The World Health Organization (WHO) declared Coronavirus disease 19 (COVID-19) a public health emergency of international concern and declared the pandemic on the 12th of March, 2020. Since the start of the pandemic, the number of reported cases in the world is 6,152,160 with 371,700 deaths as of the 1st of June 2020 [21]. The pandemic has been described as the worst health and economic threat after world war two.
The clinical picture of COVID-19 is characterized by a broad spectrum of presentations ranging from asymptomatic to life-threatening disease[22,23]. The clinical course has been divided into three stages: the first is the early (mild) infection with minor symptoms, second (moderate) stage characterized by pulmonary involvement with or without hypoxia and a third stage, the most severe, that is characterized by extra-pulmonary systemic hyper-inflammatory syndrome [24]. The first stage is characterized by the replication of SARS-CoV-2, mainly in the upper respiratory tract, and represents the best moment to start an antiviral treatment in order to avoid the progression of COVID-19.
Our in silico results suggest that lopinavir, ritonavir, darunavir, and atazanavir activated interactions with the key binding sites of SARS-CoV-2 protease with a better Ki for lopinavir, ritonavir, and darunavir. Although atazanavir appears the less effective in silico, it would not rule out pharmacokinetics and pharmacodynamics of the molecule under human trials. These four drugs have been extensively used as part of combination antiretroviral treatment of HIV infection for more than a decade [25–27]. In the clinical use in HIV-infected patients, ritonavir is currently used only at a low dose as a pharmacokinetic booster in combination with lopinavir, darunavir, and atazanavir.
A randomized, controlled, open-label study reported on the efficacy and safety of lopinavir/ritonavir for treating hospitalized adults with severe COVID-19 [28]. In this trial, 99 patients received lopinavir/ritonavir and 100 standard care for 14 days. Of note, the median interval time between symptoms onset and randomization was 13 days. No statistically significant difference in time to clinical improvement was observed between arms. However, 28 days mortality was lower in the lopinavir/ritonavir group (19.2%) compared to the control group (25%).
Furthermore, patients on lopinavir/ritonavir had significantly shorter stay (6 vs. 11 days) in the intensive care unit (ICU) in respect of the control group. Overall, faster clinical recovery and reduced mortality were observed in those treated within 12 days of symptom onset. Gastrointestinal complaints were more common in the lopinavir/ritonavir group, whereas serious adverse events did not differ between the two arms. These results suggest that lopinavir/ritonavir was associated with a faster clinical recovery in those who started early and with a shorter duration of ICU stay.
The efficacy of lopinavir/ritonavir is currently also tested in the “Recovery” trial against other three different single drug approaches (dexamethasone, hydroxychloroquine, or azithromycin) [29].
The combination of darunavir/cobicistat is currently under evaluation as monotherapy compared to conventional treatment for COVID-19 in a small open-label randomized clinical trial (30 participants in the two arms, NCT04252274). Unfortunately, no real-life data are available for darunavir and atazanavir in the treatment of COVID-19.
Remdesivir is reported to act via chain termination of nascent viral RNA in Ebola virus infection [30] and is also effective against a broad spectrum of human and pre-epidemic zoonotic CoVs and potently inhibits replication of SARS-CoV and MERS- CoV in primary human airway epithelial cultures [31,32].
One potential drawback of remdesivir monotherapy is represented by the possible selection of resistance, which is mediated by RdRp residues F480L and V557L in SARS-CoV, resulting in a 5-fold shift in half-maximal inhibitory concentration (IC50) [32]. Importantly, the two remdesivir resistance mutations, alone or together, conferred increased sensitivity to inhibition by β-D-N4-hydroxycytidine (NHC), a novel potential broad-spectrum antiviral, suggesting unique patterns of resistance for single drugs [9].
More recently, remdesivir has been shown to effectively inhibit SARS-CoV-2 replication in Vero E6 cells [33] and to interact with RdRp by forming two H-bonds with Trp509 and His381, π-cation contacts with Phe504, and hydrophobic interactions with Lys508, Leu401, Asn386, and Ser384[11]. It is currently under evaluation as a potential monotherapy treatment for COVID-19 in at least nine randomized clinical trials. Wiliamson B et al. showed in the animal model that very early administration of remdesivir to rhesus macaques was able to reduce virus titers in bronco-alveolar lavage significantly and that the viral load was significantly lower in lungs of remdesivir-treated animals than in controls [34]. However, the authors admit that the results were difficult to translate in real life of patients’ management because remdesivir was administered 12 hours after virus inoculation that in human disease is coincident with an asymptomatic incubation period. Furthermore, remdesivir is available only for intravenous administration that is logistically impossible to implement for home treatment, especially on a large scale.
Fifty-three inpatients with severe COVID-19 were enrolled in compassionate use of remdesivir study. After a median follow-up of 18 days, 36 patients (68%) showed an improvement in respiratory parameters; of these, 17 (57%) who were receiving mechanical ventilation have been extubated. Overall, 25 patients (47%) were discharged, whereas seven (13%) died. Among patients receiving invasive ventilation, the mortality was 18% and 5% among those not [35]. A recent double-blind, randomized, placebo-controlled trial enrolled adults hospitalized with Covid-19 pneumonia. Patients were randomized to receive either remdesivir or placebo for up to 10 days. Preliminary results from 1059 patients indicated that those treated with remdesivir had a median recovery time of 11 days, in respect of 15 days in those who received a placebo (rate ratio for recovery, 1.32; 95% CI, 1.12 to 1.55; P<0.001). Mortality by 14 days was 7.1% with remdesivir and 11.9% with placebo (hazard ratio for death, 0.70; 95% CI, 0.47 to 1.04) using Kaplan Meier estimates. Serious adverse events were less frequently reported in the remdesivir group compared to placebo [36].
The combination of tenofovir/emtricitabine vs. hydroxychloroquine is currently under evaluation in a randomized clinical trial conducted in Spain as a preventive regimen in healthcare professionals exposed to COVID-19 patients. Therefore, no nucleos(t)ide analogues combinations are under evaluation in clinical trials for the treatment of COVID-19.
Our in silico studies evidenced the ability of remdesivir, tenofovir, lamivudine, and emtricitabine to be incorporated, as active forms, in SARS-CoV-2 RdRp in the same protein pocket where poses the corresponding natural nucleos(t)ide substrates with comparable Ki value and activating similar interactions. In principle, the four antiviral nucleos(t)ides might be used effectively against SARS-CoV-2. To maximize the antiviral effect on SARS-CoV-2 RdRp, a cytosine-based antiviral nucleos(t)ide could be associated with an adenosine-based one.
Tenofovir, emtricitabine, and lamivudine are nucleos(t)ide analogues already widely available in the market as antiretroviral drugs. These medications have been extensively used in HIV-infected patients showing a favorable safety profile, especially when considering the limited duration of COVID-19 treatment [37–39]. Furthermore, these drugs are available for oral administration and show minimal drug-drug interactions, thus representing ideal potential candidates for both out and inpatients treatment in the early stage COVID-19. The possibility to treat patients with effective combinations at home is of crucial importance given the possible further waves of the pandemic in the upcoming months while waiting for a protective vaccine.
From lessons learned from other viral infections, including HIV and HCV, the combination of drugs with different targets may reduce more rapidly SARS-CoV-2 viral load, the probability of interhuman transmission and selection of resistance to single medications. Furthermore, early antiviral treatment has been associated with better clinical outcomes in COVID-19 patients in a large observational study conducted in China[40].