In December 2019, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in Wuhan, Hubei Province, China. No specific treatment has been established against coronavirus disease-2019 (COVID-19) so far. Therefore, it is urgently needed to identify effective antiviral agents for the treatment of this disease, and several approved drugs such as lopinavir have been evaluated. Here, we report that nelfinavir, an HIV-1 protease inhibitor, potently inhibits replication of SARS-CoV-2. The effective concentrations for 50% and 90% inhibition (EC50 and EC90) of nelfinavir were 1.13 µM and 1.76 µM respectively, the lowest of the nine HIV-1 protease inhibitors including lopinavir. The trough and peak serum concentrations of nelfinavir were three to six times higher than EC50 of this drug. These results suggest that nelfinavir is a potential candidate drug for the treatment of COVID-19 and should be assessed in patients with COVID-19.

Many variants of SARS-CoV-2 have emerged around the world. It is therefore important to understand its global viral evolution and the corresponding mutations associated with transmissibility and severity. In this study, we analyzed 112 whole genome sequences of SARS-CoV-2 collected from patients at Juntendo University Hospital in Tokyo and the genome data from entire Japan deposited in Global Initiative on Sharing Avian Influenza Data (GISAID) to examine the relationship of amino acid changes with the transmissibility and the severity of each strain/lineage. We identified 12 lineages, including B.1.1.284, B.1.1.214, R.1, AY.29, and AY.29.1, which were prevalent specifically in Japan. B.1.1.284 was most frequently detected in the second wave, but B.1.1.214 became the predominant lineage in the third wave, indicating that B.1.1.214 has a higher transmissibility than B.1.1.284. The most prevalent lineage during the fourth and fifth wave was B.1.1.7 and AY.29, respectively. In regard to the severity of identified lineages, B.1.1.214 was significantly lower than the reference lineage, B.1.1.284. Analysis of the genome sequence and other traits of each lineage/strain revealed the mutations in S, N, and NSPs that increase the transmissibility and/or severity. These mutations include S: M153T, N: P151L, NSP3: S543P, NSP5: P108S, and NSP12: A423V in B.1.1.284; S: W152L and E484K in R.1; S: H69del, V70del, and N501Y in the Alpha strain; S: L452R, T478K, and P681R in the Delta strain. Furthermore, it is suggested that the transmissibility of B.1.1.214 could be enhanced by the mutations N: M234I, NSP14: P43L, and NSP16: R287I. To address the issue of the virus evolution, it is necessary to continuously monitor the genomes of SARS-CoV-2 and analyze the effects of mutations for developing vaccines and antiviral drugs effective against SARS-CoV-2 variants.

Suspension Madin-Darby canine kidney (MDCK) cells (MDCK-N), adherent MDCK cells (MDCK-C), and adherent rhesus monkey kidney LLC-MK2 cells (LLC-MK2D) were systematically evaluated for the preparation of influenza vaccine seed viruses for humans on the basis of primary virus isolation efficiency, growth ability, genetic stability of the hemagglutinin (HA) and neuraminidase (NA) genes, and antigenic properties in hemagglutination inhibition (HI) test of each virus isolate upon further passages. All the subtypes/lineages of influenza viruses (A(H1N1), A(H1N1)pdm09, A(H3N2), B-Victoria, and B-Yamagata) were successfully isolated from clinical specimens by using MDCK-N and MDCK-C, whereas LLC-MK2D did not support virus replication well. Serial passages of A(H1N1) viruses in MDCK-N and MDCK-C induced genetic mutations of HA that resulted in moderate antigenic changes in the HI test. All A(H1N1)pdm09 isolates from MDCK-C acquired amino acid substitutions at the site from K153 to N156 of the HA protein, which resulted in striking antigenic alteration. In contrast, only 30% of MDCK-N isolates showed amino acid changes at this site. The frequency of MDCK-N isolates with less than two-fold reduction in the HI titer was as high as 70%. A(H3N2) and B-Yamagata isolates showed high antigenic stability and no specific amino acid substitution during passages in MDCK-N and MDCK-C. B-Victoria isolates from MDCK-N and MDCK-C acquired genetic changes at HA glycosylation sites that greatly affected their antigenicity. When these cell isolates were applied to passages in hen eggs, A(H1N1), B-Victoria, and B-Yamagata viruses grew well in eggs, while none of the cell isolates of A(H3N2) viruses did. Thus, we demonstrate that MDCK-N might be useful for the preparation of influenza vaccine seed viruses.

The emergence of SARS-CoV-2, responsible for the global COVID-19 pandemic, requires the rapid development of novel antiviral drugs that would contribute to an effective treatment alongside vaccines. Drug repurposing and development of new molecules targeting numerous viral targets have already led to promising drug candidates. To this end, versatile molecular scaffolds with high functionalization capabilities play a key role. Starting with the clinically used conformationally flexible HIV-1 protease inhibitors that inhibit replication of SARS-CoV-2 and bind major protease 3CL^pro, we designed and synthesized a series of rigid bicyclo[2.2.2]octenes fused to N-substituted succinimides to test whether this core scaffold could support the development of non-covalent 3CL^pro inhibitors. Inhibition assays confirmed that some compounds can inhibit the SARS-CoV-2 main protease; the most promising compound 11a inhibited 3CL^pro in micromolar range (IC₅₀ = 102.2 μM). Molecular simulations of the target-ligand complex in conjunction with dynophore analyses and endpoint free energy calculations provide additional insight and first recommendations for future optimization. The fused bicyclo[2.2.2]octenes can be used as a new potential starting point in the development of non-covalent SARS-CoV-2 3CL^pro protease inhibitors and the study also substantiates the potential of this versatile scaffold for the development of biologically active molecules.