing the need for careful patient monitoring to mitigate infection risks. Keywords:bispecific antibodies, immunotherapy, multiple myeloma, realworld data == 1. INTRODUCTION == Multiple myeloma (MM) is a hematological malignancy characterized by the proliferation of malignant plasma cells in the bone marrow, leading to various clinical manifestations and complications. Despite advances in treatment, MM remains incurable, with many patients relapsing or becoming refractory to existing therapies.1With the recent introduction of antiCD38 monoclonal antibodies (mAbs), daratumumab, immunotherapy has rapidly become indispensable in the management of the disease. AntiCD38 antibody is, according to different guidelines, recommended in the front treatment of all MM WY-135 patients, both high dose eligible and WY-135 noneligible patients, together with a proteasome inhibitor (PI) and an immunomodulatory drug (IMiD). This marked a significant advancement in MM treatment, improving patient outcomes considerably. However, the emergence of resistance to these therapies has been a growing concern, highlighting the need for new treatment strategies.2In relapsed/refractory MM (RRMM), there is no standard of care. Recommendations in the national and international guidelines are based on earlier treatment/s and the outcome of these drug combinations based on IMiDs, PIs, antiCD38 mABs, HDAC inhibitors, chemotherapy, and corticosteroids. Other emerging treatment options include CART and bispecific antibodies (BsAbs) and for patients harboring t(11;14) BCL2 inhibitors3venetoclax, alone or in combination is WY-135 sometimes recommended. Despite all available treatments for MM, most MM patients relapse and become refractory; thus, there is an unmet need for new medications with a different mode of COL5A2 action. Bispecific antibodies, especially those targeting the Bcell maturation antigen (BCMA), have recently emerged as a promising class of therapeutics in MM. These agents, by engaging both cytotoxic T cells and myeloma cells, offer a targeted approach to therapy. Teclistamab and elranatamab, two such BsAbs, have demonstrated considerable efficacy in recent clinical trials.4,5However, the clinical use of BsAbs is not without challenges, particularly regarding their safety profile. Experience with BCMAxCD3 BsAbs shows a quick decline in IgA, IgM, IgG, and free light chains in a majority of patients, putting them in a deep hypogammaglobulinemic state. This study reports on the realworld experience on efficacy, safety, and effect of introducing upfront immunoglobulin substitution. By analyzing realworld data from patients treated with BsAbs in Sweden within compassionateuse programs, we seek to provide insights into optimizing the therapeutic use of these novel agents in clinical practice. == 2. MATERIALS AND METHODS == == 2.1. Patients == This study included 58 patients treated with BsAbs targeting BCMA, specifically teclistamab WY-135 or elranatamab, as part of compassionateuse programs in Sweden. These patients were treated in nine hospitals across the country, encompassing both university and regional medical centers, between June 2021 and June 2023. Eligibility for inclusion followed disease criteria established in previously described phase II studies.4,5 == 2.2. Treatment == Initially, all patients were hospitalized for the stepup phase to closely monitor and manage immediate adverse reactions. Treatment protocols for teclistamab and elranatamab followed those outlined in the respective phase II trials.4,5These included, as earlier reported, specific dosing schedules and adjustments based on patient tolerability. After the stepup phase, patients continued their treatment on an outpatient basis, with regular followups to monitor response and manage any adverse effects. == 2.3. Study design and overview == The study was approved by the Swedish Ethics Committee (Dnr 20230312301). It adhered to the principles of the Declaration of Helsinki, the International Conference on Harmonization, and the Guidelines for Good Clinical Practice. Written informed consent was obtained from all participants. This cohort study included patients.