Of particular interest are materials that exhibit superparamagnetic behaviour when produced in the nanoscale regime. Superparamagnetic nanoparticles can magnetize strongly under the influence of an applied magnetic field; once that applied field is removed, however, the magnetic field of the materials flips back to zero. This kind of on/off magnetic switching opens up all sorts of possibilities.

A case in point is a pioneering approach to gene therapy, details of which are published this week in the Journal of the Federation of American Societies for Experimental Biology (21 2510). The proof-of-principle study involved the binding of DNA segments to iron-oxide nanoparticles, with particle and payload subsequently directed by magnetic field into arterial muscle cells (in cell cultures).

The nanoparticles carried a surface coating of DNA bound to an organic compound called polyethylenimine (PEI). It's this PEI which protected the DNA from being broken down by enzymes called endonucleases in the cell cultures (the same enzymes also occur normally in the bloodstream). The DNA was in the form of a plasmid, a circular molecule that, in this instance, carried a gene coded for a growth-inhibiting protein called adiponectin.

By applying a magnetic field, the research team was able to steer the particles into arterial smooth muscle cells. Inside each cell, the DNA separated from the particle, entered the cell nucleus, and produced enough adiponectin to significantly reduce the proliferation of new cells.

Why is this interesting? In a practical application, such nanoparticles could be magnetically directed into stents (tiny, expandable metal scaffolds inserted into a patient's partially blocked vessels to improve blood flow). Many stents eventually fail as cells grow on their surfaces and create new obstructions. So delivering anti-growth genes to stents looks like a neat way to keep the blood flowing freely.

Significantly, this gene-delivery technique might also be transferable. Study leader Robert Levy, chair of paediatric cardiology at The Children's Hospital of Philadelphia, PA, reckons the approach might ultimately find broader application - delivering gene therapy to tumours or carrying drugs instead of, or in addition to, genes.

When the drugs don't work

Other researchers are evaluating the potential of nanoparticle-assisted drug delivery. This month, for example, a German collaboration will publish results suggesting that drugs mixed with iron-oxide magnetic nanoparticles could be delivered to the lungs up to eight times more efficiently than if inhaled normally (see 'Nanomagnetosols' help drugs hit home on medicalphysicsweb).

The benefits are clear: conventional treatments for many pulmonary diseases - such as asthma, cystic fibrosis and lung cancer - require patients to gasp on an inhaler that emits particulate drugs into the windpipe. The effectiveness of these inhalers is not great, however, with typically only 4% of the drug making it through the windpipe.

A more effective approach, according to Carsten Rudolph at Ludwig-Maximilians-University in Munich and co-workers from elsewhere in Germany, is to mix the drugs with so-called "nanomagnetosols" (aerosol droplets comprising superparamagnetic iron-oxide nanoparticles dispersed in water). These nanomagnetosols can then be guided directly to problem areas using a magnetic field. The idea is not new, but the German group has shown for the first time that it can be performed in a real organism – in this case, a mouse.

Early-stage studies like these highlight the immense promise of magnetic nanoparticles in a range of therapeutic contexts. How those nanoparticles react with different types of living cell, hormone or immune factor is clearly fundamental to the long-term clinical and commercial viability of such work. Equally significant is how those nanoparticles react following biodegradation within the body - specifically, whether the particles (or their by-products) are subject to bioaccumulation within cells or organs, inducing intracellular changes or inflammatory responses.

Therein lies the challenge for the development community.