Acyclovir

Introduction Overview

Generic Name

acyclovir

Brand Name

Zovirax

Dosage Form

oral tablet, oral suspension, topical ointment, intravenous formulation

Drug Class

Purine nucleosides

Pharmacokinetics

Absorption:
Acyclovir exhibits moderate oral bioavailability, varying between approximately 15% to 30% following oral administration. This relatively low and variable absorption is attributed to incomplete gastrointestinal uptake and first-pass metabolism. Peak plasma concentrations typically occur within 1.5 to 2 hours after ingestion. The absorption can be influenced by food, although food does not significantly affect the extent of absorption, it may delay peak plasma levels. Clinical implications of this absorption profile include the need for multiple daily dosing to maintain therapeutic plasma levels necessary to inhibit herpes virus replication effectively.

Distribution:
After systemic absorption, acyclovir distributes widely into body fluids and tissues, including saliva, cerebrospinal fluid, and genital secretions, which are critical sites for herpes virus infections. The volume of distribution ranges between 0.6 to 0.8 L/kg, indicating moderate tissue penetration. Plasma protein binding is low, approximately 9% to 33%, allowing for free drug availability to exert antiviral effects. The ability to penetrate the central nervous system is clinically significant for treating herpes encephalitis. Limited data indicate active transport mechanisms may facilitate tissue distribution, but mostly distribution is governed by passive diffusion.

Metabolism:
Acyclovir undergoes minimal hepatic metabolism. Its primary metabolic pathway involves conversion to inactive metabolites via aldehyde oxidase and xanthine oxidase enzymes, but these metabolic routes are minor. The limited metabolism results in most of the drug being present as unchanged acyclovir in plasma and excreted via renal pathways. Due to this minimal metabolism, drug-drug interactions via cytochrome P450 enzymes are negligible, reducing the risk of metabolic interactions.

Elimination:
The elimination of acyclovir is predominantly renal through glomerular filtration and tubular secretion. Its plasma half-life in individuals with normal renal function is approximately 2.5 to 3 hours, necessitating frequent dosing (e.g., every 8 hours) to maintain effective antiviral concentrations. In patients with renal impairment, the half-life is significantly prolonged, requiring dose adjustments to prevent accumulation and nephrotoxicity. Clearance can be affected by concurrent nephrotoxic medications. Urinary excretion of unchanged acyclovir accounts for about 60% to 90% of the administered dose. Hemodialysis effectively removes acyclovir, which is important in overdose or renal failure scenarios.

Dosage Forms

Available Dosage Forms:
- Oral capsule: 200 mg, 400 mg, 800 mg
- Oral tablet: 200 mg, 400 mg, 800 mg
- Oral suspension (liquid): 200 mg/5 mL
- Topical ointment: 5% acyclovir
- Intravenous formulation: 50 mg/mL solution for injection

Dosages for Adults and Adolescents:
- For initial episodes of genital herpes: 200 mg orally every 4 hours, five times daily for 10 days
- For recurrent episodes of genital herpes: 200 mg orally every 4 hours, five times daily for 5 days
- For chronic suppressive therapy: 400 mg orally twice daily
- For herpes zoster (shingles): 800 mg orally five times daily for 7 to 10 days
- For chickenpox in immunocompetent children: 800 mg orally four times daily for 5 days
- Dosage adjustments are recommended based on renal function

Specific Patient Populations:
- Pediatric patients (≥2 years): dosing is weight-based and varies depending on indication; younger than 2 years use is generally not recommended due to limited safety data
- Elderly patients: dose adjustment is recommended in case of renal impairment due to decreased clearance
- Patients with renal impairment require individualized dosage reduction to prevent toxicity
- Pregnant women should use only if clearly needed, with close monitoring
- Breastfeeding mothers are advised to consult with healthcare providers due to limited safety data

Clinical guidelines emphasize the importance of initiating therapy as soon as possible after symptom onset to maximize efficacy. The oral suspension should be shaken well before use, and doses should be measured precisely with a provided syringe or measuring device to ensure dosing accuracy.

Mechanism Of Action

Acyclovir is a synthetic purine nucleoside analogue that selectively targets herpesviridae family viruses by inhibiting viral DNA replication. The drug is a guanine nucleoside analogue, structurally similar to the natural nucleoside deoxyguanosine. Upon entering infected cells, acyclovir undergoes phosphorylation to its active triphosphate form (acyclovir triphosphate) through a series of enzymatic steps.

Firstly, the viral thymidine kinase enzyme phosphorylates acyclovir to acyclovir monophosphate, a critical step conferring selectivity as this enzyme is present predominantly in herpesvirus-infected cells. Human cellular kinases then convert acyclovir monophosphate to the di- and triphosphate forms. The active acyclovir triphosphate competes with deoxyguanosine triphosphate for incorporation into viral DNA by viral DNA polymerase.

Incorporation of acyclovir triphosphate into viral DNA results in premature chain termination because it lacks the 3' hydroxyl group required for DNA chain elongation. Additionally, acyclovir triphosphate acts as a competitive inhibitor and irreversible inactivator of viral DNA polymerase, further blocking viral DNA synthesis and suppressing viral replication.

Clinically, this mechanism results in decreased viral load and reduced symptoms. Compared to other antiviral agents like valacyclovir, acyclovir has lower oral bioavailability but shares a similar mechanism of action. Research has shown that early initiation of acyclovir therapy in herpes simplex virus infections significantly improves clinical outcomes by limiting viral replication during the initial phases of infection.

Pharmacological Actions

Selective inhibition of herpesvirus DNA polymerase activity leading to suppressed viral DNA synthesis.
Chain termination in viral DNA replication by incorporation of acyclovir triphosphate into viral DNA.
Selective activation within infected cells via viral thymidine kinase, minimizing toxicity to uninfected cells.
Inhibits replication of herpes simplex virus types 1 and 2, varicella-zoster virus, Epstein-Barr virus, and cytomegalovirus to a lesser extent.
Non-immunomodulatory antiviral activity without direct effects on host immune cells.
Reduction of viral shedding and lesion formation in active herpesvirus infections.
Prophylactic suppression of recurrent genital herpes episodes when administered chronically.
Treatment of varicella-zoster virus infections such as shingles and chickenpox.
Improves clinical symptoms by limiting viral propagation in mucocutaneous and ocular infections.
Enhancement of viral clearance when combined with host immune responses.
Minimal structural similarity to human nucleotides reduces likelihood of host DNA polymerase inhibition.
Demonstrated efficacy in both immunocompetent and immunocompromised patients, albeit with dosing adjustments.
Intravenous administration allows treatment of severe or disseminated herpesvirus infections.
Topical formulations reduce lesion duration and pain in superficial herpes infections.
Limited potential for resistance development when used appropriately, but mutations in viral thymidine kinase or DNA polymerase can confer resistance.

Therapeutic Or Clinical Uses

Treatment of initial and recurrent genital herpes simplex virus (HSV) infections.
Management of herpes simplex labialis (cold sores) in adults and adolescents.
Treatment and suppression of varicella-zoster virus infections including shingles (herpes zoster).
Treatment of chickenpox (varicella) in pediatric populations.
Prophylaxis and treatment of HSV infections in immunocompromised patients, including transplant recipients and HIV-infected individuals.
Treatment of herpes simplex virus infections of the skin and mucous membranes.
Management of herpes simplex encephalitis and neonatal herpes infections when administered intravenously.
Reduction of duration and severity of HSV ocular infections (e.g., keratitis) with topical or systemic therapy.
Suppression of recurrent genital herpes episodes to reduce frequency and viral shedding.
Reduction in the risk of HSV transmission in patients receiving suppressive therapy.
Treatment of herpes simplex virus infections causing mucocutaneous lesions in immunosuppressed patients.
Adjunctive therapy for shingles to reduce pain and accelerate lesion healing.
Use in patients with contraindications or intolerance to other antiviral therapies like valacyclovir and famciclovir.
Treatment of herpes simplex virus infections in neonates and young children above 2 years with consideration of dosing.
Management of disseminated herpesvirus infections in high-risk patient populations.

Side Or Adverse Effects

Nausea and vomiting occurring in mild to moderate frequency during therapy.
Diarrhea reported occasionally, usually transient and self-limiting.
Headache, often mild in intensity and resolving without intervention.
Rash and pruritus indicating hypersensitivity or localized irritation.
Mild skin pain or burning sensation, especially with topical formulations.
Rare but severe allergic reactions including urticaria, angioedema, and anaphylaxis.
Confusion, hallucinations, and neurotoxicity particularly in elderly or renal impaired patients.
Renal toxicity manifesting as interstitial nephritis or crystalluria, with risk increased by dehydration.
Easy bruising or bleeding, along with petechiae, suggestive of hematologic adverse effects.
Fatigue and malaise in some patients during prolonged therapy.
Signs of kidney impairment including oliguria, painful urination, and peripheral edema.
Gastrointestinal upset such as abdominal discomfort or dyspepsia.
Possible elevation of liver enzymes, though clinically significant hepatotoxicity is rare.
Transient increases in serum creatinine, particularly with intravenous administration.
Local adverse effects at the site of topical or intravenous application such as irritation or phlebitis.

Precautions

Use caution in patients with pre-existing renal impairment; monitor renal function and adjust dosage accordingly.
Ensure adequate hydration to minimize risk of crystalluria and nephrotoxicity during therapy.
Monitor for signs of neurotoxicity, especially in elderly patients or those with impaired renal clearance.
Confirm no hypersensitivity to acyclovir or related drugs like valacyclovir before initiating treatment.
Avoid abrupt discontinuation to reduce risk of viral resistance development.
Use with caution in immunocompromised patients; monitor efficacy and adverse effects closely.
Pregnant women should use only if benefits outweigh potential risks; closely monitor mother and fetus.
Breastfeeding mothers should consult healthcare providers; acyclovir is excreted in breast milk but safety data are limited.
Avoid use in children under 2 years or use only if clearly indicated and under specialist guidance.
Educate patients on adherence to full course even if symptoms improve to prevent relapse and resistance.
Advise patients about contagiousness of herpes infections despite therapy; implement infection control practices.
Use cautiously in patients taking concomitant nephrotoxic drugs or those with multiple comorbidities.
Perform baseline and periodic hematologic assessments in patients receiving prolonged suppressive therapy.
Assess drug interactions prior to initiation to mitigate potential adverse effects or reduced efficacy.
Adjust dosing in elderly patients due to potential altered pharmacokinetics and comorbid conditions.

Warnings

Herpes viruses may remain contagious despite acyclovir therapy; patients must avoid contact with susceptible individuals.
Initiate treatment at the earliest signs of symptoms to maximize clinical efficacy and reduce severity.
Improper or incomplete treatment course may promote development of acyclovir-resistant viral strains.
Zovirax should not be used in patients with known hypersensitivity to acyclovir or valacyclovir due to risk of severe allergic reactions.
Risk of nephrotoxicity exists, especially with intravenous use and in dehydrated patients; monitor renal function closely.
Neurologic adverse events such as confusion, hallucinations, and seizures can occur, especially in renal impairment.
Patients should be warned that acyclovir therapy will not prevent transmission of genital herpes to sexual partners.
Avoid co-administration with other nephrotoxic agents unless necessary and under careful supervision.
Use caution in populations at risk of immunosuppression to monitor for progression or atypical presentations.
Patients should be advised to maintain adequate hydration throughout treatment to minimize renal adverse events.
Children under 2 years should generally not receive acyclovir due to limited safety data and risk profile.
Topical formulations should not be applied to large areas or extensively broken skin to prevent systemic absorption and toxicity.
Advise patients against sharing medications to prevent misuse and inadvertent exposure.
Dosage adjustments are essential in patients with renal impairment to minimize toxicity and adverse effects.
Emergency medical attention should be sought immediately for symptoms of severe allergic reactions.

Contraindication

Known hypersensitivity to acyclovir or valacyclovir.
Previous allergic or anaphylactic reaction to any component of the formulation.
Patients with a history of hypersensitivity to other purine nucleoside analogues unless risk-benefit justifies use.
Neonates or infants under 2 years when oral formulations are considered, unless under strict clinical supervision.
Use of topical acyclovir in presence of deep mucous membrane ulcerations or severely broken skin may be contraindicated.
Severe renal impairment patients without appropriate dose adjustment and monitoring.
Concurrent use with certain nephrotoxic drugs without possibility of close renal monitoring may be contraindicated.
Patients with untreated severe dehydration which increases risk for acyclovir crystalluria and toxicity.
Use in patients with a history of CNS hypersensitivity reactions to acyclovir or related antiviral drugs.
Contraindicated in patients with hypersensitivity to any inactive ingredients present in specific dosage forms.

Drug Interactions

Concurrent use with other nephrotoxic agents such as aminoglycosides, amphotericin B, or NSAIDs may increase risk of renal toxicity.
Co-administration with probenecid may reduce renal clearance of acyclovir, increasing systemic exposure and toxicity risk.
Concurrent administration with cimetidine can increase acyclovir plasma concentrations by reducing renal tubular secretion.
Use with didanosine may increase didanosine plasma concentrations, potentially enhancing toxicity.
Co-administration with mycophenolate mofetil may increase risk of adverse effects due to altered renal clearance.
Use with immunosuppressants such as cyclosporine or tacrolimus may enhance nephrotoxicity risk.
Limited interaction with cytochrome P450 metabolized agents due to minimal hepatic metabolism of acyclovir.
Co-administration with antiretrovirals requires monitoring due to overlapping renal toxicities.
Concurrent use with pyrazinamide may increase risk of nephrotoxicity and should be monitored carefully.
NSAIDs usage alongside acyclovir requires caution as both drugs can independently impair renal function.
Alcohol does not have significant metabolic interaction but should be consumed with caution to avoid dehydration.
Potential interaction with diuretics increasing the risk of dehydration and subsequent renal impairment.
Careful monitoring recommended when using acyclovir with anti-cancer agents that have renal toxicity profiles.
Interactions with over-the-counter medications and herbal supplements should be disclosed to the healthcare provider.
No significant interactions identified with vaccines, but close clinical monitoring is advised during concurrent administration.

Preparations Of The Drug

Oral tablets available in strengths of 200 mg, 400 mg, and 800 mg for systemic antiviral therapy.
Oral suspension (liquid) formulated as 200 mg per 5 mL for pediatric and patients unable to swallow tablets.
Topical ointment containing 5% acyclovir for localized treatment of herpes simplex labialis and genital lesions.
Intravenous formulation as a 50 mg/mL sterile solution for injection for severe or disseminated infections.
Topical cream and ointment formulations differ slightly in vehicle composition for skin penetration efficacy.
Oral capsules available for systemic administration including both branded and generic versions.
Advanced formulations such as buccal tablets (e.g., Sitavig) for enhanced mucosal absorption in specific indications.
Freeze-dried powder for injection to be reconstituted prior to use in hospital settings.
Extended access to compounded formulations for patients requiring unique dosing or formulations.
Unit-dose packaging available for precise dosing and improved patient compliance.
Commercial preparations stored at controlled room temperature to maintain drug stability and efficacy.
Parenteral forms require dilution prior to intravenous administration as per prescribing information.
Standardized dosing dispensing across preparation forms to allow seamless switching between formulations.
Preparations free of common allergens and preservatives designed for patients with sensitivities provided.
Manufactured under strict regulatory approval to ensure consistent potency, purity, and quality control.