Lecture 2 Flashcards
How can you define selective toxicity at a biochemical level?
- Drug target unique to the parasite, e.g. dhps in plasmodium
- Drug target with different structure and specificity in parasite and host, e.g. benzimidazoles for helminth beta-tubulin
- Drug target activity in bost host and pathogen but effect on host not significant becuase of alternative pathways or high mammalian ezyme turnover, e.g. pyruvate kinase
- Acitve drug is formed by metabolism only in parasite but not in host, e.g. 5-nitroimidazoles; in anaerobic pathogens d
How can you define selective toxicity at a cellular level?
- Drug only penetrates the parasite, due to different membrane composition
- ## Drug uptake and localisation affected if parasite intracellular
How can you define selective toxicity at the level of the host?
-Pharmacokinetics; how the drug affects the body
What must be considered for pharmacokinetics?
- Route of administration
- Adsorption
- Distribution
- metabolism
- Excretion
- Interaction with immune system
What are the key characteristics of an Antiparasitic drug target?
- Essential for parasite growth or survival in the host
- Selective inhibition possible with small molecules
- Amenable to study at the molecular level
What drugs target DHPS?
Sulphonamide drugs, e.g. sulphadoxine
What role does DHPS play in plasmodium?
Important in folate synthesis
How was DHPS validated as a drug target?
Created Plasmodium falciparum mutants with resistance to sulphadoxine due to point mutants. Transgenic P. falciparum with mutant DHPS alleles - correlated with different levels of sulphadoxine resistance
Which unique enzyme is a possible target to treat the Kinetoplastids?
Trypanothione reductase; removal of toxic oxygen radicals. 40% amino acid identity same as human variant but different enough to be targeted. Still no viable inhibitors have been found.
Give two example enzymes that are found in both host and parasite that could be targeted for treatment
1) DHFR-TS
2) NMT
What drug inhibits DHFR-TS? Why is it used on hosts?
Methotrexate
Treat arthritis, immunosuppressant and for cancer
How effective has methotrexate been in treatment of Leishmania?
In vivo activity disappointing - due to alternative folate synthetic pathway
Give an example of a piggy-back taregt
N-myristoyltransferase (NMT) inhibitors
Originally being developed for the treatment of pathogenic fungi but meant that all tools in place to study for treating parasite.
NMT is an enzyme that catalyses co-translational N-terminal addition of myrisitic acid to cellular proteins; aids sub-cellular targetting. NMT expression is essential for parasite viability in trypanosomatids.
Outline the mode of action of metabolomics
1) Metabolites are grown in the presence and absence of drug
2) Metabolism is quenched, usually by rapid cooling in a dry ice ethanol bath for paraites
3) Medium is removed and metabolites extracted
4) Metabolites chromatographically separated
5) Metabolites detected using UV, mass spec or NMR
Which drugs has metabolomics been shown to be effective in treating T. brucei?
Eflornithine, an ornithine decarboxylase (ODC) inhibitor
Nifurtimox, thought to be involved in DNA an RNA degradation
5-fluoroorotic acid and 5-fluorouracil, thought to degrade RNA
5-fluoro-2′deoxyuridine and 5-fluoro-2′deoxycytidine, inhibits the bifuntional dihydrofolate reductase-thymidylate synthase enzyme, leading to a large increase in dUMP
What has metabolomics shown about treating Leishmania with SbIII (an antimonal)?
Causes an accumulation of disulfide forms of glutathione and trypanothione, diminishing the redox capacity of the parasites
What effect has metabolomics shown miltefosine treatment to have on Leishmania?
Increase in alkaline fragments, indicating affects on lipid metabolism, and increases in nucelotides and sugars
What is a large concern over the current drugs for Leishmania?
The development of multidrug resistant strains; strains immune to SbIII are usually given miltefosine, however some are beginning to show defects in their miltefosine uptake
What effect has metabolomics shown Atovaquone treatment to have on Plasmodium falciparum?
Inhibit enzymes of the bc1 complex, causing membrane depolarization, proton pumping was disrupted and dihydroorotate dehydrogenase activity is also diminished, leading to a depletion in orotate levels, affecting pyrimidine synthesis
What are the three sub-species of T. brucei?
T. brucei brucei (non-human infective)
T. brucei gambiense (chronic disease)
T. brucei rhodesiense (acute disease)
What drugs exist to treat each stage of African trypanosomiasis?
Stage 1; pentamidine and suramin
Stage 2; Melarsoprol, eflotnithine and nifurtimox
What are the issues with current drugs for African trypanosomiasis?
Toxic, resistance a problem, not adapted to field conditions and expensive
How are antimonals metabolised to their active form?
Pentavalent antimonal given and metabolised in trivalent antimonal for acitivity
How does amphotericin B target phagocytic cells?
Given in a liposomal formation
