Nanoparticles can "home in" on particular targets in the body. Although scientists have previously designed nanoparticles to target tumours, these techniques have proved to be relatively inefficient. Now, Ruoslahti and colleagues have developed a method for amplifying this homing ability by designing a multifunctional nanoparticle that binds to a protein structure that is only found in tumours and their associated blood vessels.

The California team began by coating iron oxide nanoparticles with a peptide that binds to clotted plasma proteins, which are present in tumours but not in normal tissue. The researchers then injected the nanoparticles into mice with breast cancer tumours. They found that the peptide binds the nanoparticles to the tumour blood vessels and that the high concentration of the particles causes blood clotting in the vessels. This creates more binding sites for the nanoparticles, thus amplifying the homing of the particles to these sites in a way that is very similar to how platelets cause blood clotting.

"Our nanoparticles not only seek out tumours, they amplify their own homing to the tumours," Ruoslahti told "As a result, more particles find the tumour target than would be the case without this self-amplification."

The nanoparticles only cause clotting in tumour vessels, not surrounding tissue, so the technique could be used as a way to cut off a tumour's blood supply, either partially or fully. In the Burnham Institute experiments, as much as 20% of blood vessels in the tumour were blocked. While this is not enough to reduce the rate of tumour growth, it's a promising result for the future, says the team.

"Having identified the principle of self-amplification, we are now optimising the process, hoping to obtain a more complete shutdown of blood flow into the tumour, which would strangle it," added Ruoslahti. "We are also in the process of adding a drug-delivery function to the particles. These two approaches are synergistic: the more particles we bring into the tumour, the greater the obstruction of the blood flow and the more drug that is delivered into the tumour."

The researchers reported their work in Proc. Natl. Acad. Sci..