mTOR inhibitors are drug classes that inhibit the rapamycin (mTOR) mechanical target, which is a serine/threonine specific protein kinase belonging to the kinase-related phosphatidylinositol-3 kinase (PI3K) family. mTOR regulates cell metabolism, growth, and proliferation by forming and signaling through two protein complexes, mTORC1 and mTORC2. The most established mTOR inhibitors are so-called rapalogs (rapamycin and its analogues), which have shown tumor responses in clinical trials of different types of tumors.
Video MTOR inhibitors
Histori
The invention of mTOR was made several decades ago while investigating the mechanism of action of its inhibitor, rapamycin. Rapamycin was first discovered in 1975 in a soil sample from the South Pacific Easter Island, also known as Rapa Nui, from which it came from. Rapamycin is macrolide, produced by Streptomyces hygroscopicus microorganisms and exhibits antifungal properties. Shortly after his discovery, immunosuppressive properties were detected, which subsequently led to the formation of rapamycin as immunosuppressant. In the 1980s, rapamycin was also found to have anticancer activity although the exact mechanism of action remained unknown until many years later.
In the 1990s there was a dramatic change in this field because of the study of rapamycin's mechanism of action and the identification of drug targets. It was found that rapamycin inhibits cellular proliferation and development of cell cycle. Research on mTOR inhibitors has become a growing branch of science and has promising results.
Maps MTOR inhibitors
Protein kinase and its inhibitors
In general, protein kinase is classified into two main categories based on the specificity of the substrate, tyrosine kinase protein and serine protein/threonine kinase. Kinase of double specificity is a subclass of tyrosine kinase.
mTOR is a kinase in the family kinase-related kinase phosphatidylinositol-3 (PIKKs), which is a family of serine/threonine kinase proteins, with a sequence similarity to the kinase lipid family, PI3Ks. This Kinase has a different biological function, but all the large proteins with a common domain structure.
PIKK has four domains at the protein level, which distinguishes them from other protein kinases. From N-terminus to C-terminus, this domain is named FRAP-ATM-TRAAP (FAT), domain kinase (KD), PIKK-regulatory domain (PRD), and FAT-C-terminal (FATC). FAT domain, which consists of four? -heliks, are N-terminals to KD, but they are called FKBP12-rapamycin-binding (FRB) domains, which bind to the FKBP12-rapamycin complex. The FAT domain consists of repetition, called HEAT (Huntingtin, Elongation factor 3, A subunit of protein phosphatase 2A and TOR1). The specific protein activator regulates the PIKK kinase but its binding to the kinase complex causes conformational changes that increase substrate access to the kinase domain.
Protein kinase has become a popular drug target. They have been targeted for the discovery and design of small molecule and biological inhibitors as potential therapeutic agents. Small molecule inhibitors of protein kinase generally prevent phosphorylation either from the protein substrate or autofosforilasi from the kinase itself.
mTOR signaling path
It appears that growth factors, amino acids, ATP, and oxygen levels regulate mTOR signaling. Some downstream pathways that regulate cell development, translation, initiation, transcriptional stress response, protein stability, and cell survival signify through mTOR.
The serine/threonine kinase mTOR is a downstream effect of the PI3K/AKT pathway, and forms two different multiprotein complexes, mTORC1 and mTORC2. Both of these complexes have separate protein partner networks, feedback loops, substrates, and regulators. mTORC1 consists of mTOR and two positive regulatory subunits, raptors and LST8 mammals (mLST8), and two negative regulators, proline-rich 40 AKT substrates (PRAS40) and DEPTOR. mTORC2 consists of mTOR, mLST8, mSin1, protor, rictor, and DEPTOR.
mTORC1 is sensitive to rapamycin but mTORC2 is considered resistant and is generally insensitive to nutrients and energy signals. mTORC2 is activated by growth factors, phosphorylation of PKC, AKT and paxillin, and regulates small GTPase, Rac, and Rho activities associated with cell survival, migration and cytoskeleton actin regulation.
The mTORC1 signaling cascade is activated by a phosphorylated AKT and produces S6K1 phosphorylation, and 4EBP1, leading to mRNA translation.
mTOR signaling pathways in human cancer
Many human tumors occur because of dysregulated mTOR signaling, and may provide a higher susceptibility to mTOR inhibitors. Deregulation of various elements of mTOR pathways, such as PI3K amplification/mutation, loss of PTEN function, overexpression of AKT, and expression of S6K1, 4EBP1, and eIF4E have been associated with many types of cancer. Therefore, mTOR is an attractive therapeutic target for treating various cancers, either mTOR inhibitors alone or in combination with other pathway inhibitors.
Upstream, PI3K/AKT signaling is deregulated through a variety of mechanisms, including over-expression or activation of growth factor receptors, such as HER-2 (human epidermal growth factor 2 receptors) and IGFR (insulin-like growth factor receptors), mutations in PI3K and AKT mutation/amplification. The phosphatase suppressor and homologous tensin removed on chromosome 10 (PTEN) are negative PI3K signaling regulators. In many cancers, PTEN expression decreases and can be decreased through several mechanisms, including mutations, loss of heterozygosity, methylation, and protein instability.
Downstream, effector mTOR S6 kinase 1 (S6K1), eukaryotic initiation factor 4E 1 (4EBP1) binding protein and 4E eukaryotic initiation factor (eIF4E) associated with cellular transformation. S6K1 is the main regulator of cell growth and also phosphorylates other important targets. Both eIF4E and S6K1 are included in cellular transformation and their excessive expression has been associated with a poor prognosis of cancer.
Development of mTOR inhibitors
Since the invention of mTOR, much research has been done on the subject, using rapamycin and rapalogs to understand its biological function. The clinical results of targeting these pathways are not as straight forward as they were initially thought. These results have changed the direction of clinical research in this area.
Initially, rapamycin was developed as an antifungal drug against Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans . A few years later immunosuppressive properties are detected. Subsequent research led to the formation of rapamycin as the main immunosuppressant against transplant rejection, along with cyclosporin A. By using rapamycin in combination with cyclosporin A, it improves the prevention of rejection in renal transplantation. Therefore, it is possible to use low doses of cyclosporine which minimize drug toxicity.
In the 1980s, rapamycin was evaluated by the National Cancer Institute's Therapeutic Development Branch (NCI). It was found that rapamycin has anticancer activity and non-cytotoxic agents with cytostatic activity against some types of human cancers. However, due to the unfavorable pharmacokinetic properties, the development of mTOR inhibitors for cancer treatment was unsuccessful at the time. Since then, rapamycin has also been shown to be effective in preventing coronary artery stenosis and for the treatment of neurodegenerative diseases.
First generation mTOR inhibitors
The development of rapamycin as an anticancer agent began again in the 1990s with the discovery of temsirolimus (CCI-779). This is a new dissolved rapamycin derivative that has a favorable toxicological profile in animals. More rapamycin derivatives with improved pharmacokinetics and reduced immunosuppressive effects have since been developed for cancer treatment. These Rapalogs include temsirolimus (CCI-779), everolimus (RAD001), and ridaforolimus (AP-23573) that are being evaluated in clinical trials of cancer. Rapamycin analogues have the same therapeutic effect as rapamycin. However they have increased hydrophilicity and can be used for oral and intravenous administration. In 2012 the National Cancer Institute enrolled more than 200 clinical trials that tested anticancer activity from rapalog either as monotherapy or as part of combination therapy for many types of cancer.
Rapalogs, which are the first generation mTOR inhibitors, have proven effective in a variety of preclinical models. However, success in clinical trials is limited to only a few rare cancers. Animal and clinical studies show that rapalog is primarily cytostatic, and therefore effective as a disease stabilizer rather than for regression. The response rate in a solid tumor in which the rapalog has been used as a single agent therapy has been simple. Due to partial mTOR inhibition as mentioned earlier, the rapalog is not sufficient to achieve a broad and powerful anticancer effect, at least when used as monotherapy.
Another reason for its limited success is that there is a feedback loop between mTORC1 and AKT in certain tumor cells. It appears that the mTORC1 inhibition by the rapalog fails to suppress the negative feedback loop that results in the phosphorylation and activation of the AKT. This limitation has led to the development of a second generation of mTOR inhibitors.
Rapamycin and rapalogs
Rapamycin and rapalog (rapamycin derivatives) are small molecule inhibitors, which have been evaluated as anticancer agents. The rapalog has a more favorable pharmacokinetic profile compared to rapamycin, the parent drug, although the same binding site for mTOR and FKBP12.
Sirolimus
Natural antibiotics, rapamycin or sirolimus, cytostatic agents, have been used in combination therapy with corticosteroids and cyclosporins in patients receiving kidney transplants to prevent rejection of either organs in the US and Europe, due to unsatisfactory pharmacokinetic properties. In 2003, the US Food and Drug Administration approved a sirolimus-eluting coronary stent, which is used in patients with coronary artery narrowing, or so-called atherosclerosis.
Recently rapamycin has been shown to be effective in inhibiting the growth of some human cancers and murine cell lines. Rapamycin is a major mTOR inhibitor, but deforolymus (AP23573), everolimus (RAD001), and temsirolimus (CCI-779), is a newly developed rapamycin analogue.
Temsirolimus
The rapamycin analog temsirolimus (CCI-779) is also a non-cytotoxic agent that delayed tumor proliferation.
Temsirolimus is a pro-drug rapamycin. It is approved by the US Food and Drug Administration (FDA) and the European Drug Administration (EMA), for the treatment of renal cell carcinoma (RCC). Temsirolimus has a higher water solubility than rapamycin and is therefore administered with intravenous injection. It was approved on May 30, 2007, by the FDA for advanced RCC treatment.
Temsirolimus has also been used in Phase I clinical trials along with neratinib, a small irreversible pan-HER tyrosine kinase inhibitor. The study enrolled patients treated for HER2-amplified breast cancer, non-small celled HER2-mutant lung cancer, and other advanced solid tumors. While common toxicities include nausea, stomatitis, and anemia; responses recorded.
Everolimus
Everolimus is the second novel Rapamycin analog. From March 30, 2009 to May 5, 2011, the US FDA approved everolymus for the treatment of advanced renal cell carcinoma following treatment failure with sunitinib or sorafenib, subependymal giant cell germ (subcellular glandular astrocytoma) associated with tuberous sclerosis (TS), and progressive neuroendocrine. tumor of pancreas origin (PNET). In July and August 2012, two new indications were approved, for receptor-positive breast cancer, HER2-negative breast cancer in combination with exemestane, pediatric and adult patients with SEGA. In 2009 and 2011, it was also approved throughout the EU for advanced breast cancer, pancreatic neuroendocrine tumors, advanced kidney cell carcinoma, and SEGA in patients with tuberous sclerosis.
Ridaforolimus
Ridaforolimus (AP23573, MK-8669), or deforolimus, is the newest rapamycin analog and not a prodrug. Such temsirolimus can be administered intravenously, and oral formulations are being estimated for the treatment of sarcomas. It's not on the market in June 2012, because the FDA wants more human testing on it because of its effectiveness and safety.
Second generation mTOR inhibitors
Second generation mTOR inhibitors are known as competitive mTOR kinase inhibitors. mTORC1/mTORC2 dual inhibitors are designed to compete with ATP at the mTOR catalytic site. They inhibit all dependent kinase functions of mTORC1 and mTORC2 and therefore, block the activation of feedback from PI3K/AKT signaling, unlike the rapalogue targeting mTORC1 only. This type of inhibitor has been developed and some of which are being tested in clinical trials. Like rapalogs, they reduce protein translation, weaken the development of cell cycle, and inhibit angiogenesis in many cancer cell lines and also in human cancers. In fact they have proven to be stronger than rapalog.
Theoretically, the most important advantage of this mTOR inhibitor is a considerable reduction of AKT phosphorylation in the mTORC2 blockade and in addition to better inhibition of mTORC1. However, there are some drawbacks. Although these compounds have been effective in the non-sensitive cell lines-rapamycin, they have only demonstrated limited success in tumors controlled by KRAS. This suggests that combinational therapy may be necessary for the treatment of this cancer. Another drawback is their potential toxicity. These facts have raised concerns about the long-term efficacy of this type of inhibitor.
The close interaction of mTOR with the PI3K pathway also led to the development of a mTOR/PI3K double inhibitor. Compared with drugs that inhibit mTORC1 or PI3K, this drug has the benefit of inhibiting mTORC1, mTORC2, and all of the PI3K catalytic isoforms. Targeting both kinases at the same time reduces the increase in PI3K regulation, which is usually produced by inhibition of mTORC1. Inhibition of the PI3K/mTOR pathway has been shown to potentially inhibit proliferation by inducing G1 capture in different tumor cell lines. A strong induction of apoptosis and autophagy has also been seen. Despite promising results, there is preclinical evidence that some cancers may not be sensitive to this double inhibition. PI3K/mTOR double inhibitors also tend to increase toxicity.
Action mechanism
The study of rapamycin as an immunosuppressive agent allows us to understand its mechanism of action. This inhibits T cell proliferation and proliferative responses caused by several cytokines, including interleukin 1 (IL-1), IL-2, IL-3, IL-4, IL-6, IGF, PDGF, and colony-stimulating factors ( CSFs). ). Rapamycin inhibitors and rapalogs can target tumor growth either directly or indirectly. Their direct impact on cancer cells depends on the concentration of certain drugs and cellular characteristics. The indirect way, based on the interaction with the processes necessary for tumor angiogenesis.
Effects in cancer cells
Rapamycin and rapalog bind to immunaseiline proteins FK506 binding, tacrolimus or FKBP-12, via the methoxy group. The rapamycin-FKBP12 complex disrupts the FRB domain of mTOR. The molecular interactions between FKBP12, mTOR, and rapamycin can last for about three days (72 hours). MTOR inhibition blocks the binding of accessory protein raptors (mTOR-related regulatory proteins) to mTOR, but those necessary for downstream phosphorylation of S6K1 and 4EBP1.
As a result, S6K1 dephosphorylates, which reduces protein synthesis and decreases cell motility and size. Rapamycin induces 4EBP1 defosforilasi as well, resulting in an increase in p27 and decreased cyclin D1 expression. Which leads to late blockage of the G1/S. Rapamycin cell cycle has been shown to induce cancer cell death by stimulating autophagy or apoptosis, but the molecular mechanisms of apoptosis in cancer cells have not been fully resolved. One suggestion of the association between mTOR inhibition and apoptosis may be through the downstream target S6K1, which may phosphorylate BAD, a pro-apoptotic molecule, in Ser136. The reaction breaks down the binding of BAD to BCL-XL and BCL2, the inhibitor of mitochondrial death, resulting in BAD inactivation and decreased cell survival. Rapamycin has also been shown to induce p53-independent apoptosis in certain types of cancer.
Effects on tumor angiogenesis
The angiogenesis tumor relies on the interaction between endothelial vascular growth factors that can all activate PI3K/AKT/mTOR in endothelial cells, pericites, or cancer cells. Examples of these growth factors are angiopoietin 1 (ANG1), ANG 2, basic fibroblast growth factor (bFGF), ephrin-B2, vascular enothelial growth factor (VEGF), and a member of tumor-growth factor? (TGF?) Superfamily. One of the major stimuli of angiogenesis is hypoxia, which results in activation of hypoxia-induced transcription factors (HIFs) and expression of ANG2, bFGF, PDGF, VEGF, and VEGFR. HIF1 Inhibition? translation by preventing PDGF/PDGFR and VEGF/VEGFR can result from mTOR inhibition. A blockage of G0-G1 cell cycle may be a consequence of mTOR inactivation in activated peroksita of hypoxia and endothelial cells.
There is some evidence that extended therapy with rapamycin may have an effect on ACT and mTORC2 as well.
Activity structure relationships
The pipecolate region of the rapamycin structure appears to be necessary for rapamycin-binding to FKBP12. This step is necessary for further binding of rapamycin to mTOR kinase, which is a key enzyme in many rapamycin biological actions.
The high affinity of rapamycin binding to FKBP12 is explained by the amount of hydrogen bonding through two different hydrophobic binding pockets, and this has been revealed by the X-ray crystal structure of the protein-bound compounds. General structural characteristics for temsirolimus and sirolimus; pipecolic acid, tricarbonyl region of C13-C15, and lactone function play a key role in the binding group with FKBP12.
The most important hydrogen bond is the carbonyl lactone in C-21 to NH backbone from Ile56, carbonyl amide in C-15 to phenolic group on sidechain Tyr82, and hydroxyl protons on hemiketal carbon, C-13, to sidewhe Asp37.
Structural changes in the structure of rapamycin may affect binding to mTOR. This may include both direct and indirect binding as part of the binding to FKBP12. The interaction of the FKBP12-rapamycin complex with mTOR corresponds to the flexibility of the conformational domain of rapamycin effector. This domain comprises a molecular region that makes hydrophobic interactions with FKB domains and triene regions of C-1-C-6, methoxy groups in C-7, and methyl groups in C-33, C-27 and C-25. All macrolide ring changes can have unpredictable effects on binding and therefore, make SAR determination for problematic rapalogs.
Rapamycin does not contain ionized function groups in the pH range of 1-10 and, therefore, is rather insoluble in water. Despite its effectiveness in preclinical cancer models, its poor water solubility, stability, and long part-time elimination make parenteral use difficult, but the soluble progression of rapamycin analogs overcame many obstacles.
However, rapamycin analogues that have been approved for human use are modified in the C-43 hydroxyl group and show improved pharmacokinetic parameters and drug properties, such as solubility.
Rapamycin and temsirolimus have similar chemical structures and bind FKBP12, although the mechanisms work differently.
Temsirolimus is a dihydroxyethyl propionic acid ester of rapamycin, and its first derivative. Therefore, it is more soluble in water, and because the soluble water can be provided with intravenous formulations.
Everolimus has an O-2 substitution of the hydroxyethyl chain and deforolymus has a substitution of phosphine oxide at C-43 position in the rapamycin lactone ring.
Deforolimus (Ridaforolimus) has a secondary C43 alcohol part of the Rapamycin cyclohexyl group replaced by phosphonate and phosphate groups, preventing high affinity bonds on mTOR and FKBP. Computational modeling studies help synthesize compounds.
Bad events
Treatment with mTOR inhibitors can be complicated by side effects. The most common side effects are stomatitis, rash, anemia, fatigue, hyperglycemia/hypertriglyceridemia, decreased appetite, nausea, and diarrhea. In addition, interstitial lung disease is a very important side effect. MTORi-induced ILD is often asymptomatic (with a ground glass abnormality in chest CT) or mild symptoms (with a non-productive cough), but can be very severe as well. Even casualties have been described. Careful diagnosis and treatment are therefore essential. More recently, new diagnostic and therapeutic management approaches have been proposed.
Biomarkers
Identification of efficacy predictive biomarkers for tumor types that are sensitive to mTOR inhibitors remains a major problem. Possible predictive biomarkers for tumor response to mTOR inhibitors, as described in glioblastoma, breast and prostate cancer cells, may be the differential expression of protein pathways of mTOR, PTEN, AKT, and S6. Thus, this data is based on preclinical testing, based on the in vitro culture cell tumor cell, which suggests that the effects of mTOR inhibitors may be more pronounced in cancers that indicate loss of PTEN or PIK3CA mutations. However, the use of PTEN, PIK3CA mutations, and AKT-phosphor status to predict rapalog sensitivity have not been fully validated in the clinic. Until now, attempts to identify the rapalog response biomarker have not been successful.
Sensitivity
Clinical and translational data suggest that this type of sensitive tumor, with adequate parameters and functional apoptotic pathways, may not require high doses of mTOR inhibitors to trigger apoptosis. In most cases, cancer cells may be only partially sensitive to mTOR inhibitors due to excessive signal transduction or lack of functional apoptotic signaling pathways. In such situations, high doses of mTOR inhibitors may be necessary. In a recent study of patients with renal cell carcinoma, resistance to Temsirolimus was associated with low levels of p-AKT and p-S6K1, which played a key role in mTOR activation. This data strongly suggests the number of tumors with an activated PI3K/AKT/mTOR signaling pathway that does not respond to mTOR inhibitors. For further research it is advisable to exclude patients with low or negative PPT levels from trials with mTOR inhibitors.
Current data are not sufficient to predict tumor sensitivity to rapamycin. However, the existing data allows us to characterize tumors that may not respond to a rapalogue.
ATP-competitive mTOR kinase inhibitor
This second generation mTOR inhibitor binds to the ATP binding site at the mTOR kinase domain required for the mTORC1 and mTORC2 functions, and results in a downregulation of mTOR signaling pathways. Due to the ability of PI3K and mTORC2 to regulate AKT phosphorylation, both of these compounds play a key role in minimizing AKT feedback activation.
mTOR/PI3K dual inhibitor
Some, so-called mTOR/PI3K double inhibitors (TPdIs), have been developed and are in early-stage preclinical testing and show promising results. Their development has benefited from previous studies with PI3K-selective inhibitors. This small molecule activity of the rapalog activity differs by blocking phosphorylation depending on mTORC1 from phosphorylation of S6K1 and mTORC2 depending on the Ser473 AKT residue.
MTOR/PI3K double inhibitors include dactolisib, BGT226, SF1126, PKI-587 and more. For example, Novartis has developed the NVPBE235 compound that reportedly inhibits tumor growth in a variety of preclinical models. This increases the antitumor activity of some other drugs such as vincristine. Dactolisib appears to effectively inhibit wild-type and mutant PI3KCA forms, suggesting its use against large tumor types. Studies have shown superior antiproliferative activity against rapalog and in vivo models have confirmed this potent antineoplastic effect of multiple mTOR/PI3K inhibitors. This inhibitor targets the PI3K isoform (p110?,? And?) Along with the ATP binding sites of mTORC1 and mTORC2 by blocking PI3K/AKT signaling, even in cancer types with mutations in this pathway.
mTORC1/mTORC2 dual inhibitor (TORCdIs)
MTOR-specific inhibitors arise from screening and drug discovery. This compound blocks the activity of both mTOR complexes and is called a mTORC1/mTORC2 double inhibitor. Compounds with these characteristics such as salamisertib (codenamed INK128), AZD8055, and AZD2014 have entered clinical trials. This series of mTOR kinase inhibitors has been studied. Their structure is derived from morpholino pyrazolopyrimidine scaffold. Improvement of this type of inhibitor has been done by swapping morphine with bridged morphine in a pyrazoloprimidin inhibitor and the results show a selectivity increase in mTOR 26,000-fold.
Limited limitations of new generation mTOR inhibitors
Although a new generation of mTOR inhibitors holds great promise for anticancer therapy and is rapidly moving into clinical trials, there are many important issues that determine their success in the clinic. First of all predictable biomarkers for the benefit of this inhibitor are not available. It appears that genetic determinants affecting cancer cells become sensitive or resistant to these compounds. Tumors dependent on the PI3K/mTOR pathway should respond to this agent but it is unclear whether the compound is effective in cancer with distinct genetic lesions.
MTOR inhibition is a promising strategy for treating a number of cancers. The limited clinical activity of selective mTORC1 agents has made them unlikely to have an impact in the treatment of cancer. Development of a competitive ATP-inhibitor inhibitor has the ability to block mTORC1 and mTORC2.
Future
The limitations of the currently available rapalog has led to a new approach to mTOR targeting. Studies show that mTOR inhibitors may have anticancer activity in many cancers, such as RCC, neuroendocrine tumors, breast cancer, hepatocellular carcinoma, sarcoma, and large B-cell lymphoma. One of the major limitations to the development of mTOR inhibition therapy is that biomarkers are not currently available to predict which patients will respond. A better understanding of the molecular mechanisms involved in the cancer cell response to mTOR inhibitors is still needed so that this can be done.
How to overcome resistance and improve the efficacy of mTOR targeting agents may be by patient stratification and selection of combination drug therapy. This can lead to more effective and personalized cancer therapy. Although more research is needed, targeting mTOR remains an exciting and promising therapy option for cancer treatment.
See also
- Target rapamycin mammals (mTOR)
- PI3K/AKT/mTOR path
- Pen/PKB path path
- PI3K inhibitor
References
Source of the article : Wikipedia
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