Cancer-Detecting Nanoparticles Developed for MR Imaging

Now, a team of researchers has created the smallest magnetic nanoparticles to date that can be utilized on such seek-and-find missions. With a thinner coating, the particles also emit a stronger signal for the MRI to detect.

MRI scans use pulses of magnetic waves, and gauge the return signals to identify different types of tissue in the body, such as telling bone from muscle, or fluids from solids. Scientists have found that magnetic nanoparticles can be especially helpful in locating cancerous cell clusters during MRI scans. Like tiny guided missiles, the nanoparticles seek out tumor cells and attach themselves to them. Once the nanoparticles attach themselves to these cancer cells, the particles operate like radio transmitters, greatly aiding the MRI's detection capability.

The study's findings have been published online in May 26, 2008, in the Journal of the American Chemical Society. The research was performed by Brown University chemist Dr. Shouheng Sun and a team of researchers from Brown University (Providence, RI, USA). The graduates students, included Jin Xie, Chenjie Xu, and Sheng Peng, collaborated on the research, along with Professor Xiaoyuan Chen and his associates from Stanford University (Stanford, CA, USA).

The team created peptide-coated iron oxide nanoparticles--particles billionths of a meter in size. The researchers injected the particles into mice and assessed their ability to locate a brain tumor cell called U87MG. Dr. Sun and his coworkers concentrated specifically on the nanoparticle's size and the thickness of the peptide coating, which ensures the nanoparticle attaches to the tumor cell.

Size is important because the trick is to create a nanoparticle that is small enough to navigate through the bloodstream and reach the diseased area. Bigger particles tend to stack up, creating the circulatory system's version of a traffic jam. Dr. Sun's team developed a nanoparticle that is about 8.4 nm in overall diameter--some six times smaller than the size of particles currently used in medicinal procedures.

"We wanted to make [the nanoparticle] very small, so the body's immune system won't recognize it,” Dr. Sun explained. "That way, you let more particles interact with and attach to the tumor cell.”

Nanoparticles are important in MRI detection because they enhance what scientists refer to as the contrast between the background, such as water molecules in the body, and a solid mass, such as a tumor. The coating, while integral to the nanoparticles' attachment to the tumor cell, also is crucial to establishing the signal-to-noise ratio that an MRI uses: the thinner the coating, the stronger the emitted signal and vice versa. Dr. Sun's team outfitted their nanoparticles with a two-nm thick peptide coating--10 times thinner than the coating available in popular MRI contrast agents such as Feridex. (Feridex's generic name is ferumoxides, and is manufactured by Amag Pharmaceuticals [Cambridge, MA, USA] and is sold in the United States by Bayer Healthcare Pharmaceuticals [Wayne, NJ, USA].)

Dr. Sun's nanoparticles are similar to having a 50,000-watt radio transmitter versus a 150-watt station; it is easier for the MRI to "hear” the stronger signal and to hone in on the signal's source. Another key feature of the team's research is discovering that the RGD peptide coating binds nearly seamlessly to the U87MG tumor cell.

The team plans to test the particle's ability to bind with other tumor cells in further animal studies.


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