Treatment of Langerhans cell histiocytosis: role of BRAF/MAPK inhibition

Langerhans cell histiocytosis (LCH) is a clonally derived neoplasm with a highly variable clinical course. Although LCH was once considered a disorder of immune regulation, the identification of activating mutations in the proto-oncogene BRAF-V600E in ~50%-60% of cases and MEK and ERK phosphorylation in 100% of examined cases, has changed the definition of LCH to a dendritic cell neoplasm with a strong inflammatory component. Current international LCH trials are focused on further improving the outcome of high-risk multisystem LCH patients, by decreasing the reactivation rate, optimizing early salvage regimens, and preventing late sequelae. Anecdotal responses to vemurafenib, a BRAF-V600E inhibitor, have been reported in a few cases of LCH and Erdheim-Chester disease. However, the development of resistance, as well as the potential risks of cutaneous and pancreatic cancers in patients with BRAF-V600E-mutated melanoma treated with single inhibitors, suggest the need for prospective trials with BRAF inhibitors, alone or in combination with other inhibitors of this pathway, for patients with refractory or multiply-relapsed LCH. The recent discovery of somatic mutations in ARAF and in MAP2K1, which lead to activation of the RAS-RAF-MEK -ERK pathway in the setting of wild-type BRAF, as well as the finding that activating mutation in MAP2K1 are relatively insensitive to MEK inhibitors, suggest that a more detailed understanding of this pathway in LCH may be necessary for the development of more effective targeted therapies.