Hairy Cell Leukemia (HCL)

A rare, indolent B-cell malignancy defined by BRAF V600E — purine analog therapy (cladribine, pentostatin), rituximab consolidation, and BRAF/MEK-targeted salvage for relapsed/refractory disease

Key Points

• Hairy cell leukemia is a rare, indolent mature B-cell neoplasm (~2% of all leukemias) characterized by pancytopenia, massive splenomegaly, monocytopenia, and circulating 'hairy' lymphocytes with fine cytoplasmic projections; the near-universal BRAF V600E mutation is the disease-defining molecular lesion.
• The immunophenotype is highly characteristic: bright monoclonal surface immunoglobulin with co-expression of CD11c, CD25, CD103, and CD123, plus annexin A1 (the single most specific marker), distinguishing classic HCL from the HCL variant (HCL-v) and splenic marginal zone lymphoma.
• Not all patients require immediate treatment — asymptomatic patients with adequate blood counts are observed; therapy is indicated for cytopenias (ANC <1,000/μL, hemoglobin <11 g/dL, platelets <100,000/μL), symptomatic splenomegaly, or recurrent infections.
• A single course of a purine nucleoside analog — cladribine (2-CdA) or pentostatin — is the frontline standard, producing complete remission in ~80–90% of treatment-naïve patients with durable remissions frequently exceeding 10 years.
• Adding rituximab to cladribine markedly increases MRD-negative complete remission rates, and concurrent or sequential chemoimmunotherapy is now a preferred frontline strategy at many centers, particularly for higher-burden disease.
• Relapsed/refractory disease has excellent targeted options: the BRAF inhibitor vemurafenib (± rituximab), BRAF/MEK combinations (dabrafenib + trametinib), the anti-CD22 immunotoxin moxetumomab pasudotox, and the BTK inhibitor ibrutinib — reserving repeat purine analogs for late relapse.

Biology, Presentation, and Diagnosis

**Molecular pathogenesis:** Hairy cell leukemia is a clonal expansion of mature memory B cells. The defining genetic event is the **BRAF V600E** mutation, present in essentially 100% of classic HCL — a somatic activating mutation that constitutively drives the RAS-RAF-MEK-ERK (MAPK) signaling cascade independent of upstream receptor input. This makes BRAF both the pathognomonic diagnostic marker and the central therapeutic target. BRAF V600E is absent in the HCL variant (HCL-v) and in splenic marginal zone lymphoma, making it a key discriminator. Some IGHV4-34-expressing cases (a poor-risk…

Indications for Treatment and Frontline Therapy

**When to treat:** HCL is indolent, and not all patients require immediate therapy. Asymptomatic patients with preserved counts and no complications are observed with periodic monitoring. Treatment is indicated for any of: - Symptomatic cytopenias: ANC <1,000/μL, hemoglobin <11 g/dL, or platelets <100,000/μL - Symptomatic or massive splenomegaly - Recurrent or serious infections - Constitutional symptoms or progressive disease **Purine nucleoside analogs — the frontline standard:** A single course of a purine analog achieves complete remission in the large majority of treatment-naïve…

Relapsed/Refractory Disease and BRAF-Targeted Therapy

Most patients eventually relapse, but HCL remains highly treatable across multiple lines. Response duration to the first purine analog course guides second-line selection. **Repeat purine analog:** For patients with a durable first remission (typically >2–5 years), a second course of cladribine or pentostatin — usually now combined with rituximab — can re-induce durable remission. **BRAF-targeted therapy — exploiting the defining mutation:** Because BRAF V600E is the central oncogenic driver, BRAF inhibition is a rational and highly effective targeted strategy, and is especially valuable in…