L12 - Prion CNS infections and Antivirals

Question 1

Define Prion and its pathogenesis. What is it resistant to?

Answer 1

Infectious, misfolded “isoform” of a normal cell glycoprotein. No nucleic acid

Relatively resistant to heat (121oC, 15 min), UV light, ionizing radiation, formalin

Genetic or exogenous infection
» introduce Pathogenic prion
» prion turns normal proteins abnormal

Question 2

Difference between PrPc and PrPsc?

Answer 2

1. PrPc (c = cellular; normal) = protease-sensitive
2. PrPsc = Protease-insensitive: retains folding even in gastrointestinal tract

> > Catalyzes abnormal change in normal protein shape
PrPSc accumulates = disease

Question 3

Consequence of Prion accumulation in brain?

Answer 3

Spongiform encephalopathies

> > Loss of neurones
“Spongiform vacuolation” of brain without immune response

Question 4

Clinical presentation of Spongiform encephalopathies?

Answer 4

Rapidly progressive dementia, loss of memory and intellect, personality changes, unsteadiness / clumsiness, myoclonic jerks.

Question 5

Prion diseases manifest quickly. True or False?

Answer 5

False

Long incubation period (years).

Question 6

Prion transmission?

Answer 6

• Not by (direct / indirect) contact / droplet / airborne / fomites / environment
• No natural (non-iatrogenic) person-person spread
• Iatrogenic transmission (<1%)***** e.g. transplant, transfusion, medical equipment

Question 7

2 types of CJD? Differences? Epidemiology?

Answer 7

1) Familial (genetic predisposition): older patients
2) Variant CJD (vCJD): younger patients

No difference between CJD and vCJD in sensitivity to disinfection / sterilization

Difference in the range of tissues that are infectious

Sporadic cause (85%): older (mean 63 years; 1 per million)

Question 8

List 4 ways prion disease can be transmitted iatrogenically?

Answer 8

1) Implantation of contaminated grafts: Dura mater, cornea
2) Hormones: human growth hormone, pituitary gonadotrophin
3) Contaminated medical instruments: electrodes in brain, neurosurgical instruments
4) Blood transfusion

Question 9

List some precautions that should be taken if a CJD patient requires surgical operation?

Answer 9

 Perform in operating theatre: Last in list, Environment should be readily cleanable

 Use disposable equipment in contact with CSF, blood

 Segregate tissue by decontamination requirement

 Reusable items require special sterilization procedures

 Avoid use of high-speed drill / other aerosol-generating procedures

 Tissue biopsies: consult pathologist in advance regarding fixation

Question 10

List some ineffective disinfectants for prions?

Answer 10

 Alcohol 
 Ammonia 
 Formaldehyde
 Glutaraldehyde 
 Hydrogen peroxide
 Iodine
 Phenolics

Question 11

List some effective disinfectants for prions (<1hour) ?

Answer 11

 Chlorine >1000ppm (ideally 20,000ppm; 2%) – high concentration hypochlorite*

 Sodium hydroxide 1M*

 Proprietary enzymatic, alkaline detergents (e.g. Steris, Prionzyme)

 96% formic acid (to preserve tissue)

Question 12

List some effective disinfectanting processes for prions (>1hour) ?

Answer 12

1) Immerse in 1M NaOH for 2 hours
2) autoclave 121oC for 30 min

*Autoclave (higher temperature, longer time):
 134oC for >18 min (prevacuum)
 132oC for 60 min (gravity)

Question 13

List 4 challenges that antivirals face in managing viral infections? (Can it kill viruses? How effective? Latency issues?)

Answer 13

1. Very few virus-specific “targets”: Mechanism of virus replication involves host cell
2. Often need to commence antiviral treatment early in illness in order to make a clinical impact
3. Blocks viral replication, but not virucidal
4. Cannot eliminate virus latency

Question 14

List the 6 stages of viral replication?

Answer 14

1. Attachment
2. Penetration/ fusion
3. Uncoating
4. Transcription, Translation
5. Nucleic acid replication
6. Viral assembly, release

Question 15

List 2 drugs that target viral attachment. Give virus name in clinical use.

Answer 15

Maraviroc (CCR5) = HIV

Sialidase (e.g. DAS181) = Influenza, parainfluenza

Question 16

List 1 drug that target viral penetration/fusion . Give virus name in clinical use.

Answer 16

Enfuvirtide

HIV

Question 17

List 2 drugs that target viral uncoating. Give virus name in clinical use.

Answer 17

 Amantadine
 Rimantidine

Influenza A (most viruses now resistant)

Question 18

List 2 drugs that target viral transcription, translation. Give virus name in clinical use.

Answer 18

Ribavirin HCV, RSV

Interferon HCV, HBV

Question 19

Give 3 subtypes and examples of antivirals against nucleic acid replication?

Answer 19

1) Nucleoside analogues (e.g. acyclovir, ganciclovir): competitive DNA polymerase inhibitor
2) Directly blocks viral DNA polymerase (e.g. Foscarnet)
3) RNA polymerase inhibitors (e.g.: Baloxavir marboxyl, T705 (favipiravir))

Question 20

RNA polymerase inhibitors antivirals is given for which virus?

Answer 20

Influenza

Question 21

Give 3 subtypes and examples of antivirals against virus assembly and release?

Answer 21

1) HIV protease inhibitor = Saquinavir, Indinavir, Ritonavir, Nelfinavir
2) HCV protease inhibitors, e.g.: Boceprevir, Telaprevir
3) Neuraminidase inhibitors, e.g.: Oseltamivir,* Zanamivir, Lananimivir = Influenza

Question 22

MoA of Acyclovir?

Answer 22

Only act on virus-infected cell

Acyclovir > [Viral thymidine kinase] > ACV monophosphate > [cell enzyme] > ACV triphosphate (active)

selective inhibition of viral DNA polymerase

Question 23

When to give IV acyclovir, when to give oral?

Answer 23

HSV1, HSV2 (oral, low dose)

VZV (oral, high dose)

CMV (intravenous)

HSV enephalitis = IV

Question 24

List 2 viral mutations that cause resistance against acyclovir and 2 drugs to circumvent the resistance?

Answer 24

1) Mutant viral thymidine kinase enzyme: block
Acyclovir to ACV monophosphate conversion

> > use Cidofovir (= monophosphate nucleotide analogue, bypass TK)

2) Mutant viral DNA polymerase: No longer prefers acyclovir triphosphate
» use Foscarnet

Question 25

What form of acyclovir has improved bioavailability? Pharmacokinetics? Spectrum the same?

Answer 25

Valaciclovir: = valine ester of acyclovir = pro-drug of acyclovir:

• Increased absorption in GI = better oral bio-availability (1 g 3x/day)
• Broken down completely to acyclovir instantaneously in blood

Same spectrum as ACV

Question 26

Indication for Ganciclovir? Route of admin?

Answer 26

Treatment, prophylaxis of CMV disease in immunocompromised

Active vs HSV, VZV but no better than ACV with heavy renal/ CNS side effects

IV or Intra-ocular for retinitis
(oral = poor availability)

Question 27

Major side effect of Ganciclovir and resistance?

Answer 27

Resistance : CMV enzyme UL97, Viral DNA polymerase

Major side effect: myelosuppression (do not only target infected cells)

Question 28

What form of Ganciclovir has improved oral bioavailability? MoA? Indication?

Answer 28

Valganciclovir

Valine ester of ganciclovir rapidly converted to ganciclovir in intestinal epithelial cells.

– CMV prophylaxis (once daily dosing) in transplant Pts
– Therapy of non-severe CMV in organ transplant Pts
– Treatment of CMV retinitis in AIDS

Question 29

Foscarnet, Cidofovir can cover what virus that Acyclovir, Valaciclovir, Famciclovir cannot?

Answer 29

Acyclovir, Valaciclovir, Famciclovir: – HSV, VZV

Foscarnet, Cidofovir: – CMV**, HSV, VZV

Question 30

Foscarnet, Cidofovir advantage and disadvantage against Ganciclovir ?

Answer 30

Advantage = less myelotoxic

Disadvantage = more renal toxicity

Question 31

Mechanism of HIV replication?

Answer 31

RNA virus uses own reverse transcriptase to copy RNA genome into DNA (cDNA)

> > integrated into host cell genome (proviral DNA)

> > may remain latent / direct active viral replication

Question 32

List 2 classes of Anti-retroviral drugs and give examples.

Answer 32

1. Nucleoside / nucleotide reverse transcriptase inhibitors (NRTI):
   Zidovudine, Lamivudine, Abacavir, Tenofovir
2. Non-nucleoside inhibitors of reverse transcriptase (NNRTI):
   Nevirapine, Efavirenz

Question 33

MoA and side effect of Zidovudine?

Answer 33

MoA: ZDV is activated by both normal and infected cells:
ZDV&raquo_space; ZDV monophosphate&raquo_space; ZDV triphosphate&raquo_space; Inhibition of viral (and normal cell) polymerase & reverse transcriptase

Side effects:

• Non- specific inhibition of normal cell polymerase= myelosuppression
• RNA virus resistance

Question 34

Integrase inhibitor example?

Answer 34

raltegravir

Question 35

Define HAART therapy.

Answer 35

Highly active anti-retroviral therapy

Use multiple drugs blocking different sites of viral replication cycle = minimise development of drug resistance

Typical 1st line: reverse transcriptase inhibitors + protease inhibitors

Question 36

Indication of Ribavirin?

Answer 36

Respiratory Syncytial Virus

Indicated in RSV in pre-engraftment stem cell transplant patients

use by aerosol in patients at high risk

Question 37

MoA of interferons?

Answer 37

Direct immune system:

• Produce anti-viral protein
• Block viral RNA transcription, protein synthesis
• Augment immune response

Question 38

Which type of interferon is used as an antiviral?

Answer 38

Type 1: alpha, beta
Type 3: Lambda
Type 2: Gamma – made by immune cells, modulates immune cells.

Type 1: IFNa, B are used

Question 39

MoA of Interferon-alpha. What is the purpose of Peg-IFN over normal IFN?

Answer 39

Alpha: Blocks viral RNA transcription, protein synthesis and augments immune response

Peginterferon-α2a or 2b: interferon conjugated with polyethylene glycol/ PEG» prolongs persistence of drug in blood

Question 40

Clinical use of IFN therapy?

Answer 40

– Chronic hepatitis B (together with lamivudine)

– Hepatitis C (together with ribavirin and new protease inhibitors- Boceprevir, Telaprevir)