Acyclovir A Review of its Pharmacodynamic Properties and Therapeutic Efficacy

Publisher Website
Google Scholar
Abstract
Synopsis: Acyclovir (aciclovir)1 is a nucleoside analogue antiviral drug related to cytarabine, idoxuridine, trifluridine and vidarabine. In common with these earlier antivirals, acyclovir is active against some members of the herpesvirus group of DNA viruses. The efficacy of topical acyclovir has been convincingly demonstrated in ocular herpetic keratitis, and in initial and primary initial genital herpes infection, but little or no clinical benefit was seen when non-primary initial genital infections were assessed separately. Acyclovir ointment demonstrated little benefit in recurrent genital herpes but topical acyclovir cream decreased the course of the infection by 1 to 2 days. Orally and intravenously administered acyclovir were beneficial in initial genital herpes infections, and oral therapy shortened the duration of recurrent infections by 1 to 2 days but did not ameliorate pain. In non-immunocompromised patients with recurrent herpes simplex labialis, generally little clinical benefit was seen with the use of topical acyclovir ointment even when therapy was initiated during the prodromal phase, while topical acyclovir cream effected small but significant improvements in the clinical but not the symptomological course of the disease. However, in immunocompromised patients, both intravenous and topical acyclovir shortened the clinical course of herpes simplex virus infections occurring mainly on the lips, oral mucosa and face, and prophylaxis with either oral or intravenous acyclovir suppressed the appearance of recurrent lesions from latent virus for the period of drug administration, but acyclovir did not eradicate latent herpesviruses. In non-immunocompromised patients, intravenous acyclovir was shown to decrease the acute pain of zoster, especially in the elderly, but postherpetic neuralgia was not ameliorated. When immunocompromised patients were studied, intravenous acyclovir inhibited the progression of zoster infections and shortened the healing time and duration of viral shedding in patients with cutaneous disseminated zoster. However, acute and postherpetic pain were not significantly affected. Well designed controlled studies are underway to establish the efficacy of acyclovir in herpes simplex encephalitis and cytomegalovirus infections in immunocompromised patients, infections due to Epstein-Barr virus, and neonatal herpesvirus infections. Despite some aspects of the drug’s use which require further clarification, acyclovir will make a major impact on the treatment of herpesviral infections. Barring unexpected findings with wider clinical use, it will become the agent of choice in several conditions. Antiviral Activity: Acyclovir is active in vitro against herpes simplex virus types 1 and 2, varicella zoster virus, Epstein-Barr virus and, to a very limited extent, cytomegalovirus. Quantitative results show considerable variation depending on such factors as the strain of virus, the cell culture system and the assay method, but in general the spectrum of herpesvirus sensitivities in descending order is: herpes simplex type 1 (most sensitive), herpes simplex type 2, varicella zoster virus, Epstein-Barr virus, and cytomegalovirus. Against herpes simplex types 1 and 2, acyclovir is generally more active in vitro than idoxuridine, trifluridine or vidarabine, and of similar potency to cytarabine. The comparative potencies of acyclovir and other antivirals against varicella zoster virus varied between studies. Idoxuridine, trifluridine and vidarabine all demonstrate greater activity in vitro than acyclovir against cytomegalovirus. A few studies have demonstrated additive or synergistic antiviral activity against herpesviruses in vitro when 1 of the other nucleoside analogue antivirals is combined with acyclovir. Acyclovir has also demonstrated some antiviral activity in vitro against a few other viruses but is inactive against vaccinia virus, Friend leukaemic virus, adenovirus type 5, a range of RNA viruses, and hepatitis B virus. In in vivo animal models, acyclovir given by various routes of administration was effective against ocular infections, cutaneous infections, genital infections, encephalitis, and neonatal infections due to herpes simplex viruses. As would be expected, efficacy in these models varied somewhat with dosage, route, and frequency of administration, and correlated closely with the rapidity with which therapy was initiated following viral inoculation. When comparative studies were conducted in these models, acyclovir was at least as effective as other nucleoside analogue drugs, and in some models was more effective. Of particular interest, systemic acyclovir therapy of herpes simplex keratitis in rabbits and mice, cutaneous and genital herpes simplex infections in mice, and cutaneous herpes simplex infection in hamsters markedly reduced the death rates due to secondary encephalitis. Also, oral (200 or 400 mg/kg/day), subcutaneous (80 to 100 mg/kg/day) or intraperitoneal (80 mg/kg/day) administration, usually beginning within 24 or 48 hours after viral inoculation, increased the survival time or decreased the mortality rate in mice inoculated intracerebrally with herpes simplex virus. In animal models, acyclovir has been shown to suppress the reactivation of latent herpes simplex virus residing in neural ganglia, but the drug does not eradicate latent virus which is already established. However, early initiation of topical or systemic therapy (usually within 24 hours of inoculation) has been shown to prevent the establishment of viral latency following primary infections in some instances, but such findings have not been duplicated in clinical trials (see below). Resistant strains of herpes simplex types 1 and 2 virus and varicella zoster virus can be isolated readily in vitro; they have been isolated in animal models and clinically, in the latter case usually associated with prolonged intravenous therapy in immunocompromised patients. The number of patients with clinically resistant strains reported to date has been small, but the overall incidence of clinical resistance needs further clarification. However, because of the frequently decreased virulence of resistant strains arising during acyclovir treatment, these strains have often not been a clinical problem. As with other nucleoside analogue antiviral drugs, the mechanism of action of acyclovir involves its inhibition of viral DNA replication. Acyclovir is a highly selective inhibitor of herpesvirus DNA replication due to its enhanced uptake in herpesvirus-infected cells and phosphorylation by herpesvirus thymidine kinase, and to the substrate specificity of acyclovir triphosphate for viral, rather than cellular, DNA polymerase. The high specificity of acyclovir presumably accounts for its high therapeutic index. Pharmacokinetic Studies: Multiple intravenous doses of acyclovir 5 mg/kg every 8 hours resulted in a steady-state mean peak plasma acyclovir concentration of 9.7 μg/ml, which should be clinically useful against herpes simplex types 1 and 2, varicella zoster, and Epstein-Barr virus infections. Oral absorption of acyclovir is limited (bioavailability ranges from 15 to 30%) and may involve a saturable process, but does not appear to be altered by food. Steady-state mean peak acyclovir plasma concentrations with oral administration of the recommended dose of 200mg every 4 hours were approximately 0.5 μg/ml. Systemic absorption with topical administration appears limited, but relatively high acyclovir concentrations (1.7 μg/ml) occurred in the aqueous humour with multidose ocular application of the 3% ointment every 5 hours. In man, following multidose intravenous therapy with doses ranging from 400 to 1200 mg/m2 every 8 hours, acyclovir was detected in the kidney, lung, nervous tissue, liver, and heart. Cerebrospinal fluid and skin vesicle concentrations following intravenous therapy, and saliva concentrations following oral therapy were approximately 50%, 100% and 13% of simultaneous plasma concentrations, respectively. In vivo plasma protein binding of acyclovir varied from 9 to 24%, and was independent of plasma acyclovir concentration over the range 0.4 to 5.1 μg/ml. In patients with normal renal function, approximately 15% of a dose of acyclovir is metabolised to an inactive metabolite, and the remainder is excreted in the urine unchanged. The elimination half-life in patients with normal renal function is 2 to 3 hours. In patients with end-stage renal impairment, drug plasma concentrations are increased and the elimination half-life is increased 7-fold, necessitating dosage adjustments. The elimination half-life also varies in patients in varying stages of renal impairment. The disposition of acyclovir in children is similar to that in adults, but in neonates the total body clearance is decreased by two-thirds and the elimination half-life is increased slightly. Therapeutic Trials: In masked controlled studies in patients with herpetic ocular disease and dendritic ulcers, 3% acyclovir ointment was usually effective in over 95% of patients. Acyclovir was of comparable efficacy to 3% vidarabine ointment, and at least as effective as 0.5% and 1% idoxuridine ointment and 2% trifluridine ointment. However, acyclovir has not been compared with the usual dosage form of trifluridine (1% solution). As with other antiviral agents, acyclovir does not prevent recrudescent or recurrent ocular herpetic infection, or the development of deeper herpetic disease of the stroma and/or uvea. Although initial reports of efficacy in more difficult-to-treat geographic ulcers, and in deeper herpetic disease, are encouraging, only small numbers of such patients have been studied and confirmation of these early reports in comparative studies will be awaited with interest. A few open studies and case reports indicate that acyclovir may be useful in patients with herpetic ocular disease unresponsive to idoxuridine or vidarabine. In non-immunocompromised patients, masked placebo-controlled studies have generally shown that topical (5% in polyethylene glycol or propylene glycol cream applied 4 to 6 times daily), oral (200mg 5 times daily) and intravenous acyclovir therapy (5 mg/kg every 8 hours), initiated at a pretreatment symptom duration of 3 to 4 days, significantly decreased the duration of viral shedding and the time to lesion healing in initial and in particular, primary initial genital herpes infections. However, effects on pain were less uniformly beneficial, especially with topical therapy, and recurrence rates were unaffected by topical or systemic therapy. Where non-primary initial infections were assessed separately, acyclovir ointment was of little benefit. When administered within 48 hours of initial symptoms of a recurrent infection or during the prodromal phase, both topical and oral acyclovir shortened the duration of viral shedding. Also oral acyclovir and acyclovir cream, but not acyclovir ointment demonstrated small but significant effects on healing. However, these effects were of questionable clinical importance due to the relatively short clinical course of untreated recurrent genital herpes. More interesting, however, was the successful prophylaxis of recurrent genital herpes with oral acyclovir 200mg, 3 or 4 times daily. Topically administered acyclovir ointment was generally ineffective in treating herpes labialis infections in non-immunocompromised patients, whereas therapy with acyclovir cream resulted in a small but significant decrease in healing times, but without any accompanying reduction in pain or itching. In non-immunocompromised patients, intravenous acyclovir, 500 mg/m2 or 5 mg/kg 3 times daily, significantly decreased the acute pain of herpes zoster and was of particular benefit in elderly patients. However, pain often recurred in patients who had experienced acute pain relief, and the treatment had no effect on postherpetic neuralgia. Nevertheless, if orally administered acyclovir can be convincingly shown to be similarly effective, its use in elderly patients, on an outpatient basis, would appear justified. The efficacy of acyclovir in immunocompromised patients having herpes simplex virus infections (mainly of the lips, oral mucosa and face) has been assessed in several double-blind controlled studies. Both intravenous (250 mg/m2 every 8 hours) and topical (5% in polyethylene glycol applied 4 times daily) therapy were clinically beneficial in decreasing the duration of viral shedding and pain. However, only intravenous therapy significantly shortened the time to lesion healing. Both intravenous and oral acyclovir have been used with success prophylactically, to prevent the lesions due to reactivated latent herpes simplex virus in immunocompromised patients for the duration of drug administration. However, viral latency was not eradicated by the drug. In a controlled study in immunocompromised patients, intravenous acyclovir 500 mg/m2 3 times daily for 7 days inhibited the progression of zoster infections. Also, the healing time and duration of viral shedding were shortened in patients presenting with disseminated cutaneous infections. However, neither acute nor postherpetic pain were significantly ameliorated. A multicentre, double-blind study of primary varicella infection in immunocompromised children showed a trend towards reduced visceral complications with intravenous acyclovir therapy (500 mg/m2 every 8 hours). In a double-blind comparative study in immunocompromised patients with cytomegalovirus infections of various degrees of severity, there was a trend to reduced mortality rates with intravenously administered acyclovir (500 mg/m2 every 8 hours) compared with placebo, but the difference did not reach statistical significance. Generally acyclovir has shown little clinical therapeutic effect in the treatment of cytomegalovirus. Data are either not available or incomplete as to the efficacy of acyclovir in herpes simplex encephalitis or cytomegalovirus infections in immunocompromised patients, and in infections due to Epstein-Barr virus or neonatal herpesvirus infections. However, encouraging case reports or small open studies have appeared and large controlled studies are currently underway in several of these difficult therapeutic areas. Side Effects: Acyclovir therapy by ocular, topical, oral or intravenous routes has been well tolerated by most patients. With the 3% ophthalmic ointment, mild transient burning or stinging upon instillation (in up to 27% of patients), and reversible punctate epithelial staining with rose bengal (usually reported to occur in less than 25% of patients), occur most frequently. Also, punctal stenosis or occlusion, follicular conjunctivitis, contact blepharoconjunctivitis, palpebral allergy, or other signs of hypersensitivity have occurred occasionally, but side effects have only very rarely necessitated stopping the drug. With acyclovir 5% topical ointment, mild pain, burning and stinging (about 30%) or pruritus (about 4%) are the most frequent adverse effects, but have not necessitated discontinuation of therapy in any patients in published studies. Rash and vulvitis (0.3% each) have also been reported. Systemic absorption following topical administration is limited. Of 109 immunocompromised patients treated with oral acyclovir, the incidence, nature and severity of coryza (2 patients), dizziness, gastrointestinal discomfort, tiredness and diarrhoea (1 patient each) were similar to those reported with placebo. One case of renal colic was reported in a patient having a history of renal stones. Intravenous acyclovir therapy has been most frequently associated with inflammation or phlebitis at the injection site following tissue infiltration of the intravenous fluid (approximately 15% incidence), transient elevations of serum creatinine (approximately 5%), and rash or hives (approximately 5%). The transient abnormalities of renal function occur more frequently following rapid bolus injection (10%), in patients with renal disease, in dehydrated patients, and with concomitant administration of other nephrotoxic drugs. Approximately 1% of patients manifest encephalopathic changes characterised by lethargy, obtundation, tremors, confusion, hallucinations, agitation, seizures, or coma. Transient abnormal liver function tests have occurred occasionally. Various other ‘side effects’ have been reported rarely in patients receiving acyclovir, but their association with drug therapy is uncertain. Only rarely has an adverse reaction necessitated withdrawal of therapy with intravenous acyclovir. Dosage and Administration: For herpes simplex keratitis the recommended dosage of 3% acyclovir ophthalmic ointment is a 1cm ribbon placed inside the lower conjunctival sac every 4 hours while awake (5 times daily), and continued for at least 3 days after complete healing. Acyclovir 5% topical ointment is recommended for initial herpes genitalis and in limited non-life-threatening mucocutaneous herpes simplex virus infections in immunocompromised patients. Application with a finger cot of a ‘ribbon’ of ointment about 1cm long per 10 cm2 surface area is recommended, beginning as soon as possible following the onset of signs and symptoms. For the treatment of herpes simplex infections of the skin and mucous membranes, including initial and recurrent genital herpes, the recommended oral dosage is 200mg 5 times daily (approximately every 4 hours) for 5 days. The recommended dosage of intravenous acyclovir therapy is 5 mg/kg, infused over 1 hour every 8 hours for 5 days (severe initial clinical episodes of genital herpes) or 7 days (mucosal and cutaneous herpes simplex virus types 1 and 2 infections in immunocompromised patients). Therapy should be initiated as soon as possible following the onset of signs and symptoms. Infusion concentrations lower than 7 mg/ml are recommended because higher concentrations may produce phlebitis or inflammation at the injection site upon extravasation, and infusions must be accompanied by adequate hydration in order to establish sufficient urine flow to prevent precipitation of the drug in the renal tubules. In children under 12 years of age, more accurate dosing can be obtained by infusing 250 mg/m2 over 1 hour, every 8 hours. Dosage reductions are necessary in patients with moderate to severe renal function impairment.