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Zovirax

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. It does not eradicate latent herpes, and does not work very well against genital herpes in women. more...

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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 1 g/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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Clinical quiz
From American Family Physician, 10/15/05

Clinical Quiz questions are based on selected articles in this issue of American Family Physician. Answers appear in this issue. AFP has been reviewed by the American Academy of Family Physicians as having content acceptable for Prescribed credit. Term of approval is for one year from the beginning distribution date of January 2005. This issue has been reviewed and is acceptable for up to 4.0 Prescribed credits, of which 0.5 credit conforms to AAFP criteria for evidence-based continuing medical education (EB CME) clinical content. The amount of CME has been doubled to reflect two-for-one credit for the EB CME portion only. When reporting CME credits, AAFP members should report total Prescribed credits earned for this activity. It is not necessary for members to label credits as EB CME for reporting purposes.

The AAFP is accredited by the Accreditation Council for Continuing Medical Education to provide CME for physicians.

The AAFP designates this educational activity for a maximum of 4.0 Category 1 credits toward the American Medical Association Physician's Recognition Award (AMA/PRA). Each physician should claim only those credits that he or she actually spent in the activity.

AAFP Credit

Each copy of AFP contains a Clinical Quiz answer card. AAFP members may use this card to obtain the designated number of Prescribed credit hours for the year in which the card is postmarked.

AMA/PRA Category 1 Credit

AAFP members who satisfy the Academy's CME requirements are automatically eligible for the AMA/PRA.

Physicians who are not members of the AAFP are eligible to receive the designated number of credits in Category 1 of the AMA/PRA on completion and return of the Clinical Quiz answer card. AFP keeps a record of AMA/PRA Category 1 credits for nonmember physicians. This record will be provided on request; however, nonmembers are responsible for reporting their own Category 1 CME credits when applying for the AMA/PRA or other certificates or credentials.

For health care professionals who are not physicians and are AFP subscribers, a record of CME credit is kept by the AAFP and will be provided to you on written request. You are responsible for reporting CME credits to your professional organization.

NOTE: The full text of AFP is available online (http://www.aafp.org/afp), including each issue's Clinical Quiz. The table of contents for each online issue will link you to the Clinical Quiz. Just follow the online directions to take the quiz and, if you are an AAFP member, you can submit your answers for CME credit.

Instructions

(1) Read each article, answer all questions on the quiz pages, and transfer your answers to the Clinical Quiz answer card (bound into your copy of AFP). This will help you avoid errors and permit you to check your answers against the correct answers.

(2) Mail the Clinical Quiz answer card within one year (by October 31, 2006). The bar code on the answer card contains your identification for CME credit hours.

Before beginning the test, please note: Each Clinical Quiz includes two types of questions: Type A and Type X.

Type A questions have only one correct answer and may have four or five choices. Here is a typical Type A question:

Type X questions may have one or more correct answers. They are multiple true-false questions with four options. Here is a typical Type X question:

Clinical Quiz questions are written by the associate and assistant editors of AFP.

Answers to This Issue's Clinical Quiz

Q1. B Q2. C Q3. C Q4. C Q5. A Q6. A Q7. E Q8. E Q9. A Q10. C Q11. A, B Q12. A, B, C, D Q13. A, B, C, D Q14. A, B, D Q15. A, B, C, D

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