Inhibitions of human parainfluenza virus type 2 replication by ribavirin and mycophenolate mofetil are restored by guanosine and S-(4-nitrobenzyl)-6-thioinosine.

The antiviral activities of a nucleoside analog antiviral drug (ribavirin) and a non-nucleoside drug (mycophenolate mofetil) against human parainfluenza virus type 2 (hPIV-2) were investigated, and the restoration of the inhibition by guanosine and S-(4-nitrobenzyl)-6-thioinosine (NBTI: equilibrative nucleoside transporter 1 inhibitor) were also investigated. Ribavirin (RBV) and mycophenolate mofetil (MMF) inhibited cell fusion induced by hPIV-2. Both RBV and MMF considerably reduced the number of viruses released from the cells. Virus genome synthesis was inhibited by RBV and MMF as determined by polymerase chain reaction (PCR) and real time PCR. mRNA syntheses were also reduced. An indirect immunofluorescence study showed that RBV and MMF largely inhibited viral protein syntheses. Using a recombinant green fluorescence protein (GFP)-expressing hPIV-2 without matrix protein (rhPIV-2ΔMGFP), it was found that virus entry into the cells and multinucleated giant cell formation were almost completely blocked by RBV and MMF. RBV and MMF did not disrupt actin microfilaments or microtubules. Both guanosine and NBTI completely or partially reversed the inhibition by RBV and MMF in the viral replication, syntheses of genome RNA, mRNA and protein, and multinucleated giant cell formation. NBTI caused a little damage in actin microfilaments, but had no effect on microtubules. Both RBV and MMF inhibited the replication of hPIV-2, mainly by inhibiting viral genome RNA, mRNA and protein syntheses. The inhibition was almost completely recovered by guanosine. These results indicate that the major mechanism of the inhibition is the depletion of intracellular GTP pools.

Glycyrrhizin inhibits human parainfluenza virus type 2 replication by the inhibition of genome RNA, mRNA and protein syntheses.

The effect of glycyrrhizin on the replication of human parainfluenza virus type 2 (hPIV-2) was examined. Cell fusion induced by hPIV-2 was inhibited by glycyrrhizin, and glycyrrhizin reduced the number of viruses released from the cells. Glycyrrhizin did not change cell morphology at 1 day of culture, but caused some damage at 4 days, as determined by the effect on actin microfilaments. However, it affected the cell viability at 1 day: about 20% of the cells were not alive by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay at 1 day of culture. Real-time polymerase chain reaction (PCR) and PCR showed that virus genome synthesis was largely inhibited. mRNA synthesis was also inhibited by glycyrrhizin. Viral protein synthesis was largely inhibited as observed by an indirect immunofluorescence study. Multinucleated giant cell formation was studied using a recombinant green fluorescence protein (GFP)-expressing hPIV-2 without matrix protein (rhPIV-2ΔMGFP). A few single cells with fluorescence were observed, but the formation of giant cells was completely blocked. Taken together, it was shown that viral genome, mRNA and protein syntheses, including F and HN proteins, were inhibited by glycyrrhizin, and consequently multinucleated giant cell formation was not observed and the infectious virus was not detected in the culture medium.

[Inosine pranobex - cytotoxic activities and effect of on replication of human parainfluenza viruses (HPIV-2, HPIV-4), entroviruses (CA16, EV71) and adenoviruses (HAdV-2, HAdV-5) in vitro]. / Pranobeks inozyny - dzialanie cytotoksyczne oraz wplyw na replikacje ludzkich wirusów paragrypy (HPIV-2, HPIV-4), enterowirusów (CA16, EV71) i adenowirusów (HAdV-2, HAdV-5) w badaniu in vitro.

INTRODUCTION: There are no specific antivirals designed for many viral infections. Inosine pranobex (PI) is a purine nucleoside that is involved in a wide variety of intracellular biochemical processes. The mechanism of action in human body is still unclear but numerous studies have demonstrated that this drug inhibits viral replication and exhibit pleiotropic effect. We evaluated in vitro effect of inosine pranobex (PI) on replication of human viruses: parainfluenza viruses (HPIV-2, HPIV-4), entroviruses A (CA16, EV71) and adenoviruses C (HAdV-2, HAdV-5). MATERIALS AND METHODS: In the present study, cytotoxic effect of inosine pranobex was assessed using A549 cell line exposed to different concentrations of compound (PI: 50-800 ig/mL) for 48 hours. Cytotoxic effect of inosine pranobex was assessed visually using light, inverted microscopy Olympus CK2 under 400x magnification and by the MTT colorimetric assay. Antiviral effect was estimated according to the reduction of virus titer. The yield reduction assay (YRA), which evaluates the ability of the PI (50-800 µg/mL) to inhibit virus multiplication in cell cultures, was applied. The cytopathic effect of the virus was evaluated 48 h after infection ofA549 cell cultures with viruses by means of light, inverted microscopy. The Reed-Muench statistical method was used to determine the 50% end point (IC50) (yield reduction assay, YRA) in the presence of inosine pranobex with the controlled one. RESULTS: There were no morphological changes, as assessed visually, in cell cultures treated with PI. MTT cytotoxicity assay confirmed microscopic observations. The viability of cells in the presence of the tested compounds was average 98, 36 %. After conducting the experiments and analyzing the results we noticed that higher concentrations of PI strongly inhibited multiplication of all viruses. PI weakly reduced the titer of infectious enteroviruses and HPIV-4 as compared with the control. Adenoviruses showed the highest sensitivity to the antiviral activity of PI, however, increasing concentrations of PI up to 800 µg /ml slightly enhanced the antiviral activity of 400 µg/ml PI. CONCLUSIONS: Our study demonstrated that inosine pranobex shows no cytotoxic activity on the A549 cell line. In conducted study was observed that adenoviruses (HAdV-2 and HAdV-5) and HPIV-2 have the highest sensitivity to the antiviral activity of inosine pranobex from all tested viral strains.

Successful Treatment of Parainfluenza Virus Respiratory Tract Infection With DAS181 in 4 Immunocompromised Children.

BACKGROUND: Parainfluenza virus (PIV), a common pediatric pathogen, is associated with significant morbidity in immunocompromised (IC) hosts. DAS181, a novel sialidase fusion protein inhibitor, seems to be effective against PIV in vitro and in vivo; its use in IC children has not been evaluated. METHODS: Patients were diagnosed with PIV infection using a quantitative reverse transcription-polymerase chain reaction. DAS181 was obtained under emergency investigational new drug applications and was administered via aerosol chamber or nebulizer. Patients were assessed daily for their clinical condition and adverse outcomes. RESULTS: Four pediatric hematopoietic cell transplantation (HCT) patients with PIV detected in respiratory specimens were identified and treated with DAS 181. Patients 1 and 2 were diagnosed with PIV lower respiratory tract infection (LRTI) by bronchoalveolar lavage at 9 months and 2 days after allogeneic transplantation, respectively. Patient 3 was on chemotherapy prior to planned autologous HCT at time of PIV diagnosis from a nasal swab. Patient 4 was diagnosed with PIV via nasal wash 2 days after HCT. Patients 1-3 had clinical symptoms and chest imaging consistent with LRTI. Inhaled DAS181 was administered for 5-10 days. All 4 patients tolerated therapy well. Clinical improvement in oxygen requirement and respiratory rate was observed in all patients who required oxygen at therapy initiation. Viral load decreased in all patients within 1 week of therapy and became undetectable by day 3 of therapy in patient 3. CONCLUSION: DAS181 was used to treat 4 severely IC pediatric patients with PIV disease. The drug was well tolerated. Improvement in both viral loads and symptoms after initiation of therapy was observed in all cases. This report supports prospective, randomized studies in IC patients with PIV infection.

Ribavirin inhibits human parainfluenza virus type 2 replication in vitro.

The antiviral activities of eight nucleoside analog antiviral drugs (ribavirin, acyclovir, lamivudine, 3'-azido-3'-deoxythymidine, emtricitabine, tenofovir, penciclovir and ganciclovir) against human parainfluenza virus type 2 (hPIV-2) were investigated. Only ribavirin (RBV) inhibited both cell fusion and hemadsorption induced by hPIV-2. RBV considerably reduced the number of viruses released from the cells. Virus genome synthesis was inhibited by RBV, as determined by real time PCR. An indirect immunofluorescence study showed that RBV largely inhibited viral protein synthesis. mRNAs of the proteins were not detected, indicating that inhibition of protein synthesis was caused by transcription inhibition by RBV. Using a recombinant green fluorescence protein-expressing hPIV-2 without matrix protein, it was found that RBV did not completely inhibit virus entry into the cells; however, it almost completely blocked multinucleated giant cell formation. RBV did not disrupt actin microfilaments and microtubules. These results indicate that the inhibitory effect of RBV is caused by inhibition of both virus genome and mRNA synthesis, resulting in inhibition of virus protein synthesis, viral replication and multinucleated giant cell formation (extensive cell-to-cell spreading of the virus).

Inhibition of primary clinical isolates of human parainfluenza virus by DAS181 in cell culture and in a cotton rat model.

DAS181 is a novel drug in development for the treatment of influenza as well as human parainfluenza viruses (hPIVs). Previous studies demonstrated that DAS181 inhibited laboratory strains of hPIV, but no tests were conducted with primary clinical isolates of hPIV. To fill this gap, we studied six primary isolates including hPIV-2 and hPIV-3. First tests showed that the amplification of all viruses in vitro was reproducibly inhibited with DAS181 drug concentrations ranging between 0.1 and 1nM. An hPIV-3 primary clinical isolate was then tested in a cotton rat model for sensitivity to 0.3-1mg/kg drug treatments. Results showed that virus amplification in the lower respiratory tract was significantly and reproducibly inhibited by drug. Together, experiments demonstrated that DAS181 inhibited primary clinical isolates of hPIV in vitro and in vivo at doses similar to those previously described for inhibition of laboratory hPIV and influenza virus isolates.

Legume lectins inhibit human parainfluenza virus type 2 infection by interfering with the entry.

Three lectins with different sugar binding specificities were investigated for anti-viral activity against human parainfluenza virus type 2 (hPIV-2). The lectins, concanavalin A (Con A), lens culinaris agglutinin (LCA) and peanut agglutinin (PNA), inhibited cell fusion and hemadsorption induced by hPIV-2. Virus nucleoprotein (NP) gene synthesis was largely inhibited, but fusion (F) and hemagglutinin-neuraminidase (HN) gene syntheses were not. An indirect immunofluorescence study showed that Con A inhibited virus NP, F and HN protein syntheses, but LCA did not completely inhibit them, and that PNA inhibited only NP protein synthesis. Using a recombinant green fluorescence protein-expressing hPIV-2, without matrix protein (rghPIV-2ΔM), it was found that virus entry into the cells was not completely prevented. The lectins considerably reduced the number of viruses released compared with that of virus infected cells. The lectins bound to cell surface within 10 min, and many aggregates were observed at 30 min. Con A and LCA slightly disrupted actin microfilaments and microtubules, but PNA had almost no effect on them. These results indicated that the inhibitory effects of the lectins were caused mainly by the considerable prevention of virus adsorption to the cells by the lectin binding to their receptors.

Fucoidan inhibits parainfluenza virus type 2 infection to LLCMK2 cells.

The effects of fucoidan and L-fucose, a fundamental major component of fucoidan, on the growth of human parainfluenza virus type 2 (hPIV-2) in LLCMK(2) cells were investigated. Fucoidan inhibited cell fusion and hemadsorption, but L-fucose only partly inhibited both. Virus RNA was not detected in the hPIV-2 infected cells cultured with fucoidan. However, L-fucose did not inhibit virus RNA synthesis. Indirect immunofluorescence study showed that virus protein synthesis was inhibited by fucoidan, but not by L-fucose. Furthermore, using a recombinant, green fluorescence protein-expressing hPIV-2, it was found that virus entry was inhibited by fucoidan, but not by L-fucose. These results suggested that fucoidan inhibited virus adsorption to the surface of the cells by binding to the cell surface and prevented infection, indicating that the sulfated polysaccharide form was important for the inhibition by fucoidan.

Virucidal activities of medium- and long-chain fatty alcohols and lipids against respiratory syncytial virus and parainfluenza virus type 2: comparison at different pH levels.

Recent studies have shown that some lipids and fatty alcohols have microbicidal activities against a broad variety of pathogens. In this study, virucidal activities of fatty acids, monoglycerides and fatty alcohols were tested against respiratory syncytial virus (RSV) and human parainfluenza virus type 2 (HPIV2) at different concentrations, times and pH levels. The most active compounds were mixed with milk products and fruit juices and the mixtures tested for virucidal effects. The aim was to determine which compounds are the most active against these respiratory viruses and could possibly be used in pharmaceutical formulations or as additives to milk products or juice. Several compounds caused a significant inactivation of virus, and there was generally a good agreement between the activities against RSV and parainfluenza virus. By changing the pH from 7 to 4.2, the virucidal activities of some of the compounds were greatly increased, i.e., they inactivated virus in a shorter time and at lower concentrations. The most active compound tested was 1-monoglyceride of capric acid, monocaprin, which also showed activity against influenza A virus and significant virucidal activities after addition to milk products and fruit juices, even at a concentration as low as 0.06-0.12%. The significant virucidal activities of fatty alcohols and lipids on RSV and parainfluenza virus demonstrated in this in vitro study raise the question of the feasibility of using such compounds as ingredients in pharmaceutical dosage forms against respiratory infections caused by these viruses, and possibly other paramyxo- and myxoviruses.

Efficacy of novel hemagglutinin-neuraminidase inhibitors BCX 2798 and BCX 2855 against human parainfluenza viruses in vitro and in vivo.

Human parainfluenza viruses are important respiratory tract pathogens, especially of children. However, no vaccines or specific therapies for infections caused by these viruses are currently available. In the present study we characterized the efficacy of the novel parainfluenza virus inhibitors BCX 2798 and BCX 2855, which were designed based on the three-dimensional structure of the hemagglutinin-neuraminidase (HN) protein. The compounds were highly effective in inhibiting hemagglutinin (HA) and neuraminidase (NA) activities and the growth of hPIV-1, hPIV-2, and hPIV-3 in LLC-MK(2) cells. The concentrations required to reduce the activity to 50% of that of a control ranged from 0.1 to 6.0 micro M in HA inhibition assays and from 0.02 to 20 micro M in NA inhibition assays. The concentrations required to inhibit virus replication to 50% of the level of the control ranged from 0.7 to 11.5 micro M. BCX 2798 and BCX 2855 were inactive against influenza virus HA and NA and bacterial NA. In mice infected with a recombinant Sendai virus whose HN gene was replaced with that of hPIV-1 [rSV(hHN)], intranasal administration of BCX 2798 (10 mg/kg per day) and of BCX 2855 (50 mg/kg per day) 4 h before the start of infection resulted in a significant reduction in titers of virus in the lungs and protection from death. Treatment beginning 24 h after the start of infection did not prevent death. Together, our results indicate that BCX 2798 and BCX 2855 are effective inhibitors of parainfluenza virus HN and may limit parainfluenza virus infections in humans.

Antiviral potency of mistletoe (Viscum album ssp. album) extracts against human parainfluenza virus type 2 in Vero cells.

Various extracts from the leaves of mistletoe (Viscum album L. ssp. album) were investigated for their antiviral activity on human parainfluenza virus type 2 (HPIV-2) growth in Vero cells. Plant extracts were prepared using distilled water, 50% ethanol, petroleum ether, chloroform and acetone. The 50% effective dose (ED(50)) of aqueous extract for HPIV-2 replication was 0.53 +/- 0.12 micro g/mL, and the antiviral index (AI), which was based on the ratio of the 50% inhibitory concentration (CD(50)) for host cell viability to the ED(50) for parainfluenza virus replication, was 10.05. The aqueous extract was found to be the most selective inhibitor. Furthermore, the aqueous extract at a concentration of 1 micro g/mL was found to inhibit HPIV-2 replication and the virus production was suppressed to more than 99% without any toxic effect on host cells. The chloroform extract was also found to be moderately active. In an effort to further analyse the mechanism of antiviral activity, the effectiveness of the aqueous extract on different steps of virus replication was examined. The antiviral activity could neither be attributed to the direct inactivation of the HPIV-2 nor to the inhibition of adsorption to Vero cells. The active aqueous extract has shown a dose-dependent antiviral activity on virus replication.

Degradation of STAT1 and STAT2 by the V proteins of simian virus 5 and human parainfluenza virus type 2, respectively: consequences for virus replication in the presence of alpha/beta and gamma interferons.

Human cell lines were isolated that express the V protein of either simian virus 5 (SV5) or human parainfluenza virus type 2 (hPIV2); the cell lines were termed 2f/SV5-V and 2f/PIV2-V, respectively. STAT1 was not detectable in 2f/SV5-V cells, and the cells failed to signal in response to either alpha/beta interferons (IFN-alpha and IFN-beta, or IFN-alpha/beta) or gamma interferon (IFN-gamma). In contrast, STAT2 was absent from 2f/PIV2-V cells, and IFN-alpha/beta but not IFN-gamma signaling was blocked in these cells. Treatment of both 2f/SV5-V and 2f/PIV2-V cells with a proteasome inhibitor allowed the respective STAT levels to accumulate at rates similar to those seen in 2fTGH cells, indicating that the V proteins target the STATs for proteasomal degradation. Infection with SV5 can lead to a complete loss of both phosphorylated and nonphosphorylated forms of STAT1 by 6 h postinfection. Since the turnover of STAT1 in uninfected cells is longer than 24 h, we conclude that degradation of STAT1 is the main mechanism by which SV5 blocks interferon (IFN) signaling. Pretreatment of 2fTGH cells with IFN-alpha severely inhibited both SV5 and hPIV2 protein synthesis. However, and in marked contrast, pretreatment of 2fTGH cells with IFN-gamma had little obvious effect on SV5 protein synthesis but did significantly reduce the replication of hPIV2. Pretreament with IFN-alpha or IFN-gamma did not induce an antiviral state in 2f/SV5-V cells, indicating either that the induction of an antiviral state is completely dependent on STAT signaling or that the V protein interferes with other, STAT-independent cell signaling pathways that may be induced by IFNs. Even though SV5 blocked IFN signaling, the addition of exogenous IFN-alpha to the culture medium of 2fTGH cells 12 h after a low-multiplicity infection with SV5 significantly reduced the subsequent cell-to-cell spread of virus. The significance of the results in terms of the strategy that these viruses have evolved to circumvent the IFN response is discussed.

High resistance of human parainfluenza type 2 virus protein-expressing cells to the antiviral and anti-cell proliferative activities of alpha/beta interferons: cysteine-rich V-specific domain is required for high resistance to the interferons.

Human parainfluenza type 2 virus (hPIV-2)-infected HeLa (HeLa-CA) cells and hPIV-2 V-expressing HeLa (HeLa-V) cells show high resistance to alpha/beta interferons (IFN-alpha/beta) irrespective of whether vesicular stomatitis virus or Sindbis virus is used as a challenge virus. When Sindbis virus is used, these cells show high susceptibility to human IFN-gamma. Furthermore, the multiplication of HeLa-V cells is not inhibited by IFN-alpha/beta. HeLa cells expressing the N-terminally truncated V protein show resistance to IFN-alpha/beta, showing that the IFN resistance determinant maps to the cysteine-rich V-specific domain. A complete defect of Stat2 is found in HeLa-CA and HeLa-V cells, whereas the levels of Stat1 expression are not significantly different among HeLa, HeLa-CA, HeLa-P, and HeLa-V cells, indicating that IFN-alpha/beta resistance of HeLa-CA and HeLa-V cells is due to a defect of Stat2. HeLa-SV41V cells show high resistance to all IFNs, and no expression of Stat1 can be detected. Stat2 mRNA is fully detected in HeLa-V cells. Stat2 was scarcely pulse-labeled in the HeLa-V cells, indicating that synthesis of Stat2 is suppressed or Stat2 is very rapidly degraded in HeLa-V cells. The V protein suppresses the in vitro translation of Stat2 mRNA more extensively than that of Stat1 mRNA. An extremely small amount of Stat2 can be detected in HeLa-V cells treated with proteasome inhibitors. The half-life of Stat2 is approximately 3.5 and 2 h in uninfected and hPIV-2-infected HeLa cells, respectively. This study shows that synthesis of Stat2 may be suppressed and Stat2 degradation is also enhanced in hPIV-2-infected HeLa and HeLa-V cells.

Antiviral activity of Sanicula europaea L. extracts on multiplication of human parainfluenza virus type 2.

The antiviral activity of Sanicula europaea L. extracts against human parainfluenza virus type 2 (HPIV-2) was examined. The extract prepared from the leaves of the plant and a fraction separated from the crude extract with gel filtration chromatography were found to inhibit HPIV-2 replication without any toxic effect on Vero cells. The acidic fraction obtained from the crude extract of S. europaea leaves was found to be the most active fraction with plaque inhibition assay at non-cytotoxic concentrations. Unfortunately, antiviral activity was not detected in the molecules purified from the crude ethanol extract of Sanicula leaves.

Enhancement of human parainfluenza virus-induced cell fusion by pradimicin, a low molecular weight mannose-binding antibiotic.

Oligosaccharides, especially mannose residues, expressed on the cell surface, are thought to be important for virus-induced membrane fusion. We examined the effect of mannose-binding compounds, pradimicin derivative BMY-28,864 (PRM) and concanavalin A (Con A), on cell fusion of human parainfluenza type 2 virus (hPIV2)-infected HeLa cells. Syncytium formation of hPIV2-infected HeLa cells was suppressed in the presence of Con A. On the other hand, PRM enhanced cell fusion of hPIV2-infected HeLa cells. These effects were blocked by addition of mannose-rich mannan. However, PRM shows little effect on virus growth and the expression of viral glycoproteins on the cell surface in hPIV2-infected HeLa cells. Fluorescein-isothiocyanate-labeled pradimicin and Con A bound to both uninfected and hPIV2-infected mononuclear cells, indicating that these compounds have an affinity to several cellular component(s). In contrast to Con A, PRM had little affinity to the viral glycoproteins. It is inferred from these results that the enhancement of hPIV2-induced cell fusion is probably due to the interaction between PRM and cellular component(s).

Antiviral effect of 6-diazo-5-oxo-L-norleucine, antagonist of gamma-glutamyl transpeptidase, on replication of human parainfluenza virus type 2.

The antiviral effects of 6-diazo-5-oxo-L-norleucine (L-DON) on the replication of human parainfluenza virus type 2 (HPIV-2), mumps and vesicular stomatitis viruses were studied. L-DON suppressed growth of these viruses and, in particular, HPIV-2 in four cell types. L-DON was not toxic to the cells at the active dose and did not significantly inhibit cellular macromolecular synthesis. The L-DON-sensitive step of HPIV-2 replication was considered to be relatively early. The NP, P and M proteins were, although at a low level, clearly detectable in HPIV-2-infected Vero cells treated with L-DON, whereas the HN and F proteins were scarcely detected by either immunostaining or immunoprecipitation, indicating that L-DON mainly decreased the amounts of viral glycoproteins. Furthermore, Northern blot hybridization showed that secondary transcription of virus RNA was also inhibited.

In vitro effect of ascorbic acid on infectivity of herpesviruses and paramyxoviruses.

Suspensions of herpes simplex virus types 1 and 2, cytomegalovirus, and parainfluenzavirus type 2 were inactivated within 24 h when treated at 37 degrees C with 1 mg (5.05 mM) of copper-catalyzed sodium ascorbate per ml. The infectivity titer of respiratory syncytial virus was reduced substantially after 24 h but required 48 h for inactivation. Under these conditions, inactivation of these viruses was also successfully achieved with 5.68 mM catalyzed ascorbic acid. Copper (Cu2+), when added with the ascorbate solution at 5 micrograms/ml (0.022 mM), exhibited a catalytic effect on the inactivation of these viruses. The rate of inactivation was affected by the incubation temperature, time of exposure, and the virus concentration. Ascorbate concentrations as high as 10 mg/ml (50.5 mM) demonstrated only a minimum increase in effect on viral inactivation. The loss of infectivity did not alter either the hemagglutination or complement fixation qualities of the antigens.

Comparative inhibition of influenza and parainfluenza virus replication by ribavirin in MDCK cells.

Actinomycin D inhibited the yield of influenza virus hemagglutinin from MDCK cells infected at high multiplicity, but had little effect on the yield of parainfluenza virus hemagglutinin. In similar hemagglutinin yield experiments, ribavirin was only slightly more active (threefold) against influenza virus than against parainfluenza virus replication. In plaque inhibition experiments, ribavirin depressed influenza virus plaque formation by 50% at a concentration of approximately 3 micrograms/ml, whereas the corresponding figure for parainfluenza viruses was threefold higher. The concentration of ribavirin demonstrating anti-influenza activity was indistinguishable for that inhibiting host cell growth. It is concluded that, unlike actinomycin D, ribavirin is unlikely to have a major effect on the provision of host cell 5-germinal methylated cap structures and the subsequent priming of influenza messenger ribonucleic acid synthesis.

In vitro antiviral efficacy of ribavirin against feline calicivirus, feline viral rhinotracheitis virus, and canine parainfluenza virus.

Ribavirin had marked in vitro activity against feline calcivirus, strain 255, and canine parainfluenza virus, but showed only slight antiviral effect on feline viral rhinotracheitis virus. Antiviral activity was manifested by partial to complete suppression of viral cytopathic effect and of viral replication, depending on concentration of ribavirin in the culture medium and dosage of viral inoculum. Concentrations of ribavirin as small as 3.2 microgram/ml and 1.0 microgram/ml showed some activity against feline calcivirus and canine parainfluenza virus, respectively.