The low frequency of clinical resistance to PDGFR inhibitors in myeloid neoplasms with abnormalities of PDGFRA might be related to the limited repertoire of possible PDGFRA kinase domain mutations in vitro

A newly identified 45-kDa JAK2 variant with an altered kinase domain structure represents a novel mode of JAK2 kinase inhibitor resistance

Tyrosine-protein kinase (janus kinase; JAK)-signal transducer and activator of transcription (STAT) signaling plays a pivotal role in the development of myeloproliferative neoplasms (MPNs). Treatment with the potent JAK1/JAK2-specific inhibitor, ruxolitinib, significantly reduces tumor burden; however, ruxolitinib treatment does not fully eradicate the malignant clone. As the molecular basis for the disease persistence is not well understood, we set out to gain new insights by generating ruxolitinib-resistant cell lines. Surprisingly, these cells harbor a 45 kDa JAK2 variant (FERM-JAK2) consisting of the N-terminal FERM domain directly fused to the C-terminal kinase domain in 80% of sublines resistant to ruxolitinib. At the molecular level, FERM-JAK2 is able to directly bind and activate STAT5 in the absence of cytokine receptors. Furthermore, phosphorylation of activation-loop tyrosines is dispensable for FERM-JAK2-mediated STAT5 activation and cellular transformation, in contrast to JAK2-V617F. As a result, FERM-JAK2 is highly resistant to several ATP-competitive JAK2 inhibitors, whereas it is particularly sensitive to HSP90 inhibition. A murine model of FERM-JAK2 leukemogenesis showed an accelerated MPN phenotype with pronounced splenomegaly. Notably, most current protocols for the monitoring of emerging JAK variants are unable to detect FERM-JAK2, highlighting the urgent need for implementing next-generation sequencing approaches in MPN patients receiving ruxolitinib.

Biologic therapies for hypereosinophilic disorders: From tyrosine kinase inhibitors to monoclonal antibodies. Towards an increasingly customized management?

Hypereosinophilic syndromes (HES) encompass a wide range of disorders characterized by persistent peripheral blood hypereosinophilia (HE) (i.e., an eosinophil count ≥1.5 × 10⁹/L and ≥ 10% eosinophils preferably with a minimal duration of 6 months if documentation is available) associated with organ damage and/or dysfunction attributable to tissue eosinophilic infiltrate and release of granule contents. In most cases, HE is associated with atopic conditions/allergies, parasitic infections, medications, autoimmune disorders and/or solid tumors in most cases. More rarely, it can be one of the dominant manifestations of an underlying myeloid/lymphoid neoplasm. With regard to hematological forms, in recent decades the advances in understanding the pathogenic aspects of HES have led to a growing interest in these diseases, and in the 2016 WHO classification multiple subgroups were defined according to the molecular profile with the aim of better characterizing these syndromes and establishing which patients will benefit from specific pharmacological targeted therapies. This review article will provide a comprehensive overview of possible therapeutic approaches for HES in the light of each specific molecular alteration, considering both tyrosine kinase inhibitors and monoclonal antibodies, either implemented in clinical practice or currently still under development.

Rational Drug Design Approach of Receptor Tyrosine Kinase Type III Inhibitors

Rational drug design is accomplished through the complementary use of structural biology and computational biology of biological macromolecules involved in disease pathology. Most of the known theoretical approaches for drug design are based on knowledge of the biological targets to which the drug binds. This approach can be used to design drug molecules that restore the balance of the signaling pathway by inhibiting or stimulating biological targets by molecular modeling procedures as well as by molecular dynamics simulations. Type III receptor tyrosine kinase affects most of the fundamental cellular processes including cell cycle, cell migration, cell metabolism, and survival, as well as cell proliferation and differentiation. Many inhibitors of successful rational drug design show that some computational techniques can be combined to achieve synergistic effects.

Eosinophilia in Hematologic Disorders

Eosinophilia in the peripheral blood can be the manifestation various medical conditions, including benign or malignant disorders. There are 3 main types of eosinophilia-associated myeloid neoplasms (MN-eos): myeloid and lymphoid neoplasms, chronic eosinophilic leukemia not otherwise specified, and idiopathic hypereosinophilic syndrome (HES). Imatinib mesylate has revolutionized the treatment of molecularly defined MN-eos, and novel agents have been successfully used to treat HES. The discovery of new, recurrent molecular alterations in patients with MN-eos may improve their diagnosis and therapy. This review focuses on the hematologist's approach to a patient with eosinophilia and treatment options for those with eosinophilic myeloid neoplasms.

How I treat hypereosinophilic syndromes

Hypereosinophilic syndromes (HESs) are a group of rare disorders characterized by peripheral blood eosinophilia of 1.5 × 10(9)/L or higher and evidence of end organ manifestations attributable to the eosinophilia and not otherwise explained in the clinical setting. HESs are pleomorphic in clinical presentation and can be idiopathic or associated with a variety of underlying conditions, including allergic, rheumatologic, infectious, and neoplastic disorders. Moreover, the etiology of the eosinophilia in HESs can be primary (myeloid), secondary (lymphocyte-driven), or unknown. Although corticosteroids remain the first-line therapy for most forms of HESs, the availability of an increasing number of novel therapeutic agents, including tyrosine kinase inhibitors and monoclonal antibodies, has necessarily altered the approach to treatment of HESs. This review presents an updated treatment-based approach to the classification of patients with presumed HES and discusses the roles of conventional and novel agents in the management of these patients.

The N550K/H mutations in FGFR2 confer differential resistance to PD173074, dovitinib, and ponatinib ATP-competitive inhibitors

We sought to identify fibroblast growth factor receptor 2 (FGFR2) kinase domain mutations that confer resistance to the pan-FGFR inhibitor, dovitinib, and explore the mechanism of action of the drug-resistant mutations. We cultured BaF3 cells overexpressing FGFR2 in high concentrations of dovitinib and identified 14 dovitinib-resistant mutations, including the N550K mutation observed in 25% of FGFR2(mutant) endometrial cancers (ECs). Structural and biochemical in vitro kinase analyses, together with BaF3 proliferation assays, showed that the resistance mutations elevate the intrinsic kinase activity of FGFR2. BaF3 lines were used to assess the ability of each mutation to confer cross-resistance to PD173074 and ponatinib. Unlike PD173074, ponatinib effectively inhibited all the dovitinib-resistant FGFR2 mutants except the V565I gatekeeper mutation, suggesting ponatinib but not dovitinib targets the active conformation of FGFR2 kinase. EC cell lines expressing wild-type FGFR2 were relatively resistant to all inhibitors, whereas EC cell lines expressing mutated FGFR2 showed differential sensitivity. Within the FGFR2(mutant) cell lines, three of seven showed marked resistance to PD173074 and relative resistance to dovitinib and ponatinib. This suggests that alternative mechanisms distinct from kinase domain mutations are responsible for intrinsic resistance in these three EC lines. Finally, overexpression of FGFR2(N550K) in JHUEM-2 cells (FGFR2(C383R)) conferred resistance (about five-fold) to PD173074, providing independent data that FGFR2(N550K) can be associated with drug resistance. Biochemical in vitro kinase analyses also show that ponatinib is more effective than dovitinib at inhibiting FGFR2(N550K). We propose that tumors harboring mutationally activated FGFRs should be treated with FGFR inhibitors that specifically bind the active kinase.

Identification of Ponatinib as a potent inhibitor of growth, migration, and activation of neoplastic eosinophils carrying FIP1L1-PDGFRA

In chronic eosinophilic leukemia, the transforming oncoprotein FIP1L1-PDGFRA is a major target of therapy. In most patients, the tyrosine kinase inhibitor (TKI) imatinib induces complete remission. For patients who are intolerant or resistant, novel TKIs have been proposed. We examined the in vitro effects of 14 kinase blockers on growth and function of EOL-1 cells, a FIP1L1-PDGFRA(+) eosinophil cell line. Major growth-inhibitory effects were seen with all PDGFR-blocking agents, with IC50 values in the low nanomolar range: ponatinib, 0.1-0.2 nmol/L; sorafenib, 0.1-0.2 nmol/L; masitinib, 0.2-0.5 nmol/L; nilotinib, 0.2-1.0 nmol/L; dasatinib, 0.5-2.0 nmol/L; sunitinib, 1-2 nmol/L; midostaurin, 5-10 nmol/L. These drugs were also found to block activation of PDGFR-downstream signaling molecules, including Akt, S6, and STAT5 in EOL-1 cells. All effective TKIs produced apoptosis in EOL-1 cells as determined by microscopy, Annexin-V/PI, and caspase-3 staining. In addition, PDGFR-targeting TKIs were found to inhibit cytokine-induced migration of EOL-1 cells. In all bioassays used, ponatinib was found to be the most potent compound in EOL-1 cells. In addition, ponatinib was found to downregulate expression of the activation-linked surface antigen CD63 on EOL-1 cells and to suppress the growth of primary neoplastic eosinophils. We also examined drug effects on Ba/F3 cells expressing two clinically relevant, imatinib-resistant, mutant forms of FIP1L1-PDGFRA, namely T674I and D842V. Strong inhibitory effects on both mutants were seen only with ponatinib. In summary, novel PDGFR-targeting TKIs may be alternative agents for the treatment of patients with imatinib-resistant chronic eosinophilic leukemia. Although several different PDGFR-targeting agents are effective, the most potent drug appears to be ponatinib.

Targeting the PDGF signaling pathway in tumor treatment

Platelet-derived growth factor (PDGF) isoforms and PDGF receptors have important functions in the regulation of growth and survival of certain cell types during embryonal development and e.g. tissue repair in the adult. Overactivity of PDGF receptor signaling, by overexpression or mutational events, may drive tumor cell growth. In addition, pericytes of the vasculature and fibroblasts and myofibroblasts of the stroma of solid tumors express PDGF receptors, and PDGF stimulation of such cells promotes tumorigenesis. Inhibition of PDGF receptor signaling has proven to useful for the treatment of patients with certain rare tumors. Whether treatment with PDGF/PDGF receptor antagonists will be beneficial for more common malignancies is the subject for ongoing studies.

The Differential Diagnosis of Eosinophilia in Neoplastic Hematopathology

Eosinophilia in the peripheral blood is classified as primary (clonal) hematologic neoplasms or secondary (nonclonal) disorders, associated with hematologic or nonhematologic disorders. This review focuses on the categories of hematolymphoid neoplasms recognized by the 2008 World Health Organization Classification of Tumours and Haematopoietic and Lymphoid Tissues that are characteristically associated with eosinophilia. We provide a systematic approach to the diagnosis of these neoplastic proliferations via morphologic, immunophenotypic, and molecular-based methodologies, and provide the clinical settings in which these hematolymphoid neoplasms occur. We discuss recommendations that eosinophilia working groups have published addressing some of the limitations of the current classification scheme.

Review of current classification, molecular alterations, and tyrosine kinase inhibitor therapies in myeloproliferative disorders with hypereosinophilia

Recent advances in our understanding of the molecular mechanisms underlying hypereosinophilia have led to the development of a ‘molecular’ classification of myeloproliferative disorders with eosinophilia. The revised 2008 World Health Organization classification of myeloid neoplasms included a new category called “myeloid and lymphoid neoplasms with eosinophilia and abnormalities of PDGFRA, PDGFRB or FGFR1.” Despite the molecular heterogeneity of PDGFR (platelet-derived growth factor receptor) rearrangements, tyrosine kinase inhibitors at low dose induce rapid and complete hematological remission in the majority of these patients. Other kinase inhibitors are promising. Further discoveries of new molecular alterations will direct the development of new specific inhibitors. In this review, an update of the classifications of myeloproliferative disorders associated with hypereosinophilia is discussed together with open and controversial questions. Molecular mechanisms and promising results of tyrosine kinase inhibitor treatments are reviewed.

Eosinophilic myeloid neoplasms

PURPOSE OF REVIEW

In 2012, idiopathic hypereosinophilic syndrome (HES) is still the prevalent diagnosis in patients with persistent eosinophilia, in which a primary or secondary cause of eosinophilia has not been identified. HES is considered a provisional diagnosis until a primary or secondary cause of hypereosinophilia is established. The discovery of imatinib-sensitive fusion proteins in a subset of patients with hypereosinophilia has changed the way we approach the diagnosis and treatment of eosinophilic myeloid neoplasms [eosinophilic myeloproliferative neoplasms (MPNs)]. Despite the recent diagnostic developments, diagnosis of hypereosinophilic MPN is only made in 10-20% of patients with persistent primary hypereosinophilia.

RECENT FINDINGS

In 2008 the World Health Organization (WHO) established a semi-molecular classification of hypereosinophilic MPNs. The discovery of PDGFRA, PDGFRB, FGFR1, JAK-2, and FLT3 fusion proteins in patients with eosinophilic MPNs provide opportunities for targeted therapy. Patients with hypereosinophilic MPNs associated with PDGFRA and PDGFRB fusion genes are responsive to imatinib.

SUMMARY

Ongoing research continues to expand our understanding of the pathophysiology of persistent primary hypereosinophilia and clarify the boundaries between some of these disorders. A key challenge is to identify new targets for therapy and limit the number of patients who are classified as having HES.

Induction of endoplasmic reticulum stress by sorafenib and activation of NF-κB by lestaurtinib as a novel resistance mechanism in Hodgkin lymphoma cell lines

Hodgkin-Reed/Sternberg (HRS) cells of classical Hodgkin lymphoma show aberrant expression and activation of several receptor tyrosine kinases (RTK) in the majority of cases. Therefore, we tested whether tyrosine kinase inhibitors (TKI) already in clinical use or late stages of clinical trials have antiproliferative effects on HRS cell lines and evaluated the targets, affected signaling pathways, and mechanisms of cell death and resistance. Sorafenib and lestaurtinib had antiproliferative effects on HRS cell lines at concentrations achievable in patients. Sorafenib inhibited platelet-derived growth factor receptor (PDGFR) α, TRKA and RON, caused decreases in total and phosphorylated amounts of several signaling molecules, and provoked caspase-3-independent cell death, most likely due to endoplasmic reticulum stress as indicated by upregulation of GADD34 and GADD153 and phosphorylation of PERK. Lestaurtinib inhibited TRKA, PDGFRα, RON, and JAK2 and had only a cytostatic effect. Besides deactivation, lestaurtinib also caused activation of signaling pathways. It caused increases in CD30L and TRAIL expression, and CD30L/CD30 signaling likely led to the observed concomitant activation of extracellular signal-regulated kinase 1/2 and the alternative NF-κB pathway. These data disclose the possible use of sorafenib for the treatment of Hodgkin lymphoma and highlight NF-κB activation as a potential novel mechanism of resistance toward TKIs.

Eosinophilic myeloid disorders

The discovery of therapeutically relevant mutations involving platelet-derived growth factor receptors alpha and beta (PDGFRA and PDGFRB) changed the way we evaluate and treat patients with clonal eosinophilia. Despite our improved understanding of the pathobiology of clonal eosinophilia, more than 50% of patients are diagnosed with idiopathic disease, 10% to 20% with a clonal myeloid disorder, and the remainder with a lymphocytic variant. The World Health Organization classification of tumors recognized the importance of a semi-molecular classification of eosinophilic myeloid disorders and divided them into two major subgroups: (1) myeloid and lymphoid neoplasms with eosinophilia and abnormalities of PDGFRA, PDGFRB, or fibroblast growth factor receptor 1 (FGFR1); and (2) chronic eosinophilic leukemia, not otherwise specified. A key challenge remains the identification of tyrosine kinase responsive molecular lesions in patients in whom the pathogenesis of clonal eosinophilia remains unclear.

Sorafenib inhibits many kinase mutations associated with drug-resistant gastrointestinal stromal tumors

Sorafenib has substantial clinical activity as third- or fourth-line treatment of imatinib- and sunitinib-resistant gastrointestinal stromal tumors (GIST). Because sorafenib targets both angiogenesis-related kinases (VEGFR) and the pathogenetic kinases found in GIST (KIT or PDGFRA), the molecular basis for sorafenib efficacy in this setting remains unknown. We sought to determine the spectrum of activity of sorafenib against different mutant kinases associated with drug-sensitive and drug-resistant GIST. We compared the activity of imatinib and sorafenib against transiently expressed mutant forms of KIT and PDGFRA, including various secondary mutations that have been identified in imatinib-resistant or sunitinib-resistant GISTs. We also examined these drugs against four GIST cell lines, three of which are imatinib resistant. In our in vitro studies, we determined that sorafenib inhibited imatinib-resistant mutations in exons encoding the ATP/drug-binding pocket and in exons encoding the activation loop, with the exception of substitutions at KIT codon D816 and PDGFRA codon 842. Notably our data indicate that sorafenib is more effective than imatinib or sunitinib for inhibiting the kinase activity of drug-resistant KIT mutants (as assessed by biochemical IC(50)). We hypothesize that a major determinant of the efficacy of sorafenib for treatment of advanced GIST is the activity of this agent against KIT or PDGFRA-mutant kinases. These results have implications for the further development of treatments for drug-resistant GIST.

Therapeutic approaches to patients with hypereosinophilic syndromes

Hypereosinophilic syndromes (HES) are a heterogeneous group of disorders that range from asymptomatic eosinophilia > 1,500/mL to aggressive disease complicated by life-threatening end organ involvement, including endomyocardial fibrosis and thromboembolism. To complicate matters further, similar clinical manifestations can occur in the setting of marked eosinophilia due to helminth infection, drug hypersensitivity, and other causes. In the past, therapy was guided only by the exclusion of these secondary causes of eosinophilia and the severity of the clinical manifestations. More recently, the availability of novel targeted therapies and a better understanding of the etiologies of some subtypes of HES have necessitated a more structured approach.

Roles of the Ras/Raf/MEK/ERK pathway in leukemia therapy. Leukemia. 2011;25:1080-1094. [PMID: 21494257 DOI: 10.1038/leu.2011.66]

The Ras/Raf/mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway is often implicated in sensitivity and resistance to leukemia therapy. Dysregulated signaling through the Ras/Raf/MEK/ERK pathway is often the result of genetic alterations in critical components in this pathway as well as mutations at upstream growth factor receptors. Unrestricted leukemia proliferation and decreased sensitivity to apoptotic-inducing agents and chemoresistance are typically associated with activation of pro-survival pathways. Mutations in this pathway and upstream signaling molecules can alter sensitivity to small molecule inhibitors targeting components of this cascade as well as to inhibitors targeting other key pathways (for example, phosphatidylinositol 3 kinase (PI3K)/phosphatase and tensin homologue deleted on chromosome 10 (PTEN)/Akt/mammalian target of rapamycin (mTOR)) activated in leukemia. Similarly, PI3K mutations can result in resistance to inhibitors targeting the Ras/Raf/MEK/ERK pathway, indicating important interaction points between the pathways (cross-talk). Furthermore, the Ras/Raf/MEK/ERK pathway can be activated by chemotherapeutic drugs commonly used in leukemia therapy. This review discusses the mechanisms by which abnormal expression of the Ras/Raf/MEK/ERK pathway can contribute to drug resistance as well as resistance to targeted leukemia therapy. Controlling the expression of this pathway could improve leukemia therapy and ameliorate human health.

Eosinophilic myeloproliferative disorders

Despite recent attempts to define and classify patients with marked eosinophilia and features consistent with myeloproliferative disease, areas of controversy remain. These are particularly apparent in situations in which multiple lineages are involved in a clonal process and clinical manifestations are overlapping. Although the introduction of new molecular diagnostics and targeted therapies has begun to clarify the boundaries between some of these disorders, several questions remain with respect to the classification of patients with myeloproliferative hypereosinophilic syndrome (HES) of unknown etiology.