Sangeeta Bhatia of the University of California at San Diego and colleagues previously developed multifunctional nanoparticles that could be injected into the bloodstream. These nanoparticles home in on tumours, where they clump together. The clumped particles can then be observed using magnetic resonance imaging.
Now, the team has found that exposing these nanoparticles to low frequency electromagnetic fields of between 350 and 400 kHz causes the particles to give off heat and release active molecules, such as therapeutic drugs, that have been previously attached to the particles. The drugs are attached to the particles using strands of DNA – a heat sensitive material that breaks up in the presence of the magnetic field.
Moreover, the melting point of the DNA "tether" is tuneable – longer and differently coded strands require different amounts of heat to break. This allows a single particle to simultaneously carry different types of drug that can be released at different times or in various combinations by applying different frequencies of the electromagnetic field.
Bhatia and colleagues tested their technique by implanting a tumour-like gel saturated with nanoparticles in a mouse. They placed the animal into a cup-shaped electrical coil and applied the electromagnetic field. Without the field, the DNA strands remained attached to the nanoparticles, but with an applied field the tethers broke and released the attached drugs to the surrounding tissue.
The team, which includes researchers from the Massachusetts Institute of Technology, says that the experiment is a proof of principle, showing that nanoparticles can be activated remotely. However, much work still needs to be done before such therapies become available in the clinic.
One remaining challenge is that injected nanoparticles must clump together inside the tumour with a critical mass and the researchers are still working on achieving this. "Our overall goal is to create multifunctional nanoparticles that home to a tumour, accumulate and provide customizable remotely activated drug-delivery right at the site of disease," said Bhatia.
The work was reported in Advanced Materials.
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