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Study Shows Quanterix's Single Molecule Array Assay Measures PSA at Previously Undetectable Levels

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This story originally ran on June 9.

By Adam Bonislawski

In a study published in the June issue of Nature Biotechnology, researchers from diagnostics firm Quanterix demonstrated the ability of the company's Single Molecule Array – or SiMoA – technology to detect prostate specific antigen in the blood of radical prostatectomy patients at concentrations as low as 0.4 fM – a sensitivity 1,700 times greater than that offered by conventional ELISA-based tests, according to the firm.

The results offer the prospect for better prognostic and diagnostic tests for prostate cancer and improved detection and measurement of low concentration proteins more generally – a development that has potentially broad applications in protein biomarker research, diagnostics, and drug development, David Duffy, senior director of platform research at Quanterix, told ProteoMonitor.

The company's SiMoA platform works essentially like an ELISA, but uses arrays of femtoliter-sized reaction chambers designed to isolate single molecules, enabling each well to serve effectively as an independent assay for a single molecule. A detection system consisting of an optical fiber bundle to carry light in and out of each well, a proprietary image-capture device, and image-analysis software allow researchers to observe these assays on a single-molecule level.

"The concept is to [isolate] proteins in arrays of femtoliter wells. You can do calculations based on thermodynamics that show that if you can do that you can get really substantially increases in sensitivity," Duffy said.

Lack of sensitivity has long been a challenge with protein biomarker assays. Typical immunoassays are able to measure proteins at picomolar concentrations, but many proteins exist in blood at femtomolar concentrations and below.

"Immunoassays have only been able to detect kind of the tip of the iceberg of the proteome," Duffy said. "So our goal as a company has been to make a jump of a factor of about a thousand, from picomolar down to femtomolar and below to start picking up a lot more of these proteins."

The Nature Biotechnology study examined blood samples from 30 post-prostatectomy prostate cancer patients whose PSA levels were undetectable using conventional tests. Using SiMoA assays, Quanterix researchers detected the protein in all 30 patients, with levels ranging from 10 fg per mL up to 10 pg per mL.

"So there was a factor of 1,000 difference within those 30 patients, all of whom had essentially been told that their PSA was undetectable and the same," Duffy said. "What we hope is that the ability to measure very low levels early on will allow us to either give patients a good prognosis or identify groups that are at a higher risk of recurrence so their urologist can pay more attention to them."

Doctors typically have to wait three years after a prostate cancer patient's surgery before PSA reaches prognostically useful levels, Duffy said. The SiMoA assay could cut that time to one year.

Quanterix is now working with several urology centers including the Department of Urology at the New York University School of Medicine on studies involving several hundred patients to establish prognoses based on the low-concentration PSA readings. Next, Duffy says, the company plans to start longitudinal studies with the goal of using the low-concentration readings to provide diagnostic information.

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The company, which launched in 2007 and raised $15 million in venture capital over 2007 and 2008, is establishing a CLIA-certified laboratory at its Cambridge, Mass. headquarters with the aim of launching a commercial version of the SiMoA PSA test in 2011. Longer term, it hopes to develop an IVD version of the test that could be made available in hospitals.

"As a small company, we wanted our first product to be relatively low risk," Duffy said. "PSA is a well established biomarker and testing for recurrence is a well established diagnostic. The medical community we would be marketing this to is already using PSA tests for this application, but it's one where there's clearly a need for sensitivity."

The company is looking to apply the SiMoA technology to Alzheimer's disease, as well.

"There are several proteins – for example [amyloid-beta 42] and phosphorylated tau protein – that have been shown to be diagnostic for Alzheimer's disease when they're measured in cerebrospinal fluid," Duffy said. "The only problem is, it's a procedure that's very uncomfortable for the patient, doctors don't like doing it, and so those tests haven't been adopted by the community."

"But what we've heard is that if we could measure the same biomarkers in blood, then there's a strong chance it could become a screening tool for AD," he said. "The blood-brain barrier keeps those proteins locked in the CSF, but it's known that proteins can escape in low concentrations, so we're setting up assays to measure those proteins and other proteins to try to diagnose AD in blood."

Having had success detecting these proteins in preliminary studies, the company hopes to develop a diagnostic test for Alzheimer's by 2012, Duffy said. In the future it could move into other areas with established biomarkers like cardiac damage and chronic inflammatory diseases, he said.

The company has also partnered with several pharmaceutical companies to develop SiMoA assays to measure levels of proteins targeted by experimental drugs, Duffy said, although he declined to give any specific names.

It plans to use the SiMoA platform for biomarker discovery, as well.

"We're focused on the urology and neurology applications for the time being. Our goal is to get to revenues and break even as quickly as possible. But we see this platform as having tremendous potential for biomarker discovery," Duffy said. "People are using nucleic acids and sequencing and array technology to come up with novel putative markers, but based on tissue profiling. They take a cancer biopsy and measure those genes or proteins within that tissue and say these could be diagnostic for these diseases."

"The problem is, it's not very convenient to do that," he said. "It would be much better to have blood tests for that. But it usually turns out that those proteins aren't circulating at very high concentrations – this is always a problem people run into. So we see that as an area that's going to be very fertile in the future."

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