Testing the Combination of Anti-cancer Drugs, Tovorafenib Plus Rituximab, in Patients With Hairy Cell Leukemia

Inclusion Criteria: * Patients must have histologically or cytologically confirmed diagnosis of classical hairy cell leukemia (HCL), including demonstration of BRAF V600E mutation by immunohistochemistry, molecular diagnostic testing, or polymerase chain reaction (PCR) * PHASE 1 ONLY: Prior therapy with at least one purine nucleoside analog-containing regimen (fludarabine, pentostatin, or cladribine) unless contraindicated. Prior vemurafenib alone is allowed in the relapsed/refractory cohort * PHASE 2 ONLY: No prior HCL-directed treatment for front-line cohort. The design of this cohort is such that the patients will need to be treatment naïve * Age ≥ 18 years. Because no dosing or adverse event (AE) data are currently available on the use of tovorafenib (DAY101) or cladribine in combination with rituximab in patients \< 18 years of age, children are excluded from this study * Patients must meet indications for treatment of cHCL: * Absolute neutrophil count \< 1,000/mcL * Platelets \< 100,000/mcL * Hemoglobin \< 10 g/dL * Recurrent infections * Symptomatic and/or progressive extramedullary disease including lymph nodes and bone lesions * Progressive or symptomatic splenomegaly or hepatomegaly * Disease-related constitutional symptoms consisting of unexplained weight loss exceeding 10% body weight during the preceding 6 months, Cancer Therapy Evaluation Program (CTEP) active version of the Common Terminology Criteria for Adverse Events (CTCAE) grade 2 or 3 fati…