Chapter 51 Antiviral Agents

Abbreviations
AIDS	Acquired immunodeficiency syndrome
AZT	Zidovudine (formerly known as azidothymidine)
CMV	Cytomegalovirus
CNS	Central nervous system
CSF	Cerebrospinal fluid
DNA	Deoxyribonucleic acid
GI	Gastrointestinal
HAART	Highly active antiretroviral therapy
HIV	Human immunodeficiency virus
HPV	Human papilloma virus
HSV	Herpes simplex virus
IM	Intramuscular
IV	Intravenous
NNRTIs	Non-nucleoside reverse transcriptase inhibitors
NRTIs	Nucleoside reverse transcriptase inhibitors
RNA	Ribonucleic acid
RSV	Respiratory syncytial virus
SC	Subcutaneous

Therapeutic Overview

Viruses are responsible for significant morbidity and mortality in populations worldwide. These infectious agents consist of a core genome of nucleic acid (nucleoid) contained in a protein shell (capsid), which is sometimes surrounded by a lipoprotein membrane (envelope) (Fig. 51-1). Viruses cannot replicate independently. Rather, they must enter cells and use the energy-generating, deoxyribonucleic acid (DNA)-or ribonucleic acid (RNA)-replicating, and protein-synthesizing pathways of the host cell to replicate. Some viruses can integrate a copy of their genetic material into host chromosomes, achieving viral latency, in which clinical illness can recur without reexposure to the virus.

FIGURE 51–1 Basic components of virus particles.

Some genera of viruses that cause human infections are listed in Table 51-1. Also listed is information about which genomic material—RNA or DNA—is present and examples of clinically important diseases attributed to each virus.

TABLE 51–1 Virus Groups of Clinical Importance

Virus Genera or Groupings	Nucleic Acid	Clinical Examples of Illnesses
Adenovirus	DNA	Upper respiratory tract and eye infections
Hepadnaviridae	DNA	Hepatitis B, cancer
Herpesvirus	DNA	Genital herpes, varicella, meningoencephalitis, mononucleosis, retinitis
Papillomavirus	DNA	Papillomas (warts), cancer
Parvovirus	DNA	Erythema infectiosum
Arenavirus	RNA	Lymphocytic choriomeningitis
Bunyavirus	RNA	Encephalitis
Coronavirus	RNA	Upper respiratory tract infections
Influenzavirus	RNA	Influenza
Paramyxovirus	RNA	Measles, upper respiratory tract infections
Picornavirus	RNA	Poliomyelitis, diarrhea, upper respiratory tract infections
Retrovirus	RNA	Leukemia, AIDS
Rhabdovirus	RNA	Rabies
Togavirus	RNA	Rubella, yellow fever

The way antiviral agents act is not always known. Most currently available antiviral drugs interfere with viral nucleic acid synthesis, regulation, or both; however, some agents work by interfering with virus cell binding, interrupting viral uncoating, or stimulating the host immune system. Because viruses generally take over host cell nucleic acid and protein replication pathways before clinical infection is discovered, antiviral drugs often must penetrate cells that are already infected to produce a response. Side effects to healthy cells can occur when the drug penetrates into them and disrupts normal nucleic acid or protein synthesis. This toxicity limits clinical utility of many drugs.

In vitro susceptibility testing of antiviral compounds differs significantly from antibacterial agents, because viruses require host cells to replicate. Generally greater than a 50% reduction in plaque-forming units at an achievable serum concentration qualifies a drug to be classified as active against a given virus. In recent years the polymerase chain reaction has provided the technology

to allow detection of individual virus mutations, allowing physicians to predict viral susceptibility to many antiviral agents.

Many antiviral agents inhibit single steps in the viral replication cycle. They are considered virustatic and do not destroy a given virus, but rather, temporarily halt replication. For an antiviral agent to be optimally effective, the patient must have a competent host immune system that can eliminate or effectively halt virus replication. Patients with immunosuppressive conditions are prone to frequent and often severe viral infections that may recur when antiviral drugs are stopped.

Prolonged suppressive therapy is often necessary. Currently there is no antiviral agent that eliminates viral

Therapeutic Overview
Approaches to treatment of viral infections include:
Block viral attachment to cells
Block uncoating of virus
Inhibit viral DNA/RNA synthesis
Inhibit viral protein synthesis
Inhibit specific viral enzymes
Inhibit viral assembly
Inhibit viral release
Stimulate host immune system

latency. Strains of viruses resistant to specific drugs can also develop.

Several agents are converted in the body to active compounds (acyclovir, ganciclovir) or must be present continuously to have an antiviral effect (amantadine). Other important considerations include drug distribution, duration of infection, and difficulty of administration. Approaches to the treatment of viral infections with drugs are summarized in the Therapeutic Overview Box.

Mechanisms of Action

Viral Replication Cycle

Understanding the steps involved in virus infection and replication has led to the development of drugs that interfere with this process at various sites. This replication cycle and sites of action for the major classes of antiviral drugs are illustrated for the human immunodeficiency virus (HIV) in Figure 51-2.

FIGURE 51–2 Replication of the HIV virus and sites of action of antiviral drugs.

A virus first binds to an appropriate host cell to initiate an infection. It then penetrates the cell and promotes the synthesis of viral components by controlling host protein and nucleic acid synthesis. Virions are then formed and released to infect other cells. In the situation of HIV infection, infectious virions bind to appropriate host cell receptors. The viral genome crosses into the cell, uncoats, and disassembles. An HIV-specific reverse transcriptase converts viral RNA into DNA, and an integrase incorporates the DNA into the cell’s chromosomes. The host cell then produces a copy of the HIV genome for packaging into new virions and viral messenger RNA, which is the template for protein synthesis. An HIV-specific protease hydrolyzes a viral polyprotein into smaller subunits, which then assemble to form mature infectious virions.

Four classes of compounds have been used to interfere with the HIV reproductive cycle. These include fusion inhibitors, nucleoside reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors (NNRTIs), and protease inhibitors. Anti-HIV therapy with multiple agents has been very effective in limiting the progression to acquired immunodeficiency syndrome (AIDS) in persons carrying HIV.

Other viruses also contain unique enzymes or metabolic pathways that make them susceptible to certain drugs. For example, herpes simplex virus (HSV) encodes a thymidine kinase that monophosphorylates acyclovir significantly better than does the host cell enzyme. Because acyclovir monophosphate is trapped in cells, it becomes highly concentrated. This causes significant inhibition of viral growth with few side effects on cells that do not contain the herpes simplex thymidine kinase. Because cytomegalovirus (CMV), another herpes family virus, does not encode such a thymidine kinase, it is inhibited only by concentrations of acyclovir that are not tolerated clinically; therefore acyclovir is not effective in treating CMV.

Inhibitors of Cell Penetration

Enfuvirtide

HIV entry into cells is accomplished by a complex series of virus host interactions. Initially, virus approximates the CD4 cell by interactions of HIV surface protein and the host CD4 receptor. After approximation, host coreceptors interact with HIV surface proteins, resulting in folding of the HIV protein gp41. This folding results in fusion of the HIV membrane with the host cell membrane and insertion of the HIV nucleoid into the cell. Enfuvirtide prevents entry of HIV into cells by lying along the gp41 coils, causing steric hindrance of protein folding. Resistance occurs when mutations of gp41 occur that alter conformation and folding.

Other antiviral drugs that work by inhibiting viral entry into cells include docosanol and palivizumab. Docosanol is a topical agent used to treat HSV. Sold over-the-counter for topical use, this compound prevents virus entry by inhibiting fusion of the HSV envelope with the plasma membrane of the host cell. Palivizumab is a human monoclonal antibody directed against an epitope in the A antigen site on the F surface protein of the HSV. Clinical resistance to palivizumab has not been observed, although it has been reported in in vitro studies.

Inhibitors of Viral Uncoating

Amantadine and Rimantadine

The structure of amantadine is shown in (Fig. 51-3). Its mechanism of action is not fully established but appears to involve blocking the ion channel activity of the M₂ protein, thereby inhibiting late-stage uncoating of influenza A virions. This drug is not effective against influenza B, which lacks the M₂ protein. A single amino acid change in the M₂ protein results in amantadine resistance. Resistant virus is virulent and causes disease in exposed people. Rimantadine is a related compound with similar actions but an improved side effect profile.

FIGURE 51–3 Structures of some antiviral drugs.

Inhibitors of Viral DNA and RNA Synthesis

Acyclovir

Acyclovir (see Fig. 51-3) is a synthetic guanosine analog and is the prototypical agent for this group of anti-HSV drugs. The group includes the related drugs valacyclovir and famciclovir, a prodrug for penciclovir. All of these drugs must be phosphorylated to be active and are initially monophosphorylated by viral thymidine kinase. Because the thymidine kinases of HSV types 1 and 2 are many times more active on acyclovir than host thymidine kinase, high concentrations of acyclovir monophosphate accumulate in infected cells. This is then further phosphorylated to the active compound acyclovir triphosphate. The triphosphate cannot cross cell membranes and accumulates further. The resulting concentration of acyclovir triphosphate is 50 to 100 times greater in infected cells than in uninfected cells.

Acyclovir triphosphate inhibits virus growth in three ways. First, it competitively inhibits DNA polymerases, with human DNA polymerases being significantly less susceptible than viral enzymes. Second, it terminates DNA elongation. Third, it produces irreversible binding between viral DNA polymerase and the interrupted chain, causing permanent inactivation.

The result is a several hundred-fold inhibition of HSV growth with minimal toxic effects on uninfected cells. However, HSVs with altered thymidine kinase (acyclovir-penciclovir resistant) have developed, although they occur primarily in patients receiving multiple courses of therapy. These mutants are susceptible to the antiviral drug, foscarnet. Changes in viral DNA polymerase structures can also mediate resistance to acyclovir.

Ganciclovir

The structure of ganciclovir is shown in Figure 51-3. It is also a synthetic guanosine analog active against many herpesviruses and must also be phosphorylated to be active. Infection-induced kinases, viral thymidine kinase, or deoxyguanosine kinase of various herpesviruses can catalyze this reaction. After monophosphorylation, cellular enzymes convert ganciclovir to the triphosphorylated form, and the triphosphate inhibits viral DNA polymerase rather than cellular DNA polymerase. Ganciclovir triphosphate competitively inhibits the incorporation of guanosine triphosphate into DNA. Because of its toxicity and the availability of acyclovir for treatment of many herpesvirus infections, its use is currently restricted to treatment of CMV retinitis.

Foscarnet

Foscarnet (see Fig. 51-3) inhibits DNA polymerases, RNA polymerases, and reverse transcriptases. In vitro it is active against herpesviruses, influenza virus, and HIV. Foscarnet is used primarily in treatment of CMV retinitis. Viral resistance is attributable to structural alterations in CMV DNA polymerase. Foscarnet inhibits CMV herpesviruses that are resistant to acyclovir and ganciclovir.

Ribavirin

Ribavirin is a synthetic purine nucleoside active against many viruses, including respiratory syncytial virus, Lassa fever virus, and influenza viruses (see Fig. 51-3). Ribavirin appears to be phosphorylated in host cells by host adenosine kinase. The 5’-monophosphate subsequently inhibits cellular inosine monophosphate formation, resulting in depletion of intracellular guanosine triphosphate. In some situations ribavirin triphosphate suppresses guanosine triphosphate-dependent capping of messenger RNA, thereby inhibiting viral protein synthesis. It also acts by suppressing the initiation or elongation of viral messenger RNA. Exogenous guanosine can reverse the antiviral effects of ribavirin with some viruses.

Nucleoside Reverse Transcriptase Inhibitors and Nucleotides

These drugs include zidovudine, didanosine, lamivudine, stavudine, and others (Box 51-1), and all work through a similar mechanism. Zidovudine (AZT) is the prototype for use in HIV infection. It is a thymidine analog that is phosphorylated to monophosphate, diphosphate, and triphosphate forms by cellular kinases in infected and uninfected cells. NRTIs have two primary methods of action. First, the triphosphate form acts as a competitive inhibitor of HIV reverse transcriptase. Second, after the nucleoside is incorporated into the elongating DNA chain, the forming sugar phosphate backbone of the DNA is blocked from further elongation by substitution at the 3 position. This results in chain termination. In the case of zidovudine, this substitution is an azido (N3) group. Zidovudine inhibits HIV reverse transcriptase at much lower concentrations than those needed to inhibit cellular DNA polymerases, leading to a more targeted effect against HIV.