Use of cotton rats to evaluate the efficacy of antivirals in treatment of measles virus infections

The Nucleoside Analog GS-441524 Effectively Attenuates the In Vitro Replication of Multiple Lineages of Circulating Canine Distemper Viruses Isolated from Wild North American Carnivores

Canine distemper is a severe and lethal viral disease of dogs and wild carnivores with an urgent need for the identification of effective antiviral agents against canine distemper virus (CDV). We assessed multiple agents for their ability to block the replication of three different lineages of CDV isolated from wild carnivores in the United States. Six antiviral compounds were selected after preliminary experiments that excluded ribavirin, hesperidin and rutin: a protease inhibitor (nirmatrelvir), a polymerase inhibitor (favipiravir) and four nucleoside analogs (remdesivir, GS-441524, EIDD2801 and EIDD1931). Antiviral efficacy was determined by the attenuation of the cytopathic effect in a CDV-susceptible cell line and the inhibition of viral RNA replication. The nucleoside analog GS-441524 effectively blocked the replication of CDV at pharmacologically relevant concentrations. Four other antiviral agents inhibited CDV replication to a lesser degree (remdesivir, nirmatrelvir, EIDD2801 and EIDD1931). The replication of different viral lineages was differentially inhibited by the antivirals. Several of the nucleoside analogs have been safely used previously in carnivore species for the treatment of other viral diseases, suggesting that they may be promising candidates for the treatment of canine distemper in dogs. Our results emphasize the need to consider different viral lineages in the screening of antiviral compounds.

Measles Resurgence and Drug Development

Measles caused an estimated minimum of one million fatalities annually before vaccination. Outstanding progress towards controlling the virus has been made since the measles vaccine was introduced, but reduction of measles case-fatalities has stalled at around 100,000 annually for the last decade and a 2019 resurgence in several geographical regions threatens some of these past accomplishments. Whereas measles eradication through vaccination is feasible, a potentially open-ended endgame of elimination may loom. Other than doubling-down on existing approaches, is it worthwhile to augment vaccination efforts with antiviral therapeutics to solve the conundrum? This question is hypothetical at present, since no drugs have yet been approved specifically for the treatment of measles, or infection by any other pathogen of the paramyxovirus family. This article will consider obstacles that have hampered anti-measles and anti-paramyxovirus drug development, discuss MeV-specific challenges of clinical testing, and define drug properties suitable to address some of these problems.

Inhibitors of Measles Virus

Measles virus (MV) infections have been almost eradicated in some industrialized nations. However, MV continues to cause severe disease and mortality in the world and is responsible for clusters of exogenous-borne disease in essentially disease-free countries. Because of the ebb and flow of immunization campaigns, especially in the poverty-stricken and war-torn Third World, and the ominous potential for severe disease and mortality, it is vital that research for discovery of therapeutic countermeasures should continue. To that end, a number of compounds have been evaluated for efficacy in vitro and in animal models, and several therapeutic modalities have been tested in the clinic. The only current therapies used in the clinic include ribavirin administered orally or intravenously, alone or in combination with immune serum globulin; these therapies have demonstrated variable efficacy. Therefore, drug discovery efforts have been launched to supplement the existing treatments for MV infections. Antisense molecules, adenosine and guanosine nucleosides, including ring-expanded ‘fat’ nucleoside analogues, brassinosteroids, coumarins, peptide inhibitors, modulators of cholesterol synthesis and a variety of natural products have been screened for efficacy and toxicity both in vitro and in animals. However, none of these agents has gone into human clinical trials and most will not merit further development due to toxicity concerns and/or low potency. Thus, further research is needed to develop more potent and less toxic drugs that could be used for treating MV infections to supplement the existing MV vaccine campaigns.

Epithelial cell lines of the cotton rat (Sigmodon hispidus) are highly susceptible in vitro models to zoonotic Bunya-, Rhabdo-, and Flaviviruses

Background

Small mammals such as bats and rodents have been increasingly recognized as reservoirs of novel potentially zoonotic pathogens. However, few in vitro model systems to date allow assessment of zoonotic viruses in a relevant host context. The cotton rat (Sigmodon hispidus) is a New World rodent species that has a long-standing history as an experimental animal model due to its unique susceptibility to human viruses. Furthermore, wild cotton rats are associated with a large variety of known or potentially zoonotic pathogens.

Methods

A method for the isolation and culture of airway epithelial cell lines recently developed for bats was applied for the generation of rodent airway and renal epithelial cell lines from the cotton rat. Continuous cell lines were characterized for their epithelial properties as well as for their interferon competence. Susceptibility to members of zoonotic Bunya-, Rhabdo-, and Flaviviridae, in particular Rift Valley fever virus (RVFV), vesicular stomatitis virus (VSV), West Nile virus (WNV), and tick-borne encephalitis virus (TBEV) was tested. Furthermore, novel arthropod-derived viruses belonging to the families Bunya-, Rhabdo-, and Mesoniviridae were tested.

Results

We successfully established airway and kidney epithelial cell lines from the cotton rat, and characterized their epithelial properties. Cells were shown to be interferon-competent. Viral infection assays showed high-titre viral replication of RVFV, VSV, WNV, and TBEV, as well as production of infectious virus particles. No viral replication was observed for novel arthropod-derived members of the Bunya-, Rhabdo-, and Mesoniviridae families in these cell lines.

Conclusion

In the current study, we showed that newly established cell lines from the cotton rat can serve as host-specific in vitro models for viral infection experiments. These cell lines may also serve as novel tools for virus isolation, as well as for the investigation of virus-host interactions in a relevant host species.

Synergistic induction of interferon α through TLR-3 and TLR-9 agonists identifies CD21 as interferon α receptor for the B cell response

Maternal antibodies inhibit seroconversion and the generation of measles virus (MeV)-specific antibodies (both neutralizing and non-neutralizing antibodies) after vaccination whereas T cell responses are usually unaffected. The lack of seroconversion leaves individuals susceptible to vaccine-preventable infections. Inhibition of antibody secretion is due to the inhibition of B cells through a cross-link of the B cell receptor with the inhibitory FcγIIB receptor (CD32) by maternal antibody/vaccine complexes. Here, we demonstrate that a combination of TLR-3 and TLR-9 agonists induces synergistically higher levels of type I interferon in vitro and in vivo than either agonist alone. The synergistic action of TLR-3 and TLR-9 agonists is based on a feedback loop through the interferon receptor. Finally, we have identified CD21 as a potential receptor for interferon α on B cells which contributes to interferon α-mediated activation of B cells in the presence of maternal antibodies. The combination leads to complete restoration of B cell and antibody responses after immunization in the presence of inhibitory MeV-specific IgG. The strong stimulatory action of type I interferon is due to the fact that type I interferon uses not only the interferon receptor but also CD21 as a functional receptor for B cell activation.

Maternal antibodies provide protection against infection with pathogens early in life but also interfere with vaccination. This interference is caused by a vaccine/maternal antibody complex which links the B cell receptor to the inhibitory CD32 molecule. Here, we show that this cross-link results in impaired B cell activation and proliferation which is correlated with diminished antibody responses. We also found that induction of large amounts of type I interferon restores the neutralizing antibody response in the presence of maternal antibodies. The best induction of type I interferon was accomplished by a combination of known activators of interferon secretion (a combination of TLR-3 and TLR-9 agonists). The strong stimulation by interferon is due to the previously unappreciated role of CD21 as functional receptor for interferon alpha. Our findings demonstrate that the dual receptor usage of type I interferon receptor and CD21 is crucial for B cell activation in the presence of maternal antibodies. This study suggests that measles vaccine, and potentially other vaccines, may induce optimal antibody responses when they are reconstituted with TLR-3 and TLR-9 agonists and thus these agonists may have great potential for clinical use.

Measles control--can measles virus inhibitors make a difference?

Infection by measles virus (MV) is a major cause of human morbidity and mortality worldwide. In 2001, the WHO, UNICEF and their partners launched the Measles Initiative, the goals of which are to interrupt the transmission of MV in large geographic areas by increasing vaccination coverage and to assess the feasibility of eradicating MV worldwide. An estimated 74% reduction in mortality resulting from measles was achieved between 2000 and 2007, equivalent to a reduction of approximately 200,000 deaths annually. Despite this progress in the control of measles, the highest number of measles cases in more than a decade was observed in 2008 in several European countries and the US, and the virus was again declared endemic in the UK. In the light of this resurgence in the UK and the limitations associated with the current live-attenuated vaccine, this review discusses the means by which safe and effective measles antivirals could augment vaccination and strengthen global efforts to control measles. Important aspects of treatment are the potential to prevent infection effectively after exposure to MV, the improvement of case management, the amelioration of complications that frequently follow MV infection and the influence of antivirals on a potential strategy for global measles eradication.

Current animal models: cotton rat animal model

The cotton rat (Sigmodon hispidus) model has proven to be a suitable small animal model for measles virus pathogenesis to fill the niche between tissue culture and studies in macaques. Similar to mice, inbred cotton rats are available in a microbiologically defined quality with an ever-increasing arsenal of reagents and methods available for the study of infectious diseases. Cotton rats replicate measles virus in the respiratory tract and (depending on virus strain) in lymphoid organs. They can be infected with vaccine, wild-type, and recombinant measles viruses and have been used to study viruses with genetic modifications. Other areas of study include efficacy testing of antivirals and vaccines. The cotton rat also has been an informative animal model to investigate measles virus-induced immune suppression and suppression of vaccination by maternal antibodies. In addition, the cotton rat promises to be a useful model for the study of polymicrobial disease (interaction between measles virus and secondary pathogens).

NIAID resources for developing new therapies for severe viral infections

Severe viral infections, including hemorrhagic fever and encephalitis, occur throughout the world, but are most prevalent in developing areas that are most vulnerable to infectious diseases. Some of these can also infect related species as illustrated by the threatened extinction of gorillas by Ebola infection in west and central Africa. There are no safe and effective treatments available for these serious infections. In addition to the logistical difficulties inherent in developing a drug for infections that are sporadic and occur mainly in the third world, there is the overwhelming barrier of no hope for return on investment to encourage the pharmaceutical industry to address these unmet medical needs. Therefore, the National Institute of Allergy and infectious Disease (NIAID) has developed and supported a variety of programs and resources to provide assistance and lower the barrier for those who undertake these difficult challenges. The primary programs relevant to the development of therapies for severe viral infections are described and three case studies illustrate how they have been used. In addition, contact information for accessing these resources is supplied.

In vitro efficacy of ribavirin against canine distemper virus

Despite vaccination, canine distemper virus (CDV) remains one of the important pathogen of dogs with worldwide distribution. Ribavirin (RIB) inhibits replication of measles virus (MV), a morbillivirus closely related to CDV, both in vitro and in vivo. In this report the antiviral activity of RIB against CDV in cell cultures was assessed. Quantitative real-time RT-PCR was used to measure viral RNA in VERO cells infected by CDV and to evaluate the inhibitory effects of RIB. RIB caused a dose- and time-dependent decrease in accumulation of CDV RNA when added after virus adsorption. RIB was highly effective in preventing CDV replication at low concentrations with 50% virus-inhibitory concentrations ranging from 0.02 to 0.05 mM. Such low values were comparable to values displayed by highly susceptible strains of MV. In addition, CDV was passaged sequentially in VERO cell monolayers in the presence of RIB to trigger viral extinction. The virus was no longer detected after three passages, suggesting that error catastrophe is one of the modes of action of RIB against CDV. These findings suggest RIB as a promising tool for the therapy of CD in dogs.

Respiratory syncytial virus infections: Recent prospects for control

Respiratory syncytial virus (RSV) infections remain a significant public health problem throughout the world, although recently developed and clinically approved anti-RSV antibodies administered prophylactically to at-risk populations appear to have significantly affected the disease development. Much effort has been expended to develop effective anti-RSV therapies, using both in vitro assay systems and mouse, cotton rat, and primate models, with several products now in various stages of clinical study. Several products are also being considered for the treatment of clinical symptoms of RSV. In this review, updates on the status of the approved anti-RSV antibodies, ribavirin, and recent results of studies with potential new anti-RSV compounds are summarized and discussed.

Effect of ribavirin and glucocorticoid treatment in a mouse model of human metapneumovirus infection

Human metapneumovirus (hMPV)-infected BALB/c mice were treated with ribavirin (40 mg/kg of body weight twice a day intraperitoneally), corticosterone (0.2 mg/ml in water), or both modalities. Ribavirin significantly decreased both hMPV replication in lungs (by 5 log10) and global pulmonary inflammation on day 5 postinfection, whereas glucocorticoids reduced only alveolar and interstitial inflammation, compared to controls.

The cotton rat provides a novel model to study genital herpes infection and to evaluate preventive strategies

Prevention of genital herpes and other sexually transmitted infections (STI) is a critical health priority because of the overwhelming impact on women and infants and the epidemiological association with human immunodeficiency virus (HIV)/AIDS. Small animal models are essential for evaluating strategies for prevention or treatment of STI. Neither the murine nor the guinea pig model of genital herpes fully recapitulates human disease. We demonstrate that herpes simplex virus type 2 (HSV-2) readily infects inbred cotton rats (Sigmodon hispidus). Consistent infection does not require pretreatment with medroxyprogesterone, and primary disease resembles that observed in humans. The animals develop genital lesions and fully recover. During primary infection, viral DNA is also detected in liver, lungs, brain, and kidneys. Clinical self-limited recurrences occur spontaneously but may also be induced by dexamethasone. Pretreatment of cotton rats with PRO 2000 gel, a candidate vaginal microbicide being evaluated in clinical trials to prevent HSV and HIV, protects cotton rats from HSV. Together, these studies suggest that the cotton rat may provide an excellent model to study genital herpes and to evaluate preventive strategies.

Primary infection of mice with high titer inoculum respiratory syncytial virus: characterization and response to antiviral therapy

Intranasal infection of BALB/c mice with respiratory syncytial virus (RSV)-A2 (0.5 x 10(8) - 2.0 x 10(8) plaque-forming units, PFU) produced disease characterized by weight loss (2-3 g) and mortality (60%-100%) with the mean day of death ranging from 6-7 d after infection. The extent of RSV disease was inoculum titer-dependent and required a replication competent virus. Lung titers of virus peaked at 0.5-1 x 10(6) PFU/g wet weight. Bronchoalveolar lavage fluid (BALF) levels of IL-1beta, TNF-alpha, INF-gamma IL-12, IL-6, MIP-1alpha, RANTES, and protein were elevated, whereas IL-2, IL-4, IL-5, IL-13, and IL-10 were unchanged. Histological assessment of lungs revealed marked inflammatory pathology characterized by bronchiolitis, vasculitis, and interstitial pneumonia. Whole-body plethysmography revealed significant disease-associated deficits of respiratory function. Therapy with ribavirin administered either by the intranasal, subcutaneous, or oral route significantly reduced disease in a dose-dependent manner. Delaying the initiation of therapy resulted in a loss of activity for ribavirin. Synagis administered either intramuscularly as a single dose in prophylaxis or intranasally in prophylaxis, followed by therapy, also significantly reduced disease in a dose-dependent manner. Infection of mice with a high titer inoculum of RSV-A2 resulted in severe and fatal pulmonary disease that was responsive to treatment. This model may be useful to characterize the in vivo activity of experimental therapies for RSV infection.

Ribavirin and lethal mutagenesis of poliovirus: molecular mechanisms, resistance and biological implications

Positive strand RNA virus populations are a collection of similar but genetically different viruses. They exist as viral quasispecies due to the high mutation rates of the low fidelity viral RNA-dependent RNA polymerase (RdRp). It is thought that this genomic heterogeneity is advantageous to the population, allowing for adaptation to rapidly changing environments that present varying types and degrees of selective pressure. However, one consequence of this extensive diversity is the susceptibility to mutagens that further increase sequence variation. Since RNA viruses live at the edge of maximal variability, an increase in the mutation rate is likely to force the virus beyond the tolerable mutation frequency into 'error catastrophe'. One such mutagen, ribavirin, is an antiviral nucleoside analog that is mutagenic to several RNA viruses. Ribavirin is incorporated into the viral genome causing lethal mutagenesis and a subsequent decrease in the specific infectivity. Even so, passaging poliovirus in the presence of low to intermediate concentrations of the drug leads to the emergence of a viral population resistant to the effects of ribavirin. These viruses have a point mutation in the RdRp that increases the overall polymerase fidelity. Interestingly, as predicted by the quasispecies theory, ribavirin resistant viruses are less adaptable, as they are more susceptible to other non-mutagenic antiviral drugs and are highly attenuated in vivo. Here, we review the mechanism of action of ribavirin on poliovirus and other RNA viruses, the possibility for escape via increased fidelity of the viral polymerase, the consequences of this response on viral population dynamics, and the biological implications for the therapeutic use of mutagenic antiviral agents.

Oral efficacy of a respiratory syncytial virus inhibitor in rodent models of infection

BMS-433771 is a potent inhibitor of respiratory syncytial virus (RSV) replication in vitro. Mechanism of action studies have demonstrated that BMS-433771 halts virus entry through inhibition of F protein-mediated membrane fusion. BMS-433771 also exhibited in vivo efficacy following oral administration in a mouse model of RSV infection (C. Cianci, K. Y. Yu, K. Combrink, N. Sin, B. Pearce, A. Wang, R. Civiello, S. Voss, G. Luo, K. Kadow, E. Genovesi, B. Venables, H. Gulgeze, A. Trehan, J. James, L. Lamb, I. Medina, J. Roach, Z. Yang, L. Zadjura, R. Colonno, J. Clark, N. Meanwell, and M. Krystal, Antimicrob. Agents Chemother. 48:413-422, 2004). In this report, the in vivo efficacy of BMS-433771 against RSV was further examined in the BALB/c mouse and cotton rat host models of infection. By using the Long strain of RSV, prophylactic efficacy via oral dosing was observed in both animal models. A single oral dose, administered 1 h prior to intranasal RSV inoculation, was as effective against infection as a 4-day b.i.d. dosing regimen in which the first oral dose was given 1 h prior to virus inoculation. Results of dose titration experiments suggested that RSV infection was more sensitive to inhibition by BMS-433771 treatment in the BALB/c mouse host than in the cotton rat. This was reflected by the pharmacokinetic and pharmacodynamic analysis of the efficacy data, where the area under the concentration-time curve required to achieve 50% of the maximum response was approximately 7.5-fold less for mice than for cotton rats. Inhibition of RSV by BMS-433771 in the mouse is the result of F1-mediated inhibition, as shown by the fact that a virus selected for resistance to BMS-433771 in vitro and containing a single amino acid change in the F1 region was also refractory to treatment in the mouse host. BMS-433771 efficacy against RSV infection was also demonstrated for mice that were chemically immunosuppressed by cyclophosphamide treatment, indicating that compound inhibition of the virus did not require an active host immune response.

The cotton rat: an underutilized animal model for human infectious diseases can now be exploited using specific reagents to cytokines, chemokines, and interferons

The cotton rat represents the best or only animal model for a large number of human infectious diseases, and it may be unique among small laboratory animals in its susceptibility to several potential agents of bioterrorism. Although the cotton rat is a reliable model to define pathologic changes produced during infection with human pathogens, the lack of specific reagents has precluded a more extensive analysis of the molecular basis of pathogenesis. Here, we report the cloning of 24 cotton rat genes encoding various cytokines, chemokines, and interferons (IFNs). Analysis of the expression of most of these genes was performed by RT-PCR in cotton rat macrophages during treatment with lipopolysaccharide (LPS) and in cotton rat lungs after infection with influenza virus. The availability of these reagents will provide the tools for molecular analysis of pathogenesis and immune responses to a wide variety of pathogens and set the basis for the development of new prophylactic and therapeutic strategies against human infectious diseases.

Synthesis and anti-measles virus activity of new isoquinolin-4-one derivatives

Despite intense efforts to increase vaccine coverage, measles virus (MV) still causes significant morbidity and mortality in the world sometimes as a results of severe, chronic and lethal diseases. In an effort to develop therapies to supplement immunization strategies a number of 1-oxo-2-[[(1E)-phenylmethylene]amino]-1,2-dihydroisoquinoline-4-carboxylic acid derivatives were synthesized and evaluated for anti-measles activity. The substituents on the aromatic ring were chosen in order to evaluate the influence of electron-withdrawing or electron-donating effects on the electronic density of the aromatic moiety. We also evaluated the introduction of a vinyl chain between the exocyclic nitrogen and phenyl moiety. The biological results allow to outline some preliminary considerations on structure-activity relationship.

Comparison of the inhibition of human metapneumovirus and respiratory syncytial virus by ribavirin and immune serum globulin in vitro

Human metapneumovirus (hMPV) is a newly recognized pathogen that like its better-known relative, human respiratory syncytial virus (hRSV), appears to be ubiquitous and an important cause of respiratory disease in diverse subpopulations. No antivirals or vaccines are currently approved for the treatment or prevention of hMPV infections. However, ribavirin is licensed to treat serious hRSV-induced infections in children and immune globulin designed for intravenous administration (i.v.IG) and palivizumab (Synagis), a humanized monoclonal antibody preparation, have been utilized as alternatives to vaccines for preventing or reducing the severity of infections caused by this virus. Because both ribavirin and i.v.IG have broad viral specificities, studies were performed to compare the ability of these two agents to inhibit the replication of hRSV and hMPV in tissue culture-based assays. Two experimental chemotherapeutic agents (i.e. VP14637 and JNJ2408068) and different antibody preparations were included in this testing for comparison. Ribavirin and the i.v.IG utilized were found to have equivalent antiviral activity against hMPV and hRSV. In contrast, except for antisera specifically raised against hMPV, all of the other materials tested had marked activity only against hRSV.

Experimental vaccines against measles in a world of changing epidemiology

Vaccination with the current live attenuated measles vaccine is one of the most successful and cost-effective medical interventions. However, as a result of persisting maternal antibodies and immaturity of the infant immune system, this vaccine is poorly immunogenic in children <9 months old. Immunity against the live vaccine is less robust than natural immunity and protection less durable. There may also be some concern about (vaccine) virus spread during the final stage of an eventual measles eradication program. Opinions may differ with respect to the potential threat that some of these concerns may be to the World Health Organisation goal of measles elimination, but there is a consensus that the development of new measles vaccines cannot wait. Candidate vaccines are based on viral or bacterial vectors expressing recombinant viral proteins, naked DNA, immune stimulating complexes or synthetic peptides mimicking neutralising epitopes. While some of these candidate vaccines have proven their efficacy in monkey studies, aerosol formulated live attenuated measles vaccine are evaluated in clinical trials.

Antiviral activity of brassinosteroids derivatives against measles virus in cell cultures

Twenty-seven brassinosteroid derivatives were tested for antiviral activity against measles virus (MV) via a virus-yield reduction assay. Compounds 6b [(22S,235)-3beta-bromo-5alpha,22,23-trihydroxystigmastan-6-one], 1d [(22R,23R)-2alpha,3alpha,22,23-tetrahydroxy-beta-Homo-7-oxa-stigmastan-6-one], 8a [(22R,23R)-3beta-fluoro-22,23-dihydroxystigmastan-6-one], 9b [(22S,23S)-3beta-fluoro-5alpha,22,23-trihydroxystigmastan-6-one] and 10b [(22S,23S)-5alpha-fluor-3beta,22,23-trihydroxystigmastan-6-one], with selectivity indexes (SI) of 40, 57, 31, 37 and 53, are the derivatives with good antiviral activity against MV. These SI values are higher than those obtained with ribavirin (used as reference drug). A comparative analysis of 50% cytotoxic concentration (CC50) values, using confluent non-growing cells, gives and indication of structure-activity relationship. According to their degree of cytotoxicity the compounds were divided in three groups: low, intermediate and high cytotoxicity. By observing the chemical structures of compounds belonging to the first group we can see that less cytotoxic activities are related to the presence of a 3beta-hydroxy group on C-3 (ring A) and a double bond between C-22 and C-23 (side chain). The replacement of a 5alpha-hydroxy group by a 5alpha-fluoro group enhances cytotoxicity. Halogenated brassinosteroid derivatives in C-3 position are more cytotoxic than those with an acetoxy group in the same position. For compounds 1d, 6b, 10b and ribavirin, cytotoxicity measurements were also done with replicating cells; CC50 values were low, but they still competed favourably with ribavirin against MV.

Coumarins and pyranocoumarins, potential novel pharmacophores for inhibition of measles virus replication

A series of coumarin and pyranocoumarin analogues were evaluated in vitro for antiviral efficacy against measles virus (MV), strain Chicago. Of the 22 compounds tested for inhibition, six were found to have selectivity indices greater than 10. These were compounds 5-hydroxy-7-propionyloxy-4-propylcoumarin (2a), 5,7-bis(tosyloxy)-4-propylcoumarin (7); 5-hydroxy-4-propyl-7-tosyloxy-coumarin (8); 6,6-dimethyl-9-propionyloxy-4-propyl-2H,6H-benzo[1,2-b:3,4-b']dipyran-2-one (9); 6,6-dimethyl-9-pivaloyloxy-4-propyl-2H,6H-benzo[1,2-b:3,4-b']dipyran-2-one (10); and 7,8-cis-10,11,12-trans-4-propyl-6,6,10,11-tetramethyl-7,8,9-trihydroxy-2H,6H,12H-benzo[1 ,2-b:3,4-b':5,6-b'']tripyran-2-one (18). Three of the active drugs were propyl coumarin analogues (2a, 7 and 8), two were dipyranone or chromeno-coumarins (9 and 10), and one was a benzotripyranone with a coumarin nucleus (18). Some appeared to be rather specific and potent inhibitors of MV with EC50 values ranging from 0.2 to 50 microg/ml and the majority of the EC50 values being less than 5 pg/ml. The compounds inhibited an additional nine strains of MV, and in virucidal tests the drugs did not physically disrupt the virion to inhibit virus replication. The inhibitory activity for one of the compounds tested (7) was somewhat dependent on virus concentration and it was still active when added to cells up to 24 h after virus exposure. When used in combination with ribavirin, compound 7 appeared not to profoundly affect the antiviral efficacy of ribavirin or its cell-associated toxicity. However, a slightly antagonistic MV-inhibitory effect was observed at the highest concentration of ribavirin used in combination with most concentrations of compound 7 tested. This and related compounds may be valuable leads in the development of a potent and selective class of MV inhibitors that could be used in future in the clinic.

Inhibition of measles virus replication by 5'-nor carbocyclic adenosine analogues

Despite intense efforts to increase vaccine coverage, measles virus (MV) still causes significant morbidity and mortality in the world, sometimes as the result of severe, chronic, lethal disease. In an effort to develop therapies to supplement immunization strategies, a number of 5'-nor carbocyclic adenosine analogues were evaluated for anti-MV activity in CV-1 monkey kidney cells. Of those compounds tested, those either unsubstituted at C4 or possessing a hydroxyl, azido or amino substituent at that position were the most active, with particularly significant inhibition of MV, strain Chicago-1. The EC50 values against this strain ranged from <0.1 to 1 mg/ml, as determined by cytopathic effect reduction assay, and confirmed by neutral red uptake. By virus yield reduction assay (+)-(1S,25,3R,4S)-4-(6'-amino-9'H-purin-9'-yl)cyclopentane-1,2,3-triol (2) (-)-(1R,2S,3R)-1-(6'-amino-9'H-purin-9'-yl)-2,3-dihydoxycyclopent-4-ene (3) (-)-(1R,2S,3R)-1-(6'-amino-9'H-purin-9'-yl)cyclopentane-2,3-dihydoxycyclopentane (5) and (-)-(1R,2R,3R,4S)-4-amino-1-(6'-amino-9'H-purin-9'-yl)cyclopentane-2,3-diol (8) were the most potent compounds tested, all with EC90 values of < or =0.4 mg/ml. Compounds 3 and 5 were also tested against other MV strains, and similarly inhibited those strains except for four designated as Bil, Edmonston, SA and X-1108. Compound 8 did not potently inhibit these other MV strains. In addition, 3, 5 and 8 demonstrated synergistic (additive) inhibition of MV replication in combination with ribavirin at several concentrations. Compounds 3, 5 and 8 were also potent MV inhibitors even when added to infected cells 24 h after virus exposure. None of these three compounds was virucidal at concentrations that inhibited viral replication as determined by virus yield reduction assay. Most compounds tested were also not toxic at concentrations >100 mg/ml in actively growing and stationary-phase cells. Results suggest that these compounds may be clinically useful anti-MV virus agents.