CURRENT SCENARIO OF THERAPEUTICS FOR EBOLA VIRUS DISEASE

Currently various countries in Africa, including Li beria, Sierra Leone, Guinea, Nigeria, are facing di saster due to Ebola Virus Disease (EVD), which is primaril y caused by Ebola virus. 2014 outbreak of Ebola associated viral haemorrhagic fever has 55-60% fata lity r te. The incubation period of Ebola is below 21 days; once the appearance of symptoms starts the pe rson will be infective. As there is no specific vac cine, antiviral or drugs for treating Ebola resulting in large number of deaths. Most of the recent outbreak s occurred in remote areas of West Africa. Poverty, l ack of awareness, access to health centres, human habitats taking its toll in spreading the disease i n large scale. Few nucleotide analogues, protease i nhibitors, receptor binding, monoclonal antibodies and anticoa gulant therapies are exhibiting promising role in inhibiting the Ebola virus in various ( in vitro and in vivo) models.


INTRODUCTION
Currently various countries in Africa, including Liberia, Sierra Leone, Guinea, Nigeria, are facing disaster due to Ebola Virus Disease (EVD). First Ebola outbreak occurred in Sudan (Nzara) and Congo (Yambuku) in the year 1976, concluding in the villages situated near the Ebola River from which the viral haemorrhagic fever got its name Ebola/Ebola Virus Disease (EVD). EVD belongs to Filoviridae which causes fatal Ebola haemorrhagic fever in humans with fatality rate nearly 90%. Several out breaks of Ebola have been reported from East Africa (Sudan: 1976(Sudan: , 1979(Sudan: , 2004 Central Africa (Democratic Republic of Congo, Uganda & Gabon: 1976, 1977, 1994, 1997, 2000, 2001, 2004, 2007 and in West Africa (Ivory coast: 1995) since its discovery in 1976. The most recent outbreak started in December 2013 and 2014 in eastern sector of the Republic of Guinea resulted in more than 337 deaths and spreading to republic of Liberia (Gatherer, 2014;Muyembe-Tamfum et al., 2012; and other countries. Ebola belongs to family Filoviridae and other members in this group are Marburgvirus and Cuevavirus. Under Ebola virus genus consists of five diverse species Bundibugyo Ebolavirus (BDBV), Zaire Ebolavirus (EBOV), Reston Ebolavirus (RESTV), Sudan Ebolavirus (SUDV) and Taï Forest Ebolavirus (TAFV). BDBV, EBOV and SUDV have been associated with large EVD outbreaks near the Tropical rain forests of Central and West African distant villages, among these three EBOV causes high mortality rates in humans. RESTV and TAFV were not acquainted to cause illness or fatality in human. RESTV is mostly found in Philippines and China (Leroy et al., 2004;2005).

TRANSMISSION
Transmission of virus from the wild animals like primates (Chimpanzees, Gorillas and African green monkeys) and fruit bats (Hypsignathus monstrosus, Epomops franqueti, Myonycteris torquata and Pteropodidae) (Groseth et al., 2007) are the natural hosts for the EVD (Fig. 1) Transmission from primates to humans occurs through the activities like handling ill/infected, butchering, eating, contact through blood from these animals and subsequently viral infection spreads from humans via human to human transmission (Leroy et al., 2004;Groseth et al., 2007).

GENOME OF EBOLA
Ebola virus is 970 nm long and filamentous in nature, its diameter is around 80 nm with (-) ve stranded RNA. Genome of EVD is nearly 19kb in size which encodes for seven proteins NP (Nucleoprotien) vp35, vp40, GP (Glycoprotein), vp30, vp24 and L (Polymerase).
How Ebola virus attacks the human system is not completely elucidated. Ebola virus attaches to the host receptors through the GP (glycoprotein) mediated endocytosis into the host vesicles, fusion with the membrane leads to the release of Ribonucleo-capsid into the cytoplasm. Viral RNA dependent RNA polymerase binds to the leader region of the genome to start the sequential transcription of genes. The gene products (mRNA) are protected from the host proteins by capping and polyadenylation by the L protein during the synthesis (Stahelin, 2014;Watt et al., 2014). Replication starts only when enough nucleoprotein to encapsidated the newly synthesized genomes budding of the virus occurs with, interaction of matrix protein and nucleocapsid under plasma membrane with the help of host ESCRT complexes (McDonald and Martin-Serrano, 2009;Grove and Marsh, 2011).

Signs and Symptoms
EVD is a severe acute viral illness often characterized by the sudden onset of fever, intense weakness, muscle and joint pain, headache and sore throat. This is followed by vomiting, diarrhoea, rash, impaired kidney and liver function and in some cases, both internal and external bleeding. Laboratory findings include low white blood cell and platelet counts and elevated liver enzymes. Few patients may experience rash, red eyes, hiccups, chest pain, cough, sore throat, difficulty in swallowing, difficulty in breathing un explained internal and external bleeding. Incubation period ranges from 2 to 21 days, people are only infective once the symptomatic phase starts and viral transmission can occur through blood and secretions from the patients (Roddy et al., 2012;.

Protease Inhibitors
VP30 is essential for transcription of Ebola and it is phosphorylated at N-terminal serine clusters and threonine residues at positions 143 and 146. Host cellular Protein Phosphatase 1 (PP1) controls VP30 dephosphorylation as expression of a PP1-binding peptide cdNIPP1 increased VP30 phosphorylation. 1E7-03 compound targeted at non-catalytic site of PP1 inhibited the transcription and replication in cell culture model (Ilinykh et al., 2014). Phosphorodiamidate Morpholino Oligomers (PMOs) are short nucleotides binds to the RNA which can alter the RNA-RNA interactions and RNA-Protein interactions which effects viral transcription and translation. The AVI-7537 PMO has been effective in targeting the VP24 gene of Ebola virus (Iversen et al., 2012).

Nucleotide Inhibitors
BCX4430 a nucleoside analogue act as broad spectrum antiviral compound which can be a potential candidate in treatment of patient in delayed/later stages of infection (Warren et al., 2014).

Biologically Active Compounds/Antivirals
Fullerene sugar balls (anti adhesive agents) are the new class of biologically active compounds derived from C60 core they bind to the lectins through glycoside cluster effect which are effective against bacteria. Mannosylated fullerene sugar balls can be used as antivirals as they have shown effect on Ebola pseudotyped infection model. (Nierengarten and Nierengarten, 2014). Griffithsin (lectin) derived from red alga binds to the N-linked Glycans found on the surface of viruses like HIV, HCV, SARS and Ebola virus can be used as antiviral after proper validation (Barton et al., 2014). Favipiravir (T-705) a pyrazinecarboxamide derivateive inhibited the replication of Ebola in later stages of infection thereby decreasing the pathogenesis and disease severity and it has effective against E718 strain (Aerosol Ebola virus) in both cell culture and animal model. This is broad spectrum antiviral administered through oral route it is effective against influenza and it is under phase III clinical trials for the same Oestereich et al., 2014). Kinase inhibitors (Genistein and Tryphostin) AG1478 pre-treated host cells with this inhibited the infection of Ebola virus which can be used as broad spectrum antiviral (Kolokoltsov et al., 2012).

Targetting Host Cellular Proteins
Endoplasmic reticulum α-glucosidases I and II are essential for viral maturation and release of virion. The glycolysated viral envelope proteins uses calnexin mediated folding pathway and the compound CM-10-18 (imino sugar) and its derivatives are α-glucosidase inhibitors which have shown effect against various hemorrhagic fevers can be used as potent antiviral (Chang et al., 2013). Clomiphene and cationic amphiphiles (Ro 48-8071, Terconazole, Triparanol, AY 9944, Amorolfine and U18666A) induces defects in Niemann-Pick C1 Protein (NPC1) required for viral entry but they have to use in higher concentrations and various combinations of cationic amphiplhies to block the viral entry (Shoemaker et al., 2013). Nedd4 E3 ubiquitin ligase of host protien was utilized by viral PPxY late (L) domain of filoviruses and other RNA viruses like arena virus and rhabdo viruses in budding. L domain of PPxY can be of ideal choice for blocking the budding and filoviruses infections (Han et al., 2014). Studies on macrophages and dendritic cells with pre-treatment of pyridinyl imidazole p38 MAPK inhibitors can block the entry and infection of EBOV (Johnson et al., 2014).

Antibody/Monoclonal Antibodies Against Ebola
Recombinant adenovirus expressing the Glycoprotein (GP) of Ebola virus has elicited good humoral/antibody response against Ebola virus in wild type and interferon α/β receptor knock-out mouse (O'Brien et al., 2014). ZMAb, combination of three EBOV-GP-specific monoclonal antibodies were administered 24-48 h post exposure to Ebola virus in primates were survived after re challenged them with virus at 10 weeks and 13 weeks are also survived (Qiu et al., 2013).

FUTURE DIRECTIONS
Recent outbreak of Ebola was the outcome of complex human behavioral, environmental factors on the public. There are many drugs which are under clinical and at pre-clinical level can target both viral and human host proteins. Development of rapid diagnostic tools should be given priority. Establishment of a system for continuous surveillance of emerging and re-emerging diseases is a need of the hour. The cost effective drugs should have global acceptance, accessibility, affordability. The awareness programmers should be conducted in large scale to eradicate the disease and we need to proceed with sense of urgency.