Scientists at MIT have developed remote-controlled nano particles that, with the push of a button, can deliver drugs directly to a tumour. The same research director has also found a way to build tiny human "livers" just 500 micrometres across. This work should lead to more reliable toxicity testing for new drugs.

According to Geoff von Maltzahn, post-doctoral researcher at the Harvard-MIT division of health sciences & technology (HST), the nano particles are first persuaded to clump together, which makes it easier to track their progress through a patient's body. Then, drug molecules are attached to the clumps of nanoparticles with DNA tethers and the whole lot is injected into the patient.

The nanoparticles are then tracked with an MRI scanner (hence the clumping). When they get to their target they are pulsed with an electromagnetic field at between 350-400kHz. This is harmless to the human body, but melts the tether and releases the drugs exactly where they are needed.

The breakthrough rests on a property of the nanoparticles: superparamagnetism. This characteristic causes them to give off heat when they are exposed to a magnetic field. This heat breaks the connection with the DNA tether, and allows the system to deliver the drugs.

Using DNA as the tether has another advantage: it makes it possible to choose the EM frequency that will break the bond, since longer or differently arranged strands will have different melting points. This means one clump of nanoparticles can carry multiple doses of drugs to several sites. If each drug has differently tuned DNA tethers, doctors can use a different EM frequency to deliver each dose.

The work is reported in the 15 November online issue of Advanced Materials. Lead author Dr Sangeeta Bhatia, an associate professor in the HST, says it could lead to the improved diagnosis and targeted treatment of cancer.

Bhatia and her researchers have also developed miniature livers which could pave the way for safer and more reliable drug toxicity testing. Much of current testing is done on liver cells from rats, which don't always respond the way human livers do, or on dying human cells, which might only survive for a few days in a lab.

HST postdoctoral associate Salman Khetani describes the work in a paper published in the 18 November online issue of Nature Biotechnology. Khetani used micropatterning technology (as with building a chip) to arrange the liver cells into colonies just 500 micrometres across.

The tiny clusters of cells behave much like real livers - they secrete the blood protein albumin, synthesise urea, and produce enzymes that break down drugs and toxins. The mini livers can also survive for up to six weeks, the researchers report (http://www.eurekalert.org/pub_releases/2007-11/miot-ml111507.php). ®

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