Microbot Drug Delivery

Which model organism?

Mark Fang, Stanford iGEM

Microbots: Overview

Microbot 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 chassis

The chassis that will transport the drug should be:

Highly specific Immune response Genetically well characterized Pathogenically well characterized

Bactofection vs. Virofection

Two broad chassis classes:

Bacteria Viruses

SEM micrograph of Escherichia coli

Vesicular Somatitis Virus

Highly Specific

As with all drug delivery methods, the more specific, the fewer

the side effects.The chassis can be engineered to be specific: Quorum sensing Hypoxia Inducible control

The chassis can also be naturally specific: Listeria monocytogenes Vesicular somatitis virus rp34a

Immune response

The 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 characterized

Bacteria: 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 characterized

Non-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. Disadvantages

Viruses: 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 chassis

Listeria 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