COVID-19: Disinfectants and Virucidals (30 April 2020)
In light of recent news stories about the proposed use of disinfectants to treat patients with COVID-19, I thought it might be helpful to provide a brief overview of microbiocidal agents, focusing on the distinction between those that are intended for environmental use and those that are intended for human use. To this end, it may be helpful to first clarify some terms that are commonly confused for each other — specifically: “cleaning”, “sanitization”, “disinfection”, and “sterilization”. Although each of these words refers to a decontamination process, they are not synonymous. Simply put, cleaning refers to the removal of foreign material (e.g., soil), typically by use of water and detergents (e.g. soap) or enzymatic products. Although the process reduces the microbial burden, cleaning only implies the removal of debris. Mopping a floor with soapy water, for example, while visibly removing dirt, does not render it free of microbes. Although sometimes used interchangeably with cleaning, sanitization refers not only to the removal of visible debris, but also implies a reduction in the number of bacteria on a surface to generally safe levels. In contrast, disinfection refers to the removal of pathogenic (i.e. disease causing) microorganisms by means of chemicals or pasteurization (i.e. heating to no more than 100 degrees Celsius). Although the majority of microbes are inactivated by disinfection, some forms (e.g. bacterial spores) are resistant to disinfectants. Common disinfectants include alcohols (e.g. isopropyl or “rubbing” alcohol), aldehydes (e.g. formaldehyde), oxidizing agents (such as hydrogen peroxide), peroxy and peroxo acids, phenolics (once called carbolic acid), quaternary ammonium compounds (such as benzalkonium chloride), and numerous inorganic compounds including chlorines, iodine, acids and bases, terpenes (e.g. pine oil), etc. Some commercially available disinfectants contain more than one active ingredient (Lysol, for example, contains several alcohols, phenols, potassium hydroxide, alkyls, lactic acid, and hydrogen peroxide). Additionally, ultraviolet light has disinfecting properties, and kills microbes in a dose-dependent manner by disrupting their DNA. Lastly, sterilization refers to the utter destruction of all microbes, typically by means of physical (e.g. heat, steam, pressure) or chemical (e.g. ethylene oxide gas) methods. Surgical instruments, for example, are sterilized prior to each use by means of autoclaving or by use of ethylene oxide gas. Few infectious particles (e.g. bacterial spores and prions) remain intact after sterilization by conventional methods.
Although it may be stating the obvious, just because something destroys germs, doesn’t necessarily mean it is safe for use in humans. For example, chlorine containing compounds such as sodium hypochlorite (bleach), while effective at killing many types of germs, are both caustic and corrosive — potentially causing pulmonary edema if inhaled and esophageal perforation, burns, strictures, and cancer if ingested. Similarly, ultraviolet light non-selectively damages all DNA, whether that of bacteria or human respiratory epithelial cells (or, for that matter, the DNA of a physician’s plants after his office has been disinfected with a Tru-D device). Indeed, one of the challenges in designing a microbiocide is to identify a compound that is effective at killing bacteria (bactericidal), viruses (virucidal), fungi (fungicidal), or protozoa (protozocidal) without harming the patient. This is partly accomplished by targeting some property that is intrinsic to the pathogen but that is lacking in human cells. Consider bactericidal drugs. Presently, there are about 150 FDA approved antibiotics, comprised of about twenty distinct classes, each with its own mechanism of action. For example, beta-lactam antibiotics such as penicillins and cephalosporins work by disrupting synthesis of peptidoglycan, a vital component of bacterial cell walls. Because mammalian cells lack cell walls, they are unharmed by beta-lactam antibiotics. Similarly, aminoglycoside antibiotics such as gentamicin work by binding to bacterial ribosomes and preventing protein synthesis. Although human cells also have ribosomes, they are dissimilar enough from those of bacteria and other prokaryotes so as to be untargeted by aminoglycosides.
Development of antiviral drugs poses unique challenges because viruses are simpler than bacteria and for part of their life cycle, are metabolically inert. (Note that although I refer to the viral “life cycle”, there is no consensus on whether viruses are actually living things. For further discussion, see: https://microbiologysociety.org/…/are-viruses-alive-what-is…) Consequently, there are fewer metabolic processes to target when designing an antiviral drug. Recall from high school biology that viruses consist of genetic material and perhaps a few enzymes stored in a protein shell (called a capsid), which may or may not be covered in a lipid envelope. They cannot reproduce on their own and can only propagate by invading a susceptible host cell and then commandeering the cell’s replicative machinery. Although the precise details vary between viruses, there is a sequence of conserved steps, each of which is a potential target for drug design. The sequence begins with attachment of a virus to a host cell. This is a specific interaction that is typically mediated by binding to a receptor on the surface of the cell. It is the specificity of this binding that accounts, in part, for susceptibility to infection with certain viruses. For example, the movement of SARS-CoV-2 (the virus that causes COVID-19) from bats to humans likely reflects a mutation in the spike protein (the lollipop-shaped structure visible by electron microscopy on the virus’s surface) which conferred the ability of the virus to recognize and bind to receptors on human respiratory epithelial cells. Once the virus binds to a host cell, it releases genes (and possibly enzymes) into the cell (Note that some viruses enter the cell first and then release their genetic material). The virus then commandeers the host cell’s machinery to make copies of its own components, which are subsequently assembled into new viral particles (called virions). Once the newly assembled viruses reach a virus-specific number (called the “burst size”), the viruses are released from the cell, either by lysing the cell or through a process called “budding”. Each of these viruses then goes on to bind to and invade another host cell, thereby repeating the process. See the attached picture from Nature Reviews Microbiology for a schematic representation of the SARS-CoV-2 life cycle. [Nat Rev Microbiol 7, 226–236 (2009)]
As mentioned previously, each of the distinct steps in the viral cycle (i.e. attachment, release of genetic material, replication, assembly, and release from the host cell) represents a potential target in antiviral drug development. Some antivirals (e.g. the anti-HIV medication enfuvirtide) interfere with receptor-mediated binding and cell entry. Similarly, immune globulin also works by binding to viruses (with subsequent clearance by the immune system) before they can enter susceptible cells (For more on this topic, see my 20 April 2020 post). Other medications such as amantadine act against those viruses (e.g. influenza) that penetrate a cell and uncoat first before releasing their genetic material. Additionally, there are several antiviral medications that interfere with the ability of a virus to replicate its genome. These include nucleotide and nucleoside analogues such as the anti-herpes simplex virus drug acyclovir and the anti-Human Immunodeficiency Virus drug zidovudine (AZT). Another nucleotide analogue, remdesivir, is being studied for the treatment of patients with COVID-19, but thus far, results are inconclusive. Two examples of antivirals that prevent the release of viruses from the host cell are the anti-influenza drugs zanamivir (Relenza) and oseltamivir (Tamiflu).
In addition to synthetic antiviral drugs that target a particular step in the life cycle of a virus, there are a small number of naturally occurring biologics that work by revving up the immune system to non-specifically attack viruses. Chief among these are the interferons, cytokines which have historically been used in the treatment of patients with hepatitis B or C (before specific therapies for those infections became available). Interestingly, some interferons (e.g. Interferon-alfa2b) are also used in the treatment of patients with certain cancers such as renal cell carcinoma and melanoma, as well as the treatment of patients with the neurodegenerative disease multiple sclerosis.
Recently, I was asked for my opinion about the medicinal use of ozone in treating patients with viruses. Presumably this question derives from recent claims made by a Dallas-based company that ozone can cure and prevent COVID-19 (a claim that resulted in a lawsuit by the Justice Department). So, before concluding this post, I’ll briefly comment. By way of background, ozone (or trioxygen) is an allotrope of oxygen consisting of three oxygen atoms (O3). It is formed by the action of ultraviolet light and electricity in the upper atmosphere (stratosphere) on diatomic oxygen (O2). However, it is less stable than diatomic oxygen and breaks down in the lower atmosphere (troposphere) to O2. Ozone was discovered in 1840 by the German-Swiss chemist Christian Friedrich Schönbein. It was subsequently found to have disinfecting properties, and by the mid- to late-nineteenth century, was being used to disinfect operating rooms, surgical instruments, and drinking water. The proposed mechanism of action is disruption of the bacterial cell envelope through oxidation of the phospholipids and lipoproteins, and disruption of the viral capsid through peroxidation. Interest gradually shifted from its role as a disinfectant to potential therapeutic properties, and by the end of World War One, ozone was being studied for the treatment of tuberculosis and war wound infections. However, it quickly became apparent that ozone not only destroyed microbes, but human tissue cells as well; and gradually, the use of ozone as a therapeutic was abandoned for other therapies, including the irrigation of wounds with antiseptics. Nonetheless, adherents have persisted, maintaining that ozone is effective in the treatment of viral infections (including HIV and SARS), bacterial and fungal infections, diabetes and arteriosclerosis, gastric cancer, pain associated with osteoarthritis and disc herniation, as well as the prevention of post-operative infections. Despite these claims, the Food and Drug Administration (FDA) maintains that “Ozone is a toxic gas with no known useful medical application in specific, adjunctive, or preventive therapy.” (https://www.accessdata.fda.gov/…/cfdocs/cfcfr/CFRSearch.cfm…); and in April 2016, the FDA prohibited the medical use of ozone. In a fairly recent review of the topic, physiologist Stephanie Shore at Harvard’s T.H. Chan School of Public Health wrote that ozone damages epithelial cells and causes inflammation, which may lead to both lung disease and the metabolic syndrome (i.e. glucose intolerance and hyperlipidemia). The bottom line seems to be that in order for ozone to be effective as a germicide, it must be present in a concentration far greater than that which can be safely tolerated by people — causing me to reiterate the first line of the second paragraph of this post.
In this, the seventh of my COVID-themed posts, I attempted to explain the distinction between microbiocides that are intended for environmental use (e.g. disinfectants) and those that are intended for ingestion, injection, and sometimes, inhalation or topical application (e.g. antibiotics and antiviral drugs). As with my prior COVID-related posts, I have no agenda other than to provide information, clarify misconceptions, and whenever possible, to assuage the anxiety associated with this pandemic.
Until my next update — regards.
Michael Zapor, MD, PhD, CTropMed, FACP, FIDSA
(30 April 2020)
