4.2.2 Biology-Oriented Synthesis of Natural Product-Inspired libraries
Waldmann has developed a new concept for the design of combinatorial libraries based on natural products that he calls BIOS.(154-156) This concept is based on the recognition of fundamental and complementary properties of natural products and their protein targets. Nature, through evolution of natural products, has explored only a tiny fraction of the available small-molecule chemical space. The same holds true for the biological targets of natural products, which are mainly proteins. The number of three-dimensional protein folds have been shown to be even more conserved during evolution than the underlying sequences, since topologically similar shapes can be formed by different sequences. Estimates of the number of proteins in humans range between 100 000 and 450 000; the number of topologically different protein folds is actually much lower, with estimates of 600−8 000.(169) Since both the natural product space and the protein structure space explored by Nature are limited in size and highly conserved, these structure spaces have to be highly complementary. Thus, a natural product that is an inhibitor of a specific protein fold represents a biologically validated starting point for the development of closely related structures that may inhibit proteins with similar folds and even allow for the discovery of specificity. These concepts are fundamentally similar to the privileged structure concept,(129) but BIOS has the added dimension of using protein folding patterns as the basis for subsequent screens.
BIOS is based upon two concepts previously developed by the Waldmann group. The scaffolds of natural products can be mapped in a hierarchical manner to create a scaffold tree, a “structural classification of natural products” (SCONP).(170, 171) This allows for logical pathways for the structural simplification of scaffolds. In the second concept, “protein structure similarity clustering” (PSSC), proteins are clustered by three-dimensional shape around the ligand-binding sites, regardless of sequence similarity.(172-174) Merging these two concepts led to the BIOS approach.(156) The ligand of any member of a PSSC could be expected to exhibit some degree of complementarity toward other members of the PSSC and, thus, serve as a starting point for the development of modulators of the other members.
The success of the BIOS approach was demonstrated by a combinatorial library inspired by the marine natural product dysidiolide (60; Scheme 13). Postulating that the γ-hydroxybutenolide group of dysidiolide was the major determinant of phosphatase activity, testing of a 147-member library built around this molecule yielded a compound (61; Scheme 13) 10-fold more potent (IC50 = 350 nM) than the parent compound against Cdc25A.(175) In addition, other members of the library were identified with low micromolar activities against the enzymes acetylcholinesterase and 11β-hydroxysteroid dehydrogenase type 1, which fall within the same PSSC as Cdc25A.(176) A second example of the success of BIOS, the discovery of inhibitors of Tie-2, insulin-like growth factor 1 receptor (IGF-1R), and vascular endothelial growth factor receptor 2 (VEGFR-2 and 3), is discussed in section .
BIOS represents a refinement of combinatorial libraries based on natural product scaffolds by focusing on the most biologically relevant chemical space for the target. Furthermore, it allows the transfer of knowledge about the modulation of a target by a natural product to a whole cluster of structurally related proteins, even when those proteins catalyze mechanistically different reactions.
4.2.2 Biology-Oriented Synthesis of Natural Product-Inspired libraries
Waldmann has developed a new concept for the design of combinatorial libraries based on natural products that he calls BIOS.(154-156) This concept is based on the recognition of fundamental and complementary properties of natural products and their protein targets. Nature, through evolution of natural products, has explored only a tiny fraction of the available small-molecule chemical space. The same holds true for the biological targets of natural products, which are mainly proteins. The number of three-dimensional protein folds have been shown to be even more conserved during evolution than the underlying sequences, since topologically similar shapes can be formed by different sequences. Estimates of the number of proteins in humans range between 100 000 and 450 000; the number of topologically different protein folds is actually much lower, with estimates of 600−8 000.(169) Since both the natural product space and the protein structure space explored by Nature are limited in size and highly conserved, these structure spaces have to be highly complementary. Thus, a natural product that is an inhibitor of a specific protein fold represents a biologically validated starting point for the development of closely related structures that may inhibit proteins with similar folds and even allow for the discovery of specificity. These concepts are fundamentally similar to the privileged structure concept,(129) but BIOS has the added dimension of using protein folding patterns as the basis for subsequent screens.
BIOS is based upon two concepts previously developed by the Waldmann group. The scaffolds of natural products can be mapped in a hierarchical manner to create a scaffold tree, a “structural classification of natural products” (SCONP).(170, 171) This allows for logical pathways for the structural simplification of scaffolds. In the second concept, “protein structure similarity clustering” (PSSC), proteins are clustered by three-dimensional shape around the ligand-binding sites, regardless of sequence similarity.(172-174) Merging these two concepts led to the BIOS approach.(156) The ligand of any member of a PSSC could be expected to exhibit some degree of complementarity toward other members of the PSSC and, thus, serve as a starting point for the development of modulators of the other members.
The success of the BIOS approach was demonstrated by a combinatorial library inspired by the marine natural product dysidiolide (60; Scheme 13). Postulating that the γ-hydroxybutenolide group of dysidiolide was the major determinant of phosphatase activity, testing of a 147-member library built around this molecule yielded a compound (61; Scheme 13) 10-fold more potent (IC50 = 350 nM) than the parent compound against Cdc25A.(175) In addition, other members of the library were identified with low micromolar activities against the enzymes acetylcholinesterase and 11β-hydroxysteroid dehydrogenase type 1, which fall within the same PSSC as Cdc25A.(176) A second example of the success of BIOS, the discovery of inhibitors of Tie-2, insulin-like growth factor 1 receptor (IGF-1R), and vascular endothelial growth factor receptor 2 (VEGFR-2 and 3), is discussed in section .
BIOS represents a refinement of combinatorial libraries based on natural product scaffolds by focusing on the most biologically relevant chemical space for the target. Furthermore, it allows the transfer of knowledge about the modulation of a target by a natural product to a whole cluster of structurally related proteins, even when those proteins catalyze mechanistically different reactions.
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