Endocytic Pathway of Feline Coronavirus for Cell Entry: Differences in Serotype-Dependent Viral Entry Pathway

Lipid compartments and lipid metabolism as therapeutic targets against coronavirus

Lipids perform a series of cellular functions, establishing cell and organelles' boundaries, organizing signaling platforms, and creating compartments where specific reactions occur. Moreover, lipids store energy and act as secondary messengers whose distribution is tightly regulated. Disruption of lipid metabolism is associated with many diseases, including those caused by viruses. In this scenario, lipids can favor virus replication and are not solely used as pathogens' energy source. In contrast, cells can counteract viruses using lipids as weapons. In this review, we discuss the available data on how coronaviruses profit from cellular lipid compartments and why targeting lipid metabolism may be a powerful strategy to fight these cellular parasites. We also provide a formidable collection of data on the pharmacological approaches targeting lipid metabolism to impair and treat coronavirus infection.

[Options for treatment of feline infectious peritonitis - previously and today]

Feline infectious peritonitis (FIP) is one of the most common infectious diseases in cats that is fatal when untreated. So far, there is no legally available effective treatment in Germany. Treatment options include only symptomatic treatment (e. g. glucocorticoids, propentofylline), immunomodulatory approaches (e. g. interferons, polyprenyl immunostimulant), and antiviral chemotherapy with protease inhibitors (e. g. GC376) or nucleoside analogues (e. g. GS-441524, remdesivir). Symptomatic treatment does not cure FIP but may lead to a short-term improvement of clinical signs in a subset of cats. Immunomodulatory treatment has also not shown to be very promising. In contrary, the antiviral compounds GS-441524 and GC376 exhibited significant efficacy in several studies and their use saved the lives of many cats suffering from FIP. However, both agents are currently not licensed and thus cannot be legally administered by veterinarians in Germany. Legally, cats may only be legally treated with GS-441524 in a few countries (e.g. Great Britain and Australia). In other countries, GS-441524 is imported by cat owners via the black market and administered on their own. This article provides an overview of the available treatment options and an outlook on the legal use of effective antiviral drugs.

Ionophore Antibiotics Inhibit Type II Feline Coronavirus Proliferation In Vitro

Feline coronaviruses (FCoVs) infect cats worldwide and cause severe systemic diseases, such as feline infectious peritonitis (FIP). FIP has a high mortality rate, and drugs approved by the Food and Drug Administration have been ineffective for the treatment of FIP. Investigating host factors and the functions required for FCoV replication is necessary to develop effective drugs for the treatment of FIP. FCoV utilizes an endosomal trafficking system for cellular entry after binding between the viral spike (S) protein and its receptor. The cellular enzymes that cleave the S protein of FCoV to release the viral genome into the cytosol require an acidic pH optimized in the endosomes by regulating cellular ion concentrations. Ionophore antibiotics are compounds that form complexes with alkali ions to alter the endosomal pH conditions. This study shows that ionophore antibiotics, including valinomycin, salinomycin, and nigericin, inhibit FCoV proliferation in vitro in a dose-dependent manner. These results suggest that ionophore antibiotics should be investigated further as potential broad-spectrum anti-FCoV agents.

Coronavirus Infection and Cholesterol Metabolism

Host cholesterol metabolism remodeling is significantly associated with the spread of human pathogenic coronaviruses, suggesting virus-host relationships could be affected by cholesterol-modifying drugs. Cholesterol has an important role in coronavirus entry, membrane fusion, and pathological syncytia formation, therefore cholesterol metabolic mechanisms may be promising drug targets for coronavirus infections. Moreover, cholesterol and its metabolizing enzymes or corresponding natural products exert antiviral effects which are closely associated with individual viral steps during coronavirus replication. Furthermore, the coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 infections are associated with clinically significant low cholesterol levels, suggesting cholesterol could function as a potential marker for monitoring viral infection status. Therefore, weaponizing cholesterol dysregulation against viral infection could be an effective antiviral strategy. In this review, we comprehensively review the literature to clarify how coronaviruses exploit host cholesterol metabolism to accommodate viral replication requirements and interfere with host immune responses. We also focus on targeting cholesterol homeostasis to interfere with critical steps during coronavirus infection.

In vitro antiviral effects of GS-441524 and itraconazole combination against feline infectious peritonitis virus

Feline infectious peritonitis virus (FIPV: virulent feline coronavirus) causes a fatal disease called feline infectious peritonitis (FIP) in wild and domestic cat species. Recent studies identified several antiviral drugs that are effective against FIPV. Drug combination is one of the important strategies in the development of novel treatments for viral infections. GS-441524, a nucleoside analog, and itraconazole, a triazole antifungal drug, have been reported that have antiviral effect against FIPV. This study aims to investigate whether the combination of GS-441524 and itraconazole has synergic antiviral effect against FIPV. The antiviral effect was measured by plaque reduction assay using felis catus whole fatus-4 cell. The plaque reduction of GS-441524 against type I FIPVs increased as the concentration of itraconazole increased. The similar result was obtained for type II FIPV. In addition, the calculated combination index (CI) demonstrated that there was a strong synergy between GS-441524 and itraconazole. It is concluded that the combination of GS-441524 and itraconazole may enhance the individual effect of each drug against replication of type I FIPVs and may contribute to development more effective treatment strategy for FIP.

Nanobased Platforms for Diagnosis and Treatment of COVID-19: From Benchtop to Bedside

Human respiratory viral infections are the leading cause of morbidity and mortality around the world. Among the various respiratory viruses, coronaviruses (e.g., SARS-CoV-2) have created the greatest challenge and most frightening health threat worldwide. Human coronaviruses typically infect the upper respiratory tract, causing illnesses that range from common cold-like symptoms to severe acute respiratory infections. Several promising vaccine formulations have become available since the beginning of 2021. Nevertheless, achievement of herd immunity is still far from being realized. Social distancing remains the only effective measure against SARS-CoV-2 infection. Nanobiotechnology enables the design of nanobiosensors. These nanomedical diagnostic devices have opened new vistas for early detection of viral infections. The present review outlines recent research on the effectiveness of nanoplatforms as diagnostic and antiviral tools against coronaviruses. The biological properties of coronavirus and infected host organs are discussed. The challenges and limitations encountered in combating SARS-CoV-2 are highlighted. Potential nanodevices such as nanosensors, nanobased vaccines, and smart nanomedicines are subsequently presented for combating current and future mutated versions of coronaviruses.

In vitro activity of itraconazole against SARS-CoV-2

Although vaccination campaigns are currently being rolled out to prevent coronavirus disease (COVID-19), antivirals will remain an important adjunct to vaccination. Antivirals against coronaviruses do not exist, hence global drug repurposing efforts have been carried out to identify agents that may provide clinical benefit to patients with COVID-19. Itraconazole, an antifungal agent, has been reported to have activity against animal coronaviruses. Using cell-based phenotypic assays, the in vitro antiviral activity of itraconazole and 17-OH itraconazole was assessed against clinical isolates from a German and Belgian patient infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Itraconazole demonstrated antiviral activity in human Caco-2 cells (EC₅₀  = 2.3 µM; 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay). Similarly, its primary metabolite, 17-OH itraconazole, showed inhibition of SARS-CoV-2 activity (EC₅₀  = 3.6 µM). Remdesivir inhibited viral replication with an EC₅₀  = 0.4 µM. Itraconazole and 17-OH itraconazole resulted in a viral yield reduction in vitro of approximately 2-log₁₀ and approximately 1-log₁₀ , as measured in both Caco-2 cells and VeroE6-eGFP cells, respectively. The viral yield reduction brought about by remdesivir or GS-441524 (parent nucleoside of the antiviral prodrug remdesivir; positive control) was more pronounced, with an approximately 3-log₁₀ drop and >4-log₁₀ drop in Caco-2 cells and VeroE6-eGFP cells, respectively. Itraconazole and 17-OH itraconazole exert in vitro low micromolar activity against SARS-CoV-2. Despite the in vitro antiviral activity, itraconazole did not result in a beneficial effect in hospitalized COVID-19 patients in a clinical study (EudraCT Number: 2020-001243-15).

Possible therapeutic targets for SARS-CoV-2 infection and COVID-19

SARS-CoV-2 infection causes COVID-19, which has emerged as a health emergency worldwide. SARS-CoV-2 infects cells by binding to ACE2 receptors and enters into the cytoplasm following its escape from endolysosomes. Once in the cytoplasm, the virus replicates and eventually causes various pathological conditions including acute respiratory distress syndrome (ARDS) that is caused by pro-inflammatory cytokine storms. Thus, endolysosomes and cytokine storms are important therapeutic targets to suppress SARS-CoV-2 infection and COVID-19. Here, we discuss therapeutic targets of SARS-CoV-2 infection and available drugs that could be helpful in the suppression of the SARS-CoV-2 infection and pathological condition COVID-19. The urgency of the COVID-19 pandemic precludes the development of new drugs and increased focus on drug repurposing might provide the quickest way to finding effective medicines.

Role of Endolysosomes in Severe Acute Respiratory Syndrome Coronavirus-2 Infection and Coronavirus Disease 2019 Pathogenesis: Implications for Potential Treatments

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is an enveloped, single-stranded RNA virus. Humans infected with SARS-CoV-2 develop a disease known as coronavirus disease 2019 (COVID-19) with symptoms and consequences including acute respiratory distress syndrome (ARDS), cardiovascular disorders, and death. SARS-CoV-2 appears to infect cells by first binding viral spike proteins with host protein angiotensin-converting enzyme 2 (ACE2) receptors; the virus is endocytosed following priming by transmembrane protease serine 2 (TMPRSS2). The process of virus entry into endosomes and its release from endolysosomes are key features of enveloped viruses. Thus, it is important to focus attention on the role of endolysosomes in SARS-CoV-2 infection. Indeed, coronaviruses are now known to hijack endocytic machinery to enter cells such that they can deliver their genome at replication sites without initiating host detection and immunological responses. Hence, endolysosomes might be good targets for developing therapeutic strategies against coronaviruses. Here, we focus attention on the involvement of endolysosomes in SARS-CoV-2 infection and COVID-19 pathogenesis. Further, we explore endolysosome-based therapeutic strategies to restrict SARS-CoV-2 infection and COVID-19 pathogenesis.

Feline infectious peritonitis (FIP) and coronavirus disease 19 (COVID-19): Are they similar?

SARS‐CoV‐2 has radically changed our lives causing hundreds of thousands of victims worldwide and influencing our lifestyle and habits. Feline infectious peritonitis (FIP) is a disease of felids caused by the feline coronaviruses (FCoV). FIP has been considered irremediably deadly until the last few years. Being one of the numerous coronaviruses that are well known in veterinary medicine, information on FCoV could be of interest and might give suggestions on pathogenic aspects of SARS‐CoV‐2 that are still unclear. The authors of this paper describe the most important aspects of FIP and COVID‐19 and the similarities and differences between these important diseases. SARS‐CoV‐2 and FCoV are taxonomically distant viruses, and recombination events with other coronaviruses have been reported for FCoV and have been suggested for SARS‐CoV‐2. SARS‐CoV‐2 and FCoV differ in terms of some pathogenic, clinical and pathological features. However, some of the pathogenic and immunopathogenic events that are well known in cats FIP seem to be present also in people with COVID‐19. Moreover, preventive measures currently recommended to prevent SARS‐CoV‐2 spreading have been shown to allow eradication of FIP in feline households. Finally, one of the most promising therapeutic compounds against FIP, GS‐441524, is the active form of Remdesivir, which is being used as one therapeutic option for COVID‐19.

Potential COVID-19 therapeutics from a rare disease: weaponizing lipid dysregulation to combat viral infectivity

The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus (SARS-CoV)-2 has resulted in the death of more than 328,000 persons worldwide in the first 5 months of 2020. Herculean efforts to rapidly design and produce vaccines and other antiviral interventions are ongoing. However, newly evolving viral mutations, the prospect of only temporary immunity, and a long path to regulatory approval pose significant challenges and call for a common, readily available, and inexpensive treatment. Strategic drug repurposing combined with rapid testing of established molecular targets could provide a pause in disease progression. SARS-CoV-2 shares extensive structural and functional conservation with SARS-CoV-1, including engagement of the same host cell receptor (angiotensin-converting enzyme 2) localized in cholesterol-rich microdomains. These lipid-enveloped viruses encounter the endosomal/lysosomal host compartment in a critical step of infection and maturation. Niemann-Pick type C (NP-C) disease is a rare monogenic neurodegenerative disease caused by deficient efflux of lipids from the late endosome/lysosome (LE/L). The NP-C disease-causing gene (NPC1) has been strongly associated with viral infection, both as a filovirus receptor (e.g., Ebola) and through LE/L lipid trafficking. This suggests that NPC1 inhibitors or NP-C disease mimetics could serve as anti-SARS-CoV-2 agents. Fortunately, there are such clinically approved molecules that elicit antiviral activity in preclinical studies, without causing NP-C disease. Inhibition of NPC1 may impair viral SARS-CoV-2 infectivity via several lipid-dependent mechanisms, which disturb the microenvironment optimum for viral infectivity. We suggest that known mechanistic information on NPC1 could be utilized to identify existing and future drugs to treat COVID-19.

Broad-Spectrum Host-Based Antivirals Targeting the Interferon and Lipogenesis Pathways as Potential Treatment Options for the Pandemic Coronavirus Disease 2019 (COVID-19)

The ongoing Coronavirus Disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) signals an urgent need for an expansion in treatment options. In this study, we investigated the anti-SARS-CoV-2 activities of 22 antiviral agents with known broad-spectrum antiviral activities against coronaviruses and/or other viruses. They were first evaluated in our primary screening in VeroE6 cells and then the most potent anti-SARS-CoV-2 antiviral agents were further evaluated using viral antigen expression, viral load reduction, and plaque reduction assays. In addition to remdesivir, lopinavir, and chloroquine, our primary screening additionally identified types I and II recombinant interferons, 25-hydroxycholesterol, and AM580 as the most potent anti-SARS-CoV-2 agents among the 22 antiviral agents. Betaferon (interferon-β1b) exhibited the most potent anti-SARS-CoV-2 activity in viral antigen expression, viral load reduction, and plaque reduction assays among the recombinant interferons. The lipogenesis modulators 25-hydroxycholesterol and AM580 exhibited EC50 at low micromolar levels and selectivity indices of >10.0. Combinational use of these host-based antiviral agents with virus-based antivirals to target different processes of the SARS-CoV-2 replication cycle should be evaluated in animal models and/or clinical trials.

Antiviral Effects of Hydroxychloroquine and Type I Interferon on In Vitro Fatal Feline Coronavirus Infection

Feline infectious peritonitis (FIP) is a viral disease with a high morbidity and mortality by the FIP virus (FIPV, virulent feline coronavirus). Several antiviral drugs for FIP have been identified, but many of these are expensive and not available in veterinary medicine. Hydroxychloroquine (HCQ) is a drug approved by several countries to treat malaria and immune-mediated diseases in humans, and its antiviral effects on other viral infections (e.g., SARS-CoV-2, dengue virus) have been confirmed. We investigated whether HCQ in association with interferon-ω (IFN-ω) is effective for FIPV in vitro. A total of 100 μM of HCQ significantly inhibited the replication of types I and II FIPV. Interestingly, the combination of 100 μM of HCQ and 10⁴ U/mL of recombinant feline IFN-ω (rfIFN-ω, veterinary registered drug) increased its antiviral activity against type I FIPV infection. Our study suggested that HCQ and rfIFN-ω are applicable for treatment of FIP. Further clinical studies are needed to verify the combination of HCQ and rIFN-ω will be effective and safe treatment for cats with FIP.

Therapeutic effect of an anti-human-TNF-alpha antibody and itraconazole on feline infectious peritonitis

Feline infectious peritonitis (FIP) is a fatal disease in wild and domestic cat species. Although several drugs are expected to be useful as treatments for FIP, no drugs are available in clinical practice. In this study, we evaluated the therapeutic effect of combined use of adalimumab (an anti-human-TNF-alpha monoclonal antibody, ADA) and itraconazole (ICZ), which are presently available to veterinarians. The neutralizing activity of ADA against fTNF-alpha-induced cytotoxicity was measured in WEHI-164 cells. Ten specific pathogen-free (SPF) cats were inoculated intraperitoneally with type I FIPV KU-2. To the cats that developed FIP, ADA (10 mg/animal) was administered twice between day 0 and day 4 after the start of treatment. ICZ (50 mg/head, SID) was orally administered daily from day 0 after the start of treatment. ADA demonstrated dose-dependent neutralizing activity against rfTNF-alpha. In an animal experiment, 2 of 3 cats showed improvements in FIP clinical symptoms and blood chemistry test results, an increase in the peripheral blood lymphocyte count, and a decrease in the plasma alpha 1-AGP level were observed after the beginning of treatment. One of the cats failed to respond to treatment and was euthanized, although the viral gene level in ascites temporarily decreased after the start of treatment. ADA was found to have neutralizing activity against rfTNF-alpha. The combined use of ADA and ICZ showed a therapeutic effect for experimentally induced FIP. We consider these drugs to be a treatment option until effective anti-FIPV drugs become available.