CHEM3: Bioreductive Antimalarial Artemisinins

Question 1

What is responsible for Artemisinin’s mechanism of action?

Answer 1

Artemisinin is a ssequiterpene lactone containing an enoperoxide bridge, responsible for drug’s MOA.

Combination therapies containing artemisinin + compounds derived from it are used to treat malaria

Artemisinin is extracted from artemisisia annua L (plant) which grows in temperate climates. The plants are harvested, leaves are dried + sent to facilities where artemisinin is extracted using solvents.

Question 2

2D + 3D representations of artemisinin

Answer 2

Question 3

Total chemical + biological synthesis of artemisinin have been achieved.

Why have those routes not been used?

Answer 3

Too expensive

Question 4

What main artemisinin-related compounds are found in the plant leaf?

Answer 4

Artemisinin + arteannuic acid

Question 5

How much artemisinin is found in different varieties of A.annua plant leaf?

Answer 5

0.01% - 1.4%

Question 6

What chemical route is used to make artemisinin?

Answer 6

Chemical conversion of arteannuic acid, used in a semisynthetic route to artemisinin, increasing its total production from biomass

• Arteannuic acid reduced to dihydroarteannuic acid
• This intermediate = photo-oxidised, to the hydroperoxide in acetone, in the presence of methylene blue (photosensitiser)
• Product = air-oxidised (triplet oxygen) in hexane containing some trifluoroacetic acid to afford artemisinin in 30% yield
  
  • Transformation involves an oxidation, ring expansion, intramolecular nucleophilic attack, lactonisation.

Question 7

How can arteannuic acid form from engineered yeast?

Answer 7

1. Use baker’s yeast (saccharomyces cerevisiae) which can readily be genetically manipulated + fermented
2. Overexpress every enzyme of the mevalonate biosynthesis pathway to ERG20
3. Introduce key genes from A.annua
   
   • Amorphadiene synthase
   • Cytochrome p450 monoxygenase
4. Inhibit ergosterol biosynthesis pathway @ level of ERG9 to redirect farnesyl-PP towards ADS

Acheive fermentation yields of >40g/L arteannuic acid

Question 8

Describe hydrogenation of arteannuic acid

Answer 8

Asymmetric reduction of arteannuic acid to the desired dihydroarteannuic acid diastereomer can be achieved using hydrogenation with the use of catalysts e.g. transition metal with a bulky chiral ligand.

Question 9

In the Sanofi semisynthesis process, what catalyst is used in hydrogenation?

Answer 9

Chirally liganded RuCl₂ catalyst

This process is not satisfactory due to cost, inconvienience, + environmental burden

A convienient process is reduction of diazine

Question 10

Reactions that give single enantiomers = ______

Reactions that give diastereoisomers = ______

Answer 10

Reactions that give single enantiomers = enantioselective

Reactions that give diastereoisomers = diastereoselective

All involve in the creation of a new tetrahedral stereogenic centre @ a carbon that was planar + trigonal

Trigonal carbons that aren’t centres but made into them = prochiral

Question 11

What molecules can be prochiral?

Answer 11

1. Trigonal carbons that aren’t chiral centres but can be made into them
2. Tetrahedral carbon atoms that carry two identical groups can also be prochiralif replacement of one of these groups leads to a new chiral centre.

Question 12

How is oxidative formation of artemisinin from dihydroarteannuic acid work?

Answer 12

Question 13

What functional group modification of artemisinin leads to inactive compounds of antimalarial activity?

Answer 13

Endoperoxide bridge

The endoperoxide function is the core pharmacophore essential for the antimalarial activity of artemisinins

Question 14

What functional group modification of artemisinin is well tolerated?

Answer 14

1. Carbonyl groups
2. Methyl groups in 6-membered ring

Question 15

Artemisinins are prodrugs of what compound

Answer 15

DHA - main bioactive metabolite

In humans, artemisinin has an elimination t_1/2 of 2-18 min, whereas, DHA has a t_1/2 of 40-60 min.

Question 16

In severe malaria, how is artemisnins administered?

Answer 16

Parenterally (IV or IM - children)

Artemisinins = lipophilic compounds with low aqueous solubility

Only artesunate = water-soluble to be formulated for IV administration

Question 17

Show metabolism of artemisinins

Answer 17

Question 18

How does malaria parasite digest host haemoglobin?

Answer 18

Question 19

Artemisinins are prodrugs, however, how are they activated?

Answer 19

By reductive cleavage of endoperoxide ring

The free radicals react with groups within range of parasite proteins leading to cellular damage + killing

Artemisinins react in vitro with both haem + Fe2+ = main activators in vivo.

Presence of haematin within reducing environment of the cytoplasm would produce a pool of haem capable of activating artemisinins