Venetoclax for the Treatment of Patients With Relapsed Hairy Cell Leukemia

Inclusion Criteria: * Patients must have histologically or cytologically confirmed HCL/HCLv after purine analog therapy who are relapsed from or are ineligible for BRAF therapy and have not received prior venetoclax * Age ≥ 18 years * Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2 (Karnofsky ≥ 60%) * Total bilirubin ≤ 3 x institutional upper limit of normal (ULN) unless consistent with Gilbert's (ration between total and direct bilirubin \> 5) * Aspartate aminotransferase (AST)(serum glutamic oxaloacetic transaminase \[SGOT\])/alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase \[SGPT\]) ≤ 3 × institutional ULN * Serum creatinine ≤ 2.0 mg/dL OR creatinine clearance ≥ 45 mL/min/1.73m\^2 * Human immunodeficiency virus (HIV)-infected patients on effective anti-retroviral therapy with undetectable viral load within 6 months are eligible for this trial * For patients with evidence of chronic hepatitis B virus (HBV) infection, the HBV viral load must be undetectable on suppressive therapy, if indicated * Patients with a history of hepatitis C virus (HCV) infection must have been treated and cured. For patients with HCV infection who are currently on treatment, they are eligible if they have an undetectable HCV viral load * Patients with a prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the investigational regimen are eligible for this trial * Patie…