Antiviral Therapy

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Antiviral therapy

Definition

Antiviral therapy is often used by cancer patients to treat viral infections. Commonly used antiviral medications include acyclovir, famciclovir, ganciclovir, valacyclovir, and foscarnet.

Description

Viral infections occur in almost all people at some time in their lives. The common cold is the most easily recognizable example of a virus that can be unpleasant but generally does not cause serious problems. For people with cancer, however, viruses can often cause life-threatening illnesses.

Viral infections in cancer patients can be much more serious and debilitating than in patients without cancer. Cancer patients will often have weakened immune systems from chemotherapy or from the cancer itself. Cancer patients who have bone marrow transplants are at especially high risk for life-threatening viral infections. Immediately after the transplant, the patient will have very few, if any, white blood cells, which are the body's main infection fighters. Viral infections such as herpes simplex virus (HSV), herpes zoster virus (HZV), and cytomegalovirus are often seen in cancer patients, and all can cause serious, life-threatening infections.

Until the development of the antiviral drug acyclovir 1974, no relatively safe and effective anti-viral medications for cancer patients were available. By the mid-1980s, acyclovir was being routinely used for cancer patients with herpes infections. Besides treating the infection itself, acyclovir can be taken on a daily basis to prevent infection from occurring. This can be especially important in people with very depressed immune systems, such as cancer patients who have undergone a bone marrow transplant.

Since the introduction of acyclovir, other anti-viral medications have been developed that have been very useful in the treatment of viral illnesses. For reasons that are still unknown, certain herpes infections in certain cancer patients do not respond to acyclovir. Fortunately, two other newer medications similar to acyclovir, called famciclovir and valaciclovir, are helpful in treating herpes infections, especially ones that are resistant to acyclovir.

While antiviral drugs such as acyclovir have made a large difference in treating herpes infections in cancer patients, there are other viral infections that do not respond to acyclovir. Cytomegalovirus is a common viral infection among cancer patients, and especially common among cancer patients who have had bone marrow transplants. Some antiviral medications like acyclovir are not effective against cytomegalovirus. Fortunately, two other antiviral medications known as ganciclovir and foscarnet are both effective against cytomegalovirus.

Recommended dosage

The recommended dosage for the various antiviral medications can vary considerably, depending on the health of the patient and how the medication is administered. For the treatment of herpes simplex and herpes zoster, the drugs acyclovir, famciclovir, and valacyclovir can be used. The recommended oral dosage ranges from 500 mg twice a day for valacyclovir, 500 mg three times a day for famciclovir, to 800 mg every four hours for acyclovir. There is also a formulation for the drug to be given administered though the vein. The dose for injection is different than the dose for oral therapy.

For the treatment of cytomegalovirus, ganciclovir or foscarnet can be used. Both medications are generally given intravenously, although there is an oral formulation available for ganciclovir. The dosage is 5 mg per kg of body weight every 12 hours for 14 to 21 days, followed by maintenance therapy at a dose of 5 mg per kg per day as a single daily dose. The dosage for foscarnet ranges from 40 mg per kg to 90 mg per kg, depending on the diagnosis.

Precautions

The drugs acyclovir, famciclovir, valcyclovir, ganciclovir and foscarnet should all be used with caution by patients with kidney problems. With higher doses of these drugs, patients who do have kidney problems should have their kidney functioning monitored closely on a daily basis. The dosage is usually decreased depending on the degree of decreased kidney function. Kidney failure has been reported in patients taking high doses of foscarnet.

Ganciclovir should be used with extreme caution in women who may be pregnant, since it is teratogenic (causes abnormalities), as well as toxic, to developing embryos. There are no well-controlled studies of the other antiviral agents in pregnant women and it is not known whether these agents are excreted in breast milk. Therefore, it is not recommended that these antiviral agents be given to pregnant or nursing mothers unless the benefit outweighs the risk.

Side effects

Side effects common to all the antiviral medications include nausea, vomiting, diarrhea , headaches, and dizziness, rash, and decreased kidney function. Of the drugs used to treat herpes simplex, acyclovir seems to have more reported side effects than the other medications.

The two drugs that are used to treat cytomegalovirus, ganciclovir and foscarnet, have very different side effect profiles. Ganciclovir's major side effect is the lowering of white blood cells, a condition known as neutropenia . Because of this, a patient on ganciclovir should have their white blood cell count monitored closely. Foscarnet, while generally not causing a marked decrease in white blood cells, can cause sudden kidney failure. Patients who are taking foscarnet should make sure they maintain their fluid intake and have their kidney functions monitored closely.

Interactions

The antiviral drugs used to treat herpes simplex and zoster should be used with caution with other drugs that cause kidney problems. Also, they all interact with probenecid, a medication commonly used to treat gout.

Drug interactions with foscarnet and ganciclovir are more numerous and potentially dangerous. Both, especially foscarnet, must be used with caution with other drugs that cause kidney problems. Both must also be used with caution with other medications that lower seizure thresholds. Patients should notify their physician or consult with their pharmacist prior to starting any over the counter or herbal medications due to the numerous drug interactions that can occur with these agents.

Edward R. Rosick, DO, MPH, MS

KEY TERMS

Herpes simplex virus

—An infectious, contagious viral disease, caused by herpes simplex virus type one or two. It is characterized by fluid-filled lesions that can recur.

Herpes zoster virus

—An acute, infectious viral disease, characterized by painful, fluid-filled lesions.

Cytomegalovirus

—A viral disease caused by a herpes virus, generally rare but often seen in cancer patients. It can cause life-threatening pneumonia as well as blindness.