What is mechanism of action?

mechanism of action is DNA Polymerase Inhibitors [MoA]

Acyclovir

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Acyclovir Prescribing Information

Acute Treatment of Herpes Zoster:

800 mg every 4 hours orally, 5 times daily for 7 to 10 days.

Genital Herpes:

Treatment of Initial Genital Herpes

200 mg every 4 hours, 5 times daily for 10 days.

Chronic Suppressive Therapy for Recurrent Disease

400 mg 2 times daily for up to 12 months, followed by re-evaluation. Alternative regimens have included doses ranging from 200 mg 3 times daily to 200 mg 5 times daily. The frequency and severity of episodes of untreated genital herpes may change over time. After 1 year of therapy, the frequency and severity of the patient's genital herpes infection should be re-evaluated to assess the need for continuation of therapy with acyclovir.

Intermittent Therapy

200 mg every 4 hours, 5 times daily for 5 days. Therapy should be initiated at the earliest sign or symptom (prodrome) of recurrence.

Treatment of Chickenpox:

Children (2 years of age and older):

20 mg/kg

per dose

orally 4 times daily (80 mg/kg/day) for 5 days. Children over 40 kg should receive the adult dose for chickenpox.

Adults and Children over 40 kg:

800 mg 4 times daily for 5 days.

Intravenous acyclovir is indicated for the treatment of varicella-zoster infections in immunocompromised patients. When therapy is indicated, it should be initiated at the earliest sign or symptom of chickenpox. There is no information about the efficacy of therapy initiated more than 24 hours after onset of signs and symptoms.

Patients With Acute or Chronic Renal Impairment:

In patients with renal impairment, the dose of acyclovir capsules should be modified as shown in Table 3.

Table 3. Dosage Modification for Renal Impairment
Normal Dosage Regimen	Creatinine Clearance (mL/min/1.73 m ² )	Adjusted Dosage Regimen
Normal Dosage Regimen	Creatinine Clearance (mL/min/1.73 m ² )	Dose (mg)	Dosing Interval
200 mg every 4 hours	>10	200	every 4 hours, 5x daily
	0 to 10	200	every 12 hours
400 mg every 12 hours	>10 0 to 10	400 200	every 12 hours every 12 hours
800 mg every 4 hours	>25	800	every 4 hours, 5x daily
	10 to 25	800	every 8 hours
	0 to 10	800	every 12 hours

Hemodialysis:

For patients who require hemodialysis, the mean plasma half-life of acyclovir during hemodialysis is approximately 5 hours. This results in a 60% decrease in plasma concentrations following a 6-hour dialysis period. Therefore, the patient's dosing schedule should be adjusted so that an additional dose is administered after each dialysis.

Peritoneal Dialysis:

No supplemental dose appears to be necessary after adjustment of the dosing interval.

Bioequivalence of Dosage Forms:

Acyclovir suspension was shown to be bioequivalent to acyclovir capsules (n = 20) and one acyclovir 800 mg tablet was shown to be bioequivalent to 4 acyclovir 200 mg capsules (n = 24).

Short-Term Administration

The most frequent adverse events reported during clinical trials of treatment of genital herpes with acyclovir 200 mg administered orally 5 times daily every 4 hours for 10 days were nausea and/or vomiting in 8 of 298 patient treatments (2.7%). Nausea and/or vomiting occurred in 2 of 287 (0.7%) patients who received placebo.

Long-Term Administration:

The most frequent adverse events reported in a clinical trial for the prevention of recurrences with continuous administration of 400 mg (two 200 mg capsules) 2 times daily for 1 year in 586 patients treated with acyclovir were nausea (4.8%) and diarrhea (2.4%). The 589 control patients receiving intermittent treatment of recurrences with acyclovir for 1 year reported diarrhea (2.7%), nausea (2.4%), and headache (2.2%).

Acyclovir, USP is a synthetic nucleoside analogue active against herpesviruses. Acyclovir capsules, USP are a formulation for oral administration. Each capsule contains 200 mg of acyclovir, USP and the inactive ingredients, microcrystalline cellulose, povidone, sodium starch glycolate, pregelatinized starch and magnesium stearate. The capsule shell consists of gelatin, FD&C Blue No. 2 and titanium dioxide. Printed with edible black ink with the following components: shellac, propylene glycol, ammonia solution, black iron oxide and potassium hydroxide.

Acyclovir, USP is a white, crystalline powder with the molecular formula C₈H₁₁N₅O₃ 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, USP is 2-amino-1,9-dihydro-9-[(2-hydroxyethoxy)methyl]-6H-purin-6-one; it has the following structural 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 with 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 (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.

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.

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