Several years after its demonstration as a viable approach to interrogating ligand binding events in mixtures, FAC-MS systems, immobilization strategies and methods have advanced to the degree where FAC assays can be constructed around most molecular interactions relevant to the drug discovery industry. Aside from the utility of the frontal analysis method to support sensitive, accurate Kd measurements on single ligands, perhaps the most significant opportunity for FAC in ligand discovery is through the screening of''imperfect'' mixtures. Under equilibrium conditions, ligands elute in order of their binding strength regardless of their concentrations, thus determination of hit ''quality'' is simply by inspection. This makes the method useful for screening single-pot combinatorial synthetic mixtures, or natural product extracts where compound concentrations are nonequivalent. Most initial screening exercises do not begin with the detection of Kds < 1 mM so the ability for FAC to detect ligands in a concentration-independent manner promotes efficient use of finite compound collections. Improved LC/MS technology can array entire mixture compositions in reproducible fashion, as evidenced by applications in combinatorial library analysis and proteo-mics. This suggests that FAC-MS has a growing opportunity to assume an expanded role in high-throughput screening exercises. Although it should not be viewed as a replacement to established HTS operations, the inherent simplicity of the method suggests that FAC-MS can broaden access to chemical diversity at early stages in drug discovery.
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