Antiviral drugs

Types and differences of antiviral drugs

In addition to treating influenza, antivirals also may be used to treat other medical conditions. For example, interferons are synthetic versions of a substance naturally produced by cells in the body that help fight infections and tumors. Interferons are often used to treat hepatitis B and C and certain types of cancer.

Other antivirals include:

• Acyclovir. Drug used to heal sores or blisters caused by varicella (chickenpox), herpes zoster (shingles) and genital herpes. Acyclovir also can prevent outbreaks of genital herpes in people who are infected with the virus. In some cases, regular treatment with acyclovir has kept people symptom-free for as long as a decade, and with few reported side effects.
• Ganciclovir. Drug used to treat or prevent cytomegalovirus (CMV), a virus that infects cells and causes them to become enlarged.
• Ribavirin. Drug used with interferon to treat hepatitis C. It helps stop the virus that causes hepatitis C from spreading inside the body. Ribavirin is also sometimes used to treat viral hemorrhagic fevers and severe acute respiratory syndrome (SARS).
• Vidarabine. Drug used to treat virus infections of the eye caused by herpes infection. It sometimes causes increased sensitivity to light that can be reduced by wearing sunglasses and avoiding too much exposure to bright light.

Certain antiviral drugs also are used specifically to treat patients with HIV and AIDS. These drugs are known as antiretroviral medications because they attack the retrovirus (virus that contains RNA and reverse transcriptase) that causes AIDS. They include protease inhibitors, nucleoside-analog reverse transcriptase inhibitors and non-nucleoside reverse transcriptase inhibitors. Currently, there are more than two dozen retroviral drugs used to treat HIV/AIDS.

Other types of antivirals

There are several different kinds of antiviral agents that are prescribed to fight viral skin infections. These medications can be divided into systemic antivirals and topical antivirals.

Systemic antivirals are usually given orally in pill form. However, they sometimes may be administered intravenously (into a vein). This is most likely in situations where the patient is suffering from a suppressed immune system (due to HIV infection, organ transplantation or another condition), is unable to swallow oral medication, has impaired memory or mental capacity or does not have quick access to a hospital should complications arise during drug therapy.

Topical antivirals are usually available in various forms, including creams, ointments, gels, patches and sprays. In addition, most topical antivirals can be further subdivided into one of three major classes:

• Cytodestructive drugs (destroys infected tissue)
• Immunoenhancing drugs (enhances immune system)
• Viricidal drugs (destroys viruses)

Conditions treated with antiviral medications

Most viruses that cause skin infections can be divided into several major groups. These include:

• Human herpesvirus (HHV). This family of viruses includes strands of the herpes simplex virus that cause cold sores (herpes simplex virus type 1) and genital lesions (herpes simplex virus type 2), and the herpes zoster virus that causes chickenpox and shingles. Human herpesviruses 6 and 7 cause a childhood illness called roseola infantum that can cause high fever and rash. Systemic antiviral agents are typically used to treat HHV.
• Human papillomavirus (HPV). A group of viruses that cause various types of warts (papillomas). These include common warts, foot warts, flat warts, genital warts and filiform warts. Some types of warts require no treatment, whereas others may need treatment with topical or systemic antiviral agents.
• Poxvirus. A group of viruses that include cowpox, monkeypox, and smallpox. One of the most destructive viral infections, smallpox has been eradicated throughout the world. Since 1978, only isolated, laboratory-acquired forms of the disease have been reported. It is usually treated with a vaccine that is effective up to four days after exposure, although the drug cidofovir has shown promise in the laboratory as a potential treatment.
• Human immunodeficiency virus (HIV). HIV is the virus that causes acquired immune deficiency syndrome (AIDS). Skin lesions that appear as a result of this infection are often the first sign that the patient has HIV. These lesions often result from a separate infection with HHV, papillomavirus or other infectious agents. Systemic antiviral agents are typically used to treat HIV. Numerous other medications are available that specifically treat HIV.

Conditions of concern with antivirals

Experts generally advise against using the antiviral zanamivir in people with underlying respiratory disease, including asthma or chronic obstructive pulmonary disease (COPD). Some people with such conditions who have taken zanamivir have experienced bronchospasm.

Patients with kidney disease may need to have their dosage of oseltamivir adjusted. In addition, people who are allergic to any of these antiviral drugs or their ingredients should not take them.

In laboratory and limited clinical studies, there have been no reported interactions of the antiviral medication neuraminidase inhibitors with other drugs.

Patients should closely consult with their physician about whether certain conditions preclude taking specific antiviral agents.

Potential side effects

In general, the newer antiviral neuraminidase inhibitors (e.g., oseltamivir) are less likely to cause side effects than the older flu drugs. Side effects associated with antiviral medications include:

• Cough
• Dizziness
• Headache
• Loss of appetite
• Nausea
• Vomiting

In addition, adverse central nervous system (CNS) effects have been noted with use of some antivirals. Such side effects include lightheadedness, difficulty concentrating, nervousness, insomnia and seizures in patients with pre-existing seizure disorders.

There have been reports of serious respiratory problems in some people who use zanamivir.

In some cases, taking antiviral drugs can lead to the appearance of drug-resistant flu viruses. For example, when either amantadine or rimantadine is used for therapy, drug-resistant flu viruses may appear in about one-third of patients, according to the National Institutes of Health (NIH).

Laboratory studies indicate that influenza A and B viruses can develop resistance to zanamivir and oseltamivir. Studies are ongoing to see if this effect is duplicated in patients who take neuraminidase inhibitor medications.

Side effects associated with antivirals other than those used to treat the flu include:

• Changes in vision
• Dry, irritated or itchy skin
• Hair loss
• Memory loss
• Pain, particularly in the joints, bones or muscles
• Painful or irregular menstruation (period)
• Rash
• Sweating

Patients are urged to closely follow their physician’s instruction for taking antiviral medications. This will help both lower the risk of side effects for the patient and help prevent the development of drug-resistant flu viruses in the public at large.

Drug or other interactions

Patients should consult their physician before taking any additional prescriptions, over-the-counter medications, nutritional supplements or herbal medications. Of particular concern to individuals using antiviral agents are:

• Cimetidine (used to treat gastrointestinal disorders)
• Probenecid (used to treat chronic gout or gouty arthritis)
• Theophylline (used to treat lung problems resulting from various disorders)

Pregnancy use issues

The U.S. Food and Drug Administration (FDA) has classified most antiviral agents as belonging in either category B (which means that either animal studies have shown no risk to an animal fetus, or human studies have shown no risk to a human fetus) or category C (which means animal studies have shown a risk to animal fetuses, but risk has not been adequately studied in humans).

Patients should discuss the potential risks and benefits of using antiviral agents during pregnancy and breastfeeding with their physicians.

Child use issues

None of the antivirals used to treat influenza has been approved for use in children younger than 1 year of age. Children infected with the flu who are between 12 months and 23 months of age are most likely to benefit from antiviral medications.

Amantadine is approved for treating and preventing uncomplicated influenza A virus infection in children who are 1 year of age or older. Rimantadine is approved for preventing, but not treating, such infections in children. However these medications may not be effective every year. For example, in 2006 the U.S. Centers for Disease Control and Prevention (CDC) recommended against the use of these medications because the active strains of influenza were resistant to these drugs.

Zanamivir is approved for preventing influenza A and B virus infections in people 5 years of age and older and for treating uncomplicated influenza virus infection in people 7 years of age and older who have not had symptoms for more than two days.

Oseltamivir is approved for treating uncomplicated influenza virus infection in people 1 year of age or older who have not had symptoms for more than two days. A pediatric liquid formulation of this medication is available. Oseltamivir also is approved for preventing influenza A and B in people 1 year of age and older.