==Level 1Header== This was my revision notes for my MSc module (S807 Molecules in Medicine)
Different types of Malaria:
Plasmodium falciparum (Pf) Plasmodium vivax (Pv) Plasmodium malariae (Pm) Plasmodium yoleii (Py) Plasmodium knowlesi (Pk)
The different types of malaria
• Plasmodium falciparum is the most common strain of malaria and also the deadliest. • Found in Asia, Africa, the Middle East, the Pacific and South America, it is responsible for 60% of malarial infections and 90% of malarial deaths, worldwide. • Malignant tetian or subtertian malaria. Cycles of fever or reoccurring fever every 36 – 48 hours. • Iron ,(Fe), deficiency in Pf erythrocyte parasites bind onto receptors. • Blackwater fever – heavy excretion of haemoglobin in urine. Seen most in Pf. • Particular for blocking blood capillaries in deep tissue – particularly in the brain. This is done by erythrocytic schizants being accumulated, tissue hypoxia already there due to anaemia. • cerebral malaria is when there is a combination of circulating parasite toxins with these erythrocytic schizants, which produce metabolic disturbances in the brain. • Multiples more quickly in blood. 30 – 40% parasitemia in a few days.
• Found in Asia and Central America, with small outbreaks in West and Eastern Africa, categorized as a benign form of the disease, P. vivax is in fact the second most deadly strain of malaria. P. vivax parasites have a preference for younger red blood cells. • The incubation period is 12 – 17 days; with fever cycles of 48 hours. Recurrence of infection is common with P. vivax. • Persists in liver as hypnozoite stages – which doesn’t respond to chloroquinine. Relapses occur as a result. Hypnozoites treated with primaquine after chloroquinine has eliminated erythocytic stage of parasite.
Rodent form of the malaria infection, can be passed onto humans in the lab.
• Found in East and West Africa, Guyana, India and South East Asia, P. malariae categorized as a benign form of the disease, have a preference for mature red blood cells. • The incubation period ranges from 18 – 40 days or longer depending on the strain. • The fever cycle occurs every 72 hours and may last for 8 -10 hours at a time. (Quartan malaria) • While the first attack is usually very mild, a recurrence of the disease has been known to occur even after five decades. • Acute glomerulonephritis is common, lowering of albumin level in plasma & elevation of globulin (particularly γ – globulin) level with little change in [total plasma]. • Hypnozoites in the liver as in Pv, so chloroquinine given.
• Originally from macaque monkeys. • Found in Malaysia mainly and Bangladesh where there are jungles.
Usually oral – but in very ill or patients who can’t retain drug due to vomiting. CQ can be given intravenously via a slow infusion or intramuscular (not the preferred method). No resistance occurring in Pv or Pm.
Mechanism is not well understood – thought to inhibit haeme polymerase activity.
1. CQ enters RBC inhabiting parasite cell and digestive vacuole by simple diffusion. 2. CQ protonated to CQ2+ as the digestive vacuole known to be acidic – Ph 4.7. 3. CQ caps haemozoin molecules to prevent further biocrystallisation of haeme. Leads to haeme build up. 4. CQ binds to haeme or FP, (Fe(II) – protoporphyrin IX). To form Fe – CQ complexes. Complex highly toxic to cell and disrupts membrane function. 5. Action of toxic FP – CQ and FP results in cell lysis and ultimately parasite cell autodigestion.
In essence, the parasite drowns in own metabolic products.
Pyrimethamine and sulphadoxine. Causes severe skin reactions.
Inhibits dihydrofolate reductase of plasmodia and thereby blocks biosynthesis of purine and pyrimidine – Essential for DNA synthesis and cell multiplication.
Leads to a failure of nuclear division and time of schizant formation in erythrocytes and liver.
An antimetabolite, competes with para – aminobenzoic acid for incorporation into folic acid.
Action of sulphonamides exploits difference between mammal cells and other kinds of cells in their folic acid metabolism.
- IMPORTANT TO KNOW *****
Folic acid can’t cross protozoal (or bacterial cell walls) by diffusion or active transport. Hence the reason the folic acid MUST be synthesised by the Protozoa from the para – aminobenzoic acid.
See Pyrimethamine on last page, Dapsone below:
Inhibits folic acid synthesis in susceptible organs.
Pyrimethamine and dapasone Causes bone marrow failure, particulary in combination with CQ.
Exact mechanism is not well understood. However evidence suggests that it could generate reactive Oxygen (O2) species or by interfering with electron (e-) transport in the parasite. Another way it can work is by binding to and altering the properties of protozoal DNA.
6 – Amino antinflammatory agent, is as least as effective as CQ & effective against same strains which are resistant to CQ.
Thought to inhibit haeme polymerase activity. Results in accumulation of free haeme, which is toxic to parasites. Drug binds to free haeme so the parasite can’t convert it to a less toxic form.
Exact mechanism not known. Evidence does show that it inhibits formation of β – haematin by forming a complex with gaemin and inhibits nucleic acid and protein synthesis.
Produces swelling of Pf food vacuoles. It may also act by forming toxic complexes with free haeme that damage membranes and interact with other plasmodial compartments.
Artemisia annua or Sweet Wormwood / Sweet Annie or Qing Hao = Crop used for the base products of the artemisinin analogues.
• Increase in Pfmdr1 – Pf multidrug resistance gene, heightens susceptibility to artemisinin. • Locus affecting yield – hybridising plants to increase yield. • Resistance found most on Thailand – Cambodia border and Venezuela – Columbia borders. Used to be controlled by chloroquinine salt. • The word pfdhfr = Pf dihydrofolate reductase. • Artesunate monotherapy in [high], Absolute Neutrophil Count lower at 6 and 14 days. Risk of neutropenia (abnormally low level of WBC) in Cambodia. • Overexpression of PF.10-0355 can cause resistance to mefloquine and lumenfantrine. (2011 study) • Thailand government clinics treat with Altequin and primaquine. • Laos government clinics treat with Coartem. • Intramuscular artesunate maybe better than intramuscular artemether in severe Pf patients. • In Willcox 2011 study, A. Annua tea was used in pregnancy – 2 miscarriages but all other babies born normally. Danger is the tea would be used outwith the therapeutic time. • Dihydroartemisinin more stable and 10x more effective than artemisinin. (Tu, Nature Medicine 2011) • Artemisinin can clear rapidally parasitema in Pk, also works with severe knowlesi. • Resistance in Py = Multifunctional, two main kinds:
1. Drug accumulation of artemisinin goes down 2. TCTP (Translationally Controlled Tumor Protein) different
Comparison in studies with CQ
CQ helped the lungs in emphysema – Artemisinin only mildly helped. CQ mildly reduced thickening of alveolar wall, whilst artemisinin totally reduced it. Taken from a 2012 study on P berghei (a rodent form of Plasmodia spp.)
New therapy in development
In Thailand, Arteolane maleate + Piperaquine phosphate.
Artemisinin from plant
Artemisinin mode of action
The biological aspect of the haeme (Iron containing part of the Red Blood Cells), which the artemisinin works on. In the malaria parasite, the haemoglobin in the red blood cells are degraded by a series of protease enzymes – releasing peptides and amino acids which are used for development and frees up the space in the digestive vacuole.
The parasite has a mechanism which it is protected against the toxicity by haematin undergoes biomineralisation to form haemozin (malaria pigment). The diagram above shows this mechanism occurring and the Fe in the complex changing its oxidation state from (II) to (III).
There are two mechanisms of the mechanism of how the artemisinin works, mechanism (a) is the Reductive scission model and mechanism (b) is the Open peroxide model.
Reductive scission model
The oxygen radicals which were given off in the oxidation of the Fe rearrange to form carbon centred radicals, although the nature of the proposed radical and mechanistic pathways which formed them being different in each case.
Low valent transition ions – ferrous haeme or non haeme endogenous Fe2+, where found to bind to artemisinin and trigger e- transfer reductive scission of the peroxide bridge to produce oxygen centred radicals that rearrange to give us carbon centred radicals.
Open peroxide model
Ring opening driven by the protonation of the peroxide or by the complexation of the Fe2+. Fe acts like a Lewis acid to facilitate ionic, as opposed to radical bioactivation of the artemisinins – which have a short life too short to have any molecular interaction.
Also, alternatively, non – peroxidic oxygen plays a role in triggering the ring opening, which then generates the open hydroperoxide. The oxygen atom is providing stabilisation of the positive charge, thus which according to transition state theory – lowers the energy required for the ring opening to occur.
Heterolytic cleavage of the endoperoxide bridge & subsequent capture of H2O leads to the formation of an unsaturated hydroperoxide, capable of irreversibly modifying protein residues by direct oxidation. Fenton degredation which occurs with the hydroperoxide produces a hydroxyl radical – a species which can oxidise target amino acid residues.
To support the above theory, the artemisinin have been shown to mediate N – oxidation of 3° alkylamine derivatives via the intermediacy of the ring opening peroxide form of artemisinin.
Alkylation of a haeme model – the diethyl ester of haeme, by a 1° carbon centre radical derived from bioactivation of artemisinin. Adducts were also obtained by α and δ carbon atoms.
Medications which have artemisinin in their structure
Coartem – Artemether + Lumefantrine. Artsuamoon – Artesunate + Amodiaquine. Sulphamon plus 500 – Artesunate + Sulphadoxine/Pyrimethamine. Artequin – Artesunate + Mefloquine.