Goyal, Gaurav

Abstract Endocrinopathies are frequently the initial presentation of histiocytic neoplasms, which are rare hematologic disorders affecting multiple organ systems. Langerhans cell histiocytosis and Erdheim–Chester disease are 2 such disorders known to infiltrate the hypothalamus and/or pituitary gland, leading to arginine vasopressin deficiency (AVP-D) and anterior pituitary dysfunction (APD) in 20% to 30% of cases, often as the first manifestation. Conversely, histiocytic disorders account for a notable proportion (10-15%) of all pituitary stalk lesions. The diagnosis of histiocytoses is often delayed in such cases due to the nonspecific presentation of endocrinopathies and pituitary involvement. Consequently, endocrinologists are at the frontline and uniquely positioned to achieve early diagnosis by recognizing the varied nonendocrine features of these disorders. This article provides an overview of the endocrine manifestations of histiocytic disorders and presents a simplified algorithm to guide the diagnostic workup in cases presenting with “idiopathic” AVP-D or APD. Such cases should be evaluated for histiocytic neoplasms with additional imaging studies and biopsies of suspected disease sites. If no disease site beyond the pituitary is identified, the risks and benefits of a pituitary stalk lesion biopsy must be carefully considered. While treatments of histiocytic neoplasms are highly efficacious, endocrinopathies are considered permanent and require long-term hormone replacement. It remains unclear whether early diagnosis and novel targeted therapies can reverse these endocrine disorders. Therefore, the role of the endocrinologist role is critical in the diagnosis and management of these rare diseases.

Introduction Malignant histiocytoses (MH), encompassing histiocytic sarcoma (HS), interdigitating dendritic cell sarcoma (IDCS), and Langerhans cell sarcoma (LCS), are a group of rare, hematologic neoplasms with diverse clinical presentations and poor outcomes. There are no established or effective treatments for patients with MH. The discovery of MAPK/ERK pathway mutations in non-malignant histiocytoses have revolutionized the treatment paradigm, but it is unknown whether novel treatments are efficacious for MH. This study aimed to assess the efficacy of targeted agents (TA) and immune checkpoint inhibitors (ICI) in MH. Methods We conducted a retrospective chart review of MH patients seen at our two institutions. Only patients treated with TA and/or ICI were included. We captured the mutational status by next generation sequencing and PD-L1 expression level by immunohistochemistry, where available. We also conducted a systematic literature review using PubMed to identify additional cases of MH patients treated with TA and/or ICI. Responses were reported as complete (CR), partial (PR), or none (NR) using previously published criteria for histiocytic disorders. Results From our institutions, we identified 12 MH patients (7 HS; 4 LCS; 1 IDCS) who received 14 instances of these treatments including TA (n=9) and/or ICI (n=5). The median age at diagnosis was 55 years (range, 33-70) and 50% were males. TAs included BRAF inhibitor (vemurafenib [1]), MEK inhibitors (binimetinib [1], cobimetinib [2], trametinib [2]) and others (dasatinib [1], pazopanib [1], pexidartinib [1]), while ICIs were exclusively pembrolizumab (5). The median number of prior treatments for those who received TAs and ICIs were 4 and 2, respectively. Responses were observed in 44.4% (1 CR, 3 PRs, 5 NR) and 40% (2 CRs, 3 NRs) of patients who received TA and ICI, respectively. Only 1 patient (20%) treated with MEK inhibitor responded [DOCK8-BRAF; PR 5+ mos]. Other responders received dasatinib [PDFGRA-V608I; PR 2 mos], pexidartinib [PTPN11-A72T; PR 5 mos], and vemurafenib [BRAF-V600E; CR 10+ mos]. Two patients (40%) who received pembrolizumab responded (both CRs 12+ [10% PD-L1] and 44+ mos [40% PD-L1]). We identified 29 MH patients in the literature (27 HS; 2 LCS) treated with TAs (22) and/or ICIs (10). TAs included BRAF inhibitor only (vemurafenib [3], dabrafenib [3]), MEK inhibitor only (trametinib [5]), BRAF + MEK inhibitors (2), and others (apatinib, anlotimib, bevacizumab, daratumumab, dasatinib, imatinib, pazopanib, sorafenib, or combinations [9]), while ICIs included pembrolizumab (4), nivolumab (4), tislelizumab (1), and sintilimab (1). Responses were observed in 77% (7 CRs, 10 PRs, 5 NRs) and 100% (2 CRs, 8 PRs) of patients who received targeted agents and ICI, respectively. Most patients carrying BRAF (7/8; V600E, G596R, D594G, TRIM::BRAF) or MAP2K1 (3/3; C121S, F53I) mutations responded to treatments, irrespective of the number of mutations present. The median durations of response for ICI and targeted agents were 10.5 months (range: 1-36+) and 12+ months (range: 3-44+), respectively. After combining the datasets from our institutions and literature review, we analyzed the association between the extent of PD-L1 expression and response to ICI. Those who responded to ICI had higher PDL-1 expression (median 75% [range, 10-75] vs 25% [range, 1-30]; P=0.0097). Conclusion TAs and ICIs can be considered in the management of MH. The responses to ICI therapy may be associated with the degree of PDL1 expression

Introduction. Erdheim-Chester disease (ECD), Langerhans cell histiocytosis (LCH) and Rosai-Dorfman disease (RDD) belong to a group of histiocytic neoplasms often driven by mutations in the mitogen activated protein kinase (MAPK) pathway, most commonly in BRAF, MAP2K1 and K/NRAS. Currently, there are 2 drugs approved by the US Food and Drug Administration for the treatment of histiocytic neoplasms, cobimetinib and vemurafenib, both small molecule kinase inhibitors (SMKI) targeting mitogen-activated extracellular signal-regulated kinase (MEK) and BRAF, respectively. BRAF mutations are functionally classified as RAS-independent - either monomers (class I) or dimers (class II) - and RAS-dependent (class III). MAP2K1 mutations are functionally classified as RAF-independent, RAF-regulated, or RAF-dependent. Preclinical studies suggest class I, but not classes II-III, BRAF mutations are sensitive to RAF monomer inhibitors (dabrafenib, encorafenib, vemurafenib) while classes I-II, but not class III, BRAF mutations are sensitive to MEK inhibitors (Yaeger R, Cancer Discovery 2019). In contrast, RAF-regulated MAP2K1 mutants have variable sensitivities to allosteric MEK inhibitors (binimetinib, cobimetinib, selumetinib, trametinib) while RAF-dependent and RAF-independent MAP2K1 mutations are resistant. KRAS mutations are generally resistant to MEK inhibitors (Johnson C, Cancer Discovery 2022). Our study described the clinical efficacy of commercially available SMKIs in patients with histiocytic neoplasms harboring non-BRAFV600E mutations and compared the findings with preclinical data. Methods. We conducted a retrospective chart review of patients with histiocytic neoplasms who were treated with SMKI at Mayo Clinic. Only those who had adequate genomic testing and non-BRAFV600E mutations were included. Responses were categorized as complete (CR), partial (PR), or none (NR) based on established criteria. Results. We identified 31 non-BRAFV600E patients treated with SMKI (2 LCH; 20 ECD; 9 RDD). The median age at diagnosis was 61 years (range: 16-76) and 52% were females. The distribution of BRAF mutations was: class I (V600delinsRE [1]), class II (V495A [1], V471F [1], N486_P490del [2], GAB2-BRAF [1], UBR2-BRAF [1], RNF11-BRAF [1], UBTD2-BRAF [1]), and class III (G466E [1], D594A [1]). The only patient with class I mutation received cobimetinib and had PR. Among those with BRAF class II mutations, 2 received vemurafenib and had NR, while 9 received MEK inhibitors (binimetinib, cobimetinib, trametinib) and most responded (3 CRs, 5 PRs). The patient who had NR required dose reductions due to side effects. Both patients with class III mutation did not respond to cobimetinib. The median duration of responses was 19 months (range: 3+ to 45+) and 3 patients progressed subsequently. The distribution of MAP2K1 mutations was: RAF-independent (E102_103del [2]), RAF-regulated (I103N [1], K57T [1], Q56P [3], F53L [1], K57N [1]) and RAF-dependent (Y130C [1], D67N [1]). Both patients with RAF-independent mutations treated with cobimetinib had PRs. All patients with RAF-regulated mutations treated with allosteric MEK inhibitors responded (3 CRs, 5 PRs). Both patients with RAF-dependent mutations treated with cobimetinib responded (1 CR, 1 PR). The median duration of responses was 15 months (2 to 29+) with 3 patients progressing after treatment cessation due to intolerance. The distribution of RAS mutations was: NRAS (Q16R [1]) and KRAS (K117N [3], A146T [1], T581 [1], G13C [1], G12D [1]). Most patients treated with allosteric MEK inhibitors responded (3 CR, 4 PR, 1 NR). The median duration of response was 15 months (5 to 41) and 2 patients subsequently progressed, one of them after treatment cessation. Conclusions. Among patients with histiocytic neoplasms with non-BRAFV600E mutations receiving SMKI, our study found mixed patterns of concordance between pre-clinical predictions and clinical responses. There was concordance of efficacy in the setting of BRAF classes I-III and MAP2K1 RAF-regulated mutations. However, we found discordant findings in patients harboring RAF-dependent and RAF-independent MAP2K1 mutations as well as KRAS mutations as most responded to allosteric MEK inhibitors.

Background: The Optimized Hemophagocytic Lymphohistiocytosis (HLH) Inflammatory (OHI, Zoref-Lorenz et al, Blood, 2022) index, defined by the combined elevation in serum soluble CD25 (sCD25) and ferritin, has emerged as a promising prognostic tool for patients with newly diagnosed hematologic malignancies, demonstrating superior performance in predicting 500-day mortality compared to HLH-2004 criteria. However, previous studies were retrospective, with a substantial proportion (65%) of patients suspected clinically to have HLH. We evaluated the performance of the OHI index in an unselected, prospective cohort of newly diagnosed diffuse large B-cell lymphoma (DLBCL) patients. Methods: We analyzed the predictive utility of the OHI index in 331 newly diagnosed DLBCL patients enrolled from 2002-2015 in the prospective Iowa/Mayo Molecular Epidemiology Resource cohort. Ferritin and sCD25 levels were quantified in pre-treatment serum samples using R&D ELISA kits. OHI values were also optimized based on 500-day mortality in the cohort using receiver operating curve analysis with internal validation. Patients were classified as OHI+ using the original and newly optimized thresholds for sCD25 and ferritin. The primary endpoint was 500-day survival, and the secondary endpoints were event-free survival (EFS) at 24 months (EFS24), EFS, and overall survival (OS). The predictive capability of the OHI index was evaluated using logistic regression to determine odds ratios (OR) and 95% confidence intervals (CI) for 500-day mortality and EFS24, as well as Cox regression to calculate hazard ratios (HR) and 95% CIs for EFS and OS. Results: The median age at diagnosis of the 331 patients was 63.5 years (interquartile range, 54.5-74), and 168 (51%) were male. Using the original thresholds (sCD25 3,900 U/mL and ferritin 1,000 ng/mL), 4.2% of patients were OHI+, with a more than 5-fold higher 500-day mortality risk compared to OHI- patients (OR=5.6; CI 1.6-17, p<0.001). After optimizing thresholds for sCD25 (2,400 U/mL) and ferritin (250 ng/mL) to predict 500-day mortality, 23.6% were OHI+ and the OHI+ group had a 9-fold higher 500-day mortality risk (OR=9.2; CI 4.0-24, p<0.001), with 31% mortality in the OHI+ vs 5% in the OHI- group. OHI+ patients exhibited more B-symptoms (35% vs 20%, p=0.027), elevated LDH levels (76% vs 34%, p<0.001), poorer performance status (Eastern Cooperative Oncology Group 2+: 38% vs 12%, p<0.001), and more advanced stage disease (Stage IV: 57% vs 38%, p<0.001), although the frequency of a high International Prognostic Index (IPI) was not statistically different between the groups (IPI 3-5: 44% vs 36%, p=0.28). After adjusting for age and IPI, OHI+ remained a strong predictor of 500-day mortality (OR=5.0; CI 1.9-13, p=0.001). Further adjustment for the cell of origin did not impact the results. The association of OHI+ status with 500-day mortality was similar in IPI 0-2 (OR=5.8; CI 1.2-29, p=0.033) and 3-5 (OR=6.8; CI 0.6-73, p=0.11). OHI+ patients were also more likely to not achieve EFS24 (45% vs 13%), with a 3-fold greater risk after adjusting for age and IPI (OR=3.2; CI 1.5-6.5, p=0.0018). The OHI index also retained its predictive ability over longer-term follow-up (median 9.8 years for living patients) for both OS (HR=2.0; CI 1.0-4.0, p=0.040) and EFS (HR=2.2; CI 1.1-4.3, p=0.030) after adjusting for age and IPI. Cytokine profiling using multiplex ELISA identified elevated inflammatory markers in OHI+ patients, underscoring the role of systemic inflammation in prognosis. The distribution of cytokines was significantly different between the groups, with consistently higher means in the OHI+ vs OHI- group, including IL1a (1,628 vs 759 pg/ml, p=0.008), CCL2 (1,020 vs 724 pg/ml, p=0.028), CXCL10 (165 vs 53 pg/ml, p<0.0001), and CXCL9 (380 vs 150 pg/ml, p<0.0001). Conclusions: While the original OHI index for prediction of 500-day mortality was validated in this unselected cohort of newly diagnosed DLBCL patients, optimization of the OHI thresholds identified a larger OHI+ group (24%), enhanced the identification of hyperinflammation, improved predictive ability for early (500-day) mortality, and predicted EFS24 status and long-term OS. This underscores the clinical relevance of the OHI index, supports the need to evaluate disease-specific thresholds, and suggests its potential for use in routine management and as enrollment criteria in clinical trials for novel therapies.

Introduction: ELA026 is a first-in-class human monoclonal antibody directed against signal regulatory protein (SIRP)α/β1/γ and is under clinical investigation in secondary hemophagocytic lymphohistiocytosis (sHLH). sHLH is a life-threatening hyperinflammatory condition caused by failure to terminate the activation and proliferation of natural killer (NK), T cells and macrophages after an initial immune response. There are no approved therapies for sHLH. With a mortality rate of approximately 50% at 2 months (Lionel et al. 2023), malignancy-associated HLH (mHLH) is the deadliest subtype of sHLH. Cytotoxic, multi-agent chemotherapy (including etoposide) does not address the poor prognosis associated with mHLH (Xie et al. 2013). Directly targeting the pathogenic SIRP(+) myeloid cells and T lymphocytes represents a novel approach to resolving this severe hyperinflammatory condition. Methods: This is an open-label, single-arm, Phase 1b global multicenter study (NCT05416307) evaluating the safety, efficacy, pharmacokinetics, and pharmacodynamics of ELA026 in adult and pediatric patients with newly diagnosed and previously treated sHLH. The study is expanding enrollment to evaluate a fixed-dose regimen consisting of a priming phase of 0.1 mg/kg Day 1 and 0.3 mg/kg Days 2-4, followed by a maintenance phase of 0.5 mg/kg twice weekly beginning on Day 8. ELA026 is administered intravenously, with the option to transition to subcutaneous administration upon hospital discharge. The treatment period is 12 weeks with the option to extend in patients deriving ongoing clinical benefits. Enrollment in this cohort is restricted to frontline settings based on the experience that relapsed/refractory sHLH patients exhibited poor survival even with HLH disease control and died from complications related to severe baseline comorbidities. Response is evaluated using a modified HLH-2004 criteria (Locatelli et al. 2020) and survival is calculated from the time of HLH diagnosis. All patients receive dexamethasone as background therapy and premedication initially. Patients with mHLH may also receive cancer-directed therapies concurrently. Primary pharmacodynamic (PD) effects (monocyte and lymphocyte reduction) and secondary PD effects (C-reactive protein [CRP], ferritin, and sCD25 reduction) were best evaluated and attributed to ELA026 during windows of treatment without concurrent chemotherapy effects. Results: As of July 15, 2024, 12 treatment-naïve mHLH patients have enrolled in all cohorts of the study, with a median age of 47 years (range 21-78). The following malignancies were represented: 7 T-cell lymphoma, 2 DLBCL, 1 AML, 1 Hodgkin lymphoma, and 1 T-cell ALL. In this group, response rate was 100% with 2 mCR and 9 PR by Week 4 and survival was 88% at 2 months (7/8; 4 patients are alive and have not reached 2 months on study). Median survival has not been reached; median follow-up is 2.3 months (range 0.1-15.9 months). In terms of PD effects, monocyte and lymphocyte reduction generally occurred following the first priming dose and ferritin and CRP were reduced by an average of 40% and 50% by Day 6, respectively, for patients with windows of ELA026 treatment without concurrent chemotherapy effects. Study-wide (all patients dosed), treatment-related infusion reactions (~20%) and cytopenias (~30%) were generally manageable with supportive care and dosing modifications. One serious adverse event, a grade 4 infusion reaction, was assessed as related in a relapsed/refractory mHLH patient with acute liver failure and poor hemodynamic reserve. Infection risks are expected for this patient population, which were mitigated with growth factor support and antimicrobial prophylaxis. No deaths were related to study drug. Updated study results will be provided at the 2024 ASH meeting in San Diego. Conclusion: Data suggest ELA026 is well-tolerated to date in sHLH patients and results in a 100% response rate and improved 2-month survival in treatment-naïve mHLH patients. The study is enrolling in an expanded cohort to further assess efficacy in this patient population.

Introduction: Adults older than 60 years constitute 20-25% of newly diagnosed classic Hodgkin lymphoma (cHL) and have inferior outcomes compared to younger patients (pts). Salvage therapy followed by an autologous stem cell transplant (ASCT) is a potentially curative option for pts with relapsed/refractory (R/R cHL). There is limited data regarding outcomes of older adults with R/R cHL who undergo ASCT in the current era where novel agents are employed in the R/R setting, in comparison with younger patients. Methods: This multicenter, international, retrospective study of 18 participating institutions included adults with R/R cHL who underwent ASCT from January 1, 2010 through December 31 2020. Retrospective chart review was performed to collect data on patient and disease characteristics at relapse, including lines and type of salvage therapy, pre-ASCT response by PET, and use of brentuximab vedotin (BV) as consolidation post-ASCT. Study objectives were to assess progression-free survival (PFS), overall survival (OS), and the probability of non-relapse mortality (NRM) with relapse as a competing event. NRM was defined as death without preceding relapse. Results: Of the total 1172 adults with R/R cHL enrolled in the parent cohort, 96 (8.1%) were age 60 years or older at the time of ASCT. In this older cohort, the median age was 66 years (range: 60-72). 54 (56%) were age 60-64, 31 (32%) were of age 65-69 and 11 (11%) were of age 70-72. 59 (61%) were male, 63 (66%) had advanced stage disease, 17 (18%) had bulky disease, 37 (39%) had B symptoms and 38 (40%) had extranodal disease. All patients (pts) received ABVD or ABVD-like frontline therapy. The median time to relapse was 6 months (range: 0-174). 48 pts (50%) relapsed within a year after completion of frontline treatment and 25 (26%) had primary refractory disease. Salvage therapy prior to ASCT included platinum-based chemotherapy (65%), BV-based regimen (34%), and PD-1 inhibitor-based regimens (3%). 16 pts (16%) underwent ASCT after >1 line of salvage therapy and 75 (78%) underwent ASCT after achieving a complete metabolic response (CMR). 14 pts (14.6%) received BV consolidation. Median follow-up for entire population was 53.4 months (range: 0.3-199.0). In older adults, the 5-year PFS was 54.2% (CI95: 43.4-67.6) not statistically different compared to pts <60 years (67.7% (CI95: 64.3-71.3), p=0.2). However, 5-year OS was inferior in older adults: 71.9% (CI95: 43.4-67.6) vs. 82.6% (CI95: 43.4-67.6) in pts <60 years (p <0.0001)). In pts ≥60, 26 (27%) died including 12 (12.5%) without preceding relapse. Older adults had 3-month, 1-year and 5-year probability of NRM of 5.2% (CI95: 2.2-12.2), 7.5% (CI95: 3.6-15.3), and 10.9% (CI95: 5.8-20.8), respectively. NRM was significantly higher in older adults compared to pts <60 who had 3-month, 1-year and 5-year probability of NRM of 0.3% (CI95: 0.1-1.0), 0.9% (CI95: 0.5-1.7), and 2.5% (CI95: 1.6-3.9), respectively (p<0.0001). Older adults had 3-month, 1-year and 5-year probability of relapse of 1% (CI95: 0.2-7.3), 7.5% (CI95: 3.6-15.3), and 28.6% (CI95: 19.1-42.1), respectively, which did not differ significantly from younger pts (p=0.3) who had 3-month, 1-year and 5-year probability of relapse of 0.6% (CI95: 0.3-1.3), 5.1% (CI95: 3.8-6.6), and 29.9% (CI95: 26.6-33.5), respectively. Conclusions: Although older adults with cHL are more likely to relapse after frontline treatment, only a minority of pts (<10%) in this large multicenter cohort underwent ASCT at age ≥60. In select older adults who underwent ASCT, the risk of relapse was similar to younger pts, but NRM was significantly higher, especially in the early post-ASCT period. Detailed assessment of comorbidity scores, toxicities of salvage therapies, and prognostic factors will be presented at the conference.

Background: Erdheim-Chester disease (ECD) manifests as multi-organ infiltration of clonal histiocytes with mutations primarily in the mitogen-activated protein kinase (MAPK) pathway. The revised classification of histiocytosis (2016) categorizes ECD with typical bilateral, symmetric osteosclerosis of the lower extremity long bones as classical (C) and as non-classical (NC) if otherwise. The clinical and biological significance of this classification is unclear. This study compared the clinical and genotypic findings of the 2 subtypes within a large cohort of ECD patients. Methods: We included patients seen at our institution since 1990 meeting the following criteria: A) had an ECD diagnosis based on the revised criteria by Haroche J, et al. (Blood 2020;135:1311-1318); B) completed whole-body imaging studies including long bones of the legs. Classification as C-ECD required symmetric osteosclerosis of the distal femur and/or proximal tibia/fibula. Results: A total of 145 patients were enrolled, comprising a male-to-female ratio of 2:1 (66.2% male). C-ECD was diagnosed in 77.2% of cases, and NC-ECD in 22.8%. The median age at diagnosis was similar with 58.5 years (range, 18-82) in C-ECD and 55 years (range, 20-81) in NC-ECD, (p=0.65). The most frequently affected systems included skeletal (81.4%), renal (60.0%), vascular (45.5%), cardiac (40.0%), central nervous (38.6%), sinus (37.2%), pulmonary (36.6%), and adrenal (32.4%). The numbers of organs involved were significantly higher in C-ECD compared to NC-ECD (median 5 vs 3, p<0.01). Higher rates of skeletal (100% vs. 18.2%, p<0.01), adrenal gland (37.5% vs. 15.2%, p=0.02), and sinus (44.6% vs. 12.1%, p<0.01) involvement were observed in C-ECD. Arginine vasopressin deficiency (AVP-D) was exclusively observed in C-ECD (20.7% vs. 0%, p<0.01). NC-ECD had a higher rate of cardiac involvement (69.7% vs. 31.3%, p<0.01). No significant differences were found in other systems. The overall median time between onset of the first symptoms/signs to diagnosis was 15 months with no significant difference between C-ECD and NC-ECD (17.5 vs. 12 months, p=0.18). The median time to diagnosis of ECD in patients with AVP-D was 38.5 months, compared to 12 months without AVP-D (p=0.09). Genetic testing was conducted in 86.2% of patients and B-Raf proto-oncogene (BRAF) alterations were identified in 59.2% of the tested cases. BRAFV600E was more prevalent in C-ECD (61.3% vs. 21.9%, p<0.01), whereas non-V600 mutations were more commonly found in NC-ECD (12.5% vs. 1.1%, p<0.01). There was no significant difference in the frequency of BRAF fusions. NC-ECD patients exhibited a higher rate of MAP2K1 mutations (25.0% vs. 5.4%, p<0.01). Other frequently mutated genes included KRAS (4.0%), NRAS (2.4%), and CSF1R (2.4%), although no significant difference was observed in their frequency between C-ECD and NC-ECD. Testicular involvement appeared to be more common in patients with MAP2K1 mutations (23.1% vs. 8%, p=0.08). Two patients with pathogenic CSF1R mutations both had central nervous system (CNS) involvement. Among C-ECD patients, BRAFV600E conferred a higher rate of CNS involvement (50.9% vs. 25%, p=0.01). No significant difference was observed in NC-ECD patients (71.4% vs 36%, p=0.09). Among NC-ECD cases, MAP2K1 mutations conferred a reduced risk of cardiac involvement (37.5% vs. 79.2%, p=0.03) and no such difference was observed in patients with C-ECD (40% vs 34.1%, p=0.79). The median numbers of organs involved in patients with BRAF, MAP2K1, and KRAS/NRAS mutations were 5, 3, and 3, respectively, with no significant difference. Conclusions: C-ECD affects more organs and more frequently involves bones, adrenal glands, and sinuses. AVP-D was exclusively observed in C-ECD. Conversely, NC-ECD showed a higher prevalence of cardiac involvement. BRAFV600E was more commonly identified in C-ECD, whereas non-V600 mutations and MAP2K1 mutations were more prevalent in NC-ECD. Among C-ECD patients, BRAFV600E was correlated with a greater likelihood of CNS involvement. Conversely, NC-ECD patients with MAP2K1 mutations had a reduced risk of cardiac involvement. This study highlights the distinct clinical and molecular features of C-ECD and NC-ECD, underscoring the importance of subclassifying ECD into classical and non-classical forms. Further studies are needed to explore differences in the clinical outcomes between the 2 forms.

Introduction: Autoimmunity in Rosai-Dorfman disease (RDD) has been reported in several case reports and small case series. The recent consensus RDD guidelines even classify “immune-related RDD” as a distinct entity, but little is known about the prevalence and patterns of autoimmune conditions in this rare disease. We conducted this study to evaluate a large cohort of patients with RDD for concomitant autoimmunity. Methods: We conducted a retrospective review of cases diagnosed with RDD from five referral centers globally. Data pertaining to autoimmunity were captured in terms of serology and a clinical diagnosis of autoimmune disease (AID) and compared to previously reported prevalence of AID and autoimmunity in the general population and in patients with other lymphomas. Results: We included 157 patients diagnosed with RDD from 2003-2023. Median age at the time of RDD diagnosis was 48 years, IQR 35.5-60. This included 103 females (65.6%) and 54 male (34.4%). Racial distribution included 54 Black (34.4%), 52 White (33.1%), 40 Asian (25.5%), 7 Middle Eastern (4.5%) and 4 other (2.5%). 83 patients had multi-systemic disease (52.8%) and 74 had single organ involvement (47.2%). Most common organs involved were skin/subcutaneous tissue (78, 49.7%), lymph nodes (63, 40.1%), bone (54, 34.2%). Tissue next generation sequencing data was available for 100 patients (63.7 %) and showed KRAS mutation (19, 19 %), MAP2K/MEK (13, 13%), other mutations (27, 27 %), and no mutations (41, 41%). Mean ESR at diagnosis of RDD was 28.42 mm/Hr and mean CRP was 19.5 mg/L. In our cohort, 28 patients (17.8%) were diagnosed with an AID. 19 (67.8%) were diagnosed with the AID prior to the diagnosis of RDD. Most common AID was systemic lupus erythematosus (6, 21.4%), rheumatoid arthritis (5, 17.8%), autoimmune hemolytic anemia (3, 10.7%), polymyalgia rheumatica (2, 7.1%). AID was more common in female vs male (64.3% vs 35.7%, p 0.8) and were more commonly seen in patients with multisystem compared to single organ RDD (60.7% vs 39.3%, p 0.3). Mean ESR and CRP at the time of RDD diagnosis in patients with AID were 40.63 mm/Hr and 37.04 mg/L respectively. Autoimmune antibodies were tested in 90 patients (57.3%), of which 43 (47.8%) were positive for atleast one autoantibody which included 15 patients (16.7%) with AID and a positive autoantibody and 6 (6.6%) with AID and negative antibodies (p 0.01). Most common antibodies were ANA (29/83, 34.9%), 42.3% with a titer higher than 1:160. Other antibodies include RF (10/60, 16.67%), anti-neutrophilic cytoplasmic antibody (3/19, 15.8%), double stranded DNA (6/56, 10.7%). Positive Antibodies were more common in female compared to male patients (72% vs 23%, p 0.04) and in patients from China compared to North America (45.7% vs 22.6%, p 0.007). Autoantibody positivity was common in multi-system vs single organ RDD (53.5% vs 46.5%, p 0.9). Mean ESR and CRP in this group of patients were 34.54 mm/Hr and 23.93 mg/L respectively. Median duration of follow up for our cohort was 32 months (IQR 14-65). Five-year survival was 93.4%. There were a total 8 deaths reported in the cohort, none of which were in patients with underlying AID. In a multivariable analysis, AID was associated with race (odds ratio [OR] for Middle Eastern compared with Chinese, OR 7, p=0.02) but other factors like organ involvement, mutational status, sex and age at diagnosis were not significant. Conclusions: Our study reveals a high prevalence of autoimmunity in RDD, both in terms of AID (17.8 %) and positive autoantibodies (47.8 %), which is much higher than the general population rates of approximately 3% and 16% respectively, and higher than patients with non-Hodgkin lymphoma, 7.6% and 39%, respectively. There was a trend toward association of AID with multisystem RDD and among individuals from North America and Middle East compared to China, although auto-antibody positivity was more common in Chinese patients. Mean ESR and CRP at the time of diagnosis of RDD was higher in the patients who had AID. We did not find any statistically significant association between AID and autoantibody positivity with the underlying driver mutation. Nearly half of the patients with RDD who also had AID did not have a positive autoantibody, thereby implying the need of clinical evaluation for rheumatological symptoms rather than empiric serological testing. Overall, this study also highlights the need to explore the biological link between RDD and autoimmunity.

PURPOSE BRAF and MEK inhibitors are standard treatments in histiocytic disorders, such as Erdheim-Chester disease (ECD). Some patients lack MAPK-pathway alterations, making these treatments less effective. METHODS We describe three patients with histiocytic disorders who have novel non-MAPK pathway alterations. These alterations were studied through genomic and in silico analyses when applicable, then treated with off-label medications rationally selected on the basis of genomic alterations. RESULTS Patient 1 had rapidly progressive ECD involving the CNS. A CSF1R in-frame deletion (p.S560_P566del) was identified, and in silico modeling predicted a gain-of-function mutation. This alteration was targeted with pexidartinib, which led to a clinical complete response (CR) within 2 months, and a partial response (PR) on imaging after 3 months. After 15 months, the disease became resistant to pexidartinib and transformed to histiocytic sarcoma. Patient 2 has skin-only involvement of a xanthogranuloma disorder. A KIF5B-FGFR1 fusion was identified on RNA sequencing and targeted with pemigatinib. At 24 months of follow-up, she remains in a clinical PR. Patient 3 has ECD involving the bone marrow, gastrointestinal tract, and subcutaneous tissues. A MEF2C-FLT3 fusion was identified and targeted with sorafenib. He achieved a clinical CR and radiographic PR within 3 months, which has continued for 30 months. CONCLUSION We report three patients with histiocytic disorders harboring novel alterations who had sustained responses to off-label kinase inhibitors specific to their histiocytic disorder. Pathogenic variants outside of the MAPK pathway, including variants of unknown significant, may be targeted with readily available small molecules.

e23554 Background: Follicular dendritic cell sarcoma (FDCS) is a low-grade neoplasm arising from follicular dendritic cells. Its diagnosis is challenging due to its extreme rarity, and the optimal management remains unknown. Methods: We conducted a retrospective study of FDCS patients (pts) seen at Mayo Clinic, MN since 2018. Time to event analyses were done from the time of diagnosis. Results: Ten pts with FDCS were identified (Table). The median age was 60 years (range, 38-76) and 5 were males. Abdominal pain was the most common presenting symptom (30%) followed by constitutional symptoms, shortness of breath, and vision change. In 3 pts, the diagnosis was incidental. The 2 most common primary locations were abdomen/pelvis (50%) and mediastinum (30%). Six pts (60%) presented with advanced disease and 3 (30%) had Castleman’s disease. Most (60%) had advanced disease. Six pts had Epstein-Barr virus in situ hybridization and all tested negative. Seven pts had next generation sequencing and 5 (83%) had genetic alterations detected with CDKN2A/B alterations being most frequent (3 pts, all with advanced disease). The median follow up was 4.3 years (range, 0.2-13.3). In the 4 patients with localized disease, 3 underwent complete surgical excision resulting in complete remissions (CRs). The one who received radiation had a partial remission (PR). The median time to progression after 1st line treatment was 4 months (all with advanced disease). All 3 pts who died had advanced disease and 2 had CDKN2A/B alterations. Conclusions: In FDCS, localized diseases managed by surgical excision with or without radiation can achieve long term remissions. Advanced diseases generally do not respond to multimodality therapy including immunotherapy but can have disease stabilization with variable survival. CDKN2A/B mutations were common and potentially associated with a worse outcome. Better understanding of the impact of the FDCS mutational landscape on prognosis may provide novel approaches to therapy. [Table: see text]