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mTOR inhibitors are a class of drugs used to treat several human diseases, including cancer, autoimmune diseases, and neurodegeneration. They function by inhibiting the
mammalian target of rapamycin The mammalian target of sirolimus, rapamycin (mTOR), also referred to as the mechanistic target of rapamycin, and sometimes called FK506-binding protein 12-rapamycin-associated protein 1 (FRAP1), is a kinase that in humans is encoded by the ''M ...
(mTOR) (also known as the mechanistic target of rapamycin), which is a
serine/threonine-specific protein kinase A serine/threonine protein kinase () is a kinase enzyme, in particular a protein kinase, that phosphorylation, phosphorylates the hydroxyl, OH group of the amino acid, amino-acid residues serine or threonine, which have similar side chains. ...
that belongs to the family of phosphatidylinositol-3 kinase (PI3K) related kinases (PIKKs). mTOR regulates cellular metabolism, growth, and proliferation by forming and signaling through two
protein complexes A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multidomain enzymes, in which multiple catalytic domains are found in a single polypeptide chain. Protein c ...
,
mTORC1 mTORC1, also known as mammalian target of rapamycin complex 1 or mechanistic target of rapamycin complex 1, is a protein complex that functions as a nutrient/energy/redox sensor and controls protein synthesis. mTOR Complex 1 (mTORC1) is comp ...
and
mTORC2 mTOR Complex 2 (mTORC2) is an acutely rapamycin-insensitive protein complex formed by serine/threonine kinase mTOR that regulates cell proliferation and survival, cell migration and cytoskeletal remodeling. The complex itself is rather large, ...
. The most established mTOR inhibitors are so-called rapalogs (rapamycin and its analogs), which have shown tumor responses in clinical trials against various tumor types.


History

The discovery of mTOR was made in 1994 while investigating the
mechanism of action In pharmacology, the term mechanism of action (MOA) refers to the specific biochemical Drug interaction, interaction through which a Medication, drug substance produces its pharmacological effect. A mechanism of action usually includes mention o ...
of its
inhibitor Inhibitor or inhibition may refer to: Biology * Enzyme inhibitor, a substance that binds to an enzyme and decreases the enzyme's activity * Reuptake inhibitor, a substance that increases neurotransmission by blocking the reuptake of a neurotransmi ...
,
rapamycin Sirolimus, also known as rapamycin and sold under the brand name Rapamune among others, is a macrolide compound that is used to coat coronary stents, prevent organ rejection, organ transplant rejection, treat a rare lung disease called lymphang ...
. Rapamycin was first discovered in 1975 in a soil sample from
Easter Island Easter Island (, ; , ) is an island and special territory of Chile in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. The island is renowned for its nearly 1,000 extant monumental statues, ...
of
South Pacific The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...
, also known as Rapa Nui, from where its name is derived. Rapamycin is a
macrolide Macrolides are a class of mostly natural products with a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. Macrolides belong to the polyketide class of natural products. ...
, produced by the
microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
'' Streptomyces hygroscopicus'' and showed
antifungal An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as ...
properties. Shortly after its discovery,
immunosuppressive Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse react ...
properties were detected, which later led to the establishment of rapamycin as an 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 due to studies on the mechanism of action of rapamycin and the identification of the drug target. It was found that rapamycin inhibited cellular proliferation and cell cycle progression. Research on mTOR inhibition has been a growing branch in science and has promising results.


Protein kinases and their inhibitors

In general,
protein kinase A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them ( phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a f ...
s are classified in two major categories based on their substrate specificity, protein tyrosine kinases and protein serine/threonine kinases. Dual-specificity kinases are subclass of the tyrosine kinases. mTOR is a kinase within the family of phosphatidylinositol-3 kinase-related kinases (PIKKs), which is a family of serine/threonine protein kinases, with a sequence similarity to the family of lipid kinases, PI3Ks. These kinases have different biological functions, but are all large proteins with common domain structure. PIKKs have four domains at the protein level, which distinguish them from other protein kinases. From the
N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...
to the
C-terminus The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein Proteins are large biomolecules and macromolecules that comp ...
, these domains are named FRAP-ATM-TRAAP (FAT), the kinase domain (KD), the PIKK-regulatory domain (PRD), and the FAT-C-terminal (FATC). The FAT domain, consisting of four
α-helices An alpha helix (or α-helix) is a sequence of amino acids in a protein that are twisted into a coil (a helix). The alpha helix is the most common structural arrangement in the secondary structure of proteins. It is also the most extreme type of l ...
, is N-terminal to KD, but that part is referred to as the FKBP12-rapamycin-binding (FRB) domain, which binds the FKBP12-rapamycin complex. The FAT domain consists of repeats, referred to as
HEAT In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...
(
Huntingtin Huntingtin (Htt) is the protein coded for in humans by the ''HTT'' gene, also known as the ''IT15'' ("interesting transcript 15") gene. Mutation, Mutated ''HTT'' is the cause of Huntington's disease (HD), and has been investigated for this role an ...
, Elongation factor 3, A subunit of protein phosphatase 2A and TOR1). Specific protein activators regulate the PIKK kinases but binding of them to the kinase complex causes a conformational change that increases substrate access to the kinase domain. Protein kinases have become popular drug targets. They have been targeted for the discovery and design of
small molecule In molecular biology and pharmacology, a small molecule or micromolecule is a low molecular weight (≤ 1000 daltons) organic compound that may regulate a biological process, with a size on the order of 1 nm. Many drugs are small molecules; ...
inhibitors and
biologics A biopharmaceutical, also known as a biological medical product, or biologic, is any pharmaceutical drug product manufactured in, extracted from, or semisynthesized from biological sources. Different from totally synthesized pharmaceuticals, th ...
as potential therapeutic agents. Small-molecule inhibitors of protein kinases generally prevent either
phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...
of proteins substrates or
autophosphorylation Autophosphorylation is a type of post-translational modification of proteins. It is generally defined as the phosphorylation of the kinase by itself. In eukaryotes, this process occurs by the addition of a phosphate group to serine, threonine o ...
of the kinase itself.


mTOR signaling pathway

It appears that
growth factors A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for regu ...
,
amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...
, ATP, and
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
levels regulate mTOR signaling. Several downstream pathways that regulate cell-cycle progression,
translation Translation is the communication of the semantics, meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The English la ...
,
initiation Initiation is a rite of passage marking entrance or acceptance into a group or society. It could also be a formal admission to adulthood in a community or one of its formal components. In an extended sense, it can also signify a transformatio ...
, transcriptional stress responses,
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
stability, and survival of cells are signaling through mTOR. The
serine/threonine kinase A serine/threonine protein kinase () is a kinase enzyme, in particular a protein kinase, that phosphorylates the OH group of the amino-acid residues serine or threonine, which have similar side chains. At least 350 of the 500+ human prote ...
mTOR is a downstream effector of the PI3K/AKT pathway, and forms two distinct
multiprotein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multidomain enzymes, in which multiple catalytic domains are found in a single polypeptide chain. Protein c ...
es,
mTORC1 mTORC1, also known as mammalian target of rapamycin complex 1 or mechanistic target of rapamycin complex 1, is a protein complex that functions as a nutrient/energy/redox sensor and controls protein synthesis. mTOR Complex 1 (mTORC1) is comp ...
and
mTORC2 mTOR Complex 2 (mTORC2) is an acutely rapamycin-insensitive protein complex formed by serine/threonine kinase mTOR that regulates cell proliferation and survival, cell migration and cytoskeletal remodeling. The complex itself is rather large, ...
. These two complexes have a separate network of protein partners, feedback loops, substrates, and regulators. mTORC1 consists of mTOR and two positive regulatory subunits, raptor and mammalian LST8 ( mLST8), and two negative regulators, proline-rich AKT substrate 40 (PRAS40) and DEPTOR. mTORC2 consists of mTOR, mLST8, mSin1, protor,
rictor Rictor (Julio Esteban "Ric" Richter) is a superhero appearing in American comic books published by Marvel Comics, primarily in the ''X-Men'' family of books. The character was created by writer Louise Simonson and artist Walt Simonson and first ...
, and DEPTOR. mTORC1 is sensitive to rapamycin but mTORC2 is considered to be resistant and is generally insensitive to nutrients and energy signals. mTORC2 is activated by
growth factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for ...
s,
phosphorylates In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writt ...
PKCα, AKT and
paxillin Paxillin is a protein that in humans is encoded by the ''PXN'' gene. Paxillin is expressed at focal adhesions of non-striated cells and at costameres of striated muscle cells, and it functions to adhere cells to the extracellular matrix. Mutati ...
, and regulates the activity of the small
GTPase GTPases are a large family of hydrolase enzymes that bind to the nucleotide guanosine triphosphate (GTP) and hydrolyze it to guanosine diphosphate (GDP). The GTP binding and hydrolysis takes place in the highly conserved P-loop "G domain", a ...
,
Rac RAC or Rac may refer to: Organizations * Radio Amateurs of Canada * RATCH-Australia Corporation, electricity generator * Refugee Action Collective (Victoria), Melbourne, Australia * Religious Action Center of Reform Judaism, US * Rent-A-Cent ...
, and
Rho Rho (; uppercase Ρ, lowercase ρ or ; or ) is the seventeenth letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician alphabet, Phoenician letter resh . Its uppercase form uses the same ...
related to cell survival,
migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
and regulation of the
actin cytoskeleton Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other pr ...
. The mTORC1 signaling cascade is activated by phosphorylated AKT and results in phosphorylation of S6K1, and
4EBP1 Eukaryotic translation initiation factor 4E-binding protein 1 (also known as 4E-BP1) is a protein that in humans is encoded by the ''EIF4EBP1'' gene. inhibits cap-dependent translation by binding to translation initiation factor eIF4E. Phosphoryl ...
, which lead to mRNA translation.


mTOR signaling pathway in human cancer

Many human tumors occur because of dysregulation of mTOR signaling, and can confer higher susceptibility to inhibitors of mTOR. Deregulations of multiple elements of the mTOR pathway, like PI3K amplification/
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...
, PTEN loss of function, AKT overexpression, and S6K1, 4EBP1, and
eIF4E Eukaryotic translation initiation factor 4E, also known as eIF4E, is a protein in humans encoded by the ''EIF4E'' gene. eIF4E plays a central role in translation initiation and is involved in regulating protein synthesis. Its activity influences ...
overexpression have been related to many types of cancers. Therefore, mTOR is an interesting therapeutic target for treating multiple cancers, both the mTOR inhibitors themselves or in combination with inhibitors of other pathways. Upstream, PI3K/AKT signalling is deregulated through a variety of mechanisms, including overexpression or activation of
growth factor receptor A growth factor receptor is a receptor that binds to a growth factor. Growth factor receptors are the first stop in cells where the signaling cascade for cell differentiation and proliferation begins. Growth factors, which are ligands that bind to ...
s, such as HER-2 (human epidermal growth factor receptor 2) and IGFR (insulin-like growth factor receptor), mutations in
PI3K Phosphoinositide 3-kinases (PI3Ks), also called phosphatidylinositol 3-kinases, are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking, which i ...
and mutations/amplifications of AKT. Tumor suppressor phosphatase and tensin homologue deleted on
chromosome 10 Chromosome 10 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 10 spans about 134 million base pairs (the building material of DNA) and represents between 4 and 4.5 percent of the tota ...
(PTEN) is a negative regulator of PI3K signaling. In many cancers the PTEN expression is decreased and may be downregulated through several mechanisms, including
mutations In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosi ...
,
loss of heterozygosity In genetics, loss of heterozygosity (LOH) is a type of genetic abnormality in diploid organisms in which one copy of an entire gene and its surrounding chromosomal region are lost. Since diploid cells have two copies of their genes, one from each ...
,
methylation Methylation, in the chemistry, chemical sciences, is the addition of a methyl group on a substrate (chemistry), substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replac ...
, and protein instability. Downstream, the mTOR effectors S6 kinase 1 (S6K1), eukaryotic initiation factor 4E-binding protein 1 (4EBP1) and eukaryotic initiation factor 4E (eIF4E) are related to cellular transformation. S6K1 is a key regulator of cell growth and also phosphorylates other important targets. Both eIF4E and S6K1 are included in cellular transformation and their overexpression has been linked to poor cancer prognosis.


Development of mTOR inhibitors

Since the discovery of mTOR, much research has been done on the subject, using rapamycin and rapalogs to understand its biological functions. The clinical results from targeting this pathway were not as straight forward as thought at first. Those results have changed the course of clinical research in this field. Initially, rapamycin was developed as an antifungal drug against ''
Candida albicans ''Candida albicans'' is an opportunistic pathogenic yeast that is a common member of the human gut flora. It can also survive outside the human body. It is detected in the gastrointestinal tract and mouth in 40–60% of healthy adults. It is usu ...
'', ''
Aspergillus fumigatus ''Aspergillus fumigatus'' is a species of fungus in the genus ''Aspergillus'', and is one of the most common ''Aspergillus'' species to cause disease in individuals with an immunodeficiency. ''Aspergillus fumigatus'', a saprotroph widespread in ...
'' and ''
Cryptococcus neoformans ''Cryptococcus neoformans'' is an encapsulated basidiomycetous yeast belonging to the class Tremellomycetes and an obligate aerobe that can live in both plants and animals. Its teleomorph is a filamentous fungus, formerly referred to ''Filob ...
''. A few years later its immunosuppressive properties were detected. Later studies led to the establishment of rapamycin as a major immunosuppressant against
transplant rejection Transplant rejection occurs when transplanted tissue is rejected by the recipient's immune system, which destroys the transplanted tissue. Transplant rejection can be lessened by determining the molecular similitude between donor and recipien ...
, along with cyclosporine A. Combining rapamycin with cyclosporine A, enhanced rejection prevention in renal transplantation. Therefore, it was possible to use lower doses of cyclosporine, which minimized
toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacteria, bacterium, or plant, as well as the effect o ...
of the drug. In the 1980s the Developmental Therapeutic Branch of the National Cancer Institute (NCI) evaluated rapamycin and discovered it had an anticancer activity and was non-cytotoxic, but had
cytostatic Cytostasis (cyto – cell; stasis – stoppage) is the inhibition of cell growth and multiplication. Cytostatic refers to a cellular component or medicine that inhibits cell division and induce cell death. Cytostasis is an important prerequisite ...
activity against several human cancer types. However, due to unfavorable pharmacokinetic properties, the development of mTOR inhibitors for the treatment of cancer was not successful at that time. Since then, rapamycin has also shown to be effective for preventing coronary artery re-stenosis and for the treatment of
neurodegenerative diseases A neurodegenerative disease is caused by the progressive loss of neurons, in the process known as neurodegeneration. Neuronal damage may also ultimately result in their death. Neurodegenerative diseases include amyotrophic lateral sclerosis, mul ...
.


First generation mTOR inhibitors

The development of rapamycin as an anticancer agent began again in the 1990s with the discovery of
temsirolimus Temsirolimus, sold under the brand name Torisel, is an intravenous drug for the treatment of renal cell carcinoma (RCC), developed by Wyeth Pharmaceuticals and approved by the U.S. Food and Drug Administration (FDA) in May 2007, and was also app ...
(CCI-779). This novel soluble rapamycin derivative had a favorable toxicological profile in animals. More rapamycin derivatives with improved
pharmacokinetics Pharmacokinetics (from Ancient Greek ''pharmakon'' "drug" and ''kinetikos'' "moving, putting in motion"; see chemical kinetics), sometimes abbreviated as PK, is a branch of pharmacology dedicated to describing how the body affects a specific su ...
and reduced
immunosuppressive Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse react ...
effects have since then been developed for the
treatment of cancer Cancer treatments are a wide range of treatments available for the many different types of cancer, with each cancer type needing its own specific treatment. Treatments can include surgery, chemotherapy, radiation therapy, hormonal therapy (oncolo ...
. These rapalogs include
temsirolimus Temsirolimus, sold under the brand name Torisel, is an intravenous drug for the treatment of renal cell carcinoma (RCC), developed by Wyeth Pharmaceuticals and approved by the U.S. Food and Drug Administration (FDA) in May 2007, and was also app ...
(CCI-779),
everolimus Everolimus, sold under the brand name Afinitor among others, is a medication used as an immunosuppressant to prevent rejection of organ transplants and as a targeted therapy in the treatment of renal cell cancer and other tumours. This compoun ...
(RAD001), and ridaforolimus (AP-23573) which are being evaluated in cancer
clinical trial Clinical trials are prospective biomedical or behavioral research studies on human subject research, human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel v ...
s. Rapamycin analogs have similar therapeutic effects as rapamycin. However they have improved hydrophilicity and can be used for oral and
intravenous administration Intravenous therapy (abbreviated as IV therapy) is a medical technique that administers fluids, medications and nutrients directly into a person's vein. The intravenous route of administration is commonly used for rehydration or to provide nutr ...
. In 2012
National Cancer Institute The National Cancer Institute (NCI) coordinates the United States National Cancer Program and is part of the National Institutes of Health (NIH), which is one of eleven agencies that are part of the U.S. Department of Health and Human Services. ...
listed more than 200 clinical trials testing the anticancer activity of rapalogs both as monotherapy or as a part of
combination therapy In mathematics, a combination is a selection of items from a set that has distinct members, such that the order of selection does not matter (unlike permutations). For example, given three fruits, say an apple, an orange and a pear, there are ...
for many cancer types. Rapalogs, which are the first generation mTOR inhibitors, have proven effective in a range of preclinical models. However, the success in
clinical trials Clinical trials are prospective biomedical or behavioral research studies on human subject research, human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel v ...
is limited to only a few rare cancers. Animal and clinical studies show that rapalogs are primarily
cytostatic Cytostasis (cyto – cell; stasis – stoppage) is the inhibition of cell growth and multiplication. Cytostatic refers to a cellular component or medicine that inhibits cell division and induce cell death. Cytostasis is an important prerequisite ...
, and therefore effective as disease stabilizers rather than for regression. The response rate in solid tumors where rapalogs have been used as a single-agent therapy have been modest. Due to partial mTOR inhibition as mentioned before, rapalogs are not sufficient for achieving a broad and robust anticancer effect, at least when used as monotherapy. Another reason for the limited success is that there is a feedback loop between mTORC1 and AKT in certain tumor cells. It seems that mTORC1 inhibition by rapalogs fails to repress a
negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused ...
loop that results in
phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...
and activation of AKT. These limitations have led to the development of the second generation of mTOR inhibitors.


Rapamycin and rapalogs

Rapamycin and rapalogs (rapamycin derivatives) are small molecule inhibitors, which have been evaluated as anticancer agents. The rapalogs have more favorable pharmacokinetic profile compared to rapamycin, the parent drug, despite the same binding sites for mTOR and FKBP12.


Sirolimus

The bacterial
natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical s ...
rapamycin or
sirolimus Sirolimus, also known as rapamycin and sold under the brand name Rapamune among others, is a macrolide compound that is used to coat coronary stents, prevent organ transplant rejection, treat a rare lung disease called lymphangioleiomyomatosis ...
, a cytostatic agent, has been used in combination therapy with
corticosteroids Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex of vertebrates, as well as the synthetic analogues of these hormones. Two main classes of corticosteroids, glucocorticoids and mineralocorticoids, are invol ...
and
cyclosporine Ciclosporin, also spelled cyclosporine and cyclosporin, is a calcineurin inhibitor, used as an immunosuppressant medication. It is taken orally or intravenously for rheumatoid arthritis, psoriasis, Crohn's disease, nephrotic syndrome, ecz ...
in patients who received
kidney transplantation Kidney transplant or renal transplant is the organ transplant of a kidney into a patient with end-stage kidney disease (ESRD). Kidney transplant is typically classified as deceased-donor (formerly known as cadaveric) or living-donor transplantat ...
to prevent
organ rejection Organ and organs may refer to: Biology * Organ (biology), a group of tissues organized to serve a common function * Organ system, a collection of organs that function together to carry out specific functions within the body. Musical instruments ...
both in the US and Europe, due to its unsatisfying pharmacokinetic properties. In 2003, the
U.S. Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...
approved sirolimus-eluting coronary stents, which are used in patients with narrowing of
coronary arteries The coronary arteries are the arteries, arterial blood vessels of coronary circulation, which transport oxygenated blood to the Cardiac muscle, heart muscle. The heart requires a continuous supply of oxygen to function and survive, much like any ...
, or so-called
atherosclerosis Atherosclerosis is a pattern of the disease arteriosclerosis, characterized by development of abnormalities called lesions in walls of arteries. This is a chronic inflammatory disease involving many different cell types and is driven by eleva ...
. Recently rapamycin has shown effective in the inhibition of growth of several human cancers and murine cell lines. Rapamycin is the main mTOR inhibitor, but ridaforolimus/deforolimus (AP23573),
everolimus Everolimus, sold under the brand name Afinitor among others, is a medication used as an immunosuppressant to prevent rejection of organ transplants and as a targeted therapy in the treatment of renal cell cancer and other tumours. This compoun ...
(RAD001), and
temsirolimus Temsirolimus, sold under the brand name Torisel, is an intravenous drug for the treatment of renal cell carcinoma (RCC), developed by Wyeth Pharmaceuticals and approved by the U.S. Food and Drug Administration (FDA) in May 2007, and was also app ...
(CCI-779), are the newly developed rapamycin analogs.


Temsirolimus

The rapamycin analog
temsirolimus Temsirolimus, sold under the brand name Torisel, is an intravenous drug for the treatment of renal cell carcinoma (RCC), developed by Wyeth Pharmaceuticals and approved by the U.S. Food and Drug Administration (FDA) in May 2007, and was also app ...
(CCI-779) is also a noncytotoxic agent which delays tumor proliferation. Temsirolimus is a
prodrug A prodrug is a pharmacologically inactive medication or compound that, after intake, is metabolized (i.e., converted within the body) into a pharmacologically active drug. Instead of administering a drug directly, a corresponding prodrug can be ...
of rapamycin. It is approved by the U.S.
Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a List of United States federal agencies, federal agency of the United States Department of Health and Human Services, Department of Health and Human Services. The FDA is respo ...
(FDA) and the
European Medicines Agency The European Medicines Agency (EMA) is an agency of the European Union (EU) in charge of the evaluation and supervision of pharmaceutical products. Prior to 2004, it was known as the European Agency for the Evaluation of Medicinal Products ...
(EMA), for the treatment of renal cell carcinoma (RCC). Temsirolimus has higher water solubility than rapamycin and is therefore administered by intravenous injection. It was approved on May 30, 2007, by FDA for the treatment of advanced RCC. Temsirolimus has also been used in a Phase I clinical trial in conjunction with neratinib, a small-molecule irreversible pan-HER
tyrosine kinase inhibitor A tyrosine kinase inhibitor (TKI) is a pharmaceutical drug that inhibits tyrosine kinases. Tyrosine kinases are enzymes responsible for the activation of many proteins by signal transduction cascades. The proteins are activated by adding a phosph ...
. This study enrolled patients being treated for
HER2 Receptor tyrosine-protein kinase erbB-2 is a protein that normally resides in the membranes of cells and is encoded by the ''ERBB2'' gene. ERBB is abbreviated from erythroblastic oncogene B, a gene originally isolated from the avian genome. The ...
-amplified breast cancer, HER2-mutant non-small-cell lung cancer, and other advanced solid tumors. While common toxicities included
nausea Nausea is a diffuse sensation of unease and discomfort, sometimes perceived as an urge to vomit. It can be a debilitating symptom if prolonged and has been described as placing discomfort on the chest, abdomen, or back of the throat. Over 30 d ...
,
stomatitis Stomatitis is inflammation of the mouth and lips. It refers to any inflammatory process affecting the mucous membranes of the mouth and lips, with or without oral ulceration. In its widest meaning, stomatitis can have a multitude of different c ...
, and
anemia Anemia (also spelt anaemia in British English) is a blood disorder in which the blood has a reduced ability to carry oxygen. This can be due to a lower than normal number of red blood cells, a reduction in the amount of hemoglobin availabl ...
; responses were noted.


Everolimus

Everolimus Everolimus, sold under the brand name Afinitor among others, is a medication used as an immunosuppressant to prevent rejection of organ transplants and as a targeted therapy in the treatment of renal cell cancer and other tumours. This compoun ...
is the second novel Rapamycin analog. Compared with the parent compound
rapamycin Sirolimus, also known as rapamycin and sold under the brand name Rapamune among others, is a macrolide compound that is used to coat coronary stents, prevent organ rejection, organ transplant rejection, treat a rare lung disease called lymphang ...
, everolimus is more selective for the
mTORC1 mTORC1, also known as mammalian target of rapamycin complex 1 or mechanistic target of rapamycin complex 1, is a protein complex that functions as a nutrient/energy/redox sensor and controls protein synthesis. mTOR Complex 1 (mTORC1) is comp ...
protein complex, with little impact on the
mTORC2 mTOR Complex 2 (mTORC2) is an acutely rapamycin-insensitive protein complex formed by serine/threonine kinase mTOR that regulates cell proliferation and survival, cell migration and cytoskeletal remodeling. The complex itself is rather large, ...
complex. mTORC1 inhibition by everolimus has been shown to normalize tumor blood vessels, to increase
tumor-infiltrating lymphocytes Tumor-infiltrating lymphocytes (TIL) are white blood cells that have left the bloodstream and cell migration, migrated towards a tumor. They include T cells and B cells and are part of the larger category of ‘tumor-infiltrating immune cells’ wh ...
, and to improve adoptive cell transfer therapy. From March 30, 2009, to May 5, 2011, the U.S. FDA approved everolimus for the treatment of advanced renal cell carcinoma after failure of treatment with
sunitinib Sunitinib, sold under the brand name Sutent, is an anti-cancer medication. It is a small-molecule, multi-targeted receptor tyrosine kinase (RTK) inhibitor that was approved by the FDA for the treatment of renal cell carcinoma (RCC) and imat ...
or
sorafenib Sorafenib, sold under the brand name Nexavar, is a kinase inhibitor drug approved for the treatment of primary kidney cancer (advanced renal cell carcinoma), advanced primary liver cancer (hepatocellular carcinoma), FLT3-ITD positive AML and ra ...
,
subependymal giant cell astrocytoma Subependymal giant cell astrocytoma (SEGA, SGCA, or SGCT) is a low-grade astrocytic brain tumor (astrocytoma) that arises within the ventricles of the brain. It is most commonly associated with tuberous sclerosis complex (TSC). Although it is ...
(SEGA) associated with
tuberous sclerosis Tuberous sclerosis complex (TSC) is a rare multisystem autosomal dominant genetic disease that causes non-cancerous tumours to grow in the brain and on other vital organs such as the kidneys, heart, liver, eyes, lungs and skin. A combinatio ...
(TS), and progressive neuroendocrine tumors of pancreatic origin (PNET). In July and August 2012, two new indications were approved, for advanced hormone receptor-positive, HER2-negative breast cancer in combination with exemestane, and pediatric and adult patients with SEGA. In 2009 and 2011, it was also approved throughout the European Union for advanced breast cancer, pancreatic neuroendocrine tumours, advanced renal cell carcinoma, and SEGA in patients with tuberous sclerosis.


Ridaforolimus

Ridaforolimus (AP23573, MK-8669), or deforolimus, is another rapamycin analogue that is not a
prodrug A prodrug is a pharmacologically inactive medication or compound that, after intake, is metabolized (i.e., converted within the body) into a pharmacologically active drug. Instead of administering a drug directly, a corresponding prodrug can be ...
for sirolimus. Like temsirolimus it can be administered intravenously, and oral formulation is being estimated for treatment of
sarcoma A sarcoma is a rare type of cancer that arises from cells of mesenchymal origin. Originating from mesenchymal cells means that sarcomas are cancers of connective tissues such as bone, cartilage, muscle, fat, or vascular tissues. Sarcom ...
.


Umirolimus

Umirolimus is an immunosuppressant used in drug-eluting stents.


Zotarolimus

Zotarolimus is an immunosuppressant used in coronary drug-eluting stents.


Second generation mTOR inhibitors

The second generation of mTOR inhibitors is known as ATP-competitive mTOR kinase inhibitors. mTORC1/mTORC2 dual inhibitors such as torin-1, torin-2 and vistusertib, are designed to compete with ATP in the
catalytic Catalysis () is the increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...
site of mTOR. They inhibit all of the kinase-dependent functions of mTORC1 and mTORC2 and block the feedback activation of PI3K/AKT signaling, unlike rapalogs, which only target mTORC1. Development of these drugs has reached clinical trials, although some, such as vistusertib, have been discontinued. Like rapalogs, they decrease protein
translation Translation is the communication of the semantics, meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The English la ...
, attenuate
cell cycle The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...
progression, and inhibit
angiogenesis Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature mainly by processes of sprouting and ...
in many cancer cell lines and also in human cancer. In fact, they have been proven to be more potent than rapalogs. Theoretically, the most important advantages of these mTOR inhibitors is the considerable decrease of AKT phosphorylation on mTORC2 blockade and in addition to a better inhibition on mTORC1. However, some drawbacks exist. Even though these compounds have been effective in rapamycin-insensitive cell lines, they have only shown limited success in
KRAS ''KRAS'' ( Kirsten rat sarcoma virus) is a gene that provides instructions for making a protein called K-Ras, a part of the RAS/MAPK pathway. The protein relays signals from outside the cell to the cell's nucleus. These signals instruct the ce ...
driven tumors. This suggests that
combinational In automata theory, combinational logic (also referred to as time-independent logic) is a type of digital logic that is implemented by Boolean circuits, where the output is a pure function of the present input only. This is in contrast to seque ...
therapy may be necessary for the treatment of these cancers. Another drawback is also their potential
toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacteria, bacterium, or plant, as well as the effect o ...
. These facts have raised concerns about the long term efficacy of these types of inhibitors. The close interaction of mTOR with the PI3K pathway has also led to the development of mTOR/PI3K dual inhibitors. Compared with drugs that inhibit either mTORC1 or PI3K, these drugs have the benefit of inhibiting mTORC1, mTORC2, and all the catalytic
isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene and are the result of genetic differences. While many perform the same or similar biological roles, some isoforms have uniqu ...
of PI3K. Targeting both kinases at the same time reduces the
upregulation In biochemistry, in the biological context of organisms' regulation of gene expression and production of gene products, downregulation is the process by which a cell decreases the production and quantities of its cellular components, such as R ...
of PI3K, which is typically produced with an inhibition on mTORC1. The inhibition of the PI3K/mTOR pathway has been shown to potently block proliferation by inducing G1 arrest in different tumor cell lines. Strong induction of
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
and
autophagy Autophagy (or autophagocytosis; from the Greek language, Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-depe ...
has also been seen. Despite good promising results, there are preclinical evidence that some types of cancers may be insensitive to this dual inhibition. The dual PI3K/mTOR inhibitors are also likely to have increased toxicity.


Mechanism of action

The studies of
rapamycin Sirolimus, also known as rapamycin and sold under the brand name Rapamune among others, is a macrolide compound that is used to coat coronary stents, prevent organ rejection, organ transplant rejection, treat a rare lung disease called lymphang ...
as immunosuppressive agent enabled us to understand its
mechanism of action In pharmacology, the term mechanism of action (MOA) refers to the specific biochemical Drug interaction, interaction through which a Medication, drug substance produces its pharmacological effect. A mechanism of action usually includes mention o ...
. It inhibits
T-cell T cells (also known as T lymphocytes) are an important part of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their ce ...
proliferation and proliferative responses induced by several
cytokine Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...
s, including interleukin 1 (IL-1),
IL-2 The Ilyushin Il-2 (Russian language, Russian: Илью́шин Ил-2) is a Ground attack aircraft, ground-attack plane that was produced by the Soviet Union in large numbers during the World War II, Second World War. The word ''shturmovík'' (C ...
, IL-3, IL-4, IL-6, IGF,
PDGF Platelet-derived growth factor (PDGF) is one among numerous growth factors that regulate cell growth and division. In particular, PDGF plays a significant role in blood vessel formation, the growth of blood vessels from already-existing blood ...
, and colony-stimulating factors (CSFs). Rapamycin inhibitors and rapalogs can target tumor growth both directly and indirectly. Direct impact of them on cancer cells depend on the concentration of the drug and certain cellular characteristics. The indirect way, is based on interaction with processes required for tumor
angiogenesis Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature mainly by processes of sprouting and ...
.


Effects in cancer cells

Rapamycin and rapalogs crosslink the immunophilin
FK506 Tacrolimus, sold under the brand name Prograf among others, is an immunosuppressive drug. After allogenic organ transplant, the risk of organ rejection is moderate. To lower the risk of organ rejection, tacrolimus is given. The drug can also ...
binding protein,
tacrolimus Tacrolimus, sold under the brand name Prograf among others, is an immunosuppressive drug. After Allotransplantation, allogenic organ transplant, the risk of organ Transplant rejection, rejection is moderate. To lower the risk of organ rejectio ...
or FKBP-12, through its
methoxy group In organic chemistry, a methoxy group is the functional group consisting of a methyl group bound to oxygen. This alkoxy group has the formula . On a benzene ring, the Hammett equation classifies a methoxy substituent at the ''para'' position a ...
. The rapamycin-FKBP12 complex interferes with FRB domain of mTOR. Molecular interaction between FKBP12, mTOR, and rapamycin can last for about three days (72 hours). The inhibition of mTOR blocks the binding of the accessory protein raptor (regulatory-associated protein of mTOR) to mTOR, but that is necessary for downstream phosphorylation of S6K1 and
4EBP1 Eukaryotic translation initiation factor 4E-binding protein 1 (also known as 4E-BP1) is a protein that in humans is encoded by the ''EIF4EBP1'' gene. inhibits cap-dependent translation by binding to translation initiation factor eIF4E. Phosphoryl ...
. As a consequence, S6K1 dephosphorylates, which reduces protein
synthesis Synthesis or synthesize may refer to: Science Chemistry and biochemistry *Chemical synthesis, the execution of chemical reactions to form a more complex molecule from chemical precursors **Organic synthesis, the chemical synthesis of organi ...
and decreases cell mortality and size. Rapamycin induces dephosphorylation of 4EBP1 as well, resulting in an increase in p27 and a decrease in
cyclin D1 Cyclin D1 is a protein that in humans is encoded by the ''CCND1'' gene. Gene expression The CCND1 gene encodes the cyclin D1 protein. The human CCND1 gene is located on the long arm of chromosome 11 (band 11q13). It is 13,388 base pairs lo ...
expression. That leads to late blockage of G1/S
cell cycle The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...
. Rapamycin has shown to induce cancer cell death by stimulating
autophagy Autophagy (or autophagocytosis; from the Greek language, Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-depe ...
or
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
, but the molecular mechanism of apoptosis in cancer cells has not yet been fully resolved. One suggestion of the relation between mTOR inhibition and apoptosis might be through the downstream target S6K1, which can phosphorylate
BAD Bad or BAD may refer to: Common meanings *Evil, the opposite of moral good * Erroneous, inaccurate or incorrect * Unhealthy, or counter to well-being *Antagonist, the threat or obstacle of moral good Acronyms * BAD-2, a Soviet armored trolley ...
, a pro-apoptotic molecule, on Ser136. That reaction breaks the binding of BAD to
BCL-XL B-cell lymphoma-extra large (Bcl-xL), encoded by the BCL2-like 1 gene, is a transmembrane molecule in the mitochondria. It is a member of the Bcl-2 family of proteins, and acts as an anti-apoptotic protein by preventing the release of mitochondr ...
and
BCL2 Bcl-2, encoded in humans by the ''BCL2'' gene, is the founding member of the apoptosis regulator proteins, Bcl-2 family, Bcl-2 family of regulator proteins. BCL2 blocks programmed cell death (apoptosis) while other BCL2 family members can eithe ...
, a
mitochondrial A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...
death inhibitors, resulting in inactivation of BAD and decreased cell survival. Rapamycin has also shown to induce
p53 p53, also known as tumor protein p53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory transcription factor protein that is often mutated in human cancers. The p53 proteins (originally thou ...
-independent apoptosis in certain types of cancer.


Effects on tumor angiogenesis

Tumor angiogenesis rely on interactions between endothelial vascular growth factors which can all activate the PI3K/AKT/mTOR in endothelial cells,
pericyte Pericytes (formerly called Rouget cells) are multi-functional mural cells of the microcirculation that wrap around the endothelial cells that line the capillaries throughout the body. Pericytes are embedded in the basement membrane of blood c ...
s, or cancer cells. Example of these growth factors are angiopoietin 1 (ANG1), ANG 2, basic fibroblast growth factor (bFGF), ephrin-B2, vascular endothelial growth factor (VEGF), and members of the tumor growth factor-β (TGFβ) superfamily. One of the major stimuli of angiogenesis is hypoxia, resulting in activation of hypoxia-inducible transcription factors (HIFs) and expression of ANG2, bFGF, PDGF, VEGF, and VEGFR. Inhibition of HIF1α translation by preventing PDGF/PDGFR and VEGF/VEGFR can result from mTOR inhibition. A G0-G1 cell-cycle blockage can be the consequence of inactivation of mTOR in hypoxia-activated pericytes and endothelial cells. There is some evidence that extended therapy with rapamycin may have effect on AKT and mTORC2 as well.


Effects on chemotherapy

Pharmacologic down-regulation of (mTOR) pathway during chemotherapy in a mouse model prevents activation of primordial follicles, preserves ovarian function, and maintains normal fertility using clinically available inhibitors INK and RAD. In that way, it helps to maintain fertility while undergoing chemotherapy treatments. These mTOR inhibitors, when administered as pretreatment or co-treatment with standard gonadotoxic chemotherapy, helps to maintain ovarian follicles in their primordial state.


Effects on cognition

mTOR promotes the protein synthesis required for
synaptic plasticity In neuroscience, synaptic plasticity is the ability of synapses to Chemical synapse#Synaptic strength, strengthen or weaken over time, in response to increases or decreases in their activity. Since memory, memories are postulated to be represent ...
. Studies in cell cultures and
hippocampal The hippocampus (: hippocampi; via Latin from Greek , 'seahorse'), also hippocampus proper, is a major component of the brain of humans and many other vertebrates. In the human brain the hippocampus, the dentate gyrus, and the subiculum ar ...
slices indicate that mTOR inhibition reduces
long-term potentiation In neuroscience, long-term potentiation (LTP) is a persistent strengthening of synapses based on recent patterns of activity. These are patterns of synaptic activity that produce a long-lasting increase in signal transmission between two neuron ...
. mTOR activation can protect against certain
neurodegeneration A neurodegenerative disease is caused by the progressive loss of neurons, in the process known as neurodegeneration. Neuronal damage may also ultimately result in their cell death, death. Neurodegenerative diseases include amyotrophic lateral sc ...
associated with certain disease conditions. On the other hand, promotion of
autophagy Autophagy (or autophagocytosis; from the Greek language, Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-depe ...
by mTOR inhibition may reduce cognitive decline associated with neurodegeneration. Moderate reduction of mTOR activity by 25-30% has been shown to improve brain function, suggesting that the relation between mTOR and cognition is optimized with intermediate doses (2.24 mg/kg/day in mice, human equivalent about 0.19 mg/kg/day), where very high or very low doses impair cognition. Reduction of the inflammatory
cytokine Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...
Interleukin 1 beta Interleukin-1 beta (IL-1β) also known as leukocytic pyrogen, leukocytic endogenous mediator, mononuclear cell factor, lymphocyte activating factor and other names, is a cytokine protein that in humans is encoded by the ''IL1B'' gene."Catabolin" ...
(IL-1β) in mice by mTOR inhibition (with rapamycin in doses of 20 mg/kg/day, human equivalent about 1.6 mg/kg/day) has been shown to enhance learning and memory. Although IL-1β is required for memory, IL-1β normally increases with age, impairing cognitive function.


Structure activity relationship

The pipecolate region of rapamycin structure seems necessary for rapamycin-binding to FKBP12. This step is required for further binding of rapamycin to the mTOR kinase, which is the key enzyme in many biological actions of rapamycin. The high affinity of rapamycin binding to FKBP12 is explained by number of
hydrogen bond In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...
s through two different
hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...
binding pockets, and this has been revealed by X-ray crystal structure of the compound bound to the
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
. The structural characteristics common to temsirolimus and sirolimus; the pipecolic acid, tricarbonyl region from C13-C15, and
lactone Lactones are cyclic carboxylic esters. They are derived from the corresponding hydroxycarboxylic acids by esterification. They can be saturated or unsaturated. Lactones are formed by lactonization, the intramolecular esterification of the corresp ...
functionalities play the key role in binding groups with the FKBP12. The most important
hydrogen bonds In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, covalently bonded to a mo ...
are the lactone carbonyl oxygen at C-21 to the backbone NH of Ile56, amide carbonyl at C-15 to the phenolic group on the sidechain of Tyr82, and the hydroxyl proton at the
hemiketal In organic chemistry, a hemiacetal is a functional group the general formula , where is a hydrogen atom or an organic substituent. They generally result from the nucleophilic addition of an alcohol (a compound with at least one hydroxy group) to ...
carbon, C-13, to the sidechain of Asp37. Structural changes to the rapamycin structure can affect binding to mTOR. This could include both direct and indirect binding as a part of binding to FKBP12. Interaction of the FKBP12-rapamycin complex with mTOR corresponds with conformational flexibility of the effector domain of rapamycin. This domain consists of molecular regions that make hydrophobic interactions with the FKB domain and
triene In organic chemistry, polyenes are polyunsaturated organic compounds that contain multiple carbon–carbon double bonds (). Some sources consider dienes to be polyenes, whereas others require polyenes to contain at least three carbon–carbon d ...
region from C-1-C-6, methoxy group at C-7, and
methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated a ...
s at C-33, C-27 and C-25. All changes of the macrolide ring can have unpredictable effects on binding and therefore, make determination of SAR for rapalogs problematic. Rapamycin contains no functional groups that
ionize Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
in the pH range 1-10 and therefore, are rather insoluble in water. Despite its effectiveness in preclinic cancer models, its poor solubility in water, stability, and the long half-life elimination made its parenteral use difficult, but the development of soluble rapamycin analogs vanquished various barriers. Nonetheless, the rapamycin analogs that have been approved for human use are modified at C-43 hydroxyl group and show improvement in
pharmacokinetic Pharmacokinetics (from Ancient Greek ''pharmakon'' "drug" and ''kinetikos'' "moving, putting in motion"; see chemical kinetics), sometimes abbreviated as PK, is a branch of pharmacology dedicated to describing how the body affects a specific subs ...
parameters as well as drug properties, for example, solubility. Rapamycin and temsirolimus have similar chemical structures and bind to FKBP12, though their mechanism of action differs. Temsirolimus is a dihydroxymethyl propionic
acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
of rapamycin, and its first derivative. Therefore, it is more water-soluble, and due to its water solubility it can be given by intravenous formulation. Everolimus has O-2 hydroxyethyl chain substitution and deforolimus has a
phosphine oxide Phosphine oxide is the inorganic compound with the formula H3PO. Although stable as a dilute gas, liquid or solid samples are unstable. Unlike many other compounds of the type POxHy, H3PO is rarely discussed and is not even mentioned in major so ...
substitution at position C-43 in the lactone ring of rapamycin. Deforolimus (Ridaforolimus ) has C43 secondary alcohol moiety of the cyclohexyl group of Rapamycin that was substituted with phosphonate and phosphinate groups, preventing the high-affinity binding to mTOR and FKBP. Computational modelling studies helped the synthesise of the compound.


Adverse events

Treatment with mTOR inhibitors can be complicated by adverse events. The most frequently occurring adverse events are stomatitis, rash, anemia, fatigue, hyperglycemia/hypertriglyceridemia, decreased appetite, nausea, and diarrhea. Additionally,
interstitial lung disease Interstitial lung disease (ILD), or diffuse parenchymal lung disease (DPLD), is a group of respiratory diseases affecting the interstitium (the tissue) and space around the alveoli (air sacs) of the lungs. It concerns alveolar epithelium, pulm ...
is an adverse event of particular importance. mTORi-induced ILD often is asymptomatic (with ground glass abnormalities on chest CT) or mild symptomatic (with a non-productive cough), but can be very severe as well. Even fatalities have been described. Careful diagnosis and treatment, therefore, is essential. Recently, a new diagnostic and therapeutic management approach has been proposed.


Biomarkers

Identification of predictive
biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, ...
s of efficacy for tumor types that are sensitive to mTOR inhibitors remains a major issue. Possible predictive biomarkers for tumor response to mTOR inhibitors, as have been described in
glioblastoma Glioblastoma, previously known as glioblastoma multiforme (GBM), is the most aggressive and most common type of cancer that originates in the brain, and has a very poor prognosis for survival. Initial signs and symptoms of glioblastoma are nons ...
,
breast The breasts are two prominences located on the upper ventral region of the torso among humans and other primates. Both sexes develop breasts from the same embryology, embryological tissues. The relative size and development of the breasts is ...
and
prostate cancer Prostate cancer is the neoplasm, uncontrolled growth of cells in the prostate, a gland in the male reproductive system below the bladder. Abnormal growth of the prostate tissue is usually detected through Screening (medicine), screening tests, ...
cells, may be the differential expression of mTOR pathway proteins, PTEN, AKT, and S6. Thus, this data is based on preclinical assays, based on ''
in vitro ''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...
'' cultured tumor cell lines, which suggest that the effects of mTOR inhibitors may be more pronounced in cancers displaying loss of PTEN functions or PIK3CA mutations. However, the use of PTEN, PIK3CA
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...
s, and AKT–phospho status for predicting rapalog sensitivity has not been fully validated in clinic. To date, attempts to identify biomarkers of rapalog response have been unsuccessful.


Sensitivity

Clinical and translational data suggest that sensitive tumor types, with adequate parameters and functional
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
pathways, might not need high doses of mTOR inhibitors to trigger apoptosis. In most cases, cancer cells might only be partially sensitive to mTOR inhibitors due to redundant
signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a biochemical cascade, series of molecular events. Proteins responsible for detecting stimuli are generally termed receptor (biology), rece ...
or lack of functional apoptosis signaling pathways. In situations like this, high doses of mTOR inhibitors might be required. In a recent study of patients with
Renal cell carcinoma Renal cell carcinoma (RCC) is a kidney cancer that originates in the lining of the Proximal tubule, proximal convoluted tubule, a part of the very small tubes in the kidney that transport primary urine. RCC is the most common type of kidney cance ...
, resistance to Temsirolimus was associated with low levels of p-AKT and p-S6K1, that play the key role in mTOR activation. These data strongly suggests number of tumors with an activated PI3K/AKT/mTOR signaling pathway that does not respond to mTOR inhibitors. For future studies, it is recommended to exclude patients with low or negative p-AKT levels from trials with mTOR inhibitors. Current data is insufficient to predict sensitivity of tumors to rapamycin. However, the existing data allows us to characterize tumors that might not respond to rapalogs.


ATP-competitive mTOR kinase inhibitors

These second generation mTOR inhibitors bind to ATP-binding site in mTOR kinase domain required for the functions of both
mTORC1 mTORC1, also known as mammalian target of rapamycin complex 1 or mechanistic target of rapamycin complex 1, is a protein complex that functions as a nutrient/energy/redox sensor and controls protein synthesis. mTOR Complex 1 (mTORC1) is comp ...
and
mTORC2 mTOR Complex 2 (mTORC2) is an acutely rapamycin-insensitive protein complex formed by serine/threonine kinase mTOR that regulates cell proliferation and survival, cell migration and cytoskeletal remodeling. The complex itself is rather large, ...
, and result in
downregulation In biochemistry, in the biology, biological context of organisms' regulation of gene expression and production of gene products, downregulation is the process by which a cell (biology), cell decreases the production and quantities of its cellular ...
of mTOR signaling pathway. Due to PI3K and mTORC2 ability to regulate AKT phosphorylation, these two compounds play a key role in minimizing the feedback activation of AKT.


mTOR/PI3K dual inhibitors

Several, so-called mTOR/PI3K dual inhibitors (TPdIs), have been developed and are in early-stage preclinical trials and show promising results. Their development has been benefited from previous studies with PI3K-selective inhibitors. The activity of these small molecules from rapalog activity differs in the way by blocking both mTORC1-dependent phospholylation of S6K1 and mTORC2-dependent phosphorylation of AKT Ser473 residue. Dual mTOR/PI3K inhibitors include dactolisib, voxtalisib, BGT226, SF1126, PKI-587 and many more. For example,
Novartis Novartis AG is a Swiss multinational corporation, multinational pharmaceutical company, pharmaceutical corporation based in Basel, Switzerland. Novartis is one of the largest pharmaceutical companies in the world and was the eighth largest by re ...
has developed the compound NVPBE235 that was reported to inhibit tumor growth in various preclinical models. It enhances antitumor activity of some other drugs such as
vincristine Vincristine, also known as leurocristine and sold under the brand name Oncovin among others, is a chemotherapy medication used to treat a number of types of cancer. This includes acute lymphocytic leukemia, acute myeloid leukemia, Hodgkin lym ...
. Dactolisib seems to inhibit effectively both wild-type and mutant form of PI3KCA, which suggests its use towards wide types of tumors. Studies have shown superior antiproliferative activity to rapalogs and ''
in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, an ...
'' models have confirmed these potent antineoplastic effects of dual mTOR/PI3K inhibitors. These inhibitors target
isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene and are the result of genetic differences. While many perform the same or similar biological roles, some isoforms have uniqu ...
of PI3K (p110α, β and γ) along with ATP-binding sites of mTORC1 and mTORC2 by blocking PI3K/AKT signaling, even in cancer types with mutations in this pathway.


mTORC1/mTORC2 dual inhibitors (TORCdIs)

New mTOR-specific inhibitors came forth from screening and
drug discovery In the fields of medicine, biotechnology, and pharmacology, drug discovery is the process by which new candidate medications are discovered. Historically, drugs were discovered by identifying the active ingredient from traditional remedies or ...
efforts. These compounds block activity of both mTOR complexes and are called mTORC1/mTORC2 dual inhibitors. Compounds with this characteristics such as sapanisertib (codenamed INK128), AZD8055, and AZD2014 have entered
clinical trial Clinical trials are prospective biomedical or behavioral research studies on human subject research, human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel v ...
s. A series of these mTOR kinase inhibitors have been studied. Their structure is derived from morpholino pyrazolopyrimidine scaffold. Improvements of this type of inhibitors have been made by exchanging the morpholines with bridged morpholines in pyrazolopyrimidine inhibitors and results showed increased selectivity to mTOR by 26000 fold.


Limitations of new generation mTOR inhibitors

Although the new generation of mTOR inhibitors hold great promise for anticancer therapy and are rapidly moving into clinical trials, there are many important issues that determine their success in the clinic. First of all predictable biomarkers for benefit of these inhibitors are not available. It appears that genetic determinants predispose cancer cells to be sensitive or resistant to these compounds. Tumors that depend on PI3K/mTOR pathway should respond to these agents but it is unclear if compounds are effective in cancers with distinct genetic lesions. Inhibition of mTOR is a promising strategy for treatment of number of cancers. Limited clinical activity of selective mTORC1 agents have made them unlikely to have impact in cancer treatment. The development of competitive ATP-catalytic inhibitors have the ability to block both mTORC1 and mTORC2.


Future

The limitations of currently available rapalogs have led to new approaches to mTOR targeting. Studies suggest that mTOR inhibitors may have anticancer activity in many cancer types, such as RCC,
neuroendocrine tumors Neuroendocrine tumors (NETs) are neoplasms that arise from cells of the endocrine (hormonal) and nervous systems. They most commonly occur in the intestine, where they are often called carcinoid tumors, but they are also found in the pancreas, lu ...
,
breast cancer Breast cancer is a cancer that develops from breast tissue. Signs of breast cancer may include a Breast lump, lump in the breast, a change in breast shape, dimpling of the skin, Milk-rejection sign, milk rejection, fluid coming from the nipp ...
,
hepatocellular carcinoma Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer in adults and is currently the most common cause of death in people with cirrhosis. HCC is the third leading cause of cancer-related deaths worldwide. HCC most common ...
,
sarcoma A sarcoma is a rare type of cancer that arises from cells of mesenchymal origin. Originating from mesenchymal cells means that sarcomas are cancers of connective tissues such as bone, cartilage, muscle, fat, or vascular tissues. Sarcom ...
, and large B-cell lymphoma. One major limitation for the development of mTOR inhibition therapy is that biomarkers are not presently available to predict which patient will respond to them. A better understanding of the molecular mechanisms that are involved in the response of cancer cells to mTOR inhibitors are still required so this can be possible. A way to overcome the resistance and improve efficacy of mTOR targeting agents may be with stratification of patients and selection of drug combination therapies. This may lead to a more effective and personalized cancer therapy. Although further research is needed, mTOR targeting still remains an attractive and promising therapeutic option for the treatment of cancer.


See also

* Mammalian target of rapamycin (mTOR) *
PI3K/AKT/mTOR pathway The PI3K/AKT/mTOR pathway is an intracellular signaling pathway important in regulating the cell cycle. Therefore, it is directly related to cellular quiescence, proliferation, cancer, and longevity. PI3K activation phosphorylates and activates ...
*
Akt/PKB signaling pathway The Akt signaling pathway or PI3K-Akt signaling pathway is a signal transduction pathway that promotes survival and growth in response to extracellular signals. Key proteins involved are PI3K (phosphatidylinositol 3-kinase) and Akt (protein kinase ...
* PI3K inhibitor


References


Further reading

* * {{Major Drug Groups Signal transduction Tor signaling pathway Human proteins Antineoplastic drugs Oncology Cancer treatments