Abstract
We have reported previously that methylenecyclopropane analogs of nucleosides have excellent activity against certain members of the herpesvirus family. A second generation, the 2,2-bis-hydroxymethyl derivatives, were synthesized, and 18 compounds were tested for activity in vitro against herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), human and murine cytomegalovirus (HCMV and MCMV), varicella-zoster virus (VZV), and Epstein-Barr virus (EBV). Selected analogs were also evaluated against human herpesvirus type 6 (HHV-6) and HHV-8. None of the 18 compounds had activity against HSV-1 or HSV-2, but four were active against VZV by plaque reduction (PR) assay at 50% effective concentration (EC50) levels of ≤50 μM. Six of the 18 compounds were active against HCMV by cytopathic effect or PR assays with EC50s of 0.5 to 44 μM, and all were active against MCMV by PR (0.3 to 54 μM). Four of the compounds were active against EBV by enzyme-linked immunosorbent assay (<0.3 to 4.4 μM). Four compounds with CMV activity were also active against HHV-6A and HHV-6B (0.7 to 28 μM), and three compounds were active against HHV-8 (5.5 to 16 μM). One of these, ZSM-I-62, had particularly good activity against CMV, HHV-6, and HHV-8, with EC50s of 0.7 to 8 μM. Toxicity was evaluated in adherent and nonadherent cells, and minimal cytotoxicity was observed. Mechanism of action studies with HCMV suggested that these compounds are phosphorylated by the ppUL97 phosphotransferase and are potent inhibitors of viral DNA synthesis. These results indicate that at least one of these compounds may have potential for use in treating CMV and other herpesvirus infections in humans.
Human cytomegalovirus (HCMV) infects approximately 40 to 80% of the population, and serious life-threatening manifestations are associated with congenital infection and immunosuppression (5). Currently, therapeutic agents such as ganciclovir (GCV), foscarnet (PFA), and cidofovir (CDV) are used to treat HCMV infections, and while GCV is highly effective in treating CMV retinitis, pneumonia, and gastrointestinal disease, long-term therapy is generally required because infection often recurs upon cessation of therapy. The development of drug resistance and severe side effects due to drug toxicity may also occur during long courses of treatment and may limit the clinical use of these drugs. The increased use of immunosuppression for cancer chemotherapy and organ transplantation presents an ever-increasing need for more effective and less toxic antiviral drugs.
A relatively new series of nucleoside analogs, the methylenecyclopropanes were modeled after allene analogs, which have been shown to exhibit antiviral activity (23). The first generation can be regarded as bioisosteric analogs of acyclovir where the C-O-C moiety was replaced by the methylenecyclopropane moiety. In a similar fashion, the second generation of methylenecyclopropane analogs are related to ganciclovir (25). In both cases, the Z- and E-isomeric series of analogs were generated. We have reported previously that Z isomers of the first generation of methylenecyclopropane analogs are potent agents against some members of the herpesvirus family (6, 10, 16-21). These analogs had strong antiviral activity against HCMV, murine CMV (MCMV), human herpesvirus 6 (HHV-6), (10, 20, 24), rat CMV, rhesus monkey CMV, guinea pig CMV (20), and HHV-8 (10). Some of these compounds significantly reduced mortality and virus replication in animal models of HCMV and MCMV (2, 8, 21). More recently, the second generation of methylenecyclopropane analogs, the 2,2-bis-hydroxymethyl derivatives, were synthesized (25). One member of this new series of compounds, ZSM-I-62, which has been given the name cyclopropavir (CPV), has been reported to be very effective in reducing mortality of mice infected with MCMV and to reduce replication of virus in visceral organs from mice infected with MCMV and in human fetal tissue implanted into SCID mice and infected with HCMV (8). The purpose of the following studies was to evaluate the in vitro activity of selected new analogs against herpes simplex virus type 1 (HSV-1) and HSV-2, varicella-zoster virus (VZV), HCMV, MCMV, Epstein-Barr virus (EBV), HHV-6, and HHV-8.
MATERIALS AND METHODS
Compounds and antibodies.
The ZSM, or second generation of methylenecyclopropane analogs, and synadenol and syncytol were provided by Jiri Zemlicka (Wayne State University School of Medicine, Detroit, Mich.), through the Antiviral Substances Program, National Institute of Allergy and Infectious Diseases, National Institutes of Health. They were provided in small quantities, which limited the number of in vitro replicates. The synthesis of these compounds was reported previously (25), and their structures are presented in Fig. 1. They were provided in powder form, reconstituted in 10% dimethyl sulfoxide in tissue culture media, and diluted in RPMI 1640 containing 10% fetal bovine serum (FBS), 2 mM l-glutamine, 100 U of penicillin/ml, and 25 μg of gentamicin/ml to yield a final stock concentration of 1 mg/ml. Acyclovir (ACV), CDV, and GCV were purchased from the University of Alabama Hospital Pharmacy, prepared in minimal essential medium (MEM) with Earle's salts containing 2% FBS, l-glutamine, penicillin, and gentamicin, and stored at 4°C. Maribavir (1263 W94) was provided by K. Biron (GlaxoWellcome, Inc., Research Triangle Park, N.C.) and prepared as described above. Primary antibodies for flow cytometry were purchased from Chemicon (Temecula, Calif.), and secondary antibodies were purchased from Jackson ImmunoResearch (West Grove, Pa.). Monoclonal antibodies to IE1 (63-27), ppUL44 (28-21), and pp28 (41-18) were kindly provided by William Britt (University of Alabama School of Medicine).
FIG. 1.
Structures of second-generation methylenecyclopropane analogs.
Cell cultures.
Human foreskin fibroblasts (HFFs) and mouse embryo fibroblasts (MEFs) were prepared as primary cultures as reported previously (9, 20, 21, 22) and used in the HCMV, MCMV, HSV-1, HSV-2, and VZV assays. Cells were propagated in MEM containing 10% FBS, 2 mM l-glutamine, 100 U of penicillin/ml, and 25 μg of gentamicin/ml. Daudi cells (American Type Culture Collection, Manassas, Va.), the human T-cell lymphoblastoid line, HSB-2 cells, and body cavity-based lymphoma (BCBL-1) cells (National Institutes of Health AIDS Research and Reference Program, Rockville, Md.) were used in the EBV, HHV-6A, and HHV-8 assays, respectively. These cells were propagated in RPMI 1640 containing 10% FBS, 2 mM l-glutamine, and standard concentrations of antibiotics. Cord blood lymphocyte (CBL) cells were isolated from fresh heparinized umbilical cord blood obtained from the University of Alabama Hospital and prepared as described previously (12). CBLs were used in the HHV-6B assays and were propagated in RPMI 1640 containing 10% heat-inactivated FBS, 2 mM l-glutamine, 100 U of penicillin/ml, 25 μg of gentamicin/ml, 0.25 μg of Fungizone/ml, 0.1 U of interleukin-2 (Sigma, St. Louis, Mo.)/ml, and 0.5 μg of Phaseolus vulgaris agglutinin protein/ml.
Virus pools. (i) HSV-1, HSV-2, VZV, HCMV, and MCMV.
The viruses HSV-1 strain E-377, HSV-2 strain MS, VZV strain Ellen, HCMV strain AD169, and MCMV strain Smith were propagated by standard virological techniques, as reported previously (9, 20). The recombinant HCMV strain RCΔ97 used in the mechanism of action studies contained a large deletion in UL97 and was described previously (13). The GCV-resistant isolates were obtained from Karen Biron (GlaxoSmithKline, Research Triangle Park, N.C.) and have been described previously (3).
(ii) HHV-6A and HHV-6B.
HHV-6A strain GS was propagated in HSB-2 cells and virus stock cultures prepared by monitoring the percentage of infected cells by immunofluorescence assays followed by collection, centrifugation, and freezing at −80°C. HHV-6B strain Z-29 was propagated in CBLs and virus stock solutions prepared as described for HHV-6A (10).
(iii) EBV.
P3HR-1 cells (American Type Culture Collection) were used to propagate EBV. The cells were incubated for 2 weeks at 34°C. After incubation, cells were centrifuged with a Sorvall GSA rotor at 6,000 rpm for 15 min. The supernatant was collected, and the virus concentrated by centrifugation at 30,000 × g for 90 min. Cell-free virus stocks were resuspended in MEM containing 10% FBS and antibiotics (23).
(iv) HHV-8.
BCBL-1 cells are a continuous cell line that is latently infected with HHV-8. Lytic infection was induced by the addition of 100 ng of tetradecanoyl phorbol acetate (TPA)/ml, and virus-infected cells were propagated as described previously (10).
Determination of antiviral drug cytotoxicity and cell proliferation for adherent cell lines (HFF, MEF). (i) Neutral red uptake assay.
Cells were seeded into 96-well tissue culture plates at 2.5 × 104 cells/well. After 24 h of incubation, medium was replaced with MEM containing 2% FBS, and drug was added to the first row and then diluted serially fivefold from 100 to 0.03 μg/ml. The plates were incubated for 7 days, and cells were stained with neutral red and incubated for 1 h. Plates were washed and shaken for 15 min, and the optical density was read at 540 nm (23). The concentration of drug that reduced cell viability by 50% (CC50) was calculated by using MacSynergy II software (15).
(ii) Cell proliferation assay.
Cells were seeded in six-well plates at a concentration of 2.5 × 104 cells/well. After 24 h, the medium was aspirated, and drug that had been serially diluted 1:5 was added. The plates were incubated for 72 h at 37°C, the cells were trypsinized and counted, and 50% inhibition of cell proliferation (IC50) values were calculated by using MacSynergy II (15).
Determination of antiviral drug cytotoxicity and cell proliferation for nonadherent cell lines (HSB-2, CBL, Daudi, BCBL-1). (i) MTS tetrazolium cytotoxicity assay.
Serial fivefold dilutions of drug starting at 50 μg/ml were prepared in medium and added to 106 cells. Controls were prepared by incubating 106 cells in drug-free media. After a 4- to 6-day incubation, 200 μl of solution was transferred to a 96-well plate in duplicate. Twenty microliters of MTS was added, and the plate was wrapped in foil and incubated at 37°C for 4 h. The quantity of formazan product was measured at 490 nm in a microplate reader and was directly proportional to the number of living cells in culture (10). The drug concentration was plotted against the optical density of each sample, and CC50s were calculated as described above.
(ii) Cell proliferation assay.
Serial fivefold dilutions of drug starting at 50 μg/ml were prepared in media and added to 106 cells. Controls were prepared by incubating 106 cells in drug-free media. After an incubation period of 3 to 4 days, the number of cells for each sample was determined by using hemacytometer slides. The drug concentration was plotted against the total concentration of cells for each sample, and IC50 values were calculated as described above.
Determination of antiviral drug efficacy. (i) Cytopathic effect inhibition assay for HSV, HCMV, and VZV.
Low-passage HFFs were seeded into 96-well tissue culture plates at 2.5 × 105 cells per ml in MEM supplemented with 10% FBS. After 24 h, medium was removed, and fivefold dilutions of each drug were added to triplicate wells. ACV was used as a positive control. For HSV-1, HSV-2, and HCMV, the virus concentration utilized was 1,000 PFU per well, and for VZV, the virus concentration was 2,500 PFU per well. The plates were incubated at 37°C for 3 days for HSV-1 and HSV-2, 10 days for VZV, and 14 days for HCMV. After incubation, medium was aspirated, and cell monolayers were stained with a 0.5% crystal violet solution in ethanol and formaldehyde for 4 h. The stain was removed, and monolayers were washed, allowed to dry for 24 h, and read on a BioTek plate reader at 620 nm to quantify the cell number. EC50s (50% effective concentrations) were calculated, comparing drug-treated with untreated control wells.
(ii) Plaque reduction assay for HCMV, MCMV, and VZV.
HFFs or MEFs were placed into 6- or 12-well plates and incubated at 37°C for 2 days (HFFs) or 1 day (MEFs). ACV or GCV was used as a positive control. Virus was diluted in MEM containing 10% FBS to provide 20 to 30 plaques per well. The medium was aspirated, and 0.2 ml of virus was added to each well in triplicate with 0.2 ml of medium added to drug toxicity wells. The plates were incubated for 1 h with shaking every 15 min. Drug was serially diluted 1:5 in MEM with 2% FBS starting at 100 μg/ml and added to the appropriate wells. Following incubation for 7 days for MCMV, 8 days for HCMV, or 10 days for VZV, cells were stained for 6 h with 2 ml of 5.0% neutral red in phosphate-buffered saline (PBS). The stain was aspirated, and plaques were counted by using a stereomicroscope at a magnification of ×10. By comparing drug-treated with untreated wells, EC50s were calculated in a standard manner.
Determination of antiviral drug efficacy against EBV.
Serial fivefold dilutions of drug starting at 50 μg/ml were prepared in medium. ACV was used as a positive control. To determine antiviral efficacy, 106 Daudi cells were incubated for 1 h with sufficient EBV strain P3HR-1 to infect 10% of the cells. After infection, appropriate dilutions of drug were added, and cells were incubated for 3 days at 37°C. Negative controls were prepared by incubating 106 Daudi cells in drug-free medium for 3 days at 37°C. Virus controls were prepared by incubating 106 Daudi cells as described above in drug-free medium for 3 days at 37°C. After incubation, hemacytometer slides were used to determine the concentration of cells for each sample. Cells were rinsed thoroughly with PBS, and for each dilution of drug, 4 × 105 cells were added to three duplicate wells of a 96-well plate and allowed to dry. These plates were used for enzyme-linked immunosorbent assay (10, 12).
Enzyme-linked immunosorbent assay.
Cells were fixed in 95% ethanol-5% acetic acid for 20 min at room temperature and then incubated at 37°C with a monoclonal antibody to EBV viral capsid antigen (Chemicon) for 1 h, followed by an incubation with horseradish peroxidase-labeled goat anti-mouse immunoglobulin G1 secondary antibody (Southern Biotechnology, Birmingham, Ala.) for 30 min. Plates were rinsed thoroughly with PBS containing 0.005% Tween 20 between incubations. A tetramethyl benzidine substrate solution was added to each well to initiate the colorimetric reaction. Plates were covered and gently shaken at room temperature for 10 min. The reaction was stopped by the addition of 6 M sulfuric acid. Plates were read immediately on a microplate reader (Bio-Tek Instruments) at 450 nm. The EC50 for each drug was calculated from the plot of drug concentration versus the average optical density at 450 nm for each concentration of drug (22).
Determination of antiviral drug efficacy against HHV-6 and HHV-8.
Serial fivefold dilutions of drug starting at 50 μg/ml were prepared in medium. CDV was used as a positive control. To determine antiviral efficacy for HHV-6, 106 cells were incubated for 1 h with sufficient virus to infect approximately 10% of the cells. After infection, the appropriate dilutions of drugs were added, and cells were incubated for 4 to 6 days at 37°C. Negative controls were prepared by incubating 106 cells in drug-free medium for the designated period, and virus controls prepared by infecting cells as described above followed by incubation in drug-free medium for the designated period. To determine antiviral efficacy for HHV-8, 106 TPA-induced BCBL-1 cells were incubated with dilutions of drug for 5 days with 2 ml of fresh medium added on day 2 of incubation. TPA-induced and uninduced controls were prepared by incubating 106 TPA-induced and uninduced BCBL-1 cells in drug-free medium. After incubation, cells were rinsed with PBS and permeabilized overnight in methanol at −80°C for use in flow cytometric assays (10).
Flow cytometric assay for HHV-6 and HHV-8.
Methods for staining cells have been reported previously (10). After staining, samples were fixed in 2% paraformaldehyde in PBS and analyzed with a FACSCalibur instrument (Becton Dickinson, Franklin Lakes, N.J.). Flow cytometry data were analyzed by using the WinMDI 2.7 data analysis program (Scripps Research Institute, La Jolla, Calif.), and the EC50 was calculated from the plot of drug concentration versus the percentage of antigen-positive cells (12, 10).
Mechanism of Action Studies. (i) Inhibition of HCMV DNA synthesis.
Viral DNA synthesis was measured by a dot blot hybridization assay described previously (13). Briefly, monolayers of HFFs in six-well plates were infected at a multiplicity of infection of 3 PFU of HCMV/cell. After a 1-h incubation, the monolayers were rinsed, and fresh MEM containing CPV was added to each well. Three days postinfection, the monolayers were rinsed with PBS and solubilized with a transfer solution of 0.4 M NaOH and 1.5 M NaCl. Serial twofold dilutions of the DNA samples were performed in transfer buffer and bound to a positively charged nylon membrane by aspiration through a dot blot manifold. Membranes containing viral DNA were hybridized with a digoxigenin-labeled probe corresponding to coordinates 863 to 1113 of the AD169 genome. Specifically bound probe was detected with alkaline phosphatase-labeled antisera to digoxigenin and quantitated with the imaging software Quantity One (Bio-Rad, Hercules, Calif.).
(ii) Western blotting.
Viral proteins were prepared and assayed by western blot as described previously (13). Monolayers of HFFs were infected at a multiplicity of infection of 3 PFU/cell, and drug dilutions were added 1 h postinfection. Protein samples from equivalent numbers of infected cells were harvested at 72 h postinfection, separated on sodium dodecyl sulfate-polyacrylamide gels, and transferred to polyvinylidene difluoride membranes by using a semidry electrophoretic transfer cell. Representative viral proteins were specifically detected with antibodies to IE1, ppUL44, and pp28 and alkaline phosphatase-conjugated secondary antibodies. Membranes were developed with the luminescent substrate CDP star and visualized on Kodak Biomax film. Images were captured on a VersaDoc imaging system and quantified by using Quantity One software.
RESULTS
Activity of second-generation methylenecyclopropane analogs against multiple herpesviruses.
We have reported previously that the methylenecyclopropanes are relatively inactive against the alphaherpesviruses. To determine the activity of this new series of analogs, each of the compounds was evaluated against a representative strain of HSV-1, HSV-2, and VZV, and the antiviral activity for selected analogs is summarized in Table 1. The EC50 for ACV ranged from 5.9 to 6.9 μM for HSV and was 8.6 μM for VZV. None of the new compounds tested had significant activity against HSV-1 or HSV-2; however, one compound, ZSM-I-158, had appreciable activity against VZV, and one other, ZSM-I-287S, had moderate activity.
TABLE 1.
Activity of selected second-generation methylenecyclopropane analogs against multiple herpesviruses
| Compound | EC50 (μM) fora:
|
CC50/IC50c | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| HSV-1b | HSV-2b | VZVb | HCMVb | MCMVb | HHV-6A | HHV-6B | EBVb | HHV-8 | ||
| ACV | 6.9 ± 1.3 | 5.9 ± 1.3 | 8.6 ± 5.0 | 6.6 ± 2.0 | >444/>444 | |||||
| GCV | 3.6 ± 1.4 | 5.7 ± 0.7 | 0.4 ± 0.5 | >392/>392 | ||||||
| CDV | 5.7 ± 3.0 | 1.4 ± 1.1 | ||||||||
| ZSM-I-32F (5a)d | >404 | >404 | >404 | 12 | 10 | 18 ± 23 | 4.6 ± 3.5 | 166 | 57 ± 34 | >404/367 |
| ZSM-I-58 (5c) | >448 | >448 | >448 | 14 | 54 | 18 ± 16 | 23 ± 1.6 | >45 | 197 ± 38 | >448/386 |
| ZSM-I-62 (5b) | >380 | >380 | >380 | 1.2 ± 0.8 | 0.3 | 7.8 ± 1.8 | 0.7 ± 0.7 | 45 | 6.5 ± 0.4 | >380/70 |
| ZSM-I-64 (6b) | >380 | >380 | >380 | 123 | 42 | 116 ± 22 | 43 ± 50 | 119 | >190 | >380/>380 |
| ZSM-I-158 (5g) | >361 | >361 | 3 | 2.7 | 2.0 | 28 ± 4.1 | 2.3 ± 1.2 | 161 | 16 ± 2.8 | >361/258 |
| ZSM-I-89F (5d) | >420 | >420 | >420 | >420 | NTe | NT | NT | 4.4 | 5.5 ± 2.0 | >420/>420 |
| ZSM-I-287S (6h) | >381 | >381 | 39 | 3.3 | NT | NT | NT | <0.3 | >165 | >381/>381 |
The next series of experiments were designed to determine the in vitro activity of these analogs against four members of the betaherpesvirus subfamily, and the results for the selected analogs are summarized in Table 1. Three compounds exhibited antiviral activity against HCMV that was comparable to that of GCV, and two also had very good activity against MCMV, a surrogate virus used for animal studies. CPV was the most potent compound tested, with activity that was superior to that of GCV. Compounds that showed activity against HCMV were also tested for activity against HHV-6A and HHV-6B. Of the four active drugs tested, all exhibited activity against both types of HHV-6 at levels of 0.7 to 28 μM, and the activity tended to correlate with activity against HCMV. Overall, the active compounds were more efficacious against HHV-6B than HHV-6A. Since CPV was the most active compound in the series, it was subsequently tested against clinical and GCV-resistant isolates of HCMV. For each isolate, CPV exhibited antiviral activity that was superior to that of GCV (Table 2) and retained significant activity against GCV-resistant and PFA-resistant viruses (Table 3).
TABLE 2.
Activity of CPV against isolates of HCMV
| HCMV strain | EC50 (μM) ofa:
|
||
|---|---|---|---|
| CPV | GCV | CDV | |
| AD169 | 1.3 ± 0.7 | 5.5 ± 3.5 | 0.7 ± 0.5 |
| Davis | 1.0 ± 0.9 | 5.9 ± 1.9 | 0.5 ± 0.3 |
| Toledo | 1.3 ± 1.2 | 8.2 ± 6.3 | 9.5 ± 9.2 |
| Coffman | 1.9 ± 0.3 | 15.3 ± 11.8 | 1.9 ± 1.3 |
These results are presented as means ± standard deviations of the results from two assays.
TABLE 3.
Activity of CPV against GCV- and PFA-resistant isolates of HCMV
| HCMV strain | Mutation(s) | EC50 (μM) ofa:
|
||
|---|---|---|---|---|
| CPV | GCV or PFA | CDV | ||
| C8914-6 | UL97 | 9.9 ± 3.8 | 232 ± 125 | 3.2 ± 3.5 |
| C8805 37-1-1b | ?UL97 | 2.7 ± 0.1 | 26 ± 11 | 0.7 ± 0.2 |
| C9209 1-4-4b | ?UL97 | 1.5 ± 0.7 | 68 ± 34 | 2.1 |
| C8704 9-1-4b | ?UL97 | 0.3 | 18 | 0.4 |
| 759RD100 | UL97, UL54 | 4.5 ± 3.4 | 175 ± 127 | 1.8 ± 1.0 |
| GDGRK17 | UL97 | 2.4 | 7.1 | 1.6 ± 0.3 |
| GDGRP53 | UL54 | 0.8 ± 0.6 | 47 ± 35 | 3.5 ± 3.5 |
| VR4760R | UL54 | 1.6 ± 0.1 | >227 ± 153c | 0.6 ± 0.4 |
| VR4955R | UL54 | 1.7 ± 0.03 | 170 ± 153c | 0.5 ± 0.4 |
These results are presented as means ± standard deviations of the results from two assays.
Presumed to be UL97 mutants.
This result is for PFA.
The antiviral activity of the selected compounds also was determined against two gammaherpesviruses, EBV and HHV-8, and the results are also summarized in Table 1. Compounds with significant activity against EBV were also evaluated against HHV-8. The ACV control for EBV had an EC50 of 6.6 μM, and two compounds in this series had activity (EC50 of <0.3 to 4.4 μM) that was better than that of ACV. Three of the compounds had activity against HHV-8 ranging from 5.5 to 16 μM, and all were less active than CDV (0.4 μM). Only one, ZSM-I-89F, was active against both EBV and HHV-8, indicating that there was little correlation in the activity of these analogs between the two gammaherpesviruses, EBV and HHV-8.
Cytotoxicity of methylenecyclopropane analogs.
The cytotoxicity of these compounds in both fibroblasts and lymphoblastic cells was determined by two methods that may represent differing types of toxicity. The first was a cytotoxicity assay (CC50) that was determined by uptake of neutral red for fibroblasts or MTS for lymphoblastic cells to assess direct cytotoxicity or cell killing. The second method measured inhibition of cell proliferation (IC50) and indicates the effect a compound has on cell growth. All of the analogs were essentially noncytotoxic for HFFs and did not inhibit cell proliferation at concentrations that were at least 10 times their EC50 (Table 1). The CC50 and IC50 was also determined for the lymphoblastic cells, HSB-2, CBL, Daudi, and BCBL-1 cells used in the assay for HHV-6A, HHV-6B, EBV, and HHV-8, respectively. As indicated above for fibroblast cultures, the methylenecyclopropane analogs were generally not toxic to lymphoblastic cells either by direct cytotoxicity or inhibition of cell proliferation (data not presented).
Mechanism of action in HCMV replication.
An initial investigation of the mechanism of action of a representative compound from this series, CPV, was undertaken to understand how these compounds act specifically to inhibit viral replication. A previous report of the first-generation analogs synadenol and synguanol noted that a recombinant HCMV, strain RCΔ97, that has a large deletion in the UL97 phosphotransferase (14) was resistant to the antiviral effects of this compound (1). Thus, we hypothesized that the second-generation compounds may exhibit a similar dependence on UL97 for their activity. The susceptibility of RCΔ97 to the control CDV was not significantly reduced, since it does not require phosphorylation by ppUL97 (Table 4). However, this virus was comparatively resistant to both GCV and maribavir, with EC50s that were increased 8- and >45-fold, respectively. The deletion in UL97 also conferred resistance to both CPV and syncytol, the cytidine analog of synadenol. Thus, the new analogs presented here exhibit a dependence on UL97 similar to that of the compounds reported previously (1).
TABLE 4.
Efficacy of methylenecyclopropane analogs against a recombinant HCMV with a large deletion in UL97
| Compound | EC50 (μM) fora:
|
Fold resistance | |
|---|---|---|---|
| RCΔ97 | AD169 | ||
| CDV | 0.3 ± 0.1 | 0.7 ± 0.5 | 0.4 |
| GCV | 59.6 ± 29 | 7.1 ± 5.1 | 8 |
| Maribavir | >100 | 2.2 ± 2.1 | >45 |
| CPV | 25 ± 19.8 | 1.2 ± 0.8 | 21 |
| Synadenol | 27.6 ± 21.6 | 2.1 ± 0.7 | 13 |
| Syncytol | 75 ± 32.1 | 23.8 ± 20.2 | 3 |
These results are presented as means ± standard deviations of the results from two or more assays.
The two most likely explanations for the resistance of the UL97 knockout mutant to CPV are that it is required for the activation of the nucleoside to the monophosphate that is subsequently converted to the active triphosphate form of the drug or that the drug acts through the direct inhibition of the ppUL97 kinase and the deletion of this kinase renders it insensitive to the antiviral effects in a manner similar to that observed for maribavir (3, 22). To distinguish between these two potential mechanisms of action, the effect of CPV on viral DNA synthesis was determined in a dot blot hybridization assay. This compound efficiently inhibited the synthesis of viral DNA at submicromolar concentrations and yielded an EC50 of 0.6 μM (Fig. 2), which corresponds well to the antiviral EC50 for this compound against HCMV (Table 2). Thus, the mechanism of action appears to be closely related to GCV and inhibition of DNA synthesis was likely the primary mode of action.
FIG. 2.
Inhibition of HCMV DNA synthesis and expression of IE1, ppUL44, and pp28. Viral DNA synthesis was determined by a dot blot assay and is represented by filled squares. Expression of IE1 (open squares), ppUL44 (open circles), and pp28 (filled circles) was determined by western blotting. Images were captured, quantified, and expressed as percentages of untreated infected cells.
To confirm these data and to further characterize the block in the viral replication cycle, the expression of representative viral proteins from the α, β, and γ2 kinetic classes were measured by western blotting. The expression of IE1 was unaffected even by high concentrations of CPV, indicating that the drug did not interfere with virus entry or the expression of this immediate-early gene (Fig. 2). Expression of ppUL44 was strongly inhibited at submicromolar concentrations of CPV, which is consistent with the inhibition of DNA synthesis described above. This was confirmed with the antibody to pp28, which is expressed with γ2 kinetics and is absolutely dependent on DNA synthesis for its expression (Fig. 2). Both the inhibition of viral DNA synthesis and the inhibition of late viral protein synthesis were observed at submicromolar concentrations of CPV and are sufficient to account for the observed antiviral activity.
DISCUSSION
The availability of effective therapeutic agents for the treatment of HSV and VZV infections has significantly reduced the morbidity and mortality of these diseases. However, therapies for other herpesvirus infections including HCMV, EBV, HHV-6, and HHV-8 are inadequate, and dose-limiting toxicities associated with GCV, PFA, and CDV have impeded the clinical usefulness of these drugs. The development of resistance to these agents during the course of long-term treatments has also been observed and resulted in the cessation of or a change in treatment (4, 7, 11). Thus, it is necessary to continue to identify and develop more effective and less toxic therapies for these infections.
We have reported previously that methylenecyclopropane nucleosides are potent agents against CMV replication and a second generation of methylenecyclopropane analogs, the 2,2-bis-hydroxymethyl derivatives, was synthesized (25) and evaluated for antiherpesvirus activity. Although the results for some of the analogs were reported in the earlier study, all of the results of the most active ones are included here so their activity against all eight of the herpesviruses tested could be compared. Additionally, it is important to be able to evaluate certain structure-activity relationships among the first- and second-generation compounds. The present study examined 18 second-generation methylenecyclopropane analogs; none of the compounds tested had activity against HSV-1 or HSV-2, and only one compound had appreciable activity against VZV. Two compounds in this series had activity against HCMV that was comparable to that of GCV or CDV, and both also had very good activity against MCMV. CPV was the most potent compound tested and was about 5- to 10-fold more active than GCV against both HCMV and MCMV. This analog also exhibited the best antiviral activity against HHV-6A and HHV-6B of the five compounds tested. Thus, the antiviral activity for the betaherpesviruses correlated well. Because of its high activity against this group of viruses, CPV was tested further against clinical and GCV-resistant isolates of HCMV. In each of these strains, including GCV- and PFA-resistant isolates that contained mutations in UL97 or UL54, CPV exhibited antiviral activity that was comparable to or better than that of GCV or CDV. Three compounds in this series also had activity against EBV with EC50s 10-fold below that of ACV. Three compounds had activity against HHV-8 ranging from 5.6 to 16 μM, but all were less active than CDV. Activity against the gammaherpesviruses appeared to be virus dependent and did not appear to correlate well with other viruses in this group or with the betaherpesviruses. The relatively narrow spectrum of activity for this series of compounds is interesting and is likely related to the different mechanisms each virus may use to phosphorylate these unusual nucleoside analogs. For specific viruses, these results support the potential use of these methylenecyclopropane analogs as antiviral agents and warrant the need for future evaluation of these and similar compounds. In addition to efficacy, cellular toxicity plays a crucial role in determining a compound's potential as an antiviral agent. Neutral red uptake assays in HFFs and MEFs indicated essentially no toxicity of all the active compounds. Cell proliferation assays indicated moderate or no toxicity of most of the compounds in HFFs, MEFs, or lymphoblastic cells.
The antiviral activity spectrum of the second-generation analogues is more narrow than that of the first generation. As in the first-generation series, little activity against HSV-1 or HSV-2 was noted (Table 1). The 2-amino-6-methoxypurine analogue ZSM-I-158 was the only analogue with any significant potency against VZV, as shown by plaque reduction assay. Only two compounds, CPV (ZSM-I-62) and the 2-amino-6-methoxypurine derivative ZSM-I-158 were potent inhibitors of HCMV and MCMV, whereas the adenine derivative ZSM-I-32F and the cytosine analogue ZSM-I-158 were much weaker (Table 1). The activity against HHV-6A and HHV-6B followed a similar trend, but ZSM-I-158 was less active against HHV-6A. For the gammaherpesviruses EBV and HHV-8, the thymine Z-isomer ZSM-89F was the only analogue that was active against both viruses. CPV was active against HHV-8 but inactive in EBV assay (Table 1).
In the second generation of methylenecyclopropanes, CPV exhibited the best activity against HCMV and MCMV replication in vitro. In other studies of animal models with MCMV or HCMV, CPV was more effective than GCV in reducing MCMV replication in tissues of mice or HCMV replication in human tissue implanted into SCID mice (8). Additional studies were conducted with HCMV to help elucidate the mechanism of action for this compound and to help understand how this compound is so selective for the betaherpesviruses. The most likely explanation for this specificity was the selective phosphorylation of this compound by the UL97 kinase homologs in the betaherpesviruses that are distinct from those encoded by the alphaherpesviruses or gammaherpesvirus homologs. Previous studies supported the involvement of ppUL97 phosphotransferase in the anabolism of synguanol and synadenol (1), and studies presented here confirm these results and extended them further to syncytol, the cytidine analog of synadenol (19). Initial studies focused on a potential role for this molecule in the mechanism of action in CPV, and the activity of this compound proved to be largely dependent on the integrity of the UL97 open reading frame. The resistance to CPV conferred by the deletion of UL97 could be a result of either the elimination of this antiviral target from the genome or the prevention of the anabolism of the compound by deletion of the gene required for its phosphorylation. Since the initial genetic evidence suggested that UL97 was involved in the mechanism of action, we hypothesized that viral replication was being inhibited at the level of DNA synthesis through the inhibition of the DNA polymerase by the triphosphate form of this compound. Subsequent experiments confirmed that viral DNA synthesis was inhibited at submicromolar concentrations by this compound. To confirm these results, we looked for potential defects in the temporal cascade of viral protein expression. This analysis demonstrated that initial steps in the viral replication cycle proceeded normally, since immediate-early gene expression was unaffected by CPV. In contrast, the expression of a viral protein with early-late expression kinetics, ppUL44, was inhibited in a manner that closely paralleled the inhibition of DNA synthesis. A similar pattern was observed with antibodies to pp28, which is a viral protein that is absolutely dependent on viral DNA synthesis. These experiments suggested that the block occurred following the early stage in viral replication and prior to the expression of late viral proteins, which is characteristic of inhibitors of viral DNA synthesis. Thus, both the inhibition of viral DNA synthesis and the inhibition of late viral proteins were observed at submicromolar concentrations of CPV and are sufficient to account for the observed antiviral activity.
We propose a model for the mechanism of action of CPV and related analogs for the betaherpesviruses in which they are phosphorylated to the monophosphate by the viral phosphotransferase homologs, ppUL97 in HCMV and the U69 gene product in HHV-6. Subsequent phosphorylation events yield the active triphosphate form of the drugs that specifically inhibit the viral DNA polymerase and inhibit the synthesis of viral DNA. This model is consistent with the experimental data presented here and could also explain the rather narrow spectrum of activity of these compounds, since the betaherpesvirus kinases are different from those encoded by the other herpesviruses. These data do not exclude the possibility that CPV inhibits DNA synthesis to some degree through the direct inhibition of ppUL97, as seen with maribavir. We consider this unlikely since neither maribavir nor the UL97 deletion mutant exhibited the robust inhibition of DNA synthesis observed with CPV.
This proposed mechanism of action is similar to that reported for GCV. It is interesting, however, that GCV-resistant isolates exhibit only slightly reduced susceptibility to CPV. Since CPV and GCV both are apparently activated by the ppUL97 kinase, mutations that preclude the phosphorylation of GCV may also be expected to alter the efficiency of CPV phosphorylation depending on the specific amino acids involved. Nevertheless, it is remarkable that CPV retains significant antiviral activity against GCV-resistant isolates. These analogs are promising because they offer an excellent toxicity profile and are highly active against HCMV and some other betaherpesviruses for which few therapeutic options exist. These results along with those obtained from animal model studies indicate that at least one of these compounds, CPV, has potential for use in the treatment of HCMV infections in humans.
Acknowledgments
This work was supported by Public Health Service contracts N01-AI-85347 and N01-AI-30049 and grants 1-PO1-AI-46390 (NIAID) and 1-RO1-CA32779 (NCI) from the National Institutes of Health, Bethesda, Md.
We thank Angela Williams for excellent technical assistance.
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