Morlote, Diana M.

ImportanceRosai-Dorfman disease (RDD) is a rare histiocytic neoplasm with recent studies showing alterations in the MAPK pathway, most commonly in the KRAS and MEK genes in about 40% of patients. Reports on the use of MEK-inhibitor therapy in RDD have been limited to small case studies. There are no approved treatments for this neoplasm, and therefore patients with RDD need efficacious treatments.ObjectiveTo study the outcomes after treatment with cobimetinib based on MAPK pathway alterations in patients with RDD.Design, Setting, and ParticipantsThis retrospective cohort study conducted at 2 tertiary care centers included patients with RDD who underwent treatment with cobimetinib between January 1, 2013, and December 1, 2021. Cobimetinib was administered at a dosage of 20 to 60 mg orally once daily as a single agent for 21 days in a 28-day cycle. Pathology was centrally reviewed. Response assessment was centrally conducted and was based on the established positron emission radiography response criteria used for clinical trials of targeted therapies in histiocytosis.Main Outcomes and MeasuresMain outcomes were overall response rate (ORR), progression-free survival (PFS), adverse events (AEs) of cobimetinib in the entire cohort, and ORRs and PFS based on MAPK pathway alterations in patients with RDD.ResultsA total of 16 patients (median [range] age at cobimetinib initiation, 57 [31-74] years; 11 [69%] women) were included in the study. The median follow-up duration was 19.0 months (95% CI, 8.4-27.8 months). The ORR was 63% (n = 10), including 5 complete responses and 5 partial responses. Somatic alterations in the KRAS or MEK genes were detected in 8 (50%) patients. Patients with KRAS or MEK alterations had significantly higher ORR (88% vs 38%; P = .03), deeper responses (complete responses among responders: 71% vs 0%; P = .002), and better PFS (at 1 year, 100% vs 29% were free from progression or death, respectively; P < .001) compared with those without such alterations. Grade 2 or higher AEs occurred in 12 (75%) patients, and 9 (56%) required dose reduction or temporary/permanent treatment discontinuation due to AEs.Conclusions and RelevanceIn this cohort study, treatment with cobimetinib was associated with positive outcomes in KRAS- or MEK-variant RDD. However, AEs requiring dose modifications were common.

Abstract Background: Activation of extracellular-signal regulated kinase (ERK) is almost universal in histiocytic disorders and is driven by genomic alterations in the mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinases (PI3K) pathways. While BRAFV600E mutations are well described in histiocytic disorders, less is known about BRAF fusions. The goal of our study is to describe the frequency and clinical characteristics of patients harboring BRAF fusions. Methods: We included patients with a diagnosis of histiocytic disorder seen at our institution from November 2016-June 2021. Only those who had adequate BRAF testing were included. The following were considered adequate BRAF testing: 1) unequivocally positive BRAF V600E immunostain with or without molecular confirmation; 2) successful multigene next generation sequencing (mostly Tempus â or FoundationOne â) if BRAF V600Eimmunostain was equivocal or negative. We also searched the literature for similar cases and compared BRAF fusions found in histiocytic disorders to those found in solid tumors and other hematologic conditions. Results: One hundred and twenty-six patients were included in this study. BRAF fusions were detected in 7 (6%) patients. The median follow-up for these patients was 1.4 years (95% CI: 0.4-not reached). The frequency according to disease subtypes is as follows: Erdheim-Chester disease (ECD; 2/46 [4%]), Langerhans cell histiocytosis (LCH; 1/41 [2%]), adult or juvenile xanthogranuloma (AXG/JXG; 4/7 [57%]), and histiocytic or Langerhans cell sarcoma (HS/LCS; 0/9 [0%]). The median age at diagnosis for our study cohort was 34 years (range, 7-81 years) and 5 were females. The clinical characteristics and nature of BRAF fusions of our 7 patients plus additional 13 cases from the literature are shown in Table 1. For the final cohort of 20 patients with BRAF-fusions, the median age was 16 years (range, 0.5-81 years) and most (56%) were females. The distribution by histiocytic subtypes is as follows: AXG/JXG (45%), LCH (40%), ECD (10%), HS/LCS (5%). Two patients received cobimetinib as frontline treatment in our cohort and achieved partial responses. Of these two patients, one completed 12 cycles (12 months) of cobimetinib and had a sustained PR while the other patient received 2 cycles before having to hold therapy due to intolerable adverse effects despite dose reduction. From literature review, another patient (LR-12) received cobimetinib as second line treatment and achieved complete response. We found 15 unique BRAF gene fusions with 4 being recurrent. Except for AGAP3-BRAF, none of these BRAF fusions in histiocytosis has been reported in solid tumors or hematologic conditions to date. Conclusions: BRAF gene fusions occur rarely (<5%) in histiocytic disorders. The only exception is AXG/JXG where more than half of the patients harbor BRAF gene fusions. These gene fusions are mostly distinct to histiocytosis and not found in solid tumors or other hematologic conditions. Figure 1 Figure 1. Disclosures Bennani: Purdue Pharma: Other: Advisory Board; Daichii Sankyo Inc: Other: Advisory Board; Kyowa Kirin: Other: Advisory Board; Vividion: Other: Advisory Board; Kymera: Other: Advisory Board; Verastem: Other: Advisory Board.