Acyclovir

Abstract

Acyclovir (aciclovir) is a nucleoside antiviral drug with antiviral activity in vitro against members of the herpes group of DNA viruses. As an established treatment of herpes simplex infection, intravenous, oral and to a lesser extent topical formulations of acyclovir provide significant therapeutic benefit in genital herpes simplex and recurrent orofacial herpes simplex. The effect of acyclovir therapy is maximised by early initiation of treatment, especially in non-primary infection which tends to have a less protracted course than the primary episode. Long term prophylactic oral acyclovir, in patients with frequent episodes of genital herpes simplex, totally suppresses recurrences in the majority of subjects; as with other infections responding to acyclovir, viral latency is not eradicated and pretreatment frequencies of recurrence return after discontinuation of treatment. Caution should accompany the prophylactic use of acyclovir in the general population, due to the theoretical risk of the emergence of viral strains resistant to acyclovir and other agents whose mechanism of action is dependent on viral thymidine kinase. Intravenous acyclovir is the treatment of choice in biopsy-proven herpes simplex encephalitis in adults, and has also been successful in the treatment of disseminated herpes simplex in pregnancy and herpes neonatorium. Intravenous and oral acyclovir protect against dissemination and progression of varicella zoster virus infection, but do not protect against post-herpetic neuralgia. In immunocompromised patients, intravenous, oral and topical acyclovir shorten the clinical course of herpes simplex infections while prophylaxis with oral or intravenous dosage forms suppresses reactivation of infection during the period of drug administration. Ophthalmic application of 3% acyclovir ointment rapidly heals herpetic dendritic corneal ulcers and superficial herpetic keratitis. Thus, despite an inability to eradicate latent virus, acyclovir administered in therapeutic or prophylactic fashion is now the standard antiviral therapy in several manifestations of herpes simplex virus infection, and indeed represents a major advance in this regard. With the exception of varicella zoster virus infections, early optimism concerning the use of the drug in diseases due to other herpes viruses has generally not been supported in clinical investigations. Acyclovir exhibits in vitro activity against herpes simplex virus types 1 and 2, varicella zoster virus, Epstein-Barr virus and, to a lesser degree, cytomegalovirus. Although quantitative results vary considerably depending on the viral strain and methodological aspects of the assay system employed, in general herpes simplex virus type 1 is the most susceptible, followed in descending order of susceptibility by herpes simplex type 2, varicella zoster virus, Epstein-Barr virus and cytomegalovirus. Against herpes simplex virus types 1 and 2, acyclovir is generally more potent in vitro than the nucleoside analogues idoxuridine, trifluridine or vidarabine, of comparable potency to 2′-fluoro-5-iodoarabi-nosylcytosine (FIAC) and ganciclovir [DHPG;9-(1,3-dihydroxy-2-propoxymethyl) guanine], and more (herpes simplex type 2) or less (herpes simplex type 1) potent than bromovinyldeoxyuridine. The relative potencies of acyclovir and other antivirals against varicella zoster virus have varied between studies, but the drug is less active in vitro against cytomegalovirus than idoxuridine, trifluridine, vidarabine and ganciclovir. Acyclovir appears able to inhibit the productive cycle, but not the latent phase, of Epstein-Barr virus. Depending on the drug concentrations and cell lines involved, combinations of acyclovir plus one of the other nucleoside antivirals or one of the variously derived interferons have generally exhibited additive to synergistic activity against certain herpesviruses, including cytomegalovirus. Replication of herpes simplex virus in explant ganglionic or tissue cultures is readily interrupted during incubation with acyclovir; reversion to latency is noted after several days’ exposure to the drug. Acyclovir appears to eradicate part of the latent viral reservoir, as assessed by the reactivation rate after drug removal. Alternate exposure of explant ganglionic cultures to acyclovir-containing and drug-free media has been associated with statistically significant reductions in the proportion of reactivable virus, dependent on the frequency and duration of alternating applications. The mechanism of action of acyclovir involves highly selective inhibition of herpes virus DNA replication, via enhanced uptake in herpes virus-infected cells and phos-phorylation by viral thymidine kinase, and the substrate specificity of acyclovir triphosphate for viral, rather than cellular, DNA polymerase. Epstein-Barr virus and especially cytomegalovirus which does not encode for a thymidine kinase, have reduced susceptibility to acyclovir; the mechanism of action of acyclovir against these viruses may differ from that which operates in herpes simplex. The antiviral efficacy of acyclovir administered by various routes has been demonstrated in in vivo animal models of ocular, cutaneous, genital, CNS and neonatal infections due to herpes simplex viruses. In addition to being influenced by factors such as the animal species, size of viral inoculum, drug dosage, and route and frequency of administration employed, the efficacy of acyclovir treatment correlated closely with the rapidity of its initiation following viral inoculation. Indeed, initiation of topical or systemic acyclovir within 24 hours of viral challenge has been found to prevent the establishment of viral latency following primary infection, and reductions in the number of reactivable latent herpes simplex foci following acyclovir treatment have been observed in some in vivo studies, but acyclovir therapy has not resulted in eradication of established, latent virus from neuronal ganglia. When compared with other antiviral agents in in vivo models of herpes simplex virus infections, acyclovir was generally at least as effective as other nucleoside analogues, although research tools such as the newer 2′-fluoropyrimidines and 5′-vinylpyrimidines offered greater protection than acyclovir in certain models, when the agents were administered on an equimolar basis. Acyclovir-resistant strains of herpes simplex virus and varicella zoster virus arise chiefly from mutations in the genes affecting the production of viral thymidine kinase, and are readily produced in vitro. Such strains exhibit variable cross-resistance to other antivirals, depending on the specific mutation(s) conferring resistance and the mechanism of action of the alternative drug. Clinically, acyclovir-resistant strains have been reported very infrequently overall, and usually in association with chronic mucocutaneous lesions in severely immunocompromised patients receiving extended courses of acyclovir. The reduced in vivo pathogenicity and ability to establish latency of the vast majority of clinically isolated resistant strains indicates that they are unlikely to cause refractory or aggressive infection in immunocompetent individuals; this supposition has been supported by clinical studies to date. In addition, comparisons of the in vitro acyclovir susceptibility of viral isolates from patients receiving chronic oral suppressive acyclovir for several months have not revealed statistically significant reductions in viral susceptibility following treatment; when recurrences did ‘break through’, they almost always resolved upon administration of therapeutic or increased dosage of acyclovir. However, a recent study reported a correlation between the in vitro susceptibility of pretreatment herpes simplex virus isolates and the occurrence of mucocutaneous lesions during prophylactic acyclovir administration. Continued monitoring is required to further define the occurrence and clinical significance of viral resistance to acyclovir. Repeated 8-hourly intravenous doses of 2.5 to 15.0 mg/kg provide clinically useful steady-state mean plasma acyclovir concentrations, ranging from 6.7 to 20.6 mg/L, respectively. The bioavailability of oral acyclovir is limited at 15 to 30%. Increasing the contact time of orally administered acyclovir with the absorptive area of the gut, by administering the dose as a direct duodenal infusion over 4 hours, produced a marked increase in bioavailability, indicating that absorption may be capacity-limited, especially at higher dosages. Delivery of topical acyclovir to deeper tissues may be markedly influenced by the formulation vehicle; systemic absorption with topical administration has not been detected. Substantial intraocular penetration is evidenced by a mean acyclovir concentration of 1.7 mg/L in aqueous humour with multidose application of the 3% ointment every 5 hours, and peak concentrations of 6.7 and 5.2 mg/L, respectively, in the aqueous and vitreous humour following a single 25mg subconjunctival injection. Acyclovir was detected at autopsy in the kidney, lung, nervous tissue, liver and heart of a patient who had received high-dose intravenous therapy with the drug. Cerebrospinal fluid and skin vesicle concentrations following intravenous therapy, and saliva and tear fluid concentrations following oral therapy, were approximately 50%, 100%, 13% and 18% of simultaneous plasma concentrations, respectively. Acyclovir crosses the placenta and accumulates in breast milk such that the milk concentration in a lactating woman was more than 3 times the simultaneous plasma acyclovir concentration. In vivo protein binding of acyclovir is low (9 to 24%) and independent of the plasma drug concentration. Renal excretion is the major route of elimination of acyclovir in subjects with normal renal capacity. Depending on the creatinine clearance, up to 80% of a dose is excreted unchanged in the urine, while the remainder is metabolised to inactive derivatives. The elimination half-life in adults with normal renal function is 2 to 3 hours. As expected, dosage adjustments are required in patients with end-stage renal disease, with the elimination half-life extended in this subgroup to approximately 20 hours, and mean peak plasma concentrations increased approximately 2-fold. Acyclovir is readily haemodialysable, having a dialysis half-life in this situation of about 6 hours. However, continuous ambulatory peritoneal dialysis is much less efficient at removing the drug and the elimination half-life is extended to 14 to 18 hours. Disposition of acyclovir in children is similar to that in adults, but in neonates the relatively underdeveloped renal function results in total body clearance being reduced by two-thirds and the elimination half-life being increased to up to 4 hours. Double-blind, placebo-controlled studies in immunocompetent patients have shown intravenous (5 mg/kg 8-hourly), oral (200mg 5 times daily) and, to a lesser extent, topical acyclovir therapy (5% in polyethylene glycol ointment or propylene glycol cream applied 4 to 6 times daily) initiated within 4 days of the first appearance of signs or symptoms to produce significant reductions in the duration of viral shedding and time to complete healing of lesions in initial genital herpes infection. Statistically significant amelioration of the course of infection was most readily demonstrated in the more severe primary initial episode. Pain or dysuria symptoms tended to be less responsive to treatment, especially by topical acyclovir, which was also generally unable to produce statistically significant reductions in new lesion formation. Acyclovir treatment of initial genital herpes did not alter the chronic recurring nature of the infection. However, early, especially patient-initiated, treatment with oral acyclovir at the prodrome of recurrent genital herpes inhibits new lesion formation and viral shedding and reduces episode durations by 1 to 2 days. Nonetheless, due to the shorter duration of recurrent vs initial episodes, the beneficial effects of acyclovir on recurrent episodes are less dramatic. Although results with topical acyclovir in recurrent infection have been somewhat conflicting, the 5% cream has in some placebo-controlled studies exhibited a moderate degree of clinical efficacy, whereas the 5% ointment generally produced only marginal reductions in virological, and little or no amelioration of clinical disease parameters, even with optimal timing of treatment initiation. Prophylactic oral administration of acyclovir at dosages of 400 to 800 mg/day for 1 to 2 years led to complete suppression of recurrences of genital herpes in approximately 60 to 90% of subjects. Unfortunately recurrence rates returned to pretreatment frequencies after discontinuation of acyclovir. This form of acyclovir therapy was extremely well tolerated and was not associated with the emergence of clinically significant acyclovir resistance. Acyclovir 5 or 10% ointment has been mostly ineffective for the treatment of recurrent orofacial herpes in immunocompetent patients, although a reduction in healing time of statistical, but limited clinical significance occurred when treatment with the 5% ointment was begun during the prodromal phase. Comparisons of the results obtained in placebo-controlled, double-blind investigations indicate acyclovir 5% cream to be superior to the ointment formulation in treating recurrent orofacial outbreaks, although absolute reductions in symptom duration remain small relative to the duration of a recurrent episode. In contrast, prophylaxis with topical (5% cream) and more especially oral (200mg 4 times daily) acyclovir reduces the severity and frequency of recurrences during treatment in patients with a history of frequent outbreaks. Acyclovir 3% ophthalmic ointment applied 5 times daily cures 95 to 100% of herpetic dentritic corneal ulcers in approximately 5 days, being at least as effective as idoxuridine 0.5 and 1% ointments, trifluridine formulated as a 2% ointment, and vidarabine 3% ointment, and possibly more rapid in effect than these comparative antivirals. The larger geographic corneal ulcers also respond to acyclovir ophthalmic ointment, a double-masked study revealing no difference in efficacy with that of vidarabine in this indication. Several double-masked comparative studies showed that the combination of acyclovir 3% ophthalmic ointment applied 5 times daily plus once daily application of human α-interferon shortens the time to healing of superficial herpetic keratitis (dendritic or geographic ulcers) by approximately 3 days, from 7 to 9 days for acyclovir plus placebo. Thus, this combination may be a useful advance in the treatment of superficial herpetic keratitis. In contrast, results with the use of systemic (oral) acyclovir 400mg 5 times daily remain equivocal, insufficient duration of therapy being a potentially compromising variable. As might be expected, considering the likely immunological basis of herpetic stromal involvement, acyclovir 3% ophthalmic ointment, applied 5 times daily, is more effective in treating herpetic disciform keratitis (and in a few reported cases, necrotising stromal keratitis) when administered concomitantly with topical corticosteroids. In a double-masked comparison there was no statistically significant difference in the percentage of patients with herpetic disciform keratitis cured, the time to resolution of signs and symptoms, and the time to healing, between patients administered acyclovir or vidarabine ophthalmic ointment 5 times daily, concomitantly with betamethasone 0.1% ophthalmic drops. In herpetic kerato-uveitis, the efficacy of acyclovir 3% ophthalmic ointment applied 5 times daily was not statistically different from that of trifluridine 1% ophthalmic solution applied 6 times daily, although the mean time to healing of corneal ulcers was shorter (p < 0.05) in the trifluridine-treated group; the majority of these patients also received local injection of dexamethasone. Large collaborative studies comparing intravenous treatment using acyclovir and vidarabine have established acyclovir 10 mg/kg 8-hourly, administered for at least 10 days, to be the treatment of choice for biopsy-proven herpes simplex encephalitis. Acyclovir was found to be particularly beneficial in improving overall survival rates and reducing the incidence of serious sequelae to infection. In case reports, intravenous or oral acyclovir therapy of disseminated herpes simplex of various manifestations in near term pregnancy has been followed by survival, without complications, of mothers and infants. Acyclovir and vidarabine appear to be of comparable efficacy in the treatment of neonatal herpes simplex; antiviral prophylaxis of neonates delivered to mothers with active genital lesions is not generally warranted unless additional risk factors are present. Further case studies have reported the successful treatment with acyclovir of disseminated herpes simplex accompanied by hepatitis, disseminated primary eczema herpeticum, herpes simplex whitlow and herpes simplex-associated erythema multiforme. Intravenous acyclovir significantly attenuated the development of rash and pain, and protected against ocular involvement in double-blind placebo-controlled studies in immunocompetent patients with acute herpes zoster. Similar benefit with oral therapy usually required higher dosages (600 to 800mg 5 times daily) than are generally used for herpes simplex infections. Therapeutic efficacy was maximised with early initiation of acyclovir treatment, however the drug did not offer protection against post-herpetic neuralgia and was often associated with recurrence of pain soon after withdrawal. A marked reduction (vs placebo) in the incidence of ocular sequelae associated with trigeminal zoster occurred after 1 year’s follow-up in patients who received acyclovir 600mg 5 times daily for 10 days during an acute episode. In several case reports, acyclovir treatment of herpes zoster-associated encephalitis, varicella pneumonia, and herpes zoster oticus resulted in rapid resolution of infection. An unconfirmed potential therapeutic benefit of intravenous acyclovir (10 mg/kg 8-hourly) is suggested by a placebo-controlled study in patients with severe infectious mononucleosis. Acyclovir alone or administered sequentially with human lymphoblastoid interferon appears to offer very limited clinical benefit in chronic active hepatitis B infection, although the latter combination administered concurrently is deserving of further investigation in this indication. Earlier double-blind, placebo-controlled studies in immunocompromised patients with herpes simplex infections have shown intravenous (250 mg/m² 8-hourly) acyclovir therapy to dramatically reduce the period of viral shedding, with parallel improvements occurring in healing parameters. Topical application of the 5% ointment was effective with regard to external lesions, but failed to produce a statistically significant reduction in time to healing. More recently, oral administration of 400mg 5 times daily significantly accelerated the resolution of viral shedding and pain, and reduced time to healing by almost two-thirds, in bone marrow transplant recipients. Perhaps due to ethical difficulties concerning such studies, there remains a lack of controlled or comparative studies of acyclovir treatment of immunocompromised patients with acute varicella zoster infection. Nevertheless, intravenous dosages of 500 mg/m² or 10 mg/kg administered 8-hourly in small numbers of patients have been associated in controlled studies with a protective effect against progression and dissemination of infection, although reductions in time to healing did not generally achieve statistical significance. However, in...