A large proportion of pharmacologically relevant targets are located on the surface of cell membranes. In addition, many compounds capable of modulating cellular responses do so by directly binding to native cellular proteins on the surface of the cell. The higher the affinity that a compound or antibody has for its target, the more effective it will be at triggering the desired response at a physiologically relevant concentration.
It is, therefore, not surprising that assays investigating the interaction of potential drugs with their targets are of particular interest to the pharmaceutical industry.
Several approaches exist to measure binding between a drug compound and its target. These include ELISA, Western blot, isothermal titration calorimetry, KinExa, and flow cytometry. However, these techniques often involve radioisotope or fluorescent labeling. Such labels may inhibit the antibody-target interaction in vitro and would not be used in vivo, limiting the biological relevance of results produced using this sort of approach.
In addition, many of these equilibrium techniques lack the temporal resolution required to facilitate accurate kinetic binding analysis, while another option, surface plasmon resonance (SPR), has only been able to work with purified receptors and membrane fractions.
To address these limitations, SAW Instruments has developed the sam family of biosensor solutions for detecting and displaying biomolecular interactions in real-time. The systems use an innovative approach based on surface acoustic wave (SAW) technology, whereby an aqueous sample is guided over a proprietary sensor chip that converts a high-frequency signal into a SAW via the inverse piezoelectric effect.
The phase and amplitude of the resulting SAWs are sensitive to changes in mass loading and liquid viscosity respectively, allowing biomolecular interactions at the chip’s surface to be detected with a high level of sensitivity.
The great advantage of the SAW method is that it provides real-time data rather than relying on equilibrium-based models. This allows rapid analysis and accurate kinetic information to be captured. The system can also be used to study living cells, vesicles, antibodies, enzymes, and even LMW compounds (< 200Da in size) without the need for labeling. Molecular conformational changes, such as those that occur when a ligand binds to its target active site, can also be detected using SAW technology.
Recently, researchers at SAW Instruments and Affimed Therapeutics collaborated to investigate the interaction between a therapeutic (bispecific) antibody fragment and its specific tumor cell surface target in a live cell-based assay (Figure 1). This is an early example where real-time kinetics data has been generated to describe the binding of a label-free therapeutic antibody to cancer cells.