COVID-19: BRIEF REVIEW ON PATHOPHYSIOLOGY AND INVESTIGATIONAL DRUGS

Though coronaviruses are firstly recognized almost 6 decades back, but only reputed in 2003 and were identified that one of the members is served as the aetiologic vehicle for SARS (severe acute respiratory syndrome). Formerly coronaviruses are well-known as crucial pathogens for causing infections to ventilatory system nothing but respiratory system and enteric system nothing but digestive system of patients, causing common cold in 15% cases of all cases. The coronaviruses are probably human pathogens accomplished with pathological signs in upper and lower respiratory tract. The virological studies particularly coronavirology has been remarkably developed from the last decade. Animal coronaviruses were enlightened during the SARS wide-spread and COVID-19 pandemic brought human coronaviruses (Novel coronavirus-2019) into the spotlight.On 7th Kerkhove briefed the “evidence emerging” of the transmission of the COVID-19 through air. The potentiality of spread through air in public places particularly in closed, poorly ventilated, crowded places is high. Our review gives an account on investigational drugs and repurposed drugs that are being used for management and treatment of coronavirus disease (COVID-19) with their

The genome size of SARS-CoV-2 virus is of 29.9-32 kb which is similar to remaining β-CoVs [24]. It owns a nucleo-capsid made of phosphorylated nucleo-capsid protein (N) and genomic RNA. The nucleo-capsid protein is enfolded within the bilayered phospholipids and two distinct typed spike proteins are surrounded, the hemagglutinin-esterase (HE), the spike glycoprotein (S) which is present in all coronaviruses. The viral envelop with glycoprotein trimmer protein locates the envelope protein (E) and membrane protein (M) [23]. Typically like the β-CoVs, genome of SARS-CoV-2 is held with 50 and 30 terminal sequences with 265 and 229 nucleotides (nt) at 50 and 30 terminal regions respectively, along with a gene order 50 -replicase open reading frame (ORF) 1ab-Senvelope (E)-membrane (M)-N-30. SARS-CoV-2 has various genes such as S with 3822 nt, ORF3a with 828 nt, E with 228 nt, M with 669 nt, and N with 1260 nt. A suspected gene ORF8 is found between M and N ORF genes with 366 nt in length, which resembles SARS-CoV [23].  The viral particulate is oval shaped with 60~100 nm diameter [25]. Most of the information regarding CoVs is acquired from SARS-CoV and MERS-CoV. SARS-CoV-2 can be degraded by ultra-violet light or can be desensitized by heating at 56º C for 30 minutes and disinfectants like peracetic acid, diethyl ether, 75% ethanol, chloroform and chlorine can disinfect the virus [25]. The novel corona virus, SARS-CoV-2 has high stability on stainless steel and plastics than on cardboard and copper, the live virus was screened out 72 hours after accommodated to the surfaces. The continuance of SARS-CoV-2 is lengthier compared to SARS-CoV and lengthiest lively time is reported on plastic and stainless steel [26].

Transmission
Since the COVID-19 outburst has began and still continuously evolving the globe, everyday a new thing is spot lightened regarding the virus. SARS-CoV-2 is mainly transmitted when an infected person sneezes, coughs or come in contact directly with mucous layers. People are prone to be infected when they go in contact with contaminated surfaces and when they touch their nose, mouth with those contaminated hands.
Pre-symptomatic transmission is the transmission of virus from infected person whose is non-symptomatic till then. The time lapse between infection and occurrence of symptoms is known as incubation period, averagely 5-6 days which may hold out to 14 days. This is also considered as presymptomatic session for the reason that infected person is symptomless. And these symptomless people play a key role in the transmission of the virus which is more contagious [27][28][29][30][31][32]. There is highest possibility of transmission the virus through these symptomless people who are tested and confirmed as positive for SARS-CoV-2 [33,34]. Though it can easily be transmitted, virus need a particular route for transmitting, which is now being transmitted through droplets that come out while sneezing, coughing and why not contaminated surfaces. On 7 th July,2020 WHO technical lead head COVID-19 Dr Maria Van Kerkhove briefed the "evidence emerging" of the airborne spread of the COVID-19 after a group of scientists urged the global body to update its supervision on how the respiratory disease passes between people, but that it was not definitive. The possibility of airborne transmission in public settings -especially in very specific conditions such as closed, poorly ventilated, crowded places. Furthermore, the evidence needs to be gathered and interpreted. Symptomatic transmission is the transmission of virus from infected person with proper symptoms after the SARS-CoV-2 infection. Proper epidemiologic reports are depicting that people being infected are in contact with those who are already positive for infection [35][36][37][38][39][40][41]. A varied virological and clinical report confirms that in the early stage of infection (before onset of symptoms) virus is discharged from nose and mouth in higher amounts [42][43][44][45]. Asymptomatic transmission is defined as the condition where transmission is from confirmed positive case of SARS-CoV-2 virus infection, with no symptoms. People with absence of symptoms but infected with virus are treated as super spreaders of the virus.

Diagnosis
COVID-19 can be diagnosed by Polymerase chain-reaction (PCR) which is the main method for the detection of SARS-CoV-2 [46]. The sensitivity of testing of naso-pharyngeal swab becomes high by the onset of symptoms.
Testing of sputum samples increases sensitivity than testing the naso-pharyngeal swabs, laboratories are also accessible for sputum testing lower pulmonary tract samples and sputum samples are may be easily obtained [47]. Centers for Disease Control and Prevention (CDC) recommended that oropharyngeal swab can be used in case of unavailability of naso-pharyngeal swab [48]. On-site self-collection of nares specimens present anteriorly is recognized by the Food and Drug Administration (FDA) and accepted it as method of collection [49] which reduces the exposure of health workers for infection.   Mild patients don't have serious symptoms and have no characteristic radiographical changes in the bronchoalveolar region. Patients with moderate infection suffer with respiratory problems, fever and have characteristic change in the radiographical aspect of lungs. Patients affected severely will definitely have one of the three conditions like dyspnea with respiratory rate more than 30 times per minute where a healthy adult being needs 12-16 breaths per minute, hypoxemia where oxygen saturation less than 93 percent where the normal condition needs 95-100 percent and partial pressure of oxygen (PaO2) less than 300 mmHg when the normal range is 400-500mmHg. Critically infected patients suffer with arrest of respiration, respiratory sepsis and collective organ abnormalities. Patients suffering with simultaneous health issues such as hypertension, diabetes, chronic pulmonary diseases (emphysema and chronic bronchitis), cardiac arrests and cancer lead to report higher fatality rate when compared to patients with no such co-morbid conditions, which also implies that they are highest risk factors for patients infected with 2019-nCoV.

Pathophysiology
Entry of the virus ACE2 receptor facilitates the entry of SARS-CoV-2 host cells. Though SARS-CoV-2 is present inside the gastrointestinal cells it mainly infects the type-2 pneumocyte cell present in the lungs. Type-2 pneumocyte cell with secretes dipalmitoyl phosphatidylcholine which is a surfactant lowers the surface tension of lungs. SARS-CoV-2 with the help of S protein present on the surface binds to receptor ACE2. This association between ACE2 receptor and S protein lead for a conformational change and it allows the enzyme called serine protease, TMPRSS2 to cleave the viral structure which cleaves the S protein at S1, S2 domains facilitates the attachment of membranes of virus and host cell and virus enters into host cell. Viral replication is primarily suppose to take place in epithelium of nasal cavity and pharynx lined by a mucosa (upper respiratory tract) followed by multiplication of virus takes place in gastro-intestinal mucosa and lower respiratory tract [58], with small amounts of viremia. Some people infected with SARS-CoV-2 also experienced with acute liver injury, diarrhoea, kidney failure and heart failure which are not related to pulmonary infection [2,59-61], making clear that all organs are involved.

Pathological changes
The pathological reports found that COVID-19 patient shows a pulmonary isobilateral diffused alveolar injury with cellular fibromyxoid excretes [62]. A suspension of pneumocytes, formation of hyaline membrane in the right lung and pulmonary edema in left lung implies the acute respiratory distress syndrome (ARDS).The mono-nuclear inflammatory subverts controlled by lymphocytes are recognized in both the lungs. A cytopathical change is observed with multinucleated syncytiatic cells with unusual expansion in pneumocytes with huge nuclei, amphophilous granulated cell protoplasm and intra-alveolar spaces filled with eminent nucleoli impling the pathological changes in the function of cell. Acute Respiratory Distress Syndrome (ARDS) is a life threatened condition which the oxygen is prevented for the entry into lungs and also into circulation which leads to severe respiratory disorders and lung injuries [63]. Across 40 genes including ACE2, vascular endothelial growth factor (VEGF), tumor necrosis factor (TNF) and interleukin 10 (IL-10) are reported to be associated with the development of ARDS [64].

Dysfunctioning of immune system Reduction and hyperactivation of CD4 and CD8 T cells is observed peripherally. Increase in the concentration of the proinflammatory CD4 cells and cytotoxic granular CD8 cells triggers immune responses antivirally and T cells get overactivated [62]
. Lymphopenia emerged as a feature of COVID-19 [1,2], which suggests that it is important factor for accounting the disease severity and mortality.

Pharmacological treatment for COVID-19 Investigational Antiviral Agents Remdesivir
Remdesivir is a monophosphate prodrug officially called as GS-5734 which is metabolized to an active C-adenosine nucleoside triphosphate analogue. It was discovered when it is under screening procedure for anti-microbial activity against RNA virus family like Flaviviridae and Coronaviridae. During the peak stage of outburst of Ebola virus research activity of this agent promised for host polymerase selective activity against to Ebola virus with a very low EC50 [65] Right away, remdesivir with its broad spectrum is hopefully promising the potent therapeutic activity against SARS-CoV-2 and a in vitro action against many novel coronaviruses with EC90 value of 1.76 μM/L and EC50 value of 0.77 Μm/L [66,67].
Evaluation of the pharmacokinetic properties and safety factors of remdesivir were done in both multiple-dose and single -dose phase I trials [68]. Saphenous (intravenous) infusions halfway between 3 mg and 225 mg were accepted across the any evident for kidney and liver toxicity. Remdesivir indicated linear pharmacokinetic results in a bordered dose range with an intra-cellular half-life of higher than 35 hours. Firstly, remdesivir has been used clinically used in the treatment of Ebola virus [69]; yet, case reports suggests that a fortunate result has been reported by the use of remdesivir for 71]. Still clinical trials are under process to assess the safety parameter and activity against the virus in COVID-19 patients.

Favipiravir
Favipiravir, a prodrug of nucleotide "purine", ribofuranosyl-5′-triphosphate. Favipiravir halts replication of the virus by inhibiting the enzyme RNA polymerase viral. Yet it indicated the broad activity against RNA viruses its preclinical data is derived from anti-influenza and Ebola activity [72]. EC50 value against SARS-CoV-2 Vero E6 cells was 61.88μM/L [67].
There is probable dose variation for the reason of low EC50 value with influenza when compared to SARS-CoV-2 and Ebola [73,74]. So for COVID-19, consideration of doses should be at higher range [74]. A loading dose with 2 doses of 2400 mg to 3000 mg for every 12 hours followed by maintenance dose of 1200 mg to 1800 mg for every 12 hours are recommended. Half-life is nearly 5 hours [75]. A limited adverse affect profile with higher dose regimens is found and is well tolerable [74,[76][77][78].

Selected Repurposed Drugs Hydroxychloroquine (HCQ) and Chloroquine
Hydroxychloroquine and Chloroquine have a long history for treating and preventing malaria and several chronic inflammatory diseases which include rheumatoid arthritis and systematous lupus erythematosus (SLE) [79]. Chloroquine and HCQ blocks entry of virus into the cells by the inhibition of glycosylation of receptors, it allows proteolytic transformation, and endosomal acidification. Immunomodulatory effect by diminishing the production of cytokines and also inhibits the self-eating of host cells (autophagous effect) and inhibits the activity of lysosomes of host cells [80,81]. Chloroquine in vitro inhibits SARS-CoV-2 in low range of EC50. After one day, hydroxychloroquine shows lower EC50 when compared with chloroquine where hydroxychloroquine shows EC50 of 6.14 μM/L and chloroquine shows EC50 of 23.90 μM/L [82]. It's not evident that HCQ and chloroquine are efficient for treating SARS-CoV and MERS-CoV. China briefed in the new bulletin that chloroquine was successful in treating more than100 COVID-19 patients and radiological reports suggested that there is viral clearance and decreased disease severity [83]. Dosing of chloroquine to treat COVID-19 has consisted of 500 mg orally once or twice daily [84,85].
Nevertheless, serious adverse events are caused by both the agents that includes prolonged Q-T interval, retinopathy, hypoglycemia [86,87]. No adverse events are reported for chloroquine treatment for .

Conclusion:-
The whole universe is worried about the COVID-19 pandemic and the main groups of people getting affected by this virus are the elder group and those having severe comorbid conditions. There is still no specific therapeutic antiviral treatment or vaccine available. The novelty of the disease made scientists aim towards discovering a new therapeutic target to combat the disease. Investigation of therapeutic targets is needed and fastness and accuracy in clinical trials are demandable. Therapies emerged till date are just to manage the symptoms and save the affected people form life threat as there is no effective therapy emerged to date.