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AnalogExplorer2 – Stereochemistry sensitive graphical analysis of large analog series

AnalogExplorer is a computational methodology for the extraction and organization of series of structural analogs from compound data sets and their graphical analysis. The method is suitable for the analysis of large analog series originating from lead optimization programs. Herein we report AnalogExplorer2 designed to explicitly take stereochemical information during graphical analysis into account and describe a freely available deposition of the original AnalogExplorer program, AnalogExplorer2, and exemplary compound sets to illustrate their use.


Introduction
In medicinal chemistry, analog series are typically analyzed in R-group tables. Once analog series become so large that they are difficult to represent and study in conventional R-group tables, computational tools are indispensable for their exploration. Therefore, different computational methods have been introduced for graphical analysis of analog series [1][2][3][4][5][6][7][8][9] . Many of these approaches are based on the determination of maximum common substructures (MCS) of compound series and focus on substituents of common cores, while others employ the matched molecular pair 10 formalism to define analog series. Among these computational methods is AnalogExplorer 9 , which has been designed to systematically organize and graphically analyze analog series and associated structure-activity relationship (SAR) information. AnalogExplorer initially identifies analog series on the basis of hierarchical molecular scaffolds 11 and then determines their MCS for further analysis. Accordingly, the method is not limited to the study of individual analog series but can also be applied to extract series from structurally heterogeneous compound sets. For example, AnalogExplorer is directly applicable to late-stage lead optimization sets that often contain multiple series with large numbers of analogs.
Herein we introduce AnalogExplorer2, an extension of the approach, which explicitly considers stereoisomers during graphical analysis, providing a detailed account of stereochemistry and its influence on SARs. AnalogExplorer2 is publicly available. Hence, we also report an open access deposition of the original AnalogExplorer program and AnalogExplorer2 as well as exemplary data sets assembled to illustrate the workflow of graphical analysis and help users become familiar with the program 12 .

Methodology
Organization of analog series AnalogExplorer systematically determines substitution sites or site combinations in analog series and divides series into subsets having varying R-groups at the same site(s). Analog series are initially identified on the basis of hierarchical molecular scaffolds 11 . For a series of analogs sharing the same scaffold, the maximum common substructure (MCS) is then determined, as illustrated in Figure 1, and used for R-group decomposition in order to index and identify all substitution sites and the respective R-groups for each compound in a series.
On the basis of the MCS, an analog series is divided into subsets of compounds with varying R-groups at the same substitution site or site combination. The organization is compound-based such that each member of a series only occurs in one subset. Unique compound subsets provide the basis for graphical analysis, as discussed in the following. Further methodological details are provided in the original AnalogExplorer reference 9 .

Graphical components
AnalogExplorer consists of three graphical components. The complete graph ( Figure 2a) captures all possible substitution sites and site combinations for a series following R-group decomposition (by design it is a directed acyclic graph). Each node represents a substitution site or site combination and all compounds with varying R-groups at the site(s). The root node 0 corresponds to a (hypothetical) compound with no R-group at any site. Node 1 represents analogs that only contain R-groups at R1 and node 12 compounds with R-groups at R1 and R2 etc. Nodes are arranged in different layers. For example, layer 1 consists of all nodes with one substitution site and layer 2 of all nodes with two sites. Edges between nodes in adjacent layers indicate all possible subset relationships, i.e. an edge is drawn if the substitution site(s) represented by a node is a subset of a site combination of another node. As indicated in Figure 2a, nodes are scaled in size according to the number of analogs comprising the subset they represent and color-coded according to the mean potency of the analogs. In addition, node border thickness indicates the potency range covered by a subset. Furthermore, white (empty) nodes correspond to possible site combinations for which no analogs are currently available within a given series. In the reduced graph, all empty nodes and connecting edges are removed for clarity ( Figure 2b). Thus, the reduced graph provides a convenient format for the analysis of individual series. As the third graphical component, R-group trees are provided for each substitution site and site combination, as illustrated in Figure 2c. In the R-group tree, substitution sites for a given subset are arranged in different layers, the order of which is determined by the number of unique R-groups at each site. All R-groups are displayed in the tree. Each leaf node represents an analog (colored according to its potency). Intermediate nodes represent subsets of analogs sharing the same substituents at corresponding site(s) (and are colored by mean analog potency).
Given its design, AnalogExplorer provides a systematic hierarchical organization of all possible substitution sites or site combinations for an analog series (complete graph) and enables the elucidation of SAR patterns within the hierarchy (reduced graph) and at further increased resolution for analog subsets (R-group trees). The approach is particularly suitable for the analysis of large analog series because subsets of such series associated with interesting SAR information can be selectively displayed and analyzed.

Stereochemical information
The explicit consideration of stereochemistry during graphical analysis at the level of R-group trees is the major methodological enhancement of AnalogExplorer2 (in addition to further increased consistency of compound mapping to MCS considering intramolecular symmetry). In the original R-group tree structure, nodes located in the same layer and originated from the same parent node are associated with distinct R-groups. Therefore, stereoisomers having the same substituents are combined into a single leaf node. Hence if a terminal node is associated with more than one compound, stereoisomers are present. In AnalogExplorer2, stereoisomers are explicitly considered, as illustrated in Figure 2c. Each stereoisomer is represented by a single node and stereoisomers belonging to the same subset (i.e. compounds with different stereochemistry at the same site) are identified by a unique index (i.e. '1' for the three stereoisomers in Figure 2c). If different subsets of stereoisomers are present in an R-group tree, incremental indices are used to identify and distinguish them (i.e. '1', '2' etc.).

Implementation
Routines for scaffold, analog, and MCS identification, R-group decomposition, and indexing of substitution sites are implemented in Java using the OpenEye OEChem toolkit version 2.0.2 (Open Eye Scientific Software; http://www.eyesopen.com). Therefore, this toolkit is required to execute the program. All graphical components of AnalogExplorer and AnalogExplorer2 are implemented using the open source Java package JUNG version 2.0.1 (http://jung.sourceforge.net/). Potential inconsistencies with subsequent versions of OEChem or JUNG can be avoided by using the specified versions.

Program use
The executable program utilizes standard SD files as input and generates complete or reduced graphs for all or individual series, depending on the user's preference. The initial graph layout is produced by the DAGLayout algorithm of JUNG (http://jung. sourceforge.net/) and usually interactively modified for graphical analysis. The number of compounds assigned to each node and their mean potency can be viewed by navigating the graph. R-group trees representing compound subsets are generated together with the complete or reduced graph. In each R-group tree, the substituents associated with individual nodes, compounds (leaf nodes), and corresponding potency values can also be viewed. Subsets of stereoisomers, if available, are depicted using numerical indices, as discussed. Furthermore, an output file is generated reporting compounds belonging to individual subsets.

Exemplary applications
AnalogExplorer2 can be used for different types of SAR analysis, as illustrated by a few exemplary applications. Compound data were taken from ChEMBL 13 version 20. Figure 3 displays the reduced graph for a series of 45 alpha-1a adrenergic receptor ligands with a total of seven substitution sites and the R-group tree for an exemplary three-site combination. The tree reveals a clear SAR pattern (with increasing potency of analogs from the bottom left to the right) and identifies six (uniquely indexed) pairs of stereoisomers among these analogs. Figure 4 provides a corresponding representation for a series of 64 matrix metalloproteinase 9 inhibitors and compares R-group trees for three substitution sites. It becomes apparent that substituents attached to R4 alone or in combination with other sites consistently yield compounds having only low potency. In Figure 5, target-specific reduced graphs are compared for a series of 32 analogs with a total of nine substitution sites and activity against two dipeptidyl peptidases (DPP4 and DPP8). The graphs generated for the same analog series display different compound potency distributions, reflecting a selectivity tendency for DPP4 over DPP8. Figure 6 shows reduced graphs for six different analog series with activity against the same kinase. The graphs reveal different structural content and a different degree of chemical exploration among these series as well as differences in the SAR information they provide. In Figure 7, two of these series are combined into a new single series by re-calculating the MCS that comprises 43 analogs with a total of eight substitution sites. The reduced graph captures the structural organization and activity information of this combined series.

Software and data availability
The following tools and data sets are made publicly available without restrictions via a deposition on the ZENODO open access platform 12 . Three executable files of the original AnalogExplorer program are provided for different applications including the analysis of multiple analog series from a given compound set, analysis of an individual series, and selectivity analysis (according to     Figure 5). With the exception of the OpenEye OEChem library, jar files of the required external libraries are also provided. In addition, all compound sets analyzed in the original publication 9 are deposited. These compound sets were taken from ChEMBL version 18. Furthermore, three executable files are made available for AnalogExplorer2 (for multiple analog series, individual series, and selectivity analysis) as well as the compound sets for which graph representations are reported herein. These compounds were taken from ChEMBL version 20. A "readme" document with detailed explanations is also provided as a part of the deposition.

Conclusions
The AnalogExplorer method was designed for the systematic organization and graphical analysis of large series of analogs, which frequently originate from lead optimization efforts. Herein, an extension of the methodology has been introduced. AnalogEx-plorer2 explicitly accounts for all stereoisomers during graphical analysis and SAR exploration. The AnalogExplorer2 program is made freely available to the scientific community. The authors provide an extension to AnalogExplorer incorporating stereochemistry, and important extension of the method. This method of analysing complicated compound data sets is sophisticated and easy to use. The approach to communicating results is intuitive both from the perspective of the trends in the experimental data with respect to the compounds that have been made, but also, in complete graph view, provide insight into important regions of the dataset that have not yet been investigated. This tool will be very useful in the exploration of medicinal chemistry datasets in both industry and academia.
I have downloaded and run the software on a windows workstation using both the provided datasets and internally generated ones -the software works as described with two minor issues cropped up. First, the software depends not only on the OEChem Toolkit, but also requires OEDepict (both licenses are available from Openeye Scientific). Second, the software depends specifically on the precise OEChem Toolkit java jar file (oejava-2014.Oct.2-Windows-x64.jar) -it will fail without the correct jar file.
On balance AnalogExplorer2 is a very useful tool for exploring compound data sets. It could be made more widely useful if it were to be adapted to run on an open source chemical toolkit such as RDKit. Nonetheless, I commend the authors for making their software available to the research community.

I have read this submission. I believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard.
No competing interests were disclosed. Analysis of diverse molecular datasets is one of the most common tasks a computational chemist in pharmaceutical industry, but also in academia is facing daily. The tool presented in this manuscript -the AnalogExplorer2 supports this process and makes it easier. Enhancement to previously published AnalogExplorer presented here, namely support for handling of stereoisomers is indeed important, since in large number of complex SAR datasets molecules with the same constitution but different in large number of complex SAR datasets molecules with the same constitution but different stereochemistry are present. And in some cases differences in bioactivity for different stereoisomers are significant. Several examples shown in the manuscript illustrate nicely the powerful visualization capabilities of this tool, including maximum common substructure visualization, substituent tree visualization and selectivity analysis. These visualizations allow to understand the SAR even for the quite complex datasets.
The authors made the executable version of the tool freely available for download. This is a good news. But since the tool is based on the OEChem library only users with a valid OEChem license would be able to use it. My recommendation for the future versions of AnalogExplorer would be to make the interface to its cheminformatics processing modular, so it would be possible to plug the tool also to open source cheminformatics toolkits like RDKit or CDK and make in this way the AnalogExplorer a truly free tool for graphical analysis of complex molecular datasets.

I have read this submission. I believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard.
No competing interests were disclosed. Competing Interests: We thank both reviewers for their constructive comments and appreciate the positive feedback. We agree that a future release of this program should best make use of a publicly available toolkit.

Discuss this Article
Concerning the OEChem toolkit version dependence referred to in the paper, especially concerning the java jar file, as pointed out by Carleton Sage, please see: https://docs.eyesopen.com/toolkits/java/quickstart-java/install.html Final statement: "There is a known bug concerning OpenEye constants that occurs when compiling against an older version of the OpenEye jar and running against a newer version of the OpenEye jar. Please make sure to run against Java programs against the same version it was compiled with."

Solution:
Users are required to obtain the same OpenEye version we used (Oct 2, 2014): Visit http://eyesopen.com Login Go to support/Downloads Select Platform (e.g. Windows)