Sirolimus
Encyclopedia
Sirolimus also known as rapamycin, is an immunosuppressant
drug used to prevent rejection
in organ transplant
ation; it is especially useful in kidney
transplants. A macrolide
, sirolimus was first discovered as a product of the bacterium Streptomyces
hygroscopicus in a soil sample from Easter Island
— an island also known as Rapa Nui, hence the name. It is marketed under the trade name Rapamune by Wyeth
.
Sirolimus was originally developed as an antifungal agent. However, this use was abandoned when it was discovered to have potent immunosuppressive and antiproliferative properties.
It has since been shown to prolong the life of mice and might also be useful in the treatment of certain cancer
s.
, sirolimus is not a calcineurin inhibitor, but it has a similar suppressive effect on the immune system. Sirolimus inhibits the response to interleukin-2 (IL-2), and thereby blocks activation of T-
and B-cell
s. In contrast, tacrolimus inhibits the secretion of IL-2.
The mode of action of sirolimus is to bind the cytosol
ic protein FK-binding protein 12
(FKBP12) in a manner similar to tacrolimus. Unlike the tacrolimus-FKBP12 complex which inhibits calcineurin
(PP2B), though, the sirolimus-FKBP12 complex inhibits the mammalian target of rapamycin
(mTOR) pathway by directly binding the mTOR Complex1 (mTORC1). mTOR was also called FRAP (FKBP-rapamycin associated protein), RAFT (rapamycin and FKBP target), RAPT1, or SEP. FRAP and RAFT are older names and were coined to reflect the fact that rapamycin must bind FKBP12 first, and only the FKBP12-rapamycin complex can bind mTOR; however, mTOR is now the widely accepted name and stands for mammalian Target Of Rapamycin and based on the precedent that Tor was first discovered via genetic and molecular studies of rapamycin-resistant mutants of Saccharomyces cerevisiae that identified FKBP12, Tor1, and Tor2 as the targets of rapamycin and provided robust support that the FKBP12-rapamycin complex binds to and inhibits Tor1 and Tor2.
, as this disease is likely to recur in the transplanted kidney if a calcineurin-inhibitor is used. However, on October 7, 2008, the FDA approved safety labeling revisions for sirolimus to warn of the risk for decreased renal function associated with its use.
Sirolimus can also be used alone, or in conjunction with calcineurin inhibitors, such as tacrolimus
and/or mycophenolate mofetil
, to provide steroid-free immunosuppression regimens. Impaired wound healing and thrombocytopenia
are a possible side effects of sirolimus; therefore, some transplant centres prefer not to use it immediately after the transplant operation, but instead administer it only after a period of weeks or months. Its optimal role in immunosuppression has not yet been determined, and is the subject of a number of ongoing clinical trials.
Sirolimus is absorbed into the blood stream from the intestine variably in each patient, with some patients having up to eight times more exposure than others for the same dose. Drug levels are therefore taken to make sure patients get the right dose for their condition. This is determined by taking a blood sample before the next dose which gives the trough level. Fortunately, there is good correlation between trough concentration levels and drug exposure, known as area under the concentration-time curve, for both sirolimus (SRL) and tacrolimus (TAC) (SRL: r2 = 0.83; TAC: r2 = 0.82), so only one level need be taken to know its pharmacokinetic (PK) profile. PK profiles of SRL and of TAC are unaltered by simultaneous administration. Dose-corrected drug exposure of TAC correlates with SRL (r2 = 0.8), so patients have similar bioavailability of both.
fed rapamycin were increased between 28 and 38% from the beginning of treatment, or 9 to 14% in total increased maximum lifespan. Of particular note, the treatment began in mice aged 20 months, the equivalent of 60 human years. This suggests the possibility of an effective antiaging treatment for humans at an already-advanced age, as opposed to requiring a lifelong regimen beginning in youth. Because it strongly suppresses the immune system, though, people taking rapamycin are more susceptible to dangerous infections. It is not known whether rapamycin will have similar lifespan-lengthening effects in humans, and study authors caution the drug should not be used by the general population for this use.
s to prevent restenosis in coronary arteries following balloon angioplasty. The sirolimus is formulated in a polymer coating that affords controlled release through the healing period following coronary intervention. Several large clinical studies have demonstrated lower restenosis rates in patients treated with sirolimus-eluting stents when compared to bare metal stents, resulting in fewer repeat procedures. A sirolimus-eluting coronary stent is marketed by Cordis, a division of Johnson & Johnson
, under the tradename Cypher
. It has been proposed, however, that such stents may increase the risk of vascular thrombosis.
Additionally, sirolimus is currently being assessed as a therapeutic option for autosomal dominant polycystic kidney disease (ADPKD). Case reports indicate sirolimus can reduce kidney volume and delay the loss of renal function in patients with ADPKD.
(Novartis's RAD001) to TSC patients off-label. Numerous clinical trials using both rapamycin analogs, involving both children and adults with TSC, are under way in the United States.
Most studies thus far have noted tumors often regrew when treatment stopped. Theories that the drug ameliorates TSC symptoms such as facial angiofibromas, ADHD, and autism are a matter of current research in animal models.
in patients with renal transplants. Other mTOR inhibitors, such as temsirolimus
(CCI-779) or everolimus
(RAD001), are being tested for use in cancers such as glioblastoma multiforme
and mantle cell lymphoma
.
A combination therapy
of doxorubicin
and sirolimus has been shown to drive AKT
-positive lymphomas into remission in mice. Akt signalling promotes cell survival in Akt-positive lymphomas and acts to prevent the cytotoxic effects of chemotherapy
drugs, such as doxorubicin
or cyclophosphamide
. Sirolimus blocks Akt signalling and the cells lose their resistance to the chemotherapy. Bcl-2
-positive lymphomas were completely resistant to the therapy; eIF4E
expressing lymphomas are not sensitive to sirolimus.
. The researchers discovered sirolimus regulates one of the same proteins the TSC gene does, but in different parts of the body. They decided to treat mice three to six months old (adulthood in mice lifespans); this increased the autistic mice's intellect to about that of normal mice in as little as three days.
replication through different mechanisms, including downregulation of the coreceptor CCR5
and the inhibition of the induction of autophagy
.
Sirolimus has also been used in preliminary research to combat progeria
, a rare disorder that causes individuals to age at an exceedingly rapid pace, leading to death in the early teenage years due to causes such as heart disease
or stroke
.
of the rapamycin core is accomplished by a type I polyketide synthase
(PKS) in conjunction with a nonribosomal peptide synthetase (NRPS). The domains responsible for the biosynthesis
of the linear polyketide
of rapamycin are organized into three multienzymes, RapA, RapB and RapC, which contain a total of 14 modules (figure 1). The three multienzymes are organized such that the first four modules of polyketide chain elongation are in RapA, the following six modules for continued elongation are in RapB, and the final four modules to complete the biosynthesis
of the linear polyketide are in RapC. Then, the linear polyketide
is modified by the NRPS, RapP, which attaches L-pipecolate to the terminal end of the polyketide, and then cyclizes the molecule, yielding the unbound product, prerapamycin.
The core macrocycle
, prerapamycin (figure 2) is then modified (figure 3) by an additional five enzymes, which lead to the final product, rapamycin. First, the core macrocycle is modified by RapI, SAM-dependant O-methyltransferase (MTase), which O-methylates at C39. Next, a carbonyl is installed at C9 by RapJ, a cytochrome P-450 monooxygenases (P-450). Then, RapM, another MTase, O-methylates at C16. Finally, RapN, another P-450 installs a hydroxyl at C27 immediately followed by O-methylation by Rap Q, a distinct MTase, at C27 to yield rapamycin.
The biosynthetic genes
responsible for rapamycin synthesis have been identified. As expected, three extremely large open reading frames (ORF's) designated as rapA, rapB and rapC encode for three extremely large and complex multienzymes, RapA, RapB, and RapC respectively. The gene
rapL has been established to code for a NAD+-dependant lysine
cycloamidase, which converts L-lysine
to L-pipecolic acid
(figure 4) for incorporation at the end of the polyketide. A gene
rapP, which is embedded between the PKS genes and translationally coupled to rapC encodes for an additional enzyme
, an NPRS responsible for incorporating L-pipecolic acid, chain termination
and cyclization of prerapamycin. Additionally, genes rapI, rapJ, rapM, rapN, rapO, and rapQ have been identified as coding for tailoring enzymes which modify the macrocyclic core to give rapamycin (figure 3). Finally, rapG and rapH have been identified to code for enzymes which have a positive regulatory role in the preparation of rapamycin through the control of rapamycin PKS gene expression.
Biosynthesis of this 31-membered macrocycle begins as the loading domain is primed with the starter unit, 4,5-dihydroxocyclohex-1-ene-carboxylic acid, which is derived from the shikimate pathway. Interestingly, the cyclohexane
ring of the starting unit is reduced during the transfer to module 1. The starting unit is then modified by a series of Claisen condensation
s with malonyl or methylmalonyl substrates, which are attached to an acyl carrier protein
(ACP) and extend the polyketide by two carbons each. After each successive condensation
, the growing polyketide is further modified according to enzymatic domains which are present to reduce
and dehydrate it, thereby introducing the diversity of functionalities observed in rapamycin (figure 1). Once the linear polyketide is complete, L-pipecolic acid, which is synthesized by a lysine cycloamidase from an L-lysine, is added to the terminal end of the polyketide by an NRPS. Then, the NSPS cyclizes the polyketide, giving prerapamycin, the first enzyme-free product. The macrocyclic core is then customized by a series of post-PKS enzymes through methylation
s by MTases and oxidations by P-450s to yield rapamycin.
A US patent for an efficient production method sirolimus (3,929,992) has expired, and the drug is available in generic formulations. Generic sirolimus, manufactured in India, is available from Canadian pharmacies for $20 a day.
Immunosuppressant
An immunosuppressant is any substance that performs immunosuppression of the immune system. They may be either exogenous, as immunosuppressive drugs, or endogenous, as ,e. g., testosterone...
drug used to prevent rejection
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 recipient and by use of immunosuppressant drugs after...
in organ transplant
Organ transplant
Organ transplantation is the moving of an organ from one body to another or from a donor site on the patient's own body, for the purpose of replacing the recipient's damaged or absent organ. The emerging field of regenerative medicine is allowing scientists and engineers to create organs to be...
ation; it is especially useful in kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
transplants. A macrolide
Macrolide
The macrolides are a group of drugs whose activity stems from the presence of a macrolide ring, a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered...
, sirolimus was first discovered as a product of the bacterium Streptomyces
Streptomyces
Streptomyces is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, and have genomes with high guanine and cytosine content...
hygroscopicus in a soil sample from Easter Island
Easter Island
Easter Island is a Polynesian island in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian triangle. A special territory of Chile that was annexed in 1888, Easter Island is famous for its 887 extant monumental statues, called moai, created by the early Rapanui people...
— an island also known as Rapa Nui, hence the name. It is marketed under the trade name Rapamune by Wyeth
Wyeth
Wyeth, formerly one of the companies owned by American Home Products Corporation , was a pharmaceutical company. The company was based in Madison, New Jersey, USA...
.
Sirolimus was originally developed as an antifungal agent. However, this use was abandoned when it was discovered to have potent immunosuppressive and antiproliferative properties.
It has since been shown to prolong the life of mice and might also be useful in the treatment of certain cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
s.
Mechanism of action
Unlike the similarly named tacrolimusTacrolimus
Tacrolimus is an immunosuppressive drug that is mainly used after allogeneic organ transplant to reduce the activity of the patient's immune system and so lower the risk of organ rejection...
, sirolimus is not a calcineurin inhibitor, but it has a similar suppressive effect on the immune system. Sirolimus inhibits the response to interleukin-2 (IL-2), and thereby blocks activation of T-
T cell
T cells or T lymphocytes belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity. They can be distinguished from other lymphocytes, such as B cells and natural killer cells , by the presence of a T cell receptor on the cell surface. They are...
and B-cell
B cell
B cells are lymphocytes that play a large role in the humoral immune response . The principal functions of B cells are to make antibodies against antigens, perform the role of antigen-presenting cells and eventually develop into memory B cells after activation by antigen interaction...
s. In contrast, tacrolimus inhibits the secretion of IL-2.
The mode of action of sirolimus is to bind the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
ic protein FK-binding protein 12
FKBP
FKBP, or FK506 binding protein, is a family of proteins that have prolyl isomerase activity and are related to the cyclophilins in function, though not in amino acid sequence. FKBPs have been identified in many eukaryotes from yeast to humans and function as protein folding chaperones for proteins...
(FKBP12) in a manner similar to tacrolimus. Unlike the tacrolimus-FKBP12 complex which inhibits calcineurin
Calcineurin
Calcineurin is a protein phosphatase also known as protein phosphatase 3, PPP3CA, and calcium-dependent serine-threonine phosphatase, and formerly known as protein phosphatase 2B . It activates the T cells of the immune system and can be blocked by drugs...
(PP2B), though, the sirolimus-FKBP12 complex inhibits the mammalian target of rapamycin
Mammalian target of rapamycin
The mammalian target of rapamycin also known as mechanistic target of rapamycin or FK506 binding protein 12-rapamycin associated protein 1 is a protein which in humans is encoded by the FRAP1 gene...
(mTOR) pathway by directly binding the mTOR Complex1 (mTORC1). mTOR was also called FRAP (FKBP-rapamycin associated protein), RAFT (rapamycin and FKBP target), RAPT1, or SEP. FRAP and RAFT are older names and were coined to reflect the fact that rapamycin must bind FKBP12 first, and only the FKBP12-rapamycin complex can bind mTOR; however, mTOR is now the widely accepted name and stands for mammalian Target Of Rapamycin and based on the precedent that Tor was first discovered via genetic and molecular studies of rapamycin-resistant mutants of Saccharomyces cerevisiae that identified FKBP12, Tor1, and Tor2 as the targets of rapamycin and provided robust support that the FKBP12-rapamycin complex binds to and inhibits Tor1 and Tor2.
Prevention of transplant rejection
The chief advantage sirolimus has over calcineurin inhibitors is its low toxicity towards kidneys. Transplant patients maintained on calcineurin inhibitors long-term tend to develop impaired kidney function or even chronic renal failure; this can be avoided by using sirolimus instead. It is particularly advantageous in patients with kidney transplants for hemolytic-uremic syndromeHemolytic-uremic syndrome
Hemolytic-uremic syndrome , abbreviated HUS, is a disease characterized by hemolytic anemia, acute renal failure and a low platelet count . It predominantly, but not exclusively, affects children. Most cases are preceded by an episode of diarrhea caused by E. coli O157:H7, which is acquired as a...
, as this disease is likely to recur in the transplanted kidney if a calcineurin-inhibitor is used. However, on October 7, 2008, the FDA approved safety labeling revisions for sirolimus to warn of the risk for decreased renal function associated with its use.
Sirolimus can also be used alone, or in conjunction with calcineurin inhibitors, such as tacrolimus
Tacrolimus
Tacrolimus is an immunosuppressive drug that is mainly used after allogeneic organ transplant to reduce the activity of the patient's immune system and so lower the risk of organ rejection...
and/or mycophenolate mofetil
Mycophenolate mofetil
Mycophenolate mofetil is an immunosuppressant and prodrug of mycophenolic acid, used extensively in transplant medicine. It is a reversible inhibitor of inosine monophosphate dehydrogenase in purine biosynthesis, which is necessary for the growth of T cells and B cells...
, to provide steroid-free immunosuppression regimens. Impaired wound healing and thrombocytopenia
Thrombocytopenia
Thrombocytopenia is a relative decrease of platelets in blood.A normal human platelet count ranges from 150,000 to 450,000 platelets per microliter of blood. These limits are determined by the 2.5th lower and upper percentile, so values outside this range do not necessarily indicate disease...
are a possible side effects of sirolimus; therefore, some transplant centres prefer not to use it immediately after the transplant operation, but instead administer it only after a period of weeks or months. Its optimal role in immunosuppression has not yet been determined, and is the subject of a number of ongoing clinical trials.
Sirolimus is absorbed into the blood stream from the intestine variably in each patient, with some patients having up to eight times more exposure than others for the same dose. Drug levels are therefore taken to make sure patients get the right dose for their condition. This is determined by taking a blood sample before the next dose which gives the trough level. Fortunately, there is good correlation between trough concentration levels and drug exposure, known as area under the concentration-time curve, for both sirolimus (SRL) and tacrolimus (TAC) (SRL: r2 = 0.83; TAC: r2 = 0.82), so only one level need be taken to know its pharmacokinetic (PK) profile. PK profiles of SRL and of TAC are unaltered by simultaneous administration. Dose-corrected drug exposure of TAC correlates with SRL (r2 = 0.8), so patients have similar bioavailability of both.
Lung toxicity
Lung toxicity is a serious complication associated with sirolimus therapy, especially in the case of lung transplants. The mechanism of the interstitial pneumonitis caused by sirolimus and other macrolide mTOR inhibitors is unclear, and may have nothing to do with the mTOR pathway. The interstitial pneumonitis is not dose dependent, but is more common in patients with underlying lung disease.Lifespan extension in mice
Rapamycin was first shown to extend lifespan in eukaryotes in 2006. Powers et al. showed a dose-responsive effect of rapamycin on lifespan extension in yeast cells. Building on this and other work, in a 2009 study, the lifespans of miceMICE
-Fiction:*Mice , alien species in The Hitchhiker's Guide to the Galaxy*The Mice -Acronyms:* "Meetings, Incentives, Conferencing, Exhibitions", facilities terminology for events...
fed rapamycin were increased between 28 and 38% from the beginning of treatment, or 9 to 14% in total increased maximum lifespan. Of particular note, the treatment began in mice aged 20 months, the equivalent of 60 human years. This suggests the possibility of an effective antiaging treatment for humans at an already-advanced age, as opposed to requiring a lifelong regimen beginning in youth. Because it strongly suppresses the immune system, though, people taking rapamycin are more susceptible to dangerous infections. It is not known whether rapamycin will have similar lifespan-lengthening effects in humans, and study authors caution the drug should not be used by the general population for this use.
Antiproliferative effects
The antiproliferative effect of sirolimus has also been used in conjunction with coronary stentCoronary stent
A coronary stent is a tube placed in the coronary arteries that supply the heart, to keep the arteries open in the treatment of coronary heart disease. It is used in a procedure called percutaneous coronary intervention...
s to prevent restenosis in coronary arteries following balloon angioplasty. The sirolimus is formulated in a polymer coating that affords controlled release through the healing period following coronary intervention. Several large clinical studies have demonstrated lower restenosis rates in patients treated with sirolimus-eluting stents when compared to bare metal stents, resulting in fewer repeat procedures. A sirolimus-eluting coronary stent is marketed by Cordis, a division of Johnson & Johnson
Johnson & Johnson
Johnson & Johnson is an American multinational pharmaceutical, medical devices and consumer packaged goods manufacturer founded in 1886. Its common stock is a component of the Dow Jones Industrial Average and the company is listed among the Fortune 500....
, under the tradename Cypher
Cypher stent
Cypher is a brand of drug-eluting coronary stent from Cordis Corporation, a Johnson & Johnson company. During a balloon angioplasty, the stent is inserted into the artery to provide a "scaffold" to open the artery...
. It has been proposed, however, that such stents may increase the risk of vascular thrombosis.
Additionally, sirolimus is currently being assessed as a therapeutic option for autosomal dominant polycystic kidney disease (ADPKD). Case reports indicate sirolimus can reduce kidney volume and delay the loss of renal function in patients with ADPKD.
Tuberous sclerosis complex
Sirolimus also shows promise in treating tuberous sclerosis complex (TSC), a congenital disorder that leaves sufferers prone to benign tumor growth in the brain, heart, kidneys, skin and other organs. After several studies conclusively linked mTOR inhibitors to remission in TSC tumors, specifically subependymal giant-cell astrocytomas in children and angiomyolipomas in adults, many US doctors began prescribing sirolimus (Wyeth's Rapamune) and everolimusEverolimus
Everolimus is the 40-O- derivative of sirolimus and works similarly to sirolimus as an mTOR inhibitor....
(Novartis's RAD001) to TSC patients off-label. Numerous clinical trials using both rapamycin analogs, involving both children and adults with TSC, are under way in the United States.
Most studies thus far have noted tumors often regrew when treatment stopped. Theories that the drug ameliorates TSC symptoms such as facial angiofibromas, ADHD, and autism are a matter of current research in animal models.
Cancer
The antiproliferative effects of sirolimus may have a role in treating cancer. Recently, sirolimus was shown to inhibit the progression of dermal Kaposi's sarcomaKaposi's sarcoma
Kaposi's sarcoma is a tumor caused by Human herpesvirus 8 , also known as Kaposi's sarcoma-associated herpesvirus . It was originally described by Moritz Kaposi , a Hungarian dermatologist practicing at the University of Vienna in 1872. It became more widely known as one of the AIDS defining...
in patients with renal transplants. Other mTOR inhibitors, such as temsirolimus
Temsirolimus
Temsirolimus is an intravenous drug for the treatment of renal cell carcinoma , developed by Wyeth Pharmaceuticals and approved by the U.S. Food and Drug Administration in late May 2007, and was also approved by the European Medicines Agency on November 2007...
(CCI-779) or everolimus
Everolimus
Everolimus is the 40-O- derivative of sirolimus and works similarly to sirolimus as an mTOR inhibitor....
(RAD001), are being tested for use in cancers such as glioblastoma multiforme
Glioblastoma multiforme
Glioblastoma multiforme is the most common and most aggressive malignant primary brain tumor in humans, involving glial cells and accounting for 52% of all functional tissue brain tumor cases and 20% of all intracranial tumors. Despite being the most prevalent form of primary brain tumor, GBMs...
and mantle cell lymphoma
Mantle cell lymphoma
Mantle cell lymphoma is one of the rarest of the non-Hodgkin's lymphomas , comprising about 6% of NHL cases. There are only about 15,000 patients presently in the U.S. While it is difficult to treat and seldom considered cured, investigations into better treatments are actively pursued worldwide...
.
A combination therapy
Combination therapy
Combination therapy or polytherapy is the use of more than one medication or other therapy. In contrast, monotherapy is any therapy which is taken by itself....
of doxorubicin
Doxorubicin
Doxorubicin INN is a drug used in cancer chemotherapy. It is an anthracycline antibiotic, closely related to the natural product daunomycin, and like all anthracyclines, it works by intercalating DNA....
and sirolimus has been shown to drive AKT
AKT
Akt, also known as Protein Kinase B , is a serine/threonine protein kinase that plays a key role in multiple cellular processes such as glucose metabolism, cell proliferation, apoptosis, transcription and cell migration.-Family members:...
-positive lymphomas into remission in mice. Akt signalling promotes cell survival in Akt-positive lymphomas and acts to prevent the cytotoxic effects of chemotherapy
Chemotherapy
Chemotherapy is the treatment of cancer with an antineoplastic drug or with a combination of such drugs into a standardized treatment regimen....
drugs, such as doxorubicin
Doxorubicin
Doxorubicin INN is a drug used in cancer chemotherapy. It is an anthracycline antibiotic, closely related to the natural product daunomycin, and like all anthracyclines, it works by intercalating DNA....
or cyclophosphamide
Cyclophosphamide
Cyclophosphamide , also known as cytophosphane, is a nitrogen mustard alkylating agent, from the oxazophorines group....
. Sirolimus blocks Akt signalling and the cells lose their resistance to the chemotherapy. Bcl-2
Bcl-2
Bcl-2 is the founding member of the Bcl-2 family of apoptosis regulator proteins encoded by the BCL2 gene. Bcl-2 derives its name from B-cell lymphoma 2, as it is the second member of a range of proteins initially described in chromosomal translocations involving chromosomes 14 and 18 in...
-positive lymphomas were completely resistant to the therapy; eIF4E
EIF4E
Eukaryotic translation initiation factor 4E, also known as eIF4E, is a protein which in humans is encoded by the eIF4E gene.- Function :...
expressing lymphomas are not sensitive to sirolimus.
Risk due to immunosuppression
As with all immunosuppressive medications, rapamycin decreases the body's inherent anticancer activity and allows some cancers which would have been naturally destroyed to proliferate. Patients on immunosuppressive medications have a 10- to 100-fold increased risk of cancer compared to the general population. Furthermore, people who currently have or have already been treated for cancer have a higher rate of tumor progression and recurrence than patients with an intact immune system . These general considerations counsel caution when exploring the potential of rapamycin to combat cancer which is suggested by experiment. Rapamycin seems to lower the cancer risk in some transplant patients.Autism in mice
In a study of sirolimus as a treatment for TSC, researchers observed a major improvement regarding retardation related to autismAutism
Autism is a disorder of neural development characterized by impaired social interaction and communication, and by restricted and repetitive behavior. These signs all begin before a child is three years old. Autism affects information processing in the brain by altering how nerve cells and their...
. The researchers discovered sirolimus regulates one of the same proteins the TSC gene does, but in different parts of the body. They decided to treat mice three to six months old (adulthood in mice lifespans); this increased the autistic mice's intellect to about that of normal mice in as little as three days.
Alzheimer's in mice
Sirolimus reduced brain lesions and prevented the decline of performance in the water maze in mice with a mouse model of Alzheimer's. Because sirolimus is already approved for other indications, it should be easy to start a clinical trial to see whether it works in humans.Potential treatment for other afflictions
Studies in vitro in mice and in humans suggest sirolimus inhibits HIVHIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
replication through different mechanisms, including downregulation of the coreceptor CCR5
CCR5
C-C chemokine receptor type 5, also known as CCR5, is a protein that in humans is encoded by the CCR5 gene. CCR5 is a member of the beta chemokine receptors family of integral membrane proteins...
and the inhibition of the induction of autophagy
Autophagy
In cell biology, autophagy, or autophagocytosis, is a catabolic process involving the degradation of a cell's own components through the lysosomal machinery. It is a tightly regulated process that plays a normal part in cell growth, development, and homeostasis, helping to maintain a balance...
.
Sirolimus has also been used in preliminary research to combat progeria
Progeria
Progeria is an extremely rare genetic condition wherein symptoms resembling aspects of aging are manifested at an early age. The word progeria comes from the Greek words "pro" , meaning "before", and "géras" , meaning "old age"...
, a rare disorder that causes individuals to age at an exceedingly rapid pace, leading to death in the early teenage years due to causes such as heart disease
Heart disease
Heart disease, cardiac disease or cardiopathy is an umbrella term for a variety of diseases affecting the heart. , it is the leading cause of death in the United States, England, Canada and Wales, accounting for 25.4% of the total deaths in the United States.-Types:-Coronary heart disease:Coronary...
or stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
.
Biosynthesis
The biosynthesisBiosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
of the rapamycin core is accomplished by a type I polyketide synthase
Polyketide synthase
Polyketide synthases are a family of multi-domain enzymes or enzyme complexes that produce polyketides, a large class of secondary metabolites, in bacteria, fungi, plants, and a few animal lineages...
(PKS) in conjunction with a nonribosomal peptide synthetase (NRPS). The domains responsible for the biosynthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
of the linear polyketide
Polyketide
Polyketides are secondary metabolites from bacteria, fungi, plants, and animals. Polyketides are usually biosynthesized through the decarboxylative condensation of malonyl-CoA derived extender units in a similar process to fatty acid synthesis...
of rapamycin are organized into three multienzymes, RapA, RapB and RapC, which contain a total of 14 modules (figure 1). The three multienzymes are organized such that the first four modules of polyketide chain elongation are in RapA, the following six modules for continued elongation are in RapB, and the final four modules to complete the biosynthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
of the linear polyketide are in RapC. Then, the linear polyketide
Polyketide
Polyketides are secondary metabolites from bacteria, fungi, plants, and animals. Polyketides are usually biosynthesized through the decarboxylative condensation of malonyl-CoA derived extender units in a similar process to fatty acid synthesis...
is modified by the NRPS, RapP, which attaches L-pipecolate to the terminal end of the polyketide, and then cyclizes the molecule, yielding the unbound product, prerapamycin.
Macrocycle
A macrocycle is, as defined by IUPAC, "a cyclic macromolecule or a macromolecular cyclic portion of a molecule." In the chemical literature, organic chemists may consider any molecule containing a ring of nine or more atoms to be a macrocycle...
, prerapamycin (figure 2) is then modified (figure 3) by an additional five enzymes, which lead to the final product, rapamycin. First, the core macrocycle is modified by RapI, SAM-dependant O-methyltransferase (MTase), which O-methylates at C39. Next, a carbonyl is installed at C9 by RapJ, a cytochrome P-450 monooxygenases (P-450). Then, RapM, another MTase, O-methylates at C16. Finally, RapN, another P-450 installs a hydroxyl at C27 immediately followed by O-methylation by Rap Q, a distinct MTase, at C27 to yield rapamycin.
The biosynthetic genes
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
responsible for rapamycin synthesis have been identified. As expected, three extremely large open reading frames (ORF's) designated as rapA, rapB and rapC encode for three extremely large and complex multienzymes, RapA, RapB, and RapC respectively. The gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
rapL has been established to code for a NAD+-dependant lysine
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
cycloamidase, which converts L-lysine
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
to L-pipecolic acid
Pipecolic acid
Pipecolic acid is a small organic molecule which accumulates in pipecolic acidemia. It is the carboxylic acid of piperidine.It can be associated with some forms of epilepsy....
(figure 4) for incorporation at the end of the polyketide. A gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
rapP, which is embedded between the PKS genes and translationally coupled to rapC encodes for an additional enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
, an NPRS responsible for incorporating L-pipecolic acid, chain termination
Chain termination
Chain termination is any chemical reaction that ceases the formation of reactive intermediates in a chain propagation step in the course of a polymerization, effectively bringing it to a halt.- Mechanisms of Termination :...
and cyclization of prerapamycin. Additionally, genes rapI, rapJ, rapM, rapN, rapO, and rapQ have been identified as coding for tailoring enzymes which modify the macrocyclic core to give rapamycin (figure 3). Finally, rapG and rapH have been identified to code for enzymes which have a positive regulatory role in the preparation of rapamycin through the control of rapamycin PKS gene expression.
Biosynthesis of this 31-membered macrocycle begins as the loading domain is primed with the starter unit, 4,5-dihydroxocyclohex-1-ene-carboxylic acid, which is derived from the shikimate pathway. Interestingly, the cyclohexane
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...
ring of the starting unit is reduced during the transfer to module 1. The starting unit is then modified by a series of Claisen condensation
Claisen condensation
The Claisen condensation is a carbon–carbon bond forming reaction that occurs between two esters or one ester and another carbonyl compound in the presence of a strong base, resulting in a β-keto ester or a β-diketone...
s with malonyl or methylmalonyl substrates, which are attached to an acyl carrier protein
Acyl carrier protein
The acyl carrier protein is an important component in both fatty acid and polyketide biosynthesis with the growing chain bound during synthesis as a thiol ester at the distal thiol of a 4'-phosphopantethiene moiety...
(ACP) and extend the polyketide by two carbons each. After each successive condensation
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
, the growing polyketide is further modified according to enzymatic domains which are present to reduce
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
and dehydrate it, thereby introducing the diversity of functionalities observed in rapamycin (figure 1). Once the linear polyketide is complete, L-pipecolic acid, which is synthesized by a lysine cycloamidase from an L-lysine, is added to the terminal end of the polyketide by an NRPS. Then, the NSPS cyclizes the polyketide, giving prerapamycin, the first enzyme-free product. The macrocyclic core is then customized by a series of post-PKS enzymes through methylation
Methylation
In the chemical sciences, methylation denotes the addition of a methyl group to a substrate or the substitution of an atom or group by a methyl group. Methylation is a form of alkylation with, to be specific, a methyl group, rather than a larger carbon chain, replacing a hydrogen atom...
s by MTases and oxidations by P-450s to yield rapamycin.
Costs
Rapamune costs about $37 a day for a 4 mg dose.A US patent for an efficient production method sirolimus (3,929,992) has expired, and the drug is available in generic formulations. Generic sirolimus, manufactured in India, is available from Canadian pharmacies for $20 a day.