Med chem of antivirals

Question 1

enveloped viruses

Answer 1

surrounded by lipids

HIV, influenza, herpes

Question 2

non-enveloped viruses

Answer 2

picornavirus, adenovirus

Question 3

generic virus life cycle

Answer 3

viral attachment and entry
penetration
uncoating
early protein synthesis
nucleic acid synthesis
late protein synthesis and processing
packaging and assembly
viral release

Question 4

human immuodeficiency virus

Answer 4

Enveloped retrovirus
-Genus lentivirus
-Targets CD4+ T cells, macrophages, and dendritic cells
-Directly and indirectly destroys CD4+ T cells
-Causes immunosuppression
-Pathology associated with opportunistic infections
and cancer
-HIV-1 is more virulent and more easily transmitted - Responsible for most infections
-HIV-2 - primarily found in West Africa

Question 5

key steps in HIV life cycle

Answer 5

initial binding
uncoating of the viral RNA
reverse transcription (viral RNA - proviral DNA)
integration of proviral DNA into the host genome
transcription of proviral DNA to form mRNA
translation of viral mRNA to form viral polyprotein
cleavage of the viral polyprotein to form the mature proteins of the virus
assembly and budding

Question 6

HIV fusion/entry mechanism

Answer 6

HIV gp120 binds to CD4 on target cell
Conformational changes in gp120 exposes region
Exposed region binds to cytokine receptor (CCR5 or CXCR4 depending on the
strain of HIV)
Normal chemokine receptor ligands can interfere with HIV binding and fusion

Question 7

maraviroc

Answer 7

HIV entry inhibitor
Selective CCR5 antagonist
Binds to CCR5 and causes a conformation change that prevents gp120 binding
No effect on cell surface levels of CCR5 or on CCR5 signaling
Can only be used in patients with HIV strains that utilize CCR5*
-Potential problem: May select for viral mutants that bind to CXCR4, Almost all transmitted HIV isolates bind CCR5, Rest either bind to CXCR4 only or to both co receptors, In ~60 % of patients, strains that bind CXCR4 emerge late during infection - Emergence is correlated with rapid disease
progression

Question 8

fusion inhibitors

Answer 8

Enfuvirtide (T20)
36 amino acid peptide that inhibits fusion of virus membrane with cell membrane
Active only against HIV-1
Must be injected subcutaneously
resistance: Acquired resistance is due to mutations within a 10 amino acid motif in gp41: Forms part of the binding site of enfuvirtide, critical for viral fusion
-Easily acquired

Question 9

reverse transcriptase inhibitors

Answer 9

Reverse transcriptase (RT) has three activities
-RNA dependent DNA polymerase
-Ribonuclease H
-DNA-dependent DNA polymerase
RT does the following
-Copies plus-strand RNA to produce minu sstrand DNA
-Degrades RNA template from RNA-DNA hybrid
-Synthesizes plus-strand DNA from minus-strand DNA template
NRTIs interfere with 1st and 2nd strand DNA synthesis

Question 10

RT DNA polymerase activity

Answer 10

incorporation of dCTP into growing strand

• each nucleoside is phosphorylated 3 times
• different enxymes are used for each phosphorylation and for each nucleoside
• RT catalyzes formation of phosphodiester bond between 3’ OH of last nucleotide added and the 5’ phosphate of the next nucleotide. Pyrophosphate is released (provides energy needed for reaction)

Question 11

Nucleoside RT inhibitors mechanism

Answer 11

NRTIs are nucleoside analogs that lack the 3’ OH
-Sometimes called false nucleotides
Two effects:
-Competitive inhibitor
of reverse transcriptase
-DNA chain terminator - inhibit elongation
Active against HIV-1 and HIV-2

Question 12

NRTIs

Answer 12

7 NRTIs currently available for clinical use
Form backbone of initial therapy
Used in combination - 2 NRTIs plus NNRTI or PI or integrase inhibitor
Some combinations of NRTIs work better than others
-Tenofovir and emtracitabine
-Abacavir** and lamivudine
All must be activated by cellular kinases to triphosphate form (or equivalent)
Each NRTI is phosphorylated to triphosphate
Use cellular enzymes
Compete with normal nucleosides and NRTIs that are analogs of the same nucleoside

Question 13

Tenofovir disoproxilfumarate

Answer 13

Prodrug - converted to tenofovir (TFV)
Acyclic* nucleoside phosphonate* analog of adenosine
Requires 2 phosphorylation steps
Phosphonate cannot be cleaved by cellular esterases - catabolically stable
advantages:
-Long intracellular half life
-Different resistance profile – different mutations in RT confer resistance
-Retains some activity against mutant RT
-Highly selective for HIV RT over human cellular and mitochondrial DNA polymerases
problems:
-Plasma esterases can activate TDF to TFV
-TFV is eliminated by kidney
-Relative to other NRTIs, TDF treatment is associated with: greater loss of kidney function and a higher risk of acute renal failure, larger reduction in bone mineral density

Question 14

tenofovir alafenamide (TAF)

Answer 14

Alternative tenofovir prodrug
Activated by different pathway than TDF
-Lower plasma concentrations of TFV
-Increased accumulation in lymphocytes
-Expected to have fewer side effects
May be even better at targeting HIV
-Better accumulation in lymph nodes
-Higher intracellular concentrations
Approved for use in Nov. 2015 as a component of Genvoya (elvitegravir, cobicistat, emtricitabine, and tenofovir alafenamide)
**Different activation pathway of TAF allows for 10-fold lower dose compared to TDF**

Question 15

Cathepsin A (CatA)

Answer 15

Carboxypeptidase present in lysosomes
Expressed all cells with high levels in lymphoid cells
removes the ester from TAF (converts TAF to TVF)

Question 16

activation of NRTIs - summary

Answer 16

Nucleosides and NRTIs must be activated to their triphosphate forms by cellular kinases
Different kinases are responsible for adding the first, second and third phosphate to the same nucleoside
Different nucleosides require different kinases to activate them
A given nucleoside and its corresponding NRTI are activated by the same enzymes
NRTI analogues of the same nucleoside can compete with each other for activation
Activated NRTIs compete with dATP, dCTP, dGTP, and dTTP to be incorporated into the growing DNA chain by RT
Ratio of activated NRTI to dNTPs is important
NRTIs have a higher affinity for HIV RT than for cellular DNA polymerases
Some NRTIs inhibit mitochondrial DNA polymerase
TDF and TAF must be processed to TFV by cellular enzymes before phosphorylation
Tenofovir requires 2 phosphorylation steps

Question 17

resistance to anit-HIV drugs

Answer 17

Why do resistant mutations arise so quickly?
-HIV polymerase is error prone
-RT inhibitors are unable to suppress viral replication > 90%
-Large amounts of virus present: 10^7 to 10^8 infected cells, Half-life of infected cells is 1 - 2 days, HIV-1 genome is 9,181 nt, HIV RT has an error rate of ~1/2000
Rate at which mutations appear is inversely related to the serum drug concentration
-Need to maintain drug levels above MIC

Question 18

genetic barriers to antiviral drug resistance

Answer 18

Genetic barrier = ease with which drug
resistance develops
Low = easy for virus to become resistant
High = hard for virus to become resistant
Evolution of a viral population that overcomes the activity of a drug or drug regimen
Depends on:
-Number of mutations required
-Impact of each mutation on drug activity
-Impact of each mutation of virus replication (fitness)
-Number of pre-existing mutations in the viral population
Low genetic barrier – resistance develops easily
-single mutation with low cost to viral fitness
High genetic barrier – more difficult to develop resistance
Two common scenarios:
-A primary mutation that has a high fitness cost – i.e., makes the virus less able to replicate - one or more additional mutations needed to allow the virus to grow better
-Multiple mutations needed for complete resistance
Drugs with low genetic barriers can be used in combination to develop an effective therapy

Question 19

resistance to NRTIs

Answer 19

Discriminatory mutations: Mutations that selectively impair the ability of reverse transcriptase to incorporate analogues into DNA
Excision mutations:
-ATP molecule mediates the removal (excision) of a nucleoside analogue after it has been incorporated
-selected by thymidine analogs AZT and d4T - TAMs (Thymidine Analog Mutations)

Question 20

NRTI resistance - key points

Answer 20

Mutations are mainly near the RT active site, but can occur at more distant locations
Mutations can either help the RT to distinguish between normal dNTPs and NRTIs or can promote removal of NRTIs after they’ve been incorporated into the growing chain
Individual NRTIs have a low genetic barrier
Some mutations confer resistance to a subset of NRTIs, but make RT more susceptible to inhibition by others
-NRTIs in preferred combination for initial therapy take advantage of this phenomenon

Question 21

AEs of NRTIs - mitochondrial toxicity

Answer 21

Mitochondrial toxicity:
NRTIs are selective for HIV RT over human DNA polymerase-a and –b
Some NRTIs inhibit human DNA polymerase-g
Leads to anemia, granulocytopenia, myopathy, peripheral neuropathy, and pancreatitis
Lactic acidosis and hepatic steatosis (buildup of fat droplets in liver cells)
-More common with d4T, ZDV & ddI
Lipoatrophy – loss of body fat
-Higher incidence with stavudine

Question 22

AEs of NRTIs - hypersensitivity

Answer 22

Abacavir Hypersenstivity Reaction : Black Box Warning
Occurs in 4 - 8 % of patients - can be fatal
Symptoms: Malaise, Dizziness, Headache, GI disturbances
Discontinue immediately is symptoms develop - Prompt recovery
Highly associated with the HLA-B5701 allele
Testing for HLAB5701
is recommended before initiating treatment with abacavir

Question 23

recommended combinations of NRTIs

Answer 23

Tenofovir and emtracitabine**
-TFV has long intracellular half-life
-Once daily dosing
-Equivalent efficacy to other NRTI combinations
-Less fat maldistribution
-Different resistant mutation profiles
Abacavir and lamivudine (or emtracitabine)
-If HLA-B*5701 negative
**Lamivudine and emtricitabine are interchangeable
-Have negligible side effects
-select for the M184V mutation that can confer improved susceptibility to zidovudine or tenofovir

Question 24

antiretroviral medications - should not be offered at any time

Answer 24

Regimens not recommended:
Monotherapy** (except possibly zidovudine used to prevent perinatal HIV transmission)
Dual NRTI therapy (with no other antiretroviral)
3-NRTI regimen (except abacavir/lamivudine/ zidovudine and possibly lamivudine/zidovudine + tenofovir)

Question 25

nonnucleoside RT inhibitors (NNRTIs)

Answer 25

Bind directly to site on RT -Hydrophobic pocket near catalytic site -Near, but distinct from that of NRTIs -Binding affects flexibility of enzyme -NNRTIs do NOT compete with nucleotides for binding - noncompetitive inhibitors NNRTIs do not have to be phosphorylated Block RNA- and DNA-dependent DNA polymerase activities

Question 26

NNRTI MOA

Answer 26

In the presence of NNRTI, nucleoside triphosphate and template bind tightly to RT, but nonproductively Blocks polymerization Single mutation in binding site can promote resistance

Question 27

2nd gen NNRTIs

Answer 27

diaryl-pyrimidine–based molecule Designed to be inherently flexible** Can bind in multiple orientations -Binds to mutants that are resistant to other NNRTIs

Question 28

NNRTIs AEs

Answer 28

All NNRTIs: Rash, Drug-drug interactions Nevirapine – hepatotoxicity (may be severe and life threatening), rash including Stevens-Johnson syndrome Efavirenz - neuropsychiatric, teratogenic in nonhuman primates (FDA Pregnancy Category D)

Question 29

NNRTI metabolism

Answer 29

All are metabolized by CYP3A** -Efavirenz, nevirapine, and etravirine are moderate inducers -Efavirenz and delavirdine are inhibitors of CYP3A4 -Etravirine inhibits CYP2C9 and CYP2C19 Potential for interactions with other drugs that are metabolized by CYP3A -CYP3A4 inducers (rifampin) can reduce levels -CYP3A4 inhibitors can increase levels

Question 30

NNRTI summary

Answer 30

NNRTIs do not require activation, do not compete with dNTPs, and are not incorporated into DNA NNRTIs do not bind to cellular DNA polymerases Resistance to NNRTIs can be acquired through a single mutation Mutations that confer resistance to NNRTIs do not cause resistance to NRTIs

Question 31

integrase inhibitors

Answer 31

Raltegravir (Mk-0518) approved in 2007 Inhibits insertion of HIV DNA into the human genome Most commonly reported treatment-related adverse effects were diarrhea, nausea, fatigue, headache, and itching Orally available Compatible with other antiretroviral drugs ***Does not interact with CYP450s

Question 32

integrase function

Answer 32

``` Inserts HIV DNA into host cell DNA 2 steps: -3’-processing -Strand transfer Raltegravir inhibits the strand transfer step ```

Question 33

raltegravir mechanism

Answer 33

Integrase uses divalent metal ions to catalyze insertion - Coordinated by 3 amino acids in active site Raltegravir chelates both metal ions and stabilizes enzyme-DNA complex

Question 34

integrase inhibitor examples

Answer 34

raltegravir, elvitegravir, dolutegravir

Question 35

Integrase inhibitor (INI) resistance

Answer 35

Caused by primary mutations that reduce INI susceptibility secondary mutations further decrease virus susceptibility and/or compensate for the decreased fitness Low genetic barrier -Dolutegravir higher extensive but incomplete cross resistance -Dolutegravir less affected

Question 36

Elvitegravir

Answer 36

currently available only in coformulation with cobicistat (COBI), FTC, and TDF or TAF cobicistat is not active against HIV Role is to boost elvitegravir concentrations by inhibiting metabolism by CYP3A4

Question 37

Dolutegravir

Answer 37

Long plasma half life (14h) - once daily dosing No boosting No interactions with CYP3A4 Higher barrier for resistance

Question 38

HIV protease inhibitors

Answer 38

HIV protease is an aspartic protease - aspartic acid in active site Dimer - active site formed at interface Inhibitors are transition state mimetics** -Peptidomimetic -Nonpeptide compounds

Question 39

HIV protease

Answer 39

Viral proteins aggregate at inner surface of cell membrane One protein molecule draws the two viral RNA strands into a forming virus particle Protease cuts itself free Protease cuts the other enzymes from the long precursors and then cuts the precursors into four pieces p17 remains attached to the membrane p24, p9, and p7 form the bullet-shaped inner core of the virion

Question 40

HIV protease mechanism

Answer 40

Peptide bond cleavage is a hydrolysis** reaction Protease catalyzes addition of water to the amide Forms intermediate Breaks down to the two products, a carboxylic acid and an amine

Question 41

HIV protease inhibitors

Answer 41

Amide bond is replaced by non-cleavable linkages All PIs except tipranavir** are considered peptidomimetics Inhibitor binding causes a conformation change in protease - “flaps” close

Question 42

metabolism of protease inhibitors

Answer 42

All are substrates and some are inhibitors of CYP3A4 -High potential for drug interactions -Can cause increases in levels of other CYP3A4 metabolized drugs -Levels of PI can be influenced by other CYP3A4 inhibitors -Interactions are complicated - Delavirdine increases indinavir and saquinavir levels; Efavirenz reduces indinavir and saquinavir levels

Question 43

Saquanivir

Answer 43

First protease inhibitor approved by FDA Contains hydroxyethylamine moiety in place of peptide bond Low bioavailability as hard gel capsule (4%) Soft gel capsule increases bioavailability 3- fold

Question 44

Ritonavir

Answer 44

Higher bioavailability - 75% - Increased with food **Most potent PI inhibitor of CYP450s** Sub-therapeutic doses of ritonavir can be combined with other PIs - PI boosting Currently recommended initial treatment regimes all use ritonavir combinations

Question 45

PI boosting

Answer 45

``` Low doses of ritonavir inhibit CYP3A4** -Block metabolism of other PIs -Increases serum concentrations - Lopinavir and tipranvir are not indicated for use except in combination with ritonavir -Increases trough levels -Reduces emergence of resistant viruses -Improves compliance - reduced dosing, fewer pills, eliminates food restrictions Disadvantages -Drug-drug interactions with ritonavir -Increased risk of hyperlipidemia ```

Question 46

Atazanavir

Answer 46

Once-daily dosing and minimal lipid and glycemic effects most potent protease inhibitor in vitro prior to darunavir Different resistant mutation profile Efavirenz and tenofovir reduce ATV concentrations

Question 47

Darunavir

Answer 47

Novel peptidomimetic PI Preferred PI for initial antiretroviral combinations Two unique features: 1. Makes extensive hydrogen bonds** with protease backbone 2. Inhibits HIV protease dimerization

Question 48

peptide backbones of wildtype and mutant HIV protease

Answer 48

The peptide backbones of wildtype and mutant HIV protease have very similar structures DRV hydrogen bonds with the peptide backbone so it is less affected by changes in amino acid side chains. DRV can inhibit both wildtype and mutants that are resistant to other PIs

Question 49

Tipranavir

Answer 49

(TPV) Novel nonpeptidic** PI Retains activity against proteases in highly treated patients Selects for different set of resistance mutations CYP3A4 substrate and inducer

Question 50

resistance to protease inhibitors

Answer 50

Highest genetic barrier of antiretrovirals Mutations can be in active site or far away Modify contacts between protease and inhibitor -Reduce affinity of protease for inhibitor -Example: V82A Protease inhibitors bind tightly to protease Natural substrates have variable, less tight binding Resistance mutations have greater effect on PI than natural substrate Most mutations occur in or near substrate cleft of protease Some occur at distant sites and act indirectly Multiple mutations are usually needed to confer high level resistance DRV retains activity against most PI-resistant mutant proteases *** Some mutations confer resistance to some PIs, but increase the susceptibility to others -Example: I50L -Confers resistance to atazanavir -Increased susceptibility to other PIs

Question 51

PI AEs

Answer 51

All PIs: Hyperlipidemia, Insulin resistance and diabetes, Lipodystrophy (changes in body fat accumulation), Elevated liver function tests, Possible increased bleeding risk in hemophiliacs, Drug-drug interactions

Question 52

HAART

Answer 52

highly active antiretroviral therapy

Question 53

Initial treatment: preferred regimens - INSTI based

Answer 53

DTG/ABC/3TC; only if HLA-B*5701 negative DTG (QD) + TDF/FTC EVG/COBI/TDF/FTC; only if pre-ART CrCl >70 mL/min EVG/COBI/TAF/FTC; only if pre-ART CrCl ≥30 mL/min RAL + TDF/FTC TAF can be used in place of TDF 3TC can be used in place of FTC and vice versa TDF: caution if renal insufficiency

Question 54

Initial treatment: preferred regimens - PI based

Answer 54

DRV/r (QD) + TDF/FTC TAF can be used in place of TDF 3TC can be used in place of FTC and vice versa TDF: caution if renal insufficiency

Question 55

herpes virus

Answer 55

Large double-stranded DNA viruses Enveloped Can cause latent infections

Question 56

herpes virus latency

Answer 56

Herpes virus infections can be lytic or latent -Lytic = actively producing new virions -Latent = dormant HSV-1 enters latent state when it infects neurons -Minimal gene expression -Asymptomatic Virus reactivates in response to various signals -Usually stress-related -Can occur after very long periods of time

Question 57

human herpes simplex viruses - HSV 1

Answer 57

HSV-1 (herpes simplex virus 1) Commonly causes oral herpes, but can cause genital herpes ~60 % of adults in US are seropositive Typically infected as children usually establishes latency in the trigeminal ganglion Reactivates on face or lips

Question 58

human herpes simplex viruses - HSV-2

Answer 58

HSV-2 (herpes simplex virus 2) Commonly causes genital herpes, but can cause oral herpes ~16 % of adults in US are seropositive -More common in women than in men -More common in African Americans Typically infected as teenagers or adults usually establishes latency in the sacral ganglion Reactivates in genital area

Question 59

varicella zoster virus (VZV)

Answer 59

Causes chickenpox after primary infection Can reactivate later in life Causes shingles -Viruses migrates to ganglia in area of infection May reactivate in response to stress or decreased immune function Rash and blisters that scab over Complication – postherpetic neuralgia (PHN) Transmission - Shingles isn’t transmitted, but virus can cause chicken pox Prevention - (Zostavax®) - Recommended for people > 60

Question 60

cytomegalovirus (CMV)

Answer 60

``` Up to 80 % of adults are infected Most people have no or mild symptoms Severe disease can occur if infected: -during fetal development ---May cause of congenital abnormalities ---Most infants are not affected ---Infant can also be infected during birth -Infection of immunocompromised people ---cytomegalovirus-retinitis occurs in up to 15% of all AIDS patients ```

Question 61

epstein-barr virus (EBV)

Answer 61

human herpes virus Up to 95 % of adults are infected Targets lymphocytes causes infectious mononucleosis, especially if infected during adolescence Associated with several cancers (Burkitt's lymphoma, nasopharyngeal carcinoma, oral hairy leukoplakia) No drugs or vaccines available

Question 62

Acyclovir

Answer 62

Acyclic guanosine** derivative Lacks 3’ hydroxyl Selectively accumulates in infected cells -Results in higher concentration in infected cells -High ratio of therapeutic value to toxicity requires 3 phosphorylation events

Question 63

acyclovir MOA

Answer 63

Acyclovir is a competitive inhibitor** of viral DNA polymerase Competes with dGTP Competition occurs at a lower concentration for viral DNA pol than host DNA pol DNA pol becomes bound to template irreversibly Acyclovir is incorporated into DNA Acts as chain terminator**

Question 64

acyclovir SOA

Answer 64

Active against HSV-1, HSV-2 and VZV | Reduced activity against EBV, cytomegalovirus or HSV-6

Question 65

acyclovir PKs

Answer 65

Bioavailability is 15 to 30% | Bioavailability not affected by food

Question 66

acyclovir AEs

Answer 66

Generally very well tolerated, with no side effects | Infrequent nausea, diarrhea, rash, or headache, and very rarely, renal insufficiency or neurotoxicity have been reported

Question 67

acyclovir resistance

Answer 67

Resistance can develop by two mechanisms: -Mutations in viral thymidine kinase** - Most common – 95%, Resistant viruses are usually less pathogenic and may be less able to reactivate -Mutations in viral DNA polymerase Resistance emerges more frequently in immunocompromised people -~0.5 % in immunocompetent people -3-10% in immunocompromised

Question 68

valacyclovir

Answer 68

L-valyl* ester of acyclovir Rapidly converted to acyclovir by esterases in the intestine and liver Bioavailability is 48 to 54 % Improved efficacy compared to acyclovir for all indications Valacyclovir is transported by intestinal** amino acid transporters

Question 69

famciclovir and penciclovir

Answer 69

Famciclovir is a prodrug** of penciclovir (conversion is by first pass metabolism) -lacks intrinsic antiviral activity Famciclovir is converted to penciclovir by first pass metabolism in intestine and liver

Question 70

famciclovir and penciclovir MOA

Answer 70

Mechanism of action -Activated by viral and cellular kinases -Competitive inhibitor of viral DNA polymerase -Does NOT cause immediate chain termination - Allows for short chain elongation Viral kinase** mutants confer cross-resistance to penciclovir and acyclovir

Question 71

penciclovir vs acyclovir

Answer 71

Penciclovir has a higher affinity for HSV TK than acyclovir -Levels of penciclovir triphosphate in infected cells are much higher than the levels of acyclovir triphosphate Penciclovir triphosphate is more stable than acyclovir triphosphate in HSV-infected cells -Intracellular half-life is 10- to 20-fold longer HSV DNA polymerases have a higher affinity for acyclovir triphosphate than for penciclovir triphosphate -Net effect: both drugs have similar antiviral potencies Acyclovir triphosphate is an obligate DNA chain terminator Penciclovir triphosphate allows limited DNA chain elongation (short-chain terminator) -3’ hydroxyl group on its acyclic side chain

Question 72

clinical uses of famciclovir and penciclovir

Answer 72

``` Oral famciclovir -Primary and recurrent genital herpes -Acute herpes zoster Topical penciclovir -Recurrent herpes labialis Pharmacokinetics -70 % bioavailable Generally well tolerated ```

Question 73

ganciclovir

Answer 73

Structurally very similar to penciclovir* Mechanism of action is same as penciclovir** Better substrate for cytomegalovirus kinase than acyclovir Intracellular half-life 12 h (acyclovir = 1 - 2 h) 100 times more active against CMV Resistance is due to mutations in CMV kinase (UL97 gene) or CMV DNA pol (UL54) Mutations in kinase are not cross-resistant to cidofovir or foscarnet Mutations in DNA pol may confer resistance to cidofovir or foscarnet (less frequent)

Question 74

valganciclovir

Answer 74

Monovalyl ester of ganciclovir Increases bioavailability (60%) Rapidly hydrolyzed to ganciclovir by esterases in intestine and liver Used to treat CMV retinitis in AIDS patients

Question 75

foscarnet

Answer 75

Inorganic pyrophosphate compound (phosphonoformic acid) Inhibits viral DNA polymerase, RNA polymerase, and HIV RT Does not require phosphorylation for activity Blocks pyrophosphate binding site of the viral DNA polymerase Inhibits cleavage of pyrophosphate from dNTPs Pharmacokinetics: Poor oral bioavailability, Only administered intravenously, Up to 30 % may be deposited in bone, Renal clearance in proportion to creatinine clearance Clinical use: CMV retinitis (equivalent to ganciclovir), Dose must be titrated according to creatinine clearance, Synergistic with ganciclovir against CMV Toxicity: Renal insufficiency, Hypo- or hyperphosphatemia, Hypo- or hypercalcemia, Headaches in 25% of patients Resistance: Mutations in DNA pol or HIV RT, Resistant CMV isolates are cross-resistant to ganciclovir, Foscarnet usually still effective against ganciclovir- and cidofovir-resistant CMV

Question 76

cidofovir

Answer 76

Acyclic nucleoside phosphonate analog of cytosine* Phosphonate cannot be cleaved by cellular esterases - catabolically stable Phosphorylated by cellular kinases Intracellular half-life is 17 to 65 h -Phosphocholine metabolite has half-life of 87 h - may be intracellular reservoir Poor substrate for cellular DNA pol - 1000X less efficient than dATP Broad spectrum of activity -CMV, HSV-1, HSV-2, VZV, EBV, HHV-6, HHV-8, adenovirus, poxvirus, polyomavirus, and human papillomavirus -Highly selective for viral DNA pol -Does not require activation by viral kinases Competitive inhibitor and chain terminator - Chain termination by CMV pol requires two consecutive incorporations Adverse effects - Dose-dependent nephrotoxicity Clinical use - CMV retinitis (intravenous)

Question 77

influenza virus

Answer 77

Negative** stranded RNA virus - 8 RNA segments Enveloped Can infect humans, birds, pigs, horses, etc. Animal influenza viruses can infect humans -May not be easily transmitted from human to human -Can be more deadly New virus strains can be created by “reassortment**” - mixing of RNA segments

Question 78

influenza subtypes

Answer 78

Three types of influenza viruses - A, B, C A infects humans and many different animals, including ducks, chickens, pigs, whales, horses, and seals B widely circulates only in humans C causes a very mild disease A and B causes epidemics nearly every winter Vaccines protect against the currently circulating versions of A and B

Question 79

influenza A subtypes

Answer 79

Influenza A viruses are divided into subtypes based on two genes: Hemagglutinin (H), Neuraminidase (N) There are 16 H genes (H1 - H16) and 9 N genes (N1 - N9) known to exist Each virus has one H and one N Different subtypes can infect different organisms

Question 80

drift vs shift

Answer 80

Influenza viruses change in two ways Antigenic drift -Small changes in the virus -New virus not recognized by immune system Antigenic shift Major abrupt change New H or new H + new N viruses that can infect humans Humans have little or no protection against new viruses

Question 81

antigenic shift

Answer 81

Mutation of an avian virus to a strain that can grow in humans Reassortment -A pig is co-infected with influenza viruses from birds and humans -Progeny viruses are a mixture of the 8 RNA segments from the two viruses -Can introduce new H and new N proteins into viruses that can grow in humans

Question 82

2009 H1N1 swine flu

Answer 82

Quadruple reassortment virus - 2 genes from North American avian virus origin - 1 gene from human H3N2 virus origin - 3 genes from classical swine virus origin - 2 gene segments from the Eurasian influenza A (H1N1) swine virus lineage

Question 83

anti-influenza drugs - amantadine

Answer 83

Amantadine Tricyclic primary amine Inhibits penetration into host cells Blocks uncoating Targets the M2 protein of influenza A -M2 is in viral membrane -Ion channel -Influenza B has different protein Clinical uses: Seasonal prophylaxis with 200 mg/day is 70% to 90% effective, Can reduce duration of symptoms 1-2 days if treatment begun within 1-2 days of onset Resistance: Develops rapidly in 50 % of patients, Mutations in M2, Not cross-resistant to zanamavir and oseltamivir, Mutants are transmissable and equally pathogenic Important note: in recent years, most circulating seasonal influenza strains have been adamantane-resistant Adverse effects: GI intolerance, CNS effects (nervousness, difficulty concentrating, light-headedness)*, Effects more pronounced in people over 60 (Peak concentrations 3 times higher, Plasma half-life 12 h longer, Most likely due to reduced renal function)

Question 84

rimantadine

Answer 84

Same mechanism as amantidine 4 - 10 times more active in vitro Half-life is double (24 - 36 h) CNS effects are less severe

Question 85

infuenza virus neuraminidase

Answer 85

Essential for virus replication Located in virus membrane Cleaves glycolytic bonds between terminal sialic acids and adjacent sugars Facilitates virus dissemination -Hemaglutinin (HA) binds to terminal sialic acid residues -Cleavage by NA releases virus May prevent inactivation by respiratory mucus sialic acid, DANA, zanamavir, oseltamivir

Question 86

oseltamivir (tamiflu) overview

Answer 86

Prodrug - converted to active form by liver esterases Metabolite is an effective inhibitor of NA Active against influenza A and B - Less effective against B though

Question 87

oseltamivir PKs

Answer 87

Readily absorbed from the GI tract after oral administration Extensively converted by hepatic esterases to oseltamivir carboxylate At least 75% of an oral dose reaches the systemic circulation as oseltamivir carboxylate

Question 88

oseltamivir toxicity

Answer 88

Generally well tolerated | Nausea and vomiting in 10% of the subjects

Question 89

oseltamivir therapeutic use

Answer 89

Administration of Tamiflu® has resulted in a one-day reduction in the time to improvement of symptoms as compared to placebo Effect depends on how soon therapy is started

Question 90

oseltamivir resistance

Answer 90

resistance is associated with mutations in the active site of neuraminidase Influenza A virus with reduced susceptibility to oseltamivir carboxylate has been isolated Drug-resistant virus occurs in 3% of patients receiving oseltamivir Resistance develops more easily against oseltamivir than zanamavir

Question 91

zanamivir overview

Answer 91

Transition state analog Mechanism is same as oseltamivir** Effective against influenza A and B Administered via oral inhaler

Question 92

zanamavir PKs

Answer 92

4-17% of the inhaled dose is systemically absorbed | Excreted unchanged by the kidneys.

Question 93

zanamavir toxicicty

Answer 93

Bronchospasms have been reported in some patients | Not recommended in patients with airway diseases such as asthma and obstructive pulmonary disease

Question 94

peramivir

Answer 94

Transition state analog of sialic acid Active against A and B IV administration Emergency use authorization - ended in June 2010 Adult patients for whom therapy with an IV agent is clinically appropriate, based upon one or more of the following reasons: -patient not responding to either oral or inhaled antiviral therapy, -drug delivery by a route other than IV is not expected to be dependable or is not feasible -the clinician judges IV therapy is appropriate due to other circumstances

Question 95

hepatitis C virus (HCV)

Answer 95

Small, positive-stranded RNA virus Causes chronic liver infections Transmission is via contaminated blood - IV drug users particularly at risk Major cause of chronic hepatitis, liver cirrhosis and hepatocellular carcinoma (HCC) -~80 % of infected people develop chronic infections -60-70% of chronically infected people develop chronic liver disease

Question 96

direct acting versus host-targeted antivirals

Answer 96

Host targeted: Act on a cellular protein -May be required for virus replication (maraviroc, possibly ribavirin) -May stimulate antiviral response (interferon) Direct acting -Target a component of the virus -most currently available antiviral drugs

Question 97

anti-HCV treatment

Answer 97

Previous standard of care: Ribavirin plus pegylated interferon alpha Goal of treatment: Sustained virological response (SVR) -HCV RNA undetectable for 6 months after treatment SVR rate depends on: -HCV genotype: At least 6 HCV genotypes, with many subtypes, 3 are globally distributed, Type 1 has lower SVR rate than 2 or 3 (~50% vs 80%) -Host genetics: IL-28B polymorphism is associated with increased risk, Lower risk allele is predominant in Caucasians, but not in Africans, Encodes interferon l3 (Induced by IFN-a, viral infections, toll-like receptors, Stimulates expression of antiviral genes)

Question 98

nonspecific defenses against viral infection - interferons

Answer 98

Cytokines Block viral growth by inhibiting viral proteins Three groups - alpha, beta, and gamma -Alpha and beta are induced by viruses -Gamma is induced by antigens and is an effector of cell-mediated immunity Strongly induced by viruses and double-stranded RNA Many weak inducers Inducers are not specific Inhibitory action is not specific to particular pathogen Effects are species specific Interferons induce synthesis of cellular proteins: -More than 300 proteins are upregulated -Have antiviral effects - Effects are specific to certain viruses -Ribonuclease - degrades viral RNA but not cellular RNA -Protein kinase that phosphorylates and inactivates EIF-2 (a translation initiation factor) -Many viruses have evolved mechanisms to counteract the antiviral effects of interferons

Question 99

INF-a as an antiviral drug

Answer 99

Recombinant protein - Produced in E. coli and purified Nonglycosylated protein of ~19.5 kD Pharmacokinetics: Interferon alpha is not absorbed after oral administration, Absorption exceeds 80% following IM or SC injection Pegylated interferon alpha: Linear or branched polyethylene glycol attached, Increases half-life and reduces dosing frequency, Superior efficacy to non-pegylated interferon Combination of either interferon with ribavirin is more effective than monotherapy Clinical uses: -hepatitis C virus (HCV) - Approved for use against chronic hepatitis C -hepatitis B virus (HBV) -human herpes virus 8 (HHV-8) -papillomavirus Toxicity: -Flu-like symptoms within first 6 h in > 30% -May cause or aggravate fatal or life-threatening neuropsychiatric, autoimmune, ischemic, and infectious disorders -Patients should be monitored closely

Question 100

ribavirin

Answer 100

Guanosine analog with incomplete purine ring Phosphorylated by cellular kinases to triphosphate form Broad spectrum of activity: Influenza A & B, Hepatitis A, B, & C, Genital herpes, Herpes zoster, Measles, Hantavirus, Lassa fever virus Mechanism not definitively known: Immune modulation - promotes TH1 vs. TH2, Inhibition of Inosine monophosphate dehydrogense (IMPDH) - reduces GTP levels, Direct inhibition of viral RNA polymerase, Incorporation into viral RNA leading to error catastrophe Clinical uses -Combination therapy for hepatitis C virus -Ribavirin aerosol has been approved in the US for treatment of respiratory syncytial virus (RSV) pneumonia in hospitalized children

Question 101

HCV protease inhibitors

Answer 101

Target the HCV protease NS3 Block cleavage of the HCV polyprotein First generation - approved in 2011, discontinued in 2015 Second generation: -P1-P3 substrate analogs (Simeprevir, Paritaprevir) -P2-P4 substrate analog (Grazoprevir) Macrocyclic peptides Noncovalent inhibitors All are substrates of CYP3A4 and weak inhibitors

Question 102

2nd generation HCV PIs

Answer 102

Advantages: Once daily dosing, Better tolerated, Active against all genotypes except 3 Resistance: -Mutations in NS3 active site -Low genetic barrier -Similar, but not identical pattern of mutations for linear and macrocyclic inhibitors -Grazoprevir retains activity against some NS3 mutants that are resistant to 1st generation HCV PIs

Question 103

HCV RNA polymerase inhibitors

Answer 103

NS5B = HCV RNA polymerase Nucleoside RNA polymerase inhibitors -Sofosbuvir (Solvadi) -prodrug of 2'-deoxy-2'-fluoro-2'-C-methyluridine monophosphate Sofosbuvir mechanism: -Incorporated in viral RNA chain -Causes chain termination - 2’ Me is critical Genetic barrier for resistance - Single mutation in active site (S288T) Non-nucleoside RNA polymerase inhibitors -One FDA approved drug – Dasabuvir -7,000-fold selectivity for HCV RNA Pol -Not active against HCV genotype 2, 3 and 4 polymerases

Question 104

sofosbuvir activation

Answer 104

Converted to monophosphate by liver enzymes Triphosphorylated by cellular nucleotide kinases -uridine-cytidine monophosphate kinase (YMPK) -nucleoside diphosphate kinase (NDPK)

Question 105

HCV polymerase binding sites

Answer 105

NS5B has 5 known binding sites - catalytic site - 4 allosteric sites

Question 106

Dasabuvir

Answer 106

Mechanism -Binds to palm I site of HCV RNA polymerase -Prevents conformational changes -Blocks nucleotide incorporation into viral RNA Genetic barrier for resistance

Question 107

HCV NS5A inhibitors

Answer 107

NS5A required for HCV RNA replication, but exact mechanistic role unknown Four FDA approved drugs: Ombitasvir, Ledipasvir, Daclatasvir, Velpatasvir Bind tightly to NS5A Inhibits both viral RNA replication and assembly or release of infectious viral particles Less active against genotypes 2 and 3

Question 108

resistance to HCV NS5A inhibitors

Answer 108

``` Mutations occur in first 100 amino acids Low genetic barrier -Varies between genotypes -Single mutations confer high resistance Similar resistance pattern for all first generation NS5A inhibitors ```

Question 109

HCV combination therapies

Answer 109

``` Harvoni -Ledipasvir -Sofosbuvir Viekira Pak -Two tablets -Ombitasvir, paritaprevir, and ritonavir (Why is ritonavir included?) -Dasabuvir Zepatier -Elbasvir -Grazoprevir Epclusa -Sofosbuvir -Velpatasvir ```

Question 110

HCV inhibitor summary

Answer 110

HCV NS3 protease inhibitors (-previr): Boceprevir, Telaprevir, Simeprevir, Paritaprevir, Grazoprevir HCV NS5A inhibitors (-asvir): Ombitasvir, Ledipasvir, Daclatasvir, Velpatasvir HCV NS5B inhibitors (-buvir): Sofosbuvir, Dasabuvir

Question 111

black box warning for HCV direct acting antivirals

Answer 111

Hepatitis B virus (HBV) reactivation has occurred in patients co-infected with hepatitis C virus (HCV) while undergoing treatment with DAAs for HCV infection. Some cases have resulted in fulminant hepatitis, hepatic failure, and death. - observed with DAAs used without interferon*** to treat HCV infection. HBV reactivation is defined as abrupt increase in HBV replication - rapid increase in serum HBV DNA level, detection of hepatitis B surface antigen (HBsAg) in a person who was previously negative but was positive for HBV core antibody Reactivation of HBV replication is often followed by hepatitis, and, in severe cases, hepatic failure, and death. The mechanism through which HBV reactivation occurs is currently unknown To decrease the risk of HBV reactivation in patients co- infected with HBV and HCV, health care professionals should: -Screen all patients for evidence of current or prior HBV infection before initiating treatment with DAAs by measuring HBsAg and anti-HBc. In patients with serologic evidence of HBV infection, measure baseline HBV DNA prior to DAA treatment. -Monitor patients who show evidence of current or prior HBV infection for clinical and laboratory signs (i.e., HBsAg, HBV DNA, serum aminotransferase levels, bilirubin) of hepatitis flare or HBV reactivation during DAA treatment and post-treatment follow-up. -Consult a physician with expertise in managing hepatitis B regarding the monitoring and consideration for HBV antiviral treatment in HCV/HBV co-infected patients. Counsel patients to contact a health care professional immediately if they develop fatigue, weakness, loss of appetite, nausea and vomiting, yellow eyes or skin, or light- colored stools, as these may be signs of serious liver injury.

Question 112

hepatits B virus

Answer 112

Can cause chronic liver infections that lead to Life cycle: -Partially double stranded DNA virus -Viral genome replication includes a RNA intermediate that is converted to viral DNA by reverse transcriptase

Question 113

anti-HBV drugs

Answer 113

Anti-Retrovirals: Tenofovir and Lamivudine | Others: telbivudine, entecavir, adefovir

Question 114

nucleotide inhibitors - TDF

Answer 114

Tenofovir disoproxilfumarate Prodrug - converted to tenofovir Acyclic nucleoside phosphonate analog of adenosine Phosphonate cannot be cleaved by cellular esterases - catabolically stable

Question 115

docosanol

Answer 115

``` "other" antiviral agent Saturated 22-carbon aliphatic alcohol Inhibits herpes virus fusion Applied topically Active ingredient in Abreva ® (nonprescription anti-herpes viral medication) ```

Question 116

Zinc

Answer 116

"other" antiviral agents Inhibits rhinovirus protease Efficacy is questionable