Paper,+EFrankel

Paper, EFrankel


 * Sirolimus: Its Mechanism of Action and Role in the Medical World as a Multi-Dimensional Immunosuppressant**

Immunosuppressants carry a huge role in the medical world. Used in a wide range of applications from anti-fungal and anti-tumor growth to psoriasis and multiple sclerosis, immunosuppressant drugs act as a preventative medication [11]. Sirolimus is just one of many drugs in the immunosuppressant category, which aids in the prevention of many disorders, such as restanosis in coronary arteries, Candida growth in yeast infections, organ rejections in transplants as well as tumor growth [2]. Due to its ability to decrease the proliferation activity in cells, cells are no longer able to regenerate, which causes inflammation in a given area. Sirolimus is a reasonably accommodating compound, stable at room temperature and miscible in some inorganic solvents, making it a flexible drug with regards to its purpose [13]. Examples of this include the ability to coat a Sirolimus solution over a coronary stent to prevent rejection of the device as well as taking it orally in a tablet form for treatment of common fungal infections [2,5]. This paper will discuss just a few examples of how Sirolimus is used and its effectiveness overall for each of its applications. Overall, it is evident that Sirolimus has proven to be both successful and effective drug for a wide array of uses.
 * Abstract**

Sirolimus is a well-known immunosuppressant medication used in the prevention of organ rejection after a transplant as well as in conjunction with many medical devices to avoid post-operative infection and inflammation. Immunosuppressive drugs affect the efficacy and efficiency of the body’s immune system[14]. By deactivating the immune system, the body is no longer able to attack foreign bodies within the system, such as a new organ, skin graft or stent[11]. Functioning immune systems would normally target the foreign body and use proliferative action to remove the alien specimen. This causes both inflammation and cell generation in the infected area. While on an immunosuppressive treatment, a patient may become weak and have an elevated chance of becoming sick due to their weak immune system[13]. Sirolimus is only one compound in the vast world of immunosuppressive drugs. Other examples of immunosuppressant’s include Cyclosporine, Mycophonolate Mofetil, and Mycophenolic acid, which work to treat psoriasis, rheumatoid arthritis, scleroderma and multiple sclerosis as well as many more ailments [11]. Moreover, the wide range of immunosuppressive medications available facilitates the treatment of not only prevention of organ transplants and medical devices but also common disorders and diseases. Sirolimus has been found to be effective as a compliment to the treatment of the diseases and disorders mentioned above. Moreover, the use of sirolimus is spanning not only one-dimensional use, but also in coordination with medical devices, such as coronary stents, in the treatment of cardiovascular disease [4,5]. In general, as time goes on, there will be more instances where common medicinal drugs will be used in conjunction with medical devices to make the treatment of diseases and disorders more accurate and precise.
 * Introduction**

Sirolimus was discovered during the antimicrobial agent discovery program of the 1970s. The program included acquiring more than 70 samples of soils from Rapa Nui, also known as Easter Island in Canada [2]. One of the soil samples collected from Vai Atare location of Easter Island revealed a strain with antifungal activity by means of a mechanism screening protocol, known as //Steptomyces hygroscopicus// [2]//.// The active component responsible of the antifungal property was isolated and named rapamycin, its etymology coming from Rapa Nui-mycin. The trade name for rapamycin was declared rapamune, and generic name Sirolimus. In contrast, even though Sirolimus was discovered first and foremost to be an effective antifungal agent, it also proved to have successful antiproliferative and immunosuppressive properties. Sirolimus was first isolated in 1975, and x-ray crystallography became available in 1981[2]. The molecular interaction of sirolimus within the body is related to the intracellular FK binding proteins, often known as FKBPs[2]. FK binding proteins are known as immunophilins, which function as peptidylprolyl isomerases. Rapamycin and other immunosuppressants inhibit the peptidylprolyl isomerases (PPIs) activity upon binding with the isomerases [2]. Even though PPI inhibition is not related to immunosuppression, it is the complex formed by the rapamycin that induces immunosuppressive activity. When immunophilins are complexed with tacrolimus, sirolimus or cyclosporine, they form a complex with calcineurin, which in effect causes the inactivation of the compound [2,12]. Calcineurin is a calmodulin-dependent serine/theoronine phosphatase. Once calcineurin in no longer activated, it can no longer dephosphorylate a cytoplasmic unit of a T-cell, and therefore it can no longer travel into the nucleus, a function vital for the generation of cytokine genes needed for proliferation. The formation of this complex led to the further investigation of the regulator for cell growth and proliferation, also known as the target of rapamycin (TOR)[12]. In terms of mammals (mTOR), the target is a 289kDalton protein with several lipid kinases. Sirolimus binds directly to the mTOR, interfering with its function [2].
 * Discovery and Molecular Interaction**

TOR proteins themselves regulate the translation and transcription processes of cell generation as well as organelle production. Rapamycin works by targeting mTOR, which inhibits the translation process of mRNA to DNA, needed for cell division [12]. The cell cycle is halted because of this. TOR proteins are involved with the synthesis of ribosomes, transcription of various enzymes, take part in the initiation and elongation of translation of DNA as well as have a part in brain development. Furthermore, TOR proteins have a crucial role in balancing protein synthesis and degradation processes. Rapamycin inhibits all of these actions, and ultimately prevents new cell generation. Not only does sirolimus effect the translation and transcription processes, it has a vital role in depleting the activity with regards to various kinase complexes, specifically cdk4/cyclin D and cdk2/cyclin E [12]. These complexes usually peak in levels at the end of the G1 phase of cellular mitosis [9,12]. The depletion of these complexes off sets the stoichiometry levels between the kinases and associated inhibitors, ultimately disrupting the activation of the synthesis of important cellular proteins needed for proliferation.
 * Target of Rapamycin**

Sirolimus is a 51 carbon membered cyclic structure, containing several carbonyl and hydroxyl groups [12,19,20,21]. It has a molecular weight of approximately 914.2g/mol [18,19,20]. Rapamycin is not soluble in water, although it very soluble in various inorganic solvents, such as dimethyl sulfoxide. It is a solid at room temperature, with a physical appearance of a white, yellow crystalline powder. The prescription of sirolimus is dependent on the application it is being used for [13]. In terms of medical devices, sirolimus may be applied to the surface of the device, such as a stent and then delivered over a period of time. In this case, sirolimus is dissolved in an appropriate solvent, such as dimethyl sulfoxide and sprayed over the device as a solution [3,17]. Depending on the orientation of the device, sirolimus may be released immediately upon application into the body, or over a delayed period of time for long-term immunosuppressive capabilities [17]. In most cases, the dosage of sirolimus applied to a device is eluted over a period of 120 days. On the other hand, there have been some cases where rapamycin is eluted over a shorter period of time, for a more immediate effect [16]. When used for the treatment of organ transplants, sirolimus is often used in conjunction with other medications [7]. A patient using sirolimus begins using the medication immediately after surgery, to fight infection and rejection of the organ. For both organ transplants and other disorders including psoriasis and arthritis, sirolimus may be taken in tablet form [14]. The table is taken orally with or without food. Sirolimus is also available in the form of a solution, which should be taken with either water or orange juice. Common side effects from taking sirolimus include stomach pain, weakness, headache, diarrhea, weight gain and joint pain [13]. Moreover, unusual side effects include unusual bleeding or bruising, paleness, cough and shortness of breath.
 * Structure**

Immunotherapy has come a long way in recent years with regards to organ transplants. Various immunosuppressants are now used in the treatment against the rejection of organ transplants as well as decreasing any toxic side effects. Sirolimus is often utilized during organ transplants, due to its effects on endothelial and smooth muscle proliferation, as well as blocking various growth stimuli, which may induce inflammation [6]. Furthermore, Sirolimus inhibits the proliferation of T-cells and B-cells, which are induced by antigens [6]. The goals of using sirolimus in conjunction with other immunosuppressive agents, such as calcinuerin inhibitors, are to reduce the need for steroid use after surgery as well as increase the use of Sirolimus as a means of mono-therapy [8]. Moreover, Sirolimus is used in hopes as a substitution for calcinuerin inhibitors, which have more side effects attributed to them. Sirolimus is effective in both acute and chronic cases of solid organ rejection, making it a popular agent to use. Before Sirolimus became a well-known agent to use in organ transplants as a singular therapy in the prevention of organ rejection, it was used along side cyclosporine and prednisone [6]. This treatment had a relatively good acceptance, reducing the incidence of rejection from 32 to 7.5% [6]. When Sirolimus is used in conjunction with organ transplants, the dosage depends on the age of the patient. Children often require dosages twice a day in comparison to adults; the half-life of sirolimus is decreased in children due to their higher metabolism [6]. Patients who suffer from calcineurin nephrotoxicity, mainly due to kidney transplants, can be treated with Sirolimus in order to improve the prevention of rejection [1,6]. This holds true as well for patients whom received liver transplants. As mentioned in the article, “Sirolimus for Solid Organ Transplantation in Children” [6], 50% of the pediatric patients who experienced chronic rejection of their liver transplants had no response of rejection after treatment with Sirolimus. There are certain drawbacks attributed to using Rapamycin as an immunosuppressant with organ transplants. Patients whom were treated with rapamycin after organ transplant or graft had increased incidences of hypertriglyceridemia (increased triglycerides in the blood) and hypercholesterolemia (high cholesterol), thrombocytopenia (decrease in blood platelet levels) and leucopenia (decrease in white blood cell levels) [7]. These side effects are mainly due to the body’s inhibition to attack against harmful antigens. Sirolimus lowers the level of T-cells present in the blood, which are responsible for attacking and rejection of the organ of graft. Even though there are certain drawbacks to using Sirolimus as a monetary therapy against the rejection of organ transplants, the side effects seen with its use last a relatively short period of time, approximately two months [6]. It should be noted that treatment with Sirolimus should not begin too early, for there is an increased risk that the wound healing and graft function process may be delayed [7]. In cases where patients have diabetes or are on chemotherapeutic regimen, Sirolimus only exacerbates the delay in wound healing. Furthermore, using sirolimus too early may lead to more post-operative issues in the future [7]. In general, Sirolimus should not be used within the fist week of post-operative car in order to ensure sufficient wound healing. Moreover, a lower dosage over a period of time decreases the chance of delay in healing.
 * Activity in Organ Transplants**

Sirolimus and its various derivatives are often applied to intravascular stents to help deter postoperative inflammation and restanosis. Many medical device corporations, which produce stents, have begun applying a one of inflammatory drug onto the stent. This has become very popular with cardiovascular stents [3,4]. Cordis Corporation, Boston Scientific, and Abbott are all major manufacturers of drug eluting stents, all of which use derivatives of Rapamycin [15,16,17]. In all the corporations, the immunosuppressant drugs are used to prevent the overgrowth of cells, which are produced to help the artery heel. Boston Scientific uses the derivative Everolimus in their cardiovascular stents, Taxus® and Promus® [16]. In this case, the stents are coated with a thin matrix of solvent and drug, which is eluted over 120 days. Taxus® and Promus® differ in the actually cell geometry of the stent, which helps induce drug delivery in different ways [16]. With regards to Taxus®, the drug load varies depending on the orientation of the stent. Abbott’s Xience® is a coronary stent used to treat coronary disease by propping open the blocked artery, then releasing an immunosuppressive drug to prevent further blockage [15]. Like Boston Scientifics’ Taxus®, paclitaxel and Everolimus are the derivative used as the immunosuppressant [15]. The Cypher® stent, produced by Cordis Corporation was the first of its kind to introduce a drug into the stent process, now known as drug coated or drug eluting stents [17]. Unlike the other two stents mentioned before, the Cypher® stent uses Sirolimus in order to prevent re-blockage inside the vessel as well as decrease the chances of rejection [17]. In general there has been a lot of research involving the benefits of using a sirolimus or corresponding derivative coated stent instead of a bare metal stent [5]. In a study involving graft stents, bare metal stents and Sirolimus coated stents; there was a higher percentage of revascularization when bare metal stents were used in patients treated for chronic coronary disease [4]. Revascularization is a term that denotes the process of re-entering and fixing the artery or vessel, which was propped open by a stent, most likely due to chronic blockages. Furthermore, it is evident that drug eluting stents, such as the sirolimus coated stent studied in the research above, have fewer incidences of restanosis in the short term [4]. Restanosis is a term associated with the re-narrowing of a blood vessel, even after some time of intervention, whether stent implantation or artery balloon dilation. Due to the fact that drug-eluting stents are still fairly new in the scheme of medicine, the long-term effects have not been determined. In addition, only 5.1% of patients whom received Sirolimus coated-stents needed percutaneous coronary intervention, or reopening of the vessel, as compared to 7.7% of patients whom received a stent graft [4]. None of the patients whom received either a bare metal stent or a sirolimus coated stent died during the study, although 2.5% of the patients with a stent graft did die after 5-months post-operative [4]. In another study which analyzed the incidences of restanosis between Taxus® and Cypher® Stents, Cypher® sirolimus eluting stent proved more efficient compared to the paclitaxel-eluting stent [3]. Overall, there was a higher incidence of major adverse cardiac events with paclitaxel-coated stents than those coated with Sirolimus, approximately 10.8 and 6.2%, respectively [3].
 * Action in Cardiovascular Stents**

The effectiveness of sirolimus spans farther than in the treatment of organ transplants and application of medical devices, but also is useful in treating fungal and tumor activity. Sirolimus is extremely successful in combating Candida compounds [2,9,10]. Candida is apart of the yeast family, and ranges in a variety of species. The Candida genus can infect the intestinal tract in humans and animals as well as cause vaginal yeast infections in women. Sirolimus inhibits the fungal growth by complexing with the FK binding proteins found in the yeast [2]. Sirolimus was shown to be just as effective as amphtericin B, a common antifungal drug, when treating rats for vaginal fungal infections. In this case, sirolimus was given at 20mg/kg orally to rats, and had the same potency as amphotericin B at 1 mg/kg [2]. Although it is evident that sirolimus is an effective agent for the treatment of fungi, it is not frequently used to its degree of immunosuppressive activity [2,9]. Due to its anti-proliferative action, Sirolimus makes a sufficient drug to use against tumor growth. Although sirolimus is slightly selective when it comes to the cells in which it is effective against, the range of cell types include lymphoid (lymph-nodes), nervous system, hepatic (liver cells), osteoblastic (bone development), renal and connective tissue [2]. In most cases, sirolimus inhibits the procession of cell growth at the G1 phase of mitosis [2,8]. Although sirolimus does stop the malignant cell from dividing, it does not induce apoptosis, or the death of the cell. In this case, the cell lives and dies naturally, but is no longer able to regenerate as long as sirolimus is present within the body.
 * Action Against Tumor and Fungal Growth**

In conclusion, Sirolimus is a shown to be a proven and multi-faceted immunosuppressive agent, with a wide range of applications. Moreover, Sirolimus is just one of many immunosuppressant’s available which facilitate the treatment in the prevention of organ transplants and medical devices. Although sirolimus was first discovered to be a sufficient and successful antifungal agent, it is used as a common tumor suppressant as well as applied as a solution in coronary stents to decrease the chances of inflammation [2,5,7]. Like mentioned before, the molecular mechanism attributed with sirolimus corresponds to the intracellular binding proteins, known as FKBPs [2]. It is the complex formed with calcineurin and immunophilins that initiates the inactivation of proliferation of cells. Furthermore, rapamycin targets TOR proteins, which are mainly associated with the transcription and translation of DNA, a required factor in cell generation [12]. When utilized as a therapy in organ or graft transplant, Sirolimus can be used by itself as a monetary therapy or in conjunction with other immunosuppressant’s and steroids [4]. Studies have shown that when used as the sole therapy in the prevention of rejection of organ transplants and grafts, Sirolimus has fewer side effects then when used in combination with steroids [7]. In the case of coronary stents, Sirolimus is added to a coating overtop the stents, eluting over a period of time. Depending on the cellular geometry of the stent, the elution time of rapamycin can range over a period of weeks to several months [5]. Lastly, Sirolimus is often used as an agent in the prevention of fungi and tumor growth [2,9,10]. Originally discovered as an antifungal agent due to its combative nature against the Candida species, Sirolimus has been used for the treatment of a variety of yeast infections. Additionally, the fact that Sirolimus stops the regeneration of cells at the G1 mitotic phase, it makes for a stopping the growth of tumors. In contrast, Sirolimus has no effect in inducing apoptosis in cells, but rather inhibits the growth of the cluster. Overall, Sirolimus is a complex compound, with a ride range of capabilities and uses due in majority to its immunosuppressive character.
 * Conclusion**

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