Microbot Drug Delivery

Which model organism?

Mark Fang, Stanford iGEM

Microbots: OverviewMicrobot drug delivery involves attaching drugs to the exterior of microscopic biological chasses, such as bacteria and viruses, so that when the chasses are phagocytosed by their target cells they bring inside with them the drugs.

Choosing the chassis is thus an important design parameter, since the chassis will be responsible for which cells are targetted and how it will travel to those cells.

In order to get the chassis to localize to the desired targets, we will need to make several considerations:

Selecting a chassisThe chassis that will transport the drug

should be: Highly specific Immune response Genetically well characterized Pathogenically well characterized

Bactofection vs. VirofectionTwo broad chassis

classes: Bacteria Viruses

SEM micrograph of Escherichia coliVesicular Somatitis Virus

Highly SpecificAs with all drug delivery methods, the more

specific, the fewerthe side effects.

The chassis can be engineered to be specific: Quorum sensing Hypoxia Inducible controlThe chassis can also be naturally specific: Listeria monocytogenes Vesicular somatitis virus rp34a

Immune responseThe patient’s immune response can be a hindrance ora mechanism for inducing localization of the chassis.

Macrophage engulfing bacteria. Thechassis will need to avoid beingengulfed by immune cells to preventincidentally compromising the immunesystem.

Genetically/structurally well characterizedBacteria: many strains have entire

genome sequenced. Ex. E. coli has been used extensively in genetic engineering.

Virus: more difficult to engineer. Virus may also be too small – for example, lambda phages can only hold genomes between 75 and 105% the size of the normal genome.

Pathogenically well characterizedNon-pathogenic:

The virulence of certain bacteria species can be attenuated through knock out mutants.

The rate and extent of bacterial expansion can also be controlled.

Advantages vs. DisadvantagesViruses: More difficult to engineer May be too small Can cause undesirable mutations during

integration of viral genome into host genome Can be naturally specificBacteria: Easier to manipulate, virulence shown to be

pliable, some are also naturally localized in body

Choosing a chassisListeria monocytogenes – with the view of

targeted drug delivery to cancer cells: Has been show to localize to tumor cells

and metastases Has been engineered to exhibit

attenuated virulence Expansion can be controlled Riboswitches naturally extant in certain

Listeria species

Testing the chassis HeLa cancer cells – can conduct in vitro test

for general invasion of human cancer cells Human glioma cells - to establish possibility

for chassis to invade brain cancer cells. Further tests in animals may show whether the chassis is able to penetrate the blood brain barrier.

For a listing of 60 human cancer cell lines: http://dtp.nci.nih.gov/docs/misc/common_files/cell_list.html