Currently there are relatively few antiviral therapeutics, and most which do exist are highly pathogen-specific or have other disadvantages. We have developed a new broad-spectrum antiviral approach, dubbed Double-stranded RNA (dsRNA) Activated Caspase Oligomerizer (DRACO) that selectively induces apoptosis in cells containing viral dsRNA, rapidly killing infected cells without harming uninfected cells. We have created DRACOs and shown that they are nontoxic in 11 mammalian cell types and effective against 15 different viruses, including dengue flavivirus, Amapari and Tacaribe arenaviruses, Guama bunyavirus, and H1N1 influenza. We have also demonstrated that DRACOs can rescue mice challenged with H1N1 influenza. DRACOs have the potential to be effective therapeutics or prophylactics for numerous clinical and priority viruses, due to the broad-spectrum sensitivity of the dsRNA detection domain, the potent activity of the apoptosis induction domain, and the novel direct linkage between the two which viruses have never encountered.

Competing interests: THR is the inventor on patents and patent applications covering DRACOs: Rider TH (issued October 24, 2006) Anti-pathogen treatments. U.S. Patent 7,125,839; Rider TH (issued July 28, 2009) Anti-pathogen treatments. U.S. Patent 7,566,694; Rider TH (filed June 18, 2009) Anti-Pathogen Treatments. U.S. Patent Application 20100098680; Rider TH (filed February 7, 2003) Anti-Pathogen Treatments. European Patent Application 03716001.7; Rider TH (filed February 7, 2003) Anti-Pathogen Treatments. Canadian Patent Application 2,475,247; Rider TH (filed February 7, 2003) Anti-Pathogen Treatments. Patent Cooperation Treaty Serial No. US03/03978; Rider TH (filed February 7, 2003) Anti-Pathogen Treatments. Japanese Patent Application 2003565429; Rider TH (filed November 19, 2009) Anti-Pathogen Treatments. Japanese Patent Application 2009262426. This does not alter the authors' adherence to all the PLoS ONE policies on sharing data and materials.

Funding: This work is funded by grant AI057159 ( http://www.niaid.nih.gov/Pages/default.aspx ) from the National Institute of Allergy and Infectious Diseases and the New England Regional Center of Excellence for Biodefense and Emerging Infectious Diseases, with previous funding from the Defense Advanced Research Projects Agency, Defense Threat Reduction Agency, and Director of Defense Research & Engineering. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States government.

Copyright: © 2011 Rider et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

For delivery into cells in vitro or in vivo, DRACOs can be fused with proven protein transduction tags, including a sequence from the HIV TAT protein [28] , the related protein transduction domain 4 (PTD) [29] , and polyarginine (ARG) [30] . These tags have been shown to carry large cargo molecules into both the cytoplasm and the nucleus of all cell types in vitro and in vivo, even across the blood-brain barrier.

Because PKR activation and caspase activation function in similar ways and involve proteins that have separate domains with well-defined functions, these two processes can be combined to circumvent most viral blockades [26] – [27] . In its simplest form, a DRACO is a chimeric protein with one domain that binds to viral dsRNA and a second domain (e.g., a procaspase-binding domain or a procaspase) that induces apoptosis when two or more DRACOs crosslink on the same dsRNA. If viral dsRNA is present inside a cell, DRACOs will bind to the dsRNA and induce apoptosis of that cell. If viral dsRNA is not present inside the cell, DRACOs will not crosslink and apoptosis will not occur.

Many viruses attempt to counter these defenses. A wide variety of viruses target dsRNA-induced signaling proteins, including IPS-1, interferon response factors (IRFs), interferons and interferon receptors, JAK/STAT proteins, and eIF-2α [19] – [20] . Some viral products attempt to sequester dsRNA (e.g., poxvirus E3L [21] ) or to directly interfere with cellular dsRNA binding domains (e.g., HIV TAR RNA [19] – [20] ). Virtually all viruses that inhibit apoptosis do so by targeting early steps in the pathway, for example by inhibiting p53, mimicking anti-apoptotic Bcl-2, or interfering with death receptor signaling [22] – [23] . Among the few viral proteins that directly inhibit one or more caspases are African swine fever virus A224L (which inhibits caspase 3) [24] , poxvirus CrmA (which inhibits caspases 1, 8, and 10 but not others) [25] , and baculovirus p35 (which inhibits several caspases but is relatively ineffective against caspase 9) [25] .

The second natural process used by our approach is one of the last steps in the apoptosis pathway [14] , in which complexes containing intracellular apoptosis signaling molecules, such as apoptotic protease activating factor 1 (Apaf-1) [15] – [16] or FLICE-activated death domain (FADD) [17] – [18] , simultaneously bind multiple procaspases. The procaspases transactivate via cleavage, activate additional caspases in the cascade, and cleave a variety of cellular proteins [14] , thereby killing the cell.

Our DRACO approach combines two natural cellular processes. The first process involves dsRNA detection in the interferon pathway. Most viruses have double- or single-stranded RNA (ssRNA) genomes and produce long dsRNA helices during transcription and replication; the remainder of viruses have DNA genomes and typically produce long dsRNA via symmetrical transcription [4] – [5] . In contrast, uninfected mammalian cells generally do not produce long dsRNA (greater than ∼21–23 base pairs) [4] – [5] . Natural cellular defenses exploit this difference in order to detect and to attempt to counter viral infections [6] – [7] . For example, protein kinase R (PKR) contains an N-terminal domain with two dsRNA binding motifs (dsRBM 1 and 2) and a C-terminal kinase domain [8] – [9] . Binding of multiple PKR proteins to dsRNA with a length of at least 30–50 base pairs [5] activates the PKRs via trans-autophosphorylation; activated PKR then phosphorylates eIF-2α, thereby inhibiting translation of viral (and cellular) proteins. Other examples of proteins that detect viral dsRNA include 2′,5′-oligoadenylate (2–5A) synthetases [10] , RNase L (activated via dimerization by 2–5A produced by 2–5A synthetases in response to dsRNA [11] ), TLR 3 [12] , interferon-inducible ADAR1 [13] , and RIG-I and Mda-5 [6] – [7] .

To overcome these shortcomings of existing approaches, we have developed and demonstrated a novel antiviral approach that is effective against a very broad spectrum of viruses, nontoxic in vitro and in vivo, and potentially suitable for either prophylactic or therapeutic administration. Our approach, which we call a D ouble-stranded R NA (dsRNA) A ctivated C aspase O ligomerizer ( DRACO ), is designed to selectively and rapidly kill virus-infected cells while not harming uninfected cells.

A serious threat is posed by viral pathogens, including clinical viruses (HIV, hepatitis viruses, etc.), natural emerging viruses (avian and swine influenza strains, SARS, etc.), and viruses relevant to potential bioterrorism (Ebola, smallpox, etc.). Unfortunately, there are relatively few prophylactics or therapeutics for these viruses, and most which do exist can be divided into three broad categories [1] – [3] : (1) Specific inhibitors of a virus-associated target (e.g., HIV protease inhibitors, RNAi) generally must be developed for each virus or viral strain, are prone to resistance if a virus mutates the drug target, are not immediately available for emerging or engineered viral threats, and can have unforeseen adverse effects. (2) Vaccines also require a new vaccine to be developed for each virus or viral strain, must be administered before or in some cases soon after exposure to be effective, are not immediately available for emerging or engineered viral threats, can have unforeseen adverse effects, and are difficult to produce for certain pathogens (e.g., HIV). (3) Interferons and other pro- or anti-inflammatories are less virus-specific, but still are only useful against certain viruses, and they can have serious adverse effects through their interactions with the immune and endocrine systems.

Results and Discussion

We produced DRACOs with different dsRNA detection domains, apoptosis induction domains, and transduction tags (Figure 1). The dsRNA detection domains included PKR 1–181 , PKR 1–181 with dsRBM 1 (NTE3L), dsRBM 2 (CTE3L), or dsRBM 1 and 2 (2×E3L) replaced by the dsRNA binding motif from poxvirus E3L, and RNaseL 1–335 (which binds to 2–5A produced by endogenous cellular 2–5A synthetases in response to viral dsRNA). The apoptosis induction domains included FADD 1–90 Death Effector Domain (DED, which binds to procaspase 8), Apaf-1 1–97 caspase recruitment domain (CARD, which binds to procaspase 9), and murine Apaf-1 1–97 (mApaf) CARD. Except for mApaf, all domains refer to the human sequence. Isolated dsRNA detection domains and apoptosis induction domains were produced as negative controls. Mutant DRACOs with deleterious K64E [9] and homologous K154E mutations in the PKR domain were also produced as negative controls. Proteins were produced with TAT, PTD, or ARG tags on the N terminus, C terminus, or both termini. Proteins were expressed in BL21(DE3)pLysS Rosetta E. coli. An empty expression vector was transformed into the E. coli and the same purification protocol was followed, resulting in control extract without DRACOs.

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larger image TIFF original image Download: Figure 1. A variety of DRACOs and controls were produced. (A) DRACOs with different dsRNA detection and apoptosis induction domains were designed and produced. All domains were human except murine Apaf-1 (mApaf-1), and some dsRNA detection domains used PKR 1–181 with vaccinia E3L dsRNA binding motif replacing PKR dsRBM 1 (NTE3L), dsRBM 2 (CTE3L), or both (2×E3L). His denotes His 6 purification tag and Txd denotes PTD, TAT, or ARG transduction tag. DRACOs with transduction tags on the N-, C-, or both termini were produced. (B) This protein gel shows examples of DRACOs and negative controls that were produced. 1 µg was loaded per lane. Final yields were approximately 30 mg purified protein per liter of culture. https://doi.org/10.1371/journal.pone.0022572.g001

DRACO rapidly entered cells, persisted within cells for days, and mediated apoptosis in cells transfected with dsRNA. PKR-Apaf DRACO with PTD or TAT tags entered cells efficiently, whereas DRACO without a transduction tag did not (Figure 2A). DRACO entered cells within 10 minutes, reached a maximum after approximately 1.5 hours (Figures 2B, S1), and persisted inside cells for at least 8 days (Figure 2C). L929 cells transfected with both DRACO and poly(I)∶poly(C) dsRNA exhibited greatly increased apoptosis within 24 hours, whereas cells that received only DRACO did not (Figure 3). Pan-caspase and caspase-9 inhibitors eliminated DRACO-mediated apoptosis in the presence of dsRNA.

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larger image TIFF original image Download: Figure 2. DRACOs penetrated cells and persisted for days. (A) DRACOs with PTD or TAT tags entered H1-HeLa cells more readily than DRACO without a transduction tag. 400 nM PKR-Apaf DRACO was added to medium for 1 hour, and then cells were trypsinized and washed to remove any DRACO on the cell surface. Cells were lysed and analyzed for DRACO by westerns using anti-His 6 antibodies. Lysate from approximately 105 cells was loaded in each lane. A known amount of purified PKR-Apaf DRACO was used as a standard as indicated. (B) DRACOs entered HeLa cells within 10 minutes and reached a maximum after 1.5 hours. 400 nM TAT-PKR-Apaf DRACO was added to medium for the specified time, and then cells were analyzed as in (A). (C) DRACOs persisted within HeLa cells for at least 8 days. 500 nM PTD-PKR-Apaf DRACO was added to cell medium for 1 hour, and then cells were put into DRACO-free medium. After the specified number of days, cells were analyzed as in (A). https://doi.org/10.1371/journal.pone.0022572.g002

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larger image TIFF original image Download: Figure 3. DRACOs mediated apoptosis in cells containing dsRNA. L929 cells transfected with both DRACO and poly(I)∶poly(C) dsRNA exhibited apoptosis within 24 hours, whereas cells that received only DRACO did not. Caspase inhibitors eliminated DRACO-mediated apoptosis in the presence of dsRNA. https://doi.org/10.1371/journal.pone.0022572.g003

We measured the viability of normal human lung fibroblast (NHLF) cells that had been treated with PKR-Apaf DRACOs or negative controls and then challenged with 130 plaque forming units (pfu) per well rhinovirus 1B (Figures 4, S2, S3). Untreated cell populations succumbed to virus within days, indicating that any innate cellular responses were ineffective against the virus or blocked by the virus. DRACOs with PTD, TAT, and ARG tags prevented significant cytopathic effects (CPE) in virus-challenged cell populations by rapidly killing any initially infected cells, thereby terminating the infection in its earliest stages. DRACOs had no apparent toxicity in unchallenged cells. Isolated PKR 1–181 and Apaf-1 1–97 domains were nontoxic but not antiviral, even when added simultaneously (but not covalently linked). DRACO with deleterious amino acid changes also had little efficacy. Likewise, an amount of purified bacterial extract (without DRACOs) approximately 10-fold greater than the average volume of DRACOs added to cells was nontoxic and not efficacious, demonstrating that any remaining bacterial contaminants such as lipopolysaccharide did not affect the cells or produce antiviral activity. Thus the antiviral efficacy appears to require intact functional DRACOs. Tests using DRACOs with protein transduction tags on the N terminus, C terminus, or both termini indicated that N-terminal tags generally worked the best (data not shown). DRACOs with transduction tags penetrated cells and were antiviral when administered by themselves (Figures 2, S2A), but efficacy was enhanced by co-administration with Roche FuGene 6 to maximize uptake (Figure S2B), so FuGene was used in experiments unless otherwise noted. Cell viability measured 7 days post infection (dpi) showed little difference if DRACO-containing medium was removed 3 dpi after untreated cells had widespread CPE; there was no relapse of viral CPE in treated cells after DRACOs were withdrawn (Figure 4B).

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larger image TIFF original image Download: Figure 4. DRACOs were effective against rhinovirus 1B in NHLF cells. (A) 100 nM DRACO was effective against 130 pfu/well rhinovirus, whereas 100 nM negative controls were not (12 dpi). (B) Cell viability measured 7 dpi showed little difference if 100 nM DRACO-containing medium was removed 3 dpi when untreated cells had widespread CPE from 130 pfu/well rhinovirus 1B; there was no relapse of viral CPE in treated cells after DRACOs were withdrawn. (C) 1 dose of 25 nM PTD-PKR-Apaf DRACO was effective against rhinovirus 1B in NHLF cells when it was added from 6 days before infection to 3 days after infection. (Complete viral CPE in untreated cell populations required 3–4 days in our experiments, and for these experiments a significant fraction of cells were still uninfected 3 dpi.) Cell viability was measured 14 dpi. https://doi.org/10.1371/journal.pone.0022572.g004

DRACOs were added approximately 24 hours before virus unless otherwise noted, but other dosing times were tested (Figure 4C). One dose of PTD-PKR-Apaf DRACO was efficacious against rhinovirus 1B in NHLF cells when added up to 6 days before infection, supporting the western data (Figure 2C) that DRACO persisted inside cells for at least 8 days. Up to 3 days after infection, one DRACO dose could still rescue a significant percentage of the cell population. After 3 days, virtually all of the cells had already been killed or at least infected by the virus.

Additional DRACO designs exhibited efficacy against rhinovirus (Figure 5A). Other effective dsRNA detection domains included NTE3L, CTE3L, 2×E3L, and RNaseL 1–335 . Other effective apoptotic domains included FADD 1–90 , mApaf1 1–97 , and procaspases [26]–[27]. Although the initial performance of these alternate DRACOs was generally inferior to that of PKR-Apaf human DRACO in these experiments, better performance might be achieved with further optimization. These results demonstrate that the alternate DRACO designs are nontoxic and efficacious against virus, and they support the DRACO mechanism of action.

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larger image TIFF original image Download: Figure 5. DRACOs were effective against rhinovirus 1B and other viruses. (A) Multiple 100 nM DRACOs were effective against 130 pfu/well rhinovirus (4 dpi). Even better performance of these alternate DRACOs might be achieved with further optimization. (B) PKR-Apaf DRACOs reduced the viral titer in supernatant from NHLF cells challenged with 300 pfu/well rhinovirus 1B to undetectable levels. PKR and Apaf-1 domains not covalently linked increased viral titers somewhat, possibly by interfering with the antiviral activity of endogenous wild-type PKR and Apaf-1. Cells were treated with 100 nM DRACO or controls. Supernatants were collected 4 dpi and their viral titers determined by serial dilution onto fresh 96-well NHLF plates. (C) The EC 50 for PTD-PKR-Apaf DRACO was 2–3 nM against 130 pfu/well rhinovirus 1B in NHLF cells (measured 3 dpi), and 50 pfu/well murine encephalomyelitis (3 dpi) and 50 pfu/well murine adenovirus (11 dpi) in L929 cells. https://doi.org/10.1371/journal.pone.0022572.g005

In addition to improving survival of the cell population, DRACOs reduced viral titers from virus-challenged cells (Figures 5B, S4). One dose of PKR-Apaf DRACO administered to NHLF cells 24 hours before 300 pfu/well rhinovirus 1B eliminated any measurable viral titer in cell supernatant samples collected 4 dpi.

The median effective concentration for DRACOs with PTD, TAT, and ARG tags against a variety of viruses was 2–3 nM, as illustrated for PTD-PKR-Apaf DRACO against rhinovirus 1B, murine encephalomyelitis, and murine adenovirus (Figures 5C).

DRACOs were effective against a broad spectrum of other viruses in a variety of cell types (Tables 1–2). DRACOs were effective against rhinoviruses 2 and 30 in NHLF cells (data not shown) and rhinovirus 14 in HeLa cells (Figure S4). DRACOs were effective against murine adenovirus in L929 cells if added before or up to at least 72 hours after virus (Figures 6, S5), demonstrating efficacy against a DNA virus (Figures 6A, S5), in murine cells (using human apoptotic DRACO domains to recruit endogenous murine procaspases), when treatment is delayed until significantly after infection (Figure 6B), and with a variety of DRACO designs (Figure 6C). DRACOs were effective against murine encephalomyelitis in L929 cells regardless of whether the DRACO-containing medium was removed 3 dpi (Figure 7A), whether DRACOs were added before or after infection (Figure 7B), and which DRACOs were used (Figures 7C, S6). DRACOs were effective in Vero E6 cells against Amapari and Tacaribe, arenaviruses that are closely related to lymphocytic choriomeningitis virus (LCMV), Lassa, and Junin viruses (Figures 8A, S7, S8). Likewise, DRACOs were effective against Guama strain Be An 277 (Figures 8B, S9); comparable results were obtained for Guama strain Be Ar 12590 (data not shown). Guama virus is a significant human pathogen and is closely related to other bunyaviruses such as Rift Valley fever, hantavirus, and Crimean-Congo virus. DRACOs were similarly effective against dengue type 2 (New Guinea C) hemorrhagic fever virus, a major human pathogen that is very closely related to other flaviviruses such as West Nile virus, Yellow fever virus, and Omsk virus (Figures 8C, S10, S11). DRACOs were also effective against H1N1 influenza A/PR/8/34 in normal human hepatocytes (Figure S12 left), reovirus 3 in BALB/3T3 murine cells (Figure S12 center), and adenovirus 5 in AD293 cells (Figure S12 right).

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larger image TIFF original image Download: Figure 6. DRACOs were effective against murine adenovirus in L929 cells. (A) 100 nM DRACOs were effective against 50 pfu/well murine adenovirus, whereas all negative controls were not (16 dpi). (B) 100 nM PTD-PKR-Apaf DRACO was effective if added before or up to at least 72 hours after adenovirus (16 dpi). (C) Multiple 100 nM DRACOs were effective against 50 pfu/well murine adenovirus (11 dpi). Even better performance of these alternate DRACOs might be achieved with further optimization. https://doi.org/10.1371/journal.pone.0022572.g006

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larger image TIFF original image Download: Figure 7. DRACOs were effective against murine encephalomyelitis in L929 cells. (A) 100 nM DRACOs were effective against 50 pfu/well encephalomyelitis. Cell viability measured 6 dpi showed little difference if DRACO-containing medium was removed 3 dpi when untreated cells had widespread CPE; there was no relapse of viral CPE in treated cells after DRACOs were withdrawn. (B) 100 nM PTD-PKR-Apaf DRACO was effective if added before, simultaneously with, or up to at least 6 hours after encephalomyelitis. (C) Multiple 100 nM DRACOs were effective against 50 pfu/well murine encephalomyelitis (4 dpi). Even better performance of these alternate DRACOs might be achieved with further optimization. https://doi.org/10.1371/journal.pone.0022572.g007

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larger image TIFF original image Download: Figure 8. DRACOs were effective against arenaviruses, bunyaviruses, and flaviviruses. 200 nM DRACOs with PTD, TAT, and ARG protein transduction tags were effective in Vero E6 cells against (A) 30 pfu/well Amapari (assayed 15 dpi), (B) 30 pfu/well Guama strain Be An 277 (assayed 5 dpi), and (C) 10 pfu dengue type 2 (assayed 20 dpi). https://doi.org/10.1371/journal.pone.0022572.g008

DRACOs appeared promising in proof-of-concept trials with adult BALB/c mice. Intraperitoneal (i.p.) PKR-Apaf DRACO penetrated the liver, kidney, and lungs and persisted at least 24–48 hours (Figure 9A). Live mice and harvested mouse organs showed no apparent toxicity. PTD-PKR-Apaf and TAT-PKR-Apaf DRACOs administered i.p. from day -1 through day 3 greatly reduced the morbidity in mice challenged intranasally (i.n.) with 1.3 LD 50 influenza H1N1 A/PR/8/34 and reduced the day-2 lung viral titers by over an order of magnitude (Figure 9B). Similarly, PTD-RNaseL-Apaf, TAT-RNaseL-Apaf, and ARG-RNaseL-Apaf DRACOs administered i.p. from day -1 through day 3 prevented morbidity in mice challenged i.n. with 0.3 LD 50 influenza and reduced the day-2 viral titers by an order of magnitude or more (Figure 9C). PKR-Apaf DRACO administered i.n. to mice penetrated the lungs and persisted over 24 hours (Figure 10A). PTD-PKR-Apaf, TAT-PKR-Apaf, and ARG-PKR-Apaf DRACOs administered i.n. on day 0 reduced the morbidity in mice challenged i.n. with 1 LD 50 influenza (Figure 10B).

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larger image TIFF original image Download: Figure 9. DRACOs appeared promising when administered via intraperitoneal (i.p.) injection in proof-of-concept trials with adult BALB/c mice. (A) 2.5 mg PTD-PKR-Apaf DRACO administered i.p. penetrated the liver, kidney, and lungs and persisted for at least 48 hours. Averages of 3 mice per data point are plotted, and error bars show s.e.m. (B) PTD-PKR-Apaf and TAT-PKR-Apaf DRACOs administered i.p. from day -1 through day 3 greatly reduced the morbidity and day-2 lung viral titers in mice challenged intranasally (i.n.) with 1.3 LD 50 influenza H1N1 A/PR/8/34. (C) PTD-RNaseL-Apaf, TAT-RNaseL-Apaf, and ARG-RNaseL-Apaf DRACOs administered i.p. from day -1 through day 3 greatly reduced the morbidity and day-2 lung viral titers in mice challenged i.n. with 0.3 LD 50 influenza H1N1 A/PR/8/34. https://doi.org/10.1371/journal.pone.0022572.g009

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larger image TIFF original image Download: Figure 10. DRACOs appeared promising when administered via intranasal (i.n.) injection in proof-of-concept trials with adult BALB/c mice. (A) 0.5 mg PKR-Apaf DRACO administered i.n. to adult BALB/c mice penetrated the lungs and persisted over 24 hours. Averages of 3 mice per data point are plotted, and error bars show s.e.m. (B) PTD-PKR-Apaf, TAT-PKR-Apaf, and ARG-PKR-Apaf DRACOs administered i.n. on day 0 reduced the morbidity in mice challenged i.n. with 1 LD 50 influenza H1N1 A/PR/8/34. https://doi.org/10.1371/journal.pone.0022572.g010

Based on these encouraging initial animal trials, future work should be done to test and optimize antiviral efficacy, pharmacokinetics, and absence of toxicity in vitro and in vivo. Future experiments can further characterize and optimize dsRNA binding, apoptosis induction, cellular transduction, and other DRACO properties. More extensive trials are also needed to determine how long after infection DRACOs can be used successfully, or if DRACOs are useful against chronic viral infections without producing unacceptable levels of cell death in vivo.

DRACOs should be effective against numerous clinical and NIAID priority viruses, due to the broad-spectrum sensitivity of the dsRNA detection domain, the potent activity of the apoptosis induction domain, and the novel direct linkage between the two which viruses have never encountered. We have demonstrated that DRACOs are effective against viruses with DNA, dsRNA, positive-sense ssRNA, and negative-sense ssRNA genomes; enveloped and non-enveloped viruses; viruses that replicate in the cytoplasm and viruses that replicate in the nucleus; human, bat, and rodent viruses; and viruses that use a variety of cellular receptors (Table 1).