Responses to Combination Chemotherapy with MAPK Pathway Inhibition in Relapsed and Refractory LCH

MAPK pathway inhibition (MAPKi) is associated with high response rates in patients with Langerhans cell histiocytosis (LCH), but monotherapy may not be curative and efficacy in LCH-associated neurodegeneration (LCH-ND) is not known. Based on upregulation of Bcl-xL by MAPK activation, we hypothesize that MAPKi may render LCH cells more sensitive to chemotherapy-induced cytotoxicity.

We assessed the safety and efficacy of combination therapy in 10 patients with refractory systemic disease and LCH-ND. Records of patients treated with combination therapy at Texas Children's Hospital from October 2018 to July 2023 were reviewed for clinical responses as well as peripheral blood/tissue BRAFV600E mutation status, ataxia rating scores, and toxicity.

The median age at the start of combination therapy was 6.82 years (range, 0.48-20.45 years) with a median of 4 treatments prior to this therapy regimen (range, 1-14). Almost all Clofarabine was administered outpatient (typical starting dose 25 mg/m2/day x 5 days). MAPK inhibitors included BRAFV600E inhibitors (50%) and MEK inhibitors (50%). Overall response was 8/10 (80%), with 5/5 (100%) objective response rate (ORR) for systemic disease. Further, 5/6 (83%) demonstrated reduced or extinguished peripheral BRAFV600E+ fraction by the end of combination therapy. Ataxia rating scores were improved in 2/3 (67%) individuals with clinical neurodegeneration. Common toxicities during treatment included fever, skin rash, arthralgia, and cytopenia.

Combining therapy was well-tolerated and associated with clinical and molecular improvement in most patients with highly refractory systemic LCH and LCH-ND. Further prospective multi-center trials are required to optimize this approach, determine potential for combination therapy to achieve cure, and compare risks and benefits to chemotherapy or MAPK inhibitor monotherapy.