Ganciclovir-resistant cytomegalovirus clinical isolates: mode of resistance to ganciclovir

Cell and Tissue Specific Metabolism of Nucleoside and Nucleotide Drugs: Case Studies and Implications for Precision Medicine

Many clinically used antiviral drugs are nucleoside or nucleotide analog drugs, which have a unique mechanism of action that requires intracellular phosphorylation. This dependence on intracellular activation presents novel challenges for the discovery and development of nucleoside/nucleotide analog drugs. Contrary to many small molecule drug development programs that rely on plasma pharmacokinetics and systemic exposures, the precise mechanisms that result in efficacious intracellular nucleoside triphosphate concentrations must be understood in the process of nucleoside/nucleotide drug development. The importance is highlighted here, using the following as case studies: the herpes treatment acyclovir, the cytomegalovirus therapy ganciclovir, and human immunodeficiency virus (HIV) treatments based on tenofovir, which are also in use for HIV prophylaxis. For each drug, the specificity of metabolism that results in its activation in different cells or tissues is discussed, and the implications explored. Acyclovir's dependence on a viral enzyme for activation provides selective pressure for resistance mutations. Ganciclovir is also dependent on a viral enzyme for activation, and suicide gene therapy capitalizes on that for a novel oncology treatment. The tissue of most relevance for tenofovir activation depends on its use as treatment or as prophylaxis, and the pharmacogenomics and drug-drug interactions in those tissues must be considered. Finally, differential metabolism of different tenofovir prodrugs and its effects on toxicity risk are explored. Taken together, these examples highlight the importance of understanding tissue specific metabolism for optimal use of nucleoside/nucleotide drugs in the clinic. SIGNIFICANCE STATEMENT: Nucleoside and nucleotide analogue drugs are cornerstones in current antiviral therapy and prevention efforts that require intracellular phosphorylation for activity. Understanding their cell and tissue specific metabolism enables their rational, precision use for maximum efficacy.

Advances in the genotypic diagnosis of cytomegalovirus antiviral drug resistance

Cytomegalovirus (CMV) drug resistance mutation maps are updated with recent information for polymerase inhibitors, the terminase inhibitor letermovir and the UL97 kinase inhibitor maribavir. Newly mapped mutations and their phenotypes provide more detail on cross-resistance properties and suggest the need to expand the CMV gene regions covered in diagnostic testing. Next-generation deep sequencing technology offers a more sensitive, higher resolution view of emerging antiviral resistance and is recommended for use in clinical trials. Issues of standardization and diagnostic utility in comparison with traditional Sanger sequencing remain unresolved. Quality control is important for the accurate and reproducible detection of mutant viral populations in clinical specimens.

Incidence of Ganciclovir Resistance in CMV-positive Renal Transplant Recipients and its Association with UL97 Gene Mutations

Human cytomegalovirus (CMV) remains the most common infection affecting organ transplant recipients. Despite advances in the prophylaxis and acute treatment of CMV, it remains an important pathogen affecting the short- and long-term clinical outcome of solid organ transplant recipient. The emergence of CMV resistance in a patient reduces the clinical efficacy of antiviral therapy, complicates therapeutic and clinical management decisions, and in some cases results in loss of the allograft and/or death of the patient. Common mechanisms of CMV resistance to ganciclovir have been described chiefly with the UL97 mutations. Here we evaluate Incidence of ganciclovir resistance in 144 CMV-positive renal transplant recipients and its association with UL97 gene mutations. Active CMV infection was monitored by viral DNA quantification in whole blood, and CMV resistance was assessed by UL97 gene sequencing. Six mutations in six patients were detected. Three patients (2.6%) of 112 patients with history of ganciclovir (GCV) treatment had clinical resistance with single UL97 mutations at loci known to be related to resistance (including mutations at codon 594, codon 460, and codon 520). three patients who were anti-CMV drug naïve had single UL97 mutations (D605E) without clinical resistance. Our results confirm and extend our earlier findings on the specific mutations in the UL97 phosphotransferase gene in loci that have established role in ganciclovir resistance and also indicate that clinical ganciclovir resistance due to UL97 gene mutations is an issue in subjects with history of with ganciclovir treatment. D605E mutations remains a controversial issue that needs further investigations.

Cidofovir, a New Agent with Potent Anti-Herpesvirus Activity

Cidofovir is a potent, broad spectrum antiviral agent with activity in vitro and in vivo against cytomegalovirus and other members of the herpesvirus family, as well as certain other DNA viruses. After uptake into cells it is converted enzymatically to cidofovir diphosphate, a structural analogue of deoxycytidine triphosphate, which selectively inhibits viral DNA polymerases relative to host cell polymerases. Cross-resistance to cidofovir is not usually seen with human cytomegalovirus isolates that are foscarnet-resistant, or isolates that are ganciclovir-resistant due to a deficiency in ganciclovir phosphorylation. Cross-resistance is seen, however, with isolates that are ganciclovir resistant due to polymerase mutations. A prolonged elimination phase seen in vivo, correlates with a long intracellular half-life seen in vitro and allows for efficacy in animal models of virus infection with infrequent dosing or prophylaxis. Clinical studies of intravenous cidofovir in cytomegalovirus retinitis in patients with AIDS are claimed to show delay of retinitis progression with maintenance doses given once every 2 weeks.

Anti-HCMV Activity of Cidofovir in Combination with Antiviral Compounds and Immunosuppressive Agents: In-Vitro Analyses

Cidofovir 1-[(S)-3-hydroxy-2-(phosphonomethoxy) propyl] cytosine, HPMPC] is an acyclic cytosine nucleotide analogue with potent in-vitro and in-vivo activity against a broad spectrum of herpesviruses including human cytomegalovirus (HCMV). Cidofovir has recently been shown to delay the progression of HCMV retinitis in AIDS patients. Therefore, the effects of several antiviral compounds (GCV, AZT, ddC., ddl, d4T, 3TC and PMEA) on the anti-HCMV activity of cidofovir were investigated in vitro. Cidofovir in combination with GCV demonstrated synergistic inhibition of HCMV replication. Very little significant antiviral synergy or antagonism was measured for any of the other combinations. Furthermore, none of the combinations showed increased cytotoxicity in comparison with each drug alone. Additionally, the antiviral activity of cidofovir was determined in the presence of several immunosuppressive agents (hydrocortisone, cyclosporine A, methotrexate and mycophenolic acid) that are commonly used in the management of organ transplantation rejection in transplant patients. None of these agents altered the antiviral activity of cidofovir in vitro.

Cytomegalovirus in immunocompromised children

PURPOSE OF REVIEW

The purpose of this study is to explore the latest developments in the risk factors, prevention and treatment of cytomegalovirus (CMV) infection in immunocompromised children, including those with congenital immunodeficiency or iatrogenic immune suppression related to solid organ transplantation (SOT) or haematopoietic cell transplantation (HCT).

RECENT FINDINGS

CMV viral load measurements now have international standards, allowing for more reliable comparison across sites and within individuals. Preemptive and prophylactic therapy with routine CMV monitoring in transplant patients has yielded significant reduction in CMV morbidity and mortality in these patients. The majority of U.S. states have adopted routine newborn screening for severe combined immunodeficiency (SCID). Viral infections, including CMV, are a major obstacle preventing optimal curative transplantation in these patients. Several new antiviral agents are currently being investigated for CMV infection in immunocompromised patients. Knowledge on CMV drug resistance in children is emerging and requires further study.

SUMMARY

Conditions that diminish cell-mediated immunity impact the development of CMV infection and disease. These conditions include certain congenital immunodeficiencies and SOT and HCT. Infants identified as having SCID should be screened for CMV risk factors. A preemptive or prophylactic strategy should be chosen for CMV management in children who are high risk posttransplantation. In those who develop disease, viral loads should be monitored and resistance testing considered if response is not deemed adequate. Oral valganciclovir is being used as an alternative to ganciclovir in children, although pharmacokinetic data are limited. Other oral antiviral agents under development are promising future options for paediatric CMV therapy.

Detection of Two Drug-Resistance Mutants of the Cytomegalovirus by High-Resolution Melting Analysis

BACKGROUND

Human cytomegalovirus (CMV) is an opportunistic pathogen that can be treated with ganciclovir. Mutations in the UL97 gene of CMV render the virus ganciclovir resistance. These include H520Q and C603W mutations, against which we developed a novel genotyping assay for their identification.

METHODS

PCR reactions were performed to amplify fragments of the UL97 gene containing H520Q or C603W mutations. High resolution melting analysis (HRMA) coupled with unlabeled DNA probes was employed to identify the shift in melting temperature of the probe-template complex, which reflexes the presence of point mutations.

RESULTS

Melting point analysis performed on the dimeric DNA of PCR products of UL97 gene could not identify mutations in the gene. When coupled to unlabeled probes, point mutations in UL97 can be identified by analyzing the melting curve of probe-template complex. When WT and mutant UL97 DNAs were mixed together to mimic heterogeneous viral population in clinical samples, the genotyping assay is sensitive enough to detect H520Q and C603W mutants that constitute 10% of total DNA input.

CONCLUSION

Probe-based HRMA is effective in detecting H520Q and C603W mutations in the UL97 gene of CMV.

Genotyping cytomegalovirus UL97 mutations by high-resolution melting analysis with unlabeled probe

Human cytomegalovirus (CMV) is an opportunistic pathogen, and infections with this virus can be treated with ganciclovir (GCV). Most GCV-resistant clinical CMV isolates contain a mutation in the UL97 gene. Genotypic assays for diagnostic screening of GCV-resistant CMV have been developed. High-resolution melting analysis (HRMA) with unlabeled probe is considered a perfect tool for this purpose. In this study, we have developed an HRMA-based genotypic test for the detection of UL97 mutations. Wild type and M460V/I mutants of UL97 were constructed. HRMA with unlabeled probe was used as a genotyping method for the detection of M460V/I mutations. The melting peaks obtained directly from PCR products did not enable us to distinguish the wild type from M460 mutants. The sensitivity and accuracy of HRMA were dramatically improved by using unlabeled probe. HRMA with unlabeled probe successfully distinguished M460V from M460I and served well for the detection of M460V/I mutations in clinical samples. HRMA with unlabeled probe proves to be a sensitive and cost-effective genotyping method for the detection of M460 mutations.

Valganciclovir in the treatment of cytomegalovirus retinitis in HIV-infected patients

Oral valganciclovir is a new and highly efficacious alternative to the chronic administration of ganciclovir in the treatment of cytomegalovirus (CMV) retinitis in HIV-infected patients. In addition to its excellent bioavailability and favorable pharmacokinetic profile, valganciclovir has also proved cost effective and is the most widely used drug in the armamentarium for the treatment of CMV retinitis. Valganciclovir is a prodrug of ganciclovir, the erstwhile commonly used therapy. In March 2001, the US Food and Drug Administration approved valganciclovir for the induction and maintenance treatment of CMV disease, including CMV retinitis. Valganciclvoir has compared favorably with both oral and intravenous treatments for induction and maintenance therapy with ganciclovir. The reduced pill burden and the ease of oral administration has helped avoid the risks associated with intravenous therapy. The most serious adverse event is neutropenia, which makes the patient susceptible to infections. In the current review, we have compiled all the available evidence-based information on valganciclovir.

Stability of murine cytomegalovirus genome after in vitro and in vivo passage

While large DNA viruses are thought to have low mutation rates, only a small fraction of their genomes have been analyzed at the single-nucleotide level. Here, we defined the genetic stability of murine cytomegalovirus (MCMV) by whole-genome sequencing. Independently assembled sequences of three sister plaques showed only two single-base-pair substitutions after in vitro passage. In vivo-passaged MCMV likewise demonstrated low mutation rates, comparable to those after in vitro passage, indicating high genome stability of MCMV at the single-nucleotide level in the absence of obvious selection pressure.

Novel real-time monitoring system for human cytomegalovirus-infected cells in vitro that uses a green fluorescent protein-PML-expressing cell line

Promyelocytic leukemia (PML) bodies are discrete nuclear foci that are intimately associated with many DNA viruses. In human cytomegalovirus (HCMV) infection, the IE1 (for "immediate-early 1") protein has a marked effect on PML bodies via de-SUMOylation of PML protein. Here, we report a novel real-time monitoring system for HCMV-infected cells using a newly established cell line (SE/15) that stably expresses green fluorescent protein (GFP)-PML protein. In SE/15 cells, HCMV infection causes specific and efficient dispersion of GFP-PML bodies in an IE1-dependent manner, allowing the infected cells to be monitored by fluorescence microscopy without immunostaining. Since a specific change in the detergent solubility of GFP-PML occurs upon infection, the infected cells can be quantified by GFP fluorescence measurement after extraction. With this assay, the inhibitory effects of heparin and neutralizing antibodies were determined in small-scale cultures, indicating its usefulness for screening inhibitory reagents for laboratory virus strains. Furthermore, we established a sensitive imaging assay by counting the number of nuclei containing dispersed GFP-PML, which is applicable for titration of slow-growing clinical isolates. In all strains tested, the virus titers estimated by the GFP-PML imaging assay were well correlated with the plaque-forming cell numbers determined in human embryonic lung cells. Coculture of SE/15 cells and HCMV-infected fibroblasts permitted a rapid and reliable method for estimating the 50% inhibitory concentration values of drugs for clinical isolates in susceptibility testing. Taken together, these results demonstrate the development of a rapid, sensitive, quantitative, and specific detection system for HCMV-infected cells involving a simple procedure that can be used for titration of low-titer clinical isolates.

Rapid detection of codon 460 mutations in the UL97 gene of ganciclovir-resistant cytomegalovirus clinical isolates by real-time PCR using molecular beacons

A rapid real-time polymerase chain reaction (PCR) assay using molecular beacons has been developed for the simultaneous detection of wild-type and mutant strains of cytomegaloviruses (CMV) with respect to codon 460 of the UL97 gene has been developed. The molecular beacons were designed to complement the wild-type codon 460 or the mutant sequence arising from a single base-pair difference (point mutation). Discrimination between wild-type and mutant templates was demonstrated as the beacons did not generate fluorescence with their respective mismatch targets but only with those that they were designed to perfectly anneal with. Samples that harbor mixed populations of CMV could also be readily recognized. Applied to a small number of clinical samples, the retrospective screening by this assay are in general concordance with that obtained by PCR-RFLP. Using molecular beacons strategy, codon 460 mutation was detected in ten out o the total number of 40 samples, whereas the latter method identified nine samples as containing the mutation. The discrepant result arose from the genotyping of one clinical sample as mixed (containing both wild-type and mutant CMV strains) by molecular beacons but as wild-type by PCR-RFLP, suggesting that this real-time strategy is possibly more sensitive for mutation analysis.

Inhibitory effect of hexamethylene bisacetamide on replication of human cytomegalovirus

The effect of hexamethylane bisacetamide (HMBA), a hybrid polar compound, on gene expression and replication of human cytomegalovirus (HCMV) was studied. When HCMV-infected human thyroid papillary carcinoma (TPC-1) and human embryonic lung (HEL) fibroblast cells were maintained with medium containing 2.5 and 5 mM HMBA for 10 days, there was a greater than 2- to 3-log reduction in virus yield compared to that in untreated cells. Infection of TPC-1 cells with HCMV resulted in an establishment of persistent infection and the cells continuously produced virus with titer of over 10(5) PFU/ml, whereas HMBA prevented the infected cells from entering into the persistent infection. Moreover, treatment of the persistently infected cultures with HMBA reduced production of infectious HCMV more efficiently than did ganciclovir, and eventually ceased HCMV production. Western blotting analysis revealed that HMBA blocks accumulation of the immediate early 2 (IE2) protein in TPC-1 cells and delays synthesis of this protein in HEL cells, but has little effect on the level of the IE1 protein during the early times after infection. Synthesis of the viral early and late proteins in both cells was also substantially blocked by HMBA. The results indicate that the inhibition or the delay of the critical IE2 protein synthesis in the presence of HMBA would actually be a process that fails to proceed beyond the IE stages in HCMV replication cycle.

Synthesis and antiviral activities of 3-aralkylthiomethylimidazo[1,2-b]pyridazine derivatives

The synthesis of novel substituted 3-aralkylthiomethylimidazo[1,2-b]pyridazines is reported. All of the synthesized compounds are devoid of antiviral activity against the replication of human immunodeficiency virus. However, compounds 6-chloro-8-methyl-3-phenethylthioimidazo[1,2-b]pyridazine and 6-chloro-2-methyl-3-phenethylthioimidazo[1,2-b]pyridazine are potent inhibitors of the replication of human cytomegalovirus in vitro, while compounds 6-chloro-2-methyl-3-benzylthiomethylimidazo[1,2-b]pyridazine and 6-chloro-2-methyl-3-phenethyl-thioimidazo[1,2-b]pyridazineare inhibitors of the replication of varicella-zoster virus. The results presented here suggest that compound 10 should be considered as a new lead in the development of antiviral agents.

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.

Prevalence and clinical correlates of YMDD variants during lamivudine therapy for patients with chronic hepatitis B

YMDD variants of hepatitis B virus (HBV) emerge in some patients with chronic hepatitis B who receive lamivudine. YMDD variants were examined in 794 patients in 4 controlled studies of 1 year's duration. The long-term effects of YMDD variants were examined in a subset of patients treated up to 4 years. YMDD variants were detected by polymerase chain reaction (PCR) and restriction fragment-length polymorphism assays. After 1 year, YMDD variants were detected in 81 (24%) of 335 patients. In these patients, the median serum HBV DNA concentration at 1 year was <20% of the baseline level, and serum alanine transaminase (ALT) levels and liver histologic findings had significantly improved. In patients with YMDD variants who were treated for up to 4 years, median HBV DNA and ALT levels showed improvements. Sex, baseline body mass index, and HBV DNA level were associated with emergence of YMDD variants. Patients with YMDD variants losing clinical response with a significant increase in the HBV DNA and ALT levels may require additional therapy.

Genotypic analysis of cytomegalovirus retinitis poorly responsive to intravenous ganciclovir but responsive to the ganciclovir implant

PURPOSE

To determine whether cytomegalovirus (CMV) retinitis that responded poorly to intravenous ganciclovir therapy but responded to the ganciclovir implant was caused by virus with resistance mutations in the viral UL97 and UL54 genes.

DESIGN

Retrospective chart review and laboratory-based experimental study.

METHODS

Regions of the CMV UL97 and UL54 were amplified from the vitreous and analyzed for resistant mutations by a combination of DNA sequencing and restriction digestion. Vitreous from patients with AIDS and retinal infections other than CMV retinitis served as negative controls.

RESULTS

We amplified all target regions of UL97 DNA and most target regions of UL54 DNA from eight eyes with CMV retinitis. In one eye we found a ganciclovir resistance mutation at base 1781 of the UL97 gene, predicting an alanine to valine mutation at codon 594. In a second eye we found a ganciclovir resistance mutation at base 2960 of the UL54 gene, predicting an alanine to glycine mutation at codon 987. In two additional eyes, both from patients with bilateral retinitis, we found UL54 mutations that are likely to confer resistance to ganciclovir but have not been previously described. In both of these patients the UL54 genotype differed between the two diseased eyes.

CONCLUSIONS

Failure to control CMV retinitis with intravenous ganciclovir does not necessarily imply infection with a virus having a known mutation that confers drug resistance. The ganciclovir implant can be an effective therapy for CMV retinitis caused by virus with certain UL97 and UL54 resistance mutations. Cytomegalovirus UL54 genotypes can differ between eyes in patients with bilateral retinitis.

Phase I dose escalation trial evaluating the pharmacokinetics, anti-human cytomegalovirus (HCMV) activity, and safety of 1263W94 in human immunodeficiency virus-infected men with asymptomatic HCMV shedding

1263W94 [maribavir; 5,6-dichloro-2-(isopropylamino)-1,beta-L-ribofuranosyl-1-H-benzimidazole] is a novel benzimidazole compound for treatment of human cytomegalovirus (HCMV) infection and disease, with potent in vitro activity against HCMV and good oral bioavailability. A phase I study was conducted to determine the pharmacokinetics (PK), anti-HCMV activity, and safety of 1263W94 administered as multiple oral doses to human immunodeficiency virus type 1-infected adult male subjects with asymptomatic HCMV shedding. Subjects received one of six dosage regimens (100, 200, or 400 mg three times a day, or 600, 900, or 1,200 mg twice a day) or a placebo for 28 days. 1263W94 demonstrated linear PK, with steady-state plasma 1263W94 profiles predictable based on single-dose data. 1263W94 was rapidly absorbed following oral dosing, and values for the maximum concentration of the drug in plasma and the area under the concentration-time curve increased in proportion to the dose. 1263W94 demonstrated in vivo anti-HCMV activity in semen at all of the dosage regimens tested, with mean reductions in semen HCMV titers of 2.9 to 3.7 log(10) PFU/ml among the four regimens evaluated for anti-HCMV activity. 1263W94 was generally well tolerated; taste disturbance was the most frequently reported adverse event over the 28-day dosing period.

Alkoxyalkyl esters of cidofovir and cyclic cidofovir exhibit multiple-log enhancement of antiviral activity against cytomegalovirus and herpesvirus replication in vitro

The incidence of cytomegalovirus (CMV) retinitis is declining in AIDS patients but remains a significant clinical problem in patients with organ transplants and bone marrow transplants. Prophylaxis with ganciclovir (GCV) or valganciclovir reduces the incidence of CMV disease but may lead to the emergence of drug-resistant virus with mutations in the UL97 or UL54 gene. It would be useful to have other types of oral therapy for CMV disease. We synthesized hexadecyloxypropyl and octadecyloxyethyl derivatives of cyclic cidofovir (cCDV) and cidofovir (CDV) and found that these novel analogs had 2.5- to 4-log increases in antiviral activity against CMV compared to the activities of unmodified CDV and cCDV. Multiple-log increases in activity were noted against laboratory CMV strains and various CMV clinical isolates including GCV-resistant strains with mutations in the UL97 and UL54 genes. Preliminary cell studies suggest that the increase in antiviral activity may be partially explained by a much greater cell penetration of the novel analogs. 1-O-Hexadecyloxypropyl-CDV, 1-O-octadecyloxyethyl-CDV, and their corresponding cCDV analogs are worthy of further preclinical evaluation for treatment and prevention of CMV and herpes simplex virus infections in humans.

Potent and selective inhibition of human cytomegalovirus replication by 1263W94, a benzimidazole L-riboside with a unique mode of action

Benzimidazole nucleosides have been shown to be potent inhibitors of human cytomegalovirus (HCMV) replication in vitro. As part of the exploration of structure-activity relationships within this series, we synthesized the 2-isopropylamino derivative (3322W93) of 1H-beta-D-ribofuranoside-2-bromo-5,6-dichlorobenzimidazole (BDCRB) and the biologically unnatural L-sugars corresponding to both compounds. One of the L derivatives, 1H-beta-L-ribofuranoside-2-isopropylamino-5,6-dichlorobenzimidazole (1263W94), showed significant antiviral potency in vitro against both laboratory HCMV strains and clinical HCMV isolates, including those resistant to ganciclovir (GCV), foscarnet, and BDCRB. 1263W94 inhibited viral replication in a dose-dependent manner, with a mean 50% inhibitory concentration (IC(50)) of 0.12 +/- 0.01 microM compared to a mean IC(50) for GCV of 0.53 +/- 0.04 microM, as measured by a multicycle DNA hybridization assay. In a single replication cycle, 1263W94 treatment reduced viral DNA synthesis, as well as overall virus yield. HCMV mutants resistant to 1263W94 were isolated, establishing that the target of 1263W94 was a viral gene product. The resistance mutation was mapped to the UL97 open reading frame. The pUL97 protein kinase was strongly inhibited by 1263W94, with 50% inhibition occurring at 3 nM. Although HCMV DNA synthesis was inhibited by 1263W94, the inhibition was not mediated by the inhibition of viral DNA polymerase. The parent benzimidazole D-riboside BDCRB inhibits viral DNA maturation and processing, whereas 1263W94 does not. The mechanism of the antiviral effect of L-riboside 1263W94 is thus distinct from those of GCV and of BDCRB. In summary, 1263W94 inhibits viral replication by a novel mechanism that is not yet completely understood.

Mutations in the UL97 ORF of ganciclovir-resistant clinical cytomegalovirus isolates differentially affect GCV phosphorylation as determined in a recombinant vaccinia virus system

Mutations in the human cytomegalovirus (HCMV) UL97 phosphotransferase have been associated with ganciclovir (GCV) resistance due to an impairment of GCV monophosphorylation. Vaccinia virus recombinants (rVV) were generated that encoded different HCMV UL97 proteins (pUL97) with mutations previously detected in resistant HCMV clinical isolates at codons 460, 520, 592, 594, 595, 598 and 607. These rVVs allowed quantification of GCV phosphorylation catalyzed by the different mutated pUL97s. When compared to rVV-UL97 wild type, mean levels of residual intracellular GCV phosphorylation differed by a factor of 10 for the mutated UL97 proteins ranging from 5.2 to 51.8%. Mutations M460V (located in a UL97 region homologous to domain VIb of protein kinases) and H520Q (located in a cytomegalovirus-specific, functionally critical domain) were responsible for the lowest levels of residual GCV phosphorylation (9.3 and 5.2%). Mutations in a region homologous to the domain IX had a lower impact on GCV phosphorylation (15.8-51.8%). The relevance of pUL97 mutation G598S in inducing GCV resistance was demonstrated for the first time.

Resistance of herpesviruses to antiviral drugs: clinical impacts and molecular mechanisms

Nucleoside analogues such as acyclovir and ganciclovir have been the mainstay of therapy for alphaherpesviruses (herpes simplex virus (HSV) and varicella-zoster virus (VZV)) and cytomegalovirus (CMV) infections, respectively. Drug-resistant herpesviruses are found relatively frequently in the clinic, almost exclusively among severely immunocompromised patients receiving prolonged antiviral therapy. For instance, close to 10% of patients with AIDS receiving intravenous ganciclovir for 3 months excrete a drug-resistant CMV isolate in their blood or urine and this percentage increases with cumulative drug exposure. Many studies have reported that at least some of the drug-resistant herpesviruses retain their pathogenicity and can be associated with progressive or relapsing disease. Viral mutations conferring resistance to nucleoside analogues have been found in either the drug activating/phosphorylating genes (HSV or VZV thymidine kinase, CMV UL97 kinase) and/or in conserved regions of the viral DNA polymerase. Currently available second line agents for the treatment of herpesvirus infections--the pyrophosphate analogue foscarnet and the acyclic nucleoside phosphonate derivative cidofovir--also inhibit the viral DNA polymerase but are not dependent on prior viral-specific activation. Hence, viral DNA polymerase mutations may lead to a variety of drug resistance patterns which are not totally predictable at the moment due to insufficient information on specific drug binding sites on the polymerase. Although some CMV and HSV DNA polymerase mutants have been found to replicate less efficiently in cell cultures, further research is needed to correlate viral fitness and clinical outcome.

GCV resistance due to the mutation A594P in the cytomegalovirus protein UL97 is partially reconstituted by a second mutation at D605E

A ganciclovir (GCV)-resistant human cytomegalovirus (HCMV) was isolated from an AIDS patient. Molecular analysis of the HCMV UL97 gene revealed two point mutations, A594P and D605E, respectively. In order to evaluate quantitatively the impact of the individual mutations on GCV phosphorylation, recombinant vaccinia viruses (rVVs) were generated carrying either the two mutations (rVV-594/605) or only one mutation (rVV-594 or rVV-605, respectively). In cells infected with the rVV-594/605 double mutant, the GCV phosphorylation was decreased to 50% compared with the phosphorylation in cells infected with the rVV-UL97 wild-type. In cells infected with the rVV-594, however, the GCV phosphorylation was further decreased to 30%. Interestingly, the mutation D605E led to an even better GCV phosphorylation than that measured in cells infected with the rVV-UL97 wild type. These results were confirmed by plaque reduction assays, indicating that rVV-594 was more resistant to GCV than rVV-594/605. In contrast, rVV-605 was more sensitive to GCV than the rVV-UL97 wild type. Therefore, our results demonstrated for the first time that compensatory mutations can also occur in HCMV, as already shown for human immunodeficiency virus type 1.

(3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assays showed a lack of toxicity of ganciclovir (GCV), C.OXTs, and their derivatives, to TaY(OK) cells at 1 microM. Therefore we compared the antiviral potencies of these drugs at 1 microM by monitoring the viral loads produced during a 1-day period during the course of the drug treatment. Among the drugs tested, 3'-fluorocarbocyclic oxetanocin A (3'F-C.OXT-A) was the most effective for inhibiting the virus production, and at concentrations ranging from 0.5 microM to 10 microM, the inhibition of the viral production was dose-dependent. A comparison of the chemical structures of the derivatives with that of C.OXT-A, which is the parental molecule, suggested that the 3'-fluorine-modification might account for the higher anti-HHV-6 activity and lower cytotoxicity

An infection of TaY cells, which originated from an adult T-cell leukemia, with an HHV-6B OK isolate resulted in a chronically infected culture, termed TaY(OK). Cell cloning analysis revealed that the TaY(OK) culture consisted of a mixture of cells permissive and refractory to the infection, and that the permissive cells were continuously produced from the refractory cell population. Since the chronically infected culture has been maintained for over 2 years without the addition of uninfected TaY cells, we used it for an evaluation of the antiviral potency of nucleoside analogs, especially carbocyclic oxetanocins (C.OXTs).

Variations in the cytomegalovirus DNA polymerase and phosphotransferase genes in relation to foscarnet and ganciclovir sensitivity

BACKGROUND

Identification of human cytomegalovirus (CMV) genome variation is important for understanding mutations associated with drug resistance.

OBJECTIVES

To investigate the CMV resistance to foscarnet (PFA) and ganciclovir (GCV) in patients treated with antiviral drugs and to identify the DNA polymerase (UL54) and phosphotransferase (UL97) gene mutations inducing resistance.

STUDY DESIGN

Antiviral susceptibility of CMV strains/isolates for PFA and GCV was compared by plaque reduction assay and in situ ELISA. UL54 and UL97 gene mutations were identified by sequencing. Growth phenotype of two CMV recombinants with mutations in UL54 was studied.

RESULTS

Six of seven GCV resistant strains had alterations within the UL97. Five of them also had alterations in the UL54 (F412C, L802M or K513E), previously shown to induce GCV resistance. Seven isolates had no or reduced susceptibility to PFA, which had alterations in the UL54 (D588N, E756K, V781I or L802M). By in vitro mutagenesis, it was shown that a mutation at codon D588N of UL54 conferred 9-fold reduced susceptibility to PFA, while a mutation at codon V781I induced 4-fold reduced susceptibility to PFA and GCV. Both recombinants showed the same kinetics of protein expression (IE, E, and L antigen) and virus yields as the CMV Towne strain.

CONCLUSIONS

The recombinants containing alterations within the UL54 (D588N and V781I) showed a reduced susceptibility to antiviral drugs but no change in the replication rate compared to the CMV Towne.

Susceptibility testing. Viral pathogens

The control of the global expansion and proliferation of the AIDS pandemic has been complicated by the emergence of resistant strains of HIV-1 to the many new antiviral drugs directed to the genes coding for reverse transcriptase and protease enzymes of the virus. Similarly, new drug regimens for the management of chronic hepatitis B and C infections have been complicated by the lack of sustained clinical responses recently associated with either nucleotide mutation (HBV) or specific genotype of the virus (HCV). Commercial systems for performing and interpreting genotypic analysis will facilitate the recognition of informative mutations, standardize results between laboratories, and produce informative and interpretative result formats for optimal treatment of patients. Drug-resistant strains of herpesviruses (HSV, VZV, CMV) are generally associated with prolonged treatment of these infections in immunocompromised patients. Ultimate relevance of genotypic assays for routine clinical practice will require correlation with phenotypic results and the outcomes of long-term studies associating clinical improvement with antiviral drugs with specific mutation patterns of these viruses.

Inhibition of ganciclovir-susceptible and -resistant human cytomegalovirus clinical isolates by the benzimidazole L-riboside 1263W94

The average 50% inhibitory concentration (IC(50)) values for AD169 were 0.22 +/- 0.09 microM 1263W94 and 5.36 +/- 0.12 microM ganciclovir. For 35 human cytomegalovirus (HCMV) clinical isolates the average IC(50) was 0.42 +/- 0.09 microM 1263W94, and for 26 ganciclovir-susceptible HCMV clinical isolates the average IC(50) was 3.78 +/- 1.62 microM ganciclovir. Nine HCMV clinical isolates that were resistant to ganciclovir were completely susceptible to 1263W94.

Susceptibilities of human cytomegalovirus clinical isolates to BAY38-4766, BAY43-9695, and ganciclovir

BAY38-4766 and BAY43-9695 are nonnucleosidic compounds with activities against human cytomegalovirus (HCMV). Two phenotypic assays were used to determine the drug susceptibilities of 36 HCMV clinical isolates to the BAY compounds and ganciclovir. Using either assay, both BAY compounds at a concentration of approximately 1 microM inhibited the replication of all 36 HCMV clinical isolates, including 11 ganciclovir-resistant clinical isolates, by 50%.

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.

Human cytomegalovirus up-regulates the phosphatidylinositol 3-kinase (PI3-K) pathway: inhibition of PI3-K activity inhibits viral replication and virus-induced signaling

Infection of quiescent fibroblasts with human cytomegalovirus (HCMV) was found to cause a rapid activation of cellular phosphatidylinositol 3-kinase (PI3-K). Maximum PI3-K activation occurred from 15 to 30 min postinfection. This activation was transient, and by 2 h postinfection (hpi), PI3-K activity had declined to preinfection levels. However, at 4 hpi, a second tier of PI3-K activation was detected, and PI3-K activity remained elevated relative to that of mock-infected cells for the remainder of infection. The cellular kinases Akt and p70S6K and the transcription factor NF-kappaB were activated in a PI3-K-dependent manner at similar times following HCMV infection. Analysis using UV-irradiated virus indicated that no viral protein synthesis was necessary for the first phase of PI3-K activation, but viral protein expression was required for the second tier of PI3-K activation. Treatment of infected fibroblasts with LY294002, a potent and specific inhibitor of PI3-K kinase activity, caused a 4-log decrease in viral titers. LY294002 did not inhibit viral entry, but it did decrease viral immediate-early gene expression. In addition, the protein levels of two viral early genes required for DNA replication, UL84 and UL44, were significantly lower in the presence of LY294002. Furthermore, viral DNA replication was strongly inhibited by LY294002 treatment. This inhibition of viral DNA replication could be reversed by adding back the products of PI3-K activity (PI-3,4-P(2) and PI-3,4,5-P(3)), demonstrating that the effect of LY294002 on the viral life cycle was specifically due to the inhibition of PI3-K activity. These results are the first to suggest that PI3-K mediates HCMV-induced activation of host cell mitogenic pathways. They also provide strong evidence that PI3-K activation is important for initiation of viral DNA replication and completion of the viral lytic life cycle.

Effect of the complexation with cyclodextrins on the in vitro antiviral activity of ganciclovir against human cytomegalovirus

The toxicity of the molecules currently used in the treatment of human cytomegalovirus (HCMV) in immunocompromised hosts often causes interruption of the therapy. Cyclodextrins (Cds), oligosaccharides possessing a hydrophobic cavity, have the property of forming inclusion complexes with a great number of molecules, improving their bioavailability and their biological properties. In this study, we have tested the ability of three native Cds to improve the antiviral effect of ganciclovir (GCV) on two HCMV strains: AD169, a reference susceptible strain, and RC11, a GCV resistant strain. The efficacy of the GCV, expressed in IC50 values, showed no improvement in the presence of alpha-Cd, while the use of beta- and gamma-Cd improved by 6- and 4-fold, respectively, its antiviral activity tested on AD169 strain. The influence of beta- or gamma-Cd on GCV efficiency evaluated on RC11 strain showed a decrease of the IC50. Parallel NMR studies were undertaken in order to characterize formation of [GCV:Cd] complexes. The results showed that complexation between alpha- or gamma-Cd and GCV did not occur. In contrast, spectra proved that beta-Cd formed an inclusion complex with GCV. This complex was characterized in UV-Visible spectrophotometry and the influence of the beta-Cd on the GCV penetration in cells was measured. The use of Cds as carriers of antiviral drugs would be a good alternative to traditional treatment, because it may allow the administration of lower doses and so continuous treatment by reducing the toxic effects of drugs.

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.

Antiviral properties of a series of 1,6-naphthyridine and 7, 8-dihydroisoquinoline derivatives exhibiting potent activity against human cytomegalovirus

A series of 1,6-naphthyridine (L. Chan, H. Jin, T. Stefanac, J. F. Lavallee, G. Falardeau, W. Wang, J. Bedard, S. May, and L. Yuen, J. Med. Chem. 42:3023-3025, 1999) and isoquinoline (L. Chan, H. Jin, T. Stefanac, W. Wang, J. F. Lavallee, J. Bedard, and S. May, Bioorg. Med. Chem. Lett. 9:2583-2586, 1999) analogues exhibiting a high level of anti-human cytomegalovirus (HCMV) activity were investigated in a series of studies aimed at better understanding the mechanism of action of some representatives of this class of compounds. In vitro antiviral profiling revealed that these compounds were active against a narrow spectrum of viruses, essentially the human herpesviruses and type 2 rhinovirus. In HCMV assays, a 39- to 223-fold lower 50% inhibitory concentration was obtained for compound A1 than for ganciclovir against strains AD 169 and Towne. In addition, ganciclovir, foscarnet, cidofovir, and BDCRB (2-bromo-5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole)-resistant HCMV strains remained susceptible to 1,6-naphthyridines and 7, 8-dihydroisoquinolines tested in this study, supporting the view that a novel mechanism of action could be involved. Drug combination studies showed a small but significant synergistic antiviral effect between compound B2 and ganciclovir. Cytotoxicity profiling of representative compounds under various cell growth conditions indicated a generally similar cytotoxic effect, relative to ganciclovir, in log-phase growing cells. However, in stationary cells, a relatively higher level of toxicity was observed than that for control compound. Effect of time of drug addition showed that the anti-HCMV activity of compound A1, ganciclovir, and cidofovir was lost at approximately the same time (72 h postinfection), indicating that the compound was affecting events at the early and late stage of virus replication. This interpretation is also supported by reduction of de novo synthesis of pp65 tegument protein and lack of any effect of the compound on viral adsorption. A reduction of the HCMV enhancer-promoter-directed luciferase expression was also observed in a stably transfected cell line when compound A1 was present at relatively high concentrations.

Antiviral activity of ganciclovir elaidic acid ester against herpesviruses

A fatty acid derivative of ganciclovir (GCV), elaidic acid ganciclovir (E-GCV), has been evaluated for its inhibitory activity against laboratory and clinical strains of herpes simplex type 1 (HSV-1) and type 2 (HSV-2), varicella-zoster virus (VZV) and human cytomegalovirus (HCMV) in human embryonic lung fibroblasts. GCV, cidofovir, acyclovir (ACV), brivudin (BVDU) and foscarnet (PFA) were included as reference compounds. The viruses studied were wild-type, thymidine kinase-deficient (TK(-)) and PFA-resistant (PFA(r)) HSV strains. The IC(50) values obtained for E-GCV were 5- to 30-fold lower than those observed for GCV, the IC(50) value of E-GCV for HSV-1 strain KOS being 0.07 nM. A similarly increased activity of E-GCV (as compared to GCV) was noted for TK(-) and PFA(r) HSV-1 or HSV-2 strains. However, E-GCV did not exhibit superior activity over GCV to VZV or HCMV in vitro. The antiviral efficacy of E-GCV was also evaluated in vivo against intracerebral HSV-2 infection in NMRI mice. Animals were treated intraperitoneally or perorally with E-GCV, GCV or placebo once daily for 10 days, starting the day of infection. E-GCV compared to GCV at equimolar doses, proved markedly more efficacious than GCV in terms of reduction of mortality rate and delay of mean time of death. The elaidic acid ester of GCV should therefore be considered as a novel approach towards the treatment of HSV infections.

A standardized plaque reduction assay for determination of drug susceptibilities of cytomegalovirus clinical isolates

Twelve laboratories collaborated in formulating and testing a standardized plaque reduction assay for cytomegalovirus (CMV) cell-associated clinical isolates. Four characterized and plaque-purified CMV strains, as well as six coded clinical isolates obtained after antiviral therapy, were distributed and tested. Good agreement was obtained for four of the clinical isolates, but a broad distribution of results was obtained for two isolates. Analysis of these results indicates the problems associated with clinical isolates, including the large genetic variability and the highly cell-associated phenotype. This collaborative effort, by addressing these problems, represents a significant step toward the development of a standardized assay.

New strategies for prevention and therapy of cytomegalovirus infection and disease in solid-organ transplant recipients

In the past three decades since the inception of human organ transplantation, cytomegalovirus (CMV) has gained increasing clinical import because it is a common pathogen in the immunocompromised transplant recipient. Patients may suffer from severe manifestations of this infection along with the threat of potential fatality. Additionally, the dynamic evolution of immunosuppressive and antiviral agents has brought forth changes in the natural history of CMV infection and disease. Transplant physicians now face the daunting task of recognizing and managing the changing spectrum of CMV infection and its consequences in the organ recipient. For the microbiology laboratory, the emphasis has been geared toward the development of more sophisticated detection assays, including methods to detect emerging antiviral resistance. The discovery of novel antiviral chemotherapy is an important theme of clinical research. Investigations have also focused on preventative measures for CMV disease in the solid-organ transplant population. In all, while much has been achieved in the overall management of CMV infection, the current understanding of CMV pathogenesis and therapy still leaves much to be learned before success can be claimed.

Phosphorylation of ganciclovir phosphonate by cellular GMP kinase determines the stereoselectivity of anti-human cytomegalovirus activity

A racemic mixture of ganciclovir phosphonate was resolved by stereoselective phosphorylation using GMP kinase. The R-enantiomer of ganciclovir phosphonate was active against human cytomegalovirus but the S-enantiomer was less active. We show that enantiomeric selectivity of antiviral for ganciclovir phosphonate was conferred by stereoselective phosphorylations by mammalian enzymes, not by stereoselective inhibition of DNA polymerase from human cytomegalovirus.

Human cytomegalovirus: challenges, opportunities and new drug development

In the age of highly active antiretroviral therapy, the incidence of human cytomegalovirus (HCMV) retinitis in AIDS patients has decreased substantially. However, this change does not indicate that HCMV disease in AIDS patients and other immunocompromised patients has abated and is no longer a concern. On the contrary, HCMV disease in graft recipients, newborns, and even in AIDS patients still accounts for considerable morbidity, and drug resistance to the anti-HCMV compounds is a major problem. Furthermore, HCMV may have a role in metabolic diseases, such as atherosclerosis. Fortunately there are novel and potentially very effective new compounds undergoing pre-clinical and clinical evaluation. These developments point the way toward new therapies and also to a clearer understanding of the biology of HCMV replication, infection and disease.

Management of CMV retinitis in the era of highly active antiretroviral therapy

Although the epidemiological features of CMV retinitis is changing in patients receiving highly active antiretroviral therapy (HAART), continued attention must be paid to detect and treat earlier CMV infections in AIDS patients to prevent severe ophthalmic complications. Initial therapy must be based on characteristics of the CMV retinitis and patient conditions. Long term therapy of HAART must be pursued, even in patients with increased CD4 and undetectable HIV viral load, until results from large controlled studies are available. reserved.

Acyclovir is phosphorylated by the human cytomegalovirus UL97 protein

Acyclovir (ACV) has shown efficacy in the prophylactic suppression of human cytomegalovirus (HCMV) reactivation in immunocompromised renal transplant patients without the toxicity associated with ganciclovir (GCV). The HCMV UL97 gene product, a protein kinase, is responsible for the phosphorylation of GCV in HCMV-infected cells. This report provides evidence for the phosphorylation of ACV by UL97. Anabolism studies with the HCMV wild-type strain AD169 and with recombinant mutants derived from marker transfer experiments performed by using mutant UL97 DNA from both clinical isolates and a laboratory-derived strain resistant to GCV showed that mutations in the UL97 gene cripple the ability of recombinant virus-infected cells to anabolize both GCV and ACV. These mutant UL97 recombinant viruses were less susceptible to both GCV and ACV than was the wild-type strain. A recombinant herpes simplex virus type 1 strain, in which the thymidine kinase gene is deleted and the UL13 gene is replaced with the HCMV UL97 gene, was able to induce the phosphorylation of ACV in infected cells. Finally, purified UL97 phosphorylated both GCV and ACV to their monophosphates. Our results indicate that UL97 promotes the selective activity of ACV against HCMV.

Cidofovir: a review of its use in cytomegalovirus retinitis in patients with AIDS

Cidofovir is an antiviral nucleotide analogue with significant activity against cytomegalovirus (CMV) and other herpesviruses. The drug is indicated for the treatment of CMV retinitis, a sight-threatening condition, in patients with AIDS. Cidofovir has a long intracellular half-life which allows for a prolonged interval (2 weeks) between maintenance doses. In contrast, other intravenous treatment options for patients with CMV retinitis (i.e. ganciclovir and foscarnet) must be administered on a daily basis. The efficacy of intravenous cidofovir has been demonstrated in patients with AIDS and previously untreated CMV retinitis in multicentre randomised trials, and in a dose-finding study of cidofovir in patients with AIDS and previously treated relapsing CMV retinitis. Clinical trials have been relatively small (n < or = 100 patients) and no studies have been conducted directly comparing intravenous cidofovir with the more established intravenous agents, ganciclovir or foscarnet. Indirect comparisons of clinical trial data suggest that intravenous cidofovir may have similar efficacy to intravenous ganciclovir or foscarnet in delaying progression of CMV retinitis. However, such comparisons must be made with caution because of potential differences in patient populations, data analysis techniques and interobserver variability in the masked assessment of retinal photographs. Nevertheless, intravenous cidofovir offers a less intrusive administration regimen than intravenous ganciclovir or foscarnet because of its prolonged dosage interval. Since therapy is life-long, patients receiving daily intravenous ganciclovir or foscarnet (but not cidofovir) usually require an indwelling central venous catheter and are therefore at increased risk of serious infection. The relatively long dosage interval for cidofovir may also have favourable implications in terms of overall treatment costs and patient quality of life, although specific data are very limited. Potentially irreversible nephrotoxicity is the major treatment-limiting adverse event associated with intravenous cidofovir in patients with AIDS-related CMV retinitis. Anterior uveitis/iritis has been reported frequently with intravenous cidofovir in postmarketing reports and a small number of patients have developed hypotony. Other treatment options for CMV retinitis are also associated with serious adverse events, and selection of pharmacotherapy will depend on a number of factors including retinitis lesion characteristics, patient quality-of-life issues and efficacy and tolerability profiles of available therapies.

CONCLUSION

Although the extent of its use may be limited by its adverse event profile, cidofovir offers a useful addition to the limited number of drugs available for the treatment of CMV retinitis in patients with AIDS.

Antiviral drug susceptibility assays: going with the flow

This review describes the procedures for the use of fluorochrome labeled monoclonal antibodies and flow cytometry for the detection and quantification of virus infected cells. The application of this technology for (1) identifying virus infected cells in clinical specimens obtained from human cytomegalovirus (HCMV) and human immunodeficiency virus (HIV) infected individuals; (2) screening antiviral compounds active against HCMV, HDSV and HIV; and (3) performing drug susceptibility testing for HCMV, HSV and HIV clinical isolates are reviewed. The flow cytometry drug susceptibility assay is rapid, quantitative, and easily performed. It should be considered by anyone interested in performing drug susceptibility testing for any virus for which there are reliable monoclonal antibodies.

Antiviral drug resistance in human cytomegalovirus

Drug-resistant cytomegalovirus (CMV) should be considered when viral shedding persists after several weeks of therapy. The problem is most likely to arise in the setting of a severely immunosuppressed host with continuing or relapsing disease. Not all treatment failure can be attributed to drug resistance. The testing of CMV isolates for drug resistance in cell culture is time-consuming and labor-intensive, but recent advances in understanding of the genetics of resistance have resulted in rapid genotypic assays for specific mutations in the viral UL97 phosphotransferase or UL54 DNA polymerase genes that can predict resistance and cross-resistance to specific drugs. This information may help in the selection of alternative therapy.

Inhibitory effect of 4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)1H - imidazole on HCMV DNA replication and permissive infection

We found that Human Cytomegalovirus (HCMV) infection of human fibroblasts resulted in a dramatic increase in p38 mitogen-activated protein kinase (MAPK) phosphorylation. Recently, drug mediated inhibition of p38 has been demonstrated to exhibit anti-viral activity against HIV (Shapiro, L., Heidenreich, K.A., Meintzer, M.D. and Dinarello, C.A., 1998. Role of p38 mitogen-activated protein kinase in HIV type 1 production in vitro. Proc. Natl. Acad. Sci. USA. 95, 7422-7426). Therefore, we examined the effect of a specific p38 kinase inhibitor on HCMV infection. Inhibiting p38 activity in HCMV infected cells by treating cells with 4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)1H-imidazole; (FHPI), a p38 inhibitor drug, prevented permissive HCMV infection as measured by plaque assay. In the presence of FHPI, HCMV immediate early gene expression was slightly lower at early times of infection, but there was no inhibition of expression of the early gene UL-84, an HCMV protein essential for viral replication. However, FHPI inhibited HCMV DNA replication and late gene expression. The inhibitory effect of FHPI was reversible, as demonstrated by the induction of HCMV replication upon withdrawal of FHPI. Our data describes FHPI as a novel anti-HCMV compound that inhibits synthesis/activation of cellular and/or viral factors required for initiation of HCMV DNA replication.

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.

Comparative study of the anti-human cytomegalovirus activities and toxicities of a tetrahydrofuran phosphonate analogue of guanosine and cidofovir

Cidofovir is the first nucleoside monophosphate analogue currently being used for the treatment of human cytomegalovirus (HCMV) retinitis in individuals with AIDS. Unfortunately, the period of therapy with the use of this compound may be limited due to the possible emergence of serious irreversible nephrotoxic effects. New drugs with improved toxicity profiles are needed. The goal of this study was to investigate the anticytomegaloviral properties and drug-induced toxicity of a novel phosphonate analogue, namely, (-)-2-(R)-dihydroxyphosphinoyl-5-(S)-(guanin-9'-yl-methyl) tetrahydrofuran (compound 1), in comparison with those of cidofovir. The inhibitory activities of both compounds on HCMV propagation in vitro were similar against the AD 169 and Towne strains, with 50% inhibitory concentrations ranging from 0.02 to 0.17 microgram/ml for cidofovir and < 0.05 to 0.09 microgram/ml for compound 1. A clinical HCMV isolate that was resistant to ganciclovir and that had a known mutation within the UL54 DNA polymerase gene and a cidofovir-resistant laboratory strain derived from strain AD 169 remained sensitive to compound 1, whereas their susceptibilities to ganciclovir and cidofovir were reduced by 33- and 10-fold, respectively. Both compound 1 and cidofovir exhibited equal potencies in an experimentally induced murine cytomegalovirus (MCMV) infection in mice, with a prevention or prolongation of mean day to death at dosages of 1.0, 3.2, and 10.0 mg/kg of body weight/day. In cytotoxicity experiments, compound 1 was found to be generally more toxic than cidofovir in cell lines Hs68, HFF, and 3T3-L1 (which are permissive for HCMV or MCMV replication) but less toxic than cidofovir in MRC-5 cells (which are permissive for HCMV replication). Drug-induced toxic side effects were noticed for both compounds in rats and guinea pigs in a 5-day repeated-dose study. In guinea pigs, a greater weight loss was noticed with cidofovir than with compound 1 at dosages of 3.0 and 10.0 mg/kg/day. An opposite effect was detected in rats, which were treated with the compounds at relatively high dosages (up to 100 mg/kg/day). Compound 1 and cidofovir were nephrotoxic in both rats and guinea pigs, with the epithelium lining the proximal convoluted tubules in the renal cortex being the primary target site. The incidence and the severity of the lesions were found to be dose dependent. The lesions observed were characterized by cytoplasm degeneration and nuclear modifications such as karyomegaly, the presence of pseudoinclusions, apoptosis, and degenerative changes. In the guinea pig model, a greater incidence and severity of lesions were observed for cidofovir than for compound 1 (P < 0.001) with a drug regimen of 10 mg/kg/day.