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).
Briefly state2 facts about the PK of cyclophosphamide
well absorbed orally and needs to be metabolised in the liver by the cytochrome P450 system to become activated to a phosphoramide mustard.
Name a nitrosourea
Which features give it anticancer applications
lomustine
has a chloroethyl chain and is both a alkylating and carbamoylating agent
What does it mean to say that a drug is both an alkylating and carbamoylating agent
Which anticancer drugs have these properties
they react with a
variety of groups to attach an alkyl (R-CH2-) or a carbamoyl (R-N-CO-) moiety
nitrosoureas
Can nitrosoureas produce DNA crosslinks
yes
all of the nitrosoureas (even those with only one chloroethyl side chain) can produce interstrand cross-links in duplex DNA in which N7 and O6 positions in guanine are preferred sites of attack.
What is mitomycin C used for
both antibiotic and anticancer (inhibits DNA synthesis via cross linking and alkylation)
Give a fact about the kinetics of mitomycin C
has no effect on purified DNA in vitro unless a cell extract is added - activated by chemical or enzymatic reduction of the quinone group
What kind of anticancer drug is mitomycin C
aziridines
What is the MOA of mitomycin C
After reduction of the quinone moiety, a tertiary methoxy group is spontaneously eliminated.
The aziridine ring is then broken, creating a semiquinone radical that reacts with nucleophilic groups in DNA (most commonly guanine).
Then, intramolecular displacement of the carbamate group yields the cross-linked DNA-drug adduct.
Which drug is only active in cis form
Cis-diamine dichloroplatinum (cisplatin)
What are the principal sites of reaction of cisplatin at physiological pH
N7 atoms of guanine and adenine
True or false
cisplatin only works in vivo
false
regardless of whether it is purified DNA, intact cells, or tumour-bearing patients that are exposed to cisplatin, the principal coordinate is an intrastrand cross-link formed by binding of the drug to two neighbouring guanines (pGpG).
What was found in a trial of the DNA adducts were analysed in white cells of cancer patients treated with cisplatin?
65% of adducts represented intrastrand cross-links on pGpG, 22% on pApG (but not pGpA) sequences, and 13% a mixture of other adducts
How doese the binding of cisplatin inhibit DNA synthesis/ gene expression
intrastrand cross-linking induces major bending of the DNA duplex towards the major groove, together with the physical block provided by the platinum adduct on the template strand
Name 2 types of drugs that non covalently bind to DNA in cancer treatment
anthracyclines
actinomycins
Name 2 anthracyclines
doxorubicin, daunorubicin
Name ant actinomycin
actinomycin D
What is the in the structure of anthracyclines and actinomycins that allows them to noncovalently bond to DNA
usually have planar aromatic ring structures that allow them to intercalate between stacked nucleobases at the centre of duplex DNA
What does intercalation of anthracyclines and actinomycins cause in DNA
What does it affect (3)
local unwinding on the helix with changes in the geometry of the minor and major groove
binding of DNA and RNA pol, and transcription factors.
What is the basis of the severe side effects of anthracyclines
generation of free radicals due to the presence of
hydroxyquinone moiety
while these free radicals attack DNA and can induce DNA cleavage, radicals can also cause lipid peroxidation, e.g. in cardiac tissue
How was the free radical problem with anthracyclines overcome
Anthracycline analogues with a modified structure, (eg mitoxantrone) lack this property of quinone-type free radical generation.
What is mitoxantrone
anthracycline analogue which lacks the property of quinone free radical generation
How can antimetabolites interfere with production of DNA and RNA (2)
(i) inhibition of normal precursor production, and
(ii) substitution of purines and pyrimidines in nucleic acid synthesis.
Name a folic acid antagonist in clinical use
methotrexate
What is the antifolate action of methotrexate
inhibition of dihyrofolate reductase (DHFR)
competes with folic acid for active transport into mammalian cells
Name a) an antibiotic, b) antiprotozoal and c) an anticancer drug that inhibits DHFR
a) trimethoprim
b) pyrimethamine
c) methotrexate
Antifolate medication can lead to folate depletion in healthy cells. How can mammalian cells be saved from this
by also giving Leucovorin (N5
-formyl-tetrahydrofolate)
How does Leucovorin work
can be readily converted to other reduced folic acid derivatives (e.g. tetrahydrofolate), and thus has vitamin activity that is equivalent to folic acid.
However, since it does not require the action of dihydrofolate reductase for its conversion, its function as a vitamin is unaffected by inhibition of this enzyme by drugs eg methotrexate.
Leucovorin, therefore, allows for some pyrimidine synthesis to occur in the presence of dihydrofolate
reductase inhibition,
Why does Leucovorin not simply inhibit methatexate
Due to the difference in tetrahydrofolate requirement between normal cells and rapidly proliferating tumour cell (low versus high, respectively), leucovorin is able to reduce the toxicity of
methotrexate in normal cells only
What are the most importnat members of the group of anticancer drugs that directly block thymidylate synthetase
pyrimidine antagonists (dUMP analogues)
name a pyrimidine antagonist
How does it kill cancer cells
5-fluorouracil
Although it is also incorporated into rRNA and mRNA, with disruption of further transcription, intracellular distribution and translation, inhibition of thymidylate synthetase appears to be its primary cytotoxic mechanism
Name 2 purine antagonists
what are they analogues of
What do they cause
6-mercaptopurine and 6-thioguanine are analogues of
hypoxanthine and guanine, respectively
cause nucleotide synthesis inhibition as well as being incorporated into nucleic acids (after their activation to triphosphate nucleotides).
Name 3 inhibitors of mammalian Topo II
etoposide
daunorubicin and doxorubicin
Which drugs inhibit mammalian Topo I
Camptothecins (derived from the Chinese tree Camptotheca) and topotecan
Which drugs were extracted from the periwinkle Catharanthus roseus and the bark of the Pacific yew Taxus brevifolia
What did they cause in animals
How did they do this
Vinca alkaloids (e.g. vinblastine and vincristine) and taxol (paclitaxel) respectively
granulocytopenia
poison the mitotic spindle by acting on microtubule formation
What is the major component of microtubules
tubulin dimer arranged head to tail in linear protofilaments
What is the tubulin dimer
a protein complex containing two non-identical (alpha and beta) subunits
describe the structure of a microtubule
13 protofilaments together form a hollow structure with a minus end which is anchored to an organising centre and a plus end where growth or shrinkage of the microtubule takes place
Are microtubules stable
they are in cilia and neuronal axons but in chromosome segregation are very labile
How does the microtubule population change with changing conditions
Individual microtubules can oscillate between polymerization and depolymerisation, and the net status of the microtubules population is very sensitive to factors affecting the equilibrium between free tubulin dimers and assembled polymers
What may affect the equilibrium of microtubule formation and breakdown
availability of numerous factors including tubulin, GTP, Mg2+, and non-tubulin protein
How do vinca alkaloids work
bind to free tubulin dimers. Although these agents do not all share the same binding site, their interactions with the tubulin dimer prevent microtubule assembly, and hence, result in the disappearance of microtubules
How does taxol work
it is a taxane drug that disrupts the equilibrium between free tubulin dimers and microtubules by shifting it in the direction of assembly rather than disassembly.
As a result, taxol treatment causes both the stabilization of ordinary cytoplasmic microtubules and the formation of abnormal bundles of microtubules.
Name a cancer that responds to body hormone levels
Many breast
cancers grow more rapidly in the presence of female steroid hormones
How can you predict whether a breast cancer will respond to hormone therapy
by the amount of estrogen and progesterone receptors in the tumour tissue.
What is tamoxifen
an anti-oestrogen that competitively binds to the estrogen receptor,
but with a lower affinity than estrogen
What happens when tamoxifen binds to the oestrogen receptor
complex is translocated to the nucleus, transcription of estrogen-responsive genes involved in the development and growth of breast cancers is attenuated.
How many breast cancers can respond to tamoxifen
Approximately 70% of breast cancers are positive for estrogen and progesterone receptors and can respond to tamoxifen
What is an issue with tamoxifen
can exert oestrogen agonist effects in healthy tissue eg bone and uterus
Name a tamoxifen analogue that lacks the oestrogen agonist properties
toremifene
What is aromatase
converts androgen precursors into estradiol
Name an aromatase inhibitor
What is it used for
anastrozole
treatment of estrogen-dependent breast cancer either instead of, or after
treatment with tamoxifen.
also used in post-menopausal women with breast cancer to prevent the formation of estrogens (from androgen precursors) at
peripheral sites such as muscle and fat tissue
What does GnRH do
binds to receptors in the pituitary gland and stimulates the production of luteinising
hormone (LH) and follicle-stimulating hormone (FSH)
What happens to LH secretion with continuous GnRH production
immediate increase in LH and FSH followed complete inhibition of their release
What does goseraline do
cause a biochemical castration (no synthesis of testosterone) by binding very strongly to the GnHR receptor.
What do anti-androgens do
name one
antagonise the interaction of endogenous androgens at their nuclear receptors and are also used against prostate cancer.
flutamide
Name a drug that utilises the fact that hormones can directly inhibit growth
prednisone,
has a lymphocytolytic effect and are useful against leukemias.
What is Rituximab
a chimeric monoclonal antibody against CD20
What does Rituximab target
CD20, a protein present on
the surface of many transformed B-cells (lymphocytes) in Hodgkin’s disease and non-Hodgkin’s lymphoma
What has Rituximab been shown to do
lyses CD20 cells through antibody-dependent cell-mediated cytotoxicity, activation of the complement cascade, and introduction of apoptosis.
What was the first monoclonal Ab approved against cancer
rituximab
What is Trastuzumab
a humanised monoclonal antibody against the
human epidermal growth factor receptor 2 (HER2).
How is HER2 related to cancer
over-expressed in 25 to 30% of human breast cancers and is associated with a poorer prognosis
How is trastuzumab administered
Trastuzumab is administered intravenously, once weekly, either alone or in
combination with classical anticancer agents for metastatic breast cancer that
overexpress HER2.
Name 4 common side effects of trastuzumab
chills, asthenia, fever, and nausea.
What is a risk with trastuzumab
Rarely, trastuzumab can cause cardiac dysfunction.
However, the risk of cardiac dysfunction increases significantly if trastuzumab is given together with other cardiotoxic agents such as anthracyclines.
What is bevacizumab
a humanised monoclonal antibody that binds to and inhibits the human vascular endothelial growth factor (VEGF)
What is VEGF
Why is it important for tumours?
a soluble protein that plays an important role in inducing blood vessel formation,
thereby allowing
tumours to grow beyond a few millimetres in size.
What is the monoclonal antibody against VEGF
How is it administered
For which cancer in particular
Which other cancers has it been shown to be effective against
bevacizumab
in combination with 5-fluoro-uracil
metastatic colorectal cancer
NSCLC and breast cancer
Give some side effects of taking bevacizumab (4)
Due to its anti-angiogenic action, bevacizumab
can interfere with wound healing and increase the risk of bleeding or gastrointestinal
perforation
hyper tension
proteinuria
What kind of antibody is Cetuximab and what does it bind
a human/mouse chimeric monoclonal antibody that binds to the extracellular domain of EGFR
What does EGFR consist of
a transmembrane glycoprotein composed of an extracellular ligand-binding domain, a short transmembrane domain, and an intracellular domain that has tyrosine kinase activity
What does binding of EGF to EGFR lead to
induces conformational changes within the receptor and increases the activity of associated tyrosine kinases.
This results in autophosphorylation and increased biological activity, which might include cell proliferation and/or cells differentiation.
Why is the EGFR important for cancer understanding
Abnormal EGFR expression, either through a mutation or over-expression, has been demonstrated in many malignancies.
How can inhibition of EGFR in cancer cells be achieved
by preventing ligand binding with an anti-EGFR antibody or by inhibiting EGFR-associated tyrosine kinases.
What does Cetuximab do (3)
binds to the extracellular part of EGFR and inhibits it
reverses the resistance of colorectal cancer to the topoisomerase inhibitor, irinotecan
used in combination with irinotecan in the treatment of EGFR overexpressing metastatic colorectal cancer in patients who are refractory to irinotecan.
What are the monoclonal antibodies you need to know for cancer treatment
cetuximab
bevacizumab
Trastuzumab (sold as Herceptin)
Rituximab
What are the 4 key small molecule inhibitors that are
erlotinib
imatinib mesylate
toceranib
mastinib
What is erlotinib and gefitinib
What have they been approved to treat
What are some side effects
orally active, potent, and selective quinazoline
derivatives that inhibit EGFR tyrosine kinases
advanced NSCLC
rashes and diarrhea
Which experiments have shown the effects of erlotinib
Growth inhibition and tumour regression have been seen in human xenograft models in lung, prostate, breast and colorectal cancers
What forms the Philadelphia Chr
reciprocal exchange of genetic material between the long arms of chromosomes 9 and 22. As a result of this translocation, a fusion gene, bcr-abl, is formed
Describe the activity of BCR-ABL
tyrosine kinase activity and is
constitutively active. It is thought to be the main cause of CML
Which drug can treat CML
Imatinib mesylate
an agent that specifically inhibits the BCR-ACL protein kinase activity, and it is active in chronic and blast phases of CML
What are the 2 uses of imatinib mesylate
inhibits BCR-ABL in CML
inhibits c-kit which is overexpressed in patients with gastrointestinal stromal tumours (GISTs)
What kind of actrivty does c-kit have
tyrosine kinase activity
which is commonly over-expressed in patients with GISTs
Name 2 tyrosine kinase inhibitors that are used in veterinary medicine
what do they do
Are they relevant to real medicine
toceranib
mastinib
used in the treatment of non-resectable grade II – III mast cell tumours (mastocytomas) in connective tissue in dogs
yes - both drugs used for c-kit mutations
What are Grade II and III cancer cells
Grade II cells are intermediately differentiated with a potential for locally invasive metastasis.
Grade III cells are poorly differentiated or undifferentiated with a high potential for metastasis.
What are some other small molecular ligands that are being generated against new drug targets in
mammals
check-point protein kinases (referred to as mammalian target of rapamycin, mTOR), telomerase, and epigenetic drug targets, such as histone deacetylases (HDACs)
What was the causative agent of plague
Which antibiotics is it susceptible to
Yersinia pestis
uniformly susceptible to the antibiotics streptomycin, chloramphenicol, and tetracycline, but has been reported to have high level resistance to multiple antibiotics
Why is TB now a problem again
a result of the emergence of Mycobacterium tuberculosis strains resistant to multiple antiTB drugs.
Define drug resistance
a condition in which there is insensitivity or decreased sensitivity to drugs that ordinarily cause inhibition of cell growth or cell death
What does intrinsic resistance refer to
a microbes inherent insensitivity to a drug
eg if the organism lacks the receptor for a drug
Which antifungal drugs do bacteria have an inherent resistance to
Why
antifungal polyenes
polyenes require presence of ergosterol in the membrane, which bacteria do not have
Describe the intrinsic resistance of fungi to rifampin
How can we make the fungus more susceptible
Although fungal DNA-dependent RNA polymerase is inhibited by rifampin, this drug is not particularly effective against fungi because the drug does not readily pass through the fungal cell envelope to its site of action.
simultaneous exposure to polyene antibiotics, which facilitates drug entry
Name a bacteria that has a significant intrinsic resistance to antimicrobial agents compared to other bacteria
Why is this
TB
because of the high content of mycolic acids in a complex lipid layer outside of their peptidoglycan, which is impermeable to many drugs
What is a first line agent against TB
why
How well does this drug work on other bacteria
Isoniazid
inhibits synthesis of mycolic acids for the lipid layer
Other bacteria, which do not use mycolic acids in their cell envelope, have a high intrinsic resistance against this drug.
What is acquired antibiotic resistance to
when populations of microorganisms that are initially sensitive to a drug undergo a change so that they become less sensitive or insensitive.
Does acquired resistance always mean the patient can no longer be treated by that drug
no
resistance can be slight but often organisms become resistant to any clinically achievable concentration of
drug.
How does the abundance of resistant organisms in a population change as the treatment continues
What is this called
increases relatively as non resistant microbes are killed off
selection - continued use of the drug exerts selection pressure on the population
What are the 3 ways microbes can become insensitive to antibiotics
(i) Enzymatic inactivation of drugs,
(ii) replacement, amplification or modification of drug
targets,
(iii) decreased drug uptake or increased efflux of drugs
Which kind of antibiotic is resistance via enzymatic inactivation of drugs a real problem for?
What does this reflect
natural product antibiotic classes but has not yet been observed as a major route of resistance development for the classes of synthetic antibacterials
time of exposure of bacteria to natural products, putatively hundreds of millions of years, versus the 70 years of less for the man-made antibiotics
What do b-lactamases do
destroy the chemical warhead of b-lactam antibiotics by hydrolysing their strained b-lactam
ring, which is the chemically reactive acylating group for modifying the active-site serine residue in transpeptidases in PG cross-linking
How effective is resistance through beta-lactamase production?
Use an example
very effective as one b-lactam-resistant E. coli cell can excrete up to 105 b-lactamase molecules, each with an ability to hydrolyse ~1000 b-lactams per second.
True or false
beta lactamases have developed because of clinical antibiotic use
false
b-Lactamase activity was detected a few years before clinical use of penicillins, indicating its presence in soil bacteria that combat the natural product penicillins
How many beta-lactmases exist now
how are they classified
> 190
into A, B, C, and D
A,C and D are Ser enzymes similar to PG transpeptidases
What kind of intermediate do A, C, and D beta lactamases form
What does this suggest
same type of penicilloyl-O-Ser enzyme covalent intermediate as PG transpeptidases
evolution from these transpeptidases.
Why is resistance to aminoglycoside antibiotics different to penicillins
aminoglycoside antibiotics do not have a reactive chemical warhead comparable to the b-lactam
aminoglycoside antibiotics do not have a reactive chemical warhead comparable to the b-lactam. What do aminoglycosides do instead
read specific regions in the 16S rRNA in the 30S ribosome subunit by a hydrogen-bonding network through the various hydroxyl and amino substituents on the cyclitol rings to provide a high-affinity docking site
What is the enzymatic destruction strategy for aminoglycoside resistant bacteria
to covalently modify those specificity-conferring OH and NH2 groups in the aminoglycosides and thereby interfere with recognition by the 16 S rRNA
How does beta lactamases A, C and D confer resistance to penicillin
usually penicillin irreversibly binds to transpeptidase (half life of 90 mins)
b-lactamase binds to penicillin and can incorporate water into the active site which rapidly hydrolyses the ring in miliseconds (similar to the speed of action of transpeptidase without penicillin present)
Why do class B lactamases fail to be inhibited by A, C and D lactamase inhibitors
There is no such covalent penicilloyl enzyme intermediate in the catalytic cycle of the zinc-dependent, class B lactamases. Instead, the B class enzymes use zinc to activate a water molecule and catalyse its direct addition to the b-lactam ring
What led to the development of extended spectrum cephalosporins
he growing number of b-lactamases in gram-negative E. coli and Klebsiella pneumoniae, as well as the emergence of these enzymes in other pathogens (for example, Haemophilus influenza and Neisseria gonorrhoeae)
Why were extended spectrum cephalosporins not effective for long
selective pressure quickly fostered the emergence of extended spectrum b-lactamases
(ESBLs), which could hydrolyse many of the oxyimino-cephalosporins
How did ESBLs arise
Name an example of an ESBL
by plasmid transfer from pre-existing chromosomal ESBL genes from Kluyvera spp., which typically is a non-pathogenic commensal organism. Other ESBLs harbour point-mutations that led to single amino acid changes in existing class A b-lactamases.
CTX-M
How many ESBLs have been identified
> 200
What are the 3 kinds of enzymatic modification of OH and NH2 on aminoglycosides
what have these enzymes likely evolved from
(i) N-acetylation of NH2 groups by acetyl-CoA,
(ii) O-phosphoryl transfer of the gamma-phosphate group of ATP to a OH moiety on the aminoglycoside,
(iii) O-adenylyl transfer of the a-phosphate group of ATP, resulting in the transfer of the AMP moiety to a OH moiety on the aminoglycoside
adenylyltransferases, phosphotransferases and N-acetyltransferases that had been utilized for normal biosynthetic processes in the bacterial cells.
What are the inactivating enzyme equivalents for bacteria other than aminoglycosides
Analogous to the aminoglycoside-inactivating acetyltransferases are families of inactivating acetyltransferases for streptogramin, chloramphenicol, and others.
Give a key feature of the methicillin structure
What is it used to treat
How did it dev
a bulky 2,6-dimethoxybenzoyl substituent on the 6-aminopenicillin scaffold
Gram positive bacteria that had become resistant to penicillin via inducible b-lactamase hydrolysis of the antibiotic
What was the benefit of the bulky side chain of methicillin
enhanced the lifetime of the covalent penicillyol-O-lactamase acyl enzyme intermediate against hydrolysis, thereby effectively deactivating the b-lactamase enzyme
What is MRSA
How common is it
methicillin-resistant
Staphylococcus aureus
In hospital environment in the USA, MRSA can now reach an incidence of 20 to 40%; in Japan an incidence of up to 60% has been reported
Where is MRSA particularly prevalent
burn centres and long term facilities
What is the problem with MRSA and beta lactam administration in hospitals
MRSA is resistant to essentially all b-lactam molecules, including penicillins, cephalosporins, carbapenems, and monobactams. Therefore, b-lactam administration selects for MRSA in the clinical setting
True or false
MRSA is elaborating an improved version of beta lactamase
False
in >90% of cases MRSA has acquired mecA gene, which encodes a new b-lactam-insensitive, bifunctional transglycosylase/transpeptidase (termed penicillin-binding protein 2A).
Auxiliary genes eg fem are also important
Describe the auxiliary genes that confer beta lactam resistance in MRSA when combined with mecA
fem (factor essential for expression of methicillin resistance) genes
adds a pentaglycyl cross bridge to PG before cross linking which is a better substrate for mecA transpeptidase than the original PG strand
What causes Commuity acquired pneumonia, meningitis, otitis media and sinusitis?
Why is this bacteria relevant to drug resistance
Streptococcus pneumonia
mechanism of b-lactam resistance based on changes in the composition of PG transpeptidase and other penicillin-binding proteins has been observed
Why did the use of vancomycin increase in the 1980/90s
What did this lead to
to treat infections caused by gram positive MRSA
selected for drug-resistant enterococci, less potent pathogens than staphylococci but opportunistic in the space vacated by other bacteria and in patients with compromised immune systems
Which species accounts for >90% of clinical isolates that are resistant to vancomycin?
Which patients are particularly vulnerable to this bacteria (2)
Enterococcus faecalis
patients with indwelling catheters, including dialysis patients and those undergoing cancer chemotherapy who have chemotherapy-induced white cell depletion in the middle of treatment cycles
What is the leading cause of endocarditis
enterococci
What is VanA
The first major clinical phenotype of vancomycin-resistant enterococcus
this was followed by VanB
How does VanA differ from VanB
essentially the same molecular mechanism but differs in the continuing sensitivity to the glycopeptide teicoplanin (a vancomycin analogue).
Which genes and gene products are necessary for both VanA and VanB
5 tandemly arranged genes
the products are:
3 enzymes, VanH, VanA, and VanX, involved in reprogramming of the PG termini
from N-acyl-D-Ala-D-Ala to N-acyl-D-Ala-D-lactate,
and
2 proteins, VanS and VanR forming a two-component (sensor and response regulator) signal transduction
pathway for inducible reprogramming to vancomycin resistance
What accounts for the 1000-fold decrease in vancomycin binding in resistant bacteria
switch from D-Ala to D-lactate as the terminal residue in the pentapeptide of the uncross-linked PG
Which class of antibiotic is erythromycin involved in
What is this class used to treat
macrolide
widely used for respiratory tract infections, but erythromycin resistance has become problematic.
Which bacteria are particularly important in erythromycin resistance
Pneumococcal
MRSA
What is a major route of resistance to macrolides
methylation of a specific adenine
(A2058) in the 23S rRNA in the 50S ribosomal subunit by RNA Nmethyltransferases, which is close to the macrolide-binding site
this reduces affinity of the rRNA for lincosamides and streptogramin B, without affecting rRNA function
Reduced drug influx is an effective drug resistance mechanism for which drugs
hydrophilic drugs which hardly diffuse passively across phospholipid bilayers
Give an example of bacteria using drug efflux to promote resistance
In Gram -ve P. aeruginosa, aminoglycosides are taken into the periplasm via facilitated diffusion through porin channels in the outer membrane and are then taken across PM by oligopeptide transporters
resistance arises from a decreased porin count, modification of the lipopolysaccharide outer leaflet, and
mutations in uptake trasnporters
What drug combination is usually given to patients with an S. aureus infection
Why
Aminoglycosides are often combined with a b-lactam drug
b-lactam drug affects cell wall synthesis and increases the passive diffusion of the aminoglycoside into the cell.
enhances bactericidal activity, whereas aminoglycoside monotherapy may allow resistant staphylococci to persist during therapy and cause a clinical relapse once the antibiotic is discontinued.
Are hydrophobic drugs affected by decreased influx mechanisms
can slow down entry, but cannot not prevent it due to the non-protein mediated diffusion of the drug across cellular membranes.
Active efflux by transport proteins in the cytoplasmic membrane is then the only alternative to prevent the entry of drug in the cytosol
Name 4 types of drug where active efflux is relevant for resistance
b-lactams,
macrolides,
fluoroquinolones,
tetracyclines
Are bacterial efflux transporters dedicated to a single drug?
some are eg tetracycline transporters but some have a much broader range
Why do bacteria have efflux pumps normally
What does this allow
used physiologically for the export of specific metabolites and to pump foreign toxic substances
The integrated array of transporters with overlapping drug specificities can lead to a remarkable capacity to efflux drugs either as a chromosomally encoded metabolic capacity, which makes Pseudomonas aeruginosa intrinsically antibiotic insensitive, or by the acquisition of transport genes on plasmids and transposons.
How can drug transporters be divided bioenergetic POV
Which is more common
primary active systems (couple drug efflux to the hydrolysis of ATP) - more common in eukaryotic organisms
secondary (couple drug efflux with influx of Na+ or H+) - more common in prokaryotes
In gram-negative bacteria, what are
secondary-active drug transporters often associated with
an accessory protein, which spans the periplasm, and an outer membrane porin to allow drug transport across the cell envelope into the external environment
What is multiple drug resistance
The simultaneous expression of various antibiotic resistance mechanisms, each specific for a drug or class of drugs
What is a regulon
a ‘master switch’ which controls the co-expression of various drug resistance mechanisms, encoded by genes localised at different positions on the genome
Other than a regulon, how can multiple drug resistance arise
from the co-localization of antibiotic resistance genes on the same resistance plasmid (also called R-factor), which can be transferred from one bacterium to another by conjugation or transformation.
How can the R factor be transferred between bacteria
by conjugation or transformation
What causes multidrug resistance
expression of a multidrug efflux pump in the PM confers resistance to a wide variety of drugs due to the enormously broad specificity of the pump.
(not multiple drug resistance - be careful)
Why is multidrug resistance difficult to deal with clinically
many of the first-line classes of antibiotics can be effluxed from the cell.
Name an antifungal azole
What do these do
fluconazole
inhibit enzymes involved in ergosterol biosynthesis
What is azole resistance based upon
alterations in the activity and amount of ergosterol biosynthesis enzymes, and on active azole efflux.
What is resistance of herpes to purine analogues based upon
a change in the substrate specificity of the viral purine-activating enzyme thymidine kinase, disabling the phosphorylation of purine analogues.
Breifly what does the resistance to anti-HIV drugs arise from
Resistance to HIV reverse transcriptase inhibitors (e.g. zidovudine) or HIV protease inhibitors (e.g.,
saquinavir) is due to mutations in these enzymes that disable the interaction between enzyme and inhibitor.
How do parasites develop resistance to chloroquine
resistant parasites accumulates chloroquine in their food vacuoles
much less efficiently than chloroquine-sensitive strains, suggesting that drug resistance results mainly from exclusion of the drug from the site of action.
What might the lack of drug accumulation in chloroquine resistant parasites indicate
increased drug efflux from resistant parasites, and an ATP-dependent P-glycoprotein was implicated as the pump responsible
or
chloroquine efflux from the vacuole might be mediated by a mutated secondary-active transporter, termed CRT (chloroquine resistance transporter).
How can cancer cells use enzymes to deactivate drugs inside the cell
Drug can be detoxified by drug metabolism based on cytochrome P450 systems (CYP450) and on conjugation by glutathione S-transferase (GST) and other conjugating systems.
How can cancer cells modify drug targets to become resistant
Resistance can develop due to mutations in drug targets that alter specificity.
eg cells resistant to topo poisons (e.g. etoposides) possess modified topoisomerases.
How can altering gene expression/ enhancing gene repair increase cancer cell resistance to drugs
Nitrosourea-resistant cells have high levels of alkyltransferases that repair guanine lesions and so prevent DNA
cross-linking.
Cisplatin-resistant cells have higher levels of enzymes involved in DNA repair.
Resistance can also arise from altered gene expression. For instance, cells will not enter the apoptotic pathway if mutations result in the loss of p53 expression.
Guve an example of cancer cells using metabolic bypass to confer drug resistance
methotrexate resistance can be based on enhanced expression of dihydrofolate reductase (DHFR).
Methotrexate resistance can also result from reduced uptake due to mutations in the folate carrier, which reduces the affinity of this membrane transporter for methotrexate
How can cancer cells use drug efflux to confer resistance
use eg
Drug resistance (e.g. against vinca alkaloids, anthracyclines, and mitoxantrone) can result from enhanced drug efflux by multidrug transporters such as the multidrug resistance P-glycoprotein MDR1 (also termed ABCB1), multidrug resistance-associated proteins (MRP1 and 2, also termed ABCC1 and 2) and the breast cancer resistance protein (ABCG2).
Give 5 ways we can combat drug resistance
Identification of new drug targets
Specific inhibitors of drug resistance mechanisms
Development of new classes of antibiotics/ cancer drugs
Combination therapy
Extending antibiotic lifespan
How has genomics helped the battle against antibiotic resistance
Approaches involving gene disruption
have begun to narrow the list of genes in pathogens that are essential either for virulence or for survival to perhaps a few hundred genes
we can find chemicals that inhibit the products of these essential genes
How did the way we tackled b-lactam resistance change from when resistance was first seen to when it became a real menace
originally just tinkered with periphery of the ring so maintain effectiveness for a time
later, attention switched to approaches to neutralize the antibiotic-destroying hydrolase, both by screening against lactamase producers and by mechanism-based inhibition of the active-site serine hydrolases
What is clavulanate
How is it administered
a suicide inhibitor of lactamase
in combination with amoxicillin (as clavulanate was not an effective antibiotic by itself)
this combination is called Co-amoxiclav and it augments the range of amoxicillin
What is Unasyn
the combination fo sublactam and ampicillin
What is sublactam
a penicillin analogue with a five-ring sulphur atom oxidized to the sulphone
has a weaker C-S bond disposed the ring of the acyl-lactamase intermediate to open and create a long-lived covalent enzyme intermediate that was inactive.
What is Primaxin an example of
Another successful combination of a beta-lactamase inactivator and b-lactam antibiotic (imipenem-cilastatin)
What is an important feature of 3rd gen erythromycins currently in development
less prone to induce resistance due to methylation of the antibiotic
What combination is being designed to combat macrolide and tetracycline resistance
a separate inhibitor of efflux pumps, once identified as having sufficient potency and safety, could be combined with the macrolide antibiotic or the tetracycline.
What is Oritavancin
What is important about its structure
a semisynthetic analogue of vancomycin - used to act against vancomycin resistance enterococci
contains a hydrophobic biphenyl substituent on the vancosamine sugar and is more hydrophobic and may partition the analogue more to the membrane, as well as alter its ratio of inhibition between transpeptidases and transglycosylases
What is Synercid
What is it used to treat
combination of two non-ribosomal
peptides, quinupristin and dalfopristin, which act synergistically to inhibit protein synthesis in a bactericidal manner
treating infections by vancomycin resistant enterococci.
What is the new structural class of synthetic antibiotic molecules with a broad spectrum and acceptable potency
oxazolidinones
What is the MOA of oxazolidinones
o inhibit protein biosynthesis, specifically by interaction with the 23S ribosomal RNA of the 50S ribosomal subunit at or near the peptidyl transferase centre of the ribosome
Name an oxazolidinone
linezolid
What is ramoplanin
glycolipodepsipeptide drug against vancomycin-resistant enterococci.
What is the MOA of ramoplanin
forms a complex with the lipid pentapeptide intermediates in cellwall biosynthesis, and acts in a somewhat analogous way to vancomycin by targeting a substrate rather than an enzyme in the peptidoglycan assembly pathway
Name a natural product antibiotic produced by “uncultured”
bacteria discovered in 2015
teixobactin
Why is resistance to
teixobactin less likely to occur than with other antibiotics
binds to lipid-like
peptidoglycan precursors and gene mutations can only affect lipid structure indirectly (through alterations in enzymes in lipid synthesis pathways)
What are the different kinds of combination therapy
i) Augmentin and Synercid approaches where two components work together to neutralize a single target,
ii) combinations of distinct antibiotic classes that work on different targets concurrently (eg HAART)
Why is combination therapy eg HAART so effective
fewer cells or viruses will escape from drug action,
and the probability of mutation to clinically significant resistance is reduced
What is HAART
(highly active antiretroviral therapy
incorporates a mixture of protease inhibitors (e.g., saquinavir) and reverse transcriptase inhibitors (e.g., zidovudine and nevirapine).
What is syngergy when it comes to antibiotics
Why is it a therapeutically desired response
The characteristic that combinations of antibiotics can have a greater effect than the sum of the two individual drug effects
allows a reduction in the amount of antibiotic required to obtain a therapeutic effect.
What is an antagonistic combination of antibiotics
Why does this happen
likely to occur when a bactericidal drug (e.g., a penicillin or an aminoglycoside) is combined with a bacteriostatic drug (e.g., a tetracycline)
many bactericidal agents have a killing effect only on cells that are growing or actively synthesizing protein, and that the bacteriostatic drugs prevent growth or protein synthesis and thereby counter the effect of the bacteriostatic agent alone
Give an example of a syngerstic combination of antibiotics
an aminoglycoside and an inhibitor of cell wall synthesis, which is based on the increased entry of the aminoglycoside into the bacterium
Combinations in which two antibiotics inhibit different steps in the same critical metabolic pathway may also result in
synergism. What is an example of this?
trimethoprim and sulfamethoxazole (co-trimoxazole), which both inhibit the synthesis of tetrahydrofolate
What is an indifferent drug combination
drug combination is roughly a summation of the effects of the individual drugs
What are some ways to decrease incidence of drug resistance
judicious use of antibiotics (which has been recommended by NICE)
rotating use of antibiotics
avoid antibiotic use in farm animals
Gvive an example of how we might rotate antibiotic use to preserve efficacy
first-line therapy would be to start with one of the 3 main b-lactam categories, such as a b-lactam plus lactamase inhibitor (e.g. the amoxicillin-clavulanate combination).
After 2 months, the unit would cycle to carbapenem antibiotics as the front-line therapy.
At the end of the next 2 months, a third- or fourth-generation (expanded-spectrum and higher) cephalosporin would become the front-line choice.
Then, one would cycle back to the initial combination choice, completing a three-drug traverse in the 6 months
Which cancers can be cured by current chemotherapy (5)
testicular cancer, Hodgkin’s disease, non-Hodgkin’s lymphoma, choriocarcinoma, many childhood cancers.
How doe we screen naturally occurring products to find new treatments for canacer
New compounds are screened for activity against human and animal tumour cell lines in vitro. The most promising agents are tested further to identify the maximally tolerated dose in mice and other species
How are new anticancer drugs being developed
more drugs are being developed on the basis of know pathways involved in cancer progression and unique activity in preclinical models, rather than through largescale screening of naturally occurring compounds
What are the phase I trials in new anticancer drugs
performed in cancer patients when
no other treatment is available or when conventional therapy has been unsuccessful
What is the starting dose of a phase I anticancer drug
How does dosage change from here
based on the patient’s body
surface area, is usually equivalent to one-tenth of the maximally tolerated dose in mice
If this produces no major toxicity, the dose is increased in the next group of patients. In this way, the maximally tolerated dose is determined, which allows Phase II trials
What is the major aim of Phase II cancer trials
What happens in Phase III
Assessment of anti-tumour efficacy. Sometimes, the efficacy is also tested in combination therapy with existing agents
comparisons are made with the best available current therapy
What is D-cycloserine
what is its MOA
structural analogue of D-Ala
prevents the synthesis of
pentapeptide by inhibiting L-Ala racemase, D-Ala-D-Ala synthetase and D-Ala-D-Ala/
muramyl tripeptide ligase
What is the MOA of fosfomycin and what is it approved for
inhibits pyruvyl transferase required for the conversion of NAG into NAM.
Was recently approved in the USA for treatment of urinary tract infections.
What is bacitracin and what is its MOA (2)
What is it active against
cyclic polypeptide antibiotic;
forms a tight complex with Mg2+ and bactoprenol pyrophosphate;
inhibits the dephosphorylation to bactoprenol phosphate (lipid carrier for NAM-NAG unit).
active against Gram-positive bacteria.
What are the following and what is their MOA:
Penicillin, ampicillin, amoxicillin and methicillin
name a combination associated with these drugs
ß-lactam antibiotics; inhibit peptidoglycan-crosslinking transpeptidase enzymes by covalent and irreversible binding as a pseudosubstrate. The combination of the ß-lactam amoxicilin and the ß-lactamase inhibitor clavulanate is used as augmentin
What is vancomycin and what is its MOA
What is it effective against
glycopeptide antibiotic;
binds to D-Ala-D-Ala termini of the pentapeptide in peptidoglycan, thereby preventing its cross-linking to a
neighbouring pentapeptide in another peptidoglycan strand.
Vancomycin is effective against Clostridium difficile, and is used intravenously against
Gram-positive cocci, such as Enterococcus and Staphylococcus
What is a drug designed to be similar to vancomycin
Oritavancin
a semisynthetic analogue of vancomycin containing a hydrophobic biphenyl substituent on the vancosamine sugar. The drug is more hydrophobic and may partition more to the membrane
What is the first line treatment for TB
How does it work
isoniazid
used in the prevention and treatment of tuberculosis.
It inhibits the biosynthesis of mycolic acids in the cell envelope of Mycobacterium tuberculosis.
What is extended spectrum cephalosporin active against
active against Gram-positive bacteria with increased activity against Gram-negative bacteria.
What is Teixobactin effective against and what is its MOA
How was it discovered
gram-positive bacteria.
It inhibits bacteria by binding to Lipid II in the peptidoglycan biosynthesis pathway
was discovered using a new method of culturing bacteria in soil. This allowed researchers to grow a previously unculturable bacterium, now named Eleftheria terrae, which produces the antibiotic
Which antibiotics do we need to know that act on protein synthesis (6)
chloramphenicol tetracycline erythromycin fusidic acid streptomycin and gentamycin
What is the MOA of chloramphenicol
when is it used
locks aminoacyl tRNA binding to 50S subunit of ribosome. Very effective broad-spectrum antibiotic
use is restricted because of bone
marrow suppression in some cases. Drug is indicated in life-threatening infections eg meningitis.
What is tetracycline
what is the MOA
what is it effective against? Is it a first line drug?
is there any toxicity
polyketide antibiotic; binds to 16 rRNA in 30S subunit of ribosome and inhibits the movement of aminoacyl-tRNA into the A site.
effective against a wide range of bacteria and are first-line drugs against mycoplasma and cholera.
Drug toxicity is associated with binding of calcium in bones
and teeth.
What is erythromycin
what is its action
when is it used
14-membered macrolide antibiotic;
binds to 23S rRNA in 50S subunit, and blocks the polypeptide exit tunnel.
Drug has a similar antibacterial spectrum as penicillin and is a suitable second-line drug for patients allergic to penicillin.
What is the MOA of fusidic acid
When is it used
inhibits elongation factor G, and hence, the movement of the 30S subunit by one codon along the mRNA.
narrow spectrum and is used against staphylococcal infections
What are Streptomycin and gentamycin
MOA?
What are they used against
aminoglycoside antibiotics
target 30S subunit of ribosome and decrease fidelity of mRNA translation
Used against Gram-negative rods including Pseudomonas and Proteus. Most streptococci (Grampositive) are resistant because gentamycin cannot penetrate the cell. However, penicillin and gentamycin have a synergistic effect against some streptococci.
Compare the toxicity of gentamycin and streptomycin
Streptomycin exhibits nephrotoxicity and ototoxicity. These effects are reduced for gentamycin
What are the 5 antibiotics we learnt that act on DNA and RNA
ciprofloxacin rifampin daunorubicin bleomycin mitomycin
ciprofloxacin
type?
MOA?
effective against?
fluoroquinolone antibiotic;
inhibits type II DNA topoisomerases
(DNA gyrase and topo IV), and hence, inhibits changes in supercoiling required for replication and gene expression.
FDA-approved drug for killing Bacillus anthracis in anthrax infections. It is particularly useful for Pseudomonas infections where oral therapy is preferred, such as respiratory tract infections in patients with cystic fibrosis.
What is the MOA of rifampin
used for?
binds to b subunit of the DNA-dependent RNA polymerase and
inhibits initiation of RNA synthesis.
Is often used to treat infections by Mycobacterium.
Describe daunorubicin (3)
planar polycyclic anthracycline antibiotic;
intercalates in dsDNA and causes local unwinding.
Used as anticancer agent
Bleomycin
type of drug
MOA
metal-chelating glycopeptide antibiotic;
generates superoxide and hydroxyl radicals, causing single- and double-strand breaks in DNA.
describe mitomycin
aziridine-containing antibiotic;
alkylates and cross-links DNA, thereby preventing strand separation during DNA replication and transcription.
Name 3 antibiotics that act as antimetabolites
sulfamethoxazole
trimethoprim
co-trimoxazole
How does Sulfamethoxazole work
sulfa drug
p-aminobenzoate analogue that competitively inhibits dihydropteroate synthase in the biosynthesis of tetrahydrofolate (methyl carrier required for synthesis of dTMP).
what is the MOA of trimethoprim
inhibits dihydrofolate reductase in the biosynthesis of tetrahydrofolate.
What is Co-trimoxazole
What is it used for
combination of sulfamethoxazole and trimethoprim.
has been withdrawn as a routine anti-bacterial agent in this country, as result of its being implicated in cases of Stevens-Johnson syndrome – a severe and sometimes fatal allergic reaction. Co-trimoxazole is used in AIDS patients with fungal Pneumocystis carinii infections.
Name 5 drugs that change the bacterial membrane to kill it
valinomycin gramicidin polymixin amphotericin B fluconazole
moa of valinomycin
cyclic peptide antibiotic that binds K+ and facilitates K+ diffusion across the membrane.
structure and moa of gramicidin
linear polypeptide antibiotic
forms a homodimeric complex that acts as an ion channel in the membrane.
what is polymixin and what is its moa
cationic detergent antibiotic containing cyclic peptide and
hydrophobic tail;
binds to membrane and alters its ion permeability.
What us amphotericin
what is the moa
used for?
polyene antifungal and antiparasitic drug;
forms pores in
the membrane by an ergosterol- dependent mechanism
Is currently the drug of choice for most systemic mycoses: active against Cryptococcus, Candida, and Aspergillus
what is fluconazole
what is miconazole
What are their MOA
what are they used for
triazole antifungal drug
imidazole antifungal and antiprotozoal drug.
Both inhibit ergosterol biosynthesis.
Fluconazole is often used in the treatment of athlete’s foot and vaginal candidiasis.
name 12 anti protozoal drugs
melarsen oxide suramin amphotericin B and miconazole chloroquine and mefloquine artemisinin sulfadoxin pyrimethamine fansidar tetracycline and lincomycin
describe melarsen oxide
moa
used for
arsenic compound;
selectively accumulated in Trypanosoma species, inhibits energy metabolism.
Used for treatment of sleeping sickness and other diseases caused by trypanosomes
why was suramin introduced in 1920
what is it used for
what is a benefit of it
moa?
as a development of Ehrlich’s Trypan Red,
today still remains the drug of choice for trypanosomiasis.
Does not contain toxic metal, hence, its therapeutic index is much higher than that of melarsen.
Molecular mechanism still unknown; might act on glycolytic enzymes
What are Chloroquine and mefloquine and what do they do
antimalarial 4-aminoquinoline drug
inhibits haem polymerization in food vacuole of Plasmodium.
Describe artemisinin
prodrug of the biologically active metabolite dihydroartemisinin,
which is active during the stage when the parasite is located inside RBCs
MOA might be based on the production of oxygen radicals.
what is sulfadoxin
sulfa drug that inhibits dihydropteroate synthase in Plasmodium.
Analogue of the antibiotic sulfamethoxazole
what is the moa of pyrimethamine
inhibits dihydrofolate reductase in Plasmodium. Analogue of the antibiotic trimethoprim
what is fansidar
what is it used for
combination of sulfadoxin and pyrimethamine; analogous to
antibacterial co-trimoxazole
Used in the treatment of patients with P.
falciparum malaria when chloroquine resistance is suspected, and oral treatment is
appropriate.
name 10 antivirals
amantidine oseltamivir acyclovir ganciclovir cidofovir foscarnet zidovudine nevirapine saquinavir interferon
what is the MOA of amantadine and what is it used for
blocks the function of M2 ion channel protein involved in
nucleocapsid uncoating of influenza virus. Is used predominantly in prophylaxis and
treatment of infections by Influenza A virus. Not effective against Influenza B virus
What is the MOA of oseltamivir
what is a related drug
inhibit neuraminidase (sialidase), a glycoprotein enzyme that plays an essential role in the release of the virion progeny from infected cells, and that assists in the movement of the virus particles through the upper respiratory tract.
zanamivir
what is aciclovir
purine analogue;
upon activation by viral thymidine kinase, phosphorylated compound inhibits viral DNA polymerase.
Is present in Zovirax creams against herpes simplex infections of the skin and mucous membranes
what is ganciclovir
as acyclovir, but selective against cytomegalovirus.
what is the moa of cidofovir
nucleoside phosphonate analogue of cytosine that inhibits DNA polymerase of cytomegalovirus.
What is the moa of foscarnet
organic analogue of pyrophosphate that inhibits DNA polymerase
of cytomegalovirus and herpes virus
what is nevirapine
anti-HIV drug; non-nucleoside reverse transcriptase inhibitor.
what is AZT
moa
Zidovudine (azidothymidine)
nucleoside reverse transcriptase
inhibitor;
thymidine analogue
describe saquinavir
inhibitor of HIV protease; this protease is vital for both viral
replication within the cell and release of mature viral particles from an infected cell.
which drugs are used in HAART and when is it used
nevirapine, zidovudine and saquinavir
during serious manifestations of HIV infections in patients with AIDS
what is interferon and how is it used for antiviral therapy
immunoregulatory cytokine that is synthesized in response to viral infection.
Interferons are used as antiviral agent against hepatitis B and C, and AIDS related Kaposi sarcomas.
melphalan
class?
used for?
derivation?
nitrogen mustard alkylating agents,
is used to treat chronic myeloid leukaemia
occasionally used against melanoma.
Melphalan is a phenylalanine derivative of mechlorethamine.
cyclophosphamide
class?
PK?
use?
nitrogen mustard alkylating agent,
“prodrug”: converted in the liver to active forms that have chemotherapeutic activity.
main use of cyclophosphamide is in the treatment of lymphomas, some forms of leukemia and some solid tumours
lomustine
class?
MOA?
use and special ability?
nitrosourea class of agents,
acts by crosslinking DNA to other DNA strands or to protein in such a way that dsDNA cannot be replicated.
Lomustine is most commonly used against lymphoma (particularly cutaneous (skin) lymphoma) and melanoma, and tumours in kidney and lung.
Lomustine has the special ability to penetrate the blood/brain barrier and can be used to treat cancers in the brain.
describe cis-platin
platinum-based drug used to treat various types of cancers, including sarcomas, some carcinomas (e.g. small cell lung cancer, and ovarian cancer), lymphomas and germ cell tumours.
describe doxorubicin
anthracycline antibiotic that intercalates in DNA.
structurally closely related to daunorubicin, and also intercalates in DNA.
often administered by local injection.
mitoxantrone
class
use
anthracycline agent
used in the treatment of certain types of cancer, mostly metastatic breast cancer, acute myeloid leukemia, and non-Hodgkin’s lymphoma.
Methotrexate
class
moa
antimetabolite and antifolate drug.
It acts by inhibiting the metabolism of folic acid.
Methotrexate replaced the more powerful and toxic antifolate
aminopterin.
what is leucovorin
adjuvant used in cancer chemotherapy involving the drug methotrexate. It is also used in synergistic combination with the chemotherapy agent 5-fluorouracil.
5-fluorouracil
class
administration
use
belongs to the family of antimetabolites and is a pyrimidine analogue.
typically administered with leucovorin.
its principal use is in colorectal cancer and pancreatic cancer, in which it has been the established form of chemotherapy for decades (platinum-containing drugs are a recent addition).
etoposide
moa
use
inhibitor of the enzyme topoisomerase II.
used in the treatment of lung cancer, testicular cancer, lymphoma, and non-lymphocytic leukaemia
what is topotecan
what is it used to treat
topoisomerase I inhibitor.
Is used to treat ovarian cancer and lung cancer.
vinblastine
class
use
vinca alkaloid and an anti-mitotic drug
used to treat Hodgkin’s lymphoma, non-small cell lung cancer, breast cancer and testicular cancer
Taxol
use? (5)
belongs to the drug category of the taxanes, and is a mitotic inhibitor
used to treat patients with lung, ovarian, breast cancer, head and neck cancer, and advanced forms of Kaposi’s sarcoma.
what is tamoxifen
use
orally active oestrogen receptor antagonist
used in the treatment of breast cancer and is currently the world’s largest selling drug for that purpose.
what is toremifene
tamoxifen analogue without estrogen agonist properties
what is anastrozole
aromatase inhibitor
what is goseraline
Decapeptide analogue of gonadotropin-releasing hormone that disrupts endogenous hormonal feedback systems, resulting in the down regulation of testosterone and estrogen production
what is flutamide and what is its moa
oral anti-androgen drug primarily used to treat prostate cancer.
competes with testosterone and its powerful metabolite, dihydrotestosterone (DHT) for binding to androgen receptors in the prostate gland. By doing so, it prevents their stimulation of growth of prostate cancer cells.
what is prednisone
synthetic corticosteroid prodrug that is converted by the liver into prednisolone, which is the active agent that inhibits growth of lymphocytes in leukemias.
what are the following:
a) rituximab
b) trastuzumab
c) bevacizumab
d) cetuximab
e) erlotinib
a) anti-CD20 monoclonal antibody.
b) anti-HER2 monoclonal antibody
c) anti-VEGF monoclonal antibody.
d) anti-EGFR monoclonal antibody
e) small molecule ligand that inhibits EGFR tyrosine kinase.
what are all the monoclonal antibodies used against cancer (5)
a) rituximab
b) trastuzumab
c) bevacizumab
d) cetuximab
e) erlotinib
what is imantinib
what is it used for
what is it marketed as
2-phenylaminopyrimidine derivative that functions as a specific inhibitor of a number of tyrosine kinase enzymes.
used in treating chronic myeloid leukemia, and gastrointestinal stromal tumours, and is currently marketed as Gleevec (USA) or Glivec (Europe/Australia).
How does beta lactamases A, C and D work
How does B class differ
penicillin works by binding to transpeptidase and excluding water from the active site, meaning the complex is stable and has a half life of 90 mins When an A, C or D class lactamase is present, it brings in water to the transpeptidase-penicillin complex, causing the half life to be reduced to only 4 seconds This breaks the lactam ring and releases the transpeptidase to perform its usual task
B class has a Zn ion which breaks the lactam ring before the penicillin-transpeptidase complex is formed. It does NOT form a covalent bond with penicillin
