**3. Antiviral agent**

Antiviral drugs are a class of medication used specifically for treating viral infections. Like antibiotics for bacteria, specific antivirals are used for specific viruses. Unlike most antibiot‐ ics, antiviral drugs do not destroy their target pathogen; instead they inhibit their develop‐ ment [6].

Most of the antiviral drugs now available are designed to help deal with HIV, herpes virus‐ es (best known for causing cold sores and genital herpes, but actually the cause of a wide range of other diseases, such as chicken pox), the hepatitis B and C viruses, which can cause liver cancer, and influenza A and B viruses. Researchers are working to extend the range of antivirals to other families of pathogens.

their envelope with the target cell, or with a vesicle that transports them into the cell, before

Vaccines and Antiviral Agents http://dx.doi.org/10.5772/56866 243

A very early stage of viral infection is viral entry, when the virus attaches to and enters the host cell [8]. A number of "entry-inhibiting" or "entry-blocking" drugs are being developed to fight HIV. HIV most heavily targets the immune system's white blood cells known as "helper T cells", and identifies these target cells through T-cell surface receptors designated "CD4" and "CCR5". Attempts to interfere with the binding of HIV with the CD4 receptor have failed to stop HIV from infecting helper T cells, but research continues on trying to in‐ terfere with the binding of HIV to the CCR5 receptor in hopes that it will be more effective.

Amantadine and rimantadine, have been introduced to combat influenza. These agents act on penetration/uncoating. They are M2 inhibitors which block the ion channel formed by the M2 protein that spans the viral membrane. The influenza virus enters its host cell by re‐ ceptor-mediated endocytosis. Thereafter, acidification of the endocytotic vesicles is required for the dissociation of the M1 protein from the ribonucleoprotein complexes. Only then are the ribonucleoprotein particles imported into the nucleus via the nuclear pores. The hydro‐ gen ions needed for acidification pass through the M2 channel. Amantadine and rimanta‐

A second approach is to target the processes that synthesize virus components after a virus

One way of doing this is to develop nucleotide or nucleoside analogues that look like the building blocks of RNA or DNA, but deactivate the enzymes that synthesize the RNA or DNA once the analogue is incorporated. This approach is more commonly associated with the inhibition of reverse transcriptase (RNA to DNA) than with "normal" transcriptase

An improved knowledge of the action of reverse transcriptase has led to better nucleoside analogues to treat HIV infections. One of these drugs, lamivudine, has been approved to treat hepatitis B, which uses reverse transcriptase as part of its replication process. Research‐ ers have gone further and developed inhibitors that do not look like nucleosides, but can

Another target being considered for HIV antivirals include RNase H-which is a component

of reverse transcriptase that splits the synthesized DNA from the original viral RNA.

they can uncoat [7].

*3.1.1. Entry inhibitor*

*3.1.2. Uncoating inhibitor*

dine block the channel [9].

**3.2. During viral synthesis**

*3.2.1. Reverse transcription*

still block reverse transcriptase.

invades a cell.

(DNA to RNA).

Inhibitors of uncoating have also been investigated.

Designing safe and effective antiviral drugs is difficult, because viruses use the host's cells to replicate. This makes it difficult to find targets for the drug that would interfere with the vi‐ rus without harming the host organism's cells. Moreover, the major difficulty in developing vaccines and anti-viral drugs is due to viral variation.

**Figure 2.** Virus life cycle and targets of antivirals

#### **3.1. Before cell entry**

One anti-viral strategy is to interfere with the ability of a virus to infiltrate a target cell. The virus must go through a sequence of steps to do this, beginning with binding to a specific "receptor" molecule on the surface of the host cell and ending with the virus "uncoating" in‐ side the cell and releasing its contents. Viruses that have a lipid envelope must also fuse their envelope with the target cell, or with a vesicle that transports them into the cell, before they can uncoat [7].

## *3.1.1. Entry inhibitor*

Most of the antiviral drugs now available are designed to help deal with HIV, herpes virus‐ es (best known for causing cold sores and genital herpes, but actually the cause of a wide range of other diseases, such as chicken pox), the hepatitis B and C viruses, which can cause liver cancer, and influenza A and B viruses. Researchers are working to extend the range of

Designing safe and effective antiviral drugs is difficult, because viruses use the host's cells to replicate. This makes it difficult to find targets for the drug that would interfere with the vi‐ rus without harming the host organism's cells. Moreover, the major difficulty in developing

One anti-viral strategy is to interfere with the ability of a virus to infiltrate a target cell. The virus must go through a sequence of steps to do this, beginning with binding to a specific "receptor" molecule on the surface of the host cell and ending with the virus "uncoating" in‐ side the cell and releasing its contents. Viruses that have a lipid envelope must also fuse

antivirals to other families of pathogens.

**Figure 2.** Virus life cycle and targets of antivirals

**3.1. Before cell entry**

vaccines and anti-viral drugs is due to viral variation.

242 Current Issues in Molecular Virology - Viral Genetics and Biotechnological Applications

A very early stage of viral infection is viral entry, when the virus attaches to and enters the host cell [8]. A number of "entry-inhibiting" or "entry-blocking" drugs are being developed to fight HIV. HIV most heavily targets the immune system's white blood cells known as "helper T cells", and identifies these target cells through T-cell surface receptors designated "CD4" and "CCR5". Attempts to interfere with the binding of HIV with the CD4 receptor have failed to stop HIV from infecting helper T cells, but research continues on trying to in‐ terfere with the binding of HIV to the CCR5 receptor in hopes that it will be more effective.

#### *3.1.2. Uncoating inhibitor*

Inhibitors of uncoating have also been investigated.

Amantadine and rimantadine, have been introduced to combat influenza. These agents act on penetration/uncoating. They are M2 inhibitors which block the ion channel formed by the M2 protein that spans the viral membrane. The influenza virus enters its host cell by re‐ ceptor-mediated endocytosis. Thereafter, acidification of the endocytotic vesicles is required for the dissociation of the M1 protein from the ribonucleoprotein complexes. Only then are the ribonucleoprotein particles imported into the nucleus via the nuclear pores. The hydro‐ gen ions needed for acidification pass through the M2 channel. Amantadine and rimanta‐ dine block the channel [9].

## **3.2. During viral synthesis**

A second approach is to target the processes that synthesize virus components after a virus invades a cell.

#### *3.2.1. Reverse transcription*

One way of doing this is to develop nucleotide or nucleoside analogues that look like the building blocks of RNA or DNA, but deactivate the enzymes that synthesize the RNA or DNA once the analogue is incorporated. This approach is more commonly associated with the inhibition of reverse transcriptase (RNA to DNA) than with "normal" transcriptase (DNA to RNA).

An improved knowledge of the action of reverse transcriptase has led to better nucleoside analogues to treat HIV infections. One of these drugs, lamivudine, has been approved to treat hepatitis B, which uses reverse transcriptase as part of its replication process. Research‐ ers have gone further and developed inhibitors that do not look like nucleosides, but can still block reverse transcriptase.

Another target being considered for HIV antivirals include RNase H-which is a component of reverse transcriptase that splits the synthesized DNA from the original viral RNA.

*3.2.3. Transcription*

drug.

*3.2.4. Translation/antisense*

*3.2.5. Translation/ribozymes*

of the viral life cycle [12].

*3.2.6. Protease inhibitors*

antisense antivirals are in development.

Once a virus genome becomes operational in a host cell, it then generates messenger RNA (mRNA) molecules that direct the synthesis of viral proteins. Production of mRNA is initiat‐ ed by proteins known as transcription factors. Several antivirals are now being designed to block attachment of transcription factors to viral DNA. Kao et al. recently identified a com‐ pound called nucleozin via random screening, which was found to inhibit influenza by in‐ teracting with influenza NP. Nucleozin causes the NPs to aggregate abnormally, and consequently inhibits normal viral transcription, crippling the replication cycle by extension [11]. Examination of a nucleozin analogue revealed that the compound functions by binding to two copies of NP and forming abnormal dimers, causing the proteins to aggregate and preventing them from functioning normally. Nucleozin was also shown to inhibit influenza virus in vitro and in a mouse model, making it a promising candidate for a new antiviral

Vaccines and Antiviral Agents http://dx.doi.org/10.5772/56866 245

Genomics has not only helped find targets for many antivirals, it has provided the basis for an entirely new type of drug, based on "antisense" molecules. These are segments of DNA or RNA that are designed as complementary molecule to critical sections of viral genomes, and the binding of these antisense segments to these target sections blocks the operation of those genomes. A phosphorothioate antisense drug named fomivirsen has been introduced, used to treat opportunistic eye infections in AIDS patients caused by cytomegalovirus, and other

Yet another antiviral technique inspired by genomics is a set of drugs based on ribozymes, which are enzymes that will cut apart viral RNA or DNA at selected sites. In their natural course, ribozymes are used as part of the viral manufacturing sequence, but these synthetic

A ribozyme antiviral to deal with hepatitis C has been suggested, and ribozyme antivirals are being developed to deal with HIV. An interesting variation of this idea is the use of ge‐ netically modified cells that can produce custom-tailored ribozymes. This is part of a broad‐ er effort to create genetically modified cells that can be injected into a host to attack pathogens by generating specialized proteins that block viral replication at various phases

Some viruses include an enzyme known as a protease that cuts viral protein chains apart so they can be assembled into their final configuration, such as Saquinavir (Figure 4). HIV in‐ cludes a protease, and so considerable research has been performed to find "protease inhibi‐ tors" to attack HIV at that phase of its life cycle. Protease inhibitors became available in the 1990s and have proven effective, though they can have unusual side effects, for example

ribozymes are designed to cut RNA and DNA at sites that will disable them.

**Figure 3.** Example of the mechanisms of antivirals: Mechanism of action of azidothymidine (AZT). AZT needs to be phosphorylated, in three steps, to the triphospate form before it can interfere with the reverse transcriptase reaction

#### *3.2.2. Integrase*

Another target is integrase, which splices the synthesized DNA into the host cell genome. There appears to be no functional equivalent of the enzyme in human cells. The biochemical mechanism of integration of HIV DNA into the host cell genome involves a carefully de‐ fined sequence of DNA tailoring (3'-processing) and coupling (joining or integration) reac‐ tions [10]. In spite of some effort in this area targeted at the discovery of therapeutically useful inhibitors of this viral enzyme, there are no drugs for HIV/AIDS in clinical use where the mechanism of action is inhibition of HIV integrase. However there are several promising candidates in several classes of compounds, including nucleotides, dinucleotides, oligonu‐ cleotides and miscellaneous small molecules such as heterocyclic systems, natural products, diketo acids and sulfones, that have been discovered as inhibitors of HIV integrase.
