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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 three 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 to 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 or quantitative changes in the viral TK 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 such 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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Acyclovir is a synthetic nucleoside analog active against herpes viruses. Acyclovir for Injection USP is a sterile lyophilized powder for intravenous administration only. Each 500 mg vial contains 500 mg of Acyclovir and 49 mg of sodium, and each 1000 mg vial contains 1000 mg Acyclovir and 98 mg of sodium. Reconstitution of the 500 mg or 1000 mg vials with 10 mL or 20 mL, respectively, of Sterile Water for Injection, results in a solution containing 50 mg/mL of acyclovir. The pH of the reconstituted solution is approximately 11. Further dilution in any appropriate intravenous solution must be performed before infusion.

Acyclovir sodium is a white, crystalline powder with a molecular weight of 247.19, and a maximum solubility in water at 25°C exceeds 100 mg/mL. Each 500 mg or 1000 mg vial of Acyclovir for injection USP when reconstituted with 10 mL or 20 mL, respectively, sterile diluent yields 50 mg/mL Acyclovir (pH approximately 11). At physiologic pH, Acyclovir exists as the un-ionized form with a molecular weight of 225.21 and a maximum solubility in water at 37°C of 2.5 mg/mL. The pka’s of Acyclovir are 2.27 and 9.25.

The chemical name of Acyclovir sodium is 9-[(2-Hydroxyethoxy)methyl]guanine sodium salt.

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Once a medicine has been approved for marketing for a certain use, experience may show that it is also useful for other medical problems. Although not specifically included in product labeling, acyclovir by injection is used in certain patients with the following medical conditions:

* Herpes simplex (for prevention of repeated infections) in people with a weak immune system
* Herpes zoster infections of the eye
* Shingles (for prevention of repeated infections) in people with a weak immune system

Other than the above information, there is no additional information relating to proper use, precautions, or side effects for these uses.

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Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur:

More common — For acyclovir injection only

Pain, swelling, or redness at place of injection

Less common

Abdominal or stomach pain; decreased frequency of urination or amount of urine; increased thirst; loss of appetite; nausea or vomiting; unusual tiredness or weakness

Rare

Black, tarry stools; blood in urine or stools; chills, fever, or sore throat; confusion; convulsions (seizures); hallucinations (seeing, hearing, or feeling things that are not there); hives; pinpoint red spots on skin; trembling; unusual bleeding or bruising

Frequency not determined

Bleeding or oozing from puncture sites or mucous membranes (bowel, mouth, nose, or urinary bladder), continuing; blistering, peeling, or loosening of skin; bluish coloring, especially of the hands and feet; blurred vision; bruising at the place of injection; changes in facial skin color; changes in vision; clumsiness; coughing; decreased consciousness; difficulty in breathing or swallowing; dizziness or feeling faint, severe; fast heartbeat; irritability; itching or skin rash; large hive-like swelling on face, eyelids, lips, tongue, throat, hands, legs, feet, sex organs; mood or mental changes; muscle cramps, pain, or weakness; pale skin; red or irritated eyes; sense of agitation or uneasiness; shakiness and unsteady walk; sores, ulcers, or white spots in mouth or on lips; swelling of eyelids, face, feet, hands, lower legs or lips; swollen, painful, or tender lymph nodes (glands) in neck, armpit, or groin; unsteadiness or other problems with muscle control or coordination; yellow eyes or skin
Other side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. However, check with your doctor if any of the following side effects continue or are bothersome:

More common —  Especially seen with high doses

General feeling of discomfort or illness

Less common —  Especially seen with long-term use or high doses

Diarrhea; headache

Frequency not determined

Burning, prickling, or tingling sensations; drowsiness; loss of hair

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your doctor.

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If your symptoms do not improve within a few days, or if they become worse, check with your doctor.

The areas affected by herpes, chickenpox, or shingles should be kept as clean and dry as possible. Also, wear loose-fitting clothing to avoid irritating the sores (blisters).

It is important to remember that acyclovir will not keep you from spreading herpes to others .

Herpes infection of the genitals can be caught from or spread to your partner during any sexual activity. Even though you may get herpes if your partner has no symptoms, the infection is more likely to be spread if sores are present. This is true until the sores are completely healed and the scabs have fallen off. Therefore, it is best to avoid any sexual activity if either you or your sexual partner has any symptoms of herpes . The use of a latex condom (“rubber”’) may help prevent the spread of herpes. However, spermicidal (sperm-killing) jelly or a diaphragm will probably not help.

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Patient information about the treatment of herpes, chickenpox, or shingles is available with this medicine. Read it carefully before using this medicine.

Acyclovir is best used as soon as possible after the symptoms of herpes infection or shingles (for example, pain, burning, blisters) begin to appear .

If you are taking acyclovir for the treatment of chickenpox , it is best to start taking acyclovir as soon as possible after the first sign of the chickenpox rash , usually within one day.

Acyclovir capsules, tablets, and oral suspension may be taken with meals or on an empty stomach.

Acyclovir is best taken with a full glass (8 ounces) of water.

If you are using acyclovir oral suspension , use a specially marked measuring spoon or other device to measure each dose accurately. The average household teaspoon may not hold the right amount of liquid.

To help clear up your herpes infection, chickenpox, or shingles, keep taking acyclovir for the full time of treatment , even if your symptoms begin to clear up after a few days. Do not miss any doses . However, do not use this medicine more often or for a longer time than your doctor ordered .

Acyclovir Dosing

The dose of acyclovir will be different for different patients. Follow your doctor’s orders or the directions on the label . The following information includes only the average doses of acyclovir. Your dose may be different if you have kidney disease. If your dose is different, do not change it unless your doctor tells you to do so.

The number of capsules or tablets or teaspoonfuls of suspension that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are taking acyclovir .

* For oral dosage forms (capsules, oral suspension, or tablets):

o For treatment of genital herpes:

+ Adults and children 12 years of age and older
200 milligrams (mg) five times a day for ten days.
+ Children up to 12 years of age
Use and dose must be determined by the doctor.

o For prevention of recurrent outbreaks of genital herpes infections:

+ Adults and children 12 years of age and older
200 to 400 mg two to five times a day for five days or up to twelve months, depending on how often your outbreaks of infection occur.
+ Children up to 12 years of age
Use and dose must be determined by the doctor.

o For treatment of chickenpox:

+ Adults and children who weigh over 88 pounds (40 kilograms)
800 mg four times a day for five days.
+ Children 2 years of age and older and weighing 88 pounds (40 kilograms) or less
Dose is based on body weight and must be determined by the doctor. The usual dose is 20 mg per kilogram (kg) of body weight, up to 800 mg, four times a day for five days.
+ Children up to 2 years of age
Use and dose must be determined by the doctor.

o For treatment of shingles:

+ Adults and children 12 years of age and older
800 mg five times a day for seven to ten days.
+ Children up to 12 years of age
Use and dose must be determined by the doctor.

* For injection dosage form:

o For treatment of herpes of the brain, genitals, or mucous membranes, or for the treatment of shingles:

+ Adults and children 12 years of age and older
Dose is based on body weight and must be determined by the doctor. The usual dose is 5 to 10 mg of acyclovir per kg (2.3 to 4.5 mg per pound) of body weight, injected slowly into a vein over at least a one-hour period, and repeated every eight hours for five to ten days.
+ Children up to 12 years of age
Dose is based on body weight and must be determined by the doctor. The usual dose is 10 mg to 20 mg of acyclovir per kg (4.5 mg to 9.1 mg per pound) of body weight, injected slowly into a vein over at least a one-hour period and repeated every eight hours for seven to ten days.

o For treatment of widespread herpes virus infection in newborns:

+ Infants from birth to 3 months of age
Dose is based on body weight and must be determined by the doctor. The usual dose is 10 mg of acyclovir per kg (4.5 mg per pound) of body weight, injected slowly into a vein over at least a one-hour period and repeated every eight hours for ten days.

Acyclovir Missed dose

If you do miss a dose of this medicine, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not double doses.

Acyclovir Storage

To store Acyclovir:

* Keep out of the reach of children.
* Store away from heat and direct light.
* Do not store the capsule or tablet form of this medicine in the bathroom, near the kitchen sink, or in other damp places. Heat or moisture may cause the medicine to break down.
* Do not keep outdated medicine or medicine no longer needed. Be sure that any discarded medicine is out of the reach of children.

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In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For acyclovir, the following should be considered:

Acyclovir and allergies

Tell your doctor if you have ever had any unusual or allergic reaction to acyclovir or valacyclovir. Also tell your health care professional if you are allergic to any other substances, such as foods, sulfites or other preservatives, or dyes.

Acyclovir and Pregnancy

Acyclovir has been used in pregnant women and has not been reported to cause birth defects or other problems. However, studies have not been done in humans. Studies in rats and rabbits have shown that acyclovir given by injection may keep the fetus from becoming attached to the lining of the uterus (womb). However, acyclovir has not been shown to cause birth defects or other problems in mice given many times the usual human dose, or in rats or rabbits given several times the usual human dose.

Acyclovir and Breast-feeding

Acyclovir passes into breast milk. However, it has not been reported to cause problems in nursing babies.

Acyclovir and Children

A limited number of studies have been done using oral acyclovir in children, and it has not caused different effects or problems in children than it does in adults.

Acyclovir and Older adults

Agitation, confusion, dizziness, and drowsiness may be especially likely to occur in elderly patients who are usually more sensitive than younger adults to the central nervous system effects of acyclovir.

Acyclovir and Other medicines

Although certain medicines should not be used together at all, in many cases two different medicines may be used together even if an interaction might occur. In these cases, changes in dose or other precautions may be necessary. If you are taking acyclovir it is especially important that your health care professional know if you are taking any of the following:

* Carmustine (e.g., BiCNU) or
* Cisplatin (e.g., Platinol) or
* Combination pain medicine containing acetaminophen and aspirin (e.g., Excedrin) or other salicylates or
* Cyclosporine (e.g., Sandimmune) or
* Deferoxamine (e.g., Desferal) (with long-term use) or
* Gold salts (medicine for arthritis) or
* Inflammation or pain medicine, except narcotics, or
* Lithium (e.g., Lithane) or
* Methotrexate (Mexate) or
* Other medicine for infection or
* Penicillamine (e.g., Cuprimine) or
* Plicamycin (e.g., Mithracin) or
* Streptozocin (e.g., Zanosar) or
* Tiopronin (Thiola)—Concurrent use of these medicines with acyclovir may increase the chance for side effects, especially when kidney disease is present

Acyclovir and Other medical problems

The presence of other medical problems may affect the use of acyclovir. Make sure you tell your doctor if you have any other medical problems, especially:

* Dehydration or
* Kidney disease—Dehydration or kidney disease may increase blood levels of acyclovir, increasing the chance of side effects

* Nervous system problems—Acyclovir may make these problems worse

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Acyclovir (ay-SYE-kloe-veer) belongs to the family of medicines called antivirals, which are used to treat infections caused by viruses. Usually these medicines work for only one kind or group of virus infections.

Acyclovir is used to treat the symptoms of chickenpox, shingles, herpes virus infections of the genitals (sex organs), the skin, the brain, and mucous membranes (lips and mouth), and widespread herpes virus infections in newborns. Acyclovir is also used to prevent recurrent genital herpes infections. Although acyclovir will not cure herpes, it does help relieve the pain and discomfort and helps the sores (if any) heal faster.

Acyclovir may also be used for other virus infections as determined by your doctor. However, it does not work in treating certain virus infections, such as the common cold.

Acyclovir is available only with your doctor’s prescription, in the following dosage forms:

Oral

* Capsules (U.S.)
* Oral suspension (U.S. and Canada)
* Tablets (U.S. and Canada)

Parenteral

* Injection (U.S. and Canada)

Acyclovir Category

* Antiviral, systemic

Some commonly used brand names are:

In the U.S.—
* Zovirax

In Canada—
* Alti-Acyclovir
* Avirax
* Zovirax
* Zovirax Wellstat Pac
* Zovirax Zostab Pac

Another commonly used name is aciclovir.

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* If you have any questions about Acyclovir , please talk with your doctor, pharmacist, or other health care provider.
* Acyclovir is to be used only by the patient for whom it is prescribed. Do not share it with other people.
* If your symptoms do not improve or if they become worse, check with your doctor.

This information is summary only. It does not contain all information about Acyclovir . If you have questions about the medicine you are taking or would like more information, check with your doctor, pharmacist, or other health care provider.

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Contact 1-800-222-1222 (the American Association of Poison Control Centers), your local poison control center (http://www.aapcc.org), or emergency room immediately.

Proper storage of Acyclovir

Acyclovir is usually handled and stored by a health care provider. If you are using Acyclovir at home, store Acyclovir as directed by your pharmacist or health care provider. Keep Acyclovir out of the reach of children and away from pets.