Phosphorylation of beta-D-ribosylbenzimidazoles is not required for activity against human cytomegalovirus

Anti-CMV therapy, what next? A systematic review

Human cytomegalovirus (HCMV) is one of the main causes of serious complications in immunocompromised patients and after congenital infection. There are currently drugs available to treat HCMV infection, targeting viral polymerase, whose use is complicated by toxicity and the emergence of resistance. Maribavir and letermovir are the latest antivirals to have been developed with other targets. The approval of letermovir represents an important innovation for CMV prevention in hematopoietic stem cell transplant recipients, whereas maribavir allowed improving the management of refractory or resistant infections in transplant recipients. However, in case of multidrug resistance or for the prevention and treatment of congenital CMV infection, finding new antivirals or molecules able to inhibit CMV replication with the lowest toxicity remains a critical need. This review presents a range of molecules known to be effective against HCMV. Molecules with a direct action against HCMV include brincidofovir, cyclopropavir and anti-terminase benzimidazole analogs. Artemisinin derivatives, quercetin and baicalein, and anti-cyclooxygenase-2 are derived from natural molecules and are generally used for different indications. Although they have demonstrated indirect anti-CMV activity, few clinical studies were performed with these compounds. Immunomodulating molecules such as leflunomide and everolimus have also demonstrated indirect antiviral activity against HCMV and could be an interesting complement to antiviral therapy. The efficacy of anti-CMV immunoglobulins are discussed in CMV congenital infection and in association with direct antiviral therapy in heart transplanted patients. All molecules are described, with their mode of action against HCMV, preclinical tests, clinical studies and possible resistance. All these molecules have shown anti-HCMV potential as monotherapy or in combination with others. These new approaches could be interesting to validate in clinical trials.

Advances in the Development of Therapeutics for Cytomegalovirus Infections

The development of therapeutics for cytomegalovirus (CMV) infections, while progressing, has not matched the pace of new treatments of human immunodeficiency virus (HIV) infections; nevertheless, recent developments in the treatment of CMV infections have resulted in improved human health and perhaps will encourage the development of new therapeutic approaches. First, the deployment of ganciclovir and valganciclovir for both the prevention and treatment of CMV infections and disease in transplant recipients has been further improved with the licensure of the efficacious and less toxic letermovir. Regardless, late-onset CMV disease, specifically pneumonia, remains problematic. Second, the treatment of congenital CMV infections with valganciclovir has beneficially improved both hearing and neurologic outcomes, both fundamental advances for these children. In these pediatric studies, viral load was decreased but not eliminated. Thus, an important lesson learned from studies in both populations is the need for new antiviral agents and the necessity for combination therapies as has been shown to be beneficial in the treatment of HIV infections, among others. The development of monoclonal antibodies, sirtuins, and cyclopropovir may provide new treatment options.

Targeting the terminase: An important step forward in the treatment and prophylaxis of human cytomegalovirus infections

A key step in the replication of human cytomegalovirus (HCMV) in the host cell is the generation and packaging of unit-length genomes into preformed capsids. Enzymes required for this process are so-called terminases, first described for double-stranded DNA bacteriophages. The HCMV terminase consists of the two subunits, the ATPase pUL56 and the nuclease pUL89, and a potential third component pUL51. The terminase subunits are essential for virus replication and are highly conserved throughout the Herpesviridae family. Together with the portal protein pUL104 they form a powerful biological nanomotor. It has been shown for tailed dsDNA bacteriophages that DNA translocation into preformed capsid needs an extraordinary amount of energy. The HCMV terminase subunit pUL56 provides the required ATP hydrolyzing activity. The necessary nuclease activity to cleave the concatemers into unit-length genomes is mediated by the terminase subunit pUL89. Whether this cleavage is mediated by site-specific duplex nicking has not been demonstrated, however, it is required for packaging. Binding to the portal is a prerequisite for DNA translocation. To date, it is a common view that during translocation the terminase moves along some domains of the DNA by a binding and release mechanism. These critical structures have proven to be outstanding targets for drugs to treat HCMV infections because corresponding structures do not exist in mammalian cells. Herein we examine the HCMV terminase as a target for drugs and review several inhibitors discovered by both lead-directed medicinal chemistry and by target-specific design. In addition to producing clinically active compounds the research also has furthered the understanding of the role and function of the terminase itself.

New therapies for human cytomegalovirus infections

The recent approval of letermovir marks a new era of therapy for human cytomegalovirus (HCMV) infections, particularly for the prevention of HCMV disease in hematopoietic stem cell transplant recipients. For almost 30 years ganciclovir has been the therapy of choice for these infections and by today's standards this drug exhibits only modest antiviral activity that is often insufficient to completely suppress viral replication, and drives the selection of drug-resistant variants that continue to replicate and contribute to disease. While ganciclovir remains the therapy of choice, additional drugs that inhibit novel molecular targets, such as letermovir, will be required as highly effective combination therapies are developed not only for the treatment of immunocompromised hosts, but also for congenitally infected infants. Sustained efforts, largely in the biotech industry and academia, have identified additional highly active lead compounds that have progressed into clinical studies with varying levels of success and at least two have the potential to be approved in the near future. Some of the new drugs in the pipeline inhibit new molecular targets, remain effective against isolates that have developed resistance to existing therapies, and promise to augment existing therapeutic regimens. Here, we will describe some of the unique features of HCMV biology and discuss their effect on therapeutic needs. Existing drugs will also be discussed and some of the more promising candidates will be reviewed with an emphasis on those progressing through clinical studies. The in vitro and in vivo antiviral activity, spectrum of antiviral activity, and mechanism of action of new compounds will be reviewed to provide an update on potential new therapies for HCMV infections that have progressed significantly in recent years.

Human Cytomegalovirus pUL93 Is Required for Viral Genome Cleavage and Packaging

Human cytomegalovirus (HCMV) pUL93 is essential for virus growth, but its precise function in the virus life cycle is unknown. Here, we characterize a UL93 stop mutant virus (UL93st-TB40/E-BAC) to demonstrate that the absence of this protein does not restrict viral gene expression; however, cleavage of viral DNA into unit-length genomes as well as genome packaging is abolished. Thus, pUL93 is required for viral genome cleavage and packaging.

Benzimidazole analogs inhibit human herpesvirus 6

Several benzimidazole nucleoside analogs, including 1H-β-D-ribofuranosyl-2-bromo-5,6-dichlorobenzimidazole (BDCRB) and 1H-β-L-ribofuranosyl-2-isopropylamino-5,6-dichlorobenzimidazole (maribavir [MBV]), inhibit the replication of human cytomegalovirus. Neither analog inhibited the related betaherpesvirus human herpesvirus 6 (HHV-6). Additional analogs of these compounds were evaluated against both variants of HHV-6, and two L-analogs of BDCRB had good antiviral activity against HHV-6A, as well as more modest inhibition of HHV-6B replication.

Development of drugs for Epstein-Barr virus using high-throughput in silico virtual screening

IMPORTANCE OF THE FIELD

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that is causally associated with endemic forms of Burkitt's lymphoma, nasopharyngeal carcinoma and lymphoproliferative disease in immunosuppressed individuals. On a global scale, EBV infects > 90% of the adult population and is responsible for ∼ 1% of all human cancers. To date, there is no efficacious drug or therapy for the treatment of EBV infection and EBV-related diseases.

AREAS COVERED IN THIS REVIEW

In this review, we discuss the existing anti-EBV inhibitors and those under development. We discuss the value of different molecular targets, including EBV lytic DNA replication enzymes as well as proteins that are expressed exclusively during latent infection, such as EBV nuclear antigen 1 (EBNA-1) and latent membrane protein 1. As the atomic structure of the EBNA-1 DNA binding domain has been described, it is an attractive target for in silico methods of drug design and small molecule screening. We discuss the use of computational methods that can greatly facilitate the development of novel inhibitors and how in silico screening methods can be applied to target proteins with known structures, such as EBNA-1, to treat EBV infection and disease.

WHAT THE READER WILL GAIN

The reader is familiarized with the problems in targeting of EBV for inhibition by small molecules and how computational methods can greatly facilitate this process.

TAKE HOME MESSAGE

Despite the impressive efficacy of nucleoside analogs for the treatment of herpesvirus lytic infection, there remain few effective treatments for latent infections. As EBV latent infection persists within and contributes to the formation of EBV-associated cancers, targeting EBV latent proteins is an unmet medical need. High-throughput in silico screening can accelerate the process of drug discovery for novel and selective agents that inhibit EBV latent infection and associated disease.

The search for new therapies for human cytomegalovirus infections

Ganciclovir (GCV), the therapy of choice for human cytomegalovirus (CMV) infections and foscarnet, a drug used to treat GCV-resistant CMV infections was approved more than twenty years ago. Although cidofovir and a prodrug of GCV have since been added to the armamentarium, a highly effective drug without significant toxicities has yet to be approved. Such a therapeutic agent is required for treatment of immunocompromised hosts and infants, which bear the greatest burden of disease. The modest antiviral activity of existing drugs is insufficient to completely suppress viral replication, which results in the selection of drug-resistant variants that remain pathogenic, continue to replicate, and contribute to disease. Sustained efforts, largely in the biotech industry and academia, have identified highly active lead compounds that have progressed into clinical studies with varying levels of success. A few of these compounds inhibit new molecular targets, remain effective against isolates that have developed resistance to existing therapies, and promise to augment existing therapies. Some of the more promising drugs will be discussed with an emphasis on those progressing to clinical studies. Their antiviral activity both in vitro and in vivo, spectrum of antiviral activity, and mechanism of action will be reviewed to provide an update on the progress of potential new therapies for CMV infections.

Putative Functional Domains of Human Cytomegalovirus pUL56 Involved in Dimerization and Benzimidazole D-Ribonucleoside Activity

Background

Benzimidazole d-ribonucleosides inhibit DNA packaging during human cytomegalovirus (HCMV) replication. Although they have been shown to target pUL56 and pUL89 (the large and small subunits of the HCMV terminase, respectively) their mechanism of action is not yet fully understood. We aimed here to better understand HCMV DNA maturation and the mechanism of action of benzimidazole derivatives.

Methods

The HCMV pUL56 protein was studied by sequence analysis of the HCMV UL56 gene and herpesvirus counterparts combined with primary structure analysis of the corresponding amino acid sequences.

Results

The UL56 sequence analysis of 45 HCMV strains and counterparts among herpesviruses allowed the identification of 12 conserved regions. Moreover, comparison with the product of gene 49 (gp49) of bacteriophage T4 suggested that the pUL56 zinc finger is localized close to the dimerization site of pUL56, providing a spatial organization of the catalytic site that allows recognition and cleavage of DNA.

Conclusions

This study provides a basis to investigate the mechanism of concatemeric DNA cleavage and a biochemical basis for DNA packaging inhibition by benzimidazole derivatives.

Identification of acetylated, tetrahalogenated benzimidazole D-ribonucleosides with enhanced activity against human cytomegalovirus

DNA packaging is the key step in viral maturation and involves binding and cleavage of viral DNA containing specific DNA-packaging motifs. This process is mediated by a group of specific enzymes called terminases. We previously demonstrated that the human cytomegalovirus (HCMV) terminase is composed of the large subunit pUL56 and the small subunit pUL89. While the large subunit mediates sequence-specific DNA binding and ATP hydrolysis, pUL89 is required only for duplex nicking. An excellent inhibitor targeting HCMV terminase is 2-bromo-5,6-dichloro-1-(beta-d-ribofuranosyl)benzimidazole (BDCRB), but it was not developed as an antiviral drug due to its metabolic cleavage in experimental animals. We now have tested several new benzimidazole d-ribonucleosides in order to determine whether these compounds represent new, potent inhibitors. Analysis by bioluminometric ATPase activity assays identified two of the new compounds with a high inhibitory effect, 2-bromo-4,5,6-trichloro-1-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl) benzimidazole (BTCRB) and 2,4,5,6-tetrachloro-1-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl benzimidazole (Cl(4)RB). By using viral plaque formation, viral yield, and viral growth kinetics, we demonstrated that the two compounds BTCRB and Cl(4)RB had antiviral activities similar to that of BDCRB. Interestingly, BTCRB retained its inhibitory activity after preincubation with HFF cells. By use of electron microscopy, we observed an increase of B capsids and a lack of cytoplasmic capsids in the presence of the compounds that correlated with the virus yield. Furthermore, cleavage of concatenated DNA was inhibited by both compounds, and inhibition by BTCRB was shown to be dose dependent. These results demonstrate that the new compounds are highly active against HCMV and act by mechanisms similar but not identical to those of BDCRB.

New Functional Domains of Human Cytomegalovirus pUL89 predicted by Sequence Analysis and Three-Dimensional Modelling of the Catalytic Site DEXDc

Introduction

Benzimidazole d-ribonucleosides inhibit DNA packaging during human cytomegalovirus (HCMV) replication. Although they have been shown to target pUL56 and pUL89, the large and small subunits of the HCMV terminase respectively, their mechanism of action is not yet fully understood.

Methods and results

To better understand HCMV DNA maturation and the mechanism of action of benzimidazole derivatives, we studied the HCMV pUL89 protein by a genetic approach combined with primary structure analysis. The pUL89 sequence analysis of 25 HCMV strains and counterparts among herpesviruses allowed identification of 12 conserved regions. We also built a three-dimensional model of the pUL89 ATPasic catalytic site, including ATPase motor motifs I, II and III, that may facilitate the development of future antiviral drugs active against HCMV. Finally, we identified several putative functional domains in pUL89, such as pUL89 zinc finger (pUL89-ZF), DNA cutting sites and portal binding sites, that are probably involved in CMV DNA cleavage and packaging.

Interaction of the putative human cytomegalovirus portal protein pUL104 with the large terminase subunit pUL56 and its inhibition by benzimidazole-D-ribonucleosides

Herpesvirus DNA replication leads to unit length genomes that are translocated into preformed procapsids through a unique portal vertex. The translocation is performed by the terminase that cleaves the DNA and powers the insertion by its ATPase activity. Recently, we demonstrated that the putative human cytomegalovirus (HCMV) portal protein, pUL104, also forms high-molecular-weight complexes. Analyses now have been performed to determine the intracellular localization and identification of interaction partners of pUL104. In infected cells, HCMV pUL104 was found to be predominantly localized throughout the nucleus as well as in cytoplasmic clusters at late times of infection. The latter localization was abolished by phosphonoacetic acid, an inhibitor of viral DNA replication. Immunofluorescence revealed that pUL104 colocalized with pUL56, the large subunit of the HCMV terminase. Specific association of in vitro translated pUL104 with the carboxy-terminal half of GST-UL56C was detected. By using coimmunoprecipitations a direct interaction with pUL56 was confirmed. In addition, this interaction was no longer detected when the benzimidazole-D-nucleosides BDCRB or Cl4RB were added, thus indicating that these HCMV inhibitors block the insertion of the DNA into the capsid by preventing a necessary interaction of pUL56 with the portal. Electron microscopy revealed that in the presence of Cl4RB DNA is not packaged into capsids and these capsids failed to egress from the nucleus. Furthermore, pulsed-field gel electrophoresis showed that DNA concatemers synthesized in the presence of the compound failed to be processed.

Epstein-Barr virus infections: prospects for treatment

Epstein-Barr virus (EBV) causes infectious mononucleosis and oral hairy leucoplakia, and is associated with a number of malignancies. There are, however, no regulatory agency-approved treatments for EBV-related diseases. Several antiviral drugs inhibit replication of EBV in cell culture including acyclic nucleoside and nucleotide analogues and pyrophosphate analogues, all of which inhibit the EBV DNA polymerase. Despite their potency in vitro, these drugs have limited use in vivo for treatment of acute primary EBV infection as well as EBV-associated malignancies for several reasons. Here we discuss novel anti-EBV compounds, including maribavir, potentially useful for the treatment of acute EBV infections. A number of experimental approaches for treatment of EBV-related malignancies that are not susceptible to conventional antiviral drug treatment are also discussed.

Synthesis and antiviral properties of arabino and ribonucleosides of 1,3-dideazaadenine, 4-nitro-1,3-dideazapurine and diketopiperazine

Different arabinosides and ribosides, viz. Ara-DDA or 9(1-beta-D-arabinofuranosyl) 1,3-dideazaadenine (6), Ara-NDDP or 9(1-beta-D-arabinofuranosyl) 4-nitro-1,3-dideazapurine (7), Ara-DKP or 1(1-beta-D-arabinofuranosyl) diketopiperazine (8), Ribo-DDA or 9(1-beta-D-ribofuranosyl) 1,3-dideazaadenine (9) and Ribo-NDDP or 9(1-beta-D-ribofuranosyl) 4-nitro-1,3-dideazapurine (10) have been synthesized as probable antiviral agents. The arabinosides have been synthesized using the catalyst TDA-1 that causes stereospecific formation of beta-nucleosides while a one-pot synthesis procedure was adopted for the synthesis of the ribonucleosides where beta-anomers were obtained in higher yields. All the five nucleoside analogs have been screened for antiviral property against HIV-1 (IIIB), HSV-1 and 2, parainfluenza-3, reovirus-1 and many others. It was observed that arabinosides had greater inhibitory action than ribosides. The compound 7 or Ara-NDDP has shown maximum inhibition of HIV-1 replication than the rest of the molecules with an IC50 of 79.4 microg/mL.

Inhibition of human cytomegalovirus replication by benzimidazole nucleosides involves three distinct mechanisms

The benzimidazole nucleosides 2-bromo-5,6-dichloro-1-(beta-d-ribofuranosyl)benzimidazole (BDCRB) and 2-isopropylamino-5,6-dichloro-1-(beta-l-ribofuranosyl)benzimidazole (1263W94, or maribavir) are potent and selective inhibitors of human cytomegalovirus (HCMV) replication. These inhibitors act by two different mechanisms: BDCRB blocks the processing and maturation of viral DNA, whereas maribavir prevents viral DNA synthesis and capsid nuclear egress. In order to determine by which of these two mechanisms other benzimidazole nucleosides acted, we performed time-of-addition studies and other experiments with selected new analogs. We found that the erythrofuranosyl analog and the alpha-lyxofuranosyl analog acted late in the viral replication cycle, similar to BDCRB. In marked contrast, the alpha-5'-deoxylyxofuranosyl analog of 2,5,6-trichloro-1-(beta-d-ribofuranosyl)benzimidazole (compound UMJD1311) acted early in the replication cycle, too early to be consistent with either mechanism. Similar to other reports on early acting inhibitors of herpesviruses, compound 1311 was multiplicity of infection dependent, an observation that could not be reproduced with UV-inactivated virus. HCMV isolates resistant to BDCRB and maribavir were sensitive to compound 1311, as were viruses resistant to ganciclovir, cidofovir, and foscarnet. The preincubation of host cells with compound 1311 and removal prior to the addition of HCMV did not produce an antiviral cellular response. We conclude that this newly discovered early mode of action occurs at a stage of viral replication after entry to cells but prior to viral DNA synthesis, thereby strongly suggesting that the trisubstituted benzimidazole nucleoside series possesses three distinct biochemical modes of action for inhibition of HCMV replication.

Synthesis, Antiviral Activity, and Mode of Action of Some 3-Substituted 2,5,6-Trichloroindole 2‘- and 5‘-Deoxyribonucleosides

A series of chlorinated indole nucleosides has been synthesized and tested for activity against human cytomegalovirus (HCMV) and herpes simplex virus type-1 (HSV-1) and for cytotoxicity. The 2'- and 5'-deoxy derivatives of the reported 3-formyl-2,5,6-trichloro-1-(beta-D-ribofuranosyl)indole (FTCRI) and 3-cyano-2,5,6-trichloro-1-(beta-D-ribofuranosyl)indole (CTCRI) were synthesized by either a modification of the appropriate 3-unsubstituted sugar-modified nucleoside analogues or by a glycosylation of 3-substituted heterocycles with a protected alpha-chlorosugar. The modifications were guided in part by structural similarity to the corresponding series of chlorinated benzimidazole ribonucleosides and the fact that 5'-deoxy analogues of 2,5,6-trichloro-1-(beta-D-ribofuranosyl)benzimidazole (TCRB) are very active against HCMV. The 5'-deoxy analogues of FTCRI and CTCRI were nearly as active as FTCRI and CTCRI, suggesting that the chlorinated benzimidazole nucleosides and the chlorinated indole nucleosides act in a similar manner. This hypothesis was supported by time-of-addition studies using FTCRI and by the resistance of TCRB-resistant strains of HCMV to four different 3-substituted indole ribonucleosides. The 2'-deoxy analogues of the trichlorinated indole nucleosides also had potent antiviral activity, in contrast to decreased activity and selectivity observed for 2'-deoxy TCRB compared to TCRB. In addition, 3-acetyl-2,5,6-trichloro-1-(2-deoxy-beta-d-ribofuranosyl)indole was also active and much less cytotoxic (HCMV IC50 = 0.30 microM, HFF CC50 >100 microM) than previous analogues. None of the analogues had significant activity against HSV-1.

Synthesis of Benzodithiol-2-yl-Substituted Nucleoside Derivatives as Lead Compounds Having Anti-Bovine Viral Diarrhea Virus Activity

Nucleoside derivatives having a benzodithiol-2-yl (BDT) group were synthesized and examined for their anti-bovine viral diarrhea virus (BVDV) activities. Other substituents structurally similar to the BDT group such as 1,3-benzodioxol-2-yl, benzimidazol-2-yl and 1-oxo-benzodithiol-2-yl groups were not effective as the pharmacophore. The anti-BVDV assay revealed that 2'-O-BDT-guanosine and 2'-O-BDT-inosine had the strongest anti-BVDV activity among the nucleoside derivatives synthesized in this study. Since BVDV has been recognized as a surrogate for human hepatitis C virus (HCV), the BDT-modified nucleosides might become a new class of lead compounds to find nucleoside-type anti-HCV agents such as ribavirin.

Rhesus cytomegalovirus is similar to human cytomegalovirus in susceptibility to benzimidazole nucleosides

Rhesus and human cytomegalovirus (RhCMV and HCMV, respectively) exhibit comparable inhibition by benzimidazole nucleosides, including 2,5,6-trichloro-(1-beta-d-ribofuranosyl)benzimidazole (TCRB), and pyrrolo[2,3-d]pyrimidines. The two HCMV protein targets of TCRB, UL89 and UL56, are highly conserved with their RhCMV homologues. These data indicate that infection of rhesus macaques with RhCMV represents a useful model to test novel anti-HCMV drugs.

Mechanism of action of the ribopyranoside benzimidazole GW275175X against human cytomegalovirus

New human cytomegalovirus (HCMV) therapies with novel mechanisms of action are needed to treat drug-resistant HCMV that arises during therapy with currently approved agents. 2-Bromo-5,6-dichloro-1-beta-D-ribofuranosyl-1H-benzimidazole (BDCRB) is an effective anti-HCMV agent with a novel mechanism of action, but problems with in vivo stability preclude clinical development. A D-ribopyranosyl derivative of BDCRB, GW275175X, displays similar antiviral activity without the in vivo stability problems. We present an initial description of the activity of GW275175X against HCMV, other herpesviruses, and selected nonherpesviruses. In addition, we show that it acts as a DNA maturation inhibitor like the parent compound, BDCRB, rather than via the mechanisms of action of 1263W94 or any anti-HCMV drugs approved for marketing. GW275175X is a promising candidate for clinical development as an anti-HCMV agent.

Halogenated benzimidazoles and benzotriazoles as inhibitors of the NTPase/helicase activities of hepatitis C and related viruses

A search has been initiated for lead inhibitors of the nonstructural protein 3 (NS3)-associated NTPase/helicase activities of hepatitis C virus, the related West Nile virus, Japanese encephalitis virus and the human mitochondrial Suv3 enzyme. Random screening of a broad range of unrelated low-molecular mass compounds, employing both RNA and DNA substrates, revealed that 4,5,6,7-tetrabromobenzotriazole (TBBT) hitherto known as a potent highly selective inhibitor of protein kinase 2, is a good inhibitor of the helicase, but not NTPase, activity of hepatitis C virus NTPase/helicase. The IC50 is approximately 20 micro m with a DNA substrate, but only 60 micro m with an RNA substrate. Several related analogues of TBBT were enzyme- and/or substrate-specific inhibitors. For example, 5,6-dichloro-1-(beta-d-ribofuranosyl)benzotriazole (DRBT) was a good, and selective, inhibitor of the West Nile virus enzyme with an RNA substrate (IC50 approximately 0.3 micro m), but much weaker with a DNA substrate (IC50 approximately 3 micro m). Preincubation of the enzymes, but not substrates, with DRBT enhanced inhibitory potency, e.g. the IC50 vs the hepatitis C virus helicase activity was reduced from 1.5 to 0.1 micro m. No effect of preincubation was noted with TBBT, suggesting a different mode of interaction with the enzyme. The tetrachloro congener of TBBT, 4,5,6,7,-tetrachlorobenzotriazole (TCBT; a much weaker inhibitor of casein kinase 2) is also a much weaker inhibitor than TBBT of all four helicases. Kinetic studies, supplemented by comparison of ATP-binding sites, indicated that, unlike the case with casein kinase 2, the mode of action of the inhibitors vs the helicases is not by interaction with the catalytic ATP-binding site, but rather by occupation of an allosteric nucleoside/nucleotide binding site. The halogeno benzimidazoles and benzotriazoles included in this study are excellent lead compounds for the development of more potent inhibitors of hepatitis C virus and other viral NTPase/helicases.