Ravindran, Aishwarya

AbstractErdheim–Chester disease (ECD) is a histiocytic neoplasm that predominantly harbors mitogen‐activated protein kinase (MAPK) pathway variants. MAPK inhibitors typically are effective treatments, but mutations outside the MAPK pathway, such as CSF1R variants, may cause refractory ECD. We describe a patient with a novel somatic mutation in CSF1R (CSF1RR549_E554delinsQ) that resulted in refractory ECD affecting the central nervous system. Cell model studies, RNA sequencing analysis, and in silico protein modeling suggested that she had a gain‐of‐function mutation occurring in a region critical for autoinhibition. The patient was treated with pexidartinib, a CSF1R inhibitor, and has had a complete clinical and metabolic response lasting more than 1.5 years to date. To our knowledge, this is the first report to describe successful treatment of a patient with ECD by using an agent that specifically targets CSF1R. This case also highlights the critical role of individualized molecular profiling to identify novel therapeutic targets in ECD.

Abstract Introduction Histiocytic disorders pose significant diagnostic and management challenges for the clinicians due to diverse clinical manifestations and often non-specific histopathologic findings. Herein, we report the tumor board experience from the first-of-its-kind Histiocytosis Working Group (HWG). Materials and Methods The HWG was established in June 2017 and consists of experts from 10 subspecialties that discuss cases in a multidisciplinary format. We present the outcome of tumor board case discussions during the first 2 years since its inception (June 2017-June 2019). Results Forty cases with a suspected histiocytic disorder were reviewed at HWG during this time period. Average number of subspecialties involved in HWG case discussion was 5 (range, 2-9). Histiocytosis Working Group tumor board recommendations led to significant changes in the care of 24 (60%) patients. These included change in diagnosis (n = 11, 27%) and change in treatment (n = 13, 33%). Conclusion Our report highlights the feasibility of a multidisciplinary tumor board and its impact on outcomes of patients with histiocytic disorders.

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.

Abstract Background: Erdheim-Chester disease (ECD) is characterized by multi-organ infiltration of clonal histiocytes bearing activating mutations predominantly in the MAPK pathway. The diagnosis of ECD is clinico-pathologic; histopathologic findings alone are often non-specific. Characteristic pathognomonic finding of ECD is the symmetric osteosclerosis of the distal femur and proximal tibia/fibula, seen in >90% of cases, and referred to herein as classic ECD (C-ECD). There is a paucity of data on the phenotypic and mutational differences between C-ECD and non-classic ECD (NC-ECD). Determining phenotypic patterns may allow for earlier suspicion and diagnosis. Methods: Patients who met the revised ECD criteria proposed by Haroche J et al (Blood 2020;135:1311-1318) and had full body imaging that included the lower legs (18-FDG-PET/CT or CT/bone scan) were included. ECD diagnosis was made when >1 major criteria plus >1 minor criteria were present. Major criteria: 1) symmetric meta-diaphyseal osteosclerosis in legs; 2) "hairy kidneys"; "coated aorta", right atrial pseudotumor, xanthelasma, exophthalmos; or osteosclerosis of paranasal sinuses. Minor criteria: 1) histologic finding of typical foamy histiocytes (CD68+/CD163+/CD1a-) associated with fibrosis; 2) mutation/gene fusion of BRAF, CSF1R, or MAPK/PI3K pathways. We compared the organ involvement and BRAF V600Emutational status between C-ECD and NC-ECD. Results: A total of 105 patients were included. The median age at diagnosis was 57 years (range, 38-81) and most were males (62%). Majority had 18-FDG-PET/CT (83%) and BRAF V600E testing (65%). The main organ systems involved were skeletal (83%), renal (64%), adrenal (44%), and pulmonary (42%). Central diabetes insipidus (DI), "hairy kidneys", and "coated aorta" were present in 27 (26%), 54 (51%), and 44 (42%) patients, respectively. Among those tested, BRAF V600E mutation was found in 48/67 (72%) by immunohistochemistry. In our cohort, most patients (n=87. 83%) had C-ECD. NC-ECD had significantly lower number of organs/systems involved compared with C-ECD (median 3 vs 6, p=0.002). C-ECD had significantly higher rates of involvement of paranasal sinuses (51%/7%, p=0.002), DI (26%/0%, p=0.02), and similar rates of lung (44%/43%), cardiac (34%/14%), and skin (14%14%) involvement when compared to NC-ECD. BRAF V600E was significantly more common in C-ECD (88%/30%, p=0.004). Thirty-nine (37%) patients underwent next generation sequencing, of whom 33 (31%) had successful testing. In C-ECD, 3 patients had mutations other than BRAF V600E, these included: NRAS, MAP2K1, and MEF2C-FLT3 fusion. In NC-ECD, 5 patients had mutations other than BRAF V600E, these included: MAP2K1, KRAS, and NF1. Conclusions: Our study suggests distinct differences in clinical presentation and molecular findings exist between C-ECD and NC-ECD. C-ECD has a higher degree of organ involvement and harbor BRAF V600E more frequently than NC-ECD. Further analysis of histopathologic findings and outcomes in this cohort may provide insights into these ECD subsets that can optimize future management of this disease. Figure 1: Sites of involvement of classical Erdheim-Chester Disease (C-ECD) versus non-classical Erdheim-Chester Disease (NC-ECD) Figure 1 Figure 1. Disclosures Vassallo: Bristol-Myers-Squibb: Research Funding; Sun Pharma.: Research Funding; Pfizer: Research Funding. Tobin: Mayo Clinic Center for MS and Autoimmune Neurology: Research Funding; Mallinckrodt Pharmaceuticals: Research Funding; National Institutes of Health: Research Funding. Bennani: Verastem: Other: Advisory Board; Purdue Pharma: Other: Advisory Board; Vividion: Consultancy, Other: Advisory Board; Daichii Sankyo Inc: Other: Advisory Board; Kyowa Kirin: Other: Advisory Board.

Abstract Background Histiocytic disorders are rare hematologic neoplasms characterized by their high clinical heterogeneity. Evidence of constitutive activation of the mitogen-activated protein kinase (MAPK) pathway in a number of these disorders has led to the increasing use of BRAF- and MEK-inhibitors as a therapeutic strategy. Response to these therapies is not universal and additional effective treatment options are required. Immune checkpoint inhibitors have proven effective for a wide array of malignancies. Tumor mutational burden (TMB), the total number of somatic pathogenic variants per coding region in a tumor genome, is a clinical biomarker associated with response to immunotherapy. We have previously reported preliminary findings of low TMB in a small cohort of patients with histiocytic neoplasms (Goyal G et al. Blood 2019). In this study, our aim was to confirm the findings in a larger clinical cohort using next generation sequencing studies. Methods A retrospective review of adult patients consecutively seen at Mayo Clinic from 2017 to April 2021 and diagnosis of a histiocytic neoplasm was performed. Electronic records were queried for demographic and laboratory data of all patients who consented to undergo tumor-tissue next-generation sequencing with the Tempus-xT ® or xO ® assay (Chicago, IL), throughout the course of their evaluation. TMB was reported as the number of nonsynonymous mutations per coding area of a tumor genome. TMB across groups was compared via an independent-samples T-test or via analysis of variance and post hoc subgroup testing via Turkey's Test as required. Results Seventy-three patients were included in the study. Individual diagnoses included: Erdheim-Chester disease (ECD) in 31.5% (n=23), including 3 patients with overlap features of ECD and other histiocytoses; Rosai Dorfman disease (RDD) in 28.8% (n=21); Langerhans cell histiocytosis in 21.9% (n=16), histiocytic sarcoma (HS) in 5.5% (n=4), and other histiocytic neoplasms in 12.3% (n=10) including adult xanthogranuloma, xanthoma disseminatum, undifferentiated histiocytoses, and one case of Langerhans cell sarcoma. Median tumor percentage on analyzed samples was 30% (range 10%-90%) and was ≤20% in 29 samples. TMB was <1.0 mutations per megabase (mpMb) in 42.5% (n=31) and >5.0 mpMb in only 6 cases. Median TMB was 1.58 (range 0 - 5.26) for ECD, 1.08 (range 0 - 6.3) for LCH, 0.2 (range 0 - 5.8) for RDD, 3.19 (range 1.67 - 6.8) for HS, and 1.6 (range 0 - 7.5) for other histiocytic neoplasms (Figure 1). No clear association between increasing TMB and number of systems with histiocytic infiltration was observed (Figure 2). A significant increase in TMB was observed among the 5 cases of sarcoma as compared to the rest of the cohort (p=0.014). When not considering sarcomas, no differences were observed in TMB between patients with ECD, LCH, RDD or other histiocytoses (F=0.775; p=0.512). Conclusion In this large cohort of histiocytic disorders, TMB was low compared to that historically seen in other neoplasms though this is likely influenced by tumor purity ≤20% in 29 cases. Only 6 tumors had TMB reports with mpMb above the threshold thought to be associated with an increased likelihood of response to PD-1/PDL1-targeted therapies (>5 mpMb). Our results suggest a low likelihood of response among histiocytic disorders using immune checkpoint inhibitors. Further exploration among malignant histiocytoses (HS and Langerhans cell sarcoma) is warranted given our findings of higher TMB compared with other histiocytic neoplasms. Figure 1 Figure 1. Disclosures Bennani: Kymera: Other: Advisory Board; Vividion: Other: Advisory Board; Kyowa Kirin: Other: Advisory Board; Daichii Sankyo Inc: Other: Advisory Board; Purdue Pharma: Other: Advisory Board; Verastem: Other: Advisory Board.

Abstract INTRODUCTION Rosai-Dorfman disease (RDD) is a rare histiocytic disorder associated with histiocytes expressing high level of macrophage colony stimulating factor receptor (c-fms). Mutations in mitogen-activated protein kinase (MAPK) pathway have been reported in RDD, indicating an underlying clonal process in some patients. Aberrant activation of MAPK pathway can occur due to mutations or high c-fms expression. Data on the use of MAPK/extracellular-signal-related kinase (ERK)-inhibitor therapy in RDD are very limited. Cobimetinib is a MEK inhibitor, which is listed in the National Comprehensive Cancer Network Compendium ® for the treatment of RDD. We present our retrospective experience of cobimetinib treatment in a large series of patients with RDD. METHODS Following approval of the institutional reviewed board, patients (pts) with RDD who were consecutively seen at Mayo Clinic and treated with cobimetinib between 2013-2021 were included. Response was assessed as described by Goyal et al (Haematologica 2020;105:2). Cobimetinib was given 20-60 mg once a day by mouth for 21 days in a 28-day cycle. Adverse events (AEs) were graded using the National Cancer Institute Common Toxicity Criteria v5.0. Time-to-event analyses were calculated from the time of treatment initiation, while duration of response was calculated from the time of 1 st response. Tumor genomic profile was analyzed using Tempus-xT ® assay (Chicago, IL), which includes targeted DNA sequencing (648 oncogenes) and whole transcriptome sequencing. RESULTS Of the 54 pts with RDD, ten were treated with cobimetinib and included in this study. The median follow-up was 16 months (m) [95% confidence interval (CI): 13-not reached (NR)] and most (n=8; 80%) were females. The median age at the time of treatment was 63 years (range: 36-74). Cobimetinib was used as first line therapy in four pts. Overall response rate was 70%, with 30% and 40% achieving complete (CR) and partial (PR) responses, respectively. One had stable disease (SD) and two had progressive disease (PD) (Table 1). The median progression-free-survival was 6.4 m (95%CI: 4.9-NR) and at 6 and 12 m, 67% and 50% of pts remained free from disease progression or death, respectively (Figure 1). The median duration of response was 7.6 m (95%CI: 3.1-NR). At the time of last follow-up, nine (90%) pts were alive. One pt died due to myelodysplastic syndrome with excessive blasts. She had multiple chemotherapy treatments before initiation of cobimetinib. Cobimetinib was discontinued in this pt due to grade 3 anemia and skin toxicity after 6 m of therapy and had SD as the best response. Median duration of treatment for the entire cohort was 6 m (range: 2-14). In the seven pts who responded, the median duration of treatment was 7 m (range: 2-14). Cobimetinib was discontinued in seven pts: disease progression (n=3; one disease progression after achieving PR and two PD while on therapy), grade 3 left ventricular systolic dysfunction (n=1), grade 3 diarrhea (n=1), grade 3 anemia (n=1), and grade 2 creatinine elevation (n=1). Five (50%) pts were started at a reduced dose of cobimetinib at the discretion of treating physician [20 mg (n=4) and 40mg, (n=1)]. Two pts were started at 60 mg and dose reduced to 20 mg after one cycle of treatment due to grade 2 skin toxicity in one and grade 2 skin toxicity and diarrhea in the other. Both pts achieved a PR but progressed after four and six cycles, respectively. One pt started at 40 mg and reduced to 20 mg due to grade 2 skin toxicity after two cycles achieving a PR. The response was sustained in this pt for at least 9 months, until the time of last follow-up (Table 1). Tissue molecular testing showed five pts with MAPK pathway alterations and all of them responded to therapy (CR=3 and PR=2). Of the other five pts who either did not have a MAPK pathway alteration (n=4) or had test failure (n=1), responses were observed in two (40%) pts, both PR (Table 1). CONCLUSION Our study suggests that cobimetinib is an effective treatment in RDD, with responses observed in all pts having MAPK pathway alterations. Less than half of pts without a MAPK pathway alteration achieved a response. However, treatment discontinuations due to AEs were common despite dose reductions. Patients who were treated at low doses of cobimetinib also had remarkable clinical response. Further studies are needed in evaluating other MAPK pathway inhibitors with better toxicity profile to improve adherence and outcomes. Figure 1 Figure 1. Disclosures Tobin: National Institutes of Health: Research Funding; Mayo Clinic Center for MS and Autoimmune Neurology: Research Funding; Mallinckrodt Pharmaceuticals: Research Funding. Bennani: Vividion: Other: Advisory Board; Kyowa Kirin: Other: Advisory Board; Daichii Sankyo Inc: Other: Advisory Board; Verastem: Other: Advisory Board; Purdue Pharma: Other: Advisory Board; Kymera: Other: Advisory Board. Vassallo: Pfizer: Research Funding; Bristol-Myers-Squibb: Research Funding; Sun Pharma.: Research Funding.

Abstract Introduction: VEXAS syndrome (vacuoles, E1 ubiquitin ligase, X-linked, autoinflammatory, somatic) is a newly recognized inflammatory disorder caused by somatic mutations in the UBA1 gene. Bone marrows from these patients reveal a range of morphological changes in hematopoietic precursor cells. In this study, we aim to assess the laboratory indices and morphologic spectrum of bone marrow pathology in VEXAS syndrome. Methods: We identified 16 cases of VEXAS syndrome. All cases had confirmed UBA1 mutation. We reviewed bone marrow biopsies corresponding to the date of diagnosis. This study was approved by the Mayo Clinic Institutional Review Board. Results: All patients were male with a median age of 73 years - associated autoimmune disorders included Sweet syndrome, inflammatory arthritis, relapsing polychondritis and granulomatosis with polyangiitis. 14/16 patients had anemia with median hemoglobin of 10.4 (Range: 6.7- 14.1 g/dL). 15/16 had macrocytosis with median MCV 110.4 (Range: 94.8- 123.1 /fL). 5/16 had thrombocytopenia with median platelet count 174 (Range: 20- 500 x10^9/L). 7/16 had leukopenia with median WBC 3.65 (Range: 2.4- 11.6 x10^9/ L). The ESR and CRP medians were 61.0 mm/hr and 81.5 mg/L, respectively. Karyotype was performed in 12 patients of which 11 were normal and the remaining case showed a complex karyotype. An NGS panel targeting the most frequent myeloid disorder associated gene mutations was negative in 10/15 cases. GS for myeloid mutations revealed pathogenic mutations in 5 patients, involving genes TET2 (2/5), DNMT3A (2/5), and TP53 (1/5). Conclusions: Bone marrow findings in VEXAS syndrome, in this series of 16 patients, are individually non-specific, yet when taken altogether in the overtly abnormal cases, are very suggestive when the clinical index of suspicion is high. In such scenarios, the combined clinical and bone marrow findings should prompt discussion and consideration for UBA1 mutation testing given the significant clinical implications for patient management and prognosis. Disclosures Patnaik: StemLine: Research Funding; Kura Oncology: Research Funding. Warrington: Eli Lilly: Research Funding; Kiniksa: Research Funding.

SummaryErdheim–Chester disease (ECD) is a rare histiocytic neoplasm with frequent multiorgan involvement. An accurate diagnosis of ECD requires the correlation of clinical features, histopathologic and radiologic findings. We describe a case series of patients with a referral diagnosis of ECD, whereby the diagnosis was changed to non‐histiocytic diseases after comprehensive review at a tertiary care center. This accurate revision of the referral diagnosis of ECD enabled initiation of proper disease‐directed therapy in a timely manner for these patients and avoided unnecessary exposure to systemic cytotoxic chemotherapy or targeted agents. Our study highlights the value of a multidisciplinary team of histiocytosis experts in confirming the diagnosis of ECD and also brings attention to other conditions to consider that can mimic ECD, including osteopoikilosis, tenosynovial giant cell tumour, IgG4‐related disease, fibrous dysplasia and chronic recurrent multifocal osteomyelitis.

AbstractLangerhans cell histiocytosis (LCH) is a histiocytic neoplasm that can involve the lungs as single system (LCH‐SSL) or multisystem disease (LCH‐MSL). The role of full‐body radiographic staging to determine whether patients have LCH‐SSL or LCH‐MSL is unclear. Long‐term outcomes of LCH‐SSL versus LCH‐MSL and multisystem without lung involvement (LCH‐MSNL) are unknown. A retrospective study of adult LCH patients seen at our center from January 2000 to 2020 was performed. In Part 1, we addressed utility of whole‐body staging imaging among those presenting with isolated pulmonary signs or symptoms. Staging was defined as fluorodeoxyglucose positron emission tomography‐computed tomography (CT) or whole‐body CT obtained within 3 months of diagnosis. In Part 2, we examined the frequency of developing extra‐pulmonary disease over time and mortality in patients with LCH‐SSL. In Part 3, we compared the overall survival of LCH‐SSL, LCH‐MSL, and LCH‐MSNL. Part 1: 240 patients with LCH were identified. A total of 112 (47%) had pulmonary signs or symptoms at presentation. Thirty‐four (30%) underwent radiographic staging and only one showed evidence of extra‐pulmonary disease. Part 2: 108 (45%) were LCH‐SSL. Median follow‐up duration of 4.5 years (95% confidence interval [CI]: 2.9–6.0). None developed extra‐pulmonary disease. Part 3: 5‐year survival: 94% (95% CI: 84%–98%) for LCH‐SSL, 78% (95% CI: 59%–90%) for LCH‐MSL, and 75% (95% CI: 53%–89%) for LCH‐MSNL. LCH patients presenting with isolated pulmonary signs or symptoms rarely have extra‐pulmonary involvement at the time of diagnosis and generally do not develop extra‐pulmonary progression. LCH‐SSL has the best overall survival, while LCH‐MSL and LCH‐MSNL have similar clinical outcomes.