Human cytomegalovirus DNA replication: antiviral targets and drugs

Human cytomegalovirus: pathogenesis, prevention, and treatment

Human cytomegalovirus (HCMV) infection remains a significant global health challenge, particularly for immunocompromised individuals and newborns. This comprehensive review synthesizes current knowledge on HCMV pathogenesis, prevention, and treatment strategies. We examine the molecular mechanisms of HCMV entry, focusing on the structure and function of key envelope glycoproteins (gB, gH/gL/gO, gH/gL/pUL128-131) and their interactions with cellular receptors such as PDGFRα, NRP2, and THBD. The review explores HCMV's sophisticated immune evasion strategies, including interference with pattern recognition receptor signaling, modulation of antigen presentation, and regulation of NK and T cell responses. We highlight recent advancements in developing neutralizing antibodies, various vaccine strategies (live-attenuated, subunit, vector-based, DNA, and mRNA), antiviral compounds (both virus-targeted and host-targeted), and emerging cellular therapies such as TCR-T cell approaches. By integrating insights from structural biology, immunology, and clinical research, we identify critical knowledge gaps and propose future research directions. This analysis aims to stimulate cross-disciplinary collaborations and accelerate the development of more effective prevention and treatment strategies for HCMV infections, addressing a significant unmet medical need.

Human neural progenitor cell models to study the antiviral effects and neuroprotective potential of approved and investigational human cytomegalovirus inhibitors

Human cytomegalovirus (HCMV) is the viral leading cause of congenital defects in newborns worldwide. Many aspects of congenital CMV (cCMV) infection, which currently lacks a specific treatment, as well as the main determinants of neuropathogenesis in the developing brain during HCMV infection are unclear. In this study, we modeled HCMV infection at different stages of neural development. Moreover, we evaluated the effects of both approved and investigational anti-HCMV drugs on viral replication and gene expression in two different neural progenitor cell lines, i.e., human embryonic stem cells-derived neural stem cells (NSCs) and fetus-derived neuroepithelial stem (NES) cells. Ganciclovir, letermovir, nitazoxanide, and the ozonide OZ418 reduced viral DNA synthesis and the production of infectious virus in both lines of neural progenitors. HCMV infection dysregulated the expression of genes that either are markers of neural progenitors, such as SOX2, NESTIN, PAX-6, or play a role in neurogenesis, such as Doublecortin. Treatment with antiviral drugs had different effects on HCMV-induced dysregulation of the genes under investigation. This study contributes to the understanding of the molecular mechanisms of cCMV neuropathogenesis and paves the way for further consideration of anti-HCMV drugs as candidate therapeutic agents for the amelioration of cCMV-associated neurological manifestations.

CRM197-conjugated peptides vaccine of HCMV pp65 and gH induce maturation of DC and effective viral-specific T cell responses

Human cytomegalovirus (HCMV) infection is prevalent worldwide, and there is currently no licenced HCMV vaccine to control it. Therefore, developing an effective HCMV vaccine is a significant priority. Because of their excellent immunogenicity, the crucial components of HCMV, phosphoprotein 65 (pp65) and glycoproteins H (gH) are potential target proteins for HCMV vaccine design. In this study, we predicted and screened the dominant antigenic epitopes of B and T cells from pp65 and gH conjugated with the carrier protein cross-reacting material 197 (CRM197) to form three peptide-CRM197 vaccines (pp65-CRM197, gH-CRM197, and pp65-CRM197+gH-CRM197). Furthermore, the immunogenicity of the peptide-CRM197 vaccines and their effects on dendritic cells (DCs) were explored. The results showed that three peptide-CRM197 vaccines could induce maturation of DCs through the p38 MAPK signalling pathway and promote the release of proinflammatory factors, such as TNF-α and interleukin (IL) -6. Meanwhile, the peptide-CRM197 vaccines could effectively activate T cell and humoral immunity, which were far better than the inactivated HCMV vaccine. In conclusion, we constructed three peptide-CRM197 vaccines, which could induce multiple immune effects, providing a novel approach for HCMV vaccine design.

The human cytomegalovirus decathlon: Ten critical replication events provide opportunities for restriction

Human cytomegalovirus (HCMV) is a ubiquitous human pathogen that can cause severe disease in immunocompromised individuals, transplant recipients, and to the developing foetus during pregnancy. There is no protective vaccine currently available, and with only a limited number of antiviral drug options, resistant strains are constantly emerging. Successful completion of HCMV replication is an elegant feat from a molecular perspective, with both host and viral processes required at various stages. Remarkably, HCMV and other herpesviruses have protracted replication cycles, large genomes, complex virion structure and complicated nuclear and cytoplasmic replication events. In this review, we outline the 10 essential stages the virus must navigate to successfully complete replication. As each individual event along the replication continuum poses as a potential barrier for restriction, these essential checkpoints represent potential targets for antiviral development.

Updated clinical and glycomic features of mannosyl-oligosaccharide glucosidase deficiency: Two case reports

BACKGROUND

Mannosyl-oligosaccharide glucosidase (MOGS) deficiency is an extremely rare type of congenital disorder of glycosylation (CDG), with only 12 reported cases. Its clinical, genetic, and glycomic features are still expanding. Our aim is to update the novel clinical and glycosylation features of 2 previously reported patients with MOGS-CDG.

CASE SUMMARY

We collected comprehensive clinical information, and conducted the immunoglobulin G1 glycosylation assay using nano-electrospray ionization source quadruple time-of-flight mass spectrometry. Novel dysmorphic features included an enlarged tongue, forwardly rotated earlobes, a birth mark, overlapped toes, and abnormal fat distribution. Novel imaging findings included pericardial effusion, a deep interarytenoid groove, mild congenital subglottic stenosis, and laryngomalacia. Novel laboratory findings included peripheral leukocytosis with neutrophil predominance, elevated C-reactive protein and creatine kinase, dyslipidemia, coagulopathy, complement 3 and complement 4 deficiencies, decreased proportions of T lymphocytes and natural killer cells, and increased serum interleukin 6. Glycosylation studies showed a significant increase of hypermannosylated glycopeptides (Glc3Man7GlcNAc2/N2H10 and Man5GlcNAc2/N2H5) and hypersialylated glycopeptides. A compensatory glycosylation pathway leading to an increase in Man5GlcNAc2/N2H5 was indicated with the glycosylation profile.

CONCLUSION

We confirmed abnormal glycomics in 1 patient, expanding the clinical and glycomic spectrum of MOGS-CDG. We also postulated a compensatory glycosylation pathway, leading to a possible serum biomarker for future diagnosis.

Synthesis of New Imidazopyridine Nucleoside Derivatives Designed as Maribavir Analogues

The strong inhibition of Human Cytomegalovirus (HCMV) replication by benzimidazole nucleosides, like Triciribine and Maribavir, has prompted us to expand the structure-activity relationships of the benzimidazole series, using as a central core the imidazo[4,5-b]pyridine scaffold. We have thus synthesized a number of novel amino substituted imidazopyridine nucleoside derivatives, which can be considered as 4-(or 7)-aza-d-isosters of Maribavir and have evaluated their potential antiviral activity. The target compounds were synthesized upon glycosylation of suitably substituted 2-aminoimidazopyridines, which were prepared in six steps starting from 2-amino-6-chloropyridine. Even if the new compounds possessed only a slight structural modification when compared to the original drug, they were not endowed with interesting antiviral activity. Even so, three derivatives showed promising cytotoxic potential.

The Clinically Approved Antifungal Drug Posaconazole Inhibits Human Cytomegalovirus Replication

Posaconazole (PCZ) is a clinically approved drug used predominantly for prophylaxis and salvage therapy of fungal infections. Here, we report its previously undescribed anti-human cytomegalovirus (HCMV) activity. By using antiviral assays, we demonstrated that PCZ, along with other azolic antifungals, has a broad anti-HCMV activity, being active against different strains, including low-passage-number clinical isolates and strains resistant to viral DNA polymerase inhibitors. Using a pharmacological approach, we identified the inhibition of human cytochrome P450 51 (hCYP51), or lanosterol 14α demethylase, a cellular target of posaconazole in infected cells, as a mechanism of anti-HCMV activity of the drug. Indeed, hCYP51 expression was stimulated upon HCMV infection, and the inhibition of its enzymatic activity by either the lanosterol analog VFV {(R)-N-(1-(3,4'-difluoro-[1,1'-biphenyl]-4-yl)-2-(1H-imidazol-1-yl)ethyl)-4-(5-phenyl-1,3,4-oxadiazol-2-yl)benzamide} or PCZ decreased HCMV yield and infectivity of released virus particles. Importantly, we observed that the activity of the first-line anti-HCMV drug ganciclovir was boosted tenfold by PCZ and that ganciclovir (GCV) and PCZ act synergistically in inhibiting HCMV replication. Taken together, these findings suggest that this clinically approved drug deserves further investigation in the development of host-directed antiviral strategies as a candidate anti-HCMV drug with a dual antimicrobial effect.

The Cytomegalovirus Protein Kinase pUL97:Host Interactions, Regulatory Mechanisms and Antiviral Drug Targeting

Human cytomegalovirus (HCMV) expresses a variety of viral regulatory proteins that undergo close interaction with host factors including viral-cellular multiprotein complexes. The HCMV protein kinase pUL97 represents a viral cyclin-dependent kinase ortholog (vCDK) that determines the efficiency of HCMV replication via phosphorylation of viral and cellular substrates. A hierarchy of functional importance of individual pUL97-mediated phosphorylation events has been discussed; however, the most pronounced pUL97-dependent phenotype could be assigned to viral nuclear egress, as illustrated by deletion of the UL97 gene or pharmacological pUL97 inhibition. Despite earlier data pointing to a cyclin-independent functionality, experimental evidence increasingly emphasized the role of pUL97-cyclin complexes. Consequently, the knowledge about pUL97 involvement in host interaction, viral nuclear egress and additional replicative steps led to the postulation of pUL97 as an antiviral target. Indeed, validation experiments in vitro and in vivo confirmed the sustainability of this approach. Consequently, current investigations of pUL97 in antiviral treatment go beyond the known pUL97-mediated ganciclovir prodrug activation and henceforward include pUL97-specific kinase inhibitors. Among a number of interesting small molecules analyzed in experimental and preclinical stages, maribavir is presently investigated in clinical studies and, in the near future, might represent a first kinase inhibitor applied in the field of antiviral therapy.

Cell Line Models for Human Cytomegalovirus Latency Faithfully Mimic Viral Entry by Macropinocytosis and Endocytosis

Human cytomegalovirus (HCMV) enters primary CD34+ hematopoietic progenitor cells by macropinocytosis, where it establishes latency in part because its tegument-transactivating protein, pp71, remains associated with endosomes and is therefore unable to initiate productive, lytic replication. Here we show that multiple HCMV strains also enter cell line models used to study latency by macropinocytosis and endocytosis. In all latency models tested, tegument-delivered pp71 was found to be colocalized with endosomal markers and was not associated with the seven other cytoplasmic localization markers tested. Like the capsid-associated pp150 tegument protein, we initially detected capsid proteins in association with endosomes but later detected them in the nucleus. Inhibitors of macropinocytosis and endocytosis reduced latent viral gene expression and precluded reactivation. Importantly, we utilized electron microscopy to observe entry by macropinocytosis and endocytosis, providing additional visual corroboration of the findings of our functional studies. Our demonstration that HCMV enters cell line models for latency in a manner indistinguishable from that of its entry into primary cells illustrates the utility of these cell lines for probing the mechanisms, host genetics, and small-molecule-mediated inhibition of HCMV entry into the cell types where it establishes latency.IMPORTANCE Primary cells cultured in vitro currently provide the highest available relevance for examining molecular and genetic requirements for the establishment, maintenance, and reactivation of HCMV latency. However, their expense, heterogeneity, and intransigence to both long-term culture and molecular or genetic modification create rigor and reproducibility challenges for HCMV latency studies. There are several cell line models for latency not obstructed by deficiencies inherent in primary cells. However, many researchers view cell line studies of latency to be physiologically irrelevant because of the perception that these models display numerous and significant differences from primary cells. Here, we show that the very first step in a latent HCMV infection, entry of the virus into cells, occurs in cell line models in a manner indistinguishable from that in which it occurs in primary CD34+ hematopoietic progenitor cells. Our data argue that experimental HCMV latency is much more similar than it is different in cell lines and primary cells.

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.

Murine Cytomegalovirus Protein pM91 Interacts with pM79 and Is Critical for Viral Late Gene Expression

Viral gene expression is tightly regulated during cytomegalovirus (CMV) lytic replication, but the detailed mechanism of late gene transcription remains to be fully understood. Previous studies reported that six viral proteins (named viral transactivation factors [vTFs]) supporting late gene expression were conserved in beta- and gammaherpesviruses but not in alphaherpesviruses. Here, we performed coimmunoprecipitation experiments to elucidate the organization of these six proteins in murine CMV. Our results showed that these proteins formed a complex by both direct and indirect interactions. Specifically, pM91 strongly bound to pM79 even in the absence of other vTFs. Similar to pM79, pM91 exhibited early-late expression kinetics and localized within nuclear viral replication compartments during infection. Functional analysis was also performed using the pM91-deficient virus. Real-time PCR results revealed that abrogation of M91 expression markedly reduced viral late gene expression and progeny virus production without affecting viral DNA synthesis. Using mutagenesis, we found that residues E61, D62, D89, and D96 in pM91 were required for the pM91-pM79 interaction. Disruption of the interaction via E61A/D62A or D89A/D96A double mutation in the context of virus infection inhibited progeny virus production. Our data indicate that pM91 is a component of the viral late gene transcription factor complex and that the pM91-pM79 interaction is essential for viral late gene expression.IMPORTANCE Cytomegalovirus (CMV) infection is the leading cause of birth defects and causes morbidity and mortality in immunocompromised patients. The regulation of viral late gene transcription is not well elucidated, and understanding of this process benefits the development of novel therapeutics against CMV infection. This study (i) identified that six viral transactivation factors encoded by murine CMV form a complex, (ii) demonstrated that pM91 interacts with pM79 and that pM91 and pM79 colocalize in the nuclear viral replication compartments, (iii) confirmed that pM91 is critical for viral late gene expression but dispensable for viral DNA replication, and (iv) revealed that the pM91-pM79 interaction is required for progeny virus production. These findings give an explanation of how CMV regulates late gene expression and have important implications for the design of antiviral strategies.

Human herpesvirus portal proteins: Structure, function, and antiviral prospects

Herpesviruses (Herpesvirales) and tailed bacteriophages (Caudovirales) package their dsDNA genomes through an evolutionarily conserved mechanism. Much is known about the biochemistry and structural biology of phage portal proteins and the DNA encapsidation (viral genome cleavage and packaging) process. Although not at the same level of detail, studies on HSV-1, CMV, VZV, and HHV-8 have revealed important information on the function and structure of herpesvirus portal proteins. During dsDNA phage and herpesviral genome replication, concatamers of viral dsDNA are cleaved into single length units by a virus-encoded terminase and packaged into preformed procapsids through a channel located at a single capsid vertex (portal). Oligomeric portals are formed by the interaction of identical portal protein monomers. Comparing portal protein primary aa sequences between phage and herpesviruses reveals little to no sequence similarity. In contrast, the secondary and tertiary structures of known portals are remarkable. In all cases, function is highly conserved in that portals are essential for DNA packaging and also play a role in releasing viral genomic DNA during infection. Preclinical studies have described small molecules that target the HSV-1 and VZV portals and prevent viral replication by inhibiting encapsidation. This review summarizes what is known concerning the structure and function of herpesvirus portal proteins primarily based on their conserved bacteriophage counterparts and the potential to develop novel portal-specific DNA encapsidation inhibitors.

Characterization of phenyl pyrimidine derivatives that inhibit cytomegalovirus immediate-early gene expression

Background

Previously, we established a reporter cell line for human cytomegalovirus and screened anti-human cytomegalovirus compounds using the cell line. In this study, we characterized one of the identified compounds, 2,4-diamino-6–(4-methoxyphenyl)pyrimidine (coded as 35C10).

Methods

50% Effective concentrations (EC50s) and 50% cytotoxic concentrations (CC50s) of 35C10 and its derivatives in human fibroblasts were determined by X-gal staining of the cells infected with human cytomegalovirus Towne strain expressing β-galactosidase.

Results

EC50 and CC50 of 35C10 were 4.3 µM and >200 µM, respectively. Among several 35C10 derivatives, only one lacking 4-amino group of pyrimidine showed a similar EC50. 35C10 weakly inhibited murine cytomegalovirus, herpes simplex virus type 1, and varicella-zoster virus. A “time of addition” experiment suggested that 35C10 inhibited an early phase of the infection. Although 35C10 did not inhibit viral attachment to the cells nor the delivery of viral DNA to the nuclei, it decreased the number of infected cells expressing immediate-early 1 and 2 (IE1/IE2) proteins. 35C10 also inhibited the activation of a promoter for TRL4 in the reporter cells upon human cytomegalovirus infection, but not in the same reporter cells transfected with a plasmid expressing IE2.

Conclusion

Our findings suggest that 35C10 is a novel compound that inhibits IE gene expression in human cytomegalovirus-infected cells.

Antiviral Drugs

Viruses are major pathogenic agents causing a variety of serious diseases in humans, other animals, and plants.

Drugs that combat viral infections are called antiviral drugs. There are no effective antiviral drugs for many viral infections. However, there are several drugs for influenza, a couple of drugs for herpesviruses, and some new antiviral drugs for treatment of HIV and hepatitis C infections.

The arsenal of antivirals is complex. As of March 2014, it consists of approximately 50 drugs approved by the FDA, approximately half of which are directed against HIV. Antiviral drug creation strategies are focused on two different approaches: targeting the viruses themselves or targeting host cell factors.

Direct virus-targeting antiviral drugs include attachment inhibitors, entry inhibitors, uncoating inhibitors, protease inhibitors, polymerase inhibitors, nucleoside and nucleotide reverse transcriptase inhibitors, nonnucleoside reverse-transcriptase inhibitors, and integrase inhibitors.

Protease inhibitors (darunavir, atazanavir, and ritonavir), viral DNA polymerase inhibitors (acyclovir, valacyclovir, valganciclovir, and tenofovir), and an integrase inhibitor (raltegravir) are included in the list of Top 200 Drugs by sales for the 2010s.

The immune response to cytomegalovirus in allogeneic hematopoietic stem cell transplant recipients

Approximately, up to 70 % of the human population is infected with cytomegalovirus (CMV) that persists for life in a latent state. In healthy people, CMV reactivation induces the expansion of CMV-specific T cells up to 10 % of the entire T cell repertoire. On the contrary, CMV infection is a major opportunistic viral pathogen that remains a leading cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation. Due to the delayed CMV-specific immune recovery, the incidence of CMV reactivation during post-transplant period is very high. Several methods are currently available for the monitoring of CMV-specific responses that help in clinical monitoring. In this review, essential aspects in the immune recovery against CMV are discussed to improve the better understanding of the immune system relying on CMV infection and, thereby, helping the avoidance of CMV disease or reactivation following hematopoietic stem cell transplantation with severe consequences for the transplanted patients.

Human cytomegalovirus infection interferes with the maintenance and differentiation of trophoblast progenitor cells of the human placenta

Human cytomegalovirus (HCMV) is a major cause of birth defects that include severe neurological deficits, hearing and vision loss, and intrauterine growth restriction. Viral infection of the placenta leads to development of avascular villi, edema, and hypoxia associated with symptomatic congenital infection. Studies of primary cytotrophoblasts (CTBs) revealed that HCMV infection impedes terminal stages of differentiation and invasion by various molecular mechanisms. We recently discovered that HCMV arrests earlier stages involving development of human trophoblast progenitor cells (TBPCs), which give rise to the mature cell types of chorionic villi-syncytiotrophoblasts on the surfaces of floating villi and invasive CTBs that remodel the uterine vasculature. Here, we show that viral proteins are present in TBPCs of the chorion in cases of symptomatic congenital infection. In vitro studies revealed that HCMV replicates in continuously self-renewing TBPC lines derived from the chorion and alters expression and subcellular localization of proteins required for cell cycle progression, pluripotency, and early differentiation. In addition, treatment with a human monoclonal antibody to HCMV glycoprotein B rescues differentiation capacity, and thus, TBPCs have potential utility for evaluation of the efficacies of novel antiviral antibodies in protecting and restoring placental development. Our results suggest that HCMV replicates in TBPCs in the chorion in vivo, interfering with the earliest steps in the growth of new villi, contributing to virus transmission and impairing compensatory development. In cases of congenital infection, reduced responsiveness of the placenta to hypoxia limits the transport of substances from maternal blood and contributes to fetal growth restriction.

IMPORTANCE

Human cytomegalovirus (HCMV) is a leading cause of birth defects in the United States. Congenital infection can result in permanent neurological defects, mental retardation, hearing loss, visual impairment, and pregnancy complications, including intrauterine growth restriction, preterm delivery, and stillbirth. Currently, there is neither a vaccine nor any approved treatment for congenital HCMV infection during gestation. The molecular mechanisms underlying structural deficiencies in the placenta that undermine fetal development are poorly understood. Here we report that HCMV replicates in trophoblast progenitor cells (TBPCs)-precursors of the mature placental cells, syncytiotrophoblasts and cytotrophoblasts, in chorionic villi-in clinical cases of congenital infection. Virus replication in TBPCs in vitro dysregulates key proteins required for self-renewal and differentiation and inhibits normal division and development into mature placental cells. Our findings provide insights into the underlying molecular mechanisms by which HCMV replication interferes with placental maturation and transport functions.

Evaluation of UL99 transcript as a target for antiviral treatment efficacy

Human cytomegalovirus (HCMV) is a virus belonging to the Beta Herpes virus family. Its genome contains many different genes clustered in immediate early, early and late genes. This last cluster includes UL99, a late gene that encodes for a tegument protein called pp28. In immunocompetent patients, HCMV infection occurs asymptomatically, while its reactivation in immunocompromised patients can be a cause of pneumonia, retinitis and gastrointestinal diseases. To prevent or to contrast HCMV infection, several drugs (such as Ganciclovir, Acyclovir, Foscarnet) are available, and their efficiency is evaluated by HCMV DNA load monitoring, as also for antiviral resistance onset that may occur after the therapy. In this study is described the development of a Real Time PCR for the detection and quantification of UL99 transcript and the clearance of this target compared to HCMV DNA, both in vitro and in vivo on bronchoalveolar lavage samples.

HCMV infection of human trophoblast progenitor cells of the placenta is neutralized by a human monoclonal antibody to glycoprotein B and not by antibodies to the pentamer complex

Human cytomegalovirus (HCMV) is the major viral cause of congenital infection and birth defects. Primary maternal infection often results in virus transmission, and symptomatic babies can have permanent neurological deficiencies and deafness. Congenital infection can also lead to intrauterine growth restriction, a defect in placental transport. HCMV replicates in primary cytotrophoblasts (CTBs), the specialized cells of the placenta, and inhibits differentiation/invasion. Human trophoblast progenitor cells (TBPCs) give rise to the mature cell types of the chorionic villi, CTBs and multi-nucleated syncytiotrophoblasts (STBs). Here we report that TBPCs are fully permissive for pathogenic and attenuated HCMV strains. Studies with a mutant virus lacking a functional pentamer complex (gH/gL/pUL128-131A) showed that virion entry into TBPCs is independent of the pentamer. In addition, infection is blocked by a potent human neutralizing monoclonal antibody (mAb), TRL345, reactive with glycoprotein B (gB), but not mAbs to the pentamer proteins pUL130/pUL131A. Functional studies revealed that neutralization of infection preserved the capacity of TBPCs to differentiate and assemble into trophospheres composed of CTBs and STBs in vitro. Our results indicate that mAbs to gB protect trophoblast progenitors of the placenta and could be included in antibody treatments developed to suppress congenital infection and prevent disease.

Approaches for the generation of new anti-cytomegalovirus agents: identification of protein-protein interaction inhibitors and compounds against the HCMV IE2 protein

Human cytomegalovirus (HCMV) infection is responsible for severe, often even fatal, diseases in immunocompromised subjects and also represents the major cause of viral-associated congenital malformations in newborn children. The few drugs licensed for anti-HCMV therapy suffer from many drawbacks and none of them have been approved for the treatment of congenital infections. Furthermore, the emergence of drug-resistant viral strains represents a major concern for disease management. Thus, there is a strong need for new anti-HCMV drugs. Here we describe three different assays for the discovery of novel anti-HCMV compounds: two are in vitro assays, i.e., a fluorescence polarization (FP)-based assay and an enzyme-linked immunosorbent assay (ELISA), which are designed to search for compounds that act by disrupting the interactions between the HCMV DNA polymerase subunits, but in general can be employed to find inhibitors of any protein-protein interaction of interest; the third is a cell-based assay designed to identify inhibitors of the viral immediate-early 2 (IE2) protein activities.

Synthesis and anti-HCMV activity of 1-[ω-(phenoxy)alkyl]uracil derivatives and analogues thereof

HCMV infection represents a life-threatening condition for immunocompromised patients and newborn infants and novel anti-HCMV agents are clearly needed. In this regard, a series of 1-[ω-(phenoxy)alkyl]uracil derivatives were synthesized and examined for antiviral properties. Compounds 17, 20, 24 and 28 were found to exhibit highly specific and promising inhibitory activity against HCMV replication in HEL cell cultures with EC50 values within 5.5-12μM range. Further studies should be undertaken to elucidate the mechanism of action of these compounds and the structure-activity relationship for the linker region.

Design, synthesis, and evaluation of WC5 analogues as inhibitors of human cytomegalovirus Immediate-Early 2 protein, a promising target for anti-HCMV treatment

Although human cytomegalovirus (HCMV) infection is mostly asymptomatic for immunocompetent individuals, it remains a serious threat for those who are immunocompromised, in whom it is associated with various clinical manifestations. The therapeutic utility of the few available anti-HCMV drugs is limited by several drawbacks, including cross-resistance due to their common mechanism of action, i.e., inhibition of viral DNA polymerase. Therefore, compounds that target other essential viral events could overcome this problem. One example of this is the 6-aminoquinolone WC5, which acts by directly blocking the transactivation of essential viral Early genes by the Immediate-Early 2 (IE2) protein. In this study, the quinolone scaffold of the lead compound WC5 was investigated in depth, defining more suitable substituents for each of the scaffold positions explored and identifying novel, potent and nontoxic compounds. Some compounds showed potent anti-HCMV activity by interfering with IE2-dependent viral E gene expression. Among them, naphthyridone 1 was also endowed with potent anti-HIV activity in latently infected cells. Their antiviral profile along with their innovative mechanism of action make these anti-HCMV quinolones a very promising class of compounds to be exploited for more effective antiviral therapeutic treatment.

Targeted expression of herpes simplex virus thymidine kinase in adenovirus-infected cells reduces virus titers upon treatment with ganciclovir in vitro

BACKGROUND

Adenoviruses are a frequent cause of life-threatening infections in immunocompromised patients. Available therapeutics still cannot completely prevent fatal outcomes. By contrast, herpes viruses are well treatable with prodrugs such as ganciclovir (GCV), which are selectively activated in virus-infected cells by virus-encoded thymidine kinases. This effective group of prodrugs is not applicable to adenoviruses and other DNA viruses because they lack those kinases.

METHODS

To render adenoviruses amenable to GCV treatment, we generated an adenoviral vector-based delivery system for targeted expression of herpes simplex virus thymidine kinase (HSV-TK) in wild-type adenovirus 5 (wt Ad5)-infected cells. HSV-TK expression was largely restricted to wt virus-infected cells by transcription of the gene from the Ad5 E4 promoter. Its activity is dependent on the adenoviral E1A gene product which is not produced by the vector but is only provided in cells infected with wt adenovirus. The anti-adenoviral effect of HSV-TK expression and concomitant treatment with GCV was assessed in vitro in four different cell lines or primary cells.

RESULTS

E4 promoter-mediated HSV-TK background expression was sufficiently low to prevent cytotoxicity in the presence of low-levels GCV in cells not infected with wt Ad5. However, expression was several-fold increased in wt Ad5-infected cells and treatment with low levels of GCV efficiently inhibited wt Ad5 DNA replication. Genome copy numbers and output of infectious particles were reduced by up to > 99.99% and cell viability was greatly increased.

CONCLUSIONS

We extended the concept of enzyme/prodrug therapy to adenovirus infections by selectively sensitizing adenovirus-infected cells to treatment with GCV.

Sites and roles of phosphorylation of the human cytomegalovirus DNA polymerase subunit UL44

The human cytomegalovirus DNA polymerase subunit UL44 is a phosphoprotein, but its sites and roles of phosphorylation have not been investigated. We compared sites of phosphorylation of UL44 in vitro by the viral protein kinase UL97 and cyclin-dependent kinase 1 with those in infected cells. Transient treatment of infected cells with a UL97 inhibitor greatly reduced labeling of two minor UL44 phosphopeptides. Viruses containing alanine substitutions of most UL44 residues that are phosphorylated in infected cells exhibited at most modest effects on viral DNA synthesis and yield. However, substitution of highly phosphorylated sites adjacent to the nuclear localization signal abolished viral replication. The results taken together are consistent with UL44 being phosphorylated directly by UL97 during infection, and a crucial role for phosphorylation-mediated nuclear localization of UL44 for viral replication, but lend little support to the widely held hypothesis that UL97-mediated phosphorylation of UL44 is crucial for viral DNA synthesis.

Multiple phosphorylation sites at the C-terminus regulate nuclear import of HCMV DNA polymerase processivity factor ppUL44

The processivity factor of human cytomegalovirus DNA polymerase, phosphoprotein ppUL44, is essential for viral replication. During viral infection ppUL44 is phosphorylated by the viral kinase pUL97, but neither the target residues on ppUL44 nor the effect of phosphorylation on ppUL44's activity are known. We report here that ppUL44 is phosphorylated when transiently expressed in mammalian cells and coimmunoprecipitates with cellular kinases. Of three potential phosphorylation sites (S413, S415, S418) located upstream of ppUL44's nuclear localization signal (NLS) and one (T427) within the NLS itself, protein kinase CK2 (CK2) specifically phosphorylates S413, to trigger a cascade of phosphorylation of S418 and S415 by CK1 and CK2, respectively. Negative charge at the CK2/CK1 target serine residues facilitates optimal nuclear accumulation of ppUL44, whereas negative charge on T427, a potential cyclin-dependent 1 phosphorylation site, strongly decreases nuclear accumulation. Thus, nuclear transport of ppUL44 is finely tuned during viral infection through complex phosphorylation events.

Phosphonoformic acid inhibits viral replication by trapping the closed form of the DNA polymerase

Phosphonoformic acid (PFA, foscarnet) belongs to a class of antiviral drugs that inhibit the human cytomegalovirus DNA polymerase (UL54) by mimicking the pyrophosphate leaving group of the nucleotide transfer reaction. Difficulties expressing UL54 have hampered investigation of the precise structural requirements rendering inhibition by this drug. However, a previously engineered chimeric DNA polymerase, constructed by mutating the homologous polymerase from bacteriophage RB69 (gp43) to express several variable elements from UL54, can bypass this obstacle because of its favorable expression and acquired sensitivity to PFA (Tchesnokov, E. P., Obikhod, A., Schinazi, R. F., and Götte, M. (2008) J. Biol. Chem. 283, 34218-34228). Here, we compare two crystal structures that depict the chimeric DNA polymerase with and without PFA bound. PFA is visualized for the first time in the active site of a DNA polymerase, where interactions are resolved between the PP(i) mimic and two basic residues absolutely conserved in the fingers domain of family B polymerases. PFA also chelates metal ion B, the cation that contacts the triphosphate tail of the incoming nucleotide. These DNA complexes utilize a primer-template pair enzymatically chain-terminated by incorporation of acyclo-GMP, the phosphorylated form of the anti-herpes drug acyclovir. We postulate that the V478W mutation present in the chimera is critical in that it pushes the fingers domain to more readily adopt the closed conformation whether or not the drug is bound. The closed state of the fingers domain traps the variant polymerase in the untranslocated state and increases affinity for PFA. This finding provides a model for the mechanism of UL54 stalling by PFA.

Early inhibitors of human cytomegalovirus: state-of-art and therapeutic perspectives

Human cytomegalovirus (HCMV) infection is associated with severe morbidity and mortality in immunocompromised individuals, mainly transplant recipients and AIDS patients, and is the most frequent cause of congenital malformations in newborn children. To date, few drugs are licensed for the treatment of HCMV infections, most of which target the viral DNA polymerase and suffer from many drawbacks, including long-term toxicity, low potency, and poor bioavailability. In addition, the emergence of drug-resistant viral strains is becoming an increasing problem for disease management. Finally, none of the current anti-HCMV drugs have been approved for the treatment of congenital infections. For all these reasons, there is still a strong need for new anti-HCMV drugs with novel mechanisms of action. The first events of the virus replication cycle, including attachment, entry, immediate-early gene expression, and immediate-early functions—in particular that of Immediate-Early 2 protein—represent attractive targets for the development of novel antiviral compounds. Such inhibitors would block not only the expression of viral immediate-early proteins, which play a key role in the pathogenesis of HCMV infection, but also the host immunomodulation and the changes to cell physiology induced by the first events of virus infection. This review describes the current knowledge on the initial phases of HCMV replication, their validation as potential novel antiviral targets, and the development of compounds that block such processes.

Host-viral effects of chromatin assembly factor 1 interaction with HCMV IE2

Chromatin assembly factor 1 (CAF1) consisting of p150, p60 and p48 is known to assemble histones onto newly synthesized DNA and thus maintain the chromatin structure. Here, we show that CAF1 expression was induced in human cytomegalovirus (HCMV)-infected cells, concomitantly with global chromatin decondensation. This apparent conflict was thought to result, in part, from CAF1 mislocalization to compartments of HCMV DNA synthesis through binding of its largest subunit p150 to viral immediate-early protein 2 (IE2). p150 interaction with p60 and IE2 facilitated HCMV DNA synthesis. The IE2Q548R mutation, previously reported to result in impaired HCMV growth with unknown mechanism, disrupted IE2/p150 and IE2/histones association in our study. Moreover, IE2 interaction with histones partly depends on p150, and the HCMV-induced chromatin decondensation was reduced in cells ectopically expressing the p150 mutant defective in IE2 binding. These results not only indicate that CAF1 was hijacked by IE2 to facilitate the replication of the HCMV genome, suggesting chromatin assembly plays an important role in herpesviral DNA synthesis, but also provide a model of the virus-induced chromatin instability through CAF1.

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.

The 6-aminoquinolone WC5 inhibits human cytomegalovirus replication at an early stage by interfering with the transactivating activity of viral immediate-early 2 protein

WC5 is a 6-aminoquinolone that potently inhibits the replication of human cytomegalovirus (HCMV) but has no activity, or significantly less activity, against other herpesviruses. Here we investigated the nature of its specific anti-HCMV activity. Structure-activity relationship studies on a small series of analogues showed that WC5 possesses the most suitable pattern of substitutions around the quinolone scaffold to give potent and selective anti-HCMV activity. Studies performed to identify the possible target of WC5 indicated that it prevents viral DNA synthesis but does not significantly affect DNA polymerase activity. In yield reduction experiments with different multiplicities of infection, the anti-HCMV activity of WC5 appeared to be highly dependent on the viral inoculum, suggesting that WC5 may act at an initial stage of virus replication. Consistently, time-of-addition and time-of-removal studies demonstrated that WC5 affects a phase of the HCMV replicative cycle that precedes viral DNA synthesis. Experiments to monitor the effects of the compound on virus attachment and entry showed that it does not inhibit either process. Evaluation of viral mRNA and protein expression revealed that WC5 targets an event of the HCMV replicative cycle that follows the transcription and translation of immediate-early genes and precedes those of early and late genes. In cell-based assays to test the effects of WC5 on the transactivating activity of the HCMV immediate-early 2 (IE2) protein, WC5 markedly interfered with IE2-mediated transactivation of viral early promoters. Finally, WC5 combined with ganciclovir in checkerboard experiments exhibited highly synergistic activity. These findings suggest that WC5 deserves further investigation as a candidate anti-HCMV drug with a novel mechanism of action.

In vitro and in vivo activities of the novel anticytomegalovirus compound AIC246

Human cytomegalovirus (HCMV) remains a serious threat for immunocompromised individuals, including transplant recipients and newborns. To date, all drugs licensed for the treatment of HCMV infection and disease target the viral DNA polymerase. Although these drugs are effective, several drawbacks are associated with their use, including toxicity and emergence of drug resistance. Hence, new and improved antivirals with novel molecular targets are urgently needed. Here we report on the antiviral properties of AIC246, a representative of a novel class of low-molecular-weight compounds that is currently undergoing clinical phase II studies. The anti-HCMV activity of AIC246 was evaluated in vitro and in vivo using various cell culture assays and an engineered mouse xenograft model. In addition, antiviral properties of the drug were characterized in comparison to the current gold standard ganciclovir. We demonstrate that AIC246 exhibits excellent in vitro inhibitory activity against HCMV laboratory strains and clinical isolates, retains activity against ganciclovir-resistant viruses, is well tolerated in different cell types (median selectivity index, 18,000), and exerts a potent in vivo efficacy in a mouse xenograft model. Moreover, we show that the antiviral block induced by AIC246 is reversible and the efficacy of the drug is not significantly affected by cell culture variations such as cell type or multiplicity of infection. Finally, initial mode-of-action analyses reveal that AIC246 targets a process in the viral replication cycle that occurs later than DNA synthesis. Thus, AIC246 acts via a mode of action that differs from that of polymerase inhibitors like ganciclovir.

Antiviral prevention of sepsis induced cytomegalovirus reactivation in immunocompetent mice

INTRODUCTION

Immunocompetent patients can reactivate latent cytomegalovirus (CMV) during critical illness and reactivation is associated with significantly worse outcomes. Prior to clinical trials in humans to prove causality, we sought to determine an optimal antiviral treatment strategy.

METHODS

Mice latently infected with murine CMV (MCMV) received a septic reactivation trigger and were randomized to receive one of four ganciclovir regimens or saline. Lungs were evaluated for viral transcriptional reactivation and fibrosis after each regimen. Influences of ganciclovir on early sepsis-induced pulmonary inflammation and T-cell activation were studied after sepsis induction.

RESULTS

All ganciclovir regimens reduced measurable MCMV transcriptional reactivation, and 10mg/day for 7 or 21 days was most effective. Lower dose (5mg/kg/day) or delayed therapy was associated with significant breakthrough reactivation. Higher doses of ganciclovir given early were associated with the lowest incidence of pulmonary fibrosis, and delay of therapy for 1 week was associated with significantly worse pulmonary fibrosis. Although bacterial sepsis induced activation of MCMV-specific pulmonary T-cells, this activation was not influenced by ganciclovir.

CONCLUSION

These results suggest that antiviral treatment trials in humans should use 10mg/kg/day ganciclovir administered as early as possible in at-risk patients to minimize reactivation events and associated pulmonary injury.

The human cytomegalovirus major immediate-early proteins as antagonists of intrinsic and innate antiviral host responses

The major immediate-early (IE) gene of human cytomegalovirus (CMV) is believed to have a decisive role in acute infection and its activity is an important indicator of viral reactivation from latency. Although a variety of gene products are expressed from this region, the 72-kDa IE1 and the 86-kDa IE2 nuclear phosphoproteins are the most abundant and important. Both proteins have long been recognized as promiscuous transcriptional regulators. More recently, a critical role of the IE1 and IE2 proteins in counteracting non-adaptive host cell defense mechanisms has been revealed. In this review we will briefly summarize the available literature on IE1- and IE2-dependent mechanisms contributing to CMV evasion from intrinsic and innate immune responses.

Cytomegalovirus-specific CD8(+) T cells targeting different HLA/peptide combinations correlate with protection but at different threshold frequencies

Cytomegalovirus (CMV) causes significant morbidity and mortality in patients after haematopoietic stem cell transplantation (HSCT). Due to limitations of current antiviral therapies, alternative approaches, involving transfer of donor-derived CMV-specific CD8(+) T cells, have been considered. Levels of such cells correlating with protection against CMV infection and disease have only been reported in patients expressing HLA-A*0201 and HLA-B*0702. This is despite an increasing number of reports describing cells targeting CMV peptides presented by other human leucocyte antigens (HLAs). Considering several frequent HLA alleles, our findings suggest that HLA-A*2402/pp65 (341-349)- and HLA-B*3501/pp65 (123-131)-specific CD8(+) T cells correlate with protection from CMV reactivation at significantly lower cell levels than HLA-A*0101/pp50 (245-253)- and HLA-A*0201/pp65 (495-503)-specific CD8(+) T cells, both in HSCT recipients post-transplant and in healthy CMV seropositive volunteers. This may result from a differing efficiency of the responses restricted by the two sets of HLA alleles. These findings add to the knowledge of immunodominance and differences in antigen processing that are coordinated in individuals with different HLA alleles and have direct implications for therapy and monitoring in patients.

Molecular targets for antiviral therapy of cytomegalovirus infections

Human cytomegalovirus infections are still associated with severe morbidity and mortality in immunocompromised individuals, despite the availability of five drugs that are currently licensed for antiviral therapy. Furthermore, human cytomegalovirus is the most frequent cause of congenital infections for which antiviral treatment options are very limited. Thus, the need for a potent, safe and well-tolerated antiviral drug remains. This review focuses on target molecules that are implicated in the development of innovative anticytomegaloviral approaches, such as viral immediate-early and DNA replication proteins, as well as regulatory protein kinases. Special emphasis is given to promising host factors, in particular the receptor tyrosine kinase PDGF and cyclin-dependent protein kinases, since a combined targeting of viral and cellular factors that are critical for viral replication may alleviate the emergence of drug-resistant virus variants.

Engineering of a chimeric RB69 DNA polymerase sensitive to drugs targeting the cytomegalovirus enzyme

Detailed structural and biochemical studies with the human cytomegalovirus (HCMV UL54) DNA polymerase are hampered by difficulties to obtain this enzyme in large quantities. The crystal structure of the related RB69 DNA polymerase (gp43) is often used as a model system to explain mechanisms of inhibition of DNA synthesis and drug resistance. However, here we demonstrate that gp43 is approximately 400-fold less sensitive to the pyrophosphate analog foscarnet, when compared with UL54. The RB69 enzyme is also able to discriminate against the nucleotide analog inhibitor acyclovir. In contrast, the HCMV polymerase is able to incorporate this compound with similar efficiency as observed with its natural counterpart. In an attempt to identify major determinants for drug activity, we replaced critical regions of the nucleotide-binding site of gp43 with equivalent regions of the HCMV enzyme. We show that chimeric gp43-UL54 enzymes that contain residues of helix N and helix P of UL54 are resensitized against foscarnet and acyclovir. Changing a region of three amino acids of helix N showed the strongest effects, and changes of two segments of three amino acids in helix P further contributed to the reversal of the phenotype. The engineered chimeric enzyme can be produced in large quantities and may therefore be a valuable surrogate system in drug development efforts. This system may likewise be used for detailed structural and biochemical studies on mechanisms associated with drug action and resistance.

Function of human cytomegalovirus UL97 kinase in viral infection and its inhibition by maribavir

The serine/threonine kinase expressed by human cytomegalovirus from gene UL97 phosphorylates the antiviral drug ganciclovir, but its biological function is the phosphorylation of its natural viral and cellular protein substrates which affect viral replication at many levels. The UL97 kinase null phenotype is therefore complex, as is the mechanism of action of maribavir, a highly specific inhibitor of its enzymatic activity. Studies that utilise the drug corroborate results from genetic approaches and together have elucidated many functions of the UL97 kinase that are critical for viral replication. The kinase phosphorylates eukaryotic elongation factor 1delta, the carboxyl terminal domain of the large subunit of RNA polymerase II, the retinoblastoma tumour suppressor and lamins A and C. Each of these is also phosphorylated and regulated by cdc2/cyclin-dependent kinase 1, suggesting that the viral kinase may perform a similar function. These and other activities of the UL97 kinase appear to stimulate the cell cycle to support viral DNA synthesis, enhance the expression of viral genes, promote virion morphogenesis and facilitate the egress of mature capsids from the nucleus. In the absence of UL97 kinase activity, viral DNA synthesis is inefficient and structural proteins are sequestered in nuclear aggresomes, reducing the efficiency of virion morphogenesis. Mature capsids that do form fail to egress the nucleus as the nuclear lamina are not dispersed by the kinase. The critical functions performed by the UL97 kinase illustrate its importance in viral replication and confirm that the kinase is a target for the development of antiviral therapies.

Inhibition of human cytomegalovirus replication via peptide aptamers directed against the nonconventional nuclear localization signal of the essential viral replication factor pUL84

The UL84 open reading frame of human cytomegalovirus encodes an essential multifunctional regulatory protein that is thought to act in the nucleus as an initiator of lytic viral replication. Nuclear trafficking of pUL84 is facilitated by a complex nonconventional nuclear localization signal (NLS) that mediates its interaction with the cellular importin-alpha/beta pathway. Since binding of pUL84 to importin-alpha proteins mechanistically differs from that of cellular proteins containing a classical NLS, we assumed that specific interference with the nuclear import of pUL84 might be possible and that this could constitute a novel principle for antiviral therapy. In order to test this hypothesis, we employed peptide aptamer technology and isolated several peptide aptamers from a randomized peptide expression library that specifically bind with high affinity to the unconventional pUL84 NLS under intracellular conditions. Coimmunoprecipitation experiments confirmed these interactions in mammalian cells, and the antiviral potential of the identified peptide aptamers was determined using three independent experimental approaches. (i) Infection experiments with a recombinant human cytomegalovirus expressing green fluorescent protein demonstrated 50 to 60% decreased viral replication in primary human fibroblasts stably expressing pUL84-specific aptamers. (ii) A 50 to 70% reduction of viral plaque formation, as well as a 70 to 90% inhibition of virus release in the presence of pUL84-specific aptamers, was observed. (iii) Immunofluorescence analyses revealed a shift from an almost exclusively nuclear pUL84 staining pattern to a nucleocytoplasmic distribution upon coexpression of the identified molecules, indicating that interference with the nuclear import of pUL84 contributes to the observed antiviral activity of the identified pUL84-binding aptamer molecules.

The prevention and treatment of cytomegalovirus infection in haematopoietic stem cell transplantation

Allogeneic haematopoietic stem cell transplantation (HSCT) is an intensive medical treatment involving myeloablative chemo-radiotherapy followed by stem cell rescue using allogeneic haematopoietic stem cells harvested from HLA-matched donors, which is primarily used for the treatment of haematological malignancies. Cytomegalovirus (CMV) infection is one of the major causes of morbidity and death after HSCT. This focused research review highlights the advances made with research into CMV in the HSCT setting. It provides the reader with an overview of current CMV research into the prevention and management of CMV infection.

Assessing the economic merits of managing cytomegalovirus infection in organ and stem cell transplantation

Two preventative approaches exist to manage cytomegalovirus (CMV), a common infection in recipients of organ and stem cell transplants: prophylaxis--the prevention of viraemia--and pre-emptive therapy--the prevention of manifestation of disease in patients who have viraemia. Economic evaluation may provide a helpful framework to inform the choice between these two approaches. However, several issues arise. Direct comparisons of prophylaxis and pre-emptive therapy are rare and there are few epidemiological data that depict the full natural history of CMV infection and disease. There is a need for large, prospective randomised trials that directly compare these two strategies and are of sufficient duration to assess their overall impact on direct and indirect effects of CMV as well as patient quality of life. These methodological issues are relevant to the economic evaluation of preventative measures in other clinical settings and highlight the need for a rigorous evaluative framework to best inform decision making about the optimal strategy for patients.

Role of multivesicular bodies and their components in the egress of enveloped RNA viruses

As an enveloped virus buds, the nascent viral capsid becomes wrapped in a plasma membrane-derived lipid envelope, and a membrane fission event is thus necessary to separate the virion from the host cell. This membrane fission event is well characterised in the case of enveloped RNA viruses, where it is promoted by late assembly domains (L-domains) present at the level of specific viral structural proteins. Research conducted over the past 10 years has demonstrated that L-domains represent docking sites for cellular proteins essential for the biogenesis of a cellular organelle, the multivesicular body (MVB). In this way, enveloped RNA viruses hijack the MVB components to the cellular site where the budding is executed. This review will focus on the cellular machinery exploited by enveloped RNA viruses in order to be released from infected cells. The role of ubiquitin and lipids in viral budding will also be discussed.

5'-O-D-valyl ara A, a potential prodrug for improving oral bioavailability of the antiviral agent vidarabine

In order to improve the oral bioavailability of Adenine 9-beta-D-arabinofuranoside (Vidarabine, also called ara A), an antiviral drug which is active against herpes simplex and varicella zoster viruses and the first agent to be licensed for the treatment of systematic herpes virus infection in man, the corresponding 5'-O-D-valyl ester derivative has been synthesized. Based on their physicochemical properties, 5'-O-valyl ara A has emerged as a potential prodrug candidate to improve the oral bioavailability of vidarabine. We describe in this paper a facile synthesis route for the prodrug and its physicochemical properties.