The current Ebola virus outbreak is one of the largest outbreaks of its kind in history and the first in West Africa. By January 14, 2015, a total of 21296 probable and confirmed cases, including 8429 deaths from Ebola virus disease (EVD), had been reported from five countries in West Africa - Guinea, Liberia, Nigeria, Senegal, and Sierra Leone ( http://apps.who.int/iris/bitstream/10665/148237/2/roadmapsitrep_14Jan2015_eng.pdf?ua=1). EVD with a high case-fatality rate of 40% and with currently no approved vaccine or therapy, represents a major public health threat.

In response to the current Ebola virus outbreak, the international community has urged for accelerated development of drugs against EVD but also has endorsed the clinical use of unregistered treatments for Ebola ¹ . Conventional time and the money consuming approach of drug development (> 10 years; > 2 billions $) does not meet the current urgent need for anti-Ebola drugs. Repurposing or repositioning of existing drugs could overcome some of these obstacles and help in the rapid discovery and development of therapeutics for EVD, although this approach does not negate the need for some preclinical studies and clinical trials for validation of the proposed indications. Recently, results of two large repurposing screenings of Food and Drug Administration (FDA)-approved drugs have been reported. In the first study, Madrid and co-workers performed in vitro and in vivo (in mice) screening of 1012 FDA-approved drugs and selected 24 candidate entry inhibitors for Ebola virus ² . In the second study, 53 inhibitors of Ebola virus infection with IC ₅₀ < 10 µM and selectivity index SI > 10-fold have been identified by in vitro screening of 2816 FDA-approved drugs ³ . In the same study, an additional 95 drugs which are active against Ebola virus infection with IC ₅₀ > 10 µM and SI <10-fold were also reported.

Although in vitro and in vivo screening for repurposing/repositioning of existing drugs could significantly accelerate discovery of new drugs these approaches are time-consuming and costly for screening of large drug libraries. Recently, we proposed a novel approach for in silico screening of molecular libraries for drug candidates ^{4– 8} . This approach, which uses the average quasi valence number (AQVN) and the electron-ion interaction potential (EIIP), parameters determining long-range interaction between biological molecules, might hold a key to overcoming some of these obstacles in experimental screening by significantly reducing the number of compounds which should be in vitro and in vivo tested ⁹ .

Herein, 267 approved and 382 experimental drugs, selected by the EIIP/AQVN-based virtual screening of DrugBank ( http://www.drugbank.ca), have been proposed as candidate drugs for treatment of EVD. An open access portal allowing screening of molecular libraries for candidate drugs for treatment of EVD was established.

Previously, analyses of the EIIP/AQVN distribution of 45,010,644 compounds from the PubChem database ( http://www.ncbi.nlm.nih.gov/pccompound) revealed that 92.5% of presented compounds are homogenously distributed within EIIP and AQVN intervals (0.00 – 0.11 Ry) and (2.4 – 3.3), respectively). This domain of the EIIP/AQVN space, encompassing the majority of known chemical compounds, is referred to as the “basic EIIP/AQVN chemical space” (BCS) ⁶ . Analysis of the molecular training set ( Dataset 1), encompassing 152 small molecule inhibitors of Ebola virus infection selected by in vitro screening of 3828 FDA approved drugs ^{2, 3} , show that 79% of these compounds are placed within AQVN and EIIP region (2.3 – 2.7) and (0.0829 – 0.0954 Ry), respectively (“Ebola Virus Infection Inhibitors Space”, EVIIS). The AQVN region (2.36 – 2.54) and the EIIP region (0.0912 – 0.0924 Ry) form the part of EVIIS which encompasses 55.5% of all drugs from the learning set (core EVIIS, cEVIS). Literature data mining reveals 49 compounds with experimentally proved activity against Ebola virus infection ( Table 1) ^{13– 29} . Most of these compounds 47 (95.9%) are placed within EVIIS ( Table 1). Of note is that EVIIS and cEVIIS domains contain only 14.6% and 6.5% of compounds from PubChem, respectively. This confirms high specificity of clustering of Ebola virus infection inhibitors within the EIIP/AQVN space. Comparison of distributions of Ebola virus infection inhibitors and compounds from PubMed is given in Figure 1.

It was shown that Ebola virus glycoprotein (GP)-mediated entry and infection is subordinated with a membrane-trafficking event that translocates a GP binding partner to the cell surface, which depends on microtubules ^{30, 31} . Consistently, microtubule inhibitors which block this trafficking process could decrease infection without interfering with the direct binding and translocation of the Ebola virus into cells. AQVN and EIIP values of microtubule modulators and transcription inhibitors with reported anti-Ebola virus activity are given in Table 2. As can be seen, all these compounds, which do not directly affect binding and internalization of Ebola virus, are located outside of EVIIS. This additionally confirms the specificity of the EVIS domain.

In further analysis we used EVIIS as a filter for virtual screening for candidate Ebola virus infection inhibitors. In Dataset 2 622 approved and 1089 experimental drugs in Dataset 3 selected by EVIIS screening of 6532 drugs from DrugBank are reported. Using cEVIIS, we located 267 approved and 382 experimental drugs. This small molecular library represents a source of candidate drugs for treatment of Ebola virus disease (EVD), which can be further experimentally tested.

Madrid and co-workers selected 24 drugs by in vitro screening of 1012 FDA-approved drugs, which are effective against Ebola virus infection ² . They also showed that among these compounds, four antimalarial drugs (chloroquine, hydroxychloroquine, amodiaquine and aminoquinoline-13) also are effective against Ebola virus infection in vivo ² . Among 53 compounds which effectively inhibit Ebola virus infection in vitro, which Kouznetsova and co-workers selected from 2816 approved drugs, are also three anti-malarial drugs (mefloquione, chloroquine, amodiaquine) ³ . It was also suggested that application of chloroquine for prevention of virus transmission should be considered because this compound significantly inhibits Ebola virus infection ¹³ . Our analysis showed that 15 of 22 approved ant-malarial drugs ( http://en.wikipedia.org/wiki/Antimalarial_medication) are located in EVIIS ( Table 3). Six 2-alkylquinolines have been also included in this study. This chemical series is promising as some derivatives exhibited antiviral activity such as 2PQ, and 2QQ ^{32, 33} antimalarial activity such as 2PQ and 2PentQ2 ³⁴ , antileishmanial activity such as 2PQ ^{35, 36} and neurotrophin-like activity on dopaminergic neurons such as 2QI15 ³⁷ . These compounds exhibit some advantages in regard to their chemical synthesis with few steps and good yields as well as their chemical stability in tropical conditions of storage. Their combined effects against virus and Leishmania parasites suggested they could be an advantage for the treatment of Leishmania/HIV co-infections and they were considered as attractive enough to enter the pipeline of DNDi on 2010.

All these data strongly suggest that this class of drugs should be further investigated as a promising source of therapeutics for treatment of EVD. Anti-malarial drugs with dual activity should be of special interest because malaria represents the highest health-related disease in African countries with EVD.

Among 3828 FDA-approved drugs screened for anti-Ebola activity were six antibiotics which inhibit Ebola virus infection (azthromycin, erythromycin, spiramycin, dirithromycin, maduramicin, charitromycin) ^{2, 3} . All these antibiotics are within EVIIS and four of them are in cEVIIS. Analysis of 184 approved antibiotics ( Dataset 4) showed that only 32 (17.4%) have AQVN and EIIP values in EVIIS, and that 11 of them are located within cEVIIS. Previously we reported domains of AQVN and EIIP which characterize different classes of antibiotics ( Table 4) ⁶ . According to these data, among antibiotics some macrolides, pleuromutilins and aminoglycosides have the highest chance for inhibition of Ebola virus infection. Of note is that five of six antibiotics with experimentally proved activity against Ebola virus infection (azthromycin, erythromycin, spiramycin, dirithromycin, charitromycin) are macrolides. Antibiotics representing candidate Ebola virus infection inhibitors selected by EIIP/AQVN criterion are given in Table 5.

Previous, we determined AQVN and EIIP domains characterizing different classes of anti-HIV drugs ^{4– 9} . As can be seen in Table 6, the EIIP/AQVN domain of CCR5 HIV entry inhibitors is within EVIIS, and domains of CXCR4 HIV entry inhibitors and HIV protease inhibitors partially overlaps EVIIS. The EIIP/AQVN domains of other classes of anti-HIV agents are located outside EVIIS. This indicates that some HIV entry inhibitors and HIV protease inhibitors could also be effective drugs against Ebola virus infection.

In conclusion, the presented results show that the EIIP/AQVN criterion can be used as an efficient filter in virtual screening of molecular libraries for candidate inhibitors of Ebola virus infection. Approved ( Dataset 2) and experimental drugs ( Dataset 3), anti-malarial drugs ( Table 3) and antibiotics ( Table 5) selected by this criterion represents a valuable source of candidate therapeutics for treatment of EVD, some of which are already approved by FDA for treatment of other diseases which can be repurposed for use in EVD. We hope that these data, obtained by an in silico drug repurposing screen, will accelerate discovery of drugs for treatment of EVD, which are necessary in this ongoing emergency situation caused by the current unprecedented Ebola virus outbreak. To enable other researchers working on online EIIP/AQVN-based screening of different sources of small molecules for candidate Ebola drugs, we established an open web server ( http://www.biomedconsulting.info/ebola_screen.php).

The data referenced by this article are under copyright with the following copyright statement: Copyright: � 2015 Veljkovic V et al.

Data associated with the article are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).

The virtual screen for candidate inhibitors of EBOLA virus infection web tool is available at: http://www.biomedconsulting.info/tools/ebolascreen.php. An archived version can be accessed at: http://www.webcitation.org/6Vxtuojgx ³⁸

F1000Research: Dataset 1. FDA-approved drugs which are active against Ebola virus infection ^{2, 3} , 10.5256/f1000research.6110.d42876 ³⁹

F1000Research: Dataset 2. Approved and experimental drugs selected as candidate for treatment of EVD, 10.5256/f1000research.6110.d42877 ⁴⁰

F1000Research: Dataset 3. Experimental drugs selected as candidate for treatment of EVD, 10.5256/f1000research.6110.d42878 ⁴¹

F1000Research: Dataset 4. Approved antibiotics screened for candidate anti-Ebola drugs, 10.5256/f1000research.6110.d42879 ⁴²