mTOR kinase inhibitors as potential cancer therapeutic drugs

The mammalian target of rapamycin (mTOR) plays a critical role in the positive regulation of cell growth and survival primarily through direct interaction with raptor (forming mTORC complex 1; mTORC1) or rictor (forming mTOR complex 2; mTORC2). The mTOR axis is often activated in many types of cancer and thus has become an attractive cancer therapeutic target. The modest clinical anticancer activity of conventional mTOR allosteric inhibitors, rapamycin and its analogs (rapalogs), which preferentially inhibit mTORC1, in most types of cancer, has encouraged great efforts to develop mTOR kinase inhibitors (TORKinibs) that inhibit both mTORC1 and mTORC2, in the hope of developing a novel generation of mTOR inhibitors with better therapeutic efficacy than rapalogs. Several TORKinibs have been developed and actively studied pre-clinically and clinically. This review will highlight recent advances in the development and research of TORKinibs and discuss some potential issues or challenges in this area.     

Targeting of mTORC2 prevents cell migration and promotes apoptosis in breast cancer

Most of breast cancers are resistant to mammalian target of rapamycin complex 1 (mTORC1) inhibitors rapamycin and rapalogs. Recent studies indicate mTORC2 is emerging as a promising cancer therapeutic target. In this study, we compared the inhibitory effects of targeting mTORC1 with mTORC2 on a variety of breast cancer cell lines and xenograft. We demonstrated that inhibition of mTORC1/2 by mTOR kinase inhibitors PP242 and OSI-027 effectively suppress phosphorylation of Akt (S473) and breast cancer cell proliferation. Targeting of mTORC2 either by kinase inhibitors or rictor knockdown, but not inhibition of mTORC1 either by rapamycin or raptor knockdown promotes serum starvation- or cisplatin-induced apoptosis. Furthermore, targeting of mTORC2 but not mTORC1 efficiently prevent breast cancer cell migration. Most importantly, in vivo administration of PP242 but not rapamycin as single agent effectively prevents breast tumor growth and induces apoptosis in xenograft. Our data suggest that agents that inhibit mTORC2 may have advantages over selective mTORC1 inhibitors in the treatment of breast cancers. Given that mTOR kinase inhibitors are in clinical trials, this study provides a strong rationale for testing the use of mTOR kinase inhibitors or combination of mTOR kinase inhibitors and cisplatin in the clinic.     

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.     

New developments in mammalian target of rapamycin inhibitors for the treatment of sarcoma

Although sarcomas account for a small portion of solid malignancies, currently, there are few treatment options for sarcomas, particularly for advanced disease. The mammalian target of rapamycin (mTOR), a serine-threonine protein kinase in the phosphatidylinositol 3-kinase/serine/threonine protein kinase Akt signaling pathway, has an important role in the regulation of protein synthesis, cell proliferation, angiogenesis, and metabolism. Alterations of the mTOR signaling pathway are common in malignancies, including several types of sarcoma. Therefore, mTOR is a potentially important therapeutic target in these diseases. Rapamycin and its analogs (rapalogs) are effective anticancer agents in a broad range of preclinical models. Clinical trials with these agents alone and in combination with other anticancer agents, including chemotherapy and targeted therapies, have demonstrated potential clinical benefit in several types of sarcoma. The evidence from both preclinical and clinical studies supports further study of mTOR-targeting rapalogs in the treatment of various subtypes of sarcoma.     

Promise of rapalogues versus mTOR kinase inhibitors in subset specific breast cancer: Old targets new hope

The PI3K–AKT–mTOR network has been the major focus of attention for cancer researchers (both in the clinic and the laboratory) in the last decade. An incomplete knowledge of the molecular biology of this complex network has seen an expansion of first generation allosteric mTOR inhibitors, rapalogues, but also biomarker studies designed to identify the best responders of these agents. Currently, research in this pathway has focused on the dual nature of mTOR that is integrated by mTOR–RAPTOR complex (mTORC1) and mTOR–RICTOR complex (mTORC2). These two complexes are regulated by different upstream proteins and also regulated by multiple different compensatory feedback loops. The related advantage of feedback regulation of signaling systems is that it allows diversification in signal response. This deeper understanding has facilitated the development of a novel second generation of inhibitors that are able to affect both mTORC1 and mTORC2, and their downstream effectors, through inhibition of their catalytic activity (ATP competitive inhibitors, attacking the kinase domain of this protein) than binding to the FKBP12 regulatory proteins as for rapalogues. This article reviews the newest insight in the signaling network of the mTOR pathway, preclinical/clinical status of mTOR inhibitors (including second generation of kinase inhibitors) and then focuses on the development of a new wave of research related to combination therapies in subset specific breast tumors.     

Systematic combination screening reveals synergism between rapamycin and sunitinib against human lung cancer

Mammalian target of rapamycin (mTOR) acts as a hub integrating signals from nutrient availability and growth factors and plays central roles in regulating protein synthesis and cell growth, which has been validated as a promising target for cancer therapy. Rapamycin and its analogues have emerged as the first generation of mTOR inhibitors, but their efficacy is modest in clinical settings. Combinatorial use of rapamycin with other drugs is a promising strategy to improve its anticancer activity. Here we developed an unbiased systematic binary screening platform aiming to discover new remedy for rapamycin-based cancer therapy. We found that sunitinib emerged as one of the clinically available anticancer drugs screened that displayed significant synergy with rapamycin in NSCLC cells. Combination of rapamycin with sunitinib resulted in enhanced cell cycle arrest in G1 phase, which was accompanied with enhanced suppression of mTOR signaling and disruption of the negative feedback loop that activate AKT upon mTORC1 inhibition. Furthermore, sunitinib and rapamycin displayed synergistic activity against tube formation by human microvessel endothelial cells as well as outgrowth of endothelial tubes and microvessels both in vitro and in vivo, which is associated with down-regulation of VEGF secretion and HIF1α expression. Our study demonstrated that new combinatorial regimen could be identified via systematic drug combination screening and established a mechanistic rationale for a combination approach using rapalogs and sunitinib in the treatment of human NSCLC.     

Dual mTORC1/C2 inhibitors: gerosuppressors with potential anti-aging effect

Over the past decade, our understanding of the molecular and cellular mechanisms presiding over cellular and tissue decline with aging has greatly advanced. Classical hallmarks of aging cell include increasing levels of reactive oxygen species, DNA damage and senescence entry, which disrupt tissue architecture and function. Tissue dysfunction with aging has been shown to correlate with a cellular switch from a G0 reversible quiescence state into a G0 irreversible senescence state (geroconversion), causing a permanent proliferative block. The TOR (target of rapamycin) kinase has been shown to promote geroconversion. Rapamycin and other rapalogs specifically suppress activity of the mammalian TOR (mTOR) complex 1 (mTORC1) -but not mTOR complex 2 (mTORC2)- and decrease senescence entry, thus preserving proliferative potential. In this perspective, we briefly comment recent progress of Leontieva and colleagues showing a new class of non-rapalog drugs that target simultaneously mTORC1 and mTORC2 and prevent geroconversion in a more efficient way than rapamycin. Its potential future use as rejuvenating, anti-aging therapeutics is therefore proposed.     

New Insights into mTOR Signal Pathways in Ovarian-Related Diseases: Polycystic Ovary Syndrome and Ovarian Cancer

mTOR, the mammalian target of rapamycin, is a conserved serine/threonine kinase which belongs to the phosphatidyl-linositol kinase-related kinase (PIKK) family. It has two complexes called mTORC1 and mTORC2. It is well established that mTOR plays important roles in cell growth, proliferation and differentiation. Over-activation of the mTOR pathway is considered to have a relationship with the development of many types of diseases, including polycystic ovary syndrome (PCOS) and ovarian cancer (OC). mTOR pathway inhibitors, such as rapamycin and its derivatives, can directly or indirectly treat or relieve the symptoms of patients suffering from PCOS or OC. Moreover, mTOR inhibitors in combination with other chemical-molecular agents may have extraordinary efficacy. This paper will discuss links between mTOR signaling and PCOS and OC, and explore the mechanisms of mTOR inhibitors in treating these two diseases, with conclusions regarding the most effective therapeutic approaches.     

Mechanistic / mammalian target protein of rapamycin signaling in hematopoietic stem cells and leukemia

Mechanistic/mammalian target protein of rapamycin (mTOR) is an evolutionarily conserved kinase that plays a critical role in sensing and responding to environmental determinants such as nutrient availability, energy sufficiency, stress, and growth factor concentration. mTOR participates in two complexes, designated mTOR complex 1 (mTORC1) and 2 (mTORC2), both of which phosphorylate multiple substrates. Recent studies have revealed that the fine‐tuning activity of mTOR complexes contributes to both maintenance of hematopoietic stem cells (HSCs) and suppression of leukemogenesis. Dysregulation of mTORC1 activity results in impaired HSC homeostasis. Abnormalities of mTOR signaling are observed in many patients with leukemia and genetic studies clearly show that the leukemogenesis associated with Pten deficiency involves both mTORC1 and mTORC2. Although the several mTOR inhibitors have been developed for cancer therapy, effectiveness of the inhibitors for eradication of leukemia stem cells (LSCs) is unknown. Advances in understanding of how mTOR signaling is involved in mechanisms of normal HSC and LSC homeostasis may lead to novel therapeutic approaches that can successfully eradicate leukemia.     

Dual mTORC1/C2 inhibitors suppress cellular geroconversion (a senescence program)

In proliferating cells, mTOR is active and promotes cell growth. When the cell cycle is arrested, then mTOR converts reversible arrest to senescence (geroconversion). Rapamycin and other rapalogs suppress geroconversion, maintaining quiescence instead. Here we showed that ATP-competitive kinase inhibitors (Torin1 and PP242), which inhibit both mTORC1 and TORC2, also suppressed geroconversion. Despite inhibition of proliferation (in proliferating cells), mTOR inhibitors preserved re-proliferative potential (RP) in arrested cells. In p21-arrested cells, Torin 1 and PP242 detectably suppressed geroconversion at concentrations as low as 1-3 nM and 10-30 nM, reaching maximal gerosuppression at 30 nM and 300 nM, respectively. Near-maximal gerosuppression coincided with inhibition of p-S6K(T389) and p-S6(S235/236). Dual mTOR inhibitors prevented senescent morphology and hypertrophy. Our study warrants investigation into whether low doses of dual mTOR inhibitors will prolong animal life span and delay age-related diseases. A new class of potential anti-aging drugs can be envisioned.     

Predictive biomarkers for the activity of mammalian target of rapamycin (mTOR) inhibitors

In the quest for personalized medicine, only a few biological parameters are routinely used to select patients prior to the initiation of anticancer targeted therapies, including mTOR inhibitors. Identifying biological factors that may predict efficacy or resistance to mTOR inhibitors represents an important challenge since rapalogs may exert antitumor effects through multiple mechanisms of action. Despite the fact that no such a factor is currently available, several molecular patterns are emerging, correlating with sensitivity and/or resistance to rapalogs. While activation of the phosphatidylinositol 3 kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway, overexpression of cyclin D1, and functional apoptosis seem to sensitize tumor cells to rapalogs, Bcl2 overexpression or KRAS mutations are reported to be associated with resistance to mTOR inhibitors in several preclinical models. Translational research aimed at validating those parameters in clinical trials is ongoing.     

DNA damage-induced S and G2/M cell cycle arrest requires mTORC2-dependent regulation of Chk1

mTOR signalling is commonly dysregulated in cancer. Concordantly, mTOR inhibitors have demonstrated efficacy in a subset of tumors and are in clinical trials as combination therapies. Although mTOR is associated with promoting cell survival after DNA damage, the exact mechanisms are not well understood. Moreover, since mTOR exists as two complexes, mTORC1 and mTORC2, the role of mTORC2 in cancer and in the DNA damage response is less well explored. Here, we report that mTOR protein levels and kinase activity are transiently increased by DNA damage in an ATM and ATR-dependent manner. We show that inactivation of mTOR with siRNA or pharmacological inhibition of mTORC1/2 kinase prevents etoposide-induced S and G2/M cell cycle arrest. Further results show that Chk1, a key regulator of the cell cycle arrest, is important for this since ablation of mTOR prevents DNA damage-induced Chk1 phosphorylation and decreases Chk1 protein production. Furthermore, mTORC2 was essential and mTORC1 dispensable, for this role. Importantly, we show that mTORC1/2 inhibition sensitizes breast cancer cells to chemotherapy. Taken together, these results suggest that breast cancer cells may rely on mTORC2-Chk1 pathway for survival and provide evidence that mTOR kinase inhibitors may overcome resistance to DNA-damage based therapies in breast cancer.     

Impact of genetic alterations on mTOR-targeted cancer therapy

Rapamycin and its derivatives (rapalogs) , a group of allosteric inhibitors of mammalian target of rapamycin (mTOR), have been actively tested in a variety of cancer clinical trials, and some have been approved by the Food and Drug Administration for the treatment of certain types of cancers. However, the single agent activity of these compounds in many tumor types remains modest. The mTOR axis is regulated by multiple upstream signaling pathways. Because the genes (e.g., PIK3CA, KRAS, PTEN, and LKB1) that encode key components in these signaling pathways are frequently mutated in human cancers, a subset of cancer types may be addicted to a given mutation, leading to hyperactivation of the mTOR axis. Thus, efforts have been made to demonstrate the potential impact of genetic alterations on rapalogbased or mTOR-targeted cancer therapy. This review will primarily summarize research advances in this direction.     

The mammalian target of rapamycin signaling pathway: twists and turns in the road to cancer therapy.

The immunosuppressive drug rapamycin played a key role in the functional characterization of mammalian target of rapamycin (mTOR), an unusual protein kinase that coordinates growth factor and nutrient availability with cell growth and proliferation. Several rapamycin-related compounds are now in various stages of clinical development as anticancer agents. This article highlights recent advances in our understanding of the mTOR signaling pathway and the implications of these findings for the clinical application of mTOR inhibitors in cancer patients.     

mTOR and cancer: insights into a complex relationship

mTOR (mammalian target of rapamycin) has come a long way since its humble beginnings as a kinase of unknown function. As part of the mTORC1 and mTORC2 complexes mTOR has key roles in several pathways that are involved in human cancer, stimulating interest in mTOR inhibitors and placing it on the radar of the pharmaceutical industry. Here, I discuss the rationale for the use of drugs that target mTOR, the unexpectedly complex mechanism of action of existing mTOR inhibitors and the potential benefits of developing drugs that function through different mechanisms. The purpose is not to cover all aspects of mTOR history and signalling, but rather to foster discussion by presenting some occasionally provocative ideas.     

Mammalian target of rapamycin (mTOR) inhibitors

Current efforts in anticancer drug development are targeting key factors in cell-cycle regulation. Mammalian target of rapamycin (mTOR) is one such protein kinase that facilitates cell growth by stimulating the cell to traverse the G¹ to S phase of the cell cycle. Rapamycin is the first defined inhibitor of mTOR, and the demonstration of its antitumor activity has led to great interest in this pathway as an antitumor mechanism. Analogues with better pharmacologic properties have been developed and have entered clinical trials. Human cell lines of renal cell cancer, among several other tumors, are sensitive to growth inhibition via this pathway. Ongoing clinical trials are evaluating renal cell cancer and other malignancies using therapy with mTOR inhibitors. These agents are more likely to induce growth inhibition rather than tumor regression.     

mTOR and cancer therapy

Proteins regulating the mammalian target of rapamycin (mTOR), as well as some of the targets of the mTOR kinase, are overexpressed or mutated in cancer. Rapamycin, the naturally occurring inhibitor of mTOR, along with a number of recently developed rapamycin analogs (rapalogs) consisting of synthetically derived compounds containing minor chemical modifications to the parent structure, inhibit the growth of cell lines derived from multiple tumor types in vitro, and tumor models in vivo. Results from clinical trials indicate that the rapalogs may be useful for the treatment of subsets of certain types of cancer. The sporadic responses from the initial clinical trials, based on the hypothesis of general translation inhibition of cancer cells are now beginning to be understood owing to a more complete understanding of the dynamics of mTOR regulation and the function of mTOR in the tumor microenvironment. This review will summarize the preclinical and clinical data and recent discoveries of the function of mTOR in cancer and growth regulation.     

Predicted mechanisms of resistance to mTOR inhibitors

The serine/threonine kinase, mTOR (mammalian Target of Rapamycin) has become a focus for cancer drug development. Rapamycins are highly specific inhibitors of mTOR and potently suppress tumour cell growth by retarding cells in G1 phase or potentially inducing apoptosis. Currently, both rapamycin and several analogues are being evaluated as anticancer agents in clinical trials. Results indicate that many human cancers have intrinsic resistance and tumours initially sensitive to rapamycins become refractory, demonstrating acquired resistance. Here, we consider mechanisms of resistance to inhibitors of mTOR.     

Remarkable inhibition of mTOR signaling by the combination of rapamycin and 1,4-phenylenebis(methylene)selenocyanate in human prostate cancer cells

Preclinical studies and clinical analyses have implicated the mammalian target of rapamycin (mTOR) pathway in the progression of prostate cancer, suggesting mTOR as a potential target for new therapies. mTOR, a serine/threonine kinase, belongs to two distinct signaling complexes: mTORC1 and mTORC2. We previously showed that the synthetic organoselenium compound, p-XSC, effectively inhibits viability and critical signaling molecules (e.g., androgen receptor, Akt) in androgen responsive (AR) and androgen independent (AI) human prostate cancer cells. On the basis of its inhibition of Akt, we hypothesized that p-XSC modulates mTORC2, an upstream regulator of the kinase. We further hypothesized that combining p-XSC with rapamycin, an mTORC1 inhibitor, would be an effective combinatory strategy for the inhibition of prostate cancer. The effects of p-XSC and rapamycin, alone or in combination, on viability and mTOR signaling were examined in AR LNCaP prostate cancer cells and AI C4-2 and DU145 cells. Phosphorylation of downstream targets of mTORC1 and mTORC2 was analyzed by immunoblotting. The interaction of mTORC1- and mTORC2-specific proteins with mTOR was probed through immunoprecipitation and immunoblotting. p-XSC inhibited phosphorylation of mTORC2 downstream targets, Akt and PCKα, and decreased the levels of rictor, an mTORC2-specific protein, coimmunoprecipitated with mTOR in C4-2 cells. The combination of p-XSC and rapamycin more effectively inhibited viability and mTOR signaling in C4-2, LNCaP and DU145 cells than either agent individually.