Combinatorial library design, product-base

Make decision on monomers based on property profile of their corresponding products (Combinatorial-shaping). This leads to a fully combinatorial library design to retain efficiency in library production ... 

The Oriented Substituent Pharmacophore PRopErtY Space (OSPPREYS) approach, introduced by Martin and Hoeffel [6], is in software terms an extension of CCG s MOE package, written using SVL. The 3D oriented substituent pharmacophores are aimed towards better representation of diversity and similarity in combinatorial libraries in the 3D pharmacophore space. Combinatorial library design often operates only on substituents rather than on the final products as the complications related to the conformational coverage in the 3D space and the scaffold dependency limit the product-based approaches to smaller libraries. The 3D oriented substituent pharmacophores add two more points and the corresponding distances to each substituent pharmacophore which represent the relationship of the substituents in the product with only little additional information. The fingerprints permit the creation of property space by multidimensional scaling (MDS) and, since scaffold independent, can be stored separately and applied to different libraries [6],... 

The compound selection methods described thus far can be used to select compounds for screening from an in-house collection, or to select which compounds to purchase from an external supplier. In combinatorial library design, however, it is necessary to select subsets of reactants for actual synthesis. The two main strategies for combinatorial library design are reactant-based selection and product-based selection. In reactant-based selection, optimized subsets of reactants are selected without consideration of the products that will result and any of the compound selection methods already identified can be used. An early example of reactant-based design is that already described by Martin and colleagues which is based on experimental design and where diverse subsets of reactants were selected for the synthesis of peptoid libraries [1]. 

The choice of appropriate substituents and building blocks for hbrary synthesis depends on synthetic feasibility, availability and costs, but should also be based on an understanding of the physicochemical properties of the substituents, as well as the predicted properties of the targeted compound. Therefore, we discuss below how these choices may be made as rational as possible by considering design techniques. Indeed, there has been much debate as to whether combinatorial library design should be based on reagent (monomer) diversity or on the diversity of the final product. Some relative merits of each approach are out-fined below ... 

The chapter begins with a discussion of similarity and diversity measures and how they can be applied in a virtual screening context. The various computational filters in use are also discussed. The rest of the chapter is concerned with different approaches to combinatorial library design, beginning with reagent-based methods followed by product-based approaches of cherry picking and combinatorial subset selection. Finally, approaches to designing libraries optimized on multiple properties simultaneously are discussed. 

Product-based selection is much more computationally demanding than reagent-based selection. Typically, it requires the computational enumeration of the full virtual combinatorial library and calculation of the descriptors for all possible products, prior to the application of a subset selection method. Consider a three-component reaction with 100 reagents available at each substituent position and assume that the aim is to build a 10 x 10 x 10 combinatorial library. In reagent-based selection, this requires the calculation of descriptors for 300 compounds (100 + 100 + 100). In product-based design, however, the full library of 1 million compounds (100 x 100 x 100) must be enumerated and descriptors must be calculated for each product molecule. 

Lee, M.L. Schneider, G. (2001) Scaffold Architecture and Pharmacophoric Properties of Natural Products and Trade Drugs Application in the Design of Natural Product-Based Combinatorial Libraries. Journal of Combinatorial Chemistry, 3, 284-289. 

Lee ML, Schneider GJ. (2001) Scaffold architecture and pharmaco-phoric properties of natural products and trade drugs Application in the design of natural product-based combinatorial libraries. Comb Chem 3 284-289. 

Product-based approaches can be divided into those that take the combinatorial constraint into account such that each reactant in one pool appears in a product with every reactant from every other reactant pool, and those that merely pick product molecules without consideration of the synthetic constraint. The latter approach is often referred to as cherry-picking and is synthetically inefficient as far as combinatorial synthesis is concerned. In this chapter the emphasis is on product-based library design methods that take the combinatorial constraint into account. 

Jamois, E. A., Hassan, M., and Waldman, M. (2000) Evaluation of reactant-based and product-based strategies in the design of combinatorial library subsets. 

---