TDR Targets: a chemogenomics resource for neglected diseases

Abstract

The TDR Targets Database (http://tdrtargets.org) has been designed and developed as an online resource to facilitate the rapid identification and prioritization of molecular
targets for drug development, focusing on pathogens responsible for neglected human diseases. The database integrates pathogen
specific genomic information with functional data (e.g. expression, phylogeny, essentiality) for genes collected from various
sources, including literature curation. This information can be browsed and queried using an extensive web interface with
functionalities for combining, saving, exporting and sharing the query results. Target genes can be ranked and prioritized
using numerical weights assigned to the criteria used for querying. In this report we describe recent updates to the TDR Targets
database, including the addition of new genomes (specifically helminths), and integration of chemical structure, property
and bioactivity information for biological ligands, drugs and inhibitors and cheminformatic tools for querying and visualizing
these chemical data. These changes greatly facilitate exploration of linkages (both known and predicted) between genes and
small molecules, yielding insight into whether particular proteins may be druggable, effectively allowing the navigation of
chemical space in a genomics context.

Full text

TDR Targets: a chemogenomics resource for
neglected diseases
Marı
´a P. Magarin
˜os
1
, Santiago J. Carmona
1
, Gregory J. Crowther
2
, Stuart A. Ralph
3
,
David S. Roos
4
, Dhanasekaran Shanmugam
4
, Wesley C. Van Voorhis
2
and
Ferna
´n Agu¨ ero
1,
*
1
Instituto de Investigaciones Biotecnolo
´gicas, Universidad de San Martı
´n, San Martı
´n, Buenos Aires, Argentina,
2
Department of Medicine, University of Washington, Seattle, WA, USA,
3
Department of Biochemistry and Molecular
Biology, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Victoria, Australia and
4
Department of Biology and Penn Genomics Institute, University of Pennsylvania, Philadelphia, PA, USA
Received September 15, 2011; Revised October 24, 2011; Accepted October 25, 2011
ABSTRACT
The TDR Targets Database (http://tdrtargets.org)
has been designed and developed as an online
resource to facilitate the rapid identification and
prioritization of molecular targets for drug develop-
ment, focusing on pathogens responsible for
neglected human diseases. The database integrates
pathogen specific genomic information with func-
tional data (e.g. expression, phylogeny, essentiality)
for genes collected from various sources, including
literature curation. This information can be browsed
and queried using an extensive web interface with
functionalities for combining, saving, exporting
and sharing the query results. Target genes can be
ranked and prioritized using numerical weights
assigned to the criteria used for querying. In this
report we describe recent updates to the TDR
Targets database, including the addition of new
genomes (specifically helminths), and integration
of chemical structure, property and bioactivity
information for biological ligands, drugs and inhibi-
tors and cheminformatic tools for querying and
visualizing these chemical data. These changes
greatly facilitate exploration of linkages (both
known and predicted) between genes and small
molecules, yielding insight into whether particular
proteins may be druggable, effectively allowing the
navigation of chemical space in a genomics context.
BACKGROUND
The open access, web accessible TDR Targets database
(http://tdrtargets.org) (1), allows users to interrogate
pathogen specific genomic-scale information and to
identify and prioritize high value targets based on
whether or not they fulfill a set of user defined criteria.
The name of the database includes the initialism ‘TDR’
for Tropical Disease Research, a special program within
the World Health Organization (see Acknowledgements).
The focus of the TDR Targets database is on high priority
tropical disease pathogens (currently the top ten patho-
gens in the portfolio of this special program), and
several other phylogenetically relevant pathogens
(the Wolbachia endosymbiont of Brugia malayi and the
apicomplexan parasite Toxoplasma gondii, for example).
The database integrates information on gene products
from primary genome databases (2–6), and gathers,
from various resources and published studies, organism-
specific functional information such as information on
orthologues (7), 3D structures (8) and/or structural
models [modeling of pathogen proteins were obtained
for this work, and are now available from Modbase (9)],
enzyme/metabolic pathway classification, expression and
essentiality (1). These datasets are further supplemented
with information curated from the literature on chemical
and/or genetic validation status of targets, precedence for
druggability and assayability. Ultimately, the genomic
scale datasets compiled in TDR Targets should aid the
tropical infectious disease community in driving drug
discovery efforts. The importance of this is obvious,
given the urgent need for new drugs, the rapid emergence
of resistance and toxicity issues associated with existing
drugs, and considering that the drug discovery pipeline
for tropical infectious diseases is rather thin due to the
chronic underfunding of the field and a lack of commercial
interest from big pharmaceutical companies.
Combining genomics data with chemical data is essen-
tial for the success of discovery efforts. The availability
of large compound datasets is particularly important in
*To whom correspondence should be addressed. Tel: +54 11 4580 7255 (Ext. 310); Fax: +54 11 4752 9639; Email: fernan@unsam.edu.ar,
fernan.aguero@gmail.com
D1118–D1127 Nucleic Acids Research, 2012, Vol. 40, Database issue Published online 23 November 2011
doi:10.1093/nar/gkr1053
ßThe Author(s) 2011. Published by Oxford University Press.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/
by-nc/3.0), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

tropical infectious disease research. Until recently,
however, access to large-scale pharmacological and medi-
cinal chemistry datasets was limited (mostly due to the
proprietary nature of the data), or prohibitively expensive.
However, this landscape started to change dramatically in
the past 5 years with the advent of a number of open
access chemical resources such as PubChem, ChEMBL
and others (10), and by the release of key high-throughput
screening datasets by both academic and pharmaceutical
companies (11,12) (Novartis-GNF Malaria Box, K
Gagaring, R Borboa, C Francek, Z Chen, J Buenviaje,
D Plouffe, E Winzeler, A Brinker, T Diagana, J Taylor,
R Glynne, A Chatterjee, K Kuhen. Genomics Institute
of the Novartis Research Foundation (GNF) USA, and
Novartis Institute for Tropical Disease, Singapore).
In order to complement the genomic datasets and
target-focused functionalities available in TDR Targets
(1), we integrated in the database a number of chemical
datasets enriched in drugs and drug-like molecules
collected from various sources, and developed cheminfor-
matics components that drive a data loading pipeline
and parts of the web application, allowing users to mine
these data. In this report, we describe these new
functionalities and data, and how the integration of
chemical and genomics data can be used to formulate
relevant queries to identify either new chemical leads for
a target, or candidate new targets for orphan bioactive
molecules.
Several existing databases allow searching for target
and/or drug information, but each of these has been
designed for a different purpose. Some are focused pre-
dominantly on describing chemical entities, while others
are focused on specific aspects of proteins that make them
possible targets, such as position in a metabolic pathway
or structural features. Examples of databases focused on
chemical entities include ChemBank (13), which is focused
on small molecule activities, obtained from a wide variety
of screenings; ChEBI (Chemical Entities of Biological
Interest) (14), which contains chemical information and
properties of small molecules; and Drugbank (15) which
holds chemical data on FDA approved drugs, nutraceut-
ical and experimental drugs, together with information
about their related proteins (targets, enzymes, etc.).
Others like BindingDB (16) catalogue experimentally
validated protein–ligand interactions relevant to drug
discovery and provide detailed information on binding
affinities and inhibitory kinetics. The SuperDrug
Database (17) allows users to search and compare struc-
tures of approved drugs, while the Antimicrobial peptide
database (18) also allows users to browse or search
for peptides that target infectious agents or cancer cells
based on their activity and content. There are several
databases focused primarily on protein targets—most of
these focus on a single prioritization theme, or a single
organism. For example, the Genomic Target Database
(19) contains lists of drug targets for four human bacterial
pathogens, with specific lists of targets grouped by meta-
bolic pathway or membrane localization. The Potential
Drug Target Database (PDTD) (20), allows users to
browse solved structures of potential drug targets and to
identify potential targets of a given small molecule
through docking into many solved structures. The
Therapeutic Targets Database (21) contains a large
volume of data on known and predicted targets,
allowing users to download target validation data for
individual proteins, to view Quantitative Structure-
Activity Relationship (QSAR) data for individual
proteins, or to search targets by linked compounds.
Finally, there are a number of cheminformatics resources
such as HEOS (22) that facilitate the remote collaboration
between groups involved in a drug discovery process.
However, very few of these databases offer the same
breadth of data-types relevant to drug target characteriza-
tion as TDR Targets. ChEMBL and PubChem are two
examples of chemical databases that provide additional
links to protein targets. Nonetheless, TDR Targets
stands alone in its ability to allow users to frame
weighted, complex queries to rank and interrogate many
targets at once from different species, and to cross-relate
chemical data with target data in meaningful ways, as
described in this report. In this respect we believe TDR
Targets fills an important niche. To our knowledge, the
TDR Targets database represents a unique resource
providing comprehensive compilation of relevant data
for drug target prioritization on multiple pathogens, all
in one place.
NEW FUNCTIONALITY: INTEGRATION OF
CHEMICAL DATA IN TDR TARGETS
In order to collect chemical information that would be
useful to search for new bioactive compounds against
tropical diseases, we selected data sources enriched in
drugs and drug-like molecules. Chemical compounds
listed in our database come from: the ChEMBL
database (http://www.ebi.ac.uk/chembldb), that contains
information on small molecules together with bioactivity
information curated from the literature, and information
on associated protein targets (23) (443 602 compounds);
PubChem (24) (278 070 compounds); the DrugBank
database, which contains information on FDA approved
drugs (15) (4421 compounds). The Tres Cantos
Antimalarial TCAMS dataset (GSK) (13 469 compounds)
(11), the Novartis-GNF Malaria Box database (5388 com-
pounds), and the St. Jude Children’s Research Hospital
Malaria dataset (305 815 compounds)(12), three datasets
that contain molecules tested in high-throughput
screening assays against Plasmodium falciparum, were
obtained from ChEMBL-NTD (http://www.ebi.ac.uk/
chemblntd), a repository of screening data of molecules
directed against neglected diseases.
A total of 825 814 unique drug-like compounds
are present in the combined dataset obtained from the
above indicated sources. These were integrated into the
TDR Targets database along with data on: (i) basic infor-
mation such as chemical name, InChi and InChiKey
identifiers; (ii) chemical properties (molecular weight,
logP, hydrogen bond donors, hydrogen bond acceptors,
flexible bonds, Lipinski rule of 5 compliance); (iii) chemical
structure; (iv) bioactivities (IC50, Ki, MIC, Activity,
EC50, ED50 and percent growth inhibition, among
Nucleic Acids Research, 2012, Vol. 40, Database issue D1119

others) and (v) association with genes (curated, predicted
or any). Table 1 provides a summary of searchable fields
for chemical data. Users can access this information either
by searching for target genes, and then looking up the
chemicals linked to these genes (scenario A in Figure 1)
or by searching the chemical space itself (scenario B in
Figure 1).
In a target search, it is now possible to query for tar-
gets with associated compounds and to further filter this
search based on evidence for association (i.e. curated or
predicted, or any). After running any target search, a list
of targets is shown with links to all compounds associated
with all the targets present in the list (Figure 1).
From the compound search page (http://tdrtargets.org/
drugs) users can query chemical information using either
text-based searches or structure-based searches (Figures 1
and 2) to retrieve a list of matching compounds. Users can
then choose to either view details of individual compounds
in the list (by clicking on the relevant compound) or
retrieve all genes associated with the compounds listed
using the ‘show all/curated/predicted genes’ link. In the
first case, a new page is shown, with a detailed view for
the compound, including basic chemical information,
synonyms, structure, association with targets (and kind
of association), activities described for the compound,
external resources, and bibliographic references. In the
second case, a new query is run to search for all/
curated/predicted pathogen genes associated with all the
compounds present in the list. The retrieved genes are
shown in similar fashion to the result for a typical target
search (Figure 1). All the history functionalities, such as
combining query results using Boolean AND, OR and
NOT operators as well as saving, exporting and publish-
ing query results, that were previously available for
handling target searches are also available for handling
chemical data searches.
CONNECTING KNOWN DRUG TARGETS TO
PATHOGEN GENOMES
Drug repositioning or repurposing, i.e. finding a new
indication for existing drugs or drug-like molecules, can
greatly speed up the traditional drug discovery process,
which typically takes more than a decade to complete
(25). An open access chemogenomics resource such as
TDR Targets can now provide neglected disease
researchers and pharma companies with a basic tool for
knowledge-based drug repositioning (in a general sense).
A key element of such a chemogenomics approach is the
linking of target genes to suitable chemical inhibitors,
and the leveraging of other relationships available in
the database (e.g. sequence similarity between targets;
chemical similarity between compounds).
As mentioned above, TDR Targets now contains a
chemical database of bioactive compounds and their
targets from primary data sources such as ChEMBL,
DrugBank and PubChem, in addition to data collected
from high-throughput screenings (The Tres Cantos
Antimalarial TCAMS dataset, the Novartis-GNF
Malaria Box database, the St. Jude Children’s Research
Hospital Malaria dataset), and in-house curatorial efforts
by the TDR Targets team. Many of the compounds
included from the above sources into TDR Targets have
known target genes, mostly from non-pathogens, and this
information can be used to link the compounds to
pathogen target genes using in silico approaches such as
orthologue mapping and protein domain conservation
between the known targets and novel targets. In this
Table 1. Searchable chemical information in TDR Targets
Data types Format / Example / Observations
Structure 2D structure SDF / MOL
Identifiers InChI 1S/C7H11NO7P2/c9-7(16(10,11)12,17(13,14)15)4-6-2-1-3-8-5-6/h1-3,5,9H[...]
InChI Key IIDJRNMFWXDHID-UHFFFAOYSA-N
ChEMBL 183772
DrugBank DB00884
PubChem CID 5245
Textual information Name/ Synonyms Risedronate / 1-hydroxy-1-phosphono-2-pyridin-3-ylethyl) phosphonic acid
Data Source ChEMBL, PubChem, DrugBank
Chemical properties MW 282.104
Formula C7H10NO7P2
LogP 3.23
No. of H donors 4
No. of H acceptors 8
No. of flexible bonds 4
Activity Assay e.g. Inhibitory activity against Leishmania major Farnesyl diphosphate synthase
Readout IC50, EC50, Ki, MIC, % growth inhibition, etc. (depends on particular assay)
Target association Direct Manual curation (experimental evidence, target directly assayed)
Transitive Experimental evidence available for an ortholog/homolog
Searchable information fields for small molecules integrated in TDR Targets are shown in the Table. The 2D structure of a molecule is used for
similarity and substructure searches, and in all searches started from the JME molecule editor. All other information fields are searchable as textual
or numeric information using standard forms (see Figure 3 for some examples). Target associations are used internally to limit search results to show
only those compounds that are associated with a target (see examples in Figure 2), and to display links to targets within pages. Note: the InChI string
in the table has been truncated for presentation purposes.
D1120 Nucleic Acids Research, 2012, Vol. 40, Database issue

way, users of the TDR Targets resource can find potential
pathogen drug targets, linked to a set of chemical com-
pounds with measured activity against a related protein.
From here, chemical scaffolds of the proposed compounds
can be used as starting points to identify new chemical
entities as potential drugs for the novel target. A schematic
view of this approach is presented in Figure 1.
The strategy of mapping gene functional data and
chemical bioactivity data across orthologues is a key
mechanism by which TDR Targets establishes links
between chemicals and target genes. Orthologues are a
set of genes from two or more species that originated by
vertical descent from a single gene in the last common
ancestor. Orthologues are often functionally and structur-
ally similar. Thus, they may be modulated by identical
or similar molecules, making orthology assessment a
powerful tool to connect a known druggable target with
a potential novel target [see (26) for an example of this
strategy]. However, non-orthologous genes can share
homologous druggable domains. Methods to predict
orthology such as COG (27) and OrthoMCL (7) are
based on a reciprocal best BLAST hit step (i.e. the first
sequence finds the second sequence as its best hit in the
second species, and vice versa). Protein coding genes
containing multiple domains with one of them being
‘druggable’ could pass undetected because of the recipro-
cal best hit requirement of orthology-based methods
(even if the other domains are conserved, but rearranged
in the protein sequence). Thus, we implemented a second
mapping strategy in TDR Targets using BLAST, in which
a sequence similarity search is used to identify drug targets
with high similarity to pathogen genes (although not
necessarily reciprocal), with the requirement that the simi-
larity span should almost completely cover the ‘druggable’
target (80% coverage, E-value <10
10
). With this simple
approach we are able to connect members of a protein
family, which may be grouped in different ortholog
clusters, although they are structurally similar. Using
this strategy in the TDR Targets database, 3575 known
druggable targets (primary drug-target association is
based on manual curation) were assigned to 1509
OrthoMCL clusters of orthologous genes. About 50% of
Figure 1. Schematic view of chemogenomic searches and navigation supported in TDR Targets. (A) Targets search. Query #1 retrieves 170 genes
from Leishmania major,Trypanosoma cruzi and Trypanosoma brucei that were associated to compounds by manual curation. Clicking on gene
LmjF06.0860 (dihydrofolate reductase-thymidylate synthase from L. major) shows the corresponding gene page, and allows users to inspect the
associated compounds. In TDR Targets a target resultset can be used to generate the corresponding compound resultset by clicking on the ‘Show
associated compounds’ link (and vice versa for compound resultsets). Query #3 was generated in this way, and produces a list of 902 compounds
associated to trypanosomatid genes by manual curation. (B) Compound search (textual). Query #2 was performed from the compounds search page,
retrieving 1321 compounds that meet all 4 of Lipinski’s rules, and that were associated to genes by manual curation. The combination of queries #2
and #3 (INTERSECTION) can be calculated at the history page, returning 574 compounds that meet all specified criteria. (C) Compound similarity
search. In Query #3, a 2,4-diaminoquinazoline was found associated to 2 trypanosomatid genes by manual curation. In order to find additional
related compounds with potential activity against this target, a similarity search can be performed (Query #4), retrieving another 14 compounds at
Tanimoto similarity 0.8 (chemical analogs).
Nucleic Acids Research, 2012, Vol. 40, Database issue D1121

these groups (798) contain 4529 genes from tropical
disease pathogens (and are therefore candidate druggable
genes). The additional sequence similarity step performed
with BLAST allowed us to identify 2087 pathogen genes
that were not detected by strict orthology. A number of
example cases are depicted in Figure 3. In TDR Targets,
these similarity links between targets provide additional
navigation routes in the database, facilitating the assess-
ment of available evidence (e.g. activity of compounds)
for a related group of homologues.
QUERYING THE AVAILABLE CHEMICAL
INFORMATION IN TDR TARGETS
The new chemical data available in TDR Targets can be
queried in a number of different ways. Every compound
entry in TDR Targets is associated with a number
of searchable parameters such as basic information
(name, synonyms, InChi and InChi Key identifiers, data
source), chemical properties (formula and atomic compos-
ition, molecular weight, solubility [logP], Lipinski rule of
5 compliance), bioactivity (if applicable), target genes (if
Figure 2. A query strategy designed to find active compounds from a defined chemical class. TDR Targets allows searches of the activity of
compounds. Query #1 uses this functionality to retrieve reasonably active chemical leads that have been associated to targets. TDR Targets also
allows users to perform substructure searches. Query #2 implements such a search, retrieving compounds containing the drawn structure as part of
the molecule. The intersection of these two queries finds 37 active compounds from this class (2-iminobenzimidazoles). One example molecule from
this list is shown at the bottom, with a few selected panels of information from the corresponding compound page.
D1122 Nucleic Acids Research, 2012, Vol. 40, Database issue

applicable), and chemical structure (Table 1). At the top of
the compound search page (Figure 2), the user can
perform queries based on basic information, chemical
properties, number and type of atoms, activities, associ-
ation with genes and type of association (curated,
predicted or any), and information source (Table 1). The
InChi and InChiKey identifiers are unique representations
of a molecule that can be used to look for a specific
compound of interest. Alternatively, chemical properties
like the molecular weight, logP, etc., are useful to retrieve
a collection of compounds that meet user-defined criteria.
Users can search for desirable compounds using a
combination of search parameters. For example, one can
search for molecules that have a molecular weight below
500, and ‘any’ described activity, or for compounds that
are associated with genes by manual curation of literature,
and meet all four Lipinski rules. An illustration of this
type of chemical data search using various parameters
is shown in Figure 3.
Searching for specific compounds using names or
synonyms is possible in TDR Targets, however, these
types of searches often do not produce the expected
results, in many cases due to the different names and
synonyms available for a molecule, as well as possible
alternative spellings (e.g. metrifonate versus metripho-
nate). A more useful approach would be to use the 2D
chemical structure of the compound to run an ‘exact
match’ search to retrieve only the compound being
searched for. In TDR Targets these type of searches can
be initiated by drawing a molecule in the ‘Structure-based
searches’ section of the compounds search page (http://
tdrtargets.org/drugs). Users can draw molecule structures
using the JME Java applet integrated in TDR Targets
(JME molecule editor courtesy of Peter Ertl, Novartis
http://www.molinspiration.com/jme/index.html), and run
‘exact’, ‘similarity’, or ‘substructure’ searches. This latter
type of search will find any compound that contains
the drawn structure as a part of it, and is usually a good
way to find molecules that belong to a given chemical class
(e.g. 2-iminobenzimidazoles, see Figure 3 for an example),
for example to analyze and compare their bioactivities.
Another potential use of this tool is to exclude certain
compounds from a search, for example those containing
undesirable functional groups (e.g. reactive/toxic in vivo,
metabolically unstable or known to cause problems
in screening for a particular assay). In such a case, the
users could first query the database using an initial set
of criteria (e.g. search for all compounds that have a
measured IC50 below 2 mM). Then, a second query
would retrieve all the compounds that contain an undesir-
able substructure. Finally, the second query can be sub-
tracted from the first at the query history page, therefore
obtaining a list of compounds that meet the first criteria
(IC50 below 2 mM) but lack the undesirable substructure.
Another way of finding potentially active molecules
starting from a known bioactive compound is by similarity
searching. To measure the similarity between the mol-
ecules, TDR Targets implements the Tanimoto (Jaccard)
association coefficient which is a commonly used metric
for chemical similarity of small molecules (28,29).
Molecules that have a Tanimoto index equal to or
greater than 0.8 to the query molecule are retrieved.
If the molecule is too large to be easily drawn, but the
structure is available as a molfile (in SDF/MOL format),
the same searches can be initiated by pasting the content
of the molecule’s molfile (text) into the corresponding
input textbox.
Altogether, these implemented query modes allow
users to find either individual compounds or groups of
compounds for later inspection of the information
associated with them, including targets. Links to other
resources such as ChEMBL, PubChem and ChemSpider
(30) allow users to gather additional information
on compounds.
USE CASES: SOME QUESTIONS THAT CAN BE
ANSWERED USING THE NEW FUNCTIONALITIES
The new chemical functionalities developed and the recent
integration of chemical data allow users to answer ques-
tions about compounds directly (e.g. in the compound
search page), and also formulate questions about
pathogen targets (from the targets search page, where
the chemical information can be used as an additional
criteria to restrict genes searches). Relevant questions
that are now possible in TDR Targets include the
Figure 3. Links between non-homologous genes in TDR Targets.
Many druggable target sequences can be completely aligned to
pathogen genes for which no druggable orthologs can be detected
using stricter orthology methods (generally due to large differences in
protein length between homologs). The figure depicts a number of sche-
matic views of alignments between druggable targets (names shown in
red) and helminth genes (% id = percentage identity). Genes repre-
sented from top to bottom (OrthoMCL Ortholog Cluster Identifier in
brackets, IC50 of most active compound in square brackets) are: (A)S.
mansoni Smp_159890 (OG4_13640), H. sapiens P15169 (OG4_27945)
[2nM, PubChem CID 194328]; (B) Smp_155200 (OG4_12097), P62136
(OG4_10262) [0.1 nM, PubChem CID 445434]; (C) Bm1_17240
(OG4_12720), Q00526 (OG4_10184) [5 mM, PubChem CID 4369491];
(D) Bm1_52100 (OG4_12799), P62937 (OG4_10089) [2 nM, PubChem
CID 9855081].
Nucleic Acids Research, 2012, Vol. 40, Database issue D1123

following: find compounds with activity against
Leishmania that are also associated with genes based on
manual curation of the literature. This question can be
answered by performing a single query in the compounds
search page, selecting those compounds with assay
descriptions matching ‘leishmania’ (‘Description’ box in
the ‘Activities’ section), and choosing ‘Curated’ in the
‘Gene Associations’ section. This query results in a
list of 63 compounds, that includes drugs such as
pamidronate, risedronate, artemisinine, oryzalin and
suramin.
Another example helps to illustrate how the integration
of genomics and chemical data can be used to select
candidate druggable targets. In this case, the following
question can be easily translated into a search strategy
in TDR Targets: which P. falciparum genes have
evidence of essentiality in any species (through orthology),
and have associated compounds by manual curation of
literature? The corresponding search strategy would start
by querying the database from the target search page,
first choosing the species (Plasmodium), then proceeding
to the Essentiality section of the search form and asking
for genes with ‘Any evidence of essentiality in any species’.
Finally, in the same page, under Druggability, one can
further restrict the search by requesting only genes with
‘Associated compounds: Curated’. Such a query would
produce a list of 39 genes. The user can then obtain
a list of the compounds associated with these genes by
clicking on ‘Show curated compounds’ from the results
page.
THE TDR TARGETS CURATION EFFORT
A target-based drug discovery process is guided by the
incremental gathering of data about a target, usually
with the final goal of validating the target. Key data
about the target’s essentiality for the parasite, its expres-
sion in a relevant life cycle stage, and its chemical tract-
ability are all available in the literature, and can be
extracted and integrated in the database using controlled
vocabularies (ontologies) that facilitate querying and
cross-relation to other database objects. As previously
described (1), the TDR Targets team has compiled exten-
sive literature data on phenotypic responses of targets or
whole pathogens to genetic or chemical (pharmacological)
perturbations. These literature data nicely complement
the genome-scale databasets in providing additional
target validation-related information on individual
targets or groups of targets that have been the subject
of more focused research. For chemicals, our structured
representation of the effects of compounds as phenotypes
(e.g. decreased cell growth, abnormal morphology, inhib-
ition of catalytic activity, etc.) is distinct from others’
(e.g. ChEMBL’s) reporting of these effects as ‘activities’
(e.g. IC50’s, % growth inhibition, etc.), which we have
also imported into our database. These inconsistencies
lead to certain challenges in querying the curated data
(see ‘Caveats’ below). Nevertheless, it makes sense
to combine our own curated datasets with others
because there is relatively little overlap among them.
For example, of the 450 pathogen targets we have
curated internally for association with compounds, only
20% have also been curated in the ChEMBL dataset,
which is larger but focused mostly on non-infectious
diseases. This limited overlap is explained in part by the
different journals used in these curation efforts; ChEMBL
draws mostly from medicinal chemistry journals such as
the Journal of Medicinal Chemistry and Bioorganic &
Medicinal Chemistry Letters, while TDR Targets draws
heavily from pathogen-specific journals like Molecular
and Biochemical Parasitology and Antimicrobial Agents
and Chemotherapy. The curatorial work of the TDR
Targets team is an ongoing effort and in the future
will include inputs solicited from the tropical infectious
disease community.
In the current release of TDR Targets, we incorporated
data on curated validation credentials for Schistosoma
mansoni and Trypanosoma cruzi, therefore completing
curation for six key tropical disease causing organisms
(species targeted for curation in previous releases of
TDR Targets include: Mycobacterium tuberculosis,
Trypanosoma brucei,Leishmania major and P. falcip-
arum). The new data contains descriptions of 303 pheno-
types (associated with 143 targets and 322 compounds),
derived from genetic experiments (for 39 targets)
and chemical experiments (for 113 targets). These data
complement the data from previous releases for other
diseases/organisms, bringing the number of targets with
curated validation credentials to 448; and the number
of compounds curated internally to 968.
Additionally, we have recently integrated genome-wide
phenotyping data for T. brucei derived from the work of
Alsford et al. (31).These data, covering different life cycle
stages of the parasite, and from different culture condi-
tions, contributed genetic validation information for
7400 (80%) of the annotated protein coding genes in
the genome, in the form of ‘loss of fitness’or‘gain of
fitness’ phenotypes. The integrated phenotype descriptions
derived from this work are a significant addition to
TDR Targets. These correspond to 30 000 annotations,
about half of all phenotype annotations previously
available in the database. These annotations are now
searchable and readily available for target prioritizations.
Together with gene knockout datasets for M. tuberculosis
integrated previously in TDR Targets, they represent the
only two examples of genome-wide genetic validation
data for WHO target organisms, which are the focus of
TDR Targets.
INCORPORATION OF DATA TO ASSESS
ASSAYABILITY OF TARGETS
For the purposes of this database, a target is considered
assayable if it is an enzyme included in Sigma-Aldrich’s
collection of assays, or if it has been assayed according to
the BRENDA database (32). The BRENDA database
contains categories for cloned and purified genes but not
assayed genes per se, so to create our ‘assayed’ category we
combined entries from the Km and Specific Activity
categories, which give the clearest picture of whether a
D1124 Nucleic Acids Research, 2012, Vol. 40, Database issue

protein has actually been enzymatically assayed. The
mapping of the BRENDA entries to genes in TDR
Targets was carried out as follows: (i) Mapping by
Enzyme Commission (EC) number: EC numbers in
BRENDA were used to map the entries to those TDR
genes with identical EC numbers; (ii) for BRENDA
entries where there was no match to a pathogen gene by
EC, the gene was identified by name in the species-specific
database (e.g. PlasmoDB) and mapped to that gene; (iii) if
there was no gene in the species-specific database with the
same EC or name as in the BRENDA entry, the gene was
identified by sequence similarity using the sequence
from the associated BRENDA literature reference as the
query. For each TDR species all entries for the genus were
mapped; for example, genes that were assayed/purified/
cloned in Plasmodium knowlesi were mapped to P. falcip-
arum and Plasmodium vivax. The source species for the
data is specified in the ‘Assayability’ section on the cor-
responding gene pages.
Aside from having a convenient readout of activity,
another aspect of assayability is being able to produce
and purify a recombinant form of the protein in
question. The largest-scale attempt to express enzymes
from TDR Targets species has come through the
Structural Genomics of Pathogenic Protozoa (SGPP)
and Medical Structural Genomics of Pathogenic
Protozoa (MSGPP) project at the University of
Washington. Therefore each protein that has been success-
fully expressed in recombinant form by (M)SGPP is
annotated as such in the database, and links are
provided for users to access additional information avail-
able at these resources (progress in obtaining diffracting
crystals, availability of plasmid clones, etc.)
OTHER DATA UPDATES IN TDR TARGETS:
NEW GENOMES
Since its launch in 2007 (1), and through several releases,
the database has integrated a number of additional
genomes. Most important amongst recent additions was
the incorporation of helminth genomes (Brugia malayi,
a nematode, the causative agent of Filariasis; and
Schistosoma mansoni, a trematode flatworm that causes
Schistosomiasis). Data from these organisms, including
essential annotation were derived from GenBank (5),
SchistoDB (33) and GeneDB (3). Other genomes of
interest for those studying parasites are T. gondii,
the causative agent of Toxoplasmosis, sometimes used
as a model organism for studying some aspects of
apicomplexan biology that are more difficult to study in
e.g. malaria parasites; and P. vivax, an important human
malaria parasite which cannot be yet cultured and studied
under laboratory conditions. These genomes have also
been integrated in TDR Targets, allowing the full
range of operations to be performed on their targets.
In addition to integrating the genomes of various patho-
genic organisms, TDR targets also includes the genomes
of various phylogenetically useful species from which
different kinds of datasets can be mapped. The genomes
of vertebrates (human, mouse), plants (Arabidopsis
thaliana,Oryza sativa) invertebrates (Drosophila
melanogaster), nematodes (Caenorhabditis elegans) and
closely related species of pathogens already represented
in TDR Targets (e.g. Leishmania braziliensis,infantum,
and mexicana). These complete proteomes are part of
the OrthoMCL database of orthologue groups (7),
and allow users to formulate questions such as: ‘search
for L. major proteins that are/are not present in
L. braziliensis’, or ‘find P. falciparum proteins that are
also present in plants or bacteria’ (e.g. when looking
for apicoplast associated targets).
CAVEATS: THINGS TO LOOK OUT FOR WHEN
SEARCHING TDR TARGETS
A number of important clarifications can be made to help
users of the database. These are related to the way
chemical information has been curated and integrated in
the database. Knowing about these issues will help users
make sense of search results, and understand why com-
pounds fail to appear in a result set against all expect-
ations. First, when curating the literature, if a
compound has been assayed against an organism (e.g.
for inhibition of growth of Leishmania amastigotes), this
will be recorded by a curator, even if the activity/pheno-
type is nil (e.g. no growth inhibition). Therefore when
searching the database for compounds with ‘any activity’
a user is actually searching for ‘compounds with any in-
formation about their activity’, as the query will return
both active and inactive compounds. Second, the activities
of compounds are recorded in different forms and units, as
specified by the original authors in published papers. Even
when a significant effort is invested to standardize the in-
formation in the database (e.g. as done by the ChEMBL
team), the number of different ways in which activity is
reported prevents the formulation of simple filtering
queries such as ‘show me all compounds with activity
<5 uM’. This type of query can be easily applied to
activities reported as IC50s, but does not make sense for
activities reported as ‘% inhibition’ (see Figure 3 for a
limited list of available activity types). In the latter case,
the concentration of the compound used to assess
inhibition is usually attached to the assay description
(e.g. ‘% inhibition at 1 ug/ml’), and is therefore difficult
to query separately. Finally, it is also worthwhile to note
here that when substructure or similarity searches are run
using a very minimal chemical scaffolds or a very ubiqui-
tous chemical fragment, the number of results retrieved
can be enormous and therefore the database currently
limits these types of searches.
CONCLUSION
The TDR Targets Database (http://tdrtargets.org) is an
online open-access resource developed for the purpose of
facilitating target prioritization using a comprehensive
collection of data describing gene function, structure,
essentiality, assayability and druggability. An extensive
set of informatic tools facilitates mining of this resource.
The current focus of the database is on pathogenic
Nucleic Acids Research, 2012, Vol. 40, Database issue D1125

organisms that are causative agents of tropical infectious
diseases as prioritized by the World Health Organization’s
Special Programme for Research and Training in Tropical
Diseases (TDR) and include M. leprae, M. tuberculosis,
P. falciparum, P. vivax, S. mansoni, T. gondii, T. brucei,
T. cruzi, L. major, and B. malayi and its endosymbiont
Wolbachia. Starting with release 4 of the TDR Targets
database, chemical information has been integrated
into TDR Targets along with cheminformatic tools to
run chemical searches taking advantage of a variety of
data describing chemical properties and 2D structure of
small molecules. These new developments provide a
chemogenomics platform that allows users to query the
available chemical data, and investigate associations
between targets and compounds. The web interface imple-
mented in TDR Targets allows users to seamlessly move
from a list of target genes to list of compounds known to
interact with these genes and vice versa. As pointed to
above, several different databases are now available for
browsing and searching chemical datasets and drug
targets, but each of them focuses on only a subset of the
functionalities available in TDR Targets, and almost none
of them is focused on tropical diseases. TDR Targets,
therefore, addresses a need that is somewhat neglected
by comparable databases and resources. Initially de-
veloped to integrate in one place data from parasitic
genomes, evaluation of gene function, essentiality and
suitability for drug development of targets, TDR Targets
now has extended coverage of another key component of
the drug discovery process: chemical data including
assays, activities, and associations with targets.
A number of key improvements are necessary to keep
TDR Targets useful, up to date and relevant for the
community of scientists working on tropical diseases.
Development of web services and other computational
tools to facilitate reuse of data is one area that will be a
major focus in the future. Incorporating information on
the commercial availability of compounds, and providing
links to providers is another key aspect that will be
incorporated in future releases. But more importantly
perhaps, a sustained curation effort is also required to
keep valuable target validation data and compound
activity data up to date, and to identify valuable medicinal
chemistry data for integration in TDR Targets’ chemical
database. As mentioned above, the focus of the TDR
Targets curation effort has been largely put on the
gathering of information on validating credentials for
targets. However, now that a substantial investment has
been made into the integration of compound data,
curation should be extended to gather other supporting
information, such as data on assays, and on the reported
activities of compounds (in the form of IC50s, %inhib-
ition, phenotypes, etc.) Adequate funding is much
needed to sustain these activities.
ACKNOWLEDGEMENTS
We would like to thank John Overington (EBI) for
providing an early release of the Starlite/ChEMBL
database for integration into TDR Targets, and Peter
Ertl (Novartis) for providing the JME applet for
structure-based searches. This work was supported by
the Special Programme for Research and Training in
Tropical Diseases (TDR), and in part by a grant from
the National Agency for the Promotion of Science
and Technology (ANPCyT, Argentina, PICT-
2010-1479). FA and SJC are fellows of the National
Research Council (CONICET, Argentina). MPM is sup-
ported by fellowship from a Fogarty International
Research Collaboration Award, NIH (FIRCA Grant
Number D43TW007888).
FUNDING
Funding for open access charge: Agencia Nacional de
Promocion Cientifica y Tecnologica (ANPCyT,
Argentina) PICT-2010-1479.
Conflict of interest statement. None declared.
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