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Catalytic mammalian target of rapamycin inhibitors as antineoplastic agents b
Nisha A. Mohindra & Leonidas C. Platanias
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a
Robert H. Lurie Comprehensive Cancer Center, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA b
Division of Hematology-Oncology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA c
Division of Hematology-Oncology, Department of Medicine, Jesse Brown Veterans Affairs Medical Center, Chicago, IL, USA Published online: 23 Apr 2015.
Click for updates To cite this article: Nisha A. Mohindra & Leonidas C. Platanias (2015): Catalytic mammalian target of rapamycin inhibitors as antineoplastic agents, Leukemia & Lymphoma To link to this article: http://dx.doi.org/10.3109/10428194.2015.1026816
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Leukemia & Lymphoma, 2015; Early Online: 1–6 © 2015 Informa UK, Ltd. ISSN: 1042-8194 print / 1029-2403 online DOI: 10.3109/10428194.2015.1026816
Emerging Drug Profile
Catalytic mammalian target of rapamycin inhibitors as antineoplastic agents Nisha A. Mohindra2 & Leonidas C. Platanias1,2,3 1Robert H. Lurie Comprehensive Cancer Center and 2Division of Hematology-Oncology, Feinberg School of Medicine,
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Northwestern University, Chicago, IL, USA and 3Division of Hematology-Oncology, Department of Medicine, Jesse Brown Veterans Affairs Medical Center, Chicago, IL, USA
As aberrant activation of the phosphatidylinositol 3-kinase (PI3K)/AKT/mTOR pathway promotes tumorigenesis [3,4,13–15], mTOR cascades have been considered attractive targets for the development of antitumor approaches for many years. Early investigations and efforts to inhibit the mTOR pathway for clinical applications utilized the agent rapamycin (sirolimus) [16,17]. However, rapamycin has not emerged as an antineoplastic agent because of unfavorable pharmacokinetics and limited antitumor effects [18]. Analogs to rapamycin were developed to overcome these limitations and are referred to as rapalogs [19]. Rapamycin and the rapalogs allosterically inhibit the FRB domain of mTORC1 by binding to the intracellular protein FKBP-12 [11]. Although these agents do not target mTORC2, there has been some limited evidence suggesting that prolonged continuous exposure to rapamycin can disrupt mTORC2 function as well [20]. However, such limited effects do not appear to be of clinical significance or relevance.
Abstract The mammalian target of rapamycin (mTOR) pathway is a major therapeutic target in the treatment of hematological malignancies, as it controls cellular events of high importance for regulation of mRNA translation and protein production. Rapalogs, or first-generation mTOR inhibitors, have produced only modest clinical benefits so far. Limitations to rapalogs likely result from the partial inhibition of mTORC1 substrates and lack of effects on mTORC2. Efforts toward the development of agents with more potent and complete inhibitory effects on the mTOR pathway have resulted in the development of catalytic mTOR inhibitors. Key preclinical and early clinical investigations of several catalytic mTOR inhibitors and potential resistance mechanisms to their activities are summarized here. Keywords: mTOR, leukemia, lymphoma
Introduction: mammalian target of rapamycin pathway and rapalogs
Rapalogs
The mammalian target of rapamycin (mTOR) pathway plays an integral role in cellular growth and metabolism by controlling protein synthesis in response to a variety of stimuli and signals [1–5]. mTOR is present in two distinct protein complexes, mTORC1 and mTORC2 [6–10]. mTORC1 is composed of the mTOR kinase and is associated with regulatory proteins Raptor, PRAS40, DEPTOR and mLST8. mTORC1 plays a central role in regulating cellular metabolism, mRNA translation and other downstream effects through S6K [6–10], 4E-BP1 [6–10] and Grb10 [6,11,12]. mTORC2 also utilizes mTOR kinase as its main subunit; however, it is associated with proteins such as Rictor, SIN1 and PROTOR [6,7,10]. mTORC2 is involved in cell survival and proliferation through activation of kinases such as AKT, specifically through phosphorylation of Ser473 [6,7,10] and protein kinase Ca [6,7,10]. The specific interactions between mTORC1 and mTORC2 have not been fully elucidated, but there is evidence of cross-talk between these complexes [10].
The rapalogs include temsirolimus (CCI779) [21], everolimus (RAD001) [22] and ridaforolimus (AP23573) [23]. These agents have been investigated in a number of malignancies and are now approved for the treatment of mantle cell lymphoma [24], advanced renal cell carcinoma [25,26], pancreatic neuroendocrine tumors [27] and hormone receptor-positive breast cancer [28]. Despite some clinical success, rapalogs are not universally efficacious and resistance to therapy can develop quickly. Because of the modest antitumor effects of rapalogs, efforts were initiated that have led to the development of catalytic inhibitors of mTOR kinase, with the hope that such agents may address some of the shortcomings of the first-generation agents. In addition, because of such limitations, searches for other mTOR inhibitors with unique mechanisms of action are ongoing. Below we review the problems and limitations associated with the clinical use of rapalogs, the rationale for dual inhibitors and
Correspondence: Leonidas C. Platanias, Robert H. Lurie Comprehensive Cancer Center, Feinberg School of Medicine, Northwestern University, Chicago, IL, 60611, USA. E-mail: [email protected] Received 6 January 2015; revised 18 February 2015; accepted 3 March 2015
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2 N. A. Mohindra & L. C. Platanias the catalytic mTOR inhibitors currently undergoing clinical investigation.
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Limitations of rapalogs Despite promising preclinical evidence, clinical benefits have not been as dramatic with the clinical use of various rapalogs [29]. Some of the limitations associated with rapalogs arise from lack of effects on mTORC2 complexes; incomplete mTORC1 inhibition and associated downstream signals; and activation of pro-survival feedback loops [29]. Rapalogs can frequently lead only to inhibition of S6K1, but not 4E-BP1 phosphorylation, and this may be one of the major reasons for the clinical limitations of these drugs [30]. Although SK61 phosphorylation is inhibited by the rapalogs, rapalogs have less activity and inconsistent effects on the phosphorylation of 4E-BP1, an event important for cap-dependent mRNA translation and subsequent activation of oncogenic pathways. When mTORC1 phosphorylates 4E-BP1, eukaryotic translation initiation factor 4E (eIF4E) is dissociated from 4E-BP1 and can form complexes with other translation-initiation factors, which are involved in recruitment of ribosomes to mRNA, leading to cap-dependent translation [31]. In certain cell types treated by rapamycin, there is a recovery of 4E-BP1 phosphorylation at some point during treatment, which can allow resumption of events promoting cap-dependent translation [30,32]. Additionally, phosphorylated 4E-BP1 appears to be a convergence point for several oncogenic signals that lead to proliferation and survival [33]. Rapalogs can also paradoxically stimulate PI3K/AKT and mitogen activated protein kinase (MAPK) activities through activation of feedback loops and signaling from receptor tyrosine kinases (RTKs) [34– 40]. Further, despite their effectiveness on certain mTORC1 related functions, short-term use of rapalogs produces very limited effects on mTORC2 function and can lead to changes in phosphorylation of subunits within the mTORC2 complex [20,41–44]. The stability of mTORC2 is dependent on the presence of the core subunits Rictor and SIN1. Both Rictor and SIN1 are highly phosphorylated, indicating that their phosphorylation status may modulate their activities and thereby mTORC2 functions. Also, there is some evidence that S6K1 can phosphorylate Rictor and play a role in modulating mTORC2 [43,44]. While the function of this phosphorylation event remains to be precisely defined, it does not affect the structure or direct kinase activity of mTORC2 [44]. Nonetheless when mutated Rictor cannot be phosphorylated it leads to increased mTORC2 phosphorylation of AKT [44]. Thus, rapamycin mediated loss of S6K1 activity may further promote AKT activity through this mechanism.
Rationale for dual mTORC1/mTORC2 inhibition The complexity of the mTOR pathway and the multiple feedback loops that result from isolated mTORC1 inhibition have led to the development of unique agents that address at least some of these mechanisms [8,45,46]. There have been intensive efforts to develop mTOR selective kinase inhibitors (TOR-KIs) that suppress both mTORC1 and mTORC2 through blockade of this catalytic site. TOR-KIs have been
shown to produce greater effects in inhibiting protein synthesis, inducing autophagy and apoptosis, causing cell cycle arrest, and affecting lipid and glucose metabolism critical to tumor survival [47–57]. Although several TOR-KIs have been evaluated pre-clinically, only a few agents have entered clinical development, namely AZD8055, AZD2014, OSI-027 and INK128.
AZD8055 AZD8055 is an orally available adenosine triphosphate (ATP)-competitive inhibitor of mTOR kinase [47]. AZD8055 was optimized from Ku-0063794, a molecule originally identified from a screening library as an inhibitor of mTOR kinase activity [58]. Despite the high affinity of Ku-0063794 for the mTOR kinase, its low aqueous solubility and high activity against the hERG (human ether-a-go-go-related gene) ion channel made further clinical development difficult [58]. AZD8055 then emerged through molecular modifications to Ku0065794 that addressed solubility and effects on hERG [58]. AZD8055 inhibits the mTOR kinase with a 50% inhibitory concentration (IC50) of 0.8 nmol/L. It is highly selective for mTOR and exhibits a 1000-fold decrease in potency when screened against class I and class III PI3K lipid kinases, as well as members of the PI3K-like kinase family. Additionally it has shown no activity against a panel of 260 kinases at a concentration of 10 mmol/L [47]. AZD8055 inhibits phosphorylation of both major mTORC1 substrates, S6K and 4E-BP1, in vitro. This includes phosphorylation of key sites of 4E-BP1, at Ser65, Thr70 and Thr37/46, producing potent effects on cap-dependent mRNA translation. Simultaneous inhibition of mTORC1 and mTORC2 prevents feedback phosphorylation of AKT on Ser473, which often occurs with rapalogs due to release of the negative feedback loop between S6K and IRS1. In addition, AZD8055 induces autophagy in vitro as well as in a variety of tumor xenograft models [47,48]. In acute myeloid leukemia (AML) cell lines, AZD8055 was shown to significantly suppress growth of leukemic clone progenitors; however, it does not affect the differentiation of CD34 progenitor cells [57]. Further, AZD8055 produced caspase-dependent apoptosis of leukemic cells but not of normal immature CD34 cells, making it an attractive targeting agent [57]. The promising preclinical evidence led to the investigation of AD8055 in two phase I trials. The first study assessed the safety and tolerability of AZD8055 in patients with advanced solid tumors or lymphomas [59]. Forty-nine patients were evaluated in the safety, pharmacokinetics and efficacy analyses after receiving at least one dose of study treatment. There were seven treatment cohorts consisting of three dose levels of the oral solution and four dose levels of the tablet form. Median duration of treatment was 62 days across all seven cohorts. The most frequent adverse events related to AZD8055 treatment were increases in liver function tests and fatigue. Other adverse events included nausea, decreased appetite and diarrhea. The most common dose limiting toxicity was elevation in transaminases; however, in the majority of cases, transaminases resolved back to baseline or normal values with dose
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Catalytic mTOR inhibitors 3 reduction or treatment interruption. Pharmacokinetic analysis revealed that AZD8055 was quickly absorbed and eliminated. On the other hand, pharmacodynamic analysis was relatively challenging as there was high variability in pretreatment marker levels. In fact, most patients had very low pre-dose levels of pAKT, making detection of treatment related decreases difficult. There were no complete or partial responses seen among treated patients. However, seven patients had stable disease for greater than or equal to 4 months, including three patients with head/neck cancer and one patient each with renal cell carcinoma, thymic carcinoma, thyroid cancer and melanoma. Compared historically to rapalogs, AD8055 produced less mucositis and skin rashes but was more notably associated with rises in liver transaminases. Alternative dosing schedules were proposed to overcome the liver function test abnormalities but were not pursued further. The second phase I study of AZD8055 was done in 17 Japanese patients with advanced solid tumors and the results mirrored the results found in the previous study [60]. Dose limiting toxicity was due to elevations in aspartate transaminase (AST) and alanine transaminase (ALT). The most common adverse events were stomatitis, rash, decreased appetite, nausea and increases in transaminases. There were similar limitations to pharmacodynamic analysis in this study, as there were variable biomarker levels prior to treatment. However, mean pAKT and p4EBP1 levels decreased in most treatment cohorts, indicating mTORC1/2 inhibition. No complete or partial responses to therapy were seen in this population and only two patients had stable disease. A multinational phase I/II trial of AZD8055 in Asian patients with advanced hepatocellular carcinoma (HCC) and mild to moderate hepatic impairment has been completed, but the results are not yet available. However, further clinical development of AZD8055 is not being pursued due to unpredictable pharmacokinetics that led to inconsistent exposure in rodents and the high rate of metabolism in human hepatocytes [58]. Efforts to optimize the AZD8055 compound led to the identification of AZD2014 that is now under clinical investigation [58].
AZD2014 By removing the methoxy group from AZD8055, the resultant compound, AZD2014 (Table I), achieves potent inhibition of mTOR, with good aqueous solubility and low hepatic turnover [58]. AZD2014 displays high selectivity against the PIK Table I. AZD2014 quick profile. Drug name Company Other names MoA MoR MTD DLT Schedule Plasma concentration Plasma half-life
AZD2014 AstraZeneca, London, England None mTORC1 and mTORC2 inhibitor Not known 50 mg twice a day Grade 3 mucositis, grade 2 lethargy Twice daily 1.7 mg/mL (Cmax) ∼3 h
oA, mechanism of action; MoR, mechanism of resistance; MTD, maximum M tolerated dose; DLT, dose limiting toxicity.
family kinases and no activity against a panel of 200 other kinases at 10 mM. In vitro, AZD2014 is five times less potent than AZD8055 but still produces anti-proliferative effects and even cell death in a variety of tumor cell lines [56]. In addition, AZD2014 has produced growth inhibition in several xenograft models, including an animal model of endocrine refractory estrogen receptor-positive breast cancer [56]. Clinical evaluation of AZD2014 is under way, with interim analysis from a phase I study regarding safety and clinical activity. In this phase I dose escalation and expansion study, 50 patients with advanced solid tumors were given AZD2014 in either single doses or twice daily, up to 100 mg twice a day [61]. The maximum tolerated dose was determined to be 50 mg twice a day. The most common adverse reactions were fatigue, stomatitis, anorexia, nausea and diarrhea. Reductions in pAKT and p4E-BP1 were noted in platelet rich plasma at 2 h and in peripheral blood mononuclear cells at 8 h. At a dose of 50 mg twice a day, AZD2014 led to a decrease in cytoplasmic phosphorylated S6 in eight of 10 paired tumor biopsies and decreased phosphorylated 4E-BP1 in three of nine paired biopsies, indicating mTORC1 inhibition. pAKT was reduced in three out of six samples, indicating mTORC2 inhibition. Two patients achieved a partial response with AZD2014 and four patients had stable disease [62]. Further clinical evaluation of AZD2014 is being pursued, both as monotherapy and in combination with other therapeutic agents.
OSI-027 OSI-027 is highly selective for mTOR, with 100 times less potency against PI3K and almost no activity against a panel of other kinases at a concentration of 1 mmol/L [55]. OSI-027 affects both mTORC1 and mTORC2 downstream targets in a dose-dependent manner [49]. In contrast to rapamycin treatment, OSI-027 blocks mTORC2-mediated phosphorylation of AKT at Ser473 and mTORC1 substrate 4E-BP1 on Thr37/46. The effects on pAKT and p4E-BP1 in vitro appear to correlate with greater inhibition of cell proliferation and cell death in vivo [55]. OSI-027 produced greater antileukemic effects than rapamycin in chronic myeloid leukemia (CML) and AML cell lines [52,54]. These studies underscored the importance of mTORC2 in leukemogenesis and demonstrated that dual blockade of mTORC1 and mTORC2 is needed for effective antileukemic responses. Further, OSI-027 was able to induce apoptosis in Philadelphia chromosome positive (Ph) hematologic malignancies harboring the T315I mutation [54]. Among lymphoid malignancies, OSI-027 induced apoptosis in acute lymphocytic leukemia (ALL), mantle cell lymphoma, marginal zone lymphoma and Sezary cell lines [51]. Similar success was noted in pre-clinical solid tumor models in which OSI-027 inhibited in vitro breast cancer proliferation and further enhanced chemotherapy-induced apoptosis [50]. When used in combination with vascular endothelial growth factor receptor (VEGFR) tyrosine kinase inhibitors in lung and ovarian xenograft models, the addition of OSI-027 potentiated antiangiogenic effects and produced greater growth inhibition and tumor regression than either agent alone [49].
4 N. A. Mohindra & L. C. Platanias A phase I study evaluated three dose schedules of OSI-027 in 31 patients with advanced solid tumors and lymphoma [63]. Dose limiting toxicities were due to changes in left ventricular ejection fraction and fatigue. The maximum tolerated dose (MTD) had not been reached at the time of presentation of the data. Other drug related toxicities included nausea, vomiting, diarrhea, fatigue, elevated creatinine and reversible increases in QTc. Pharmacokinetic analysis revealed dose-dependent drug exposure. Phosphorylation of 4E-BP1 was reduced in peripheral blood mononuclear cells in 13 of 23 patients, correlating with mTORC1 inhibition. There were no responses to therapy, but eight patients were able to achieve stable disease for greater than 12 weeks. The trial has now been closed to accrual and updated results are awaited (NCT00698243).
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Limitations to OSI-027 Among certain leukemia cell lines, treatment with OSI-027 resulted in induction of autophagy, which appeared to attenuate apoptosis unless an autophagy inhibitor was used concurrently [53,54]. Thus, autophagy induced by OSI-027 may be protective and a limitation to achieving full therapeutic benefit from this agent when used as monotherapy. In lymphoid malignancies, BCL-2 overexpression reduced induction of apoptosis by OSI-027 [51]. These studies indicated that OSI-027 relied on the transcriptional activity of the PUMA and BIM genes to achieve cell death, both of which were reduced when BCL-2 was overexpressed. Therefore, BCL-2 expression may represent a mechanism of resistance to OSI-027 in certain malignancies [51].
INK128/MLN0128 INK128 (MLN0128, Table II) was discovered through rational drug design. This catalytic inhibitor blocks the mTOR kinase at sub-nanomolar concentrations [64]. It has been shown to be highly selective for the mTOR kinase when tested against a panel of more than 400 kinases. Phosphorylation of mTORC1 substrates S6 and 4E-BP1 and mTORC2 substrate AKT are inhibited in vitro and in vivo [64]. Significant activity against these substrates in cell lines resistant to rapamycin and pan-PI3K inhibitors has been also documented [64]. INK128 produced antileukemic effects in B-ALL cell lines in vitro and reduced colony formation in patient derived Table II. INK128 quick profile. Drug name Company Other names MoA MoR MTD DLT Schedule Plasma concentration Plasma half-life
INK128 Takeda Pharmaceutical Company, Osaka, Japan MLN0128 mTORC1 and mTORC2 inhibitor Not known 6 mg daily (MTD for 3 days a week every week was not reached at time of data presentation) Uriticaria, mucositis/stomatitis Daily; weekly dose not published Not available ∼8 h
oA, mechanism of action; MoR, mechanism of resistance; MTD, maximum M tolerated dose; DLT, dose limiting toxicity.
non-Ph and Ph leukemic cells. INK128 also reduced leukemic growth in B-ALL xenografts without affecting bone marrow function and further enhances the efficacy of dasatinib in Ph B-ALL cell lines and murine models. In nonPh ALL, INK128 produced a cytostatic effect when used as a single agent [65]. In multiple myeloma (MM) cell lines, INK128 significantly inhibited proliferation in vitro even in the presence of growth stimulation from bone marrow stromal cells or interleukin-6 (IL-6) [66]. Likewise, decreased phosphorylation of AKT and 4E-BP1 were noted in vivo, as well as activation of caspase 3 necessary for apoptosis [66]. Interestingly, unlike rapamycin, INK128 significantly inhibited multiple myeloma cell adhesion to bone marrow stromal cells [66]. INK128 did not suppress normal lymphocytes or granulocytes, making this agent an attractive therapeutic agent for monotherapy or combination therapy [65,66]. As c-myc driven oncogenesis is thought to be derived in part by protein synthesis via mTORC1-dependent 4E-BP1 phosphorylation [67], the effects of INK128 in myc-driven MM and lymphoma cell lines were assessed in one study [67]. INK128 treatment significantly reduced 4E-PB1 phosphorylation in myc-driven MM and lymphoma cell lines, providing a rationale for use of this agent in these malignancies as well [67]. Apart from hematologic malignancies, there is strong evidence of INK128 activity in cell lines and xenograft models from several tumors, underscoring the potential importance of this agent in clinical oncology [68–73]. Preliminary results are available from a phase I dose escalation study of INK128 in patients with relapsed/refractory MM, Waldenström macroglobulinemia (WM) and nonHodgkin lymphoma [74]. Thirty-seven patients were enrolled and treated with six dose levels of INK128 (either daily or daily 3 days on and 4 days off in 28-day cycles). The MTD daily dose was 6 mg; however, the MTD for the three times a week dosing had not been reached at the time the study was presented. Dose limiting toxicities were related to thrombocytopenia, mucositis/stomatitis and urticaria among both cohorts. Some 27% of patients discontinued the drug due to adverse events, while 43% of patients experienced grade 3 or 4 adverse events, the most common of which included thrombocytopenia, fatigue, mucositis and neutropenia. Of the 27 patients evaluable for response, one patient with MM achieved a minor response, while 13 patients with MM and two with WM had stable disease [74]. Several other clinical investigations of INK128 are ongoing in both hematologic and solid malignancies.
Conclusions Several large clinical investigations have failed to demonstrate dramatic responses using rapalog therapy [29]. Limitations to rapalogs result from partial inhibition of mTORC1 substrates and lack of meaningful effects on mTORC2. Preclinical investigation of catalytic inhibitors of mTOR kinase has demonstrated anti-proliferative effects in a variety of tumor models, and these agents have now entered clinical investigation. Preliminary evidence suggests that some of these drugs may be effective as monotherapy or in combination with chemotherapy or other targeted agents. The full therapeutic potential of this class of drugs remains to be
Catalytic mTOR inhibitors 5 defined, and the results of ongoing clinical trials will provide an answer to these issues. Potential conflict of interest: Disclosure forms provided by the authors are available with the full text of this article at www.informahealthcare.com/lal.
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6 N. A. Mohindra & L. C. Platanias Puma-dependent apoptosis in lymphoid malignancies. Blood 2012;119:476–487. [52] Altman JK, Sassano A, Kaur S, et al. Dual mTORC2/mTORC1 targeting results in potent suppressive effects on acute myeloid leukemia (AML) progenitors. Clin Cancer Res 2011;17:4378–4388. [53] Vakana E, Sassano A, Platanias LC. Induction of autophagy by dual mTORC1-mTORC2 inhibition in BCR-ABL-expressing leukemic cells. Autophagy 2010;6:966–967. [54] Carayol N, Vakana E, Sassano A, et al. Critical roles for mTORC2and rapamycin-insensitive mTORC1-complexes in growth and survival of BCR-ABL-expressing leukemic cells. Proc Natl Acad Sci USA 2010;107:12469–12474. [55] Bhagwat SV, Gokhale PC, Crew AP, et al. Preclinical characterization of OSI-027, a potent and selective inhibitor of mTORC1 and mTORC2: distinct from rapamycin. Mol Cancer Ther 2011;10: 1394–1406. [56] Guichard S, Howard Z, Heathcote D. AZD2014, a dual mTORC1 and mTORC2 inhibitor is differentiated from allosteric inhibitors of mTORC1 in ER breast cancer. Cancer Res 2012;72(Suppl.): Abstract 917. [57] Willems L, Chapuis N, Puissant A, et al. The dual mTORC1 and mTORC2 inhibitor AZD8055 has anti-tumor activity in acute myeloid leukemia. Leukemia 2012;26:1195–1202. [58] Pike KG, Malagu K, Hummersone MG, et al. Optimization of potent and selective dual mTORC1 and mTORC2 inhibitors: the discovery of AZD8055 and AZD2014. Bioorg Med Chem Lett 2013;23:1212–1216. [59] Naing A, Aghajanian C, Raymond E, et al. Safety, tolerability, pharmacokinetics and pharmacodynamics of AZD8055 in advanced solid tumours and lymphoma. Br J Cancer 2012;107:1093–1099. [60] Asahina H, Nokihara H, Yamamoto N, et la. Safety and tolerability of AZD8055 in Japanese patients with advanced solid tumors; a dosefinding phase I study. Invest New Drugs 2013;31:677–684. [61] Banerji U, Dean E, Gonzalez M. First-in-human phase I trial of the dual mTORC1 and mTORC2 inhibitor AZD2014 in solid tumors. J Clin Oncol 2012;30(15 Suppl.): Abstract 3004. [62] AZD2014. AstraZeneca. Available from: www.ncats.nih.gov/files/ AZD2014.pdf [63] Tan DS, Dumez H, Olmos D, et al. First in-human phase I study exploring three schedules of OSI-027, a novel small molecule
TORC1/TORC2 inhibitor, in patients with advanced solid tumors and lymphoma. J Clin Oncol 2010;28(15 Suppl.): Abstract 3006. [64] Jessen K, Wang S, Kessler L, et al. INK128 is a potent and selective TORC1/2 inhibitor with broad oral anti-tumor activity. Presented at: AACR 2009 Molecular Targets and Cancer Therapeutics Meeting, November 2009, Boston, MA; poster B148. [65] Janes MR, Vu C, Mallya S, et al. Efficacy of the investigational mTOR kinase inhibitor MLN0128/INK128 in models of B-cell acute lymphoblastic leukemia. Leukemia 2013;27:586–594. [66] Maiso P, Liu Y, Morgan B, et al. Defining the role of TORC1/2 in multiple myeloma. Blood 2011;118:6860–6870. [67] Pourdehnad M, Truitt ML, Siddiqi IN, et al. Myc and mTOR converge on a common node in protein synthesis control that confers synthetic lethality in Myc-driven cancers. Proc Natl Acad Sci USA 2013;110:11988–11993. [68] Zhang H, Dou J, Yu Y, et al. mTOR ATP-competitive inhibitor INK128 inhibits neuroblastoma growth via blocking mTORC signaling. Apoptosis 2015;20:50–62. [69] Lou HZ, Weng XC, Pan HM, et al. The novel mTORC1/2 dual inhibitor INK-128 suppresses survival and proliferation of primary and transformed human pancreatic cancer cells. Biochem Biophys Res Commun 2014;450:973–978. [70] Kang MH, Reynolds CP, Maris JM, et al. Initial testing (stage 1) of the investigational mTOR kinase inhibitor MLN0128 by the pediatric preclinical testing program. Pediatr Blood Cancer 2014;61:1486–1489. [71] Ingels A, Zhao H, Thong AE, et al. Preclinical trial of a new dual mTOR inhibitor, MLN0128, using renal cell carcinoma tumorgrafts. Int J Cancer 2014;134:2322–2329. [72] Hayman TJ, Wahba A, Rath BH, et al. The ATP-competitive mTOR inhibitor INK128 enhances in vitro and in vivo radiosensitivity of pancreatic carcinoma cells. Clin Cancer Res 2014;20:110–119. [73] Gild ML, Landa I, Ryder M, et al. Targeting mTOR in RET mutant medullary and differentiated thyroid cancer cells. Endocr Relat Cancer 2013;20:659–667. [74] Ghobrial IM, Siegel D, Vij R, et al. MLN0128 (INK128), an investigational oral dual TORC1/2 inhibitor, in patients with relapsed or refractory multiple myeloma, non-Hodgkin’s lymphoma, or Waldenstrom macroglobulinemia: preliminary results from a phase I dose-escalation study. Blood 2012;120(Suppl. 1): Abstract 4038.
Leukemia & Lymphoma Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ilal20
Catalytic mammalian target of rapamycin inhibitors as antineoplastic agents b
Nisha A. Mohindra & Leonidas C. Platanias
abc
a
Robert H. Lurie Comprehensive Cancer Center, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA b
Division of Hematology-Oncology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA c
Division of Hematology-Oncology, Department of Medicine, Jesse Brown Veterans Affairs Medical Center, Chicago, IL, USA Published online: 23 Apr 2015.
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Leukemia & Lymphoma, 2015; Early Online: 1–6 © 2015 Informa UK, Ltd. ISSN: 1042-8194 print / 1029-2403 online DOI: 10.3109/10428194.2015.1026816
Emerging Drug Profile
Catalytic mammalian target of rapamycin inhibitors as antineoplastic agents Nisha A. Mohindra2 & Leonidas C. Platanias1,2,3 1Robert H. Lurie Comprehensive Cancer Center and 2Division of Hematology-Oncology, Feinberg School of Medicine,
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Northwestern University, Chicago, IL, USA and 3Division of Hematology-Oncology, Department of Medicine, Jesse Brown Veterans Affairs Medical Center, Chicago, IL, USA
As aberrant activation of the phosphatidylinositol 3-kinase (PI3K)/AKT/mTOR pathway promotes tumorigenesis [3,4,13–15], mTOR cascades have been considered attractive targets for the development of antitumor approaches for many years. Early investigations and efforts to inhibit the mTOR pathway for clinical applications utilized the agent rapamycin (sirolimus) [16,17]. However, rapamycin has not emerged as an antineoplastic agent because of unfavorable pharmacokinetics and limited antitumor effects [18]. Analogs to rapamycin were developed to overcome these limitations and are referred to as rapalogs [19]. Rapamycin and the rapalogs allosterically inhibit the FRB domain of mTORC1 by binding to the intracellular protein FKBP-12 [11]. Although these agents do not target mTORC2, there has been some limited evidence suggesting that prolonged continuous exposure to rapamycin can disrupt mTORC2 function as well [20]. However, such limited effects do not appear to be of clinical significance or relevance.
Abstract The mammalian target of rapamycin (mTOR) pathway is a major therapeutic target in the treatment of hematological malignancies, as it controls cellular events of high importance for regulation of mRNA translation and protein production. Rapalogs, or first-generation mTOR inhibitors, have produced only modest clinical benefits so far. Limitations to rapalogs likely result from the partial inhibition of mTORC1 substrates and lack of effects on mTORC2. Efforts toward the development of agents with more potent and complete inhibitory effects on the mTOR pathway have resulted in the development of catalytic mTOR inhibitors. Key preclinical and early clinical investigations of several catalytic mTOR inhibitors and potential resistance mechanisms to their activities are summarized here. Keywords: mTOR, leukemia, lymphoma
Introduction: mammalian target of rapamycin pathway and rapalogs
Rapalogs
The mammalian target of rapamycin (mTOR) pathway plays an integral role in cellular growth and metabolism by controlling protein synthesis in response to a variety of stimuli and signals [1–5]. mTOR is present in two distinct protein complexes, mTORC1 and mTORC2 [6–10]. mTORC1 is composed of the mTOR kinase and is associated with regulatory proteins Raptor, PRAS40, DEPTOR and mLST8. mTORC1 plays a central role in regulating cellular metabolism, mRNA translation and other downstream effects through S6K [6–10], 4E-BP1 [6–10] and Grb10 [6,11,12]. mTORC2 also utilizes mTOR kinase as its main subunit; however, it is associated with proteins such as Rictor, SIN1 and PROTOR [6,7,10]. mTORC2 is involved in cell survival and proliferation through activation of kinases such as AKT, specifically through phosphorylation of Ser473 [6,7,10] and protein kinase Ca [6,7,10]. The specific interactions between mTORC1 and mTORC2 have not been fully elucidated, but there is evidence of cross-talk between these complexes [10].
The rapalogs include temsirolimus (CCI779) [21], everolimus (RAD001) [22] and ridaforolimus (AP23573) [23]. These agents have been investigated in a number of malignancies and are now approved for the treatment of mantle cell lymphoma [24], advanced renal cell carcinoma [25,26], pancreatic neuroendocrine tumors [27] and hormone receptor-positive breast cancer [28]. Despite some clinical success, rapalogs are not universally efficacious and resistance to therapy can develop quickly. Because of the modest antitumor effects of rapalogs, efforts were initiated that have led to the development of catalytic inhibitors of mTOR kinase, with the hope that such agents may address some of the shortcomings of the first-generation agents. In addition, because of such limitations, searches for other mTOR inhibitors with unique mechanisms of action are ongoing. Below we review the problems and limitations associated with the clinical use of rapalogs, the rationale for dual inhibitors and
Correspondence: Leonidas C. Platanias, Robert H. Lurie Comprehensive Cancer Center, Feinberg School of Medicine, Northwestern University, Chicago, IL, 60611, USA. E-mail: [email protected] Received 6 January 2015; revised 18 February 2015; accepted 3 March 2015
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2 N. A. Mohindra & L. C. Platanias the catalytic mTOR inhibitors currently undergoing clinical investigation.
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Limitations of rapalogs Despite promising preclinical evidence, clinical benefits have not been as dramatic with the clinical use of various rapalogs [29]. Some of the limitations associated with rapalogs arise from lack of effects on mTORC2 complexes; incomplete mTORC1 inhibition and associated downstream signals; and activation of pro-survival feedback loops [29]. Rapalogs can frequently lead only to inhibition of S6K1, but not 4E-BP1 phosphorylation, and this may be one of the major reasons for the clinical limitations of these drugs [30]. Although SK61 phosphorylation is inhibited by the rapalogs, rapalogs have less activity and inconsistent effects on the phosphorylation of 4E-BP1, an event important for cap-dependent mRNA translation and subsequent activation of oncogenic pathways. When mTORC1 phosphorylates 4E-BP1, eukaryotic translation initiation factor 4E (eIF4E) is dissociated from 4E-BP1 and can form complexes with other translation-initiation factors, which are involved in recruitment of ribosomes to mRNA, leading to cap-dependent translation [31]. In certain cell types treated by rapamycin, there is a recovery of 4E-BP1 phosphorylation at some point during treatment, which can allow resumption of events promoting cap-dependent translation [30,32]. Additionally, phosphorylated 4E-BP1 appears to be a convergence point for several oncogenic signals that lead to proliferation and survival [33]. Rapalogs can also paradoxically stimulate PI3K/AKT and mitogen activated protein kinase (MAPK) activities through activation of feedback loops and signaling from receptor tyrosine kinases (RTKs) [34– 40]. Further, despite their effectiveness on certain mTORC1 related functions, short-term use of rapalogs produces very limited effects on mTORC2 function and can lead to changes in phosphorylation of subunits within the mTORC2 complex [20,41–44]. The stability of mTORC2 is dependent on the presence of the core subunits Rictor and SIN1. Both Rictor and SIN1 are highly phosphorylated, indicating that their phosphorylation status may modulate their activities and thereby mTORC2 functions. Also, there is some evidence that S6K1 can phosphorylate Rictor and play a role in modulating mTORC2 [43,44]. While the function of this phosphorylation event remains to be precisely defined, it does not affect the structure or direct kinase activity of mTORC2 [44]. Nonetheless when mutated Rictor cannot be phosphorylated it leads to increased mTORC2 phosphorylation of AKT [44]. Thus, rapamycin mediated loss of S6K1 activity may further promote AKT activity through this mechanism.
Rationale for dual mTORC1/mTORC2 inhibition The complexity of the mTOR pathway and the multiple feedback loops that result from isolated mTORC1 inhibition have led to the development of unique agents that address at least some of these mechanisms [8,45,46]. There have been intensive efforts to develop mTOR selective kinase inhibitors (TOR-KIs) that suppress both mTORC1 and mTORC2 through blockade of this catalytic site. TOR-KIs have been
shown to produce greater effects in inhibiting protein synthesis, inducing autophagy and apoptosis, causing cell cycle arrest, and affecting lipid and glucose metabolism critical to tumor survival [47–57]. Although several TOR-KIs have been evaluated pre-clinically, only a few agents have entered clinical development, namely AZD8055, AZD2014, OSI-027 and INK128.
AZD8055 AZD8055 is an orally available adenosine triphosphate (ATP)-competitive inhibitor of mTOR kinase [47]. AZD8055 was optimized from Ku-0063794, a molecule originally identified from a screening library as an inhibitor of mTOR kinase activity [58]. Despite the high affinity of Ku-0063794 for the mTOR kinase, its low aqueous solubility and high activity against the hERG (human ether-a-go-go-related gene) ion channel made further clinical development difficult [58]. AZD8055 then emerged through molecular modifications to Ku0065794 that addressed solubility and effects on hERG [58]. AZD8055 inhibits the mTOR kinase with a 50% inhibitory concentration (IC50) of 0.8 nmol/L. It is highly selective for mTOR and exhibits a 1000-fold decrease in potency when screened against class I and class III PI3K lipid kinases, as well as members of the PI3K-like kinase family. Additionally it has shown no activity against a panel of 260 kinases at a concentration of 10 mmol/L [47]. AZD8055 inhibits phosphorylation of both major mTORC1 substrates, S6K and 4E-BP1, in vitro. This includes phosphorylation of key sites of 4E-BP1, at Ser65, Thr70 and Thr37/46, producing potent effects on cap-dependent mRNA translation. Simultaneous inhibition of mTORC1 and mTORC2 prevents feedback phosphorylation of AKT on Ser473, which often occurs with rapalogs due to release of the negative feedback loop between S6K and IRS1. In addition, AZD8055 induces autophagy in vitro as well as in a variety of tumor xenograft models [47,48]. In acute myeloid leukemia (AML) cell lines, AZD8055 was shown to significantly suppress growth of leukemic clone progenitors; however, it does not affect the differentiation of CD34 progenitor cells [57]. Further, AZD8055 produced caspase-dependent apoptosis of leukemic cells but not of normal immature CD34 cells, making it an attractive targeting agent [57]. The promising preclinical evidence led to the investigation of AD8055 in two phase I trials. The first study assessed the safety and tolerability of AZD8055 in patients with advanced solid tumors or lymphomas [59]. Forty-nine patients were evaluated in the safety, pharmacokinetics and efficacy analyses after receiving at least one dose of study treatment. There were seven treatment cohorts consisting of three dose levels of the oral solution and four dose levels of the tablet form. Median duration of treatment was 62 days across all seven cohorts. The most frequent adverse events related to AZD8055 treatment were increases in liver function tests and fatigue. Other adverse events included nausea, decreased appetite and diarrhea. The most common dose limiting toxicity was elevation in transaminases; however, in the majority of cases, transaminases resolved back to baseline or normal values with dose
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Catalytic mTOR inhibitors 3 reduction or treatment interruption. Pharmacokinetic analysis revealed that AZD8055 was quickly absorbed and eliminated. On the other hand, pharmacodynamic analysis was relatively challenging as there was high variability in pretreatment marker levels. In fact, most patients had very low pre-dose levels of pAKT, making detection of treatment related decreases difficult. There were no complete or partial responses seen among treated patients. However, seven patients had stable disease for greater than or equal to 4 months, including three patients with head/neck cancer and one patient each with renal cell carcinoma, thymic carcinoma, thyroid cancer and melanoma. Compared historically to rapalogs, AD8055 produced less mucositis and skin rashes but was more notably associated with rises in liver transaminases. Alternative dosing schedules were proposed to overcome the liver function test abnormalities but were not pursued further. The second phase I study of AZD8055 was done in 17 Japanese patients with advanced solid tumors and the results mirrored the results found in the previous study [60]. Dose limiting toxicity was due to elevations in aspartate transaminase (AST) and alanine transaminase (ALT). The most common adverse events were stomatitis, rash, decreased appetite, nausea and increases in transaminases. There were similar limitations to pharmacodynamic analysis in this study, as there were variable biomarker levels prior to treatment. However, mean pAKT and p4EBP1 levels decreased in most treatment cohorts, indicating mTORC1/2 inhibition. No complete or partial responses to therapy were seen in this population and only two patients had stable disease. A multinational phase I/II trial of AZD8055 in Asian patients with advanced hepatocellular carcinoma (HCC) and mild to moderate hepatic impairment has been completed, but the results are not yet available. However, further clinical development of AZD8055 is not being pursued due to unpredictable pharmacokinetics that led to inconsistent exposure in rodents and the high rate of metabolism in human hepatocytes [58]. Efforts to optimize the AZD8055 compound led to the identification of AZD2014 that is now under clinical investigation [58].
AZD2014 By removing the methoxy group from AZD8055, the resultant compound, AZD2014 (Table I), achieves potent inhibition of mTOR, with good aqueous solubility and low hepatic turnover [58]. AZD2014 displays high selectivity against the PIK Table I. AZD2014 quick profile. Drug name Company Other names MoA MoR MTD DLT Schedule Plasma concentration Plasma half-life
AZD2014 AstraZeneca, London, England None mTORC1 and mTORC2 inhibitor Not known 50 mg twice a day Grade 3 mucositis, grade 2 lethargy Twice daily 1.7 mg/mL (Cmax) ∼3 h
oA, mechanism of action; MoR, mechanism of resistance; MTD, maximum M tolerated dose; DLT, dose limiting toxicity.
family kinases and no activity against a panel of 200 other kinases at 10 mM. In vitro, AZD2014 is five times less potent than AZD8055 but still produces anti-proliferative effects and even cell death in a variety of tumor cell lines [56]. In addition, AZD2014 has produced growth inhibition in several xenograft models, including an animal model of endocrine refractory estrogen receptor-positive breast cancer [56]. Clinical evaluation of AZD2014 is under way, with interim analysis from a phase I study regarding safety and clinical activity. In this phase I dose escalation and expansion study, 50 patients with advanced solid tumors were given AZD2014 in either single doses or twice daily, up to 100 mg twice a day [61]. The maximum tolerated dose was determined to be 50 mg twice a day. The most common adverse reactions were fatigue, stomatitis, anorexia, nausea and diarrhea. Reductions in pAKT and p4E-BP1 were noted in platelet rich plasma at 2 h and in peripheral blood mononuclear cells at 8 h. At a dose of 50 mg twice a day, AZD2014 led to a decrease in cytoplasmic phosphorylated S6 in eight of 10 paired tumor biopsies and decreased phosphorylated 4E-BP1 in three of nine paired biopsies, indicating mTORC1 inhibition. pAKT was reduced in three out of six samples, indicating mTORC2 inhibition. Two patients achieved a partial response with AZD2014 and four patients had stable disease [62]. Further clinical evaluation of AZD2014 is being pursued, both as monotherapy and in combination with other therapeutic agents.
OSI-027 OSI-027 is highly selective for mTOR, with 100 times less potency against PI3K and almost no activity against a panel of other kinases at a concentration of 1 mmol/L [55]. OSI-027 affects both mTORC1 and mTORC2 downstream targets in a dose-dependent manner [49]. In contrast to rapamycin treatment, OSI-027 blocks mTORC2-mediated phosphorylation of AKT at Ser473 and mTORC1 substrate 4E-BP1 on Thr37/46. The effects on pAKT and p4E-BP1 in vitro appear to correlate with greater inhibition of cell proliferation and cell death in vivo [55]. OSI-027 produced greater antileukemic effects than rapamycin in chronic myeloid leukemia (CML) and AML cell lines [52,54]. These studies underscored the importance of mTORC2 in leukemogenesis and demonstrated that dual blockade of mTORC1 and mTORC2 is needed for effective antileukemic responses. Further, OSI-027 was able to induce apoptosis in Philadelphia chromosome positive (Ph) hematologic malignancies harboring the T315I mutation [54]. Among lymphoid malignancies, OSI-027 induced apoptosis in acute lymphocytic leukemia (ALL), mantle cell lymphoma, marginal zone lymphoma and Sezary cell lines [51]. Similar success was noted in pre-clinical solid tumor models in which OSI-027 inhibited in vitro breast cancer proliferation and further enhanced chemotherapy-induced apoptosis [50]. When used in combination with vascular endothelial growth factor receptor (VEGFR) tyrosine kinase inhibitors in lung and ovarian xenograft models, the addition of OSI-027 potentiated antiangiogenic effects and produced greater growth inhibition and tumor regression than either agent alone [49].
4 N. A. Mohindra & L. C. Platanias A phase I study evaluated three dose schedules of OSI-027 in 31 patients with advanced solid tumors and lymphoma [63]. Dose limiting toxicities were due to changes in left ventricular ejection fraction and fatigue. The maximum tolerated dose (MTD) had not been reached at the time of presentation of the data. Other drug related toxicities included nausea, vomiting, diarrhea, fatigue, elevated creatinine and reversible increases in QTc. Pharmacokinetic analysis revealed dose-dependent drug exposure. Phosphorylation of 4E-BP1 was reduced in peripheral blood mononuclear cells in 13 of 23 patients, correlating with mTORC1 inhibition. There were no responses to therapy, but eight patients were able to achieve stable disease for greater than 12 weeks. The trial has now been closed to accrual and updated results are awaited (NCT00698243).
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Limitations to OSI-027 Among certain leukemia cell lines, treatment with OSI-027 resulted in induction of autophagy, which appeared to attenuate apoptosis unless an autophagy inhibitor was used concurrently [53,54]. Thus, autophagy induced by OSI-027 may be protective and a limitation to achieving full therapeutic benefit from this agent when used as monotherapy. In lymphoid malignancies, BCL-2 overexpression reduced induction of apoptosis by OSI-027 [51]. These studies indicated that OSI-027 relied on the transcriptional activity of the PUMA and BIM genes to achieve cell death, both of which were reduced when BCL-2 was overexpressed. Therefore, BCL-2 expression may represent a mechanism of resistance to OSI-027 in certain malignancies [51].
INK128/MLN0128 INK128 (MLN0128, Table II) was discovered through rational drug design. This catalytic inhibitor blocks the mTOR kinase at sub-nanomolar concentrations [64]. It has been shown to be highly selective for the mTOR kinase when tested against a panel of more than 400 kinases. Phosphorylation of mTORC1 substrates S6 and 4E-BP1 and mTORC2 substrate AKT are inhibited in vitro and in vivo [64]. Significant activity against these substrates in cell lines resistant to rapamycin and pan-PI3K inhibitors has been also documented [64]. INK128 produced antileukemic effects in B-ALL cell lines in vitro and reduced colony formation in patient derived Table II. INK128 quick profile. Drug name Company Other names MoA MoR MTD DLT Schedule Plasma concentration Plasma half-life
INK128 Takeda Pharmaceutical Company, Osaka, Japan MLN0128 mTORC1 and mTORC2 inhibitor Not known 6 mg daily (MTD for 3 days a week every week was not reached at time of data presentation) Uriticaria, mucositis/stomatitis Daily; weekly dose not published Not available ∼8 h
oA, mechanism of action; MoR, mechanism of resistance; MTD, maximum M tolerated dose; DLT, dose limiting toxicity.
non-Ph and Ph leukemic cells. INK128 also reduced leukemic growth in B-ALL xenografts without affecting bone marrow function and further enhances the efficacy of dasatinib in Ph B-ALL cell lines and murine models. In nonPh ALL, INK128 produced a cytostatic effect when used as a single agent [65]. In multiple myeloma (MM) cell lines, INK128 significantly inhibited proliferation in vitro even in the presence of growth stimulation from bone marrow stromal cells or interleukin-6 (IL-6) [66]. Likewise, decreased phosphorylation of AKT and 4E-BP1 were noted in vivo, as well as activation of caspase 3 necessary for apoptosis [66]. Interestingly, unlike rapamycin, INK128 significantly inhibited multiple myeloma cell adhesion to bone marrow stromal cells [66]. INK128 did not suppress normal lymphocytes or granulocytes, making this agent an attractive therapeutic agent for monotherapy or combination therapy [65,66]. As c-myc driven oncogenesis is thought to be derived in part by protein synthesis via mTORC1-dependent 4E-BP1 phosphorylation [67], the effects of INK128 in myc-driven MM and lymphoma cell lines were assessed in one study [67]. INK128 treatment significantly reduced 4E-PB1 phosphorylation in myc-driven MM and lymphoma cell lines, providing a rationale for use of this agent in these malignancies as well [67]. Apart from hematologic malignancies, there is strong evidence of INK128 activity in cell lines and xenograft models from several tumors, underscoring the potential importance of this agent in clinical oncology [68–73]. Preliminary results are available from a phase I dose escalation study of INK128 in patients with relapsed/refractory MM, Waldenström macroglobulinemia (WM) and nonHodgkin lymphoma [74]. Thirty-seven patients were enrolled and treated with six dose levels of INK128 (either daily or daily 3 days on and 4 days off in 28-day cycles). The MTD daily dose was 6 mg; however, the MTD for the three times a week dosing had not been reached at the time the study was presented. Dose limiting toxicities were related to thrombocytopenia, mucositis/stomatitis and urticaria among both cohorts. Some 27% of patients discontinued the drug due to adverse events, while 43% of patients experienced grade 3 or 4 adverse events, the most common of which included thrombocytopenia, fatigue, mucositis and neutropenia. Of the 27 patients evaluable for response, one patient with MM achieved a minor response, while 13 patients with MM and two with WM had stable disease [74]. Several other clinical investigations of INK128 are ongoing in both hematologic and solid malignancies.
Conclusions Several large clinical investigations have failed to demonstrate dramatic responses using rapalog therapy [29]. Limitations to rapalogs result from partial inhibition of mTORC1 substrates and lack of meaningful effects on mTORC2. Preclinical investigation of catalytic inhibitors of mTOR kinase has demonstrated anti-proliferative effects in a variety of tumor models, and these agents have now entered clinical investigation. Preliminary evidence suggests that some of these drugs may be effective as monotherapy or in combination with chemotherapy or other targeted agents. The full therapeutic potential of this class of drugs remains to be
Catalytic mTOR inhibitors 5 defined, and the results of ongoing clinical trials will provide an answer to these issues. Potential conflict of interest: Disclosure forms provided by the authors are available with the full text of this article at www.informahealthcare.com/lal.
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