Evaluation of antileukaemic effects of rapamycin in patients with imatinib-resistant chronic myeloid leukaemia

Integrated systems biology analysis of acute lymphoblastic leukemia: unveiling molecular signatures and drug repurposing opportunities

Acute lymphoblastic leukemia (ALL) is a hematological malignancy characterized by aberrant proliferation and accumulation of lymphoid precursor cells within the bone marrow. The tyrosine kinase inhibitor (TKI), imatinib mesylate, has played a significant role in the treatment of Philadelphia chromosome-positive ALL (Ph + ALL). However, the achievement of durable and sustained therapeutic success remains a challenge due to the development of TKI resistance during the clinical course.The primary objective of this investigation is to propose a novel and efficacious treatment approach through drug repositioning, targeting ALL and its Ph + subtype by identifying and addressing differentially expressed genes (DEGs). This study involves a comprehensive analysis of transcriptome datasets pertaining to ALL and Ph + ALL in order to identify DEGs associated with the progression of these diseases to identify possible repurposable drugs that target identified hub proteins.The outcomes of this research have unveiled 698 disease-related DEGs for ALL and 100 for Ph + ALL. Furthermore, a subset of drugs, specifically glipizide for Ph + ALL, and maytansine and isoprenaline for ALL, have been identified as potential candidates for therapeutic intervention. Subsequently, cytotoxicity assessments were performed to confirm the in vitro cytotoxic effects of these selected drugs on both ALL and Ph + ALL cell lines.In conclusion, this study offers a promising avenue for the management of ALL and Ph + ALL through drug repurposed drugs. Further investigations are necessary to elucidate the mechanisms underlying cell death, and clinical trials are recommended to validate the promising results obtained through drug repositioning strategies.

Overcoming Drug Resistance in Advanced Prostate Cancer by Drug Repurposing

Prostate cancer (PCa) is the second most common cancer in men. Common treatments include active surveillance, surgery, or radiation. Androgen deprivation therapy and chemotherapy are usually reserved for advanced disease or biochemical recurrence, such as castration-resistant prostate cancer (CRPC), but they are not considered curative because PCa cells eventually develop drug resistance. The latter is achieved through various cellular mechanisms that ultimately circumvent the pharmaceutical’s mode of action. The need for novel therapeutic approaches is necessary under these circumstances. An alternative way to treat PCa is by repurposing of existing drugs that were initially intended for other conditions. By extrapolating the effects of previously approved drugs to the intracellular processes of PCa, treatment options will expand. In addition, drug repurposing is cost-effective and efficient because it utilizes drugs that have already demonstrated safety and efficacy. This review catalogues the drugs that can be repurposed for PCa in preclinical studies as well as clinical trials.

Drug Repositioning in Oncology

BACKGROUND

The worldwide increase in the occurrence of cancer associated with the limitations of immunotherapy and the emergence of resistance have impaired the prognosis of cancer patients, which leads to the search for alternative treatment methods. Drug repositioning, a well-established process approved by regulatory agencies, is considered an alternative strategy for the fast identification of drugs, because it is relatively less costly and represents lower risks for patients.

AREAS OF UNCERTAINTY

We report the most relevant studies about drug repositioning in oncology, emphasizing that its implementation faces financial and regulatory obstacles, making the creation of incentives necessary to stimulate the involvement of the pharmaceutical industry.

DATA SOURCES

We present 63 studies in which 52 non-anticancer drugs with anticancer activity against a number of malignancies are described.

THERAPEUTIC INNOVATIONS

Some have already been the target of phase III studies, such as the Add-Aspirin trial for nonmetastatic solid tumors, as well as 9 other drugs (aprepitant, artesunate, auranofin, captopril, celecoxib, disulfiram, itraconazole, ritonavir, and sertraline) in the CUSP9* clinical trial for the treatment of recurrent glioblastoma. Others have already been successful in repositioning such as thalidomide, zoledronic acid, celecoxib, methotrexate, and gemcitabine.

CONCLUSIONS

Therefore, drug repositioning represents a promising alternative for the treatment of oncological disorders; however, the support from funding agencies and from the government is still needed, the latter regarding regulatory issues.

Overcoming cancer therapeutic bottleneck by drug repurposing

Ever present hurdles for the discovery of new drugs for cancer therapy have necessitated the development of the alternative strategy of drug repurposing, the development of old drugs for new therapeutic purposes. This strategy with a cost-effective way offers a rare opportunity for the treatment of human neoplastic disease, facilitating rapid clinical translation. With an increased understanding of the hallmarks of cancer and the development of various data-driven approaches, drug repurposing further promotes the holistic productivity of drug discovery and reasonably focuses on target-defined antineoplastic compounds. The "treasure trove" of non-oncology drugs should not be ignored since they could target not only known but also hitherto unknown vulnerabilities of cancer. Indeed, different from targeted drugs, these old generic drugs, usually used in a multi-target strategy may bring benefit to patients. In this review, aiming to demonstrate the full potential of drug repurposing, we present various promising repurposed non-oncology drugs for clinical cancer management and classify these candidates into their proposed administration for either mono- or drug combination therapy. We also summarize approaches used for drug repurposing and discuss the main barriers to its uptake.

A medicinal chemistry perspective of drug repositioning: Recent advances and challenges in drug discovery

Drug repurposing is a strategy consisting of finding new indications for already known marketed drugs used in various clinical settings or highly characterized compounds despite they can be failed drugs. Recently, it emerges as an alternative approach for the rapid identification and development of new pharmaceuticals for various rare and complex diseases for which lack the effective drug treatments. The success rate of drugs repurposing approach accounts for approximately 30% of new FDA approved drugs and vaccines in recent years. This review focuses on the status of drugs repurposing approach for various diseases including skin diseases, infective, inflammatory, cancer, and neurodegenerative diseases. Efforts have been made to provide structural features and mode of actions of drugs.

Old drugs, new uses: Drug repurposing in hematological malignancies

Discovery and development of novel anti-cancer drugs are expensive and time consuming. Systems biology approaches have revealed that a drug being developed for a non-cancer indication can hit other targets as well, which play critical roles in cancer progression. Since drugs for non-cancer indications would have already gone through the preclinical and partial or full clinical development, repurposing such drugs for hematological malignancies would cost much less, and drastically reduce the development time, which is evident in case of thalidomide. Here, we have reviewed some of the drugs for their potential to repurpose for treating the hematological malignancies. We have also enlisted resources that can be helpful in drug repurposing.

Targeting PI3K/AKT/mTOR-mediated autophagy for tumor therapy

Autophagy is a highly conserved catabolic process and participates in a variety of cellular biological activities. The phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) pathway, as a critical regulator of autophagy, is involved in the initiation and promotion of a series of pathological disorders including various tumors. Autophagy also participates in regulating the balance between the tumor and the tumor microenvironment. Natural products have been considered a treasure of new drug discoveries and are of great value to medicine. Mounting evidence has suggested that numerous natural products are targeting PI3K/AKT/mTOR-mediated autophagy, thereby suppressing tumor growth. Furthermore, autophagy plays a "double-edged sword" role in different tumors. Targeting PI3K/AKT/mTOR-mediated autophagy is an important therapeutic strategy for a variety of tumors, and plays important roles in enhancing the chemosensitivity of tumor cells and avoiding drug resistance. Therefore, we summarized the roles of PI3K/AKT/mTOR-mediated autophagy in tumorigenesis, progression, and drug resistance of tumors, which may be utilized to design preferably therapeutic strategies for various tumors.

Efficacy of the dual PI3K and mTOR inhibitor NVP-BEZ235 in combination with imatinib mesylate against chronic myelogenous leukemia cell lines

BACKGROUND

Phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin (PI3K/Akt/mTOR) pathway is a therapy target of cancer. We aimed to confirm the effect of dual PI3K/mTOR inhibitor NVP-BEZ235 on proliferation, apoptosis, and autophagy of chronic myelogenous leukemia (CML) cells and sensitivity of tyrosine kinase inhibitor in vitro.

METHODS

Two human CML cell lines, K562 and KBM7R (T315I mutant strain), were used. The proliferation of CML cells was detected by MTS (Owen's reagent) assay. Cell cycle and apoptosis assay were examined by flow cytometric analysis. The phosphorylation levels and the expression levels were both evaluated by Western blot analysis. NVP-BEZ235 in combination with imatinib was also used to reveal the effect on proliferation and apoptosis.

RESULTS

NVP-BEZ235 significantly inhibited the proliferation in a time- and dose-dependent manner, and the half-maximal inhibitory concentration values of NVP-BEZ235 inhibiting the proliferation of K562 and KBM7R were 0.37±0.21 and 0.43±0.27 μmol/L, respectively, after 48 h. Cell apoptosis assay showed that NVP-BEZ235 significantly increased the late apoptotic cells. Cell cycle analysis indicated that the cells were mostly arrested in G1/G0 phase after treatment by NVP-BEZ235. In addition, results also found that, after treatment by NVP-BEZ235, phosphorylation levels of Akt kinase and S6K kinase significantly reduced, and the expression levels of cleaved caspase-3 significantly increased; meanwhile, the expression levels of caspase-3, B-cell lymphoma-2, cyclin D1, and cyclin D2 significantly decreased, and the ratio of LC3II/LC3I was significantly increased with increased LC3II expression level. Moreover, imatinib in combination with NVP-BEZ235 induced a more pronounced colony growth inhibition than imatinib alone.

CONCLUSION

NVP-BEZ235 effectively inhibited cell proliferation by G0/G1 cell cycle arrest and induced apoptosis through deregulating PI3K/Akt/mTOR pathway in CML cells; in addition, NVP-BEZ235 can enhance cell autophagy, and is conducive to raising CML cell sensitivity to imatinib to inhibit the growth of imatinib-resistant cells.

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.

Targeting the mTOR Pathway in Leukemia

Optimal function of multiple intracellular signaling pathways is essential for normal regulation of cellular transcription, translation, growth, proliferation, and survival. Dysregulation or aberrant activation of such cascades can lead to inappropriate cell survival and abnormal cell proliferation in leukemia. Successful treatment of chronic myeloid leukemia (CML) with tyrosine kinase inhibitors targeting the BCR-ABL fusion gene is a prime example of effectively inhibiting intracellular signaling cascades. However, even in these patients resistance can develop via emergence of mutations or feedback activation of other pathways that cause refractory disease. Constitutive activation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR) signaling pathway has been observed in different types of leukemia, including CML, acute myeloid leukemia, and acute lymphoblastic leukemia. Abnormal mTOR activity may contribute to chemotherapy resistance, while it may also be effectively targeted via molecular means and/or development of specific pharmacological inhibitors. This review discusses the role of PI3K/Akt/mTOR dysre-gulation in leukemia and summarizes the emergence of preliminary data for the development of novel therapeutic approaches. J. Cell. Biochem. 117: 1745-1752, 2016. © 2016 Wiley Periodicals, Inc.

Cytokine Regulation of Microenvironmental Cells in Myeloproliferative Neoplasms

The term myeloproliferative neoplasms (MPN) refers to a heterogeneous group of diseases including not only polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), but also chronic myeloid leukemia (CML), and systemic mastocytosis (SM). Despite the clinical and biological differences between these diseases, common pathophysiological mechanisms have been identified in MPN. First, aberrant tyrosine kinase signaling due to somatic mutations in certain driver genes is common to these MPN. Second, alterations of the bone marrow microenvironment are found in all MPN types and have been implicated in the pathogenesis of the diseases. Finally, elevated levels of proinflammatory and microenvironment-regulating cytokines are commonly found in all MPN-variants. In this paper, we review the effects of MPN-related oncogenes on cytokine expression and release and describe common as well as distinct pathogenetic mechanisms underlying microenvironmental changes in various MPN. Furthermore, targeting of the microenvironment in MPN is discussed. Such novel therapies may enhance the efficacy and may overcome resistance to established tyrosine kinase inhibitor treatment in these patients. Nevertheless, additional basic studies on the complex interplay of neoplastic and stromal cells are required in order to optimize targeting strategies and to translate these concepts into clinical application.

Post-transplantation Cyclophosphamide and Sirolimus after Haploidentical Hematopoietic Stem Cell Transplantation Using a Treosulfan-based Myeloablative Conditioning and Peripheral Blood Stem Cells

Haploidentical hematopoietic stem cell transplantation (HSCT) performed using bone marrow (BM) grafts and post-transplantation cyclophosphamide (PTCy) has gained much interest for the excellent toxicity profile after both reduced-intensity and myeloablative conditioning. We investigated, in a cohort of 40 high-risk hematological patients, the feasibility of peripheral blood stem cells grafts after a treosulfan-melphalan myeloablative conditioning, followed by a PTCy and sirolimus-based graft-versus-host disease (GVHD) prophylaxis (Sir-PTCy). Donor engraftment occurred in all patients, with full donor chimerism achieved by day 30. Post-HSCT recovery of lymphocyte subsets was broad and fast, with a median time to CD4 > 200/μL of 41 days. Cumulative incidences of grade II to IV and III-IV acute GVHD were 15% and 7.5%, respectively, and were associated with a significant early increase in circulating regulatory T cells at day 15 after HSCT, with values < 5% being predictive of subsequent GVHD occurrence. The 1-year cumulative incidence of chronic GVHD was 20%. Nonrelapse mortality (NRM) at 100 days and 1 year were 12% and 17%, respectively. With a median follow-up for living patients of 15 months, the estimated 1-year overall and disease-free survival (DFS) was 56% and 48%, respectively. Outcomes were more favorable in patients who underwent transplantation in complete remission (1-year DFS 71%) versus patients who underwent transplantation with active disease (DFS, 34%; P = .01). Overall, myeloablative haploidentical HSCT with peripheral blood stem cells (PBSC) and Sir-PTCy is a feasible treatment option: the low rates of GVHD and NRM as well as the favorable immune reconstitution profile pave the way for a prospective comparative trial comparing BM and PBSC in this specific transplantation setting.

Novel drug therapies in myeloid leukemia: a patent review

Both acute myeloid leukemia and chronic myeloid leukemia are thought to arise from a subpopulation of primitive cells, termed leukemic stem cells that share properties with somatic stem cells. Leukemic stem cells are capable of continued self-renewal, and are resistant to conventional chemotherapy and are considered to be responsible for disease relapse. In recent years, improved understanding of the underlying mechanisms of myeloid leukemia biology has led to the development of novel and targeted therapies. This review focuses on clinically relevant patent applications and their relevance within the known literature in two areas of prevailing therapeutic interest, namely monoclonal antibody therapy and small molecule inhibitors in disease-relevant signaling pathways.

Prediction of drug indications based on chemical interactions and chemical similarities

Discovering potential indications of novel or approved drugs is a key step in drug development. Previous computational approaches could be categorized into disease-centric and drug-centric based on the starting point of the issues or small-scaled application and large-scale application according to the diversity of the datasets. Here, a classifier has been constructed to predict the indications of a drug based on the assumption that interactive/associated drugs or drugs with similar structures are more likely to target the same diseases using a large drug indication dataset. To examine the classifier, it was conducted on a dataset with 1,573 drugs retrieved from Comprehensive Medicinal Chemistry database for five times, evaluated by 5-fold cross-validation, yielding five 1st order prediction accuracies that were all approximately 51.48%. Meanwhile, the model yielded an accuracy rate of 50.00% for the 1st order prediction by independent test on a dataset with 32 other drugs in which drug repositioning has been confirmed. Interestingly, some clinically repurposed drug indications that were not included in the datasets are successfully identified by our method. These results suggest that our method may become a useful tool to associate novel molecules with new indications or alternative indications with existing drugs.

Calcineurin inhibitor--free graft-versus-host disease prophylaxis with post-transplantation cyclophosphamide and brief-course sirolimus following reduced-intensity peripheral blood stem cell transplantation

Calcineurin inhibitors (CNIs) form the foundation of current graft-versus-host disease (GVHD) prophylaxis regimens. We hypothesized that a CNI-free regimen consisting of post-transplantation cyclophosphamide (PTCy) and brief-course sirolimus would reduce chronic GVHD and nonrelapse mortality (NRM) after reduced-intensity conditioning allogeneic peripheral blood stem cell transplantation (PBSCT). Twenty-six patients (median age, 61 years) underwent unmanipulated PBSCT from an 8/8 locus-matched donor (matched related donor, n = 17; natched unrelated donor, n = 9). GVHD prophylaxis consisted of PTCy and brief-course sirolimus. Donor engraftment occurred in all patients. The cumulative incidence (CI) of grade II-IV acute GVHD, grade III-IV acute GVHD, and chronic GVHD was 46%, 15%, and 31% respectively. One-year NRM was 4%. The median time to immunosuppression discontinuation was day +138. With a median follow-up of 20 months, the estimated 2-year overall survival was 71%, estimated disease-free survival was 64%, and estimated relapse incidence was 32%. In patients with a lymphoid malignancy (eg, chronic lymphoblastic leukemia, non-Hodgkin lymphoma, Hodgkin disease), 2-year disease-free survival was 100%, and there were no relapses. Good immune reconstitution was evidenced by low cytomegalovirus reactivation rate of 21% (4 of 19 at-risk patients). GVHD prophylaxis with PTCy and sirolimus achieves consistent donor engraftment, low rates of chronic GVHD and NRM, and excellent outcomes in recipients of HLA-identical related and unrelated donor allogeneic PBSCT.

Rapamycin combined with celecoxib enhanced antitumor effects of mono treatment on chronic myelogenous leukemia cells through downregulating mTOR pathway

Chronic myelogenous leukemia is a neoplasm of myeloid progenitor cells. We recently found that rapamycin could induce G0/G1 phase arrest and apoptosis and inhibit proliferation of K562 cells through inhibiting mammalian target of rapamycin (mTOR) pathway. However, whether rapamycin has synergistic effects with other drugs in chronic myelogenous leukemia (CML) therapies remain unclear. Therefore, we examined the effect of rapamycin combined with celecoxib on K562 cells in vitro. The survival rates showed a significant decrease in rapamycin + celecoxib treatment group. The combination treatment also increased the G0/G1 phase cells as compared to rapamycin or celecoxib treatment alone (P < 0.05), accompanied with the decreased population of S phase cells. Meanwhile, the rate of apoptosis was 15.87 ± 2.21 % in rapamycin + celecoxib treatment group, significantly higher than that in mono treatment group (P < 0.05). Western blot and reverse transcription PCR (RT-PCR) analysis showed that the expressions of mTOR, 4E-BP1, and p70S6K were all significantly decreased in K562 cells after rapamycin + celecoxib treatment (P < 0.05). In conclusion, rapamycin combined with celecoxib could induce cell cycle arrest and apoptosis and decrease the expressions of mTOR, 4E-BP1, and p70S6K. It suggested that the combination could enhance the antitumor effects of mono treatment on CML cells through downregulating mTOR pathway.

Targeting survival pathways in chronic myeloid leukaemia stem cells

Chronic myeloid leukaemia (CML) is a clonal myeloproliferative disorder characterized by the presence of a fusion oncogene BCR-ABL, which encodes a protein with constitutive TK activity. The implementation of tyrosine kinase inhibitors (TKIs) marked a major advance in CML therapy; however, there are problems with current treatment. For example, relapse occurs when these drugs are discontinued in the majority of patients who have achieved a complete molecular response on TKI and these agents are less effective in patients with mutations in the BCR-ABL kinase domain. Importantly, TKI can effectively target proliferating mature cells, but do not eradicate quiescent leukaemic stem cells (LSCs), therefore allowing disease persistence despite treatment. It is essential that alternative strategies are used to target the LSC population. BCR-ABL activation is responsible for the modulation of different signalling pathways, which allows the LSC fraction to evade cell death. Several pathways have been shown to be modulated by BCR-ABL, including PI3K/AKT/mTOR, JAK-STAT and autophagy signalling pathways. Targeting components of these survival pathways, alone or in combination with TKI, therefore represents an attractive potential therapeutic approach for targeting the LSC. However, many pathways are also active in normal stem cells. Therefore, potential targets must be validated to effectively eradicate CML stem cells while sparing normal counterparts. This review summarizes the main pathways modulated in CML stem cells, the recent developments and the use of novel drugs to target components in these pathways which may be used to target the LSC population.

LINKED ARTICLES

This article is part of a themed section on Emerging Therapeutic Aspects in Oncology. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.169.issue-8.

Efficacy of the dual PI3K and mTOR inhibitor NVP-BEZ235 in combination with nilotinib against BCR-ABL-positive leukemia cells involves the ABL kinase domain mutation

Imatinib, an ABL tyrosine kinase inhibitor (TKI), has shown clinical efficacy against chronic myeloid leukemia (CML). However, a substantial number of patients develop resistance to imatinib treatment due to the emergence of clones carrying mutations in the protein BCR-ABL. The phosphoinositide 3 kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway regulates various processes, including cell proliferation, cell survival, and antiapoptosis activity. In this study, we investigated the efficacy of NVP-BEZ235, a dual PI3K and mTOR inhibitor, using BCR-ABL-positive cell lines. Treatment with NVP-BEZ235 for 48 h inhibited cell growth and induced apoptosis. The phosphorylation of the AKT kinase, eukaryotic initiation factor 4-binding protein 1 (4E-BP1), and p70 S6 kinase were decreased after NVP-BEZ235 treatment. The combination of NVP-BEZ235 with a BCR-ABL kinase inhibitor, imatinib, or nilotinib, induced a more pronounced colony growth inhibition, whereas the combination of NVP-BEZ235 and nilotinib was more effective in inducing apoptosis and reducing the phosphorylation of AKT, 4E-BP1, and S6 kinase. NVP-BEZ235 in combination with nilotinib also inhibited tumor growth in a xenograft model and inhibited the growth of primary T315I mutant cells and ponatinib-resistant cells. Taken together, these results suggest that administration of the dual PI3K and mTOR inhibitor NVP-BEZ235 may be an effective strategy against BCR-ABL mutant cells and may enhance the cytotoxic effects of nilotinib in ABL TKI-resistant BCR-ABL mutant cells.

PI3K/mTOR pathway inhibitors sensitize chronic myeloid leukemia stem cells to nilotinib and restore the response of progenitors to nilotinib in the presence of stem cell factor

Nilotinib is a second-generation tyrosine kinase inhibitor, designed to specifically inhibit break-point cluster region (BCR)-Abelson (ABL) and developed to treat chronic myeloid leukemia (CML) in patients showing a resistance to imatinib. We previously demonstrated that nilotinib-induced apoptosis was reduced by stem cell factor (SCF) addition. Here, the SCF-activated survival pathway was investigated. BCR-ABL expression was accompanied by the activation of the SCF receptor: c-KIT. Nilotinib inhibited this activation that was restored by SCF binding. Parallel variations were observed for mammaliam target of rapamycin (mTOR) kinase and mTOR complex 1 substrate S6K. The inhibition of mTORC1 restored the response of BCR-ABL cell lines to nilotinib in the presence of SCF. PI3K inhibition restored nilotinib-induced apoptosis. On hematopoietic progenitors from CML patient's bone marrows, mTORC1 inhibition also restored nilotinib sensitivity in the presence of SCF, confirming its involvement in SCF-activated survival pathway. However, this pathway seems not to be involved in the nilotinib-induced resistance of the CML stem cell population. Conversely, PI3K inhibition sensitized both CML progenitors and stem cells to nilotinib, suggesting that, downstream PI3K, two different kinase pathways are activated in CML progenitor and stem cell populations.

Cancer drug discovery by repurposing: teaching new tricks to old dogs

Progressively increasing failure rates, high cost, poor bioavailability, poor safety, limited efficacy, and a lengthy design and testing process associated with cancer drug development have necessitated alternative approaches to drug discovery. Exploring established non-cancer drugs for anticancer activity provides an opportunity rapidly to advance therapeutic strategies into clinical trials. The impetus for development of cancer therapeutics from non-cancer drugs stems from the fact that different diseases share common molecular pathways and targets in the cell. Common molecular origins of diverse diseases have been discovered through advancements in genomics, proteomics, and informatics technologies, as well as through the development of analytical tools that allow researchers simultaneously to screen large numbers of existing drugs against a particular disease target. Thus, drugs originally identified as antitussive, sedative, analgesic, antipyretic, antiarthritic, anesthetic, antidiabetic, muscle relaxant, immunosuppressant, antibiotic, antiepileptic, cardioprotective, antihypertensive, erectile function enhancing, or angina relieving are being repurposed for cancer. This review describes the repurposing of these drugs for cancer treatment.

Targeting the PI3K/AKT/mTOR signaling axis in children with hematologic malignancies

The phosphatidylinositiol 3-kinase (PI3K), AKT, mammalian target of rapamycin (mTOR) signaling pathway (PI3K/AKT/mTOR) is frequently dysregulated in disorders of cell growth and survival, including a number of pediatric hematologic malignancies. The pathway can be abnormally activated in childhood acute lymphoblastic leukemia (ALL), acute myelogenous leukemia (AML), and chronic myelogenous leukemia (CML), as well as in some pediatric lymphomas and lymphoproliferative disorders. Most commonly, this abnormal activation occurs as a consequence of constitutive activation of AKT, providing a compelling rationale to target this pathway in many of these conditions. A variety of agents, beginning with the rapamycin analogue (rapalog) sirolimus, have been used successfully to target this pathway in a number of pediatric hematologic malignancies. Rapalogs demonstrate significant preclinical activity against ALL, which has led to a number of clinical trials. Moreover, rapalogs can synergize with a number of conventional cytotoxic agents and overcome pathways of chemotherapeutic resistance for drugs commonly used in ALL treatment, including methotrexate and corticosteroids. Based on preclinical data, rapalogs are also being studied in AML, CML, and non-Hodgkin's lymphoma. Recently, significant progress has been made using rapalogs to treat pre-malignant lymphoproliferative disorders, including the autoimmune lymphoproliferative syndrome (ALPS); complete remissions in children with otherwise therapy-resistant disease have been seen. Rapalogs only block one component of the pathway (mTORC1), and newer agents are under preclinical and clinical development that can target different and often multiple protein kinases in the PI3K/AKT/mTOR pathway. Most of these agents have been tolerated in early-phase clinical trials. A number of PI3K inhibitors are under investigation. Of note, most of these also target other protein kinases. Newer agents are under development that target both mTORC1 and mTORC2, mTORC1 and PI3K, and the triad of PI3K, mTORC1, and mTORC2. Preclinical data suggest these dual- and multi-kinase inhibitors are more potent than rapalogs against many of the aforementioned hematologic malignancies. Two classes of AKT inhibitors are under development, the alkyl-lysophospholipids (APLs) and small molecule AKT inhibitors. Both classes have agents currently in clinical trials. A number of drugs are in development that target other components of the pathway, including eukaryotic translation initiation factor (eIF) 4E (eIF4E) and phosphoinositide-dependent protein kinase 1 (PDK1). Finally, a number of other key signaling pathways interact with PI3K/AKT/mTOR, including Notch, MNK, Syk, MAPK, and aurora kinase. These alternative pathways are being targeted alone and in combination with PI3K/AKT/mTOR inhibitors with promising preclinical results in pediatric hematologic malignancies. This review provides a comprehensive overview of the abnormalities in the PI3K/AKT/mTOR signaling pathway in pediatric hematologic malignancies, the agents that are used to target this pathway, and the results of preclinical and clinical trials, using those agents in childhood hematologic cancers.

Mechanisms and novel approaches in overriding tyrosine kinase inhibitor resistance in chronic myeloid leukemia

Chronic myeloid leukemia is a stem cell-initiated but progenitor-driven disease induced by the BCR-ABL oncogene. Tyrosine kinase inhibitors (TKIs) were introduced in the late 1990s and have revolutionized the management of chronic myeloid leukemia in chronic phase. The majority of patients can now expect to live a normal life as long as they continue to comply with TKI treatment. However, in a significant proportion of cases TKI resistance develops over time, requiring a switch of therapy. The most frequent mechanism for drug resistance is the development of kinase domain mutations that reduce or completely ablate drug efficacy. Fortunately, the last 10 years have seen an impressive array of new drugs, some modeled on the mechanism of action of imatinib, others employing more novel approaches, for these 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.

Molecular biology of chronic myeloid leukemia

Detailed information on the crystal structure of the pharmacologically targeted domains of the BCR-ABL molecule and on its intracellular signaling, which are potentially involved in growth, anti-apoptosis, metabolism and stemness, has made the study of chronic myeloid leukemia the most successful field in tumor biology. However, we now face the issue of drug resistance due to deregulation in the quality control of both DNA and protein. BCR-ABL is basically a misfolded protein with intrinsically disordered regions, which not only produces endoplasmic reticulum stress followed by unfolded protein response in some settings, but also conformational plasticity that may affect the structure of the whole molecule. The intercellular signaling derived from the leukemic cell microenvironment may influence the intracellular responses that take place in a manner both dependent on and independent of BCR-ABL tyrosine kinase activity.

An overview of the mTOR pathway as a target in cancer therapy

INTRODUCTION

The mammalian target of rapamycin (mTOR) signaling cascade is a key regulatory pathway controlling initiation of mRNA translation in mammalian cells. The mTOR inhibitor rapamycin and its derivatives have shown potent antineoplastic activities in many preclinical models and clinical trials. First-generation mTOR inhibitors are now FDA-approved for the treatment of renal cell carcinoma.

AREAS COVERED

This article reviews the components of the mTOR pathway and their normal functions, highlighting the most common alterations in the pathway, seen in various human malignancies. It also discusses elements and effectors of this signaling cascade and reviews the therapeutic relevance of pharmacological inhibitors of the pathway in several malignancies, including lymphomas, leukemias, sarcomas, renal cell carcinoma, and breast cancer.

EXPERT OPINION

mTOR targeting is a highly promising therapeutic approach. First-generation mTOR inhibitors have already shown substantial activity in the treatment of certain tumors, while the emergence of second-generation catalytic mTOR inhibitors provides a better approach to target the pathway in malignant cells and has raised the potential for better clinical outcomes in the future.

The PI3K/PKB signaling module as key regulator of hematopoiesis: implications for therapeutic strategies in leukemia

An important mediator of cytokine signaling implicated in regulation of hematopoiesis is the PI3K/protein kinase B (PKB/c-Akt) signaling module. Constitutive activation of this signaling module has been observed in a large group of leukemias. Because activation of this signaling pathway has been demonstrated to be sufficient to induce hematologic malignancies and is thought to correlate with poor prognosis and enhanced drug resistance, it is considered to be a promising target for therapy. A high number of pharmacologic inhibitors directed against either individual or multiple components of this pathway have already been developed to improve therapy. In this review, the safety and efficacy of both single and dual-specificity inhibitors will be discussed as well as the potential of combination therapy with either inhibitors directed against other signal transduction molecules or classic chemotherapy.

Successful treatment of a chronic-phase T-315I-mutated chronic myelogenous leukemia patient with a combination of imatinib and interferon-alfa

The T315I BCR-ABL mutation in chronic myelogenous leukemia (CML) patients is responsible for up to 20% of all clinically observed resistance. This mutation confers resistance not only to imatinib, but also to second-generation BCR-ABL tyrosine kinases, such as nilotinib and dasatinib. A number of strategies have been implemented to overcome this resistance, but allogeneic stem cell transplantation remains the only established therapeutic option for a cure. A 61-year-old male was diagnosed with Philadelphia chromosome-positive chronic-phase CML in 2002. He was initially treated with imatinib and complete cytogenetic response (CCyR) was achieved 12 months later. However, after 18 months, a loss of CCyR was observed and a molecular study at 24 months revealed a T315I mutation of the BCR-ABL gene. At 30 months, imatinib/interferon-alfa (IFNα) combination therapy was initiated in an effort to overcome the resistance. Thirty months later, he re-achieved CCyR, and the T315I BCR-ABL mutation disappeared at 51 months. To our knowledge, this is the first case report showing the effectiveness of imatinib/IFNα combination therapy for CML patients bearing the T315I BCR-ABL mutation.

[m-TOR inhibitors: biology and use in the treatment of haematological diseases]

Mammalian target of rapamycyin (mTOR) is a downstream serine/threonine kinase of the PI3K/AKT pathway that integrates signals from the microenvironment such as cytokines, growth factors, and nutriments to regulate multiple cellular processes, including mRNA translation, autophagy, metabolism, growth and survival. mTOR operates in two distinct multi-protein complexes: mTORC1 and mTORC2; sharing mTOR kinase as a common catalytic subunit, mTORC1 controls cell growth and mTORC2 modulates cell survival and drug resistance. mTOR signalling pathway has been found to be deregulated in many haematological malignancies, and has been designed as an attractive anti-tumor target. Thereby, mTOR inhibition with rapamycin (sirolimus) or its derivates (rapalogs) represents promising treatments, either alone or in combination with strategies to target other pathways that may overcome resistance. At present time, numerous clinical trials with mTOR inhibitors are ongoing for treatment of haematological diseases with modest or promising results. The aim of this review is to present the rationale for using mTOR inhibitors in haematology, first via biological explanations and secondly, by focusing on each haematological malignancies with new perspective of treatment.

Recent advances in anti-angiogenic therapy of cancer

Since angiogenesis is critical for tumor growth and metastasis, anti-angiogenic treatment is a highly promising therapeutic approach. Thus, for over last couple of decades, there has been a robust activity aimed towards the discovery of angiogenesis inhibitors. More than forty anti-angiogenic drugs are being tested in clinical trials all over the world. This review discusses agents that have approved by the FDA and are currently in use for treating patients either as single-agents or in combination with other chemotherapeutic agents.

Targeting the Mammalian Target of Rapamycin (mTOR) in Cancer Therapy: Lessons from Past and Future Perspectives

Over the last decade, extensive studies have been made to understand the role played by the mammalian target of rapamycin (mTOR) in cancer. Knowledge in this field has been gained from discoveries in basic research as well as from observations made in patients treated with allosteric mTOR inhibitors such as rapamycin. Despite promising preclinical studies, targeting mTOR in cancer therapy has shown limited clinical benefits so far. However, recent findings have revealed the complexity of the functions of mTOR in cancer and have helped develop new strategies to improve the anticancer efficacy of mTOR inhibitors. In particular, a complex network between mTOR and other signaling pathways has been identified that influences the anticancer efficacy of mTOR inhibitors. In addition, an emerging role of mTOR in the tumor microenvironment has been suggested. In this review, we confront the major findings that have been made in the past, both in experimental settings as well as in clinical trials. We further review the strategies that have been designed to further improve the efficacy of therapies targeting mTOR.

Regulation of mammalian target of rapamycin and mitogen activated protein kinase pathways by BCR-ABL

A large body of evidence has established that BCR-ABL regulates engagement and activation of mammalian target of rapamycin (mTOR) and mitogen activated protein kinase (MAPK) signaling cascades. mTOR-mediated signals, as well as signals transduced by ERK, JNK, and p38 MAPK, are important components of the aberrant signaling induced by BCR-ABL. Such deregulation of mTOR or MAPK pathways contributes to BCR-ABL leukemogenesis, and their targeting with selective inhibitors provides an approach to enhance antileukemic responses and/or overcome leukemic cell resistance in chronic myeloid leukemia (CML) and Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL). This review explores recent advances in our understanding of mTOR and MAPK signaling in BCR-ABL-expressing leukemias and discusses the potential therapeutic targeting of these pathways in CML and Ph+ ALL.

BCR-ABL1-independent PI3Kinase activation causing imatinib-resistance

BACKGROUND

The BCR-ABL1 translocation occurs in chronic myeloid leukemia (CML) and in 25% of cases with acute lymphoblastic leukemia (ALL). The advent of tyrosine kinase inhibitors (TKI) has fundamentally changed the treatment of CML. However, TKI are not equally effective for treating ALL. Furthermore, de novo or secondary TKI-resistance is a significant problem in CML. We screened a panel of BCR-ABL1 positive ALL and CML cell lines to find models for imatinib-resistance.

RESULTS

Five of 19 BCR-ABL1 positive cell lines were resistant to imatinib-induced apoptosis (KCL-22, MHH-TALL1, NALM-1, SD-1, SUP-B15). None of the resistant cell lines carried mutations in the kinase domain of BCR-ABL1 and all showed resistance to second generation TKI, nilotinib or dasatinib. STAT5, ERK1/2 and the ribosomal S6 protein (RPS6) are BCR-ABL1 downstream effectors, and all three proteins are dephosphorylated by imatinib in sensitive cell lines. TKI-resistant phosphorylation of RPS6, but responsiveness as regards JAK/STAT5 and ERK1/2 signalling were characteristic for resistant cell lines. PI3K pathway inhibitors effected dephosphorylation of RPS6 in imatinib-resistant cell lines suggesting that an oncogene other than BCR-ABL1 might be responsible for activation of the PI3K/AKT1/mTOR pathway, which would explain the TKI resistance of these cells. We show that the TKI-resistant cell line KCL-22 carries a PI3Kα E545G mutation, a site critical for the constitutive activation of the PI3K/AKT1 pathway. Apoptosis in TKI-resistant cells could be induced by inhibition of AKT1, but not of mTOR.

CONCLUSION

We introduce five Philadelphia-chromosome positive cell lines as TKI-resistance models. None of these cell lines carries mutations in the kinase domain of BCR-ABL1 or other molecular aberrations previously indicted in the context of imatinib-resistance. These cell lines are unique as they dephosphorylate ERK1/2 and STAT5 after treatment with imatinib, while PI3K/AKT1/mTOR activity remains unaffected. Inhibition of AKT1 leads to apoptosis in the imatinib-resistant cell lines. In conclusion, Ph+ cell lines show a form of imatinib-resistance attributable to constitutive activation of the PI3K/AKT1 pathway. Mutations in PIK3CA, as observed in cell line KCL-22, or PI3K activating oncogenes may undelie TKI-resistance in these cell lines.

PI3K as a target for therapy in haematological malignancies

Although classical mutations in genes such as PIK3CA and PTEN occur at a relatively low frequency in haematological malignancies, activation of PI3K signalling is often detected in these tumours. In some conditions, for example acute myeloid leukaemia (AML), this is due to activating mutations of upstream regulators such as the FLT3 tyrosine kinase or RAS. Primary tumour cells taken from patients with AML, acute lymphoblastic leukaemia, chronic lymphocytic leukaemia and multiple myeloma show varying levels of sensitivity to PI3K and mTOR inhibitors. The challenge now is to conduct high quality trials with novel agents that target these pathways to establish the level of clinical response and to identify those subsets of patients that are more likely to respond.

An investigation of reversal of imatinib resistance in the Bcr-Abl positive imatinib-resistant cell line K562r by dasatinib, nilotinib, rapamycin and bortezomib

INTRODUCTION

We investigated the second-generation tyrosine kinase inhibitors, dasatinib and nilotinib, for their potential to overcome resistance in the imatinib-resistant K562 cell line, and evaluated whether rapamycin, an mammalian target of rapamycin (mTOR) inhibitor, and bortezomib, a proteasome inhibitor, could increase imatinib sensitivity in resistant cell lines.

METHODS

Cell lines, including K562 (chronic myeloid leukaemia in blast crisis) and K562r (imatinib-resistant K562), were exposed to dasatinib, nilotinib, rapamycin, bortezomib, and rapamycin plus imatinib. Cell proliferation was measured by 3-[4, 5-dimethylthia-zol-2-yl]-2, 5-diphenyl tetrazolium bromide (MTT) assay and mTOR signaling pathways were assessed by Western blotting.

RESULTS

Dasatinib, nilotinib, and bortezomib inhibited proliferation of K562 and K562r cell lines at nm concentrations. Resistance of K562r due to duplication of autophosphorylation of wild-type Bcr-Abl was not overcome by dasatinib and nilotinib, but was sensitive to bortezomib. Rapamycin partially inhibited proliferation of K562 and K562r cell lines. Rapamycin plus imatinib did not have a more inhibitory effect on the proliferation of K562 and K562r cell lines.

CONCLUSION

K562r due to duplication of autophosphorylation of wild-type Bcr-Abl induced by imatinib was still partially resistant to dasatinib and nilotinib, but this was overcome by incremental dosing. Rapamycin did not enhance imatinib sensitivity. The blockade of the ubiquitin-proteasome pathway could be effective in overcoming resistance in the K562r imatinib-resistant cell line.

Perspectives on inhibiting mTOR as a future treatment strategy for hematological malignancies

Mammalian target of rapamycin (mTOR) is a protein kinase implicated in the regulation of various cellular processes, including those required for tumor development, such as the initiation of mRNA translation, cell-cycle progression and cellular proliferation. In a wide range of hematological malignancies, the mTORC1 signaling pathway has been found to be deregulated and has been designed as a major target for tumor therapy. Given that pre-clinical studies have clearly established the therapeutic value of mTORC1 inhibition, numerous clinical trials of rapamycin and its derivates (rapalogs) are ongoing for treatment of these diseases. At this time, although disease stabilization and tumor regression have been observed, objective responses in some tumor types have been modest. Nevertheless, some of the mechanisms underlying cancer-cell resistance to rapamycin have now been described, thereby leading to the development of new strategy to efficiently target mTOR signaling in these diseases. In this review, we discuss the rationale for using mTOR inhibitors as novel therapies for a variety of hematological, malignancies with a focus on promising new perspectives for these approaches.

RAD 001 (everolimus) prevents mTOR and Akt late re-activation in response to imatinib in chronic myeloid leukemia

The mammalian target of rapamycin (mTOR) is one target of BCR-ABL fusion gene of chronic myeloid leukemia (CML). Moreover, it drives a compensatory route to Imatinib mesylate (IM) possibly involved in the progression of leukemic progenitors towards a drug-resistant phenotype. Accordingly, mTOR inhibitors are proposed for combined therapeutic strategies in CML. The major caveat in the use of mTOR inhibitors for cancer therapy comes from the induction of an mTOR-phosphatidylinositol 3 kinase (PI3k) feedback loop driving the retrograde activation of Akt. Here we show that the rapamycin derivative RAD 001 (everolimus, Novartis Institutes for Biomedical Research) inhibits mTOR and, more importantly, revokes mTOR late re-activation in response to IM. RAD 001 interferes with the assembly of both mTOR complexes: mTORC1 and mTORC2. The inhibition of mTORC2 results in the de-phosphorylation of Akt at Ser(473) in the hydrophobic motif of C-terminal tail required for Akt full activation and precludes Akt re-phosphorylation in response to IM. Moreover, RAD 001-induced inhibition of Akt causes the de-phosphorylation of tuberous sclerosis tumor suppressor protein TSC2 at 14-3-3 binding sites, TSC2 release from 14-3-3 sigma (restoring its inhibitory function on mTORC1) and nuclear import (promoting the nuclear translocation of cyclin-dependent kinase [CDK] inhibitor p27(Kip1), the stabilization of p27(Kip1) ligand with CDK2, and the G(0)/G(1) arrest). RAD 001 cytotoxicity on cells not expressing the BCR-ABL fusion gene or its p210 protein tyrosine kinase (TK) activity suggests that the inhibition of normal hematopoiesis may represent a drug side effect.

Targeting the translational machinery as a novel treatment strategy for hematologic malignancies

The dysregulation of protein synthesis evident in the transformed phenotype has opened up a burgeoning field of research in cancer biology. Translation initiation has recently been shown to be a common downstream target of signal transduction pathways deregulated in cancer and initiated by mutated/overexpressed oncogenes and tumor suppressors. The overexpression and/or activation of proteins involved in translation initiation such as eIF4E, mTOR, and eIF4G have been shown to induce a malignant phenotype. Therefore, understanding the mechanisms that control protein synthesis is emerging as an exciting new research area with significant potential for developing innovative therapies. This review highlights molecules that are activated or dysregulated in hematologic malignancies, and promotes the transformed phenotype through the deregulation of protein synthesis. Targeting these proteins with small molecule inhibitors may constitute a novel therapeutic approach in the treatment of cancer.

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

BACKGROUND

The mammalian target of rapamycin (mTOR) has recently been identified as a potential target in acute myeloid leukemia (AML).

METHODS

We treated 5 patients with chemotherapy-refractory AML with the mTOR-inhibitor rapamycin at 2mg per os daily for 14 days, with dose adjustment allowed to reach a target serum rapamycin concentration of 10-20 ng/mL. Four of five patients received additional hydroxyurea at constant dose during treatment with rapamycin.

RESULTS

Two patients achieved a leukocyte response, in one of them, a prolonged response was seen. In the other patients, blast counts remained stable or increased during rapamycin therapy. We did not observe severe hematologic or non-hematologic side effects of rapamycin.

CONCLUSION

Rapamycin at 2mg per day acts mildly cytoreductive in a subgroup of patients with refractory AML. Higher doses and drug combinations may be required to obtain long lasting anti-leukemic effects in these patients.

Immunosuppression and atypical infections in CML patients treated with dasatinib at 140 mg daily

BACKGROUND

The multikinase inhibitor dasatinib exerts growth-inhibitory effects in patients with imatinib-resistant chronic myeloid leukaemia (CML). In first clinical trials, side effects of dasatinib, 140 mg daily, were reported to be mild and tolerable.

PATIENTS AND METHODS

We examined the side effect profile in 16 patients with imatinib-resistant CML who received 140 mg dasatinib daily in our center.

RESULTS

Dasatinib produced substantial and sometimes severe or even life-threatening side effects with > or = 10% body weight loss (6/16 patients), pleural effusions grade II or higher (12/16) and infectious complications (12/16), including atypical infections not seen in imatinib-treated patients. One patient developed Epstein-Barr-Virus-positive mucosal leucoplakia, one died from pneumonia caused by pneumocystis carinii and three patients developed a skin-cancer. Most events were recorded within the first 2 years of therapy, only skin tumours developed after the second year. In ex vivo experiments performed in dasatinib-treated patients, transient suppression of IgE-dependent activation of blood basophils and TcR-dependent activation of T-lymphocytes was found. Moreover, in drug-binding studies, dasatinib was found to bind to several key kinase-targets of the immune system including Lyn and Btk, in mast cell, basophil, B-cell and T-cell lines.

CONCLUSION

Dasatinib acts not only anti-neoplastic in CML but may also act as an immunosuppressive agent when applied at 140 mg daily, and produces frequent pleural effusions and weight loss in advanced CML.

PI3K/PTEN signaling in angiogenesis and tumorigenesis

Phosphatidylinositol 3-kinase (PI3K) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) signaling pathway play an important role in multiple cellular functions such as cell metabolism, proliferation, cell-cycle progression, and survival. PI3K is activated by growth factors and angiogenesis inducers such as vascular endothelial growth factor (VEGF) and angiopoietins. The amplification and mutations of PI3K and the loss of the tumor suppressor PTEN are common in various kinds of human solid tumors. The genetic alterations of upstream and downstream of PI3K signaling molecules such as receptor tyrosine kinases and AKT, respectively, are also frequently altered in human cancer. PI3K signaling regulates tumor growth and angiogenesis by activating AKT and other targets, and by inducing HIF-1 and VEGF expression. Angiogenesis is required for tumor growth and metastasis. In this review, we highlight the recent studies on the roles and mechanisms of PI3K and PTEN in regulating tumorigenesis and angiogenesis, and the roles of the downstream targets of PI3K for transmitting the signals. We also discuss the crosstalk of these signaling molecules and cellular events during tumor growth, metastasis, and tumor angiogenesis. Finally, we summarize the potential applications of PI3K, AKT, and mTOR inhibitors and their outcome in clinical trials for cancer treatment.

A phase I/II study of the safety and efficacy of the addition of sirolimus to tacrolimus/methotrexate graft versus host disease prophylaxis after allogeneic haematopoietic cell transplantation in paediatric acute lymphoblastic leukaemia (ALL)

Sirolimus has been shown to have activity against human acute lymphoblastic leukaemia at serum levels used for immunosuppression. We hypothesized that the addition of sirolimus to a tacrolimus/methotrexate graft-versus-host disease (GVHD) prophylaxis regimen would decrease relapse after haematopoietic stem cell transplantation and initiated a phase I/II study to demonstrate safety, feasibility, and efficacy. The study cohort included 18 patients in high-risk (HR) first complete remission (CR1), 16 in HR CR2, 17 in intermediate risk (IR) CR2, and 12 in CR3+. The 2-year event-free survival (EFS) of the cohort was 66% (standard error 6.4). EFS of risk groups was 74%, 81%, 44% and 46% for CR1, IR CR2, HR CR2 and CR3+ patients respectively, and did not differ by stem cell source. Cumulative incidence of acute GVHD grade II-IV and III-IV was 38% and 21% respectively, while the cumulative incidence of chronic GVHD was 32%. Cumulative incidence of transplant-related mortality and relapse was 10% and 25% respectively. Significant toxicities included veno-occlusive disease [seven patients (11%)], transplant-associated microangiopathy (three patients), and idiopathic pneumonitis (one patient). In summary, sirolimus-based GVHD prophylaxis can be given safely in this population and early survival results are promising. A phase III trial to test whether sirolimus decreases relapse and improves outcome after transplantation for ALL is ongoing.

Mammalian target of rapamycin inhibitors and their potential role in therapy in leukaemia and other haematological malignancies

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that functions as a key regulator of cell growth, protein synthesis, and cell-cycle progression through interactions with a number of signalling pathways, including PI3K/AKT, ras, TCL1, and BCR/ABL. Many haematological malignancies have aberrant activation of the mTOR and related signalling pathways. Accordingly, mTOR inhibitors, a class of signal transduction inhibitors that were originally developed as immunosuppressive agents, are being investigated in preclinical models and clinical trials for a number of haematological malignancies. Sirolimus and second-generation mTOR inhibitors, such as temsirolimus and everolimus, are safe and relatively well-tolerated, making them potentially attractive as single agents or in combination with conventional cytotoxics and other targeted therapies. Promising early clinical data suggests activity of mTOR inhibitors in a number of haematological diseases, including acute lymphoblastic leukaemia, chronic myeloid leukaemia, mantle cell lymphoma, anaplastic large cell lymphoma, and lymphoproliferative disorders. This review describes the rationale for using mTOR inhibitors in a variety of haematological diseases with a focus on their use in leukaemia.