Mechanistic Studies of Viral Entry: An Overview of Dendrimer‐Based Microbicides As Entry Inhibitors Against Both HIV and HSV‐2 Overlapped Infections

This review provides an overview of the development of different dendrimers, mainly polyanionic, against human immunodeficiency virus (HIV) and genital herpes (HSV‐2) as topical microbicides targeting the viral entry process. Vaginal topical microbicides to prevent sexually transmitted infections such as HIV and HSV‐2 are urgently needed. To inhibit HIV/HSV‐2 entry processes, new preventive targets have been established to maximize the current therapies against wild‐type and drug‐resistant viruses. The entry of HIV/HSV‐2 into target cells is a multistep process that triggers a cascade of molecular interactions between viral envelope proteins and cell surface receptors. Polyanionic dendrimers are highly branched nanocompounds with potent activity against HIV/HSV‐2. Inhibitors of each entry step have been identified with regard to generations and surface groups, and possible roles for these agents in anti‐HIV/HSV‐2 therapies have also been discussed. Four potential binding sites for impeding HIV infection (HSPG, DC‐SIGN, GSL, and CD4/gp120 inhibitors) and HSV‐2 infection (HS, gB, gD, and gH/gL inhibitors) exist according to their mechanisms of action and structures. This review clarifies that inhibition of HIV/HSV‐2 entry continues to be a promising target for drug development because nanotechnology can transform the field of HIV/HSV‐2 prevention by improving the efficacy of the currently available antiviral treatments.     

The next ten stories on antiviral drug discovery (part E): advents,advances, and adventures

This review article presents the fifth part (part E) in the series of stories on antiviral drug discovery. The ten stories belonging to this fifth part are dealing with (i) aurintricarboxylic acid; (ii) alkenyldiarylmethanes; (iii) human immunodeficiency virus (HIV) integrase inhibitors; (iv) lens epithelium‐derived growth factor as a potential target for HIV proviral DNA integration; (v) the status presens of neuraminidase inhibitors NAIs in the control of influenza virus infections; (vi) the status presens on respiratory syncytial virus inhibitors; (vii) tricyclic (1,N‐2‐ethenoguanine)‐based acyclovir and ganciclovir derivatives; (viii) glycopeptide antibiotics as antivirals targeted at viral entry; (ix) the potential (off‐label) use of cidofovir in the treatment of polyoma (JC and BK) virus infections; and (x) finally, thymidine phosphorylase as a target for both antiviral and anticancer agents. © 2009 Wiley Periodicals, Inc. Med Res Rev, 31, No. 1, 118–160, 2010     

Antiviral Chemistry & Chemotherapy's current antiviral agents FactFile 2008 (2nd edition): RNA viruses

Among the RNA viruses, other than the retroviruses (that is, HIV), which are dealt with separately in the current FactFile, the most important targets for the development of antiviral agents at the moment are the orthomyxoviruses (that is, influenza), the hepaciviruses (that is, hepatitis C virus [HCV]) and, to a lesser extent, the picornaviruses. Although the uncoating inhibitors amantadine and rimantadine were the first known inhibitors of influenza A, the neuraminidase inhibitors oseltamivir, zanamivir and peramivir have now become the prime antiviral drugs for the treatment of influenza A and B virus infections. For HCV infections, standard treatment consists of the combination of pegylated interferon-alpha with ribavirin, but several other antivirals targeted at specific viral functions such as the HCV protease and/ or polymerase may be expected to soon take an important share of this important market. Still untapped is the potential of a variety of uncoating inhibitors, as well as protease and/or polymerase inhibitors against the wide spectrum of picornaviruses. While ribavirin has been available for 35 years as a broad-spectrum anti-RNA virus agent, relatively new and unexplored is favipiravir (T-705) accredited with activity against influenza as well as flaviviruses, bunyaviruses and arenaviruses.     

Antiviral drugs in the immunocompetent host: part I. Treatment of hepatitis,cytomegalovirus, and herpes infections

Since the release of amantadine in 1966, other agents designed to fight a diverse range of viral infections have been released. Part I of this two-part article focuses on agents used to manage hepatitis, cytomegalovirus, and herpes infections. In patients with chronic hepatitis B, interferon alfa-2b or lamivudine is the treatment of choice. Pegylated interferon alfa-2a or -2b, along with ribavirin, is standard treatment for patients with chronic hepatitis C. Although treatment of cytomegalovirus infections generally is supportive, there have been reports of severely ill patients who improved after receiving ganciclovir or foscarnet. Oral antiviral agents for initial and recurrent herpes simplex virus infections have been shown to shorten the duration of lesions. Treatment of herpes zoster infections with antiviral drugs shortens the course of infection and decreases symptoms. Studies have shown that antiviral treatment can prevent prolonged post-herpetic neuralgia, although this use remains controversial.     

Broad-spectrum antiviral activity of the IMP dehydrogenase inhibitor VX-497: a comparison with ribavirin and demonstration of antiviral additivity with alpha interferon

The enzyme IMP dehydrogenase (IMPDH) catalyzes an essential step in the de novo biosynthesis of guanine nucleotides, namely, the conversion of IMP to XMP. The major event occurring in cells exposed to competitive IMPDH inhibitors such as ribavirin or uncompetitive inhibitors such as mycophenolic acid (MPA) is a depletion of the intracellular GTP and dGTP pools. Ribavirin is approved as an inhaled antiviral agent for treatment of respiratory syncytial virus (RSV) infection and orally, in combination with alpha interferon (IFN-alpha), for the treatment of chronic hepatitis C virus (HCV) infection. VX-497 is a potent, reversible uncompetitive IMPDH inhibitor which is structurally unrelated to other known IMPDH inhibitors. Studies were performed to compare VX-497 and ribavirin in terms of their cytotoxicities and their efficacies against a variety of viruses. They included DNA viruses (hepatitis B virus [HBV], human cytomegalovirus [HCMV], and herpes simplex virus type 1 [HSV-1]) and RNA viruses (respiratory syncytial virus [RSV], parainfluenza-3 virus, bovine viral diarrhea virus, Venezuelan equine encephalomyelitis virus [VEEV], dengue virus, yellow fever virus, coxsackie B3 virus, encephalomyocarditis virus [EMCV], and influenza A virus). VX-497 was 17- to 186-fold more potent than ribavirin against HBV, HCMV, RSV, HSV-1, parainfluenza-3 virus, EMCV, and VEEV infections in cultured cells. The therapeutic index of VX-497 was significantly better than that of ribavirin for HBV and HCMV (14- and 39-fold, respectively). Finally, the antiviral effect of VX-497 in combination with IFN-alpha was compared to that of ribavirin with IFN-alpha in the EMCV replication system. Both VX-497 and ribavirin demonstrated additivity when coapplied with IFN-alpha, with VX-497 again being the more potent in this combination. These data are supportive of the hypothesis that VX-497, like ribavirin, is a broad-spectrum antiviral agent.     

Update on new antivirals under development for the treatment of double-stranded DNA virus infections

All the currently available antiviral agents used in the treatment of double-stranded (ds) DNA viruses, with the exception of interferon-α, inhibit the same target, the viral DNA polymerase. With increasing reports of the development of resistance of herpes simplex virus (HSV), cytomegalovirus (CMV), and hepatitis B virus (HBV) to some of these drugs, new antiviral agents are needed to treat these infections. Additionally, no drugs have been approved to treat several DNA virus infections, including those caused by adenovirus, smallpox, molluscum contagiosum, and BK virus. We report the status of 10 new antiviral drugs for the treatment of dsDNA viruses. CMX-001 has broad activity against dsDNA viruses; 3 helicase-primase inhibitors, maribavir, and FV-100 have activity against certain herpesviruses; ST-246 inhibits poxviruses; GS-9191 inhibits papillomaviruses; and clevudine and emtricitabine are active against HBV. Most of these drugs have completed at least phase I trials in humans, and many are in additional clinical trials.     

Serine protease inhibitors as anti-hepatitis C virus agents

Approximately 3% of the worldwide population (i.e., more than 170 million people) are chronically infected with the hepatitis C virus (HCV). An estimated 20% of these patients will develop liver cirrhosis within a mean of 20 years, and 2–5% of cirrhotic patients will die of end-stage liver disease or hepatocellular carcinoma. The currently approved antiviral therapy with pegylated interferon (pegIFN) and ribavirin induces a sustained virological response (SVR) in 40–50% of patients infected with genotype 1, the most prevalent HCV type. In this review, we focus on the development and clinical application of serine protease inhibitors as anti-HCV agents. Although highly active in inducing a significant decline of serum HCV RNA, the rapid development of resistance must be counteracted in combination with other antiviral agents, currently pegIFN-α and ribavirin. Two serine protease inhibitors have reached clinical Phase III trials, increasing SVR rates and shortening treatment duration when combined with pegIFN and ribavirin. Trials of interferon-free targeted combination therapies are currently underway.     

MK-5172, a selective inhibitor of hepatitis C virus NS3/4a protease with broad activity across genotypes and resistant variants

HCV NS3/4a protease inhibitors are proven therapeutic agents against chronic hepatitis C virus infection, with boceprevir and telaprevir having recently received regulatory approval as add-on therapy to pegylated interferon/ribavirin for patients harboring genotype 1 infections. Overcoming antiviral resistance, broad genotype coverage, and a convenient dosing regimen are important attributes for future agents to be used in combinations without interferon. In this communication, we report the preclinical profile of MK-5172, a novel P2-P4 quinoxaline macrocyclic NS3/4a protease inhibitor currently in clinical development. The compound demonstrates subnanomolar activity against a broad enzyme panel encompassing major hepatitis C virus (HCV) genotypes as well as variants resistant to earlier protease inhibitors. In replicon selections, MK-5172 exerted high selective pressure, which yielded few resistant colonies. In both rat and dog, MK-5172 demonstrates good plasma and liver exposures, with 24-h liver levels suggestive of once-daily dosing. When administered to HCV-infected chimpanzees harboring chronic gt1a or gt1b infections, MK-5172 suppressed viral load between 4 to 5 logs at a dose of 1 mg/kg of body weight twice daily (b.i.d.) for 7 days. Based on its preclinical profile, MK-5172 is anticipated to be broadly active against multiple HCV genotypes and clinically important resistance variants and highly suited for incorporation into newer all-oral regimens.     

Antiviral drugs for viruses other than human immunodeficiency virus

Most viral diseases, with the exception of those caused by human immunodeficiency virus, are self-limited illnesses that do not require specific antiviral therapy. The currently available antiviral drugs target 3 main groups of viruses: herpes, hepatitis, and influenza viruses. With the exception of the antisense molecule fomivirsen, all antiherpes drugs inhibit viral replication by serving as competitive substrates for viral DNA polymerase. Drugs for the treatment of influenza inhibit the ion channel M(2) protein or the enzyme neuraminidase. Combination therapy with Interferon-α and ribavirin remains the backbone treatment for chronic hepatitis C; the addition of serine protease inhibitors improves the treatment outcome of patients infected with hepatitis C virus genotype 1. Chronic hepatitis B can be treated with interferon or a combination of nucleos(t)ide analogues. Notably, almost all the nucleos(t) ide analogues for the treatment of chronic hepatitis B possess anti-human immunodeficiency virus properties, and they inhibit replication of hepatitis B virus by serving as competitive substrates for its DNA polymerase. Some antiviral drugs possess multiple potential clinical applications, such as ribavirin for the treatment of chronic hepatitis C and respiratory syncytial virus and cidofovir for the treatment of cytomegalovirus and other DNA viruses. Drug resistance is an emerging threat to the clinical utility of antiviral drugs. The major mechanisms for drug resistance are mutations in the viral DNA polymerase gene or in genes that encode for the viral kinases required for the activation of certain drugs such as acyclovir and ganciclovir. Widespread antiviral resistance has limited the clinical utility of M(2) inhibitors for the prevention and treatment of influenza infections. This article provides an overview of clinically available antiviral drugs for the primary care physician, with a special focus on pharmacology, clinical uses, and adverse effects.     

Mechanisms of action of ribavirin in antiviral therapies

Although ribavirin was originally synthesized over 30 years ago and has been used to treat viral infections as monotherapy (respiratory syncytial virus and Lassa fever virus) or with interferon-alpha (IFN-alpha) as combination therapy (hepatitis C virus), the precise mechanism of its therapeutic activities remains controversial. In this review we focus on two main biological properties of ribavirin: its indirect and direct antiviral activities (with particular emphasis on its efficacy against chronic hepatitis C infection). Each property could individually or collectively account for its clinical efficacy against viral infections. First, with emphasis on the evidence for indirect activities of ribavirin, we will review the clinical observations that suggest that the immunomodulatory properties of ribavirin can in part account for its antiviral activities in vivo. We will then describe the mode of ribavirin's direct antiviral activities. These direct activities can be ascribed to several possible mechanisms, including the recently described activity as an RNA mutagen, a property that may be important in driving a rapidly mutating RNA virus over the threshold to 'error catastrophe'.     

Ribavirin Inhibits In Vitro Hepatitis E Virus Replication through Depletion of Cellular GTP Pools and Is Moderately Synergistic with Alpha Interferon

Hepatitis E virus (HEV) is a common cause of acute hepatitis that results in high mortality in pregnant women and may establish chronic infections in immunocompromised patients. We demonstrate for the first time that alpha interferon (IFN-α) and ribavirin inhibit in vitro HEV replication in both a subgenomic replicon and an infectious culture system based on a genotype 3 strain. IFN-α showed a moderate but significant synergism with ribavirin. These findings corroborate the reported clinical effectiveness of both drugs. In addition, the antiviral activity of ribavirin against wild-type genotype 1, 2, and 3 strains was confirmed by immunofluorescence staining. Furthermore, the in vitro activity of ribavirin depends on depletion of intracellular GTP pools, which is evident from the facts that (i) other GTP-depleting agents (5-ethynyl-1-β-d-ribofuranosylimidazole-4-carboxamide [EICAR] and mycophenolic acid) inhibit viral replication, (ii) exogenously added guanosine reverses the antiviral effects, and (iii) a strong correlation (R² = 0.9998) exists between the antiviral activity and GTP depletion of ribavirin and other GTP-depleting agents.     

Antiviral therapy for chronic hepatitis C in patients with inherited bleeding disorders: an international, multicenter cohort study

Summary. Background: Hepatitis C is a major co‐morbidity in patients with hemophilia. However, there is little information on the efficacy of antiviral therapy and long‐term follow‐up after treatment.Objectives: To assess the effect of interferon‐based (IFN‐based) therapy on hepatitis C virus (HCV) eradication, to identify determinants associated with treatment response, and to assess the occurrence of end‐stage liver disease (ESLD) after completing antiviral therapy.Patients and methods: In a multicenter cohort study, 295 treatment‐naïve hemophilia patients chronically infected with HCV were included. The effect of therapy was expressed as sustained virological response (SVR). Determinants associated with treatment response were expressed as odds ratios (ORs). Cumulative incidence of ESLD was assessed using a Kaplan–Meier survival table.Results: Among human immunodeficiency virus (HIV) negative patients (n = 235), SVR was 29% (29/101) for IFN monotherapy, 44% (32/72) for IFN with ribavirin, and 63% (39/62) for pegylated IFN (PegIFN) with ribavirin. In patients co‐infected with HIV (n = 60), IFN monotherapy, IFN with ribavirin, and PegIFN with ribavirin eradicated HCV in 7/35 (20%), 1/2 (50%), and 11/23 (48%), respectively. SVR increased with genotype 2 and 3 [OR 11.0, 95% CI: 5.8–20.5], and combination therapy (IFN and ribavirin OR 3.7, 95% CI: 1.7–8.4), PegIFN and ribavirin (OR 4.2, 95% CI: 1.8–9.5). Up to 15 years after antiviral treatment, none of the patients with a SVR relapsed and none developed ESLD. In contrast, among unsuccessfully treated patients the cumulative incidence of ESLD after 15 years was 13.0%.Conclusions: Successful antiviral therapy appears to have a durable effect and reduces the risk of ESLD considerably.     

Protease inhibitors: silver bullets for chronic hepatitis C infection?

Recent trials evaluated the safety and efficacy of two protease inhibitors, boceprevir (Victrelis) and telaprevir (Incivek), added to standard care with pegylated interferon and ribavirin, in patients with chronic hepatitis C virus (HCV) infection. These drugs open the door for triple therapy and other new therapies involving combinations of other direct-acting antiviral agents to become the new standard of care for this population.     

New pharmacotherapy for hepatitis C

Chronic hepatitis C infection remains a major global public health burden associated with substantial morbidity and mortality. Recent advances in antiviral therapy with the US Food and Drug Administration (FDA) approval of the oral protease inhibitors boceprevir and telaprevir introduce a new era of treatment for hepatitis C based on directly acting antiviral agents, which are associated with significant improvements in viral eradication rates in combination with pegylated interferon plus ribavirin. Newer classes targeting the hepatitis C virus (HCV) protease, polymerase, NS5A, and other components of the viral genome demonstrate great promise to further enhance viral eradication with superior efficacy, improved tolerability, shorter duration of therapy, and diminished requirement for interferon. Current and future strategies for HCV pharmacotherapy are reviewed.     

Amphipathic DNA polymers are candidate vaginal microbicides and block herpes simplex virus binding, entry and viral gene expression

BACKGROUND: Amphipathic DNA polymers are promising therapies for the prevention of HIV and genital herpes infections. Recent studies on a panel of such compounds indicated potent activity against HIV binding and entry. This current study was conducted to explore the anti-herpes simplex virus (HSV) activity of the same panel of compounds and to determine their mechanism of activity. METHODS: The anti-HSV activity of a 40-nucleotide degenerate polymer (REP 9), a 40-nucleotide polycytidine amphipathic DNA polymer (REP 9C) and an analogue lacking amphipathic activity (Randomer 3) were compared in plaque reduction assays in the absence or presence of human genital tract secretions; the mechanisms of anti-HSV activity were explored. RESULTS: REP 9 inhibited HSV infection 10,000-fold, whereas Randomer 3 displayed no anti-HSV activity. The antiviral activity was independent of sequence but was dependent on size: the most potent activity was observed for analogues of 40 nucleotides in length. Mechanistic studies indicated that REP 9 and REP 9C blocked HSV-2 binding and entry, were active when added post-entry, inhibited viral gene expression and blocked HSV-induced apoptosis. Confocal microscopy studies showed rapid delivery of fluorescently tagged REP 9 and REP 9C into human epithelial cells, and delivery was significantly greater in infected cells as compared with uninfected cells. REP 9 exhibited no cytotoxicity and retained anti-HSV activity in the presence of cervicovaginal secretions and when virus was introduced in seminal plasma. CONCLUSIONS: REP 9 and REP 9C represent a novel class of antiviral agents that act by multiple mechanisms. These compounds warrant further development for systemic or topical delivery for the prevention and treatment of HIV and HSV.     

Future therapies for hepatitis C

Although pegylated interferon-alpha plus ribavirin has become the standard for treating chronic hepatitis C virus infection, a substantial number of patients do not tolerate therapy and require dose reduction or discontinuation, or do not respond to this combination therapy. Thus, new therapeutic options are needed. An increased knowledge of the hepatitis C virus and an understanding of its replication cycle, as well as advances in biotechnology, have stimulated the development of numerous new antiviral treatments for patients with hepatitis C virus infection. This review focuses on four classes of new agents: new interferons, ribavirin-like molecules, specific small-molecule hepatitis C virus inhibitors and new immune therapies, with particular emphasis on medications in the later stages of development.     

Antiviral drug discovery: Ten more compounds,and ten more stories (part B)

This review article that complements the previous review article on “The discovery of antiviral agents: ten different compounds, ten different stories” presents ten more compounds and ten more stories in which I have been closely involved at one or another point of my scientific career: (i) interferon (IFN) (in particular, IFN‐β); (ii) poly(I).poly(C); (iii) suramin; (iv) novel acyclic nucleoside phosphonates; (v) the double prodrug of [9‐(2‐phosphonomethoxyethyl)guanine]; (vi) cyclic nucleoside phosphonates; (vii) picornavirus inhibitors; (viii) human immunodeficiency virus (HIV) co‐receptor inhibitors; (ix) nonimmunosuppressive cyclosporin A analogues; and (x) bicyclic (furanopyrimidine) nucleoside analogues. With the exception of the HIV co‐receptor CCR5 inhibitor none of the compounds described here have already been marketed (for the indication they were initially developed). Successful antiviral drug development depends on the interplay of three disciplines, chemistry, biology/medicine, and industry, crucial factors being open mindedness for the unexpected, preparedness to explore serendipitous observations, and perseverance (in trying) to overcome the hurdles or setbacks inevitably compounding any drug development. © 2009 Wiley Periodicals, Inc. Med Res Rev     

Ribavirin derivatives with a hexitol moiety: synthesis and antiviral evaluation

Current standard therapy for the treatment of chronic infections with hepatitis C virus consists of combination therapy with (pegylated) interferon-alpha and ribavirin. 1,5-Anhydrohexitol nucleoside analogues are constrained congeners known to mimic the ribonucleoside conformation. Within this series some analogues are endowed with strong antiviral properties, particularly against herpesviruses. The six-membered anhydrohexitol ring was, therefore, combined with the triazolyl carboxamide moiety of ribavirin, thus providing a new series of ribavirin analogues. None of the newly synthesized compounds elicited any substantial antiviral activity, neither against herpes simplex virus type 1 and 2, nor against bovine viral diarrhoea virus (a surrogate for hepatitis C virus) or the yellow fever virus.