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  <front>
    <journal-meta>
      <journal-title-group>
        <journal-title>Care and Life Sciences, February</journal-title>
      </journal-title-group>
    </journal-meta>
    <article-meta>
      <title-group>
        <article-title>drug repurposing workflow for rare diseases</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Carmen A.T. Reep</string-name>
          <email>c.reep@erasmusmc.nl</email>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Katherine Wolstencroft</string-name>
          <email>k.j.wolstencroft@liacs.leidenuniv.nl</email>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Eleni Mina</string-name>
          <email>e.mina@lumc.nl</email>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Núria Queralt-Rosinach</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Leiden University Medical Centre, Department of Human Genetics</institution>
          ,
          <addr-line>Einthovenweg 20, 2333 ZC Leiden</addr-line>
          ,
          <country country="NL">The Netherlands</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>The Leiden Institute of Advanced Computer Science (LIACS)</institution>
          ,
          <addr-line>Niels Bohrweg 1, 2333 CA Leiden</addr-line>
          ,
          <country country="NL">The Netherlands</country>
        </aff>
      </contrib-group>
      <pub-date>
        <year>2024</year>
      </pub-date>
      <volume>2</volume>
      <fpage>6</fpage>
      <lpage>29</lpage>
      <abstract>
        <p>There are over 7000 known rare diseases. Each one afects fewer than 1 in 2000 individuals, but collectively, they afect approximately 10% of the European and American populations. Developing treatment options for rare diseases is essential for those with such conditions, but as drug development is a time-consuming and costly process, developing new treatments is not often economically viable. The result is that fewer than 6% of rare disease have approved treatment options. The rare disease research community are adopting new approaches to this problem, where the focus is not on developing novel treatments, but on identifying approved drugs which could be repurposed to treat other conditions. These computational drug repurposing approaches require data and knowledge integration, to establish links between diseases, their symptoms, associated genes and drugs. Representing these concepts and relationships as a knowledge graph of machine-readable nodes and edges, enables predictions to be made about missing edges that may represent new drug target interactions.</p>
      </abstract>
      <kwd-group>
        <kwd>drug repurposing</kwd>
        <kwd>workflow</kwd>
        <kwd>knowledge graph</kwd>
        <kwd>rare diseases</kwd>
        <kwd>Huntington's disease</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Background</title>
      <p>
        Rare diseases are low-prevalent disorders caused by pathogenic mutations or harmful
environmental factors that can have chronic, debilitating, or life-threatening efects [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ]. Currently,
there are over 7000 rare diseases that afect approximately 10% of the European and American
populations, yet fewer than 6% have an approved treatment option [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ]. This highlights the
pressing need for developing therapies targeting rare diseases. However, the development of a
new drug can be a time-consuming and costly process, taking up to 15 years and costing as
much as US$2.5 billion [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ]. Consequently, the development of novel drugs for rare diseases,
which afect only a small number of individuals, is not pursued frequently, as it is less likely to
provide a return on investment for pharmaceutical companies [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ]. A cost-eficient and faster
way to provide drugs for rare diseases is via computational drug repurposing.
      </p>
      <p>
        Drug repurposing is the process of identifying for an already approved or investigational
drug a new use outside the scope of the original medical indication. For example, a drug could
be repurposed for a diferent disease, based on the knowledge that drugs target particular
pathways and disease mechanisms that may be shared by multiple diseases [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ]. Computational
drug repurposing aims to predict novel drug-disease associations, which can be achieved by
predicting drug-target interactions (DTIs) [
        <xref ref-type="bibr" rid="ref4">4</xref>
        ]. The computational prediction of new DTIs can
provide insights into potential pathological and drug mechanisms, as well as drug repurposing
and design, helping researchers to generate testable hypotheses in the lab [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ]. Network-based
data integration and machine learning-based methods for DTIs prediction can mitigate costly
and time consuming experimental verifications and are the current state of the art approaches
in computational drug repurposing [
        <xref ref-type="bibr" rid="ref10 ref6 ref7 ref8 ref9">6, 7, 8, 9, 10</xref>
        ].
      </p>
      <p>
        The landscape of biomedical information resources is heterogeneous and broad, yet most
current methods for predicting DTIs are limited to homogeneous networks or bipartite models,
failing to account for the intricate relationships among diverse data sources [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ]. To fully
exploit the potential of computational drug repurposing, we propose an automated workflow
for predicting DTIs that does take complex relationships among diverse data sources into
account. For our DTI prediction, we use the biophysical drug repurposing approach, which
is based on the hypothesis that structurally similar drug molecules share similar targets. We
extract biological data from multiple online databases using the BioKnowledge reviewer library
(https://github.com/SuLab/bioknowledge-reviewer), a tool developed by Queralt-Rosinach et
al. [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ]. This library integrates heterogenous knowledge and data into a knowledge graph,
which is a machine-readable semantic representation of relational information, where concepts
are encoded as nodes and relationships between them as edges. Now the prediction of DTIs
can be framed as a link prediction problem, where the goal is to identify missing edges in the
knowledge graph between drugs and targets that represent potential DTIs. To address this
challenge, our proposed automated drug repurposing workflow leverages both network-based
analysis and machine learning methods. Network-based methods help to identify potential
interactions based on network topology and structural features, while machine learning methods
can use more complex data features to make predictions. By combining these approaches, our
automated workflow is able to discover new DTIs that can be further used by biologists to
generate drug repurposing hypotheses that can be tested in the lab.
      </p>
      <p>
        The drug repurposing workflow generalises to any rare disease, but as a proof-of-concept,
we focus on Huntington’s disease (HD). HD is a rare neurodegenerative disorder of the central
nervous system characterized by dementia, involuntary movements due to the movement
disorder chorea and behavioural and psychiatric disturbances [
        <xref ref-type="bibr" rid="ref12">12</xref>
        ]. There are some symptomatic
treatments available but because their efects are limited, there is a constant need for better,
modifying drugs to treat symptoms of the disease [
        <xref ref-type="bibr" rid="ref12">12</xref>
        ].
      </p>
      <p>We present the automated data mining workflow as a web application, using Flask, which
makes the workflow accessible for researchers with no technical expertise. The Python code for
running this app is accessible as a Docker container, available at https://github.com/carmenreep/
DrugRepurposing, which runs on a laptop with minimal specifications (at least CPU 2.80 GHz).</p>
    </sec>
    <sec id="sec-2">
      <title>2. Methods</title>
      <sec id="sec-2-1">
        <title>2.1. Workflow steps</title>
        <p>Figure 1 provides an overview of the proposed drug repurposing workflow. The workflow
comprises four main steps: (1) creation of the knowledge graph, (2) embedding of the graph,
(3) creation of edge representations, and (4) training of a supervised machine learning model.
Besides finding missing edges (potential DTIs) in the knowledge graph, the machine learning
model also predicts the interaction types of these missing edges. Good embeddings for this link
prediction task were achieved through enriched information on the drugs and target sites.</p>
      </sec>
      <sec id="sec-2-2">
        <title>2.2. Data sources</title>
        <p>
          To obtain both human and animal biological data and metadata, the workflow uses the Monarch
Biolink API version 1.1.14 (https://api.monarchinitiative.org/api). The Monarch Initiative, a
collaborative, open science project, seeks to semantically integrate genotype-phenotype information
from numerous sources and species [
          <xref ref-type="bibr" rid="ref13">13</xref>
          ]. To integrate drugs into the knowledge graph, the
workflow utilizes the Drug-Gene Interaction Database (DGIdb), a web resource that aggregates
information on drug-gene interactions and druggable genes from various sources, including
publications, databases, and web-based resources [
          <xref ref-type="bibr" rid="ref14">14</xref>
          ]. We obtain drug-gene information from
DGIdb using its API (version v2) available at https://dgidb.org/api.
        </p>
      </sec>
      <sec id="sec-2-3">
        <title>2.3. Knowledge graph construction</title>
        <p>
          The workflow leverages the BioKnowledge reviewer library to extract and integrate data from
online sources into a knowledge graph. Starting with a list of seed nodes, a Monarch network
is created by including the first layer of neighbours and relations from Monarch for each
seed node, along with their ortholog-phenotype nodes. A seed node can take the form of
a disease phenotype MIM number, such as ’143100’ representing Huntington’s disease. The
edges are formatted as triples, where each triple includes additional information such as the
reference Uniform Resource Identifier (URI), the date when the information was obtained, and
more information about the semantics of the relation. Nodes in the graph are identified using
diferent biomedical ontologies in the OBO Foundry [https://doi.org/10.1093/database/baab069]
maintained or used by Monarch and contain other attributes such as semantic group, URI, label,
name, synonyms, and description. The URI serves as a link to a web page that provides a more
detailed description of the ontology term representing the node [
          <xref ref-type="bibr" rid="ref11">11</xref>
          ].
        </p>
        <p>
          To obtain drug-target information, we use DGIdb and take all genes (targets) in the Monarch
graph as seeds. First, we need to map the Monarch genes to Entrez Gene identifiers (Entrez
ID), which are used as a standard gene identifier system [
          <xref ref-type="bibr" rid="ref15">15</xref>
          ]. We accomplish this using the
BioThings MyGene.info API, accessed with the Python wrapper biothings_client version v0.2.6
[
          <xref ref-type="bibr" rid="ref16">16</xref>
          ]. For each gene, we obtain a list of drugs (ID, name) that interact with the gene, along
with the type of interaction and interaction source. The drug identifiers are from either the
ChEMBL Database [
          <xref ref-type="bibr" rid="ref17">17</xref>
          ] or the Wikidata knowledge base [
          <xref ref-type="bibr" rid="ref18">18</xref>
          ]. There are various interaction
types, such as ’activator’, ’blocker’, but to improve our predictions, we used the interaction
direction (inhibits or activates) instead of the interaction types themselves [
          <xref ref-type="bibr" rid="ref14">14</xref>
          ]. Because some
relations lack direction, we introduced a third category called ’regulates’. Table 1 shows each
interaction direction category along with the interaction types belonging to that category.
We mapped these three interaction groups to URIs using the OBO Relations Ontology (RO)
https://www.ebi.ac.uk/ols/ontologies/ro, version 2022-05-23.
        </p>
        <p>
          To enable the biophysical drug repurposing approach, it is necessary to identify structurally
similar drugs. In our workflow, we use the Tanimoto coeficient [
          <xref ref-type="bibr" rid="ref19">19</xref>
          ] to measure the similarity
between drugs. To achieve this, we first retrieve the SMILES chemical structure notation for
each drug in our graph using the BioThings MyGene.info API accessed with the biothings_client
version v0.2.6 [
          <xref ref-type="bibr" rid="ref16">16</xref>
          ]. Subsequently, we convert the SMILES structures into RDKit molecule
objects using the RDKit Python package version 2022.3.2 with the Chem module [
          <xref ref-type="bibr" rid="ref20">20</xref>
          ]. The
RDKit molecule objects are then transformed into Morgan fingerprints using the
GetMorganFingerprintAsBitVect() function of the AllChem RDKit module. Using the BulkTanimotoSimilarity()
function of the DataStruct module from RDKit, we can calculate Tanimoto coeficients between
every possible pair of drugs in the graph. This results in a large number of weighted edges,
which can lead to a complex network. To mitigate this, we adopt a method by Thafar et al. [
          <xref ref-type="bibr" rid="ref21">21</xref>
          ],
where all similarity scores are ranked in descending order and only the top-10 most similar
drugs are retained, similar to the k-nearest neighbours algorithm. Finally, we label all similarity
edges with the ‘CHEMINF:000481’ ID, ‘http://semanticscience.org/resource/CHEMINF_000481’
URI, and the human readable string description ‘similar to’.
        </p>
        <p>
          The final graph is transformed into a Resource Description Framework (RDF) graph [
          <xref ref-type="bibr" rid="ref22">22</xref>
          ],
A drug is a chemical substance that contains one or more
active ingredients that regulate one or more biological
processes.
        </p>
        <p>A gene is part of a nucleic acid that contains all the
necessary elements to encode a functional transcript.</p>
        <p>A disease is the outward manifestation of one or more
disorders.</p>
        <p>A genotype is a functional specification of a biological
entity in terms of its genetic composition (or lack thereof).</p>
        <p>A genomic sequence variant is part of a nucleic acid
which is compositionally diferent than another reference
genomic part.</p>
        <p>A phenotype is an observable characteristic of an
individual.</p>
        <p>
          A pathway is an efective specification that outlines a
set of actions that forms a way to achieve an objective.
count
1352
284
194
127
106
71
49
where each entity, relationship, and entity class (gene, drug, etc.) is represented as an ontology
term by its URI. As Monarch did not provide identifiers for the entity classes, we manually
mapped the entity class labels to terms in ontologies and used their URIs. To achieve this,
we utilized the Semanticscience Integrated Ontology (SIO) [
          <xref ref-type="bibr" rid="ref23">23</xref>
          ]
(http://semanticscience.org/ontology/sio.owl) version 1.53 and obtained URIs using the URI resolution service identifiers.org
(https://registry.identifiers.org/registry/sio , accessed June 2022). Table 2 presents the specific URIs
we used for the entity classes. To perform this transformation, we extended the BioKnowledge
reviewer by using the RDFLib Python package version 6.1.1 [24] and ensured that the graph
was stored in Turtle format [25].
        </p>
      </sec>
      <sec id="sec-2-4">
        <title>2.4. Graph embedding</title>
        <p>To prepare the knowledge graph for embedding, we first remove all known drug-gene
interactions from the graph. This is important to prevent bias in the prediction task, as keeping these
edges would make the embedding vectors of the drugs very similar to the embedding vectors of
the genes they interact with. We therefore split the graph into two separate graphs, one for
drugs and one for genes. Each graph is then embedded separately using a graph embedding
algorithm. After the embedding, the drug vectors are fused with the gene vectors to obtain
drug-gene edges, which are used for training the machine learning model, as explained in more
detail in the XGBoost prediction model section below.</p>
        <p>Celebi et al. [26] compared diferent knowledge graph embedding methods for drug-drug
interaction prediction, and found that RDF2Vec with Skip-Gram generally outperforms other
methods. Therefore, this workflow employs RDF2Vec for graph embedding. RDF2Vec adapts
the language modelling approach of Word2Vec to RDF graph embeddings [26]. First, random
walks are performed over the graph to generate sequences of entities and relations. Then, the
Skip-Gram model is used to learn one embedding for each entity/relationship in the graph. After
training, semantically and syntactically similar entities/relationships have similar embeddings
[26]. For the prediction task in this study, only the drug and gene vectors are of interest, and
therefore, only these vectors were selected for further computation.</p>
        <p>For RDF2Vec, the Python function RDF2VecTransformer from the rdf2vec module of the
package pyrdf2vec version 0.2.3 is used [27]. The maximum depth of one walk is set to 4 and
for each entity in the graph, the maximum number of walks is set to 10.</p>
      </sec>
      <sec id="sec-2-5">
        <title>2.5. Fused embeddings for link prediction</title>
        <p>To train a supervised machine learning model, it is essential to have both positive and negative
samples of data [26]. The positive samples are all known interactions (regulates, inhibits, or
activates). The negative samples can be obtained from unknown interactions between drugs
and genes. Edge embeddings for positive and negative samples are generated by adopting a
node embedding fusion approach. For every possible drug-gene combination, we obtain one
embedding by fusing the drug embedding and the gene embedding with the Hadamard operator,
which is a strong operator for learning edge features in link prediction tasks [28]. We then add
the class of the interaction (inhibits, activates, or regulates) to the resulting embedding. For
edges that do not exist in the graph, we assign the label ”unknown” to represent the unknown
interaction class.</p>
        <p>The prediction data for our machine learning model includes all unknown drug-gene
interactions involving genes that contribute to the disease phenotype of interest. The negative samples
of the training data are all unknown interactions that are not the prediction data. However, the
number of negative samples significantly outweighs the number of positive samples. Including
all of these negative samples could result in data imbalance and afect the performance of our
model [26]. To address this issue, we decided to downsample the negative cases by randomly
selecting negative samples with a sample size equal to the class in the positive set with the
largest number of interactions (regulates, inhibits, or activates).</p>
      </sec>
      <sec id="sec-2-6">
        <title>2.6. XGBoost prediction model</title>
        <p>
          For our machine learning model, we utilized the XGBoost classifier proposed by Thafar et al.
in 2021 [
          <xref ref-type="bibr" rid="ref21">21</xref>
          ]. To implement the model, we used the XGBClassifier() function from the Python
package xgboost (version 1.3.3) [29]. We set the learning objective to ‘multi:softmax’, which
allows XGBoost to optimize the likelihood of each class label and assign a probability to each
possible class.
        </p>
        <p>To address minor class imbalance in our positive sample, we computed sample weights using
the compute_sample_weight() function from sklearn [30] version 1.1.1. These weights are then
used for training the model, which provides some bias towards the minority classes during
training.</p>
        <p>To optimize the hyperparameters of our model, we conducted a randomized search on
the search space presented in Table 3 using the RandomizedSearchCV() function from the
model_selection module of the sklearn Python package (version 1.1.1) [30]. We set the number
of parameter setting combinations to be tested (n_iter) to 20.</p>
        <p>Given the challenge of identifying negative examples of drug-target pairs, as unlinked drugs
and targets may simply represent drug-target pairs that have not been identified yet, we opted
against conducting an error analysis. Instead, the model performance is assessed using the
repeated stratified k-fold cross-validation technique alongside the F1-score metric.. The number
of subsets for the k-fold cross validation is set to 10 and the number of repeats is set to 5. During
each iteration of the process, the F1-score is calculated and averaged for each class X. Finally,
the average F1-score over all k iterations and number of repeats is computed to obtain the final
evaluation metric.</p>
        <p>The best hyperparameters are used to build the final model. The confidence of each prediction
is obtained using the predict_proba() function of xgboost version 1.3.3, which returns the
probability of an interaction belonging to its predicted class.</p>
      </sec>
      <sec id="sec-2-7">
        <title>2.7. DTI ranking and validation</title>
        <p>For every gene in the graph that is associated with the symptom of interest, an interaction type
and score is predicted for every drug in the dataset, provided that this interaction does not
exist in the graph. To prioritize the most promising drug candidates for further investigation,
we perform a ranking step based on the predicted positive interactions. First, we remove
all predictions with a confidence score lower than 0.9 to focus only on the most confident
predictions. Next, we rank the drugs based on the number of positive interactions they have
with the genes associated with the symptom. This ranking approach is based on the hypothesis
that drugs with more positive interactions with genes that cause a symptom are more likely
to be efective in alleviating that symptom. In the case of drugs with the same number of
interactions, we use the sum of prediction confidence scores as a secondary ranking criterion,
with drugs having higher sums being ranked higher.</p>
      </sec>
    </sec>
    <sec id="sec-3">
      <title>3. Results</title>
      <p>Our workflow was initially run with the terms ”huntington’s disease” (OMIM number ‘143100’)
and ”chorea” (‘HP:0002072’) as seeds representing the disease and symptom fields respectively,
for constructing the knowledge graph. The graph was created on 2022-06-27 and has in total
2189 nodes and 17467 edges. Figure 2 provides an overview of all entities and relationships in
the graph and Table 4 shows the identifiers and descriptions of each relation between nodes
in this graph. The graph includes 1352 drugs and 284 genes, resulting in a total of 383,968
edge representations, of which 1753 are known (1301 regulates, 391 inhibits, and 61 activates).
This graph has 200 genes that are associated with the symptom chorea, which are the genes
of interest, and there are 1077 known drug-gene edges with these 200 genes, indicating that
the prediction data consists of 269,323 unknown interactions of potential interest. To deal
with the imbalance between the larger number of negative samples and the comparatively
smaller number of positive samples, 1301 negative samples were randomly selected, to balance
the number of the largest interactions in the positive class (regulates). The best XGBoost
hyperparameters can be found in Table 5, and the F1 score with these hyperparameters is
0.867. The trained XGBoost model was used to predict the classes of the unknown drug-gene
interactions of interest.</p>
      <p>Table 6 shows the predicted top ten ranked drugs that interact with genes that are associated
with the phenotype chorea. We manually explored the top ranked predictions for HD that
are associated with chorea. Below we present the top three candidates. Table 7 presents the
two highest ranked drugs and the genes that these drugs have a positive predicted interaction
with. The top predicted ranked drug is CHEMBL29097. CHEMBL29097 (synonym MK-886)
is an inhibitor of 5-lipoxygenase-activating protein activity, currently in preclinical phase. It
has been found that 10 microM MK-886 can abolish the biosynthetic production of cysteinyl
leukotrienes (CysLTs), which is suggested to be involved in brain inflammation and neurological
diseases [33].In addition to its anti-inflammatory activity, MK-886 has been shown to activate
the proteasome which is known to have a causative role in HD [34]. Impaired function of the
proteasome leads to the formation of intracellular aggregates in the nucleus as the proteasome
cannot clear eficiently misfolded huntingtin proteins [ 35].</p>
      <p>The second highest ranked drug is baicalein. Baicalein (CHEMBL8260) is a flavonoid isolated
from the traditional Chinese medicinal herbal Scutellaria baicalensis Georgi, currently on Phase
2. Baicalein has known anti-inflammatory and neuroprotective eficacy in neurodegenerative
disease models [36]. Rui et al. [36] studied the efects of baicalein on inflammasome-induced
neuroinflammation in Parkinson’s disease(PD) and found that baicalein can suppresses
MPTPinduced nigral dopaminergic neuron death, glial activation, and motor dysfunction in mice
by suppressing the NLRP3/caspase-1/GSDMD pathway. In addition, several studies have have
demonstrated that baicalein protects neurons in animal models of Alzheimer’s disease (AD) and
PD by inhibiting neuroinflammation [ 37].</p>
      <p>Amphotericin b (CHEMBL267345) was another drug on our list that ranked very high.
Ampotericin b is an approved antifungal drug used to treat serious fungal infections. Experimental
evidence shows that some antibiotics and antifungal medication have neuroprotective action
through anti-aggregating activity on disease-associated proteins [38]. Although this drug has
been shown to cause a delay in the formation of amyloid- , it was also found to induce toxicity
[39]. However, Soler et al., [40] developed a derivative of amphotericin that has anti-aggregating
action but lacks toxicity and antimicrobial activity [38].</p>
      <sec id="sec-3-1">
        <title>3.1. Other rare diseases</title>
        <p>To demonstrate the reusability of our approach, we also applied our methodology to another
rare disease that currently lacks treatment; Spinocerebellar ataxia type 1 (SCA1). We used as
seeds the terms ”SCA1” (OMIM number ’164400’) and the symptom ”hyporeflexia” (HP:0001265)
to run our drug repurposing workflow and below we describe few of the top hits.</p>
        <p>The first prioritized drug by our workflow was Dovitinib (CHEMBL522892) currently in phase
3. Dovitinib is a pan receptor tyrosine kinase (RTK) inhibitor that has anti-tumor activity in pre
clinical models of several cancers [41]. It has been recently suggested as a candidate treatment
for AD because it normalizes  amyloid mediated transcriptional responses by targeting the
CREB3L2-ATF4 heterodimerization which is responsible for the majority of the transcriptional
changes occuring in AD neurons [42]. Its well tolerated safety profile and the ability to cross
the blood brain barrier [42] makes it an interesting candidate for AD but also potentially for
other neurodegenerative disorders that exhibit similar disease mediated changes like AD.</p>
        <p>The second predicted drug on the list was broquinaldol (CHEMBL1394319), a small molecule
that has antifungal and antibacterial activity. This is an investigational drug that was found to
have activity against thyroid cancer in a high throughput screening experiment [43]. However,
there is currently no evidence for being associated with neurodegenerative diseases.</p>
        <p>Number three on the candidate drug list for SCA1 was an interesting compound, astemizole
(CHEMBL296419). Astemizole is an approved second generation antihistamine drug [44] that
has been found to rescue motor phenotype in a Drosophila model of PD [45].</p>
      </sec>
    </sec>
    <sec id="sec-4">
      <title>4. Discussion</title>
      <p>
        This work presents a novel disease-drug profiling approach to identifying potential candidate
compounds that could alleviate the symptoms of a rare disease. It combines two established, and
widely accepted approaches, of mining rare disease-specific data from multiple public databases
into a knowledge graph [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ], and graph-based machine learning approaches to identifying
drugtarget interactions [
        <xref ref-type="bibr" rid="ref21">21</xref>
        ]. The result is an automated workflow which makes disease-drug profile
predictions targeted to specific rare diseases. We demonstrated its utility using predictions from
Huntington’s disease and SCA1.
      </p>
      <p>
        The advance that this work provides to the field is the use of rare disease specific knowledge
graphs. Using BioKnowlegde reviewer in a drug repurposing automated workflow enables
to learn from a comprehensive view of the underlying druggable rare disease biology and
pathogenesis of interest. This is advantageous over current integration methods used in rare
disease research, which use information about thousands of complex disorders [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ], because
it leverages knowledge for precision medicine. Another advantage is that by comparing to
existing solutions [
        <xref ref-type="bibr" rid="ref10 ref8 ref9">8, 9, 10</xref>
        ], our method harnesses heterogeneous and expressive semantic
graphs for DTI prediction beyond bipartite networks. Integrating new types of entities with
Semantic Web technologies enables us to represent more complex relations around drugs and
targets, and it opens the possibility of learning from them and exploiting the semantics by
means of methods such as RDF2Vec graph embedding methods.
      </p>
      <p>
        Through sophisticated graph-based algorithms, we can traverse the knowledge graph to
identify patterns in the data and predict potential new relationships between drugs and diseases.
We demonstrated that knowledge graphs and graph machine learning used streamlined in an
automated workflow gives testable DTI hypotheses for drug repurposing in the rare disease
area. This can support researchers to systematically generate compound prioritization coupled
with well-designed validation experiments to discover treatments for rare diseases in a timely
and cost-efective manner. One limitation is that we did not integrate domain expert knowledge
on disease pathobiology with patient data, which can be the basis for highly innovative drugs.
While [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ] gave a solution to include expert knowledge in graphs, access to patient data is
a serious problem in health research. However, projects such as the EJP-RD1 are providing
Semantic Web based solutions for patient data sharing.
      </p>
      <p>Our results provide some interesting candidates that could potentially be of great value for
the rare disease community. Some of our prioritized drugs are already associated with other
neurodegenerative disorders (AD and PD) targetting neuroinflammation, which is a hallmark of
the HD pathology. Other candidates (Broquinaldol and Amphotericin b) belong to the class of
antifungal and antibacterial medication. These types of drugs have drawn a lot of attention and
although they are mainly used to treat infections new applications are being discovered. It has
been reported that antibiotics, for example Doxycycline and minocycline, have neuroprotective
efects due to their anti-inflammatory properties [ 46].</p>
      <p>The workflow is presented as a web application and yields promising results in a reusable and
reproducible way for the rare diseases community. In the future, it could be extended and
improved by the addition of experimentally validated negative interactions from reliable databases.
Our current approach uses unknown drug-gene pairs as negative samples for classification.
It is therefore possible that this set includes currently unknown positive interactions, which
may adversely afect model training [ 47]. Additionally, the knowledge graph could be extended
to include further information about each drug, such as side-efects and drug interactions,
and additional input seeds could be obtained from the Monarch database, such as genes and
metabolites associated with the particular disease, or related diseases. Moreover, integrating the
predicted embeddings into the knowledge graph would enable the evaluation of performance
across diverse prediction methods, ofering valuable insights into model eficacy and versatility.
Lastly, it is important to note that the RDF graphs are stored in turtle format at the location
where the code is executed, to enable further use in code execution and analysis. The RDF
graph is currently not served as a live RDF/SPARQL endpoint, which presents an opportunity
for improvement in our approach. This aligns with broader eforts in the field to enhance the
transparency and accessibility of machine learning outcomes.</p>
    </sec>
    <sec id="sec-5">
      <title>5. Conclusion</title>
      <p>Integrating data into knowledge graphs with state-of-the-art graph-based machine learning
methods results in a novel automated drug repurposing workflow, specifically suited for rare
diseases, where data tends to be sparse and distributed. The workflow produces a ranked
list of candidate compounds, which serve as new hypotheses for drug treatments. The drug
repurposing workflow generalises to any rare disease, but as a proof-of-concept, we focused on
Huntington’s disease, and a related condition, SCA1. We identified several promising candidate
drugs for Huntington’s Disease for the symptom chorea, demonstrating the potential of our
approach. With further testing and validation, these candidates could be explored as potential
treatments for the disease. The workflow is provided as a web application, in a publicly available
Docker container. It is therefore accessible for researchers with no technical expertise and is a
reusable and reproducible application for the rare disease community.
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