Langerhans cell histiocytosis (LCH) is the most common histiocytic disorder in children, caused by excessive proliferation and accumulation of Langerhans cells (a type of immune cell) in various body tissues. The annual incidence is about 2.6-8.9 cases per million children.Clinical presentation varies widely.

Mild (low-risk) cases may resolve spontaneously or cause minimal issues with excellent outcomes. Severe multisystem LCH involves multiple organs, particularly high-risk sites such as liver, spleen, or bone marrow, leading to poorer prognosis and potential life-threatening complications without appropriate treatment.Standard first-line therapy for many children is prednisone (a corticosteroid) plus vinblastine (chemotherapy). Trials like LCH-III show near-100% survival in low-risk disease, but long-term survival drops to ~80% in high-risk cases. Reactivation occurs in ~37% of low-risk patients post-treatment, and ~50% of children eventually develop resistance, resulting in progression or relapse. Treatment failure heightens risks of long-term sequelae, including growth retardation, endocrine dysfunction, and neurological damage, severely impacting quality of life.

More than half of LCH cases harbor the BRAF V600E mutation, activating the MAPK pathway abnormally. This has driven development of targeted MAPK inhibitors (e.g., vemurafenib, dabrafenib, trametinib), which demonstrate strong efficacy and acceptable safety (mainly manageable skin rash) in relapsed/refractory pediatric cases, with no reported secondary malignancies to date. These agents provide rapid symptom relief and durable control, though monotherapy often fails to eradicate abnormal cells in multisystem disease, leading to relapse after discontinuation.

No MAPK inhibitors were previously approved specifically for LCH. In 2025, luvometinib (developed by Fosun Pharma, China; a selective MEK1/2 inhibitor) received approval in China for adult LCH and histiocytic neoplasms. Adult studies showed ~83% objective response rate and ~74% progression-free at ≥12 months, with mostly mild side effects (skin issues, hypertriglyceridemia) and no discontinuations due to serious toxicity.

Laboratory evidence indicates MAPK overactivation confers apoptosis resistance to LCH cells; combining MAPK inhibitors with chemotherapy may enhance cell killing and leverage chemotherapy-induced immune microenvironment changes for better clearance.

Small studies and real-world data in refractory LCH support this: combination regimens yielded low relapse rates (especially with prolonged therapy), 100% responses in some pediatric cohorts with sustained remission and no added severe toxicity, and notably lower relapse (20% vs 75% with inhibitor alone) in our center's early experience with LCH-III backbone plus MAPK inhibitor.

This multicenter randomized trial will enroll children with multisystem LCH, assigning them to modified standard LCH-III chemotherapy alone or the same regimen combined with luvometinib, to evaluate whether adding this targeted agent improves outcomes.

Lab studies show that when the MAPK pathway is overactive, it makes LCH cells harder to kill (more resistant to dying naturally). Combining an MAPK inhibitor with chemotherapy might make the abnormal cells easier to destroy. Chemotherapy also changes the immune environment in the body, which could help clear out the bad cells more completely.

Several small studies and real-world experiences have already tested this idea in children and adults with hard-to-treat LCH:

One study combined MAPK inhibitors with certain chemotherapy drugs and saw very few relapses, especially when treatment lasted longer.

Another study using a similar approach achieved 100% response in a group of children (including some as first treatment), with most staying in remission afterward, and no extra serious side effects from the combination.

A Chinese study using prednisone, another chemo drug, plus an MAPK inhibitor showed good results with low relapse after stopping.

Early data from our own center found that children who got an MAPK inhibitor together with the standard LCH-III treatment had much lower relapse rates (only 20%) compared to those who got the inhibitor alone (75% relapse).

Based on this growing evidence, this study plans to run a large, carefully designed trial at multiple hospitals. It will randomly assign children with multisystem LCH to receive either the improved (modified) version of the standard LCH-III chemotherapy alone, or the same chemotherapy combined with luvometinib. The goal is to compare how well each approach works and to learn more about whether adding this targeted drug to chemotherapy can improve treatment for children with this disease.

Inclusion Criteria:

1. Children aged 0-18 years, of either sex.
2. Pathologically confirmed diagnosis of Langerhans cell histiocytosis (LCH) with positive staining for CD1a and/or CD207 (Langerin), and no prior treatment specifically directed against LCH.
3. Multisystem involvement of LCH, as determined by clinical and imaging evaluation.
4. Provision of written informed consent (by parent/legal guardian and, where appropriate, assent from the child), with willingness to comply with the study treatment regimen and follow-up assessments.

Exclusion Criteria:

1. Presence of any other significant underlying medical condition, including but not limited to primary immunodeficiency disorders, congestive heart failure, renal insufficiency, chronic viral hepatitis, HIV infection, or status post solid organ transplantation.
2. History of a second (secondary) malignancy.
3. QTcF interval > 0.47 seconds on electrocardiogram performed prior to enrollment.
4. Ophthalmologic screening prior to enrollment revealing retinal vein occlusion, retinal pigment epithelial detachment, or other clinically significant ocular abnormalities that, in the opinion of the investigator, contraindicate participation.
5. LCH harboring Class 3 MEK pathway mutations, specifically the following alterations: L98_I103del, L98_K104del, P105_A106del, P105_I107delinsL, L101_I103delinsF, E102_I103delinsF, E102_I103del, E102_I103delinsV, E102_I103delinsVN, E102_K104delinsQ, or I103_A106del.
6. Refusal or inability to provide written informed consent (or assent, as applicable).

IPD might be shared in the future upon approval by the ethics committee and under a data use agreement.