Acyclovir

ACYCLOVIR- acyclovir capsule
AvPAK

Rx only

DESCRIPTION

Acyclovir is a synthetic nucleoside analogue active against herpesviruses. Acyclovir capsules are formulations for oral administration.

Each capsule contains 200 mg of acyclovir and the inactive ingredients: lactose monohydrate, sodium lauryl sulfate, sodium starch glycolate, and magnesium stearate. The capsule shell consists of FD&C Blue #1, gelatin, and titanium dioxide. The imprinting ink contains, alcohol, FD&C Blue #2, FD&C Red #40, FD&C Yellow #10, iron oxide black, shellac Glaze.

Acyclovir is a white, crystalline powder with the molecular formula C 8 H 11 N 5 O 3 and a molecular weight of 225. The maximum solubility in water at 37°C is 2.5 mg/mL. The pka’s of acyclovir are 2.27 and 9.25.

The chemical name of acyclovir is 2-amino-1,9-dihydro-9- [(2-hydroxyethoxy) methyl]-6 H -purin-6-one; it has the following structural formula:

Structural FormulaStructural Formula

VIROLOGY

Mechanism of Antiviral Action: Acyclovir is a synthetic purine nucleoside analogue with in vitro and in vivo inhibitory activity against herpes simplex virus types 1 (HSV-1), 2 (HSV-2), and varicella-zoster virus (VZV). The inhibitory activity of acyclovir is highly selective due to its affinity for the enzyme thymidine kinase (TK) encoded by HSV and VZV. This viral enzyme converts acyclovir into acyclovir monophosphate, a nucleotide analogue. The monophosphate is further converted into diphosphate by cellular guanylate kinase and into triphosphate by a number of cellular enzymes. In vitro, acyclovir triphosphate stops replication of herpes viral DNA. This is accomplished in 3 ways: 1) competitive inhibition of viral DNA polymerase, 2) incorporation into and termination of the growing viral DNA chain, and 3) inactivation of the viral DNA polymerase. The greater antiviral activity of acyclovir against HSV compared to VZV is due to its more efficient phosphorylation by the viral TK.

Antiviral Activities: The quantitative relationship between the in vitro susceptibility of herpes viruses to antivirals and the clinical response to therapy has not been established in humans, and virus sensitivity testing has not been standardized. Sensitivity testing results, expressed as the concentration of drug required to inhibit by 50% the growth of virus in cell culture (IC 50 ), vary greatly depending upon a number of factors. Using plaque-reduction assays, the IC 50 against herpes simplex virus isolates ranges from 0.02 to 13.5 mcg/mL for HSV-1 and from 0.01 to 9.9 mcg/mL for HSV-2. The IC 50 for acyclovir against most laboratory strains and clinical isolates of VZV ranges from 0.12 to 10.8 mcg/mL. Acyclovir also demonstrates activity against the Oka vaccine strain of VZV with a mean IC 50 of 1.35 mcg/mL.

Drug Resistance: Resistance of HSV and VZV to acyclovir can result from qualitative and quantitative changes in the viral TK and/or DNA polymerase. Clinical isolates of HSV and VZV with reduced susceptibility to acyclovir have been recovered from immunocompromised patients, especially with advanced HIV infection. While most of the acyclovir-resistant mutants isolated thus far from immunocompromised patients have been found to be TK-deficient mutants, other mutants involving the viral TK gene (TK partial and TK altered) and DNA polymerase have been isolated. TK-negative mutants may cause severe disease in infants and immunocompromised adults. The possibility of viral resistance to acyclovir should be considered in patients who show poor clinical response during therapy.

CLINICAL PHARMACOLOGY

Pharmacokinetics: The pharmacokinetics of acyclovir after oral administration have been evaluated in healthy volunteers and in immunocompromised patients with herpes simplex or varicella-zoster virus infection. Acyclovir pharmacokinetic parameters are summarized in Table 1.

Table 1. Acyclovir Pharmacokinetic Characteristics (Range)

* Bioavailability decreases with increasing dose.

Parameter Range
Plasma protein binding 9% to 33%
Plasma elimination half-life 2.5 to 3.3 h
Average oral bioavailability 10% to 20%*

In one multiple-dose, crossover study in healthy subjects (n = 23), it was shown that increases in plasma acyclovir concentrations were less than dose proportional with increasing dose, as shown in Table 2. The decrease in bioavailability is a function of the dose and not the dosage form.

Table 2: Acyclovir Peak and Trough Concentrations at Steady State
Parameter 200 mg 400 mg 800 mg
C SS max 0.83 mcg/mL 1.21 mcg/mL 1.61 mcg/mL
C SS trough 0.46 mcg/mL 0.63 mcg/mL 0.83 mcg/mL

There was no effect of food on the absorption of acyclovir (n = 6); therefore, acyclovir capsules may be administered with or without food.

The only known urinary metabolite is 9-[(carboxymethoxy) methyl ] guanine.

Special Populations: Adults with Impaired Renal Function:

The half-life and total body clearance of acyclovir are dependent on renal function. A dosage adjustment is recommended for patients with reduced renal function (see DOSAGE AND ADMINISTRATION).

Geriatrics: Acyclovir plasma concentrations are higher in geriatric patients compared to younger adults, in part due to age-related changes in renal function. Dosage reduction may be required in geriatric patients with underlying renal impairment (see PRECAUTIONS: Geriatric Use).

Pediatrics: In general, the pharmacokinetics of acyclovir in pediatric patients is similar to that of adults. Mean half-life after oral doses of 300 mg/m 2 and 600 mg/m 2 in pediatric patients aged 7 months to 7 years was 2.6 hours (range 1.59 to 3.74 hours).

Drug Interactions: Coadministration of probenecid with intravenous acyclovir has been shown to increase the mean acyclovir half-life and the area under the concentration-time curve. Urinary excretion and renal clearance were correspondingly reduced.

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