Researchers from USP (University of São Paulo) elucidated the biochemical mechanism by which hypertonic saline solution inhibits the replication of the SARS-CoV-2 virus, which causes covid-19. The study, published in the journal ACS Pharmacology & Translational Science, was performed on lung epithelial cells infected with the new coronavirus.
If efficacy is proven in clinical trials, the finding may contribute to the development of new prophylactic strategies or even treatments for covid-19.
“Given the severity of the pandemic, we believe it would be important to advance in this study and carry out clinical trials to verify the effectiveness of the use of spray and nebulization with hypertonic sodium chloride solution [NaCl] as a form of prophylaxis, helping to reduce the spread of the virus in the infected organism and to reduce the chances of more severe inflammation”, says Cristiane Guzzo, a researcher at the Institute of Biomedical Sciences at the University of São Paulo (ICB-USP).
The investigation was supported by FAPESP and had the participation of researchers Edison Durigon, also from ICB-USP, and Henning Ulrich, from the Institute of Chemistry (IQ-USP).
The authors point out that, although evidence suggests that the use of sodium chloride solution inhibits virus replication, the finding does not represent complete protection against infection – much less a cure for the disease. “This is a very simple and inexpensive measure, already used as prophylaxis for other respiratory diseases and that could minimize the severity of COVID-19 by reducing the viral load. It could be added to safety protocols, without replacing the use of masks, social distance or the need for vaccination”, highlights Guzzo.
By comparing different concentrations of the product, the researchers found that using the 1.5% NaCl solution inhibited the replication of SARS-CoV-2 by 100% in vero cells – monkey kidney cell line used as a model to study the new coronavirus. In tests with human lung epithelial cells, the 1.1% solution was sufficient to inhibit virus replication by 88%.
Hypertonic sodium chloride solution has been used as an additional prophylactic measure in cases of flu, bronchiolitis, rhinitis, sinusitis, and a variety of airway problems. Treatment with sprays has an effect on the upper airways, while nebulization also affects the lungs. Although these measures show good results, minimizing the effects of diseases, little is known about their mechanism of action.
“Once we were able to explain this intracellular mechanism of response to the hypertonic solution, we carried out a basic science study with clear applications in health and in the understanding of different respiratory diseases. What was observed in the case of SARS-CoV-2 is likely to be repeated with other viruses, as it is a mechanism of the host cell”, says Ulrich.
To understand the action of saline solution on the infected cell, it is necessary to take into account that, for the virus to replicate its genetic material and advance to other cells and organs, it uses host cell tools, such as proteins and energy sources. “We identified that NaCl does not interfere with the interaction between the spike of SARS-CoV-2 (protein spike) and the ACE-2 protein (receptor responsible for the entry of the virus into human cells), but it affects the cycle of the virus after infection”, explains Guzzo.
Another study published by the ICB-USP researcher in The Journal of Physical Chemistry Letters shows how the complex involving the protein spike and the human ACE-2 receptor is maintained at different concentrations of NaCl. “Probably, the virus evolves in order to compensate for fluctuations in ionic strength and thus maintain an effective means of cell invasion”, explains the researcher.
Thus, when NaCl molecules enter the cell, a polarization of the cytoplasmic membrane occurs due to the increase in sodium ions (Na+). This energy imbalance causes a considerable amount of potassium (K+) that was present in the cell to be expelled out of the cytoplasm. These are called potassium pumps, which serve to establish the balance of charges in the cytoplasmic membrane.
Saturation due to the exchange of sodium and potassium makes the cell use up ATP (adenosine triphosphate) – one of the main sources of energy for carrying out cellular processes. As ATP molecules are used in this process of cell depolarization, the virus cannot use them to replicate.
“The cells have to throw away the sodium via the sodium and potassium pump. With this, they are using up their energy stores. Consequently, there is no ATP left for the virus to replicate”, explains Ulrich.
The study also showed that salt has no effect on the activity of mitochondria – an organelle involved in the process of cellular respiration and high metabolic activity. “At these concentrations, the salt does not cause damage to the cell. We observed that the mitochondria remains healthy throughout the process”, says Guzzo.
In the study, researchers suggest testing two types of use of hypertonic NaCl solution. One in the form of a nasal spray for airway prophylaxis, the gateway for SARS-CoV-2 into the body. “This type of spray can be found in any pharmacy and could be used as prophylaxis for people who are on the front lines and with greater possibility of contact with the virus. If proven, it could reduce the replication of the virus in the nose and of the throat,” says Guzzo.
The other proposed strategy is nebulization to deliver the serum to the lung. In this case, the use of the right NaCl concentrations is essential and the effectiveness of the method can only be evaluated through clinical trials in patients with covid-19 – it is noteworthy that nebulization with hypertonic solution is a strategy already used to treat children with bronchiolitis , for example.
In the case of respiratory syncytial virus, which causes bronchiolitis, it is known that hypertonic saline solution causes a reduction in infection and inflammatory responses in cultures of human respiratory epithelial lineages.
“This is not a single solution and would be a treatment used in the first days of infection. Reducing virus replication means reducing the severity of the disease and the inflammatory worsening. COVID-19 is a complex disease, it has a part of viral replication – that the saline solution would have an effect – and also that of systemic inflammation, which goes beyond. This second phase, when started, can be intense and generate a series of other complications in different organs”, warns Guzzo.