Phosphonylmethoxyalkyl purine and pyrimidine derivatives for treatment of opportunistic cytomegalovirus and herpes simplex virus infections in murine AIDS

Inhibition of HIV-1 Protease by Carpobrotus edulis (L.)

Carpobrotus edulis (L.) is a plant commonly found in the Eastern Cape Province of South Africa and is used for the general treatment of infections relating to the human immunodeficiency virus (HIV). HIV-1 protease plays an important role during HIV replication and maturation to its infectious form, and therefore inhibition of the enzyme is one of the main focus areas in drug development. The inhibitory effect of a water extract of C. edulis leaves against HIV-1 protease activity was determined using the SensoLyte® 520 HIV-1 protease assay fluorimetric kit and employing a HiLyte Fluor™488/QXL™520 fluorescence resonance energy transfer (FRET) peptide. Cytotoxicity of the extract towards HeLa Chang cell lines was determined using an in vitro MTT assay, and the phytochemical profile of the extract was determined with FT-IR and LC-MS. HIV-1 protease activity was inhibited 83.06% (IC₅₀ 1.6 mg/ml) (p < 0.0001) by the pepstatin A inhibitor control. Treatment with all C. edulis extract concentrations (16, 1.6, 0.16, and 0.016 mg/ml) inhibited HIV-1 protease activity significantly (p < 0.0001) in a typical dose response manner. With regards to cytotoxicity, the negative controls containing untreated HeLa Chang cells exhibited high formazan formation rates in contrast with the positive controls, containing curcumin, which reduced formazan formation significantly (p < 0.001), exhibiting cytotoxicity towards the cells. There was no significant (p > 0.05) difference in the formazan formation rates between the negative controls and 1, 0.5, 0.125, 0.065, 0.031, and 0.015 mg/ml plant extract, confirming no toxicity of C. edulis extracts towards HeLa Chang cells. Major functional phytochemical compounds identified included alcohols, phenols, alkanes, amines, carboxylic acids, and esters. LC-ESI-TOF/MS analysis revealed the putative identities of main compounds present in the aqueous leaves extract, including some that contribute to its anti-HIV-1 protease action.

Inhibitory Effects of Acyclic Nucleoside Phosphonate Analogues on Hepatitis B Virus DNA Synthesis in HB611 Cells

By using an assay system based on a human hepatoblastoma cell line (HB611) that continuously synthesizes hepatitis B virus (HBV) DNA, 56 acyclic nucleoside phosphonate analogues were examined for their inhibitory effects on HBV DNA synthesis. The following compounds were found to inhibit HBV DNA synthesis at concentrations that were significantly lower than their minimum cytotoxic concentrations; 9-(2-phosphonylmethoxyethyl)adenine (PMEA), 9-(2-phosphonylmethoxyethyl) guanine(PMEG), 9-(2-phosphonylmethoxyethyl) guanine ethyl ester (PMEGEE), 9 - (2 - phosphonylmethoxyethyl) - 1 - deazaadenine (PMEC1A), 9-(2-phosphonylmethoxyethyl)-2,6-diaminopurine (PMEDAP), ( S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine (HPMPA), 9-(3-isopropoxy-2-phosphonylmethoxypropyl)adenine (IPPMPA), 9-( RS)-(2-phosphonylmethoxypropyl)adenine (PMPA) and 9-(3-hydroxy-2-phosphonylmethoxypropyl)-2, 6-diaminopurine (HPMPDAP). The most selective compounds (with indexes greater than 100) were PMEDAP, PMEA, IPPMPA, and PMPA. Acyclic pyrimidine nucleoside phosphonate analogues did not prove markedly selective as anti-HBV agents. Diphosphoryl derivatives of some acyclic purine nucleoside phos-phonates (i.e. PMEA, PMEDAP, HPMPA) were prepared. They proved inhibitory to HBV DNA polymerase but not cellular DNA polymerase α.

HPMPC (cidofovir), PMEA (adefovir) and Related Acyclic Nucleoside Phosphonate Analogues: A Review of their Pharmacology and Clinical Potential in the Treatment of Viral Infections

The acyclic nucleoside phosphonate (ANP) analogues are broad-spectrum antiviral agents, with potent and selective antiviral activity in vitro and in vivo. The prototype compounds are: ( S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC, cidofovir), which is active against a wide variety of DNA viruses; 9-(2-phosphonylmethoxyethyl)adenine (PMEA, adefovir), which is active against retro-, herpes- and hepadnaviruses, and ( R)-9-(2-phosphonylmethoxypropyl) adenine (PMPA), which is active against retro- and hepadnaviruses. The antiviral action of the ANP analogues is based on a specific interaction of the active diphosphorylated metabolite with the viral DNA polymerase. The long intracellular half-life of the active metabolite accounts for the optimal efficacy in infrequent dosing schedules. The potential of HPMPC as a broad-spectrum anti-DNA virus agent, as originally observed in vitro and in vivo, has been confirmed in clinical trials. HPMPC has recently been commercially released in the USA for the treatment of cytomegalovirus retinitis in AIDS patients. In addition, topical systemic HPMPC is being (or will be) explored for use against other herpesviruses (i.e. herpes simplex virus, Epstein-Barr virus, or varicella-zoster virus), by adenoviruses, or by human papilloma- or polyomaviruses. Intravenous HPMPC is associated with dose-dependent nephrotoxicity, that should be counteracted by prehydration and concomitant administration of probenecid, and by the application of an infrequent dosing schedule. The oral prodrug of PMEA, bis(pivaloyloxymethyl)-PMEA, is currently being evaluated in patients infected with human immunodeficiency virus (HIV) or hepatitis B virus. Finally, preclinical data on the efficacy of PMPA in animal retrovirus models point to its potential usefulness against HIV infections, when given either prophylactically or therapeutically in the treatment of established HIV infections.

Ganciclovir Treatment of Murine Cytomegalovirus Infection in Mice Immunosuppressed by Prior Infection with Friend Leukaemia Virus

Since many of the severe cytomegalovirus infections in humans occur in individuals immunosuppressed by the human immunodeficiency virus, we developed an analogous murine model for studying this disease. BALB/c mice infected with the Friend retrovirus complex (FV) were immunosuppressed (i.e., exhibited reduced spleen-cell mitogenic responses and natural killer cell activity) by 21 days after FV inoculation. Challenge with murine cytomegalovirus (MCMV) at that time led to mortality at doses generally non-lethal to normal mice. Superinfection of FV-infected mice with MCMV reduced spleen cell FV infectious centres and splenomegaly, and extended the lives of mice surviving the MCMV infection. Once-daily ganciclovir treatments of 12.5,25, and 50 mg kg−1 given to dually-infected mice for 5 days starting 24 h after MCMV inoculation resulted in 90–100% survival at 14 days (relative to MCMV inoculation) compared to 15% survival in the placebo group. By 70 days, survival in the drug-treated and placebo groups were 0–5%, these deaths being attributed to FV disease. Ganciclovir treatments reduced MCMV titres in spleen, liver, and kidney during treatment (day 4 of the infection), but lung and salivary gland titres rose between days 7 and 13 in surviving animals. Improved concanavalin A-induced mitogenic responses were noted on day 4 in mice treated with 25 and 50 mg kg−1. These results indicate that the FV/MCMV dual infection in mice can be used as a model for evaluating antiviral agents. Because of the complex nature of the interaction between FV and MCMV, the model may be more appropriate for advanced studies of well-defined viral inhibitors than for routine screening of potential new compounds.

Adefovir dipivoxil: A new antiviral agent for the treatment of hepatitis B virus infection

BACKGROUND

Hepatitis B virus (HBV) infection is a major problem worldwide and an important cause of chronic liver disease and cirrhosis. Currently available treatments include interferon alfa-2b, lamivudine, and adefovir dipivoxil. Adefovir dipivoxil is an acyclic nucleotide analogue that was developed in part to improve on the limitations of earlier therapies.

OBJECTIVE

This article is a review of available data on the clinical pharmacology, virology, efficacy, tolerability, and clinical use of adefovir dipivoxil.

METHODS

A search of the English-language literature indexed on MEDLINE from 1966 to July 2003 was performed using the terms adefovir, PMEA, and Bis-POM PMEA. Pertinent abstracts from scientific meetings on infectious diseases and hepatology were also included. The manufacturer of adefovir dipivoxil provided additional information. These materials were supplemented by US Food and Drug Administration briefing documents and other unpublished materials. In vitro and preclinical studies were included in the review, as were Phase II and III clinical trials.

RESULTS

In vitro, adefovir dipivoxil concentrations exceed those necessary to inhibit both wild-type and lamivudine-resistant isolates of HBV. In clinical trials, adefovir dipivoxil was clinically and virologically effective in patients in whom lamivudine therapy had failed due to the presence of lamivudine-resistant HBV. The drug was generally well tolerated. The risk of nephrotoxicity, the most notable adverse effect of adefovir dipivoxil at previously used higher doses, has been substantially reduced at the currently recommended dosage of 10 mg/d.

CONCLUSION

Based on the data reviewed adefovir dipivoxil is an effective and well-tolerated alternative for the treatment of HBV infection, including disease that is lamivudine resistant.

Current Options for the Therapy of Chronic Hepatitis B Infection

Until recently the only available treatment for chronic hepatitis B was interferon-alpha. Over the past few years a new class of antiviral has become available, the reverse transcriptase inhibitors. Lamivudine is a nucleoside analogue reverse transcriptase inhibitor that was recently approved in many countries for the treatment of hepatitis B. Despite the potent action of lamivudine, the development of new antivirals and new strategies to treat hepatitis B are still the major goal. We review the latest options for therapy of chronic hepatitis B, including combination strategies that could be an approach to improving the response rate of antivirals.

In vitro activities of methylenecyclopropane analogues of nucleosides and their phosphoralaninate prodrugs against cytomegalovirus and other herpesvirus infections

Human cytomegalovirus (HCMV) infection does not generally cause problems in the immunocompetent adult but can result in severe clinical disease in the fetus, neonate, and immunocompromised host. Ganciclovir (GCV), the agent currently used to treat most HCMV infections, has resulted in much therapeutic success; however, efficacy remains suboptimal. Therefore, there is still a need to develop new compounds for use against HCMV infections. In the present study, several Z- and E-series methylenecyclopropane analogues and their phosphoroalaninate prodrugs were tested initially for activity against HCMV, strain AD169, and murine cytomegalovirus (MCMV) in vitro. Many were found to exhibit efficacy comparable to that of GCV against HCMV in plaque assays and were active against MCMV as well. The compounds were also tested for efficacy against herpes simplex virus types 1 and 2, varicella-zoster virus, and Epstein-Barr virus, and some had levels of activity that were comparable to that of acyclovir. In addition, the compounds synguanol (QYL-438) and 2-amino-6-cyclopropylamino analogue (QYL-769) were chosen for further evaluation and were found to be effective against additional laboratory and clinical isolates of HCMV and GCV-resistant isolates. QYL-438 and QYL-769 were found to be nontoxic in human and mouse fibroblasts and were considerably less toxic than GCV in granulocyte macrophage CFUs and erythroid burst-forming units. These results provide evidence for the high activity of some of these methylenecyclopropane analogues against various herpesviruses, particularly HCMV, in tissue culture and suggest that further evaluation is warranted to determine their potential for use in future clinical studies.

Effective treatment of murine cytomegalovirus infections with methylenecyclopropane analogues of nucleosides

A number of new nucleoside analogues with a Z- or E-methylenecyclopropane structure exhibited significant activity against human and murine cytomegaloviruses (HCMV, MCMV) in tissue culture that was generally comparable to, or greater than, 9-[(1-3-dihydroxy-2-propoxy)methyl]guanine (ganciclovir, GCV). Several of these analogues were chosen for further evaluation of therapeutic efficacy utilizing a MCMV infection. Intraperitoneal (i.p.) inoculation of 3-week-old Balb/c mice with 2.0 x 10(5) plaque forming units (pfu) of MCMV results in an acute, lethal infection with rapid virus replication in visceral and glandular tissue, thus, making it an ideal model for identifying compounds that have potential for use in humans. Synadenol (QYL-284A) and synguanol (QYL-438) were administered i.p. once daily for 5 days initiated 6, 24, or 48 h post-viral infection. Significant protection was demonstrated at 50 and 16.7 mg/kg compared to placebo, with efficacy comparable to GCV. When delivered orally once or twice daily at 100 mg/kg per day, QYL-438 was active, but less effective than GCV. In addition, 2-amino-6-methoxypurine analogue (QYL-941) was active at 60 mg/kg administered orally twice daily, comparable to GCV, while it's prodrug (QYL-972) was as effective as GCV at 40 mg/kg when delivered twice daily for 5 days. Additionally, analogue 2-amino-6-cyclopropylaminopurine (QYL-769) was found to be highly efficacious when given orally twice daily for 5 days. Mortality of 0% and 13% was observed at 60 and 20 mg/kg, respectively, which was similar to GCV. Oral treatment with QYL-769 or GCV reduced virus replication in target organs, but neither resulted in complete clearance of MCMV. These data indicate that these new analogues have activity comparable to GCV when given orally to mice and should be evaluated further to assess their potential for use in humans.

Susceptibility of lamivudine-resistant hepatitis B virus to other reverse transcriptase inhibitors

The emergence of resistant hepatitis B virus (HBV), with mutations in the YMDD motif of the polymerase gene after treatment with lamivudine, is becoming an important clinical problem. In this study, susceptibility of wild-type and lamivudine-resistant HBV M552I, M552V, and L528M/M552V mutants to other reverse transcriptase inhibitors was investigated by transient transfection of full-length HBV DNA into human hepatoma cells. HBV DNA replication was monitored by Southern blot hybridization, which showed the presence of a single-stranded band (representative of the HBV replicative intermediates) in the drug-free, wild-type HBV-transfected cells. This band was diminished in the samples of wild-type HBV DNA treated with either lamivudine, adefovir, or lobucavir. The band intensities from the lamivudine-resistant mutants were not decreased by treatment with lamivudine, but were decreased by the treatments with adefovir or lobucavir. In contrast, penciclovir and nevirapine did not diminish the intensity of the single-stranded band of wild-type HBV or the lamivudine-resistant mutants. These results demonstrate that lamivudine-resistant HBV is susceptible to adefovir and lobucavir. Lamivudine-resistant HBV should be treated with adefovir or lobucavir, and combination therapy with lamivudine and adefovir/lobucavir may prevent the emergence of lamivudine-resistant HBV.

Selective inhibition of human papillomavirus-induced cell proliferation by (S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine

(S)-1-[3-Hydroxy-2-(phosphonylmethoxy)propyl]cytosine (HPMPC) is a nucleoside phosphonate analog which in its active diphosphorylated form is known to inhibit herpesvirus DNA polymerase. In this study, we have demonstrated that, in a dose-dependent manner, this compound irreversibly suppressed proliferation of cells infected with human papillomavirus (HPV), which does not possess a viral DNA polymerase. To elucidate the mechanism of cell growth inhibition, cell cycle indicator-regulator expression, thymidine incorporation, transcript levels of apoptosis factors, and anabolic products of HPMPC following drug treatment were evaluated. HPMPC treatment reduced WAF1 (p21) levels independent of those of p53, while proliferating cell nuclear antigen increased. However, in comparison to controls, HPMPC-treated cells displayed a decrease in thymidine incorporation, indicating an inhibition of host DNA polymerase activity. In normal primary keratinocytes, HPMPC predominantly accumulated in the form of the choline adduct HPMPCp-choline. However, in HPV type 16-transformed keratinocytes, HPMPCpp was the most abundant anabolic product, with little HPMPCp-choline having formed. The data imply that an unrecognized viral factor is modulating the conversion of nucleotides, including HPMPC, to the triphosphorylated form.

Effect of Weekly Adefovir (PMEA) Infusions on HIV-1 Virus Load: Results of a Phase I/II Study

The compound 9-(2-phosphonylmethoxyethyl)adenine (adefovir; PMEA) is a potent inhibitor of a number of viruses in vitro, such as human immunodeficiency virus (HIV) type 1 and 2, herpes simplex virus (HSV) type 1 and 2, human papillomavirus virus (HBV) and Epstein–Barr virus (EBV). Adefovir also proved to be effective in vivo against feline immunodeficiency virus (FIV) in cats and simian immunodeficiency virus (SIV) in rhesus monkeys. In an open, non-placebo-controlled trial the antiviral activity of weekly doses of adefovir in nine patients with AIDS or AIDS-related complex was studied for a period of 11 weeks. CD4 cell counts at baseline were between 10 and 450 cells/mm3, HIV-1 RNA levels at baseline were between 24210 copies/ml and 406197 copies/ml. The drug was administered intravenously at a dose of 1000 mg every week and plasma viral load was assessed at multiple points during the study. Administration of adefovir was tolerated well and no severe side effects were seen. The response to adefovir treatment differed widely between patients. The increase in CD4 cell count at end point ranged from -40 to 120 cell/mm3. The lowest HIV RNA levels were measured after 3–5 days, showing an increase thereafter. The nadir in viral load was achieved after 2 weeks, with a mean viral load decline of 0.7 from baseline. The decrease of the HIV RNA level at end point ranged from -0.3 log10 to 1.8 log10, with a mean decrease of 0.4 log10. Our results indicate that adefovir given intravenously once weekly has a short-lasting initial antiviral effect. The effect of more frequent dosing requires further evaluation. If adefovir is to be useful clinically, it needs to be combined with other antiviral agents.

In vitro characterization of the anti-human cytomegalovirus activity of PMEA (Adefovir)

PMEA [9-[2-(phosphonomethoxy)ethyl]adenine; adefovir] has shown anti-cytomegalovirus activity in animal models and in preliminary human trials. PMEA diphosphate (PMEApp), the active antiviral metabolite of PMEA, is a potent inhibitor of human cytomegalovirus (HCMV) DNA polymerase. PMEA is efficiently taken up and phosphorylated to PMEApp in numerous human cell lines. In vitro replication of wild type and drug resistant HCMV clinical isolates is effectively inhibited by PMEA. PMEA in combination with other anti-HCMV agents shows additive inhibition of HCMV replication.

An in vivo model of human cytomegalovirus retinal infection

PURPOSE

To develop an animal model system in which human retina implanted in the anterior chamber of the eyes of rats would support human cytomegalovirus replication. Cytomegalovirus retinitis currently represents the most common cause of posterior uveitis in many urban areas in North America. Despite the tremendous interest in cytomegalovirus retinitis as a result of the acquired immunodeficiency syndrome (AIDS) epidemic, human cytomegalovirus infection has been difficult to model in vivo because of its extreme species-specificity.

METHODS

Human retina was introduced into the anterior chamber of athymic rats and allowed to attach to the rat iris. A human cytomegalovirus mutant carrying a beta-galactosidase indicator gene was then injected into the anterior chamber to infect the implanted tissue. After 4 weeks, the eyes were removed, sectioned, and developed with a chromogenic substrate to demonstrate the presence and location of beta-galactosidase expression.

RESULTS

Multiple spreading foci of beta-galactosidase expression were found in the retinal implants, indicating that human cytomegalovirus replication had occurred within the human tissue. There was no infection of rat tissue.

CONCLUSIONS

This model allows human cytomegalovirus infection of human retina to be established in vivo and sustained long enough to permit multiple cycles of viral replication to occur. The model thus has potential for evaluating antiviral therapies directed against human cytomegalovirus retinal disease.

Efficacy of BMS-180194 against experimental cytomegalovirus infections in immunocompromised mice

A new antiviral nucleoside, BMS-180194 [1R-(1 alpha, 2 beta, 3 alpha)]- 2-amino-9[2,3-bis(hydroxymethyl)cyclobutyl]-1,9dihydro-6H-purin-6- one, is a broad spectrum antiviral agent. The antiviral effectiveness of BMS-180194 against murine cytomegalovirus (MCMV) infection in immunocompromised C57BL/6 mice was investigated and was compared to that of ganciclovir (GCV). LP-BM5 murine retrovirus complex-induced immunocompromised C57BL/6 mice were challenged with MCMV then treated intraperitoneally or per os with various doses of BMS-180194 ranging from 30 to 3 mg/kg/day. When administered intraperitoneally, BMS-180194 was effective against MCMV-mediated mortality in a dose-dependent manner demonstrating a 50% protective dose (PD50) of 3.12 mg/kg/day which was comparable to that of GCV. There was a marked reduction in organ MCMV titers in BMS-180194-treated animals (10-10,000- fold lower than the placebo controls). Similar findings were observed when the compound was administered orally. Interestingly, oral BMS-180194 demonstrated a similar antiviral efficacy as that obtained by the parental route of administration suggesting a high oral bioavailability of the compound. Oral ganciclovir treatment, however, required more than a 4-fold higher amount of GCV to confer the same degree of protection obtained by a parenteral route of administration. Oral BMS-180194 was also effective in reducing the organ MCMV titer in genetically severe combined immunodeficient (SCID) mice. The parenteral or oral antiviral efficacy of BMS-180194 was comparable to that of parenteral ganciclovir against MCMV infection in the present study. Doses of BMS-180194 employed in the present study showed no toxicity to mice. These results suggest that BMS-180194 may be of value as an oral antiviral agent for treatment of opportunistic CMV infections in immunocompromised individuals.

Safety of 9-(2-phosphonylmethoxyethyl)adenine (PMEA) in patients with human immunodeficiency virus infection: a pilot study

The compound 9-(2-phosphonylmethoxyethyl)adenine (PMEA) is a potent inhibitor of a number of viruses in vitro such as human immunodeficiency virus types 1 and 2, herpes simplex virus types 1 and 2, hepatitis B virus, cytomegalovirus, and Epstein-Barr virus. PMEA also proved to be effective in vivo against feline immunodeficiency virus in cats and simian immunodeficiency virus in rhesus monkeys. In an open, non-placebo-controlled trial, the safety of weekly doses of PMEA in 10 patients with acquired immunodeficiency syndrome (AIDS) or AIDS-related complex was studied for a period of 11 weeks. CD4+ T-cell counts at baseline were between 10 and 450/mm(3). The drug was administered intravenously at a dose of 1000 mg. No serious side-effects were seen. On one occasion one patient showed alanine aminotransferase and aspartate aminotransferase levels 5 times higher than the upper limit of normal and another patient showed on one occasion aspartate aminotransferase levels 5 times higher than the upper limit of normal. In another patient serum amalyse levels increased, on one occasion 1.5 times above the upper limit of normal. An improvement in general well-being was reported by all patients. For patients with a CD4+ T-cell count > 100/mm(3) at baseline, the CD4+ T-cell count increased from a mean of 283/mm(3) at baseline to a mean of 448/mm(3) at the end of the study. Repeat infusions of PMEA at a dose of 1000 mg were safe and well tolerated. Our results suggest that PMEA, administrated according to this treatment schedule, may be effective in treating patients with human immunodeficiency virus infection.

Effects of phosphonylmethoxyalkyl derivatives studied with a murine model for abortion induced by equine herpesvirus 1

(S)-9-(3-Hydroxy-2-phosphonylmethoxypropyl)adenine (HPMPA) and (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC) were tested in a mouse model for equine herpesvirus 1-induced abortion. HPMPA, given twice daily, reduced virus replication, but the compound was embryotoxic. A single dose of HPMPC, however, reduced the incidence of abortion and transfer of virus to the fetuses while producing no obvious toxic effects.

Protection against murine cytomegalovirus infection in aged mice and mice with severe combined immunodeficiency disease with the biological response modifiers polyribosinic-polycytidylic acid stabilized with L-lysine and carboxymethylcellulose, maleic anhydride divinyl ether and colony stimulating factor 1

A variety of biological response modifiers (BRMs) have provided antiviral protection to immunocompetent mice, and this prompted us to determine their efficacy against murine cytomegalovirus (MCMV) infection in immunocompromised mice-including the profoundly immunocompromised SCID mice and C57Bl/6 and B6D2F1 aged mice. SCID mice showed a marked decrease (> 20-fold) in resistance to MCMV, while there was a slight decrease (3-fold) in aged mice. In BRM antiviral protection studies, SCID mice were almost completely protected against MCMV infection by the pleiotropic immunomodulators, MVE-2 and pICLC, but much less by the more selective CSF-1. pICLC-induced IFN and NK cell cytotoxicity were maintained in SCID mice, suggesting that pleiotropic immunomodulatory effects may be required for antiviral protection in such a profoundly immunocompromised model. pICLC also effectively protected aged mice against lethal MCMV infection and effectively induced IFN. These results emphasize the potential for BRM treatment in immunocompromised hosts.

Antiviral agents: characteristic activity spectrum depending on the molecular target with which they interact

The target protein (enzyme) with which antiviral agents interact determines their antiviral activity spectrum. Based on their activity spectrum, antiviral compounds could be divided into the following classes: (1) sulfated polysaccharides (i.e., dextran sulfate), which interact with the viral envelope glycoproteins and are inhibitory to a broad variety of enveloped viruses (i.e., retro-, herpes-, rhabdo-, and arenaviruses): (2) SAH hydrolase inhibitors (i.e., neplanocin A derivatives), which are particularly effective against poxvirus, (-)RNA viruses (paramyxovirus, rhabdovirus), and (+/-)RNA virus (reovirus); (3) OMP decarboxylase inhibitors (i.e., pyrazofurin) and CTP synthetase inhibitors (i.e., cyclopentenylcytosine), which are active against a broad range of DNA, (+)RNA, (-)RNA, and (+/-)RNA viruses; (4) IMP dehydrogenase inhibitors (i.e., ribavirin), which are also active against various (+)RNA and (-)RNA viruses and, in particular, ortho- and paramyxoviruses; (5) acyclic guanosine analogs (i.e., ganciclovir) and carbocyclic guanosine analogs (i.e., cyclobut-G), which are particularly active against herpesviruses (i.e., HSV-1, HSV-2, VZV, CMV); (6) thymidine analogs (i.e., BVDU, BVaraU), which are specifically active against HSV-1 and VZV because of their preferential phosphorylation by the virus-encoded thymidine kinase; (7) acyclic nucleoside phosphonates (i.e., HPMPA, HPMPC, PMEA, FPMPA), which, depending on the structure of the acyclic side chain, span an activity spectrum from DNA viruses (papova-, adeno-, herpes-, hepadna-, and poxvirus) to retroviruses (HIV); (8) dideoxynucleoside analogs (i.e., AZT, DDC), which act as chain terminators in the reverse transcriptase reaction and thus block the replication of retroviruses as well as hepadnaviruses; and (9) the TIBO, HEPT, and other TIBO-like compounds, which interact specifically with the reverse transcriptase of HIV-1 and thus block the replication of HIV-1, but not of HIV-2 or any other retrovirus.

Treatment of murine cytomegalovirus infections in severe combined immunodeficient mice with ganciclovir, (S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine, interferon, and bropirimine

Severe combined immunodeficient (SCID) mice were found to be highly susceptible to murine cytomegalovirus (MCMV) infection. Treatment of infected mice with ganciclovir (12.5, 25, and 50 mg/kg of body weight for 10 days) starting 24 h after virus challenge resulted in delays in death by 2 to 8 days, and no animals survived the infection. (S)-1-[3-Hydroxy-2-(phosphonylmethoxy)propyl]cytosine (HPMPC) was much more potent, with doses of 1, 3.2, and 10 mg/kg/day (for 10 days) increasing the mean survival time by 15 to 30 days. Twenty-day treatments with HPMPC starting 5 days after virus inoculation increased the mean survival time by 24 to 32 days, with once-weekly (50-mg/kg) treatments being equivalent to daily (10-mg/kg) treatments. Delays in the development of liver, lung, and spleen virus titers in ganciclovir- and HPMPC-treated groups correlated with extensions in the mean survival times relative to the survival times of the placebo controls. The two compounds were approximately equally toxic to uninfected BALB/c mice treated for 10 days, causing 80 to 100% mortality after a dose of 150 mg/kg and 0% mortality after a dose of 75 mg/kg. Thus, the relative therapeutic index of HPMPC was 50-fold greater than that of ganciclovir. Recombinant alpha interferon delta 4 alpha 1/alpha 2 (1 x 10(4) and 5 x 10(4) units per mouse per day) and bropirimine (100 and 300 mg/kg/day) provided no protection from the lethal MCMV infection. The severe combined immunodeficient mouse MCMV infection is an important new model that will permit chemotherapy regimens to be studied over several months.

Chemotherapy of the acquired immune deficiency syndrome (AIDS): acyclic nucleoside phosphonate analogues

The acyclic nucleoside phosphonate analogues (HPMPA, HPMPC, PMEA, FPMPA) show great promise for the treatment of infections with such important human pathogens as adeno, pox (vaccinia) and hepadna (hepatitis B) viruses (HPMPA), herpes (herpes simplex, varicella-zoster, cytomegalo, Epstein-Barr) viruses (HPMPC), and retro (human immunodeficiency) viruses (PMEA, FPMPA). All these compounds seem to be targeted at the viral DNA polymerase, with which they interact, as either competitive inhibitors or alternative substrates (or chain terminators), following their intracellular phosphorylation to the diphosphoryl derivatives. Of particular interest is the prolonged anti-viral action, lasting for several days or even weeks, that has been noted both in vitro and in vivo after a single administration of the acyclic nucleoside phosphonates.