Chemotherapy Flashcards
What is the concept of an antibiotic
typically antibacterial or antifungal drugs, interfering with some structure or process that is essential to growth or survival of these organisms without harm to the eukaryotic host harbouring the infecting cells.
How have most of the antibiotics in the last 60 years come about
natural products, elaborated by one microorganism in a particular habitat and set of environmental conditions to affect neighbouring microorganisms, either to regulate their growth or to trigger their elimination.
What are the 3 main classes of synthetic antibiotic
Give an example of each and their introduction date
sulfa drugs (such as sulfamethoxazole), introduced in the 1930s,
fluoroquinolones (such as ciprofloxacin), introduced in the 1960s,
oxazolidinone (linezolid), approved in the USA in 2000.
What dichotomy does the existence and clinical development of both synthetic and natural product antibiotics reflect in the 20th century
At one end of the spectrum was the medicinal chemistry view and the
classical “magic bullet” approach, initiated by Ehrlich, that pure
compounds could be made with therapeutic specificity and utility
At the other end of the spectrum, and from a separate track, came the isolation of penicillin (by Fleming in 1929), a natural product, as a potent antibacterial agent
Which type of antibiotic was a triumph for Ehrlich’s ‘magic bullet’ approach
sulfa drugs
Why was the isolation of penicillin important
led to the recognition of the paradigm that microbes wage war against each other with antibiotics
Name 8 antibiotic drug classes which stemmed from the realization that microbes wage war on each other
penicillins and cephalosporins,
tetracyclins,
streptomycins
and later generations of aminoglycosides,
chloramphenicol, rifamycins,
glycopeptides
the erythromycin class of macrolide antibiotics
What does broad or narrow range mean?
those that kill or inhibit a wide range of Gram-positive and Gram-negative bacteria are said to be broad spectrum.
If effective mainly against Gram-positive or Gramnegative bacteria, they are narrow spectrum.
If effective against a single organism or disease, they are referred to as limited spectrum.
When did the 2 strands of antibiotic discovery converge
with the introduction of later generations of semisynthetic variants of b-lactam antibiotics and macrolides, in which chemistry is used to engineer some desired new property, such as oral bioavailability, increased stability, broader spectrum of activity (referred to as extended spectrum), or efficacy against resistant microorganisms.
From 2000 to 2010, how did the use of last resort antibiotics change
increased consumption of carbapenems (45%) and polymixins (13%)
Antibiotic use increased by 36% from 2000 to 2010, where was this mainly accounted for? How can we stop this?
Brazil, Russia, India, China, and South Africa accounted for 76% of this increase
programmes that promote rational use through coordinated efforts by the international community should be a priority
bactericidal vs bacteriostatic
antibiotics that stop bacteria or fungi from growing are bacteriostatic, exemplified by chloramphenicol.
Antibiotics that cause cell death are
bactericidal; they lower the cell count of the infecting organism, as shown for penicillin
When would you use bactericidal drugs over bacteriostatic antibiotics?
when the immune system is compromised as bacteriostatic antibiotics require the bacteria to be eliminated by the immune system
Can a drug be both bacteriostatic and bactericidal
Some antibiotics can display bacteriostatic activity in some circumstances and bactericidal activity in others, where sufficient damage to one or more metabolic pathways or cellular structures occurs such that a net bactericidal response is triggered
What is the MIC and MBC for antibiotics
minimal inhibitory concentration (MIC): lowest concentration of a drug that prevents growth of a particular bacterium.
The minimal bactericidal concentration (MBC) is the lowest concentration that kills the bacterium
Discriminate between selective toxicity and therapeutic index
selective toxicity: growth of the infecting organism is selectively inhibited or killed without damage to the cells of the host
therapeutic ratio: the ratio of the maximum non-toxic dose over the minimum effective dose of a drug.
What is the selective toxicity and therapeutic index of penicillin
What drug has a selective toxicity and therapeutic index that is the opposite of this
high degree of selective toxicity, which corresponds with a large therapeutic index
Polyene antibiotic (eg amphotericin B) have both a low degree of selective toxicity and a low therapeutic index
Are the examples of penicillin and polyene antibiotics representative of most antibiotics’ selective toxicity and therapeutic index relationship?
Give an example
no
there is often little relationship between selective
toxicity and therapeutic index.
aminoglycosides
Describe the selective toxicity and therapeutic index of aminoglycosides
very selective with respect to killing bacterial versus host cells, but for some members of this class, unrelated effects on the patient’s nervous system, kidneys, or inner ear (hearing, balance) result in a much lower margin for therapeutic error that would be predicted on the basis of their selective action on cell viability
Where does the selectivity of anthracyclines come from
differential accumulation:
some tumour cells have enhanced rates of glycolysis, and as a result, reduced pH. Drugs (e.g., anthracyclines) that are trapped in the cell through protonation (thus becoming cationic) are accumulated more in tumour cells than in normal cells.
Give an example of differential activation in cancer chemotherapy
drugs that are activated by reduction (e.g. alkylating agent Mitomycin C), have enhanced toxicity in hypoxic tumour cell in solid tumours
How can cancer drug selectivity be based on differential importance
Alkylating and cross-linking agents, antimicrotubule drugs and antifolates are effective because cancer cells have a high demand in DNA replication and cell
division.
Antibodies and small molecule inhibitors target signaling pathways that are permanently switched on in cancer cells.
What are the 4 major targets for antibiotic drugs
(i) cell wall biosynthesis (sometimes membrane is targeted),
(ii) protein biosynthesis,
(iii) DNA replication, repair and expression
(iv) folate coenzyme
biosynthesis
One guiding concept for selectively killing bacteria while sparing the mammalian
host would be for the antibiotic to act against a target present in bacteria but not
found in animals or humans. Which antibiotic types is this true for
bacterial cell wall biosynthesis targetters
Enzymes in protein biosynthesis, DNA replication, repair and expression, and folic acid synthesis clearly have mammalian counterparts, but there are enough structural differences between the prokaryotic and eukaryotic synthesis machineries that selective inhibition is achievable.
What does the bacterial cell wall do in essence
Why can the cell membrane not do this
encases a volume hypertonic to the environment of the organism
Although it is critical to the maintenance of osmotic gradients between the cytosol and the extracellular environment, it is not strong enough to keep the hypertonic sac from rupturing by osmotic shock
What do gram positive and negative bacteria both have in common in their cell walls
How does this feature differ between them
a peptidoglycan (PG) layer
thicker and mutlilayered in gram +ve
gram +ve also has polymers of teichoic acids associated with it.
Describe the crosslinking of the PG layer of the cell wall in bacteria
What is the purpose
Which types of bacteria does this happen in
PG, with orthogonal glycan and peptide strands, undergoes enzymatic cross-linking of the glycan strands, by transglycosylase action, and of the peptide strands, by transpeptidase action.
introduce covalent connectivity to the meshwork, impart mechanical strength, and provide the major structural barrier in the bacterial cell wall to osmotic pressure forces
both Gram + and -ve
What is the first of the 3 stages of PG synthesis for the bacterial cell wall
UDP-NAG + L-Ala + D-Glu + L-Lys (or DAP) + PEP —> UDP N-acetyl
muramyl-tripeptide
D-Ala-D-Ala dipeptide is joined to produce UDP N-acetyl muramyl pentapeptide.
How can the formation of UDP-NAM-pentapeptide be inhibited by antibiotics
D-cyclosporine (analogue of D-Ala):
inhibits L-alanine racemase, D-Ala-D-Ala synthetase and ligase
Fosfomycin:
inhibits pyruvyl transferase
What do the following enzymes do
L-Ala racemase
Pyruvyl transferase
L-Ala racemase:
converts L-alanine into D-alanine
Pyruvyl transferase:
that catalyses the transfer of the PEP group to UDP NAG in the production of UDP NAM-tripeptide
What is the 2nd of the 3 stages in bacterial PG biosynthesis
UDP-NAM-pentapeptide is bound to bactoprenol via a pyrophosphate bridge and then a UDP-NAG is added to it, forming Lipid II
this can then be transferred from the cytosol to the outer leaflet of the phospholipid bilayer
on the outside the the basic repeating subunits can be put together into a polymer by the action of PG transglucosylase and are cross linked by peptidoglycan transpeptidase
Where does the first stage of PG synthesis take place
inside the cell, at the inner leaflet of the phospholipid bilayer
What is bactoprenol phosphate
a lipid anchor also referred to as C55 undecaprenyl phosphate
involved in PG synthesis
What does the final stage of PG synthesis involve
reorientation of bactoprenol pyrophosphate to the inner membrane surface, and its dephosphorylation to
bactoprenol phosphate to be recycled
Which antibiotic interferes with the final stage of PG synthesis
how
bacitracin
forms a very tight complex with Mg2+ and bactoprenol pyrophosphate, this cyclic peptide antibiotic inhibits the regeneration of the lipid carrier, and hence, the biosynthesis of PG.
Which antibiotics inhibit transpeptidation of PG strand cross linking
Penicillins, cephalosporins and other beta-lactam antibiotics
How does the pentapeptide in PG synthesis differ between different bacteria
contains diaminopimelic acid (DAP) in Escherichia coli, and L-lysine in Staphylococcus species.
In the transpeptidation
reaction DAP can be used directly, whereas L-lysine is first linked to a pentapeptide interbridge
composed of glycine, alanine and/or serine residues
What type of enzyme are transpeptidases
serine hydrolases
active site has serine as a nucleophile
What is the mechanism for the transpeptidase reaction (3)
- serine attacks D-Ala-D-Ala amide bond -> acyl-O-Ser formed and one is D-Ala released
- The acyl-O-Ser enzyme intermediate has the glycan-tetrapeptidyl moiety as the transiently linked acyl group. Hence, the transpeptidase enzyme forms an intermediate in which it is covalently linked to PG
- acyl transfer to amino moiety of DAP (or L-Lysine) in a neighbouring pentapeptide, resulting in cross-linking of the two peptide strands with the regeneration of the enzyme for another catalytic cycle.
How do transpeptidases interact with beta lactam antibiotics
Ser adds into the strained four-ring lactam carbonyl and generates an acyl enzyme intermediate in which the b-lactam ring has opened.
The covalent penicilloyl enzyme, thus formed, is very slow to hydrolyse, as H2O is excluded from the active site of transpeptidase enzymes
What happens without PG biosynthesis in bacteria treated with beta-lactam antibiotics
continued activity of cell wall autolytic enzymes (autolysins), which normally enable re-shaping of PG during cell growth and division, induce lytic death of the cell
Hence, only proliferating cells (in
which autolysins are active) are sensitive to β-lactam antibiotics
How does vancomycin block cell wall biosynthesis
binds to pentapeptidyl tails in the PG repeating unit terminating in D-Ala4-D-Ala5
This substrate sequestration shuts down transpeptidation by making the D-Ala-D-Ala terminus
unavailable to the transpeptidase enzyme
also reduces the accessibility of the PG repeating unit to the transglycosylase enzyme
Which drugs contain both a beta lactam ring and a thiazolidine ring
penicillins and cephalosporins.
How do carbapenems differ from penicillin in structure
different stereochemical configuration of constituents on the beta lactam ring
What is the ratio of protein to RNA in a ribosome
1: 2
protein: RNA
What are the subunits of the bacterial ribosome
A bacterial 70S ribosome consists of two subunits, 30S and 50S
What is the first stage of protein synthesis
formation of the initiation complex
What is the first N terminal amino acid for all bacterial proteins
formylmethionine
How does the bacterial ribosome attach to the bacterial RNA (2)
How is the initiation complex completed
mRNA becomes attached to the 30S subunit (requires initiation factor 3).
The formylmethionine-charged tRNA then combines with the mRNA-30S ribosomal
complex - requires two additional initiation factors (1 and 2) and GTP
the 50S ribosomal subunit becomes bound to the mRNA-30S-tRNA amino acid complex, the bound GTP is hydrolysed, and the initiation factors are released.
What can the ribosome do once the initiation complex is complete
translate the reading frame associated with the initiation codon, with which the formylmethionine-charged tRNA was associated.
Describe protein synthesis in the bacterial ribosome once the initiation complex has been completed
The first two tRNAs orient themselves appropriately towards the mRNA, with their attached amino acids adjacent to each other on the surface of the 50S portion of the ribosomes. The two amino acids then become linked by a peptide bond by the peptidyltransferase activity associated with the 50S subunit. The carboxyl group of formylmethionine is linked to the amino group of the second amino acid, and the dipeptide is attached to the second tRNA, which is occupying the A (aminoacyl) site. The tRNA for formylmethionine is now moved from the P (peptidyl) site to the E (exit) site and is released; the tRNA with the attached dipeptide moves from the A to the P site, and the 30S subunit moves one codon along the mRNA. The process of elongation continues with the addition of single amino acid units until a termination sequence signals that the protein is complete
Name 2 drugs that target the 30S bacterial ribosomal subunit
tetracycline
streptomycin
Name 2 drugs that target the 50S ribosomal subunit
chloramphenicol
erythromycin
Where does streptomycin bind to the ribosome
which other drug is associated with this site
streptomycin bind to the 30S subunit near the A
site for aminoacyl-tRNA binding
Paromomycin also binds near the A site and these drugs’ binding is cooperative
What happens to bacterial translation in the presence of streptomycin
disruption in decoding and translational accuracy, resulting in a decrease in the fidelity of translation.
Where does tetracycline bind
what are the chemical reactions involved
on the 30S subunit
contains rRNA, not protein, in a groove of 20 Å wide and 7 Å deep
oxygens of internucleotide phosphodiester links in 16S rRNA form electrostatic interactions, directed through a Mg2+ ion to the bottom edge of tetracycline.
How does tetracycline interfere with translation
rotation of aminoacyl-tRNA into the A site would be blocked by tetracycline. The aminoacyl-tRNA would then be prematurely released, terminating that cycle without peptide bond formation.
Where does erythromycin bind
to E site of the 50S subunit
and interacts with 23S rRNA, allowing about a 6- to 8-oligopeptidyl-tRNA build-up before elongation is blocked and prematurely terminated
What are macrolide antibiotics
Name 3
bind to E site on 50S subunit
Erythromycin
clarithromycin and roxithromycin
Which antibiotics do macrolide antibiotics interact with
why is this surprising
competitive with lincosamide antibiotics (such as lincomycin) that are direct peptidyltransferase inhibitors.
macrolide antibiotics do not directly block the peptide bond-forming step at the peptidyltransferase centre of the 50S subunit
Which antibiotics are affected by mutations in the 23S RNA of the bacterial ribosome
alter the binding of macrolides, lincosamides and streptogramin B family members,
suggesting the partial physical overlap of binding sites of these antibiotics.
What does clindamycin interact with
both the A site and the P site of the peptidyltransferase centre, competing with the binding of loaded tRNA molecules at these positions
Where does chloramphenicol bind
to 50S subunit
blocks aminoacyl-tRNA interaction with the A site of the peptidyl transferase centre
What does fusidic acid do
binds to and inhibits the elongation factor G, thereby blocking translocation in bacteria
What is synercid
a synergistic nonribosomal peptide combination (quinupristin plus dalfopristin) that blocks polypeptide translocation at the 50S subunit of the ribosome by binding to 23S rRNA sites partially overlapping with those targeted by macrolides.
Why does stopping transcription of proteins not kill bacteria
bacteria are unable to influence the composition or nutrient availability in the extracellular environment. The temporary inability to synthesize cellular proteins is therefore not unusual, and bacteria have evolved stress response mechanisms to survive periods of starvation
Are aminoglycosides bactericidal
yes
Name 2 aminoglycoside antibiotics
streptomycin (or the newer, less toxic gentamycin)
How do streptomycin and gentamycin affect translation
decreases the fidelity of mRNA translation, growing bacteria frequently insert the “wrong” amino acid with a consequent alteration in the activity of proteins. When this happens in membrane proteins, the permeability of the membrane will be affected, resulting in the leakage of small ions followed by larger molecules and eventually by whole proteins from the bacterial cell prior to aminoglycoside-induced death
Is erythromycin bacteriostatic or bactericidal
both:
depends on the bacterial species, the drug concentration and the bacterial density.
Which class of enzymes have been used for many years and led to the discovery of Topo II enzymes (specifically DNA gyrase)
the coumarins, represented by streptomycete metabolites such as novobiocin and coumermycin A1
Name a fluoroquinolone
ciprofloxacin
Why is ciprofloxacin so widely used
because of its
activity against both gram-negative and gram-positive bacteria in urinary tract
infections, osteomyelitis, community-acquired pneumonia, and gastroenteritis.
How do newer quinolones differ from older fluoroquinolones
newest generation of quinolones, such as gatifloxacin, have increased potency against gram-positive pathogens.
Why are Topo enzymes essential for cell viability
changes in supercoiling of DNA are required during replication and gene expression
What are etoposide and camptothecin used for
mammalian Topo II inhibitors
used in cancer to kill rapidly dividing tumour cells
what is topo IV
Like DNA gyrase, topo IV is a bacterial type II DNA topoisomerase, but unlike the DNA gyrase it cannot supercoil DNA.
Instead, topo IV carries out the ATP-dependent relaxation of DNA and is a more potent decatenase than DNA gyrase.
How do quinolones affect DNA enzymes
affect the double-strand
cleavage/double-strand religation equilibrium in DNA gyrase and topo IV catalytic cycles, such that the cleaved complex accumulates
may speed up the double-cleavage step of bound DNA or selectively slow the double-religation step
What is the killing action of quinolones
As the quinolone-covalent DNA gyrase-doubly cut DNA intermediate accumulates,
it blocks the progression of the replication forks,
DNA repair machinery is recruited which, upon failure, turns on signalling pathways that lead to rapid, directed cell death (apoptosis).
What does rifampin do
binds in a very
tight, but non-covalent manner to an allosteric site on the b subunit of the DNAdependent RNA polymerase at a ratio of one mole of drug per mole of enzyme.
directly blocks the elongating RNA chain at the di- or tri-nucleotide stage by binding in the DNA/RNA tunnel associated with the b subunit (analogous to the binding of macrolide antibiotics to the protein exit tunnel in the 50S ribosomal subunit). As a result of the binding, rifampin inhibits the initiation of RNA synthesis, but synthesis in progress at the time of drug exposure is not affected.
What are the 2 categories of antibiotic that interact with DNA
those that interact noncovalently with DNA and those that form covalent bonds with DNA
Name some drugs which intercalate DNA
Many rigid planar polycyclic antibiotics (e.g., daunorubicin and actinomycin D) and synthetic compounds (e.g. ethidium)
Why is the planar character of daunorubicin important
allows intercalation between the adjacent stacked base-pairs of the double DNA helix
How do intercalating drugs disrupt replication and DNA transcription
insertion event is based on a preliminary local unwinding of the double helix to produce spaces between the stacked pairs into which the planar polycyclic molecules can move
this partial unwinding affects the molecular dimensions of the major and minor grooves in the DNA, and therefore the interaction of template DNA with DNA polymerases, RNA polymerases and transcription factors
What are Bleomycins
Which organisms do they particularly affect
a family of metal-chelating glycopeptide antibiotics
especially toxic to Gram-positive bacteria and mammalian cells.
What does mitomycin C do in bacteria
alkylates DNA (at guanine bases at GC positions in complementary DNA strands)
Thus, mitomycin induces the cross-linking of two Gs, one in each strand of the double helix, and prevents strand separation during DNA replication and transcription.
What is the chemical interactions underpinning bleomycins’ action
based on the interaction between O2 and the bound iron,
which generates superoxide and hydroxyl radicals causing single- and DSB in DNA
What is Co-trimoxazole
a combination drug of sulfamethoxazole and trimethoprim
What is the class of synthetic chemicals in the longest use as effective antibiotics
sulfa drugs, first tested in the 1930s as bacteria-killing molecules
What does sulfamethoxazole block
dihydropteroate synthase (DHPS)
What does trimethoprim inhibit
dihydrofolate reductase (DHFR)
What are DHPS and DHFR important for
synthesis of tetrahydrofolate, a methyl carrier coenzyme required for the biosynthesis of dTMP, and hence, DNA
Why is folate metabolism different in eukaryotes and bacteria
how is this reflected in their enzymes
Bacteria have to make the folate skeleton de novo, while eukaryotes can scavenge
folate from dietary sources and transport it into cells
DHPS is totally absent from mammals while bacterial versus mammalian DHFR have enough structural differences that selective inhibition can be achieved
Why is it useful for a DHPS inhibitor and a DHFR inhibitor to be used in combination
while sulfonamides shut off de novo synthesis of folate, folate pool levels would take several bacterial generations to decline, giving a slow killing mechanism.
Addition of trimethoprim traps the folate coenzyme molecules in the useless dihydrofolate form after each cycle of dTMP synthesis, leading to a rapid depletion of the tetrahydrofolate form of the coenzyme
Name 3 bacterial nonribosomal peptide antibiotics
What do they all act on
valinomycin,
gramicidin A,
polymixin
act on the cytoplasmic membrane.
How is valinomycin made
How is it toxic
produced by Streptomyces species,
exerts toxicity by its
ionophoretic capacity
Describe the MOA of valinomycin
contains three repeating units of (L-lactate)-(Lvaline)-(D-hydroxyisovalerate)-(D-valine), which form a circular structure
A dehydrated K+ ion is coordinated precisely and specifically to carbonyl groups in the hydrophilic interior of valinomycin.
Because valinomycin is electroneutral, it carries the single (+) charge of the bound K+ ion.
As valinomycin diffuses across the membrane, it functions as a K+ uniporter.
What is the MOA of monensin
The ionophores nigericin and monensin lose a proton as they bind
K+ or Na+, respectively, and function as K+-H+ or Na+-H+ antiporters
(similar to valinomycin)
Describe gramicidin A
What is its MOA
a hydrophobic linear polypeptide antibiotic consisting of 15 aa and a carboxyterminal ethanolamine
Upon dimerisation in the membrane, this molecule forms a transmembrane ion channel that permits passive diffusion of monovalent cations with diameters of up to 5 Å
What is the net charge of polymixin
What is its structure
+5
a cyclic amphipatic protein
How is polymixin proposed to work
associate with the negatively charged phosphate head group region on the outer surface of the membrane, and then, to aggregate into micelle-like complexes which bind lipids and affect the permeability of the cytoplasmic membrane
It is possible that all lipopeptide antibiotics have some component of
membrane penetration and membrane disruption to their bioactivity. Give an example
the cyclic lipopeptidolactone daptomycin, in complex with Ca2+ ions, may exert part of its antibacterial activity through such membrane-seeking, surface-active behaviour.
What makes antibiotics that interact with the bacteria cell membrane toxic
What is a problem with a lot of these drugs
dissipation of transmembrane ion gradients, which disturb ion homeostasis and the energy metabolism, and induce leakage of macromolecules from the bacteria affected
lack of selectivity,
(presence of a phospholipid bilayer is not restricted to bacteria.)
Which antifungal agents target cell membranes
How are they selective
polyenes amphotericin B and nystatin
exploit differences in the sterol composition of the fungal plasma membrane (ergosterol) and mammalian plasma membrane (mainly cholesterol)
bind preferentially to
ergosterol, facilitating the formation of pores for ions and macromolecules
Name some antibiotics which indirectly affect the barrier function of membranes
Antifungal synthetic triazoles (eg fluconazole) and imidazoles (eg miconazole) inhibit enzymes involved in ergosterol biosynthesis.
Depletion of ergosterol alters fluidity of the membrane, thereby affecting permeability and the activity of membrane-associated enzymes
Where are parasitic diseases most prevalent
under conditions of crowding, poverty, and poor sanitation
What is the most important distinction of pharmacological significance to the
treatment of parasitic infections
How do drugs act between these groups
whether the infecting organism is a unicellular protozoan, or a multicellular helminth
Most antiparasitic agents have a relatively broad spectrum of activity within one of these groups, but virtually no crossover activity in the other group
Why are the molecular mechanisms of antiparasitic drugs often not known
difficulties in culturing parasites under controlled experimental conditions
What are the 2 broad categories for the MOA of anti-parasitic drugs
acting on (i) cellular integrity and (ii) biosynthesis of essential cofactors and macromolecules.
What do trivalent organic arsenicals react with
Why are they toxic
hiol groups (especially dithiols) in cofactors (e.g., lipoic acid; this
inhibits lipoic acid-dependent enzymes) and enzymes (e.g. pyruvate kinase and
phospofructokinase),
hence, ultimately inhibit ATP synthesis
What is melarsen oxide
an arsenical generated from prodrug melarsoprol are preferentially toxic for Trypanosoma species.
What makes melarsen oxide selective against Trypanosoma species
probably due to selective uptake in these organisms
Which parasites does suramin attack
How does it work
Trypanosoma
inhibits glycerol-3-phosphate oxidase
and NAD+-dependent glycerol-3-phosphate dehydrogenase
interfere with the reoxidation of NADH and inhibit ATP synthesis.
Which antifungals act against Leishmania (2)
Why
amphotericin B and
miconazole
membrane of certain Leishmania species contains ergosterol, generated
in an ergosterol biosynthesis pathway
What do trophozoites of Plasmodium do when in RBCs
digest hemoglobin to obtain amino acids for biosynthesis (haem is released as a byproduct)
Where does digestion of haemoglobin occur in trophozoites
inside the food vacuole of the parasite
What is the toxic waste problem that trophozoites face
If the released heme were allowed to accumulate within the food vacuole, the heam level could easily reach 200 – 500 mM!
oxidation of haem iron results in the production of ROS
(superoxide anion, H2O2, and hydroxyl radicals), which would harm the parasite.
How do trophozoites prevent haem accumulation
by polymerizing the haem into nontoxic
crystals of hemozoin
How does chloroquine work
which drug works in a similar way
inhibits haem polymerisation reaction in trophozoite food vacuoles, causing haem to build up
mefloquine
How does artemisinin extert antimalarial activity
y through the generation of highly reactive
organic free radicals.
How do causal prophylactic drugs block malaria development
block the link between the exo-erythrocytic stage and the erythrocytic stage, and thus prevent the development of malarial attacks
What is true causal prophylaxis in malaria
How do we do this
the prevention of infection by the killing
of the sporozoites on entry into the host
not feasible with the drugs at present in use, although it may be achieved in the future with vaccines.
What is the antiparasitic drug sulfadoxin
p-aminobenzoate analogue
competitively inhibit the action of dihydropteroate synthase (DHPS)
What is sulfadoxin usually given in combination with when treating a parasitic infection
What is this combination called
Which parasites can it treat
trimethoprim analogue pyrimethamine (blocks DHFR)
Fansidar
Plasmodium falciparum
Why is the potential for finding drugs that target parasitic protein synthesis limited
However 2 antibiotics show activity against certain protozoa. What are these drugs?
their protein synthesis is essentially similar to mammals
tetracyclines and lincomycins
Why is it difficult to develop antiviral drugs that act only on infected cells
Viral replication depends on many of the same cellular processes that operate in normal uninfected mammalian cells
What is the genome of influenza like
what contains the genome
eight segments of negative-sense single-stranded RNA
nucleocapsid
How is the influenza genome affected by pH
The nucleocapsid encasing the genome contains an ion channel
protein, M2, that triggers uncoating of the genome when the virus is exposed to low pH.
What are the roles of influenza’s neuraminidase
helps prevent viral aggregation,
facilitates release from the host cells, and may have a role as a virulence factor.
Name 2 major drugs used against influenza A
What are their MOAs
amantadine and rimantadine
At an early step in viral replication, they
block the function of the M2 channel protein. At a later stage, they interfere with hemagglutinin processing
How does amantadine work
At an early step in viral replication, block the function of the M2 channel protein.
At a later stage, interfere with hemagglutinin processing
What is sialidase
another name for neuraminidase
What is zanamivir used to treat
What is its MOA
What is another drug that works the same way
Influenza A and possibly B
Inhibits neuraminidase, enhancing viral aggregration and inhibiting release from host cells
also reduces viral movement in the upper respiratory tract
oseltamivir
Compare oseltamivir and zanamivir
both inhibit neuraminidase in Influenza A and (possibly) B
Zanamivir is an active drug whereas oseltamivir is an ethyl ester pro-drug, which is cleaved by esterases in the plasma and in cells of the gut upon the adsorption of oseltamivir.
What do all available anti-herpes agents target
the virally encoded DNA polymerases
that replicate the double-stranded DNA genome of these viruses
How do viral DNA pol work
in the same manner as cellular DNA polymerases, (i.e., join the 5’-OH group of the base being added to the 3’-OH group of a 2’-deoxyribose sugar in the polymerized strand of DNA)
What is the origin of the ‘acyclo’ in acyclovir
purine analogues used against herpes viruses all lack the cyclic sugar of 2’- deoxyguanosine and acyclovir was the first drug of this ring-lacking class
What happens when acyclovir is taken up by cells (3)
monophosphorylated by a herpesvirus-encoded thymidine kinase. Cellular enzymes then convert it to the triphosphate form.
Aciclovir triphosphate competitively inhibits viral DNA polymerase.
In addition, it can be added by DNA polymerase to the 3’-OH of a strand of DNA, but it has no corresponding 3’-OH to which additional nucleotides may be added. This terminates the DNA strand and permanently inactivates the enzyme
Why is acyclovir selective against viral cells
(1) only virally infected cells have the thymidine kinase required to monophosphorylate the drug,
(2) the drug preferentially binds to the virally encoded DNA polymerase.
Does acyclovir affect healthy cells
yes but is 30 times more potent against the viral DNA polymerase that the host
enzyme
What is ganciclovir
What happens to it in infected cells
competitive inhibitors of DNA polymerase, but they have 3’-OH moieties and will permit chain extension
monophosphorylated by a phosphotransferase encoded by cytomegalovirus (CMV)
What is the drug of choice against CMV
Ganciclovir is more effective that aciclovir against CMV-infected cells
Name a pyrimidine analogue used as an antiviral
Cidofovir
How does cidofovir work
is a nucleoside phosphonate analogue of cytosine, which is converted to a diphosphoryl derivative that selectively inhibits the DNA polymerase of CMV
What is the antiviral drug foscarnet
How does it work
an organic analogue of pyrophosphate
selectively binds to viral DNA pol of CMV and herpes simplex virus (HSV) and others and prevents the cleavage of pyrophosphate from nucleoside triphosphate during DNA
polymerization.
What is contained in a HIV virion (for MoDA purposes)
two copies of a single-stranded RNA genome, a reverse transcriptase, and an aspartic protease
What happens to the HIV RNA early in the viral replication cycle
reverse
transcriptase converts the RNA into double-stranded DNA, which is then integrated
(via integrase) into the host cell DNA
What is the fidelity of the HIV reverse transcriptase like
What does this lead to
poor
frequent transcription errors, and a high degree of sequence variation among the viral genome copies that are produced
How are HIV reverse transcriptase inhibitors divided
into Non-nucleoside reverse transcriptase inhibitors (NNRTI) or Nucleoside reverse transcriptase inhibitors (NRTI)
Name a NNRTI
what is the MOA
nevirapine
binds to the target enzyme near the catalytic site and denature it.
Name a NRTI
zidovudine
azidothymidine, AZT - a thymidine analogue
How do NRTI drugs work
bind to the target enzyme by mimicking naturally occurring nucleosides
How do NRTIs and NNRTIs differ in action
NRTIs (but not NNRTIs) must be phosphorylated by host cell enzymes before becoming active
How do NRTIs terminate synthesis
What part of the viral life cycle are they important for
When incorporated into DNA, these compounds lack 3’-OH groups.
Reverse transcription is an early event in the replication cycle, so these agents have no effect on a cell in later stages of viral infection
What is often used in conjunction with AZT
Why is this given with pyrimidine analogues
Hydroxyurea
inhibits ribonucleotide reductase.
This decreases the intracellular pool of pyrimidine nucleotides, and thereby potentiates the effect of pyrimidine analogues.
How many HIV protease inhibitors are there currently
Name 2
What are they approved for
6
saquinavir and ritonavir
clinical use against HIV-1 and HIV-2
What is the purpose of the Asp protease in HIV
the mRNA transcribed from the provirus is translated into two biochemically inert polypeptides, termed gag proteins. HIV protease then converts these polypeptides into functional proteins by cleavage at the appropriate positions
Why is the HIV protease a good target
this protease does not occur in the host
When does HIV protease usually perform its function
What would happen without it
as the virus is budding from the cell membrane or shortly
thereafter.
Without these cleavages, the newly produced virus is not infectious.
Name something of note that is present in the HIV protease active site
HIV protease contains two aspartyl residues in its active site
What is viral interference
biochemical changes that inhibit viral propagation, brought about by inteferons
What are the 3 classes of interferon
Which have antiviral activity
IFN-a, IFN-b, and IFN-g
a and b
What do antiviral Interferons bind to and what does this cause
specific ganglioside receptors on host cell membranes and promote in host cell ribosomes the production of enzymes that inhibit the translation of viral mRNA.
Name 2 inteferons and state their use in treating viral infections
IFN-a-2a: used in treatment of hepatitis B infections and AIDS related Kaposi sarcomas.
IFN-a-2b: used for hepatitis C.
Name 3 uncommon cancers that have whose treatment has rapidly advanced through development of cytotoxic drugs
childhood cancers,
lymphomas
teratomas
When was nitrogen mustard introduced into clinical practice
What was quickly established
1946
The effectiveness of this class of compounds in producing regression in lymphomas
also gastrointestinal and haematological toxicity they produced
What was aminopterin shown to do in 1947
What was this quickly followed by
produces remission in acute leukaemia
(it is an analogue of folate)
production of methotrexate in ‘49
How do most anticancer drugs work generally
reducing rate of cell growth and division in the tumour
Give 3 ways to block cancerous cell proliferation
- By preventing effective DNA replication through direct binding to nucleobases or impairing the DNA synthesis machinery,
- By damaging the mechanisms of cell division such as formation of the mitotic spindle,
- By blocking the pathways involved in cell growth that are activated by signals such as growth factors or hormones.
Name 2 nitrogen mustards
melphalan
cyclophosphamide
Which anticancer agents bind directly to DNA
name a drug of each type
nitrogen mustards (melphalan),
nitrosoureas (lomustine),
aziridines (mitomycin C),
Pt compounds
(cisplatin).
What is the important chemical feature of nitrogen mustard
its 2 chloroethyl side chains
How does nitrogen mustard crosslink DNA
One of the
chloroethyl side chains undergoes a cyclization and forms an immonium-ion
intermediate with the release of a chloride
This strained three-membered ring is highly reactive and can attack the 7-nitrogen group of guanine.
This repeats with the other chloroethyl side chain with another G in DNA
What are the possible results of the reaction of nitrogen mustard with DNA
may result in cross-linking between DNA strands
or linking between bases within the same strand of DNA - inhibiting replication and gene expression
If the second side chain reacts with
H2O instead of a guanine, a monoalkylated guanine is produced, which bonds incorrectly with T, causing a GC>AT transition
In addition, the DNA containing the drug adduct is recognized by DNA repair systems, and strand scission may occur as a result of endonuclease attack when the cell attempts to repair the alkylated DNA.
Where does the general toxicity of nitrogen mustards come from
Alkylating agents have less favoured reactions with other nucleophilic
groups in DNA, RNA and protein
What is melphalan
L-phenylalanine mustard
a synthetic nitrogen mustard
What is the benefit of using melphalan
as phenylalanine is a precursor of melanin, it is accumulated in melanomas and thereby produces a relatively selective effect.
What is the most commonly used anticancer alkylating agent
Why is this
cyclophosphamide
has a a broad application in cancer chemotherapy (e.g. lymphoid tumours, and carcinomas of the breast, lung, ovary and endometrium).
