Antimalarial Drugs Flashcards
How many fatalities from malaria are caused by P. falciparum?
- 90% of fatalities are from P. falciparum
- 75% of infections are with P. falciparum
What factors are considered in drug design in how to target the parasite, P. falciparum?
Selective toxicity:
Exploit metabolic differences between host and parasite.
- Unique target in parasite
> Unique pathway or expression profile - Discriminate between host and parasite targets:
> Molecular differences
> Location
> Access - Target more important to parasite:
> Secondary or redundant to man - Greater drug accumulation by parasite:
> E.g. role of BBB in man, vs. FV (food vacuole) in parasite - Drug activation by parasite
> E.g. pro-drug
How is treatment determined for P. falciparum? What is the process?
- Pretreatment
> 72 hours prior to arrival in hospital; take bloods, culture parasites and determine drug sensitivity - Drug resistance vs. drug compliance vs. dosage errors:
> Measure drug levels in blood samples HPLC (high-performance liquid chromatography)
> Molecular characterisation of identified markers that are associated w/resistance - Mechanisms of treatment and resistance
> Guidance; WHO
> Reviews; Olliaro 2001
What modes of action are there to treat P. falciparum, and their corresponding drugs?
Folate antagonists; hypnozoite/liver stage)
- Proguanil hydrochloride
Antimitochondrial; hypnozoite/liver stage)
- Atovaquone
Antibiotic; both stages
- Doxycycline (prophylaxis AND treatment)
Parasitic food vacuole; Schizont + RBCs (Schizonticides, Intra-erythrocytics, Quinolones) - Quinine (quinidine) - Chloroquine - Mefloquine (Larium) - Sesquiterpene lactone - Natural product
What agents are used in the treatment of acute, uncomplicated P. falciparum, and the prophylaxis of?
Malarone (combination of:)
- Atovaquone
- Proguanil hydrochloride
How does atovaquone work?
- Naphthoquinone class
- Effective against sporozoites (sporelike-stage inoculated into humans)
> Often used in children w/sickle cell disease
> CQ resistant (Chloroquine-resistant) and MDR (Multiple Drug Resistant) strains of P. falciparum and P. vivax
How does atovaquone work?
Analogue of ubiquinone:
- In mitochondrial electron transport chain (ETC)
- Blocking ATP-synthesis and mitochondrial function
- Interacts w/cytochrome bc1 complex (Complex III) of ETC
- Binding to ubiquinol oxidation pocket of cyto bc1 complex
Why does the ubiquinone analogue atovaquone have low mammalian toxicity?
- Drug target is mitochondria
- Specificity for parasite by structural features of its Complex III; ubiquinone binding sites divergent from those of other species
- 1000x more potent at malarial than mammalian Complex III
What is the caveat with targeting malarial mitochondria as per atavaquone?
Erythrocytic stages do not use mitochondria for energy:
- Maybe less susceptible; but mainly used in hypnozoites/sporozoite, hence prophylaxis
- May relate to branch chain respiration of parasite
- Resistance can arise due to point mutations in bc1 complex ubiquinol binding pocket
How does proguanil work?
- Chloroguanide class
- Effective against sporozoites
> Prophylaxis
> Often used in children w/sickle cell disease
> Combination therapy w/atovaquone (Malarone)
Two proposed mechanisms:
- Antifolate
- Synergistic w/atovaquone; impair respiration of parasite?
How does the antifolate activity of proguanil come about?
Proguanil is a pro-drug:
- Converted to cycloguanil (CYP450)
- Inhibits DHFR (dihydrofolate reductase), which normally catalyses formation of THF from DHF, required for purine base (DNA) synthesis, and some AA synthesis.
> Thus inhibits cell proliferation (cycle), cell growth
Parasite has high rate of replication; folate synthesis required by rapidly proliferating cells
Humans cannot synthesise folate de novo; dietary intake
Thus lack of effect of drug in man (and greater affinity for malarial DHFR), and parasite cannot utilise exogenous folate to synthesise
What does the theory of proguanil’s mode of action being synergistic w/atovaquone propose?
- Proguanil is a pro-drug
- But with humans with CYP450 polymorphism who cannot convert it to its active metabolite (cycloguanil), it still works, even on cycloguanil-resistant [point mutation in DHFR] parasites (when given w/atovaquone)
- Proguanil has alternative target to DHFR too; does not act to affect mitochondria alone
Why is atovaquone (Malarone combi) used for P. falciparum prophylaxis?
Active against liver stages; sporozoites
Why is parasite death slower w/atovaquone therapy than with Artemisinin or Chloroquine?
- Delay due to acting on late trophozoites?
- Lipophilic and slow uptake; leads to development of parasite resistance
- Humans and mammals insensitive to drug; different bc1 complex
»> General property of mitochondrial-acting antimalarials
How is Malarone (Atovaquone + Proguanil) to be taken in malaria prophylaxis?
- 1-2 days before entering endemic area, continue for 1 week after leaving
- Adult and child over 40kg; 1 tablet OD
What are the CIs for Malarone?
Cautions:
- N&V (reduced absorption of atovaquone?)
- Diarrhoea
S/Es (many):
- Dizziness
- Depression
- Insomnia
CI:
- Breast-feeding women
- Use of machinery; effects on psychomotor performance (hands)
- Many drug-drug interactions e.g. ‘those that act on’ CYP450 (Artemisinin, moclobemide, tetracyclines, warfarin)
What is quinine? What stage does it act on? (16:45)
- First effective Western treatment for malaria
- Natural product of the Cinchona tree
- Used until 1940s; advent of chloroquine
»> Acts on erythrocyctic (RBC) stage
What are the analogues of quinine (the OG from the Cinchona tree) that are used today as antimalarials?
- Chloroquine (contains Cl)
- Mefloquine (contains Fl)
How do quinoline-containing compounds (e.g. Chloroquine/Mefloquine) work?
- Weak bases; accumulate in the acidic food (digestive) vacuole of parasite
> Impairs action of food vacuole
> Prevents hemozoin production
> Haem toxic; lytic (acts on membrane), ROS production; oxidising parasitic proteins.
What is the mechanism of base trapping in the food vacuole w/quinoline analogues?
- pH 4.5 in food vacuole
- Chloroquine; pKa = 8.1;
- Mefloquine; pKa = 8.7 (accumulates less well; more is ionised at physiological pH 7.4)
> E.g. 90% NH+ at physiological pH
> But then 99.8% NH+ (ionisation) at lysosomal pH 4.6; drug trapped inside food vacuole
How can P. falciparum develop resistance against quinoline-containing compounds?
Multiple genes:
Decreased drug accumulation in food vacuole:
- Accelerated drug efflux
> Expression of ATP-dependent P-glycoprotein in FVM (food vacuole membrane)
> PfMDR1 & PfMDR2 (P. falciparum Multiple Drug Resistant); code for Pgh1 protein expressed in FV membrane
- PfMDR1 imparts mefloquine resistance (MQ); CQ resistance is unclear, but there is reduced uptake.
> PfCRT gene product; Chloroquine Resistant Transporter
- Anion channel expressed in FVM; pumps out CQ
- Perfect correlation w/CQ resistance
- Transfection confers CQ resistance
When is Mefloquine (Larium) used for anti-malarial therapy?
- Use in prophylaxis; where high risk of CQ-resistant P. falciparum (country specific)
- Not for general treatment otherwise; P. falciparum resistance, better tolerated alternatives for non-P. falciparum
What ADR is Mefloquine associated with?
Potentially serious neuropsychiatric reaction:
- High affinity in the brain for 5-HT2A receptors; partial 5-HT2A agonist, full 5-HT2C agonist:
> Insomnia, anxiety, psychosis, depression, suicidal ideation, suicide.
- A2A antagonist too
- ADR can persist up to several months post-treatment; long half-life (due to lipophilicity)
»> BUT, not everyone experiences these S/Es…
What are the pros of CQ (chloroquine) use?
- Not associated w/potentially serious neuropsychiatric reaction (as MQ is; CQ does not bind readily to NT receptors in CNS)
- Recommended for use for treatment of non-P. falciparum:
> P. vivax, P. ovale
> Due to little resistance
> 3 day oral dosage
What are the cons of CQ (chloroquine) use?
- P. falciparum resistance (thus mostly used for P. vivax, P. ovale)
- CIs; glucose-6-phosphate dehydrogenase deficiency
- High Vd (volume of distribution):
> Lipid soluble, Log P = 3.0, enters adipose tissue
> But Log D at pH 7.4 = 1
> Potentially base-trapped in cells - Toxic in overdose; accumulates in other acidic vacuoles
- Retinal toxicity; accumulation may cause blurred vision and even blindness.
When is CQ (chloroquine) used in antimalarial therapy?
- Prophylaxis; where risk of CQ-resistant P. falciparum is low (country-specific)
- RARELY used for treatment; P. falciparum resistance
> However, not associated w/potentially serious neuropsychiatric reaction
> Thus recommended in treatment of non-P. falciparum malaria; P. vivax, P. ovale.
Where is quinine’s (OG) place in antimalarial treatment?
- Widely used and effective
> PO Quinine Sulphate 600mg every 8 hours for 5-7 days
> Ensure complete eradication/ addition of doxycycline 200mg OD (simultaneously or sequentially; quinine not 100% effective as sole agent) - Initially used in pregnancy
- Base trapping in food vacuole
What occurs in quinine overdose?
Hypoglycaemia:
> Hyperinsulinemia can result in coma + death
- Directly stimulates insulin secretion
- Acts like sulphonylureas
What is doxycycline, and where is its place in antimalarial treatment?
- Tetracyclic antibiotic
- Prophylaxis NOT treatment of malaria (apart from in combination w/quinine)
What is doxycycline only used in malarial prophylaxis, and not the treatment of?
- Mode of action TOO DELAYED for treatment
But can be used in conjunction w/quinine to ensure eradication
How does doxycycline work?
Impairs progeny (replication) of the apicoplast (plastid thing) genes; results in abnormal cell division
What is the apicoplast, and which therapeutic agent targets it?
- Non-photosynthetic plastid; organelle stores pigment
- Found in most Apicomplexa, includes malaria parasites
- Vital to parasite survival:
> Lipid metabolism
> Host cell invasion
> > > Doxycycline impairs progeny of apicoplast genes
What are the dosage instructions for doxycycline for the prophylaxis and the treatment of malaria respectively?
Prophylaxis:
- 1-2 days before entering endemic area (esp. high risk areas)
Treatment:
- Only when WITH quinine
- Over 12 y/o; 200 mg OD for 7 days
»> NOT in pregnancy; impair teeth and skeletal development
What are the various mechanisms of drug resistance?
Combination of:
- Mutations in target gene/protein
> Decrease selectivity
- Increased production of target
- Decreased accumulation of drug
> Increased efflux of drug
> Inactivation of drug - Parasites w/mutation or genetic polymorphisms
> Eneole and confer resistance under drug pressure
> Unichemotherapy often leads to rapid selection of resistant mutations
> Simultaneous resistance (to several drugs that are used simultaneously)
(treat w/combination therapy) - Cross-resistance (CQ/MQ)
> Resistance to agent imparts resistance to other similar acting drugs
> Also indicates common mode of action
Why is there no viable, approved malaria vaccine on the market?
- Malaria is a protozoa (unlike viruses)
»> Which malaria?
»> Active antigenic variation; there are > 50 variants of the var allele (would need > 50 types of vaccine) - Limited protection; even for those vaccines currently trialled e.g. Mosquirix; only 55% protection in children, 33% in infants, as well as adjuvant risks.
What are the logistical and financial problems with a malaria vaccine?
Whom:
- Sufficient for herd immunity (cost)
- Which population communities?
Delivery:
- Multiple inoculation (4 - 6 doses)
Costs:
- Affordability
What is an example of a new antimalarial therapy?
Why is it preferable?
Artemisinin-based combination therapies (ACT)
- Current GOLD STANDARD
- Fixed dose (3)
> Limiting number of doses improves compliance, and cheaper too.
> Gives prophylactic protection; long plasma half-life duration (hence fewer doses too; > 1 week plasma lifetime)
How does ACT work?
Artemisinin-based combination therapies (ACT):
- Reduces parasite load by 10,000 over 48 hour life cycle
- To ensure eradication, activity over 3-4 life cycles
- Plasma lifetime: > 1 week
What are the aims of new antimalarial therapy?
Eradication
- Prevention of transmission
- Screen (treat) whole population whether infected or not
New targets
- Enzymes structurally specific to Plasmodium
Natural products
- Herbal (traditional medicines)
- Fungal
How are the different antimalarials availible classed?
Casual Prophylaxis:
- Destroys parasites in liver, prevents RBC invasion
> Malarone
Suppressive Prophylaxis
- Suppresses RBC phase
> Chloroquine, mefloquine proguanil, doxycycline (NOT quinine)
Clinical cure:
- Erythrocyte schizonticides
- Terminate malarial fever
- Slow acting, low efficacy:
> Proguanil - Fast acting, high efficacy
> Quinolines, artemisinin (hence gold standard)
