Exam 4 Flashcards
What is the replication cycle of HIV?
- A mature extracellular virion attaches/fuses with the cell (just needs to have a CD4)
- Penetration & uncoating - Nucleocapsid enters the cell
- Reverse transcription - RT makes copy strand, then RNA is degraded, DNA is synthesized
- Integration - DNA attempts to integrate into the nucleus
- Transcription & translation - viral RNA and proteins are made in nucleus
- Budding and release - proteases allow infection HIV to form & infect more cells
What is a genetic barrier to resistance? Which antiretroviral drugs have high vs. low genetic barriers?
NRTIs have a low genetic barrier to resistance, meaning mutations are pretty common due to their structure/mechanism. Integrase inhibitors also have a low barrier.
High genetic barrier means that the mutations that cause resistance are pretty hard to develop.
What are the human or viral targets, reasons the class is selective for the virus/viral cells, affected step in viral cycle, and MOA of these drugs: entry/fusion inhibitors, nucleoside reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors
Entry/fusion inhibitors - enfuvirtide binds to HIV GP41 & blocks GP41 conformational change that is needed for fusion, maraviroc binds to human CCR5 and blocks GP120 binding. Overall this blocks the first step of the cycle by inhibiting the virus to enter the cell.
NRTIs - RTs can do these 3 things: (1) RNA dependent DNA polymerase, (2) Ribonuclease H, (3) DNA dependent DNA polymerase. RT copies plus-strand RNA and produces minus-strand RNA. It then degrades the RNA template and synthesizes plus-strand DNA from the minus-strand DNA template. NRTIs have 2 effects: (1) compete w/ nucleotides for reverse transcriptase, (2) DNA chain terminator
NNRTIs - These block RNA and DNA-dependent DNA polymerase activities by binding to an allosteric site on the reverse transcriptase.
What are the targets and mechanisms by which maraviroc and enfuvirtide block HIV entry?
Maraviroc - Selective CCR5 antagonist that binds CCR5 and causes a conformational change that prevents gp120 binding. Has no effect on cell surface levels of CCR5, so there’s little/no side effects. Can only be used in pts with HIV strains that utilize CCR5. Potentially can select for viral mutants that bind to CXCR4 (bad).
Enfuviritide - Only active against HIV-1. Causes a conformational change that inhibits the necessary binding insertion, thus inhibiting viral fusion. Mutations are easily acquired
How are nucleoside RT inhibitors activated metabolically before enzyme inhibition can occur?
The DNA must be tri-phosphorylated in order for viral RT to do it’s functions.
All of the nucleoside RT inhibitors are prodrugs that need to be activated by cellular kinases to the triphosphate form. (These enzymes are thymidine kinase, thymidylate kinase, and NDP kinase)
What are the key structural differences between nucleoside analogues and the normal nucleosides used by reverse transcriptase?
NRTIs look almost exactly like nucleotides, but they don’t have an OH group that would be needed to make a new bond in the DNA.
How can you describe the structure on tenofovir, emtricitabine, and zidovudine?
tenofovir - deoxyadenosine analog, but acyclic
emtricitabine - deoxycytidine analog with no OH, does have a sulfur in the ring and a fluorine at the top
zidovudine -deoxythymidine analog with N=N=N
What are the differences in structure and activation between tenofovir and the other nucleoside RT inhibitors? How does this contribute to the longer half-life of tenofovir?
Azidothymidine - instead of OH on normal deoxythymidine, it has an azole group (three N)
Stavudine - instead of OH on normal deoxythymidine, it has no OH, it does have a double bond in the ring structure
Tenofovir - deoxyadenosine analog, but acyclic; activated by cellular enzymes; already monophosphorylated
- Not sure how this contributes to a longer half-life of tenofovir
What are the differences in activation pathways for tenofovir disoproxil fumarate and tenofovir alafenamide? What are the effects of these differences on dosage, plasma and intracellular tenofovir concentrations, and tenofovir side effects?
tenofovir disoproxil fumarate (TDF) - Converted to tenofovir (TFV) by cellular esterases, then two more phosphorylations need to occur.
- Long intracellular half life
- plasma esterases (not cellular) can activate TDF -> TFV, which can cause kidney toxicity & reduction in bone mineral density
tenofovir alafenamide - activated by a different pathway than TDF. Plasma esterases don’t metabolize TAF.
- Lower concentrations of TFV -> less toxicity
- Increased accumulation of lymphocytes
- May be better at targeting HIV!
- Less side effects
- High lipid levels than TDF
Why are tenofovir and emtracitabine preferred NRTI for initial antiretroviral therapy? (5)
TFV has a long intracellular half-life
once daily dosing
equivalent to other NRTI combinations w/ less toxic effects
less fat maldistribution
different resistant mutation profiles
What hypersensitivity reaction can be caused by abacavir? What is the role of the HLA-B*5701 polymorphism?
Black Box Warning: hypersensitivity reaction -> can be fatal
- symptoms: malaise, dizziness, headache, GI disturbances
*must d/c immediately if symptoms develop
It’s highly associated with the HLA-B*5701 allele, so testing for this is recommended before initiating treatment with abacavir.
How can HIV become resistant to the RT inhibitors (discriminatory vs. excision mutations)? Does resistance to one RT inhibitor confer resistance to all drugs in this class? How can combinations of RT inhibitors be used to manage resistance?
HIV polymerase is error prone and there’s large amounts of virus present. The rate at which mutations appear is inversely related to the serum drug concentration.
- Discriminatory mutations: selectively impair the ability of reverse transcriptase to incorporate analogues into the DNA
- Excision mutations: ATP molecule mediates the removal of a nucleoside analogue after its been incorporated (happens to thymidine analogues)
Why are NRTI monotherapy and some combos of NRTIs not recommended?
The combinations need to be different analogs (T analog and C analog, not two T analogs).
NRTIs are preferred in combination for initial therapy to take advantage of some mutations making RT more susceptible to inhibition by some analogues over others.
Monotherapy not recommended due to mutations that may arise
Dual NRTI therapy w/ no other antiretroviral not used, usually use another agent as well
3-NRTI regmimen not normally done, but there are some options.
How do the non-nucleoside RT inhibitors inhibit RT?
These bind directly to the site on RT near the catalytic site. This is near but different than where the NRTIs bind. These are noncompetitive inhibitors (allosteric) that block RNA and DNA dependent DNA polymerase activities.
**FYI these have CYP450 interactions
How do the second generation NNRTIs (ertavirine and rilviripine) differ from the first generation NNRTIs with regards to their structural changes and interactions with HIV RT?(+ what are 2 1st gen and 2 2nd gen agents)
1st gen - nevirapine, efavirenz
2nd gen - ertavirine, rilpivirine; diaryl-pyrimidine-based molecule. These are designed to be inherently flexible, so they can bind even if the site is mutated.
How does HIV become resistant to the non-nucleoside RT inhibitors?
Resistance to NNRTIs can be acquired through a single mutation. Mutations that confer resistance to NNRTIs do not cause resistance of NRTIs
By which mechanism do raltegravir, dolutegravir, and elvitegravir inhibit HIV integrase?
Integrase inhibitors - These inhibit the insertion of HIV DNA into the human genome. These block the strand transfer step of integrase.
Integrase uses divalent metal ions to catalyze insertion. Raltegravir chelates both of the metal ions and stabilizes the enzyme-DNA complex, which prevents it from binding to the chromosomal DNA.
- Elvitegravir is metabolized by CYP3A4, so it’s given with cobicistat to inhibit the CYP3A4 and increase elvitegravir effect
- Dolutegravir has once daily dosing and doesn’t require cobicistat boosting. Also has higher barrier of resistance.
How does HIV become resistant to integrase inhibitors?
Resistance can be caused by primary and secondary mutations.
These have a low genetic barrier to resistance (dolutegravir has higher barrier). They also have extensive, but incomplete cross resistance (dolutegravir less affected).
How can you recognize the structure of HIV protease inhibitors atazanavir and darunavir, and which are the noncleavable bonds? How does the structure of tipranavir, a non-peptidomimetic PI, differ?
HIV protease inhibitors are transition state mimetics (except tipranavir). PIs have non-cleavable linkages (hydroxyethylene bond) instead of the amide bond in HIV protease.
atazanavir -
- most potent PI (except darunavir)
- different resistant mutation profile
darunavir - preferred PI for initial antiretroviral combinations
- makes extensive hydrogen bonds w/ protease backbone
- inhibits HIV protease dimerization
tipranavir - nonpeptidic
- Doesn’t need ritonavir (PI boosting)
- Retains activity against proteases in highly treated patients, even those resistant to darunavir
By which mechanism do protease inhibitors block HIV protease activity?
PIs have non-cleavable linkages (hydroxyethylene bond) instead of the amide bond in HIV protease. The inhibitors bind at the interface of the protease active site. This causes a conformational change in the protease to reduce its activity (inhibit substrates from entering).
How does resistance to protease inhibitors arise and how can the development of resistant viral strains be minimized? How do darunavir and atazanavir differ from other PIs in regards to PI resistance mutations? What is the basis for these differences for darunavir?
PIs have the highest genetic barrier of antiretrovirals. Mutations can be in the active site or far away and these modify the contact between protease and inhibitor. Some mutations could actually increase the susceptibility of others.
Use CYP3A4 inhibitors like ritonavir and cobicistat to reduce mutations
Atazanavir shows a different resistance mutation profile than the other PIs.
Darunavir can display strong hydrogen bonds with the peptide backbone of HIV protease, so it’s less affected by changes in amino acid side chains. It also inhibits HIV protease dimerization.
What are the main toxic effects of protease inhibitors? (6)
Hyperlipidemia
Insulin resistance and diabetes
Lipodystrophy
Elevated liver function tests
Possible increased bleeding risk in hemophiliacs
Drug-drug interactions
What are the effects of nucleoside reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, integrase inhibitors, and protease inhibitors on CYP3A4?
NRTIs - no info on these with CYPs
NNRTIs - All metabolized by CYP3A4, potential for interactions with other drugs that are metabolized by CYP3A. Ertavirine inhibits 2C9 and 2C19.
Integrase inhibitors - Elitegravir metabolized by CYP3A4. Dolutegravir has no CYP3A4 interactions
Protease inhibitors - all are substrates and some are inhibitors of CYP3A4. The levels of PI can be influenced by other CYP3A4 inhibitors
- Delavirdine (NNRTI) increases indinavir and saquinavir levels
- Efavirenz (NNRTI) reduces indinavir and saquinavir levels
Why do the structures of ritonavir and cobicistat act on CYP3A4 to act as PI boosters?
Ritonavir
- Ritonavir is the most potent PI inhibitor of CYP450s
- All initial PI treatment regimens use ritonavir combinations
- Low doses used
- Also reduces emergence of resistant viruses by keeping the serum drug concentrations high
Cobicistat
- Peptidomimetic that was designed to inhibit CYP3A4
What is the currently recommended initial anti-retroviral drug regimen?
HAART
preferred: dolutegravir (integrase inhibitor) + abacavir/lamivudine (NRTI)
What is the MOA, mechanism of resistance, and spectrum of activity of these: acyclovir, valacyclovir, penciclovir, famciclovir, ganciclovir, and valganciclovir
These bind to DNA polymerase to inhibit DNA replication.
acyclovir - selectively accumulates in infected cells, competitive inhibitor of viral DNA polymerase. It competes w/ dGTP, incorporates into DNA irreversibly, and acts as a chain terminator. Resistance due to mutations in viral thymidine or DNA polymerase. Spectrum is HSV1&2 and VZV
valacyclovir - prodrug of acyclovir -> improved efficacy
penciclovir & famciclovir - famciclovir is the prodrug of penciclovir. Competitive inhibitor of viral DNA polymerase, but does NOT cause immediate chain termination b/c it still has the OH group (is a short-chain terminator). Similar antiviral potency to acyclovir
ganciclovir - similar structure to penciclovir, same MOA as penciclovir, better substrate for cytomegalovirus kinase than acyclovir, more toxic than acyclovir
valganciclovir - monovalyl ester of ganciclovir
What is the MOA, mechanism of resistance, and spectrum of activity of these: foscarnet, cidofovir, and letermovir
foscarnet - pyrophosphate compound that inhibits viral DNA polymerase, RNA polymerase, and HIV RT. It doesn’t require phosphorylation and blocks the pyrophosphate binding site of the viral DNA polymerase (different binding site than the others) to inhibit cleavage of pyrophosphate from dNTPs.
cidofovir - cytosine analog. Already has 1 phos, so only needs to be phosphorylated 2 more times. Has a broad spectrum of activity, is highly selective for viral DNA pol. It’s a competitve inhibitor and chain terminator
letermovir - non-nucleoside that is used for prophylaxis of CMV. Well tolerated due to selective toxicity. Inhibits the terminase complex to prevent herpes virus genomes to be cut out properly.
Which herpes meds are DNA chain terminators? (acyclovir, valacyclovir, penciclovir, famciclovir, ganciclovir, valganciclovir, foscarnet, cidofovir, and letermovir)
acyclovir/valacyclovir
cidofovir
NOT:
- famciclovir/penciclovir
- ganciclovir/valgancyclovir
- foscarnet (?)
- letermovir
Which herpes meds can be incorporated into DNA? (acyclovir, valacyclovir, penciclovir, famciclovir, ganciclovir, valganciclovir, foscarnet, cidofovir, and letermovir)
acyclovir/valacyclovir
penciclovir/famciclovir
ganciclovir/valganciclovir
cidofovir
Which herpes meds are prodrugs? (acyclovir, valacyclovir, penciclovir, famciclovir, ganciclovir, valganciclovir, foscarnet, cidofovir, and letermovir)
*they are all prodrugs b/c they need to be triphosphorylated before being active
acyclovir - activated by viral thymidine, which selects for viral cells, then diphophorylated by cellular kinases
valacyclovir - prodrug of acyclovir
famciclovir - prodrug of penciclovir that gets converted through first pass metabolism in the intestine and liver
valganciclovir - monovalyl ester of ganciclovir that is rapidly hydrolyzed to ganciclovir by esterase in intestine and liver
cidofovir - requires 2 phosphorylations (not 3)
NOT: foscarnet, letermovir
How are these activated & how does that contribute to their specificity?: acyclovir, penciclovir/famciclovir
acyclovir - activated by viral thymidine, which selects for viral cells
famciclovir/penciclovir - activated by viral and cellular kinases
Which herpes drugs are able to cause cross-resistance? (acyclovir, valacyclovir, penciclovir, famciclovir, ganciclovir, valganciclovir, foscarnet, cidofovir, and letermovir)
famciclovir/penciclovir - some cross-resistance with acyclovir (think of viral thymidine activation)
ganciclovir - assuming there’s cross resistance with penciclovir b/c they have very similar structures and the same MOA
foscarnet - resistance due to mutations in DNA polymerase or HIV RT. Resistant CMV isolates are cross-resistant to ganciclovir, probably because there are multiple mutations, or a single mutation could effect the pyrophosphate AND active site. Usually still effective against cidofovir-resistant CMV though.
letermovir - no cross resistance to other CMV drugs
What is the binding side of foscarnet on DNA polymerase?
Foscarnet blocks the pyrophosphate binding site of the viral DNA polymerase, which traps the polymerase in a closed formation, inhibiting the cleavage of pyrophosphate from dNTPs. Therefore, the DNA is unable to translocate.
Why are viral thymidine kinase mutants that are resistant to ganciclovir not resistant to cidofovir or foscarnet? Why are foscarnet-resistant versus likely to be cross-resistant to ganciclovir?
Viral thymidine kinase mutants that are ganciclovir-resistant are not resistant to cidofovir because they are different analogues (?). Not resistant to foscarnet because it’s a different binding site.
Acyclovir - mutations in viral thymidine kinase inhibits the production of the active acyclovir molecule
Ganciclovir - resistance due to mutations in CMV or CMV DNA polymerase. The mutations in the viral kinase are not cross-resistant to cidofovir or foscarnet.
Foscarnet - resistance due to mutations in DNA polymerase or HIV RT. Resistant CMV isolates are cross-resistant to ganciclovir, probably because there are multiple mutations, or a single mutation could effect the pyrophosphate AND active site. Usually still effective against cidofovir-resistant CMV though.
How is cidofovir metabolized to form the biologically active species?
Cidofovir is phosphorylated by cellular kinases to form the biologically active species
Why do valacyclovir and valganciclovir have greater bioavailability than acyclovir and ganciclovir, respectively?
Valacyclovir is the L-valyl ester of acyclovir, which makes it easier to absorb. Then it gets converted to acyclovir by esterases in the intestine and liver.
Valganciclovir is the monovalyl ester of ganciclovir. It’s hydrolyzed to ganciclovir by esterases in the intestines and liver.
What is the influenza virus life cycle? Which steps are targeted by anti-influenza drugs?
Influenza enters through endosomal pathway. Virus uncoating occurs and the virus enters the nucleus. Replication of the virus occurs in the nucleus (unlike other viruses where replication happens in the cytoplasm). Synthesis of viral mRNAs also in nucleus. Proteins synthesized in ER. Proteins/virus relased from the cells.
Amantadine - blocks viral uncoating (M2 protein that allows uncoating)
Neuraminidase inhibitors - Inhibits release of new virions (inhibits cleaving from the sialic acid containing receptor)
Baloxavir - inhibit transcription
What is the MOA, mechanism of resistance, and spectrum of these influenza drugs: oseltamivir, zanamavir, peramivir, baloxavir
Neuraminidase inhibitors - Transition state analogues that inhibit the release of new virions (inhibits neuraminidase cleaving from the sialic acid containing receptor).
oseltamivir - prodrug converted to active form by liver. Binds to active site of neuraminidase. Active against influenza A and B (less). Resistance is associated with mutations in the active site of neuraminidase. Resistance develops more easily against oseltamivir than zanamavir because the binding with oseltamivir is more dependent on certain proteins compaired to zanamivir
zanamavir - same MOA as oseltamivir. Effective against influenza A and B.
peramivir - newest neuraminidase inhibitor that cannot be absorbed PO. Active against A and B.
baloxavir - inhibits viral “cap-snatching,” meaning it blocks transcription. (influenza steals mRNA cap from host cell mRNA to make its own mRNA.) baloxavir binds to PB2 subunit of RNA polymerase of the virus to inhibit the cap-snatching.
What are the life cycles of Hep B and Hep C? Which steps are targeted by anti-HBV and HCV drugs?
Hep C - HCV lipoparticle binds and is endocytosed. Viral RNA is released. Translation of viral RNA in the cytoplasm generates an HCV polyprotein. The polyprotein is eventually processed by the viral protease. As the virus matures, it forms a replication complex (uses NS5A). RNA polymerase replicates the genome (NS5B). The virion is assembled and released.
- Anti HCV: interferon (nonspecific), ribavirin (RNA polymerase), RNA polymerase inhibitors, NS5A inhibitors
Hep B - partially double-stranded DNA virus. The viral genome replications includes an RNA intermediate that is converted to viral DNA by reverse transcriptase.
- tenovoir, lamivudine, telbivudine, entecavir, adefovir
What is the MOA, mechanism of resistance, and spectrum of activity of these HCV inhibitors: ribavirin, simeprevir, paritaprevir, grazoprevir, voxilaprevir, sofosbuvir, dasabuvir, daclatasvir, velpatasvir, pibrentasvir
ribavirin - guanosine analog. Broad spectrum of activity (flu, Hep A/B/C, etc.). Inhibits Inosine monphosphate dehydrogenese (host enzyme), which reduces GTP levels. It directly inhibits viral RNA polymerase.
HCV protease inhibitors: Resistance if mutations in NS3 active site (low genetic barrier)
- simeprevir (2nd gen P1-P3)
- paritaprevir (2nd gen P1-P3)
- grazoprevir (2nd gen P2-P4)
- voxilaprevir (2nd gen P2-P4)
sofosbuvir - HCV RNA polymerase inhibitor. (Nucleoside) Converted to monophosphate by liver enzymes, then di and tri phosphorylated. It’s incorporated into viral RNA chain and causes chain termination. Single mutation (S288T) causes resistance.
dasabuvir - non-nucleoside RNA polymerase inhibitor. Doesn’t bind to active site, binds to palm I site of RNA polymerase to prevent conformation changes, thus blocking nucleotide incorporation into viral RNA. Not active against all HCV genotypes. Pretty good genetic barrier for resistance.
HCV NS5A inhibitors: binds tighly to NS5A, inhibits both viral RNA replication and assembly or release of infectious viral particles. Mutations more common for 1st gen. 2nd gen retains activity against common resistance-associated substituations
- daclatasvir (1st gen)
- velpatasvir (2nd gen)
- pibrentasvir (2nd gen)
What is the MOA, mechanism of resistance, and spectrum of activity of these HBV inhibitors: lamivudine, tenofovir
lamivudine - antiretroviral (NRTI), incorporated into RT to cause chain termination
tenofovir - bind to active site of RT to cause chain termination
Which HCV protease inhibitors and which HCV NS5A inhibitors are second generation? What are the advantages of second gen inhibitors vs. the first gens?
HCV protesase inhibitors target the HCV protease NS3. This blocks the cleavage of the HCV polyprotein.
1st generation - linear peptide mimics. Form covalent bonds. Discontinued now.
- Telaprevir
- Boceprevir
2nd generation - Macrocyclic peptides. non-covalent inhibitors. Advantages: These have once daily dosing, are better tolerated, and are activated against all genotypes.
- Paritaprevir (P1-P3)
- Simeprevir (P1-P3)
- Voxilaprevir (P2-P4)
- Grazoprevir (P2-P4)
- Gelcaprevir (P2-P4)
What the CYP interactions with HCV protease inhibitors?
Second gen HCV Protease Inhibitors are all substrates and weak inhibitors of CYP3A4
What are the proposed MOAs of ribavirin? What viruses are susceptible to ribavirin? How is ribavirin like the NRTIs? Would any NRTIs be expected to compete with ribavirin? If so, how?
Ribavirin is a guanosine analog that causes inhibition of inosine monphosphate dehydrogenase (IMPDH) that reduces GTP levels, causing direct inhibition of viral RNA polymerase. It’s also incorporated into viral RNA, leading to error catastrophe.
Lots of viruses are susceptible to ribavirin (ex. influenza A & B, Hep A/B/C, genital herpes, herpes zoster, etc.)
Similar to NRTIs becuase it is a nucleoside analog that gets incorporated into the DNA to cause chain termination.
The guanosine analogs would likely compete with ribavirin because they are the same analog.
What are the basics of the possibility of HBV reactivation during HCV treatment in patients with a known prior HBV infection?
HBV reactivation - abrupt increase in HBV replication
HBV reactivation has been seen in patients co-infected with HCV which undergoing treatment with direct acting antivirals (DAAs) for HCV. This is observed with DAAs used without interferon.
Reactivation of HBV replication is often followed by hepatitis, and potentially hepatic failure and death.
The mechanism is unknown.
To decrease the risk of HBV reactivation, pts should be screened to see if HBV infection previously occurred, monitor for symptoms, and consult physicians with expertise in managing hep B.
What are the MOAs, prodrug/metabolism, and side effects of these SARS-CoV-2 drugs: remdesivir, nirmatrelvir, molnupiravir
remdesivir - prodrug that is bio-transformed to a ribonucleotide analog that can inhibit viral RNA polymerase (adenosine analog). Approved for emergency use for COVID-19.
nirmatrelvir - Peptidomimetic that inhibits active site of SARS-CoV-2 3CLpro to inhibit production of nonstructural proteins. This was approved for emergency use for COVID-19.
molnupiravir - prodrug of synthetic nucleoside derivative that serves as a polymerase inhibitor and chain terminator. Emergency use authorization for COVID-19.
What are the differences between opportunistic fungal pathogens and those considered to be true pathogens?
True pathogens - cutaneous infective agents, subcutaneous infective agents, systemic infective agents
Opportunistic infections require the host to be impaired before causing an infections.
What are the definitions of dermatophytosis and onchomycosis? What fungi are associated with these conditions?
dermatophytosis - classic skin and hair infections like ringworm, athlete’s foot, jock itch, etc. These involve 3 genera of mold that grow on keratin on a living host.
- ex. Epidermophyton, Trichophyton, and Microsporum
- Tinea capitis is ringworm of the scalp, Tinea pedis is athlete’s foot
onychomycosis - non-dermatophyte nail infections or any fungal nail infection caused by any fungus
What are the differences between fungal and human cells? How are these differences exploited in antifungal drugs?
Fungal cells need ergosterol (not cholesterol) to survive. If we can inhibit this, the membrane can be weakened and cause it to become leaky
b-glucan synthesis occurs in fungal cells but not human cells
we can target fungal microtubules
Which steps are inhibited by the allylamines and the azole antifungals in ergosterol synthesis?
allylamines - inhibit conversion of squalene to lanosterole
azole - inhibit conversion of lanosterol to ergosterol
Family, MOA, basis for selective inhibition of fungi, major toxicities, spectrum: echinocandins, griseofulvin, tavaborole
echinocandins (caspofungin, micafungin, anidulafungin) - These inhibit the synthesis of b(1-3) glucan in the fungi cell wall (not in human cells, so this is the reason for specificity). These will cause leaky membranes -> fungicidal. Can be used in combination if needed.
griseofulvin - disrupts fungal microtubles
tavaborole - inhibits leucyl transfer RNA synthetase (leuRS), which inhibits protein synthesis
How can you recognize these structures: Amphotericin B, terbinafine, ketoconazole, itraconazole, fluconazole, voriconazole, posaconazole, tavaborole, caspofungin, anidulafungin, flucytosine
amphotericin B - large molecule with polar side, that forms H-bonds with warer, lipophilic side that liked the lipid bilayer. Also has a mycosamine group that binds to cholesterol.
terbinafine - small molecule with 2 rings and a carbon-carbon triple bond
Azoles - 5-membered aromatic azole ring (key nitrogen that helps to inhibit the 14 a-demethylase)
- ketoconzaole has dioxalane ring on asymmetric carbon
- itraconazole is a triazole instead of imidazole
- fluconazole has 2 azole rings
- voriconazole has a fluoropyrimidine ring instead of the 2nd triazole
- posaconazole has a furan ring
- isavuconazole is structurally similar to voriconazole but is a prodrug
echinocandins - large lipopeptides that are all delivered IV
Flucytosine - pyrimidine analog
griseofulvin - 3 ringed molecule, nothing special
tavaborole - small molecule with a Boron that is essential for activity
What regions of amphotericin B are lipophilic and hydrophilic?
The top chain is lipophobic (no double bonds, lots of OH), the bottom chain is lipophilic (double bonds)
What are the solubility properties of amphotericin B and how have these problems been overcome to increase bioavailability? What type of delivery is needed for treatment of fungal meningitis?
Very poorly absorbed (need IV for systemic infections). Can use PO if treating a GI infection. Need intrathecal therapy for fungal meningitis.
Lipid formulations have been used to reduce nephrotoxicity.
What is the toxicity of amphotericin B? How does this relate to flucytosine therapy?
Infusion-related: fever, chills, muscle spasms, vomiting, headache, hypotension -> reduce rate of infusion to help
Renal damage: occurs in nearly all patients.
- reversible: reduced renal perfusion
- irreversible: renal tubular injury (usually after prolonged administration >4g)
Since flucytosine is renally excreted, you need to make sure that the amphotericin B isn’t causing renal damage that results in a toxic level of flucytosine.
What are the CYP450 interactions with the azole agents? How does this affect other drug therapies? Why can this be a problem?
Azoles are metabolized extensively by CYP450s. This means we can’t use them systemically unless the drug doesn’t get metabolized by CYP450s.
Triazole concentrations can be increased by other drugs metabolized by this pathway. Concentrations of other drugs that use CYP enzymes may be elevated. Inducers of CYPs can decrease triazole levels.
- Ketoconazole - 3A4 inhibitor
- Itraconazole - metabolized by 3A4
- Voriconazole - metabolized most by 2C19
All are inhibitors of CYP3A4.
Why can some azole antifungals only be used topically whereas others can be used systemically?
Only the azoles with reduced metabolism are used for systemic infection (because we need to avoid the CYP metabolism)
- ketoconazole, fluconazole, itraconazole, voriconazole, posaconazole, isavuconazole
What is the azole functional group and what is its significance for the MOA?
5-membered aromatic azole ring (key nitrogen that helps to inhibit the 14 a-demethylase). The nitrogen is what binds to the Fe3+ in the CYP450 to inhibit the conversion to ergosterol.
How does the metabolism of flucytosine in fungal cells differ from that in animal cells?
Animal cells are unable to convert flucytosine to the active metabolite.
What reaction is catalyzed by thymidylate synthase? How does flucytosine inhibit this reaction?
dUMP is converted to dTMP by thymidylate synthase. Flucytosine inhibits this by being converted to %-GU, then 5-FdUMP, which inhibits thymidylate synthase.
In 5-FdUMP, there is an F in place of an H that is in dUMP. This traps the thymidylate in the inactive form.
What antifungal drugs are able to penetrate the cerebrospinal fluid?
- intrathecal amphotericin B
- flucytosine
What are the two reasons why amphotericin B and 5-fluorocytosine are often used in combination?
He didn’t say much about this
- 5-fluorocytosine is synergistic with amphotericin B (maybe because they both effect the cell wall?)
By which mechanisms do fungi become resistant to antifungal drugs?
Natural resistance (intrinsic) vs. Induced (acquired)
- Candida krusei, Candida glabrata, and Aspergillus terreus all have intrinstic resistance.
- Acquired resistance is not transferred between strains as in bacteria.
Azoles - target site alterations accounts for most resistant strains, reduced drug concentration via efflux pumps, target enzyme upregulation, development of bypass pathways
Polyenes - reduced ergosterol content
Echinocandins - rare, but target site mutations
Flucytosine - cytosine deaminase or UPRT (cytosine permease) so it can’t get into the cell/get activated
Allylamines and griseofulvin - not well characterized yet
What fungal classes/drugs cause these toxicities: hepatic (4), renal (2), CNS (1), photopsia (1), cutaneous (2), GI (3), cardiac (2), infusion reactions (2), bone marrow suppression (2)
hepatic - all azoles, amphotericin B, 5-FC, echinocandins
renal - amphotericin B, IV voriconazole
CNS - voriconazole
Photopsia - voriconazole
Cutaneous - rash (all), photosensitivity/malignancy (voriconazole)
GI - Itraconzaole, posaconazole, 5-FC
Cardiac - cardiomyopathy (itraconazole), QTc prolongations (all azoles, esp. with drug interactions)
Infusion reactions - amphotericin B, echinocandins
Bone marrow suppression - 5-FC, amphotericin B
What is the MOA, spectrum (HSV, VZV, CMV, influenza, SARS-CoV-2, etc.), PK (absorption, 1/2 life, elimination, dose adjustments, etc.) for acyclovir?
MOA - After being converted to the monophosphate form by viral enzymes, acyclovir is triphosphorylated, then inhibits viral DNA polymerase resulting in inhibition of viral replication. It’s also incorporated into viral DNA to cause premature chain termination.
Spectrum - Herpes simplex virus (HSV) 1 and 2, Varicella zoster virus (VZV)
- HSV-1 > HSV-2 > VZV > EBV > CMV
PK - 10-20% PO bioavailability, dose-dependent oral absorption; 1/2 life 2.5-3.5hrs; widely distributed in tissues/body fluids, can penetrate CSF if dose is doubled, adjust for renal dysfunction
What are the therapeutic indications, adverse events, and monitoring for acyclovir?
Therapeutic indications -
1. Herpes simplex virus infections (inc. HSV encephalitis [IV dose 10mg/kg q8h], mucocutaneous disease in immunocompromised host)
2. Varicella zoster virus infections (severe VZV in immunocompromised host 10mg/kg q8h IV)
Adverse events -
- Nephrotoxicity (crystalline nephropathy)
- Thrombophlebitis
Monitor -
- Renal function (prevent nephrotoxicity)
What is the MOA, spectrum (HSV, VZV, CMV, influenza, SARS-CoV-2, etc.), PK (absorption, 1/2 life, elimination, dose adjustments, etc.) for valacyclovir?
MOA - same as acyclovir (inhibit DNA polymerase to inhibit replication/chain termination)
Spectrum - HSV, VZV
PK - 3-5x greater PO absorption than acyclovir, adjust for renal dysfunction
What are the therapeutic indications and adverse events for valacyclovir?
Indications -
1. Varicella zoster virus (preferred over PO acyclovir)
Adverse events - same as acyclovir
1. Nephrotoxicity
2. Thrombophlebitis
What is unique about famciclovir?
Oral famciclovir undergoes rapid and extensive conversion to penciclovir. Penciclovir is phosphorylated by viral thymidine kinase to penciclovir monphosphate and eventually to penciclovir triphosphate by cellular enzymes, which then inhibits viral replication
(other than this he said he doesn’t ever use this)