Effective treatments

Drugs, an herbal remedy, and a vitamin were documented; 24 were used alone, and 12 in combination with others. These successfully reduced mortality, hospitalization, respiratory failure, viral load, disease progression, or recovery time in patients with COVID-19. Most are available internationally and have been identified in repositioning studies. Table 2 lists the drugs, vitamins, and remedies that were administered individually, while table 3 lists the combined therapies. For each treatment, the identifier and type of trial, the severity of the disease, the sponsor and country in which the trial was conducted, and the trial results are presented.

Among the therapies that promoted clinical improvement, we highlight those that reduced the rate or probability of death by 50% or more in patients with severe, very severe, or critical illness. Aviptadil reduced mortality by 50% in critically ill patients. While in patients with very severe disease, the reduction in the rate of mortality was in first place bevacizumab (no mortality), in second place the combination of ivermectin, dexamethasone, aspirin, and enoxaparin (71%), and in third place pyridostigmine bromide (63.3%). In severe patients, Nigella sativa seeds with honey also reduced mortality (~70%), followed by low-molecular-weight heparin (64%), the combination therapy of hydroxychloroquine with favipiravir, darunavir, and ritonavir (16-4%), baricitinib (50% and 58% with corticosteroids) and anakinra (50%), as well as the combination of hydroxychloroquine, favipiravir, darunavir, and ritonavir (25%). We also highlight tocilizumab as it significantly reduced the time to clinical discharge (at day 4) (Tables 2 and 3).

However, given that the disease develops in two phases, an initial phase of high viral replication and a second phase of inflammation⁷,⁸, controlling the infection before the hyperinflammatory response is triggered is important. Therefore, we highlight the activity of those treatments that, in mild-to-moderate disease, promoted patient recovery before seven days or significantly reduced viral load, such as danoprevir with ritonavir, hydroxychloroquine with oseltamivir, the combination of baricitinib with lopinavir and ritonavir, multidrug therapy with lopinavir, ritonavir, interferon (IFN) β-1b, and ribavirin, in addition to, umifenovir, favipiravir, novaferon, tofacitinib, and colchicine (Tables 2 and 3).

Mechanism of action

Regarding the mechanism of action of the drugs, vitamins, and natural products that have shown efficacy in clinical trials have been found to act by inhibiting cytokine release, inflammation, tubulin polymerization, complement system, tyrosine kinases, viral replication, thrombosis, chemotaxis, or promote bronchodilation, or are regulators of innate and adaptive immunity. They are presented below according to their biological activity.

Anti-inflammatory drugs that regulate the release of proinflammatory cytokines

Aviptadil (Zyesami) is a vasodilator neuropeptide with anti-inflammatory and immunomodulatory activity. It binds to the vasoactive intestinal peptide (VIP) receptor type 1 in type II pneumocytes and blocks chromatin condensation and fragmentation by caspases, thus preventing cell apoptosis (Fig. 2)⁹; it also blocks the release of IL-6 and TNFα¹⁰. When used in COVID-19, Aviptadil reduced respiratory failure and mortality by ~50% when patients were treated with the maximum standard of care in tertiary care hospitals but not in regional secondary care hospitals¹⁰.

Figure 2 Drugs with greater potential against SARS-CoV-2 and COVID-19. A: type II pneumocyte. B: macrophage. ACE2: angiotensin 2 receptor; TMPRSS2: transmembrane serine protease 2; AAK1: AP2-associated protein kinase-1; VEGFR2/1: vascular endothelial growth factor receptor; ABL2: ABL tyrosine kinase; JAK: Janus kinase; STAT: signal transducer and activator of transcription; IFN-α2: interferon-alpha-2; AChE: acetylcholinesterase; ACh: acetylcholine; IL1-R: IL-1 receptor; NF-κB: nuclear factor enhancer of activated B-lymphocyte kappa light chains; VE-Cadherin: vascular endothelial cadherin. Natural products in green, biological in blue, synthetic in gray. Figure created on BioRender.com. 

Anakinra (Kineret) is a recombinant form of the IL-1 receptor antagonist. It is a disease-modifying antirheumatic drug that inhibits nuclear factor-kappa B (NFκB) translocation (Fig. 2), thereby controlling the production of proinflammatory mediators¹¹. It reduced the 30-day mortality by 50% and improved patients' ability to breathe. Pyridostigmine bromide (Mestinon) also inhibits NFκB because it is an acetylcholinesterase inhibitor, and its activity results in an increase in the half-life of acetylcholine, which is critical in the cholinergic anti-inflammatory pathway¹²-¹⁴ (Fig. 2); its administration reduced the need for mechanical ventilation by 2 days and reduced mortality by 63%¹⁵.

Thalidomide is an anti-inflammatory, immunomodulatory, and anti-angiogenic drug that inhibits the NFκB and interferon regulatory factor 3 (IRF3) pathways¹⁶. Thalidomide, in combination with glucocorticoids, reduced hospital stay and viral load by ~50%, decreased IFN-γ and IL-6 levels, and the need for mechanical ventilation¹⁷. In addition, chloroquine, an antimalarial and arthritis drug, inhibits the inflammatory response mediated by IL-6, IL-17, and IL-22 and prevents viral entry by interacting with ACE2¹⁸. Chloroquine (400 mg, 2 times daily for 4 days) showed a mortality rate of 15% versus 39% in the high-dose group (600 mg, 2 times daily for 10 days)¹⁹. The mortality rate was similar to that of the thalidomide control group (16%), so its efficacy is questionable, and the use of high doses in critically ill patients is not recommended.

Fluvoxamine is an antidepressant that pharmacologically functions as a selective serotonin reuptake inhibitor. It inhibits the production of TNF-a and IL-6 through the S1R-IRE1 pathway²⁰. Patients treated with fluvoxamine experienced no clinical worsening compared to 8.3% in the placebo group²¹, while tocilizumab (Actemra), an IL-6 receptor inhibitor, induced recovery of patients requiring supplemental oxygen in a median of 4 days in tertiary care²², compared to 10 days for patients treated with remdesivir²³. However, several clinical trials have suggested that tocilizumab may not be effective against COVID-19¹¹,²⁴,²⁵ because, unlike the drugs above, which inhibit the upstream inflammatory response, tocilizumab only inhibits IL-6 activity.

Treatment with N. sativa (Ranunculaceae) seeds and honey was evaluated in a study conducted at a tertiary care center in Pakistan. It reduced the time to symptom relief by 50%, accelerated viral clearance, and reduced the mortality rate fivefold (4% vs. 18.8% for placebo)²⁶, while N. sativa seed oil increased the percentage of recovered patients (62%) compared to the control (36%) and reduced the recovery time in a study conducted at a tertiary care center in Saudi Arabia²⁷.

In both studies, most patients were under 60 years of age, while the average age in the Saudi Arabian group was 36 years. Both studies showed a significant benefit from using N. sativa seeds, resulting in remission of symptoms. There were differences in the baseline characteristics of the patients in terms of comorbidities, as the Pakistani group had a higher percentage of hypertension and obesity. In addition, various concomitant treatments were used due to the greater severity of the disease compared to the patients in the Saudi Arabian study, where the use of other medications was not reported (Table 4).

The main phytopharmaceutical of this seed is thymoquinone, which has anti-inflammatory effects by suppressing expression of enzymes that produce prostaglandins and leukotrienes and also blocks ACE2 (Fig. 2)²⁸. This species also contains nigellone, which blocks histamine release²⁹. The oil showed antiviral activity in a murine cytomegalovirus and H9N2 model and decreased proinflammatory cytokines in a murine allergic asthma model²⁸.

Antimitotics

Colchicine, an alkaloid derived from the plant Colchicum autumnale (Colchicaceae), used in the treatment of gout, inhibits tubulin polymerization in leukocytes and acts on the NLRP3 inflammasome³⁰. Colchicine-reduced mortality and hospitalization by 24% compared with placebo in patients over 40 years of age with COVID-19 confirmed by polymerase chain reaction (PCR)³¹.

Complement system inhibitors

Eculizumab (Soliris) is a monoclonal antibody used to treat autoimmune diseases; it is an inhibitor of complement protein C5b-9³² and reduces the levels of IL-1, IL-6, and TNFa³³. In one study, it contributed to the improvement of 6 out of 8 patients³². Ravulizumab (Ultomiris), on the other hand, is a complete inhibitor of C5 convertase, which is the initiator of the terminal phase of the complement system³⁴, so in addition to preventing complement-mediated inflammation, it also blocks cell activation and lysis³⁵.

Tyrosine kinase inhibitors

Imatinib mesylate, a c-ABL kinase inhibitor used to treat certain cancers, inhibits VE-cadherin dissociation (Fig. 2), preventing capillary leakage and alveolar edema; it also reduces IL-6 and IL-8 secretion³⁶. Imatinib mesylate reduced the median duration of mechanical ventilation from 12 to 7 days and reduced the likelihood of death in patients by 49%³⁷.

Baricitinib (Olumiant), an inhibitor of Janus kinase (JAK) 1 and 2 kinases used in the treatment of rheumatoid arthritis, decreases the expression of ACE2 in human liver cells and the expression of proinflammatory cytokines induced by IFN-α2 through the JAK/signal transducer and activator of transcription (STAT) pathway³⁸,³⁹. In addition, it has affinity for the AP2-associated protein kinase-1 and reduces SARS-CoV-2 endocytosis (Fig. 2)⁴⁰. Baricitinib reduced respiratory failure, mortality, and disease progression by 50%³⁸, and in another study where it was administered with corticosteroids, supplemental oxygen requirements were reduced (25.8% vs. 62% in the methylprednisolone control group), and mortality was ~4% in both groups⁴¹, suggesting that corticosteroid activity reduced mortality. Baricitinib in combination with lopinavir and ritonavir (Kaletra) improved patient status and prevented disease progression⁴², and in another study, inhibited mortality, reduced disease progression by 95% (0.88% vs. 17.9% control), and increased hospital discharge rate (9.7% vs. 1.3%)⁴³.

Tofacitinib as baricitinib, is an inhibitor of JAK1 and 3 kinases and the JAK/STAT pathway used to treat rheumatic diseases. Tofacitinib alone reduced mortality at day 28 by 49% (2.8% vs. 5.5% for placebo) and the cumulative incidence of death or respiratory failure by 37.6% (18% vs. 29% for placebo)⁴⁴. Meanwhile, Novaferon, an antitumor/antiviral protein that interacts with the IFN2 receptor, a JAK/STAT45 signaling pathway⁴⁵, with or without lopinavir and ritonavir, had a higher viral clearance rate at day 6 than the lopinavir/ritonavir group (50.0% vs. 24.1% and 60.0% vs. 24.1%, respectively)⁴⁶.

Acalabrutinib improved lung function and reduced inflammation by targeting BTK tyrosine kinase; BTK is important in activating the innate immune response of blood monocytes⁴⁷ by promoting inflammasome and phagocytic receptor activation⁴⁸. In addition, BTK inhibition also blocks nuclear translocation of NFκB, which results in a reduction of the synthesis of proinflammatory cytokines. Also, the activity of opaganib, which is an inhibitor of sphingosine kinase-2. This kinase has been proposed to be a factor in viral replication⁴⁹,⁵⁰, and its inhibition also decreases TNF-α and IL-6. In patients, it reduced the need for supplemental oxygen (61.6% vs. 46.7% placebo) and accelerated hospital discharge⁵¹.

Antivirals

Danoprevir is an inhibitor of hepatitis C virus NS3/4a protease, which has high structural similarity to Mpro of SARS-CoV-2⁵², while ritonavir increases danoprevir exposure by inhibiting cytochrome P450 isoenzyme 34A. Danoprevir with ritonavir suppressed viral replication in < 1 week and reduced ground glass opacity⁵³.

Lopinavir and ritonavir are HIV-1 protease inhibitors; ribavirin inhibits normal viral replication, and IFN β-1b induces the synthesis of antiproliferative and immunomodulatory proteins⁵⁴. Meanwhile, lopinavir and ritonavir had no clinical benefit when administered alone²⁴ or with Arbidol⁵⁵. Treatment with lopinavir and ritonavir with ribavirin and IFN β-1b, administered 7 days after symptom onset, reduced the median time for negative nasopharyngeal swab results compared to the control group (from 12 to 7 days), suppressed viral load at 8 days, relieved symptoms at 4 days, and reduced IL-6 levels⁵⁶.

Remdesivir reduced the median recovery time by 10 days (vs. 15 days for placebo) and also reduced mortality (6.7% vs. 11.9% for placebo) in a study conducted in 60 tertiary care hospitals in Europe, Asia, and the Americas in patients with moderate to severe illness²³. In another study conducted in 105 secondary and tertiary care hospitals in the U.S., Europe, and Asia in patients with moderate illness, 5 days of remdesivir improved the condition of patients compared to placebo. However, the clinical significance was uncertain due to the study design⁵⁷. In another study conducted in 71 centers in eight countries in Europe, Asia, and the Americas, remdesivir was administered alone or in combination with baricitinib, and in the second case, the median recovery time was reduced from 8 to 7 days⁵⁸.

The protocols of the three studies were the same in terms of doses. However, there was a difference in the duration evaluated, as the first study found that the 10-day treatment was sufficient to shorten patients' recovery time. In contrast, the second study found that a 5-day treatment was more beneficial than a 10-day treatment. From these data, it can be concluded that a 10-day course provides greater benefit in patients with more advanced diseases (Tables 4 and 5).

We also found two articles that are not included in the results tables. In one, remdesivir was administered for 5 or 10 days and produced a clinical improvement of 2 points or more on the 7-point ordinal scale at day 14 in 64% of patients; however, they found no significant differences between the two groups, in patients with severe disease who did not require mechanical ventilation⁵⁹. In another study in China with critically ill patients, remdesivir did not affect clinical improvement⁶⁰. Therefore, the benefit of remdesivir is uncertain; it appears that its individual activity may not be sufficient to improve clinical status significantly.

Favipiravir is an influenza virus RNA polymerase inhibitor. It has been evaluated in various studies in patients with mild to severe disease in which it did not reduce viral load; it only showed a reduction in the recovery time of patients from 5 to 3 days in a study conducted in India⁶¹ and in a study conducted in China it contributed to 71% of patients recovering before 7 days versus 55% in the group with umifenovir⁶². The latter study used lower doses, with a difference of 200 mg. Both studies were conducted in tertiary care hospitals, and the patients evaluated had similar characteristics in terms of disease status. However, the standard of care differed in that the Chinese study used herbal medicines and additional antiviral treatment. Both studies concluded that favipiravir therapy accelerated patient recovery but had no effect on viral load reduction (Table 4).

On the other hand, umifenovir, a broad-spectrum antiviral⁶¹,⁶³ that prevents the hemagglutinin from switching to its fusion state, thereby inhibiting the entry of the virus, also reduced the recovery time of symptoms such as fever (4.9 vs. 6 days) and dry cough (4.3 vs. 5 days)⁶³.

Oseltamivir inhibits influenza virus neuraminidases, limiting the spread of infection and inflammation. Patients who received hydroxychloroquine with oseltamivir had a negative reverse transcriptase-PCR (RT-PCR) test in an average of 8.1 days⁶⁴ (article retracted due to questionable results). In another study, patients had a negative RT-PCR test 4 days earlier than the control group (7.5 vs. 11.5 days)⁶⁵. This is important because it reduces the period of infectiousness and the risk of disease progression.

Antithrombotics

Low-molecular-weight heparin is an anticoagulant that enhances antithrombin III activity, reduces the risk of coagulopathy and thromboembolism, and interacts with NF-kB by mediating the production of proinflammatory cytokines⁶⁶. It reduced the likelihood of death by 64% and the risk of disease progression and death by 39%²⁴.

Chemotaxis inhibitors

It is possible that the effect of bevacizumab is related to the secretion of vascular endothelial growth factor (VEGF) by leukocytes in response to a hypoxic environment⁶⁷ and that inhibition of VEGF (Fig. 2) prevents its function as a chemoattractant for monocytes and macrophages through its receptor VEGF receptor 1⁶⁸. Bevacizumab reduced fever in the first 72 h, improved respiration in populations from China and Italy, and no patients died. However, a decrease in respiratory capacity was observed only in the control group in Italy throughout the study⁶⁹. This is probably due to the use of Chinese herbal medicine, including Jinhua Qinggan, Lianhua Qingwen, and Xuebijing⁷⁰, in the treatment of all control patients in Wuhan, unlike those in Italy⁶⁹ (Table 4 for more data on concomitant treatments).

Bronchodilators

Inhaled nitric oxide reduced respiratory rate, improved oxygenation, and prevented hospital readmissions after discharge⁷¹. In addition to its bronchodilator activity, it is thought to have antiviral activity, as it reduced viral replication by 99% in in vitro studies by inhibiting the Mpro protease of SARS-CoV-2⁷².

Vitamins

Regular supplementation with Vitamin D₃ during the year before illness significantly reduced mortality; the clinical benefit was not significant in patients who received it after diagnosis⁷³. In an extension of this trial, Vitamin D₃ supplementation before, during, or after hospitalization reduced mortality in geriatric patients⁷⁴. Vitamin D regulates innate and adaptive immunity, as its hormonal metabolite (calcitriol) mainly suppresses the production of inflammatory cytokines, induces regulatory T lymphocytes, and has a possible antiviral effect⁷⁵.

Multi-target combination therapies

The combination of hydroxychloroquine with favipiravir, darunavir, and ritonavir reduced mortality by 25% compared with patients receiving oseltamivir, lopinavir, and ritonavir (4% vs. 16%)⁶⁵. The individual effect of the drugs in this regimen against COVID-19 is inconclusive. However, he efficacy of this regimen is related to the inhibition of different stages of the viral cycle, as it includes antiviral agents, protease inhibitors, polymerase inhibitors, and an antiparasitic agent that may interfere with viral entry.

The combination of ivermectin, dexamethasone, acetylsalicylic acid (aspirin), and enoxaparin covers different aspects of the disease, as ivermectin blocks the IMPa/b1 import heterodimer on which nuclear trafficking of RNA virus proteins depends (Fig. 2)⁷⁶. Aspirin is an antithrombotic agent in mild cases, enoxaparin is an anticoagulant⁷⁷, and dexamethasone inhibits transcription factors such as AP-1, NF-kB, and IRF⁷⁸, prevents the synthesis of cytokines and chemokines, and induces apoptosis of T lymphocytes and neutrophils⁷⁹. This therapy inhibited disease progression and reduced mortality by 71% compared to the mortality rate in Argentina and 87% compared to the mortality rate in Spain and Italy for hospitalized patients⁷⁷.

Fig. 3 shows the molecular structure of the drugs included in clinical trials that were shown to inhibit the processes involved in SARS-CoV-2 infection.

Figure 3 Molecular structure of drugs included in clinical studies that were shown to inhibit the processes implicated in SARS-CoV-2 infection.