4. Viral Characteristics
Intracellular life
No organelles
Require specific receptors to attach (viral
tropism)
Exploit host enzymes for replication
Virus-specific targets do exist
High level of mutation
5. Target: Life Cycle
Stages
1. Attachment
2. Fusion
3. Uncoating
4. Reverse transcription
5. Integration
6. Transcription
7. Translation to protein
8. Protein modification
9. Viral Assembly
10. Viral release
6. HIV Replication Video
HIV Replication 3D Medical Animation (5min)
https://www.youtube.com/watch?v=RO8MP3wMvqg
19. 2. Fusion inhibitors
Enfuvirtide
Binds HIV gp41 hence preventing fusion with host cell
membrane
Palivizumab
Monoclonal antibody - binds F protein of RSV preventing
fusion
Docosanol
Prevents fusion of HSV envelope with host cell membrane
20. 3. Agents that block
Uncoating
Amantadine/ Rimantadine
Block M2 ion channels – a viral protein
Prevents acidification of the endosome (required for
uncoating)
Active on Influenza A only, not B or C
Resistance is widespread especially in H1N1 and H3N2
strains
21. 4. Nucleic acid
synthesis inhibitors
A. DNA Synthesis Inhibitors
DNA polymerase (DdDp) inhibitors
Reverse transcriptase (RdDp) inhibitors
B. DNA Integration inhibitors
C.RNA Synthesis inhibitors
22. 4(a) DNA Synthesis
Inhibitors
(i). DNA Polymerase Inhibitors (DdDp)
Incorporated into DNA chain by DNA polymerase causing
termination
i. Nucleoside analogues
Acyclovir, Idoxuridine
ii. Nucleotide analogues
Cidofovir
Pyrophosphate (PPi) analogues:
Block the PPi binding site of the viral DNA polymerase
Foscarnet (HSV & CMV)
DNA Synthesis Inhibitors
23. Idoxyuridine
First antiviral agent (1963)
A pyrimidine analogue
Inhibits viral DNA polymerase
Non-selective – also inhibits host DNA
polymerase
Largely replaced by less toxic drugs e.g.
DNA Synthesis Inhibitors
24. Acyclovir
Acyclic nucleoside analogue
Active against HSV-1, HSV-2 and VZV
A pro-drug, undergoes 3 phosphorylation steps to its active
form
Initial phosphorylation by Viral thymidine kinase
Subsequent di & tri phosphorylation by host-cell enzymes
Acyclovir
DNA Synthesis Inhibitors
25. Acyclovir…
MoA: Inhibition of DNA Polymerase
Acyclovir triphosphate competes with endogenous
nucleotides for DNA Polymerase
Incorporated into the growing DNA chain (viral genome)
Causes chain termination once incorporated
DNA Synthesis Inhibitors
26. Acylovir
Mechanisms of Resistance:
Reduced production of Viral Thymidine kinase
(TK)
Altered TK hence reduced affinity for Acyclovir
Altered DNA polymerase
DNA Synthesis Inhibitors
27. (ii) Reverse Transcriptase Inhibitors
Block DNA synthesis from RNA by Reverse
transcriptase (aka RNA-dependent DNA
polymerase)
i. Nucleoside Reverse Transcriptase Inhibitors
(NRTIs)
Lamivudine, Zidovudine, Abacavir
ii. Nucleotide RT Inhibitors (NtRTIs)
Tenofovir (HIV)
iii. Non-Nucleoside RT Inhibitors
Nevirapine, Efavirenz, Etravirine
Tenofovir and Lamivudine (+DTG) are part of the 1st line ARV regimen in adults
DNA Synthesis Inhibitors
28. MoA: RT Inhibitors
HIV: Mechanisms of Action of NRTIs, e.g.
Tenofovir
https://www.youtube.com/watch?v=XK52oKGekeA
HIV: Mechanisms of Action of NNRTIs, e.g.
Efavirenz
https://www.youtube.com/watch?v=AQkhBAQcSrE
DNA Synthesis Inhibitors
29. 4 (b).DNA Integration
inhibitors
Prevent integration of viral DNA into host
genome by binding Integrase enzyme
Dolutegravir (Part of 1st lineARVs in adults)
Raltegravir
Cabotegravir
All are ARVs
30. 4(c). RNA Synthesis
Inhibitors
Block action of RNA polymerase
Ribavirin
Inhibits RNA polymerase of several RNA viruses (HCV,
Influenza)
Prevents capping of mRNA in Influenza viruses
Favipiravir – Inhibits Influenza RNA polymerase
Sofosbuvir - Inhibits Hepatitis C virus RNA
Polymerase
Remdesevir – SARS-CoV-2 RNA Polymerase
31. 5. Protein Synthesis
Inhibitors
Fomivirsen
An anti-sense oligonucleotide
Binds the complementary Cytomegalovirus
mRNA
Prevents translation of the mRNA into protein
Stabilized to prevent degradation by nucleases
5'-GCG TTT GCT CTT CTT CTT GCG-3'
32. 6. Protease Inhibitors
Bind Protease preventing protein
cleavage hence blocking viral
maturation
Hepatitis C protease inhibitors:
Simeprevir, Paritaprevir, etc.
HIV protease inhibitors:
Lopinavir, Ritonavir, etc.
SARS-CoV-2 protease inhibitors:
Nirmatrelvir/ritonavir
33. 7. Capsid Inhibitor
Lenacapavir
Blocks formation and function of HIV capsid
By binding the interface between capsid protein (p24)
subunits
Also:
Blocks capsid-mediated nuclear uptake of HIV
proviral DNA
34. 8. Viral release inhibitors
Oseltamivir/Zanamivir/Peramivir
Block viral neuraminidase of Influenza A & B
viruses
NA clears sialic acid from the infected cell surface and
mucus secretions allowing the Influenza virus to spread to
other cells
35. 9. Interferons
Cytokines with antiviral, immunomodulatory and
antiproliferative activity
Induce gene expression through the JAK-STAT
signaling pathway > synthesis of antiviral proteins
IFNs induce synthesis of proteins that prevent viral
Transcription, translation, protein modification, maturation,
release
Numerous side effects
36.
37. Available forms
Natural IFN
Recombinant IFN
Pegylated IFN
PEG – PolyEthylGlycol – large inert molecule
slows metabolism allowing for lower, less frequent
doses
Antiviral Activities of Interferons
https://www.youtube.com/watch?v=o64S1VNVUPc&ab_channel=Dr.GBhanuPrakashAnimatedMedicalVide
38. 10. Monoclonal
Antibodies (MAbs)
MAbs are usually designed neutralize
viruses by binding viral surface glycoproteins
Ebola:
Atoltivimab/maftivimab/odesivimab
Covid-19
Casirivimab + Imdevimab
Tixagevimab + Cilgavimab
Challenge in developing these agents is because proteins are rapidly broken down in circulation
Other agents that prevent uncoating
Pleconaril – prevents nucleocapsid release from RNA
Arildone - blocks ion transport
Both have some activity against Picornaviridae – Enterovirus &Rhinovirus
Foscarnet: blocks the pyrophosphate binding site, preventing cleavage of pyrophosphate from deoxynucleotide triphosphates
First antibiotic = Prontosil (Sulfamidochrysoïdine hydrochloride) developed in 1930s by Bayer™
Guanosine analogue
Weak in-vitro activity against EBV, CMV & HHV-6
Cross-resistance with its analogues – Valacyclovir, Gancyclovir but not Cidofovir, Foscarnet
NtRTI - Adefovir (HBV)
Favipiravir – currently being tried for Ebola in Guinea
In prokaryotes, the 5′ cap (cap-0), found on the 5′ end of an mRNA molecule, consists of a guanine nucleotide connected to mRNA via an unusual 5′ to 5′ triphosphate linkage. It is methylated on the 7 position directly after capping in vivo by a methyltransferase
The 5′ cap has four main functions:
Regulation of nuclear export;
Prevention of degradation by exonucleases
Promotion of translation
Promotion of 5′ proximal intron excision
Vidarabine
Interferes with mRNA capping (polyadenylation) in HSV & VZV
Fomivirsen – blocks CMV immediate early 2 protein preventing its translation into protein
Stabilized by phosphorothioate
Neuraminic acid = Sialic acid is the influenza virus receptor.
1. Type I: IFN-α (leukocytes), IFN-β (fibroblasts)
2. Type II – IFN – γ (immune cells)
3. ?? Type III – IFN-λ
Transcription – blocks mRNA synthesis
Translation – activates methylase> reduced RNA cap methylation. Activates protein kinase> blocks elf-2a function> inhibits initiation of mRNA translation. Activates phosphodiesterase> blocks tRNA function
Post-translational modification – inhibits glycosyltransferase> reduced glycosylation of proteins
Virus maturation – inhibits glycosyltransferase> reduced glycoprotein maturation
Virus release – causes membrane changes> blocks budding
IFNAR: interferon-α/β receptor
IRF-9: Interferon Regulatory Factor 9
ISG: Interferon stimulated Genes
The JAK-STAT signaling pathway is a chain of interactions between proteins in a cell, and is involved in processes such as immunity, cell division, cell death and tumour formation. The pathway communicates information from chemical signals outside of a cell to the cell nucleus, resulting in the activation of genes through a process called transcription. There are three key parts of JAK-STAT signalling: Janus kinases (JAKs), signal transducer and activator of transcription proteins (STATs), and receptors (which bind the chemical signals).[1] Disrupted JAK-STAT signalling may lead to a variety of diseases, such as skin conditions, cancers, and disorders affecting the immune system
Cytokine Signaling:
Many cytokines, which are signaling proteins involved in immune responses and inflammation, bind to specific receptors on the cell surface.
JAKs are often associated with these receptors. When a cytokine binds to its receptor, it triggers a conformational change in the receptor that activates the associated JAKs.
Phosphorylation Cascade:
Activated JAKs then phosphorylate themselves (autophosphorylation) and also phosphorylate tyrosine residues on the receptor, creating docking sites for signaling proteins called STATs (Signal Transducers and Activators of Transcription).
STAT Activation:
STATs are then recruited to the phosphorylated receptors, and JAKs phosphorylate the STATs.
Phosphorylated STATs form dimers and translocate to the cell nucleus.
Gene Expression Regulation:
In the nucleus, STAT dimers bind to specific DNA sequences, regulating the transcription of target genes.
This process influences various cellular activities, including immune responses, cell growth, and differentiation.
Cellular Responses:
The ultimate outcome of JAK-STAT signaling is the modulation of cellular responses, such as the activation or inhibition of immune cells, cell proliferation, and differentiation.
Implications in Disease:
Dysregulation of JAK-STAT signaling has been implicated in various diseases, including autoimmune disorders, inflammatory diseases, and certain cancers.
Drugs known as JAK inhibitors are used to modulate JAK activity and are employed in the treatment of conditions like rheumatoid arthritis.
