INTRODUCTION

The COVID-19 pandemic might be anything but unforeseeable. Throughout history, mankind has faced many devastating pandemics, such as the Middle Age bubonic plague (´Black death`), the 20 ^th century Spanish flu (H1N1 first pandemic) and AIDS (HIV), and the swine flu (H1N1 second pandemic) in the past decade. Notwithstanding a similar deadly viral infection (SARS-CoV-1) had lit a warning light in 2002-4, no vaccine was created, nor were drugs against coronaviruses developed ¹.

If vaccines are not available, strategies to curb the spread of contagious illnesses rely on quarentine, a traditional health practice dating back to 1377 ², disease-specific preventive actions and medications.

When swine flu (H1N1) emerged in 2009, there existed neuraminidase-inhibiting antiviral drugs (oseltamivir, zanamivir) for treatment and prophylaxis of influenza infections ³. Neuraminidase blockage prevents virion release from the surface of infected cells thereby halting their replication ³. Although expectations on oseltamivir for prophylaxis and treatment of swine flu were largely unmet ^{4, 5, 6, 7}, a vaccine was developed and H1N1 was finally tamed.

Contrasting to the poor performance of antiviral medicines in H1N1 pandemic, the extensive use of effective antiretroviral therapies (ART) was a notable public health triumph. ART combines three or more drugs (new molecular entities, NME) acting on distinct molecular targets, and by doing so it maximally suppresses HIV replication. The combination of antiretroviral drugs not only stopped disease progression in HIV-infected patients, but it also prevented onward transmission of the virus ⁸. It took decades, however, to develop such a set of antiretrovirals with complementary and synergistic modes of action, including inhibition of virus reverse-transcriptidase, protease, integrase and cell entry/fusion. This lenght of time is not available under the current scenario of COVID-19 pandemic progression.

Development of NME drugs from the bench to the bedside is a long and costly endeavor the success of which can not be taken for granted. Certainly, it is not on the table when we are facing COVID-19, a fast-spreading viral infection that, within a few days, may progress from relatively mild symptoms to a life-threatening Acute Respiratory Distress Syndrome (ARDS).

Tackling such a challenging contagious disease, drug repurposing or repositioning (DR) seems to be the most viable approach to find effective therapies in a timely manner. DR implies in identifying new medical uses for existing (in use, discontinued, shelved or experimental) drugs. It requires conducting clinical trials of drug effectiveness and safety for new and still unapproved therapeutic indications ⁹.

The advantage of DR over NME drug development is a reduction of development time, costs and uncertainty. Since data on manufacturing process, quality control and analyticaI methods, as well as nonclinical safety, pharmacokinetics, pharmaceutical formulation and first-use in humans are available for existing medications, these time-consuming steps of drug development are circumvented ⁹.

In the quest for life-saving COVID-19 therapies, time is certainly the most valuable commodity. Those researchers and managers who are committed to developing COVID-19 drugs hear the clock ticking constantly while pandemic death toll steadily rises. It is not surprising that the pandemic had broken the emergency glass on all possible options and many drugs are rushing into compassionate use and clinical investigation even when enough preliminary evidence of safety and efficacy for COVID-19 is missing.

This study was performed to identify and analyze drug clinical trials (on April 20 ^th, 2020) addressing the treatment of COVID-19 and infection-related Acute Respiratory Distress Syndrome (ARDS). A special focus was placed on identifying which drugs are being considered for repurposing and their pharmacological targets as well as the drawbacks and strengths of ongoing DR studies. Moreover, this report addresses prospects and perils ahead in the desperate race to find a drug useful to attenuate COVID-19 (and ARDS) morbidity and death toll.

METHOD

On April 20 ^th, 2020, a clinical trials database (https://clinicaltrials.gov) was searched for identifying which drug treatments (DR, drug repurposing or repositioning) for COVID-19 are under investigation. The foregoing clinical trials registry is run by the United States National Library of Medicine at the US National Institutes of Health. It is the largest international registry of clinical trials and holds registrations from over two hundred countries. The searching terms used to identify studies of pharmacological therapies for COVID-19 were as follows: Status (“All”); Condition or Disease (“Covid”); Other terms (“Treatment”); Countries (no selection). All retrieved trials that investigated therapeutic options other than drugs (e.g., hyperbaric O ₂, mesenchymal stem cells, plasma of convalescent patients and others) were excluded and so were non-interventional trials (i.e., observational, cohort or case-control designs). Information extracted from all potentially relevant studies (records) found in the searched database were: Registry ID number, status (recruiting, not yet recruiting, active, complete, terminated, suspended), drug treatment, clinical indication, study design features (arms, randomization, masking, type of comparator, i.e., placebo or active treatment comparator, number of patients enrolled), estimated date of completion, and, if available, study results. Extracted data were consolidated in spreadsheets for further analysis and qualitative synthesis.

All authors screened the retrieved records for trials which were relevant for the study and took part in data extraction. Data were extracted independently by each investigator and crosschecked by the others.

RESULTS AND DISCUSSION

Unsurprisingly, the emergence of COVID-19, a fast-spreading respiratory infection much deadlier than influenza, has unleashed a worldwide race for finding effective pharmacological therapies. This survey (on 20 April 2020) identified 516 studies addressing therapeutic interventions for COVID-19 ( Table 1). About 57.0% of these studies were trials of potentially repurposable drugs while the remaining ones were of hyperbaric O ₂, mesenchymal stem cells, plasma of convalescent patients, heat-killed Mycobacterium, medical devices and observational (prospective cohorts or case-control) designs (n = 37, 17.0% of excluded studies). It was amazing to find such an expressive number of observational studies in ClinicalTrials.gov database because it supposedly should contain only trials, i.e., interventional clinical studies ( Figure).

The majority of drugs under clinical investigation for COVID-19 are antiviral agents developed for and used to treat other viruses. Two structurally related old antimalarial compounds (chloroquine and hydroxychloroquine) with mild immunossupressive properties and putative antiviral activity, and a diversity of drugs belonging to other therapeutic classes are being tested as well (Tables 2 and 3).

Survey results revealed that pharmacological interventions against COVID-19 generally pursue one of two distinct therapeutic goals: 1) to accelerate resolution and/or to prevent worsening of oligosymptomatic or mild COVID-19 infections (i.e., proactive prophylaxis) or 2) to relieve symptoms and to reduce mortality in severe infections and ARDS.

Drugs that effectively inhibit SARS-CoV-2 replication in humans are likely to be of benefit to patients with mild symptoms as well as to those with severe manifestations of COVID-19. Based on the pathophysiology of COVID-19 pneumonia and ARDS ¹⁰, it is plausible to think that drugs other than typically antiviral compounds, such as immunosuppressive and anti-inflammatory agents, might also be useful to alleviate the respiratory symptoms and to reduce disease fatality rate. Immunosuppression, on the other hand, is likely to facilitate viral proliferation thereby aggravating mild, oligo and asymptomatic infections. Physicians should be aware that risks of drug adverse events (AE) that might be tolerable for critically ill patients are not necessarily acceptable for those who exhibit only mild symptoms of COVID-19.

A recent report by the World Health Organization (WHO) estimated that approximately 80.1% of patients with laboratory confirmed COVID-19 infection had only mild to moderate symptoms and showed spontaneous resolution of the disease, while 13.8% developed ARDS and 6.1% had infections that progressed to a critical clinical condition (respiratory failure, septic shock and multiple organ dysfunctions) ¹¹.

Obviously, patients showing only mild symptoms of COVID-19 should not receive drugs that are capable of causing moderate to severe AE. In mild COVID-19 infections, depending on the drug toxicity profile, AE might eventually be worse than the disease. A different picture emerges when drugs are prescribed to treat severe and life-threatening manifestations of COVID-19. In severely ill patients, one might presume that expected (but still undemonstrated) clinical benefits of a repurposable drug are likely to outweigh risks of AE. So far, there exists no approved treatment for COVID-19, nor are there sufficient data to recommend for or against the use of drugs outside of clinical trials ¹². In other words, there are no proven beneficial effects to support COVID-19-specific pharmacological interventions. Along this line, compassionate (or expanded) use of repositionable medicines for COVID-19 must be cautious and based on robust scientific evidence. Narrow margin-of-safety (MOS) medicines, for instance, must be avoided in oligosymptomatic or mild clinical presentations of the infection.

Antiviral agents

Clinical trials on drug repurposing for COVID-19 address a diversity of existing and experimental (new) antiviral drugs with distinct mechanisms of action, such as inhibition of viral protease – Lopinavir+Ritonavir (LOP+RIT), Darunavir, DRV, ASC09 ^{13, 14}; RNA replicase – Favipiravir (FAV), Remdesivir (RDV) ^{15, 16, 17}; neuraminidase (OSV) ³; RNA synthesis and mRNA capping (Ribavirin, RBV) ¹⁸; and membrane fusion, a key step for enveloped viruses entry into cells (Umifenovir, UMV) ^{19, 20} ( Table 2). The boosted protease inhibitors form an integral part of the current ART for HIV infections. Guanosine analog inhibitors of RNA synthesis (RBV) are used in the treatment of respiratory syncytial virus (RSV) and hepatitis C virus (HCV), and a few other infections ^{18, 21}. The remaining antiviral agents under investigation for COVID-19 (UMV and OSV) are predominantly used to treat influenza infections. It is of note that OSV and RBV had been used experimentally in 2003’s outbreak of SARS. Poutanen et al. ²² reported that five of seven SARS Canadian patients treated with RBV improved with the therapy. However, since RBV-treated patients also received an array of other drugs, it is unclear whether RBV did in fact affect the clinical outcome ²².

Among the tested antivirals, RDV is perhaps researchers’ best guess in an effective anti-COVID-19 drug, and, therefore, there is a great deal of expectation regarding the results of ongoing clinical trials. In in vitro assays, RDV strongly inhibited replication of SARS-CoV-1 and MERS-CoV viruses in several cell lines ^{23, 24}. Furthermore, data from a cohort of patients hospitalized for severe COVID-19 who had received RDV in a compassionate-use basis, indicated that 36 (68%) drug-treated patients showed a clear-cut clinical improvement ²⁵. Early results (29 April 2020) of an ongoing US NIH-sponsored large (> 1,000 participants) placebo-controlled randomized controlled trial (RCT) suggested that RDV cut recovery time for hospitalized COVID-19 patients by four days, or 31% (i.e., about 11 days in RDV-treated against 15 days in the placebo group) ²⁶. A nonsignificant reduction in death rate (8% in RDV-treated patients against 11% in the placebo group) was also noted. Although these preliminary figures suggest a relatively modest clinical benefit, they were enthusiastically celebrated as a first reliable clinical indication of efficacy of a COVID-19 drug and a “proof-of-concept” ²⁶ regarding this antiviral mode of action for SARS-CoV-2.

Antibiotics

It is hard to see the rationale behind the clinical trials on the potential benefits of macrolide antibiotics such as carrimycin (CRM) and azithromycin (AZM) in COVID-19 pneumonia. Antibiotics are known to be ineffective against viruses and their use for treatment or prevention of acute viral infections of the (lower and/or upper) respiratory tract is not only unnecessary but also inapropriate ^{27, 28, 29}. The most recently issued UK National Institute for Health and Care Excellence (NICE) guidelines recommend “not to offer an antibiotic for treatment or prevention of pneumonia if COVID-19 is likely to be the cause and symptoms are mild” ³⁰. In theory, use of antibiotics in patients with a diagnosis of COVID-19 pneumonia might have one of three explanations: physicians’ uncertainty about the viral etiology of the pneumonia, to prevent a bacterial secondary infection if immunosupressive agents are employed as adjunct therapies, or a difficulty in ruling out the co-existence of viral and bacterial infections what considerably worsens the prognosis of critically ill patients ^{31, 32, 33}.

Antiparasitic agents

Antimalarials

An array of antiparasitic drugs with putative antiviral activity (found in in vitro assays) are under investigation in COVID-19 clinical trials. The most well-known potentially repurposable drugs are the old antimalarials chloroquine (CQ) and hydroxychloroquine (HCQ) ( Table 2). The hypothesis that CQ and HCQ might be useful to treat SARS-CoV infections can be traced back to 2003 ³⁴. Based on the reports that CQ inhibited replication of enveloped RNA virus, Savarino et al. ³⁵ proposed that it could be useful to treat the disease caused by SARS-CoV, a positive-stranded RNA virus. Further in vitro studies in African green monkey kidney (Vero) cells showed that CQ and HCQ changed terminal glycosylation of the cellular receptor Angiotensin Converting Enzyme 2 (ACE2) and spike proteins thereby blocking SARS-CoV cell infection at entry and post-entry stages ^{24, 36}. Moreover, both antimalarials also have (mild) immunosuppressive properties and had been successfully repurposed for treatment of rheumatic and auto-immune diseases. Therefore, CQ and its derivative HCQ may have a dual therapeutic effect because one of the distinctive features of the pathophysiology of COVID-19 pneumonia (and ARDS) involves a massive release of cytokines (cytokine storm) leading to lung hyperinflammation ¹⁰. The combined action on two disease-related targets, i.e., inhibition of virus replication (demonstrated in vitro) and immunosuppressive/anti-inflammatory action (shown in humans), could potentially make CQ and HCQ unique drugs for severe COVID-19. The severe and life-threatening adverse effects of CQ and HCQ (e.g., retinopathy and irreversible vision lost, cardiac arrhythmias, cardiomiopathy, hearing deficits and tinnitus, shortness of breath, mental disturbances and others) and the narrow margin of safety, however, are a hurdle for their widespread use, particularly to treat the less severe cases of COVID-19 ³⁴.

Results from a recently completed small study (pilot open-label RCT) involving 30 patients with confirmed COVID-19 showed no discernible difference in clinical improvement between patients treated with HCQ and those who received conventional therapy only ³⁷. This study has a number of methodological shortcomings and it is definitely underpowered to evaluate the effectiveness and safety of HCQ for COVID-19.

An observational study (published on 7 May 2020) compared the clinical outcomes in 811 COVID-19 patients who received HCQ with those in COVID-19 (unmatched) patients who did not. A Cox proportional-hazards regression model analysis showed that HCQ was not associated with a significantly higher or lower risk of intubation or death (hazard ratio: 1.04, 95% CI: 0.82 to 1.32) ³⁸. Results from this observational investigation do not support the use of CQ/HCQ in COVID-19 patients. Nonetheless, owing to the inherent limitations of studies with an observational design (e.g., unmeasured/uncontrolled confoundings and bias) this investigation is not sufficient to ascertain whether or not HCQ is in fact of benefit for COVID-19 patients. Patients treated with HCQ, for instance, might have been those with the most severe manifestations of COVID-19 and poorest prognosis.

The antiviral activities of CQ and HCQ in humans, and their effectiveness in the treatment of COVID-19 and ARDS remain to be proven by large, randomized and placebo-controled studies with masking and concealment of allocation.

Facing a widespread prescription of HCQ and CQ for COVID-19 patients, on April 24 ^th 2020, the US Food and Drug Administration (FDA) issued a warning that it had received reports of serious heart-related adverse events and death in patients with COVID-19 receiving hydroxychloroquine and chloroquine, either alone or combined with azithromycin or other QT prolonging medicines. These adverse events included QT interval prolongation, ventricular tachycardia and ventricular fibrillation, and in some cases, death.

FDA authorized ( Emergency Use Authorization) CQ-HCQ temporary use only in hospitalized patients with COVID-19 when clinical trials are not available or participation is not feasible ³⁹. Along the same line, the Brazilian Ministry of Health and National Agency of Sanitary Surveillance (Anvisa) had authorized (on March 27 ^th, 2020) use of CQ and HCQ (strictly under medical prescription) for patients with the most severe manifestations of COVID-19 ^{40, 41}.

Anthelmintics

Clinical trials on the repurposing of some anthelmintics (Nitazoxanide, NTZ; Ivermectin, IVT; Niclosamide, NCL) for COVID-19 seem to be based on in vitro data showing that these compounds inhibit replication of a variety of viruses in cell culture assays. NTZ, for instance, was active in cell culture assays against a broad range of influenza A and B, as well as other RNA and DNA viruses, such as RSV, parainfluenza, coronavirus, rotavirus, norovirus, hepatitis B and C viruses, dengue, yellow fever, Japanese encephalitis and HIV ⁴². Likewise, in in vitro tests, IVM inhibited the replication of a broad range of viruses (dengue, West Nile virus, HIV, simian SV-40, influenza and others) and strongly repressed SARS-CoV-2 virus replication in Vero-hSLAM cells ⁴³. Also in in vitro assays, NCL proved to be a potent inhibitor (nanomolar to micromolar range) of replication of SARS-CoV, MERS-CoV, zika virus, hepatitis C virus and human adenovirus ⁴⁴. NCL had been reported to be active ( in vitro) against SARS-CoV at concentrations as low as 1.56 µM in 2003 ⁴⁵.

Contrasting to a direct inhibition of viral replication by NTZ, IVM and NCL, the hypothesis that Levamisole (LVM) could be useful in the prophylaxis and therapy of viral infections is based on its putative immuno-stimulant properties ⁴⁶. Several clinical trials (as to 1980) showed no benefit from LVM as compared to placebo in the treatment of herpes simplex virus recurrent infections ^{46, 47, 48}. A recent study in piglets, however, indicated that LVM could be useful to prevent intestinal damage in porcine rotavirus diarrhea ⁴⁹. At any rate, the scientific evidence supporting the conduction of clinical trials of LVM for COVID-19 is poor. Moreover, the use of LVM as anthelmintic and/or immunomodulator has been discouraged by their immunotoxic side effects and induction of agranulocytosis and neutropenia ⁵⁰. A number of cases of agranulocytosis have been reported in users of LVM-tainted cocaine ⁵¹.

Immunomodulators and anti-inflammatory agents

Since COVID-19 may elicit massive release of cytokines and pulmonary hyperinflammation, the use of immunosuppressive and anti-inflammatory drugs as adjuvant therapies could be expected ^{10, 52}. Immunosuppression, however, might be a double-edged sword in these cases. Although it is likely to suppress the massive cytokine release syndrome and to mitigate lung inflammation, it may also facilitate viral proliferation, if it is not combined with effective antiviral therapy. Along this line, the open research questions are: how effective the tested anti-inflammatory interventions (with or without concomitant antiviral therapy) are in COVID-19 pneumonia, which is the most effective and safe immunosuppressive and/or anti-inflammatory compound and at which dosage regimen does it produce the best overall clinical response?

As shown in Table 3, a multiplicity of immunosuppressive drugs or therapies have been used to reverse the cytokine release syndrome in COVID-19 trials. The tested immunomodulating and/or anti-inflammatory drugs cover a broad range of compounds and modes of action such as classical glucocorticoids or agonists of glucocorticoid receptor (dexamethasone, methylprednisolone, budesonide), biologicals including some of the newest ones (anti-IL-6 monoclonal antibodies tocilizumab and siltuximab, the antibody against CCR5 receptors on T-lymphocytes leronlimab ⁵³, and an antibody against cytokine fusion protein CD24Fc), ruxolitinib (Janus kinase JAK1 and JAK2 selective inhibitor and blocker of cytokine signalling) ⁵⁴, thalidomide (inhibitor of production of TNF-α and activation of NF-кB) ^{55, 56}, colchicine (tubulin disruption, inhibition of neutrophil chemotaxis, adhesion and mobilization, inhibition of inflammasomes and IL-1β processing and release) ⁵⁷, sirolimus (a macrolide inhibitor of cytokines transcription and synthesis) ⁵⁸, piclidenoson (A3 adenosine receptor agonist) ⁵⁹, tetrandrine (Ca ² channel blocker anti-inflammatory) ⁶⁰ and non-steroidal antinflammatory agents which are nonselective inhibitors of Cox1 and Cox2 (naproxen, ibuprofen, aspirin). Another immunosuppresive drug tested for COVID-19 is fingolimod, a sphingosine-1-phosphate receptor modulator, which sequesters lymphocytes in lymph nodes, preventing them from contributing to autoimmune reactions. It is mostly used to treat the relapsing form of muliple sclerosis ⁶¹. It is of note that fingolimod-mediated immunodepression has been reported to enhance risks of viral, fungal and bacterial infections, and concerns have been raised regarding influenza infections, reactivation of herpes and varicela-zoster as well as John Cunningham virus, a polyomavirus related to the Progressive Multifocal Leukoencephalopathy ⁶².

A RCT study (NCT04268537) was designed to investigate whether treatment with thymosin or with PD-1 antibody would attenuate lung injury and improve prognosis of COVID-19 patients with respiratory failure and lymphocytopenia. Thymosin β4 (a hormone from thymus) stimulates T-lymphocyte production and it is thought to possibly decrease mortality in sepsis via regulation of actin expression and anti-inflammatory actions ⁶³. Programmed Cell Death Ligand 1 protein (PD-1) is expressed on activated T-cells and PD-1-related blockage (PD-1 antibody) is believed to potentially decrease mortality in sepsis as well ^{64, 65}. This research on the possible beneficial effects of thymosin or PD-1 antibody on critically ill COVID-19 patients seems to be based on the notion that sepsis is often secondary to excessive inflammatory response syndrome and that PD-1 and PDL-1 are key mediators of T-cell depletion in sepsis ^{64, 65}.

Moreover, two cytokines (interferons) playing a key role in innate immunity and control of viral infections, i.e., Interferon-lambda (IFN-λ) and Interferon beta (INF-β), were tested in COVID-19 patients as well. INF-β regulates the expression of a plethora of genes through the classical JAK/STAT and other pathways thereby eliciting antiviral, antiproliferative and immunomodulatory activities on numerous cell types. It is used to treat hepatitis C infection, multiple sclerosis and other conditions ⁶⁶. While acting on the control of viral infections (e.g., chronic hepatitis C) and also establishing a robust innate immunity against cancer, IFN-λ has a restricted cell response pattern and thus it was associated with fewer AE ⁶⁷. It has been proposed that administration of pegylated IFN-λ in influenza infections improves respiratory function and survival by reducing overabundance of neutrophils in the lungs. A recent study ⁶⁸, however, called attention on the fact that IFN-λ, by decreasing neutrophyl motility, may impair bacterial clearance during influenza superinfection and by doing this it might increase the likelihood of a secondary bacterial pneumonia.

Miscellaneous drugs

The use of drugs from a variety of other pharmacological classes are also being tested in COVID-19 clinical trials.

Antifibrinolytics and antithrombotic agents

There have been several reports of strong associations between elevated levels of D-dimer (being a degradation product of cross-linked fibrina, D-dimer reflects blood clot formation and its subsequent fibrinolysis ) and poor prognosis in COVID-19 and, therefore, thrombotic complications (e.g., venous thromboembolism, disseminated intravascular coagulation, thrombosis) are a cause for deep concern ⁶⁹. Within this context, use of antithrombotic and fibrinolytic drugs (e.g., defibrotide) or heparin-like anticoagulants (e.g., enoxparin, a low molecular heparin) may be necessary to prevent thromboembolism, particularly, if levels of D-dimer are high ^{69, 70, 71}. The rationale for COVID-19 trials investigating the benefits of tranexamic acid (TXA), an antifibrinolytic agent used to prevent or control postsurgical or posttraumatic bleeding ⁷², however, is not so obvious. One trial of TXA in COVID-19 is based on a hypothesis that an endogenous protease plasmin acts on SARS-CoV-2 cleaving a newly inserted furin site in the virus S protein portion, what could (theoretically) increase its infectivity and virulence (see trial NCT04338126 in www.ClinicalTrials.gov). If so, the suppression of conversion of plasminogen to plasmin by TAX could blunt this process thereby decreasing the infectivity and virulence of SARS-CoV-2 in infected patients. Even though this hypothesis sounds plausible, nonclinical empiric evidence to adequately support it is missing.

Inhibitors of angiotensin-converting enzyme and angiotensin receptor blockers

Two apparently conflicting pharmacological interventions on the renin-angiotensin system are under investigation in distinct COVID-19 trials. A quadruple-blinded RCT trial evaluates whether administration of losartan, a selective competitive angiotensin II receptor (AT ₁) blocker (a drug widely used for hypertension), might be beneficial for patients infected by COVID-19. A second RCT (single-blinded) study investigates whether discontinuation of chronic treatment with inhibitors of angiotensin-converting enzyme 2 (ACEI), or with angiotensin-2 receptor blockers (ARB), could improve outcomes in symptomatic SARS-CoV-2-infected patients. Both experimental interventions are ultimately based on the observation that SARS-CoV-2 uses angiotensin-converting enzyme 2 (ACE2) as the receptor binding domain for its spike protein ^{73, 74}. Thus, it is believed that angiotensin-converting enzyme 2 (ACE2) is likely to be a functional receptor for SARS-CoV-2 to enter host target cells.

Experimental studies showed that continued treatments with ARB, as losartan, or ACEI, as captopril and enalapril, upregulate the expression of ACE2 receptors and theoretically might increase the morbidity and mortality of COVID-19 ⁷⁵. However, studies in mice also indicated that, paradoxically, ARB could also have a protective effect against COVID-19 pneumonia and ARDS because ARB prevented aggravation of lung injury in mice infected with a similar virus (SARS-CoV-1 involved in 2002-2003 outbreak) ^{75, 76, 77}. It should be pointed out that, to date, there is no clinical or experimental indication that ARB or ACEI either increase the susceptibility to SARS-CoV-2 or aggravate the severity of clinical outcomes of COVID-19.

Also, based on this notion, another trial investigates the effectiveness of a recombinant antibody against human angiotensin-converting enzyme 2 (rhACE2) to block SARS-CoV-2 entry into cells and to inhibit viral replication in COVID-19 patients.

Antiarrhythmics

The antiarrhythmic drugs (ion channel blockers) verapamil and amiodarone were reported to block Filoviridae (e.g., Ebola and Marburg viruses) cell entry in cell culture tests ⁷⁸. Based on the foregoing non-clinical evidence, a clinical trial is in progress to investigate whether they are effective against SARS-CoV-2 as well ( Table 3).

Mucolytics and bronchodilators

Mucolytics as bromhexine and compounds that decrease the resistance in the respiratory airway, thereby increasing airflow to the lungs (e.g., long-acting selective β ₂-adrenergic agonists, as formeterol), are commonly used to treat pulmonary obstructive conditions such as asthma, chronic obstructive pulmonary disease (COPD) and others ^{79, 80}. The therapeutic potential of bromhexine and formeterol as well as that of inhaled nitric oxide (NO), a reported selective pulmonary vasodilator ⁸¹, were tested in COVID-19 patients.

Traditional Chinese Medicine and other interventions

A few RCT COVID-19 trials investigate the effectiveness of traditional chinese medicine (TCM) remedies as adjunct therapies to the standard of care. A variety of other modern medicine drugs are under investigation for COVID-19 as well. Serine protease inhibitors (camostat mesylate and nafamostat mesylate) are under clinical testing for COVID-19. Serine protease inhibitors exhibit anti-inflammatory activity (through blockage of NF-кB signalling pathways), anticoagulant, anticancer and potential antiviral (against Ebola virus) properties ^{82, 83, 84}. Diurectics (spironolactone), symvastatin (inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, a cholesterol-lowering statin), nintedanid (intracelular tyrosine kinase inhibitor used to slow progression of idiopathic pulmonary fibrosis), thyroid hormone T3 (triiodothyronine), high doses of vitamins D and C, deferoxamine (medication that binds iron and aluminium in the blood and enhance their elimination via urine), cobicistat (potent inhibitor of CYP3A subfamily enzymes; although lacking antiviral activity, cobicistat is combined to anti-HIV compounds to slow down their clearance) ⁸⁵ and vagezepant (a small molecule calcitonin gene-related peptide-CGRP-receptor antagonist used in the treatment of migraine) ^{86, 87}. Manufacturers of vagezepant have claimed that their drug might mitigate lung hyperimmune response in COVID-19 and thus this hypothesis is under investigation in a study (by intranasal route) in hospitalized patients requiring supplemental oxygen.

Strengths and weaknesses of COVID-19 drug trials

As aforementioned, this analysis of clinical trials, addressing drugs potentially repurposable for COVID-19, covers only studies involving more than one arm and that are randomized and controlled. Information avaliable on the ClinicalTrials.gov registry database is rather limited and does not allow in depth and meticulous evaluations of the design and methodological quality of the study. Nonetheless, in a first-round to separate the wheat from the chaff, to keep only the wheat, we examined some study-design key features such as masking, type of comparators and number of participants. Of 294 included trials, 150 (51.0%) were open label (no masking) studies and 26 (8.8%) were single-blinded (participant, investigator or assessor only), so that only 40.1% seemed to have been properly masked (i.e., double-, triple- or quadruple-blinded) ( Table 1). Open label studies and those with insufficient or inadequate masking and concealment of randomization entail a high risk of bias. Since under these study conditions clinical outcomes are likely to be influenced by investigators’ and or participants’ expectations, these not adequately blinded approaches do not yield robust evidence on drug effectiveness and safety. Another drawback of most controlled clinical trials of drugs for COVID-19 is the type of comparator chosen by investigators. Whenever there is no proven effective treatment for the disease (i.e., there is no “active” comparator), trials are expected to be placebo-controlled. Many included studies, however, used standard of care (no intervention arm) with no placebo for the drug as the inactive comparator. Even worse is to compare the intervention under testing with another drug of undemonstrated effectiveness against COVID-19 (e.g., chloroquine/hydroxychloroquine). Therapies of unproven efficacy are by no means suitable “active” comparators for a drug monotherapy or a drug combination under testing. In the set of clinical trials examined in this study, the “standard of care” or no intervention arm at times included drugs of unproven or questionable efficacy for COVID-19.

Adequate sample size estimation in clinical trials is crucial for the robustness of the evidence on drug efficacy and safety for a given therapeutic indication. As shown in Tables 2 and 3, sample sizes of COVID-19 trials ranged from a couple of tenths to thousands. The ClinicalTrials.gov registry data did not provide study-design details needed for examining the adequacy of the estimated trial sample size. However, some of the studies enrolled so few participants that they are definitely underpowered to produce any robust evidence of drug effectiveness and safety in COVID-19 patients.

In summary, notwithstanding the multiplicity of clinical studies of medications for COVID-19, only a minority of them (RCT), which are sufficiently large, randomized, placebo-controlled and designed with masking and concealment of allocation, are likely to yield robust evidence on potential benefits for infected patients.

Ethical issues

Ethically, there is a great divide between conducting expeditiously clinical studies on drug effectiveness (and safety) and rushing into trials in COVID-19 patients without a plausible hypothesis and appropriate evidence of safety. The COVID-19 health emergency is by no means a carte blanche for neglecting the ethical standards for clinical research.

An article by Emanuel et al. ⁸⁸ listed seven conditions that need to be met to make ethical a clinical trial. The analysis of registered study protocols led to the conclusion that at least two of these conditions (scientific validity and favourable risk-benefit ratio) are not fully met in most COVID-19 clinical trials. As explained in the previous section, many trials on drugs for COVID-19 suffer from methodological shortcomings that weaken their power to demonstrate that the drugs are effective and safe for this disease. For instance, trials not testing a clear and scientifically founded hypothesis that are poorly designed (e.g., single arm, nonramdomized, open label) and do not have enough power to definitely respond the research question, do not meet scientific validity requirements. Moreover, of particular concern is the apparently unfavorable drug risk-benefit ratio in several of planned and ongoing trials. This is illustrated by some trials of CQ and HCQ in COVID-19. Both are drugs with narrow MOS that may cause serious AE such as irreversible vision loss, cardiac arrhythmias and death. Since most COVID-19 patients (80.0%) are oligosymptomatic achieving spontaneous cure within two weeks and antiviral activity of these antimalarials in humans remains unproven, it is fair to think that risk-benefit ratio of CQ/HCQ in preventive (prophylatic) intervention trials is unfavorable. For mild COVID-19 patients, risks of severe adverse effects would certainly outweigh the potential health benefits of disease prevention or treatment, even if CQ/HCQ were in fact effective antiviral drugs.

CONCLUSIONS

Ongoing and planned clinical trials of drug repurposing for COVID-19 address the primary treatment of the disease (inhibitors of viral replication) as well as adjunct therapies for resolution of hyperinflammation in pneumonia and thromboembolism associated with SARS-CoV-2 infection.

So far, no antiviral pharmacological intervention has proved to be effective against SARS-CoV-2 in humans. Virtually all existing antiviral compounds, and a multiplicity of modes of action which work against other viruses are under investigation. Antiparasitic drugs which inhibited viral replication in cell culture assays and new SARS-CoV-2 specific modes of action (e.g., rhACE2, TXA) are being tested as well.

As far as adjunctive immunomodulatory, anti-inflammatory and antithrombotic therapies are concerned, a number of drugs with different pharmacological targets are being used and tested in clinical trials mostly in severe cases of COVID-19. Strictly speaking, most of these drugs are not being repurposed for COVID-19, because their therapeutic effectiveness (e.g., glucocorticoids and others) has already been demonstrated for inflammation and thromboembolism in a wide variety of diseases and medical conditions and it is fair to assume that they shall work in COVID-19 infection complications as well. Collectively, the outcomes of these trials are expected to contribute to find out the best timing for the intervention, the most effective and safe drugs and dose regimens to be used, and to evaluate the relevance of the intervention for resolution of severe COVID-19. Certainly, they will yield empiric information of value to update evidence-based clinical guidelines for COVID-19.

Finally, although a large number of clinical studies of drugs for primary treatment of COVID-19 are planned or in progress, only a minority of them are large, randomized and placebo-controlled trials with masking and concealment of allocation. Therefore, only a few of these studies are likely to produce robust clinical evidence of antiviral drug efficacy and safety for dealing with COVID-19 pandemic. Five months or so after SARS-CoV-2 emergence as a deadly pandemic, antiviral drug therapy is more and more unlikely to “change the game” in a timely manner along this first wave of COVID-19. Effective and safe vaccines seem to be the best guess for the coming years. Currently, old and proven effective behavioral approaches such as social-distancing and quarantine continue to be public health scientists’ most powerful weapons to fight against the pandemic.

Acknowledgments

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. LRP is a PhD student of the Post Graduate Program on Sanitary Surveillance of the National Institute for Health Quality Control (INCQS) of the Oswaldo Cruz Foundation (Fiocruz). FJRP is the recipient of a research productivity fellowship from the Brazilian National Council for Scientific and Technological Development (CNPq).

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Author notes

* E-mail:paum@ensp.fiocruz.br** E-mail:isabella.delgado@fiocruz.br

Conflict of interest declaration