Antiretrovirals - klöhn

Producing antiretroviral drugs in plants In 1983 Luc Montagnier who worked at the Pasteur Institute in France discovered that the acquired immune deficiency syndrome (AIDS) is caused by a retrovirus that is nowadays known as the human immunodeficiency virus (HIV). According to estimates of the Joint United Nations Program on HIV/AIDS (UNAIDS) and the World Health Organization (WHO) 25 million people have died of the disease so far ( Montagnier´s work provided a basis for research concerning the treatment of AIDS and scientists started immediately looking for drugs with an effect on the virus. In 1985 a group of scientists at the National Cancer Institute discovered that the drug Zidovudine has an impact on the virus by reducing the viral load. Today Zidovudine is one of several known antiretroviral drugs that are used in combination therapies. HIV-carriers in developed countries benefit from these therapies whereas the considerable number of AIDS-infected people in developing countries cannot afford the costs of the drugs. Producing antiretroviral drugs in genetically modified plants is a way to approach this The HI-virus is a spherical retrovirus with a diameter of about 120 nm that can be transferred in blood, semen, vaginal fluid and breast milk. Inside the virus particle there is a capsid that consists of two protein components and contains the single stranded RNA-genome. The genome contains 9000 nucleotides that encode for nine genes. In addition the virus particle contains the enzymes reverse transcriptase, protease and integrase. The capsid is surrounded by a membrane that is composed of a lipid bilayer and transmembrane proteins gp120 and gp41. An infection begins with the specific binding of gp120 to receptors within human CD4 T-cells. The glycoprotein gp41 promotes the fusion of the virus envelope and the host cell. This process enables viral RNA to enter the host cell where the RNA is translated into complementary DNA (cDNA) by viral reverse transcriptase. The process of integration of the viral cDNA into the human genome is catalyzed by the enzyme integrase. The cDNA encodes for viral proteins that are transcribed and translated by human enzymes. The newly formed viruses are released and infect more CD4 T-cells hence the number of CD4 T-cells is reduced during an HIV-infection. As the CD4 T-cells are important parts of the human immune system, their decimation causes a loss of the body’s defences. Due to the fact that reverse transcriptase has no proofreading function mutations occur 106 times more often during transcription of viral RNA than during transcription of human DNA ( Therefore the genome of the HI-virus is not stable. That is why up until now the development of an effective preventive antiretroviral drug has failed. But there are several post-infection drugs on the market that These antiretroviral drugs can be divided into the following groups as they interfere with the process of infection at different stages: entry inhibitors, protease inhibitors, fusion inhibitors, integrase inhibitors and reverse transcriptase inhibitors. These drugs are used in a combination therapy called HAART (highly active antiretroviral therapy). A few years ago the diagnosis of AIDS was a death sentence and the patient had a life expectancy of about two years after diagnosis. Due to antiretroviral drugs that reduce the viral load, this situation has changed and life expectancy has risen to more than twenty years after infection. But only HIV-carriers in developed countries can benefit from this progress whereas most of the HIV-carriers in developing countries cannot afford the costs of antiretroviral drugs. At the present time antiretroviral drugs are produced by chemical synthesis. The synthesis can only be accomplished in modern factories in developed countries as it is a high-tech process. The high costs of production are reflected in the price for antiretroviral drugs. The costs of medical care for a HIV-patient add up to $2,100 per month or $618,000 in a lifetime ( Hence only HIV-carriers in developed countries can afford the money for an adequate treatment of the disease. Only every tenth AIDS-infected person lives in a developed country whereas 90% of all HIV-carriers live in developing countries. Sub-Saharan Africa is the worst effected region with about 25 million people living with the virus. Only 810,100 HIV-carriers in Sub-Saharan Africa receive antiretroviral therapy ( [1]). Thus the mortality after an HIV-infection in developing countries is much higher than it is in developed countries. One of the few practical solutions to approach the problem is to lower the costs of production as it would bring down the price of the product to make it affordable for people living in developing countries. The use of genetically modified plants as production platforms could help to bring down the costs of production as it is an inexpensive process. The first plant-derived pharmaceutical enzyme was human serum albumin that was produced in transgenic tobacco (Sijmons et al, 1990). Genetic modification of plants has been further developed in recent years so it is nowadays possible to transform a wide range of plants and the first technical proteins produced in genetically modified plants are sold. Experiments regarding the production of antiretroviral drugs focus on the use of maize, wheat, tobacco, barley and rice plants as production platforms because they have the following advantages. All of these plants are easily transformable and produce reasonable amounts of the product in a short period. Moreover most of these plants can be grown in developing regions such as South and South East Asia or South America, as their climatic environmental requirements are similar to the climate in these regions. Maize plants can store recombinant proteins in their seeds if the gene encoding for the product is connected with a promoter for storage proteins. Stored in the seed the protein is stable for many years. To use tobacco for the production of drugs is advantageous because tobacco can produce reasonable amounts of the product which is located in the leaf. In addition tobacco is a non-food crop therefore there is no danger for unintentional uptake of antiretroviral drugs produced in the plant. Compared to chemical synthesis of antiretroviral drugs the production in plants is a low-tech process. Therefore the process could not only be conducted in pharmaceutical plants in developed countries but also in developing countries. This would guarantee that the drugs come on the market where they are required and it would save transportation charges. Another advantage of producing antiretroviral drugs in plants is the fact that reasonable amounts can be produced. The fact that only 2% of all HIV-carriers receive antiretroviral drugs shows that the current amount of drugs produced by chemical synthesis is not enough to meet the demand ( Examples for antiretroviral drugs producible in plants are the HIV neutralizing antibodies 2F5 and 2G12. These antibodies bind to the glycoprotein gp41 which is essential for the fusion of the virus membrane and the host cell. If an antibody is bound to the gp41 the infection is interrupted hence 2F5 and 2G12 belong to the group of fusion inhibitors. In 1989 the first antibodies have been produced in tobacco plants (Hiatt et al, 1989). Antibodies are complex proteins that consist of four subunits. Producing antibodies in plants is possible as plants are able to perform complex protein folding, whereas most bacteria are only able to synthesize simple proteins. Not only the correct folding but also post-translational modifications such as N- glycolisation are performed in plants. This argues for the use of plants as production platforms instead of bacteria. Another advantage of producing drugs in plants is the fact that phytopathogenic viruses are no danger for human beings. Therefore there is no risk of an infection whereas bacteria and animal cells used in a production process can carry viruses that infect humans. Although using plants to produce antiretroviral drugs offers lots of advantages, there are safety issues that have to be considered. Of course the risk of escape of transgenic pollen has to be minimized to ensure that antiretroviral drugs do not enter the food chain. This can be accomplished in different ways. For example the production can be carried out in greenhouses. According to estimates the required amount of hepatitis B virus could be produced on 250 acres of greenhouse space (Ma et al, 2005). This demonstrates that it is possible to produce reasonable amounts in the controlled environment of greenhouses. Another way to control genetically modified plants is to mark them. The gene for the marker has to be linked to the gene for the antiretroviral drug. Examples for markers that can report transgenic activity are fluorescent proteins such as the green fluorescent protein or DsRed that are derived from When it comes to the production of antiretroviral drugs in plants these safety aspects have to be taken into consideration by manufacturing companies to avoid unintentional uptake of drugs and the escape of transgenic pollen, plants or seeds that could affect the gene pool of wild species. Using plants as production platforms to increase the amount of antiretroviral drugs could help to meet the demand for antiretroviral drugs of 40 million AIDS-infected people worldwide. HIV-carriers in developing countries could benefit from the low costs of production that affect the price for medication. The effect of the drugs is shown by the decrease of mortality after an infection in developed countries. However the mortality after an infection is stagnating in developing countries. UNAIDS, the program of the United Nations, estimates that if the number of people having access to antiretroviral drugs does not increase there will be 6.5 million dying of the disease in 2030, compared to 2.8 million in 2002 ( References:, retrieved/accessed December 28, 2006. Hiatt, A., Cafferkey, R. and Bowdish, K. (1989): Production of antibodies in transgenic plants. Nature, 342, 76−78. [1], [2]:, retrieved/accessed December 28, 2006. Ma, J., Barros, E., Bock, R., Christou, P., Dale, P., Dix, P., Fischer, R., Irwin, J., Mahoney, R., Pezzotti, M., Schillberg, S., Sparrow, P., Stoger, E., Twyman, R. (2005): Molecular farming for new drugs and vaccines. EMBO reports, 6, 594., retrieved/accessed December 11, 2006. Sijmons, P., Dekker, B., Schrammeijer, B., Verwoerd, T., van den Elzen, P., Hoekema, A. (1990): Production of correctly processed human serum albumin in transgenic plants. Bio/Technology, 8, 217 – 221., retrieved/accessed December 28, 2006., retrieved/accessed December 28, 2006.



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