Viral infections are among the most formidable conditions in the primary care setting, causing a wide range of illnesses that are difficult to treat. Developing antiviral medicines has been difficult because most drugs that kill viruses also damage the host's cells. However, since the first antiviral drug, amantadine (Symmetrel), was released in 1966, encouraging progress has been made in this area. Part I of this two-part article focuses on antiviral agents used to treat hepatitis, cytomegalovirus (CMV), and herpes infections in nonimmunosuppressed patients. Important issues in the management of viral infections in patients with human immunodeficiency virus (HIV) infection and in solid-organ or bone-marrow transplant patients are not reviewed.

Hepatitis Viruses

HEPATITIS B

Chronic hepatitis B virus (HBV) infection affects 5 percent of the worldwide population and may lead to cirrhosis and hepatocellular carcinoma. HBV infection is considered chronic when surface antigen persists for more than six months. Three criteria for treating chronic HBV include alanine aminotransferase (ALT) level greater than two times normal and positive tests for HBV DNA and hepatitis B e antigen (HbeAg) (Table 1). All three criteria should be present before patients are considered for anti-HBV therapy. The treatment of choice in these patients is interferon alfa-2b (Intron A)¹ [Evidence level B, cohort study] or lamivudine (Epivir-HBV).² More recently, adefovir dipivoxil (Hepsera) was approved for use in chronic HBV infection.³

Interferon Alfa-2b

Response to interferon alfa-2b (defined as loss of HBeAg and HBV DNA) occurs in 30 to 40 percent of patients. Unfortunately, interferon is associated with significant side effects, including flu-like symptoms (e.g., headaches, fevers, myalgias, fatigue), thrombocytopenia, leukopenia, depression, weight loss, rash, cough, hypo- or hyperthyroidism, tinnitus, auto-antibody formation, and retinopathy.

Lamivudine

Lamivudine is a nucleoside reverse transcriptase inhibitor that is used to treat HIV and HBV infections. Response rates to lamivudine are similar to those obtained with interferon alfa-2b, and lamivudine is typically much better tolerated.² [Evidence level A, randomized controlled trial (RCT)] However, drug resistance is a major difficulty associated with lamivudine. Resistance develops after one year of treatment in 15 to 30 percent of patients. Unlike interferon alfa-2b, lamivudine is approved for use in patients with decompensated cirrhosis resulting from HBV infection.

Adefovir Dipivoxil

Adefovir dipivoxil is a nucleotide reverse transcriptase inhibitor with a mechanism of action similar to that of lamivudine. In early studies, resistance to adefovir dipivoxil appears to be uncommon. Adefovir dipivoxil also can be used effectively in patients who are resistant to lamivudine.^3,4 It is likely that future treatment protocols for HBV infection will use multiple combination regimens, although such regimens are currently experimental.

HEPATITIS C

Hepatitis C virus (HCV) is the most frequent cause of end-stage liver disease in the United States and the leading indication for liver transplant.⁵ Therefore, as with chronic HBV, it is important that physicians consider treatment options in any HCV-infected patient.

Therapy for chronic HCV infection is indicated in patients with a detectable HCV RNA viral load and a persistently elevated ALT level. Findings of cirrhosis, fibrosis, or even moderate inflammation on liver biopsy support the choice of therapeutic intervention, but biopsy is not mandatory before initiating therapy.⁵

Pegylated Interferon Alfa and Ribavirin

The standard treatment regimen for chronic HCV infection is outlined in Table 1. Pegylated interferon alfa-2a (Pegasys) and pegylated interferon alfa-2b (PEG-Intron) are modified forms of interferon alfa with much longer half-lives, which allow these drugs to be taken once a week. In addition, they are significantly more effective against HCV, either alone or in combination with ribavirin (Rebetol), compared with unmodified interferon alfa, and they have side effect profiles similar to that of unmodified interferon alfa.^6,7 [References ⁶ and ⁷—Evidence level A, RCT]

The HCV viral load should be assessed after 24 weeks of therapy. Recent studies suggest that patients infected with HCV genotype 2 or 3 should receive a total course of 24 weeks of therapy, while patients with genotypes 1 or 4 have higher sustained virologic response rates if they are treated for 48 weeks. In patients with genotype 1a or 1b infection, the detection of viremia at 24 weeks predicts viral persistence despite therapy. Thus, it is recommended that patients with viremia at 24 weeks discontinue therapy. Because viremia at 12 weeks now appears to predict persistent HCV infection, some experts recommend discontinuing therapy at 12 weeks if HCV viremia is detected. In patients with genotype 1a or 1b (and probably genotype 4, as well) and an undetectable HCV viral load at 24 weeks, continuation of therapy for the full 48-week course is indicated.

Treatment of newly established HCV infection with unmodified interferon alfa-2b for 24 weeks was recently found to eliminate the virus in the vast majority of patients studied.⁸ [Evidence level B, nonrandomized clinical trial] Unfortunately, few patients present to their physicians with acute HCV infection, although it is sometimes found during screening (after a needlestick exposure, for example). Comparable data have not been collected for the response of acute HCV infection to pegylated interferons, but it is reasonable to anticipate a similar benefit.

HEPATITIS D

Hepatitis D virus affects only patients who are already infected with HBV. Co-infected patients are more likely to develop cirrhosis and hepatocellular carcinoma. Treatment for hepatitis B and D co-infection is similar to treatment for chronic HBV infection, although some protocols use higher interferon dosages.⁹

Cytomegalovirus

CMV infection generally causes an asymptomatic or mildly symptomatic acute illness in immunocompetent adults. Approximately 10 percent of patients develop a self-limited, mononucleosis-like syndrome.¹⁰ Treatment of CMV mononucleosis is generally supportive. Severe CMV disease in immunocompetent patients is rare and has been associated with a nearly 50 percent mortality rate.¹⁰

Ganciclovir and Foscarnet

There are no established recommendations for treating severe CMV infection in immunocompetent patients. However, case reports of severely ill patients who received either intravenous ganciclovir (Cytovene) or foscarnet (Foscavir) describe an improved outcome compared with untreated patients.¹⁰ Patients were treated with various dosages for various durations.

The major toxic effect of ganciclovir is myelosuppression. Ganciclovir also can cause fever, rash, and abnormal liver function. Foscarnet is associated with nausea, vomiting, anemia, electrolyte abnormalities, central nervous system disturbances, and renal impairment. Both ganciclovir and foscarnet should be used with caution in patients with concomitant renal impairment.

Herpes Viruses

Herpes viruses are DNA viruses that can lie dormant in sensory neurons after initial infection, then later reactivate and cause disease. Viruses in this family include herpes simplex virus (HSV) and varicella zoster virus (VZV). The dominant manifestations of these viruses are dermatologic and sensory. Antiviral agents used in the treatment of herpes viruses are listed in Table 2.¹¹

GENITAL HERPES

Initial and recurrent episodes of genital HSV can be treated, and recurrent episodes (more than six per year) can be suppressed with antiviral medications. Suppressive treatment is much more effective than episodic treatment.

Acyclovir

Acyclovir (Zovirax) is a guanosine analog that inhibits DNA polymerase. It has poor bioavailability and a short half-life. Treatment with daily oral acyclovir decreases episodes from 11.4 to 1.8 per year.^12,13 [Reference ¹³—Evidence level A, RCT] Topical acyclovir is not an effective treatment for episodic genital HSV.

Valacyclovir

Valacyclovir (Valtrex) is a pro-drug that metabolizes to acyclovir. It has better bioavailability and less frequent dosing than acyclovir. After one year of daily treatment with valacyclovir, 40 to 50 percent of patients are episode free, and the mean rate of occurrence is 0.8 episodes per year.^14,15 Compared with placebo, valacyclovir decreases the length of episodes and mean healing time by two days.¹⁶ It is as effective as acyclovir for initial and episodic treatment and for suppression of genital HSV.

Famciclovir and Penciclovir

Famciclovir (Famvir) is a prodrug of penciclovir (Denavir), a purine analog. It has high bioavailability and quickly metabolizes to penciclovir. In episodic treatment of genital HSV, famciclovir decreases time to healing.¹⁷ A study of its use in the suppression of recurrent genital HSV infection showed an average of 1 to 1.8 episodes per year in treated patients versus 5.1 episodes per year in patients who received placebo.¹⁸ Topical penciclovir decreases time to crusting by one day.¹⁹

Acyclovir, valacyclovir, and famciclovir have similar side effects, which include nausea, vomiting, headache, and diarrhea. When used in high dosages as an intravenous medication, acyclovir can crystallize the renal tubules, causing acute renal failure.

OROLABIAL HERPES

Initial orolabial HSV infection (gingivostomatitis) often affects young children, and treatment with oral antiviral medications may be helpful in certain populations (i.e., skiers with a sunlight trigger). Few studies have investigated the use of oral antiviral drugs for episodic or recurrent orolabial HSV infection.

Acyclovir

Treatment with oral acyclovir significantly shortens the course of initial orolabial HSV infection in children (by approximately six days) and decreases symptoms and duration of viral shedding.²⁰

Famciclovir

When taken within 48 hours of exposure to the precipitating stimulus, oral famciclovir decreases time to healing by two days in ultraviolet radiation–induced orolabial herpes.²¹

Valacyclovir

Valacyclovir was recently approved by the U.S. Food and Drug Administration for the treatment of orolabial HSV infections in adults and children 12 years of age and older.

Topical Medications

Topical penciclovir and docosanol (Abreva) decrease time to healing by 0.7 days.^22,23

VARICELLA ZOSTER VIRUS

Studies have linked treatment with antiviral agents to a decrease in the duration of post-herpetic neuralgia, although the use of drugs for this purpose is controversial.

Acyclovir

In a randomized, double-blind study, acyclovir accelerated healing, decreased frequency of rash dissemination, and reduced acute pain.²⁴

Valacyclovir

In patients older than 50 years, valacyclovir decreases the duration of acute pain and post-herpetic neuralgia compared with acyclovir.²⁵

Famciclovir

Famciclovir is effective in decreasing time to resolution of VZV lesions and decreasing duration of post-herpetic neuralgia.²⁶ There is no difference in effectiveness between famciclovir and valacyclovir in the treatment of VZV.²⁷