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Pharmacists_ coffee magazine
Electronic Edition
Edition No. 423, year 4
September 16, 2011
Today's edition sent to: 25,385 Arabian Pharmacists

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Today in Hot pharma news

Medicine of the week

Aciclovir

Aciclovir (INN) (pronounced /eɪˈsaɪklɵvɪər/) or acyclovir (USAN, former BAN), chemical name acycloguanosine, abbreviated as ACV,^[1] is a guanosine analogue antiviral drug, marketed under trade names such as Cyclovir, Herpex, Acivir, Acivirax(Mash-Premier), Zovirax, Aciclovir (Sanofi-Aventis) and Zovir (GSK). One of the most commonly-used antiviral drugs, it is primarily used for the treatment of herpes simplex virus infections, as well as in the treatment of herpes zoster (shingles).
Aciclovir was seen as the start of a new era in antiviral therapy,^[1] as it is extremely selective and low in cytotoxicity. Pharmacologist Gertrude B. Elion was awarded the 1988 Nobel Prize in Medicine, partly for the development of aciclovir. Dr. Richard Whitley, a University of Alabama at Birmingham researcher and pioneer in antiviral therapy, was the first to successfully use the drug in humans.

Pharmacology:

Mechanism of action

Aciclovir differs from previous nucleoside analogues in that it contains only a partial nucleoside structure: the sugar ring is replaced by an open-chain structure. It is selectively converted into acyclo-guanosine monophosphate (acyclo-GMP) 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, acyclo-guanosine triphosphate (acyclo-GTP), by cellular kinases. Acyclo-GTP is a very potent inhibitor of viral DNA polymerase; it has approximately 100 times greater affinity for viral than cellular polymerase. As a substrate, acyclo-GTP is incorporated into viral DNA, resulting in chain termination. Although acyclovir resembles a nucleotide, it has no 3' end. Therefore, after its incorporation into a growing DNA strand, no further nucleotides can be added to this strand. It has also been shown that viral enzymes cannot remove acyclo-GTP 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.
In sum, aciclovir can be considered a prodrug: it is administered in an inactive (or less active) form and is metabolised into a more active species after administration.

Microbiology

Aciclovir is active against most known species in the herpesvirus family. In descending order of activity:^[2]

Herpes simplex virus type I (HSV-1)
Herpes simplex virus type II (HSV-2)
Varicella zoster virus (VZV)
Epstein-Barr virus (EBV)
Cytomegalovirus (CMV) -- least activity

Activity is predominantly against HSV, and to a lesser extent VZV. It is only of limited efficacy against EBV and CMV. It is inactive against latent viruses in nerve ganglia.

Resistance

Resistance to aciclovir is rare, but is more common in patients on chronic antiviral prophylaxis (transplant recipients, people with acquired immunodeficiency syndrome due to HIV infection). Mechanisms of resistance in HSV include deficient viral thymidine kinase; and mutations to viral thymidine kinase and/or DNA polymerase, altering substrate sensitivity.^[3] Acyclovir has also shown cross-resistance with Valacyclovir and Famcyclovir.

Pharmacokinetics

Aciclovir is poorly water soluble and has poor oral bioavailability (15-30%), hence intravenous administration is necessary if high concentrations are required. When orally administered, peak plasma concentration occurs after 1–2 hours. Aciclovir has a high distribution rate; only 30% is protein-bound in plasma. The elimination half-life of aciclovir is approximately 3 hours. It is renally excreted, partly by glomerular filtration and partly by tubular secretion.
The poor oral bioavailability may also be improved by administering Valaciclovir, which has an oral bioavailability of about 55%. Valaciclovir is then converted to Aciclovir by esterases via hepatic first-pass metabolism.

Clinical use:

Indications

Aciclovir is indicated for the treatment of HSV and VZV infections, including:^[4]

Genital herpes simplex (treatment and prophylaxis)
Herpes simplex labialis (cold sores)
Herpes zoster (shingles)
Acute chickenpox in immunocompromised patients
Herpes simplex encephalitis
Acute mucocutaneous HSV infections in immunocompromised patients
Herpes simplex keratitis (ocular herpes)
Herpes simplex blepharitis (not to be mistaken with ocular herpes)
Bell's Palsy

HIV-1 progression can be delayed by using Aciclovir, according to study led by Dr Jairam Lingappa. Effective in 16% of cases, can delay the HAART treatment by 1–2 years. University of Washington, Seattle. During a 2 year trial, 284 people progressed with the HIV-1, versus 324 who had not been treated with Aciclovir.^[5] It has been claimed that the evidence for the effectiveness of topically applied cream for recurrent labial outbreaks is weak.^[6] An earlier review of scientific literature showed that there is some effect in reducing the number and duration of lesions if aciclovir is applied at an early stage of an outbreak.^[7] However, it was concluded that oral therapy for episodes is inappropriate for most non-immunocompromised patients based on costs and benefits, presumably in countries where aciclovir is only available on prescription. It was concluded that there is evidence for an oral prophylactic role in preventing recurrences.

Dosage forms

Aciclovir is commonly marketed as tablets (200 mg, 400 mg, 800 mg and 1 gram), topical cream (5%), intravenous injection (25 mg/mL) and ophthalmic ointment (3%). Cream preparations are used primarily for labial herpes simplex. The intravenous injection is used when high concentrations of aciclovir are required. The ophthalmic ointment preparation is only used for herpes simplex keratitis. In Singapore, it is available as a 400 mg preparation known as Avorax and a 800 mg preparation known as Herperax.

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