Evaluation of in vivo antineoplastic effects of rapamycin in patients with chemotherapy-refractory AML

Bone Marrow Microenvironment as a Source of New Drug Targets for the Treatment of Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is a heterogeneous disease with one of the worst survival rates of all cancers. The bone marrow microenvironment is increasingly being recognised as an important mediator of AML chemoresistance and relapse, supporting leukaemia stem cell survival through interactions among stromal, haematopoietic progenitor and leukaemic cells. Traditional therapies targeting leukaemic cells have failed to improve long term survival rates, and as such, the bone marrow niche has become a promising new source of potential therapeutic targets, particularly for relapsed and refractory AML. This review briefly discusses the role of the bone marrow microenvironment in AML development and progression, and as a source of novel therapeutic targets for AML. The main focus of this review is on drugs that modulate/target this bone marrow microenvironment and have been examined in in vivo models or clinically.

Phosphoproteomics of primary AML patient samples reveals rationale for AKT combination therapy and p53 context to overcome selinexor resistance

Acute myeloid leukemia (AML) is a heterogeneous disease with variable patient responses to therapy. Selinexor, an inhibitor of nuclear export, has shown promising clinical activity for AML. To identify the molecular context for monotherapy sensitivity as well as rational drug combinations, we profile selinexor signaling responses using phosphoproteomics in primary AML patient samples and cell lines. Functional phosphosite scoring reveals that p53 function is required for selinexor sensitivity consistent with enhanced efficacy of selinexor in combination with the MDM2 inhibitor nutlin-3a. Moreover, combining selinexor with the AKT inhibitor MK-2206 overcomes dysregulated AKT-FOXO3 signaling in resistant cells, resulting in synergistic anti-proliferative effects. Using high-throughput spatial proteomics to profile subcellular compartments, we measure global proteome and phospho-proteome dynamics, providing direct evidence of nuclear translocation of FOXO3 upon combination treatment. Our data demonstrate the potential of phosphoproteomics and functional phosphorylation site scoring to successfully pinpoint key targetable signaling hubs for rational drug combinations.

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Phosphoproteomics with functional scoring uncovers context for selinexor sensitivity

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Functional p53 correlates with selinexor sensitivity, which is enhanced by nutlin-3a

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Dysregulated AKT-FOXO3 drives selinexor resistance, which is overcome with MK-2206

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Spatial proteomics reveals selinexor-induced nucleocytoplasmic protein shuttling

The PI3K-Akt-mTOR Signaling Pathway in Human Acute Myeloid Leukemia (AML) Cells

Acute myeloid leukemia (AML) is a heterogeneous group of diseases characterized by uncontrolled proliferation of hematopoietic stem cells in the bone marrow. Malignant cell growth is characterized by disruption of normal intracellular signaling, caused by mutations or aberrant external signaling. The phosphoinositide 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) pathway (PI3K-Akt-mTOR pathway) is among one of the intracellular pathways aberrantly upregulated in cancers including AML. Activation of this pathway seems important in leukemogenesis, and given the central role of this pathway in metabolism, the bioenergetics of AML cells may depend on downstream signaling within this pathway. Furthermore, observations suggest that constitutive activation of the PI3K-Akt-mTOR pathway differs between patients, and that increased activity within this pathway is an adverse prognostic parameter in AML. Pharmacological targeting of the PI3K-Akt-mTOR pathway with specific inhibitors results in suppression of leukemic cell growth. However, AML patients seem to differ regarding their susceptibility to various small-molecule inhibitors, reflecting biological heterogeneity in the intracellular signaling status. These findings should be further investigated in both preclinical and clinical settings, along with the potential use of this pathway as a prognostic biomarker, both in patients receiving intensive curative AML treatment and in elderly/unfit receiving AML-stabilizing treatment.

Autophagy in endothelial cells regulates their haematopoiesis-supporting ability

Background

Endothelial cells (ECs) function as an instructive platform to support haematopoietic stem cell (HSC) homeostasis. Our recent studies found that impaired bone marrow (BM) ECs are responsible for the defective haematopoiesis in patients with poor graft function (PGF), which is characterised by pancytopenia post-allotransplant. Although activated autophagy was reported to benefit ECs, whether EC autophagy plays a critical role in supporting HSCs and its effect on PGF patients post-allotransplant remain unclear.

Methods

To evaluate whether the autophagy status of ECs modulates their ability to support haematopoiesis, human umbilical vein endothelial cells (HUVECs) and primary BM ECs derived from healthy donors were subjected to knockdown or overexpression of Beclin-1 (an autophagy-related protein). Moreover, BM ECs derived from PGF patients were studied.

Findings

Beclin-1 knockdown significantly reduced the haematopoiesis-supporting ability of ECs by suppressing autophagy, which could be restored by activating autophagy via Beclin-1 upregulation. Moreover, autophagy positively regulated haematopoiesis-related genes in HUVECs. Subsequently, a prospective case-control study demonstrated that defective autophagy reduced Beclin-1 expression and the colony-forming unit (CFU) plating efficiency in BM ECs from PGF patients compared to matched patients with good graft function. Rapamycin, an autophagy activator, quantitatively and functionally improved BM ECs from PGF patients in vitro and enhanced their ability to support HSCs by activating the Beclin-1 pathway.

Interpretation

Our results suggest that the autophagy status of ECs modulates their ability to support haematopoiesis by regulating the Beclin-1 pathway. Defective autophagy in BM ECs may be involved in the pathogenesis of PGF post-allotransplant. Rapamycin provides a promising therapeutic approach for PGF patients.

Funding

Please see funding sources.

Rapamycin downregulates NKG2D ligands in acute myeloid leukemia cells via an activation of the STAT3 pathway: a potential mechanism for rapamycin-induced immune escape in leukemia

Background

The constitutive activation of the mammalian target of rapamycin (mTOR) is involved in the pathogenesis of many cancers. Rapamycin (RAPA), a specific inhibitor of mTOR, has been applied to the clinical treatment of tumors, and its anti-leukemia effect has also been confirmed.

Methods

We detected apoptosis and the NKG2D ligands expression in acute myeloid leukemia (AML) cells using flow cytometry and investigated the cytotoxicity of AML cells that had been co-cultured with natural killer (NK) cells using CFSE staining. We evaluated the signal pathways with a western blot assay.

Results

In this study, we found that RAPA can significantly inhibit the proliferation of AML cells. Further studies showed that the use of RAPA alone reduced the expression of NKG2D ligands on the membranes of HL-60 and THP-1 AML cells. Also, RAPA blocked the upregulation of the NKG2D ligand when AML cells were cultured with the demethylation drug decitabine (DAC). We found that RAPA decreased the expression of the NKG2D ligands by inducing the STAT3 phosphorylation of AML cells.

Conclusions

The discovery of this mechanism might further optimize the clinical use of RAPA for the treatment of AML.

Targeting of mTORC1/2 by dihydroevocarpine induces cytotoxicity in acute myeloid leukemia

Interactions between the tumor cells and bone marrow (BM) microenvironment promote survival, growth, and chemoresistance of acute myeloid leukemia (AML). The mTOR pathway plays a key role in mediating the AML-BM microenvironment interactions. Here, we report the anti-AML activity of a natural monomer extracted from the Chinese medicinal herb Evodia rutaecarpa, dihydroevocarpine. Our results showed that dihydroevocarpine-induced cytotoxicity, apoptosis, and G0/G1 arrest in AML cells, and inhibited the tumor growth in an AML xenograft model. Importantly, our study revealed that the dihydroevocarpine treatment inhibited the mTOR pathway via suppressing the mTORC1/2 activity, and thus overcame the protective effect of the BM microenvironment on AML cells. Taken together, our findings suggest that dihydroevocarpine could be used as a potential anti-AML agent alone or a therapeutic adjunct in AML therapy, particularly in the presence of the BM microenvironment.

Dueling for dual inhibition: Means to enhance effectiveness of PI3K/Akt/mTOR inhibitors in AML

The phosphatidylinositol 3-kinase/protein kinase B (Akt)/mechanistic target of rapamycin (PI3K/Akt/mTOR) pathway is amplified in 60-80% of patients with acute myelogenous leukemia (AML). Since this complex pathway is crucial to cell functions such as growth, proliferation, and survival, inhibition of this pathway would be postulated to inhibit leukemia initiation and propagation. Inhibition of the mTORC1 pathway has met with limited success in AML due to multiple resistance mechanisms including direct insensitivity of the mTORC1 complex, feedback activation of the PI3k/Akt signaling network, insulin growth factor-1 (IGF-1) activation of PI3K, and others. This review explores the role of mTOR inhibition in AML, mechanisms of resistance, and means to improve outcomes through use of dual mTORC1/2 inhibitors or dual TORC/PI3K inhibitors. How these inhibitors interface with currently available therapies in AML will require additional preclinical experiments and conduct of well-designed clinical trials.

Targeting the Akt, GSK-3, Bcl-2 axis in acute myeloid leukemia

Over the last few decades, there has been significant progress in the understanding of the pathogenetic mechanisms of the Acute Myeloid Leukemia (AML). However, despite important advances in elucidating molecular mechanisms, the treatment of AML has not improved significantly, remaining anchored at the standard chemotherapy regimen "3 + 7", with the prognosis of patients remaining severe, especially for the elderly and for those not eligible for transplant procedures. The biological and clinical heterogeneity of AML represents the major obstacle that hinders the improvement of prognosis and the identification of new effective therapeutic approaches. To date, abundant information has been collected on the genetic and molecular alterations of AML carrying prognostic significance. However, not enough is known on how AML progenitors regulate proliferation and survival by redundant and cross-talking signal transduction pathways (STP). Furthermore, it remains unclear how such complicated network affects prognosis and therapeutic treatment options, although many of these molecular determinants are potentially attractive for their druggable characteristics. In this review, some of the key STP frequently deregulated in AML, such as PI3k/Akt/mTOR pathway, GSK3 and components of Bcl-2 family of proteins, are summarized, highlighting in addition their interplay. Based on this information, we reviewed new targeted therapeutic approaches, focusing on the aberrant networks that sustain the AML blast proliferation, survival and drug resistance, aiming to improve disease treatment. Finally, we reported the approaches aimed at disrupting key signaling cross-talk overcoming resistances based on the combination of different targeting therapeutic strategies.

Inhibition of mTOR kinase as a therapeutic target for acute myeloid leukemia

INTRODUCTION

Acute myeloid leukemia (AML), the most common acute leukemia in adults, remains a therapeutic challenge. The phosphatidylinositol 3-kinase/AKT/mammalian target of rapamycin (PI3K/AKT/mTOR) signaling pathway is one of the key aberrant intracellular axes involved in AML. Areas covered: mTOR plays a critical role in sensing and responding to environmental determinants such as nutrient availability, stress, and growth factor concentrations; and in modulating key cellular functions such as proliferation, metabolism, and survival. Although abnormalities of mTOR signaling are strongly associated with neoplastic leukemic proliferation, the role of pharmacologic inhibitors of mTOR in the treatment of AML has not been established. Expert opinion: Inhibition of mTOR signaling has in general modest growth-inhibitory effects in preclinical AML models and clinical trials. Yet, combination of allosteric mTOR inhibitors with standard chemotherapy or targeted agents has a greater anti-leukemia efficacy. In turn, dual mTORC1/2 inhibitors, and dual PI3K/mTOR inhibitors show greater activity in pre-clinical AML models. Further, understanding the role of mTOR signaling in stemness of leukemias is important because AML stem cells may become chemoresistant by displaying aberrant signaling molecules, modifying epigenetic mechanisms, and altering the components of the bone marrow microenvironment.

m-TOR inhibitors and their potential role in haematological malignancies

It is widely demonstrated that the PI3K-AKT-mTOR signalling is critical in normal myeloid and lymphoid development and function. Thus, it is not strange that this pathway is often deregulated in haematological tumours, providing a strong preclinical rationale for the use of drugs targeting the PI3K-AKT-mTOR axis in haematological malignancies. The main focus of this review is to examine the mammalian target of rapamycin (mTOR, also termed mechanistic target of rapamycin [MTOR]) signalling pathways and to provide a brief overview of rapalogs and second-generation mTOR inhibitors used to target its aberrant activation in cancer treatment. We will also discuss the results obtained with the use of these agents in patients with acute leukaemia, Hodgkin lymphoma, non-Hodgkin lymphomas, multiple myeloma and Waldenström macroglobulinaemia. Ongoing clinical trials in haematological malignancies that are investigating first- and second-generation mTOR inhibitors as single agents and as components of combination regimens are also presented.

The mTOR inhibitor everolimus in combination with azacitidine in patients with relapsed/refractory acute myeloid leukemia: a phase Ib/II study

Therapeutic options are limited in relapsed/refractory acute myeloid leukemia (AML). We evaluated the maximum tolerated dose (MTD) and preliminary efficacy of mammalian target of rapamycin (mTOR) inhibitor, everolimus (days 5–21) in combination with azacitidine 75 mg/m² subcutaneously (days 1–5 and 8–9 every 28 days) in 40 patients with relapsed (n = 27), primary refractory (n = 11) or elderly patients unfit for intensive chemotherapy (n = 2). MTD was not reached following everolimus dose escalation (2.5, 5 or 10 mg; n = 19) to the 10 mg dose level which was expanded (n = 21). Major adverse events (grade > 2) were mostly disease-related: neutropenia (73%), thrombocytopenia (67%), mucositis (24%) and febrile neutropenia (19%). Overall survival (OS) of the entire cohort was 8.5 months, and overall response rate (ORR; including CR/CRi/PR/MLFS) was 22.5%. Furthermore, a landmark analysis beyond cycle 1 revealed superior OS and ORR in patients receiving 2.5 mg everolimus with azoles, compared to those without azoles (median OS 12.8 vs. 6.0 months, P = 0.049, and ORR 50% vs. 16%, P = 0.056), potentially due to achievement of higher everolimus blood levels. This study demonstrates that everolimus in combination with azacitidine is tolerable, with promising clinical activity in advanced AML.

Targeting mTOR signaling pathways and related negative feedback loops for the treatment of acute myeloid leukemia

An accumulating understanding of the complex pathogenesis of acute myeloid leukemia (AML) continues to lead to promising therapeutic approaches. Among the key aberrant intracellular signaling pathways involved in AML, the phosphatidylinositol 3-kinase/AKT/mammalian target of rapamycin (PI3K/AKT/mTOR) axis is of major interest. This axis modulates a wide array of critical cellular functions, including proliferation, metabolism, and survival. Pharmacologic inhibitors of components of this pathway have been developed over the past decade, but none has an established role in the treatment of AML. This review will discuss the preclinical data and clinical results driving ongoing attempts to exploit the PI3K/AKT/mTOR pathway in patients with AML and address issues related to negative feedback loops that account for leukemic cell survival. Targeting the PI3K/AKT/mTOR pathway is of high interest for the treatment of AML, but combination therapies with other targeted agents may be needed to block negative feedback loops in leukemia cells.

Acute myeloid leukemia: potential for new therapeutic approaches targeting mRNA translation pathways

Despite advances in molecular research related to acute myeloid leukemia (AML) and a better understanding of the mechanisms of leukemogenesis and pathophysiology of the disease, the pharmacological agents used in the treatment of AML have remained essentially unchanged for the last three decades. Advances in the clinical management of AML patients have been achieved by defining better molecular prognostic markers, but there remains a need for new targeted drugs that disrupt non-overlapping pathways in leukemia cells. The mTOR cellular cascade is critical for cell metabolism, growth, proliferation and survival. Extensive preclinical work suggests that targeting mTOR may provide a powerful approach to block AML precursor cells, while other findings suggest enhanced antileukemic effects by combining mTOR inhibitors with traditional chemotherapy. Such combinations may increase antileukemic responses further, offering unique ways to overcome leukemic cell resistance and to eliminate primitive leukemic precursors.

The prevalent predicament of relapsed acute myeloid leukemia

A majority of patients with acute myeloid leukemia (AML) will relapse after achieving complete remission. At relapse, patients should be risk stratified and a decision made about the appropriateness of intensive therapy and whether a potentially curative allogeneic stem cell transplantation (allo-SCT) is possible. Risk factors include duration of first complete remission and adverse cytogenetics, as well as age and FLT3 mutation status. Available therapies are steadily increasing, but for the most part should be regarded as either best palliation or as a bridge to allo-SCT. Simple symptomatic therapies for patients with extreme age or the worst prognosis should also be considered. Newer therapeutic options include novel cytotoxic chemotherapies including clofarabine, immunomodulatory agents, targeted therapies against FLT3 and mTOR, and immunoconjugates. All patients with relapsed AML should be considered for an appropriate clinical trial.

Two hits are better than one: targeting both phosphatidylinositol 3-kinase and mammalian target of rapamycin as a therapeutic strategy for acute leukemia treatment

Phosphatidylinositol 3-kinase (PI3K) and mammalian target of rapamycin (mTOR) are two key components of the PI3K/Akt/mTOR signaling pathway. This signal transduction cascade regulates a wide range of physiological cell processes, that include differentiation, proliferation, apoptosis, autophagy, metabolism, motility, and exocytosis. However, constitutively active PI3K/Akt/mTOR signaling characterizes many types of tumors where it negatively influences response to therapeutic treatments. Hence, targeting PI3K/Akt/mTOR signaling with small molecule inhibitors may improve cancer patient outcome. The PI3K/Akt/mTOR signaling cascade is overactive in acute leukemias, where it correlates with enhanced drug-resistance and poor prognosis. The catalytic sites of PI3K and mTOR share a high degree of sequence homology. This feature has allowed the synthesis of ATP-competitive compounds targeting the catalytic site of both kinases. In preclinical models, dual PI3K/mTOR inhibitors displayed a much stronger cytotoxicity against acute leukemia cells than either PI3K inhibitors or allosteric mTOR inhibitors, such as rapamycin. At variance with rapamycin, dual PI3K/mTOR inhibitors targeted both mTOR complex 1 and mTOR complex 2, and inhibited the rapamycin-resistant phosphorylation of eukaryotic initiation factor 4E-binding protein 1, resulting in a marked inhibition of oncogenic protein translation. Therefore, they strongly reduced cell proliferation and induced an important apoptotic response. Here, we reviewed the evidence documenting that dual PI3K/mTOR inhibitors may represent a promising option for future targeted therapies of acute leukemia patients.

mTOR kinase inhibitors as a treatment strategy in hematological malignancies

The mammalian target of rapamycin (mTOR) kinase is a key element of intracellular signal transduction, responsible for the regulation of cell growth and proliferation. Since abnormal activation of the mTOR pathway was found in several tumors, including human malignancies, it may be an attractive target for antineoplastic treatment. The first identified mTOR inhibitor was rapamycin (sirolimus). Subsequently, the most potent rapamycin analogues (rapalogues), such as everolimus, temsirolimus and deforolimus, have been developed. After encouraging preclinical experiments, several clinical trials testing the rapalogues in monotherapy or in combinations with other cytotoxic agents have been conducted in patients with hematological malignancies. Results of these studies, described in this review, indicate that inhibition of the mTOR pathway may be a very promising strategy of anti-tumor treatment in several types of lymphomas and leukemias. Recently, a second generation of more effective mTOR inhibitors has been developed. These are currently being assessed in preclinical, Phase I or I/II clinical studies.

Exploring long-term protection of normal human fibroblasts and epithelial cells from chemotherapy in cell culture

Killing of proliferating normal cells limits chemotherapy of cancer. Several strategies to selectively protect normal cells were previously suggested. Here we further explored the protection of normal cells from cell cycle-specific chemotherapeutic agents such as mitotic inhibitors (MI). We focused on a long-term cell recovery (rather than on a short-term cell survival) after a 3-day exposure to MI (paclitaxel and nocodazole). In three normal human cell types (RPE, NKE, WI-38t cells) but not in cancer cells with mutant p53, pre-treatment with nutlin-3a, a non-genotoxic inducer of wt p53, caused G1 and/or G2 arrest, thus preventing lethal mitotic arrest caused by MI and allowing normal cells to recover after removal of MI. Rapamycin, an inhibitor of the nutrient-sensing mTOR pathway, potentiated the protective effect of nutlin-3a in normal cells. Also, a combination of rapamycin and metformin, an anti-diabetic drug, induced G1 and G2 arrest selectively in normal cells and thereby protected them from MI. A combination of metformin and rapamycin also protected normal cells in low glucose conditions, whereas in contrast it was cytotoxic for cancer cells. Based on these data and the analysis of the literature, we suggest that a rational combination of metformin and rapamycin can potentiate chemotherapy with mitotic inhibitors against cancer, while protecting normal cells, thus further increasing the therapeutic window.

Utility of mTOR inhibition in hematologic malignancies

The mammalian target of rapamycin (mTOR) is an intracellular serine/threonine kinase that exists as a downstream component of numerous signaling pathways. The activation of mTOR results in the production of proteins involved in cell metabolism, growth, proliferation, and angiogenesis. Aberrant activation of mTOR signaling has been identified in a number of cancers, and targeted inhibition of mTOR has been successful in achieving tumor responses, prolonging progression-free survival, and increasing overall survival in various oncologic patient populations. In particular, persistent activation of mTOR signaling has been identified in cell lines and patient samples with leukemias, Hodgkin's lymphoma (HL), non-Hodgkin's lymphoma (NHL), multiple myeloma (MM), and Waldenström's macroglobulinemia (WM). In vitro and preclinical studies using agents that inhibit mTOR signaling have demonstrated cytostatic and cytotoxic effects in these hematologic malignancies, suggesting that mTOR is a rational target for therapy in these disease states. In addition, the combination of mTOR inhibitors with traditional therapies may help to overcome the development of resistance and may improve response rates over those seen with established regimens through synergistic or additive effects. Inhibitors of mTOR signaling currently are being investigated in clinical trials of hematologic malignancies as single agents and as components of combination regimens. Thus far, promising results have been seen with the application of mTOR inhibitors as single agents in patients with relapsed or refractory leukemia, HL, NHL, MM, and WM.