Empty virus-like particles (eVLPs) as bio-compatible targeted drug-delivery vehicles

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A major challenge in pharmacology is to devise methods whereby drugs can be delivered specifically to target tissues. This is a particular issue in the case of anti-cancer drugs which usually discriminate between cancerous and normal cells by the fact that the cancer cells are dividing more rapidly. However anti-cancer drugs are toxic to all cells and thus often have severe side-effects. To avoid this, it would clearly be desirable to target the drug molecule specifically to the cancerous tissue. A potential means of achieving this would be to package or encapsulate the drug molecules inside a particle which is designed to bind solely to the cancerous tissue. Such encapsulation would have the additional advantage of protecting the drug from breakdown in blood plasma. For this to become a reality it will be necessary to develop particles which can be modified on their outer surface to achieve the desired targeting and which can contain drug molecules. The particles need to be small enough to be able to move in the bloodstream, are not toxic and to be able to enter cells. One type of particle which has all these characteristics is the plant virus, cowpea mosaic virus (CPMV), which can be produced in large quantities by infecting plants. Previous research has shown that it is possible to 'stick' molecules on the surface of CPMV particles which enable them to be targeted to specific cells. Despite these advantages, CPMV particles have not, to date, been exploited as a drug-delivery vehicle. The reason for this is that particles produced by the infection of plants are already filled with the virus' own genetic material. This means that there is little or no room to put anything else, such as drug molecules, inside the particles. Even if such molecules could somehow be squeezed in, there would still be concerns about administering particles containing viral genetic material, even though it is from a plant virus that cannot infect animals. This project addresses the issue of the genetic material within the virus particles by exploiting the recent discovery at the John Innes Centre of a method of producing large quantities of pure empty (lacking genetic material) virus-like particles (eVLPs) of CPMV in plants. The method involves simultaneously expressing genes coding for a precursor of the viral coat proteins and the enzyme used to process it using a recently developed highly efficient plant transient expression system. Applying this approach we will produce particles which are modified so that they will specifically bind to proteins expressed on the surface of cancerous, but not normal, cells. This modification will be done either chemically or by making genetic fusions to the virus coat protein. We will investigate the best way of loading the targeted particles with the anti-cancer drug, gemcitabine. To do this we will make use of pores in the virus particles which allow small molecules to enter under certain conditions. The ability of the targeted particles to deliver the drug to cancer cells will be investigated by testing their ability to bind to and kill cancers in culture. This will be the first important step in potentially developing brand new therapeutic agents to tackle human cancer.