Expanding the Toolbox: Utility of HistioTrak for Minimal Residual Monitoring in Pediatric Patients with Langerhans Cell Histiocytosis Treated with Targeted Therapy

Background/Objectives: Langerhans cell histiocytosis (LCH) is a rare myeloid neoplasm characterized by the clonal proliferation of Langerhans-like dendritic cells and constitutive activation of the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK-ERK) signaling pathway. Nearly 80% of ERK pathway activation can be attributed to B-Raf proto-oncogene, serine/threonine kinase (BRAF V600E), and mitogen-activated protein kinase kinase 1 (MAP2K1) variants, with BRAF V600E specifically detected in approximately 50% of pediatric LCH cases and associated with a higher risk of severe disease and treatment failure. The use of the HistioTrak clinical assay to detect the presence of BRAF V600E mutations in peripheral blood mononuclear cells (PBMCs) has emerged as a useful diagnostic tool and biomarker. Methods: This study is a single-center retrospective study that explores the favorable outcomes of treatment with trametinib on a small number of patients with LCH. We retrospectively analyzed the records of 11 children with LCH treated with trametinib at diagnosis as front-line therapy (n = 6), due to progressive disease (n = 3) or intolerance (n = 1) to chemotherapy, or at relapse (n = 1). Results: HistioTrak identified the presence of BRAF V600E PBMCs in five patients. In this small single-center retrospective cohort, trametinib was associated with favorable short-term outcomes in all patients, and serial HistioTrak testing appeared feasible in selected patients. Conclusions: Prospective studies are needed before routine diagnostic or monitoring use can be recommended.