ATP-competitive mTOR Inhibitor Research Articles

Ligand-based 3-D pharmacophore generation and molecular docking of mTOR kinase inhibitors

The 3-D structure of the human mTOR kinase domain was modeled based on the crystal structure of PI3Kγ using comparative modeling methods, and the ATP-binding site of mTOR was characterized. The mTOR kinase 3-D model structure is similar to the structure of the PI3Kγ kinase domain, and exhibits great similarity to PI3Kγ at the active site of the kinase. Pharmacophore generation, the docking of mTOR inhibitors, and molecular dynamics (MD) simulations of mTOR-inhibitor docked complexes were carried out in this work. The best pharmacophore model generated from 27 ATP-competitive mTOR inhibitors comprised two hydrogen-bond acceptors, one aromatic ring, and one hydrophobic feature. These 27 inhibitors were docked into the ATP-binding site comprising the DFG motif, and the interactions in each protein-inhibitor complex were characterized. Mapping the pharmacophore model onto the docked inhibitors explained the specificity of the features of the pharmacophore and how they were arranged inside the active site of mTOR kinase. MD studies revealed important structural features, such as the large hydrophobic pocket "HP" and hydrophilic pocket "A1," and the solvent-exposed hydrophilic pocket "A2" at the active site of mTOR. Our results provide structural models of mTOR-inhibitor complexes and clues that should aid in the design of better mTOR kinase inhibitors.

ATP-Competitive Inhibitors of mTOR: An Update

mTOR (mammalian target of rapamycin) is a serine-threonine kinase belonging to the PI3K/Akt/mTOR signalling pathway that is involved in several cell functions, including growth, proliferation, apoptosis and autophagy. mTOR hyperactivation has been detected in several human cancers, thus representing, together with its upstream effectors, an important target for cancer therapy. mTOR exists in two different complexes in cells, mTORC1 and mTORC2 which could both be targeted by potential anticancer agents. Rapamycin, the selective and allosteric inhibitor of mTOR, inhibits the enzyme in mTORC1, but not in mTORC2. In the last few years a number of mTOR ATP-competitive inhibitors has been reported acting on mTOR in both complexes and possessing a more complete anticancer activity in comparison with that of rapamycin and its derivatives. mTOR shares high sequence homology in the hinge-region with PI3K that is a lipid kinase upstream to mTOR in the same signaling pathway; for this reason some compounds originally developed as PI3K inhibitors later showed to also target mTOR. As indicated by preclinical and clinical studies, compounds acting on more than one target could result in a better biological response and in enhanced therapeutic potential and also dual PI3K/mTOR inhibitors result of great interest as potential antitumor agents. This review mainly reports the recently discovered mTOR ATP-competitive inhibitors in terms of medicinal chemistry, classified by their chemical structures, focusing on SAR and modelling studies that led to the discovery of very potent and selective agents, such as AZD-8055, OSI-027 and INK128, already entered clinical trials, or WYE-132, Torin1 and others in preclinical studies. Also some examples of dual PI3K/mTOR inhibitors, including PI-103, GNE477, WJD008 and GSK2126458 are reported together with their biological and clinical data.

MTORC2 Protein-mediated Protein Kinase B (Akt) Serine 473 Phosphorylation Is Not Required for Akt1 Activity in Human Platelets

Protein kinase B (PKB, Akt) is a Ser/Thr kinase involved in the regulation of cell survival, proliferation, and metabolism and is activated by dual phosphorylation on Thr308 in the activation loop and Ser473 in the hydrophobic motif. It plays a contributory role to platelet function, although little is known about its regulation. In this study, we investigated the role of the mammalian target of rapamycin complex (mTORC)-2 in Akt regulation using the recently identified small molecule ATP competitive mTOR inhibitors PP242 and Torin1. Both PP242 and Torin1 blocked thrombin and insulin-like growth factor 1-mediated Akt Ser473 phosphorylation with an IC50 between 1 and 5 nm, whereas the mTORC1 inhibitor rapamycin had no effect. Interestingly, PP242 and Torin1 had no effect on Akt Thr308 phosphorylation, Akt1 activity, and phosphorylation of the Akt substrate glycogen synthase kinase 3β, indicating that Ser473 phosphorylation is not necessary for Thr308 phosphorylation and maximal Akt1 activity. In contrast, Akt2 activity was significantly reduced, concurrent with inhibition of PRAS40 phosphorylation, in the presence of PP242 and Torin1. Other signaling pathways, including phospholipase C/PKC and the MAPK pathway, were unaffected by PP242 and Torin1. Together, these results demonstrate that mTORC2 is the kinase that phosphorylates Akt Ser473 in human platelets but that this phosphorylation is dispensable for Thr308 phosphorylation and Akt1 activity.

Relieving Autophagy and 4EBP1 from Rapamycin Resistance

The mammalian target of rapamycin complex 1 (mTORC1) is a multiprotein signaling complex regulated by oncogenes and tumor suppressors. Outputs downstream of mTORC1 include ribosomal protein S6 kinase 1 (S6K1), eukaryotic translation initiation factor 4E (eIF4E), and autophagy, and their modulation leads to changes in cell growth, proliferation, and metabolism. Rapamycin, an allosteric mTORC1 inhibitor, does not antagonize equally these outputs, but the reason for this is unknown. Here, we show that the ability of rapamycin to activate autophagy in different cell lines correlates with mTORC1 stability. Rapamycin exposure destabilizes mTORC1, but in cell lines where autophagy is drug insensitive, higher levels of mTOR-bound raptor are detected than in cells where rapamycin stimulates autophagy. Using small interfering RNA (siRNA), we find that knockdown of raptor relieves autophagy and the eIF4E effector pathway from rapamycin resistance. Importantly, nonefficacious concentrations of an ATP-competitive mTOR inhibitor can be combined with rapamycin to synergistically inhibit mTORC1 and activate autophagy but leave mTORC2 signaling intact. These data suggest that partial inhibition of mTORC1 by rapamycin can be overcome using combination strategies and offer a therapeutic avenue to achieve complete and selective inhibition of mTORC1.

The Role of mTOR Inhibitors for the Treatment of B-Cell Lymphomas

Despite the fact that the majority of lymphomas initially respond to treatment, many patients relapse and die from disease that is refractory to current regimens. The need for new treatment strategies in lymphomas has led to the investigation and evaluation of novel agents that target cellular pathways. The mammalian target of rapamycin (mTOR) is a representative pathway that may be implicated in lymphomagenesis. Rapamycin and especially its derivatives (temsirolimus, everolimus, and deforolimus) represent the first described mTOR inhibitors. These agents have shown promising results in the treatment of lymphoid malignancies. On the other hand, new ATP-competitive mTOR inhibitors that provoke a broader inhibition of mTOR activity are in early stages of clinical development. The purpose of this paper is to summarize the existing knowledge about mTOR inhibitors and their use in the treatment of B-cell lymphomas. Relevant issues regarding mTOR biology in general as well as in B-cell lymphoid neoplasms are also discussed in short.

Targeting mTOR Signaling Pathway in Ovarian Cancer

The mammalian target of rapamycin (mTOR) is frequently activated in epithelial ovarian cancer, and is regarded as an attractive therapeutic target for therapy. Preclinical investigations using rapamycin and its analogs have demonstrated significant growthinhibitory effects on the growth of ovarian cancer both in the setting of monotherapy and in combination with cytotoxic agents. Based on promising preclinical data, mTOR inhibitors are currently being evaluated in several phase I/II trials in patients with ovarian cancer. In an effort to overcome resistance to rapamycin and its analogs, the novel ATP-competitive mTOR inhibitors have recently been developed. In this report, we review the scientific rationale and evidence for the potential clinical benefits provided by mTOR inhibitor therapy for patients with epithelial ovarian cancer.

Recent advances in the discovery of small-molecule ATP competitive mTOR inhibitors: a patent review

Introduction: The mammalian target of rapamycin (mTOR) is a protein kinase and a key component of the PI3K/Akt/mTOR signaling pathway, and is deregulated in half of all human cancers. Rapamycin and its analogs (rapalogs) are allosteric inhibitors of one functional mTOR complex, mTORC1, and are clinically proven therapeutic agents for the treatment of certain cancers. However, rapalogs mainly partially inhibit mTORC1, while ATP competitive inhibitors suppress both mTORC1 and mTORC2, and therefore may offer advantages in the clinic. Recently, small-molecule inhibitors have entered clinical trials that are mTOR-selective or dual mTOR/PI3K inhibitors.Areas covered: This review focuses on ATP-competitive mTOR inhibitors that have appeared in the patent literature in 2010. Many inhibitors with new structural motifs have been discovered as well as inhibitors that are related to previously disclosed structures. This review endeavors to put into perspective the diverse structural elements that make up these compounds. Patent applications are covered that include either selective mTOR inhibitors or dual mTOR/PI3K inhibitors.Expert opinion: The PI3K/mTOR signaling pathway is an exciting target for the development of pharmaceuticals to treat cancer and other diseases, due to the unique combination of a clinically and commercially validated pathway approach (i.e., rapalogs), combined with a biological rationale for further increased efficacy (i.e., ATP-competitive inhibitors). With the number of candidate drugs currently in development or at earlier stages of the drug discovery pipeline, we are bound to see small-molecule inhibitors reach pivotal trials, and hopefully the market, in the near future.

The inhibition of MAPK potentiates the anti-angiogenic efficacy of mTOR inhibitors

The mammalian target of rapamycin (mTOR) which is part of two functionally distinct complexes, mTORC1 and mTORC2, plays an important role in vascular endothelial cells. Indeed, the inhibition of mTOR with an allosteric inhibitor such as rapamycin reduces the growth of endothelial cell in vitro and inhibits angiogenesis in vivo. Recent studies have shown that blocking mTOR results in the activation of other prosurvival signals such as Akt or MAPK which counteract the growth inhibitory properties of mTOR inhibitors. However, little is known about the interactions between mTOR and MAPK in endothelial cells and their relevance to angiogenesis. Here we found that blocking mTOR with ATP-competitive inhibitors of mTOR or with rapamycin induced the activation of the mitogen-activated protein kinase (MAPK) in endothelial cells. Downregulation of mTORC1 but not mTORC2 had similar effects showing that the inhibition of mTORC1 is responsible for the activation of MAPK. Treatment of endothelial cells with mTOR inhibitors in combination with MAPK inhibitors reduced endothelial cell survival, proliferation, migration and tube formation more significantly than either inhibition alone. Similarly, in a tumor xenograft model, the anti-angiogenic efficacy of mTOR inhibitors was enhanced by the pharmacological blockade of MAPK. Taken together these results show that blocking mTORC1 in endothelial cells activates MAPK and that a combined inhibition of MAPK and mTOR has additive anti-angiogenic effects. They also provide a rationale to target both mTOR and MAPK simultaneously in anti-angiogenic treatment.

ATP-competitive inhibitors of mTOR: new perspectives in the treatment of renal cell carcinoma

Targeting mTOR (mammalian target of rapamycin) is an effective approach in the treatment of advanced RCC (renal cell carcinoma). Rapamycin-like drugs (rapalogues) have shown clinical activities and have been approved for the treatment of RCC. Recently, with the development of ATP-competitive inhibitors of mTOR, therapies targeting mTOR have entered a new era. Here, we discuss the biological relevance of blocking mTOR in RCC and review the mechanisms of action of rapalogues in RCC. We also advance some perspectives on the use of ATP-competitive inhibitors of mTOR in RCC.

Allosteric and ATP-competitive kinase inhibitors of mTOR for cancer treatment

Over the past few years a number of components of the PI3K/mTOR pathway have been the subject of intense drug discovery activities both in pharmaceutical companies and in academia. This review article summarizes progress made in the identification and development of allosteric and ATP-competitive kinase inhibitors of mTOR and their potential therapeutic use in oncology.

Abstract LB-70: In vitro anti-cancer activity of PF-04691502, a potent dual inhibitor of phosphatidylinositol 3-kinase (PI3K) and mammalian target of rapamycin (mTOR) in a panel of human breast cancer cell lines

Abstract Introduction: Activating mutations in the PIK3CA gene (encoding for the P110 catalytic subunit of PI3K) are found in more than 25% of human breast cancers [Saal LH Cancer Res 2005]. PF-04691502 is a potent orally-available ATP-competitive and reversible dual inhibitor of PI3K and mTOR. This study evaluates the anti-proliferative and cytotoxic activity of PF-04691502 across a large panel of breast cancer cell lines that recapitulate the clinical spectrum of the disease. Methods: Over 40 human breast cancer cell lines were exposed in vitro to PF-0469102 over 9 concentrations to generate dose-response curves. These assays were performed over a 6-day incubation period using direct cell counts. The ability of PF-04691502 to inhibit proliferation and induce cell death was calculated using IC50 and LD50 values, respectively. PI3K mutation status was available from public databases. In addition, baseline gene expression profiles were generated using the Agilent platform. A subset of breast cancer cell lines with varying levels of sensitivity to the compound were incubated with PF-04691502 concentrations ranging from 10 to 1000 nM, and processed for Western blotting. Results: Twenty-two cell lines had IC50 values < 100 nM, with a trend towards increased sensitivity to PF-04691502 in breast cancer cell lines harboring PIK3CA mutations, and in those belonging to the luminal subtype. Lethality (LD50 < 1 M) was observed in 10 out of 12 breast cancer cell lines harboring PI3KCA mutations, and was prevalent in cell lines of luminal subtype. Inhibition of the downstream target pAKT was observed. Conclusions: PF-04691502 has anti-cancer activity across a spectrum of breast cancer cell lines, and demonstrates lethality against a subset of cell lines. A first-in-human phase I trial of PF-04691502 is ongoing. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-70.

Chemical Inducers of Autophagy That Enhance the Clearance of Mutant Proteins in Neurodegenerative Diseases

Many of the neurodegenerative diseases that afflict people are caused by intracytoplasmic aggregate-prone proteins. These include Parkinson disease, tauopathies, and polyglutamine expansion diseases such as Huntington disease. In Mendelian forms of these diseases, the mutations generally confer toxic novel functions on the relevant proteins. Thus, one potential strategy for dealing with these mutant proteins is to enhance their degradation. This can be achieved by up-regulating macroautophagy, which we will henceforth call autophagy. In this minireview, we will consider the reasons why autophagy up-regulation may be a powerful strategy for these diseases. In addition, we will consider some of the drugs and associated signaling pathways that can be used to induce autophagy with these therapeutic aims in mind.

Discovery of 2-ureidophenyltriazines bearing bridged morpholines as potent and selective ATP-competitive mTOR inhibitors

Incorporation of bridged morpholines in monocyclic triazine PI3K/mTOR inhibitors gave compounds with increased mTOR selectivity relative to the corresponding morpholine analogs. Compounds with ureidophenyl groups gave highly potent and selective mTOR inhibitors. Potency and selectivity was demonstrated both in vitro and in vivo through biomarker suppression studies. Select compounds exhibited potent inhibition of tumor growth in nude mouse xenograft assays upon PO and IV administration.

2-Arylureidophenyl-4-(3-oxa-8-azabicyclo[3.2.1]octan-8-yl)triazines as highly potent and selective ATP competitive mTOR inhibitors: Optimization of human microsomal stability

Isosteric replacement of one of the 3,5-ethylene-bridged morpholines in 2-arylureidophenyl-4,6-di(3-oxa-8-azabicyclo[3.2.1]octan-8-yl)triazines led to significant improvements in human microsomal stability. 3- R-Me-morpholine and tetrahydropyran were identified as preferred isosteres for the bridged morpholine. Combination of tetrahydropyran substitution with an N-Me-piperazinophenylureido group led to 27, that selectively suppressed mTOR biomarkers in vivo and possessed excellent efficacy in a murine xenograft model.

Abstract B141: Effects of AZD8055, a novel mTOR kinase inhibitor, in human cancer cells developing resistance to rapamycin

Abstract Background: Cancer cells are thought to adapt to rapamycin, an inhibitor of mTORC1, by activating AKT via mTORC2 and others mTORC1-independent survival pathways. In this study, we explored the effects of AZD8055, an ATP competitive inhibitor of mTOR, inhibiting both mTORC1 and mTORC2, in cancer cells with intrinsic and developing resistance to rapamycin. Materials and Methods: Antiproliferative effects and cellular growth inhibition of AZD8055 and rapamycin were evaluated in a panel of 20 human cancer cell lines by MTT assay. Baseline and phosphorylated protein levels were assessed usingWestern blot. mRNA expression was studied by qRT-PCR. Results: AZD8055 and rapamycin (48h exposures) displayed antiproliferative effects in our cell panel at IC50s ranging 0.005–>3µM and 0.04–20µM, respectively. CAKI1 and SK-HEP1 cells expressed high mRNA levels of mesenchymal markers (vimentin, N-cadherin, ZEB1, SIP1, SLUG, ACTA2, and HMGA2) and low levels of E-cadherin associated with intrinsic resistance to several receptor tyrosine kinase inhibitors. AZD8055 induced antiproliferative effects in CAKI1 and SK-HEP1 cells at concentrations <0.1µM. In CAKI1 and SK-HEP1 cells exposed to 0.1µM rapamycin for 5–24h, we observed the inhibition of p-S6 Ser235/236 but an activation of p-AKTser473. Consistent with its mechanism of action, exposure to 0.1µM AZD8055 led to the inhibition to p-S6, and p-4EBP1Ser65, without p-AKT ser473 activation. Downstream to mTORC2, PKC Ser657 was transiently activated by both rapamycin and AZD8055 in CAKI1 cells but not in SK-HEP1 cells. To study the acquired resistance to rapamycin, SK-HEP1 cells were grown with increased concentrations of rapamycin for >6 months, making these cells tolerant to ≥4-fold IC50 concentrations. In SK-RAP cells, we observed the transcriptional activation of IGF1R, TGFB2, the anti-apoptotic factors BCL2 and CDKN1Ap21, and the multidrug resistance ABCB1mdr1. In SK-RAP cells, sustained exposure to rapamycin increased pAKTser473 and decreased p-S6 and p-PKC. Consistent with the inhibition of mTOR kinase activity, exposure to AZD8055 for 5h almost completely abrogated rapamycin-induced AKTser473 activation in SK-RAP cells. Furthermore, SK-RAP cells were more sensitive to AZD8055 than to the parental SK-HEP1 cells (IC50: 0.1 versus >3µM, p=0.044), possibly due to the extended functions of AZD8055 against mTORC1 and inhibition of mTORC2. Conclusions: AZD8055, by inhibiting mTOR kinase activity, decreases mTORC1 substrates pS6 and p-4EBP1Ser65 and mTORC2 substrate AKTser473. AZD8055 decreases cancer cell proliferation in cell lines expressing mesenchymal markers displaying intrinsic and developing resistance to rapamycin, abrogating the hyperactivation of AKT observed in these cells after rapamycin exposure. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B141.

Abstract B145: Highly selective pyrazolopyrimidine mTOR inhibitors with profound in vivo antitumor activity

Abstract The mammalian target of rapamycin (mTOR) is an important target for cancer chemotherapy due to the deregulation of its signaling pathway in a wide spectrum of human tumors. mTOR exists in at least two functional protein complexes: mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2). While rapamycin and its analogs (rapalogs) inhibit only mTORC1, ATP-competitive inhibitors of mTOR target both mTORC1 and mTORC2 and therefore have the potential for improved anticancer efficacy. We have previously reported on the pyrazolopyrimidine series of ATP-competitive mTOR inhibitors (J. Med. Chem. 2009, 52, 5013 – 5016). We now report that derivatization of the morpholine ring in these inhibitors leads to greatly enhanced mTOR selectivity versus PI3K. Selective mTOR inhibitors have the potential to achieve a higher therapeutic index in the clinic due to better tolerability. Testing these inhibitors in tumor cells and evaluating the most potent analogs in human tumor xenograft models in the nude mouse led to the identification of WYE-125132, a candidate for clinical development. WYE-125132 is also selective versus a wide range of other kinases including the closely-related PIKKs. Detailed structure activity relationships in the pyrazolopyrimidine series will be presented. In conclusion, modification of the morpholine ring of a series of pyrazolopyrimidines leads to analogs with enhanced mTOR selectivity over PI3K and potent efficacy in human tumor xenograft models in the nude mouse. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B145.

Biochemical, Cellular, and In vivo Activity of Novel ATP-Competitive and Selective Inhibitors of the Mammalian Target of Rapamycin

The mammalian target of rapamycin (mTOR) is centrally involved in cell growth, metabolism, and angiogenesis. While showing clinical efficacy in a subset of tumors, rapamycin and rapalogs are specific and allosteric inhibitors of mTOR complex 1 (mTORC1), but they do not directly inhibit mTOR complex 2 (mTORC2), an emerging player in cancer. Here, we report chemical structure and biological characterization of three pyrazolopyrimidine ATP-competitive mTOR inhibitors, WAY-600, WYE-687, and WYE-354 (IC(50), 5-9 nmol/L), with significant selectivity over phosphatidylinositol 3-kinase (PI3K) isofoms (>100-fold). Unlike the rapalogs, these inhibitors acutely blocked substrate phosphorylation by mTORC1 and mTORC2 in vitro and in cells in response to growth factor, amino acids, and hyperactive PI3K/AKT. Unlike the inhibitors of PI3K or dual-pan PI3K/mTOR, cellular inhibition of P-S6K1(T389) and P-AKT(S473) by the pyrazolopyrimidines occurred at significantly lower inhibitor concentrations than those of P-AKT(T308) (PI3K-PDK1 readout), showing mTOR selectivity in cellular setting. mTOR kinase inhibitors reduced AKT downstream function and inhibited proliferation of diverse cancer cell lines. These effects correlated with a strong G(1) cell cycle arrest in both the rapamycin-sensitive and rapamycin-resistant cells, selective induction of apoptosis, repression of global protein synthesis, and down-regulation of angiogenic factors. When injected into tumor-bearing mice, WYE-354 inhibited mTORC1 and mTORC2 and displayed robust antitumor activity in PTEN-null tumors. Together, our results highlight mechanistic differentiation between rapalogs and mTOR kinase inhibitors in targeting cancer cell growth and survival and provide support for clinical development of mTOR kinase inhibitors as new cancer therapy.

MTOR mediated anti-cancer drug discovery

The mammalian target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase and the founding member of a signaling pathway that regulates many fundamental features of cell growth and division. In cells, mTOR acts as the catalytic subunit of two functionally distinct complexes, called mTOR Complex 1 (mTORC1) and mTOR Complex 2 (mTORC2). Together, these complexes coordinate a variety of processes that include protein translation, autophagy, proliferation, survival and metabolism in response to nutrient, energy and growth factor signals. Consistent with its role as a growth-promoting pathway, numerous studies have found that Mtor signaling is hyper-activated in a broad spectrum of human cancers. In particular, mTORC2 is considered a primary effector of the phosphatidylinositol-3-kinase (PI3K) signaling pathway, which is mutated in a majority of human cancers, in part through its ability to phosphorylate and regulate the proto-oncogene Akt/PKB. Many biological functions of mTOR have been pharmacologically explored using the natural product rapamycin, an allosteric inhibitor that has been reviewed extensively elsewhere. This review will focus specifically on the development of small molecule ATP-competitive inhibitors of mTOR and their prospects as a targeted therapy.

An ATP-competitive Mammalian Target of Rapamycin Inhibitor Reveals Rapamycin-resistant Functions of mTORC1

The mammalian target of rapamycin (mTOR) kinase is the catalytic subunit of two functionally distinct complexes, mTORC1 and mTORC2, that coordinately promote cell growth, proliferation, and survival. Rapamycin is a potent allosteric mTORC1 inhibitor with clinical applications as an immunosuppressant and anti-cancer agent. Here we find that Torin1, a highly potent and selective ATP-competitive mTOR inhibitor that directly inhibits both complexes, impairs cell growth and proliferation to a far greater degree than rapamycin. Surprisingly, these effects are independent of mTORC2 inhibition and are instead because of suppression of rapamycin-resistant functions of mTORC1 that are necessary for cap-dependent translation and suppression of autophagy. These effects are at least partly mediated by mTORC1-dependent and rapamycin-resistant phosphorylation of 4E-BP1. Our findings challenge the assumption that rapamycin completely inhibits mTORC1 and indicate that direct inhibitors of mTORC1 kinase activity may be more successful than rapamycin at inhibiting tumors that depend on mTORC1.

Biomarkers and surrogate endpoints in clinical research – some statistical challenges

The mammalian target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase and the founding member of a signaling pathway that regulates many fundamental features of cell growth and division. In cells, mTOR acts as the catalytic subunit of two functionally distinct complexes, called mTOR Complex 1 (mTORC1) and mTOR Complex 2 (mTORC2). Together, these complexes coordinate a variety of processes that include protein translation, autophagy, proliferation, survival and metabolism in response to nutrient, energy and growth factor signals. Consistent with its role as a growth-promoting pathway, numerous studies have found that mTOR signaling is hyper-activated in a broad spectrum of human cancers. In particular, mTORC2 is considered a primary effector of the phosphatidylinositol-3-kinase (PI3K) signaling pathway, which is mutated in a majority of human cancers, in part through its ability to phosphorylate and regulate the proto-oncogene Akt/PKB. Many biological functions of mTOR have been pharmacologically explored using the natural product rapamycin, an allosteric inhibitor that has been reviewed extensively elsewhere. This review will focus specifically on the development of small molecule ATP-competitive inhibitors of mTOR and their prospects as a targeted therapy.