Mutations in the human cytomegalovirus UL27 gene that confer resistance to maribavir

Human cytomegalovirus DNA replication: antiviral targets and drugs

Human cytomegalovirus (HCMV) infection is associated with severe morbidity and mortality in immunocompromised individuals, in particular transplant recipients and AIDS patients, and is the most frequent congenital viral infection in humans. There are currently five drugs approved for HCMV treatment: ganciclovir and its prodrug valganciclovir, foscarnet, cidofovir and fomivirsen. These drugs have provided a major advance in HCMV disease management, but they suffer from poor bioavailability, significant toxicity and limited effectiveness, mainly due to the development of drug resistance. Fortunately, there are several novel and potentially very effective new compounds which are under pre-clinical and clinical evaluation and may address these limitations. This review focuses on HCMV proteins that are directly or indirectly involved in viral DNA replication and represent already established or potential novel antiviral targets, and describes both currently available drugs and new compounds against such protein targets.

Protein kinase inhibitors of the quinazoline class exert anti-cytomegaloviral activity in vitro and in vivo

Cytomegalovirus infection is associated with severe disease in immunocompromised individuals. Current antiviral therapy faces several limitations. In a search of novel drug candidates, we describe here the anti-cytomegaloviral properties of two compounds of the chemical class of quinazolines, gefitinib (Iressa) and Ax7396 (RGB-315389). Both compounds showed strong inhibitory effects in vitro against human and animal cytomegaloviruses with IC(50)s in a low micromolar range. Cytotoxicity did not occur at these effective concentrations. The antiviral mode of action was based on the inhibition of protein kinase activity, mainly directed to a viral target kinase (UL97/M97) in addition to cellular target candidates. This was demonstrated by a high sensitivity of the respective protein kinases in vitro and by infection experiments with viral mutants carrying genomic alterations in the ORF UL97/M97 modulating viral drug sensitivity. In a guinea pig model, gefitinib showed inhibition of cytomegaloviral loads in blood and lung tissue. Importantly, the rate of mortality of infected animals was reduced by gefitinib treatment. In contrast to the in vitro data, Ax7396 showed no significant antiviral activity in a mouse model. Further in vivo analyses have to assess the potential use of gefitinib in the treatment of cytomegalovirus disease.

Conserved herpesvirus protein kinases

Conserved herpesviral protein kinases (CHPKs) are a group of enzymes conserved throughout all subfamilies of Herpesviridae. Members of this group are serine/threonine protein kinases that are likely to play a conserved role in viral infection by interacting with common host cellular and viral factors; however, along with a conserved role, individual kinases may have unique functions in the context of viral infection in such a way that they are only partially replaceable even by close homologues. Recent studies demonstrated that CHPKs are crucial for viral infection and suggested their involvement in regulation of numerous processes at various infection steps (primary infection, nuclear egress, tegumentation), although the mechanisms of this regulation remain unknown. Notwithstanding, recent advances in discovery of new CHPK targets, and studies of CHPK knockout phenotypes have raised their attractiveness as targets for antiviral therapy. A number of compounds have been shown to inhibit the activity of human cytomegalovirus (HCMV)-encoded UL97 protein kinase and exhibit a pronounced antiviral effect, although the same compounds are inactive against Epstein-Barr virus (EBV)-encoded protein kinase BGLF4, illustrating the fact that low homology between the members of this group complicates development of compounds targeting the whole group, and suggesting that individualized, structure-based inhibitor design will be more effective. Determination of CHPK structures will greatly facilitate this task.

Accelerated evolution of maribavir resistance in a cytomegalovirus exonuclease domain II mutant

A human cytomegalovirus (CMV) UL54 pol exonuclease domain II mutation, D413A, found in a clinical specimen, conferred ganciclovir (GCV) and cidofovir resistance but not foscarnet resistance when incorporated into laboratory strain T2294. After several passages without drug, mutation was observed in five of eight plaques of T2294, and its plating efficiency under foscarnet was increased approximately 30-fold over that of a control strain. When T2294 was serially passed under maribavir (MBV), phenotypic changes and viral UL97 mutations were detected by passage 5, much earlier than previously reported for other CMV strains. By passage 15, mutations included two cases of H411Y, one each of H411L and H411N, three of T409M, five of V353A, and one of L397R. Five instances of codon 409 or 411 mutations evolved into double mutations including V353A. Marker transfer experiments showed H411N/Y/L to confer 9- to 70-fold-increased MBV resistance and combinations of H411L/Y and V353A to confer >150-fold-increased MBV resistance, but no GCV resistance. These findings are consistent with defective exonuclease activity of the pol D413A mutant T2294, leading to the accelerated evolution of UL97 mutations under MBV. This recapitulated the known resistance mutations V353A, L397R, and T409M; suggested their relative frequency; and identified new ones at codon 411. These UL97 mutations predict an MBV binding region overlapping the kinase ATP binding site and located upstream of known GCV resistance mutations. The existence of viable pol D413A mutants may facilitate the selection of additional drug resistance mutations in vivo and the study of these mutations in vitro.

Maribavir antagonizes the antiviral action of ganciclovir on human cytomegalovirus

The cytomegalovirus (CMV) UL97 kinase inhibitor maribavir antagonized the anti-CMV effect of ganciclovir, increasing the ganciclovir 50% inhibitory concentration against a sensitive strain by up to 13-fold. Antiviral activities of foscarnet and cidofovir were unaffected by maribavir.

Maribavir sensitivity of cytomegalovirus isolates resistant to ganciclovir, cidofovir or foscarnet

BACKGROUND

The cytomegalovirus (CMV) UL97 inhibitor drug maribavir (MBV) is undergoing clinical antiviral trials.

OBJECTIVES

To assess the MBV sensitivity of CMV strains and isolates containing mutations that confer resistance to current antiviral drugs ganciclovir, cidofovir or foscarnet.

STUDY DESIGN

Resistant clinical isolates and laboratory strains containing UL97 and or UL54 DNA polymerase mutations were tested for sensitivity to all four drugs by standard plaque reduction assay and a reporter-based yield reduction assay. Sensitive control strains were also tested.

RESULTS

Eleven CMV strains or isolates resistant to GCV, four resistant to FOS and two resistant to CDV, were all sensitive to MBV. These viruses represent four UL97 mutations and three UL54 DNA polymerase mutations. The laboratory derived UL97 L397R mutant was highly MBV-resistant but remained sensitive to the other three drugs.

CONCLUSIONS

No cross-resistance has been detected between viruses resistant to MBV and those resistant to one or more of the current CMV antiviral drugs, consistent with differences in their mechanisms of action.

Antiviral drugs for cytomegalovirus diseases

Cytomegalovirus infections are associated with severe morbidity and mortality is patients at risk for disease because of immune system disabilities; in particular, recipients of stem cell (HSCT) or solid organ (SOT) transplants. There are three systemic drugs approved for CMV treatment: ganciclovir, or its prodrug valganciclovir, foscarnet, and cidofovir. An anti-sense therapeutic, ISIS 2922, is also approved specifically as in intravitreal treatment for CMV retinitis. Ganciclovir, and more recently, valganciclovir, have been useful in proactive approaches of CMV disease management; in both prophylactic and preemptive regimens in HSCT and SOT populations. The major anti-herpes agent valacyclovir has also been approved for prophylaxis of renal transplant recipients, or SOTs outside of the US. These drugs have provided major advances in CMV disease management, although they are limited by intolerable toxicities, oral bioavailability and efficacy, and risk of drug resistance with extended use. Several drugs are in early clinical development which may address these limitations; this review will provide an overview of our current arsenal of available drugs, and of those in the early clinical development pipeline.

Human cytomegalovirus UL27 is not required for viral replication in human tissue implanted in SCID mice

Inhibition of the human cytomegalovirus UL97 kinase by maribavir is thought to be responsible for the antiviral activity of this compound. Some mutations that confer resistance to maribavir map to UL97, however additional mutations that also confer resistance to the drug were mapped to UL27. These open reading frames share a low level of homology, yet the function of pUL27 remains unknown. A recombinant virus with a deletion in the UL27 open reading frame was reported previously to exhibit a slight replication deficit, but a more important function in vivo was hypothesized given its homology to the UL97 kinase. The potential for an important function in vivo was investigated by determining if these knockout viruses could replicate in human tissue implanted in SCID mice. None of the AD169 derived viruses replicated well in the implanted thymus/liver tissue, and is consistent with previous observations, although all of the viruses replicated to some degree in retinal tissue implants. Replication of the parent viruses was observed at 7 days post inoculation, whereas no replication was detected with any of the recombinant viruses with deletions in UL27. By day 14, replication was detected in two of the three knockout viruses and in all of the viruses by day 42. These data are consistent with minimal defects observed in cell culture, but are not consistent with an important role for UL27 in vivo. We conclude that UL27 is not required for viral replication in vivo.

Human cytomegalovirus UL97 Kinase is required for the normal intranuclear distribution of pp65 and virion morphogenesis

Recombinant human cytomegaloviruses that do not express UL97 kinase activity exhibit a distinctive plaque morphology characterized by the formation of highly refractile bodies late in infection. These structures were also observed in infected cells treated with the UL97 kinase inhibitor maribavir. Nuclear inclusions were purified to near homogeneity, and the constituent proteins were identified by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. This analysis demonstrated that the aggregates were formed principally of the tegument proteins pp65 and ppUL25 but also contained additional virion structural proteins including the major capsid protein. Immunoblotting experiments confirmed these results and identified a number of additional viral proteins present in the purified tegument aggregates. Interestingly, the formation of these structures appeared to be dependent on pp65, since it was not induced in cells infected with a recombinant virus with this open reading frame deleted. Morphologically similar aggregates could be reproduced in nuclei of uninfected cells by overexpressing pp65, and their formation was prevented by coexpressing the UL97 kinase. Inhibition of UL97 kinase activity with maribavir or mutation of an essential amino acid in the kinase abolished its ability to prevent aggregate formation. These data taken together suggest that the UL97 kinase impacts the aggregation of pp65 in the nuclei of infected cells. We propose that the kinase plays an important role in the acquisition of tegument during virion morphogenesis in the nucleus and that this activity represents an important step in the production of mature virus particles.

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.

[New antivirals for herpesviruses]

The long-term treatment of herpesvirus infections with current antivirals leads to the development of drug-resistant viruses. Because currently available antivirals finally target the viral DNA polymerase, mutant resistant to one drug often shows cross-resistance to other drugs. This evidence highlights the need for the development of new antivirals that have the different viral targets. Recently, high-through-put screening of large compound collections for inhibiting specific viral enzymes, or in vitro cell culture assay, has identified several new antivirals. These include the inhibitors of helicase/primase complex, terminase complex, portal protein and UL97 protein kinase. This review will focus on these new compounds that directly inhibit viral replication.