Good as gold

Researchers at the Massachusetts Institute of Technology (

) believe that tiny gold particles could target and destroy tumours in a patient’s body with minimal side effects.

Such particles, known as gold nanorods, would absorb energy from near-infrared light and emit it as heat to destroy the tumours.

The nanorods could diagnose as well as treat tumours, according to MIT graduate student Geoffrey von Maltzahn, who developed the tumour-targeting particles with Sangeeta Bhatia, professor in the Harvard-MIT Health Sciences and Technology (HST) division.

Cancer is expected to affect up to 15 million people by 2020. Currently, most patients are treated with chemotherapy or radiotherapy, which are often effective but can have debilitating side effects because it is difficult to preciscely target tumour tissue.

With chemotherapy treatment, 99 per cent of drugs administered do not typically reach the tumour, said von Maltzahn. In contrast, the gold nanorods can specifically focus heat on tumours.

‘This class of particles provides the most efficient method of specifically depositing energy in tumours,’ he said.

Gold nanoparticles can absorb different frequencies of light, depending on their shape. Rod-shaped particles, such as those used by von Maltzahn and Bhatia, absorb light at near-infrared frequency. Such light heats the rods but passes harmlessly through human tissue.

The team performed experiments on mice to prove their technology. According to their report, tumours in mice that received an intravenous injection of nanorods and near-infrared laser treatment disappeared within 15 days.

Once the nanorods are injected, they disperse uniformly throughout the bloodstream. The MIT team developed a polymer coating for the particles to allow them to survive in the bloodstream longer than any other gold nanoparticles. The half-life is greater than 17 hours.

In designing the particles, the researchers took advantage of the fact that blood vessels located near tumours have tiny pores just large enough for the nanorods to enter. Nanorods accumulate in the tumours and, within three days, the liver and spleen clear any that do not reach the tumour.

During a single exposure to a near-infrared laser, the nanorods heat up to 70^oC - hot enough to kill tumour cells. Additionally, the team reported that heating them to a lower temperature weakens tumour cells enough to enhance the effectiveness of existing chemotherapy treatments, raising the possibility of using the nanorods as a supplement to those treatments.

Von Maltzahn claims that the nanorods could also be used to kill tumour cells left behind after surgery. The nanorods can be 1,000 times more precise than a surgeon's scalpel, he said, so they could potentially remove residual cells that the surgeon cannot get.

The MIT research report states that the nanorods' ability to find a tumour also make them a promising diagnosing tool. After the particles are injected, they can be imaged using a technique known as Raman scattering. Any tissue that lights up, other than the liver or spleen, could harbour an invasive tumour.

The researchers showed in their report that they could enhance the nanorods' imaging abilities by adding molecules that absorb near-infrared light to their surface. As a result of this surface-enhanced Raman scattering, very low concentrations of nanorods - to only a few parts per trillion in water - can be detected.

The MIT team claims that another advantage of the nanorods is that, by coating them with different types of light-scattering molecules, they can be designed to simultaneously gather multiple types of information. Therefore, theoretically, they could be used to determine not only if there is a tumour but whether it is at risk of invading other tissues or if it is a primary or secondary tumour, as well as its origin.

Bhatia and von Maltzahn are looking into commercialising the technology. Before the gold nanorods can be used in humans, they must undergo clinical trials and be approved by the Food and Drug Administration (FDA), which von Maltzahn said will be a multi-year process.

The research was funded by the National Institutes of Health, the Whitaker Foundation and the National Science Foundation in the US. Nanopartz Inc supplied gold nanoparticles, gold nanowires and the precursor gold nanorods used in this work.