Lecture 20: antiviral drugs Flashcards
What are some characteristics of viruses?
Obligate intracellular parasites
No cell wall or plasma membrane
No metabolism (tough to target)
Few drugs block reproduction selectively
Pharmacology focused on late symptoms
Few virus groups can be effectively treated with drugs
Virus classification
(+)ssRNA - HIV, Hep A and C (can give direct protein synthesis)
dsDNA - Hep B, Herpes
(-) ssRNA -Influenza
See figure
What are the cytopathic effects of viruses?
Host cell metabolism hijacked
Viral-induced suppression of host homeostasis
Viral proteins induce lysis or apoptosis
Viral proteins trigger host immune response
Inflammatory reaction kills host
All above involve lytic cycle (generation of new virus)
Viruses can become latent, host cell survives
No overt immune response
What cells does Herpes simplex virus target?
Neurons (neurotropic)
What type of genome does HSV have?
Complex dsDNA
What symptoms does HSV cause?
HSV-1: cold sores
HSV-2: genital herpes
Chicken pox (varicella zoster - VZV), shingles, cytomegalovirus
How is HSV transmitted?
Close contact
HSV life cycle
Lytic cycle: in epithelial cells (80 genes in cascade)
Viral progeny spread to sensory neurons
Virus is retrogradedly transported to the cell body of the neuron
Latent infection: circular episomal DNA (not incorporated into genome)
No immune signature, no cytotoxic effect
Stress-related reactivation
Anterograde transport (similar to how neurotransmitter is transported)
See figure
HSV replication
Viral DNA enters nucleus and circularizes
Immediate early genes (IEG) are transcripts by the host RNA polymerase (2-4 hours post infection).
This transcription uses host transcription factors and VP16, a viral activator protein, which binds host cell factor that activates OCT1 (host transcription factor)
IEG proteins trigger early genes
E proteins control viral DNA replication
DNA replication initiates late genes, which are responsible for viral structure and assembly
See figure
How does acyclovir work?
HSV treatment
The viral thymidine kinase converts acyclovir to acyclo-GMP.
Host enzymes convert acyclo-GMP to acyclo-GTP.
Acyclo-GTP is incorporated into the growing DNA, and causes chain termination.
It is an inhibitor of the viral DNA polymerase.
See figure
Where is cytomegalovirus (CMV) a problem?
In immuno-compromised patients (organ transplantation)
What are the complications of CMV?
Liver failure
Colitis
Retinitis (inflammation of retina, can cause blindness)
What type of virus is HIV?
Lentivirus (HIV-1 and HIV-2)
Retrovirus family (+) ssRNA virus
Fast replication cycle and multiple infection
What enzymes is most responsible for drug resistance in HIV?
Reverse transcriptase
Enzyme is error prone = drug resistance
What does HIV cause?
AIDS
90% of people progress to AIDS within 10-15 years
How is HIV transmitted?
Blood/fluid
What cells does HIV invade?
T helper cells (CD4+)
Macrophages, dendritic cells
What receptors does HIV target?
CD4 receptors and chemokine co-receptors (CCR5, CCR4)
What types of immunity are lost in HIV infection?
Cell-mediated immunity
Loss of CD4+ cells
Development of aids - CD4+ count and HIV RNA copies
See figure
Initially, during primary infection, the CD4+ cell count is high. The virus starts to replicate rapidly, which causes the CD4+ cell count to drop
During clinical latency, the virus replicates slowly, and the CD4+ cell count drops
Eventually, there are very low CD4+ cells in the blood, and the virus takes over
This predisposes the host to opportunistic diseases etc.
Epidemiology of HIV
High in Africa
Origin of HIV
Started in Africa
Person from Haiti travelled to Africa and likely contracted HIV from a sex worker
Haitian then returned to haiti and transmitted virus by donating blood
People visiting Haiti for vacation contract the virus
See figure
Treatment options for HIV
No cure
Post-exposure prophylaxis
Highly active antiretroviral therapy (HAART) - 3 drugs belonging to at least 2 classes
Newer integrase and entry inhibitors can be added
Aggressive treatment in children
Points of intervention in HIV replication cycle
See figure
Fusion inhibitors
CCR5 antagonists
Reverse transcriptase inhibitors
Integrase inhibitors
Protease inhibitors
NRTI mechanism of action
Nucleoside/nucleotide reverse transcriptase inhibitors
Analog of native ribosides, but lack 3’ hydroxyl.
Phosphorylated to triphosphate by host enzymes. Incorporated into viral DNA.
Lack of 3’ hydroxyl leads to DNA chain termination.
How do NRTI’s affect mitochondria?
Each mitochondria has it’s own DNA
The normal process of energy production produces free radicals
Free radicals can act on mt DNA and damage it
Mitochondria get around this by producing new mitochondria
However, AZT inhibitors the DNA polymerase in mitochondria, so they get destroyed and cannot regenerate
Cell death
Which NRTI is recommended in pregnant women?
Lamivudine
Which NRTIs have long half lives?
Emtricitabine (39 hours)
Tenofovir
Mechanism of NNRTIs
Non-competitive inhibitors of HIV-1 reverse transcriptase
No activation required
No effect on bone marrow or mitochondrial DNA polymerase
Mechanism of HIV protease inhibitors
Reversible inhibitors of HIV aspartyl protease
Prevents proteolysis of viral polyprotein = virus does not get released
Prevents maturation of viral particles
Non infectious virus produced
Mechanism of action of HIV integrate inhibitors
Normally: integrase binds to viral DNA and catalytically processes 3’ ends. Integrase then joins viral and cellular DNA.
Integrase inhibitors block strand transfer into the host DNA
See figure
Mechanism of co-receptor and fusion inhibitors
See figure
What HIV protein mediates cell fusion?
gp41
36 amino acid
What are the most common influenza viruses?
Flu A and Flu B
What carries flu A?
Aquatic birds (domestic birds)
Which flu is the most severe
Flu A
Spanish flu pandemic 1918-1919
40-100 million died
How does resistance against influenza drugs arise?
No RNA proof reading in the flu virus
What type of genome does the influenza virus have?
(-)ssRNA
How is influenza transmitted?
Aerosol passage
What cells does influenza attack?
Epithelial cells
What are the key proteins of influenza virus? functions?
Hemagglutin (Hag) - binds to sialic sugars on host cells
Neuraminidase cleaves sialic residues to release virus
M2 ion channel - proton channel that modulates pH (uncoating)
See figure
What is the influenza virus life cycle?
Hag binds to cell surface
Mediates endocytosis of particle
M2 regulates uncoating
M2 controls Hag processing
Release of particles from buds requires neuraminidase
See figure
Mechanism of neuraminidase inhibitor - influenza
Viral neuraminidase: glycoside hydrolase enzyme
Cleaves glycosidic linkages on neuraminic (sialic) acid
Permits release of viral particle from host cell
Neuraminidase inhibitors block this
Mechanism of action of inhibitors of viral uncoating - influenza
Block M2 channel. Prevents acidification of viral particle. Stop releasing of viral genome and uncoating
What type of genome does hepatitis B have?
dsDNA
But uses RT
See figure
How are hepatitis B and C transferred?
B: Blood-blood, 2 billion people infected
C: blood-blood, 200 million infected
What is the most common form of hepatitis virus?
B
Is vaccination against hepatitis B possible?
yes
What occurs in persistent Hep B infection?
cccDNA
covalently closed circular DNA (circular DNA that exists outside the genome)
Type of genome in hepatitis C virus?
(+)ssRNA
Symptom detection in hepatitis C
Difficult to detect
When is interferon treatment used in Hepatitis infection?
Chronic infection only
Mechanism of interferon treatment in hepatitis
Raises cell resistance in liver
Cells are in an “antiviral state”
Elevates MHC1 - presentation to cytotoxic CD8 T cells (enhanced cell mediated immunity)
Increase p53 - apoptosis
How does innate induction of interferon occur?
By high levels of dsRNA or foreign RNA
What is standard therapy for HCV?
Ribavirin plus peg interferon alfa
Which viruses use reverse transcriptase?
HIV and HBV
Anti HBV drugs
NRTIs = DNA chain termination
Lamivudine, tenofovir
Combined with interferon
