Rift Valley fever virus (RVFV) is a mosquito-borne pathogen endemic to sub-Saharan Africa and the Arabian Peninsula. There are no approved antiviral therapies or vaccines available to treat or prevent severe disease associated with RVFV infection in humans. The adenosine analog, galidesivir (BCX4430), is a broad-spectrum antiviral drug candidate with in vitro antiviral potency (EC of less than 50 μM) in more than 20 different viruses across eight different virus families. Here we report on the activity of galidesivir in the hamster model of peracute RVFV infection. Intramuscular and intraperitoneal treatments effectively limited systemic RVFV (strain ZH501) infection as demonstrated by significantly improved survival outcomes and the absence of infectious virus in the spleen and the majority of the serum, brain, and liver samples collected from infected animals. Our findings support the further development of galidesivir as an antiviral therapy for use in treating severe RVFV infection, and possibly other related phleboviral diseases.

BackgroundJunín virus (JUNV), the etiologic agent of Argentine hemorrhagic fever (AHF), is classified by the NIAID and CDC as a Category A priority pathogen. Presently, antiviral therapy for AHF is limited to immune plasma, which is readily available only in the endemic regions of Argentina. T-705 (favipiravir) is a broadly active small molecule RNA-dependent RNA polymerase inhibitor presently in clinical evaluation for the treatment of influenza. We have previously reported on the in vitro activity of favipiravir against several strains of JUNV and other pathogenic New World arenaviruses.Methodology/Principal FindingsTo evaluate the efficacy of favipiravir in vivo, guinea pigs were challenged with the pathogenic Romero strain of JUNV, and then treated twice daily for two weeks with oral or intraperitoneal (i.p.) favipiravir (300 mg/kg/day) starting 1–2 days post-infection. Although only 20% of animals treated orally with favipiravir survived the lethal challenge dose, those that succumbed survived considerably longer than guinea pigs treated with placebo. Consistent with pharmacokinetic analysis that showed greater plasma levels of favipiravir in animals dosed by i.p. injection, i.p. treatment resulted in a substantially higher level of protection (78% survival). Survival in guinea pigs treated with ribavirin was in the range of 33–40%. Favipiravir treatment resulted in undetectable levels of serum and tissue viral titers and prevented the prominent thrombocytopenia and leucopenia observed in placebo-treated animals during the acute phase of infection.Conclusions/SignificanceThe remarkable protection afforded by i.p. favipiravir intervention beginning 2 days after challenge is the highest ever reported for a small molecule antiviral in the difficult to treat guinea pig JUNV challenge model. These findings support the continued development of favipiravir as a promising antiviral against JUNV and other related arenaviruses.

Favipiravir is approved in Japan to treat novel or re-emerging influenza viruses, and is active against a broad spectrum of RNA viruses, including Ebola. Ribavirin is the only other licensed drug with activity against multiple RNA viruses. Recent studies show that ribavirin and favipiravir act synergistically to inhibit bunyavirus infections in cultured cells and laboratory mice, likely due to their different mechanisms of action. Convalescent immune globulin is the only approved treatment for Argentine hemorrhagic fever caused by the rodent-borne Junin arenavirus. We previously reported that favipiravir is highly effective in a number of small animal models of Argentine hemorrhagic fever. We now report that addition of low dose of ribavirin synergistically potentiates the activity of favipiravir against Junin virus infection of guinea pigs and another arenavirus, Pichinde virus infection of hamsters. This suggests that the efficacy of favipiravir against hemorrhagic fever viruses can be further enhanced through the addition of low-dose ribavirin.

Favipiravir (T-705) is a new anti-influenza drug approved for human use in Japan and progressing through Phase 3 clinical trials in the U.S. In addition to its potent inhibitory effects against influenza virus infection, the compound has been shown to be broadly active against RNA viruses from 9 different families, including the Arenaviridae. Several members of the Arenaviridae family of viruses are significant human pathogens that cause viral hemorrhagic fever, a severe systemic syndrome where vascular leak is a cardinal feature. Because arenaviral infections are unlikely to be diagnosed and treated until the illness has progressed to a more advanced state, it is important to understand the effects of the disease state on favipiravir pharmacokinetics (PK) and biodistribution to help guide therapeutic strategy. During acute arenavirus infection in hamsters, we found reduced plasma favipiravir concentrations and altered kinetics of absorption, elimination and time to maximum drug concentration. In addition, the amounts of the favipiravir M1 primary metabolite were higher in the infected animals, suggesting that favipiravir metabolism may favor the formation of this inactive metabolite during viral infection. We also discovered differences in favipiravir and M1 PK parameters associated with arenavirus infection in a number of hamster tissues. Finally, analysis at the individual animal level demonstrated a correlation between reduced plasma favipiravir concentration with increased disease burden as reflected by weight loss and viral load. Our study is the first to show the impact of active viral infection and disease on favipiravir PK and biodistribution, highlighting the need to consider alterations in these parameters when treating individuals with viral hemorrhagic fever of arenavirus or other etiology.

BackgroundA growing number of arenaviruses can cause a devastating viral hemorrhagic fever (VHF) syndrome. They pose a public health threat as emerging viruses and because of their potential use as bioterror agents. All of the highly pathogenic New World arenaviruses (NWA) phylogenetically segregate into clade B and require maximum biosafety containment facilities for their study. Tacaribe virus (TCRV) is a nonpathogenic member of clade B that is closely related to the VHF arenaviruses at the amino acid level. Despite this relatedness, TCRV lacks the ability to antagonize the host interferon (IFN) response, which likely contributes to its inability to cause disease in animals other than newborn mice.Methodology/Principal FindingsHere we describe a new mouse model based on TCRV challenge of AG129 IFN-α/β and -γ receptor-deficient mice. Titration of the virus by intraperitoneal (i.p.) challenge of AG129 mice resulted in an LD50 of ∼100 fifty percent cell culture infectious doses. Virus replication was evident in the serum, liver, lung, spleen, and brain 4–8 days after inoculation. MY-24, an aristeromycin derivative active against TCRV in cell culture at 0.9 µM, administered i.p. once daily for 7 days, offered highly significant (P<0.001) protection against mortality in the AG129 mouse TCRV infection model, without appreciably reducing viral burden. In contrast, in a hamster model of arenaviral hemorrhagic fever based on challenge with clade A Pichinde arenavirus, MY-24 did not offer significant protection against mortality.Conclusions/SignificanceMY-24 is believed to act as an inhibitor of S-adenosyl-L-homocysteine hydrolase, but our findings suggest that it may ameliorate disease by blunting the effects of the host response that play a role in disease pathogenesis. The new AG129 mouse TCRV infection model provides a safe and cost-effective means to conduct early-stage pre-clinical evaluations of candidate antiviral therapies that target clade B arenaviruses.

Several arenaviruses can cause viral hemorrhagic fever, a severe disease with case-fatality rates in hospitalized individuals ranging from 15-30%. Because of limited prophylaxis and treatment options, new medical countermeasures are needed for these viruses classified by the National Institutes of Allergy and Infectious Diseases (NIAID) as top priority biodefense Category A pathogens. Recombinant consensus interferon alpha (cIFN-α) is a licensed protein with broad clinical appeal. However, while cIFN-α has great therapeutic value, its utility for biodefense applications is hindered by its short in vivo half-life, mode and frequency of administration, and costly production. To address these limitations, we describe the use of DEF201, a replication-deficient adenovirus vector that drives the expression of cIFN-α, for pre- and post-exposure prophylaxis of acute arenaviral infection modeled in hamsters. Intranasal administration of DEF201 24 h prior to challenge with Pichindé virus (PICV) was highly effective at protecting animals from mortality and preventing viral replication and liver-associated disease. A significant protective effect was still observed with a single dosing of DEF201 given two weeks prior to PICV challenge. DEF201 was also efficacious when administered as a treatment 24 to 48 h post-virus exposure. The protective effect of DEF201 was largely attributed to the expression of cIFN-α, as dosing with a control empty vector adenovirus did not protect hamsters from lethal PICV challenge. Effective countermeasures that are highly stable, easily administered, and elicit long lasting protective immunity are much needed for arena and other viral infections. The DEF201 technology has the potential to address all of these issues and may serve as a broad-spectrum antiviral to enhance host defense against a number of viral pathogens.

Punta Toro virus (PTV; Bunyaviridae, Phlebovirus) is related to Rift Valley fever virus (RVFV), a pathogenic agent which causes severe disease in humans and livestock primarily in the sub-Saharan region of Africa. The recent range expansion of RVFV and the potential for its intentional release into naïve populations pose a significant threat to public health and agriculture. Studies modeling disease in rodents and nonhuman primates have shown that PTV and RVFV are highly sensitive to the antiviral effects of alpha interferon (IFN-␣), an important component of the innate antiviral host response. While recombinant IFN-␣ has high therapeutic value, its utility for the treatment of neglected tropical diseases is hindered by its short in vivo half-life and costly production of longer-lasting pegylated IFNs. Here, we demonstrate extended preexposure protection against lethal PTV challenge following a single intranasal administration of DEF201, which is a replication-deficient human adenovirus type 5 vector engineered to constitutively express consensus IFN-␣ (cIFN-␣) from transduced host cells. DEF201 was also efficacious when administered within 24 h as a postexposure countermeasure. Serum concentrations of cIFN-␣ could be detected as early as 8 h following treatment and persisted for more than 1 week. The prolonged antiphlebovirus prophylactic effect, low production costs, and ease of administration make DEF201 a promising agent for intervention during natural disease outbreaks and for countering possible bioterrorist acts. R ift Valley fever virus (RVFV; Bunyaviridae, Phlebovirus) has been the cause of numerous devastating epizootics throughout sub-Saharan Africa and, more recently, the Arabian Peninsula (3). It is a mosquito-borne virus that causes significant losses in livestock characterized by dramatic "abortion storms" resulting in near-complete mortality in newborn animals (5). RVFV transmission to humans occurs through the bites of infected mosquitoes or contact with tissue from infected animals. Because the virus is also infectious by the airborne route, it poses a potential bioterrorism threat, which is amplified by the fact that mosquitoes native to the United States can readily transmit RVFV and serve as vectors (24). Presently, there are no FDA-approved vaccines or antivirals to prevent or treat RVFV infection, which underscores the urgent need to develop new antiviral therapies.Several reports suggest that RVFV is sensitive to the effects of alpha interferon (IFN-␣) (15,16,18), a potent cytokine essential to the control of viral replication and dissemination (20). Studies employing the closely related Punta Toro virus (PTV), a less biohazardous, more accessible model for RVFV infection, have also demonstrated sensitivity toward agents that elicit type I IFN responses (8,22). In addition, a recent report described the antiphlebovirus activity of human consensus IFN-␣ (cIFN-␣) in cell culture and its prophylactic and therapeutic efficacy in hamsters challenged with PTV (10). Because recombinant IFN prote...

Five New World mammarenaviruses (NWMs) cause life-threatening hemorrhagic fever (HF). Cellular entry by these viruses is mediated by human transferrin receptor 1 (hTfR1). Here, we demonstrate that an antibody (ch128.1/IgG1) which binds the apical domain of hTfR1, potently inhibits infection of attenuated and pathogenic NWMs in vitro. Computational docking of the antibody Fab crystal structure onto the known structure of hTfR1 shows an overlapping receptor-binding region shared by the Fab and the viral envelope glycoprotein GP1 subunit that binds hTfR1, and we demonstrate competitive inhibition of NWM GP1 binding by ch128.1/IgG1 as the principal mechanism of action. Importantly, ch128.1/IgG1 protects hTfR1-expressing transgenic mice against lethal NWM challenge. Additionally, the antibody is well-tolerated and only partially reduces ferritin uptake. Our findings provide the basis for the development of a novel, host receptor-targeted antibody therapeutic broadly applicable to the treatment of HF of NWM etiology.