Treatment of Herpes simplex virus infections with topical antiviral agents

ARTICLE

Approximately 2/3 patients in a family medicine clinic were seropositive for Herpes simplex virus (HSV) infections in a 1995 study conducted in Seattle, Washington. The majority, 277 patients (56%), were seropositive for HSV-1 antibody, while 114 patients (23%) were found to be seropositive for HSV-2 antibody and 59 patients (12%) were seropositive for both antibodies [1]. Extrapolation from this randomly tested population suggests a large number of individuals exposed to HSV, and the prevalence of genital HSV infection alone has been recently estimated to be 55 million people in the United States. Interestingly, only 20 to 25% of those with genital HSV infection are aware of their condition [2]. It has been reported that 53% of patients showing signs of HSV recurrences reported one or more per month [3]. Furthermore, recurrent HSV type 1 infection, which predominantly affects the orolabial region, causes distressing cutaneous and mucous membrane lesions that have been estimated to occur in 20 to 45% of the US population, and approximately 23% of those infected experience two or more recurrences per year [4].

After causing a productive infection, HSV spreads from the epithelium by retrograde, intra-axonal transport on sensory nerves to regional sensory ganglia establishing a latent stage of infection. Viral reactivation occurs in response to theoretical "ganglionic" or "skin" triggers. The former theory proposes that a stimulus such as menses, fever, stress or ganglionic manipulation re-activates the latent infection in the ganglion, causing the virus to travel distally down the peripheral nerve to the epidermal cells to form a skin lesion. The latter theory proposes that the virus is being continuously replicated in the ganglion and travels to the skin, but host defenses normally block clinically evident cutaneous infection. However, a skin stimulus such as trauma, heat or sunburn may cause the minor skin infection to become symptomatic by either stimulating further virus production and/or by suppressing host defenses. Latent infection, which may persist throughout one's entire life, leads to recurrent productive infections [3] localized to cutaneous or mucosal surfaces in immunocompetent individuals. These productive infections usually last 4 to 10 days; however, immunocompromised patients may remain symptomatic for over 30 days and may develop extensive necrosis [5].

Beyond the physical discomforts, infection with HSV may precipitate psycho-social difficulties. Female patients who acquire genital herpes fear the development of cervical cancer which is associated with HSV [3], and they also struggle with the prospect of passing the infection to their offspring. Both male and female patients may have psychological and social problems associated with the potential of spreading this disease sexually [3].

Characteristics of an ideal antiviral medication would include 1) immediate and complete termination of viral replication in order to stop infection of new cells; 2) restoration of previously infected cells; and 3) inactivation of free virions. Unfortunately, today no one drug possesses all three of these features. In addition, to treat a cutaneous HSV infection, it may be advantageous to administer therapy topically. Reduced systemic exposure to drug side effects, greater levels of the drug targeted to the site of infection (relative to drug levels achieved by a systemic route), specific targeting of the drug to the site of infection, and convenience are among the benefits afforded by topical administration of drugs [6]. Because of the many potential advantages of topical treatment when infection is limited to the skin, we have briefly included discussion of nonspecific topical antiviral treatments for HSV infection which is then followed by a more in depth description of topical nucleoside analogs and other topical antiviral agents, including proposed mechanisms of action and evidence of their efficacy.

Nonspecific antiviral agents

Glutaraldehyde

This reactive aldehyde that has been formerly used as a cold sterilant can be buffered to a 2% alkine glutaraldehyde, capable of completely inhibiting HSV infectivity within 10 min. Gordon observed that lesions dried more quickly and duration of symptoms were reduced. Additionally, he felt an in vivo autogenous vaccine may result, causing lower recurrence rates [7].

Povidone-iodine

Iodine in a molar concentration of 10^­3 tM inactivates 100% of an HSV inoculum [8]. Povidone-iodine solutions are commonly used for skin antisepsis [9]. Genital herpes patients were given this topical agent, and the expected duration of symptoms and healing times were shortened [9].

Butylated hydroxytoluene (BHT)

A drug affecting lipid membranes called butylated hydroxytoluene has activity against many enveloped viruses including HSV. In a small study of recurrent herpes labialis, treatment with 15% BHT in mineral oil offered little clinically relevant benefit. Topical therapy with BHT was well tolerated, with no evidence of local or systemic toxicity [10].

Ether

Ether, like BHT, alters cellular lipid membranes. However, ether was found to have no significant therapeutic effect on herpes labialis or herpes genitalis, while causing "extreme pain" upon application to genital lesions [10]. Topical application of diethyl ether reduced the mean number of days of new genital herpetic lesion formation after initiation of therapy. Unfortunately, the drug did not shorten the duration of primary or recurrent genital herpes nor did it prevent or delay recurrent episodes [11].

Ascorbic acid-containing solution

Since the 1960's, Scandinavia has marketed ascorbic acid as a 100 mg tablet that is dissolved in 3 ml of tap water to treat gingivitis. Antibacterial, antimycotic, and antiviral activity has been documented. The exact mechanism of action of ascorbic acid solution is unknown; however, it is speculated that there is a virucidal effect on HSV and a possible direct effect on the inflammatory reaction in the lesion due to the short-lived oxidation products of ascorbic acid [12]. In 1995, it was reported that patients who underwent treatment with ascorbic acid solution reported a reduction in the number of days with scab, symptoms, and worsening symptoms after treatment, and nurses records showed that scabs lasted a shorter duration [12].

Nucleoside analog drugs

Nucleoside analogs are incorporated into viral DNA and competitively inhibit the activity of DNA polymerase; however, they do not inactivate free virus and it is not known whether they cause complete cessation of viral replication or prevent the death of an already infected cell [6].

Acyclovir (ACV)

Acyclovir is an inactive guanosine analog which must be phosphorylated by virally encoded thymidine kinase to its monophoshate form, acycloGMP. It is further phosphorylated to its active triphosphate form, acycloGTP, by host cell enzymes. ACV inhibits HSV DNA polymerase 10-30 times more than it does host cell polymerase [13], but it is also used as a substrate by the DNA polymerase so that it is incorporated into the growing DNA chain and terminates further synthesis because ACV lacks a hydroxyl group in the 3' position [14].

ACV has been tested topically in two forms, an ointment and a cream. Data from studies on both of these preparations indicate that the ointment has nominal therapeutic efficacy in genital herpes while the cream may be beneficial for orolabial herpes [6, 15-19]. Overall, topical ACV is a minimally effective treatment for immunologically competent patients with a first episode of genital herpes. According to Gold et al., topical ACV must be supplemented by oral and intravenous administration to decrease the duration of genital lesions by 29%, the duration of viral shedding by 55%, and the duration of pain by 26% [5].

Adenine arabinoside (ara-A; 9-beta-D-arabinofuranosyladenine)

ACV-resistant HSV variants predominantly exist in the form of thymidine kinase deficient strains, but also as thymidine kinase-altered variants. Testing in mice has shown that these types of HSV episodes are completely refractory to ACV therapy; however, ara-A treatment or a combination of ara-A with ACV prevented chronic disease or death in the animals [20]. The observed effectiveness is due to the fact that unlike ACV, which is transformed into its active form by using viral thymidine kinase, ara-A utilizes both cellular adenosine kinase and cellular deoxyadenosine kinase for activation [21] and subsequent inhibition of HSV DNA polymerase [22].

Later studies with topically applied ara-A, unfortunately, have failed to detect therapeutic efficacy in either herpes labialis or herpes genitalis, using 3% ara-A ointment and gel and a 10% ara-A dose in a cream base [23-25]. In a more recent study with topical ara-A in AIDS patients, all participants discontinued treatment due to lack of efficacy [14].

Edoxudine (EDU)

Significant in vitro and in vivo anti-HSV activity has been demonstrated with edoxudine, a deoxythymidine analog that, when activated by selective phosphorylation by thymidine kinase, inhibits DNA polymerase and, hence, HSV replication. Edoxudine is better absorbed from its aqueous cream base than acyclovir from its polyethylene glycol base, and surprisingly, the rate of absorption of edoxudine increases with longer time allowed after application. Degradation of edoxudine in circulation occurs through pyrimidine nucleoside phosphorylase resulting in the production of 5-ethyluracil [26]. Edoxudine is an effective treatment for genital herpes by having antiviral effects and clinically reducing symptom severity [26].

(S)-1-(3-hydroxy-2-phosphonylmethoxypropyl) cytosine (HPMPC)

Potent and selective activity against DNA viruses including HSV types 1 and 2 have been demonstrated with HPMPC. Activity of this acyclic nucleoside-phosphonate derivative is not dependent on thymidine kinase activity, and, therefore, was of potential usefulness in treating ACV-resistant strains of HSV. Unfortunately, the only study was non-placebo controlled and was limited to only two immunocompromised patients. The limited data suggest that HPMPC may be effective in the treatment of ACV or ACV/PFA-resistant HSV infections in immunocompromised patients [27].

Idoxuridine (IDU)

Studies of this drug date back to 1966; however, the small number of patients enrolled in the study did not allow the differences to reach statistical significance [28]. In a larger study, clinical efficacy was shown, especially in those patients who began treatment in the prodromal or erythematous stages [29]. Although some adverse reactions such as transient stinging, nausea, and nonspecific aftertaste did occcur in patients from the 80% dimethyl sulfoxide (DMSO) used in both studies, 15% IDU in 80% DMSO is currently marketed in Europe as an effective treatment for herpes labialis and herpes genitalis [6, 29].

Trifluorothymidine (TFT)

Some doctors prescribe a 5% ophthalmic solution of trifluorothymidine (TFT) for HSV infections, despite no controlled clinical trials proving efficacy and the observation that women respond less well than men. This is especially true of ACV-resistant strains of HSV. One physician reported that one of his patients with perianal herpes resistant to ACV responded to topical TFT [14]. This flourinated nucleoside analog is used as a topical treatment for HSV-associated epithelial keratitis and ophthalmic HSV. In two AIDS patients with HSV, reepithelialization resulted from TFT treatment. Synergistic effects of TFT with interferon were seen in another study. In vitro studies also revealed strong synergy between TFT and interferon-alpha for ACV-resistant HSV isolates and for HSV strains with wild-type, drug-sensitive profiles [30]. Larger studies are required to confirm that TFT has a beneficial therapeutic effect, especially in combination with interferon.

Penciclovir

The newest HSV therapy that has been tested is a 1% penciclovir cream. It is converted into its intracellular triphosphate form and retained in vitro inside HSV infected cells for 10-20 hours. This is an advantage over ACV which is only active for 0.7 to 1 hr [31]. Penciclovir cream was extremely effective in both early and late initiation of therapy of herpes labialis. Impressive results of the 1997 study demonstrate its powerful benefits.

Other antiviral drugs

Arildone (4-[(2-chloro-4-methoxy)phenoxy]hexyl-3,5-heptanedione)

Both RNA and DNA viruses have been found to be susceptible to arildone, an aryl diketone. It inhibits replication of poliovirus at an early step by preventing the uncoating of the virus. Less is known about the mechanism this compound uses against HSV, although it exerts its antiviral activity during early infection and prevents synthesis of both viral DNA and proteins. Importantly, arildone is equally effective against ACV-sensitive and ACV-resistant strains of HSV. In human studies, however, arildone only showed clinical efficacy in men. In addition, after application of the 8% arildone cream, localized burning and pain lasting up to 15 minutes occurred [32].

Interferon alpha and beta

Interferon production is induced by viral infection as part of the human host defense. This endogenously synthesized antiviral has a great effect against DNA and RNA viruses by inhibiting viral protein translation and creating an antiviral state in the cell, causing resistance to infection. First, the common receptor for interferon alpha and beta is coupled to a tyrosine kinase which phoshporylates signal-transducing activators of transcription. This promotes host-cell protein formation that inhibits viral replication. Second, a serine/threonine kinase called P1 kinase is activated and phosphorylates the eukaryotic protein synthesis initiation factor eIF-2, contributing to the inhibition of viral replication. Third, interferons strongly activate natural killer cells which are a good defense against the intracellular virus. And, fourth, interferons increase expression of MHC class I molecules, transporter proteins, and components of the proteasome. These events allow presentation of viral peptides to CD8 T cells, and increased MHC class I prevents attack of uninfected host cells by natural killer cells [33]. All of the actions of interferon play an important role in host defense against viruses; therefore, researchers have tried testing the efficacy of topical interferon.

Although the stratum corneum operates as a formidable barrier to penetration of a large protein, topically applied alpha and beta interferon have shown some therapeutic benefits for HSV [34-39]. Treatment with topical interferon is valuable therapy for HSV infection. Better efficacy may be obtained when it is used in combination with other substances such as nonoxynol-9, DMSO, and caffeine, and, as described previously, Birch et al. demonstrated clinical efficacy of interferon with trifluorothymidine (TFT) in 1992. Furthermore, prophylactic therapeutic benefits may result from the use of beta-interferon gel.

Ribonucleotide reductase inhibitors

Ribonucleotide reductase catalyzes the synthesis of viral DNA from cellular ribonucleotides. HSV-specific ribonucleotide reductase is inhibited by the thiocarbonohydrazone class of irreversible ribonucleotide reductase inhibitors. These inhibitors have a significantly greater affinity against the viral enzyme than the mammalian counterpart, allowing for greater selectivity and reduced adverse effects on host cells. A1110U (2-acetylpyridine-5-[(dimethylamino) thiocarbonyl] thiocarbonohydrazone) potentiates the effects of acyclovir by increasing the pools of intracellular acyclovir triphosphate as dGTP levels decrease. Adding A1110U to acyclovir causes a reduction in the 50% inhibitory concentration for HSV types 1 and 2. Studies with mice show that combination therapy was more effective in healing lesions [40].

The antiviral activity of acyclovir is potentiated as effectively by topical 348U87 (2-acetylpyridine-5-[2-chloroanilinothiocarbonyl] thiocarbonohydrazone) as topical A1110U; however, it has no hematological toxicity. When 348U87 was added to acyclovir in tissue culture, it caused a reduction in the 90% inhibitory concentration to acyclovir from 10 to 2.1 µM for HSV-1 and from 8 to 0.09 µM for HSV-2 in mice. Unfortunately, evaluation of a topical cream containing 3% 348U87 and 5% acyclovir showed little effective therapuetic value [40].

SP-303

SP-303 is an antiviral phenolic polymer isolated from the latex of the plant Croton iechler which acts by inhibiting virus penetration into cells. SP-303T is an ointment containing 15% SP-303; however, no therapeutic significance for the treatment of ACV-resistant HSV could be proven [41].

Trisodium phosphonoformate (PFA; Foscarnet)

A pyrophosphate analog, trisodium phosphonoformate, inhibits HSV DNA polymerase without requiring activation by viral thymidine kinase; therefore, this drug is effective against ACV-resistant thymidine kinase deficient HSV strains [22]. PFA attaches to the pyrophosphate binding sites of viral DNA and RNA polymerases, causing reversible inhibition of polymerase activity [14].

Although in Sweden, PFA was shown to have important clinically relevant reduction in time to healing in patients with genital herpes [42], a larger study in the United Kingdom and the Netherlands failed to show efficacy in treating recurrent genital HSV infection [43]. Since studies in treating HSV infections in a guinea pig model suggest that a higher concentration of cream is more effective, this was undertaken in patients with orolabial herpes. Effective therapy was only proven in the subgroup of patients who began treatment in the prevesiciular stage [44]. It appears that earlier treatment with PFA may result in greater clinical significance in HSV treatment.

Tromantadine (N-adamantyl-N-[2-(dimethylamino) ethoxyl]acetamide hydrochloride)

Tromantadine, an amantadine derivative, inhibits HSV-induced cytopathic effect and viral replication possibly by blocking the absorption of HSV at the cellular surface. A 1% tromantadine ointment is licensed in several European countries for HSV therapy. According to other studies, tromantadine has some significant therapeutic effects on labial herpes, but is not effective for genital herpes [45-47].

Discussion

Many clinical trials of different topical antiviral drugs have been performed to assess their efficacy against HSV, but the different methods researchers have used make direct comparison difficult. Vehicles, times of initiation of therapy after prodromal symptoms, application frequencies, and duration of therapy differ from study to study. However, 15% IDU in DMSO, interferons, and penciclovir appear to exert the greatest clinical efficacy. IDU, studied in 301 patients reduced pain duration and decreased time to loss of crust. Alpha-interferon shows synergism with other anti-HSV drugs such as caffeine, TFT, DMSO, and nonoxynol-9. Finally, in a study of over 2,000 patients, penciclovir treatment resulted in decreased duration of lesions, pain, and viral shedding in both early and late application of cream. ACV-resistant strains of HSV may be treated with HPMPC, and ascorbic acid shows promising effects against HSV.

An increase in absorption of an effective drug through the skin may occur if different vehicles are used. Topical therapy may incorporate substances to alter the permeability of skin using mechanical, electrical, or chemical mechanisms. This has been referred to as "active" topical therapy. The stratum corneum is the most difficult layer to penetrate in the skin; it is primarily made of lipids and functions to prevent water loss and absorption of foreign material. "Chemophoresis" has been the most clinically effective method of changing skin permeability, while DMSO, an excellent solvent, has probably been the most studied [29]. Another vehicle that increases skin penetration is propylene glycol [48], and a study with Azone, a penetration enhancer, failed to increase the absorption of lipophilic arildone through the stratum corneum probably because of the difficulty in transversing hydrophilic regions of the epidermis and upper dermis [49].

Combined therapy has become increasingly popular in combatting HSV, and it has been shown that some drugs may potentiate the antiviral activity of other drugs so that not only an additive, but also a synergistic effect can be demonstrated. TFT with interferon-alpha, caffeine with interferon, and DMSO with IDU are three examples of effective combination therapy. Nonoxynol 9 is a nonionic surfactant that inactivates HSV in vitro by breakdown of the viral envelope, but no clinical effect on herpes genitalis was observed [10]. However, studies using nonoxynol 9 to aid uniform dispersion of interferon have shown some added benefit [34].

New therapies such as modelin-1 (mod-1), vaccination, and gene therapy are on the horizon. Mod-1 is an amphiphilic synthetic peptide with significant antiviral activity against HSV-1 and 2. Its mechanism of action against HSV differs from that of ACV in that it takes its toll before the cell becomes infected, suggesting direct interaction with the viral envelope, rather than inhibition of DNA replication or gene expression [50].

Experimental vaccines have been tested in order to attempt to completely eradicate HSV symptoms. Subcutaneously injected, heat-inactivated HSV-1 is known as Lupidon-H and HSV-2 as Lupidon-G [3]. With Lupidon-H, cell-mediated immunity is enhanced, increased intervals between recurrent episodes were seen, and these recurrent episodes showed more attenuated symptomology than episodes before vaccination [51]. Finally, gene therapy for HSV infection has also been investigated; however, minimal efficacy has been shown [52].

Perhaps further attempts to combine drugs can provide the future therapy that will effectively prevent HSV episodes. Subsequent work must be done in optimizing vehicles, concentrations, application schedules and testing mixtures with other agents. Alternatively, it is possible that a topical vaccine or a strongly effective topical form of gene therapy may evolve in the near future.

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