Nanospheres leave cancer no place to hide
21 June 2007
NewScientist.com news service
Celeste Biever
GOLD-coated glass "nanoshells" can reveal the location of tumours and then destroy them minutes later in a burst of heat.

Using these particles to detect and destroy tumours could speed up cancer treatment and reduce the use of potentially toxic drugs. It could also make treatment cheaper, says Andre Gobin of Rice University in Houston, Texas, who helped to create the particles.

In 2003 Gobin's supervisor Jennifer West showed that gold-coated silica nanospheres could destroy tumours in mice, while leaving normal tissue intact. The blood vessels surrounding tumours are leakier than those in healthy tissue, so spheres injected into the bloodstream tend to accumulate at tumour sites. Illuminating the tumour with a near-infrared laser then excites a "sea" of loose electrons around the gold atoms via a process called plasmon resonance. This creates heat, killing all the nearby cells.

However, before this can happen doctors first have to make sure they find all the tumour sites, which requires an MRI or CT scan. This extra stage can mean multiple hospital visits and more drugs for the patient.

Now the team has shown how to tweak the size of the nanoshells so that they also scatter some of the radiation. That means any cancer sites will "light up" under low-intensity infrared, so they can then be zapped with the laser. "We can use one single particle to accomplish two tasks and neither feature is diminished greatly," says Gobin.

To achieve this, the team had to carry out a delicate balancing act. Smaller spheres convert more radiation to heat, which makes them better at destroying tumours, but larger ones scatter more radiation, which is vital for the imaging stage. Previously, the spheres were 120 nanometres in diameter, which meant they only scattered 15 per cent of the light shone on them, and converted the rest to heat. West's team increased their size to 140 nanometres, causing them to convert 67 per cent of the light to heat, and to scatter the remaining 33 per cent.

The team injected the new particles into mice with colon carcinoma tumours and used a technique called optical coherence tomography to test their ability to act as an imaging agent. This involves shining low-power near-infrared light onto the tissue and then measuring where the scattered light bounces back. They found that the nanoparticles caused tumour tissue to light up 56 per cent more strongly than healthy tissue.

The team then applied a higher-power infrared laser to each tumour site for 3 minutes to heat the tissue. After a few weeks, they found the tumours had been almost completely destroyed. Eighty per cent of the mice treated survived for more than seven weeks, while all the control mice, who did not receive the nanoshells, died after three weeks.

Since optical coherence tomography only penetrates up to 2 millimetres, the imaging step will only be useful for locating tumours near the surface, such as cervical, mouth and skin cancers, says Gobin. However, the team plans to modify the nanoshells so that they work with more deeply penetrating radiation, such as X-rays. Houston-based Nanospectra Biosciences, which West co-founded, will begin trials of the spheres in humans in the next two months.

From issue 2609 of New Scientist magazine, 21 June 2007, page 28

Printed on Thu Jun 21 21:16:15 BST 2007