ABBV-383 for the Treatment of Relapsed Refractory Waldenström Macroglobulinemia

Inclusion Criteria: * PRE-REGISTRATION: Age ≥ 18 years * PRE-REGISTRATION: Histological confirmation of relapsed and/or refractory Waldenstrom macroglobulinemia, with known prior exposure to a Bruton tyrosine kinase (BTK) inhibitor unless medically contraindicated. Note: although not preferred, archival bone marrow tumor tissue that was collected within 12 weeks prior to screening and without intervening anti- BCMA treatment may be used if the patient is unwilling to provide a fresh pretreatment marrow biopsy * PRE-REGISTRATION: Measurable disease as defined as serum immunoglobulin M (IgM) levels ≥ 0.5 g/dL (500 mg/dL) * PRE-REGISTRATION: Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0, 1 or 2 * PRE-REGISTRATION: Subject is naïve to treatment with ABBV-383 or anti-BCMA bispecific antibodies * PRE-REGISTRATION: Hemoglobin ≥ 8.0 g/dL (obtained ≤ 14 days prior to pre-registration) * PRE-REGISTRATION: Absolute neutrophil count (ANC) ≥ 1000/mm\^3 \[neutropenia due to marrow infiltration may be supported by granulocyte colony-stimulating factor (GCSF)\] (obtained ≤ 14 days prior to pre-registration). Transfusion and/or growth factor support is permitted prior to assessment, but neutrophils, platelets, and hemoglobin must be stable for ≥ 72 hours after transfusion and/or growth factor administration for the subject to be eligible * PRE-REGISTRATION: Platelet count ≥ 75,000/mm\^3 (obtained ≤ 14 days prior to pre-registration) EXCEPTION: If thrombocytopenia deemed…