Prolonged and potent therapeutic and prophylactic effects of (S)-1-[(3-hydroxy-2-phosphonylmethoxy)propyl]cytosine against herpes simplex virus type 2 infections in mice

Antiviral Therapies for Herpesviruses: Current Agents and New Directions

PURPOSE

The objective of this review was to summarize the recent literature describing the current burden of disease due to herpesviruses in the antiviral and transplant era; describe mechanisms of action of antiviral agents and the development of resistance; summarize the literature of recent antiviral agents brought to market as well as agents under development; and to present literature on future strategies for herpesvirus therapeutics.

METHODS

An extensive search of the medical literature related to antiherpesviral therapy was conducted to compose this narrative review. Literature searches were performed via PubMed and ultimately 137 articles were included as most relevant to the scope of this article.

FINDINGS

Herpesviruses are a family of DNA viruses that are ubiquitous throughout human populations and share the feature of establishing lifelong infections in a latent phase with the potential of periodic reactivation. With the exception of herpes simplex virus, varicella zoster virus, and Epstein-Barr virus, which have a significant disease burden in individuals with normal immune function, the morbidity and mortality of the remaining viruses are primarily associated with the immunocompromised host. Over the last half-century, several agents have been tested in large randomized, placebo-controlled trials that have resulted in safe and effective antiviral agents for the treatment of many of these infections.

IMPLICATIONS

With increasing use of antiherpesviral agents for extended periods, particularly in immunocompromised hosts, the emergence of resistant viruses has necessitated the development of newer agents with novel targets and better side-effect profiles.

Antiviral Drugs in Newborn and Children

This article focuses on the clinically relevant approved antiviral medications available for the treatment of infants and children. A brief overview of drug categories, mechanism of action, resistance, pharmacokinetics, and side effects is provided for the more commonly prescribed antivirals. The patient categories addressed are treatment and prophylaxis of influenza, neonatal herpes simplex virus and congenital cytomegalovirus, treatment and prophylaxis of viral disease in the immunocompromised host, and a brief introduction to the antivirals available to treat hepatitis B and hepatitis C in children.

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.

Brincidofovir (CMX001) inhibits BK polyomavirus replication in primary human urothelial cells

BK polyomavirus (BKPyV)-associated hemorrhagic cystitis (PyVHC) complicates 5 to 15% of allogeneic hematopoietic stem cell transplantations. Targeted antivirals are still unavailable. Brincidofovir (BCV; previously CMX001) has shown inhibitory activity against diverse viruses, including BKPyV in a primary human renal tubule cell culture model of polyomavirus-associated nephropathy. We investigated the effects of BCV in BKPyV-infected and uninfected primary human urothelial cells (HUCs), the target cells of BKPyV in PyVHC. The BCV concentrations causing 50 and 90% reductions (EC50 and EC90) in the number of intracellular BKPyV genome equivalents per cell (icBKPyV) were 0.27 μM and 0.59 μM, respectively. At 0.63 μM, BCV reduced viral late gene expression by 90% and halted progeny release. Preinfection treatment for only 24 h reduced icBKPyV similarly to treatment from 2 to 72 h postinfection, while combined pre- and postinfection treatment suppressed icBKPyV completely. After investigating BCV's effects on HUC viability, mean selectivity indices at 50 and 90% inhibition (SI50 and SI90) calculated for cellular DNA replication were 2.7 and 2.9, respectively, those for mitochondrial activity were 8.9 and 10.4, those for total ATP were 8.6 and 8.2, and those for membrane integrity were 25.9 and 16.7. The antiviral and cytostatic effects, but less so the cytotoxic effects, were inversely related to cell density. The cytotoxic effects at concentrations of ≥10 μM were rapid and likely related to BCV's lipid moiety. After carefully defining the antiviral, cytostatic, and cytotoxic properties of BCV in HUCs, we conclude that a preemptive or prophylactic approach in PyVHC is likely to give the best results.

Transporter-targeted lipid prodrugs of cyclic cidofovir: a potential approach for the treatment of cytomegalovirus retinitis

Cidofovir (CDF) and its cyclic analogue (cCDF) have shown potential in vitro and in vivo antiviral activity against cytomegalovirus (CMV) retinitis. However, hydrophilic nature of CDF may affect cell permeation across lipophilic epithelium and thus limit its effectiveness in the treatment of CMV retinitis. In the present study, we have tested a novel hypothesis, which involves chemical derivatization of cCDF into lipophilic transporter-targeted prodrug [via conjugation with different carbon chain length of lipid raft and targeting moiety (biotin) for sodium-dependent multivitamin transporter (SMVT)]. We have synthesized and characterized three derivatives of cCDF including biotin B-C2-cCDF, B-C6-cCDF, and B-C12-cCDF. Physicochemical properties such as solubility, partition coefficient (n-octanol/water and ocular tissue), bioreversion kinetics, and interaction with SMVT transporter have been determined. Among these novel conjugates, B-C12-cCDF has shown higher interaction to SMVT transporter with lowest half maximal inhibitory concentration value, higher cellular accumulation, and high tissue partitioning. Improvement in physicochemical properties, lipophilicity, and interaction with transporter was observed in the trend of increasing the lipid chain length, that is, B-C12-cCDF > B-C6-cCDF > B-C2-cCDF. These results indicate that transporter-targeted lipid analogue of cCDF exhibits improved cellular accumulation along with higher transporter affinity and hence could be a viable strategy for the treatment of CMV retinitis.

Oral hexadecyloxypropyl-cidofovir therapy in pregnant guinea pigs improves outcome in the congenital model of cytomegalovirus infection

Cytomegalovirus (CMV) infection is the leading cause of congenital infection, producing both sensorineural hearing loss and mental retardation. We evaluated the in vivo efficacy of an orally bioavailable analog of cidofovir, hexadecyloxypropyl-cidofovir (HDP-CDV), against guinea pig CMV (GPCMV) in a guinea pig model of congenital CMV infection. HDP-CDV exhibited antiviral activity against GPCMV with a 50% effective concentration (EC(50)) of 0.004 μM ± 0.001 μM. To evaluate in vivo efficacy, pregnant Hartley guinea pigs were inoculated with GPCMV during the late second/early third trimester of gestation. Animals were administered 20 mg HDP-CDV/kg body weight orally at 24 h postinfection (hpi) and again at 7 days postinfection (dpi) or administered 4 mg/kg HDP-CDV orally each day for 5 days or 9 days. Virus levels in dam and pup tissues were evaluated following delivery, or levels from dam, placenta, and fetal tissues were evaluated following sacrifice of dams at 10 dpi. All HDP-CDV regimens significantly improved pup survival, from 50 to 60% in control animals to 93 to 100% in treated animals (P ≤ 0.019). Treatment with 20 mg/kg HDP-CDV significantly reduced the viral load in pup spleen (P = 0.017) and liver (P = 0.029). Virus levels in the placenta were significantly reduced at 10 dpi following daily treatment with 4 mg/kg HDP-CDV for 5 or 9 days. The 9-day treatment also significantly reduced the viral levels in the dam spleen and liver. Although the 4-mg/kg treatment improved pup survival, virus levels in the fetal tissues were similar to those in control tissues. Taken together, HDP-CDV shows potential as a well-tolerated antiviral candidate for treatment of congenital human CMV (HCMV) infection.

Oral and perioral herpes simplex virus type 1 (HSV-1) infection: review of its management

Herpes simplex virus type 1 (HSV-1) gives rise to a variety of clinical disorders and is a major cause of morbidity and mortality worldwide. HSV-1 infections are common in oral and perioral area. The aim of the present report was to critically examine the published literature to evaluate the advantages and limitations of therapy of HSV-1 infection in both immunocompetent and immunocompromised patients. Systemic antiviral therapy has been widely accepted as effective for primary herpetic gingivostomatitis. Aciclovir (ACV) 5% cream seems to be the accepted standard topical therapy for herpes labialis, being both effective and well tolerated, although penciclovir 1% cream has been proposed as a potentially useful treatment. Systemic ACV may be effective in reducing the duration of symptoms of recurrent HSV-1 infection, but the optimal timing and dose of the treatment are uncertain. Aciclovir and famciclovir may be of benefit in the acute treatment of severe HSV-1 disease in immunocompromised patients. There is also evidence that prophylactic oral ACV may reduce the frequency and severity of recurrent attack of herpetic infection in immunocompromised patients, but the optimal timing and duration of treatment is uncertain and can vary in different situations.

In vitro efficacy of ganciclovir, cidofovir, penciclovir, foscarnet, idoxuridine, and acyclovir against feline herpesvirus type-1

OBJECTIVE

To establish the in vitro efficacy of 4 novel drugs (ie, ganciclovir, cidofovir, penciclovir, and foscarnet) against feline herpesvirus type-1 (FHV-1) and compare their antiviral efficacy with that of acyclovir and idoxuridine.

SAMPLE POPULATION

Cultured Crandell-Reese feline kidney (CRFK) cells and FHV-1 strain 727

PROCEDURE

For each drug, antiviral effect was estimated by use of conventional plaque-reduction assays, and inhibitory concentration 50 (IC50; drug concentration at which plaque numbers were reduced by 50% relative to the number of plaques for nontreated control wells) was calculated. To determine whether observed antiviral effects were related to alterations in the number or viability of CRFK cells, cytotoxicity assays were performed at 1, 2, and 10 times the median IC50 for each antiviral drug.

RESULTS

Median IC50 for each drug was as follows: ganciclovir, 5.2 microM; cidofovir, 11.0 microM; penciclovir, 13.9 microM; foscarnet, 232.9 microM; idoxuridine, 4.3 microM; and acyclovir, 57.9 microM. Obvious changes in morphologic characteristics, confluence, or viability of CRFK cells were not observed at concentrations up to and including 2 times the IC50 for each drug.

CONCLUSIONS AND CLINICAL RELEVANCE

In vitro efficacy of idoxuridine and ganciclovir against FHV-1 was approximately equivalent and about twice that of cidofovir and penciclovir. Foscarnet appeared to be comparatively ineffective. Given the reasonable clinical efficacy of idoxuridine in cats infected with FHV-1, clinical trials of ganciclovir, cidofovir, and penciclovir or their prodrug forms appear to be warranted.

Clinical potential of the acyclic nucleoside phosphonates cidofovir, adefovir, and tenofovir in treatment of DNA virus and retrovirus infections

The acyclic nucleoside phosphonates HPMPC (cidofovir), PMEA (adefovir), and PMPA (tenofovir) have proved to be effective in vitro (cell culture systems) and in vivo (animal models and clinical studies) against a wide variety of DNA virus and retrovirus infections: cidofovir against herpesvirus (herpes simplex virus types 1 and 2 varicella-zoster virus, cytomegalovirus [CMV], Epstein-Barr virus, and human herpesviruses 6, 7, and 8), polyomavirus, papillomavirus, adenovirus, and poxvirus (variola virus, cowpox virus, vaccinia virus, molluscum contagiosum virus, and orf virus) infections; adefovir against herpesvirus, hepadnavirus (human hepatitis B virus), and retrovirus (human immunodeficiency virus types 1 [HIV-1] and 2 [HIV-2], simian immunodeficiency virus, and feline immunodeficiency virus) infections; and tenofovir against both hepadnavirus and retrovirus infections. Cidofovir (Vistide) has been officially approved for the treatment of CMV retinitis in AIDS patients, tenofovir disoproxil fumarate (Viread) has been approved for the treatment of HIV infections (i.e., AIDS), and adefovir dipivoxil (Hepsera) has been approved for the treatment of chronic hepatitis B. Nephrotoxicity is the dose-limiting side effect for cidofovir (Vistide) when used intravenously (5 mg/kg); no toxic side effects have been described for adefovir dipivoxil and tenofovir disoproxil fumarate, at the approved doses (Hepsera at 10 mg orally daily and Viread at 300 mg orally daily).

Immunobiological activity of N-[2-(phosphonomethoxy)alkyl] derivatives of N6-substituted adenines, and 2,6-diaminopurines

Acyclic nucleoside phosphonates are novel class of virostatics effective against replication of both DNA-viruses and retroviruses. We found recently, that in addition to the antimetabolic mode of action, some acyclic nucleoside phosphonates such as 9-[2-(phosphonomethoxy)propyl]adenine [(R)-PMPA; tenofovir], which is used in treatment of human immunodeficiency virus (HIV) infection, possess immunostimulatory and immunomodulatory activities known to interfere with replication of viruses. The present experiments analyzed immunobiological effects of more than 70 novel derivatives of acyclic nucleoside phosphonates. They comprise substitutions at the N6-amino function of adenine (A) or 2,6-diaminopurine (DAP) by monoalkyl, dialkyl, cycloalkyl, alkenyl, alkynyl or substituted alkyl group, and at the N9-side chain represented by (R)- or (S)-enantiomeric 9-[2-(phosphonomethoxy)ethyl] (PME) and 9-[2-(phosphonomethoxy)propyl] (PMP) moieties. Their biological effects were investigated in vitro using mouse resident peritoneal macrophages. A number of the compounds under scrutiny, mainly the N6-cycloalkyl derivatives of 9-[2-(phosphonomethoxy)ethyl]2,6-diaminopurine (PMEDAP) and (R)-enantiomeric 9-[2-(phosphonomethoxy)propyl]adenine [(R)-PMPDAP] stimulate secretion of cytokines [tumor necrosis factor-alpha (TNF-alpha), interleukin-10 (IL-10)] and chemokines ["regulated-upon-activation, normal T expressed and secreted" (RANTES), macrophage inflammatory protein-1alpha (MIP-1alpha)]. Moreover, they substantially augment production of nitric oxide (NO) triggered by interferon-gamma. The effects are produced in a dose-dependent fashion. The most potent derivatives, i.e. N6-isobutyl-PMEDAP, N6-cyclopentyl-PMEDAP, N6-cyclooctyl-PMEDAP, N6-dimethylaminoethyl-(R)-PMPDAP, N6-cyclopropyl-(R)-PMPDAP, and N6-cyclopentyl-(R)-PMPDAP are more effective than (R)-PMPA (tenofovir) itself. They exhibit immunostimulatory effects at concentrations as low as 1 to 5 microM. It is suggested that these compounds might be prospective candidates for antiviral therapeutic exploitation.

Management of acyclovir-resistant herpes simplex virus

In immunocompetent patients, HSV is controlled rapidly by the human host's immune system, and recurrent lesions are small and short lived. When treated with antiviral agents, these patients rarely develop resistance to these drugs. In contrast immunocompromised patients might not be able to control HSV infection. Thus, frequent and severe reactivations are often seen and might lead to fatal herpetic encephalitis or disseminated HSV infection. Treatment in these patients is limited because immunocompromised hosts often develop severe herpes disease refractory to antiviral drug therapy. It is therefore imperative that physicians develop regimens to deal with both receptive and refractory HSV disease. The following treatment protocol (modified from Balfour and colleagues) might serve as a guide until further investigation of new drugs is performed. In all patients standard oral ACV therapy should be initiated at a dose of 200 mg orally, five times a day for the first 3 to 5 days. Prior to treatment, cultures the lesions should be obtained to verify HSV etiology. If the response is poor, the dose of oral ACV should be increased to 800 mg five times a day. If no response seen after 5 to 7 days, it is unlikely that the lesion will respond to intravenous ACV (or chemically and structurally related drugs such as VCV or famciclovir), so an alternative regimen must be assigned. First, repeat cultures for vital, fungal, and bacterial pathogens must be performed. In addition, ACV susceptibility studies should be ordered, if available. If the mucocutaneous lesion is accessible for topical treatment, TFT (as ophthalmic solution) should be applied to the area three to four times a day until the lesion is completely healed. If the lesion is inaccessible or if the response to TFT is poor, therapy with intravenous foscarnet should be given for 10 days or until complete resolution of the lesions. The dosage of foscarnet should be 40 milligrams per kilogram three times per day or 60 milligrams per kilogram twice daily. If foscarnet fails to achieve clinical clearing, consideration should be given to use of intravenous cidofovir (or application of compounded 1% to 3% topical cidofovir ointment). Vidarabine is reserved for situations in which all of these therapies fail. If lesions reoccur in the same location following clearing, the patient should started on high-dose oral ACV (800 mg, five times daily) or intravenous foscarnet (40 mg/kg tid or 60 mg/kg bid) as soon as possible. When lesions occur in a different location, the patient should be treated initially with standard doses of oral ACV (200 mg, five times daily) and the above protocol should be followed should there be clinical failure. In the future, new treatment options for patients with documented HSV resistance will be important in reducing the clinical impact of HSV.

Antiviral therapy for cytomegalovirus infections in pediatric patients

Appreciation of the spectrum of illness caused by cytomegalovirus (CMV) infections has increased markedly during the past 2 decades. The number of immunosuppressed patients also has increased during the same time period, reflecting the central tenet that CMV disease is most severe in this patient population. Fortunately, antiviral therapies with activity against CMV also have been identified during this same time course, and they include ganciclovir, foscamet, and cidofovir. Although all 3 of these therapies can have significant toxicities associated with them, nonetheless they are employed with relative frequency to treat potentially life-threatening CMV disease. Ganciclovir is the first-line compound used, followed by foscarnet and cidofovir. This article summarizes those CMV infections that require antiviral therapy and outlines therapeutic options for each.

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.

Herpes simplex virus zosteriform lesions with adoptive transfer of immune cells: a murine model which mimics human recurrent disease

Existing murine models for cutaneous herpes simplex virus type 1 (HSV-1) infection have limited relevance to recurrent disease in humans, since the infection is usually primary rather than reactivated and infection occurs in the absence of an established immune response. To obtain a reproducible model to study the effects of topical antiviral therapy on recurrent disease we have adapted a mouse model which employs zosteriform spread of HSV-1 in the presence of adoptive transfer of immunity (ATI) which mimics human recrudescent lesions. Mice were infected with HSV-1 by scarification at the lateroventral line of the neck; 2 days later, the mice received adoptive transfer of immune cells from the cervical lymph nodes of syngeneic mice that had been infected in the ear pinna with the same strain of virus 7 days earlier. ATI resulted in a heightened inflammatory response in the target tissues for virus replication. Virus was cleared more quickly from the infected tissues in comparison with mice similarly inoculated without ATI, however, the intensity and duration of the inflammation was greater. The model was then used to test the effect of a topical formulation of foscarnet. The results presented demonstrate that the ATI model can provide useful data concerning the efficacy of topical antiviral chemotherapy in man.

Immunomodulatory properties of antiviral acyclic nucleotide analogues: cytokine stimulatory and nitric oxide costimulatory effects

Acyclic nucleotide analogues exhibit strong activity against a broad range of viruses, including HIV-1 and -2. We have investigated their effects on in vitro secretion of cytokines and production of nitric oxide (NO) by murine peritoneal macrophages, factors known to play a role in virus replication. Included in the study were the most prominent compounds of the series: 9-(2-phosphonomethoxyethyl)adenine, 9-(2-phosphonomethoxyethyl)-2,6-diaminopurine, the (R)- and (S)-enantiomers of 9-(2-phosphonomethoxypropyl) adenine [(R)- or (S)-PMPA], (R)- and (S)-enantiomers of 9-(2-phosphonomethoxypropyl)-2,6-diaminopurine [(R)- or (S)-PMPDAP], 9-(2-phosphonomethoxyethyl)guanine (PMEG), and (S)-enantiomer of 1-(3-hydroxy-2-phosphonomethoxypropyl)cytosine [(S)-HPMPC]. PMEG, (R)-PMPA, and (S)-PMPA greatly enhanced the secretion of both tumour necrosis factor-alpha (TNF-alpha) and interleukin-10 (IL-10), (R)-PMPDAP stimulated only TNF-alpha, other test compounds were ineffective. None of them influenced the secretion of IL-2 or interferon-gamma (IFN-gamma). Both TNF-alpha and IL-10 have been found to be major factors determining enhancing effects of PMEG, (R)-PMPA, and (S)-PMPA on production of NO generated by exogenous IFN-gamma. The study points to a possible implication of immunomodulatory properties in the antiviral effects of some acyclic nucleotide analogues. In addition, our data support the view that endogenous IL-10 can stimulate certain macrophage functions.

Effective treatment of experimental cytomegalovirus-induced encephalo-meningitis in immunocompromised rats with HPMPC

Cytomegalovirus (CMV)-induced encephalomeningitis is a dramatic complication in patients with the acquired immunodeficiency syndrome (AIDS) and treatment of this infection remains a major clinical problem. In order to study the pathogenesis and treatment of CMV-induced encephalomeningitis, we experimentally induced intracranial rat CMV (RCMV) infection in rats that were immunosuppressed by total body X-irradiation. CMV infection was monitored by viral plaque assay for estimation of the viral load. CMV-induced pathology, the presence of CMV-infected cells, as well as the presence of T-lymphocytes and monocytes/macrophages were studied by histopathologic and immunohistochemical staining techniques. The meninges showed CMV infection in mononuclear infiltrative cells and in endothelium of small blood vessels 8 days after intracerebral inoculation. This was accompagnied by multiple haemorraghes and inflammatory cell infiltration. The infection and inflammatory response persisted for at least 21 days p.i. Animals were treated with (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC), 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG), hyperimmune serum (HIS) and both DHPG and HIS combined. Treatment with one dosage of HPMPC at 20 mg/kg effectively reduced virus titers. However, all other treatment modalities were not effective. In conclusion, the pathology of RCMV-induced encephalomeningitis in immunocompromised rats closely resembles that of AIDS patients. The infection is effectively treated by HPMPC.

Herpes simplex: evolving concepts

A large body of molecular biologic research has begun to clarify some basic aspects of viral latency and reactivation. The clinical definition of herpes simplex virus infection is expanding, with the recognition that the disease is largely asymptomatic and that most transmission occurs during periods of asymptomatic viral shedding. With this awareness, serologic diagnosis has become increasingly important. New treatment modalities are now available, and other promising treatments are in development.

Evaluation of Cidofovir (HPMPC, GS-504) against adenovirus type 5 infection in vitro and in a New Zealand rabbit ocular model

The antiviral inhibitory activity of Cidofovir [1-[(S)-3-hydroxy-2-(phosphonomethoxy)propyl]cytosine dihydrate, HPMPC, GS-504] against adenovirus type 5 (Ad5) in the New Zealand rabbit ocular replication model was evaluated. The 50% inhibitory dose (ID50) of Cidofovir was determined to be 4.7-9.5 micrograms/ml against four adenoviruses (two Ad5, Ad8 and Ad14) by plaque reduction assay in A549 cells. Twenty-four New Zealand rabbits received intrastromal inoculation and topical application of 2 x 10(6) plaque-forming units (PFU) per eye of Ad5 McEwen, a clinical isolate. Cidofovir was administered topically at three different concentrations twice per day, beginning 16 h postinoculation and continuing for 20 consecutive days. The inhibitory effects were determined by measuring suppression of virus replication and by observation of the clinical effects. Compared to the placebo group, the 1% and 0.5% Cidofovir-treated groups showed significantly reduced Ad5 ocular titers, fewer days of viral shedding and less severe subepithelial opacities (P = 0.0001). The 1% Cidofovir group had the lowest humoral antibody titer against adenovirus antigens, but the difference was not significant (P = 0.24). Cidofovir proved to have potent antiviral activity against adenovirus replication and may have great promise for the treatment of adenovirus infection. Further investigation is recommended.

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.

Effect of BV-araU and acyclovir on varicella-zoster virus replication with various length and timing of drug exposure

We studied antiviral effects of 1-beta-D-arabinofuranosyl-5-[(E)-2-bromovinyl]uracil (BV-araU) and acyclovir against varicella-zoster virus (VZV) multiplication varying the length or timing of drug exposure. First, residual anti-VZV effect of drugs, exposed to cells for various periods followed by incubation in drug-free medium, was determined by the plaque inhibition assay. None of the drugs showed activity when removed within 24 hr of incubation. Weakened efficacy of BV-araU was seen in 2 days of treatment. When it was removed after 3 or 4 days, the ED50 was as low as that for cultures in which the drug was not removed. Still, plaque inhibition was not complete even at high concentrations. Acyclovir inhibited plaque formation only by 50% or less in 2 days of treatment. It gave a much higher ED50 in 3 days of treatment than that observed without drug removal. In the experiments, in which BV-araU was added to VZV-infected cells 1 day after infection, BV-araU immediately suppressed increase in the number of infective centers at a concentration of 0.001 microgram/ml, and reduced it at concentrations of 0.01 microgram/ml or higher. The reduction of infective centers was seen with a dose-dependent manner when added 2 or 3 days after infection. BV-araU stimulated the decrease in the number of infective centers when added 4 days after infection. This inhibitory effect of acyclovir was very weak. Microscopic observations supported the above results. BV-araU was still much superior to acyclovir in the anti-VZV effect when the length and timing of drug exposure were varied.

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.

Mechanism of uptake of the phosphonate analog (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC) in Vero cells

The cellular uptake of phosphonylmethoxypropyl cytosine (HPMPC) was characterized to gain insight into the molecular properties that allow this anticytomegalovirus drug to permeate cell membranes. The time course of uptake of HPMPC into Vero cells was linear between 10 and 75 min and proportional to the concentration in the medium from 10(-6) to 10(-2) M. HPMPC uptake was temperature sensitive and the rate of uptake was considerably lower at 27 degrees than at 37 degrees and almost totally inhibited at 4 degrees. In competition studies with naturally occurring nucleosides, nucleotides or the phosphonylmethoxyethyl derivatives, none affected the uptake of HPMPC at concentrations up to 2000-fold molar excess. The uptake of [3H]HPMPC into Vero cells was compared with that of [14C]sucrose, a probe for fluid-phase endocytosis. Kinetics for both compounds were very similar, as were the effects of the microtubule antagonist colchicine and the tumor promoting agent phorbol myristate acetate. Colchicine and the phorbol ester are known to, respectively, inhibit and stimulate endocytosis. It is concluded from these data that HPMPC enters Vero cells by fluid-phase endocytosis and that once internalized it may accumulate in the lysosome. Protonation of the negative charge on the phosphonyl group in HPMPC may allow its diffusion across the lysosome membrane and eventual activation to its putative active diphosphorylated form in the cell cytoplasm.

Activity of the anti-HIV agent 9-(2-phosphonyl-methoxyethyl)-2,6-diaminopurine against cytomegalovirus in vitro and in vivo

9-(2-phosphonylmethoxyethyl)-2,6-diaminopurine (PMEDAP), a potent inhibitor of human immunodeficiency virus (HIV) replication, was evaluated for its activity against human cytomegalovirus (HCMV) in vitro, and murine cytomegalovirus (MCMV) and rat CMV (RCMV) in vivo. PMEDAP strongly inhibited HCMV-induced cytopathicity in human embryonic lung (HEL) cell cultures (EC50 11 microM) and caused a concentration-dependent suppression of viral DNA synthesis (IC50 20 microM) [corrected]. PMEDAP had no effect on the expression of HCMV-specific immediate early antigens (IEA) as measured on day 1 post-infection, but inhibited the expression of HCMV late antigens as measured on day 6 post-infection (EC50 20 microM) [corrected]. The diphosphate derivative of PMEDAP (PMEDAPpp) selectively inhibited HCMV-induced DNA polymerase (IC50 0.1 microM). PMEDAP proved markedly effective in reducing the mortality rate of NMRI mice that had been infected intraperitoneally or intracerebrally with a lethal dose of MCMV. PMEDAP exhibited greater anti-MCMV activity when administered as a single dose immediately after infection than when this dose was divided over repeated administrations. 9-(2-phosphonylmethoxyethyl)-adenine (PMEA) also prevented MCMV-induced mortality, but only at a dose ten-fold higher than that of PMEDAP. PMEDAP also delayed death in severe combined immune deficiency (SCID) mice that had been infected with MCMV. The effect of PMEDAP on RCMV infections in rats was less pronounced.

Evaluation of HPMPC therapy for primary and recurrent genital herpes in mice and guinea pigs

The nucleoside analogue (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC) inhibited the replication of herpes simplex virus (HSV) types 1 and 2 in tissue culture cells at about 1.0 micrograms/ml, whereas Acyclovir (ACV) had an EC50 of about 0.10-0.50 micrograms/ml. The purpose of these studies was to evaluate the efficacy of topically applied HPMPC in animal models of primary and recurrent genital HSV-2 infections. Mice treated with 5%, 1% or 0.5% HPMPC three times daily, beginning 6 or 24 h after virus inoculation had reduced vaginal viral replication regardless of time of initiation of therapy. ACV at 5% also reduced vaginal viral replication, but not as effectively as HPMPC. In primary infection of guinea pigs, therapy with 5% or 1% HPMPC beginning at 24 h but not 72 h significantly altered lesion development. However, 5% HPMPC was highly toxic to guinea pigs. Vaginal viral replication was reduced significantly with either 1% or 0.3% HPMPC initiated at 24 h. In these studies, HPMPC was also more efficacious than 5% ACV. Topical treatment with 1% HPMPC did not reduce the incidence or severity of spontaneous or UV-induced recurrent genital lesions. These results indicate that topical therapy with 1%, 0.5% or 0.3% HPMPC was more effective than 5% ACV in the treatment of primary genital HSV-2 infections of guinea pigs and mice and suggest that HPMPC should be considered for topical use in humans.

Efficacy of (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)-cytosine and 9-(1,3-dihydroxy-2-propoxymethyl)-guanine in the treatment of intracerebral murine cytomegalovirus infections in immunocompetent and immunodeficient mice

The efficacy of HPMPC and DHPG against systemic and intracerebral murine cytomegalovirus (MCMV) infections was examined in immunocompetent NMRI mice and in mice with severe combined immunodeficiency (SCID). HPMPC proved to be far superior to DHPG in preventing mortality and growth retardation in MCMV-infected NMRI mice even when given as a single dose on the day of infection. In intraperitoneally infected SCID mice, HPMPC administered as a single dose of 2, 10, 20 or 50 mg/kg per week increased the survival period of the mice by 22, 49, 77 and 156 days, respectively. In contrast, DHPG at daily doses of 10, 20 or 50 mg/kg for five consecutive days every week did not delay death by more than 13, 17 and 21 days, respectively. About one week before the MCMV-infected SCID mice (treated with either DHPG or HPMPC) died, they developed signs of neurological disease and intranuclear inclusion-bearing cells were found in their brains. The virus that was recovered from the brains of these mice did not prove to be resistant to HPMPC or DHPG. Only the virus recovered from the brains of mice treated with HPMPC at a dosage of 50 mg/kg/week had a slightly decreased susceptibility to HPMPC. When HPMPC (50 mg/kg for 4 consecutive days) was administered to SCID mice at the time when neurological symptoms became apparent, death of the animals could be delayed by another 35 to 40 days.

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.