Table 1. Drug susceptibility of influenza A, B, C, and D viruses by viral yield reduction assay in MDCK cells*.
| Type | Virus | Virus titer, log10 TCID50/mL† |
EC90, nmol/L |
|||||
|---|---|---|---|---|---|---|---|---|
| Baloxavir, mean ± SD |
Favipiravir |
|||||||
| 24 hpi | 48 hpi | 24 hpi | 48 hpi | 48 hpi | ||||
| A |
A/Texas/138/2018 (H1N1)pdm09 | 6.4 ± 0.5 | 9.1 ± 0.1 | 0.8 ± 0.2 | 1.2 ± 0.1 | NT | ||
| A/Illinois/08/2018 (H1N1)pdm09 |
5.8 ± 0.5 |
8.9 ± 0.3 |
|
2.7 ± 0.5 |
3.3 ± 0.2 |
|
3,005 |
|
| B |
B/Maryland/29/2018 | 3.5 ± 0.5 | 7.1 ± 0.6 | 8.9 ± 1.6 | 5.8 ± 1.1 | 1,789 | ||
| B/Iowa/18/2018 |
3.5 ± 0.5 |
7.6 ± 0.4 |
|
13.8 ± 2.0 |
7.8 ± 1.7 |
|
1,635 |
|
| C |
C/Taylor/1233/47 | <1.5 | 5.9 ± 0.5 | NA | 13.0 ± 3.3 | 27,476 | ||
| C/Aomori/74 |
<1.5 |
4.9 ± 0.4 |
|
NA |
18.4 ± 6.5 |
|
31,603 |
|
| D | D/swine/Oklahoma/13334/2011 | 4.4 ± 0.3 | 7.7 ± 0.1 | 110.2 ± 27.6 | 98.3 ± 23.9 | 2,764 | ||
| D/bovine/Oklahoma/660/2013 | 4.8 ± 0.0 | 8.0 ± 0.5 | 105.6 ± 37.0 | 64.3 ± 16.2 | 3,106 | |||
*Cell monolayers were inoculated at a multiplicity of infection of 0.0005 and virus was allowed to adsorb for 1 h. Virus inoculum was removed, serially diluted drug (baloxavir: 0.5–500 nmol/L; favipiravir: 310–318,000 nmol/L) was added, and cells were incubated at 33°C in a 5% CO2 incubator. At 24 and 48 hpi, cell culture supernatants were harvested to determine infectious virus titers. Replication of influenza A and B viruses was detected by neuraminidase activity (Fluor-NA Kit; Applied Biosystems, https://www.thermofisher.com), and replication of influenza C and D viruses by esterase activity (3-acetyl-umbelliferone in 20 mmol/L Tris-HCl, pH 8.0, reaction buffer (Sigma-Aldrich, https://www.sigmaaldrich.com). The EC90 corresponds to a drug concentration causing a 90% reduction in virus titer compared with control wells without drug. The EC90 for each virus and drug were determined by using nonlinear regression analysis (GraphPad, https://www.graphpad.com). For baloxavir, results are shown as mean ± SD of 3 independent experiments; favipiravir results are shown as single or average of 2 independent experiments. EC90, 90% effective concentration; hpi, hours postinfection; NA, not applicable; NT, not tested; TCID50, 50% tissue culture infectious dose. †Virus titers were determined in control wells without drug.