Antiviral Agents

Symposium on Anti-Infective Therapy

Antiviral Age.nts Raymond F. Schinazi, Ph.D.,* and William H. Prusoff, Ph.D. t

Viral infections are among the greatest causes of human morbidity and occasional fatality. The use of viral vaccines has prevented or eradicated some viral infections. However, there are important viral diseases for which no vaccine is as yet available and for which effective treatment would be particularly helpful. Almost two decades after the introduction of penicillin in clinical practice, 5-iodo-2' -deoxyuridine (IdUrd; IOU; idoxuridine; Stoxil; Idoxene) was shown in 1964 to be effective in the treatment of herpes simplex virus (HSV) keratitis, a viral disease of the cornea that may lead to blindness. The success achieved with IdUrd stimulated the study of many additional drugs. However, to date, only five antiviral drugs are approved for clinical use in the United States: 1-adamantanamine hydrochloride (amantadine HCl; Symmetrel) for prophylaxis or treatment of influenza A virus infection; topical IdUrd and 5-trifluorothymidine (TFT; trifluridine; Viroptic) for HSV keratitis, topical 9-~-D-arabino-furanosyladenine (ara-A; Vira-A; adenine arabinoside; vidarabine) for ocular herpes as well as an intravenous preparation for HSV encephalitis (its approval for neonatal herpes is under consideration), and most recently topical 9-(2-hydroxymethyl) guanine (ACV, acyclovir, Zovirax) for primary genital herpes. Isatin-3-thiosemicarbazone (methisazone) has been evaluated for the treatment of smallpox and vaccinia infections, but will not be discussed further, since smallpox has now been "eradicated" and routine vaccination is no longer recommended. The effectiveness of currently available antivirals as well as the potential and problems with some of the newer drugs will now be discussed. Amantadine This drug is effective in the prevention and treatment of influenza A infections. 39· 52 Interference with virus penetration or uncoating is believed

*Department of Pediatrics, Division of Infectious Diseases, Allergy and Immunology, Emory University School of Medicine, Atlanta Georgia tDepartment of Pharmacology, Yale University School of Medicine, New Haven, Connecticut Supported in part by grants No. DE07074 and All8600 from the National Institutes of Health, and by U.S. Public Health Service Grant CA-05262 from the National Cancer Institute. The literature survey for this chapter was completed in March 1982.

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to be the main mechanism of action of the drug. Although it also has been reported to inhibit the initiation of RNA synthesis by virion-RNA transcriptase, the direct effect on the enzyme has not been confirmed. 39 The drug is given for a period of 4 to 6 weeks at a dose of 200 mg/day for adults. In children less than 9 years old, the total dose should be calculated on the basis of 2 to 4 mg per pound of body weight per day, not to exceed 150 mg/day. This drug is particularly effective in shortening the course of illness (or in lessening symptomatology) when treatment is initiated early. However, there are some caveats for physicians: (1) amantadine has not been proved effective against other respiratory viruses, including influenza B; (2) resistance to amantadine has developed in vitro and in humans, 3 • 35 and indiscriminate use may lead to outgrowth of resistant strains of influenza A; (3) amantadine has not been demonstrated as either safe or effective in children under one year of age or in high-risk elderly patients for whom the drug might be the most helpful; and (4) confusion, dizziness, nervousness, and insomnia are the more common side effects of the drug, occurring especially in the elderly and in those with renal insufficiency. These are reversible when the drug is discontinued. Amantadine is not metabolized and is mainly excreted in the urine. The first and most effective line of protection against influenza A still remains vaccination. Amantadine's main usefulness could prove to be in nursing homes in the event of a widespread outbreak in the community. Amantadine has been reported to be embryotoxic and teratogenic in rats at 50 mg/kg/day, but not at 37 mg/kg/day or in rabbits receiving up to 25 times the recommended adult dose. Rimantadine is a compound closely related to amantadine that was reported not only to have greater activity in humans than amantadine but also to have a lower frequency of neurologic side effects. 52 Rimantadine is still under investigation in the United States but is marketed in the Soviet Union.

5-Iodo-2' -Deoxyuridine This drug is effective in the topical treatment of ocular herpes infection. IdUrd selectively inhibits viral replication by being more rapidly anabolized in the infected cells, relative to uninfected cells, to the iodo analog of thymidylate, and is subsequently incorporated into viral DNA. Along the way, the drug or its metabolites effect competitive inhibition and also cause feedback inhibition of regulatory enzymes. 72 Viral DNA into which IdUrd has been incorporated also shows increased lability to stress, mutation in DNA synthesis, errors in protein formation, and outright inhibition of replication. A direct correlation has been found between the extent of incorporation of IdUrd into herpesvirus-DNA and inhibition of viral replication. 28 The drug is available as a 0.1 per cent ophthalmic solution and as a 0.5 per cent ophthalmic ointment. It is recommended that one drop of solution be placed in the infected eye every hour during daytime and every 2 hours at night. Treatment should be discontinued if no response is noted after 7 or 8 days. If the ointment is used, it should be administered 5 times a day every 4 hours. A combination of these two procedures may also be used in which drops are used during the day and ointment at night.

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In spite of its usefulness, IdUrd has three potential problems of antiviral chemotherapy other than possible toxic effects. The first is the development of "clinical" resistance, which is a potential problem with all antiviral pyrimidine and purine nucleoside analogs. Resistance to Id U rd by herpesvirus develops rapidly in cells culture, 5 and this is attributed to a decrease in the formation of herpesvirus encoded thymidine kinase. 25· 55 Hirano et al. 38 isolated four strains of herpes simplex virus type-1 from two patients with recurrent herpes keratitis of which two were highly resistant to IdUrd in cell culture and the other two were susceptible to this drug. All four viruses induced thymidine kinase. Although the basis for resistance was not established, under consideration was a decreased affinity of IdUrd for thymidine kinase or of IdUTP for herpes simplex virus-DNA polymerase. Resistant strains of HSV have been isolated from patients previously; 50· 68 however IdUrd resistance in vivo does not appear very easilyM and clinically resistance may be due to pharmacokinetic problems. Thus Jawetz et al. 44 found that of 12 strains of HSV isolated from "clinically-IdUrd-resistant" patients, ten were equally sensitive to IdUrd and two had a 10-fold to 30-fold increase in resistance. Similar findings were made by Nordenfelt and Nordenfelt. 64 The second potential problem is that, in vitro, IdUrd has been shown to reactivate latent retroviruses and to increase the yield of cytomegalovirus31 (there is as yet no evidence that it occurs in humans); the third is that IdUrd interferes with DNA synthesis of the host cells and prolonged topical IdUrd therapy may slow down the normal healing process. The use of systemic IdUrd for viral infections is limited because of its myelosuppression6 and lack of transport to the brain. 53 British researchers reported that IdUrd prepared in dimethyl sulfoxide (DMSO), a topical formulation available in Europe, was effective in the management of cold sores, genital herpes, herpetic whitlow, and varicella-zoster skin lesions. 56· 67. 95 However, the use of this regimen is controversial.l7 • 80 • 89 In addition, it should be noted that IdUrd and DMSO are teratogenic in several species of animals. 6· 1• 69 In high concentration and in active solvents there also seems to be a risk of sensitization to IdUrd in humans. 87 5-Ethyl-2' -deoxyuridine (Aedurid) and 5-iodo-2' -deoxycytidine (CebeViran) are two analogs of IdUrd that are available in Europe for the treatment of ocular herpes. Clinical efficacy in open trials only has been reported for these drugs.26, 58 Adenine Arabinoside (Ara-A) This compound has in cell culture a broad spectrum of activity against several clinically important DNA viruses as well as on animal RNA viruses that induce an RNA-dependent DNA polymerase (reverse transcriptase). There is still considerable debate about its primary site of inhibition. The agent is widely believed to target viral DNA polymerase preferentially, and it is incorporated into DNA.4, 12.65 , The U.S. National Institute of Allergy and Infectious Diseases (NIAID) Collaborative Antiviral Group evaluated adenine arabinoside in three clinical double-blind placebo-controlled protocols for the treatment of (a) herpes simplex virus (HSV) encephalitis; (b) varicella-zoster virus (VZV)

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infection in immunocompromised patients; and (c) neonatal herpes simplex virus infections. Reported in 1977, the results of the encephalitis study demonstrated the potential efficacy of the drug. Mortality of biopsy-proven HSV encephalitis was 70 per cent in the placebo group; treatment with ara-A (via slow-drip infusion at a dosage of 15 mg/kg/day over 12 to 24 hours for 10 days) reduced it to 28 per cent. 93 Over 50 per cent of the drug-treated survivors suffered no major neurological sequelae; damage was less evident if patients had been treated early in the course of the infection, but, since half of the ara-A-treated patients either died or had severe disability, they felt that there is need for improved therapy. Results in a much larger number of patients have confirmed the drug's efficacy, and those obtained by a British study corroborate the high rate of mortality in patients given either placebo or an ineffective drug (cytosine arabinoside). Most important in predicting ultimate outcome in the ara-A treated patients was the level of consciousness at the time of initiating therapy, and this was at least partly related to the duration of disease. Until more simple and reliable diagnostic tests became available, it is important to continue brain biopsy in adults suspected of having herpes encephalitis, since biopsy-negative patients would not be expected to benefit from araA therapy. In addition, the brain biopsy often leads to the correct diagnosis and appropriate therapy for herpes negative patients. 94 Ara-A also has shown in a controlled, randomized cross-over study to diminish virus shedding and the morbidity of herpes zoster infection in immunocompromised patients, but not mortality or postherpetic neuralgia. 8 In a pilot study of immunosuppressed children with chickenpox, ara-A was also shown to be effective. r12 A new, controlled study comparing ara-A and placebo (no cross-over), besides confirming the effectiveness of ara-A in accelerating healing, showed that the incidence of cutaneous dissemination, visceral complications, and duration of postherpetic neuralgias were also decreased. 91 However, its efficacy in chickenpox pneumonia or encephalitis still needs to be ascertained. Although believed to be relatively uncommon (about one in 3000 to 10,000 deliveries), neonatal herpes has a high mortality and morbidity depending on the extent of viral involvement. In patients with disseminated infection, ara-A reduced the death rate from 74 per cent in those given placebo to 38 per cent. 92 More localized infections as well as early therapy for the more severe cases carried the best prognosis. Thus ara-A reduced the mortality from 50 per cent to 10 per cent in patients with localized involvement of the central nervous system. It is expected that ara-A will be recommended for this disease shortly at a dose of 30 mg/kg/day for 10 days (continuous infusion). Ara-A has not yet proved effective for the management of cytomegalovirus infection, herpes genitalis, or mucocutaneous herpes. In a study of immunosuppressed and non-immunosuppressed adult patients and congenitally infected newborns, ara-A treatment resulted in decreased urinary excretion of cytomegalovirus but in no clear clinical improvement. 92 In addition, in patients in whom ara-A eliminated viruria, excretion resumed after cessation of treatment. A limitation of ara-A is its lack of solubility at high doses. It would, therefore, be desirable to have a more soluble form; one such candidate is

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the 5' -monophosphate of ara-A (ara-AMP), which can also be administered intravenously. Whether the two drugs are equally effective remains to be resolved. As with ara-A, topical administration of ara-AMP against oral mucocutaneous herpes showed no efficacy. 84 A recent double-blind placebo-controlled study with ara-AMP ointment for recurrent genital herpes showed statistically significant reduction in virus titer, but no clinical significance was noted. 33 Perhaps the use of a different vehicle may provide greater transcutaneous diffusion and therapeutic effect. Ara-A or ara-AMP has also been evaluated in the treatment of chronic active hepatitis B virus (HBV) infections. 40• 71 The therapy resulted in a sustained improvement in the levels of hepatitis B virus markers and in indices of disease activity. Evaluation in a randomized, controlled study is still needed to confirm the effectiveness of these drugs. Clinical trials of topical 3 per cent ara-A ointment in the treatment of herpetic keratitis have indicated that it is at least as effective as IdUrd. The ointment should be applied 5 times daily every 3 to 4 hours. Ara-A appears to be effective also against some herpes simplex virus strains that are resistant to IdUrd, 51 and cross-resistance to acyclovir and phosphonoformate has been observed in cell culture. Topical applications of ara-A may result (like IdUrd) in signs and symptoms of drug toxicity such as punctate keratitis, follicular and papillary conjunctivitis, punctal occlusion, and allergic reaction. Ara-A is rapidly deaminated in humans to the water-soluble arabinosylhypoxanthine (ara-B), which has antiviral activity lower than that of the parent compound in cell culture, but ara-H is 25 times less toxic than araA. 30 Study of compounds that are resistant to deamination such as 2-fluoroara-A, cyclo-ara-A, and 2' -azido-ara-A are in progress, as well as the coadministration of 2' -deoxycoformycin to inhibit the enzymic deamination of ara-A. In mice inoculated intracerebrally with herpes simplex virus, araA and ara-H were equally effective in reducing mortality, 75a suggesting that the in vivo deamination of ara-A may not be as serious a limitation as had been previously thought. Nausea and other gastrointestinal symptoms are the most common adverse effects of ara-A. Neurological symptoms such as tremor, dizziness, and confusion have also been observed, but these may be difficult to distinguish from the symptoms of the disease or the effects of concomitant medication, such as human leukocyte interferon. 73 The latter appears to potentiate the toxicity of ara-A and results in accumulation of ara-Hx. 74 Decreases in hemoglobin, white cells, and platelets have occurred, particularly at high ara-A doses. Ara-A had teratogenic effects in rats and rabbits4 and is carcinogenic in mice and rats. In two patients who had received ara-A for severe viral infections, in vitro lymphocyte preparations showed a significant increase in chromosome breaks and gaps. 4 On the positive side, and a most important consideration for any antiviral, is that ara-A is not immunosuppressive. 5-Trifluorothymidine (TFT) Although this drug is highly active against herpes simplex virus, most studies on its m~chanism of action have been done with vaccinia virus. The 5' -monophosphate derivative is known to be an inhibitor of thymidylate

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synthetase and can also be further phosphorylated to the triphosphate, which becomes incorporated into DNA. 34 Uptake into DNA is responsible for its antiviral activity because late virus messenger RNA is not properly made. In several double-blind clinical studies of patients with herpes keratitis, TFT treatments were equally if not more effective than IdU rd or ara-A treatments. TFT treatment (1 per cent ophthalmic solution) appeared to heal corneal and geographic ulcers more reliably than previously available drugs; ulcers that had failed to heal with either IdUrd or ara-A, healed on treatment with TFT. 15 · 88 This perhaps is related in part to the finding that herpes simplex virus resistant to IdUrd and ara-A may be sensitive to TFT, however the possibility of cross-resistance has been suggested by other studies. TFT, unlike IdUrd or ara-A, rarely caused hypersensitivity, although burning or stinging and palpebral edema occurs frequently and contact dermatitis of the eyelids can also result. The drug should not be used continuously for more than 21 days because of potential ocular toxicity. Of particular interest is the recent observation that TFT has a high therapeutic index in vitro against human cytomegalovirus, suggesting that this drug may be of some promise as a clinically useful anti-cytomegalovirus agent. 96 This drug is stable under acidic conditions, but is rapidly hydrolyzed under physiologic (half-life in plasma is 18 minutes) and alkaline conditions to 5-carboxy-2'-deoxyuridine, a metabolite that is devoid of antiviral but not cytotoxic activity. TFT is not for systemic use because of its toxicity to bone marrow and to the gastrointestinal tract. TFT is mutagenic in vitro, and teratogenicity has been described. High doses. of TFT were not teratogenic42 when applied to the eyes of pregnant rabbits (IdUrd was teratogenic in the same experiment, but this finding has not been confirmed). 5-Fluoro-2'-deoxyuridine but not TFT or IdUrd produced oncogenic transformation of mammalian cells; 47 however, its oncogenic potential in vivo is unknown.

Ribavirin (Virazole) Unlike the previously noted antivirals, which affect primarily some of the DNA herpesviruses, ribavirin has in vitro a broad antiviral spectrum affecting also RNA viruses. 9• 82 This compound structurally resembles guanosine and is converted to its active form by phosphorylation. In the cell, ribavirin is metabolized to the 5'-monophosphate; this product inhibits inosinate dehydrogenase, which ordinarily converts inosinate to xanthylate. In other words, ribavirin interferes with the formation of guanosine monophosphate, upon which both RNA and DNA synthesis depend. In addition, after conversion to the 5' -triphosphate derivative inhibition of influenza virus RNA polymerase occurs, and this may be responsible for the antiviral activity against this virus. Controlled clinical trials of ribavirin given orally (usually 1 gm/day for 5 days) for influenza A and B virus, hepatitis B virus, and genital herpes infections have been conflicting. In most of these studies, the drug had either no beneficial effect or resulted in slight clinical improvement. Viral shedding was reduced in some studies but this could not be confirmed in others. In a recent controlled study, an aerosol preparation has been

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shown to have some effectiveness in reducing the duration of virus shedding and symptoms in patients infected with influenza A virus. 49 A similar aerosol preparation was effective in a respiratory syncytial virus (RSV) animal model study. 41 Efficacy in humans infected with respiratory syncytial virus has not yet been carried out in a carefully controlled trial. Limited open trials with oral preparations against herpes zoster, orolabial herpes, measles, and Lassa fever have yielded favorable, but still unconfirmed, results. 9 The major toxicologic effects in rats, monkeys, and humans at highdose levels administered on a prolonged basis is development of anemia; this effect is reversible on termination of therapy. In a double-blind study with a placebo, a significantly higher number of ribavirin-treated patients had gastrointestinal symptoms two days after onset of therapy as well as high bilirubin levels. 57 Ribavirin is teratogenic in rats and hamsters and caused fetal resorptions in rabbits. However, pregnant baboons treated with ribavirin during the first trimester of pregnancy delivered normal offspring. 9 Some concern has been expressed that ribavirin is immunosuppressive, but such effects have been seen at relatively high doses and prolonged treatments or in animal models. Ribavirin was not mutagenic or carcinogenic in vitro and to date resistance to this drug has not been reported. Because ribavirin is lipid-insoluble, it does not cross the bloodbrain barriers, and this may pose significant problems in the application of this drug to serious neurologic infections. Inosiplex (Isoprinosine) Although this immunodulating drug is a weak inhibitor of herpes simplex virus, adenovirus, poliovirus, and influenza virus in cell culture systems, human trials have so far been conflicting. 9 The majority of these studies have various deficiencies including lack of true double-blind placebo-controlled trials, small number of patients, and unconfirmed diagnosis. Even in controlled studies, experimental infection of volunteers with rhinovirus or influenza A virus demonstrated that although virus shedding was reduced in inosiplex treated individuals, clinical illness was not delayed in onset, lessened in severity, nor shortened in course. 9 • 48 In open trials with small numbers of patients with subacute sclerosing panencephalitis (SSPE), inosiplex demonstrated some beneficial effect in a third of the patients. 43 A recent retrospective analysis of 98 patients who had received inosiplex therapy for SSPE for less than one month to over nine years showed that survival was greatly increased compared with three historical control groups. 46 In another recent "controlled" study that used a historical control group, inosiplex therapy did not appear to alter the clinical course of SSPE. 32 Due to the low incidence and high mortality of this disease, evaluation of treatment by double-blind placebo-controlled studies has not been carried out. There has been no serious toxicity encountered in the chronic use of inosiplex in humans over extended periods of time. At high doses, occasional transient nausea and rise of serum and urinary uric acid were observed. Teratology, fertility, and reproductive studies in rats and rabbits showed no abnormality. 9

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SECOND GENERATION ANTIVIRALS Recently, much work in the development of antivirals has focused on compounds that use a viral coded enzyme either to activate a drug preferentially in the infected cell or as a specific target for inhibition. These enzymes have some unique properties not shared by the corresponding cell enzymes. Thus, the appeal of this approach lies in its exquisite selectivity, which theoretically could attain (or at least approach) the ideal. Prominent examples of this diverse group of compounds that selectively inhibit HSV and VZV are the experimental compounds: 5-iodo-5' -amino2' ,5' -dideoxyuridine (AldUrd; AIU); 5-ethyl-2' -deoxyuridine (EdUrd); 5-n-propyl-2' -deoxyuridine (PdU rd); 1-13-D-arabino-furanosylthymine (ara-T); 9-(2-hydroxyethoxymethyl) guanine (ACV; acyclovir; zovirax); phosphonoformate (PFA; Foscarnet); E-5-(2-bromovinyl)-2' -deoxyuridine (BVDU); 1-(2-deoxy-2-fluoro-13-D-arabinosyl)-5-iodocytosine (FIAC) and its thymine derivative (FMAU), and 9-[2-hydroxy-1-(hydroxymethyl) ethoxy methyl] quanine (BIOLF-62). AldUrd, EdUrd, PdUrd, ara-T, ACV, BVDU, FIAC, FMAU, and BIOLF-62 are all phosphorylated preferentially by the virus specific thymidine kinase, and further phosphorylated to the respective triphosphates by cellular enzymes. The triphosphates of these drugs act by either inhibiting the viral DNA polymerase, or by being a substrate for this enzyme and are then incorporated into viral DNA or by both mechanisms. PFA is unique in that it does not require metabolic conversion in order to exert its inhibition of DNA polymerase. Clinical experience with some of these drugs in controlled studies and their effectiveness in the treatment of herpesvirus infection will be briefly reviewed. Acyclovir (ACV) This drug is an acyclic analog of 2' -deoxy-guanosine. Considering that synthesis of the drug was first published in 1978, 76 a remarkably large number of controlled clinical trials have already been completed. Acyclovir (3 per cent ointment) has been shown to be as effective as ara-A for the treatment of dendritic corneal epithelial ulcers in humans. 45 In a recent large double-blind multicenter clinical trial, acyclovir was as effective as IdUrd for the treatment of dendritic lesions and geographic epithelial lesions.13· 60 Like the other drugs available for ocular herpes, acyclovir is not effective for deep ocular herpesvirus infections. Acyclovir appears to be also effective for the treatment of herpes zoster ocular infections. 61 Recent trials with topical treatment of recurrent orolabial or genital herpes indicate that virus shedding is consistently reduced by the drug, whereas there is limited or no effect on the duration of lesions and the severity of pain. 14· 85 An ongoing controlled study of self-administered drug as soon as prodrome is noted should permit a determination of the possible effectiveness of acyclovir in recurrent infections. Trials with topical or intravenous acyclovir for the treatment of primary genital herpes infections (that is, in patients in whom genital herpes .represents the first experience ever with either herpes simplex viruses type 1 or type 2) have indicated that the drug is effective in significantly reducing virus shedding and symptoms, but the treatment did not reduce the rate of recurrence. 27 Topical acyclovir (5 per cent in polyethylene gly-

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col) was licensed in the United States in March 1982 for the treatment of primary genital herpes. The ointment should be applied in sufficient quantity to adequately cover all lesions every three hours, six times a day, for seven days. The NIAID Collaborative Study group is currently evaluating acyclovir in parallel with ara-A for treatment of herpes encephalitis and neonatal herpes. Other studies in patients infected with cytomegalovirus or Epstein-Barr virus are ongoing. Since immunosuppressed patients are particularly vulnerable to herpesvirus infections, parental acyclovir therapy has been given in open trials to cancer patients (adults and children) with cutaneous and/or systemic herpes zoster or herpes simplex virus infections. 66, 78 The drug arrested the progress of the infections and was most effective when given early. More recent double-blind placebo-controlled studies with systemic acyclovir for mucocutaneous herpes simplex virus infections in immunocompromised patients63 and heart-transplant patients11 have shown that the median times to cessation of new lesion formation, lesion crusting, lesion healing, cessation of pain and termination of viral shedding were shorter in the acyclovir-treated group. In a doubleblind placebo-controlled study of systemic acyclovir prophylaxis against herpes simplex virus infection in seropositive recipients of bone-marrow transplant, acyclovir prevented the development of herpes simplex virus lesions during the course of treatment. In contrast, most of the placebo group developed lesions. 75 It thus appears that acyclovir can provide effective prophylaxis against reactivated infections in the compromised host. A comparison of systemic acyclovir and placebo in non-immunocompromised patients with acute herpes zoster showed that acyclovir produced a small but significantly improved rate of healing of the skin lesions. 7o The drug also shortened the period of pain, particularly in patients older than 67 years. Acyclovir did not influence the rate of pain at one-month follow-up. The urinary recovery of unmetabolized acyclovir in patients ranged from 33 to 58 per cent of the total dose administered. 83 Animal and human studies have shown that 2 to 14 per cent of the dose administered was oxidized to 9-carboxy-methoxymethylguanine and about 1 per cent to 8-hydroxy-9-(2-hydroxyethoxymethyl)-guanine. 24 Both these metabolites are inactive in vitro against herpesvirus. No adverse reactions attributable to acyclovir were noted in these trials. However, renal blockade or acyclovir crystallization has recently been reported in kidneys of dogs and patients receiving bolus systemic acyclovir, as well as drug accumulation in patients with altered renal function.10· u. 66 Slow infusion or reduced dosage of the drug has now been recommended in ongoing studies. A potential advantage of acyclovir is its use in an oral formulation, since sufficiently high drug blood levels are achieved by this route. Herpes simplex virus resistant to acyclovir emerge rapidly in vitro. Since the two loci involved are thymidine kinase and DNA polymerase, it is not surprising that cross-resistance to ara-A, phosphonoformate, or idoxuridine can occur. Of possible clinical importance is the isolation of acyclovir-resistant herpes simplex virus strains from untreated patients or from two children who had received multiple systemic acyclovir treatments.16· 62· 79.81Judicious choice of treatments with antivirals in combination should not only reduce the likelihood of resistant mutants developing

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but should also be potentially valuable in case of life-threatening infection with herpes simplex virus strains that are resistant to one of the agents. 77• Phosphonoformate (PF A) This compound is one of a new class of antivirals that inhibits the virus-encoded DNA polymerase produced by DNA viruses, such as herpes simplex virus, cytomegalovirus, and heptatitis B. 36• 90 It is currently being investigated for the topical treatment of genital and orolabial herpes (3 per cent phosphonoformate cream). In a self-initiated, crossover, placebo-controlled clinical trial of recurrent orolabial herpes, significant benefit was observed when the time to crusting and the duration of vesicles were analyzed. 37 The patients' assessment of efficacy indicated a significant preference for drug over placebo despite the fact that no prodromal symptoms occurred in more than 50 per cent of the episodes. However, since a wide variation occurs in the severity and duration of lesion between patients or between episodes with the same patients, the clinical benefit demonstrated with phosphonoformate needs to be confirmed with carefully defined objective parameters. No adverse dermal toxicity was reported in that study. In a rabbit model for herpes simplex virus ocular infections, topical phosphonoformate was as effective as IdUrd. 1 Systemic phosphonoformate has been found to accumulate in bones of animals; however this process appears to be reversible. Although phosphonoformate-resistant mutants have so far not been isolated in humans, they can be produced in cell culture and may be cross-resistant to ara-A and acyclovir. E-5-(2-Bromovinyl)-2'-Deoxyuridine (BVD U) In vitro testing has shown that BVDU is very active against herpes simplex virus-1 and even more so against varicella-zoster virus. 18 It is about 100- to 200-fold less active against herpes simplex virus-2 than against herpes simplex virus-1, and thus this compound may not be useful for the treatment of the majority of genital or neonatal herpes infections. Preliminary experiments have shown that oral or intraperitoneal BVDU was ineffective in mice infected intracerebrally with herpes simplex virus-2 (Schinazi, R. F., unpublished results) but was effective when the drug was given intracerebrally simultaneously with herpes simplex virus-1 inoculation.20 The drug was effective in mice inoculated with herpes simplex virus-1 intracutaneously. 21 · 23 In guinea pigs, oral and topical BVDU has been shown to suppress the development of herpes simplex virus-1 skin lesions. 21 It is effective in the treatment of herpetic keratitis in rabbits and appears to be effective in therapy of both epithelial and stromal herpes of the eyes in humans, but the latter needs to be confirmed in controlled study. 59 In a limited open trial, oral BVDU enhanced the recovery of adults and children from severe herpes zoster. 19 BVDU-resistant herpes simplex virus mutants are easily produced in cell culture and these are also resistant to acyclovir (Schinazi, R. F., unpublished results); presumably the thymidine kinase locus is here involved. BVDU is phosphorylated by the herpesvirus thymidine kinase, and subsequently cellular enzymes further phosphorylate to the triphosphate derivative. The latter is an inhibitor and/or substrate for DNA polymerase, and is also extensively incorporated into herpes simplex virus-DNA. 2

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No toxic local or systemic side effects have been observed in patients treated with the drug. Furthermore, BVDU does not appear to be mutagenic or teratogenic and, unlike IdUrd, BVDU (and acyclovir) does not stimulate the release of retroviruses in vitro. 22 1-(2- Deoxy-2-fluoro-~- D-arabinofuranosyl)-5-idocytosine (FIAC) This compound and a metabolic derivative, 2' -fluoro-ara-T (FMA U), are active in cell culture against herpes simplex virus-1, herpes simplex virus-2 and VZV replication29 • 54 and also have potential anticancer activity. In uncontrolled clinical studies of individuals with varicella-zoster virus or herpes simplex virus infections who also had underlying malignancies, FIAC appeared to be effective (C. Lopez, personal communication). In mice inoculated intraperitoneally or intracerebrally with herpes simplex virus, FIAC and FMA U were at least as effective as acyclovir in prolonging life and reducing mortality. 29• 77 Radioactivity from [2- 14C] FIAC is incorporated into DNA. FMAU appears to be more physiologically active and stable than FIAC. FIAC, FMAU, and the 5-ethyluracil 2'-fluoro-analog are under study as oral, topical, and injectable antiviral and anticancer agents. Additional Antiviral Agents Owing to limitations of the scope of this review, only brief comments can be made on the following antiviral agents. 1-~-D-Arabinofuranosylthymine (ara-T). In cell culture ara-T inhibits herpes simplex virus-1, herpes simplex virus-2, and varicella-zoster virus, and efficacy against herpes keratitis in rabbits and herpes encephalitis has been reported. No studies in humans have been reported. 5-n-Propyl-2' -deoxyuridine (PdUrd). This nucleoside inhibits herpes simplex virus-1, herpes simplex virus-2, and varicella-zoster virus in cell culture. It is a unique substrate for the herpesvirus-encoded thymidine kinase and inhibits the induction of herpes DNA polymerase and herpes deoxyribonuclease. Incorporation into DNA occurs but this is believed not to be the mechanism of action. (S)-9-(2,3-dihydroxypropyl)adenine [(S)-DHPA]. (S)-DHPA has a broad spectrum of antiviral activity being inhibitory to both RNA and DNA viruses. Antiviral activity in mice has been reported against rabies virus infection as well as varicella-zoster virus encephalitis. This agent inhibits the enzyme S-adenosylhomocysteine hydrolase thereby causing accumulation of adenosylhomocysteine with resultant inhibition of S-adenosylmethionine-dependent methylation reactions. 2-Deoxy-D-glucose. This agent affects both RNA and DNA viruses that have envelopes. The virions are produced but have a decreased infectivity because of a defect in their ability to penetrate into the cell or to be uncoated. Deoxyglucose replaces glucose in the nucleotide guanosinediphosphate-glucose, and the deoxyglucose analog binds to a lipid carrier, dolichol pyrophosphate, thereby preventing glycosylation of proteins. Clinical efficacy has been reported in the therapy of herpes genitalis, but confirmation is required because of questions raised about the experimental design. Zinc Sulfate, Tannic Acid, Urea ("Herpigon"). Efficacy in the therapy of herpes genitalis has been claimed by the application of Herpigon

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and ultrasound (1 W/cm 2 for 60 seconds for 3 consecutive days). Confirmation is required. L-Lysine (Hydrochloride Salt). The amino acid lysine in an uncontrolled test was reported to suppress herpes infection of the lips and mouth in 45 patients. Arginine deficiency suppresses growth of herpes simplex virus in cell culture, and lysine is an analog of arginine. Thus dietary arginine was restricted during administration of lysine. Arildone, (4-[6-(2 -chloro-4-methoxylphenoxy}hexylJ -3 ,5-heptanedione). This compound, also known as Win 38020, has been reported to be effective against both RNA and DNA viruses: rhinovirus, herpesvirus, parainfluenza virus and respiratory syncytial viruses. Activity has been reported against experimental cutaneous herpes infection in guinea pigs and genital herpes in mice by topical administration in DMSO or as a cream. Win 41258-3 is an analog of arildone that has also been reported to be effective in mice and guinea pigs. Enviroxime, Zinviroxime and 4' ,6-Dichloroflavan. These agents have been reported to be inhibitory to rhinoviruses in vitro, and clinical trials are reported to be in progress. PROSPECTUS We have now entered an exciting era in the development of selective antiviral drugs. The extent to which those developed thus far have met or have failed to meet the criteria set to measure efficacy in double-blind placebo trials has been instructive. Future clinical studies with antiviral drugs will benefit from the knowledge gained from these trials. There are a host of newly developed compounds that have a high therapeutic index (ratio of efficacy-to-toxicity) in cell culture and in animal models against DNA and RNA viruses, including herpes, polio, rhino, picorna, and respiratory syncytial virus. As various interferon preparations86 are being developed and tested against different infections, the clinician's armamentarium will correspondingly increase, particularly if combinations are shown to be even more beneficial. With this will come a corresponding need for improved viral diagnosis and appreciation of the drug's potential toxicity and development of resistant strains. The finding of antivirals of proven efficacy will undoubtedly hasten the development of newer drugs for the treatment of various viral infections. REFERENCES l. Alenius, S., Laurent, U., and Oberg, B.: Effect of trisodium phosphonoformate and idoxuridine on experimental herpes simplex keratitis in immunized and non-immunized rabbits. Acta. Ophthalmol., 58:167-173, 1980. 2. Allaudeen, H. S., Chen, M. S., Lee, J. J., DeClercq, E., and Prusoff, W. H.: Incorporation of E-5-(2-halovinyl)-2' -deoxyuridine into deoxyribonucleic acids of herpes simplex virus type-1 infected cells. J. Bioi. Chern., 257:603-606, 1982. 3. Appleyard, G.: Amantadine-resistance as a genetic marker for influenza viruses. J. Gen. Virol., 36:24~253, 1977. 4. Buchanan, R. A., and Hess, F.: Vidarabine (Vira-A): Pharmacology and clinical experience. Pharmac. Ther., 8:143-171, 1980. 5. Buthala, D. A.: Cell culture studies on antiviral agents. I. Action of cytosine arabinoside

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30. Gephart, J. F., and Lerner, A. M.: Comparison of the effect of arabinosyladenine, arabinosylhypoxanthine, and arabinosyladenine 5' -monophosphate against herpes simplex virus, varicella-zoster virus, and cytomegalovirus with their effect on cellular deoxyribonucleic acid synthesis. Antimicrob. Agents. Chemother., 19:170-178, 1981. 31. Green, J. A., and Baron, S.: 5-Iododeoxyuridine potentiation of the replication in vitro of several unrelated RNA and DNA viruses. Science, 190:1099-1101, 1975. 32. Haddad, F. S., and Risk, W. S.: Isoprinosine treatment in 18 patients with subacute sclerosing panencephalitis: a controlled study. Ann. Neurol., 7:185-188, 1980. 33. Hatcher, V., Friedman-Kien, A. E., and Klein, R. J.: Arabinosyladenine monophosphate: a double-blind placebo controlled study. (Abstract 867.) 21st ICAAC. Chicago, Ill., 1981. 34. Heidelberger, C., and King, D. H.: Trifluorothymidine. Pharmacol. Ther., 6:427-442, 1979. 35. Heider, H., Adamczyk, B., Presber, H. W., et al.: Occurrence of amantadine- and rimantadine-resistant influenza-A virus strain during the 1980 epidemic. Acta Virol., 25:395-400, 1981. 36. Helgstrand, E., Eriksson, B., Johansson, N. G., et al.: Trisodium phosphonoformate, a new antiviral compound. Science, 201:819-821, 1978. 37. Helgstrand, E., Flodh, H., Lernestedt, J. 0., et al.: Trisodium phosphonoformate: antiviral activities, safety, evaluation and preliminary clinical results. In Collier, L. H., and Oxford, J. (eds.): Developments in Antiviral Therapy. London, Academic Press, 1980, pp. 62-83. 38. Hirano·, A., Yumura, K., Kurimura, T., et al.: Analysis of herpes simplex virus isolated from patients with recurrent herpes keratitis exhibiting "treatment resistance" to 5iodo-2'-deoxyuridine. Acta. Virol., 23:226-230, 1979. 39. Hoflinan, C. E.: Structure, activity and mode of action of amantadine HCl and related compounds. Antibiot. Chemother., 27:23~250, 1980. 40. Hoofnagle, J., Straus, S., Johnson, R., et al.: Treatment of chronic type B hepatitis with adenine arabinoside monophosphate (Ara-AMP). (Abstract.) Gastroenterology, 80:1337, 1981. 41. Hruska, J. F., Morrow, P. E., Suffin, S. C.: In vivo inhibition of respiratory syncytial virus by ribavirin. Antimicrob. Agents Chemother., 21:125-130, 1982. 42. Itoi, M., Gefter, J. ·w., Kaneko, N., et al.: Teratogenicities of ophthalmic drugs. Arch. Ophthalmol., 93:46-51, 1975. 43. Jabbour, J. T., Van der Zwaag, R., Baldridge, R., et al.: A comparative analysis of the therapeutic effect of isoprinosine on the course of SSPE. 4th International Congress of Immunology, Paris, France, 1980. 44. Jawetz, E., Coleman, V. R., Dawson, C. K., and Thygeson, P.: The dynamics of IUDR action in herpetic keratitis, and the emergence ofiUDR resistance in vitro. Ann. N.Y. Acad. Sci., 173:282-289, 1970. 45. Jones, B. R., Fison, P. N., Coho, L. M., et al.: Efficacy of acycloguanosine (Welcome 248U) against herpes simplex corneal ulcers. Lancet, 2:24~244, 1979. 46. Jones, C. E., Dyken, P. R., Huttenlocher, P. R., et al.: Inosiplex therapy in subacute sclerosing panencephalitis (SSPE): a multicenter study of 98 patients. Lancet, 2:10341036, 1982. 47. Jones, P. A., Benedict, W. F., Baker, M. S., et al.: Oncogenic transformation of C3HI lOT 112 clone 8 mouse embryo cells by halogenated pyrimidine nucleosides. Cancer Res., 36:101-107, 1976. 48. Khakoo, R. A., Watson, G. W., Waldman, R. H., et al.: Effect ofinosiplex (isoprinosine) on induced human influenza A infection. J. Antimicrob. Chemother., 7:389-397, 1981. 49. Knight, V., McClung, H. W., Wilson, S. Z., et al.: Ribavirin small-particle aerosol treatment of influenza. Lancet, 2:945-949, 1981. 50. Laibson, P.R., Sery, T. W., and Leopold, I. H.: The treatment of herpetic keratitis with 5-iodo-2'-deoxyuridine (IDU). Arch. Ophthalmol. (Chicago), 70:52-58, 1963. 51. Laibson, P.R., and Krachmer, J. H.: Controlled comparison of adenine arabinoside and idoxuridine therapy of human superficial dendritic keratitis. In Adenine arabinoside: an antiviral agent, edited by Pavan-Langston, D., Buchanan, R. A., and Alford, C. A. (eds.): Raven Press, N.Y., 1975, pp. 3~0. 52. LaMontagne, J. R., and Galasso, G. J.: Report of a workshop on clinical studies of the efficacy of amantadine and rimantadine against influenza virus. J. Infect. Dis., 138:928-931, 1978.

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