In the 1970’s the first genetically engineering bacteria were used to produce human insulin on a commercial scale. Since then, several other protein-synthesising DNA fragments have been inserted into bacterial and yeast cells to enable large scale in vitro production of drugs, antibodies and other therapeutics. Nevertheless, barriers to optimisation still exist: most biosynthetic pathways are complex and not easily transferable between species, and the build up of metabolic by-products can often prove toxic to the new host.
Synthetic biology uses the disciplines of biology and engineering to understand and synthesise standardized biological parts that can be reconstructed to form biological devices, systems, or even whole chromosomes. This technology has the potential not only to alter existing metabolic pathways, but also to create new ones, and to design tailor-made cells that churn out drugs or antibodies with maximum efficiency.
In 2012, the first anti-malarial drug produced using synthetic biology will be available on the global market. The story of artemisinic acid, a precursor of the anti-malarial drug artemisinin, shows how synthetic biology strategies can be successfully translated from the laboratory to an industrial scale.
Every year nearly one million people, mostly children, die of malaria. Although effective anti-malarial therapies are available, their average cost (approximately £6.50) puts it beyond the reach of the most vulnerable populations. Artemisinin, a key ingredient for the therapy, can be readily extracted from its plant source, but yields are low, and the cost of the chemically synthesised derivatives is several times higher than the original plant material.
In 2003, Jay Keasling and his group at UC Berkeley constructed a new metabolic pathway for an artemisinin precursor by combining 10 genes from three different organisms (bacteria, yeast and plant) into a single bacterial chassis. These genes produced enzymes, that acted sequentially to turn a compound called acetyl coenzyme A, found in abundance in microbial cells, into artemisinic acid. Artemisinic acid can be readily purified from the cells and converted into artemisinin or one of its synthetic derivatives, using a relatively inexpensive process.
Following this initial success, Keasling set up Amyris Biotechnologies, to optimise the scale-up to an industrial process, and bring down the cost of synthesis. In 2004, Amyris Biotechnologies and OneWorld Health, a nonprofit drugmaker, received a grant of $42.6 million (approximately £27 million in today’s currency) from the Bill and Melinda Gates foundation, to produce artemisinin for global distribution, using synthetic biology tools. When this semisynthetic artemisinin is launched in 2012, it will represent an affordable, non-seasonal source of artemisinin – the cost of a course of anti-malarial treatment now costing less that 60p.
Synthetic biological circuits have also helped identify new anti-tuberculosis compounds. Tuberculosis (TB), a bacterial infection, affects over 9 million people each year, and if left untreated can prove fatal. One of the major problems with tuberculosis in the development of drug-resistance in TB bacteria.
Ethionamide is a drug which is currently used as the last-line-of-defence against tuberculosis. Once inside infected cells, ethanomide is converted by an enzyme EthA into a more lethal form that kills cells. However some bacteria are capable of producing a protein, EthR, which can block the production of EthA.
In 2008, a study published by Wilfried Weber et al. showed how a synthetic gene circuit could help screen for drugs able to block the activity of EthR protein. This gene circuit, linked to an enzyme that would change colour in response to an inhibitor, was able to successfully identify a non-toxic substance called phenylethyl butyrate, that could be administered as part of anti-tuberculosis therapy. In the future, such standardised genetic circuits may be able to screen for anti-cancer drugs.
Advances in gene engineering also makes it possible to use bacteria and viruses as therapies themselves. Microorganisms could be tailored to act as nanomachines, capable of selectively targeting pathogenic organisms or cancer cells, and disrupting pathogenic pathways. For example, a study published by Timothy Lu & James Collins shows how is it possible to engineer viruses to act as adjuncts to antibiotic therapy. Antibiotics partially damage the cell wall of the bacteria, then the engineered virus inserts a gene for an enzyme into the damaged cells, preventing them from initiating self-repair mechanisms. This boosts the killing power of the antibiotic by around 30,000 times, and significantly reduces the number of drug-resistant bacteria that survive a course of treatment.
Here is video from Timothy Lu, talking about bioengineered phages.
Synthetic biology is also coming to its own in cancer therapy. The biotechnology firm Intrexon is collaborating with the pharmaceutical company Ziopharm to develop DNA-based anti-cancer drugs. Their DNA delivers a gene for Interleukin-2 (IL-12), a potent anti-cancer agent, to the site of the tumour. The gene for IL-12 is linked to a genetic toggle switch that is only activated by an orally administered activator molecule. By controlling the amount of activator administered, it is possible to control the amount of IL-12 released into the tumour and minimise damage to healthy tissue. This strategy is currently being tested on skin cancer in phase I clinical trials.
The advances highlighted here may one day make it possible to develop off-the-shelf parts to create new synthetic pathways for drugs or custom-made “live therapeutics” at will. At the same time, these developments throw up some challenging ethical questions. Should genes for medically-important drugs be patented or freely available to all? Will patenting lead to an increased dependence of economically-deprived countries on their rich cousins? Even in the case of success stories, one needs to be aware of other implications – farmers in South East Asia depend on the artemisininc crop for their livelihoods, and flooding of the market with a cheap, synthetically produced drug could drastically alter their economy.
And, finally, what happens if only those with means can afford to purchase the most expensive, tailor-made drugs? Will the pharmaceutical industry be driven by demand and profit, or by need? As we move rapidly ahead with developing new therapeutics, we need to be careful about not short-changing those already at the economic margins of society.