Study to Assess Change in Disease Activity of Oral Venetoclax in Adult Participants With Recurring Relapsed or Refractory (R/R) Waldenström Macroglobulinemia (WM)/Lymphoplasmacytic Lymphoma (LPL)

Lymphoplasmacytic Lymphoma (LPL) is a rare type of low-grade B-cell lymphoma. The purpose of this study is to assess the change in disease activity of adult participants with relapsed or refractory Waldenström macroglobulinemia(WM)/LPL receiving venetoclax. Venetoclax is being investigated in the treatment of WM/LPL. Participants will receive oral venetoclax at doses ramping up to the target dose, as part of treatment. Approximately 14 adult participants with WM/LPL will be enrolled in the study at approximately 20 sites in Japan. Participants will receive oral venetoclax at doses ramping up to the target dose. The total study duration is approximately 28 months. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, and checking for side effects.

Inclusion Criteria: * Documented diagnosis of Waldenström macroglobulinemia(WM) /lymphoplasmacytic lymphoma (LPL) according to the 5th edition of the World Health Organization (WHO) classification and/or documented clinicopathological diagnosis of WM in accordance with the consensus panel of the second International Workshop on WM (IWWM). * At least one prior standard therapy for WM/LPL. * Measurable disease, defined as follows: * WM type LPL population: Immunoglobulin M (IgM) \>= 500 mg/dL per central laboratory (approximately 14 participants) * Non-IgM type LPL population or IgM \< 500 mg/dL: A measurable node having a longest diameter (LDi) greater than 1.5 cm, or a measurable extranodal disease having a LDi greater than 1.0 cm, according to contrast-enhanced computed tomography (CT) scan. (up to 2 participants) * Requires systemic anti-cancer treatment for WM/LPL, according to the investigator. * Eastern Cooperative Oncology Group Performance Status \< = 2 * Adequate organ and bone marrow function Exclusion Criteria: * History of prior exposure to venetoclax or BCL-2 targeted therapy. * Uncontrolled active systemic infection. * Known bleeding disorders (e.g., von Willebrand's disease or hemophilia).