Synergy between BRD9- and IKZF3-Targeting as a Therapeutic Strategy for Multiple Myeloma

Progress in the treatment of multiple myeloma (MM) has resulted in improvement in the survival rate. However, there is still a need for more efficacious and tolerated therapies. We and others have shown that bromodomain-containing protein 9 (BRD9), a member of the non-canonical SWI/SNF chromatin remodeling complex, plays a role in MM cell survival, and targeting BRD9 selectively blocks MM cell proliferation and synergizes with IMiDs. We found that synergy in vitro is associated with the downregulation of MYC and Ikaros proteins, including IKZF3, and overexpression of IKZF3 or MYC could partially reverse synergy. RNA-seq analysis revealed synergy to be associated with the suppression of pathways associated with MYC and E2F target genes and pathways, including cell cycle, cell division, and DNA replication. Stimulated pathways included cell adhesion and immune and inflammatory response. Importantly, combining IMiD treatment and BRD9 targeting, which leads to the downregulation of MYC protein and upregulation of CRBN protein, was able to override IMiD resistance of cells exposed to iberdomide in long-term culture. Taken together, our results support the notion that combination therapy based on agents targeting BRD9 and IKZF3, two established dependencies in MM, represents a promising novel therapeutic strategy for MM and IMiD-resistant disease.

Repurposing clinically available drugs and therapies for pathogenic targets to combat SARS-CoV-2

The coronavirus disease 2019 (COVID-19) pandemic has affected a large portion of the global population, both physically and mentally. Current evidence suggests that the rapidly evolving coronavirus subvariants risk rendering vaccines and antibodies ineffective due to their potential to evade existing immunity, with enhanced transmission activity and higher reinfection rates that could lead to new outbreaks across the globe. The goal of viral management is to disrupt the viral life cycle as well as to relieve severe symptoms such as lung damage, cytokine storm, and organ failure. In the fight against viruses, the combination of viral genome sequencing, elucidation of the structure of viral proteins, and identifying proteins that are highly conserved across multiple coronaviruses has revealed many potential molecular targets. In addition, the time- and cost-effective repurposing of preexisting antiviral drugs or approved/clinical drugs for these targets offers considerable clinical advantages for COVID-19 patients. This review provides a comprehensive overview of various identified pathogenic targets and pathways as well as corresponding repurposed approved/clinical drugs and their potential against COVID-19. These findings provide new insight into the discovery of novel therapeutic strategies that could be applied to the control of disease symptoms emanating from evolving SARS-CoV-2 variants.

Anti-SARS-CoV-2 activity of targeted kinase inhibitors: Repurposing clinically available drugs for COVID-19 therapy

Coronavirus disease 2019 (COVID-19) remains a major public health concern, and vaccine unavailability, hesitancy, or failure underscore the need for discovery of efficacious antiviral drug therapies. Numerous approved drugs target protein kinases associated with viral life cycle and symptoms of infection. Repurposing of kinase inhibitors is appealing as they have been vetted for safety and are more accessible for COVID-19 treatment. However, an understanding of drug mechanism is needed to improve our understanding of the factors involved in pathogenesis. We tested the in vitro activity of three kinase inhibitors against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), including inhibitors of AXL kinase, a host cell factor that contributes to successful SARS-CoV-2 infection. Using multiple cell-based assays and approaches, gilteritinib, nintedanib, and imatinib were thoroughly evaluated for activity against SARS-CoV-2 variants. Each drug exhibited antiviral activity, but with stark differences in potency, suggesting differences in host dependency for kinase targets. Importantly, for gilteritinib, the amount of compound needed to achieve 90% infection inhibition, at least in part involving blockade of spike protein-mediated viral entry and at concentrations not inducing phospholipidosis (PLD), approached a clinically achievable concentration. Knockout of AXL, a target of gilteritinib and nintedanib, impaired SARS-CoV-2 variant infectivity, supporting a role for AXL in SARS-CoV-2 infection and supporting further investigation of drug-mediated AXL inhibition as a COVID-19 treatment. This study supports further evaluation of AXL-targeting kinase inhibitors as potential antiviral agents and treatments for COVID-19. Additional mechanistic studies are needed to determine underlying differences in virus response.

BRD9 degraders as chemosensitizers in acute leukemia and multiple myeloma

Bromodomain-containing protein 9 (BRD9), an essential component of the SWI/SNF chromatin remodeling complex termed ncBAF, has been established as a therapeutic target in a subset of sarcomas and leukemias. Here, we used novel small molecule inhibitors and degraders along with RNA interference to assess the dependency on BRD9 in the context of diverse hematological malignancies, including acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL), and multiple myeloma (MM) model systems. Following depletion of BRD9 protein, AML cells undergo terminal differentiation, whereas apoptosis was more prominent in ALL and MM. RNA-seq analysis of acute leukemia and MM cells revealed both unique and common signaling pathways affected by BRD9 degradation, with common pathways including those associated with regulation of inflammation, cell adhesion, DNA repair and cell cycle progression. Degradation of BRD9 potentiated the effects of several chemotherapeutic agents and targeted therapies against AML, ALL, and MM. Our findings support further development of therapeutic targeting of BRD9, alone or combined with other agents, as a novel strategy for acute leukemias and MM.

Repurposing of Kinase Inhibitors for Treatment of COVID-19

The outbreak of COVID-19, the pandemic disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has spurred an intense search for treatments by the scientific community. In the absence of a vaccine, the goal is to target the viral life cycle and alleviate the lung-damaging symptoms of infection, which can be life-threatening. There are numerous protein kinases associated with these processes that can be inhibited by FDA-approved drugs, the repurposing of which presents an alluring option as they have been thoroughly vetted for safety and are more readily available for treatment of patients and testing in clinical trials. Here, we characterize more than 30 approved kinase inhibitors in terms of their antiviral potential, due to their measured potency against key kinases required for viral entry, metabolism, or reproduction. We also highlight inhibitors with potential to reverse pulmonary insufficiency because of their anti-inflammatory activity, cytokine suppression, or antifibrotic activity. Certain agents are projected to be dual-purpose drugs in terms of antiviral activity and alleviation of disease symptoms, however drug combination is also an option for inhibitors with optimal pharmacokinetic properties that allow safe and efficacious co-administration with other drugs, such as antiviral agents, IL-6 blocking agents, or other kinase inhibitors.

Current therapies under investigation for COVID-19: potential COVID-19 treatments

In response to the outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), researchers are expeditiously searching for antiviral treatments able to alleviate the symptoms of infection, which can be life-threatening. Here, we provide a general overview of what is currently known about the structure and characteristic features of SARS-CoV-2, some of which could potentially be exploited for the purposes of antiviral therapy and vaccine development. This minireview also covers selected and noteworthy antiviral agents/supportive therapy out of hundreds of drugs that are being repurposed or tested as potential treatments for COVID-19, the disease caused by SARS-CoV-2.

Correction: Evaluation of ERK as a therapeutic target in acute myelogenous leukemia

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Effects of the multi-kinase inhibitor midostaurin in combination with chemotherapy in models of acute myeloid leukaemia

Recently, several targeted agents have been developed for specific subsets of patients with acute myeloid leukaemia (AML), including midostaurin, the first FDA‐approved FLT3 inhibitor for newly diagnosed patients with FLT3 mutations. However, in the initial Phase I/II clinical trials, some patients without FLT3 mutations had transient responses to midostaurin, suggesting that this multi‐targeted kinase inhibitor might benefit AML patients more broadly. Here, we demonstrate submicromolar efficacy of midostaurin in vitro and efficacy in vivo against wild‐type (wt) FLT3‐expressing AML cell lines and primary cells, and we compare its effectiveness with that of other FLT3 inhibitors currently in clinical trials. Midostaurin was found to synergize with standard chemotherapeutic drugs and some targeted agents against AML cells without mutations in FLT3. The mechanism may involve, in part, the unique kinase profile of midostaurin that includes proteins implicated in AML transformation, such as SYK or KIT, or inhibition of ERK pathway or proviability signalling. Our findings support further investigation of midostaurin as a chemosensitizing agent in AML patients without FLT3 mutations.

The combination of FLT3 and SYK kinase inhibitors is toxic to leukaemia cells with CBL mutations

Mutations in the E3 ubiquitin ligase CBL, found in several myeloid neoplasms, lead to decreased ubiquitin ligase activity. In murine systems, these mutations are associated with cytokine-independent proliferation, thought to result from the activation of hematopoietic growth receptors, including FLT3 and KIT. Using cell lines and primary patient cells, we compared the activity of a panel of FLT3 inhibitors currently being used or tested in AML patients and also evaluated the effects of inhibition of the non-receptor tyrosine kinase, SYK. We show that FLT3 inhibitors ranging from promiscuous to highly targeted are potent inhibitors of growth of leukaemia cells expressing mutant CBL in vitro, and we demonstrate in vivo efficacy of midostaurin using mouse models of mutant CBL. Potentiation of effects of targeted FLT3 inhibition by SYK inhibition has been demonstrated in models of mutant FLT3-positive AML and AML characterized by hyperactivated SYK. Here, we show that targeted SYK inhibition similarly enhances the effects of midostaurin and other FLT3 inhibitors against mutant CBL-positive leukaemia. Taken together, our results support the notion that mutant CBL-expressing myeloid leukaemias are highly sensitive to available FLT3 inhibitors and that this effect can be significantly augmented by optimum inhibition of SYK kinase.

A non-covalent inhibitor XMU-MP-3 overrides ibrutinib-resistant BtkC481S mutation in B-cell malignancies

BACKGROUND AND PURPOSE

Bruton's tyrosine kinase (BTK) plays a key role in B-cell receptor signalling by regulating cell proliferation and survival in various B-cell malignancies. Covalent low-MW BTK kinase inhibitors have shown impressive clinical efficacy in B-cell malignancies. However, the mutant Btk^C481S poses a major challenge in the management of B-cell malignancies by disrupting the formation of the covalent bond between BTK and irreversible inhibitors, such as ibrutinib. The present studies were designed to develop novel BTK inhibitors targeting ibrutinib-resistant Btk^C481S mutation.

EXPERIMENTAL APPROACH

BTK-Ba/F3, BTK(C481S)-Ba/F3 cells, and human malignant B-cells JeKo-1, Ramos, and NALM-6 were used to evaluate cellular potency of BTK inhibitors. The in vitro pharmacological efficacy and compound selectivity were assayed via cell viability, colony formation, and BTK-mediated signalling. A tumour xenograft model with BTK-Ba/F3, Ramos and BTK(C481S)-Ba/F3 cells in Nu/nu BALB/c mice was used to assess in vivo efficacy of XMU-MP-3.

KEY RESULTS

XMU-MP-3 is one of a group of low MW compounds that are potent non-covalent BTK inhibitors. XMU-MP-3 inhibited both BTK and the acquired mutant BTKC481S, in vitro and in vivo. Further computational modelling, site-directed mutagenesis analysis, and structure-activity relationships studies indicated that XMU-MP-3 displayed a typical Type-II inhibitor binding mode.

CONCLUSION AND IMPLICATIONS

XMU-MP-3 directly targets the BTK signalling pathway in B-cell lymphoma. These findings establish XMU-MP-3 as a novel inhibitor of BTK, which could serve as both a tool compound and a lead for further drug development in BTK relevant B-cell malignancies, especially those with the acquired ibrutinib-resistant C481S mutation.

Comparison of effects of midostaurin, crenolanib, quizartinib, gilteritinib, sorafenib and BLU-285 on oncogenic mutants of KIT, CBL and FLT3 in haematological malignancies

Mutations in two type-3 receptor tyrosine kinases (RTKs), KIT and FLT3, are common in both acute myeloid leukaemia (AML) and systemic mastocytosis (SM) and lead to hyperactivation of key signalling pathways. A large number of tyrosine kinase inhibitors (TKIs) have been developed that target either FLT3 or KIT and significant clinical benefit has been demonstrated in multiple clinical trials. Given the structural similarity of FLT3 and KIT, it is not surprising that some of these TKIs inhibit both of these receptors. This is typified by midostaurin, which has been approved by the US Food and Drug Administration for mutant FLT3-positive AML and for KIT D816V-positive SM. Here, we compare the in vitro activities of the clinically available FLT3 and KIT inhibitors with those of midostaurin against a panel of cells expressing a variety of oncogenic FLT3 or KIT receptors, including wild-type (wt) FLT3, FLT3-internal tandem duplication (ITD), FLT3 D835Y, the resistance mutant FLT3-ITD+ F691L, KIT D816V, and KIT N822K. We also examined the effects of these inhibitors in vitro and in vivo on cells expressing mutations in c-CBL found in AML that result in hypersensitization of RTKs, such as FLT3 and KIT. The results show a wide spectrum of activity of these various mutations to these clinically available TKIs.

Spotlight on midostaurin in the treatment of FLT3-mutated acute myeloid leukemia and systemic mastocytosis: design, development, and potential place in therapy

The Fms-like tyrosine kinase-3 (FLT3; fetal liver kinase-2; human stem cell tyrosine kinase-1; CD135) is a class III receptor tyrosine kinase that is normally involved in regulating the proliferation, differentiation, and survival of both hematopoietic cells and dendritic cells. Mutations leading it to be constitutively activated make it an oncogenic driver in ~30% of acute myeloid leukemia (AML) patients where it is associated with poor prognosis. The prevalence of oncogenic FLT3 and the dependency on its constitutively activated kinase activity for leukemia growth make this protein an attractive target for therapeutic intervention. Of the numerous small molecule inhibitors under clinical investigation for the treatment of oncogenic FLT3-positive AML, the N-benzoyl-staurosporine, midostaurin (CGP41251; PKC412; Rydapt^®; Novartis Pharma AG, Basel, Switzerland), is the first to be approved by the US Food and Drug Administration for the treatment, in combination with standard chemotherapy, of newly diagnosed adult AML patients who harbor mutations in FLT3. Here, we describe the early design of midostaurin, the preclinical discovery of its activity against oncogenic FLT3, and its subsequent clinical development as a therapeutic agent for FLT3 mutant-positive AML.

Abstract PR04: Degradation of leukemia oncogenes: A novel approach to therapy of leukemia

AML is a heterogeneous group of malignancies with significant variability in phenotype and mechanisms of leukemogenesis. Despite our increased understanding of the genetic abnormalities that drive AML, very few targeted therapeutic agents have yet been developed. The prevailing approach to drugging oncoproteins is direct targeting of the mutant protein with a small molecule or antibody. Unfortunately, this strategy has not proven particularly fruitful in the case of AML as many mutant driver proteins are “undruggable” and the targetable enzymes (i.e., oncogenic FLT3) rapidly acquire resistance to inhibitors in the clinic. We are pursuing a novel and innovative approach that focuses on promoting degradation of AML oncoproteins through small-molecule inhibition of the cellular machinery that regulates turnover. Many oncoproteins are tagged for degradation by the proteasome or lysosome by post-translational addition of polyubiquitin chains. The process is, however, highly dynamic and reversible by deubiquitylating enzymes (DUBs), which cleave ubiquitin from substrate proteins. A subset of AML mutant drivers, including RAS, FLT3, and PML-RARα, have been found to undergo ubiquitin-mediated degradation; however, the DUB enzymes stabilizing them are unknown.

In order to identify novel targets and compounds that regulate protein homeostasis of leukemic oncoproteins, we employed whole-cell phenotypic screens of most reported small-molecule DUB inhibitors, annotated for inhibitory activity across a broad panel of DUBS, using oncogene-dependent and control cell lines followed by hit validation and target deconvolution. Using this approach, we identified inhibitors, and candidate DUBs, involved in regulation of protein levels of mutant FLT3, mutant JAK2, mutant KRAS, and PML-RARα;. Follow-up work that focused on FLT3-ITD identified USP10 as a DUB that stabilizes the oncoprotein via removal of a degradative ubiquitin tag. Furthermore, we show that pharmacologic inhibition of USP10 promotes degradation of FLT3-ITD but not wild-type (wt) FLT3, leads to selective killing of oncogenic FLT3-expressing AML cells in vitro and in vivo, and overrides resistance to FLT3 kinase inhibitors caused by tyrosine kinase domain (TKD) mutations and other mechanisms. Our studies hence validate USP10 as a therapeutic target for FLT3 mutant-positive AML. Building on this work, we executed a medicinal chemistry optimization effort that has yielded USP10 inhibitors with improved potency, and selectivity and mechanism studies have yielded insights into the origin of specificity for mutant versus wt FLT3. Furthermore, we have validated multiple DUB inhibitors that promote selective degradation of V617F JAK2 and intriguingly, identified compounds efficacious, in terms of inhibition of proliferation, across multiple genetic backgrounds. To the best of our knowledge, this is the first identification of a novel DUB substrate using a DUB-targeting small-molecule library screen and the first demonstration of stabilization of a mutant driver oncoprotein in AML by a DUB enzyme.

DUB inhibitors have the potential to eliminate oncoproteins, but it is early yet in the field. The first DUB inhibitor clinical trial is starting this year for multiple myeloma. Our work establishing the therapeutic potential of DUB inhibitors for oncogenes including PML-RARα; and constitutively activated FLT3, JAK2, and RAS, as well as potentially other oncoprotein-driven AML patient populations, will lay the groundwork for development of first-in-class drugs for AML.

This abstract is also being presented as Poster 10.

Citation Format: Sara Buhrlage, Ellen Weisberg, Ilaria Lamberto, Jing Yang, Nathan Schauer, Martin Sattler, Atushi Nonami, Amanda Christie, David Weinstock, Stella Ritorto, Virginia DeCesare, Matthias Trost, Richard Stone, Nathanael Gray, James Griffin. Degradation of leukemia oncogenes: A novel approach to therapy of leukemia [abstract]. In: Proceedings of the Second AACR Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; May 6-9, 2017; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(24_Suppl):Abstract nr PR04.

Discovery of a Highly Potent and Selective Indenoindolone Type 1 Pan-FLT3 Inhibitor

For a subpopulation of acute myeloid leukemia (AML) patients, the mutationally activated tyrosine kinase FLT3, has emerged as a promising target for therapy. The development of drug resistance due to mutation is a growing concern for mutant FLT3 inhibitors, such as PKC412, Quizartinib, PLX3397, and Crenolanib. Thus, there is a need to develop novel FLT3 inhibitors that overcome these mutations. Here we report the development of a novel type I ATP competitive inhibitor, JH-IX-179, that is extremely potent and selective for FLT3. JH-IX-179 also has the highest affinity for three constitutively active isoforms of FLT3 (FLT3-ITD, FLT3-N841I, and FLT3-D835V) compared to a panel 456 other kinases. The unique and specific kinase inhibition profile suggests that this chemotype may represent an attractive starting point for the development of further improved FLT3 inhibitors with therapeutic potential in tumors harboring deregulated FLT3 activity.

Identification of ILK as a novel therapeutic target for acute and chronic myeloid leukemia

Current treatment options as well as clinical efficacy are limited for chronic myelogenous leukemia (CML), Ph+ acute lymphoblastic leukemia (ALL), and acute myeloid leukemia (AML). In response to the pressing need for more efficacious treatment approaches and strategies to override drug resistance in advanced stage CML, Ph+ ALL, and AML, we investigated the effects of inhibition of ILK as a potentially novel and effective approach to treatment of these challenging malignancies. Using the small molecule ILK inhibitor, Cpd22, and ILK knockdown, we investigated the importance of ILK in the growth and viability of leukemia. Our results suggest that the ILK inhibition may be an effective treatment for CML, Ph+ ALL, and AML as a single therapy, with ILK expression levels positively correlating with the efficacy of ILK inhibition. The identification of ILK as a novel target for leukemia therapy warrants further investigation as a therapeutic approach that could be of potential clinical benefit in both acute and chronic myeloid leukemias.

Inhibition of Wild-Type p53-Expressing AML by the Novel Small Molecule HDM2 Inhibitor CGM097

The tumor suppressor p53 is a key regulator of apoptosis and functions upstream in the apoptotic cascade by both indirectly and directly regulating Bcl-2 family proteins. In cells expressing wild-type (WT) p53, the HDM2 protein binds to p53 and blocks its activity. Inhibition of HDM2:p53 interaction activates p53 and causes apoptosis or cell-cycle arrest. Here, we investigated the ability of the novel HDM2 inhibitor CGM097 to potently and selectively kill WT p53-expressing AML cells. The antileukemic effects of CGM097 were studied using cell-based proliferation assays (human AML cell lines, primary AML patient cells, and normal bone marrow samples), apoptosis, and cell-cycle assays, ELISA, immunoblotting, and an AML patient-derived in vivo mouse model. CGM097 potently and selectively inhibited the proliferation of human AML cell lines and the majority of primary AML cells expressing WT p53, but not mutant p53, in a target-specific manner. Several patient samples that harbored mutant p53 were comparatively unresponsive to CGM097. Synergy was observed when CGM097 was combined with FLT3 inhibition against oncogenic FLT3-expressing cells cultured both in the absence as well as the presence of cytoprotective stromal-secreted cytokines, as well as when combined with MEK inhibition in cells with activated MAPK signaling. Finally, CGM097 was effective in reducing leukemia burden in vivo. These data suggest that CGM097 is a promising treatment for AML characterized as harboring WT p53 as a single agent, as well as in combination with other therapies targeting oncogene-activated pathways that drive AML.

Abstract B01: Improve MEK/ERK targeting in mutant KRAS lung cancer

KRAS activating mutations are detected in 30% Non-Small Cell Lung Cancer (NSCLC) cases, and represent the subset of patients with the worst prognosis. Current efforts to develop treatments for this type of lung cancer are partially focused on inhibiting the Raf/MEK/ERK signaling cascade downstream of KRAS. Inhibition of the Raf/MEK/ERK pathway results in death of many mutant KRAS-driven lung cancer cell lines in vitro, and we have previously reported a murine active Kras lung cancer model that showed significant tumor regression upon treatment with the MEK inhibitor, selumetinib, when it was combined with other agents, such as PI3K/mTOR dual inhibitor BEZ235 or the cytotoxic agent docetaxel. The activity of the latter combination was validated in a recent phase II clinical trial in NSCLC patients harboring KRAS mutations. However, the responses were not durable in either human patients or mice, as resistance develop rapidly following chronic MEK inhibition. We have developed two approaches to reduce or delay the emergence of resistant clones. First, we have identified a number of novel treatment strategies that would directly improve the initial efficacy of MEK/ERK inhibition to dramatically enhance the initial response to MEK inhibition. Second, we have designed chemical screening strategies that identify compounds that act against active Kras lung cancer independently from the Raf/MEK/ERK pathway. We are using the first approach to delay the development of chronic resistance, while using the second approach to overcome resistance to MEK/ERK inhibition.

Citation Format: Zhao Chen, Ellen Weisberg, Katherine Cheng, Andrew L. Kung, James Bradner, James Griffin, Kwok-Kin Wong. Improve MEK/ERK targeting in mutant KRAS lung cancer. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr B01. doi: 10.1158/1557-3125.RASONC14-B01

Upregulation of IGF1R by mutant RAS in leukemia and potentiation of RAS signaling inhibitors by small-molecule inhibition of IGF1R

PURPOSE

Activating mutations in the RAS oncogene occur frequently in human leukemias. Direct targeting of RAS has proven to be challenging, although targeting of downstream RAS mediators, such as MEK, is currently being tested clinically. Given the complexity of RAS signaling, it is likely that combinations of targeted agents will be more effective than single agents.

EXPERIMENTAL DESIGN

A chemical screen using RAS-dependent leukemia cells was developed to identify compounds with unanticipated activity in the presence of an MEK inhibitor and led to identification of inhibitors of IGF1R. Results were validated using cell-based proliferation, apoptosis, cell-cycle, and gene knockdown assays; immunoprecipitation and immunoblotting; and a noninvasive in vivo bioluminescence model of acute myeloid leukemia (AML).

RESULTS

Mechanistically, IGF1R protein expression/activity was substantially increased in mutant RAS-expressing cells, and suppression of RAS led to decreases in IGF1R. Synergy between MEK and IGF1R inhibitors correlated with induction of apoptosis, inhibition of cell-cycle progression, and decreased phospho-S6 and phospho-4E-BP1. In vivo, NSG mice tail veins injected with OCI-AML3-luc+ cells showed significantly lower tumor burden following 1 week of daily oral administration of 50 mg/kg NVP-AEW541 (IGF1R inhibitor) combined with 25 mg/kg AZD6244 (MEK inhibitor), as compared with mice treated with either agent alone. Drug combination effects observed in cell-based assays were generalized to additional mutant RAS-positive neoplasms.

CONCLUSIONS

The finding that downstream inhibitors of RAS signaling and IGF1R inhibitors have synergistic activity warrants further clinical investigation of IGF1R and RAS signaling inhibition as a potential treatment strategy for RAS-driven malignancies.

Identification of Wee1 as a novel therapeutic target for mutant RAS-driven acute leukemia and other malignancies

Direct targeting of rat sarcoma (RAS), which is frequently mutated, has proven to be challenging, and inhibition of individual downstream RAS mediators has resulted in limited clinical efficacy. We designed a chemical screen to identify compounds capable of potentiating mammalian target of rapamycin (mTOR) inhibition in mutant RAS-positive leukemia, and identified a Wee1 inhibitor. Synergy was observed in both mutant neuroblastoma RAS viral oncogene homolog (NRAS)- and mutant kirsten RAS viral oncogene homolog (KRAS)-positive acute myelogenous leukemia (AML) cell lines and primary patient samples. The observed synergy enhanced dephosphorylation of AKT, 4E-binding protein 1 and s6 kinase, and correlated with increased apoptosis. The specificity of Wee1 as the target of MK-1775 was validated by Wee1 knockdown, as well as partial reversal of drug combination-induced apoptosis by a cyclin-dependent kinase 1 (CDK1) inhibitor. Importantly, we also extended our findings to other mutant RAS-expressing malignancies, including mutant NRAS-positive melanoma, and mutant KRAS-positive colorectal cancer, pancreatic cancer and lung cancer. We observed favorable responses with combined Wee1/mTOR inhibition in human cancer cell lines from multiple malignancies, and inhibition of tumor growth in in vivo models of mutant KRAS lung cancer and leukemia. The present study introduces for the first time Wee1 inhibition combined with mTOR inhibition as a novel therapeutic strategy for the selective treatment of mutant RAS-positive leukemia and other mutant RAS-expressing malignancies.

The gene dosage of class Ia PI3K dictates the development of PTEN hamartoma tumor syndrome

The PTEN hamartoma tumor syndrome (PHTS) is a complex disorder caused by germline inactivating mutations of the tumor suppressor gene PTEN. Loss of PTEN function leads to unimpeded phosphatidylinositol-3'-kinase (PI3K) activity and PI3K-driven cell division. Individuals with PHTS develop benign hamartomas in various tissues and have an increased risk of developing malignant diseases. Notably, no effective therapy currently exists for this disorder. Using both genetic mouse models and pharmacological approaches, we recently demonstrated that PI3K p110α and p110β isoforms play spatially distinct but concerted roles in the skin that are required for the development and maintenance of PHTS. We also show that treatment with a pan-PI3K inhibitor prevents the development of skin PHTS and reverses advanced-stage skin hamartomas in vivo. Here, we report that genetic ablation of only 3 out of 4 p110 alleles is sufficient to block the development of skin hamartomas resulting from the complete loss of Pten in mice. Similar to our findings in skin, we now also show that mammary gland neoplastic lesions can be prevented or reversed upon PI3K inhibition in our PHTS mouse model. Our data suggest a possible route to chemoprevention using reduced doses of PI3K inhibitors for PTEN-deficient carrier patients.

Discovery of a selective irreversible BMX inhibitor for prostate cancer

BMX is a member of the TEC family of nonreceptor tyrosine kinases. We have used structure-based drug design in conjunction with kinome profiling to develop a potent, selective, and irreversible BMX kinase inhibitor, BMX-IN-1, which covalently modifies Cys496. BMX-IN-1 inhibits the proliferation of Tel-BMX-transformed Ba/F3 cells at two digit nanomolar concentrations but requires single digit micromolar concentrations to inhibit the proliferation of prostate cancer cell lines. Using a combinatorial kinase inhibitor screening strategy, we discovered that the allosteric Akt inhibitor, MK2206, is able to potentiate BMX inhibitor's antiproliferation efficacy against prostate cancer cells.

The STAT5 Inhibitor Pimozide Displays Efficacy in Models of Acute Myelogenous Leukemia Driven by FLT3 Mutations

Activation of the transcription factor STAT5 is essential for the pathogenesis of acute myelogenous leukemia (AML) containing the FLT3 internal tandem duplication (ITD) mutation. FLT3 ITD is a constitutively active tyrosine kinase that drives the activation of STAT5, leading to the growth and survival of AML cells. Although there has been some success in identifying tyrosine kinase inhibitors that block the function of FLT3 ITD, there remains a continued need for effective treatment of this disease. We have identified the psychotropic drug pimozide as an effective inhibitor of STAT5 function. Pimozide inhibits the tyrosine phosphorylation of STAT5, leading to the death of AML cells through the induction of apoptosis. Pimozide shows a combinatorial effect with the tyrosine kinase inhibitors midostaurin (PKC412) and sunitinib in the inhibition of STAT5 tyrosine phosphorylation and the induction of apoptosis. Significantly, pimozide reduces the tumor burden in a mouse model of FLT3-driven AML. Therefore, identifying STAT5 inhibitors may provide a new avenue for the treatment of AML, and these may be effective alone or in combination with tyrosine kinase inhibitors.

Development of 'DFG-out' inhibitors of gatekeeper mutant kinases

HG-7-85-01(22) and HG-7-86-01(26) are thiazolo[5,4-b]pyridine containing type II tyrosine kinase inhibitors with potent cellular activity against both wild-type and 'gatekeeper' mutant T315I- Bcr-Abl. Here we report on the 'hybrid design' concept and subsequent structure activity guided optimization efforts that resulted in the development of these inhibitors.

An amino-indazole scaffold with spectrum selective kinase inhibition of FLT3, PDGFRα and kit

Here we describe the synthesis and characterization of a number of 3-amino-1H-indazol-6-yl-benzamides that were designed to target the 'DFG-out' conformation of the kinase activation loop. Several compounds such as 4 and 11 exhibit single-digit nanomolar EC(50)s against FLT3, c-Kit and the gatekeeper T674M mutant of PDGFRα.

Inhibition of CXCR4 in CML cells disrupts their interaction with the bone marrow microenvironment and sensitizes them to nilotinib

Drug resistance is a growing area of concern. It has been shown that a small, residual pool of leukemic CD34+ progenitor cells can survive in the marrow microenvironment of chronic myeloid leukemia (CML) patients after years of kinase inhibitor treatment. Bone marrow (BM) stroma has been implicated in the long-term survival of leukemic cells, and contributes to the expansion and proliferation of both transformed and normal hematopoietic cells. Mechanistically, we found that CML cells expressed CXCR4, and that plerixafor diminished BCR-ABL-positive cell migration and reduced adhesion of these cells to extra cellular-matrix components and to BM stromal cells in vitro. Moreover, plerixafor decreased the drug resistance of CML cells induced by co-culture with BM stromal cells in vitro. Using a functional mouse model of progressive and residual disease, we demonstrated the ability of the CXCR4 inhibitor, plerixafor, to mobilize leukemic cells in vivo, such that a plerixafor-nilotinib combination reduced the leukemia burden in mice significantly below the baseline level suppression exhibited by a moderate-to-high dose of nilotinib as single agent. These results support the idea of using CXCR4 inhibition in conjunction with targeted tyrosine kinase inhibition to override drug resistance in CML and suppress or eradicate residual disease.

Antileukemic Effects of Novel First- and Second-Generation FLT3 Inhibitors: Structure-Affinity Comparison

Constitutively activated mutant FLT3 has emerged as a promising target for therapy for the subpopulation of acute myeloid leukemia (AML) patients who harbor it. The small molecule inhibitor, PKC412, targets mutant FLT3 and is currently in late-stage clinical trials. However, the identification of PKC412-resistant leukemic blast cells in the bone marrow of AML patients has propelled the development of novel and structurally distinct FLT3 inhibitors that have the potential to override drug resistance and more efficiently prevent disease progression or recurrence. Here, we present the novel first-generation "type II" FLT3 inhibitors, AFG206, AFG210, and AHL196, and the second-generation "type II" derivatives and AST487 analogs, AUZ454 and ATH686. All agents potently and selectively target mutant FLT3 protein kinase activity and inhibit the proliferation of cells harboring FLT3 mutants via induction of apoptosis and cell cycle inhibition. Cross-resistance between "type I" inhibitors, PKC412 and AAE871, was demonstrated. While cross-resistance was also observed between "type I" and first-generation "type II" FLT3 inhibitors, the high potency of the second-generation "type II" inhibitors was sufficient to potently kill "type I" inhibitor-resistant mutant FLT3-expressing cells. The increased potency observed for the second-generation "type II" inhibitors was observed to be due to an improved interaction with the ATP pocket of FLT3, specifically associated with introduction of a piperazine moiety and placement of an amino group in position 2 of the pyrimidine ring. Thus, we present 2 structurally novel classes of FLT3 inhibitors characterized by high selectivity and potency toward mutant FLT3 as a molecular target. In addition, presentation of the antileukemic effects of "type II" inhibitors, such as AUZ454 and ATH686, highlights a new class of highly potent FLT3 inhibitors able to override drug resistance that less potent "type I" inhibitors and "type II" first-generation FLT3 inhibitors cannot.

Reversible resistance induced by FLT3 inhibition: a novel resistance mechanism in mutant FLT3-expressing cells

Objectives

Clinical responses achieved with FLT3 kinase inhibitors in acute myeloid leukemia (AML) are typically transient and partial. Thus, there is a need for identification of molecular mechanisms of clinical resistance to these drugs. In response, we characterized MOLM13 AML cell lines made resistant to two structurally-independent FLT3 inhibitors.

Methods

MOLM13 cells were made drug resistant via prolonged exposure to midostaurin and HG-7-85-01, respectively. Cell proliferation was determined by Trypan blue exclusion. Protein expression was assessed by immunoblotting, immunoprecipitation, and flow cytometry. Cycloheximide was used to determine protein half-life. RT-PCR was performed to determine FLT3 mRNA levels, and FISH analysis was performed to determine FLT3 gene expression.

Results and Conclusions

We found that MOLM13 cells readily developed cross-resistance when exposed to either midostaurin or HG-7-85-01. Resistance in both lines was associated with dramatically elevated levels of cell surface FLT3 and elevated levels of phosphor-MAPK, but not phospho-STAT5. The increase in FLT3-ITD expression was at least in part due to reduced turnover of the receptor, with prolonged half-life. Importantly, the drug-resistant phenotype could be rapidly reversed upon withdrawal of either inhibitor. Consistent with this phenotype, no significant evidence of FLT3 gene amplification, kinase domain mutations, or elevated levels of mRNA was observed, suggesting that protein turnover may be part of an auto-regulatory pathway initiated by FLT3 kinase activity. Interestingly, FLT3 inhibitor resistance also correlated with resistance to cytosine arabinoside. Over-expression of FLT3 protein in response to kinase inhibitors may be part of a novel mechanism that could contribute to clinical resistance.

The JAK2V617F oncogene requires expression of inducible phosphofructokinase/fructose-bisphosphatase 3 for cell growth and increased metabolic activity

Myeloproliferative neoplasms are characterized by overproduction of myeloid lineage cells with frequent acquisition of oncogenic JAK2V617F kinase mutations. The molecular mechanisms that regulate energy requirements in these diseases are poorly understood. Transformed cells tend to rely on fermentation instead of more efficient oxidative phosphorylation for energy production. Our data in JAK2V617F-transformed cells show that growth and metabolic activity were strictly dependent on the presence of glucose. Uptake of glucose and cell surface expression of the glucose transporter Glut1 required the oncogenic tyrosine kinase. Importantly, JAK2V617F as well as active STAT5 increased the expression of the inducible rate-limiting enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3), which controls glycolytic flux through 6-phosphofructo-1-kinase. PFKFB3 was required for JAK2V617F-dependent lactate production, oxidative metabolic activity and glucose uptake. Targeted knockdown of PFKFB3 also limited cell growth under normoxic and hypoxic conditions and blocked in vivo tumor formation in mice. Overall, these data suggest that inducible PFKFB3 is required for increased growth, metabolic activity and is regulated through active JAK2 and STAT5. Novel therapies that specifically block PFKFB3 activity or expression would, therefore, be expected to inhibit JAK2/STAT5-dependent malignancies and related cancers.

Smac mimetics: implications for enhancement of targeted therapies in leukemia

Drug resistance is a growing concern with clinical use of tyrosine kinase inhibitors. Utilizing in vitro models of intrinsic drug resistance and stromal-mediated chemoresistance, as well as functional mouse models of progressive and residual disease, we attempted to develop a potential therapeutic approach designed to suppress leukemia recurrence following treatment with selective kinase inhibitors. The novel IAP inhibitor, LCL161, [corrected] was observed to potentiate the effects of tyrosine kinase inhibition against leukemic disease both in the absence and presence of a stromal-protected [corrected] environment. LCL161 enhanced the proapoptotic effects of nilotinib and PKC412, against leukemic disease in vitro and potentiated the activity of both kinase inhibitors against leukemic disease in vivo. In addition, LCL161 synergized in vivo with nilotinib to reduce leukemia burden significantly below the baseline level suppression exhibited by a moderate-to-high dose of nilotinib. Finally, LCL161 displayed antiproliferative effects against cells characterized by intrinsic resistance to tyrosine kinase inhibitors as a result of expression of point mutations in the protein targets of drug inhibition. These results support the idea of using IAP inhibitors in conjunction with targeted tyrosine kinase inhibition to override drug resistance and suppress or eradicate residual disease.

Discovery and characterization of novel mutant FLT3 kinase inhibitors

For a subpopulation of acute myeloid leukemia (AML) patients, the constitutively activated tyrosine kinase, mutant FLT3, has emerged as a promising target for therapy. The development of drug resistance, however, is a growing concern for mutant FLT3 inhibitors, such as PKC412. Potential therapeutic benefit can arise from the combination of two structurally diverse inhibitors that target-but bind differently to-the same protein or from two inhibitors with completely different mechanisms of action. Thus, there is a need for identification and development of novel FLT3 inhibitors that have the ability to positively combine with PKC412 or standard chemotherapeutic agents used to treat AML as a way to suppress the development of drug resistance and consequently prolong disease remission. Here, we report the effects of the novel type II ATP-competitive inhibitors, HG-7-85-01 and HG-7-86-01, which potently and selectively target mutant FLT3 protein kinase activity and inhibit the proliferation of cells harboring FLT3-ITD or FLT3 kinase domain point mutants via induction of apoptosis and cell cycle inhibition. Antileukemic activity of HG-7-85-01 was shown in vivo to be comparable with that observed with PKC412 in a bioluminescence assay using NCr nude mice harboring Ba/F3-FLT3-ITD-luc+ cells. HG-7-85-01 was also observed to override PKC412 resistance. Finally, HG-7-85-01 and HG-7-86-01 synergized with PKC412 and standard chemotherapeutic agents against mutant PKC412-sensitive and some PKC412-resistant, FLT3-positive cells. Thus, we present a structurally novel class of FLT3 inhibitors that warrants consideration for clinical testing against drug-resistant disease in AML patients.

Combining the FLT3 inhibitor PKC412 and the triterpenoid CDDO-Me synergistically induces apoptosis in acute myeloid leukemia with the internal tandem duplication mutation

Mutations of the FLT3 receptor tyrosine kinase consisting of internal tandem duplications (ITD) have been detected in blasts from 20% to 30% of patients with acute myeloid leukemia (AML) and are associated with a poor prognosis. FLT3/ITD results in constitutive autophosphorylation of the receptor and factor-independent survival in leukemia cell lines. The C-28 methyl ester of the oleane triterpenoid (CDDO-Me) is a multifunctional molecule that induces apoptosis of human myeloid leukemia cells. Here, we report that CDDO-Me blocks targeting of NFkappaB to the nucleus by inhibiting IkappaB kinase beta-mediated phosphorylation of IkappaBalpha. Moreover, CDDO-Me blocked constitutive activation of the signal transducer and activator of transcription 3. We report the potent and selective antiproliferative effects of CDDO-Me on FLT3/ITD-positive myeloid leukemia cell lines and primary AML cells. The present studies show that CDDO-Me treatment results in caspase-3-mediated induction of apoptosis of FLT3/ITD-expressing cells and its antiproliferative effects are synergistic with PKC412, a FLT3-tyrosine kinase inhibitor currently in clinical trials. Taken together, our studies indicate that CDDO-Me greatly enhanced the efficacy of the FLT3 inhibitor PKC412, suggesting that combining two separate pathway inhibitors might be a viable therapeutic strategy for AML associated with a FLT3/ITD mutation.

Tumor cell-specific bioluminescence platform to identify stroma-induced changes to anticancer drug activity

Conventional anticancer drug screening is typically performed in the absence of accessory cells of the tumor microenvironment, which can profoundly alter antitumor drug activity. To address this limitation, we developed the tumor cell-specific in vitro bioluminescence imaging (CS-BLI) assay. Tumor cells (for example, myeloma, leukemia and solid tumors) stably expressing luciferase are cultured with nonmalignant accessory cells (for example, stromal cells) for selective quantification of tumor cell viability, in presence versus absence of stromal cells or drug treatment. CS-BLI is high-throughput scalable and identifies stroma-induced chemoresistance in diverse malignancies, including imatinib resistance in leukemic cells. A stroma-induced signature in tumor cells correlates with adverse clinical prognosis and includes signatures for activated Akt, Ras, NF-kappaB, HIF-1alpha, myc, hTERT and IRF4; for biological aggressiveness; and for self-renewal. Unlike conventional screening, CS-BLI can also identify agents with increased activity against tumor cells interacting with stroma. One such compound, reversine, shows more potent activity in an orthotopic model of diffuse myeloma bone lesions than in conventional subcutaneous xenografts. Use of CS-BLI, therefore, enables refined screening of candidate anticancer agents to enrich preclinical pipelines with potential therapeutics that overcome stroma-mediated drug resistance and can act in a synthetic lethal manner in the context of tumor-stroma interactions.

Endometrial and vaginal effects of low-dose estradiol delivered by vaginal ring or vaginal tablet

AIMS

The major aims of the study were to compare the safety of a continuous low-dose estradiol-releasing vaginal ring (ESTring) to that of a vaginal estradiol tablet (Vagifem) on the endometrium and the relief of subjective symptoms and signs of urogenital estrogen deficiency. Quality of life and acceptability of treatment delivery were also assessed.

STUDY DESIGN

A prospective, randomized study in which women were assigned in a 2:1 ratio to ESTring and Vagifem and followed for 12 months. The primary endpoint was endometrial safety, based on the results of ultrasound measurement of endometrial thickness and a progestogen challenge test at baseline and week 48. Efficacy was determined by subjective assessment of urogenital estrogen deficiency symptoms at baseline and weeks 3, 12, 24, 36 and 48 and assessment of signs of vaginal epithelial atrophy by the clinician at baseline, 12 and 48 weeks. In addition, pelvic floor strength, vaginal cytological evaluation and pH, bacteruria and patient acceptability were assessed. Quality of life was assessed using a menopause-specific quality-of-life questionnaire and a 2-day bladder diary at baseline and 12 and 48 weeks. The comparability of the two groups was assessed using ANOVA, chi2 or Fisher's exact tests.

RESULTS

A total of 126 women were randomized to ESTring and 59 to Vagifem. There was no statistical difference between the groups in the alleviation of symptoms and signs of urogenital estrogen deficiency. Maturation indices increased in both groups, from generally atrophic at baseline to proliferative or highly proliferative at 48 weeks. After 48 weeks of treatment, there was no statistically significant difference in endometrial thickness between the two groups. A statistically smaller proportion of bleeding/spotting occurred in the ESTring group (n = 0) compared to the Vagifem users (n = 4). Estradiol and total estrone serum levels increased during treatment in both groups but remained within the normal postmenopausal range. General health status in both groups was unchanged but the urogenital component of health burden was significantly improved in both groups. Bladder diary variables showed no differences between treatment groups.

CONCLUSION

Equivalent endometrial safety and efficacy in the relief of the symptoms and signs of urogenital estrogen deficiency were demonstrated for the 12 months' use of a low-dose estradiol-releasing vaginal ring and a vaginal estradiol tablet.

FLT3 inhibition and mechanisms of drug resistance in mutant FLT3-positive AML

An appealing therapeutic target in AML is constitutively activated, mutant FLT3, which is expressed in a subpopulation of AML patients and is generally a poor prognostic indicator in patients under the age of 65. There are currently several FLT3 inhibitors that are undergoing clinical investigation. However, the discovery of drug-resistant leukemic blast cells in FLT3 inhibitor-treated AML patients has prompted the search for novel, structurally diverse FLT3 inhibitors that could be alternatively used to circumvent drug resistance. Here, we provide an overview of FLT3 inhibitors under preclinical and clinical investigation, and we discuss mechanisms whereby AML cells develop resistance to FLT3 inhibitors, and the ways in which combination therapy could potentially be utilized to override drug resistance. We discuss how the cross-talk between major downstream signaling pathways, such as PI3K/PTEN/Akt/mTOR, RAS/Raf/MEK/ERK, and Jak/STAT, can be exploited for therapeutic purposes by targeting key signaling molecules with selective inhibitors, such as mTOR inhibitors, HSP90 inhibitors, or farnesyltransferase inhibitors, and identifying those agents with the ability to positively combine with inhibitors of FLT3, such as PKC412 and sunitinib. With the widespread onset of drug resistance associated with tyrosine kinase inhibitors, due to mechanisms involving development of point mutations or gene amplification of target proteins, the use of a multi-targeted therapeutic approach is of potential clinical benefit.

A randomized controlled trial of treatment options for troublesome uterine bleeding in Implanon users

BACKGROUND

Pilot data have indicated that both doxycycline alone and mifepristone combined with ethinyl estradiol (EE) are effective in stopping episodes of bleeding in Implanon users with troublesome bleeding. We compared four treatments against a placebo in Implanon users and tested whether repeated treatment improved subsequent bleeding patterns.

METHOD

Implanon users aged 18-45 years were randomized to treatment with (i) mifepristone 25 mg given twice on day 1 followed by 4 days of EE 20 microg; (ii) doxycycline 100 mg twice daily for 5 days; (iii) mifepristone 25 mg given twice on day 1 plus doxycycline 100 mg twice daily for 5 days; (iv) doxycycline 100 mg twice daily with EE 20 microg daily; and (v) placebo twice daily for 5 days. The primary end-point was the number of days of bleeding/spotting immediately following initiation of the first 5-day course of each therapy, compared with placebo.

RESULTS

There were 204 women assigned to treatment. Mifepristone in combination with either EE or doxycycline was significantly more effective in stopping an episode of bleeding (mean 4.0 days (CI 3.5-4.6) and 4.4 days (CI 3.8-5.2), respectively) than doxycycline alone or in combination with EE, or placebo (6.4 days (CI 4.4-9.2), 6.4 days (CI 4.8-8.6) and 6.4 days (CL 5.1-8.0), respectively).

CONCLUSION

Mifepristone combined with either EE or doxycycline was significantly more effective than placebo in terminating an episode of bleeding in Implanon users. However there was no improvement in subsequent bleeding patterns.

TRIAL REGISTRATION NUMBER

ACTR # 012605000206628.

Stromal-mediated protection of tyrosine kinase inhibitor-treated BCR-ABL-expressing leukemia cells

Clinical studies of patients with chronic myeloid leukemia revealed that a common pattern of response is a dramatic fall in the circulating population of blast cells, with a minimal or delayed decrease in marrow blasts, suggesting a protective environment. These observations suggest that a greater understanding of the interaction of stromal cells with leukemic cells is essential. Here, we present an in vivo system for monitoring relative tumor accumulation in leukemic mice and residual disease in leukemic mice treated with a tyrosine kinase inhibitor and an in vitro system for identifying integral factors involved in stromal-mediated cytoprotection. Using the in vivo model, we observed high tumor burden/residual disease in tissues characterized as significant sources of hematopoiesis-promoting stroma, with bone marrow stroma most frequently showing the highest accumulation of leukemia in untreated and nilotinib-treated mice as well as partial protection of leukemic cells from the inhibitory effects of nilotinib. These studies, which showed a pattern of leukemia distribution consistent with what is observed in imatinib- and nilotinib-treated chronic myeloid leukemia patients, were followed by a more in-depth analysis of stroma-leukemia cell interactions that lead to protection of leukemia cells from nilotinib-induced cytotoxicity. For the latter, we used the human BCR-ABL-positive cell line, KU812F, and the human bone marrow stroma cell line, HS-5, to more closely approximate the bone marrow-associated cytoprotection observed in drug-treated leukemia patients. This in vitro system helped to elucidate stromal-secreted viability factors that may play a role in stromal-mediated cytoprotection of tyrosine kinase inhibitor-treated leukemia cells.

Potentiation of antileukemic therapies by Smac mimetic, LBW242: effects on mutant FLT3-expressing cells

Members of the inhibitor of apoptosis protein (IAP) family play a role in mediating apoptosis. Studies suggest that these proteins may be a viable target in leukemia because they have been found to be variably expressed in acute leukemias and are associated with chemosensitivity, chemoresistance, disease progression, remission, and patient survival. Another promising therapeutic target, FLT3, is mutated in about one third of acute myelogenous leukemia (AML) patients; promising results have recently been achieved in clinical trials investigating the effects of the protein tyrosine kinase inhibitor PKC412 on AML patients harboring mutations in the FLT3 protein. Of growing concern, however, is the development of drug resistance resulting from the emergence of point mutations in targeted tyrosine kinases used for treatment of acute leukemia patients. One approach to overriding resistance is to combine structurally unrelated inhibitors and/or inhibitors of different signaling pathways. The proapoptotic IAP inhibitor, LBW242, was shown in proliferation studies done in vitro to enhance the killing of PKC412-sensitive and PKC412-resistant cell lines expressing mutant FLT3 when combined with either PKC412 or standard cytotoxic agents (doxorubicin and Ara-c). In addition, in an in vivo imaging assay using bioluminescence as a measure of tumor burden, a total of 12 male NCr-nude mice were treated for 10 days with p.o. administration of vehicle, LBW242 (50 mg/kg/day), PKC412 (40 mg/kg/day), or a combination of LBW242 and PKC412; the lowest tumor burden was observed in the drug combination group. Finally, the combination of LBW242 and PKC412 was sufficient to override stromal-mediated viability signaling conferring resistance to PKC412.

Second generation inhibitors of BCR-ABL for the treatment of imatinib-resistant chronic myeloid leukaemia

Imatinib, a small-molecule ABL kinase inhibitor, is a highly effective therapy for early-phase chronic myeloid leukaemia (CML), which has constitutively active ABL kinase activity owing to the expression of the BCR-ABL fusion protein. However, there is a high relapse rate among advanced- and blast-crisis-phase patients owing to the development of mutations in the ABL kinase domain that cause drug resistance. Several second-generation ABL kinase inhibitors have been or are being developed for the treatment of imatinib-resistant CML. Here, we describe the mechanism of action of imatinib in CML, the structural basis of imatinib resistance, and the potential of second-generation BCR-ABL inhibitors to circumvent resistance.

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.

Aggresome induction by proteasome inhibitor bortezomib and alpha-tubulin hyperacetylation by tubulin deacetylase (TDAC) inhibitor LBH589 are synergistic in myeloma cells

Histone deacetylase (HDAC) inhibitors have shown cytotoxicity as single agents in preclinical studies for multiple myeloma (MM) cells. LBH589 is a novel hydroxamic acid derivative that at low nanomolar concentrations induces apoptosis in MM cells resistant to conventional therapies via caspase activation and poly-(ADP-ribose) polymerase (PARP) cleavage. Significant synergistic cytotoxicity was observed with LBH589 in combination with bortezomib against MM cells that were sensitive and resistant to dexamethasone (Dex), as well as primary patient MM cells. LBH589 at low nanomolar concentrations also induced alpha-tubulin hyperacetylation. Aggresome formation was observed in the presence of bortezomib, and the combination of LBH589 plus bortezomib induced the formation of abnormal bundles of hyeracetylated alpha-tubulin but with diminished aggresome size and apoptotic nuclei. These data confirm the potential clinical benefit of combining HDAC inhibitors with proteasome inhibitors, and provide insight into the mechanisms of synergistic anti-MM activity of bortezomib in combination with LBH589.

Beneficial effects of combining nilotinib and imatinib in preclinical models of BCR-ABL+ leukemias

Drug resistance resulting from emergence of imatinib-resistant BCR-ABL point mutations is a significant problem in advanced-stage chronic myelogenous leukemia (CML). The BCR-ABL inhibitor, nilotinib (AMN107), is significantly more potent against BCR-ABL than imatinib, and is active against many imatinib-resistant BCR-ABL mutants. Phase 1/2 clinical trials show that nilotinib can induce remissions in patients who have previously failed imatinib, indicating that sequential therapy with these 2 agents has clinical value. However, simultaneous, rather than sequential, administration of 2 BCR-ABL kinase inhibitors is attractive for many reasons, including the theoretical possibility that this could reduce emergence of drug-resistant clones. Here, we show that exposure of a variety of BCR-ABL+ cell lines to imatinib and nilotinib results in additive or synergistic cytotoxicity, including testing of a large panel of cells expressing BCR-ABL point mutations causing resistance to imatinib in patients. Further, using a highly quantifiable bioluminescent in vivo model, drug combinations were at least additive in antileukemic activity, compared with each drug alone. These results suggest that despite binding to the same site in the same target kinase, the combination of imatinib and nilotinib is highly efficacious in these models, indicating that clinical testing of combinations of BCR-ABL kinase inhibitors is warranted.

Effectiveness of trichostatin A as a potential candidate for anticancer therapy in non-small-cell lung cancer

BACKGROUND

A well-known histone deacetylase inhibitor, trichostatin A, was applied to non-small-cell lung cancer cells to determine whether inhibition of histone deacetylase leads to the production of proteins that either arrest tumor cell growth or lead to tumor cell death.

METHODS

Trichostatin A (0.01 to 1.0 micromol/L) was applied to one normal lung fibroblast and four non-small-cell lung cancer lines, and its effect was determined by flow cytometry, annexin-V staining, immunoprecipitation, and Western blot analysis.

RESULTS

Trichostatin A demonstrated tenfold greater growth inhibition in all four non-small-cell lung cancer lines compared with normal controls, with a concentration producing 50% inhibition ranging from 0.01 to 0.04 micromol/L for the tumor cell lines and 0.7 micromol/L for the normal lung fibroblast line. Trichostatin A treatment reduced the percentage of cells in S phase (10% to 23%) and increased G1 populations (10% to 40%) as determined by flow cytometry. Both annexin-V binding assay and upregulation of the protein, gelsolin (threefold to tenfold), demonstrated that the tumor cells were apoptotic, whereas normal cells were predominantly in cell cycle arrest. Trichostatin A increased histone H4 acetylation and expression of p21 twofold to 15-fold without significant effect on p16, p27, CDK2, and cyclin D1.

CONCLUSIONS

Collectively, these data suggest that inhibition of histone deacetylation may provide a valuable approach for lung cancer treatment. We evaluated trichostatin A as a potential candidate for anticancer therapy in non-small-cell lung cancer.