A phase 2 clinical study of SU5416 in patients with refractory acute myeloid leukemia

Quizartinib (AC220) is a potent second generation class III tyrosine kinase inhibitor that displays a distinct inhibition profile against mutant-FLT3, -PDGFRA and -KIT isoforms

Background

Activating mutations of class III receptor tyrosine kinases (RTK) FLT3, PDGFR and KIT are associated with multiple human neoplasms including hematologic malignancies, for example: systemic mast cell disorders (KIT), non-CML myeloproliferative neoplasms (PDGFR) and subsets of acute leukemias (FLT3 and KIT). First generation tyrosine kinase inhibitors (TKI) are rapidly being integrated into routine cancer care. However, the expanding spectrum of TK-mutations, bioavailability issues and the emerging problem of primary or secondary TKI-therapy resistance have lead to the search for novel second generation TKIs to improve target potency and to overcome resistant clones.

Quizartinib was recently demonstrated to be a selective FLT3 inhibitor with excellent pharmacokinetics and promising in vivo activity in a phase II study for FLT3 ITD + AML patients. In vitro kinase assays have suggested that in addition to FLT3, quizartinib also targets related class III RTK isoforms.

Methods

Various FLT3 or KIT leukemia cell lines and native blasts were used to determine the antiproliferative and proapoptotic efficacy of quizartinib. To better compare differences between the mutant kinase isoforms, we generated an isogenic BaF3 cell line expressing different FLT3, KIT or BCR/ABL isoforms. Using immunoblotting, we examined the effects of quizartinib on activation of mutant KIT or FLT3 isoforms.

Results

Kinase inhibition of (mutant) KIT, PDGFR and FLT3 isoforms by quizartinib leads to potent inhibition of cellular proliferation and induction of apoptosis in in vitro leukemia models as well as in native leukemia blasts treated ex vivo. However, the sensitivity patterns vary widely depending on the underlying (mutant)-kinase isoform, with some isoforms being relatively insensitive to this agent (e.g. FLT3 D835V and KIT codon D816 mutations). Evaluation of sensitivities in an isogenic cellular background confirms a direct association with the underlying mutant-TK isoform – which is further validated by immunoblotting experiments demonstrating kinase inhibition consistent with the cellular sensitivity/resistance to quizartinib.

Conclusion

Quizartinib is a potent second-generation class III receptor TK-inhibitor – but specific, mutation restricted spectrum of activity may require mutation screening prior to therapy.

Successful treatment with low-dose decitabine in acute myelogenous leukemia in elderly patients over 80 years old: Five case reports

The incidence of acute myelogenous leukemia (AML) in patients over 80 years old is >20 times greater than that observed in younger patients. Previously, no standard treatment protocol for elderly patients with AML existed, however the development of hypomethylating agents, including decitabine, has brought about promising results in AML. In the present study, we report on the usage of a lower than routine dosage of decitabine in patients over 80 years old with AML. Since January 2010, 5 patients diagnosed with AML over the age of 80 years old received treatment with decitabine in our hospital. Decitabine was administered at a dose of 10–15 mg/m² and repeated every other day for a total of 5 days. This cycle was repeated for ∼6 weeks. The 5 patients received a total of 19 cycles of treatment with decitabine. No patient achieved complete or partial remission. An antileukemic effect was observed in 25% of courses (3/12). An increase in platelet count of >20×10⁹/l was observed in 26.3% (5/19) of cycles compared with previous treatment. An increase in hemoglobin concentration of >20 g/l was observed in 36.8% (7/19) of cycles in comparison to previous treatment, four of which achieved normal hemoglobin levels. One patient became red blood cell transfusion-independent. The median survival time was 19.8±4.8 months. Survival time from decitabine administration to mortality was 13.2±5.1 months. The main side-effect was bone marrow suppression with grade III–IV thrombocytopenia, grade III–IV leukocytopenia, grade III–IV neutropenia and anemia accounting for 94.7% (18/19), 47.4% (9/19), 89.5% (17/19) and 21.1% (4/19), respectively. Severe infection or bleeding was not observed and no patient stopped treatment due to adverse effects. In conclusion, extremely low-dose decitabine may be used safely in elderly patients and achieved longer survival times than reported previously in AML patients aged 80 and above. It is suggested that complete remission may not be the primary objective, while improvement of quality of life may be a better choice in AML patients over 80 years old. The cases observed in our study were limited, so more cases are required for further study.

The VEGF pathway in cancer and disease: responses, resistance, and the path forward

Antiangiogenesis was proposed as a novel target for the treatment of cancer 40 years ago. Since the original hypothesis put forward by Judah Folkman in 1971, factors that mediate angiogenesis, their cellular targets, many of the pathways they signal, and inhibitors of the cytokines and receptors have been identified. Vascular endothelial growth factor (VEGF) is the most prominent among the angiogenic cytokines and is believed to play a central role in the process of neovascularization, both in cancer as well as other inflammatory diseases. This article reviews the biology of VEGF and its receptors, the use of anti-VEGF approaches in clinical disease, the toxicity of these therapies, and the resistance mechanisms that have limited the activity of these agents when used as monotherapy.

Targeting CDK1 promotes FLT3-activated acute myeloid leukemia differentiation through C/EBPα

Mutations that activate the fms-like tyrosine kinase 3 (FLT3) receptor are among the most prevalent mutations in acute myeloid leukemias. The oncogenic role of FLT3 mutants has been attributed to the abnormal activation of several downstream signaling pathways, such as STAT3, STAT5, ERK1/2, and AKT. Here, we discovered that the cyclin-dependent kinase 1 (CDK1) pathway is also affected by internal tandem duplication mutations in FLT3. Moreover, we also identified C/EBPα, a granulopoiesis-promoting transcription factor, as a substrate for CDK1. We further demonstrated that CDK1 phosphorylates C/EBPα on serine 21, which inhibits its differentiation-inducing function. Importantly, we found that inhibition of CDK1 activity relieves the differentiation block in cell lines with mutated FLT3 as well as in primary patient-derived peripheral blood samples. Clinical trials with CDK1 inhibitors are currently under way for various malignancies. Our data strongly suggest that targeting the CDK1 pathway might be applied in the treatment of FLT3ITD mutant leukemias, especially those resistant to FLT3 inhibitor therapies.

The mechanisms that regulate the localization and overexpression of VEGF receptor-2 are promising therapeutic targets in cancer biology

The vascular endothelial growth factor (VEGF) family has been proposed to be the most important signaling protein family in vessel formation and maturation. VEGF receptor-2 (VEGFR-2) plays an abundant role in the most common forms of cancer. The localization of VEGFR-2 expression is important in cancer pathogenesis; however, so far, little attention has been paid to this phenomenon. Induced cytoplasmic VEGFR-2 transition from the nucleus is associated with poor prognostic cancer stages. Current VEGFR-2-targeted therapy approaches are effective in inhibiting or arresting tumor growth. Moreover, VEGFR-2-targeted therapy was demonstrated to restore the abnormal vasculature in tumors, enhancing their susceptibility toward conventional therapy. Most effects can be found when VEGFR-2-targeted therapy inhibits not only the induced angiogenesis but also the cancer cells that sometimes overexpress VEGFR-2. Nevertheless, we still have little knowledge about the mechanisms that regulate VEGFR-2 expression and how its localization is exactly involved in cancer prognosis. Further research and evaluation of VEGFR-2 regulation and its nuclear transition is necessary to develop more accurate therapeutic strategies to improve the patients' quality of life and their survival.

FLT3 inhibition: a moving and evolving target in acute myeloid leukaemia

Internal tandem duplication (ITD) of the fms-like tyrosine kinase 3 (FLT3) gene is a gain-of-function mutation common in acute myeloid leukaemia (AML). It is associated with inferior prognosis and response to chemotherapy. Single base mutations at the FLT3 tyrosine kinase domain (TKD) also leads to a gain of function, although its prognostic significance is less well defined because of its rarity. The clinical benefits of FLT3 inhibition are generally limited to AML with FLT3-ITD. However, responses are transient and leukaemia progression invariably occurs. There is compelling evidence that leukaemia clones carrying both ITD and TKD mutations appear when resistance to FLT3 inhibitors occurs. Interestingly, the emergence of double ITD and TKD mutants can be recapitulated in vitro when FLT3-ITD+ leukaemia cell lines are treated with mutagens and FLT3 inhibitors. Furthermore, murine xenotransplantation models also suggest that, in some cases, the FTL3-ITD and TKD double mutants actually exist in minute amounts before treatment with FLT3 inhibitors, expand under the selection pressure of FLT3 inhibition and become the predominant resistant clone(s) during the drug-refractory phase. On the basis of this model of clonal evolution, a multipronged strategy using more potent FLT3 inhibitors, and a combinatorial approach targeting both FLT3-dependent and FLT3-independent pathways, will be needed to improve outcome.

Sorafenib treatment of FLT3-ITD(+) acute myeloid leukemia: favorable initial outcome and mechanisms of subsequent nonresponsiveness associated with the emergence of a D835 mutation

Internal tandem duplication (ITD) of the fms-related tyrosine kinase-3 (FLT3) gene occurs in 30% of acute myeloid leukemias (AMLs) and confers a poor prognosis. Thirteen relapsed or chemo-refractory FLT3-ITD(+) AML patients were treated with sorafenib (200-400 mg twice daily). Twelve patients showed clearance or near clearance of bone marrow myeloblasts after 27 (range 21-84) days with evidence of differentiation of leukemia cells. The sorafenib response was lost in most patients after 72 (range 54-287) days but the FLT3 and downstream effectors remained suppressed. Gene expression profiling showed that leukemia cells that have become sorafenib resistant expressed several genes including ALDH1A1, JAK3, and MMP15, whose functions were unknown in AML. Nonobese diabetic/severe combined immunodeficiency mice transplanted with leukemia cells from patients before and during sorafenib resistance recapitulated the clinical results. Both ITD and tyrosine kinase domain mutations at D835 were identified in leukemia initiating cells (LICs) from samples before sorafenib treatment. LICs bearing the D835 mutant have expanded during sorafenib treatment and dominated during the subsequent clinical resistance. These results suggest that sorafenib have selected more aggressive sorafenib-resistant subclones carrying both FLT3-ITD and D835 mutations, and might provide important leads to further improvement of treatment outcome with FLT3 inhibitors.

Angiogenesis in acute myeloid leukemia and opportunities for novel therapies

Acute myeloid leukemia (AML) arises from neoplastic transformation of hematopoietic stem and progenitor cells, and relapsed disease remains one of the greater challenges in treating this hematologic malignancy. This paper focuses on angiogenic aspects of AML including the significance and prognostic value of bone marrow microvessel density and circulating cytokine levels. We show three general mechanisms whereby AML exploits angiogenic pathways, including direct induction of angiogenesis, paracrine regulation, and autocrine stimulation. We also present early evidence that leukemia cells contribute directly to vascular endothelia. Novel treatment strategies are proposed, and a review of relevant antiangiogenic clinical trials is presented. By understanding how blood vessels can serve as a reservoir for refractory and relapsed AML, new diagnostics and promising treatment strategies can be developed.

FLT3 inhibition as therapy in acute myeloid leukemia: a record of trials and tribulations

Acute myeloid leukemia (AML) is a hematologic malignancy with a poor prognosis. Approximately one quarter of the patients with AML also carry an internal tandem duplication (ITD) mutation in the gene encoding FMS-like tyrosine kinase 3 (FLT3), which has a significantly deleterious impact on prognosis. The ITD mutation renders FLT3 constitutively active and leads to uncontrolled proliferation of the leukemic blast. Over the course of the last decade, a variety of compounds have been developed in preclinical and clinical studies as potent inhibitors of FLT3. Many of the earlier agents under investigation, such as lestaurtinib, midostaurin, and sunitinib, were initially developed as inhibitors of other tyrosine kinases and as targeted therapies in a variety of malignancies. These compounds have been demonstrated to have some efficacy in clinical trials of AML, mainly manifesting as transient decreases in circulating blasts correlating with effective in vivo suppression of the FLT3 target. Nevertheless, the cumbersome pharmacokinetics of some compounds and the suboptimal specificity and potency of others have limited their therapeutic efficacy. In the last few years, newer, more potent and specific agents have been under investigation, with the leading example being AC220. This agent has shown significant promise in early phases of clinical investigation, and is currently in more advanced clinical trials. Hope remains that FLT3 inhibition will be become an effective therapeutic adjunct to our current treatment approach to AML.

FLT3 inhibitors: a story of the old and the new

PURPOSE OF REVIEW

Ever since the recognition that FMS-like tyrosine kinase 3 (FLT3) mutations exert a profound negative prognostic impact on the clinical outcome of patients with acute myeloid leukemia (AML), researchers have sought to find effective small-molecule inhibitors of this receptor tyrosine kinase. This review will attempt to provide a survey of the FLT3 inhibitors currently under investigation and provide a discussion on their current status in clinical trials.

RECENT FINDINGS

Over the past 10 years, a number of different compounds have been studied in vitro and clinically as FLT3 inhibitors. The first inhibitors studied were hampered by cumbersome pharmacokinetics and a general lack of potency. However, some agents have shown promise in clinical trials with transient responses in AML. Newer compounds, such as AC220, have demonstrated profound selectivity and potency against the FLT3 target, and are currently being investigated in clinical trials.

SUMMARY

Clinical trials have so far demonstrated that inhibitors of FLT3 do have clinical activity in patients with FLT3-mutant AML, although this activity is often transient and correlates with effective in-vivo suppression of the FLT3 target. As newer, more potent agents are now entering advanced clinical trials, opportunities will emerge for real progress against this grim disease.

Acute myeloid leukaemia in the elderly: a review

The majority of patients with acute myeloid leukaemia (AML) are elderly. Advancements in supportive care and regimen intensification have resulted in improvements in clinical outcomes for younger AML patients, but analogous improvements in older patients have not been realized. While outcomes are compromised by increased comorbidities and susceptibility to toxicity from therapy, it is now recognized that elderly AML represents a biologically distinct disease that is more aggressive and less responsive to therapy. Some patients tolerate and benefit from intensive remission-induction approaches, while others are best managed with less aggressive strategies. The challenge is to differentiate these groups based on host-related and biological features, in order to maximize the therapeutic benefit and minimize toxicity. As more is understood about the complicated pathogenesis and molecular basis of AML, there are more opportunities to develop and test targeted therapies. Elderly patients, with their narrow therapeutic window, are well positioned to derive a benefit from these novel agents, and therefore, despite a difficult past, there are reasons to be optimistic about the future of elderly AML.

Dose study of the multikinase inhibitor, LY2457546, in patients with relapsed acute myeloid leukemia to assess safety, pharmacokinetics, and pharmacodynamics

Background:

Acute myeloid leukemia (AML) is a life-threatening malignancy with limited treatment options in chemotherapy-refractory patients. A first-in-human dose study was designed to investigate a safe and biologically effective dose range for LY2457546, a novel multikinase inhibitor, in patients with relapsed AML.

Methods:

In this nonrandomized, open-label, dose escalation Phase I study, LY2457546 was administered orally once a day. Safety, pharmacokinetics, changes in phosphorylation of target kinases in AML blasts, and risk of drug–drug interactions (DDI) were assessed.

Results:

Five patients were treated at the starting and predicted minimal biologically effective dose of 50 mg/day. The most commonly observed adverse events were febrile neutropenia, epistaxis, petechiae, and headache. The majority of adverse events (81%) were Grade 1 or 2. One patient had generalized muscle weakness (Grade 3), which was deemed to be a dose-limiting toxicity. Notably, the pharmacokinetic profile of LY2457546 showed virtually no elimination of LY2457546 within 24 hours, and thus prevented further dose escalation. No significant DDI were observed. Ex vivo flow cytometry studies showed downregulation of the phosphoproteins, pcKIT, pFLT3, and pS6, in AML blasts after LY2457546 administration. No medically relevant responses were observed in the five treated patients.

Conclusion:

No biologically effective dose could be established for LY2457546 in chemotherapy-resistant AML patients. Lack of drug clearance prevented safe dose escalation, and the study was terminated early. Future efforts should be made to develop derivatives with a more favorable pharmacokinetic profile.

FLT3 inhibitors in acute myeloid leukemia

The fms-like tyrosine kinase 3 (FLT3) plays an important role in both normal and malignant hematopoiesis. Activating mutations in the FLT3 receptor can be detected in approximately 30% of acute myeloid leukemias (AMLs) and are associated with a distinctly poor clinical outcome for patients. There are now several classes of FLT3 inhibitors in development with varying degrees of potency and selectivity for the target, including several in late-phase clinical trials in combination with chemotherapy. Major clinical responses in AML patients receiving single-agent FLT3 inhibitors have been rare, although transient peripheral blood blast reduction is common. Given such biological suggestion and preclinical activity, FLT3 inhibitors hold promise in improving the outcome of patients with mutant FLT3 AML. This review summarizes the current attempts to target this molecule, with emphasis on the validity of the target, the results of the clinical trials evaluating the FLT3 inhibitors in AML, the optimal use of these compounds and the mechanisms of resistance.

High acute myeloid leukemia derived VEGFA levels are associated with a specific vascular morphology in the leukemic bone marrow

Background

Acute Myeloid Leukemia (AML) bone marrow biopsies at diagnosis display enhanced angiogenesis and increased VEGFA expression. In a xenograft mouse model it was described that availability of free VEGFA versus bound VEGFA is related to different vascular morphology. In this study we investigate the relationship between vascular morphology within AML bone marrow biopsies and AML derived VEGFA levels.

Methods

Vessel count and surface area (Chalkley count) were calculated in AML bone marrow biopsies at diagnosis (n = 32), at remission (n = 8) and Normal Bone Marrow (n = 32) using immunohistochemical staining for FVIII, CD31, CTIV, SMA and VEGFA. VEGFA protein levels were measured.

Results

High vessel count was associated with an immature vessel status. Combining vessel count and Chalkley count different vessel morphology patterns were quantified within AML bone marrow biopsies. Three different subgroups could be distinguished. The subgroup (37.5% of the samples) exhibiting a high vessel count and vessels with predominantly large lumen (normal Chalkley count) was associated with high secreted VEGFA protein levels.

Conclusion

Different vasculature patterns are seen in AML bone marrow biopsies, defined by combining number and size of vessel. These quantified morphology patterns, combined with VEGFA levels, might be of value in the success of VEGF/VEGFR-signaling interference approaches.

Tyrosine kinase inhibitors: Multi-targeted or single-targeted?

Since in most tumors multiple signaling pathways are involved, many of the inhibitors in clinical development are designed to affect a wide range of targeted kinases. The most important tyrosine kinase families in the development of tyrosine kinase inhibitors are the ABL, SCR, platelet derived growth factor, vascular endothelial growth factor receptor and epidermal growth factor receptor families. Both multi-kinase inhibitors and single-kinase inhibitors have advantages and disadvantages, which are related to potential resistance mechanisms, pharmacokinetics, selectivity and tumor environment. In different malignancies various tyrosine kinases are mutated or overexpressed and several resistance mechanisms exist. Pharmacokinetics is influenced by interindividual differences and differs for two single targeted inhibitors or between patients treated by the same tyrosine kinase inhibitor. Different tyrosine kinase inhibitors have various mechanisms to achieve selectivity, while differences in gene expression exist between tumor and stromal cells. Considering these aspects, one type of inhibitor can generally not be preferred above the other, but will depend on the specific genetic constitution of the patient and the tumor, allowing personalized therapy. The most effective way of cancer treatment by using tyrosine kinase inhibitors is to consider each patient/tumor individually and to determine the strategy that specifically targets the consequences of altered (epi)genetics of the tumor. This strategy might result in treatment by a single multi kinase inhibitor for one patient, but in treatment by a couple of single kinase inhibitors for other patients.

Targeting non-malignant disorders with tyrosine kinase inhibitors

Receptor and non-receptor tyrosine kinases are involved in multiple proliferative signalling pathways. Imatinib, one of the first tyrosine kinase inhibitors (TKIs) to be approved, revolutionized the treatment of chronic myelogenous leukaemia, and other TKIs with different spectra of kinase inhibition are used to treat renal cell carcinoma, non-small-cell lung cancer and colon cancer. Studies also support the potential use of TKIs as anti-proliferative agents in non-malignant disorders such as cardiac hypertrophy, and in benign-proliferative disorders including pulmonary hypertension, lung fibrosis, rheumatoid disorders, atherosclerosis, in-stent restenosis and glomerulonephritis. In this Review, we provide an overview of the most recent developments--both experimental as well as clinical--regarding the therapeutic potential of TKIs in non-malignant disorders.

Molecular genetics in acute myeloid leukemia

PURPOSE OF REVIEW

Acute myeloid leukemia (AML) is a highly heterogeneous disorder being composed of various genetically defined subtypes. In recent years, molecular research provided the basis for a more differentiated characterization of AML patients, for example, of the large subgroup with normal karyotypes. This review summarizes the current status of molecular diagnostics in AML and refers to the diagnostic techniques being most suitable for the individual markers.

RECENT FINDINGS

A molecular data set based on mutations of the NPM1, FLT3, and CEBPA genes and the MLL-PTD provides a prognostically relevant risk stratification that can support the decision pro or con an allogeneic hematopoietic stem cell transplantation in first remission. The panel of known molecular markers is continuously increasing, for example, considering the recently described TET2 and IDH1 mutations. The introduction of next generation sequencing will certainly catalyze the molecular characterization of AML. Monitoring of the minimal residual disease load with quantitative real-time PCR can be performed for NPM1 and MLL-PTD-mutated cases.

SUMMARY

Targeted therapy studies with FLT3 inhibitors for patients with FLT3-mutated AML as single agents or combined with chemotherapy illustrate the translation of the molecular techniques into clinical practice already being realized in distinct subgroups of AML.

VEGF targeted therapy in acute myeloid leukemia

The cooperation of two classes of mutations in hematopoietic cells is hypothesized in a multistep pathogenesis model of acute myeloid leukemia (AML). Class I mutations confer a proliferative and/or survival advantage, whereas Class II mutations block hematopoietic differentiation and impair apoptosis in AML cells. In addition to these two classes of mutations, a relevant role for angiogenesis in the pathophysiology of AML has been recently proposed. The recognition that the vascular endothelial growth factor (VEGF) pathway is a key regulator of angiogenesis has led to the development of several VEGF-targeted approaches. These include neutralizing antibodies, VEGF traps or selective tyrosine kinase inhibitors for VEGFRs. Other drugs that indirectly affect VEGF pathway, such as statins or arsenic trioxide, also have been shown to possess antiangiogenic activity in leukemias. The benefits of these VEGF targeted agents and their current stage of development as novel anti-antiangiogenic therapies in AML are discussed in this review.

FLT3 as a therapeutic target in AML: still challenging after all these years

Mutations within the FMS-like tyrosine kinase 3 (FLT3) gene on chromosome 13q12 have been detected in up to 35% of acute myeloid leukemia (AML) patients and represent one of the most frequently identified genetic alterations in AML. Over the last years, FLT3 has emerged as a promising molecular target in therapy of AML. Here, we review results of clinical trials and of correlative laboratory studies using small molecule FLT3 tyrosine kinase inhibitors (TKIs) in AML patients. We also review mechanisms of primary and secondary drug resistance to FLT3-TKI, and from the data currently available we summarize lessons learned from FLT3-TKI monotherapy. Finally, for using FLT3 as a molecular target, we discuss novel strategies to overcome treatment failure and to improve FLT3 inhibitor therapy.

Targeting the c-kit receptor in the treatment of acute myelogenous leukemia

Acute myelogenous leukemia (AML) is a difficult disease to treat. Novel treatment strategies, including molecular targeted therapy, are being explored. The c-kit receptor represents a potential therapeutic target for AML. The receptor is expressed on more than 10% of blasts in 64% of patients with de novo AML and 95% of those with relapsed AML. It mediates proliferation and anti-apoptotic effects in AML. This review discusses the biology of c-kit in normal and malignant hematopoiesis and the recent clinical trials targeting c-kit in AML.

Clinical trials with anti-angiogenic agents in hematological malignancies

New blood vessel formation (angiogenesis) is not only essential for the growth of solid tumors but there is also emerging evidence that progression of hematological malignancies like multiple myeloma, acute leukemias, and myeloproliferative neoplasms, also depends on new blood vessel formation. Anti-angiogenic strategies have become an important therapeutic modality for solid tumors. Several anti-angiogenic agents targeting angiogenesis-related pathways like monoclonal antibodies, receptor tyrosine kinase inhibitors, immunomodulatory drugs, and proteasome inhibitors have been entered clinical trials or have been already approved for the treatment of hematological malignancies as well and in some instances these pathways have emerged as promising therapeutic targets. This review summarizes recent advances in the basic understanding of the role of angiogenesis in hematological malignancies and clinical trials with novel therapeutic approaches targeting angiogenesis.

VEGF blockade inhibits lymphocyte recruitment and ameliorates immune-mediated vascular remodeling

RATIONALE

There are conflicting data on the effects of vascular endothelial growth factor (VEGF) in vascular remodeling. Furthermore, there are species-specific differences in leukocyte and vascular cell biology and little is known about the role of VEGF in remodeling of human arteries.

OBJECTIVE

We sought to address the role of VEGF blockade on remodeling of human arteries in vivo.

METHODS AND RESULTS

We used an anti-VEGF antibody, bevacizumab, to study the effect of VEGF blockade on remodeling of human coronary artery transplants in severe combined immunodeficient mice. Bevacizumab ameliorated peripheral blood mononuclear cell-induced but not interferon-gamma-induced neointimal formation. This inhibitory effect was associated with a reduction in graft T-cell accumulation without affecting T-cell activation. VEGF enhanced T-cell capture by activated endothelium under flow conditions. The VEGF effect could be recapitulated when a combination of recombinant intercellular adhesion molecule 1 and vascular cell adhesion molecule-1 rather than endothelial cells was used to capture T cells. A subpopulation of CD3+ T cells expressed VEGF receptor (VEGFR)-1 by immunostaining and FACS analysis. VEGFR-1 mRNA was also detectable in purified CD4+ T cells and Jurkat and HSB-2 T-cell lines. Stimulation of HSB-2 and T cells with VEGF triggered downstream ERK phosphorylation, demonstrating the functionality of VEGFR-1 in human T cells.

CONCLUSIONS

VEGF contributes to vascular remodeling in human arteries through a direct effect on human T cells that enhances their recruitment to the vessel. These findings raise the possibility of novel therapeutic approaches to vascular remodeling based on inhibition of VEGF signaling.

Leukemia regression by vascular disruption and antiangiogenic therapy

Acute myelogenous leukemias (AMLs) and endothelial cells depend on each other for survival and proliferation. Monotherapy antivascular strategies such as targeting vascular endothelial growth factor (VEGF) has limited efficacy in treating AML. Thus, in search of a multitarget antivascular treatment strategy for AML, we tested a novel vascular disrupting agent, OXi4503, alone and in combination with the anti-VEGF antibody, bevacizumab. Using xenotransplant animal models, OXi4503 treatment of human AML chloromas led to vascular disruption in leukemia cores that displayed increased leukemia cell apoptosis. However, viable rims of leukemia cells remained and were richly vascular with increased VEGF-A expression. To target this peripheral reactive angiogenesis, bevacizumab was combined with OXi4503 and abrogated viable vascular rims, thereby leading to enhanced leukemia regression. In a systemic model of primary human AML, OXi4503 regressed leukemia engraftment alone and in combination with bevacizumab. Differences in blood vessel density alone could not account for the observed regression, suggesting that OXi4503 also exhibited direct cytotoxic effects on leukemia cells. In vitro analyses confirmed this targeted effect, which was mediated by the production of reactive oxygen species and resulted in apoptosis. Together, these data show that OXi4503 alone is capable of regressing AML by a multitargeted mechanism and that the addition of bevacizumab mitigates reactive angiogenesis.

Angiogenesis inhibition in cancer therapy: platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) and their receptors: biological functions and role in malignancy

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen in vitro and an angiogenic inducer in a variety of in vivo models. VEGF gene transcription is induced in particular in hypoxic cells. In developmental angiogenesis, the role of VEGF is demonstrated by the finding that the loss of a single VEGF allele results in defective vascularization and early embryonic lethality. Substantial evidence also implicates VEGF as a mediator of pathological angiogenesis. In situ hybridization studies demonstrate expression of VEGF mRNA in the majority of human tumors. Platelet-derived growth factor (PDGF) is mainly believed to be an important mitogen for connective tissue, and also has important roles during embryonal development. Its overexpression has been linked to different types of malignancies. Thus, it is important to understand the physiology of VEGF and PDGF and their receptors as well as their roles in malignancies in order to develop antiangiogenic strategies for the treatment of malignant disease.

Investigational drugs targeting FLT3 for leukemia

FMS-like tyrosine kinase-3 (FLT3) is a member of the class III membrane receptor tyrosine kinase family and is important in survival, proliferation and differentiation of hematopoietic cells. FLT3 is mutated in approximately 30% of acute myelogenous leukemia patients. These mutations involve internal tandem duplications in the juxtamembrane domain of the receptor and tyrosine kinase point mutations in the activation loop. Over the past decade, due to the incidence and poor prognosis associated with FLT3, numerous agents have been developed to directly inhibit the activity of wild type and mutated FLT3. In this review, we focus on the preclinical data demonstrating in vitro activity, inhibition of downstream signaling pathways and potential synergy with traditional chemotherapeutic agents. Also, early clinical trial data specifically focusing on drug toxicity, clinical efficacy and future directions of FLT3-directed anticancer therapy are discussed.

Expression profiling of a hemopoietic cell survival transcriptome implicates osteopontin as a functional prognostic factor in AML

Deregulated cell survival programs are a classic hallmark of cancer. We have previously identified a serine residue (Ser585) in the βc subunit of the granulocyte-macrophage colony-stimulating factor receptor that selectively and independently promotes cell survival. We now show that Ser585 phosphorylation is constitutive in 20 (87%) of 23 acute myeloid leukemia (AML) patient samples, indicating that this survival-only pathway is frequently deregulated in leukemia. We performed a global expression screen to identify gene targets of this survival pathway and report a 138-gene βc Ser585-regulated transcriptome. Pathway analysis defines a gene network enriched for PI3-kinase target genes and a cluster of genes involved in cancer and cell survival. We show that one such gene, osteopontin (OPN), is a functionally relevant target of the Ser585-survival pathway as shown by siRNA-mediated knockdown of OPN expression that induces cell death in both AML blasts and CD34+CD38−CD123+ leukemic progenitors. Increased expression of OPN at diagnosis is associated with poor prognosis with multivariate analysis indicating that it is an independent predictor of overall patient survival in normal karyotype AML (n = 60; HR = 2.2; P = .01). These results delineate a novel cytokine-regulated Ser585/PI3-kinase signaling network that is deregulated in AML and identify OPN as a potential prognostic and therapeutic target.

ABT-869, a promising multi-targeted tyrosine kinase inhibitor: from bench to bedside

Tyrosine Kinase Inhibitors (TKI) have significantly changed the landscape of current cancer therapy. Understanding of mechanisms of aberrant TK signaling and strategies to inhibit TKs in cancer, further promote the development of novel agents.ABT-869, a novel ATP-competitive receptor tyrosine kinase inhibitor is a potent inhibitor of members of the vascular endothelial growth factor (VEGF) and platelet derived growth factor (PDGF) receptor families. ABT-869 showed potent antiproliferative and apoptotic properties in vitro and in animal cancer xenograft models using tumor cell lines that were "addicted" to signaling of kinases targeted by ABT-869. When given together with chemotherapy or mTOR inhibitors, ABT-869 showed at least additive therapeutic effects. The phase I trial for ABT-869 was recently completed and it demonstrated respectable efficacy in solid tumors including lung and hepatocellular carcinoma with manageable side effects. Tumor cavitation and reduction of contrast enhancement after ABT-869 treatment supported the antiangiogenic activity. The correlative laboratory studies conducted with the trial also highlight potential biomarkers for future patient selection and treatment outcome.Parallel to the clinical development, in vitro studies on ABT-869 resistance phenotype identified novel resistance mechanism that may be applicable to other TKIs. The future therapeutic roles of ABT-869 are currently been tested in phase II trials.

Survivin mediates aberrant hematopoietic progenitor cell proliferation and acute leukemia in mice induced by internal tandem duplication of Flt3

Internal tandem duplication mutations in the Flt3 tyrosine kinase gene (ITD-Flt3) and overexpression of Survivin are frequently found in patients with acute myeloid leukemia (AML). We investigated whether Survivin mediates the enhanced survival of primary hematopoietic progenitor cells (HPCs) resulting from ITD-Flt3 signaling. Ectopic ITD-Flt3 mutants increased Survivin expression in Ba/F3 cells downstream of PI3-kinase/Akt. Treatment of ITD-Flt3(+) human MV4-11 leukemia cells with the ITD-Flt3 inhibitor SU5416 reduced Survivin expression and inhibited cell proliferation. ITD-Flt3 dramatically increased the number of primary mouse marrow c-kit(+), Sca-1(+), Lin(Neg) cells and colony-forming unit granulocyte-macrophages (CFU-GMs) able to proliferate in the absence of growth factors, whereas Survivin deletion significantly reduced growth factor-independent proliferation and increased apoptosis, which was further accentuated by SU5416. Ectopic ITD-Flt3 reduced differentiation of Lin(Neg) marrow cells cultured with granulocyte-macrophage colony-stimulating factor (GM-CSF) plus stem cell factor, which was partially blocked by Survivin deletion. In addition, Survivin deletion decreased secondary colony formation induced by ITD-Flt3. Dominant-negative (dn)-Survivin delayed development of acute leukemia in mice that received a transplant of Ba/F3 cells expressing ITD-Flt3. These results suggest that Survivin regulates expansion of ITD-Flt3-transformed HPCs with self-renewal capability and development of ITD-Flt3(+) acute leukemia and that antagonizing Survivin may provide therapeutic benefit for patients with acute leukemia expressing ITD-Flt3.

Acute myelogenous leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease with outcomes dependent upon several factors, including patient age, karyotype, mutational status, and comorbid conditions. For younger patients, approximately 60% to 80% achieve complete remission with standard therapy involving cytarabine and an anthracycline. However, only 20% to 30% have long-term disease-free survival. For adults older than 60 years of age, only 40% to 55% achieve a complete remission, with dismal long-term survival rates. Unfortunately, the median age at diagnosis for AML is 70 years. Significant advances in our understanding of the molecular biology of AML have led to newer therapies that specifically target molecular abnormalities. Examples of such therapies include the immunoconjugate gemtuzumab ozogamicin, FMS-like tyrosine kinase 3 inhibitors, farnesyl transferase inhibitors, histone deacetylase inhibitors, DNA hypomethylating agents, multidrug-resistance inhibitors, BCL-2 inhibitors, antiangiogenesis agents, and various nucleoside analogs. This review summarizes the standard treatments for AML and discusses the role of novel therapies.

Micro-vessel density and the expression of vascular endothelial growth factor (VEGF) and platelet-derived endothelial cell growth factor (PdEGF) in classical Hodgkin lymphoma (HL)

There is little information to date regarding the role of angiogenesis in Hodgkin lymphoma (HL). The present study examines micro-vessel density and the expression of vascular endothelial growth factor (VEGF) and platelet-derived endothelial growth factor (PdEGF) in lymph node biopsies of patients with HL at presentation and relapse. Using immunohistochemistry, the degree of new blood vessel formation and the expression of VEGF and PdEGF was assessed in Hodgkin-rich tissue. The micro-vessel density (MVD) increased with disease progression in seven out of 11 cases. Expression of VEGF was observed in endothelial cells (EC) of some micro-vessels and also in follicular dendritic cells. The Hodgkin/Reed-Sternberg (H-RS) cells as well as the inflammatory lymphocytes were negative for VEGF. Cytoplasmic or cytoplasmic and nuclear expression of PdEGF by the H-RS cells was observed in five of the 11 presentation cases. The expression of PdEGF increased with disease progression in seven cases. In conclusion, Hodgkin tissue shows prominent vascularization. The increased MVD and PdEGF expression with disease progression merits further investigation.

Disease-specific expression of VEGF and its receptors in AML cells: possible autocrine pathway of VEGF/type1 receptor of VEGF in t(15;17) AML and VEGF/type2 receptor of VEGF in t(8;21) AML

Various angiogenic factors, such as vascular endothelial growth factor (VEGF) and an associated molecule, placenta growth factor (PlGF), are thought to be important for normal and malignant hematopoiesis. This study examined mRNA expression of VEGF, PlGF and receptors for these molecules in AML cells and identified the disease-specific patterns of expression. AML M3 having t(15;17) abnormality showed highest expression of VEGF and VEGF receptor type 1 (VEGFR1), suggesting the autocrine pathway of VEGF-VEGFR1. Then, t(8;21) AML demonstrated augmented expression of VEGF and VEGF receptor type 2 (VEGFR2), suggesting VEGF-VEGFR2 autocrine pathway. Then, addition of VEGFR2 kinase inhibitor in Kasumi-1, a t(8;21) AML cell line, resulted in marked inhibition of cell growth, although growth inhibitory effect of R2 kinase inhibitor to HL-60 was marginal. In addition, cell cycle analysis study showed S-phase cell population reduction by R2 kinase inhibitor in Kasumi-1, but not in HL-60. This observation is thought to be the rationale for novel molecular target therapy directed to angiogenic molecules.

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.

Patient-derived acute myeloid leukemia (AML) bone marrow cells display distinct intracellular kinase phosphorylation patterns

Multiparametric analyses of phospho-protein activation in patients with acute myeloid leukemia (AML) offers a quantitative measure to monitor the activity of novel intracellular kinase (IK) inhibitors. As recent clinical investigation with FMS-like tyrosine-3 inhibitors demonstrated, targeting IK with selective inhibitors can have a modest clinical benefit. Because multiple IKs are active in patients with AML, multikinase inhibitors may provide the necessary inhibition profile to achieve a more sustained clinical benefit. We here describe a method of assessing the activation of several IKs by flow cytometry. In 40 different samples of patients with AML we observed hyper-activated phospho-proteins at baseline, which is modestly increased by adding stem cell factor to AML cells. Finally, AML cells had a significantly different phospho-protein profile compared with cells of the lymphocyte gate. In conclusion, our method offers a way to determine the activation status of multiple kinases in AML and hence is a reliable assay to evaluate the pharmacodynamic activity of novel multikinase inhibitors.

Depletion of L-ascorbic acid alternating with its supplementation in the treatment of patients with acute myeloid leukemia or myelodysplastic syndromes

PURPOSE

L-ascorbic acid (LAA) modifies the in vitro growth of leukemic cells from approximately 50% of patients with acute myeloid leukemia (AML) or myelodysplastic syndromes (MDS). To test the hypothesis that depletion of LAA, alternating with supplementation to prevent scurvy, would provide therapeutic benefit, a single-arm pilot trial was conducted (ClinicalTrials.gov identifier: NCT00329498). Experimental results: During depletion phase, patients with refractory AML or MDS were placed on a diet deficient in LAA; during supplementation phase, patients received daily intravenous administration of LAA. An in vitro assay was performed pretherapy for LAA sensitivity of leukemic cells from individual patients.

RESULTS

Of 18 patients enrolled, eight of 16 evaluable patients demonstrated a clinical response. Responses were obtained during depletion (four patients) as well as during supplementation (five patients) but at a pharmacologic plasma level achievable only with intravenous administration. Of nine patients for whom the in vitro assay indicated their leukemic cells were sensitive to LAA, seven exhibited a clinical response; compared with none of six patients who were insensitive to LAA.

CONCLUSIONS

The clinical benefit, along with a conspicuous absence of significant adverse events, suggests that further testing of LAA depletion alternating with pharmacologic dose intravenous supplementation in patients with these and other malignancies is warranted.

Genomic approaches to small molecule discovery

With the sequencing of the human genome and the development of new genomic technologies, biomedical discovery has been transformed. The applications of these new approaches are ever-expanding from disease classification, to identification of new targets, to outcome prediction. A logical next step is the integration of genomic approaches into small molecule discovery. This review will focus on the application of genomics to compound discovery, with an emphasis on the hematological malignancies. It will focus on the use of genomic tools to discover cancer targets and the development and application of both cell-based and in silico gene expression-based approaches to small molecule discovery.

Mechanisms of resistance to FLT3 inhibitors

The success of the small molecule tyrosine kinase receptor inhibitor (TKI) imatinib mesylate (Gleevec) in the treatment of chronic myeloid leukemia (CML) constitutes an eminent paradigm shift advocating the rational design of cancer therapeutics specifically targeting the transformation events that drive tumorigenicity. In acute myeloid leukemias (AMLs), the most frequent identified transforming events are activating mutations in the FLT3 receptor tyrosine kinase that constitutively activate survival and proliferation pathways. FLT3 TKIs that are in various phases of clinical trials are showing some initial promise. However, primary and secondary acquired resistance stands to severely compromise long-term and durable efficacy of these inhibitors as a therapeutic strategy. Here, we discuss the mechanisms of resistance to FLT3 inhibitors and possible strategies to overcome resistance through closer examination of the events of leukemogenesis and design of combination therapy.

Phase I study of SU5416, a small molecule inhibitor of the vascular endothelial growth factor receptor (VEGFR) in refractory pediatric central nervous system tumors

SU5416 is a novel small molecule tyrosine kinase inhibitor of the VEGF receptors 1 and 2. A phase I dose escalation study stratified by concurrent use (stratum II) or absence (stratum I) of enzyme-inducing anticonvulsant drugs was undertaken to estimate the maximum-tolerated dose (MTD) and to describe the toxicity profile of SU5416 in pediatric patients with refractory brain tumors. Dose escalations were conducted independently for stratum I starting at 110 mg/m(2) while stratum II started at 48 mg/m(2). Thirty-three eligible patients were treated on stratum I (n = 23) and stratum II (n = 10). Tumor types included 23 glial tumors, 4 neural tumors, 4 ependymomas, and 2 choroid plexus carcinomas. The MTD in stratum I was initially estimated to be 110 mg/m(2). The protocol was amended to determine the MTD after excluding transient AST elevation. Re-estimation of the MTD began at the 145 mg/m(2) dose level but due to development of SU5416 being stopped by the sponsor, the trial was closed before completion. The most serious drug-related toxicities were grade 3 liver enzyme abnormalities, arthralgia, and hallucinations. The plasma pharmacokinetics of SU5416 was not significantly affected by the concurrent administration of enzyme-inducing anticonvulsant drugs. Mean values of the total body clearance, apparent volume of distribution, and terminal phase half-life of SU5416 for the 19 patients in stratum I were 26.1 +/- 12.5 l/hr/m(2), 41.9 +/- 21.4 l/m(2), and 1.11 +/- 0.41 hr, respectively. The plasma pharmacokinetics of SU5416 in children was similar to previously reported findings in adult cancer patients. Prolonged disease stabilization was observed in 4 of 16 stratum I patients.