Figure 2. Chemical structures of available drugs that have been investigated for the treatment of COVID-19.
3.1. Potential Candidates
3.1.1. Antimalarial Agents
Chloroquine (CQ) and Hydroxychloroquine (HCQ): Both are amino acidotropic forms of quinine and are FDA-approved to treat or prevent malaria and autoimmune diseases
[115][34]. They have shown to prevent the entry and transport of coronaviruses by increasing the endosomal pH required for virus and cell fusion and interfering with the glycosylation of cell receptors
[116][35]. CQ and HCQ were expected to prevent the replication and invasion of the virus and inhibit T-cell activation, reducing the risk of a cytokine storm
[117][36]. On 15 June 2020, the FDA determined that CQ and HCQ are unlikely to be effective in treating COVID-19 due to serious cardiac adverse events and other serious side effects, concluding that these medicines showed no benefit for decreasing the likelihood of death or expediting recovery for hospitalized patients and ultimately revoking the emergency use of CQ and HCQ
[118][37].
3.1.2. Antiviral Agents
Remdesivir (GS-5734) is an adenosine triphosphate nucleoside analog that interferes with virus replication and has been recognized as a promising antiviral drug against a plethora of RNA viruses, including Ebola, SARS, and MERS. However, its safety and efficacy are currently being tested in multiple clinical trials
[119,120][38][39]. Remdesivir has been reported to inhibit viral RdRp activity at early stages of the infection cycle, and the incorporation of its active triphosphate form by the SARS-CoV-2 RdRp complex is higher than that of the competing natural ATP substrate, disrupting replication of the virus
[121][40]. In June 2020, the FDA approved remdesivir for the treatment of COVID-19 requiring hospitalization. This approval was supported by analysis of data from three randomized, controlled clinical trials that included patients hospitalized with mild to severe COVID-19
[122,123][41][42].
Ribavirin is a guanosine nucleoside analog known as a broad-spectrum antiviral that is effective against HIV, hepatitis C, herpes viruses, and other viruses
[124,125][43][44]. Ribavirin interferes with RNA polymerase and RNA capping, interrupting the viral cells’ ability to replicate, causing recognizable degradation. Treatment plans typically combine ribavirin with interferon-α2a or β1
[124][43]. Ribavirin monotherapy for MERS has shown little positive effect on patients with severe symptoms. There was hope that combination therapy of ribavirin with lopinavir/ritonavir against hCoV infections would be effective
[126,127][45][46]. A phase II trial in March 2020 found that such a triple therapy was successful at early stages of COVID-19 in reducing the length of hospital treatment
[128][47]. However, in July 2020, the NIH withdrew support of the use of ribavirin treatment as it did not reduce COVID-19 mortality
[129][48].
Interferons (IFNs) are secreted antiviral cytokines that are produced by virus-infected cells. IFNs are broad-spectrum antivirals with
α,
β, and λ subtypes that have been used for the treatment of hepatitis and have been previously tested to treat MERS and SARS-CoV
[130][49]. However, trials for IFN therapy (
α or
β) for hCoVs have shown variable results. Two clinical trials for SARS patients with IFN treatment determined that while IFN-α was able to inhibit SARS replication, IFN-β had similar results when used as a combination therapy
[131][50]. Another study deemed IFN-β1b the preferred treatment for MERS and IFN-α2b/ribavirin for SARS, while several trials suggested that IFN-α2b/ribavirin could also be effective against MERS
[132,133][51][52]. In a large randomized controlled trial of hospitalized patients with COVID-19, the combination of IFN-β1a plus remdesivir showed no clinical benefit when compared to remdesivir alone
[134,135][53][54]. Trials evaluating IFN-α were not sufficient to determine whether this agent provides a clinical benefit for patients
[136,137][55][56].
Lopinavir/Ritonavir (LPV/RTV) are antiviral agents that act as HIV protease inhibitors that have been used for the treatment of SARS and MERS
[138][57]. Lopinavir is commonly combined with ritonavir to increase its half-life and reduce side effects
[139][58]. LPV and RTV along with interferon-β1 and ribavirin have shown greater effects in inhibiting coronavirus replication as compared to LPV or RTV alone
[140][59]. Due to the lack of efficacy and increased incidence of adverse events, the clinical use of LPV/r in hospitalized COVID-19 patients was not recommended
[141][60].
Umifenovir (Arbidol), also known as ARB, is an antiviral drug that can inhibit several DNA and RNA viruses, including the West Nile virus, Zika virus, and hepatitis C virus. Simulations showed that umifenovir inserts into the spike protein of the virus, blocking viral fusion to the cell
[142][61]. Reaching phase IV of clinical trials, angiotensin II receptor blockers (ARBs) and ACE inhibitors have been used to help in treating moderate to severe infections in patients with pneumonia
[143][62]. Another study showed that ARB combined with LPV/r was better than lopinavir alone
[144][63]. However, arbidol has shown little to no effect on patients with mild or moderate symptoms of COVID-19
[145,146][64][65].
Favipiravir (Avigan/T-705) is a pyridinecarboxamide that was initially developed as an anti-influenza substance and is classified as an antiviral drug that selectively inhibits the RNA polymerase of RNA viruses
[147][66]. Favipiravir was shown to be effective against the influenza strains A(H1N1) and A(H7N9), and it later was used as a potential treatment for the Ebola virus, West Nile viruses, arenaviruses, and other viruses
[148][67]. Once inside the cell, favipiravir is ribosylated and phosphorylated into its active form, T-705RTP
[149][68]. Enzyme kinetic analysis demonstrated that T-705RTP inhibited the incorporation of ATP and GTP in a competitive manner, preventing the strand extension of the virus
[150][69]. Initial trials for use in response to COVID-19 suggested that favipiravir administration in individuals with mild to moderate infection has a strong potential to improve clinical outcomes
[151][70]. However, recent studies showed that favipiravir does not improve clinical response in all COVID-19 patients admitted to hospital, and further high-quality studies of antiviral agents and their potential treatment combinations are warranted
[152][71].
Gemcitabine (Gemzar): Gemcitabine hydrochloride is a pyrimidine nucleoside analog that is commonly used as a chemotherapeutic agent against several forms of cancer, such as pancreatic cancer
[153][72]. Previous investigations of the effect of gemcitabine on MERS and SARS reported that the drug inhibits both HCoVs at a low micromolar EC
50 of 1.22 and 4.96 μM, respectively
[154][73]. One report suggested that the difluoro group of gemcitabine is critical to its antiviral activity and could be a desirable option to treat SARS-CoV-2 in combination with other antiviral drugs, such as remdesivir
[155][74].
Nafamostat Mesylate (Fusan) is a broad-spectrum serine protease inhibitor that can act as an anticoagulant and is used to treat pancreatitis and other inflammatory diseases
[156][75]. In 2016, nafamostat mesylate was identified as a treatment option for MERS, successfully inhibiting the host cell’s TMPRSS2 receptor during viral S protein–cell membrane fusion, setting it up for potential inhibition of other HCoVs
[113][31]. In April 2020, when comparing serine protease inhibitors, one study showed that nafamostat mesylate had a 15-fold increase when compared with camostat mesylate in inhibiting SARS-CoV-2 S protein fusion mediated by the host cell ACE2/TMPRSS2 receptor complex
[157][76]. Moreover, a case study reported the efficiency of both favipiravir and nafamostat mesylate as a joint treatment in SARS-CoV-2 patients, noting that the anti-blood clotting properties of nafamostat mesylate were likely beneficial in treating SARS-CoV-2 patients
[158][77]. Additional clinical trials are needed to provide more robust data on the safety and efficacy of nafamostat as a treatment for COVID-19
[159][78].
Sofosbuvir/Daclatasvir: Both drugs are classified as direct-acting antivirals (DAAs) and are FDA-approved to treat the hepatitis C virus (HCV) due to their combined ability to inhibit the functions of HCV nonstructural protein 5 (NS5B), an essential component in viral replication
[160][79]. Like HCV, SARS-CoV-2 is a positive-stranded RNA virus, making it the potential primary target of sofosbuvir/daclatasvir
[161][80]. However, no significant reduction in the SARS-CoV-2 viral load was observed in patients hospitalized with COVID-19 and receiving this treatment plan when compared with a control group
[162][81]. Larger clinical trials are warranted.
EIDD-2801 (molnupiravir) is an orally bioavailable ribonucleoside analog of β-D-
N4-hydroxycytidine (NHC, EIDD-1931)
[163][82]. NHC has been found to participate in viral replication by causing mutations from G to A and C to U, which is deadly to the new replication of the virus
[164][83]. Studies have shown that EIDD-2801 is highly effective against SARS-CoV in Vero cells (IC
50 of 0.3 µM) and in mice infected with MERS, and it was also effective against SARS-CoV-2 RdRp mutations that developed as a result of remdesivir treatment
[165][84]. In November 2021, the U.K. became the first country to approve this drug for the treatment against mild and moderate SARS-CoV-2. Within a month, the U.S. FDA approved EIDD-2801 for emergency use
[166][85]. EIDD-2801 is the first oral antiviral drug that demonstrated efficacy in decreasing the viral RNA amounts in nasopharyngeal swabs
[167][86]. Relatedly, recent studies provide evidence of the efficacy of molnupiravir, which compliments the ongoing clinical trials of EIDD-2801
[168][87].
Galidesivir (BCX4430) is an adenosine nucleoside analog shown to have effective antiviral properties against several RNA viruses, including the Zika virus, HCV, and Ebola virus
[169][88]. Two studies showed that galidesivir has an ability to inhibit SARS-CoV-2 RdRp and thus has potential as a therapeutic agent
[170,171][89][90]. In cells where phosphorylation occurs, premature chain termination can be achieved. Structural modeling shows a potential allosteric inhibition of RdRp as galidesivir binds to the noncatalytic site of the enzyme
[172][91]; however, early stages of clinical trials showed no benefit for COVID-19 patients
[173][92].
3.1.3. Other Agents and Therapies
Vitamin C: Studies have shown that intravenous (IV) vitamin C may help COVID-19 patients by reducing inflammation or possibly by restoring the antioxidant protection of the body
[174][93]. Overall, evidence from randomized controlled trials suggests a survival benefit for vitamin C in patients with severe COVID-19; however, more data with definitive outcomes are needed before it is recommended to provide high-dose vitamin C therapy to prevent or treat COVID-19
[175][94]. It is currently advised to maintain a normal physiologic range of plasma vitamin C through diet or supplements for adequate prophylactic protection against the virus
[176][95].
Corticosteroids are synthetic analogs of steroid hormones naturally produced by the adrenal cortex that have broad anti-inflammatory activities
[177][96]. Dexamethasone is a fluorinated corticosteroid that relieves inflammation and is commonly used to treat arthritis
[178][97]. In a clinical trial of hospitalized patients with COVID-19, the use of dexamethasone resulted in lower mortality among those receiving either mechanical ventilation or oxygen
[179][98].
Bamlanivimab and etesevimab are monoclonal antibodies that target the receptor binding domain of the SARS-CoV-2 spike protein and thus prevent the entry of the virus into human cells
[182][99]. Identifying the specific antibodies from a patient that recovered from SARS-CoV-2 infection expedited the development of this treatment in eight months
[183][100].
3.1.4. Antibiotic/Antibacterial
Azithromycin (AZM) is a macrolide antibiotic that has been approved by the FDA to treat enteric and respiratory tract bacterial infections. It was investigated in combination therapy with hydroxychloroquine (HCQ) to treat hospitalized COVID-19 patients with bacterial infections
[187][101]. A clinical trial of HCQ and AZM consisting of 504 COVID-19 patients randomized at a 1:1:1 ratio compared the outcomes of one group receiving standard care, another group treated only with HCQ, and the third group receiving an AZM/HCQ mixture. The results showed that the use of HCQ with or without AZM did not improve clinical status at 15 days when compared to the standard care treatment
[188][102]. Several other studies with AZM as a therapy showed no increase in clinical benefit and that there was potential risk for development of antibacterial resistance
[189][103]. The results from the COALITION II Trial, RECOVERY Trial, and PRINCIPLE trial showed that AZM was not better than standard care alone and did not provide any benefits in recovery. Following these findings, the NIH and WHO recommended against the use of antibacterial medicine for COVID-19 patients
[190][104].
Cefuroxime, a broad-spectrum antibiotic that is commonly used in the treatment of respiratory and gastrointestinal tract infections, has been investigated as anti-SARS-CoV-2 agent
[191][105]. In silico studies reported the antibiotic as a strong inhibitor against SARS-CoV-2’s major protease
[192,193,194][106][107][108]. As such, these in silico studies showed that cefuroxime could potentially be successful in clinical trials due to its bioavailability and viral inhibitory activity. However, the gratuitous use of antibiotics for COVID-19 treatment without proper clinical rationale raised concerns about the global problem of the emergence of antimicrobial resistance
[195][109].
Teicoplanin is a glycopeptide antibiotic used to treat Gram-positive bacterial infections
[196][110] that has been of interest in the drug-repurposing efforts against SARS-CoV-2. Teicoplanin and its derivatives were shown to inhibit cathepsin L activity in Ebola, SARS, and MERS infections. Teicoplanin does not block cellular viral receptors or target viral particles, but its inhibitory action against cathepsin L leads to reduced low-pH-associated cleavage of the SARS-CoV S protein, preventing the S glycoprotein trimer from fusing with the host cell membrane
[197][111].
3.1.5. ACE2 Inhibitors
Captopril, a high blood pressure medication primarily marketed for patients affected by diabetes, congestive heart failure, or hypertension
[200][112], was selected as a potential treatment against SARS-CoV-2 infection due to its angiotensin-converting enzyme (ACE) inhibitory action
[201][113]. Experimental data suggested that captopril increased the level of ACE2 expression in lung cells, and this up-regulation was counteracted by drug-induced mechanisms that reduced SARS-CoV-2 spike protein entry
[202][114]. The ACE2-centered therapeutic approaches to prevent and treat COVID-19 now need to be tested in clinical trials to combat current cases of COVID-19, including all SARS-CoV-2 variants and other emerging zoonotic coronaviruses exploiting ACE2 as their cellular receptor
[203][115].
hrsACE2/rhACE2c: Human recombinant soluble ACE2 (hrsACE2) is a novel compound listed for clinical trials to test its efficacy in regulating a patient’s imbalance in the renin–angiotensin system (RAS)
[204][116]. While one report that hrsACE2 successfully prevented SARS-CoV-2 entry and decreased viral RNA levels suggests that hrsACE2 might provide an effective SARS-CoV-2 treatment, further clinical trial data are needed to validate this finding
[205][117].
Ramipril is an antihypertensive drug that has been considered as a potential treatment against SARS-CoV-2
[206][118]. Ramipril works by preventing counter-regulatory ACE from cleaving decapeptide angiotensin I into angiotensin II. Angiotensin II plays an important role in the renin–angiotensin–aldosterone system (RAAS), which regulates blood pressure, vasoconstriction, and extracellular volume
[207][119]. Assessment of hypertensive drugs has been conducted extensively, and guidance for the use of ACE and ARBs (hypertensive drugs that operate in the same biochemical pathway as ACE2, i.e., the RAAS system) suggested that the use of ACEIs/ARBs did not significantly influence either mortality or severity in comparison to not taking ACEIs/ARBs in COVID-19-positive patients
[208,209][120][121].
3.1.6. Anti-Inflammatories
COVID-19 can trigger a cytokine storm through hyperactivation of the immune system and the uncontrolled release of cytokines. This uncontrolled immune response can lead to severe tissue damage and contribute to the progression of the disease. Thus, there has been significant interest in strategies to modulate the immune response in COVID-19 cases, with a focus on suppressing the cytokine storm. This can be achieved through the use of immunomodulatory drugs like corticosteroids or specific drugs that target cytokines or their receptors. However, it is important to note that while these drugs can be effective in managing such conditions, they also suppress the immune system, which can make patients more susceptible to infections. Therefore, patients receiving this medication are typically carefully monitored for signs of infections while on treatment.
Tocilizumab, which is marketed under the brand name Actemra, is an immunosuppressive humanized monoclonal antibody drug. It is used to treat a range of inflammatory conditions, including rheumatoid arthritis and juvenile idiopathic arthritis
[210,211][122][123]. Tocilizumab is the first FDA-approved monoclonal antibody for treating patients with severe COVID-19
[212][124]. It works by targeting and inhibiting the activity of the interleukin-6 receptor (IL-6), a cytokine that plays a key role in various autoimmune and inflammatory diseases
[213][125]. By blocking the IL-6 receptor, tocilizumab helps reduce the inflammatory response and can provide relief to patients. Tocilizumab has shown to decrease the duration of hospitalization, the risk of being placed on mechanical ventilation, and the risk of death for patients with severe COVID-19
[214][126].
Anakinra is an immunomodulatory drug that has been studied for its potential to modulate the immune response in COVID-19 cases. It antagonizes inflammation mediated by interleukin-1 (IL-1) via binding to the corresponding receptors, thereby preventing the cascade of sterile inflammation seen in various pathological states. Additionally, anakinra interferes with the assembly of the inflammasome, a multiprotein complex that plays a key role in initiating and regulating the inflammatory response
[215][127]. Studies showed that anakinra might be beneficial in the early phase of inflammation in patients at risk for progression to respiratory failure; however, its effectiveness in patients already suffering from respiratory failure is not suggested
[216][128]. A recent systematic review and meta-analysis concluded that anakinra has no effect on adult hospitalized patients with SARS-CoV-2 infection regarding mortality, clinical improvement, and worsening or on safety outcomes compared to a placebo or standard care alone
[217][129].