Fragment-based lead discovery
Fragment-based lead discovery (FBLD) also known as fragment-based drug discovery (FBDD) is a method used for finding lead compounds as part of the drug discovery process. It is based on identifying small chemical fragments, which may bind only weakly to the biological target, and then growing them or combining them to produce a lead with a higher affinity. FBLD can be compared with high-throughput screening (HTS). In HTS, libraries with up to millions of compounds, with molecular weights of around 500 Da, are screened, and nanomolar binding affinities are sought. In contrast, in the early phase of FBLD, libraries with a few thousand compounds with molecular weights of around 200 Da may be screened, and millimolar affinities can be considered useful.
In analogy to the rule of five, it has been proposed that ideal fragments should follow the 'rule of three' (molecular weight < 300, ClogP < 3, the number of hydrogen bond donors and acceptors each should be < 3 and the number of rotatable bonds should be < 3). Since the fragments have relatively low affinity for their targets, they must have high water solubility so that they can be screened at higher concentrations.
Advantages over traditional libraries
Advantages of screening low molecular weight fragment based libraries over traditional higher molecular weight chemical libraries are several. These include:
- More hydrophilic hits in which hydrogen bonding is more likely to contribute to affinity (enthalpically driven binding). It is generally much easier to increase affinity by adding hydrophobic groups (entropically driven binding), starting with a hydrophilic ligand increases the chances that the final optimized ligand will not be too hydrophobic (log P < 5).
- Higher ligand efficiency so that the final optimized ligand will more likely be relatively low in molecular weight (MW < 500).
- Since two to three fragments in theory can be combined to form an optimized ligand, screening a fragment library of N compounds is equivalent to screening N2 - N3 compounds in a traditional library.
- Fragments are less likely to contain sterically blocking groups that interfere with an otherwise favorable ligand-protein interaction, increasing the combinatorial advantage of a fragment library even further.
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