Displacement Competition - Search for drugs with the most modern techniques


How well does a potential drug bind to its target (such as an enzyme)? In the search for new pharmaceuticals, as well as in other areas of bioscience, the affinity of ligands for their receptors is of central importance. Klaus Theodor Wanner and Georg Höfner at the Ludwig-Maximilians University of Munich have developed a new method for the verification of affinity. Their technique depends on the competition between ligands, and mass spectrometry is the instrumental technique of choice.

Competitive bonding experiments are nothing new. They require a marker, a ligand that binds the desired site on the "target" with high affinity and selectivity. Ligands under investigation must compete to displace the marker from the target. The higher the affinity, the more ligand, and thus less marker, is bound to the receptor. The amount of bound marker is then quantified. Until now, only radioactivity and fluorescence methods have been sensitive enough to do this. The marker has therefore had to contain the corresponding radioactive isotope (such as tritium, sulfur-35, iodine-125) or fluorescing group. This involves additional synthetic complexity, safety precautions, disposal problems, and the possible inhibition of the properties of the marker because of the fluorescing group. In future, we may be able to avoid all this. Mass spectrometry (MS) has now attained a sufficiently high level of sensitivity for binding experiments. The MS experiment works with unaltered, native markers - and is therefore simple and universally applicable.

For a competitive MS binding study à la Wanner and Höfner, the receptor, native marker, and ligand under investigation are incubated together. Once it has been separated from the mixture through centrifugation, the free marker can be isolated by liquid chromatography and quantified by MS. In contrast to traditional methods, the amount of unbound, rather than bound, marker is measured, for simplification. "Tests with dopamine receptors and known ligands correlated well with affinity data from radiological binding experiments," say Wanner and Höfner. Competitive MS binding studies are more than an attractive alternative to the traditional method; an entire substance library could initially be quickly searched for interesting ligands in one screening. The successful candidates could then be separated from the receptor and identified by mass spectrometry.

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