Furthermore, KEGG DRUG contains information about two types of molecular networks. The first network is a molecular interaction network representing interactions and/or relations with target molecules (often in the context of pathway maps), drug metabolizing enzymes, drug transporters and other drugs (especially those causing adverse effects). The second network is a network of chemical structure changes in small molecules, which includes series of chemical modifications introduced by medicinal chemists in the history of drug development (in the KEGG drug structure maps), secondary metabolic pathways for biosynthesis of druggable natural products and drug metabolism (both in the KEGG pathway maps). We have analyzed the chemical architecture of marketed drugs and the patterns of chemical structure transformations in the history of drug development (15), in a similar spirit to the RDM patterns of chemical structure transformations in enzyme-catalyzed reactions. As illustrated in Figure 2, the second network may be used to analyze the chemical architecture of natural products and the chemical architecture of marketed drugs towards drug discovery from the genomes of plants and microorganisms. Furthermore, the second network may have relevance
