Spasers seek and destroy metastasized cancer cells

The nanolaser – dubbed spaser - can be used as an optical probe. When released into the body (possibly through an injection or drinking a solution), it can find circulating tumour cells (CTCs), adhere to them and destroy these cells by breaking them apart to prevent cancer metastases. The spaser is claimed to work by absorbing laser light, which causes it to heat up, causing shock waves in the cell and destroying the cell membrane. The findings are published in Nature Communications.

The spaser - surface plasmon amplification by stimulated emission of radiation - effect is caused by a 20nm nanoparticle with folic acid attached to its surface that allows selective molecular targeting of cancer cells.

According to the research team – from Georgia State University, the University of Arkansas for Medical Sciences, the University of Arkansas at Little Rock and the Siberian Branch of the Russian Academy of Science - the folate receptor is commonly overexpressed on the surface of most human cancer cells and is weakly expressed in normal cells.

"There is no other method to reliably detect and destroy CTCs," said Dr Mark Stockman, director of the Center for Nano-Optics and professor of physics at Georgia State. "This is the first. This biocompatible spaser can go after these cells and destroy them without killing or damaging healthy cells. Any other chemistry would damage and likely kill healthy cells. Our findings could play a pivotal role in providing a better, life-saving treatment option for cancer patients."

Metastatic cancer occurs when cancer spreads to distant parts of the body, often to the bone, liver, lungs and brain. Once cancer spreads, it can be difficult to control, and most metastatic cancer can't be cured with current treatments, according to the US National Institute of Health's National Cancer Institute. One of the most dangerous ways metastasising occurs is through the CTCs, which this study aims to detect and destroy using spasers.

The spasers used in this study measure 22nm, setting the record for the smallest nanolasers. Most results were obtained with a gold, spherical nanoparticle surrounded by a silica shell and covered with a uranine tracer dye.

The researchers studied the spaser's capabilities in vitro in human breast cancer cells with high folate receptor expression and endothelial cells with low folate receptor expression, plus mouse cells in vivo.

They found cells with spasers demonstrated high image contrasts with one or many individual "hot spots" at different laser energies above the spasing threshold. The presence of spasers was confirmed with several optical and electron microscopy techniques, which revealed an initial accumulation of individual spasers on the cell membrane followed by their entrance into the cell cytoplasm.

The study also found low toxicity of the spasers for human cells. At the same time, the spasers subjected to laser irradiation selectively killed the tumour cells without damaging the healthy ones.

Based on the study's results, spaser-based therapeutic applications with high-contrast imaging is a promising field. The data suggest spasers have high potential as therapeutic and diagnostic agents that integrate optical diagnosis and photothermal-based cell killing, using a few laser pulses to kill cancer cells.