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Parasites and infectious disease > week 6 - drugs of malaria > Flashcards

week 6 - drugs of malaria Flashcards

1
Q

what are the 2 main types of drugs

A

quinine and non-quinine

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2
Q

when treating malaria the drugs generally are aimed towards which stage of the parasite?

A

RBC trophozoite asexual stage

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3
Q

quinine, chloroquine and mefloquine target which part of the parasite?

A

the food vacuole

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4
Q

what was the first drug used to treat malaria and where did it come from?

A

quinine

plant in peru

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5
Q

what sort of structure is quinine?

A

4-methanol quinoline

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6
Q

what is one side effect of quinine?

A

it rarely causes cinchonism that can lead to blindness

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7
Q

against which stage is quinine effective?

A

the RBC asexual trophozoite stage

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8
Q

can quinine be used against chloroquine resistant malaria?

A

yes the mechanisms of resistance gained by plasmodium sp. to quinine and chlorquine differ

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9
Q

how is quinine administered?

A

orally

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10
Q

how much of quinine is blood bound and how much is protein bound?

A

70% protein

30% blood

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11
Q

how is quinine eliminated from the body?

A

readily metabolised by the liver
20% excreted in urine
problem with this drug is that the group right at the end can be metabolised in the liver and by removing this ethylene group stops the drug from working

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12
Q

what is the half life of quinine?

A

8-14 hours

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13
Q

what is the structure of chloroquine like?

A

4-aminoquinoline

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14
Q

where does chloroquine come from?

A

it is an artificial analogue of quinine

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15
Q

against which stage is chloroquine effective?

A

against the RBC trophozoite stage

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16
Q

Chloroquine was used extensively int he 1960s-1070s in an eradication campaign in conjunction with DDT however now _______

A

resistance is a major problem

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17
Q

how is chloroquine taken?

A

orally

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18
Q

what is the distribution of blood and protein like for chloroquine and what sorts of tissues does it prefer?

A

50-60% protein

preference for adipose tissue

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19
Q

how is chloroquine eliminated from the body?

A

partially metabolised in liver to active de-ethylated metabolites - these still work against the parasite unlike quinine where chopping off end group stops the drug working

excreted in urine unchanged (45%) but very slowly

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20
Q

what is one of the reasons chloroquine is so effective at treating malaria?

A

due to its long half life of 1-2 months and hence can be maintained at therapeutic levels within the blood plasma for a long time

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21
Q

quinine and chloroquine target for the mode of action is the food vacuole but specifically which process?

A

the method of feeding

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22
Q

As the parasite feeds the haemoglobin from the RBC is taken up by ____ and then through a modified structure called the ___ in the parasites cell surface

A

phagocytosis

cytosome

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23
Q

Haemoglobin is broken down via proteolytic processes. Releasing ___ and ______

A

amino acids

haematin

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24
Q

Haematin is toxic, what happens to it?

A

crystallised to form haemozoin

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25
Q

which part of haematin is capable of generating free radicals and damaging membranes?

A

the iron atom (Fe 3+)

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26
Q

What helps detoxify the haematin?

A

the haematin molecules stick together and the iron and propionate groups stick together. They neutralise each other

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27
Q

What is haemozoin formed of?

A

4 beta haematin molecules are joined by hydrogen bonding

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28
Q

Quinine and chloroquine mode of action: drugs enter the parasite how?

A

drugs enters cell by diffusion and accumulates in food vacuole

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29
Q

what do quinine and chloroquine prevent?

A

formation of haemozoin, so haematin forms and kills the parasite by generating free radicals

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30
Q

How do the quinine based drugs interact with the haematin?

A

the quinoline aromatic ring will interact with the aromatic porphyrin ring of haematin.
this interaction blocks the iron and prevents haemin detoxication.

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31
Q

What stops the chloroquine once in the food vacuole coming out again?

A

the pH

acidity in the food vacuole causes chloroquine / quinine to accumulate - they become protonated.

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32
Q

Malaria eradication was regarded as possible how?

A

prior to resistance of parasite to chloroquine

malaria eradication regarded as feasible using a combination of DTT to attack mosquito and chloroquine to kill the parasite

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33
Q

Describe the spread of Chloroquine resistant malaria.

A

Chloroquine resistant started to emerge in SE asia and S.America in the 1960s probably because of the misuse of the drug. Mainly it’s the parasite in these regions have the ability to adapt to drugs.

Most resistance starts around vietnam, laos.
In 1959 it emerges a problem in south america, and whether this was an imported case? Or was it completely separate event.

Chloroquine has spread to all of the regions of malaria in the world.
Chloroqune is more or less redundant now a days

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34
Q

what suggests that the mechanism of resistance to chloroquine is complex?

A

because the resistance develops slowly. can get spectrum of resistances - low to high

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35
Q

where is the resistance to chloroquine in terms of drug or target?

A

unlikely to be resistance at target level as the drug target is haematin which is synthesised by the human host and cannot be altered by the parasite.

more likely to be resistance at the drug level - less chloroquine in food vacuole of resistant parasites could be due to reduced uptake, and drug effluz.

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36
Q

chloroquine resistance involves _____ protein. A protein which contains ___ transmembrane domain and is localised to the membrane of the ______. It functions as amino acid/peptide transporter

A

PfCRT
10
food vacuole

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37
Q

How was the actual mechanism to chloroquine discovered?

A

The parasites can undergo genetic exchange in mosquito. Took wild type populations mixed with a non resistant population in the lab and having them pass through the mosquito and then into a RBC environment and looking at the offspring at that point and monitoring the offspring for chloroquine resistance.

They managed to identify 1 loci of around 35kb associated with chloroquine resistance.

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38
Q

In clinical chloroquine resistant strains, mutation detected in the ____ gene

A

pfcrt

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39
Q

key chloroquine resistance conferring mutation is localised in a region of the protein implicated in _______

A

substrate activity

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40
Q

what is the point mutation called which is implicated in chloroquine resistance?

A

K76T

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41
Q

___ is a diagnostic tool for chloroquine resistance

A

K76T

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42
Q

What occurs in chloroquine sensitive strains?

A

chloroquine is protonated (due to low pH) hence is +vely charged

side chain on lysine (K) amino acid is a +vely charged

lysine repels the +vely charged chloroquine 
(ie it is retained in food vacuole)
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43
Q

What occurs in chloroquine resistant strains?

A

chloroquine is protonated (due to low pH) hence is +vely charged

lysine (K) has been altered to threonine (uncharged)

chloroquine can now interact with PfCRT and is transported out of 	the food vacuole (efflux pump)
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44
Q

In addition to the K76T mutations, some strains show high levels of chloroquine resistance have acquired other mutations in ___

A

pfCRT

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45
Q

What could lead to higher levels of chloroquine resistance?

A

resistance in K76T PfCRT strains can be enhanced by mutations in a second gene – pfmdr

(pfmdr = P. falciparum multi-drug resistance )

PfMDR is an ATP-binding cassette transporters (ABC-transporter) group of membrane proteins

MDRs on other organisms function to export hydrophobic drugs & indirectly regulate ionic gradients

MDRs responsible for drug resistance in cancer cells

the calcium channel blocker verapamil which reverses resistance of mammalian cells to anti-cancer cells reverses chloroquine resistance

46
Q

pfMDR is localised to the membrane of the ______

A

food vacuole

47
Q

Precise mechanisms underlying PfMDR in resistance remains unknown but what is known?

A

that different geographical regions show different mutations

48
Q

Resistance to chloroquine develops ____

A

slowly

49
Q

mutation in 2 genes (___&____) have been implicated in resistance

A

pfcrt

pfmdr

50
Q

All chloroquine resistance strains contain a mutation in ______ leads to low / medium levels of resistance

A

pfcrt (K76T)

51
Q

high levels of chloroquine resistance can occur in which 2 ways?

A

acquisition of other mutation in pfcrt

acquisition of mutations in a 2nd gene pfmdr

52
Q

Mefloquine is another derivative of quinine. How is it administered?

A

orally

53
Q

what is the trade name of mefloquine?

A

Lariam

54
Q

When was mefloquine used and why is it not used much anymore?

A

used in the treatment of chloroquine resistant falciparum malaria.
crosses the blood brain barrier

severe and permanent side effects including neurological disorders

55
Q

what is the mode of action of mefloquine?

A

food vacuole

56
Q

Is there resistance to mefloquine?

A

resistance is emerging in south east asia - but different mechanism to chloroquine

57
Q

resistance to mefloquine depends on _____ pfmdr1

A

wildtype

mutation of pfmdr confers mefloquine sensitivity

58
Q

high levels of mefloquine resistance is associated with what?

A

over expression of wildtype PfMDR

59
Q

so why is mefloquine resistance different to chloroquine resistance?

A

amplification of efflux pump = mefloquine

mutation of efflux pump = chloroquine

60
Q

Sulfadoxine pyrimethamine has the trade name ______

A

combinational therapy

fansidar

61
Q

which life stage does sulfadoxine pyrimethamine target?

A

active against the asexual RBC cycle

62
Q

when is sulfadoxine pyrimethamine used?

A

second line drug combination where chloroquine resistance is prevalent.
not used in UK since 2008

63
Q

what are the side effects of sulfadoxine pyrimethamine?

A

fatalities have occured due to drug induced, dermatological conditions.
skin rash, photosensitivity, blood disorders

64
Q

Pharmacokinetics of sulfadoxine pyrimethamine = absorption?

A

single oral dose
adult - 3 tablets
1 tablet contains 500mg sulfadoxin and 25mg pyrimethamine

65
Q

Pharmacokinetics of sulfadoxine pyrimethamine = distribution?

A

90% drug binds to proteins
cross placental barrier and pass into breast milk

pyrimethamine concentrates in red/white blood cells and crosses into CNS fluids

66
Q

Pharmacokinetics of sulfadoxine pyrimethamine = elimination

A

low levels of each drug is metabolised

both excreted in urine - unmetabolised drug in urine

long half lives- sulfadoxine = 170hrs
pyrimethamine = 110hrs

67
Q

What is the mode of action of sulfadoxine-pyrimethamine?

A

drugs target different enzyme in folate biosynthesis pathway

drugs act synergistically (cooperative)

function to reduce folate levels

many bacterial and anti cancer drugs target enzymes in this pathway

68
Q

why does reducing folate levels kill parasite?

A

folate is essential for DNA synthesis and metabolism of certain amino acids

69
Q

why is the folate biosynthesis pathway particularly attractive to target drugs at in malaria parasites?

A

because we do not have a folate biosynthesis pathway as we have to obtain folate in our diet

70
Q

Sulfadoxine component is responsible for _____

A

most side effects observed in treatment

71
Q

Sulfadoxine is an analogue of _____

A

p-amino benzoic acid

72
Q

what is the target of sulfadoxine and how?

A

dihyropteroate synthase - this enzyme is missing in humans. Its a competitive inhibitor

73
Q

pyrimethamine is a _______ drug that targets???

A

pyrimidine containing

targets dihydrofolate reductase this is present in humans
also competitive inhibitor

74
Q

is there resistance for sulfadoxine pyrimethamine?

A

Arose in s.e.asia then spread rapidly. it is still used but not as widely as it used to be.

75
Q

Sulfadoxine-pyrimethamine basis for resistance due to differences in _____activities.

A

antifolate

76
Q

Type II antifolates (pyrimethamine) are _____

Type I antifolates (sulfadoxine) not as __

A

highly active

active

77
Q

Sulfadoxine-pyrimethamine resistance occurs due to point mutations in which genes?

A

Firstly in DHFR-TS (target for type II)

then in PPPK-DHPS (target for type I)

78
Q

Explain DHFR-TS mutation further

A

Only DHFR domain is affected. A diagnostic mutation in DHFR (S108N) is always seen first confers 100 fold increase in resistance to pyrimethamine.

Acquisition of secondary DHFR mutations progressively enhances resistance.

79
Q

Explain PPK-DHPS mutation further

A

After alteration of DHFR at all 4 positions, mutations in DHPS arise to give resistance to sulfadoxine

80
Q

What is the trade name of atovaquine and what sort of drug is it?

A

malarone

part of a combinational therapy

81
Q

Which stage is atovaquine active against?

A

active against the liver and erythrocyte stages.

active against strains resistant to other anti malarials

very few side effects

82
Q

Atovaquone pharmacokinetics - absorption

A

oral

1 tablet = 100mg proguanil and 250mg of atovaquone

83
Q

Atovaquone pharmacokinetics - distribution

A

highly lipophilic
99% drug binds to
serum albium

84
Q

Atovaquone pharmacokinetics - elimination

A

not metabolised
excretes slowly in faeces, little in urine
half life - 48-72hours

85
Q

what is the active site of atovaquone?

A

the ubiquinone structure which is a quinone base structure

86
Q

what does ubiquinone do?

A

shuttles electrons from dehydrogenase that form complex I and II of the oxidative phosphorylation pathway onto complex III (cytochrome b)

87
Q

Is there resistance to atovaquone?

A

resistance with atovaquone alone can easily arise by a point mutation in the gene encoding cytochrome b.

Resistance is rare when using the Malarone combinational therapy. First failure reported in 2002

88
Q

Atovaquone targets what?

A

mitochondrial electron transport

89
Q

Which drug targets the apicoplast?

A

doxycycline

90
Q

What is doxycycline?

A

tetracycline antibiotic
it is a broad spectrum antibiotic
prophylaxis to prevent malaria

91
Q

which stage does doxycycline target?

A

active against the asexual cycle

92
Q

what are the side effects of doxycycline?

A

stomach upset and nausea

vaginal yeast infection

93
Q

Pharmacokinetics of doxycycline - absorption

A

oral

100-200mg/day for 7-14 days

94
Q

Pharmacokinetics of doxycycline - distribution

A

90% drug remains in plasma

cross placental barrier and pass into breast milk

95
Q

Pharmacokinetics of doxycycline - elimination

A

not metabolised
excreted in urine / faeces
half life 18hrs

96
Q

what is the mode of action of doxycycline?

A

target in apicoplast - non photosynthetic organelle related to plastids such as chloroplasts that evolved via secondary endosymbiosis

97
Q

The apicoplast is the bacterial ______ mechanism, doxycycline inhibits cell growth by inhibiting ________

A

translation

translation in the apicoplast

98
Q

How does doxycycline inhibit translation in the apicoplast?

A

Doxy blocks the acceptance site in the ribosome and so the charged tRNA cannot come in and bind effectively with the ribosome so the protein doesnt form

99
Q

What is the problem with doxy?

A

it prevents parasite growth (static agent) rather than killing (-cidal agents) and therefore the immune system is required to activate and wipe the parasite out

100
Q

Is there resistance to doxy?

A

no but matter of time.

3 mechanisms of resistance reported in bacteria - enzyme inactivation, efflux, and ribosomal protection

101
Q

Which part of the parasite do the artemisinins affect?

A

no idea!

102
Q

what is artemisinin?

A

from leaves of a plant that have been used by Chinese herbalists, the active compound was purified in 1972

103
Q

Artemisinin is made from a series of _______units that contain a ______bond

A

isoprene

peroxide

104
Q

Which stage is artemisinin active against?

A

asexual ring stage

105
Q

When is artemisinin used?

A

to treat multi drug resistant plasmodium strains

106
Q

Is there resistance to artemisinin?

A

documented in south wast asia.

The molecular marker for resistance is KELCH protein

107
Q

what are the 3 suggested mode of actions for artemisinin?

A

inhibition of food vacuole cysteine protease activity

damage parasites electron transport chain in the mitochondria to oxidative stress

irreversible inhibition with an ATP-ase that pumps Calcium from the cytoplasm into the ER

108
Q

What are the problems with Artemisinin?

A

problems with supply cannot grow enough plant material

semi-synthetic compounds developed

artemisinin use as a monotherapy explicitly discouraged by the WHO treatment failure because it has been used on the Thai Laos boarder and it should always be a part of a combinatorial therapy

109
Q

what has been suggested as the way forward with Artemisinin?

A

Artemisinin Combinational Therapies

110
Q

What is Artesunate and Amodiaquine?

A

ASAQ

Active against all forms of malaria and against all chloroquine resistant strain of Plasmodium falciparum

111
Q

What has ACT accomplished?

A

in a short period ACTs main treatment targeting malaria in africa.

Parasites and infectious disease (15 decks)

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