Antiviral drugs for treatment herpes
The major antiviral medications available in pill form that have been specifically developed for the treatment of genital herpes include acyclovir, valacyclovir, and famciclovir. They may significantly lessen the severity of a primary outbreak and reduce the time it takes genital herpes outbreaks to heal.
Antiviral medication is most effective if it is taken when the prodromal symptoms (tingling and pain) of a recurrent genital herpes outbreak are first noticed and if they are taken for the next 5 to 7 days or until symptoms go away. Some people with frequent recurrent outbreaks (more than 6 recurrences a year) take antiviral medication every day to help reduce the frequency and duration of recurrent outbreaks. Antiviral medication can reduce the number of outbreaks by 70% to 80%.
Acyclovir (brand name Zovirax) is the oldest of the antiviral medications. It has been available since 1982 in a topical form (as an ointment) and sold since 1985 in pill form. Now acyclovir is available in a generic form. Acyclovir is the only antiviral medication available for intravenous administration.
Valacyclovir (Valtrex) was the second antiviral medication to come to market in the United States, and it was approved by the FDA on December 15, 1995. Valtrex is manufactured by GlaxoSmithKline. Valtrex is a prodrug of acyclovir, meaning that the body converts it to acyclovir after it has been absorbed. This medication delivers acyclovir more efficiently so that the body absorbs much of the drug, which has the advantage of taking the medication fewer times during the day.
Famciclovir (Famvir) is a prodrug manufactured by Novartis. When taken, the body converts it to the long acting antiviral drug penciclovir. Like valacyclovir, it is well absorbed, persists for a longer time in the body, and can be taken less frequently than acyclovir.
Mechanism of action
Antiviral agents reduce viral replication by inhibiting viral DNA synthesis, needed to reproduce itself. This helps to keep the virus inactive, or "sleeping".
Approved indications and uses
Acyclovir is indicated for:
- treatment of initial episodes and the management of recurrent episodes of genital herpes
- acute treatment of herpes zoster (shingles)
- treatment of chickenpox (varicella)
Valacyclovir is indicated for:
- treatment or suppression of genital herpes and for the suppression of recurrent genital herpes
- reduction of transmission of genital herpes
- treatment of herpes zoster
- treatment of cold sores (herpes labialis)
Famciclovir is indicated for:
- treatment or suppression of recurrent genital herpes
- treatment of acute herpes zoster
Efficacy
Clinical studies have not found any significant differences in effectiveness among the three medications. All are quite safe, very rarely producing any side effects at all. All work by disrupting the virus's reproductive ability.
All three of these oral antiviral drugs can be taken either episodically - when a person has an outbreak or feels one coming on, or suppressively - daily to help prevent the recurrence of outbreaks.
For the treatment of first genital herpes infections, oral acyclovir or valacyclovir is preferable to famciclovir. The efficacy of famciclovir for initial episode genital herpes infection has not been established.
For the treatment of recurrent infections, clinical trials have demonstrated that acyclovir, valacyclovir, and famciclovir have equivalent efficacy.
Intravenous acyclovir is used to treat serious flare-ups or outbreaks that effect internal organs.
Adverse reactions and side effects
All these agents are well tolerated and have excellent safety records. Possible side effects include nausea, vomiting, headache, fatigue, tremor, and very rarely, seizures. They can effect the kidney, however, and people with kidney problems should use them with caution and at lower doses. Intravenous administration increases the risk for kidney problems and can cause blood clots at the injection site. In rare cases, it can cause central nervous system complications.
Pharmacokinetics
Acyclovir. It has poor bioavailability of about 20% and a short half life, which necessitates frequent dosing. Acyclovir speeds healing of lesions and suppress viral shedding if taken within 24 hours of the first indication of a recurrent episode.
Valacyclovir. Valacyclovir provides a unique mechanism of enhancing the oral bioavailability of the parent compound, acyclovir. Valacyclovir is a prodrug converted to acyclovir in the intestine and liver. It has better bioavailability (about 55%) and a longer duration of action than acyclovir. Thus, valacyclovir requires less frequent dosing than acyclovir. It is available in a one-day regimen for oral herpes, a once-daily dose to suppress genital herpes, and a three-day treatment for recurrent herpes. Valacyclovir is most effective if taken within 24 hours of the first signs of an outbreak.
When used as an episodic treatment, valacyclovir can help the sores heal faster, shorten the period of pain during the outbreak and cut down the time during which the virus is detected on genital skin surfaces. When taken as soon as the first signs of an outbreak are noticed, such as tingling, itching or redness, valacyclovir may be able to completely prevent the development of painful blisters.
Famciclovir. Famciclovir is a prodrug for the active metabolite penciclovir. Famciclovir is converted into its active compound within the infected cell by contact with an enzyme from the virus. It has high bioavailability of 77%. It remains active in the body longer than acyclovir and, like valacyclovir, requires less frequent dosing (usually two or three times a day). It is most effective if taken within six hours of onset of symptoms.
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