Response-adapted Luvometinib With or Without Cytarabine in Langerhans Cell Histiocytosis

• Luvometinib (Drug): Luvometinib is administered orally once daily in 35-day cycles. Adult participants receive 8 mg once daily. Pediatric participants receive body-surface-area-adjusted dosing at 5 mg/m² once daily, rounded according to protocol with a maximum single dose of 8 mg. Luvometinib is used during induction and maintenance according to the assigned response path.
• Luvometinib, Cytarabine. (Drug): Luvometinib is administered as described for induction and maintenance. In the B response path, cytarabine is administered at 100 mg/m² by subcutaneous injection on days 1-5 of each 35-day cycle for 12 cycles, followed by luvometinib maintenance for 6 additional 35-day cycles.

Primary outcomes

• Objective response rate after six cycles of luvometinib induction by blinded independent central review [At completion of six 35-day cycles, approximately week 30 / target C7D1 +/- 7 days]: Proportion of full analysis set participants achieving complete metabolic response (CMR) or partial metabolic response (PMR) by PET response criteria at the cycle 6 assessment window (target C7D1 +/- 7 days), as determined by blinded independent central review. No subsequent confirmatory PET/CT is required. The analysis is descriptive and will report the point estimate with an exact two-sided 95% confidence interval; no confirmatory hypothesis test is planned. Mild out-of-window assessments may be included with protocol deviation documentation if no major treatment or disease-status change affects interpretation. Clearly out-of-window, non-evaluable, or unreliable assessments, death, progression, treatment discontinuation due to toxicity, withdrawal from study treatment, or non-evaluable imaging before the cycle 6 assessment are counted as non-responders.
• Incidence of adverse events and serious adverse events [Routine AE recording: consent to 30 days after last study treatment; TEAE summaries from first dose; SAEs, study-related AEs, pregnancy, and important safety information followed per protocol]: AEs, TRAEs, grade >=3 AEs, SAEs, deaths, and clinically significant laboratory abnormalities summarized by NCI-CTCAE v5.0 and MedDRA SOC/PT. All AEs after informed consent will be recorded; treatment-emergent summaries will start at first study treatment and be summarized by actual exposure period, including luvometinib monotherapy, luvometinib plus cytarabine, and post-progression or salvage treatment descriptions as applicable. SAEs, study-related AEs, pregnancy, and important safety information after study treatment discontinuation may be followed during continued follow-up unless follow-up consent is withdrawn.

Secondary outcomes

• Kaplan-Meier estimated progression-free survival rate at 24 months [24 months after first dose]: Kaplan-Meier estimated proportion of participants without PRC-defined PMD, investigator-confirmed clinical progression supported by clinically performed imaging or documentation after study treatment discontinuation, or death from any cause. Clinically performed data may be used to record PFS progression events but are not included in ORR/DCR/CBR endpoint analyses. Such PFS events may be investigator-adjudicated, with BICR review or audit when feasible; if BICR review is not feasible, the source, assessment date, adjudicator, rationale, and reason will be documented. A sensitivity analysis will include only BICR-confirmed progression, protocol-window imaging-confirmed progression meeting PRC/BICR requirements, or death from any cause.
• Time to response among CMR/PMR responders [From first dose through the first documented CMR/PMR, up to 24 cycles (each cycle is 35 days)]: Time from first dose to first documented CMR or PMR. TTR will be calculated only for participants who achieve CMR or PMR. SMD is not considered a response for TTR; if SMD precedes later PMR/CMR, TTR is measured from first dose to first documented PMR/CMR. Participants with SMD only are not assigned a TTR value.
• Kaplan-Meier estimated overall survival rate at 12 and 24 months [12 and 24 months after first dose]: Kaplan-Meier estimated survival rate from first dose to death from any cause. Participants who transition to combination treatment, receive salvage therapy, or discontinue study treatment remain in survival follow-up unless follow-up consent is withdrawn.
• Objective response rate at cycles 12, 18, and 24 [At the end of Cycle 12, 18, and 24 (each cycle is 35 days)]: Proportion of evaluable participants achieving CMR or PMR by PRC/BICR at the cycle 12, 18, and 24 assessment windows. Later ORR is summarized descriptively using the evaluable response set at each time point; the FAS denominator, number evaluable, and reasons for missing or non-evaluable assessments will also be reported. These later visits evaluate durability and subsequent response patterns and do not confirm the primary ORR endpoint.
• Disease control rate at cycles 12, 18, and 24 [At the end of Cycle12, 18, and 24 (each cycle is 35 days)]: Proportion of evaluable participants achieving CMR, PMR, or stable metabolic disease (SMD) by PRC/BICR at the cycle 12, 18, and 24 assessment windows. DCR is summarized descriptively using the evaluable response set at each time point; the FAS denominator, number evaluable, and missing or non-evaluable reasons will be listed.
• Clinical benefit rate at cycles 12, 18, and 24 [At the end of Cycle 12, 18, and 24 (each cycle is 35 days)]: Proportion of evaluable participants who achieve CMR, PMR, or SMD and maintain disease control for at least 24 weeks. The 24-week duration is counted from the first assessment date documenting CMR, PMR, or SMD to first PMD, death, or loss of disease control, whichever occurs first. Node-specific CBR at cycles 12, 18, and 24 will count only participants who have met the at-least-24-week disease control duration requirement by that assessment node. Participants pending confirmation or not yet meeting the duration requirement remain in the denominator and will be listed separately.

Other outcomes

• Duration of response among CMR/PMR responders [From first documented CMR/PMR through progression, death, or last evaluable disease assessment, up to 24 cycles (each cycle is 35 days)]: Duration of response will be analyzed among responders only, defined as participants who achieve CMR or PMR. DOR is measured from the first documented CMR/PMR to first documented PMD or death from any cause; responders without progression or death will be censored at the date of last evaluable disease assessment. SMD is not considered a response for DOR; if SMD precedes later PMR/CMR, DOR starts at the first documented PMR/CMR. Participants with SMD only are not included in the DOR analysis.
• Spearman Correlation Between Changes in MAPK VAF in cfDNA and peripheral blood cell DNA [Baseline and the end of cycles 6, 12, 18, and 24 as applicable (each cycle is 35 days)]: Change from baseline in the proportion of MAPK pathway variant allele fraction (VAF) in plasma cell-free DNA (cfDNA) and peripheral blood cell DNA, including tumor-derived cfDNA (ctDNA) when detectable, with Spearman's rank correlation coefficient calculated between these proportional changes. These biomarkers will not independently determine treatment assignment or response assessment. After study treatment discontinuation, additional research cfDNA testing is not mandatory; clinically performed or voluntarily completed relevant testing may be recorded descriptively and will not be included in protocol-defined efficacy endpoint analyses unless it occurs within a protocol-defined window and meets applicable protocol requirements.
• Spearman Correlation Coefficient for Changes in MAPK VAF in cfDNA/Peripheral Blood Cell DNA and SUVmax change [Baseline and cycles 6, 12, 18, and 24 as applicable (each cycle is 35 days)]: Spearman's rank correlation coefficient between change from baseline in MAPK pathway VAF in plasma cfDNA/peripheral blood cell DNA and change from baseline in SUVmax of target lesions. Exploratory only, not for treatment assignment or response assessment.
• Quality-of-life scores (adults) [Baseline and the end of Cycle 6, 12, 18, and 24 as applicable (each cycle is 35 days)]: EORTC QLQ-C30 for adults
• Quality-of-life scores (participants aged 10-17) [Baseline and the end of Cycle 6, 12, 18, and 24 as applicable (each cycle is 35 days)]: PedsQL 4.0 for participants aged 10-17.

Sex: All, ages: 10 Years to any

Inclusion Criteria:

* Histologically confirmed Langerhans cell histiocytosis (LCH).
* Age 10 years or older.
* Systemic treatment indication and at least one PET response criteria-evaluable lesion.
* Expected survival of at least 12 weeks, as judged by the investigator, and able to undergo protocol-specified treatment, assessments, and follow-up.
* ECOG performance status 0-2 or Lansky score >=60.
* Adequate organ function as defined in the protocol.
* Written informed consent from adult participants or legal guardians, with participant assent when applicable.

Exclusion Criteria:

* Hypersensitivity to luvometinib or any excipient.
* Concurrent other malignant tumor.
* Pregnancy or breastfeeding.
* Failure to meet protocol contraception requirements.
* Active bacterial, fungal, or viral infection.
* Significant retinal disease or glaucoma.
* NYHA class >=3 heart failure or LVEF <50%.
* Psychiatric disease or other condition preventing protocol compliance.
* Investigator judgment that participation is unsuitable.