New variants of the causative pathogen, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), continue to emerge, and the ‘new normal’ appears to be a scenario where human beings coexist with the virus.

However, many individuals are at higher risk of severe COVID-19 due to comorbidities like obesity, hypertension, type 2 diabetes mellitus, advancing age, and a lower socioeconomic stratum. Antiviral drugs need to be developed to deal with this threat.

A recent paper describes the use of aprotinin, a protease inhibitor that works across several pathways, to reduce the severity of the condition.

Introduction

Several drugs have been approved for use in COVID-19, including molnupiravir, Paxlovid (a nirmatrelvir/ritonavir combination), and lopinavir/ritonavir. However, these interact with other medications, are relatively expensive, and have significant side effects. Moreover, they are not useful once COVID-19-related inflammation sets in, or may not help prevent COVID-19.

In the light of these limitations, aprotinin is being explored for its potential utility as an antiviral drug. This compound is a broad-spectrum inhibitor of host proteases, including fibrinogen and the kinin-kallikrein pathway, which are required for the cleavage-activation of the viral spike protein. This is essential for the recognition of the spike by the host cell surface angiotensin-converting enzyme 2 (ACE2) receptors, which mediates viral attachment and entry.

The authors of the current study conducted a phase III trial called TAC (Aprotinin Treatment Against COVID-19). It demonstrated the safety and efficacy of nebulized aprotinin in moderate COVID-19, leading to better clinical outcomes with inhaled aprotinin in moderate COVID-19 patients hospitalized with pneumonia. These patients required less supplemental oxygen and their treatment period was lower than the control group, which received a placebo.

Aprotinin is convenient to use in clinical practice, may be used to prevent COVID-19, is useful for outpatients with mild disease, and is thus suitable for use in low-resource settings. Nebulization avoids systemic administration and thus reduces the risk of adverse reactions.

As people have returned to intermingling socially and in other settings, the chances of secondary infections and coinfections with other pathogens, such as the seasonal flu, are likely to cause more severe infections. In this hypothetical setting, too, aprotinin is an interesting drug because of its ability to act against a broad spectrum of viruses.

In the previous study, the researchers could not measure the viral load in all patients. The present study, published online in the European Journal of Clinical Investigation, set out to measure the reduction in viral load in the preserved tracheobronchial samples from a group of moderately ill COVID-19 patients, which would indicate its ability to prevent the progression of the disease as well as treat severe cases.

The researchers used reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) to measure the viral load in a group of patients with moderate COVID-19. Two measurements were obtained, one at baseline and the second at 5 days after standard COVID-19 management with aprotinin treatment. This group contained 28 patients, while a control group of 32 patients was given only standard management.

The study covered four Spanish medical centers, with all patients having tested positive for the virus within 48 hours of randomization to the treatment or control groups. All had moderate COVID-19 pneumonia.

While the PCR results were similar between groups at baseline, the viral loads dropped significantly in the group treated with aprotinin. Moreover, the aprotinin group had a shorter treatment period overall, at 5.5 days vs. 7.4 days, respectively.

In the aprotinin group, the viral load decreased more markedly, by -2.8 log copies/mL, vs. only -0.8 in the control group. Platelets and D-dimer levels were lower by 1.5 times and over seven times in the aprotinin group compared to the controls. Meanwhile, fibrinogen levels dropped comparably in both groups.

These results were independent of age, sex, or body mass index (BMI).

Implications

The current study showed a reduction in viral load with aprotinin inhalation in patients with moderate COVID-19. Earlier research indicated that aprotinin inhalation coupled with its intravenous use was useful, in combination with favipiravir, in such patients. This combination led to a decrease in viral load, fewer admissions to the intensive care unit, and a shorter period of hospitalization with improved lung damage markers by day 14 of treatment. However, in that study, the antiviral activity was attributed to favipiravir, while aprotinin was thought to produce clinical improvement.

Aprotinin is superior to camostat mesylate, an inhibitor of the host protease TMPRSS2 that reduced SARS-CoV-2 entry into target cells in vitro. This is due to the broader spectrum of activity of the former, which allows it to target the virus effectively.

Aprotinin prevents thrombosis and inflammation via its effects on the host proteases and coagulation regulators like kallikreins, which are activated by SARS-CoV-2 capsid proteins. The former prevents SARS-CoV-2 activation and cell entry, and replication within the host cells.

Kallikrein activation causes neutrophilia, with neutrophil extracellular traps (NETosis), which trigger microthrombosis. Besides kallikrein inhibition, aprotinin inhibits inflammatory mediators, reduces the expression of white cell adhesion molecules, reduces tracheobronchial secretions, and prevents complement activation.

Our findings may reflect immunomodulatory/anti-inflammatory effects that could be either secondary to the reduction in viral load or aprotinin-mediated and might participate in the rapid discharge and shorten treatment in patients who had aprotinin + standard of care.”

Aprotinin is thus a potential antiviral drug for COVID-19, and further study is indicated to demonstrate its ability to prevent infection or modulate disease severity in this condition.