In a recent publication, a group of authors headed by Quentin Pankhurst, director of the Davy-Faraday Research Laboratory at the Royal Institution and professor at University College London, take a look at the latest developments in the field (J. Phys. D: Appl. Phys. 42 224001). The article is part of a cluster of review articles examining the uses of magnetic nanoparticles in biomedicine.

One appealing application is targeted hyperthermia, in which implanted magnetic nanoparticles are used to ablate cancerous tissue while sparing nearby normal tissue. The procedure involves dispersing nanoparticles throughout the target, and then applying an alternating magnetic field of sufficient strength and frequency to heat the particles. In areas where the temperature is maintained above the therapeutic threshold of 42 °C for 30 min or more, the cancer is destroyed.

The major advance in this field, say Pankhurst and co-authors, was the start of human clinical trials. In 2007, researchers at Charité Hospital in Berlin, Germany, reported results from the first clinical investigation of thermotherapy using magnetic nanoparticles. The study included 14 brain-cancer patients treated with a combination of fractionated radiotherapy and thermotherapy using aminosilane-coated iron oxide nanoparticles.

The Charité researchers injected the particles (dispersed in water) directly into multiple sites throughout each tumour, and then exposed the target region to an alternating magnetic field. The study demonstrated that this form of thermotherapy could be safely applied to treat brain tumours and that hyperthermic temperatures could be achieved. Clinical outcomes were promising, with the treatment well tolerated by all patients.

The project is ongoing, with a complete evaluation of clinical outcome being undertaken in a phase II study of 65 patients. The researchers are also examining the application of nanoparticle-based thermotherapy for the treatment of prostate cancer, and have set up a company called MagForce Nanotechnologies to commercialize the technology.

An alternative approach to direct injection of nanoparticles, and one that's received much attention of late, is conjugation of the magnetic nanoparticles with monoclonal antibodies for targeted delivery. The article cites studies headed up at the University of California, Davis, in which monoclonal antibodies linked to iron oxide beads were injected into a tail vein in tumour-bearing mice. Exposure to an alternating magnetic field three days later led to a decrease in tumour growth rate compared with controls.

The group reported a mean concentration of around 0.315 mg of bioprobes per gram of tumour. This is around 30 times less than that achieved via direct injection, but was compensated for by applying high magnetic field strengths (between 56 and 104 kA/m). This approach is, however, intrinsically limited by the maximum magnetic field strength and frequency that can be used without stimulation of peripheral nerves or cardiac tissue.

Another challenge is the fact that current magnetic nanoparticles cannot heat anything smaller than a 10 mm diameter tumour, due to a larger surface area-to-volume ratio increasing heat loss into surrounding tissue. To heat a 3 mm cluster of cells, for example, requires particles with a specific loss power several orders of magnitude greater than the best currently reported, even with high target concentrations. As such, work is ongoing on the chemical synthesis of magnetic nanoparticles with improve the intrinsic heating properties.

Broad scope

Targeted hyperthermia is just one of the applications examined in the review article. Pankhurst and co-authors also describe the use of magnetic nanoparticles as carriers for in vivo drug targeting and gene delivery, as well as their application in tissue engineering and regenerative medicine. They also details recent advances in the development of magnetic-nanoparticle-based MRI contrast agents, and highlights the technique of magnetic particle imaging, introduced by Philips Research in Hamburg and now under investigation by other groups.

For the full story, see: Progress in applications of magnetic nanoparticles in biomedicine. "Healthcare biomagnetics is booming, and there's so much to keep up with," said Pankhurst. "Hopefully this paper will help us all in this, writing it certainly helped me."

• This article is part of a cluster of three review articles, published in Journal of Physics D: Applied Physics. The reviews are free to download until November 2010.

To find out more about this field of research, check out our video Q&A with Kevin O'Grady professor of physics at the University of York, UK. O'Grady puts the technology in perspective by explaining how advances in chemistry and a better understanding of the underlying physics of magnetic nanoparticles have led to a boom in clinical applications.