In February, Quadraspec (www.quadraspec.com) delivered the first shipment of its Spinning Disc Interferometry™ (SDI™) label-free immunoassay system. The underlying technology, which came out of Purdue University in 2004, will first be commercially available to a large veterinary reference laboratory as a diagnostic test for canine heartworm. Using the SDI system, the lab will be able to run 260 samples in less than an hour, the company reports.
In parallel development at Quadraspec is the Assay Development Kit, designed for the research and drug development market. It contains a disposable SDI disc on which a researcher can deposit an array of antibodies—onto an existing carpet of proteins—and screen those against an antigen of interest. These discs can replace existing ELISA tests or be the basis for developing multiplexed assays.
The SDI label-free technology is based on the phenomenon of interferometry, in which light shines down on the discs’ reflective surface. The thin surface layer is composed of an upper surface and a lower surface, and the light waves that reflect off of these interact, either attenuating or extinguishing each other. At the outset, the light source and detector are positioned to achieve maximal interferometric light intensity. Placing something on the surface, such as bound antibodies, “detunes” the interferometric intensity, and, as a result, the intensity of the reflected light will decrease.
“That is a sensitive measure of the mass of the object deposited on the surface,” explained Joerg Schreiber, Ph.D., COO of Quadraspec. Bound antigens change the interferometric response again. By measuring the resulting mass change, the system detects binding of the target protein without the need for a second antibody or label. Thus the technology enables label-free detection of the immune reaction.
The SDI disc enables multiplexing, offering the ability to measure up to 100 unique analytes in 260 patient samples. As the disk spins, a laser tracks the position and contents of each 125-micron spot (containing the immobilized antibodies). Although there is some noise in the system, noise-reduction methods account for this by subtracting out the background signal and averaging measurements within a spot and between spots.
Maven Biotechnologies’ (www.mavenbiotech.com) LFIRE™ is a label-free, high-density imaging system that measures molecular binding reactions in a microarray or well-plate format. LFIRE achieves real-time monitoring of reactions through the use of total internal reflection ellipsometry, a technique that measures changes in the polarization of light reflected from the interface between materials, providing optical properties of thin films. The light enters the underside of the microarray and reflects off the solid-liquid interface between the substrate and the sample solution. The presence of bound protein on the array and the resulting mass change due to specific binding of analytes is detected by measuring the changes in the polarization state of the reflected light.
Shane Dultz, Ph.D., CSO at Maven, described “the increased mass from the binding reaction” as an “internal label that permits sensitive measurement of biomarkers in biological fluids.” According to the company, “LFIRE has been validated with gene and protein microarray densities of 2,500 spots/cm2 and is capable of detecting surface attachment of molecules as small as 150 Daltons.” For antibodies that attach to the substrate surface, it can detect roughly 12 molecules/µm2.
The main advantages of LFIRE over surface plasmon resonance (SPR) technologies are that it does not require metal films, has more spatial resolution, and is inherently more sensitive than imaging SPR, explained Dr. Dultz.
“Furthermore LFIRE has been validated against commercial single-plex diagnostic kits (ELISA) and fluorescence multiplexed diagnostics (Luminex) to demonstrate capability in markets that SPR technologies have not been designed to address,” he added. “These commercial immunoassay markets require economical, high-throughput testing for single to multiple tests per patient, with an open-systems approach using disposables compatible with most robotic sample-handling technologies.”
LFIRE technology is in the early stages of commercial development—a commercial instrument is about a year away from launch. The company plans to deliver both slide-based and plate-based systems that would replace labeled assays used in diagnostics, research, and drug discovery and development applications and that would be compatible with disposable formats already on the market.