Antiviral drugs: Introduction, classification and mode of action


Antiviral agents interfere with virus-specific events in replication, including viral attachment, uncoating, assembly, and virus-directed macromolecular synthesis. As a result, antiviral agents typically have a restricted spectrum of activity. In contrast to most antibacterial drugs, antiviral drugs can also affect the function of host cell machinery and thus antiviral drugs have a greater risk of toxicity. In addition to acute toxicities such as marrow suppression, many antiviral agents are immunosuppressive, carcinogenic, and teratogenic. Many are given topically in order to minimize systemic toxicity. Antiviral agents are more likely to be effective when an intact host immune response is present. Combinations of antiviral drugs with different mechanisms of action and combinations of antiviral and immunomodulatory drugs are increasingly being used to treat human viral infections, especially HIV infection, with an associated decrease in toxicity and reduction in selection for drug-resistant mutants.

Although all antiviral drugs used to treat small animals were originally developed for treatment of human viral infections, the application of these drugs to treat viral infections of small animals has not proven straightforward. This relates to differences in the composition of human and companion animal viruses (which affect their susceptibility to inactivation by antiviral drugs), or problems relating to toxicity of human antiviral drugs in dogs and cats.

Antiviral drugs can be classified based on their spectrum of activity (i.e., family of viruses that are inhibited) and their mode of action. Antiviral drugs used in small animal medicine are primarily effective against herpesviruses or retroviruses, and most are nucleoside analogues. Nucleoside analogues resemble host nucleosides, which are nitrogenous bases with an attached sugar molecule used as a building block for the formation of DNA or RNA. Use of nucleoside analogues by viruses during replication leads to the formation of abnormal nucleic acids or termination of nucleic acid synthesis. Latent viral infections are not affected because the replication of latent virus is suspended. However, reactivation of these infections can be reduced in frequency or prevented. Other antiviral drugs that have been used in clinical veterinary medicine include amino acids (l-lysine) and neuraminidase inhibitors (oseltamivir). The value of other treatments such as small inhibitory RNA molecules for viral infections of small animals is also under investigation.

Antiviral Drugs in Use in Small Animals

FHV-1, feline herpesvirus-1; HSV, herpes simplex virus; TK, thymidine kinase.

Chemical structures of deoxyguanosine (a nucleoside) and antiherpesviral nucleoside analogues. the grey portion is missing in the nucleoside analogues.


A, Deoxyguanosine. B, Acyclovir. C, Penciclovir. D, Ganciclovir.

Antiherpesviral Drugs

The standard antiviral treatments for herpesviral infections in human patients are the nucleoside analogues acyclovir and penciclovir, and their prodrugs valacyclovir and famciclovir, respectively. These drugs are activated by the herpesviral enzyme thymidine kinase (TK), which phosphorylates them to a monophosphate form. Host cell enzymes then phosphorylate the drugs further to triphosphate forms, which concentrate in virus-infected cells and interfere with viral DNA replication via inhibition of the viral DNA polymerase enzyme. Because the DNA polymerases of different herpesviruses are inhibited to varying degrees by acyclovir triphosphate, not all herpesviral infections are equally susceptible. Drug resistance results from reduced viral TK activity, altered viral TK, or altered viral DNA polymerase. The antiherpesviral drugs cidofovir, idoxuridine, trifluridine, and vidarabine are not dependent on viral TK for phosphorylation and so have greater host cell toxicity. In small animal medicine, these four drugs have been used topically to treat ocular feline herpesviral infections.

Acyclovir and Valacyclovir

Acyclovir is a synthetic analogue of the purine nucleoside deoxyguanosine. In human patients, acyclovir is widely used to treat herpes simplex and varicella-zoster virus infections, both of which are α-herpesviruses. In small animal medicine, acyclovir has primarily been used to treat feline herpesvirus-1 (FHV-1) infections, although FHV-1 has a much lower susceptibility to the drug compared with the human herpesviruses. Acyclovir has also been used to treat canine herpesvirus-1 (CHV-1) infections in puppies. Other nucleoside analogues (trifluridine, idoxuridine, cidofovir, ganciclovir, and penciclovir) show much greater activity than acyclovir against FHV-1 in vitro.

Topical acyclovir has been used with limited success to treat feline herpetic dermatitis and keratitis. Topical treatment requires frequent (>4 times a day) application for beneficial effect. The oral bioavailability of acyclovir is low in cats, and high doses are required to achieve adequate serum drug concentrations. Valacyclovir, the prodrug for acyclovir, is rapidly converted to acyclovir by a first-pass effect after oral administration, and administration of the prodrug results in improved oral bioavailability of acyclovir. Unfortunately, administration of high doses of acyclovir or valacyclovir to cats has resulted in significant toxicity, including myelosuppression, renal tubular necrosis, and hepatic necrosis, without effective suppression of viral replication. Thus the use of systemic acyclovir and valacyclovir to treat cats with herpesvirus infections is not recommended.

Penciclovir and Famciclovir

Of all antiviral drugs used to treat small animal patients, penciclovir and famciclovir have shown the greatest promise. Penciclovir is another guanosine analogue. It is present at much higher concentrations and for longer duration in cells than is acyclovir, which permits less frequent dosing. In humans, the prodrug famciclovir is well absorbed orally and rapidly converted to penciclovir, leading to increased oral bioavailability of penciclovir. Most of the drug is eliminated unchanged in the urine, and so dose reduction may be required for animals with decreased kidney function. In human patients, the concurrent administration of food decreases peak plasma concentrations without affecting overall bioavailability. In humans with herpes simplex virus and herpes zoster infections, famciclovir is as effective as acyclovir. In contrast, penciclovir (and thus famciclovir) have been shown to be potent inhibitors of FHV-1 replication (which is not the case for acyclovir). Penciclovir-resistant mutants of FHV-1 with altered TK enzymes have been described.

Administration of famciclovir to cats at dosages comparable to those used in other species results in much lower plasma concentrations and a longer time to development of peak plasma concentrations of penciclovir when compared with other animal species, which suggests altered absorption or metabolism of famciclovir in cats. High doses of famciclovir have been required to achieve adequate plasma drug concentrations. Nevertheless, famciclovir is well tolerated when administered orally to cats at doses that produce clinical responses. Treatment of cats with experimentally induced FHV-1 conjunctivitis with famciclovir at 90 mg/kg PO q8h for 21 days resulted in lower clinical and pathologic disease scores and decreased viral shedding when compared with placebo-treated cats. Clinical responses also occur in naturally infected cats with both acute and chronic manifestations of disease with negligible adverse effects. Because of saturation of the metabolism of famciclovir to penciclovir, equivalent serum and tear penciclovir concentrations can be achieved in cats with 40 or 90 mg/kg PO q8h of famciclovir, so 40 mg/kg PO q8h is considered equally efficacious. Clinical improvement often occurs within a week of treatment.


Ganciclovir resembles acyclovir except that it has an additional hydroxymethyl group on its acyclic side chain. In human patients, ganciclovir is widely used specifically for the treatment of human cytomegalovirus infections, which can be life threatening in the immunocompromised. Cytomegaloviruses are β-herpesviruses. Systemic administration of ganciclovir to human patients is associated with a high prevalence of adverse drug reactions, especially cytopenias and central nervous system (CNS) signs, and so the use of ganciclovir is limited to patients with life-threatening or sight-threatening infections. A topical ophthalmic gel formulation of ganciclovir (0.15%) is now available for treatment of keratitis caused by herpes simplex virus-1. Ganciclovir is highly inhibitory to FHV-1 replication in vitro. Pharmacokinetic, safety, and efficacy studies in cats have not yet been performed, but topical ganciclovir holds promise for treatment of feline ocular herpesviral infections.


Cidofovir is a nucleotide analogue of deoxycytidine monophosphate. Because the drug already has a monophosphate group, its metabolism to the active diphosphate form by host cellular enzymes is not dependent on viral TK, and so it has been used to treat acyclovir- and penciclovir-resistant infections. In human patients, cidofovir is primarily used to treat cytomegalovirus retinitis. Cidofovir also has efficacy against other DNA virus infections, such as poxvirus and papillomavirus infections, and has been used topically as a cream to treat viral warts in people. Because the oral bioavailability of cidofovir is extremely low (<5%), it is always administered intravenously, intravitreally, or topically. Cidofovir has a prolonged intracellular half-life, which enables infrequent parenteral dosing regimens in human patients (once weekly, and then every other week for maintenance therapy).

Like ganciclovir, cidofovir is highly active in vitro against FHV-1.  Twice-daily administration of a 0.5% cidofovir ophthalmic solution significantly decreases viral shedding and the severity of clinical disease in cats with experimentally induced ocular FHV-1 infection. Local irritation and scarring of the nasolacrimal duct has been reported with topical administration of cidofovir to humans and rabbits with keratoconjunctivitis.

Antiviral Drugs Used for Treatment of Feline Herpesvirus Infections

Idoxuridine and Trifluridine

Idoxuridine and trifluridine are halogenated thymidine analogues that interfere with the replication of FHV-1 in vitro, although they are more potent inhibitors of herpes simplex virus replication. They have been used to treat herpesviral keratitis in humans and in cats. These drugs are highly toxic when given systemically, because host cell and viral DNA synthesis are equally affected. Frequent topical application is required (five to six times daily), and prolonged use can cause corneal irritation or ulceration. Trifluridine has better corneal penetration than idoxuridine and is available as a 1% ophthalmic solution. Unfortunately, trifluridine is expensive, and ocular administration of trifluridine is often extremely irritating to cats. In contrast, topical idoxuridine administration is generally well tolerated.


Vidarabine is an adenosine analogue that is phosphorylated by host cellular enzymes to vidarabine triphosphate, which interferes with DNA synthesis by both the virus and host cells. It is effective against idoxuridine-resistant herpesviral strains because its mechanism of action differs from that of idoxuridine. It is reportedly well tolerated by cats when administered five to six times daily to cats as a 3% ophthalmic ointment.


Lysine is an amino acid that interferes with herpesviral replication by a poorly understood mechanism. Antagonism of arginine may somehow be involved, because a high lysine-to-arginine ratio appears to be important for efficacy. However, arginine itself was also shown to interfere with the replication of herpes simplex virus in vitro.

Lysine has shown efficacy when administered as tablets to cats with FHV-1 conjunctivitis and, in another study, it reduced shedding of reactivated virus by latently infected cats. When administered as tablets to cats in a shelter, there was no reduction in upper respiratory tract disease. There was concern that the stress of tablet administration may have contributed to disease in these cats. However, in two other studies, dietary supplementation with lysine was not effective for management of upper respiratory tract disease in a shelter. In fact, cats that received the lysine-supplemented diet had more severe disease and more frequent viral shedding than cats that received a non-supplemented ration, despite having increased plasma lysine concentrations. The lysine-supplemented diet did not affect plasma arginine concentration, so altered arginine levels did not appear to contribute to the increased severity of disease.

Antiretroviral Drugs

All antiviral drugs used to treat cats with retrovirus infections have been nucleoside analogues, which inhibit the DNA polymerase function of the retroviral reverse transcriptase (RT) enzyme. Unfortunately, many drugs used for treatment of HIV infections (such as protease inhibitors) are not effective for treatment of feline retrovirus infections, because they only act on HIV enzymes. The only antiretroviral that has shown benefit in naturally infected cats is zidovudine (AZT). At the time of writing, an integrase inhibitor known as raltegravir has shown early promise for treatment of FeLV infections both in vitro and in vivo. Many drugs that have activity against feline retroviruses in vitro, such as ribavirin and adefovir (PMEA), are toxic when given to cats, which limits their use in practice.

Zidovudine and Fozivudine

Zidovudine (azidothymidine; AZT; Retrovir) is a thymidine analogue and was one of the first drugs shown to be effective against HIV. Inside host cells, it is converted to the active triphosphate form. AZT inhibits the replication of FIV, reduces plasma viral load, improves stomatitis, and increases CD4/CD8 ratios in cats naturally infected with FIV. Some FIV isolates are resistant as a result of mutations in the RT enzyme. Although AZT is active against FeLV in vitro, when compared with FIV-infected cats, it has not performed as well for treatment of naturally infected, sick cats with chronic FeLV infection. Nevertheless, improvement in stomatitis, reduced antigenemia, and reduction in development of lymphoma have been reported in studies of naturally and experimentally FeLV-infected cats that were treated with AZT.

AZT is available as a10 mg/mL syrup and a 10 mg/mL injection. It has good oral bioavailability and is well distributed to tissues, including the CNS. It is metabolized to an inactive form by the liver and excreted by the kidneys. Dosage reduction has been recommended for cats with renal failure. Unfortunately, some cats treated with AZT can develop dose-related hematologic adverse effects, most commonly nonregenerative anemia and neutropenia, so the CBC must be monitored during treatment. Adverse effects may be confused with retrovirus-induced cytopenias. In human patients with HIV infection, cytopenias are more likely to occur when disease is advanced.

Fozivudine is a thioether lipid-zidovudine conjugate that undergoes intracellular cleavage to zidovudine monophosphate and subsequent phosphorylation to the active triphosphate form. Cleavage preferentially occurs in lymphocytes and monocytes compared with RBC and marrow stem cells, and so hematologic toxicity is less likely to occur. Fozivudine reduces viremia in cats experimentally infected with FIV, without significant adverse effects. Further study is required to evaluate this drug for treatment of chronic FIV and FeLV infections in cats.


Lamivudine (3TC), a cytidine analogue, is synergistic when combined with AZT for treatment of HIV infection, and the combination is in common use in human medicine. AZT/3TC prevented FIV infection when given to cats shortly after experimental inoculation, but did not appear to be beneficial for treatment of cats with chronic FIV infection. In addition, severe hematologic adverse effects and fever occurred in some cats.

Other Antiretroviral Drugs

Raltegravir inhibits the retroviral integrase enzyme. Integrase incorporates transcribed viral DNA into the host chromosome. In human patients, raltegravir is approved for treatment of HIV infections and is used in combination with other antiretroviral drugs. Although expensive, raltegravir has shown great promise for treatment of FeLV infections in vitro and also in vivo. Other drugs that have shown promise in vitro for treatment of FeLV infections, with no evidence of toxicity to cell cultures, are the nucleoside analogues tenofovir, decitabine, and gemcitabine. Tenofovir is used as part of combination therapy to treat HIV infections. It is administered as a prodrug. Decitabine and gemcitabine are cytidine analogues used in human patients to treat myelodysplastic syndromes and carcinomas, respectively. The use of combinations of gemcitabine and carboplatin to treat carcinomas in cats has been reported, but cytopenias and gastrointestinal toxicity occurred in some of the cats.

Plerixafor is a bicyclam derivative that selectively blocks the chemokine receptor, CXCR4. This receptor is used by FIV to enter cells. In a placebo-controlled, masked clinical trial, administration of plerixafor to cats with FIV infection for 6 weeks reduced proviral load but did not lead to improvement in clinical or immunologic variables.

Antiinfluenza Viral Drugs

The main drugs used in human medicine to treat influenza virus infections are neuraminidase inhibitors, such as oseltamivir and zanamivir, and the tricyclic amines amantadine and rimantadine, which inhibit the M2 ion channel protein that is present in influenza A viruses. Only oseltamivir has been used to any great extent in small animals. Amantadine, which also has antagonistic effects at the NMDA (N-methyl-d-aspartate) receptor, has been used to treat osteoarthritis in animals in combination with other drugs.

Structure of an influenza virus. The M2 protein, which is only present in influenza A viruses, is inhibited by tricyclic amines such as amantadine. Oseltamivir inhibits the viral neuraminidase.


Oseltamivir (Tamiflu) is the prodrug of oseltamivir carboxylate (GS4071), a potent inhibitor of influenza virus neuraminidase. Oseltamivir was developed because of the poor oral bioavailability of zanamivir, a neuraminidase inhibitor that is structurally similar to GS4071. Neuraminidase is a surface glycoprotein of both influenza A and influenza B viruses. The viral neuraminidase cleaves sialic acid residues on the surface of infected cells, which allows new virus particles to be released from host cells. It also prevents aggregation of virus particles after they are released and facilitates spread of the virus through the mucus of the respiratory tract by cleaving sialic acid residues in mucin.

In humans and dogs, oseltamivir has high oral bioavailability. Esterase enzymes in the liver then convert oseltamivir to its active form, which is well distributed to most body fluids, including surface epithelial cells throughout the upper and lower respiratory tract. Elimination of the drug relies on renal excretion. The drug is well tolerated in humans, with gastrointestinal signs being the most frequently reported adverse effects.

The prevalence of resistance to oseltamivir among influenza virus isolates is generally low (<5%). High-level resistance results from mutations in the viral neuraminidase. Neuraminidase inhibitors could play a critical role in prevention of mortality in human influenza virus pandemics, so strategies that minimize the selection of resistant mutants are important. The Centers for Disease Control and Prevention recommends prioritizing antiviral treatment to human patients at risk of complications of influenza, such as the very young and very old.

Oseltamivir has been used to treat canine parvovirus (CPV) enteritis in puppies, with anecdotal reports of improved outcome. A single prospective, randomized, masked, placebo-controlled trial of 35 dogs with CPV enteritis showed that dogs treated with oseltamivir (2 mg/kg PO q12h) had no significant drop in their white blood cell count, whereas untreated dogs had a significant drop in their white blood cell count in the first 5 days of hospitalization. Treated dogs also gained weight during hospitalization, whereas untreated dogs lost weight. However, there was no difference in hospitalization time, treatments needed, clinical scores, morbidity, or mortality between the two groups, and the number of dogs in each group was small. No significant adverse drug effects were observed, but oseltamivir was administered as a 1:1 dilution with water, in order to reduce reactions to the taste of the drug and vomiting shortly after drug administration. The authors acknowledged that there were potential concerns that related to administration of an oral medication to dogs with enteritis, with variability in drug absorption.

As CPV has no neuraminidase, it was hypothesized that oseltamivir instead may act on the neuraminidases of bacteria that are normally responsible for secondary bacterial infections in CPV enteritis, which are primarily those of the gastrointestinal tract. The role of bacterial neuraminidases in the pathogenesis of enteric bacterial infections and bacterial translocation is unknown. Bacterial neuraminidase enzymes may play a role in biofilm formation and help bacteria to invade mucin layers of the respiratory tract. It has been suggested that gut bacteria may use neuraminidase enzymes to cleave sialic acid residues on gastrointestinal epithelial cells, which exposes receptor sites for bacterial adherence.

A major concern that relates to treatment of CPV enteritis with oseltamivir is the possibility of selection for resistant mutants among influenza viruses if widespread use of the drug occurs in veterinary clinics. Given the restrictions on the use of this drug for treatment of human influenza virus infections, further investigation is required before the use of oseltamivir can be recommended for treatment of CPV enteritis.