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Magnetic Biosensor Could Enable Simultaneous Monitoring of Thousands of Protein-Ligand Bindings


By Adam Bonislawski

Stanford University researchers have developed a magnetic nanotag-based biosensor that they said is potentially capable of simultaneously monitoring thousands of protein-binding events and offers improvements in cost, speed, and sensitivity over conventional surface plasmon-resonance assays.

The sensor is aimed primarily at drug-screening applications, allowing pharmaceutical firms to test compounds against a wide number of proteins for off-target binding, said Shan Wang, director of Stanford's Center for Magnetic Nanotechnology and leader of the research effort.

The device, which was detailed in an article in the April edition of Nature Nanotechnology, uses giant magnetoresistive sensors similar to those in the read heads of computer hard drives to analyze protein-ligand interactions.

Using ligands pre-labeled with magnetic nanoparticles, the researchers are able to monitor real-time protein-ligand binding kinetics by tracking changes in the electrical resistance of the GMR sensors caused by the presence of the nanoparticles.

The technique offers significant improvements over surface plasmon resonance assays, currently the standard method of monitoring protein binding, Wang told ProteoMonitor. SPR instruments have limits of detection — around 25 ng per mL — and dynamic range limits of roughly 2 logs, he said, compared to sub-pg per mL LOD and 6-log dynamic range limits for the GMR-based device.

Because the model of binding kinetics used by the device doesn't require the reactions being studied to proceed to equilibrium, assays take only minutes compared to hours for SPR.

Additionally, while SPR devices are able to monitor only several protein-ligand bindings at once, the GMR device could potentially measure tens of thousands, Wang said. To date his team has monitored up to 64 interactions simultaneously, but they've fabricated sensor arrays with 1,008 sensors on a chip area 1 mm2 and, he said, "we don't see any problems in going further up" in size.

The instrument could prove considerably cheaper than SPR as well, he added, noting that SPR can cost several hundred dollars per protein measurement, while the current version of the GMR chip costs around $35 to produce, a number that Wang said he expects will come down as they are made in larger volumes.

Wang is the founder and board member of array company MagArray, which he launched in 2005 to commercialize the GMR biosensor technology. According to CEO Luis Carbonell, the main focus of the Sunnyvale, Calif.-based firm has been to develop an earlier version of the platform for immunoassay-based diagnostics work, and it is currently in negotiations with several potential partners. He declined to name them.

The Nature Nanotechnology study represents "a broadening application for the technology," he told ProteoMonitor, expanding its possible uses to the drug-screening space.

The earlier GMR device combines a conventional sandwich antibody assay with magnetic nanoparticles to detect antigens of interest via the same sort of changes in electrical resistance used by the protein-binding platform.

In a paper describing the device in the December 2008 edition of PNAS, Wang estimated that it could detect the tumor biomarker chorionic gonadotropin at levels about 400 times lower than a commercial ELISA. He also noted that his team used it to detect seven cancer biomarkers at levels of one pg per mL.

Currently, Wang is using the instrument in collaboration with Sanjiv Gambhir, director of Stanford's Molecular Imaging Program, to validate protein biomarkers for ovarian, lung, and prostate cancers. The ultimate aim, he said, is to commercialize the system through MagArray as either an alternative or a complement to conventional ELISA-based diagnostics.

"We believe our advantage [compared to ELISAs] is the sensitivity and also the reliability of the platform," he said, noting that CVs for the device are typically 10 percent or less.

The new protein-binding version of the platform will initially be targeted at basic research applications, Wang said. "If people want to find out affinities and on- and off-rates in proteins of interest, they can use our chip in their research."

"But ultimately this technology will be married to drug [research]," he added, adding that he has been approached by several undisclosed pharmaceutical companies about the system.

"If we want to find on-target binding affinity [of a drug] and then the side effects, which are the off-target binding affinities, we can put the target protein on the chip and the non-target proteins also on the chip, and see how that drug binds to all of these proteins," said Wang.

Studying off-target binding is key to drug research in terms of identifying unwanted side effects, identifying new off-label uses for established drugs. and developing compounds capable of targeting multiple targets in complex diseases like cancer.

For instance, in a study published last week in the online edition of PLoS Computational Biology, a research team led by University of California, San Diego, pharmacology professor Philip Bourne proposed that the anti-cancer effects of Pfizer's HIV drug Viracept stem from weak binding to multiple protein kinases upstream of the PI3K/AKT signaling pathway (PM 04/29/2011).

The finding, the authors said, "supports the notion that multiple weak drug-target interactions may have a more profound impact in biological systems than a single strong interaction." This suggests that platforms like Wang's capable of simultaneously monitoring thousands of protein-ligand bindings could play a significant role in pharma development.

Have topics you'd like to see covered in ProteoMonitor? Contact the editor at abonislawski [at] genomeweb [.] com.

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