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Rheumatoid arthritis (RA) is a form of arthritis, which normally is felt by patients as pain or stiffness of the joints with swelling of the joints leading to restricted movement also occurring in some cases.
RA can occur at any age and in almost any joint in the body, even those with little environmental stress such as finger joints. RA also normally occurs in pairs of joints such as ankles, fingers and toes of both hands etc. It is also more painful or stiff after periods of rest, such as first thing in the morning whereas osteoarthritis is made worse with increased activity.
RA is an autoimmune disease and no single factor can be said to be the cause but there are factors which may in combination bring on the disease. The main factor is genetics, if it is "in the family" your risk level will be higher, other factors including stress, infection and injury may trigger it.
Women are more likely to suffer from RA (70%) and hormone changes including those associated with childbirth may be a factor (RA in a sufferer may go into remission during pregnancy but it normally returns after about 1 year)
The symptoms felt by a sufferer are caused by localised inflammation of the synovium (the sac containing the synovial fluid) at the joint., Where an immune response has been triggered this inflammation may lead to the destruction of the cartilage, bone and ligaments causing deformity of the joints The symptoms of RA are not only localized , as the immune response is triggered it may also cause other effects on other organs in the body such as the heart , lungs and eyes and may also cause tiredness.
Because it can affect other organs of the body, rheumatoid arthritis is referred to as a systemic illness.
Enbrel is a fusion protein, that is it is made of two portions of protein fused together to produce a new protein. The two proteins fused in Enbrel are, a portion of human Immunoglobulin IgG and two portions of the tumour necrosis factor receptor (TNFR) protein. In fig 2 the two blue "arms" of the molecule are the TNFR while the bottom section is the IgG portion. Both parts of the molecule are naturally occurring and the final molecule was designed specifically to act as an immunosuppressant.
Fig 2: the etranercept (Enbrel®) molecule. http://www.biomedcentral.com/graphics/email/images/general/IndieEdNewsletter25_JoNI_Media.jpg
Autoimmune diseases are caused by the body or cells of the body reacting to a stimulus which should not cause an immune response such as a normal component of the body.
Enbrel is used in the treatment of various autoimmune type diseases including Rheumatoid Arthritis (RA) Psoriatic Arthritis (PsA) ,Ankylosing Spondylitis (AS) Plaque Psoriasis and Juvenile Idiopathic Arthritis (JIA).
Over production of TNF¡ has been seen in all of the diseases mentioned above and this caused the interest in blocking this signal pathway as a way to regulate or control these diseases. A chemical which can block the TNF¡ production should decrease or eliminate this response therefore reducing or eliminating the symptoms. .
TNF¡ initiates an inflammatory response in the cells of the affected tissue. It is the inflammation of the cells and tissues that cause the symptoms of these diseases.
In the normal pathway the TNF¡ is produced in response to a stimulus, it then binds to TNF receptors on the surfaces of cells , when the cells receive this signal they turn on to an inflammatory mode which produces an effect known as a cascade, producing more TNF¡ which in turn switches on more cells into inflammatory mode. A number of drugs have been developed to try and regulate the initiation of the inflammatory response they are known as biologics they include Etanercept (aka Enbrel), Inflaximab (aka Remicade) and Adalimumab (aka Humira) and some other drugs which inhibit or block other pathways involved in the immune response.
It was found that Soluble TNF Receptors (sTNFRs) could act as inhibitors in the path that leads to the inflammatory response
It is thought that Enbrel may act like sTNFRs and bind to TNF¡ or to the receptors of TNF ¡ which then will not initiate the inflammatory response.
Remicade and Humira act in almost exactly the same way and these drugs differ from Enbrel mostly in the methods (or hosts) of production.
Enbrel is produced by recombinant DNA technology in a Chinese hamster ovary (CHO) mammalian cell expression system.
Remicade (inflaximab) is produced using murine( mouse) protien antibody fused
to human constant protein domains: whereas Adalimumab (Humira) is made from the fusion of the same human constant region but combined to a recombinant human protein (fig 3)
Fig 3. Types of TNF¡ inhibitors. (http://molinterv.aspetjournals.org/cgi/reprint/3/6/310.pdf)
Having known that increased levels of TNF¡ occurs in patients with autoimmune diseases such as RA , a team of scientists at a company called Immunex targeted their research on the gene that produced a natural inhibitor to the TNF¡. They found that there were natural inhibitors to the reception of the signal initiated by the TNF¡ and these were called TNF receptors (TNFR)
They identified the gene responsible for the TNFR and. They then isolated the gene and cloned it. The p75 TNF receptor portion of the Enbrel molecule was cloned from a human fibroblast cell line. Whereas the IgG portion was cloned by PCR amplification. Early versions of the drug had only a single p75 receptor and had only low effectiveness. The later versions of the drug had the Dimeric form (2- p75 domains- the two yellow portions in fig 3) and was found to be much more effective. Once the effectiveness of the drug was established the gene responsible was introduced into the carrier cell, in this case Chinese Hamster Ovary (CHO) cell line.
History of Immunex the company who developed Enbrel
The company Immunex was formed by two scientists called Steven Gillis and Christopher Henney, began as a biotechnology company in the early 1980s and began by developing drugs which it then licensed to other companies to fund further research mainly into drugs aimed at immune response mediation.
The research into the drug that eventually became Enbrel began in the late 80's and continued up to and including clinical trials when, in the mid 90's Immunex had brought another company called Wyeth -Ayerst, in as partners in the development of the production process and marketing of Enbrel as they had greater expertise in the large scale manufacture of drugs. The drug was approved for use for the treatment of RA in 1998. The drug was so successful that in the first week of sales they sold $13 millions worth of the drug. Production of the drug could not be scaled up quickly enough for the market and demand outstripped supply for the first few years. Enbrel was also approved for other uses and it is now approved for use on juvenile arthritis (JIA), Psoriatic Arthritis (PSA), Ankylosing Spondylitis (AS) and Plaque Psoriasis.
In 2002 the immunex brand was bought out by Amgen who are now the license holder and main producer of the drug, Wyeth corp. who were early partners of Immunex and helped them to develop the manufacturing process are also joint license holders of the drug and are licensed to produce the drug outside the U.S.A. notably in their Dublin plant.
Mode of administration
Enbrel is normally administered as a subcutaneous injection in doses of either 25 or 50mg per week depending on the condition being treated and the weight/age of the patient. Sometimes a 50mg dose per week will be administered in two 25mg injections.
Enbrel in supplied in one of two forms either a dry powder which is reconstituted and then injected or more recently and more widespread now a pre filled syringe.
The powdered form of the drug is a lyophilized single dose of the drug which is made up with a supplied vial of Water For Injection (WFI).
The powder contains the drug and a few "excipients" which are there to help in the reconstitution and to protect the drug during lyophisation and storage. The excipients in the powder form are mannitol, sucrose and trometamol.
In the pre-filled syringe the excipients are , Sucrose, sodium chloride, L-arginine hydrochloride, sodium phosphate monobasic dihydrate, sodium phosphate dibasic dihydrate and water for injections.The pre filled syringes have to be stored at 2 to 7°C and the excipients are there to prevent the degradation of the active ingredient.
Fig. 4: a month's supply of 4 pre-filled syringes of Enbrel
For RA and various other types of arthritis in the in-vivo studies it was found that Enbrel reduced the incidence of arthritis and the severity of the inflammation or consequent damage of the joints. One of the types of arthritis it was found to be also effective for was Psoriatic Arthritis and it was also found to be effective against Plaque Psoriasis which is almost always present in sufferers of PsA. Plaque psoriasis can be treated in other ways, such as topical creams and phototherapy, as it is a disease which manifests itself as lesions on the skin. Plaque psoriasis responds to a number of other treatments in most cases but it normally becomes resistant to these treatments, some of the treatments for plaque psoriasis also have high risk factors associated with them (e.g. skin cancer with prolonged phototherapy), so trials began for the use of Enbrel for plaque Psoriasis. It was also approved for use in paediatric cases of Psoriasis in 2002.
The following graph represents the PASI 75 response of patients on the paediatric trial of the drug. In the first section (up to 12 weeks) a steady improvement can be seen (blue line) in the patients on the Enbrel whereas the placebo group showed little change. As soon as the second group (the placebo group) were given the drug the same improvement could be seen in the same time frame with the effect remaining as long as the trial lasted. So far most patients have not gone into remission with this treatment with only a few patients having developed antibodies to the active ingredient, etranercept.
Fig 5. : improvement rating of patients on double blind trial followed by an open label trial of Enbrel (etranercept), in juvenile patients suffering from severe plaque psoriasis. (EMEA/H/C/262/II/94).
As Enbrel blocks some of the signal pathways involved in the immune response, patients may suffer from increased risk of infections as their ability to fight them is reduced.
On of the problems associated with any immunosuppressant drug is that latent infections may appear when they are administered. Because of this risk a patient who is being proscribed Enbrel must undergo a number of tests including chest/lung x-rays to ensure there are no latent infections especially Tuberculosis. which can be latent in the lung tissue.
Another problem sometimes seen is that patients may suffer from some localised infections at the injection sites. As Enbrel is administered in most cases by the patient themselves it is thought that a combination of bad aseptic technique and immuno-suppression are combined factors here.
Patients on Enbrel are warned by the physician and on the patient information leaflet of the risk of serious infections and are told to report abnormalities , even cold or flu symptoms as even these can become more serious if the immune system is suppressed.
Mode of production
The drug which was designed had the gene responsible for its production inserted using recombinant technology into a Chinese Hamster Ovary (CHO) cell line.
Enbrel is produced as a monoclonal antibody in a process where the original host (an animal) is challenged by the antigen, which causes the production of the antibody, at this stage the B-cells from the hosts' spleen are removed and then fused with the CHO cells. When the cell line has been proven to be stable and free from viral or other contamination a master cell bank is constructed.
This produces a cell line which will produce the antibody. .From this initial cell population a working cell bank population was cloned or grown. From the working cell bank a vial is taken to prepare each batch of production.
The cells are grown in tightly controlled conditions in vats of increasing size until the final production stage which is a high volume (>20,000 Litre)
The product (Enbrel) is produced extracellularly, which means that the cells secrete it into the broth in which they are grown. In-process testing during cell culture includes tests for microbial growth, microscopic contamination, endotoxin presence, cell density, viability, pH, osmolality, CO2, glucose, lactate and etranercept production.
If the batch meets all the conditions set down for production it is then passed on to the purification stage. Purification normally begins with simple filtration to remove the cells and cell debris from the liquid followed by chromatography and ultrafiltration to isolate the Enbrel from all the other chemicals in the broth then more filtration to remove and remaining particulate or viral material.
The purification stages are as closely controlled as the production process and if again all steps and tests are clear the batch moves to the next stage which is either Lyophilisation if the Enbrel is to be used as a powder or concentration and make up to the final concentration for the pre filled syringes.
Enbrel was one of the first of a group of drugs called "biologics" to come to market and demand for the drugs was very high, given that it was the first to come to market it had the lead and continued to be dominant in the market taking almost 75% of total market share. The market has expanded since then and although it is still the market leader it has not maintained its 75% position.
(Exact figures are hard to obtain but this has been reported without exact figures to back it up)
Fig 6: Enbrel market share of "biologics" Enbrel is the blue part of the columns and the red part is all others (years 1999-2001) www.123wise.net/enbrelPpt.pdf