by ecancer reporter Clare Sansom

Targeted monoclonal antibodies are a relatively new type of cancer treatment that have recently had significant success, with Herceptin^® for HER2 receptor-positive breast cancer still probably the best known example of the class. 

However, the development of these drugs does still present some significant technical challenges, among them the problem of how to deliver the antibody selectively to the tumour cells. 

Recent developments in the emerging discipline of nanotechnology may help shed light on and eventually provide solutions to this problem.

A technique known as “DNA origami” (named after the Japanese art of paper folding) has been developed in which single-stranded DNA can be folded into customised shapes and assemblies through interaction with hundreds of oligonucleotide strands. 

George Church and his co-workers of Harvard Medical School, Boston, MA, USA are using this technique to develop DNA-based nano-robots or “nanobots” that can deliver a molecular “target” such as an antibody or a signalling molecule specifically to the surfaces of cells of a particular type.

They first used a computer-based design tool for DNA origami known as cadnano to build a hexagonal-shaped barrel structure from an over 7000-base DNA scaffold derived from a phage. 

They then designed a “lock” for this DNA box using DNA aptamers, which are oligonucleotides that bind to specific target molecules, usually proteins. An aptamer and its complement were attached to the sides of the barrel so that a conformational change of the whole device is triggered when these oligonucleotides recognise and bind to their targets.

“Cargo” or “payload” molecules including antibody fragments were then modified by binding to oligonucleotide linkers and loaded inside the barrels, with at least two payload molecules per nanobot.  

The function of the nanobots was tested by choosing a fluorescently labelled fragment of an anti-HLA antibody as the cargo, mixing the loaded nanobots with a variety of cell types including some expressing the HLA molecules targeted, and analysing the observed fluorescence using flow cytometry.

 An increase in fluorescence was observed when the nanobots were exposed to cells expressing the targeted molecules, indicating that they had recognised their targets and released the payload. 

The nanobots were designed with two aptamer lock sites, making two basic types of lock and key mechanism possible. If both lock sites are loaded with the same aptamer the box will open in response to only one type of key. 

However, if they are loaded with different aptamers, both keys will need to be recognised and both locks opened simultaneously before the box is opened. 

This second type of mechanism is therefore equivalent to a logical AND gate in which both cell surface antigens targeted must be recognised to trigger a response.

 Church and his co-workers then designed six different nanobots using pairs of aptamers taken from a set of three, all well-characterised and known to recognise cell surface antigens associated with cancer. 

They used these to probe cancer cell lines expressing different combinations of the antigens, and found that each cell line triggered release of cargo from a different combination of nanobots. 

The robots were also found to be able to select a single cell population from a mixture of several cell types. In one example that mimics physiological conditions, nanobots bearing two identical aptamers were able to discriminate between NK-type leukaemia cells and healthy leukocytes, binding only to the cancer cells. 

Taken together, these results show that this technology can be used to discriminate between cell types and deliver a “molecular payload” to a single type of cell with high precision. This may have important implications for improving the delivery of monoclonal antibodies and other drugs to tumour cells.

Reference

Douglas, S.M., Bachelet, I. and Church, G.M. (2012). A Logic-Gated Nanorobot for Targeted Transport of Molecular Payloads. Science 335, 831-834. doi:  10.1126/science.1214081