Modified nanoparticles that can identify prostate cancer could track a wide variety of diseases.

New gold and magnetic nanoparticles developed by Northwestern University scientists can detect prostate specific antigens (PSA), the leading indicator of prostate cancer, at far lower levels than is possible with existing blood tests.

The new technology could not only save the lives of millions of men but also be adapted to detect or monitor a wide variety of diseases, from HIV to mad cow.

"When a man is diagnosed with prostate cancer, and the cancer is removed, his PSA level drops to zero," said Chad Mirkin, a professor at Northwestern University and a co-author of the paper that appears in the current issue of the Proceedings of the National Academy of Sciences.

"The real number isn't really zero, but it's below what is currently detectable," said Mirkin. "What we are doing is redefining zero."

Doctors currently test for prostate cancer using two methods. The first is a manual examination of the prostate. The second involves removing a small amount of blood and testing it for the presence of PSA, a protein produced by the prostate. Men with prostate cancer typically have a higher level of PSA in their blood.

The old test directly measured the amount of PSA in the blood. The new nanoparticle test will indirectly measure PSA levels.

For this procedure, two types of nanoparticles are added to a man's blood sample. First, spherical gold nanoparticles tipped with antibodies bind to one side of the PSA proteins. Next, magnetic nanoparticles bind to the other side, basically sandwiching the PSA.

A magnet then draws the PSA and nanoparticle sandwich out of the blood. The nanoparticles then separate, which in turn can spur the release thousands of DNA strands.

Since every single PSA particle can result in thousands of DNA pieces, even the tiniest amounts of PSA can be easily detected. Compared with the existing PSA blood tests, this new method is 300 percent more sensitive.

Typically, a patient who undergoes prostate surgery may have to wait up to seven years before he definitively knows whether he is cancer free. The new test would shave years off that wait.

PSA, however, isn't even an ideal marker for early stage prostate cancer, says Shana Kelley, a scientist building nanostructures for disease diagnosis at the University of Toronto. "To get an elevated PSA level that we can detect usually means (a patient) already has a good sized tumor," said Kelley.

"If we have a new tool that provides this level of sensitivity, then maybe we can revamp the PSA test to find cancer sooner."

Diagnosing cancer is just the first step for Mirkin and his colleagues. By changing the antibodies that coat gold and magnetic nanoparticles, this technology can be easily adapted to detect or monitor virtually any other disease, says Mirkin.

"For anyone with a residual disease, this will be quite important," said Mirkin. "The same assay can be retrofitted for HIV, cancers, prior diseases (such as mad cow) and other diseases."