Broad spectrum antiviral activity of favipiravir (T-705): protection from highly lethal inhalational Rift Valley Fever

Abstract

Background: Development of antiviral drugs that have broad-spectrum activity against a number of viral infections would be of significant benefit. Due to the evolution of resistance to currently licensed antiviral drugs, development of novel anti-influenza drugs is in progress, including Favipiravir (T-705), which is currently in human clinical trials. T-705 displays broad-spectrum in vitro activity against a number of viruses, including Rift Valley Fever virus (RVFV). RVF is an important neglected tropical disease that causes human, agricultural, and economic losses in endemic regions. RVF has the capacity to emerge in new locations and also presents a potential bioterrorism threat. In the current study, the in vivo efficacy of T-705 was evaluated in Wistar-Furth rats infected with the virulent ZH501 strain of RVFV by the aerosol route.

Methodology/principal findings: Wistar-Furth rats are highly susceptible to a rapidly lethal disease after parenteral or inhalational exposure to the pathogenic ZH501 strain of RVFV. In the current study, two experiments were performed: a dose-determination study and a delayed-treatment study. In both experiments, all untreated control rats succumbed to disease. Out of 72 total rats infected with RVFV and treated with T-705, only 6 succumbed to disease. The remaining 66 rats (92%) survived lethal infection with no significant weight loss or fever. The 6 treated rats that succumbed survived significantly longer before succumbing to encephalitic disease.

Conclusions/significance: Currently, there are no licensed antiviral drugs for treating RVF. Here, T-705 showed remarkable efficacy in a highly lethal rat model of Rift Valley Fever, even when given up to 48 hours post-infection. This is the first study to show protection of rats infected with the pathogenic ZH501 strain of RVFV. Our data suggest that T-705 has potential to be a broad-spectrum antiviral drug.

Figure 1. Survival of ZH501 RVFV-infected Wistar-Furth rats after treatment with T-705.

(A) Results from T-705 dose-determination study. Groups of 12 rats were infected with RVFV by aerosol exposure, with an average presented dose of 50 pfu/rat. Within 1 hour after infection, twice-daily (BID) T-705 administration was given at the indicated doses. After 14 days (shaded area), drug treatment was removed, and the rats were monitored for another 28 days, as indicated by the split x-axis. (B) Results from T-705 delayed-treatment study. Groups of 6–12 rats were infected by aerosol exposure, with an average presented dose of 20 pfu/rat. All treated rats received 100 mg/kg of T-705 starting at the indicated times post-infection and continuing for 14 days (shaded area), followed by an additional 28 days of monitoring for survival (split x-axis). For ease of visual identification, the T-705 treatment period is indicated by gray shading from days 0–14 post-infection. For the delayed treatment groups, the treatment periods are shifted by 12, 24, or 48 hours, which is not indicated on the graph. In both experiments, all groups of T-705 treated rats survived significantly longer when compared to the untreated control rats (Log-Rank test p<0.0001).

Figure 2. Weight loss and fever responses in Wistar-Furth rats treated with T-705.

Results from the dose-determination study (A, C) and the delayed-treatment study (B, D) are shown. For each graph, the shaded area represents the 14 days of T-705 treatment. The black lines represent the mean +/− SEM of the infected, treated surviving rats (n = 33 in A and C; n = 39 in B and D). The red line represents the mean +/− SEM of the infected, untreated control rats (n = 12 in A and C; n = 6 in B and D). Three rats from each experiment were infected, treated, and succumbed to infection. Their weight loss and temperature data are plotted separately on each panel (blue, orange, and green lines). The x-axis is plotted through day 16 only in order to make the earlier time points more clearly visible.

Figure 3. Tissue viral loads during T-705 efficacy studies in Wistar-Furth rats.

Plaque assays were used to measure levels of infectious virus within the indicated tissues. Data from both the dose-determination study and the delayed-treatment study are combined in this figure. The black circles represent the tissue viral load from rats that were infected with RVFV but did not receive T-705 treatment (n = 6–12 per tissue; horizontal line is the mean). For the T-705-treated rats, 92% survived (n = 66), and tissues taken at necropsy from the surviving rats did not have detectable tissue viral loads by plaque assay (data not shown). A total of 6 T-705 treated rats from both studies died. The tissue viral loads from these 6 rats are shown as individual data points (red stars; red horizontal line is the mean).

Figure 4. IgG antibody responses during T-705 efficacy studies in Wistar-Furth rats.

Total IgG levels were measured using an indirect antigen-capture ELISA, and the data are expressed as the SumOD of each sample. The cutoff value is indicated by the horizontal dotted line, which represents 3 standard-deviations above the control wells. (A) Dose-determination study, and (B) Delayed-treatment study. For the untreated groups on each panel, serum was taken at necropsy when the rats were moribund (between 4–6 days post-infection). For the surviving rats within the T-705 treated groups (black circles), serum samples were taken at necropsy at the end of the study (day 42). Serum was available for 4 of the 6 T-705 treated rats that died (indicated by the red squares on each graph). Unpaired t-tests were used to compare each T-705 treated group with the untreated control group. Level of significance is indicated by the number of asterisks.