Mini-beacon design a major development for smart drug delivery

A nanoscopic beacon used by Melbourne researchers will help to enhance the design of smart gene and drug delivery systems.

A team from the University of Melbourne’s Department of Chemical and Biomolecular Engineering has used a molecular beacon made from single DNA strands to measure how easily DNA (e.g. genes) can pass through the wall of drug delivery particles.

Federation Fellow Professor Frank Caruso, who heads the Centre for Nanoscience and Nanotechnology, says, “The past number of years has seen major advances in the design of ‘molecular vehicles’ – particles that can be filled with a medicine or new genes. The vehicles then ferry their contents to the site in the body where they are needed.”

“One of the major roadblocks that we have encountered in designing these molecular transport systems is how to get the vehicle contents out of their container once they reach the site where they are needed.”

In order to achieve this effectively, the researchers need to know how big the pores in the vehicle’s membranes are and how easily the contents can pass through them. This has proved quite difficult.

Dr Angus Johnston who works with Professor Caruso says, “Scientists designing these drug-delivery vehicles need to be able to measure the very small number of molecules which pass through the membrane. Normally, we could label the molecules, so we can see them as they pass through. The problem with this is that adding a label alters the size, so the ability to pass through the pore will change when the label is removed.”

Dr Johnston and Professor Caruso have developed a clever technique that overcomes this problem which allows scientists to rapidly and accurately determine the permeability of DNA through films.

The beacons the researchers used are single DNA strands which have a light-emitting molecule (a fluorophore) at one end and a quencher at the other. A fluorophore is simply a molecule that emits light and a quencher is a molecule that stops the fluorophore from emitting light.

The DNA strand self assembles so that the two end segments pair up, forming a loop in the centre – much like the shape of a round-bottomed flask (see illustration). This is the closed molecular beacon.

When the beacon is closed the fluorophore on one end of the DNA strand is close to the “quencher” on the other end, which stops the fluorophore from giving off light.

To determine the permeability of the capsule, the molecular beacons are placed inside the delivery vehicle. If DNA passes through the capsule wall, the beacon opens and the fluorophore emits light. So when DNA passes through the capsule, the beacon is switched ‘on’. If no DNA passes through the capsule, the beacon remains switched off.

The researchers used this technique with different length strands of DNA and were able to use the beacons to successfully determine whether or not the strands were able to pass through the membrane.

Professor Caruso says, “We hope to now use the technique in the design of intelligent drug-delivery systems which can transport medicine to target locations and release the contents in a controlled way.”

The research was recently published in the Journal of the American Chemical Society and highlighted in the July 15 Issue of the journal Science. This research was funded by the Australian Research Council.