In patients with cHCL, excellent results including complete remissions have been reported with the use of BRAF inhibitors, both as a single agent and when combined with anti-CD20 therapy. The 2 commercially available BRAF inhibitors are vemurafenib and dabrafenib, and both have been tested in relapsed cHCL.38,39 The first study of vemurafenib was reported by Tiacci and colleagues, who found an overall response rate of 96% after a median of 8 weeks and a 100% response rate after a median of 12 weeks, with complete response rates up to 42%.38 The median relapse-free survival was 23 months (decreasing to only 6 months in patients who achieved only a partial remission), indicating that these agents will likely need to be administered in combination with other effective therapies with non-overlapping toxicities. Vemurafenib has been administered concurrently with rituximab, and preliminary results of this combination therapy showed early rates of complete responses.40 Dabrafenib has been reported for use as a single agent in cHCL and clinical trials are underway evaluating its efficacy when administered with trametinib, a MEK inhibitor.39,41 Of note, patients receiving BRAF inhibitors frequently develop cutaneous complications of RAF inhibition including cutaneous squamous cell carcinomas and keratoacanthomas, and close dermatologic surveillance is required.
Variant HCL does not harbor the BRAF V600E mutation, but up to half of patients have been found to have mutations of MAP2K1, which upregulates MEK1 expression.42 Trametinib is approved by the US Food and Drug Administration for the treatment of patients with melanoma at a dose of 2 mg orally daily, and has been successfully used to treat 1 patient with vHCL.43 Further evaluation of this targeted therapy is underway.
Ibrutinib, a Bruton tyrosine kinase inhibitor, and moxetumomab pasudotox, an immunotoxin conjugate, are currently being studied in National Institutes of Health–sponsored multi-institutional trials for patients with HCL. Ibrutinib is administered orally at 420 mg per day until relapse.44 Moxetumomab pasudotox was tested at different doses between 5 and 50 μg/kg intravenously every other day for 3 doses for up to 16 cycles unless they experienced disease progression or developed neutralizing antibodies.45 Both agents have been shown to have significant activity in cHCL and vHCL and will likely be included in the treatment armamentarium once trials are completed. Second-line therapy options are summarized in Table 4.
Complications and Supportive Care
The complications of HCL may be separated into the pre-, intra-, and post-treatment periods. At the time of diagnosis and prior to the initiation of therapy, marrow infiltration by HCL frequently leads to cytopenias which cause symptomatic anemia, infection, and/or bleeding complications. Many patients develop splenomegaly, which may further lower the blood counts and which is experienced as abdominal fullness or distention, with early satiety leading to weight loss. Patients may also experience constitutional symptoms with fatigue, fevers in the absence of infection, and unintentional weight loss even without splenomegaly.
For patients who initiate therapy with purine nucleoside analogs, the early part of treatment is associated with the greatest risk of morbidity and mortality. Chemotherapy leads to both immunosuppression (altered cellular immunity) as well as myelosuppression. Thus, patients who are already in need of treatment because of disease-related cytopenias will experience an abrupt and sometimes significant decline in the peripheral blood counts. The treatment period prior to recovery of neutrophils requires the greatest vigilance. Because patients are profoundly immunocompromised, febrile neutropenia is a common complication leading to hospital admission and the cause is often difficult to identify. Treatment with broad-spectrum antibiotics, investigation for opportunistic and viral infections, and considerations for antifungal prophylaxis or therapy are required in this setting. It is recommended that all patients treated with purine nucleoside analogs receive prophylactic antimicrobials for herpes simplex virus and varicella zoster virus, as well as prophylaxis against Pneumocystis jirovecii. Unfortunately, growth factor support has not proven successful in this patient population but is not contraindicated.46
Following successful completion of therapy, patients may remain functionally immunocompromised for a significant period of time even with a normal neutrophil count. Monitoring of the CD4 count may help to determine when prophylactic antimicrobials may be discontinued. A CD4 count greater than 200 cells/µL is generally considered to be adequate for prevention of opportunistic infections. Although immunizations have not been well studied in HCL, it is recommended that patients receive annual influenza immunizations as well as age-appropriate immunizations against Streptococcus pneumoniae and other infectious illnesses as indicated. Live viral vaccines such as the currently available herpes zoster vaccine can lead to infections in this patient population and are not recommended.