antiviral / antiparasites

Question 1

Normal stages of fusion are as follows: gp41, gp120>

Answer 1

1. Binding of HIV surface protein gp120 to the CD4 receptor
2. Conformational change in gp120, which both increases its affinity for a co-receptor and exposes gp41
3. Binding of gp120 to a co-receptor either CCR5 or CXCR4
4. Penetration of cell membrane by gp41, which approximates the membrane of HIV and the T cell, promoting their fusion (importance of gp41)
5. The HIV viral core then enters the cells

Question 2

what are the 2 fusion inhibitors and what do they do

Answer 2

enfuvirtide
maroviroc

binds to gp41, causing steric hindrance, so the gp41 cannot alter its conformation and prevents gp 41 from creating an entry pore for the capsid of the virus. This stage that is inhibited is the final stage of the host-virus fusion and leaves them trapped at the binding stage.

maraviroc blocks the CCR5 coreceptor that works together with gp41 to facilitate viral entry

Question 3

what is one ion channel blovker, and how does it work?

Answer 3

amatadine

Influenza contains: NA, HA, M2 proton channels, sialic acid receptor / protein
Of which, the M2 proton channels are essential for viral uncoating as once the virus is endocytosed, it is within an acidic vacuole, and M2 channel opens to permit the entry of protons, hence causing the inside of the cell to become acidified too. This acidification results in the dissociation of ribonucleoproteins and the onset of replication.

MOA: blocks the M2 proton channels, resulting in the virus not being able to be acidified, viral core not broken down and cannot replicate.

Indicated for influenza, but increasing resistance rising.

Question 4

what are the different types/examples of viral synthesis inhibitors

Answer 4

NRTI (nucleoside RT inhibitor) - AZT Zidovudine (azidothymidine)
NNRTI (non-nucleoside RT inhibitor) - efavirenz

Acyclovir (ACV)

Question 5

what does NRTI do and what is one example

Answer 5

Zidovudine (azidothymidine) AZT

MOA: AZT is an analog of thymidine, making it a very good substrate for cellular thymidine kinase, which is the enzyme that adds its first phosphate. The 5’ triphosphate compound eventually gets incorporated into the growing viral DNA, resulting in obligatory chain termination as it has no 3’-OH group, for the addition of more nucleotides, so we get incomplete DNA. AZT is a more potent competitive inhibitor of viral RT than of cellular DNA polymerase (selectivity), although at higher concentration it can affect DNA polymerase also!! (around 100 times more potent for RT)

Question 6

what is one NNRTI and its MOA

Answer 6

Efavirenz

What: non-competivie, non-nucleotide reverse transcriptase inhibitors (NNRTIs)

MOA: binds near the active site of RT, altering its conformation, They allow RT to still bind to dNTP, and a primer template, but inhibit the actual joining of the 2. Unlike NRTIs, they do not need to be phosphorylated / activated.

Question 7

what is acyclovir and how does it work

Answer 7

works by inhibiting viral synthesi s

What: competitive inhibitor of viral DNA polymerase, which is used to treat the herpes simplex virus and varicella zoster. It is a guanosine derivative, which needs to be triphosphorylated in order to become active.

MOA: Drug competes with GTP for binding ot DNA polymerase and then gets incorporated into the synthesising DNA, terminating further synthesis as it lacks a 3’-OH, thich causes irreversible binding between DNA polymerase and interrupted chain.

Why is it selective” the viral DNA polymerase is much more sensitive to triphosphate ACV than cellular DNA polymerase is.

Indicated for: viruses which it is used to treat both encode their own thymidine kinase, which does the first phopsohyrlaton, around 30 fold faster than the mammalian TK, meaning that it accumulates much faster in infected cells.

Question 8

what are the inhibitors of viral maturation

Answer 8

protease inhibitors (e.g. Paxlovid, Ritonavir)

Question 9

what is ritonavir and what does it do>

Answer 9

Protease inhiibtors,

MOA: inhibits the cleavage of gag/pol HIV polyproteins, meaning that replicated viruses still get released but are immature and so noninfectious.

Drug interactions - numerous, unpredictable.
Interacts with P450, CYP3A4
Inhibitors of P450 enhance SE of protease inhibitors
Inducers of P450 can lower plasma levels of protease inhibitors, and encourage viral escape

Question 10

what inhbitors of viral relase

Answer 10

Zanamivir / Oseltamivir

What: homologue of sialic acid, which is a surface sugar on cells (esp the respiratory epithelium), of which HA of the influenza binds to

MOA: NA would normally cleave SA from cells such that the virus can be released, but zanamivir binds to the active site of NA to prevent this action.

Question 11

what are the 6 targets of antiviral drugs

Answer 11

1. fusion inhibitors (viral attachment)
2. viral uncoating (ion channel blockers)
3. viral DNA integration
4. viral RNA/DNA synthesis (polymerase)
5. viral protein synthesis
6. viral release

Question 12

what are the 4 classes of drugs used in the treatment of malaria (p. falciparum)

Answer 12

1. haem metabolism - chloroquine, artemisinin
2. ETC - atovaquone, primaquine
3. protein translation - doxycycline
4. folate metabolism - proguanil

Question 13

What is used to treat E. histolytica

Answer 13

metronidazole

Question 14

what is used to treat T. cruzi

Answer 14

early stage (non CNS)
suramine, pentamidine

late state (CNS)
melarsoprolw

Question 15

what is use to treat O. volvulus

Answer 15

ivermectin
diethylcarbemazine
albendazole / mebendazole
praziquantel

Question 16

what does chloroquine do

Answer 16

plasmodium falciparum

Chloroquine / mefloquine is a weak base, which gets trapped in the acidic parasitic food vacuole, where it accumulates 100x fold. In protonated form, it inhibits heme polymerase as it binds to ferriprotoporphyrin 9 (FP9) decreasing the conversion of toxic FP9 into non-toxin hemozoin, leading to toxic heme polymerisation, killing the parasite. Additionally, chloroquine also exhausts the enzyme superoxide dismutase (SOD), which is responsible for clearing the free radicals produced in the process, leading to free radical buildup which is also toxic to the plasmodium. Chloroquine is used to treat infection by P. fal, vivax, ovale, malariae, knowlesi. However, parasites can develop resistance, due to the K76T mutation in the Plasmodium falciparum chloroquine resistant transporter (PfCRT), leading to increased drug efflux from the food vacuole.

Question 17

what does artemisinin do

Answer 17

p. falciparum

Artemisinin / artesunate have an unusual peroxide bridge that reacts with the iron in haem, causing bond breaking and releasing free radicals, and leading to free radical mediated alkylation of proteins within the plasmodium, such as PfATP-6 (an Ca2+ ATPase), killing the parasite. Artemisinin is used as the first line treatment of chloroquine resistant P. falciparum, however, a mutation in PfATP-6 can allow the plasmodium to also develop resistance. `

Question 18

what does atovaquone do

Answer 18

Atovaquone is a structural analogue of ubiquinone, inhibits the interaction between ubiquinone and the next step in the electron transport chain, cytochrome bc1, disrupting the ETC. The ETC is required to regenerate DHOD (dihydroorotate dehydrogenase), decreasing conversion of dihydroorotate to orotate, decreasing orotate, hence decreasing pyrimidine production, decreasing plasmodial DNA replication, and killing the parasites.

Atovaquone is 100x more selective for plasmodium over human ubiquinone, as there are differences in amino acid sequences at the cytochrome bc1 - ubiquinone binding site. Resistance against atovaquone can be achieved by point mutations in the cytochrome bc1, hence atovaquone is often administered as combination therapy with either doxycycline (protein translation inhibitor) or proguanil (folate metabolism inhibitor)

Question 19

what does doxycycline and proguanil do

Answer 19

p falciparum

Doxycycline inhibits protein translation of the plasmodium by binding to the 30S ribosomal subunit of plasmodium ribosome, inhibit the interaction between the ribosome subunit and the aminoacyl tRNA, leading to decreased RNA translation and hence protein synthesis and parasite dies. It is an indication for combination therapy with artemisinin for treatment of chloroquine resistant plasmodium falciparum.

Proguanil inhibits folate metabolism, and is a biguanide derivative. It is converted into its active metabolite cycloguanil, which inhibits DHFR-TS complex (dihydrofolate reductase thymidylate synthase complex), leading to decreased tetrahydrofolate production (THF), decreased purine and thymidine production, and decreasing DNA and RNA production, causing the parasite to die. Resistance can develop if there are mutations in the plasmodial DHFR.

Question 20

what does metronidazole do

Answer 20

e. histolytica

Metronidazole is mainly used in the treatment of E. histolytica, a protozoal infection characterised by dysentery.

Metronidazole contains a nitro group, which is converted into the toxic radical active form by reduction of the nitro group, allowing it to act as an electron acceptor, which then binds to proteins, membranes and DNA of target cells, causing severe damage to E. histolytica. The activity of metronidazole is directly related to the pyruvate ferredoxin oxidoreductase (PFOR) activity, as PFOR metabolises the drug into its active form. PFOR is only found in amoeba and anaerobes, as they lack pyruvate decarboxylase to convert pyruvate into acetyl coA, hence the drug exhibits selective toxicity for these organisms. Metronidazole is used to treat invasive amoebiasis, and is able to kill trophozoites in the tissues, but not in the GIT lumen due to high GIT uptake, hence has to be co-administered with an agent that high intraluminal activity like iodoquinol.

Resistance against metronidazole can be developed when there is a E. histolytica co-infection with H. pylori, which leads to a null mutation in the rdxA gene encoding for O2 insensitive NADPH nitroreductase.

Question 21

what does ivermectin do

Answer 21

Ivermectin is used as an anti-filarial drug, especially in onchocerca volvulus infections, which causes river blindness.

The exact mechanism of Ivermectin is still unknown, but it is thought to:
Activate and open glutamate gated Cl- channels, causing Cl- influx and leading to hyperpolarization of the neuromuscular cells, resulting in pharyngeal paralysis of the worm.
Activates GABA receptors, which also increases Cl- influx, further hyperpolarizing the cell. Does not affect the human GABA receptors as there is low affinity and inability to cross the BBB. However, in meningitis, the patients BBB is permeable.
Binds to the allosteric site of nAChR, increasing ACh transmission and leading to motor paralysis of the worm.
Inhibits the nutrient uptake of the worm hence this kills the larvae, but not the adult worm.
Ivermectin is used to kill the O. volvulus parasite larvae, but not the adult, and hence is non-curative for onchocerciasis. It is administered every 6 months for the lifespan of the worm. One main side effect is Manzzotti reactions, to which the dying larvae induce an inflammatory / allergic response to the dying microfilariae, such as oedema, headache, itchiness, abdominal pain, hypotension. O. volvulus doesn’t develop resistance in humans, but may do so in animals due to the expression of P-glycoproteins.

Question 22

what does praziquantel do

Answer 22

o volvuvlus

Drug of choice for treatment of adult cestode (tapeworm) and trematode (fluke) infections, e.g. schistosomiasis, which causes significant morbidity and mortality

MOA:
Binds to protein kinase C binding site in the beta subunit of schistosome voltage gated Ca2+ channels, leading to contracture and musculature paralysis.
Disrupts tegument, unmasking novel antigens, making it more susceptible to host immune response
Phagocytosed in the liver.

Question 23

what does albendazole / mebendazole do

Answer 23

MOA: binds to beta tubulin, inhibiting polymerisation and microtubule assembly. This disrupted nematode motility and DNA replication, leading to degenerative changes in the integument and intestinal cells of helminth - immobilisation, starvation and death.
Selective for nematodal isoform of beta-tubulin (250-400 fold)