Mechanistic target of rapamycin inhibitors: successes and challenges as cancer therapeutics
Abstract
Delineating the contributions of specific cell signalling cascades to the development and maintenance of tumours has greatly informed our understanding of tumorigenesis and has advanced the modern era of targeted cancer therapy. It has been revealed that one of the key pathways regulating cell growth, the phosphatidylinositol 3-kinase/mechanistic target of rapamycin (PI3K/mTOR) signalling axis, is commonly dysregulated in cancer. With a specific, well-tolerated inhibitor of mTOR available, the impact of inhibiting this pathway at the level of mTOR has been tested clinically. This review highlights some of the promising results seen with mTOR inhibitors in the clinic and assesses some of the challenges that remain in predicting patient outcome following mTOR-targeted therapy.
Keywords
Introduction
The mechanistic target of rapamycin (mTOR) is a 289-kDa serine/threonine protein kinase that belongs to the class IV phosphoinositide 3-kinase (PI3K)-related kinase (PIKK) family. Considered a principal regulator of cell growth and metabolism, this multifunctional kinase coordinates upstream signals from growth factors, and nutrient and oxygen resources to control cell growth in both physiological and pathological settings. mTOR does this through regulating fundamental biological activities including cell proliferation and survival, cell cycle, protein synthesis, and glucose metabolism[1].
mTOR functions as a central catalytic subunit to two complexes, mTOR complex 1 (mTORC1) and 2 (mTORC2). mTORC1 and mTORC2 are structurally and functionally different, are stimulated by different mechanisms and play different roles in controlling cell bioactivity[2]. They also express individual substrate specificity.
mTORC1 signalling is responsive to extracellular stimulation via growth factor, amino acid and energy resources in order to stimulate protein biosynthesis, cell proliferation and angiogenesis. At its core, mTORC1 is comprised of mTOR, regulatory-associated protein of mTOR (RAPTOR) and mammalian lethal with Sec13 protein 8 (mLST8)[1]. Other proteins also associate with mTORC1, depending on species and specific conditions, including proline-rich AKT Substrate 40 (PRAS40)[3], DEP domain-containing mTOR interacting protein (DEPTOR)[4], GRP58[5], telomere maintenance 2 (TEL2)-interacting protein 1 (TTI1), TEL2[6] and Ras family small GTPase 1 (Rac1)[7].
mTORC2 is largely stimulated by upstream growth factor binding. This complex is composed of mTOR, rapamycin-insensitive companion of mTOR (RICTOR), mLST8, proline-rich protein 5 (PRR5, also known as Protor-1), heat shock protein 70, DEPTOR, GRP58, TTI1-TEL2, Rac1, mammalian stress-activated protein kinase interacting protein 1 and protein observed with RICTOR (PROTOR). mTORC2 plays key roles in cytoskeletal organisation and facilitating cell survival by its activation of multiple related due to sequence similarity (AGC) family kinases, including serum and glucocorticoid kinase, protein kinase B (AKT), and protein kinase C[8].
Cellular processes controlled by mTORC1
Both complexes incorporating mTOR play distinct physiological roles, but we only provide a brief overview of mTORC1 signalling in this review. mTORC1 signalling is promoted by growth factors and amino acid sufficiency. Growth factor or insulin binding to receptors at the cell surface triggers PI3K/AKT signalling pathway activation[9,10]. Once activated, AKT phosphorylates tuberous sclerosis complex 2 (TSC2) protein, inducing its dissociation from the tumour suppressor complex it forms with TSC1[11]. Dissociation of this heterodimer TSC1/2 complex causes loss of GTPase activity of the complex. Thus, its inhibitory effect over Ras homolog enriched in brain (RHEB) ceases, allowing the activation of mTORC1 signalling on lysosomes[12][Figure 1]. Additionally, insulin-activated AKT phosphorylates mTORC1-associated PRAS40, causing its dissociation from RAPTOR, which serves as a means to activate mTORC1 signalling independently of TSC1/2[3]. These upstream signalling mechanisms are reviewed in more detail in ref.[13]. Amino acid sufficiency is sensed by the Rag GTPases and the Ragulator complex and is required for full activity of mTORC1. Amino acid sensing by mTORC1 has recently been comprehensively reviewed in ref.[14].
Figure 1. An overview of the mTORC1 signalling pathway. IRS1: insulin receptor substrate 1; PI3K: phosphoinositide 3-kinase; PTEN: phosphatase and TENsin homolog; AKT: AKT8 virus oncogene cellular homolog; AMPK: AMP-activated protein kinase; TSC1: tuberous sclerosis complex 1; TSC2: tuberous sclerosis complex 2; RHEB: ras homolog enriched in brain; GDP: guanosine diphosphate; GTP: guanosine triphophate; mTORC1: mammalian target of rapamycin complex 1; S6K1/2: ribosomal protein S6 kinase 1/2; 4E-BP1: eIF4E binding protein 1; eIF4E: eukaryotic initiation factor 4E; STAT3: signal transducer and activator of transcription 3; HIF-1α: hypoxia inducible factor 1 alpha; VEGF: vascular endothelial growth factor; SREBP: sterol regulatory element-binding protein; ULK1: Unc-51-like autophagy activating kinase 1
Once activated, mTORC1 signalling drives increases in cell size and proliferation. To achieve this, mTORC1 signals to important downstream effectors, p70S6K1 and 4E-BP1. Both of these proteins express a common mTORC1 signalling motif, permitting their respective recognition by RAPTOR and their subsequent phosphorylation by activated mTORC1[15]. Their phosphorylation enhances protein synthesis through two means. Hypophosphorylated 4E binding proteins bind tightly to eukaryotic translation initiation factor (eIF4E) at the 5’ cap of mRNA, preventing interaction with eIF4G and thus preventing translation. Once phosphorylated at multiple sites by mTORC1, 4E-BP1 releases from eIF4E, allowing eIF4G and eIF4A to bind free eIF4E (collectively forming the eIF4F complex) at the 5’ cap of mRNA, thus increasing mRNA translation, including proteins necessary for G1-to-S phase transition[16]. Enhanced protein translation is also facilitated when p70S6K1 becomes phosphorylated at its Thr389 site by activated mTORC1. Through a variety of pathways, this controls effective ribosomal biogenesis, thereby potentiating translation[13].
In addition to protein synthesis, mTORC1 activation controls several other growth promoting processes. When active, mTORC1 induces the upregulation of hypoxia-inducible factor-1 alpha (HIF-1α) and vascular endothelial growth factor (VEGF) to promote both angiogenic potential[17] and an increase in glycolysis[18]. mTORC1 also positively regulates glutamine metabolism by inhibiting sirtuin 4 (SIRT4)[19], as well as facilitating lipid synthesis and influencing the pentose phosphate pathway through its action on sterol regulatory element binding protein 1 (SREBP-1)[18,20,21]. mTORC1 also cross-talks with autophagy through ULK1 to maintain a homeostatic balance between anabolic and catabolic processes[22].
mTOR inhibitors
The discovery of rapamycin and its early use
Rapamycin, known clinically as sirolimus, is a natural macrolide isolated originally from soil samples of Rapa Nui (Easter Island) by Sehgal and colleagues[23]. Produced by Streptomyces hygroscopicus, it was discovered serendipitously in a screen for antimicrobial activity against Candida albicans. It was initially recognised for its potent anti-fungal activity, but was later shown to be a potent immunosuppressant and anti-proliferation agent. Today, rapamycin is used clinically to prevent kidney and cardiac graft rejection in post-transplantation patients[24,25]. However, it is its role as an anti-proliferative agent that is the focus of this review.
Functional studies on the mechanism of action of rapamycin identified the cellular target of rapamycin, named TOR. Rapamycin binds to immunophilin 12-kDa protein FKBP12, and this drug-protein complex causes contraction of the mTORC1 dimer active site cleft from 20 Å to 10 Å[26,27], thus inhibiting mTORC1 activity selectively and allosterically.
The development of rapalogs
The development of rapamycin analogues, termed rapalogs, began in an effort to improve the solubility, stability and poor pharmacokinetic characteristics of rapamycin [Figure 2]. First generation rapalogs included temsirolimus (CCI-779), everolimus (RAD001) and ridaforolimus (AP23573). Rapalogs elicit the same mechanism of action as rapamycin. They bind to FKBP12 and complex with the FKBP12-rapamycin binding (FRB) domain on mTOR, constricting its site of action and thus inhibiting substrate docking and phosphorylation[28]. As with rapamycin, their downstream effects include inactivation of p70S6K1 and 4E-BP1, preventing ribosomal biogenesis and mRNA translation[29]. The rapalogs inhibit cell cycle progression by arresting cells at the G1-S interface, thereby achieving anti-proliferative effects.
Figure 2. Timeline of rapamycin and rapalogs from discovery to clinic. Rapalogs include temsirolimus, everolimus and ridaforolimus. RCC: renal cell carcinoma; SEGA: subependymal giant cell astrocytoma; BC: breast cancer; PNET: pancreatic neuroendocrine tumours; FDA: Food and Drug Administration (United States of America)
Despite the efforts to improve pharmacokinetics using rapalogs, bioavailability is still considered a limitation of mTOR inhibitors. For example, everolimus has a bioavailability of 16% vs. that of 10% for sirolimus, as determined in an in vivo model[30]. Furthermore, rapamycin and rapalogs have therapeutic limitations given their inability to inhibit mTORC1 signalling completely, due to activation of compensatory pathways (discussed in more detail in “Pitfalls of mTOR inhibitor use in the clinic” section) and their differential inhibitory effects on 4E-BP1 vs. S6Ks[31]. To overcome this, ATP-competitive inhibitors of mTOR were developed, which inhibit both mTORC1 and mTORC2. These showed great potential in pre-clinical studies, demonstrating high potency and selectivity for mTOR inhibition and suppressing cell growth and survival[32-34]. They are currently being evaluated in clinical trials[35,36].
mTOR and cancer
mTOR signalling in cancer
With its key role in regulating of a number of anabolic cellular processes, it is unsurprising that cancer cells hijack the mTORC1 pathway as part of tumour development and growth. Aberrant activation of mTORC1 signalling permits the cancer cells to sustain their proliferative drive even when nutrients and growth factor stimulation are lacking.
mTORC1 pathway activation has been reported in a wide variety of solid tumours and commonly occurs due to aberrant expression of upstream regulators of the pathway. For example, the tumour suppressor, phosphate and tensin homolog deleted from chromosome ten (PTEN), regulates the PI3K/AKT pathway which converges on mTOR [Figure 1]. PTEN is lost or mutated in at least seven cancer types, including breast and prostate cancers[37]. The mitogen-activated protein kinase (MAPK) pathway can also cross-talk with mTORC1 signalling [Figure 1], through phosphorylation of TSC2[38] and RAPTOR[39]. The RAS and RAF components of the pathway are frequently mutated in cancer, with gain-of-function missense mutations in RAS genes detected in around 25% of human cancers[40]. Activating BRAF mutations are found in around half of melanomas and 60% of thyroid cancers[41]. Upstream amplification of growth factor receptors and substrates, such as epidermal growth factor receptor (EGFR), insulin-like growth factor receptor and insulin receptor substrate (IRS), promotes the PI3K and MAPK pathways, which cross-talk with mTORC1, and thus enhance tumour-promoting processes.
Less commonly, mTORC1 activation can occur through mutations to the mTORC1 pathway components themselves, such as TSC1, TSC2 and mTOR. Loss of function mutations of the tumour suppressor TSC1 are seen in bladder cancer[42], while 25 different TSC2 mutation sites, mainly in or near the RAP-GAP domain, have been reported in pancreatic neuroendocrine tumours (PNET)[43]. TSC2 mutations are also found in hepatocellular carcinoma[44]. A recent study analysing The Cancer Genome Atlas (TCGA) pan-cancer cohort looked in detail at molecular alterations involving the PI3K/AKT/mTOR pathway[45]. In their pan-cancer analyses, the authors reported 2% mutation rate in TSC1 and 4% mutation rate in mTOR. Additionally, some cases showed TSC1 or TSC2 rearrangements[45]. Various mTOR mutations were found in another study which used partial genome sequencing data from the TCGA, CCLE, International Cancer Genome Consortium (ICGC), and Catalogue of Somatic Mutations in Cancer (COSMIC) databases. These authors reported that, when normalised for gene length, mTOR had the highest percentage of recurring mutations out of all the mTOR pathway genes, and they identified 33 novel mTOR pathway-activating mutations[46].
The promise of mTOR inhibitors in cancer
Due to the major involvement of mTORC1 signalling in cancer described above, drugs which bind mTORC1 selectively and specifically were anticipated to hinder cancer cell metabolism and downstream protein and lipid synthesis, thereby eliciting anti-cancer effects. In the hamartoma syndrome, tuberous sclerosis complex (TSC), loss of function of TSC1 or TSC2 leads to mTORC1 hyperactivation and the formation of cysts and tumours in multiple organs. In this condition, where the defined genetic cause of mTORC1 activation is known, rapamycin and rapalogs have shown great promise. Kidney cysts, known as angiomyolipomas, regress while on therapy[47-49], while there is also an improvement in neurological outcomes, with a marked reduction in the volume of subependymal giant-cell astrocytomas and seizure frequency[50-52]. Recently, benefits from mTOR inhibitors were also reported for cardiac rhabdomyomas, arrhythmias and neurological outcomes in very young TSC patients[53].
With the frequency of dysregulation of mTORC1 signalling observed in cancer, it might be expected that reductions in tumour volume similar to that seen in TSC would be apparent in sporadic cancers following rapamycin treatment. In renal cell carcinoma (RCC), phosphorylated ribosomal protein S6, a marker of mTORC1 activation, was found to stain strongly in 85% of RCC tissues examined[54]. In accordance with this finding, the RECORD-1 randomised, Phase III study of the rapalog, everolimus, in metastatic RCC found a reduction in the risk of disease progression or death relative to placebo. This positive result prompted an early termination of this study, with patients permitted to cross from the placebo to the everolimus study arms[55,56]. While progression-free survival was improved with everolimus, the tumour volume response was low and follow-up analysis indicated that this may be partly due to resistant lesions that continue to grow despite everolimus treatment[57]. The large prospective Phase II RAPTOR study in papillary metastatic RCC also demonstrated some clinical benefit of everolimus treatment for this patient population[58].
The rare PNET have also been found to have mTOR hyperactivity, with one study demonstrating that 85% of primary tumours had altered protein levels of the tumour suppressors TSC2 and PTEN[59]. The authors found that mTORC1 activity in PNET cell lines could be inhibited by rapamycin[59]. Several studies have now evaluated the clinical impact of mTOR inhibitors in PNET. In RADIANT-1, which assessed everolimus in advanced PNET, everolimus, with or without concomitant octreotide long-acting release, demonstrated antitumor activity, with 59.3% of patients on everolimus monotherapy showing tumour shrinkage. Overall survival also compared favourably with large institutional series[60]. A further Phase III study (RADIANT-3) showed that everolimus, as compared with placebo, significantly prolonged progression-free survival among patients with progressive advanced PNET[61]. As with the RCC trials, tumour shrinkage was only seen in a small proportion of patients, so the benefit from everolimus treatment was seen primarily in the stabilisation of disease or minor tumour shrinkage and in the lower incidence of progressive disease[61]. These positive findings about prolonged progression-free survival in PNET patients on everolimus treatment were further supported by the RADIANT-4 study[62] and everolimus has now been approved worldwide for the treatment of patients with advanced, progressive, well-differentiated, non-functional PNET[63].
mTOR inhibitors as monotherapy have also shown some clinical activity in biliary tract cancer (BTC). In an exploratory Phase II trial, favourable anti-tumour activity was observed in a subset of patients with advanced BTC, although the study narrowly failed to meet its primary endpoint of disease control rate (RADiChol Study, NCT00973713)[64]. Additionally, the use of rapalogs as a maintenance therapy is being explored. For example, ridaforolimus (AP23573) is under investigation as a maintenance therapy for metastatic soft tissue or bone sarcomas in patients with stable disease or improved disease following four cycles of chemotherapy. In the SUCCEED trial (NCT00538239), this rapalog induced a 28% reduction in the risk of disease progression in this patient cohort[65], showing sustained anti-proliferative effects of ridaforolimus. It is thought that, while statistically significant, the absolute magnitude of clinical disease control may have been small due to the innate aggressiveness of this disease.
From this evidence, it appears that rapalogs, used as single agents, have the potential to reduce tumour volume and improve patient care in some conditions. However, despite the common activation of mTORC1 signalling in cancer, trials with rapamycin and its analogues as monotherapies in many cancer types have not been as effective as had been hoped [Table 1].
A summary of the clinical successes and failures of rapalog treatment
| Cancer/Disease type | Study and reference | |
|---|---|---|
| Food and Drug Administration approved | Renal cell carcinoma | RECORD-1 trial[55]; [56,113] |
| HR+, HER2- breast cancer | [118]; TAMRAD[119]; BOLERO-2[120]; PrE0102[121] | |
| Pancreatic neuroendocrine tumours | RADIANT-1 (NCT00363051)[60]; RADIANT-3 (NCT00510068)[61]; RADIANT-4 (NCT01524783)[62,63] | |
| Tuberous sclerosis complex | NCT00457808[47]; NCT00490789[48,49]; NCT00411619[50]; NCT00126672[51]; EXIST-1 (NCT00789828)[52]; [53] | |
| Potential clinical benefit | Biliary tract cancer | RADiChol study (NCT00973713)[64]; EUDRACT 2008-007152-94[125] |
| Thyroid cancer (in patients refractory to other agents) | NCT01263951[126]; NCT00936858[127] | |
| No clinical benefit seen | Gastric cancer | GRANITE-1, NCT00879333[67] |
| Non-small-cell lung carcinoma | [68-70] | |
| Metastatic colorectal cancer | [71,73] | |
| Prostate cancer | [74,75] |
Pitfalls of mTOR inhibitor use in the clinic
Many studies demonstrate that the initial promise of rapalog therapy has not been realised. A common pattern seen in trial data is of a modest response to rapalog monotherapy which does not lead to a significant improvement in patient outcomes. For example, while everolimus monotherapy initially looked promising in a Phase II gastric cancer study, with a decrease in tumour size from baseline observed in 45% of patients[66], a follow up Phase III study (GRANITE-1) showed everolimus did not significantly improve overall survival compared with best supportive care[67]. A Phase I trial of everolimus in solid tumours indicated that some non-small-cell lung cancer (NSCLC) patients could achieve a partial response to therapy[68]. However, another small study using temsirolimus as a single agent in NSCLC demonstrated clinical benefit but did not meet the primary objective of confirmed response rate[69]. A Phase II trial which looked for a benefit of adding everolimus to erlotinib treatment did not lead to any substantial improvement in NSCLC disease stabilisation[70].
In colorectal cancer, 40%-60% of tumours show mTOR activation[71] and everolimus showed some efficacy in patients with metastatic colorectal cancer in Phase I studies[68,72]. However, in a Phase II study which enrolled 199 heavily pre-treated metastatic colorectal cancer patients who had progressive disease, single-agent everolimus showed minimal activity, with disease stabilisation the best response seen[71]. Supporting this, a second Phase II study in metastatic colorectal cancer was unable to demonstrate a clinically meaningful anti-tumour activity of temsirolimus[73]. Although stabilisation of disease was seen in a proportion of patients, no objective responses according to RECIST criteria were observed[73].
Rapalogs have also been trialled in prostate cancer, where it was hoped that the high level of PI3K-mTORC1 signalling, often due to PTEN loss, would make these tumours susceptible to mTORC1 inhibition. However, the studies to date have not shown sufficient clinical activity of rapalogs as monotherapy. One Phase II trial of temsirolimus in men with castration-resistant metastatic prostate cancer was stopped prematurely because of a lack of efficacy[74]. Another Phase II trial of everolimus in a chemotherapy-naïve metastatic castration-resistant prostate cancer population revealed that mTORC1 inhibition in unselected patients only had a moderate effect[75].
Reasons for rapalog resistance
It is not immediately clear why rapalogs have not lived up to their expected promise in many solid cancers, although several suggestions have been put forward. One of the likely reasons is that it is caused by reactivation of mTORC1-regulated negative feedback loops. For example, active mTORC1 phosphorylates S6K1, which in turn promotes degradation of IRS-1 leading to downregulation of PI3K/AKT signalling. Following mTORC1 inhibition by rapamycin, IRS-1 expression is induced and the repression is relieved, thus reactivating PI3K signalling and promoting cell growth and survival[76]. A mTORC1-MAPK feedback loop has also been identified, with mTORC1 inhibition inducing MAPK cascade activation in tumour samples taken from patients treated with everolimus. Interestingly, mTORC1 inhibition seemed to elicit a differential MAPK activation depending on the specific dose and administration schedule of everolimus[77]. Both PI3K/AKT and MAPK signalling are also regulated by the mTORC1 substrate, growth factor receptor-bound protein 10 (Grb10). mTORC1-mediated phosphorylation stabilises Grb10, leading to negative regulation of PI3K/AKT and MAPK signalling. Rapamycin treatment relieves this feedback inhibition as Grb10 becomes destabilised[78,79].
mTORC1 activity also cross-talks with, as well as negatively regulates, the catabolic process of autophagy. While rapamycin treatment is often less effective at re-activating autophagy in mammalian cells than in yeast[80], mTORC1 inhibition could permit some reactivation of autophagy in cancer cells, thus allowing them to recycle macromolecules more effectively, maintain homeostasis and survive. In addition to the incomplete re-activation of autophagy, rapamycin treatment also incompletely inhibits protein synthesis, due to differential regulation of S6K1 and 4E-BP1 phosphorylation[31]. Furthermore, due to similar architecture, other AGC kinase family members such as p90 ribosomal S6 kinase (RSK), can compensate for S6K1 inhibition[81]. As RSK family members are activated via classical MAPK signalling, they are largely unaffected by rapamycin and so can permit rpS6 phosphorylation and translation initiation in an mTORC1-independent manner[81].
A further pitfall of rapalogs is their cytostatic mode of action. This has been clearly shown during trials in TSC, where cessation of everolimus treatment permitted rapid regrowth of angiomyolipomas[47]. This weakness of rapalogs has similarly been seen in sporadic cancer, where stabilisation of disease, rather than partial or complete remission, is often the best response for many patients[61,71].
Additionally, it is common for treatment-resistant cancer cells to have acquired secondary mutations in the kinase being targeted. Some mTOR inhibitor-naïve tumours have been reported to contain mTOR mutations[82,83], but mTOR has also been reported to become mutated in cancers treated with mTOR inhibitors. An anaplastic thyroid cancer patient demonstrated a sustained 18-month response to everolimus, with a substantial decrease in tumour size. The original tumour cells were found to contain an inactivating mutation in TSC2, explaining the acute sensitivity of the tumour to mTORC1 inhibition. However, resistance to treatment developed. Although the TSC2 mutation was still present in the resistant cells, a new mTOR mutation was also detected in these cells. This mutation was within the FRB domain and was shown to confer rapamycin resistance in cell culture experiments[84]. Mutations within the wider mTORC1 signalling network have also been proposed to contribute to rapamycin resistance. Epigenetic silencing of the PP2A regulatory B55b subunit (PPP2R2B) has been reported in over 90% of colorectal cancers[85]. Through mechanistic analysis, the authors proposed that loss of PPP2R2B resulted in PDK1-dependent myc phosphorylation in response to rapamycin, resulting in rapamycin resistance[85].
Tumour heterogeneity may be another contributing factor to rapalog resistance. Bulk tumours may comprise of groups of cells with differing sensitivities to treatment due to the presence of distinct molecular signatures. With the development of powerful genomic profiling techniques, researchers have been able to probe the genetic heterogeneity of tumours. An interesting study used exon-capture multiregion sequencing to profile spatially separated samples from a renal cell carcinoma[86]. Amongst the 101 non-synonymous point mutations identified across the different tumour regions, they reported a heterogeneous mTOR mutation which rendered the kinase constitutively active. The regions containing this mutation corresponded to the areas with increased S6 phosphorylation[86]. This identification of spatially-separated somatic mutations, which can alter signalling activity, suggests that single tumour-biopsy specimens will only reveal a minority of the genetic variation present in a tumour and so could be poor predictors of treatment response.
Overcoming resistance
Strategies to improve rapalog efficacy
What can be done to overcome these issues? There are two main approaches that can be taken to enhance the efficacy of rapalogs in the clinic. The first is to use them in combined therapies. For example, in an effort to overcome the feedback loop to autophagy, mTOR inhibitors have been combined with the autophagy inhibitor, hydroxychloroquine. This has been shown to be safely tolerated in patients with lymphangioleiomyomatosis, with the higher dose level of hydroxychloroquine showing potential for a durable effect following treatment cessation[87]. The combination of everolimus and hydroxychloroquine is also promising in renal cell carcinoma, where a Phase I trial showed disease control in 67% of evaluable patients[88], although a trial in soft tissue sarcoma was less promising[89]. Ongoing and completed trials are testing autophagy and mTORC1 inhibition in breast cancer (NCT03032406), advanced malignancies (NCT01266057 and NCT00909831) and myeloma (NCT01396200).
To counteract the re-activation of Akt and MAPK signalling following mTORC1 inhibition, dual PI3K/mTOR inhibitors and combination therapies of mTOR and MEK inhibitors have been trialled. As the hinge region of mTOR shares high sequence homology with that of PI3K[90], compounds originally designed to target PI3K also show binding affinity for mTOR. These ATP-competitive inhibitors compete for the ATP binding pocket on both mTOR and PI3K kinases, and it was hoped that a broader anti-cancer effect could be achieved with this dual targeting. A number of dual PI3K/mTOR inhibitors looked very promising in the pre-clinical stage. However, results from clinical trials have been mixed. In PNET patients, the dual inhibitor BEZ235 was poorly tolerated. Although there was some evidence of disease stability, the study did not proceed to the second stage[91]. A similar outcome was seen in a multicentre Phase I/Ib trial in advanced solid tumours, including breast cancer, where PI3K/mTOR inhibition by BEZ235 was not sufficient to achieve an adequate antitumor effect with a favourable safety profile[92]. Limitations were also seen with another dual inhibitor, PF-04691502 when tested in a variety of solid cancers. Although there was evidence of PI3K down-regulation, no evaluable tumour responses were seen[93]. However, some more encouraging results of this approach have been observed. Using the dual inhibitor LY3023414, more than half of the advanced cancer patients treated at or above the maximum tolerated dose in a Phase I study demonstrated a decrease in the sum of target lesions. Interestingly, one endometrial cancer patient had a partial response that lasted for over 18 months. This patient was found to have a PIK3R1 mutation and a truncating PTEN mutation[94].
A very similar story emerges with mTOR and MEK inhibition. Pre-clinical data are promising[95], but clinical trials have shown the combination is not very well tolerated[96,97]. Although some patients have shown partial responses and stable disease, the overall clinical activity of the trametinib and everolimus combination in patients with advanced solid tumours is modest, with a dose providing acceptable tolerability and adequate drug exposure not achievable[96].
Other trials have combined rapalogs with conventional chemotherapy drugs. As with the studies combining mTOR with PI3K or MEK inhibitors, these have often proved disappointing. In high-risk acute myeloid leukaemia patients, everolimus combined with mitoxantrone, etoposide and cytarabine showed substantial clinical activity and was reported as an effective, well-tolerated regimen for salvage or initial therapy of high-risk acute myeloid leukaemia[98]. However, the UK NCRI AML17 trial, where patients received everolimus sequentially with chemotherapy, reported no clinical benefit[99]. A Phase II trial of sirolimus with cyclophosphamide in sarcoma found that progression-free survival was similar to other active treatment regimens[100]. In clear cell RCC, pre-clinical trials suggested synergy between histone deacetylase inhibitors and mTOR inhibitors. Phase I data show this combination was tolerated, although no apparent clinical benefit was seen in the advanced clear cell RCC cohort under study[101]. Anti-VEGF therapy in combination with everolimus was tested in a Phase II trial of recurrent ovarian, fallopian tube or peritoneal carcinoma but the combination did not significantly prolong progression-free survival compared to bevacizumab alone[102]. More promising were two case reports in the pancreatic cancer setting which indicated that the addition of sirolimus to a gemcitabine, capecitabine and docetaxel regimen resulted in tumour regression[103]. Modest efficacy was also reported in a recent Phase I trial in triple negative breast cancer, which combined everolimus with the cell cycle inhibitor, eribulin. Triple negative breast cancer has poor clinical outcomes and more precise and personalised treatments are needed. This combination was shown to be safe, with a disease response rate of 36%[104].
The second option is to molecularly stratify patients to only treat those likely to benefit from rapalogs based on the genetics of their cancer. This strategy is supported by case reports of patients who have demonstrated marked and durable responses to rapalog monotherapy. For example, next generation sequencing analysis of a liquid biopsy from a patient with advanced breast cancer, who failed to respond to conventional chemotherapy, showed mutations in PIK3CA, PTEN and mTOR. Everolimus monotherapy resulted in partial remission after two months and sustained stable disease after 18 months[105]. Similarly, a near-complete and sustained response to everolimus was also reported in a metastatic breast cancer patient with a STK11 point mutation[106]. A durable and ongoing complete response to everolimus was also reported in a patient with metastatic bladder cancer, even though the trial that they were enrolled in (NCT00805129) failed to achieve its progression-free survival end point. This super-responder was found to have a TSC1 mutation[107].
There are also indications from larger trials that selecting only patients with mTORC1 signalling dysregulation for rapalog therapy could be an effective strategy. A small Phase II trial of everolimus as a single agent in castration-resistant prostate cancer (mCRPC) showed that, although everolimus activity in unselected patients with mCRPC was only moderate, there was a trend towards longer progression-free survival in patients with PTEN deletion[75]. In a study of hormone receptor-positive breast cancer patients treated with everolimus, patients with the PIK3CA/H1047R mutation had longer progression-free survival than patients with wild-type or other mutant forms of PIK3CA. This led the authors to suggest that the PIK3CA/H1047R mutation could be a potential biomarker of sensitivity to everolimus[108]. In the metastatic urothelial carcinoma setting, combination treatment with everolimus and the VEGF-TKI pazopanib showed significant clinical benefit in genomically selected patients with mutations in the mTOR pathway or FGFR[109]. An ongoing clinical trial is evaluating whether mutational status correlates with the response to a rapalog in colorectal cancer (NCT03439462)[110]. However, caution must be applied when selecting cancer patients for rapalog treatment based on their mutational profile. In a cohort of metastatic renal cell carcinoma patients, those who benefitted clinically from rapalogs more commonly had mutations in mTOR, TSC1 or TSC2 than patients who progressed. However, analysis of the extent and duration of response failed to suggest a correlation between truly exceptional responses and TSC1/TSC2/mTOR mutation. Indeed, 56% of responders had no mTOR pathway mutation identified[111]. Therefore, it appears that unlike, for example, lung cancer where there is a correlation between specific EGFR mutations and clinical responsiveness to EGFR tyrosine kinase inhibitors[112], in many cases response to rapalogs is not so easy to define based on genotype.
Promise of mTOR inhibitors as therapies to overcome resistance in patients refractory to other agents
Rapalogs have shown promise as second- and subsequent-line therapies in cancers that have become resistant to the targeting of other oncogenic pathways. Some early evidence for this came in the setting of RCC, where studies indicated that rapalog therapy following the development of VEGF receptor tyrosine kinase inhibitor (TKI) resistance (for example, progression on sunitinib) could be beneficial[55,113]. Indeed, the initial Food and Drug Administration approval for everolimus was for RCC where patients had progressed despite treatment with sunitinib, sorafenib, or both. The approval was based on the RECORD-1 trial (detailed in “The promise of mTOR inhibitors in cancer” section). Further evidence supports the use of everolimus in combination with another VEGF-targeted therapy, lenvatinib, with a high level of efficacy associated with the combined regimen in a Phase II study[114] and anti-tumour activity also reported in a small Japanese cohort[115]. However, while everolimus is approved and effective in RCC, it may not be the only or optimal treatment for all patients. The INTORSECT trial found that second-line rapalog treatment in patients who had progressed following first-line VEGFR inhibition therapy did not demonstrate a progression-free survival advantage compared with treatment with an alternative VEGFR inhibitor[116]. This suggests that agents other than rapalogs may be equally or more effective in renal cell carcinoma that has progressed after first-line VEGFR TKI therapy[117]. Additionally, the risk-benefit balance needs to be considered, as RCC patients receiving lenvatinib plus everolimus reported more adverse events than those receiving either single-agent drug[114].
Further evidence for the ability of rapalogs to show success in resistant cancers comes from studies of breast cancer patients. One study in endocrine-resistant hormone receptor(HR)-positive, HER2-negative metastatic breast cancer indicated that everolimus can potentially reverse resistance to endocrine therapies[118]. This supports earlier combination therapy studies in similar patient cohorts where the addition of everolimus to the treatment regime improved outcomes in patients who had shown aromatase inhibition resistance. In summary, these studies showed that: (1) combining everolimus with tamoxifen may reverse hormone resistance and lead to an increased clinical benefit rate, time to progression and overall survival compared with tamoxifen alone (TAMRAD trial)[119]; (2) the addition of everolimus to exemestane significantly improves progression-free survival (BOLERO-2)[120]; and (3) everolimus enhances the efficacy of fulvestrant (PrE0102 trial)[121]. Based on this evidence, everolimus was approved in 2012 to treat HR-positive, HER2-negative breast cancer patients. A further trial is now underway to determine whether concomitant administration of everolimus with endocrine therapy further prolongs progression-free survival in breast cancer patients who have not yet developed hormone resistance (Chloe trial)[122]. The recent MANTA Phase II trial is the first to report on the impact of adding the dual mTORC1/2 inhibitor, vistusertib (AZD2014), to endocrine therapy (fulvestrant) in an oestrogen-receptor positive advanced breast cancer cohort. Similar to the PrE0102 trial, MANTA demonstrated everolimus in combination with fulvestrant demonstrated significantly longer progression-free survival than fulvestrant alone[123]. However, the trial failed to show a benefit of adding the dual mTOR inhibitor to fulvestrant, despite pre-clinical promise. It remains to be investigated whether sub-optimal mTORC1 inhibition by vistusertib or some other factor could be responsible for the poor response seen[123].
The results of trials using rapalogs as second-line therapy in other cancer types have been mixed. While little benefit of rapalog treatment has been seen in refractory gastric cancer[124], rapalog treatment as a means of controlling refractory cancers has been seen more widely than just in RCC and breast cancer. For example, in an Italian Phase II study of patients with BTC which was progressing despite previous chemotherapy (EUDRACT 2008-007152-94), everolimus showed encouraging anti-tumour activity, with one patient demonstrating a complete response which was sustained for eight months[125]. Positive outcomes were also seen in thyroid cancer, where one trial aimed to determine whether the addition of everolimus to sorafenib is beneficial to patients who progress on sorafenib monotherapy (NCT01263951). Initial results indicate that the addition of everolimus allowed patients a median additional 13.7 months of disease stability[126]. Another active thyroid cancer trial (NCT00936858) indicates that everolimus therapy for patients who are radioactive iodine-refractory has significant benefits[127]. A Phase II study of everolimus with tivozanib in refractory colorectal cancer achieved stabilisation of disease in 50% of patients[128]. Evidence from the RADIANT-3 trial in PNET indicates that everolimus can prolong progression-free survival regardless of whether patients have had prior chemotherapy[129].
Conclusion
The targeting of mTOR signalling as a front-line therapy or as an alternative treatment to overcome resistance is of interest in many cancer types. While rapalogs may not have lived up to their initial promise suggested by in vitro data, they have had notable success in TSC and PNET, and are still actively being explored in other settings. With over 1000 clinical trials listed on clinicaltrials.gov involving the use of everolimus, temsirolimus or rapamycin in the cancer setting (as at November 2019), it is clear that substantial interest still exists in their clinical potential. As to the pitfalls of rapamycin resistance, steps are being taken to develop a new generation of mTOR inhibitors. The third generation, bivalent mTOR inhibitor, RapaLink, inhibits breast cancer cell growth at a level comparable to rapamycin, is effective against hyperactive mTOR-mutant cells and did not allow the evolution of resistance during the course of the study[130]. A follow-up study showed RapaLink-1 was also a potent inhibitor of both wild-type and active mTOR mutants in glioblastoma cell lines[131]. The pre-clinical in vivo aspect of the study showed RapaLink-1 had anti-tumour efficacy in both intracranial xenograft and spontaneously arising brain tumour models, indicating potential translational promise. However, a note of caution was that the initial tumour regression was followed by re-growth[131]. Therefore, the mechanisms behind this re-growth and whether this third generation of inhibitors will translate to the clinical setting remain to be evaluated. However, as the PI3K/Akt/mTOR signalling axis is one of the most commonly activated in human cancers, effective targeting of this pathway will remain of high interest to scientists and clinicians in their hunt for improved cancer therapies for patients.
Declarations
Authors’ contributionsBoth authors contributed to the writing and editing of the review.
Availability of data and materialsNot applicable.
Financial support and sponsorshipOur research is supported by the Tuberous Sclerosis Association (2018-S02, 2019-P01) and the US Department of Defense Congressionally Directed Medical Research Program (TS180037).
Conflicts of interestAll authors declared that there are no conflicts of interest.
Ethical approval and consent to participateNot applicable.
Consent for publicationNot applicable.
Copyright© The Author(s) 2019.
REFERENCES
2. Loewith R, Jacinto E, Wullschleger S, Lorberg A, Crespo JL, et al. Two TOR complexes, only one of which is rapamycin sensitive, have distinct roles in cell growth control. Mol Cell 2002;10:457-68.
3. Sancak Y, Thoreen CC, Peterson TR, Lindquist RA, Kang SA, et al. PRAS40 is an insulin-regulated inhibitor of the mTORC1 protein kinase. Mol Cell 2007;25:903-15.
4. Peterson TR, Laplante M, Thoreen CC, Sancak Y, Kang SA, et al. Deptor is an mTOR inhibitor frequently overexpressed in multiple myeloma cells and required for their survival. Cell 2009;137:873-86.
5. Ramirez-Rangel I, Bracho-Valdes I, Vazquez-Macias A, Carretero-Ortega J, Reyes-Cruz G, et al. Regulation of mTORC1 complex assembly and signaling by GRp58/ERp57. Mol Cell Biol 2011;31:1657-71.
6. Kaizuka T, Hara T, Oshiro N, Kikkawa U, Yonezawa K, et al. Tti1 and Tel2 are critical factors in mammalian target of rapamycin complex assembly. J Biol Chem 2010;285:20109-16.
7. Saci A, Cantley LC, Carpenter CL. Rac1 regulates the activity of mTORC1 and mTORC2 and controlscellular size. Mol Cell 2011;42:50-61.
8. Gaubitz C, Prouteau M, Kusmider B, Loewith R. TORC2 structure and function. Trends Biochem. Sci 2016;41:532-45.
9. Inoki K, Li Y, Zhu T, Wu J, Guan KL. TSC2 is phosphorylated and inhibited by Akt and suppresses mTOR signalling. Nat Cell Biol 2002;4:648-57.
10. Manning BD, Tee AR, Logsdon MN, Blenis J, Cantley LC. Identification of the tuberous sclerosis complex-2 tumor suppressor gene product tuberin as a target of the phosphoinositide 3-kinase/akt pathway. Mol Cell 2002;10:151-62.
11. Potter CJ, Pedraza LG, Xu T. Akt regulates growth by directly phosphorylating Tsc2. Nat Cell Biol 2002;4:658-65.
14. Kim J, Guan KL. mTOR as a central hub of nutrient signalling and cell growth. Nat Cell Biol 2019;21:63-71.
15. Nojima H, Tokunaga C, Eguchi S, Oshiro N, Hidayat S, et al. The mammalian target of rapamycin (mTOR) partner, raptor, binds the mTOR substrates p70 S6 kinase and 4E-BP1 through their TOR signaling (TOS) motif. J Biol Chem 2003;278:15461-4.
16. Gingras AC, Kennedy SG, O’Leary MA, Sonenberg N, Hay N. 4E-BP1, a repressor of mRNA translation, is phosphorylated and inactivated by the Akt(PKB) signaling pathway. Genes Dev 1998;12:502-13.
17. Dodd KM, Yang J, Shen MH, Sampson JR, Tee AR. mTORC1 drives HIF-1α and VEGF-A signalling via multiple mechanisms involving 4E-BP1, S6K1 and STAT3. Oncogene 2015;34:2239-50.
18. Düvel K, Yecies JL, Menon S, Raman P, Lipovsky AI, et al. Activation of a metabolic gene regulatory network downstream of mTOR complex 1. Mol Cell 2010;39:171-83.
19. Csibi A, Fendt SM, Li C, Poulogiannis G, Choo AY, et al. The mTORC1 pathway stimulates glutamine metabolism and cell proliferation by repressing SIRT4. Cell 2013;153:840-54.
20. Porstmann T, Santos CR, Griffiths B, Cully M, Wu M, et al. SREBP activity is regulated by mTORC1 and contributes to Akt-dependent cell growth. Cell Metab 2008;8:224-36.
21. Li S, Brown MS, Goldstein JL. Bifurcation of insulin signaling pathway in rat liver: mTORC1 required for stimulation of lipogenesis, but not inhibition of gluconeogenesis. PNAS 2010;107:3441-6.
22. Dunlop EA, Tee AR. The kinase triad, AMPK, mTORC1 and ULK1, maintains energy and nutrient homoeostasis. Biochem Soc Trans 2013;41:939-43.
23. Vezina C, Kudelski A, Sehgal SN. Rapamycin (AY-22,989), a new antifungal antibiotic. I. Taxonomy of the producing streptomycete and isolation of the active principle. J Antibiot (Tokyo) 1975;28:721-6.
24. Asleh R, Briasoulis A, Kremers WK, Adigun R, Boilson BA, et al. Long-term sirolimus for primary immunosuppression in heart transplant recipients. J Am Coll Cardiol 2018;71:636-50.
25. Viana SD, Reis F, Alves R. Therapeutic use of mTOR inhibitors in renal diseases: advances, drawbacks, and challenges. Oxid Med Cell Longev 2018;2018:3693625.
26. Aylett CH, Sauer E, Imseng S, Boehringer D, Hall MN, et al. Architecture of human mTOR complex 1. Science 2016;351:48-52.
28. Kolos JM, Voll AM, Bauder M, Hausch F. FKBP ligands - where we are and where to go? Fron Pharm 2018;9:1425.
29. Kirchner GI, Meier-Wiedenbach I, Manns MP. Clinical pharmacokinetics of everolimus. Clin Pharmacokinet 2004;43:83-95.
30. Crowe A, Bruelisauer A, Duerr L, Guntz P, Lemaire M. Absorption and intestinal metabolism of SDZ-RAD and rapamycin in rats. Drug Metab Dispos 1999;27:627-32.
31. Choo AY, Yoon SO, Kim SG, Roux PP, Blenis J. Rapamycin differentially inhibits S6Ks and 4E-BP1 to mediate cell-type-specific repression of mRNA translation. Proc Natl Acad Sci U S A 2008;105:17414-9.
32. Cassell A, Freilino ML, Lee J, Barr S, Wang L, et al. Targeting TORC1/2 enhances sensitivity to EGFR inhibitors in head and neck cancer preclinical models. Neoplasia 2012;14:1005-14.
33. Lou HZ, Weng XC, Pan HM, Pan Q, Sun P, 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;450:973-8.
34. Li C, Cui JF, Chen MB, Liu CY, Liu F, et al. The preclinical evaluation of the dual mTORC1/2 inhibitor INK-128 as a potential anti-colorectal cancer agent. Cancer Biol Ther 2015;16:34-42.
35. Basu B, Dean E, Puglisi M, Greystroke A, Ong M, et al. First-in-human pharmacokinetic and pharmacodynamics study of the dual mTORC1/2 inhibitor AZD2014. Clin Cancer Res 2015;21:3412-9.
36. Powles T, Wheater M, Din O, Geldart T, Boleti E, et al. A randomised phase 2 study of AZD2014 versus everolimus in patients with VEGF-refractory metastatic clear cell renal cancer. Eur Urol 2016;69:450-6.
37. Álvarez-Garcia V, Tawi Y, Wise HM, Leslie NR. Mechanisms of PTEN loss in cancer: It’s all about diversity. Semin Cancer Biol 2019. Epub ahead of print [PMID: 30738865 DOI: 10.1016/j.semcancer.2019.02.001]
38. Ma L, Chen Z, Erdjument-Bromage H, Tempst P, Pandolfi PP. Phosphorylation and functional inactivation of TSC2 by Erk implications for tuberous sclerosis and cancer pathogenesis. Cell 2005;121:179-93.
39. Carrière A, Cargnello M, Julien LA, Gao H, Bonneil E, et al. Oncogenic MAPK signalling stimulates mTORC1 activity by promoting RSK-mediated raptor phosphorylation. Curr Biol 2008;18:1269-77.
40. Hobbs GA, Der CJ, Rossman KL. RAS isoforms and mutations in cancer at a glance. J Cell Sci 2016;129:1287-92.
41. Holderfield M, Deuker MM, McCormick F, McMahon M. Targeting RAF kinases for cancer therapy: BRAF-mutated melanoma and beyond. Nat Rev Cancer 2014;14:455-67.
42. Guo Y, Chekaluk Y, Zhang J, Du J, Gray NS, et al. TSC1 involvement in bladder cancer: diverse effects and therapeutic implications. J Pathol 2013;230:17-27.
43. Yuan F, Shi M, Ji J, Shi H, Zhou C, et al. KRAS and DAXX/ATRX gene mutations are correlated with the clinicopathological features, advanced diseases, and poor prognosis in Chinese patients with pancreatic neuroendocrine tumors. Int J Biol Sci 2014;10:957-65.
44. Huynh H, Hao HX, Chan SL, Chen D, Ong R, et al. Loss of tuberous sclerosis complex 2 (TSC2) is frequent in hepatocellular carcinoma and predicts response to mTORC1 inhibitor everolimus. Mol Cancer Ther 2015;14:1224-35.
45. Zhang Y, Ng PK, Kucherlapati M, Chen F, Liu Y, et al. A pan-cancer proteogenomic atlas of PI3K/AKT/mTOR pathway alterations. Cancer Cell 2017;31:820-32.
46. Grabiner BC, Nardi V, Birsoy K, Possemato R, Shen K, et al. A diverse array of cancer-associated MTOR mutations are hyperactivating and can predict rapamycin sensitivity. Cancer Discov 2014;4:554-63.
47. Bissler JJ, McCormack FX, Young LR, Elwing JM, Chuck G, et al. Sirolimus for angiomyolipoma in tuberous sclerosis complex or lymphangioleiomyomatosis. N Engl J Med 2008;358:140-51.
48. Davies DM, de Vries PJ, Johnson SR, McCartney DL, Cox JA, et al. Sirolimus therapy for angiomyolipoma in tuberous sclerosis and sporadic lymphangioleiomyomatosis: a phase 2 trial. Clin Cancer Res 2011;17:4071-81.
49. Cabrera López C, Martí T, Catalá V, Torres F, Mateu S, et al. Effects of rapamycin on angiomyolipomas in patients with tuberous sclerosis. Nefrologia 2011;31:292-8.
50. Krueger DA, Care MM, Holland K, Agricola K, Tudor C, et al. Everolimus for subependymal giant-cell astrocytomas in tuberous sclerosis. N Engl J Med 2010;363:1801-11.
51. Dabora SL, Franz DN, Ashwal S, Sagalowsky A, DiMario FJ Jr, et al. Multicenter phase 2 trial of sirolimus for tuberous sclerosis: kidney angiomyolipomas and other tumors regress and VEGF- D levels decrease. PLoS One 2011;6:e23379.
52. Franz DN, Belousova E, Sparagana S, Bebin EM, Frost M, et al. Efficacy and safety of everolimus for subependymal giant cell astrocytomas associated with tuberous sclerosis complex (EXIST-1): a multicentre, randomised, placebo-controlled phase 3 trial. Lancet 2013;381:125-32.
53. Saffari A, Brosse I, Wiemer-Kriel A, Wilken B, Kreuzaler P, et al. Safety and efficacy of mTOR inhibitor treatment in patients with tuberous sclerosis complex under 2 years of age - a multicenter retrospective study. Orphanet J Rare Dis 2019;14:96.
54. Pantuck AJ, Seligson DB, Klatte T, Yu H, Leppert JT, et al. Prognostic relevance of the mTOR pathway in renal cell carcinoma: implications for molecular patient selection for targeted therapy. Cancer 2007;109:2257-67.
55. Motzer RJ, Escudier B, Oudard S, Hutson TE, Porta C, et al; RECORD-1 Study Group. Efficacy of everolimus in advanced renal cell carcinoma: a double-blind, randomised, placebo-controlled phase III trial. Lancet 2008;372:449-56.
56. Motzer RJ, Escudier B, Oudard S, Hutson TE, Porta C, et al; RECORD-1 Study Group. Phase 3 trial of everolimus for metastatic renal cell carcinoma : final results and analysis of prognostic factors. Cancer 2010;116:4256-65.
57. Stein A, Bellmunt J, Escudier B, Kim D, Stergiopoulos SG, et al; RECORD-1 Trial Study Group. Survival prediction in everolimus-treated patients with metastatic renal cell carcinoma incorporating tumor burden response in the RECORD-1 trial. Eur Urol 2013;64:994-1002.
58. Escudier B, Molinie V, Bracarda S, Maroto P, Szczylik C, et al. Open-label phase 2 trial of first-line everolimus monotherapy in patients with papillary metastatic renal cell carcinoma: RAPTOR final analysis. Eur J Cancer 2016;69:226-35.
59. Missiaglia E, Dalai I, Barbi S, Beghelli S, Falconi M, et al. Pancreatic endocrine tumors: expression profiling evidences a role for AKT-mTOR pathway. J Clin Oncol 2010;28:245-55.
60. Yao JC, Lombard-Bohas C, Baudin E, Kvols LK, Rougier P, et al. Daily oral everolimus activity in patients with metastatic pancreatic neuroendocrine tumors after failure of cytotoxic chemotherapy: a phase II trial. J Clin Oncol 2010;28:69-76.
61. Yao JC, Shah MH, Ito T, Lombard Bohas C, Wolin EM, et al; RAD001 in Advanced Neuroendocrine Tumors, Third Trial (RADIANT-3) Study Group. Everolimus for advanced pancreatic neuroendocrine tumors. N Engl J Med 2011;364:514-23.
62. Yao JC, Fazio N, Singh S, Buzzoni R, Carnaghi C, et al. Everolimus for the treatment of advanced, non-functional neuroendocrine tumours of the lung or gastrointestinal tract (RADIANT-4): a randomised, placebo-controlled, phase 3 study. Lancet 2016;387:968-77.
63. Fazio N, Buzzoni R, Delle Fave G, Tesselaar ME, Wolin E, et al. Everolimus in advanced, progressive, well-differentiated, non-functional neuroendocrine tumors: RADIANT-4 lung subgroup analysis. Cancer Sci 2018;109:174-81.
64. Lau DK, Tay RY, Yeung YH, Chionh F, Mooi J, et al. Phase II study of everolimus (RAD001) monotherapy as first-line treatment in advanced biliary tract cancer with biomarker exploration: the RADiChol Study. Br J Cancer 2018;118:966-71.
65. Demetri GD, Chawla SP, Ray-Coquard I, Le Cesne A, Staddon AP, et al. Results of an international randomized phase III trial of the mammalian target of rapamycin inhibitor ridaforolimus versus placebo to control metastatic sarcomas in patients after benefit from prior chemotherapy. J Clin Oncol 2013;31:2485-92.
66. Doi T, Muro K, Boku N, Yamada Y, Nishina T, et al. Multicenter phase II study of everolimus in patients with previously treated metastatic gastric cancer. J Clin Oncol 2010;28:1904-10.
67. Ohtsu A, Ajani JA, Bai YX, Bang YJ, Chung HC, et al. Everolimus for previously treated advanced gastric cancer: results of the randomized, double-blind, phase III GRANITE-1 study. J Clin Oncol 2013;31:3935-43.
68. O’Donnell A, Faivre S, Burris HA, Rea D, Papadimitrakopoulou V, et al. Phase I pharmacokinetic and pharmacodynamic study of the oral mammalian target of rapamycin inhibitor everolimus in patients with advanced solid tumors. J Clin Oncol 2008;26:1588-95.
69. Reungwetwattana T, Molina JR, Mandrekar SJ, Allen-Ziegler K, Rowland KM, et al. Brief report: a phase II “window-of-opportunity” frontline study of the mTOR inhibitor, temsirolimus given as a single agent in patients with advanced NSCLC, an NCCTG study. J Thorac Oncol 2012;7:919-22.
70. Besse B, Leighl N, Bennouna J, Papadimitrakopoulou VA, Blais N, et al. Phase II study of everolimus-erlotinib in previously treated patients with advanced non-small-cell lung cancer. Ann Oncol 2014;25:409-15.
71. Ng K, Tabernero J, Hwang J, Bajetta E, Sharma S, et al. Phase II study of everolimus in patients with metastatic colorectal adenocarcinoma previously treated with bevacizumab-, fluoropyrimidine-, oxaliplatin-, and irinotecan-based regimens. Clin Cancer Res 2013;19:3987-95.
72. Tabernero J, Rojo F, Calvo E, Burris H, Judson I, et al. Dose- and schedule-dependent inhibition of the mammalian target of rapamycin pathway with everolimus: a phase I tumor pharmacodynamic study in patients with advanced solid tumors. J Clin Oncol 2008;26:1603-10.
73. Spindler KL, Sorensen MM, Pallisgaard N, Andersen RF, Havelund BM, et al. Phase II trial of temsirolimus alone and in combination with irinotecan for KRAS mutant metastatic colorectal cancer: outcome and results of KRAS mutational analysis in plasma. Acta Oncol 2013;52:963-70.
74. Armstrong AJ, Shen T, Halabi S, Kemeny G, Bitting RL, et al. A phase II trial of temsirolimus in men with castration-resistant metastatic prostate cancer. Clin Genitourin Cancer 2013;11:397-406.
75. Templeton AJ, Dutoit V, Cathomas R, Rothermundt C, Bärtschi D, et al; Swiss Group for Clinical Cancer Research (SAKK). Phase 2 trial of single-agent everolimus in chemotherapy-naive patients with castration-resistant prostate cancer (SAKK 08/08). Eur Urol 2013;64:150-8.
76. O’Reilly KE, Rojo F, She QB, Solit D, Mills GB, et al. mTOR inhibition induces upstream receptor tyrosine kinase signaling and activates Akt. Cancer Res 2006;66:1500-8.
77. Carracedo A, Ma L, Teruya-Feldstein J, Rojo F, Salmena L, et al. Inhibition of mTORC1 leads to MAPK pathway activation through a PI3K-dependent feedback loop in human cancer. J Clin Invest 2008;118:3065-74.
78. Yu Y, Yoon SO, Poulogiannis G, Yang Q, Ma XM, et al. Phosphoproteomic analysis identifies Grb10 as an mTORC1 substrate that negatively regulates insulin signalling. Science 2011;332:1322-6.
79. Hsu PP, Kang SA, Rameseder J, Zhang Y, Ottina KA, et al. The mTOR-regulated phosphoproteome reveals a mechanism of mTORC1-mediated inhibition of growth factor signalling. Science 2011;332:1317-22.
80. Kim YC, Guan KL. mTOR: a pharmacologic target for autophagy regulation. J Clin Invest 2015;125:25-32.
81. Roux PP, Shahbazian D, Vu H, Holz MK, Cohen MS, et al. RAS/ERK signaling promotes site-specific ribosomal protein S6 phosphorylation via RSK and stimulates cap-dependent translation. J Biol Chem 2007;282:14056-64.
82. Sato T, Nakashima A, Guo L, Coffman K, Tamanoi F. Single amino-acid changes that confer constitutive activation of mTOR are discovered in human cancer. Oncogene 2010;29:2746-52.
83. The Cancer Genome Atlas Research Network. Comprehensive molecular characterization of clear cell renal cell carcinoma. Nature 2013;499:43-9.
84. Wagle N, Grabiner BC, van Allen EM, Amin-Mansour A, Taylor-Weiner A, et al. Response and acquired resistance to everolimus in anaplastic thyroid cancer. N Engl J Med 2014;371:1426-33.
85. Tan J, Lee PL, Li Z, Jiang X, Lim YC, et al. B55β-associated PP2A complex controls PDK1-directed myc signaling and modulates rapamycin sensitivity in colorectal cancer. Cancer Cell 2010;18:459-71.
86. Gerlinger M, Rowan AJ, Horswell S, Math M, Larkin J, et al. Intratumor heterogeneity and branched evolution revealed by multiregion sequencing. N Engl J Med 2012;366:883-92.
87. El-Chemaly S, Taveira-Dasilva A, Goldberg HJ, Peters E, Haughey M, et al. Sirolimus and autophagy inhibition in lymphangioleiomyomatosis: results of a phase i clinical trial. Chest 2017;151:1302-10.
88. Haas NB, Appleman LJ, Stein M, Redlinger M, Wilks M, et al. Autophagy inhibition to augment mTOR inhibition: a phase i/ii trial of everolimus and hydroxychloroquine in patients with previously treated renal cell carcinoma. Clin Cancer Res 2019;25:2080-7.
89. Chi MS, Lee CY, Huang SC, Yang KL, Ko HL, et al. Double autophagy modulators reduce 2-deoxyglucose uptake in sarcoma patients. Oncotarget 2015;6:29808-17.
90. Schenone S, Brullo C, Musumeci F, Radi M, Botta M. ATP-competitive inhibitors of mTOR: an update. Curr Medicin Chem 2011;18:2995-3014.
91. Fazio N, Buzzoni R, Baudin E, Antonuzzo L, Hubner RA, et al. A phase ii study of bez235 in patients with everolimus-resistant, advanced pancreatic neuroendocrine tumours. Anticancer Res 2016;36:713-9.
92. Rodon J, Pérez-Fidalgo A, Krop IE, Burris H, Guerrero-Zotano A, et al. Phase 1/1b dose escalation and expansion study of BEZ235, a dual PI3K/mTOR inhibitor, in patients with advanced solid tumors including patients with advanced breast cancer. Cancer Chemother Pharmacol 2018;82:285-98.
93. Britten CD, Adjei AA, Millham R, Houk BE, Borzillo G, et al. Phase I study of PF-04691502, a small-molecule, oral, dual inhibitor of PI3K and mTOR, in patients with advanced cancer. Invest New Drugs 2014;32:510-7.
94. Bendell JC, Varghese AM, Hyman DM, Bauer TM, Pant S, et al. A first-in-human phase 1 study of LY3023414, an oral PI3K/mTOR dual inhibitor, in patients with advanced cancer. Clin Cancer Res 2018;24:3253-62.
95. Roberts PJ, Usary JE, Darr DB, Dillon PM, Pfefferle AD, et al. Combined PI3K/mTOR and MEK inhibition provides broad antitumor activity in faithful murine cancer models. Clin Cancer Res 2012;18:5290-303.
96. Tolcher AW, Bendell JC, Papadopoulos KP, Burris HA, Patnaik A, et al. A phase IB trial of the oral MEK inhibitor trametinib (GSK1120212) in combination with everolimus in patients with advanced solid tumors. Ann Oncol 2015;26:58-64.
97. Liu X, Lorusso P, Mita M, Piha-Paul S, Hong DS, et al. Incidence of mucositis in patients treated with temsirolimus-based regimens and correlation to treatment response. Oncologist 2014;19:426-8.
98. Kasner MT, Mick R, Jeschke GR, Carabasi M, Filicko-O’Hara J, et al. Sirolimus enhances remission induction in patients with high risk acute myeloid leukemia and mTORC1 target inhibition. Invest New Drugs 2018;36:657-66.
99. Burnett AK, Das Gupta E, Knapper S, Khwaja A, Sweeney M, et al; UK NCRI AML Study Group. Addition of the mammalian target of rapamycin inhibitor, everolimus, to consolidation therapy in acute myeloid leukemia: experience from the UK NCRI AML17 trial. Haematologica 2018;103:1654-61.
100. Schuetzea SM, Zhao L, Chugh R, Thomas DG, Lucas DR, et al. Results of a phase II study of sirolimus and cyclophosphamide in patients with advanced sarcoma. Eur J Cancer 2012;48:1347-53.
101. Wood A, George S, Adra N, Chintala S, Damayanti N, et al. Phase I study of the mTOR inhibitor everolimus in combination with the histone deacetylase inhibitor panobinostat in patients with advanced clear cell renal cell carcinoma. Invest New Drugs 2019. Epub ahead of print [DOI: 10.1007/s10637-019-00864-7]
102. Tew WP, Sill MW, Walker JL, Secord AA, Bonebrake AJ, et al. Randomized phase II trial of bevacizumab plus everolimus versus bevacizumab alone for recurrent or persistent ovarian, fallopian tube or peritoneal carcinoma: An NRG oncology/gynecologic oncology group study. Gynecol Oncol 2018;151:257-263.
103. Sherman WH. Sirolimus can reverse resistance to gemcitabine, capecitabine and docetaxel combination therapy in pancreatic cancer. JOP 2009;10:393-5.
104. Lee JS, Yost SE, Blanchard S, Schmolze D, Yin HH, et al. Phase I clinical trial of the combination of eribulin and everolimus in patients with metastatic triple-negative breast cancer. Breast Cancer Res 2019;21:119.
105. Shi Y, Zhang W, Ye Y, Cheng Y, Han L, et al. Benefit of everolimus as a monotherapy for a refractory breast cancer patient bearing multiple genetic mutations in the PI3K/AKT/mTOR signaling pathway. Cancer Biol Med 2018;15:314-21.
106. Parachoniak CA, Rankin A, Gaffney B, Hartmaier R, Spritz D, et al. Exceptional durable response to everolimus in a patient with biphenotypic breast cancer harboring an STK11 variant. Cold Spring Harb Mol Case Stud 2017;3:a000778.
107. Iyer G, Hanrahan AJ, Milowsky MI, Al-Ahmadie H, Scott SN, et al. Genome sequencing identifies a basis for everolimus sensitivity. Science 2012;338:221.
108. Yi Z, Ma F, Liu B, Guan X, Li L, et al. Everolimus in hormone receptor-positive metastatic breast cancer: PIK3CA mutation H1047R was a potential efficacy biomarker in a retrospective study. BMC Cancer 2019;19:442.
109. Bellmunt J, Lalani AA, Jacobus S, Wankowicz SA, Polacek L, et al. Everolimus and pazopanib (E/P) benefit genomically selected patients with metastatic urothelial carcinoma. Br J Cancer 2018;119:707-12.
110. Sharma S, Becerra CR, Matrana MR, Alistar AT, Chiorean EG, et al. A phase I/II multicenter study of ABI-009 (nab-sirolimus) combined with FOLFOX and bevacizumab as first-line (1L) therapy in patients (pts) with metastatic colorectal cancer (mCRC) with or without PTEN loss. J Clin Oncol 2019;37.
111. Kwiatkowski DJ, Choueiri TK, Fay AP, Rini BI, Thorner AR, et al. Mutations in TSC1, TSC2, and MTOR are associated with response to rapalogs in patients with metastatic renal cell carcinoma. Clin Cancer Res 2016;22:2445-52.
112. Lynch TJ, Bell DW, Sordella R, Gurubhagavatula S, Okimoto RA, et al. Activating mutations in the epidermal growth factor receptor underlying responsiveness of non-small-cell lung cancer to gefitinib. N Engl J Med 2004;350:2129-39.
113. Juengel E, Kim D, Makarevic J, Reiter M, Tsaur I, et al. Molecular analysis of sunitinib resistant renal cell carcinoma cells after sequential treatment with RAD001 (everolimus) or sorafenib. J Cell Mol Med 2015;19:430-41.
114. Motzer RJ, Hutson TE, Glen H, Michaelson MD, Molina A, et al. Lenvatinib, everolimus, and the combination in patients with metastatic renal cell carcinoma: a randomised, phase 2, open-label, multicentre trial. Lancet Oncol 2015;16:1473-82.
115. Matsubara N, Naito Y, Nakano K, Fujiwara Y, Ikezawa H, et al. Lenvatinib in combination with everolimus in patients with advanced or metastatic renal cell carcinoma: a phase 1 study. Int J Urol 2018;25:922-8.
116. Hutson TE, Escudier B, Esteban E, Bjarnason GA, Lim HY, et al. Randomized phase III trial of temsirolimus versus sorafenib as second-line therapy after sunitinib in patients with metastatic renal cell carcinoma. J Clin Oncol 2014;32:760-7.
117. Zarrabi K, Fang C, Wu S. New treatment options for metastatic renal cell carcinoma with prior anti-angiogenesis therapy. J Hematol Oncol 2017;10:38.
118. Paplomata E, Zelnak A, Santa-Maria CA, Liu Y, Gogineni K, et al. Use of everolimus and trastuzumab in addition to endocrine therapy in hormone-refractory metastatic breast cancer. Clin Breast Cancer 2019;19:188-96.
119. Bachelot T, Bourgier C, Cropet C, Ray-Coquard I, Ferrero JM, et al. Randomized phase II trial of everolimus in combination with tamoxifen in patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative metastatic breast cancer with prior exposure to aromatase inhibitors: a GINECO study. J Clin Oncol 2012;30:2718-24.
120. Baselga J, Campone M, Piccart M, Burris HA, Rugo HS, et al. Everolimus in postmenopausal hormone-receptor-positive advanced breast cancer. N Engl J Med 2012;366:520-9.
121. Kornblum N, Zhao F, Manola J, Klein P, Ramaswamy B, et al. Randomized phase II trial of fulvestrant plus everolimus or placebo in postmenopausal women with hormone receptor-positive, human epidermal growth factor receptor 2-negative metastatic breast cancer resistant to aromatase inhibitor therapy: results of PrE0102. J Clin Oncol 2018;36:1556-63.
122. Shimoi T, Shimomura A, Shien T, Uemura Y, Kato H, et al. Open-label Phase II study of everolimus plus endocrine therapy in postmenopausal women with ER-positive and HER2-negative metastatic breast cancer (Chloe trial). Open Access J Clin Trials 2018;10:13-8.
123. Schmid P, Zaiss M, Harper-Wynne C, Ferreira M, Dubey S, et al. Fulvestrant plus vistusertib vs fulvestrant plus everolimus vs fulvestrant alone for women with hormone receptor-positive metastatic breast cancer: the MANTA phase 2 randomized clinical trial. JAMA Oncol 2019. Epub head of print [DOI: 10.1001/jamaoncol.2019.2526]
124. Yoon DH, Ryu MH, Park YS, Lee HJ, Lee C, et al. Phase II study of everolimus with biomarker exploration in patients with advanced gastric cancer refractory to chemotherapy including fluoropyrimidine and platinum. Br J Cancer 2012;106:1039-44.
125. Buzzoni R, Pusceddu S, Bajetta E, De Braud F, Platania M, et al. Activity and safety of RAD001 (everolimus) in patients affected by biliary tract cancer progressing after prior chemotherapy: a phase II ITMO study. Ann Oncol 2014;25:1597-603.
126. Brose MS, Troxel AB, Yarchoan M, Cohen AB, Harlacker K, et al. A phase II study of everolimus (E) and sorafenib (S) in patients (PTS) with metastatic differentiated thyroid cancer who have progressed on sorafenib alone. J Clin Oncol 2015;33:6072.
127. Hanna GJ, Busaidy NL, Chau NG, Wirth LJ, Barletta JA, et al. Genomic correlates of response to everolimus in aggressive radioiodine-refractory thyroid cancer: a phase ii study. Clin Cancer Res 2018;24:1546-53.
128. Wolpin BM, Ng K, Zhu AX, Abrams T, Enzinger PC, et al. Multicenter phase II study of tivozanib (AV-951) and everolimus (RAD001) for patients with refractory, metastatic colorectal cancer. Oncologist 2013;18:377-8.
129. Lombard-Bohas C, Yao J, Hobday T, Van Cutsem E, Wolin E, et al. Impact of prior chemotherapy use on the efficacy of everolimus in patients with advanced pancreatic neuroendocrine tumors: a subgroup analysis of the phase III RADIANT-3 trial. Pancreas 2015;44:181-9.
130. Rodrik-Outmezguine VS, Okaniwa M, Yao Z, Novotny CJ, McWhirter C, et al. Overcoming mTOR resistance mutations with a new-generation mTOR inhibitor. Nature 2016;534:272-6.
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Bhaoighill, M. N.; Dunlop E. A. Mechanistic target of rapamycin inhibitors: successes and challenges as cancer therapeutics. Cancer. Drug. Resist. 2019, 2, 1069-85. http://dx.doi.org/10.20517/cdr.2019.87
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