PKC412 overcomes resistance to imatinib in a murine model of FIP1L1-PDGFRα-induced myeloproliferative disease

Diagnosis and treatment of hypereosinophilic syndromes

PURPOSE OF REVIEW

The aim of this article is to provide an update of causes of hypereosinophilia, including advances in knowledge of eosinophilic leukemia, and to outline an approach to investigation. We also aim to discuss in more detail the diagnosis and management of various hypereosinophilic syndromes including the clonal eosinophilias and those driven by abnormal cytokine-secreting T cells.

RECENT FINDINGS

Our understanding of the causative genetic abnormalities in eosinophilic leukemia is increasing, as is the repertoire of techniques available to detect them. New treatments on the horizon include further tyrosine kinase inhibitors for use in eosinophilic leukemia, which should provide an alternative to imatinib for those patients who develop resistance. These may also prove useful for other hypereosinophilic syndromes without PDGFRA or PDGFRB rearrangements. Other new therapies including anti-IL5 monoclonal antibodies are proving beneficial for some patients, especially those with abnormal T-cell populations.

SUMMARY

As our understanding of the various hypereosinophilic syndromes increases, and we are able to characterize many of the causative genetic lesions in the clonal eosinophilias, we are increasingly able to select appropriate therapy for an individual patient. New therapies based on this knowledge should serve to further improve the prognosis for many patients with hypereosinophilia.

PKC 412 small-molecule tyrosine kinase inhibitor

BACKGROUND

PKC412 is a kinase inhibitor that blocks protein kinase C (PKC), vascular endothelial growth factor receptors, platelet-derived growth factor receptor FLT3, and other class III receptor tyrosine kinases. The enthusiasm for this compound is based on its inhibitory effect even in the case of FLT3 mutations. The aim of this study was to analyze the role of FLT3 in pancreatic cancer and to study the biological activity of combined inhibition of neovascularization and mitogenesis in this disease.

METHODS

FLT3 expression was analyzed in 18 pancreatic cancer specimens by real-time quantitative polymerase chain reaction (RTQ-PCR) and immunohistochemistry. Sixteen pancreatic cancer cell lines were screened for ITD and D835 point mutations of the FLT3 gene. MTT assays and anchorage-independent growth assays were used to study cell growth. Flow cytometry was used for cell cycle analysis and apoptosis quantification. In vivo AsPC-1 and HPAF-II cells were used for orthotopic tumor modeling. Immunohistochemistry was used to quantify tumor angiogenesis.

RESULTS

FLT3 expression is down-regulated in pancreatic cancer. Activating FLT3 mutations (ITD, D835) were not detectable in any of the pancreatic cancer cell lines. Cell growth was significantly inhibited as cell-cycle progression was reduced and programmed cell death increased. In vivo PKC412 therapy resulted in a significant inhibition of orthotopic tumor growth with abrogation of tumor angiogenesis.

CONCLUSIONS

These data highlight that PKC412 may be a new compound in target therapy of inoperable pancreatic cancer patients and suggest a potential role for the combined use of broad spectrum kinase inhibitors in the management of these patients.

A critical appraisal of conventional and investigational drug therapy in patients with hypereosinophilic syndrome and clonal eosinophilia

Hypereosinophilic syndrome (HES) is a rare disorder characterized by persistent and marked eosinophilia, leading to end-organ damage. Over the last decade, great progress has been made in unraveling the molecular basis of HES that has resulted in the characterization of specific genetic alterations linked to clonal eosinophilia. The most frequently encountered genetic aberrancy is the cryptic FIP1-like 1/platelet-derived growth factor receptor alpha (FIP1L1-PDGFRA) fusion transcript, which results in an eosinophilic, myeloproliferative disorder. In addition, in a subset of patients with HES, a population of aberrant T cells that secretes interleukin-5 can be identified, indicating the existence of lymphocyte-mediated hypereosinophilia. These new insights have led to both a genetically based (re)classification of eosinophilic blood disorders and to effective therapies with targeted agents, such as small-molecule tyrosine kinase inhibitors (eg, imatinib, nilotinib, PKC412) and, more recently, monoclonal antibodies (eg, mepolizumab, alemtuzumab). These targeted therapies hold great promise for improving the clinical outcomes of patients with HES and clonal eosinophilia, and they have exhibited relatively safe toxicity profiles.

Effects of PKC412, nilotinib, and imatinib against GIST-associated PDGFRA mutants with differential imatinib sensitivity

BACKGROUND & AIMS

Activating mutations in platelet-derived growth factor receptor alpha (PDGFRA) have been reported in a subset of gastrointestinal stromal tumor (GIST) patients who do not express the mutant stem cell factor receptor c-kit. The responsiveness of mutant PDGFRA-positive GIST to imatinib depends on the location of the PDGFRA mutation; for example, the V561D juxtamembrane domain mutation is more sensitive to imatinib than the D842V kinase domain mutation. In this study, we compare the effects of 3 tyrosine kinase inhibitors, PKC412 and nilotinib, and imatinib, on 2 GIST-related PDGFRA mutants, V561D and D842V, which possess differential sensitivity to imatinib.

METHODS

The effects of PKC412, nilotinib, and imatinib, alone and in combination, were evaluated via in vitro proliferation studies performed with V561D- or D842V-PDGFRA mutants. The effects of nilotinib and PKC412, alone and combined, were investigated in vivo.

RESULTS

PKC412 potently inhibited the V561D-PDGFRA mutant in vitro and the D842V-PDGFRA mutant in vitro and in vivo. Both imatinib and nilotinib displayed potent activity in vitro against the V561D-PDGFRA mutant but were significantly less efficacious against D842V-PDGFRA. However, when combined with either imatinib or PKC412, nilotinib showed no evidence for antagonism and acted in a cooperative fashion against D842V-PDGFRA.

CONCLUSIONS

Our findings support the clinical testing of PKC412 for treatment of mutant PDGFRA-GIST. The data also support the use of nilotinib as a treatment option for V561D-PDGFRA-associated GIST, although the reduced sensitivity of D842V-PDGFRA probably limits the potential of nilotinib monotherapy for D842V-PDGFRA-associated GIST.

Epidermal growth factor receptor activation in glioblastoma through novel missense mutations in the extracellular domain

BACKGROUND

Protein tyrosine kinases are important regulators of cellular homeostasis with tightly controlled catalytic activity. Mutations in kinase-encoding genes can relieve the autoinhibitory constraints on kinase activity, can promote malignant transformation, and appear to be a major determinant of response to kinase inhibitor therapy. Missense mutations in the EGFR kinase domain, for example, have recently been identified in patients who showed clinical responses to EGFR kinase inhibitor therapy.

METHODS AND FINDINGS

Encouraged by the promising clinical activity of epidermal growth factor receptor (EGFR) kinase inhibitors in treating glioblastoma in humans, we have sequenced the complete EGFR coding sequence in glioma tumor samples and cell lines. We identified novel missense mutations in the extracellular domain of EGFR in 13.6% (18/132) of glioblastomas and 12.5% (1/8) of glioblastoma cell lines. These EGFR mutations were associated with increased EGFR gene dosage and conferred anchorage-independent growth and tumorigenicity to NIH-3T3 cells. Cells transformed by expression of these EGFR mutants were sensitive to small-molecule EGFR kinase inhibitors.

CONCLUSIONS

Our results suggest extracellular missense mutations as a novel mechanism for oncogenic EGFR activation and may help identify patients who can benefit from EGFR kinase inhibitors for treatment of glioblastoma.

Efficacy of the kinase inhibitor SU11248 against gastrointestinal stromal tumor mutants refractory to imatinib mesylate

PURPOSE

The majority of gastrointestinal stromal tumors harbor mutations in the receptor tyrosine kinases KIT or platelet-derived growth factor receptor A (PDGFRA), and respond to treatment with the tyrosine kinase inhibitor imatinib. Some tumors, however, show primary resistance to imatinib treatment, and most others become resistant during treatment. The most common mechanism of imatinib resistance involves specific mutations in the kinase domains of KIT or PDGFRA. We tested the activity of SU11248, an orally active small-molecule tyrosine kinase inhibitor, to inhibit important imatinib-resistant KIT and PDGFRA mutants.

EXPERIMENTAL DESIGN

Primary imatinib-resistant tumor cells and cell lines expressing clinically identified imatinib-resistant KIT-V654A, KIT-T670I, or PDGFRA-D842V mutant isoforms were evaluated for sensitivity to SU11248 by Western immunoblotting and proliferation assays. Three patients with the KIT-V654A mutation were treated with SU11248.

RESULTS

Based on ex vivo assays, SU11248 potently inhibits KIT kinase activity of V654A and T670I mutants and suppresses proliferation of the cells expressing these mutations. Sensitivity of KIT-V654A and KIT-T670I mutants to SU11248 was confirmed using cell lines expressing these mutants. In contrast, SU11248 did not potently inhibit the PDGFRA-D842V mutant. In agreement with these results, two of the three imatinib-resistant patients with the KIT-V654A mutation responded to SU11248 treatment.

CONCLUSIONS

These studies suggest that SU11248 may be a useful therapeutic agent to treat gastrointestinal stromal tumors harboring the imatinib-resistant KIT-V654A or KIT-T670I mutations, but it has no effect on the activity of the PDGFRA-D842V mutant. Specific kinase inhibitors should be designed to inhibit the constitutive activating PDGFRA mutation at codon 842.

Eosinophilic disorders: molecular pathogenesis, new classification, and modern therapy

Before the 1990s, lack of evidence for a reactive cause of hypereosinophilia or chronic eosinophilic leukemia (e.g. presence of a clonal cytogenetic abnormality or increased blood or bone marrow blasts) resulted in diagnosticians characterizing such nebulous cases as 'idiopathic hypereosinophilic syndrome (HES)'. However, over the last decade, significant advances in our understanding of the molecular pathophysiology of eosinophilic disorders have shifted an increasing proportion of cases from this idiopathic HES 'pool' to genetically defined eosinophilic diseases with recurrent molecular abnormalities. The majority of these genetic lesions result in constitutively activated fusion tyrosine kinases, the phenotypic consequence of which is an eosinophilia-associated myeloid disorder. Most notable among these is the recent discovery of the cryptic FIP1L1-PDGFRA gene fusion in karyotypically normal patients with systemic mast cell disease with eosinophilia or idiopathic HES, redefining these diseases as clonal eosinophilias. Rearrangements involving PDGFRA and PDGFRB in eosinophilic chronic myeloproliferative disorders, and of fibroblast growth factor receptor 1 (FGFR1) in the 8p11 stem cell myeloproliferative syndrome constitute additional examples of specific genetic alterations linked to clonal eosinophilia. The identification of populations of aberrant T-lymphocytes secreting eosinophilopoietic cytokines such as interleukin-5 establish a pathophysiologic basis for cases of lymphocyte-mediated hypereosinophilia. This recent revival in understanding the biologic basis of eosinophilic disorders has permitted more genetic specificity in the classification of these diseases, and has translated into successful therapeutic approaches with targeted agents such as imatinib mesylate and recombinant anti-IL-5 antibody.

Signal transduction therapy in haematological malignancies: identification and targeting of tyrosine kinases

Tyrosine kinases play key roles in cell proliferation, survival and differentiation. Their aberrant activation, caused either by the formation of fusion genes by chromosome translocation or by intragenic changes, such as point mutations or internal duplications, is of major importance in the development of many haematological malignancies. An understanding of the mechanisms by which BCR-ABL contributes to the pathogenesis of chronic myeloid leukaemia led to the development of imatinib, the first of several tyrosine kinase inhibitors to enter clinical trials. Although the development of resistance has been problematic, particularly in aggressive disease, the development of novel inhibitors and combination with other forms of therapy shows promise.

Classification of chronic myeloid disorders: From Dameshek towards a semi-molecular system

Hematological malignancies are phenotypically organized into lymphoid and myeloid disorders, although such a distinction might not be precise from the standpoint of lineage clonality. In turn, myeloid malignancies are broadly categorized into either acute myeloid leukemia (AML) or chronic myeloid disorder (CMD), depending on the presence or absence, respectively, of AML-defining cytomorphologic and cytogenetic features. The CMD are traditionally classified by their morphologic appearances into discrete clinicopathologic entities based primarily on subjective technologies. It has now become evident that most CMD represent clonal stem cell processes where the primary oncogenic event has been characterized in certain instances; Bcr/Abl in chronic myeloid leukemia, FIP1L1-PDGFRA or c-kit(D816V) in systemic mastocytosis, rearrangements of PDGFRB in chronic eosinophilic leukemia, and rearrangements of FGFR1 in stem cell leukemia/lymphoma syndrome. In addition, Bcr/Abl-negative classic myeloproliferative disorders are characterized by recurrent JAK2(V617F) mutations, whereas other mutations affecting the RAS signaling pathway molecules have been associated with juvenile myelomonocytic leukemia. Such progress is paving the way for a transition from a histologic to a semi-molecular classification system that preserves conventional terminology, while incorporating new information on molecular pathogenesis.

Pathogenesis, clinical features, and treatment advances in mastocytosis

Systemic mastocytosis (SM) is characterized by the abnormal growth and accumulation of mast cells (MC) in one or more organs. The interaction between the cytokine stem cell factor (SCF) and its cognate receptor, the c-kit receptor tyrosine kinase (KIT), plays a central role in regulating MC growth and differentiation. Whereas germline and somatically acquired activating mutations of KIT have been identified in SM, the issue as to whether individual KIT mutation(s) are necessary and sufficient to cause MC transformation remains unclear based on currently available data. Activating mutations of platelet-derived growth factor receptor-alpha (FIP1 L1-PDGFRA) are identified in a significant number of SM cases that have associated eosinophilia. To date, as with gastrointestinal stromal tumors, activating mutations of KIT and PDGFRA appear to be alternative and mutually exclusive genetic events in SM. The World Health Organization has specified criteria for classification of SM into six major subtypes: cutaneous mastocytosis, indolent systemic mastocytosis (ISM), systemic mastocytosis with an associated clonal hematological non-mast-cell disorder (SM-AHNMD), aggressive systemic mastocytosis (ASM), mast cell leukemia, and mast cell sarcoma. The ability to molecularly classify individual SM cases based on the presence or absence of specific mutations allows for molecularly targeted therapy in a growing number of cases. Imatinib mesylate therapy might result in complete remission of SM cases with wild-type KIT, certain KIT mutations, such as F522C, or the FIP1L1-PDGFRA fusion gene, but not of D816V-KIT-bearing SM. For the latter, interferon-alpha and 2-CdA are potential first- and second-line therapeutic options. Other drugs under investigation include novel tyrosine kinase inhibitors, as well as NF-kappaB inhibitors, which might display greater selectivity towards D816V-KIT as compared to wild type KIT. The pathogenesis of mastocytosis, its major clinical subtypes, and recent treatment advances are discussed in this chapter.

KIT Mutations in Mastocytosis and Their Potential as Therapeutic Targets

Deregulation of the KIT receptor TK by the prevalent activation loop mutation D816V has served as a focal point in therapeutic strategies aimed curbing neoplastic mast cell growth. Perhaps the most important development in this era of targeted therapy, and certainly relevant to KIT-driven diseases like mastocytosis, is the realization that small molecule inhibitors with varied chemical structure (eg, PKC412, dasatinib, AP23464) can circumvent the resistance of TKs to first-generation agents such as imatinib. Genuine opportunity now exists to effectively treat mastocytosis, and the arsenal consists of several orally bioavailable drugs with promising preclinical activity against D816V and other KIT mutants that promote mast cell growth. Because KIT mutations may not act as fully transforming oncogenic events in SM, it is prudent to evaluate combinations of TK inhibitors with drugs with activity in mast cell disease, such as cladribine, interferon-alpha, and corticosteroids. The identification of novel "drug-able" targets within mast cells should aid in the development of complementary therapies that promote enhanced cytotoxicity of mast cells through blockade of nonredundant signaling pathways. In addition, the generation of murine models that recapitulate human mastocytosis should accelerate preclinical testing of novel agents.

Dasatinib (BMS-354825) inhibits KITD816V, an imatinib-resistant activating mutation that triggers neoplastic growth in most patients with systemic mastocytosis

Mastocytosis is associated with an activating mutation in the KIT oncoprotein (KITD816V) that results in autophosphorylation of the KIT receptor in a ligand-independent manner. This mutation is inherently resistant to imatinib and, to date, there remains no effective curative therapy for systemic mastocytosis associated with KITD816V. Dasatinib (BMS-354825) is a novel orally bioavailable SRC/ABL inhibitor that has activity against multiple imatinib-resistant BCR-ABL isoforms in vitro that is presently showing considerable promise in early-phase clinical trials of chronic myeloid leukemia (CML). Pharmacokinetic analysis suggests that high nanomolar concentrations of dasatinib can be achieved safely in humans. In this study, we demonstrate significant inhibitory activity of dasatinib against both wild-type KIT and the KITD816V mutation in the nanomolar range in in vitro and cell-based kinase assays. Additionally, dasatinib leads to growth inhibition of a KITD816V-harboring human masto-cytosis cell line. Significantly, dasatinib selectively kills primary neoplastic bone marrow mast cells from patients with systemic mastocytosis while sparing other hematopoietic cells. Computer modeling suggests that the KITD816V mutation destabilizes the inactive conformation of the KIT activation loop to which imatinib binds, but it is not predicted to impair binding of KIT by dasatinib. Based upon our results, further evaluation of dasatinib for the treatment of systemic masto-cytosis in clinical trials is warranted. Moreover, dasatinib may be of clinical utility in other disease settings driven by activating KIT mutations. (Blood. 2006;108:286-291)

MPLW515L is a novel somatic activating mutation in myelofibrosis with myeloid metaplasia

BACKGROUND

The JAK2V617F allele has recently been identified in patients with polycythemia vera (PV), essential thrombocytosis (ET), and myelofibrosis with myeloid metaplasia (MF). Subsequent analysis has shown that constitutive activation of the JAK-STAT signal transduction pathway is an important pathogenetic event in these patients, and that enzymatic inhibition of JAK2V617F may be of therapeutic benefit in this context. However, a significant proportion of patients with ET or MF are JAK2V617F-negative. We hypothesized that activation of the JAK-STAT pathway might also occur as a consequence of activating mutations in certain hematopoietic-specific cytokine receptors, including the erythropoietin receptor (EPOR), the thrombopoietin receptor (MPL), or the granulocyte-colony stimulating factor receptor (GCSFR).

METHODS AND FINDINGS

DNA sequence analysis of the exons encoding the transmembrane and juxtamembrane domains of EPOR, MPL, and GCSFR, and comparison with germline DNA derived from buccal swabs, identified a somatic activating mutation in the transmembrane domain of MPL (W515L) in 9% (4/45) of JAKV617F-negative MF. Expression of MPLW515L in 32D, UT7, or Ba/F3 cells conferred cytokine-independent growth and thrombopoietin hypersensitivity, and resulted in constitutive phosphorylation of JAK2, STAT3, STAT5, AKT, and ERK. Furthermore, a small molecule JAK kinase inhibitor inhibited MPLW515L-mediated proliferation and JAK-STAT signaling in vitro. In a murine bone marrow transplant assay, expression of MPLW515L, but not wild-type MPL, resulted in a fully penetrant myeloproliferative disorder characterized by marked thrombocytosis (Plt count 1.9-4.0 x 10(12)/L), marked splenomegaly due to extramedullary hematopoiesis, and increased reticulin fibrosis.

CONCLUSIONS

Activation of JAK-STAT signaling via MPLW515L is an important pathogenetic event in patients with JAK2V617F-negative MF. The bone marrow transplant model of MPLW515L-mediated myeloproliferative disorders (MPD) exhibits certain features of human MF, including extramedullary hematopoiesis, splenomegaly, and megakaryocytic proliferation. Further analysis of positive and negative regulators of the JAK-STAT pathway is warranted in JAK2V617F-negative MPD.

Apoptosis Induction by Retinoids in Eosinophilic Leukemia Cells: Implication of Retinoic Acid Receptor-α Signaling in All-Trans-Retinoic Acid Hypersensitivity

Hypereosinophilic syndrome (HES) has recently been recognized as a clonal leukemic lesion, which is due to a specific oncogenic event that generates hyperactive platelet-derived growth factor receptor-alpha-derived tyrosine kinase fusion proteins. In the present work, the effect of retinoids on the leukemic hypereosinophilia-derived EoL-1 cell line and on primary HES-derived cells has been investigated. We show that all-trans-retinoic acid (ATRA) inhibits eosinophil colony formation of HES-derived bone marrow cells and is a powerful inducer of apoptosis of the EoL-1 cell line. Apoptosis was shown in the nanomolar concentration range by phosphatidylserine externalization, proapoptotic shift of the Bcl-2/Bak ratio, drop in mitochondrial membrane potential, activation of caspases, and cellular morphology. Unlike in other ATRA-sensitive myeloid leukemia models, apoptosis was rapid and was not preceded by terminal cell differentiation. Use of isoform-selective synthetic retinoids indicated that retinoic acid receptor-alpha-dependent signaling is sufficient to induce apoptosis of EoL-1 cells. Our work shows that the scope of ATRA-induced apoptosis of malignancies may be wider within the myeloid lineage than thought previously, that the EoL-1 cell line constitutes a new and unique model for the study of ATRA-induced cell death, and that ATRA may have potential for the management of clonal HES.

Activity of dual SRC-ABL inhibitors highlights the role of BCR/ABL kinase dynamics in drug resistance

Mutation in the ABL kinase domain is the principal mechanism of imatinib resistance in patients with chronic myelogenous leukemia. Many mutations favor active kinase conformations that preclude imatinib binding. Because the active forms of ABL and SRC resemble one another, we tested two dual SRC-ABL kinase inhibitors, AP23464 and PD166326, against 58 imatinib-resistant (IM(R)) BCR/ABL kinase variants. Both compounds potently inhibit most IM(R) variants, and in vitro drug selection demonstrates that active (AP23464) and open (PD166326) conformation-specific compounds are less susceptible to resistance than imatinib. Combinations of inhibitors suppressed essentially all resistance mutations, with the notable exception of T315I. Guided by mutagenesis studies and molecular modeling, we designed a series of AP23464 analogues to target T315I. The analogue AP23846 inhibited both native and T315I variants of BCR/ABL with submicromolar potency but showed nonspecific cellular toxicity. Our data illustrate how conformational dynamics of the ABL kinase accounts for the activity of dual SRC-ABL inhibitors against IM(R)-mutants and provides a rationale for combining conformation specific inhibitors to suppress resistance.

Activation of FIP1L1-PDGFRalpha requires disruption of the juxtamembrane domain of PDGFRalpha and is FIP1L1-independent

Genetic abnormalities that result in expression of chimeric tyrosine kinase proteins such as BCR-ABL1 and ETV6-PDGFRbeta are common causes of hematopoietic malignancies. The paradigm for constitutive activation of these fusion tyrosine kinases is enforced homodimerization by self-association domains present in the fusion partner proteins. The unique interstitial deletion on chromosome 4q12 that leads to expression of the FIP1L1-PDGFRalpha fusion tyrosine kinase was recently identified as a cause of chronic eosinophilic leukemia. In this report, we demonstrate that FIP1L1 is completely dispensable for PDGFRalpha activation in vitro and in vivo. Instead, truncation of PDGFRalpha between two conserved tryptophan residues in the juxtamembrane (JM) domain is required for kinase activation and transforming potential of FIP1L1-PDGFRalpha. The presence of a complete JM domain in FIP1L1-PDGFRalpha is inhibitory, but this autoinhibition can be overcome by enforced homodimerization. Similar effects of the JM domain in the context of PDGFRbeta were observed. These results suggest that disruption of the autoinhibitory JM domain is an alternative, dimerization-independent mechanism by which chimeric tyrosine kinases are constitutively activated and induce leukemogenesis.

Activity of AMN107, a novel aminopyrimidine tyrosine kinase inhibitor, against human FIP1L1-PDGFR-alpha-expressing cells

Idiopathic hypereosinophilic syndrome (HES) is a myeloproliferative disorder characterized by tissue involvement and organ dysfunction due to abnormal eosinophil proliferation. In a subset of patients, this is caused by the FIP1L1-PDGFR-alpha fusion tyrosine kinase. Cumulative evidence indicates that the Bcr-Abl tyrosine kinase inhibitor imatinib mesylate (Gleevec) is active for the treatment of patients with HES, particularly those expressing the FIP1L1-PDGFR-alpha oncoprotein. The novel tyrosine kinase inhibitor AMN107 was initially developed as a potent Bcr-Abl inhibitor based on the molecular structure of imatinib. We tested the in vitro efficacy of imatinib and AMN107 in the EOL-1 cell line and in cells from a patient with HES harboring the FIP1L1-PDGFR-alpha fusion kinase. AMN107 was as potent as imatinib in inducing apoptosis and inhibiting proliferation of EOL-1 cells, with IC(50) values of 0.54 and 0.20 nM, respectively. In addition, both drugs inhibited the phosphorylation of PDGFR-alpha tyrosine kinase with equivalent efficacy. We conclude that AMN107 and imatinib are active and equipotent against cells expressing the FIP1L1-PDGFR-alpha fusion gene.

Sorafenib is a potent inhibitor of FIP1L1-PDGFRalpha and the imatinib-resistant FIP1L1-PDGFRalpha T674I mutant

The FIP1L1-PDGFRA oncogene is a common cause of chronic eosinophilic leukemia (CEL), and encodes an activated tyrosine kinase that is inhibited by imatinib. FIP1L1-PDGFRA-positive patients with CEL respond to low-dose imatinib therapy, but resistance due to acquired T674I mutation has been observed. We report here the identification of sorafenib as a potent inhibitor of the FIP1 like 1-platelet-derived growth factor receptor alpha (FIP1L1-PDGFRalpha) (T674I) mutant. Sorafenib inhibited the proliferation of FIP1L1-PDGFRalpha and FIP1L1-PDGFRalpha(T674I)-transformed Ba/F3 cells and induced apoptosis of the EOL-1 cell line at a low nanomolar concentration. Western blot analysis confirmed that these effects were due to a direct effect on FIP1L1-PDGFRalpha and FIP1L1-PDGFRalpha(T674I). Sorafenib was recently approved for the treatment of renal cell carcinoma. Our data suggest that low doses of sorafenib could be efficient for the treatment of FIP1L1-PDGFRA-positive CEL and could be used to overcome resistance to imatinib associated with the T674I mutation.

Atypical myeloproliferative disorders: diagnosis and management

Myeloid disorders constitute a subgroup of hematological malignancies that is separate from lymphoid disorders. The World Health Organization system for classification of tumors of the hematopoietic system divides myeloid disorders into acute myeloid leukemia and chronic myeloid disorders based on the presence or absence, respectively, of acute myeloid leukemia--defining morphological and cytogenetic features including the presence of 20% or more myeloblasts in either the bone marrow or the peripheral blood. A recently proposed semimolecular classification system for chronic myeloid disorders recognizes 3 broad categories: the myelodysplastic syndrome, classic myeloproliferative disorders (MPD), and atypical MPD. Classic MPD includes polycythemia vera, essential thrombocythemia, myelofibrosis with myeloid metaplasia, and chronic myeloid leukemia. Both myelodysplastic syndrome and BCR/ABL-negative classic MPD were previously discussed as part of the current ongoing symposium on hematological malignancies. The current review focuses on the diagnosis and treatment of both molecularly defined and clinicopathologically assigned categories of atypical MPD: chronic myelomonocytic leukemia, juvenile myelomonocytic leukemia, chronic neutrophilic leukemia, chronic basophilic leukemia, chronic eosinophilic leukemia, idiopathic eosinophilia including hypereosinophilic syndrome, systemic mastocytosis, unclassified MPD, and eosinophilic/mast cell disorders associated with mutations of platelet-derived growth factor receptors alpha (PDGFRA) and beta (PDGFRB), FGFR1, and KIT.

Multilineage involvement of the fusion gene in patients with FIP1L1/PDGFRA-positive hypereosinophilic syndrome

Myeloproliferative hypereosinophilic syndrome (MHES) is a disorder characterised by male predominance, marked eosinophilia, splenomegaly, tissue fibrosis, elevated serum tryptase and the presence of the FIP1L1/PDGFRA fusion gene in peripheral blood mononuclear cells. The characteristic hypercellular bone marrow with dysplastic eosinophils and spindle-shaped mast cells suggest that multiple lineages may be involved in the clonal process. To determine which haematopoietic lineages are involved in MHES, we purified cells of specific lineages from patients with MHES and used nested reverse transcription polymerase chain reaction (RT-PCR), quantitative RT-PCR and fluorescence in situ hybridisation to analyse the purified cell populations for the presence of the fusion gene. The fusion gene was detected in eosinophils, neutrophils, mast cells, T cells, B cells and monocytes. These results suggest that the mutation arises in a pluripotential haematopoietic progenitor cell capable of giving rise to multiple lineages. The basis for the preferential expansion of eosinophils and mast cells remains unclear.

KIT-D816 mutations in AML1-ETO-positive AML are associated with impaired event-free and overall survival

Mutations in codon D816 of the KIT gene represent a recurrent genetic alteration in acute myeloid leukemia (AML). To clarify the biologic implication of activation loop mutations of the KIT gene, 1940 randomly selected AML patients were analyzed. In total, 33 (1.7%) of 1940 patients were positive for D816 mutations. Of these 33 patients, 8 (24.2%) had a t(8;21), which was significantly higher compared with the subgroup without D816 mutations. Analyses of genetic subgroups showed that KIT-D816 mutations were associated with t(8;21)/AML1-ETO and other rare AML1 translocations. In contrast, other activating mutations like FLT3 and NRAS mutations were very rarely detected in AML1-rearranged leukemia. KIT mutations had an independent negative impact on overall (median 304 vs 1836 days; P = .006) and event-free survival (median 244 vs 744 days; P = .003) in patients with t(8;21) but not in patients with a normal karyotype. The KIT-D816V receptor expressed in Ba/F3 cells was resistant to growth inhibition by the selective PTK inhibitors imatinib and SU5614 but fully sensitive to PKC412. Our findings clearly indicate that activating mutations of receptor tyrosine kinases are associated with distinct genetic subtypes in AML. The KIT-D816 mutations confer a poor prognosis to AML1-ETO-positive AML and should therefore be included in the diagnostic workup. Patients with KIT-D816-positive/AML1-ETO-positive AML might benefit from early intensification of treatment or combination of conventional chemotherapy with KIT PTK inhibitors.

The FIP1L1-PDGFRA fusion gene cooperates with IL-5 to induce murine hypereosinophilic syndrome (HES)/chronic eosinophilic leukemia (CEL)-like disease

Dysregulated tyrosine kinase activity by the Fip1-like1 (FIP1L1)-platelet-derived growth factor receptor alpha (PDGFRA) (F/P) fusion gene has been identified as a cause of clonal hypereosinophilic syndrome (HES), called F/P-positive chronic eosinophilic leukemia (CEL) in humans. However, transplantation of F/P-transduced hematopoietic stem cells/progenitors (F/P(+) HSCs/Ps) into mice results in a chronic myelogenous leukemia-like disease, which does not resemble HES. Because a subgroup of patients with HES show T-cell-dependent interleukin-5 (IL-5) overexpression, we determined if expression of the F/P fusion gene in the presence of transgenic T-cell IL-5 overexpression in mice induces HES-like disease. Mice that received a transplant of CD2-IL-5-transgenic F/P(+) HSC/Ps (IL-5Tg-F/P) developed intense leukocytosis, strikingly high eosinophilia, and eosinophilic infiltration of nonhematopoietic as well as hematopoietic tissues, a phenotype resembling human HES. The disease phenotype was transferable to secondary transplant recipients of a high cell dose, suggesting involvement of a short-term repopulating stem cell or an early myeloid progenitor. Induction of significant eosinophilia was specific for F/P since expression of another fusion oncogene, p210-BCR/ABL, in the presence of IL-5 overexpression was characterized by a significantly lower eosinophilia than IL-5Tg-F/P recipients. These results suggest that F/P is not sufficient to induce a HES/CEL-like disease but requires a second event associated with IL-5 overexpression.

Targeting mutated tyrosine kinases in the therapy of myeloid leukaemias

Myeloid leukaemias are frequently associated with translocations and mutations of tyrosine kinase genes. The products of these oncogenes, including BCR-ABL, TEL-PDGFR, Flt3 and c-Kit, have elevated tyrosine kinase activity and transform haematopoietic cells, mainly by augmentation of proliferation and enhanced viability. Activated ABL kinases are associated with chronic myeloid leukaemia. Mutations in platelet-derived growth factor receptor beta are associated with chronic myelomonocytic leukaemia. Flt3 or c-Kit cooperate with other types of oncogenes to create fully transformed acute leukaemias. Elevated activity of these tyrosine kinases is crucial for transformation, thus making the kinase domain an ideal target for therapeutic intervention. Tyrosine kinase inhibitors for various kinases are currently being evaluated in clinical trials and are potentially useful therapeutic agents in myeloid leukaemias. Here, the authors review the signalling activities, mechanism of transformation and therapeutic targeting of several tyrosine kinase oncogenes important in myeloid leukaemias.

FGFR3 as a therapeutic target of the small molecule inhibitor PKC412 in hematopoietic malignancies

Reccurent chromosomal translocation t(4;14) (p16.3;q32.3) occurs in patients with multiple myeloma (MM) and is associated with ectopic overexpression of fibroblast growth factor receptor 3 (FGFR3) that sometimes may contain the activation mutations such as K650E thanatophoric dysplasia type II (TDII). Although there have been significant advances in therapy for MM including the use of proteasome inhibitors, t(4;14) MM has a particularly poor prognosis and most patients still die from complications related to their disease or therapy. One potential therapeutic strategy is to inhibit FGFR3 in those myeloma patients that overexpress the receptor tyrosine kinase due to chromosomal translocation. Here we evaluated PKC412, a small molecule tyrosine kinase inhibitor, for treatment of FGFR3-induced hematopoietic malignancies. PKC412 inhibited kinase activation and proliferation of hematopoietic Ba/F3 cells transformed by FGFR3 TDII or a TEL-FGFR3 fusion. Similar results were obtained in PKC412 inhibition of several different t(4;14)-positive human MM cell lines. Furthermore, treatment with PKC412 resulted in a statistically significant prolongation of survival in murine bone marrow transplant models of FGFR3 TDII-induced pre-B cell lymphoma, or a peripheral T-cell lymphoma associated TEL-FGFR3 fusion-induced myeloproliferative disease. These data indicate that PKC412 may be a useful molecularly targeted therapy for MM associated with overexpression of FGFR3, and perhaps other diseases associated with dysregulation of FGFR3 or related mutants.

Molecularly targeted therapies in myelodysplastic syndromes and acute myeloid leukemias

Although there has been significant progress in acute myeloid leukemia (AML) treatment in younger adults during the last decade, standard induction therapy still fails to induce remission in up to 40% of AML patients. Additionally, relapses are common in 50-70% of patients who achieve a complete remission, and only 20-30% of patients enjoy long-term disease-free survival. The natural history of myelodysplastic syndrome (MDS) is variable, with about half of the patients dying from cytopenic complications, and an additional 20-30% transforming to AML. The advanced age of the majority of MDS patients limits the therapeutic strategies often to supportive care. To address these shortcomings, much effort has been directed toward the development of novel treatment strategies that target the evolution and proliferation of malignant clones. Presented here is an overview of molecularly targeted therapies currently being tested in AML and MDS patients, with a focus on FMS-like tyrosine kinase 3 inhibitors, farnesyltransferase inhibitors, antiangiogenesis agents, DNA hypomethylation agents, and histone deacetylase inhibitors.

FLT3 internal tandem duplication mutations induce myeloproliferative or lymphoid disease in a transgenic mouse model

Activating FMS-like tyrosine kinase 3 (FLT3) mutations have been identified in approximately 30% of patients with acute myelogenous leukemia (AML), and recently in a smaller subset of patients with acute lymphoblastic leukemia (ALL). To explore the in vivo consequences of an activating FLT3 internal tandem duplication mutation (FLT3-ITD), we created a transgenic mouse model in which FLT3-ITD was expressed under the control of the vav hematopoietic promoter. Five independent lines of vav-FLT3-ITD transgenic mice developed a myeloproliferative disease with high penetrance and a disease latency of 6-12 months. The phenotype was characterized by splenomegaly, megakaryocytic hyperplasia, and marked thrombocythemia, but without leukocytosis, polycythemia, or marrow fibrosis, displaying features reminiscent of the human disease essential thrombocythemia (ET). Clonal immature B- or T-lymphoid disease was observed in two additional founder mice, respectively, that could be secondarily transplanted to recipient mice that rapidly developed lymphoid disease. Treatment of these mice with the FLT3 tyrosine kinase inhibitor, PKC412, resulted in suppression of disease and a statistically significant prolongation of survival. These results demonstrate that FLT3-ITD is capable of inducing myeloproliferative as well as lymphoid disease, and indicate that small-molecule tyrosine kinase inhibitors may be an effective treatment for lymphoid malignancies in humans that are associated with activating mutations in FLT3.

Chronic myeloproliferative disorders: a tyrosine kinase tale

Chronic myeloproliferative diseases (CMPDs) are characterized by the abnormal proliferation and survival of one or more myeloid cell types. The archetype of this class of hematological diseases is chronic myeloid leukemia (CML), characterized by the presence of the Philadelphia (Ph) chromosome, the result of t(9;22)(q34;q11), and the associated BCR-ABL1 oncogene. Some of the Ph-negative myeloproliferative diseases are characterized by other chromosomal translocations involving a variety of tyrosine kinase genes, including ABL1, ABL2, PDGFRA, PDGFRB, FGFR1, and JAK2. The majority of Ph-negative CMPDs, however, such as chronic eosinophilic leukemia, polycythemia vera, essential thrombocythemia, and idiopathic myelofibrosis are not characterized by the presence of recurrent chromosomal abnormalities. Recent studies have identified the FIP1L1-PDGFRA fusion gene, generated due to a small cryptic deletion on chromosome 4q12, and the activating V617F mutation in JAK2 in a significant fraction of Ph-negative CMPDs. These results show that abnormalities in tyrosine kinase genes are central to the molecular pathogenesis of CMPDs. Genome-wide screenings to identify novel tyrosine kinase abnormalities in CMPDs may contribute to further improvement of the diagnosis and the treatment of these diseases.

Resistance to tyrosine kinase inhibitors: calling on extra forces

Over the past 5 years, small molecule tyrosine kinase inhibitors have been successfully introduced as new cancer therapeutics. The pioneering work with the ABL inhibitor imatinib (Glivec, Gleevec) was rapidly extended to other types of leukemias as well as solid tumors, which stimulated the development of a variety of new tyrosine kinase inhibitors. Unfortunately, oncogenic tyrosine kinases seem to have little problem to develop resistance to these inhibitors, and there is good evidence that this is not limited to imatinib, but also occurs with other inhibitors, such as FLT3 and EGFR inhibitors. Based on studies with imatinib, mutation and amplification of the target kinase seem to be the most important mechanisms for the development of resistance, but these mechanisms alone cannot explain all cases of resistance. A better understanding of the resistance mechanisms will be required to design improved treatment strategies in the future. In this review, we summarize the current insights in the different mechanisms of resistance to small molecule tyrosine kinase inhibitors, and discuss future improvements that might limit or even overcome resistance.

Activity of the tyrosine kinase inhibitor PKC412 in a patient with mast cell leukemia with the D816V KIT mutation

The majority of patients with systemic mast cell disease express the imatinib-resistant Asp816Val (D816V) mutation in the KIT receptor tyrosine kinase. Limited treatment options exist for aggressive systemic mastocytosis (ASM) and mast cell leukemia (MCL). We evaluated whether PKC412, a small-molecule inhibitor of KIT with a different chemical structure from imatinib, may have therapeutic use in advanced SM with the D816V KIT mutation. We treated a patient with MCL (with an associated myelodysplastic syndrome (MDS)/myeloproliferative disorder [MPD]) based on in vitro studies demonstrating that PKC412 could inhibit D816V KIT-transformed Ba/F3 cell growth with a 50% inhibitory concentration (IC50) of 30 nM to 40 nM. The patient exhibited a partial response with significant resolution of liver function abnormalities. In addition, PKC412 treatment resulted in a significant decline in the percentage of peripheral blood mast cells and serum histamine level and was associated with a decrease in KIT phosphorylation and D816V KIT mutation frequency. The patient died after 3 months of therapy due to progression of her MDS/MPD to acute myeloid leukemia (AML). This case indicates that KIT tyrosine kinase inhibition is a feasible approach in SM, but single-agent clinical efficacy may be limited by clonal evolution in the advanced leukemic phase of this disease.

The hypereosinophilic syndromes: still more heterogeneity

In 1968, the term hypereosinophilic syndromes (HES) was coined to refer to a spectrum of eosinophil-associated diseases presumed to be caused by an underlying immunological pathology. In the 1990s, the identification of an HES subset with T lymphocyte clonality and production of cytokines, particularly IL-5, validated this concept. Then, in 2002, imatinib mesylate, which was introduced for the treatment of chronic myelogenous leukemia, effectively controlled another subgroup of HES patients. Imatinib's target is a novel constitutively-active kinase. Most imatinib-responsive HES patients show an increased number of bone marrow mast cells and elevated serum tryptase; mast cells, lymphocytes and neutrophils express the novel kinase. This new information critically modifies our view of HES and indicates that several cell lines are altered and likely to contribute to HES pathophysiology.

Genetics of Myeloid Malignancies: Pathogenetic and Clinical Implications

Myeloid malignancies are clonal disorders that are characterized by acquired somatic mutation in hematopoietic progenitors. Recent advances in our understanding of the genetic basis of myeloid malignancies have provided important insights into the pathogenesis of acute myeloid leukemia (AML) and myeloproliferative diseases (MPD) and have led to the development of novel therapeutic approaches. In this review, we describe our current state of understanding of the genetic basis of AML and MPD, with a specific focus on pathogenetic and therapeutic significance. Specific examples discussed include RAS mutations, KIT mutations, FLT3 mutations, and core binding factor rearrangements in AML, and JAK2 mutations in polycythemia vera, essential thrombocytosis, and chronic idiopathic myelofibrosis.

Imatinib mesylate as a novel treatment option for hypereosinophilic syndrome: two case reports and a comprehensive review of the literature

Hypereosinophilic syndromes (HES) are a heterogenous group of rare disorders characterized by sustained and otherwise unexplained overproduction of eosinophils with organ involvement and consecutive dysfunction. Recent reports document the efficacy of imatinib mesylate in a large proportion of HES patients (65%). Rearrangements involving the platelet-derived growth factor receptor genes (PDGFRA and PDGFRB), both tyrosine kinase receptors, have been demonstrated to be pathogenetically linked to the dysregulated clonal overproduction of eosinophils. This refined hypothesis has been confirmed by the discovery of the novel FIP1L1-PDGFRA fusion gene, which is a gain-of-function gene on chromosome 4q12. Its product is an imatinib-sensitive tyrosine kinase, which can be found in a subset of patients with HES, particularly in those responding to treatment with imatinib mesylate. Here, we sum up recent knowledge of clinical features, pathophysiology and novel treatment aspects of HES by performing a comprehensive search of the available literature and report on 94 patients. We particularly address the issue of organ involvement and specific characteristics of the variable clinical pictures. In addition, two cases will be presented, which illustrate typical clinical scenarios and treatment outcome.

The small molecule tyrosine kinase inhibitor AMN107 inhibits TEL-PDGFRbeta and FIP1L1-PDGFRalpha in vitro and in vivo

AMN107 is a small molecule tyrosine kinase inhibitor developed, in the first instance, as a potent inhibitor of breakpoint cluster region-abelson (BCR-ABL). We tested its effectiveness against fusion tyrosine kinases TEL-platelet-derived growth factor receptorbeta (TEL-PDGFRbeta) and FIP1-like-1 (FIP1L1)-PDGFRalpha, which cause chronic myelomonocytic leukemia and hypereosinophilic syndrome, respectively. In vitro, AMN107 inhibited proliferation of Ba/F3 cells transformed by both TEL-PDGFRbeta and FIP1L1-PDGFRalpha with IC50 (inhibitory concentration 50%) values less than 25 nM and inhibited phosphorylation of the fusion kinases and their downstream signaling targets. The imatinib mesylate-resistant mutant TEL-PDGFRbeta T681I was sensitive to AMN107, whereas the analogous mutation in FIP1L1-PDGFRalpha, T674I, was resistant. In an in vivo bone marrow transplantation assay, AMN107 effectively treated myeloproliferative disease induced by TEL-PDGFRbeta and FIP1L1-PDGFRalpha, significantly increasing survival and disease latency and reducing disease severity as assessed by histopathology and flow cytometry. In summary, AMN107 can inhibit myeloid proliferation driven by TEL-PDGFRbeta and FIP1L1-PDGFRalpha and may be a useful drug for treatment of patients with myeloproliferative disease who harbor these kinase fusions.

Activation mutations of human c-KIT resistant to imatinib mesylate are sensitive to the tyrosine kinase inhibitor PKC412

Constitutively activated forms of the transmembrane receptor tyrosine kinase c-KIT have been associated with systemic mast cell disease, acute myeloid leukemia, and gastrointestinal stromal tumors. Reports of the resistance of the kinase domain mutation D816V to the adenosine triphosphate (ATP)-competitive kinase inhibitor imatinib mesylate prompted us to characterize 14 c-KIT mutations reported in association with human hematologic malignancies for transforming activity in the murine hematopoietic cell line Ba/F3 and for sensitivity to the tyrosine kinase inhibitor PKC412. Ten of 14 c-KIT mutations conferred interleukin 3 (IL-3)-independent growth. c-KIT D816Y and D816V transformed cells were sensitive to PKC412 despite resistance to imatinib mesylate. In these cells, PKC412, but not imatinib mesylate, inhibited autophosphorylation of c-KIT and activation of downstream effectors signal transducer and transcriptional activator 5 (Stat5) and Stat3. Variable sensitivities to PKC412 or imatinib mesylate were observed among other mutants. These findings suggest that PKC412 may be a useful therapeutic agent for c-KIT-positive malignancies harboring the imatinib mesylate-resistant D816V or D816Y activation mutations.

Strategies to overcome resistance to targeted protein kinase inhibitors

Selective inhibition of protein tyrosine kinases is gaining importance as an effective therapeutic approach for the treatment of a wide range of human cancers. However, as extensively documented for the BCR-ABL oncogene in imatinib-treated leukaemia patients, clinical resistance caused by mutations in the targeted oncogene has been observed. Here, we look at how structural and mechanistic insights from imatinib-insensitive Bcr-Abl have been exploited to identify second-generation drugs that override acquired target resistance. These insights have created a rationale for the development of either multi-targeted protein kinase inhibitors or cocktails of selective antagonists as antitumour drugs that combine increased therapeutic potency with a reduced risk of the emergence of molecular resistance.

Characterization of AMN107, a selective inhibitor of native and mutant Bcr-Abl

The Bcr-Abl tyrosine kinase oncogene causes chronic myelogenous leukemia (CML) and Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL). We describe a novel selective inhibitor of Bcr-Abl, AMN107 (IC50 <30 nM), which is significantly more potent than imatinib, and active against a number of imatinib-resistant Bcr-Abl mutants. Crystallographic analysis of Abl-AMN107 complexes provides a structural explanation for the differential activity of AMN107 and imatinib against imatinib-resistant Bcr-Abl. Consistent with its in vitro and pharmacokinetic profile, AMN107 prolonged survival of mice injected with Bcr-Abl-transformed hematopoietic cell lines or primary marrow cells, and prolonged survival in imatinib-resistant CML mouse models. AMN107 is a promising new inhibitor for the therapy of CML and Ph+ ALL.

Mechanisms of resistance to imatinib mesylate in gastrointestinal stromal tumors and activity of the PKC412 inhibitor against imatinib-resistant mutants

BACKGROUND AND AIMS

Resistance is a major challenge in the treatment of patients with gastrointestinal stromal tumors (GISTs). We investigated the mechanisms of resistance in patients with progressive GISTs with primary KIT mutations and the efficacy of the kinase inhibitor PKC412 for the inhibition of imatinib-resistant mutants.

METHODS

We performed a cytogenetic analysis and screened for mutations of the KIT and PDGFRA kinase domains in 26 resistant GISTs. KIT autophosphorylation status was assessed by Western immunoblotting. Imatinib-resistant GIST cells and Ba/F3 cells expressing these mutant proteins were tested for sensitivity to imatinib and PKC412.

RESULTS

Six distinct secondary mutations in KIT were detected in 12 progressive tumors, with V654A and T670I found to be recurrent. One progressive tumor showed acquired PDGFRA -D842V mutation. Amplification of KIT or KIT / PDGFRA was found in 2 patients. Eight of 10 progressive tumors available for analysis showed phosphorylated KIT. Two remaining progressive tumors lost KIT protein expression. GIST cells carrying KIT -del557-558/T670I or KIT -InsAY502-503/V654A mutations were resistant to imatinib, while PKC412 significantly inhibited autophosporylation of these mutants. Resistance to imatinib and sensitivity to PKC412 of KIT -T670I and PDGFRA -D842V mutants was confirmed using Ba/F3 cells.

CONCLUSIONS

This study shows the high frequency of KIT/PDGFRA kinase domain mutations in patients with secondary resistance and defines genomic amplification of KIT / PDGFRA as an alternative cause of resistance to the drug. In a subset of patients, cancer cells lost their dependence on the targeted tyrosine kinase. Our findings show the sensitivity of the imatinib-resistant KIT -T670I and KIT -V654A and of PDGFRA -D842V mutants to PKC412.

PDGF receptors-mediators of autocrine tumor growth and regulators of tumor vasculature and stroma

PDGFs and their cognate tyrosine kinase alpha- and beta-receptors are involved in multiple tumor-associated processes including autocrine growth stimulation of tumor cells, stimulation of tumor angiogenesis and recruitment and regulation of tumor fibroblasts. The recent development of clinically useful PDGF antagonists, like STI571/Glivec, has increased the interest in PDGF receptors as cancer drug targets. Autocrine PDGF receptor signaling occurs in certain malignancies characterized by mutational activation of PDGF or PDGF receptors, for instance, dermatofibrosaracoma protuberans, gastrointestinal stromal tumors, and hypereosinophilic syndrome. The roles of PDGF in regulation of tumor angiogenesis and tumor fibroblasts are more general, and probably occur in most common solid tumors. Concerning tumor angiogenesis recent studies have predominantly focused on the importance of PDGF receptor signaling for tumor pericyte recruitment. PDGF receptors in the tumor stroma have also attracted attention as interesting drug targets because of their function as regulators of tumor interstitial fluid pressure, tumor transvascular transport and tumor drug uptake. In summary, the improved understanding of the role of PDGF signaling in tumor biology, and the introduction of PDGF antagonists, has set the stage for a continued development of PDGF antagonists as novel cancer drugs.

Targeted treatment of hypereosinophilic syndromes and chronic eosinophilic leukemias with imatinib mesylate

Idiopathic hypereosinophilic syndrome (HES) and chronic eosinophilia leukemia (CEL) represent the most recent additions to the list of molecularly defined chronic myeloproliferative disorders. Beginning with the observation that imatinib mesylate (Gleevec) could elicit rapid and complete hematologic remissions in a proportion of patients with HES, a reverse bedside-to-bench translational research effort led to the discovery of FIP1L1-PDGFRA, a novel fusion gene on chromosome 4q12 whose product is an imatinib-sensitive protein tyrosine kinase. FIP1L1-PDGFRA is the first description of a gain-of-function fusion gene derived from an interstitial chromosomal deletion rather than a reciprocal translocation. Empiric use of imatinib in HES and CEL provides a dramatic example of how the development of targeted therapeutics can provide tremendous insight into the molecular etiology of what appear to be a diverse and otherwise indecipherable collection of diseases. In this review, we discuss the role of imatinib in HES/CEL and other malignancies characterized by constitutively activated tyrosine kinases, and examine molecular features of the FIP1L1-PDGFRA fusion.

Molecular classification and pathogenesis of eosinophilic disorders: 2005 update

Use of the term "idiopathic hypereosinophilic syndrome (HES)" has highlighted our basic lack of understanding of the molecular pathophysiology of eosinophilic disorders. However, over the last 10 years, the study of hypereosinophilia has enjoyed a revival. This interest has been rekindled by two factors: (1) the development of increasingly sophisticated molecular biology techniques that have unmasked recurrent genetic abnormalities linked to eosinophilia, and (2) the successful application of targeted therapy with agents such as imatinib to treat eosinophilic diseases. To date, most of these recurrent molecular abnormalities have resulted in constitutively activated fusion tyrosine kinases whose phenotypic consequence is an eosinophilia-associated myeloid disorder. Most notable among these are rearrangements of platelet-derived growth factor receptors alpha and beta (PDGFRalpha, PDGFRbeta), which define a small subset of patients with eosinophilic chronic myeloproliferative disorders (MPDs) and/or overlap myelodysplastic syndrome/MPD syndromes, including chronic myelomonocytic leukemia. Discovery of the cryptic FIP1L1-PDGFRA gene fusion in cytogenetically normal patients with systemic mast cell disease with eosinophilia or idiopathic HES has redefined these diseases as clonal eosinophilias. A growing list of fibroblast growth factor receptor 1 fusion partners has similarly emerged in the 8p11 myeloproliferative syndromes, which are often characterized by elevated eosinophil counts. Herein the focus is on the molecular gains made in these MPD-type eosinophilias, and the classification and clinicopathological issues related to hypereosinophilic syndromes, including the lymphocyte variant. Success in establishing the molecular basis of a group of once seemingly heterogeneous diseases has now the laid the foundation for establishing a semi-molecular classification scheme of eosinophilic disorders.

Prediction of resistance to small molecule FLT3 inhibitors: implications for molecularly targeted therapy of acute leukemia

Mutations in the receptor tyrosine kinase FLT3 occur frequently in patients with acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL). Small molecules that selectively inhibit FLT3 kinase activity induce apoptosis in blasts from AML patients with FLT3 mutations and prolong survival in animal models of FLT3-induced myeloproliferative disease. A spectrum of structurally different small molecules with activity against FLT3 have been described, and their efficacy for treatment of AML and ALL is now being investigated in clinical trials. Here, we describe the results of an in vitro screen designed to identify mutations in the ATP-binding pocket of FLT3 that confer resistance to tyrosine kinase inhibitors. Mutations at four different positions (Ala-627, Asn-676, Phe-691, and Gly-697) were identified that confer varying degrees of resistance to PKC412, SU5614, or K-252a. FLT3 proteins mutated at Ala-627, Asn-676, or Phe-691 remained sensitive to higher concentrations of the inhibitors, but the G697R mutation conferred high-level resistance to each of these inhibitors as well as to six additional experimental inhibitors. These data provide insights into potential mechanisms of acquired resistance of FLT3 to small molecule inhibitors and indicate that the G697R mutation may be a clinically problematic resistance mutation that warrants proactive screening for additional inhibitors.

PKC412 inhibits the zinc finger 198-fibroblast growth factor receptor 1 fusion tyrosine kinase and is active in treatment of stem cell myeloproliferative disorder

Human stem cell leukemia-lymphoma syndrome usually presents itself as a myeloproliferative disorder (MPD) that evolves to acute myeloid leukemia and/or lymphoma. The syndrome associated with t(8;13)(p11;q12) results in expression of the ZNF198-fibroblast growth factor receptor (FGFR) 1 fusion tyrosine kinase. Current empirically derived cytotoxic chemotherapy is inadequate for treatment of this disease. We hypothesized that small-molecule inhibitors of the ZNF198-FGFR1 fusion would have therapeutic efficacy. We characterized the transforming activity of ZNF198-FGFR1 in hematopoietic cells in vitro and in vivo. Expression of ZNF198-FGFR1 in primary murine hematopoietic cells caused a myeloproliferative syndrome in mice that recapitulated the human MPD phenotype. Transformation in these assays, and activation of the downstream effector molecules PLC-gamma, STAT5, and phosphatidylinositol 3-kinase/AKT, required the proline-rich domains, but not the ZNF domains, of ZNF198. A small-molecule tyrosine kinase inhibitor, PKC412 (N-benzoyl-staurosporine) effectively inhibited ZNF198-FGFR1 tyrosine kinase activity and activation of downstream effector pathways, and inhibited proliferation of ZNF198-FGFR1 transformed Ba/F3 cells. Furthermore, treatment with PKC412 resulted in statistically significant prolongation of survival in the murine model of ZNF198-FGFR1-induced MPD. Based in part on these data, PKC412 was administered to a patient with t(8;13)(p11;q12) and was efficacious in treatment of progressive myeloproliferative disorder with organomegaly. Therefore, PKC412 may be a useful therapy for treatment of human stem cell leukemia-lymphoma syndrome.

The biology of Kit in disease and the application of pharmacogenetics

C-kit encodes a transmembrane protein with intrinsic tyrosine kinase activity, which functions as the receptor for stem cell factor. It is expressed on a variety of cell types, including mast cells, hematopoietic progenitor cells, melanocytes, germ cells, and gastrointestinal pacemaker cells. Mutations resulting in alteration of Kit function are associated with diseases involving each of these cells. Recent development of tyrosine kinase inhibitors led to their evaluation as novel therapies for diseases associated with Kit activation. This review will discuss the pathobiology of Kit in human disease, with a particular emphasis on implications for potential targeted treatment strategies in mast cell disease.

Treatment of cockroach allergen asthma model with imatinib attenuates airway responses

In the present study it was determined whether a pharmacologic approach to blocking receptor tyrosine kinase-mediated activation during allergic airway responses could be beneficial. To examine these responses, allergic mice were given a single oral dose of imatinib at clinically relevant concentrations, ranging from 0.05 to 50 mg/kg, by oral gavages just before allergen challenge. The reduction in the allergen-induced responses was significant and centered on reducing overall inflammation as well as pulmonary cytokine levels. In particular, the treatment of the mice with imatinib significantly attenuated airway hyperreactivity and peribronchial eosinophil accumulation, and significantly reduced Th2 cytokines, interleukin-4 and interleukin-13. In addition, chemokines previously associated with allergen-induced pulmonary disease, CCL2, CCL5, and CCL6, were significantly reduced in the lungs of the imatinib-treated animals. Together these data demonstrate that the pharmacologic inhibitor imatinib may provide a clinically attractive therapy for allergic, asthmatic responses.

Relationship between idiopathic hypereosinophilic syndrome, eosinophilic leukemia, and systemic mastocytosis

Chronic eosinophilic leukemia is a neoplastic condition with persistent eosinophilia as the major hematological abnormality and with the eosinophils being part of the neoplastic clone. Some cases can be recognized by traditional hematological criteria, but many can be recognized only when a clonal cytogenetic or molecular genetic abnormality is demonstrated. A range of cytogenetic and molecular genetic abnormalities has been recognized, including both those seen in other myeloid malignancies (such as trisomy 8, monosomy 7, and 20q-) and those that are particularly linked to eosinophil differentiation (such as rearrangements of PDGFRB, FGFR1, and PDGFRA, the latter with formation of a FIP1L1-PDGFRA fusion gene). The discovery of the FIP1L1-PDGFRA fusion gene has led to the recognition that many patients who would previously have been regarded as having idiopathic hypereosinophilia actually have chronic eosinophilic leukemia. The same fusion gene has also been found in patients with hypereosinophilia and atypical bone marrow mast cells but whether this syndrome should be regarded as a variant of eosinophilic leukemia or as a variant of systemic mastocytosis remains to be established.

The FIP1L1-PDGFRalpha kinase in hypereosinophilic syndrome and chronic eosinophilic leukemia

PURPOSE OF REVIEW

The idiopathic hypereosinophilic syndrome is a rare hematologic disorder characterized by sustained unexplained eosinophilia with associated end-organ damage and by a striking male predominance. The first insights into the molecular etiology of this heterogeneous disease were obtained from a "bedside-to-bench" approach. Successful empiric treatment of patients with the hypereosinophilic syndrome with the selective tyrosine kinase inhibitor imatinib mesylate (Gleevec, Novartis) ultimately led to the discovery of the FIP1L1-PDGFRalpha fusion kinase in about half of the hypereosinophilic syndrome cases.

RECENT FINDINGS

The FIP1L1-PDGFRA fusion gene is generated by a cryptic interstitial chromosomal deletion, del(4)(q12q12), which indicates that these cases are clonal hematopoietic malignancies and should be reclassified as chronic eosinophilic leukemias based on current World Health Organization recommendations. In addition, the FIP1L1-PDGFRA fusion gene was also identified in cases with systemic mast cell disease. In vitro and in vivo studies confirmed that FIP1L1-PDGFRalpha is a therapeutic target of imatinib, forming a rational basis for the treatment of FIP1L1-PDGFRA positive chronic eosinophilic leukemia and mastocytosis with imatinib. Similar to BCR-ABL-positive leukemias, resistance to imatinib due to point mutations in the PDGFRalpha kinase domain may develop. We have explored strategies to circumvent resistance to imatinib using alternative tyrosine kinase inhibitors such as PKC412.

SUMMARY

The discovery of the FIP1L1-PDGFRA fusion gene in the hypereosinophilic syndrome is an example of the power of clinical translational research and identifies interstitial chromosomal deletion as a novel mechanism to generate oncogenic tyrosine kinase fusion genes.

Stable expression of small interfering RNA sensitizes TEL-PDGFbetaR to inhibition with imatinib or rapamycin

Small molecule inhibitors, such as imatinib, are effective therapies for tyrosine kinase fusions BCR-ABL-TEL-PDGFbetaR-mediated human leukemias, but resistance may develop. The unique fusion junctions of these molecules are attractive candidates for molecularly targeted therapeutic intervention using RNA interference (RNAi), which is mediated by small interfering RNA (siRNA). We developed a retroviral system for stable expression of siRNA directed to the unique fusion junction sequence of TEL-PDGFbetaR in transformed hematopoietic cells. Stable expression of the siRNA resulted in approximately 90% inhibition of TEL-PDGFbetaR expression and its downstream effectors, including PI3K and mammalian target of rapamycin (mTOR). Expression of TEL-PDGFbetaR-specific siRNA (TPsiRNA) significantly attenuated the proliferation of TEL-PDGFbetaR-transformed Ba/F3 cells or disease latency and penetrance in mice induced by intravenous injection of these Ba/F3 cells. Although a 90% reduction in TEL-PDGFbetaR expression was insufficient to induce cell death, stable siRNA expression sensitized transformed cells to the PDGFbetaR inhibitor imatinib or to the mTOR inhibitor rapamycin. TPsiRNA also inhibited an imatinib-resistant TEL-PDGFbetaR mutant, and the inhibition was enhanced by siRNA in combination with PKC412, another PDGFbetaR inhibitor. Although siRNA delivery in vivo is a challenging problem, stable expression of siRNA, which targets oncogenic fusion genes, may potentiate the effects of conventional therapy for hematologic malignancies.