Low Dose Vemurafenib and Rituximab in Hairy Cell Leukemia

Inclusion Criteria: * ≥ 18 years of age * Histologically confirmed HCL that are BRAF V600E positive by IHC or NGS * Patient's must meet the standard treatment initiation criteria, as defined by ANC ≤1.0, Hgb ≤ 10.0 or PLT ≤100K * Patients can either have (1) not received any prior therapy for the disease or have had (2) failure to achieve any response to the initial purine analog-based therapy or (3) subsequent relapse after any prior therapy. * ECOG performance status of 0-2 * Acceptable pre-study organ function during screening not exacerbated by Hairy Cell Leukemia. General thresholds should be a total bilirubin ≤ 1.5 times the upper limit of normal (ULN), aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5x ULN, and serum creatinine ≤ 1.5x ULN * For women of childbearing potential, agreement to use acceptable methods of contraception * For men with female partners of childbearing potential, agreement to use barrier contraception * Negative serum pregnancy test within 7 days of commencement of treatment in premenopausal women. * Ability to understand and willingness to sign a written informed consent document. * Willingness and ability to comply with scheduled visits, treatment plans, laboratory tests, and other study procedures. Exclusion Criteria: * Pregnant or breast-feeding or intending to become pregnant during the study * Have had chemotherapy (including purine analogs), rituximab, and other investigational agents within six weeks prior to en…