Aciclovir chemical structure
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Acyclovir

Aciclovir (INN) or acyclovir (USAN), marketed as Zovirax®, is one of the main antiviral drugs. Its discovery has been seen as the start of a new era in antiviral therapy, as it is extremely selective and low in cytotoxicity. However, it has a very narrow spectrum, only effective against certain viruses such as HSV-1, HSV-2, and VZV, with limited effectiveness against active EBV, and has hardly any effect against human cytomegalovirus (CMV). It is about 10 times more potent against HSV than VZV. more...

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It does not eradicate latent herpes, and does not work very well against genital herpes in women. Aciclovir is rather different from other nucleoside analogues, for it contains only a partial nucleoside structure as the sugar ring is replaced by an open-chain structure.

Mode of action

Aciclovir is converted into monophosphate form only by viral thymidine kinase, which is far more effective (3000 times) in phosphorylation than cellular thymidine kinase. Subsequently, the monophosphate form is further phosphorylated into the active triphosphate form, aciclo-GTP, by cellular kinases. Aciclo-GTP is a very potent inhibitor of viral DNA polymerase; it has approximately 100 times higher affinity to viral than cellular polymerase. Its monophosphate form also incorporates into the viral DNA, resulting in chain termination. It has also been shown that the viral enzymes cannot remove acyclo-GMP from the chain, which results in inhibition of further activity of DNA polymerase. Acyclo-GTP is fairly rapidly metabolised within the cell, possibly by cellular phosphatases.

Pharmacokinetics

Aciclovir suffers from low water solubility, and also from poor oral absorption, which is only 20%. Orally administered the peak plasma concentration will be reached in 1-2 hours. If large doses are required, aciclovir must be administered intravenously. Aciclovir has a high distribution rate, only 30 % is protein-bound in plasma. The half-life of aciclovir is approximately 3 hours. Aciclovir can also be given topically for treatment of herpes infections of mucous membrane and skin, such as genital herpes or recurrent herpes labialis (cold sore). Prophylactic administration is possible, and is often used for patients who are under immunosuppressant drugs or radiotherapy or for those who are suffering from recurrent genital infection herpes simplex.

Metabolism

The excretion of aciclovir takes place via the renal system, partly by glomerular filtration and partly by tubular secretion. Renal problems have been reported when given in large, fast doses intravenously, due to the crystallisation of aciclovir in the kidneys.

Side effects

Since aciclovir can be incorporated also into the cellular DNA, it is a chromosome mutagen, therefore, its use should be avoided during pregnancy. However it has not been shown to cause any teratogenic nor carcinogenic effects. The acute toxicity (LD50) of aciclovir when given orally is greater than 1mg/kg, due to the low absorption rate from the gastrointestinal tract. Single cases have been reported, where extremely high (up to 80mg/kg) doses have been accidentally given intravenously without causing any side effects. The most common adverse effects are local irritation at the site of injection, headache when given orally, and a stinging and burning sensation when administered topically. The resistance towards aciclovir evolves rather rapidly, although this has not much hindered its clinical use. Resistant forms are most likely viruses that have a mutation in their thymidine kinase or DNA polymerase.

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Short course of acyclovir for genital herpes - Tips from Other Journals - Brief Article
From American Family Physician, 8/15/02 by Bill Zepf

Patients with recurrent genital herpes may experiment with shorter courses of antiviral medication as they gain experience with the disease. Studies of viral replication have shown that active viral reproduction ends in the first few days of recurrent infections, well before the resolution of genital lesions or clinical symptoms. Wald and associates conducted a randomized, double-blind, placebo-controlled trial of higher-dose, shorter-course acyclovir in the treatment of recurrent genital herpes.

The study enrolled 131 patients with recurrent genital herpes (i.e., at least three documented outbreaks in the year before the study). Patients were randomized to treatment with either acyclovir at 800 mg or placebo, three times daily for two days. At least one recurrence occurred during follow-up in 64 percent of participants, and 50 percent of participants had two or more episodes of infection. With each recurrence, lesions were cultured for herpes simplex virus every other day until resolution.

Acyclovir was more than twice as likely as placebo (27 percent versus 10.6 percent) to completely abort recurrence of the herpes virus infection. In patients who developed active lesions, viral shedding was reduced by more than one half with acyclovir (25 hours versus 58.5 hours with placebo), and the duration of skin lesions also decreased (four days versus six days with placebo). These results were comparable with results published elsewhere for standard antiviral regimens. There was no significant difference in the likelihood of a subsequent herpes outbreak with the shorter-course regimen. The higher-dose, shorter-course regimen of acyclovir (800 mg three times daily for two days) promoted better compliance than standard dosing (200 mg five times daily for five days) but had the same overall cost because the total drug dosage was similar.

The authors conclude that a shorter course of higher-dose acyclovir for recurrent genital herpes is effective in aborting recurrences, decreasing viral shedding, and speeding clinical resolution of outbreaks.

COPYRIGHT 2002 American Academy of Family Physicians
COPYRIGHT 2002 Gale Group

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