Combined antiviral effects of paired nucleosides against guinea pig cytomegalovirus replication in vitro

Activity of Penciclovir in Combination with Azido-Thymidine, Ganciclovir, Acyclovir, Foscarnet and Human Interferons against Herpes Simplex Virus Replication in Cell Culture

Combinations of penciclovir (PCV) with other antiviral agents (acyclovir, ACV; ganciclovir, GCV; foscarnet, PFA; azido-thymidine, AZT) or with human interferons (HulFN-α,β,γ) were tested for inhibitory activity against herpes simplex virus type 1 (HSV-1) or type 2 (HSV-2) in cell culture. The antiviral interactions observed between combinations of PCV with ACV or GCV were purely additive. Combinations of PCV with HulFNs demonstrated highly synergistic anti-herpesvirus activity; some synergy was also detected between PCV and PFA against HSV-1. High concentrations of AZT inhibited the antiviral activity of PCV; this antagonism was competitive. In more detailed studies it was demonstrated that high concentrations of AZT also inhibited the antiviral activity of ACV, and that ACV was more sensitive to this antagonism than PCV. It was concluded that the antagonism was unlikely to have clinical significance.

Zidovudine Antagonizes the Antiviral Effects of Ganciclovir against Cytomegalovirus Infection in Cultured Cells and in Guinea Pigs

The antiviral effects of ganciclovir (DHPG) combined with zidovudine (AZT) at several dosages against cytomegalovirus infection were evaluated in cultured cells and in Hartley guinea pigs. Combinations of DHPG and AZT at fixed ratios ranging from 1:0.1 to 1:1 showed reduced antiviral effects of DHPG in cultured human lung fibroblast (HEL) cells and guinea pig embryo (GPE) cells infected with human cytomegalovirus and guinea pig cytomegalovirus, respectively. Synergistic cytotoxicity (CI values < 1) was noted in HEL and GPE cell cultures at all DHPG/AZT combinations tested. In vivo experiments using a fixed ratio at three dosage levels for treatment of GPCMV infected guinea pigs for 5 days did not show significant antagonistic antiviral nor synergistic toxic effects at lower dosages. When GPCMV infected guinea pigs were treated with DHPG and AZT in combinations at 40/20, 40/40 and 40/80 mg kg−1 day 1 for 7 days, a significant increase of GPCMV infectivity titres in the salivary gland, lung and spleen were noted when compared with those animals treated with DHPG 40mg kg−1 day−1 alone. In addition, histopathological findings showed more cytotoxicity in the bone marrow of infected and non-infected animals treated wth DHPG/AZT combinations than animals treated with each drug alone. These results suggest that AZT antagonizes the antiviral effects of DHPG against HCMV and GPCMV replication in cultured cells and GPCMV infection in guinea pigs with increased cytotoxicity in cultured cells and in bone marrow of animals receiving the drug combinations.

Sulphated polysaccharides from Ulva clathrata and Cladosiphon okamuranus seaweeds both inhibit viral attachment/entry and cell-cell fusion, in NDV infection

Sulphated polysaccharides (SP) extracted from seaweeds have antiviral properties and are much less cytotoxic than conventional drugs, but little is known about their mode of action. Combination antiviral chemotherapy may offer advantages over single agent therapy, increasing efficiency, potency and delaying the emergence of resistant virus. The paramyxoviridae family includes pathogens causing morbidity and mortality worldwide in humans and animals, such as the Newcastle Disease Virus (NDV) in poultry. This study aims at determining the antiviral activity and mechanism of action in vitro of an ulvan (SP from the green seaweed Ulva clathrata), and of its mixture with a fucoidan (SP from Cladosiphon okamuranus), against La Sota NDV strain. The ulvan antiviral activity was tested using syncytia formation, exhibiting an IC50 of 0.1 μg/mL; ulvan had a better anti cell-cell spread effect than that previously shown for fucoidan, and inhibited cell-cell fusion via a direct effect on the F0 protein, but did not show any virucidal effect. The mixture of ulvan and fucoidan showed a greater anti-spread effect than SPs alone, but ulvan antagonizes the effect of fucoidan on the viral attachment/entry. Both SPs may be promising antivirals against paramyxovirus infection but their mixture has no clear synergistic advantage.

Theoretical basis, experimental design, and computerized simulation of synergism and antagonism in drug combination studies

The median-effect equation derived from the mass-action law principle at equilibrium-steady state via mathematical induction and deduction for different reaction sequences and mechanisms and different types of inhibition has been shown to be the unified theory for the Michaelis-Menten equation, Hill equation, Henderson-Hasselbalch equation, and Scatchard equation. It is shown that dose and effect are interchangeable via defined parameters. This general equation for the single drug effect has been extended to the multiple drug effect equation for n drugs. These equations provide the theoretical basis for the combination index (CI)-isobologram equation that allows quantitative determination of drug interactions, where CI < 1, = 1, and > 1 indicate synergism, additive effect, and antagonism, respectively. Based on these algorithms, computer software has been developed to allow automated simulation of synergism and antagonism at all dose or effect levels. It displays the dose-effect curve, median-effect plot, combination index plot, isobologram, dose-reduction index plot, and polygonogram for in vitro or in vivo studies. This theoretical development, experimental design, and computerized data analysis have facilitated dose-effect analysis for single drug evaluation or carcinogen and radiation risk assessment, as well as for drug or other entity combinations in a vast field of disciplines of biomedical sciences. In this review, selected examples of applications are given, and step-by-step examples of experimental designs and real data analysis are also illustrated. The merging of the mass-action law principle with mathematical induction-deduction has been proven to be a unique and effective scientific method for general theory development. The median-effect principle and its mass-action law based computer software are gaining increased applications in biomedical sciences, from how to effectively evaluate a single compound or entity to how to beneficially use multiple drugs or modalities in combination therapies.

Interactions among antiviral drugs acting late in the replication cycle of human cytomegalovirus

This study describes the extent of cross-resistance and interactions for selected inhibitors of human cytomegalovirus (HCMV) DNA synthesis and DNA processing. HCMV isolates resistant to the benzimidazole D-ribonucleoside viral DNA processing inhibitors TCRB and BDCRB were sensitive to BAY 38-4766, a non-nucleoside inhibitor of viral DNA processing. This indicates that these two drug types have distinct interactions with the products of HCMV genes UL56 and UL89 required for viral DNA cleavage and packaging. These virus isolates also were sensitive to ganciclovir (GCV) but slightly resistant to the L-benzimidazole ribonucleoside viral DNA synthesis inhibitor 1263W94. Virus resistant to 1263W94 remained sensitive to BDCRB, GCV, and BAY 38-4766. Examination of drug-drug interactions in cell culture assays measuring inhibition of HCMV replication revealed strong synergism for the combination of BDCRB with 1263W94, and for combinations of 1263W94 with cidofovir (CDV) and foscarnet (PFA), but not with GCV. Combinations of GCV with CDV and PFA were synergistic as well. The combination of GCV with 1263W94 showed additive antiviral interactions, whereas, a combination of BAY 38-4766 with GCV showed antagonism. Interaction of BDCRB with BAY 38-4766 showed a mixed pattern of synergy and antagonism. The antiviral synergy observed between GCV and PFA or CDV serves to validate clinical combination therapies for these drugs. Antagonism seen for BAY 38-4766 with GCV indicates that these two drugs are unlikely to be useful for combination therapies. Notably, 1263W94 demonstrated greater synergy in combination with PFA or CDV than did GCV, suggesting some promise for this benzimidazole L-riboside in such combination therapies.

Cidofovir in the treatment of cytomegaloviral disease

OBJECTIVE

To review the clinical pharmacology and microbiology of cidofovir in the therapy of cytomegalovirus (CMV) disease.

DATA SOURCES

Pertinent literature was identified via a MEDLINE search (October 1986-February 1997), and data from abstracts presented at recent scientific meetings were also included; unpublished information was provided by the manufacturer.

STUDY SELECTION

Antiviral activity data were included if widely accepted methodology was used. All clinical data currently available from human studies were also included.

DATA SYNTHESIS

Cidofovir is similar to ganciclovir in mechanism of action; however, cidofovir does not require viral enzymes for activation. Although the half-life of cidofovir in plasma is only 2.6 hours, the intracellular half-life may be much longer, allowing efficacy with biweekly maintenance dosing. In vitro, cidofovir appears to be equally or more effective than the other agents currently available for the treatment of CMV. In vivo, cidofovir appears to be effective in delaying the progression of CMV retinitis, although no clinical trials to date have directly compared cidofovir with either ganciclovir or foscarnet. Current intravenous dose recommendations are 5 mg/kg once weekly for two doses (induction), and then 5 mg/kg once every other week (maintenance). Since cidofovir is cleared almost entirely by the kidneys, dosage adjustments must be made in patients with impaired renal function. Disadvantages of cidofovir primarily include its risks of adverse drug reactions, such as nephrotoxicity, which is likely to occur in up to 50% of patients if appropriate preventative measures are not taken. Neutropenia and constitutional reactions to probenecid are also commonly encountered during the course of cidofovir therapy.

CONCLUSIONS

Cidofovir is the first acyclic phosphonate nucleoside antiviral agent to be approved for general use in the US. In addition to delaying the progression of CMV retinitis, cidofovir may provide some protective benefits to patients at risk for developing the disease and may be active against certain strains of virus resistant to other currently available therapies. Another advantage of cidofovir is its infrequent dosage schedule, which may prove beneficial in patients who are not compliant with daily intravenous dosing regimens. When determining the appropriate treatment for a patient with CMV retinitis, the benefits of using cidofovir must be weighed carefully against the risk of potentially serious adverse effects.

Combined treatment with 2'-nor-cGMP and ganciclovir against cytomegalovirus infection in a guinea pig model

The combination 2'-nor-cGMP/DHPG at fixed ratios 1:5, 1:10 and 1:20 showed synergistic antiviral effects against GPCMV replication in vitro with CI value < 1. In vivo, a fixed ratio of 1:10 at three different dosage levels of 1.25/12.5 mg, 2.5/25 mg and 5/50 mg/kg/day 2'-nor-cGMP/DHPG combination showed only additive results when compared with each drug alone. However, synergistic antiviral effects were obtained when infected guinea pigs were treated with 2'-nor-cGMP/DHPG combination 2.5/10 mg/kg/day (1:4). A significantly lower GPCMV infectivity titer was noted in the salivary gland, lung and spleen of infected guinea pigs treated with the combination of 2'-nor-cGMP/DHPG 2.5/10 mg/kg/day, as compared to animals treated with a corresponding dose of each drug alone. In addition, GPCMV-infected animals treated with the latter combination showed increased body weight than when either drug was used alone. Histopathologically, each drug alone reduced the viral induced changes in the lung and spleen, but the combination therapy reduced these changes still further. Toxic changes seen in the kidney and bone marrow of infected animals treated with 2'-nor-cGMP, 2.5 mg/kg/day were not significantly increased when DHPG 10 mg/kg/day was added to the regimen. Therefore, combined treatment with 2'-nor-cGMP/DHPG in appropriate concentration is more helpful for acute cytomegalovirus infection in guinea pigs than when either drug was used alone.

Ganciclovir antagonizes the anti-human immunodeficiency virus type 1 activity of zidovudine and didanosine in vitro

In studies examining potential interactions between ganciclovir (GCV) and either zidovudine (AZT) or didanosine (DDI) in H9 cells, GCV was found to consistently reduce the anti-human immunodeficiency virus type 1 potency of both AZT and DDI. In the presence of GCV, the 50% effective doses of AZT and DDI were increased three- to sixfold, depending on the molar ratio of drugs and the measure of human immunodeficiency virus type 1 replication (p24 antigen, reverse transcriptase activity, or infectious virus yield). Multiple dose-effect analysis revealed strong antagonism between GCV and either AZT or DDI (combination indices, 2.2 to 6.7). This antagonistic effect occurred at drug concentrations that were well below the cytotoxic range. At higher drug concentrations, the combination of GCV and AZT was synergistically cytotoxic (combination indices, less than 1.0), whereas GCV and DDI were only additively cytotoxic (combination indices, ca. 1.0). Thus, the combination of GCV with AZT or DDI may result in antiviral antagonism and either synergistic (AZT-GCV) or additive (DDI-GCV) cytotoxicity.