Mechanisms of Antivirals

Question 1

Learning outcomes

Answer 1

• Describe aspects of viral biology that makes treatment possible
• List the main classes of anti-viral drugs and their mechanisms of action
• List the main indications for anti-viral treatments
• Demonstrate understanding of the challenges of anti-viral treatment in clinical practice

Question 2

What is a virus

Answer 2

Sub-microscopic infectious agents – 80-1400nm diameter
• Obligate intracellular parasites
– rely on the metabolic processes of host cells

Question 3

How do we classify viruses?

Answer 3

Some descriptive classifications based on: Route of transmission (e.g. arboviruses)
The diseases they cause (e.g. viral haemorrhagic fevers) Size / shape (e.g. filoviruses)
Appearance of the capsid (e.g. icosahedral vs. helical) Presence or absence of a lipid envelope

Baltimore classification system:
based on the mechanism of mRNA production

Question 4

Discuss some classes of viruses based on nucleic acid

Answer 4

Retroviruses: are ‘reverse transcribed’ to DNA and integrated to the host genome

DNA viruses: require transcription to mRNA

Positive sense (+) RNA viruses: contain RNA in 5’-3’ orientation which can be directly used as mRNA for translation into proteins

Negative sense (-) RNA viruses: contain RNA in 3’-5’ orientation which requires conversion to 5’-3’ before translation into proteins

Question 5

Discuss the mechanism of action of anti viral therapy

Answer 5

Virucides
Detergents, organic solvents, UV light • Cryotherapy, laser, podophyllin
Anti-viral drugs
Ineffective vs. non-replicating / latent viruses
Immunomodulators
Replace deficient host response Enhance endogenous response
(Reduce harmful host immune response)

Question 6

Discuss the stages in viral life cycle

Answer 6

All potential targets for anti-viral drugs

1. Attachment 2. Penetration
2. Disassembly 4. Transcription* 5. Translation
3. Replication
4. Assembly
5. Release

Question 7

Discuss some targets for anti viral drugs

Answer 7

Host cell entry (HIV
CCR5 antagonist – e.g. maraviroc Fusion inhibitor – e.g.enfuviritide (T20))

Viral uncoating /disassembly (Influenza
M2 ion channel inhibitors – e.g. amantadine / rimantadine)

Viral replication (Several
Viral polymerase inhibitors – e.g. aciclovir for herpes simplex, NRTI, NNRTIs for HIV
Integrase inhibitors & protease inhibitors for HIV)

Viral release (Influenza
Neuraminidase inhibitors – e.g. oseltamivir, zanamivir)

Question 8

Discuss nucleoside analogues as antiviral drugs

Answer 8

A common way to prevent viral replication is to interfere with nucleic acid replication, transcription and translation
This is often done using ‘analogue’ compounds which compete with essential nucleosides for binding sites in these processes

Question 9

Discuss considerations for anti-viral therapy

Answer 9

Effectiveness and aim of therapy
Viral suppression or eradication?
Does the medication improve clinical outcomes?
Does every patient need it or are there ‘high risk’ groups?
Toxicity and side effects
Drug-drug interactions
Emergence of resistance
Depends on the virus and the drug Possibly overcome by multi-drug therapy

Question 10

Discuss anti viral drug resistance

Answer 10

Resistance results from mutations within the viral genome
Selective drug pressure → resistant viral population
Development of resistance favoured by:
– High viral load
– High intrinsic viral mutation rate (error prone, esp. RNA viruses) – Degree of selective drug pressure
– ‘Resistance’ barrier of drug class / individual agent
– Antiviral target that can mutate without affecting fitness

Question 11

Discuss the basic virology of HIV

Answer 11

(+)ss RNA-RT retrovirus
Contains Reverse Transcriptase (RT) - an RNA-dependent DNA polymerase, which makes a DNA copy of the viral RNA
The DNA copy is integrated into the genome of the host cell (often CD4 cells)
This provirus DNA is transcribed into both new genomic RNA and mRNA for translation into viral proteins using host cell machinery
CD4 cells killed by invading virus and host becomes dangerously immune-suppressed
Another human retroviruses is Human T Lymphtrophic Virus (HTLV). Some RNA retroviruses can transform normal cells into malignant cells

Main routes of transmission Sexual, Parenteral, Vertical
Since the 1980s:
76 million people infected 33 million people have died
~38 million people currently live with HIV Africa accounts for 2/3 of those

Question 12

Make a slide abt life cycle of HIV (slide 22)

Answer 12

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Question 13

Discuss HIV entry inhibitors

Answer 13

Fusion inhibitor
Enfuviritide - T20
Synthetic peptide
Subcutaneous administration Side effects – rarely used first line

CCR5 antagonist
Maraviroc Binds to CCR5

Question 14

What are the two classes of reverse transcriptase inhibitors

Answer 14

Nucleoside Reverse Transcriptase Inhibitors (NRTIs)
Nucleoside analogues which compete with reverse transcription preventing viral pro-DNA synthesis
Also affect host cell DNA synthesis causing toxicity

Non-Nucleoside Reverse Transcriptase Inhibitors (NRTIs)
Drugs which bind directly to RT causing conformational change which stops the enzyme from working

Question 15

Provide examples of NRTIs

Answer 15

This class includes:
• Tenofovir(TDF/TAF)–analogueofadenosine
• Emtricitabine(FTC)–analogueofdeoxycytidine • Abacavir(ABC)–analogueofguanosine
• Lamivudine(3TC)–analogueofcytosine
• Zidovudine(AZT)–analogueofthymidine
They are prodrugs and require intracellular phosphorylation by viral and/or cellular kinases to convert them from the 5’-monophosphate form to active 5’-triphosphates

Question 16

Give some examples of NNRTIs

Answer 16

This class includes:
• Efavirenz
• Nevirapine 
• Rilpivirine 
• Doravirine 
• Etravirine

First generation (Efavirenz and Nevirapine) have a low barrier to resistance 
Drug interactions are common
EFV and RPV can result in neurologic and psychiatric AE

Question 17

Discuss integrate inhibitors

Answer 17

HIV integrase mediates two critical reactions:
• 3’end processing of the double-stranded viral DNA ends
• Strand transfer which joins the viral DNA to the host chromosomal DNA forming a provirus
Four drugs in clinical use:
Raltegravir
Dolutegravir
Elvitegravir
Bictegravir

Question 18

Discuss HIV protease inhibitors

Answer 18

• Host mRNAs code directly for functional proteins
• In HIV, the mRNA is translated into biochemically inert proteins
• Virus-specific protease→various functional proteins
• Since the protease does not occur in the host, it is a good selective- toxicity target
• Examples:
– Darunavir, atazanavir, lopinavir
– Administered with PK enhancer (ritonavir, cobicistat)
• High barrier to resistance, drug interactions frequent
• Diarrhoea common

Question 19

Discuss antiretroviral therapy (ART)

Answer 19

Typically give 3 drugs: often two NRTIs and an additional drug from another class (NNRTI, PI or Integrase Inhibitor)
More recently consideration of fewer drugs from newer, more effective drug classes
Previously “defer ART until low CD4 count”
Now – early ART initiation improves long-term outcomes1-3
• Reduction in sexual and vertical transmission of HIV
• Restore and preserve immunological function

Question 20

Discuss Hepatitis C

Answer 20

Flaviviridae family (+) ss RNA virus
Primarily transmitted through parenteral route, sexual transmission also recognised
Causes acute and chronic hepatitis, cirrhosis and hepatocellular carcinoma
Mainstay of therapy for many years was pegIFN-α & ribavirin for up to a year These treatments were toxic and poorly efficacious
Much more effective therapies are now available

Key principles:
Treatment is now short & all oral Stratification by
• Viral genotype
• Severity of liver disease
Don’t try to remember the detail!

Question 21

Discuss Hepatitis B virus

Answer 21

Hepadnavirus
Enveloped, partially dsDNA virus, genome
maintained in circular confirmation
Replication occurs via RNA intermediate (template for HBV polymerase)
Causes acute and chronic hepatitis, cirrhosis and hepatocellular carcinoma

Question 22

Discuss treatment of chronic HBV

Answer 22

Immunomodulatory - pegylated IFNα
– Recombinant version of human IFN
– Variable response
– Finite treatment – 48 weeks
– AE = flu like illness, myalgia, depression, autoimmunity

Nucleos(t)ide therapies
– Agents active against RT step
• Tenofovir – analogue of adenosine 5’ monophosphate
• Entecavir – deoxyguanosine analogue
• Lamivudine – cytidine analogue
– Complete clearance of chronic HBV is rare: the aim is usually long-term control

Question 23

Discuss herpesviruses

Answer 23

dsDNA viruses - 8 which infect humans
• HSV-1 and HSV-2
• VZV – chickenpox and shingle
(these two are neurotropic, skin and mucous membranes)
• EBV and CMV – infectious mononucleosis
• HHV-6 and HHV-7 – febrile exanthem
• HHV-8 (similar to EBV – lymphoproliferative)
All can persist in latent state following primary infection

Question 24

Discuss aciclovir

Answer 24

Deoxyguanosine analogue – active against herpesvirus infections HSV1/HSV2 > VZV&raquo_space; CMV/EBV
Uptake and phosphorylation facilitated by viral thymidine kinase (TK) Competitively inhibits viral DNA polymerase
Incorporated into viral DNA → chain termination → block viral DNA synthesis AE – CNS toxicity, renal impairment
Not active against latent virus – reactivation can occur

Question 25

Discuss uses of aciclovir

Answer 25

• Aciclovir is used for the treatment of herpes simplex virus and varicella zoster virus infections, including:
• Genital herpes simplex (treatment and prevention)
• Herpes simplex labialis (cold sores)
• Shingles
• Acute chickenpox in immunocompromised patients
• Herpes simplex and varicella zoster encephalitis
• Acute mucocutaneous HSV infections in immunocompromised patients
• Herpes of the eye and herpes simplex blepharitis (a chronic form of herpes eye infection)
• Prevention of herpes viruses in immunocompromised people (such as people undergoing cancer chemotherapy)

Question 26

What are some other herpesvirus antivirals

Answer 26

Valaciclovir – prodrug of aciclovir – Higher oral bioavailability

Ganciclovir/valganciclovir
– Deoxyguanosine analogue – differs from aciclovir as has additional hydroxymethyl group on side chain
– Potent inhibitor of CMV replication
– AE = myelosuppression, CNS toxicity

Foscarnet
– Directly inhibits herpesvirus DNA polymerase or HIV RT – AE = nephrotoxicity, electrolyte abnormalities

Question 27

Discuss influenza

Answer 27

Orthomyxoviridae family
Enveloped (-) ssRNA viruses 3 distinct types: A, B and C

Seasonal epidemics and sporadic pandemics Variable severity
mild coryza → life-threatening pneumonia Elderly & immune-compromised at highest risk

Question 28

Discuss neuraminidase inhibitors

Answer 28

Oseltamivir - oral Zanamivir – inhalational

NA – cleaves a sialic acid receptor on host cell enabling viral release

NA inhibitors – competitively bind the NA binding site, preventing viral release

Question 29

Discuss other anti-influenza drugs

Answer 29

Amantadine/rimantadine
block M2 ion channel necessary for
– fusionofviralmembraneand endosome membrane
– latestageofassemblyand release of new virions
Not used: high-level resistance

Baloxavir
Viral polymerase complex inhibitor Not currently used in UK

Question 30

Discuss SARS CoV-2

Answer 30

Fuck this virus man

(+) ssRNA virus
4 structural proteins: S,E,M,N
Spike (S) protein facilitates host cell entry via ACE2 receptor Nucleocapsid (N) protein contains viral genome
Responsible for ongoing global COVID-19 pandemic
Oxygen & ventilatory support is key in severe disease Ongoing therapeutics research

Question 31

What are some medicines for COVID-19

Answer 31

Remdesivir
Adenosine analogue Initial optimism
WHO now advise against

Molnupiravir
Promotes errors in viral replication
May be active in early disease to prevent hospitalisation

Monoclonal antibodies Regeneron
(casirivimab + imdevimab)

Question 32

Summary

Answer 32

Viruses are a major cause of infectious disease morbidity and mortality
Modern classification is based on nucleic acids and the mechanism of mRNA production
Approaches to treatments include virucides, anti-viral drugs and immunomodulators
Anti-viral drugs can be targeted at each stage of the viral life cycle
Aims of therapy
Viral eradication - ? Hepatitis C, influenza, SARS-CoV2
Long-term suppression – ? HIV, Hepatitis B
Management of flares / reactivations of disease - ? herpesviruses