Outcomes after interruption of targeted therapy in patients with histiocytic neoplasms

Transcriptome Analysis of Canine Histiocytic Sarcoma Tumors and Cell Lines Reveals Multiple Targets for Therapy

Background: Histiocytic sarcoma (HS) is a highly aggressive malignancy characterized by the excessive proliferation of histiocytes in dogs and humans. A subset of dog breeds, including the Bernese Mountain Dog (BMD), show a remarkably high prevalence of HS. Previous work by us and others has identified somatic driver mutations of HS in the PTPN11 and KRAS genes that activate the MAPK pathway in about 60% of canine HS. However, no somatic driver mutations have been identified in the remaining 40%. Objectives: Our goals are to study HS in BMDs to gain insight into the molecular pathogenesis of the disease, and identify rational approaches to therapy. Methods: Here, we report our whole transcriptome analysis of 18 well-characterized BMD HS tumor tissues, as well as three HS cell lines. Results: Our analysis reveals the significant upregulation of molecular pathways involving the FOXM1, AURKB, PLK1, and E2F genes, in HS as well as hemophagocytic HS, providing new information regarding pathways that may be targeted with inhibitors. In addition, we document the expression of multiple checkpoint genes, suggesting the option of treatment with small-molecule inhibitors together with checkpoint inhibitors. Further, we show that the transcriptomes of three canine HS cell lines mirror those of canine patient tumors, further highlighting their potential use in drug discovery and efficacy studies. Finally, we demonstrate, for the first time, that aurora kinase inhibitors are effective in curtailing the growth of HS cells in vitro and show synergism with MAPK inhibition. Conclusions: This study provides the most detailed analysis of the canine HS transcriptome to date, highlighting key pathways in its pathogenesis and suggesting new avenues for both single and combination treatment strategies, which may be pertinent to the treatment of human HS.

Current State of Targeted Therapy in Adult Langerhans Cell Histiocytosis and Erdheim-Chester Disease

The mitogen-activated protein kinase (MAPK) pathway is a key driver in many histiocytic disorders, including Langerhans cell histiocytosis (LCH) and Erdheim-Chester disease (ECD). This has led to successful and promising treatment with targeted therapies, including BRAF inhibitors and MEK inhibitors. Additional novel inhibitors have also demonstrated encouraging results. Nevertheless, there are several problems concerning targeted therapy that need to be addressed. These include, among others, incomplete responsiveness and the emergence of resistance to BRAF inhibition as observed in other BRAF-mutant malignancies. Drug resistance and relapse after treatment interruption remain problems with current targeted therapies. Targeted therapy does not seem to eradicate the mutated clone, leading to inevitable relapes, which is a huge challenge for the future. More fundamental research and clinical trials are needed to address these issues and to develop improved targeted therapies that can overcome resistance and achieve long-lasting remissions.

Canine Histiocytic and Hemophagocytic Histiocytic Sarcomas Display KRAS and Extensive PTPN11/SHP2 Mutations and Respond In Vitro to MEK Inhibition by Cobimetinib

Histiocytic sarcoma (HS) is a rare and highly aggressive cancer in humans and dogs. In dogs, it has a high prevalence in certain breeds, such as Bernese mountain dogs (BMDs) and flat-coated retrievers. Hemophagocytic histiocytic sarcoma (HHS) is a unique form of HS that presents with erythrophagocytosis. Due to its rareness, the study of HHS is very limited, and mutations in canine HHS patients have not been studied to date. In previous work, our research group identified two major PTPN11/SHP2 driver mutations, E76K and G503V, in HS in dogs. Here, we report additional mutations located in exon 3 of PTPN11/SHP2 in both HS and HHS cases, further supporting that this area is a mutational hotspot in dogs and that mutations in tumors and liquid biopsies should be evaluated utilizing comprehensive methods such as Sanger and NextGen sequencing. The overall prevalence of PTPN11/SHP2 mutations was 55.8% in HS and 46.2% in HHS. In addition, we identified mutations in KRAS, in about 3% of HS and 4% of HHS cases. These findings point to the shared molecular pathology of activation of the MAPK pathway in HS and HHS cases. We evaluated the efficacy of the highly specific MEK inhibitor, cobimetinib, in canine HS and HHS cell lines. We found that the IC50 values ranged from 74 to 372 nM, which are within the achievable and tolerable ranges for cobimetinib. This finding positions cobimetinib as a promising potential candidate for future canine clinical trials and enhances our understanding of the molecular defects in these challenging cancers.

Advancements in the understanding and management of histiocytic neoplasms

Histiocytic neoplasms are rare diseases involving macrophages, dendritic cells, and monocytes. They include Langerhans cell histiocytosis (LCH), Erdheim-Chester disease (ECD), Rosai-Dorfman disease (RDD), juvenile xanthogranuloma (JXG), and histiocytic sarcoma. Histiocytic neoplasms are characterized by varied clinical courses and prognoses, necessitating a nuanced understanding of their classification, epidemiology, and clinical manifestations. Genetic studies have revealed somatic mutations, predominantly in the MAPK pathway, suggesting a clonal neoplastic nature. This review covers the current understanding of histiocytic neoplasms, molecular pathophysiology, with a particular focus on mutations in genes such as BRAF, MAP2K1, and the PI3K-AKT signaling pathways, and evolving treatment strategies, especially focusing on LCH, ECD, RDD, and JXG. The treatment landscape has evolved with advancements in targeted therapies. BRAF inhibitors, such as vemurafenib and dabrafenib, have shown efficacy, especially in high-risk LCH cases; however, challenges remain, including relapse post-treatment discontinuation, and adverse effects. MEK inhibitors have also demonstrated effectiveness, and cobimetinib has recently been approved for use in adults. Further research is required to determine the optimal treatment duration and strategies for managing therapy interruptions. Advancements in molecular genetics and targeted therapies have revolutionized the management of histiocytic neoplasms. However, ongoing research is crucial for optimizing patient outcomes.

Advances in our understanding of genetic markers and targeted therapies for pediatric LCH

INTRODUCTION

Langerhans cell histiocytosis (LCH) is a rare myeloid neoplasm, encompassing a diverse clinical spectrum ranging from localized bone or skin lesions to a multisystemic life-threatening condition. Over the past decade, there has been an expansion in understanding the molecular biology of LCH, which translated into innovative targeted therapeutic approaches.

AREAS COVERED

In this article, we will review the molecular alterations observed in pediatric LCH and the relationship between these molecular changes and the clinical phenotype, as well as targeted therapies in LCH.

EXPERT OPINION

Mitogen-activated protein kinase (MAPK) pathway mutation is a hallmark of LCH and is identified in 80% of the cases. Notably, BRAFV600E mutation is seen in ~50-60% of the cases, ~30% has other MAPK pathway mutations, while 15-20% have no detected mutations. While the first line therapeutic approach is vinblastine and prednisone, targeted therapies - specifically BRAF/MEK inhibitors - emerged as a promising second-line salvage strategy, particularly when a mutation is identified. Most patients respond to BRAF/MEK inhibitors but at least 75% reactivate after stopping, however, most patients respond again when restarting inhibitors.

First-line MAPK inhibition in pediatric histiocytosis: are we ready?

Not available.

Phase 2 study using low dose cytarabine for adult patients with newly diagnosed Langerhans cell histiocytosis

Langerhans cell histiocytosis (LCH) lacks a standardized first-line therapy. This single-center, phase 2 prospective study (NCT04121819) enrolled 61 newly diagnosed adult LCH patients with multisystem or multifocal single system disease from October 2019 to June 2022. Subcutaneous cytarabine (100 mg/m2 for 5 days) was administered in 35-day cycles for 12 total cycles. The primary endpoint was event-free survival (EFS). The median age was 33 years (range 18-66). Twelve patients (19.7%) had liver involvement, of which 2 also had spleen involvement. Among 43 patients undergoing next-generation sequencing, BRAF alterations (44.2%) were most frequent, followed by TP53 (16.3%), MAP2K1 (14.0%) and IDH2 (11.6%). MAPK pathway alterations occurred in 28 patients (65.1%). The overall response rate was 93.4%, with 20 (32.7%) achieving complete response and 37 (60.7%) partial response. After a median 30 months follow-up, 21 (34.4%) relapsed without deaths. Estimated 3-year OS and EFS were 100.0% and 58.5%, respectively. Multivariate analysis identified ≥3 involved organs (P = 0.007; HR 3.937, 95% CI: 1.456-9.804) and baseline lung involvement (P = 0.028; HR 2.976, 95% CI: 1.126-7.874) as poor prognostic factors for EFS. The most common grade 3-4 toxicities were neutropenia (27.9%), thrombocytopenia (1.6%), and nausea (1.6%). In conclusion, cytarabine monotherapy is an effective and safe regimen for newly diagnosed adults, while baseline lung or ≥3 involved organs confers poor prognosis.

Histiocytic neoplasms: Going, going, but not quite gone

BRAF and MEK inhibitors have revolutionised the treatment of patients with high-risk histiocytic neoplasms but does a complete response mean that treatment can be withdrawn? Commentary on: Reiner et al. Outcomes after interruption of targeted therapy in patients with histiocytic neoplasms. Br J Haematol 2023;203:389-394.

Erdheim-Chester Disease Due to a Novel Internal Duplication of NRAS: Response to Targeted Therapy with Cobimetinib

Histiocytoses encompass a group of exceptionally rare disorders characterized by the abnormal infiltration of tissues by histocytes. Among these, Erdheim-Chester disease (ECD) stands out as a multisystem histiocytosis that typically affects bones and various other tissues. Historically, the treatment of ECD has been challenging. However, recent breakthroughs in our understanding, particularly the discovery of somatic mutations in the RAS-MAPK pathway, have opened new opportunities for targeted therapy in a significant subset of patients with ECD and other histiocytoses. In this report, we present the case of a patient with ECD harboring a previously unidentified microduplication in the NRAS gene in a small fraction of skin cells. This discovery played a pivotal role in tailoring an effective therapeutic approach involving kinase inhibitors downstream of NRAS. This case underscores the crucial role of deep sequencing of tissue samples in ECD, enabling the delivery of personalized targeted therapy to patients.