PSA 'Nanotest' May Spot Prostate Cancer's Return After Surgery

MONDAY Oct. 19, 2009 -- A new test that could revolutionize the treatment of men following prostate cancer surgery has worked well in a small, early trial, researchers report.

Using nanotechnology, the researchers were able to detect prostate-specific antigen (PSA), the cancer-associated protein, at levels far below what is now possible with older technology, explained Chad A. Mirkin, lead author of a report published online Oct. 19 in Proceedings of the National Academy of Sciences.

"This could knock years off our current ability to detect a recurrence of cancer," said Mirkin, a professor of chemistry at Northwestern University in Chicago.

But it will be "a number of years" before the assay, or test, undergoes the larger-scale testing and vetting needed for approval of its use in medical practice, said Dr. C. Shad Thaxton, assistant professor of urology at Northwestern's Feinberg School of Medicine, one of Mirkin's collaborators in development of the test.

The ultra-sensitive PSA test uses gold particles that are just 30 nanometers in diameter and have antibodies to PSA as well as strands of DNA, the basic genetic molecule, attached to them. The combination allows detection of PSA at levels 300 times lower than is now possible, Mirkin said.

PSA testing is controversial when it is used to screen for prostate cancer, he acknowledged. "But no one is arguing that a rising PSA level is not a marker of recurrence," Mirkin said.

He envisions use of the PSA nanotest routinely in men after prostate surgery. "It can stratify the population post-operatively," Mirkin said. "About 42 percent of the time there will be no recurrence, and this test will allow us to determine that years earlier than we can with conventional tools."

If rapidly rising PSA levels are detected, "there is no definitive therapy, but we can offer experimental chemotherapy or radiotherapy and can validate whether those therapies are working," he said.

That knowledge could be invaluable, the researchers believe. "If the treatment is not working, we can try a different approach," Thaxton said.

The journal report describes use of the nanotest for 18 men who had prostate cancer surgery. A larger study, with 260 participants, is also under way at Northwestern. "We hope to have data by the end of the year," Mirkin said.

Even larger studies that are necessary to validate the assay could start "within a year or two," Thaxton said.

Both Mirkin and Thaxton are stockholders in Nanosphere, Inc., the company that has licensed the nanotechnology from Northwestern and is applying it to a wide variety of medical tests.

"They have six FDA approvals already," Mirkin said of the company's medical nanotechnology tests. Approval by the U.S. Food and Drug Administration is required for a test to be used in medical practice in the United States.

For now, caution is needed until widespread studies are done, said Dr. Durado Brooks, director of prostate and colorectal cancers at the American Cancer Society.

"We're talking about only 18 patients, but more importantly, we still don't know if this is clinically significant," Brooks said. "If we find a recurrence earlier, does it make a difference if we treat it earlier? We don't know if it will make a difference long-term."

While he called the work "exciting," Brooks said "it is way too early in the game to tell whether it will have clinical significance."

More information

For more on PSA testing, see the U.S. National Institutes of Health.

Posted: October 2009


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