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Figure 5<br />

Generation of a virtual combinatorial library <strong>by</strong> finding<br />

substituents of a custom scaffold that can be<br />

accommodated in the binding site of a molecular target<br />

or meet other 3D structural criteria. Once the virtual<br />

library is synthesized in the computer, individual<br />

members can be selected using structural or additional<br />

criteria and synthesized using automated equipment.<br />

Page 532<br />

The first step in the process is the generation of a chemical template or scaffold that can be derivatized<br />

at multiple sites using reliable chemical reactions to produce a large combinatorial library (Figure 5).<br />

This custom chemical template is designed <strong>based</strong> on the structure of the target using the same heuristic<br />

set of rules used for traditional structure-<strong>based</strong> drug design. Useful 3-dimensional pharmacophore<br />

models are best derived from crystallographic or nuclear magnetic resonance structures of the target, but<br />

can also be derived from homology models <strong>based</strong> on the structures of related targets or 3-dimensional<br />

quantitative structure-activity relationships (3D QSAR) derived from a previously discovered series of<br />

active compounds [20]. In addition, the mechanism of action of the target or any other information that<br />

exists regarding the target or the target class can be used in the design to maximize the chances of<br />

finding hits [21,22]. The combinatorial libraries are designed so that a few thousand to millions of<br />

discrete molecules can be produced <strong>by</strong> reaction of the custom-designed template with appropriate<br />

proprietary and commercially available chemical building blocks.<br />

The next step in the process involves the implementation of the automated synthesis and generation of<br />

the library. Significant lead time is anticipated before a library can be produced because even the most<br />

reliable chemical reactions require optimization if they are to be carried out <strong>by</strong> a robot, particularly if<br />

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