What Should Be Learned From Repurposed Antivirals Against SARS-CoV-2?

Abstract

Since severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first identified in 2019, more than 270 million infections have been reported, and the death toll from the associated coronavirus disease 2019 (COVID-19) has reached 5.5 million. The magnitude of this pandemic has encouraged the repurposing of numerous drugs, with the aim of rapidly curbing the morbidity, mortality, and spread of this new disease. However, this approach has had limited success. Only vaccine development and the repurposing of dexamethasone have impacted COVID-19 severity. Drug repurposing is, perhaps, an effective strategy for identifying antivirals, but there are no shortcuts for drug development. The identification of effective drugs, regardless of previous approval, requires time and funding to confirm and understand the drug target, the drug's toxicity, and its proper use. The lessons learned with COVID-19 should provide a roadmap for approaching drug discovery, when confronting SARS-CoV-2 and other current and future viral threats. From the beginning of the SARS-CoV-2 pandemic in China, it was clear that severe COVID-19 was an inflammatory acute respiratory distress syndrome (ARDS). Indeed, early in the pandemic, COVID-19 progression from mild to moderate to severe was associated with immune dysfunction, hyper-inflammatory response, and sepsis (Zhou et al., 2020). Later studies showed that some patients infected with SARS-CoV-2 developed severe pneumonia and ARDS (Grant et al., 2021). Consequently, both antiviral and immunomodulatory drugs were contemplated and administered early in the pandemic (Martinez, 2020; Cross et al., 2021). Early studies (Tay et al., 2020) searched for parallelisms between SARS-CoV-2-related pneumonia and pneumonias connected to other human coronavirus diseases, such as severe acute respiratory syndrome (SARS-CoV) and Middle East respiratory syndrome (MERS-CoV). However, the pathobiology of SARS-CoV-2-related pneumonia seems to be distinct from pneumonia caused by other respiratory viral and bacterial pathogens; the SARS-CoV-2-related pneumonia displays unusual clinical features and has a longer clinical course than severe pneumonia (Gattinoni et al., 2020; Grant et al., 2021). An accurate clinical characterization of SARS-CoV-2 infections, based on known clinical syndromes, is critical for the identification of effective therapies (Cross et al., 2021). Understandably, early in the SARS-CoV-2 pandemic, several repurposed drugs were quickly translated to the clinic. However, over the 2 last years, the results have been rather poor (Edwards, 2020; Edwards and Hartung, 2021; Martinez, 2021). The repurposed drugs for combating COVID-19 were selected either without a hypothetical basis or with mechanistic hypothetical approaches. Unfortunately, when those candidates were tested in clinical studies, none proved effective against SARS-CoV-2. Given that the repurposed drugs were optimized for a different target, dosage, or tissue, their pharmacology was probably not appropriate for the new indication (Edwards, 2020; Dahlin et al., 2021). In addition, the cellular assays designed for repurposing antivirals, including phenotypic and high-throughput screening assays, are frequently subject to interfering or undesirable bioactive mechanisms (Dahlin et al., 2021). Remarkably, most of the repurposed drugs tested during this pandemic were administered off-label and outside of properly designed observational or clinical trials. However, unsuccessful approaches might reduce the public trust in evidence-based medicine (Cross et al., 2021). Therefore, clinical trials and methodologies for identifying effective drugs should be designed based on methodologies with the highest likelihood of success. Antiviral drugs target key processes in the virus life cycle. These processes include virus cell entry, genome replication, translation, protein processing, and virus particle generation. More than two-thirds of all antivirals approved for human use target viral enzymes, such as viral RNA/DNA polymerases and viral proteases (De Clercq and Li, 2016; Tompa et al., 2021). Importantly, in the last 30 years, no antiviral drug has been approved in the absence of a specific viral target. Thus, perhaps the main drawback of the repurposed antiviral drugs that have been tested unsuccessfully against SARS-CoV-2 has been the absence of an evident mechanism of action (Table 1). Table 1. Most relevant repurposed antiviral drugs against SARS-CoV-2. Hits from antiviral screening based on biochemical or cell-culture assays must be treated with skepticism. Chloroquine and hydroxychloroquine represent a clear example of misinterpreted cell culture results. The efficacy of chloroquine and hydroxychloroquine against SARS-CoV-2 in tissue cultures was determined with African green monkey kidney-derived Vero cells. However, when Vero cells were engineered to express TMPRSS2, a cellular protease that activates SARS-CoV-2 for entry into lung cells, the genetic manipulation rendered SARS-CoV-2–infected Vero cells insensitive to chloroquine (Hoffmann et al., 2020). Accordingly, chloroquine could not block SARS-CoV-2 infections in TMPRSS2-expressing human lung Calu-3 cells; thus, chloroquine targets a viral activation pathway that is not present in human lung cells. Therefore, the drug is unlikely to protect against the spread of SARS-CoV-2. Another example of a misinterpretation of cell culture results was recently reported (Tummino et al., 2021) in a study that demonstrated that phospholipidosis was a shared mechanism underlying the antiviral activity of many repurposed drugs for SARS-CoV-2 (Tummino et al., 2021). Conversely, drugs that were active against the same targets, but did not induce phospholipidosis, were not effective against viruses. Those results strongly suggested that a failure to induce phospholipidosis could explain why most drugs that were selected for repurposing to date lacked...