ACYCLOVIR — acyclovir tablet
State of Florida DOH Central Pharmacy
Acyclovir is a synthetic nucleoside analogue active against herpesviruses. Each capsule, for oral administration, contains 200 mg of acyclovir. In addition, each capsule contains the following inactive ingredients: corn starch, lactose monohydrate, magnesium stearate and sodium lauryl sulfate. The capsule shell consists of gelatin, FD&C blue No. 1, D&C red No. 28, D&C red No. 33 and titanium dioxide. Printed with edible black ink that contains FD&C blue No. 1, FD&C blue No. 2, FD&C red No. 40 and D&C yellow No. 10. Each tablet, for oral administration, contains 400 mg or 800 mg of acyclovir. In addition, each tablet contains the following inactive ingredients: colloidal silicon dioxide, lactose monohydrate, magnesium stearate, microcrystalline cellulose, povidone, pregelatinized starch and sodium starch glycolate. The 400 mg tablets also contain FD&C blue No. 2.
Acyclovir is a white to off-white, crystalline powder. 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]-6H- purin-6-one; it has the following structural formula:
C8 H11 N5 O3 M.W. 225
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.
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 (IC50 ), vary greatly depending upon a number of factors. Using plaque-reduction assays, the IC50 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 IC50 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 IC50 of 1.35 mcg/mL.
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.
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)|
|Plasma protein binding||9% to 33%|
|Plasma elimination half-life||2.5 to 3.3 hr|
|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|
|Css max||0.83 mcg/mL||1.21 mcg/mL||1.61 mcg/mL|
|Css 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 and tablets may be administered with or without food.
The only known urinary metabolite is 9-[(carboxymethoxy)methyl]guanine.
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).
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).
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