Mechanisms of Antivirals

Question 1

Describe and explain the concepts of selective toxicity in relation to viral infections

Answer 1

Selective toxicity is a concept that refers to the ability of a drug or treatment to selectively kill or inhibit the disease-causing organism without causing significant harm to the host

When it comes to viral infections, achieving selective toxicity is a challenging task due to the nature of viruses

Viruses are intracellular parasites that rely on host cells to reproduce

They utilize the host’s cellular machinery for their replication, which means they do not have as many unique targets for therapeutics as bacteria or fungi

1) Inhibition of viral replication:

• Antiviral drugs often work by inhibiting steps in the viral replication cycle that are not utilised by host cells
• For instance, nucleoside analogues, like acyclovir used for herpes simplex virus, act by being incorporated into the viral DNA during replication and prematurely terminating the growing DNA chain
• These drugs are selectively toxic because human DNA polymerase enzymes have a proofreading ability and can remove the analogues, while viral DNA polymerases cannot

2) Inhibition of viral enzymes:

• target enzymes that are critical for the virus but are not present in the host
• An example of this is protease inhibitors used in the treatment of HIV, which block the activity of HIV-1 protease

3) Blocking viral entry or release:

• preventing the virus from entering host cells or from releasing newly formed viruses
• These drugs are selective as they target viral components or specific interactions between the virus and the host cell
• An example is the influenza drug oseltamivir (Tamiflu), which inhibits the neuraminidase enzyme on the surface of the virus, preventing the release of new viruses from infected cells

Question 2

Describe and explain the generalised life cycles of viruses as obligate intracellular organisms

Answer 2

Viruses are obligate intracellular parasites - they must enter a host cell in order to replicate and propagate

1) Attachment:

• The virus attaches to specific receptors on the surface of a susceptible host cell
• This process is highly specific, with viruses able to bind only to cells with the appropriate receptors
• Determines the host range (which types of organisms the virus can infect) and tissue tropism (which types of cells within an organism the virus can infect)

2) Penetration (Entry):

• penetrates the host cell and delivers its genome inside the cell
• various mechanisms, such as fusion with the cell membrane (in the case of enveloped viruses) or endocytosis

3) Uncoating:

• Once inside the host cell, the viral capsid (protein coat) is removed or degraded to release the viral genetic material

4) Replication:

• The viral genome hijacks the host cell’s machinery to replicate its own genetic material and produce viral proteins
• DNA viruses typically replicate in the nucleus of the host cell, while RNA viruses usually replicate in the cytoplasm
• This process involves transcription (synthesis of mRNA from the viral genome), translation (synthesis of viral proteins using the host’s ribosomes), and replication of the viral genome

5) Assembly (Maturation):

• Newly synthesized viral genomes and viral proteins are assembled into new virus particles

6) Release:

• through budding (where the virus acquires a membrane envelope as it pushes out through the cell membrane)
• or cell lysis (where the cell breaks open and releases the virus particles)

Question 3

Discuss the consideration of potential targets for antivirals

Answer 3

1) Viral Attachment and Entry:

• attachment is facilitated by viral surface proteins that interact with specific receptors on the host cell
• Drugs that block this interaction can prevent viral entry
• E.g. Maraviroc is a CCR5 antagonist that blocks HIV-1 entry into cells by binding to the CCR5 co-receptor on the cell surface, preventing the virus from using this co-receptor for entry

2) Viral Uncoating:

• After the virus has entered the cell, it must uncoat its capsid to release its genomic material for replication
• Inhibitors that target this process can disrupt the uncoating process, thereby halting viral replication

3) Nucleic Acid Synthesis:

• Replication of the viral genome is a crucial step in the viral life cycle
• Nucleoside analogues, for instance, incorporate themselves into the growing viral DNA or RNA chain, causing premature termination of these chains, E.g. Tenofovir and Zidovudine
• Non-nucleoside reverse transcriptase inhibitors (NNRTIs), like Efavirenz and Nevirapine, also inhibit HIV-1 replication by binding to a different site on the reverse transcriptase

4) Integration of Viral DNA:

• In retroviruses such as HIV, the viral RNA genome is reverse-transcribed into DNA, which is then integrated into the host cell’s genome by the viral integrase enzyme
• Integrase inhibitors, such as Raltegravir and Dolutegravir, bind to this enzyme and prevent the integration of the viral DNA into the host genome

5) Protein Processing and Maturation:

• After the production of viral proteins, some viruses require the cleavage of these proteins into their functional forms by viral proteases
• drugs that inhibit these proteases can prevent the virus from producing functional proteins
• E.g. Saquinavir and Ritonavir in HIV treatments

6) Virus Assembly and Release:

• inhibit the assembly and release of the newly synthesized viruses
• Neuraminidase inhibitors, like Oseltamivir and Zanamivir, prevent the release of new influenza virus particles from the infected host cell

Question 4

Describe common examples of anti-viral drugs: e.g. flu, HIV, HepB, and HepC; herpesviruses

Answer 4

1) Influenza Virus:

• Nueraminidase inhibitors e.g. Oseltamivir (Tamiflu) and Zanamivir (Relenza), prevent the release of new virus particles from the infected cells

2) HIV:

• Antiretroviral therapy (ART) is used for treating HIV infection, which is a combination of drugs that prevent the virus from replicating
• Nucleoside reverse transcriptase inhibitors (NRTIs) such as Zidovudine (Retrovir) and Tenofovir (Viread) inhibit the reverse transcriptase enzyme, which the virus uses to convert its RNA into DNA
• Non-nucleoside reverse transcriptase inhibitors (NNRTIs), like Efavirenz (Sustiva), also block the reverse transcriptase enzyme
• Protease inhibitors (PIs) such as Saquinavir (Invirase) and Ritonavir (Norvir) inhibit the HIV protease enzyme for virus replication
• Integrase inhibitors like Raltegravir (Isentress) and Dolutegravir (Tivicay) blocks viral integrase enzyme, preventing the integration of viral DNA into the host’s DNA

3) Hepatitis B and C:

• Commonly prevents the virus from replicating
• For Hepatitis B, antivirals such as Lamivudine (Epivir) and Adefovir (Hepsera) are used
• For Hepatitis C, antivirals such as Ledipasvir/Sofosbuvir (Harvoni) and Ribavirin are used

Herpesviruses:

• Acyclovir (Zovirax) and Valacyclovir (Valtrex) are commonly used to treat both HSV and VZV infections
• Viral DNA polymerase inhibitors

Question 5

Describe the mechanism of action of common antivirals and new direct-acting antivirals

Answer 5

1) Acyclovir:

• Used to treat HSV and VZV infections
• guanosine analogue and, after being phosphorylated by viral thymidine kinase, is incorporated into the viral DNA chain during replication
• leads to premature termination of the DNA chain; inhibiting viral replication

2) HAART:

• includes three drugs from at least two different classes
• nucleoside reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors (NNRTIs), protease inhibitors (PIs), and integrase strand transfer inhibitors (INSTIs)

3) Amantadine:

• treat certain influenza A virus infections, blocks uncoating
• targets the M2 ion channel protein, preventing the acid-mediated dissociation of the viral RNA from the M1 protein, an essential step in the uncoating phase of viral replication

4) Ribavirin:

• synthetic nucleoside analogue with broad antiviral activity
• commonly used in combination with other drugs to treat HCV infection
• phosphorylated in cells to its active triphosphate form, which inhibits the HCV RNA-dependent RNA polymerase, resulting in a reduction in viral replication

5) HCV Protease Inhibitor:

• Direct-acting antivirals like boceprevir, telaprevir, and simeprevir are part of this class
• These drugs inhibit the NS3/4A serine protease, an enzyme required for the proteolytic cleavage of the HCV encoded polyprotein into mature forms

Question 6

Describe and explain the concepts of anti-viral resistance

Answer 6

Use of single agents leads to rapid development of resistance

the drug binding site is altered in structure by as few as one amino acid substitution; high mutation rate & high viral load = resistance

1) Genetic Mutation:

• Viruses have high mutation rates because many do not have the proofreading ability when replicating their genetic material
• This leads to the accumulation of random mutations, some of which may confer resistance to antiviral drugs

2) Selective Pressure:

• When a population of viruses is exposed to an antiviral drug, those that randomly acquire resistance through mutation have a survival advantage and are “selected” over time
• They replicate more efficiently in the presence of the drug, gradually replacing the drug-sensitive viral population

3) Viral Fitness:

• This is the ability of a virus to reproduce effectively.
• Even when resistance mutations occur, they often come at a “cost” of reduced viral fitness but some can maintain high fitness level

4) Compensatory Mutation:

• when a secondary mutation restores the loss of fitness caused by a resistance mutation
• Compensatory mutations can further enhance the spread and survival of drug-resistant viruses

5) Drug concentration and Adherence to Treatment:

• Inadequate drug levels in the body can facilitate the development of resistance, as sub-optimal drug concentrations may not be enough to effectively suppress viral replication
• This can be due to poor drug absorption, drug interactions, or poor adherence to treatment by the patient

Question 7

Describe and explain the concepts of use of antivirals in pre-and post-exposure prophylaxis

Answer 7

Antiviral drugs can be used both for the prevention (prophylaxis) and the treatment of viral infections

Pre-Exposure Prophylaxis (PrEP):

• PrEP, antiviral medication is administered to high risk individuals for a particular viral infection but have not yet been exposed to the virus
• helps to prevent the virus from establishing a productive infection in case an exposure event occurs
• This approach requires continuous use of the antiviral agent and regular health check-ups to assess its efficacy and monitor side effects
• A common example of PrEP is for HIV prevention
• emtricitabine and tenofovir disoproxil fumarate are often combined in a single pill (Truvada), which is taken once daily
• work by inhibiting the reverse transcriptase enzyme, which HIV uses to replicate its genome, hence blocking its life cycle
• PrEP has been shown to reduce the risk of HIV infection via sex by about 99% when taken daily
• the potential for side effects (like nausea, loss of appetite, and potential kidney issues), and the risk of developing drug-resistant strains is a concern

Post-exposure Prophylaxis (PEP):

• use of antiviral medication immediately after potential exposure to a virus, aiming to prevent the establishment and propagation of the virus in the body
• used in emergencies, such as accidental exposure to HIV (e.g., through a needlestick injury in a healthcare setting) or potential exposure to viruses such as hepatitis B and rabies
• For HIV, a combination of three antiretroviral drugs is usually prescribed within 72 hours of potential exposure and must be taken for 28 days
• work by inhibiting the replication of HIV, stopping the virus from establishing a permanent infection
• success depends on early initiation and strict adherence to the medication regimen