EXEL-0862, a novel tyrosine kinase inhibitor, induces apoptosis in vitro and ex vivo in human mast cells expressing the KIT D816V mutation

KIT and mastocytosis

KIT is a receptor tyrosine kinase that is functionally relevant for hematopoiesis, mast cell development and function, gametogenesis and melanogenesis. Normal KIT signaling requires binding to stem cell factor, and PI3K-Akt is one of the putative effector pathways. In humans, germline loss-of-function KIT mutations have been associated with piebaldism - an autosomal dominant condition characterized by depigmented patches of skin and hair. Gain-of-function KIT mutations are usually acquired and have been associated with myeloid malignancies including core binding factor acute myeloid leukemia and systemic mastocytosis (SM), germ cell tumors, gastrointestinal stromal tumors and sinonasal T cell lymphomas. KITD816V is the most prevalent KIT mutation in mast cell disease and occurs in more than 90% of the cases that fulfill the World Health Organization diagnostic criteria for SM. However, its precise pathogenetic contribution is not well understood. In clinical practice, SM is considered either indolent or aggressive depending on the respective absence or presence of symptomatic target organ dysfunction aside from skin disease. In general, conventional therapy for SM is suboptimal, and efforts are under way to develop and employ small molecule drugs that target mutant KIT.

The JAK kinase inhibitor CP-690,550 suppresses the growth of human polycythemia vera cells carrying the JAK2V617F mutation

The somatic activating janus kinase 2 mutation (JAK2)(V617F) is detectable in most patients with polycythemia vera (PV). Here we report that CP-690,550 exerts greater antiproliferative and pro-apoptotic activity against cells harboring JAK2(V617F) compared with JAK2(WT). CP-690,550 treatment of murine factor-dependent cell Patersen-erythropoietin receptor (FDCP-EpoR) cells harboring human wild-type or V617F JAK2 resulted in inhibition of cell proliferation with a 50% inhibitory concentration (IC(50)) of 2.1 microM and 0.25 microM, respectively. Moreover, CP-690,550 induced a significant pro-apoptotic effect on murine FDCP-EpoR cells carrying JAK2(V617F), whereas a lesser effect was observed for cells carrying wild-type JAK2. This activity was coupled with inhibition of phosphorylation of the key JAK2(V617F)-dependent downstream signaling effectors signal transducer and activator of transcription (STAT)3, STAT5, and v-akt murine thymoma viral oncogene homolog (AKT). Furthermore, CP-690,550 treatment of ex-vivo-expanded erythroid progenitors from JAK2(V617F)-positive PV patients resulted in specific, antiproliferative (IC(50) = 0.2 microM) and pro-apoptotic activity. In contrast, expanded progenitors from healthy controls were less sensitive to CP-690,550 in proliferation (IC(50) > 1.0 microM), and apoptosis assays. The antiproliferative effect on expanded patient progenitors was paralleled by a decrease in JAK2(V617F) mutant allele frequency, particularly in a patient homozygous for JAK2(V617F). Flow cytometric analysis of expanded PV progenitor cells treated with CP-690,550 suggests a possible transition towards a pattern of erythroid differentiation resembling expanded cells from normal healthy controls.

Activity of tyrosine kinase inhibitors against human NUP214-ABL1-positive T cell malignancies

Amplification of the NUP214-ABL1 oncogene can be detected in patients with T cell acute lymphoblastic leukemia (T-ALL). We screened 29 patients with T cell malignancies for the expression of NUP214-ABL1 by reverse transcription-polymerase chain reaction (RT-PCR). NUP214-ABL1 was detected in three (10%) patients. These results were confirmed by fluorescence in situ hybridization techniques. We also studied the activity of imatinib, nilotinib and dasatinib against the human NUP214-ABL1-positive cell lines PEER and BE-13. All three tyrosine kinase inhibitors decreased the viability of PEER and BE-13 cells, but nilotinib and dasatinib had >1-log lower IC(50) values than imatinib (P<0.001). In contrast, the NUP214-ABL-negative T-ALL cell line Jurkat, was remarkably resistant to tyrosine kinase inhibition. The inhibition of cellular proliferation was associated with time-dependent induction of apoptosis and inhibition of ABL, CrKL and STAT5 phosphorylation. Moreover, dasatinib was active in a NUP214-ABL1-positive leukemia xenograft murine model and in marrow lymphoblasts from a patient with NUP214-ABL1-positive T-ALL. On the basis of these results, ABL1 kinase inhibitors warrant clinical investigation in patients with NUP214-ABL1-positive T-cell malignancies.

Pharmacological targeting of the KIT growth factor receptor: a therapeutic consideration for mast cell disorders

KIT is a member of the tyrosine kinase family of growth factor receptors which is expressed on a variety of haematopoietic cells including mast cells. Stem cell factor (SCF)-dependent activation of KIT is critical for mast cell homeostasis and function. However, when KIT is inappropriately activated, accumulation of mast cells in tissues results in mastocytosis. Such dysregulated KIT activation is a manifestation of specific activating point mutations within KIT, with the human D816V mutation considered as a hallmark of human systemic mastocytosis. A number of other activating mutations in KIT have recently been identified and these mutations may also contribute to aberrant mast cell growth. In addition to its role in mast cell growth, differentiation and survival, localized concentration gradients of SCF may control the targeting of mast cells to specific tissues and, once resident within these tissues, mast cell activation by antigen may also be amplified by SCF. Thus, KIT inhibitors may have potential application in multiple conditions linked to mast cells including systemic mastocytosis, anaphylaxis, and asthma. In this review, we discuss the role of KIT in the context of mast cells in these disease states and how recent advances in the development of inhibitors of KIT activity and function may offer novel therapies for the treatment of these disorders.

Concurrent inhibition of kit- and FcepsilonRI-mediated signaling: coordinated suppression of mast cell activation

Although primarily required for the growth, differentiation, and survival of mast cells, Kit ligand (stem cell factor) is also required for optimal antigen-mediated mast cell activation. Therefore, concurrent inhibition of Kit- and FcepsilonRI-mediated signaling would be an attractive approach for targeting mast cell-driven allergic reactions. To explore this concept, we examined the effects of hypothemycin, a molecule that we identified as having such properties, in human and mouse mast cells. Hypothemycin blocked Kit activation and Kit-mediated mast cell adhesion in a similar manner to the well characterized Kit inhibitor imatinib mesylate (imatinib). In contrast to imatinib, however, hypothemycin also effectively inhibited FcepsilonRI-mediated degranulation and cytokine production in addition to the potentiation of these responses via Kit. The effect of hypothemycin on Kit-mediated responses could be explained by its inhibition of Kit kinase activity, whereas the inhibitory effects on FcepsilonRI-dependent signaling were at the level of Btk activation. Because hypothemycin also significantly reduced the mouse passive cutaneous anaphylaxis response in vivo, these data provide proof of principle for a coordinated approach for the suppression of mast cell activation and provide a rationale for the development of compounds with a similar therapeutic profile.

Sorafenib inhibits imatinib-resistant KIT and platelet-derived growth factor receptor beta gatekeeper mutants

PURPOSE

Targeting of KIT and platelet-derived growth factor receptor (PDGFR) tyrosine kinases by imatinib is an effective anticancer strategy. However, mutations of the gatekeeper residue (T670 in KIT and T681 in PDGFRbeta) render the two kinases resistant to imatinib. The aim of this study was to evaluate whether sorafenib (BAY 43-9006), a multitargeted ATP-competitive inhibitor of KIT and PDGFR, was active against imatinib-resistant KIT and PDGFRbeta kinases.

EXPERIMENTAL DESIGN

We used in vitro kinase assays and immunoblot with phosphospecific antibodies to determine the activity of sorafenib on KIT and PDGFRbeta kinases. We also exploited reporter luciferase assays to measure the effects of sorafenib on KIT and PDGFRbeta downstream signaling events. The activity of sorafenib on interleukin-3-independent proliferation of Ba/F3 cells expressing oncogenic KIT or its imatinib-resistant T670I mutant was also tested.

RESULTS

Sorafenib efficiently inhibited gatekeeper mutants of KIT and PDGFRbeta (IC(50) for KIT T670I, 60 nmol/L; IC(50) for PDGFRbeta T681I, 110 nmol/L). Instead, it was less active against activation loop mutants of the two receptors (IC(50) for KIT D816V, 3.8 micromol/L; IC(50) for PDGFRbeta D850V, 1.17 micromol/L) that are also imatinib-resistant. Sorafenib blocked receptor autophosphorylation and signaling of KIT and PDGFRbeta gatekeeper mutants in intact cells as well as activation of AP1-responsive and cyclin D1 gene promoters, respectively. Finally, the compound inhibited KIT-dependent proliferation of Ba/F3 cells expressing the oncogenic KIT mutant carrying the T670I mutation.

CONCLUSIONS

Sorafenib might be a promising anticancer agent for patients carrying KIT and PDGFRbeta gatekeeper mutations.

Recent advances in the understanding of mastocytosis: the role of KIT mutations

Mastocytosis is a heterogeneous disorder characterised by the expansion and accumulation of mast cells in different organs and tissues. Mast cell physiology is closely dependent on activation of the stem cell factor/Kit signalling pathways and accumulating evidences confirm the physiopathological key role of activating KIT mutations (typically D816V) in mastocytosis and their relationship with the clinical manifestations of the disease. This paper reviews the most recent advances in the understanding of the molecular mechanisms associated with KIT mutations in mastocytosis, including recent data about the use of new therapies targeting the Kit molecule and its associated downstream signalling pathways.

The novel tyrosine kinase inhibitor EXEL-0862 induces apoptosis in human FIP1L1-PDGFR-α-expressing cells through caspase-3-mediated cleavage of Mcl-1

The FIP1-like-1 (FIP1L1)-platelet-derived growth factor receptor-alpha (FIP1L1-PDGFR-alpha) fusion kinase causes hypereosinophilic syndrome (HES) in a defined subset of patients. Imatinib mesylate is a potent inhibitor of ABL but also of PDGFR-alpha, and has been associated with durable hematologic responses in patients with HES. However, development of mutations in the tyrosine kinase domain may hamper the activity of tyrosine kinase inhibitors (TKIs), which suggests that novel agents are warranted to prevent or overcome resistance. We evaluated the efficacy of the novel TKI EXEL-0862 in FIP1L1-PDGFR-alpha-expressing cell lines and in cells from a patient with HES harboring the FIP1L1-PDGFR-alpha gene. EXEL-0862 inhibited the proliferation of EOL-1 and imatinib-resistant T674I FIP1L1-PDGFR-alpha-expressing cells and resulted in potent inhibition of the phosphorylation of PDGFR-alpha and downstream proteins STAT3 and Erk1/2, both in vitro and ex vivo. Moreover, EXEL-0862 induced apoptotic death in EOL-1 cells and imatinib-resistant T674I FIP1L1-PDGFR-alpha-expressing cells, and resulted in significant downregulation of the antiapoptotic protein Mcl-1 through a caspase-dependent mechanism. Our data establish EXEL-0862 as a solid candidate for the targeted treatment of patients with FIP1L1-PDGFR-alpha-positive HES.