Antiviral Agents that Prevent Virus Uncoating or Release
Different approches are used to target the initial and final steps of a virus life cycle.
Compare the mechanisms of the discussed antiviral drugs
- The attachment step is targeted by molecules that will block the receptor on the host cell surface, or on the viral capsid region responsible for binding to the host receptor.
- Drugs that target the uncoating step bind to, and inactivate, proteins on the capsid surface responsible for the uncoating.
- The release step is targeted by drugs that inhibit the activity of neuraminidase, an enzyme on the viral surface.
- sialic acid: A derivative of neuraminic acid (a nine-carbon monosaccharide) that is often the sugar part of glycoproteins.
A viral infection starts with entry of the virus into the cell. The entry mechanism is complex, consists of multiple steps and involves host cell structures.
Targeting the Attachment Step
Virus infection starts with a virus attaching to the host cell by binding to a receptor molecule. There are two main strategies used to design antiviral drugs at this step:
- Using molecules that will bind to the cell receptor and inactivate it; thus preventing the virus from attachment. Examples include anti-receptor antibodies or natural ligands that can bind to the receptor.
- Using receptor-like molecules to bind to the virus and inactivate it before it meets the cell. These include anti-virus antibodies (with specificity against the viral structure that binds to the receptor) or synthetic molecules that mimic the receptor.
The search for such drugs, however, is very expensive and time-consuming.
Targeting the Uncoating Step
Another drug target is the uncoating step during viral infection. Uncoating is the process of capsid disintegration, which leads to the release of the genomic material. This step is performed by viral or host enzymes, or by capsid dissociation alone. Drugs that can perform such functions are used against the influenza virus, rhinoviruses (the cause of the common cold), and enteroviruses (gastrointestinal infections, meningitis, etc.). It is believed that such drugs prevent the virus from uncoating by blocking the proteins on the capsid responsible for uncoating, such as ion channel proteins. An example of such a drug is Rimantadine, which blocks the ion channel in the influenza virus. The ion channel has an important role in disintegrating the viral capsid.
Targeting the Release of the Newly Formed Viral Particles
The last step in the virus life cycle—release from the cell—has been targeted by drugs as well. Neuraminidase is an enzyme on the capsid of influenza virus. It cleaves sialic acid from glycoproteins on the surface of the host cell and allows the viral particles to leave the cell. Tamiflu and Relenza are trend names of two drugs used to treat influenza infections by targeting neuraminidase.
Since viruses use many structures in the host cells to replicate, designing or discovering good antiviral drugs that will not affect the eukaryotic cells is a challenging task. Serious side effects are often observed with the use of antiviral drugs, as is resistance against the drugs. Developing drugs that inhibit different steps in the virus life cycle is of critical importance.
Antiviral DNA Synthesis Inhibitors
Inhibiting DNA synthesis during viral replication is another key approach in battling viral infections.
Review the mechanism of action for antiviral DNA synthesis inhibitors and recognize the types of these inhibitors
- Drugs such as acyclovir, are nucleoside analogues that lack a free 3′ group that is needed for the addition of the next nucleotide. When added into a growing DNA chain they stop its synthesis.
- Another drug, foscarnet, mimics pyrophosphates and inactivates the activity of the viral DNA polymerase.
- Resistance can develop against both of these groups of drugs.
- CMV retinitis: An inflammation of the eye’s retina caused by CMV. It can lead to blindness.
Inhibiting DNA synthesis during viral replication is another approach to battle viral infections.
The most common strategy used for this approach is to use molecules that mimic the structure of a nucleoside. The similarity is good enough to ensure its incorporation into the newly synthesized DNA chain. However, the nucleoside analogue lacks free 3′ end needed for the addition of the next nucleotide. This prevents the incorporation of the next nucleotide and terminates the elongation of the DNA chain.
One of the most often used antiviral drugs that works with the described mechanism is acyclovir (aciclovir), a guanosine analogue. It is used to treat herpes simplex virus infections (type 1 and type 2) as well as chicken pox and shingles. It was designed based on nucleosides isolated from a Caribbean sponge. After administration, the molecule gets activated by phosphorylation both by viral and host cell kinases and the resulting nucleotide incorporated into the newly synthesized DNA resulting in premature chain termination. The drug has very low cytotoxicity and there is low resistance to it.
Other drugs that are also nucleoside analogues and have the same mode of actions are ganciclovir (a synthetic analogue of 2′-deoxy-guanosine) and vidarabine(an adenosine analog). However, both drugs are more toxic and have more serious side effects than acyclovir.
Another type of drug that is a DNA synthesis inhibitor is foscarnet. It mimics pyrophosphate and inactivates the activity of the DNA polymerase. This inhibitor is active against the viral DNA polymerases at doses much lower than the ones needed to inhibit the human polymerases. This drug is used in cases of resistance against acyclovir and ganciclovir nucleoside analogue chemicals. It is also used to treat cytomegalovirus infection (CMV) and specifically CMV retinitis.
Another antiviral drug that targets DNA synthesis is hydroxycarbamide, commonly referred to as a hydroxyurea. Hydroxycarbamide can be used an antiretroviral drug against HIV/AIDS. The mechanism of hydroxycarbamide is thought to be based on the reduction of production of deoxyribonucleotides; therefore, inhibiting DNA synthesis. Hydroxycarbamide is thought to inhibit the enzyme ribonucleotide reductase.
Nucleotide and Nonnucleotide Reverse Transcriptase Inhibitors
Reverse transcriptase in viruses is inhibited by nucleoside (nucleotide) analogues or drugs that change the conformation of the enzyme.
Summarize the mechanism of action for reverse transcriptase inhibitors
- Nucleoside and nucleotide inhibitors are competitive substrate inhibitors that mimic the structure of a normal nucleotide but lack the 3′ hydroxyl group needed for the addition of the next nucleotide for DNA elongation.
- Non-nucleotide inhibitors bind to a site different than the active one and cause rearrangements of the protein domains needed for DNA polymerization.
- Mutations in the reverse transcriptase gene can cause resistance to both types of drugs.
- competitive substrate inhibitors: Molecules that bind to the active site of an enzyme and prevent the real substrate from binding to it.
- non-competitive inhibitors: Molecules that bind to sites other than the active site of an enzyme while still being able to indirectly inhibit its function.
- nucleotide: the monomer comprising DNA or RNA biopolymer molecules, consisting of a nitrogenous heterocyclic base; a five-carbon pentose sugar; and a phosphate group
Reverse transcriptase is an enzyme that has the ability to transcribe single-stranded DNA from a single-stranded RNA chain. This is the reverse of the usual flow of information when RNA is synthesized from DNA. Viruses that use reverse transcriptase to convert their genetic material (RNA) into DNA are called retroviruses. One of the most prominent representative of a retrovirus is HIV. Due to the high prevalence of HIV/AIDS in the world, it is important to have drugs that will prevent or cure the infection. This enzyme is also found in tumors and cancer cells.
Drugs that inhibit the function of this enzyme are divided into three groups:
- nucleoside analog reverse transcriptase inhibitors
- nucleotide analog reverse transcriptase inhibitors
- non-nucleoside reverse transcriptase inhibitors
The first two inhibitors act on the same principle. They mimic, respectively, nucleosides or nucleotides but lack a free hydroxyl group at the 3′ end. The major difference between them is that the nucleosides need to be phosphorylated by cellular kinases. The enzyme reverse transcriptase recognizes them as regular nucleotides and inserts them into the newly synthesized DNA chain. But once inserted the elongation stops at them because no more nucleotides can be added due to the lack of the 3′ hydroxyl group and the inability of the formation of 5′-3′ phosphodiester bond. This process is called chain termination. Nucleoside and nucleotide inhibitors are also called competitive substrate inhibitors. Examples of such drugs are Zidovudine (AZT) and Lamivudine. AZT was the first FDA approved drug for the treatment of HIV. Lamivudine is used for the treatment of both HIV and hepatitis B. Since some viruses, such as hepatitis B, carry RNA-dependent DNA polymerases reverse transcriptase inhibitors can be used to treat these infections as well.
Non-nucleotide reverse transcriptase inhibitors bind to a different site, not the active one, of the reverse transcriptase enzyme. That leads to conformational changes that distort the position of the DNA binding sites in the enzyme and lead to halt in DNA polymerization. Non-nucleotide inhibitors are non-competitive inhibitorsof reverse transcriptase. Such drugs are Efavirenz and Nevirapine.
Resistance occurs to all drug groups. The mechanisms for resistance against the nucleoside (nucleotide) inhibitors are two. The first one is due to mutations in the N-terminal polymerase domain of the reverse transcriptase that makes it less likely to incorporate the analogues. The second mechanism is caused by mutations in the transcriptase that allow the removal of the incorporated inhibitor and hence restart of DNA replication.
Resistance to the non-nucleotide inhibitors is caused by mutations in the inhibitor binding site of the enzyme. Such mutations prevent the binding of the inhibitor to the enzyme.
Protease inhibitors target viral proteases which are key enzymes for the completion of viral maturation.
Describe the mechanism of action for protease inhibitors
- Protease inhibitors mimic peptides or are chemicals that can be inserted in the active site of a protease. They prevent it from binding the viral polyproteins.
- Such drugs were one of the first to be used against HIV. They are an inseparable part of the HIV/AIDS therapy.
- Mutations in the enzyme active site and other sites, which cause conformational changes, can cause resistance.
- cross-resistance: Bacterial or viral resistance to a chemical which causes resistance to other chemicals of the same group.
Proteases are enzymes that have the ability to cut proteins into peptides. They are used by some viruses (e.g., HIV) to cleave precursor long protein chains into individual proteins. This allows the completion of the assembly step in the viral life cycle where the proteins and the viral RNA come together to form virion particles ready to exit the cell.
The design of protease inhibitors, that could be used to battle HIV, started soon after the discovery of the virus. The first approved protease inhibitor drug was released on the market in 1995, only 10 years after the discovery of HIV. These drugs are an inseparable part of an HIV therapy. Natural protease inhibitors are found in Shiitake mushrooms. The experimental protease inhibitor drugs Zmapp and Brincidofovir are currently being tested to treat the ebola virus disease.
Protease inhibitors are short peptide-like molecules that are competitive inhibitors of the enzyme. Instead of -NH-CO- peptide link, they contain -(CH2-CH(OH)-). When such a peptide gets into the enzyme active site, the protease is unable to cut the linkage and gets inactivated. This leads to a lack of cleavage of the polypeptide chains of two crucial viral proteins, Gag and Pol, which are essential structural and enzymatic proteins of HIV. Their absence blocks the formation of mature virion particles.
Saquinavir is the first clinically used peptide-like inhibitor. Some protease inhibitors do not mimic peptides in their structure. One such drug is Nelfinavir. In general, protease inhibitors exhibit the unusual side effect of fat storage in non-typical organs and tissues. The reasons for this are still unclear.
Mutations in the enzyme active site and other sites, which cause conformational changes, can cause resistance. Quite often one mutation can lead to resistance to many different drugs simultaneously since they all share the same mode of action. This is called cross-resistance. It is one of the major drawbacks of protease inhibitors therapy.