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 entering the study. The patients cannot have received BRAF inhibitor therapy within 6 months of entering the study. * Major surgery within 4 weeks prior to entering the study * Invasive malignancy within the past 2 years prior to first study drug administration, except for adequately treated (with curative intent) basal or squamous cell carcinoma, melanoma, in situ carcinoma of the cervix, in situ ductal adenocarcinoma of the breast, in situ prostate cancer, or limited stage bladder cancer or other cancers from which the patient has been disease-free for at least 2 years * Active HIV, hepatitis B and hepatitis C or any clinically significant history of liver disease. Hepatitis B prior infection is not a contraindication though will require therapy. * Known hypersensitivity to any of the study drugs * Patients with HCL that are BRAF V600E mutation negative