Cytomegalovirus: genetics of drug resistance

The Herpesvirus Family in the 21st Century

Eight members of the human herpesvirus (HHV) family have been discovered to date. All the viruses have a characteristic virion structure and share a number of other features. From the perspective of potential treatments, these include the encoding of a protease enzyme which is essential for virus assembly, a DNA polymerase and at least one kinase enzyme which can phosphorylate nucleosides. The herpesviruses also demonstrate a number of mechanisms for avoiding the host immune system, including interference with different stages of the presentation of viral peptides on the cell surface by class I human leukcocyte antigen (HLA) proteins. Three members of the herpesvirus family (HHV-6, HHV-7 and HHV-8) have been discovered within the last 10 years. Both HHV-6 and HHV-7 cause exonthem subitum, although HHV-7 seems to be less pathogenic than HHV-6. HHV-6 also appears to be associated with a number of other disorders. HHV-8 was discovered by the newly developed technique of representational difference analysis, and was identified in Kaposi's sarcoma (KS) tissue. Although the studies to date do not prove that HHV-8 causes KS, they show that the virus is closely associated with the disease. Since discovery of these HHV has been facilitated by the development of new techniques, further technological advances could help to uncover new members of the family.

Current and potential therapies for the treatment of herpes-virus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease-causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, an overview is provided of the diseases caused by the herpesviruses as well as the replication strategy of the better studied members of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

PD0084430: a non-nucleoside inhibitor of human cytomegalovirus replication in vitro

Human cytomegalovirus (HCMV) is a major opportunistic pathogen in immunocompromised individuals. Current therapies target viral DNA replication and accumulate mutations that yield cross-resistance among the approved drugs. A novel, non-nucleoside inhibitor of HCMV replication, PD0084430, was identified in a screening assay using the HCMV beta-galactosidase recombinant RC256. The EC(50) for PD0084430 by inhibition of beta-galactosidase production is 1+/-0.7 microM. This antiviral activity was confirmed by yield reduction and plaque reduction assays using HCMV strain AD169. The TC(50) of PD0084430 as measured by (4C)thymidine incorporation is approximately 30 microM and by XTT is approximately 90 microM. The TC(50) for inhibition of cellular proliferation is approximately 20 microM. Time of addition experiments displayed a similar drop in efficacy for both PD0084430 and GCV when added after the onset of viral DNA replication. The transcomplementation assay for viral DNA replication, using a transfected ori(Lyt) containing plasmid, confirmed that viral DNA synthesis was inhibited at the same concentrations that showed antiviral activity. Western blots showed no apparent block of immediate early or early gene expression. Two ganciclovir (GCV) resistant isolates of HCMV tested showed no cross-resistance to PD0084430. These data suggested a potentially promising novel compound that inhibited HCMV at or before viral DNA replication. However, in vivo testing in mice dosed either orally or intraperitoneally showed rapid glucuronidation on the -OH group. SAR studies on this backbone showed that the -OH group was essential for the antiviral activity in vitro.

Current and potential therapies for the treatment of herpesvirus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella-zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, we provide an overview of the diseases caused by the herpesviruses as well as the replication strategy of the better studiedmembers of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

Current and potential therapies for the treatment of herpesvirus infections

Human herpesviruses are found worldwide and are among the most frequent causes of viral infections in immunocompetent as well as in immunocompromised patients. During the past decade and a half a better understanding of the replication and disease causing state of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella-zoster virus (VZV), and human cytomegalovirus (HCMV) has been achieved due in part to the development of potent antiviral compounds that target these viruses. While some of these antiviral therapies are considered safe and efficacious (acyclovir, penciclovir), some have toxicities associated with them (ganciclovir and foscarnet). In addition, the increased and prolonged use of these compounds in the clinical setting, especially for the treatment of immunocompromised patients, has led to the emergence of viral resistance against most of these drugs. While resistance is not a serious issue for immunocompetent individuals, it is a real concern for immunocompromised patients, especially those with AIDS and the ones that have undergone organ transplantation. All the currently approved treatments target the viral DNA polymerase. It is clear that new drugs that are more efficacious than the present ones, are not toxic, and target a different viral function would be of great use especially for immunocompromised patients. Here, we provide an overview of the diseases caused by the herpesviruses as well as the replication strategy of the better studied members of this family for which treatments are available. We also discuss the various drugs that have been approved for the treatment of some herpesviruses in terms of structure, mechanism of action, and development of resistance. Finally, we present a discussion of viral targets other than the DNA polymerase, for which new antiviral compounds are being considered.

Resistance of human cytomegalovirus to antiviral drugs

Resistance of cytomegalovirus (CMV) to antiviral agents is a well-recognized phenomenon that has been observed in the laboratory and in the clinical setting. Infections caused by antiviral-resistant CMV have been found exclusively among immunocompromised individuals, including patients with AIDS, bone marrow and solid-organ transplant recipients, and patients with hematologic malignancies, and in individuals with primary immunodeficiencies. The majority of these infections have been described to occur in patients with AIDS receiving prolonged antiviral therapy for CMV end-organ disease. Antiviral agents currently licensed for the treatment of CMV infections include ganciclovir, foscarnet, and cidofovir. Resistance of CMV to ganciclovir is related to mutations in the UL97 region of the viral genome and/or mutations in the viral DNA polymerase. Resistance to foscarnet and cidofovir is associated with mutations in the viral DNA polymerase. Antiviral susceptibility of CMV strains containing DNA polymerase mutations is dependent on the region of the DNA polymerase where the mutations are located. Some DNA polymerase mutant viruses are cross-resistant to ganciclovir, foscarnet, and cidofovir. The recognition that specific UL97 and UL54 mutations are associated with resistance to antiviral agents has led to the development of molecular methods for detection of mutant viruses. This article reviews the mechanisms of resistance of CMV to antiviral agents, the laboratory methods for detection of resistant CMV, and the clinical aspects of infections caused by antiviral-resistant CMV.

Cytomegalovirus polymerase chain reaction viraemia in patients receiving ganciclovir maintenance therapy for retinitis

OBJECTIVES

To determine whether recurrence of polymerase chain reaction (PCR) viraemia during maintenance ganciclovir for cytomegalovirus (CMV) retinitis correlates with (i) CMV disease at a new anatomical site, (ii) progression of the presenting retinitis, or (iii) acquisition of genetic changes in gene UL97 associated with resistance to ganciclovir.

DESIGN

A previously described cohort of 45 patients presenting with first episode retinitis was followed clinically using ophthalmoscopy and serial tests for PCR viraemia for a median of 7 months. CMV viral load and genetic markers of ganciclovir resistance were measured in PCR-positive samples.

METHODS

PCR amplification of the glycoprotein B region of CMV and quantitative competitive PCR assays were employed. Genetic changes in UL97 were identified by sequencing/point mutation assay.

RESULTS

PCR viraemia correlated significantly with new episodes of CMV disease (P=0.011) and a trend was seen for the association with progression of retinitis (P=0.07). Amongst the 14 patients PCR-positive during maintenance ganciclovir, 10 (71%) had genetic markers of resistance. None of these patients became PCR-negative in blood after reinduction ganciclovir therapy compared with three out of four without markers of resistance (P=0.022).

CONCLUSIONS

CMV PCR viraemia correlated strongly with the development of new episodes of CMV disease. Most patients with progression of retinitis remained PCR-negative in blood, consistent with therapeutic failure due to poor intraocular penetration of ganciclovir. However, the minority who were PCR-positive in blood may have reinfected their eye, and frequently had markers of ganciclovir resistance. The implications of these findings for the management of patients with CMV disease are discussed.

Lobucavir is phosphorylated in human cytomegalovirus-infected and -uninfected cells and inhibits the viral DNA polymerase

Lobucavir (LBV) is a deoxyguanine nucleoside analog with broad-spectrum antiviral activity. LBV was previously shown to inhibit herpes simplex virus (HSV) DNA polymerase after phosphorylation by the HSV thymidine kinase. Here we determined the mechanism of action of LBV against human cytomegalovirus (HCMV). LBV inhibited HCMV DNA synthesis to a degree comparable to that of ganciclovir (GCV), a drug known to target the viral DNA polymerase. The expression of late proteins and RNA, dependent on viral DNA synthesis, was also inhibited by LBV. Immediate-early and early HCMV gene expression was unaffected, suggesting that LBV acts temporally coincident with HCMV DNA synthesis and not through cytotoxicity. In vitro, the triphosphate of LBV was a potent inhibitor of HCMV DNA polymerase with a Ki of 5 nM. LBV was phosphorylated to its triphosphate form intracellularly in both infected and uninfected cells, with phosphorylated metabolite levels two- to threefold higher in infected cells. GCV-resistant HCMV isolates, with deficient GCV phosphorylation due to mutations in the UL97 protein kinase, remained sensitive to LBV. Overall, these results suggest that LBV-triphosphate halts HCMV DNA replication by inhibiting the viral DNA polymerase and that LBV phosphorylation can occur in the absence of viral factors including the UL97 protein kinase. Furthermore, LBV may be effective in the treatment of GCV-resistant HCMV.