 Research
 Open Access
A physaruminspired prizecollecting steiner tree approach to identify subnetworks for drug repositioning
 Yahui Sun†^{1},
 Pathima Nusrath Hameed†^{1, 2, 3},
 Karin Verspoor^{4} and
 Saman Halgamuge^{5}Email author
https://doi.org/10.1186/s1291801603713
© The Author(s) 2016
 Published: 5 December 2016
Abstract
Background
Drug repositioning can reduce the time, costs and risks of drug development by identifying new therapeutic effects for known drugs. It is challenging to reposition drugs as pharmacological data is large and complex. Subnetwork identification has already been used to simplify the visualization and interpretation of biological data, but it has not been applied to drug repositioning so far. In this paper, we fill this gap by proposing a new Physaruminspired PrizeCollecting Steiner Tree algorithm to identify subnetworks for drug repositioning.
Results
Drug Similarity Networks (DSN) are generated using the chemical, therapeutic, protein, and phenotype features of drugs. In DSNs, vertex prizes and edge costs represent the similarities and dissimilarities between drugs respectively, and terminals represent drugs in the cardiovascular class, as defined in the Anatomical Therapeutic Chemical classification system. A new Physaruminspired PrizeCollecting Steiner Tree algorithm is proposed in this paper to identify subnetworks. We apply both the proposed algorithm and the widelyused GW algorithm to identify subnetworks in our 18 generated DSNs. In these DSNs, our proposed algorithm identifies subnetworks with an average Rand Index of 81.1%, while the GW algorithm can only identify subnetworks with an average Rand Index of 64.1%. We select 9 subnetworks with high Rand Index to find drug repositioning opportunities. 10 frequently occurring drugs in these subnetworks are identified as candidates to be repositioned for cardiovascular diseases.
Conclusions
We find evidence to support previous discoveries that nitroglycerin, theophylline and acarbose may be able to be repositioned for cardiovascular diseases. Moreover, we identify seven previously unknown drug candidates that also may interact with the biological cardiovascular system. These discoveries show our proposed PrizeCollecting Steiner Tree approach as a promising strategy for drug repositioning.
Keywords
 Steiner tree problem
 Subnetwork identification
 Drug similarity network
 Big data
 Physarum polycephalum
Background
Drug repositioning aims to identify new therapeutic effects for known drugs. By repositioning known drugs, drug development time, costs and risks can be reduced significantly [1–3]. There are mainly two challenges to reposition drugs. First, pharmacological data is usually big and difficult to analyze [4, 5]. Second, pharmacological data is highly complex and involves various drug characteristics, including their chemical structures, molecular targets and induced gene expression signatures [6].
Existing drug repositioning methods can be divided into three categories; datadriven methods [1–3, 6], textmining methods [7, 8], and networkbased methods [3, 9–11]. The datadriven methods reposition drugs by analyzing pharmacological data using statistical and machine learning concepts such as statistical estimations, classification and clustering [1, 6, 10]. Because of the overlapping nature of pharmacological data [3, 11], the evaluation process of the datadriven methods is complicated [11]. On the other hand, text mining methods use efficient text analytics and semantic inference approaches to reposition drugs [7, 8], but their application is limited by the availability of relevant biomedical publications and reports. Networkbased methods are emerging methods that use networks to represent pharmacological data [10]. These methods typically reposition drugs by identifying drug candidates in multiple decomposed subnetworks [10–12]. Even though multiple therapeutic effects are expected to be found, it requires a long time to analyze these multiple decomposed subnetworks.
Subnetwork identification is a technique to identify a single smallscale subnetwork from a largescale network. It differs from previous networkbased methods in that we only need to analyze a single identified subnetwork. This method has already been proven to be efficient to simplify the visualization and interpretation of proteinprotein interaction networks [13–16], proteinDNA interaction networks [17], generegulatory networks [18] and metabolic networks [19]. However, to our knowledge, no one has applied subnetwork identification to pharmacological networks so far. This paper will fill this gap by exploring the application of subnetwork identification to drug repositioning for the first time.
The PrizeCollecting Steiner Tree (PCST) approach is gaining traction in subnetwork identification, but has not been tried with pharmacological data yet. Existing methods are slow and nondeterministic, chance based. This method is heuristic, i.e. it is not an exact solution, but it is deterministic. The definition of PrizeCollecting Steiner Tree Problem (PCSTP) is given as follows: let G=(V,E,p,c) be a connected, undirected graph, where V is the set of vertices, E is the set of edges, p is a function which maps each vertex in V to a nonnegative number called the prize, and c is a function which maps each edge in E to a positive number called the cost. Let T be a subset of V called terminals. The aim of PCSTP is to find a connected subgraph G ^{′}=(V ^{′},E ^{′}),V ^{′}⊆V,E ^{′}⊆E which contains all the terminals while minimizing the objective function \(c(G')=\sum _{e \in E'}{c(e)}\sum _{v \in V'}{p(v)}\), and the optimal solution of PCSTP is called Steiner Minimum Tree (SMT) in G for T.
The algorithms for PCSTP can be divided into two groups: exact algorithms and heuristic algorithms. Exact algorithms can find SMT, but are slow in large graphs [20]. On the contrary, heuristic algorithms can find solutions faster, but they may only find close approximations to SMT [21]. The Drug Similarity Networks (DSN) we used in this paper are large graphs with 548 vertices and thousands of edges. Thus, it is necessary for us to use heuristic algorithms in DSNs. Many heuristic algorithms have been proposed to solve PCSTP; the GW algorithm (named for Michel X. Goemans and David P. Williamson) is the most popular one [22–25]. However, in our simulations we observe that GW algorithm does not perform well in DSNs. Physaruminspired algorithms are emerging heuristic algorithms that have already been used to solve PCSTP [26]. In this paper, we propose a new Physaruminspired algorithm called Physaruminspired Subnetwork Identification Algorithm (PSIA) to identify subnetworks in DSNs. Our proposed PSIA outperforms the popular GW algorithm by identifying more suitable subnetworks for drug repositioning. Furthermore, by analyzing the identified subnetworks, we find evidence to support previous discoveries that some drugs could be repositioned for cardiovascular diseases. These discoveries show that our proposed PrizeCollecting Steiner Tree approach is effective and efficient to reposition drugs.
Methods
Generation of drug similarity networks
We build Drug Similarity Networks (DSNs) to represent the similarities between drugs. There are several pharmacological databases at present, such as PharmGKB [27], DrugBank [5, 28], SIDER [29], etc. We generate DSNs using the data following the work of Zhang et al. [30], which includes data from DrugBank and SIDER. Similarities between drugs are quantified in DSNs based on their chemical, therapeutic, protein and phenotype features. There are 881 chemical features, 719 therapeutic features, 775 protein features, and 1385 phenotype features considered for each drug. Therefore, 3760 (881+719+775+1385) features in total are considered for each drug.

vertex: Each vertex represents a drug. There are 548 drugs included in each of our generated DSNs [30]. Each drug is associated with a 1×3760 feature vector where binary numbers represent the presence or absence of each individual feature that we consider. Note that, binary numbers have already been widely used to describe drug features [6, 30, 31].

edge: Each edge represents the association between two drugs.

terminal: Each terminal represents a vertex which must be contained in the identified subnetworks of DSNs. In each DSN, the terminal set represents a cardiovascular subclass of drugs in the Anatomical Therapeutic Chemical (ATC) classification system [32]. ATC is used for the classification of active ingredients of drugs according to the organ or system on which they act and their therapeutic, pharmacological and chemical properties. There are 9 subclasses in the cardiovascular class (C); cardiac therapy (C01), antihypertensives (C02), diuretics (C03), peripheral vasodilators (C04), vasoprotectives (C05), beta blocking agents (C07), calcium channel blockers (C08), agents acting on the reninangiotensin system (C09), and lipid modifying agents (C10). There are 104 drugs in total in these subclasses. (Notably, there is no C06 in the ATC classification system).

edge cost: Each edge cost represents the quantified dissimilarity between two drugs. The bigger the edge cost is, the more dissimilar the two drugs are. The edge cost is calculated using the Jaccard coefficient, as shown in the formula below.$$ \begin{aligned} c_{ij}=1\frac{\sum_{k=1}^{n}{\upsilon_{i}(k) \cap \upsilon_{j}(k)}}{\sum_{k=1}^{n}{\upsilon_{i}(k) \cup \upsilon_{j}(k)}} \end{aligned} $$(1)
where i and j are indexes of two different drugs, c _{ ij } is the cost of edge (i,j), n is the total number of features considered for each drug, which is 3760, and υ _{ i } is the feature vector of drug i.

vertex prize: A prize is associated with each vertex to signify the similarity between the drug represented by this vertex and all the drugs represented by terminals. The vertex prize is calculated using the following equation.$$ p_{i}=\frac{\sum_{j \in T, j \neq i}{\frac{1}{1+c_{ij}}}}{T} $$(2)
where p _{ i } is the prize of vertex i, T is the set of terminals, and T is the total number of terminals.
The objective of PCSTP is to minimize the netcost of edge costs and vertex prizes. Thus, the subnetwork identified using the PCST approach tends to include edges with small costs and vertices with big prizes. In our generated DSNs, edges with small costs connect drugs with big similarities, and vertices with big prizes represent drugs that are similar to the drugs represented by terminals. Hence, a subnetwork of DSN that includes drugs similar to the drugs represented by terminals is expected to be identified using the PCST approach.
Complete graphs with different sets of terminals can be generated using the five graph components defined above. Since the sets of vertices are identical, the sets of edge costs are also the same in different complete graphs. However, the sets of vertex prizes are different as the sets of terminals are different in different complete graphs. PCSTP algorithms perform better in sparse graphs than in complete graphs [22]. Therefore, we propose two sparse graph generation algorithms to prune the complete graphs to produce sparse graphs for DSNs.
DSNs in sparse graphs are generated using the two proposed algorithms. Because no vertex has been deleted in any of these sparse graphs, subnetworks containing similar drugs can be identified in the sparse graphs generated by both proposed algorithms. Nevertheless, in our simulations we find that PCSTP algorithms have better performances in DSNs generated using the second proposed algorithm than in DSNs generated using the first proposed algorithm.
The proposed physaruminspired subnetwork identification algorithm
Physarum polycephalum is a large amoeboid organism that has displayed many intelligent behaviors [35–37]. The Physaruminspired Subnetwork Identification Algorithm (PSIA) is proposed in this paper to identify subnetworks in DSNs. The proposal of PSIA is inspired by the Lowestcost Network Physarum Optimization algorithm (LNPO) [26]. LNPO is designed to find PCSTP solutions as close to SMT as possible. There are two iteration processes in LNPO, the inner iteration process and the outer iteration process. A feasible PCSTP solution can be found in each inner iteration process. The outer iteration process is used to find multiple solutions and choose the solution which is closest to SMT as the final solution. However, SMT or close approximations to SMT may not be suitable for drug repositioning. There is no need to use the outer iteration process in PSIA. Thus, only the inner iteration process is included in PSIA. Moreover, the subnetwork identified in the inner iteration process may not be a tree. Hence, a postprocessing technique is used in PSIA to ensure that the identified subnetwork is a tree.
where D _{ ij } is the edge conductivity, C _{ ij } is a netcost for edge (i,j), P r _{ i } and P r _{ j } are pressures at vertex i and j, c _{ ij } is the cost of edge (i,j), p _{ i } and p _{ j } are the prizes of vertex i and j, d _{ i } and d _{ j } are the degrees of vertex i and j, and N=m a x(p _{ k }),k⊆V.
where V(j) is the set of vertices linked to vertex j, and I _{0} is the flux flowing into each source node. Let the pressure at the sink node be 0, and other pressures can be calculated by solving the network Poisson equation. In our simulations, we find that the netcosts of edges in DSNs are quite close to each other. In this case, if all the edge conductivities are the same, then the network Poisson equation may not be solvable. Thus, we give each edge conductivity a random initial value to make the network Poisson equation solvable.
Because the sink node is chosen probabilistically in PSIA, different subnetworks can be identified in a single DSN by employing PSIA for multiple times. To reposition drugs, we employ PSIA for multiple times in each DSN to identify multiple subnetworks. Then, we select the most suitable subnetwork from them for drug repositioning.
GW algorithm
Besides the proposed PSIA, we also use the popular GW algorithm to identify subnetworks in DSNs. GW algorithm was proposed by Michel X. Goemans and David P. Williamson [22], and it is widely used to solve PCSTP [23–25]. However, GW algorithm is designed to solve PCSTP instances with a single terminal, which is called the root. While in DSNs, there are multiple terminals. In this paper, we apply GW algorithm to DSNs by randomly choosing a single terminal to be the root and give other terminals big prizes.
We first choose a single terminal to be the root. Then, we give each of the other terminals a big prize M, and \(M > \sum _{(i,j) \in E} c_{ij}\). This big prize ensures that all the terminals will be included in the subnetwork identified by GW algorithm.
To identify a subnetwork, we initially set each vertex as a component. Each component has a surplus (initially the vertex prize). A component is active when its surplus is bigger than 0. However, the root component will always be inactive. In addition, each edge has a deficit (initially the edge cost), and an edge is active when it is not connecting two vertices in the same component.
Setting a constant Δ, we iteratively do this: the surplus of all active components are reduced by Δ, the deficit of any active edge adjacent to a single active component is reduced by Δ, and the deficit of any active edge adjacent to two active components is reduced by 2Δ. After the update of surpluses and deficits, we check that: if an edge’s deficit is not above 0, we merge the two components linked by this edge and give the new merged component the sum of surpluses of the two components being merged; if a component’s surplus is not above 0, we deactivate this component. The iteration will end until there is no active component disconnected with the root component.
After the iteration, the vertices and the edges in the root component will be a tree. Then, we delete some vertices and edges by strong pruning the tree. The strong pruning idea was proposed by Johnson et al. in 2000 [25]. In the general GW algorithm, MST of the strong pruned tree is recommended to be found to increase the total netprize of the identified subnetwork. However, in this paper, the aim of identifying subnetworks is to identify drug candidates, which are vertices in DSNs. Therefore, it is not necessary to find MST of the strong pruned tree, and we can directly use the strong pruned tree as the identified subnetwork for drug repositioning. The MATLAB coding of GW algorithm is publicly available at https://github.com/YahuiSun/GWtoidentifysubnetworks.
Subnetwork evaluation for drug repositioning
As described above, we select each of the 9 cardiovascular subclasses individually as the terminal set, and all the other drugs in the DSN are considered as nonterminal vertices. We then apply two sparse graph generation algorithms to generate two sparse graphs for each cardiovascular subclass, resulting in 18 DSNs. We name each DSN as D_i_a or D_i_b, in which i represents the origin of the terminal set (subclass C01, C02, C03, C04, C05, C07, C08, C09, or C10), a or b represents the first or the second sparse graph generation algorithm that is used to generate that particular DSN.
where I _{ c } is the number of nonterminal vertices that represent drugs that are in both the identified subnetwork and the cardiovascular class (C; including drugs in all 9 cardiovascular subclasses), N _{ nc } is the number of vertices that represent drugs that are neither in the identified subnetwork nor in the cardiovascular class, V is the number of vertices in DSN (V=548 in this paper), T is the number of terminals in DSN. Notably, our computational trials show that identifying true positives (I _{ c }) and true negatives (N _{ nc }) are both important to subnetwork identification for drug repositioning.
We evaluate all the subnetworks identified by PSIA and GW algorithm. Then, we select the subnetworks with high RI as the suitable subnetworks for drug repositioning. Most drugs in these selected subnetworks have already been classified into the cardiovascular class. However, there may still be drugs in these selected subnetworks that have not been classified into the cardiovascular class yet. We consider the ‘notclassifiedyet’ drugs that have frequently occurred in these selected subnetworks as candidates for drug repositioning.
Results
There are two groups of DSNs generated in this paper. Each group contains 9 DSNs that are generated using 9 cardiovascular subclasses (C01, C02, C03, C04, C05, C07, C08, C09, C10). The DSNs in the first group (D_01_a to D_10_a) are generated using the first proposed sparse graph generation algorithm (Fig. 1), while the DSNs in the second group (D_01_b to D_10_b) are generated using the second proposed sparse graph generation algorithm (Fig. 2). These DSNs are publicly available at https://github.com/YahuiSun/DrugSimilarityNetwork.
Both PSIA and GW algorithm are used to identify subnetworks in two groups of DSNs. Since PSIA can identify multiple subnetworks in a single DSN, we employ PSIA for three times in each DSN to identify three subnetworks.
Subnetwork identification results in drug similarity network: D_01_a to D_10_a
DSN  Identified subnetwork  

ID  V  E  T  TOrigin  Algorithm  V’   E’   I _{ c }  Rand Index 
D_01_a  548  1500  22  C01  PSIA  60  59  7  79.8 
GW  354  353  53  41.4  
D_02_a  548  1500  12  C02  PSIA  37  36  10  81.9 
GW  339  338  62  45.0  
D_03_a  548  1500  13  C03  PSIA  35  34  4  80.4 
GW  330  329  61  46.5  
D_04_a  548  1500  4  C04  PSIA  9  8  1  81.1 
GW  322  321  66  47.4  
D_05_a  548  1500  9  C05  PSIA  25  24  4  80.9 
GW  281  280  52  51.2  
D_07_a  548  1500  15  C07  PSIA  25  24  1  81.8 
GW  301  300  55  50.3  
D_08_a  548  1500  8  C08  PSIA  23  22  2  80.2 
GW  320  319  63  47.8  
D_09_a  548  1500  16  C09  PSIA  29  28  4  82.5 
GW  322  321  56  47.0  
D_10_a  548  1500  8  C10  PSIA  18  17  1  80.7 
GW  354  353  66  42.6 
Subnetwork identification results in Drug Similarity Network: D_01_b to D_10_b
DSN  Identified subnetwork  

ID  V  E  T  TOrigin  Algorithm  V’   E’   I _{ c }  Rand Index 
D_01_b  548  1391  22  C01  PSIA  41  40  2  81.6 
GW  32  31  1  82.9  
D_02_b  548  1391  12  C02  PSIA  22  21  2  81.7 
GW  25  24  1  80.8  
D_03_b  548  1391  13  C03  PSIA  18  17  2  82.8 
GW  20  19  1  82.1  
D_04_b  548  1391  4  C04  PSIA  9  8  2  81.4 
GW  10  9  1  80.9  
D_05_b  548  1391  9  C05  PSIA  12  11  1  82.2 
GW  17  16  1  81.3  
D_07_b  548  1391  15  C07  PSIA  23  22  2  82.6 
GW  24  23  1  82.0  
D_08_b  548  1391  8  C08  PSIA  19  18  1  80.6 
GW  19  18  0  80.2  
D_09_b  548  1391  16  C09  PSIA  22  21  1  82.7 
GW  26  25  1  82.0  
D_10_b  548  1391  8  C10  PSIA  54  53  5  75.6 
GW  14  13  1  81.5 
The identified subnetwork with a higher RI in each DSN has been highlighted in Tables 1 and 2. It can be seen that every highlighted subnetwork has a smaller number of vertices than the other subnetwork in the same DSN. Thus, we observe that
Observation 1: In each DSN, the identified subnetwork which has a higher RI is generally smaller than the other identified subnetwork.
It is preferable to choose small subnetworks than large subnetworks for drug repositioning as analysis can be done more efficiently in small subnetworks. Most drugs included in our generated DSNs are not in the cardiovascular class. Hence, it is important for subnetworks to identify true negatives (N _{ nc } in Eq. (8)), and then avoid false positives (drugs that are not in the cardiovascular class). One counterexample is that the subnetworks identified by GW algorithm in D_01_a to D_10_a contain many false positives, and thus are large and not suitable for drug repositioning.
In D_01_a to D_10_a, all the highlighted subnetworks are identified by PSIA. In D_01_b to D_10_b, 7 out of 9 highlighted subnetworks are identified by PSIA. In 18 DSNs, the average RI of the subnetworks identified by PSIA is 81.1%, while the average RI of the subnetworks identified by GW algorithm is 64.1%. Therefore, the conclusion below can be made.
Conclusion 1: In our generated DSNs, PSIA generally outperforms GW algorithm in identifying subnetworks for drug repositioning.
D_01_a to D_10_a are generated using the first proposed sparse graph generation algorithm (Fig. 1), while D_01_b to D_10_b are generated using the second proposed sparse graph generation algorithm (Fig. 2). 8 out of 9 highlighted subnetworks in D_01_b to D_10_b (except D_02_b) have higher RI than the corresponding highlighted subnetworks in D_01_a to D_10_a (two DSNs corresponds to each other when they use the same cardiovascular subclass as the terminal set; see Tables 1 and 2). Hence, the conclusion below can be made.
Conclusion 2: The second proposed sparse graph generation algorithm is more suitable than the first proposed sparse graph generation algorithm for DSN generation.
Discussion
Due to the long time, large costs and high risks to develop new drugs, drug repositioning is important since it finds new therapeutic effects for known drugs. In this paper, we propose subnetwork identification as a new method to reposition drugs. Because cardiovascular health contributes significantly to the burden of illness and injury in the Australian community [39], and the PrizeCollecting Steiner Tree (PCST) approach is a good way to identify subnetworks, we focus on using the PCST approach to reposition drugs for cardiovascular diseases.
To identify subnetworks for drug repositioning, we generate Drug Similarity Networks (DSN) including five components, which are vertices, vertex prizes, edges, edge costs, and terminals. The PCSTP algorithm tends to identify a subnetwork constructed by vertices with big prizes and edges with small costs. In our DSNs, the vertex prizes represent similarities between drugs, and the edge costs represent dissimilarities between drugs. Moreover, terminals represent drugs in the cardiovascular class. Therefore, a subnetwork of drugs that are closely related to the cardiovascular system is expected to be identified using the PCST approach.
18 DSNs are generated using 9 cardiovascular subclasses and 2 sparse graph generation algorithms. After generating DSNs, PCSTP algorithms are used to identify subnetworks. GW algorithm is one of the most popular PCSTP algorithm. However, GW algorithm is designed for the singleterminal (root) case, while there are multiple terminals in DSNs. Therefore, we first adapt GW algorithm for the multipleterminal case and then use it to identify subnetworks in DSNs. Nevertheless, GW algorithm can only identify a single subnetwork in each DSN, and this subnetwork may not be suitable for drug repositioning. Hence, we propose a new PCSTP algorithm, Physaruminspired Subnetwork Identification Algorithm (PSIA), to identify subnetworks in DSNs as well, and PSIA can identify multiple subnetworks in each DSN.
We employ both PSIA and GW algorithm in 18 DSNs. In each DSN, one subnetwork is identified by GW algorithm, and three subnetworks are identified by PSIA. Since Rand Index gives equal weight to the identification of true positives and true negatives, it can be used to select suitable subnetworks for drug repositioning. Thus, we evaluate these subnetworks using their Rand Index. Furthermore, the subnetwork identified by GW algorithm and the best subnetwork identified by PSIA are compared with each other in each DSN.
Based on the comparison results shown in Tables 1 and 2, we first observe that smaller subnetworks always have higher Rand Index than larger subnetworks in the same DSN. Then, we conclude that PSIA outperforms GW algorithm in DSNs. Moreover, we conclude that the second proposed sparse graph generation algorithm is more suitable than the first proposed sparse graph generation algorithm for DSN generation.
Drug repositioning for cardiovascular diseases
Newly identified drugs in the selected subnetworks
Index  Drug Name  Freq  S01  S02  S03  S04  S05  S06  S07  S08  S09 

368  nitroglycerin  7  X  X  X  X  X  X  X  
496  theophylline  5  X  X  X  X  X  
32  arsenic trioxide27  3  X  X  X  
261  isocarboxazid  3  X  X  X  
287  lincomycin  3  X  X  X  
2  acarbose  2  X  X  
7  adapalene  2  X  X  
239  haloperidol  2  X  X  
298  malathion  2  X  X  
359  neomycin  2  X  X  
10  alclometasone  1  X  
14  amcinonide  1  X  
39  azathioprine  1  X  
70  caffeine  1  X  
74  carbachol  1  X  
93  ceftazidime  1  X  
135  desflurane  1  X  
165  droperidol  1  X  
217  formoterol  1  X  
241  hexachlorophene  1  X  
367  nitrofurantoin  1  X  
417  pramipexole  1  X  
422  prednisone  1  X  
429  procyclidine  1  X  
449  repaglinide  1  X  
466  selegiline  1  X  
497  thiabendazole  1  X  
513  topiramate  1  X  
518  tranexamic acid  1  X  
526  triiodothyronine  1  X 
It can be seen from Table 3 that ten newly identified drugs have occurred more than once in the selected subnetworks, while the other 20 drugs have occurred only once in the selected subnetworks. We consider the ten drugs which have occurred more than once as candidates for drug repositioning. These ten drug candidates are nitroglycerin, theophylline, arsenic trioxide, isocarboxazid, lincomycin, acarbose, adapalene, haloperidol, malathion, and neomycin.
We believe that these ten drug candidates could be repositioned for cardiovascular diseases. Thus, we evaluate each drug candidate using published pharmacological discoveries. The existing discoveries on three candidates (nitroglycerin, theophylline and acarbose) are introduced below.
As to nitroglycerin, Koch et al. [40] found that nitroglycerin can produce a sharp fall in the cardiac filling pressures and the pulmonary arterial pressures. Moreover, the vasodilatory effects of nitroglycerin also have the potential to be used in cardiovascular therapeutics [41]. As to theophylline, Sollevi et al. [42] found that theophylline can act as an adenosine antagonist to antagonize cardiovascular responses. As to acarbose, Chiasson et al. [43] found that treating impaired glucose tolerance patients with acarbose is associated with a significant reduction in the risk of cardiovascular diseases and hypertension.
It can be seen from these discoveries that nitroglycerin, theophylline and acarbose have already been suspected for their potential therapeutic effects for cardiovascular diseases. Therefore, we provide evidences to support these previous discoveries. As to the other seven drug candidates, we believe that they also may interact with the biological cardiovascular system. These evidences have shown the effectiveness and efficiency of our proposed PCST approach for drug repositioning.
Different types of drug similarities
In our generated DSNs, the edge cost represents the quantified dissimilarity between drugs, and the vertex prize represents the similarity between the drug represented by this vertex and all the drugs represented by terminals. There are different types of drug similarities with physical meanings, such as chemical similarity, therapeutic similarity, phenotype similarity, and similarity based on their interacting targets (such as proteins) [44].
In our generated DSNs, drug similarities are calculated using four types of drug features, which are the chemical, therapeutic, protein, and phenotype features. In this section, we generate new DSNs based on new drug similarities, and show that the initial drug similarities calculated using four types of drug features are the best drug similarities for drug repositioning.
We generate four new types of DSNs, and in each of them the drug similarities are calculated using a single type of drug features. The used drug features are chemical, therapeutic, protein, and phenotype features. We compare the standard deviations of vertex prizes and edge costs in the initial type of DSNs and four new types of DSNs.
Standard deviations of vertex prizes and edge costs
DSN_C  DSN_T  DSN_Pr  DSN_Ph  DSN_01_a/b to  

DSN_10_a/b  
SD_VP  3.68  3.34  2.32  2.24  2.42 
SD_EC  15.63  7.72  7.43  8.90  10.14 
It can be seen from Table 4 that SD_VP and SD_EC of DSN_C are higher than that of other types of DSNs. It is recommended to select DSNs with high standard deviations for drug repositioning as it is hard to identify drug repositioning candidates in DSNs with low standard deviations. However, many drugs undergo complex and largely uncharacterized metabolic transformations, and the physiological effects of drugs may not be able be predicted by their chemical properties alone [45]. Therefore, it is not appropriate to only consider chemical similarities for drug repositioning. Similarly, it is not appropriate to only consider any other homogeneous drug similarity either [11]. The initial drug similarities are heterogeneous as they are calculated using multiple types of drug features. It can also be seen from Table 4 that SD_VP and SD_EC of DSN_01_a/b to DSN_10_a/b are also relatively high. Therefore, the initial heterogeneous drug similarities calculated using four types of drug features are the best drug similarities for drug repositioning.
The running time in large drug similarity networks
We use the PCST approach to identify subnetworks for drug repositioning. The PrizeCollecting Steiner Tree Problem is NPhard [46], which means that the time required to solve it may increase exponentially as the graph size increases. Large DSNs with thousands of vertices can be generated using the existing pharmacology data. Thus, it is necessary to ensure that we can use the PCST approach to identify subnetworks in large DSNs.
The running time of PSIA and GW algorithm in DSNs with different sizes
DSN_100  DSN_548  DSN_1000  DSN_3000  

GW  0.001 min  0.039 min  0.196 min  7.345 min 
PSIA  0.036 min  0.638 min  2.102 min  19.169 min 
It can be seen from Table 5 that both PSIA and GW algorithm can identify subnetworks in large DSNs with up to 3000 vertices in a reasonable time. Moreover, the running time above can be further shortened by using a lowlevel programming language. Thus, we can use the PCST approach to identify subnetworks in large DSNs. Notably, even though the running time of PSIA is longer than that of GW algorithm, PSIA is considered better as it can identify more suitable subnetworks for drug repositioning.
Conclusions
Drug repositioning is important for drug development. In this paper, the subnetwork identification method is used to reposition drugs for the first time. A new PriceCollecting Steiner Tree algorithm is proposed in this paper to identify subnetworks. The popular GW algorithm is also used to compare with our proposed algorithm. Drug Similarity Networks are generated, in which vertex prizes and edge costs represent the similarities and dissimilarities between drugs respectively, and terminals represent drugs in the cardiovascular class, as defined in the Anatomical Therapeutic Chemical classification system. In the generated Drug Similarity Networks, our proposed algorithm identifies subnetworks with higher Rand Index than the popular algorithm. Furthermore, nine most suitable subnetworks are selected for drug repositioning, and ten drug candidates are identified from these subnetworks. We find evidence to support previous discoveries that nitroglycerin, theophylline and acarbose may be able to be repositioned for cardiovascular diseases. Moreover, we identify seven previously unknown drug candidates that also may interact with the biological cardiovascular system. Therefore, our proposed PrizeCollecting Steiner Tree approach is shown to be a promising strategy for drug repositioning.
Declarations
Declarations
This article has been published as part of BMC Systems Biology Volume 10 Supplement 5, 2016. 15th International Conference On Bioinformatics (INCOB 2016): systems biology. The full contents of the supplement are available online http://bmcsystbiol.biomedcentral.com/articles/supplements/volume10supplement5.
Funding
Both YS and PNH are supported by the PhD scholarships of The University of Melbourne. PNH is also partially supported by NICTA scholarship of National ICT Australia, now Data61 since merging CSIRO’s Digital Productivity team. This work is partially funded by Australian Research Council grant DP1096296.
Availability of data and materials
The data supporting the results of this article are included and cited within the article.
Authors’ contributions
YS proposed the idea of applying subnetwork identification to pharmacology networks. PNH proposed the idea of repositioning drugs for cardiovascular diseases using subnetwork identification. PNH collected ATC classification (2016) data. Both YS and PNH designed and generated the Drug Similarity Networks. YS proposed the PSIA algorithm for drug positioning. YS applied both PSIA and GW algorithm to identify subnetworks. PNH evaluated the identified subnetworks. Both YS and PNH analyzed the identified subnetworks and the drug candidates. YS and PNH drafted the manuscript. Both SH and KV provided assistance on this work. All authors edited and approved the final draft.
Competing interests
The authors declare that they have no competing interests.
Consent for publication
Not applicable.
Ethics approval and consent to participate
Not applicable.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Authors’ Affiliations
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