NMR-based screening methods for lead discovery.

Diversity and robustness of NMR based screening methods make these techniques highly attractive as tools for drug discovery. Although not all screening techniques discussed here may be applicable to any given target, there is however a good chance that at least one of the described methods will prove productive in finding several medium affinity ligands. A comparison of each of the methods is given in Table 1. For drug targets of molecular weight < 30 kDa SAR by NMR appears to be the method of choice since it yields detailed information about the location of the binding site. It remains to be seen whether 15N-1H-TROSY based screening techniques will prove useful for larger protein targets, especially considering the added effort needed for spectral assignment and the increased complexity due to spectral overlap. Nevertheless, with the application of new cryo-cooled NMR probes, 15N-1H-HSQC based screening can now be considered a high throughput method. Ligand-based NMR screening methods can be used for protein targets of virtually any size, but are restricted in the ligand's binding affinity range. Because sufficient ligand-protein dissociation rates are needed, only binding of ligands with low (milimolar) to intermediate (micromolar) affinities is detectable. It is expected that cryo-cooled NMR probe technology will also advance ligand detected NMR screening to the high throughput level. Certainly protein and ligand concentrations can be lowered drastically and experiment times can be shortened with increased sensitivity. However, spectral overlap will be of major concern when mixtures of up to 100 compounds are to be screened. For such applications only techniques for which the signals of bound ligands survive will be useful, and sophisticated software will be needed to deconvolute the spectra of multiple bound ligands. Although only ligands with medium to low affinities can be found, ligand based NMR screening has been used as an effective prescreening tool for assay based high throughput screening. Identifying a large ensemble of medium affinity ligands may not only aid in building a binding site pharmacophore model (see Chapter 11), but also may yield crucial information for overcoming tissue availability, toxicity, or even intellectual property related problems. Although NMR based screening is only one of the more recent additions to the bag of tools used in drug discovery [1, 2], its simplicity and wide range of application (including protein-protein and protein-nucleic acid interactions) has attracted much attention. Advances in NMR instrumentation and methodology have already paved the road for NMR based screening to become a high throughput technique. In addition to this, NMR is exceptional in the amount of detailed structural [table: see text] information it can provide. Not only can NMR readily reveal the binding site (15N-1H-HSQC screening) or the conformation of the bound ligand (transfer NOE), but it can also supply information that enables precise docking of the ligand to the protein's binding pocket (isotope-filtered NOESY). NMR data can therefore provide a natural connection between experimental HTS and combinatorial chemistry techniques with computational methods such as 3D-database searching (see Chapter 10), virtual screening (docking) and structure-based ligand design (see also Chapter 8).