Drug Targeting and Resistance Flashcards
What is drug targeting?
Is ensuring that your inhibitor has a very high affinity for the target (enzyme)
What is drug resistance?
Is the result of mutations in the target (enzyme)
What are some factors that need to be considered when considering if your drug will reach the target cell?
Oral or parenteral administration
Metabolism by liver, plasma enzymes etc. (is it stable?)
Excretion in the kidneys
Segregation in specific tissues, binding to plasma proteins
Blood-brain barrier; placenta; blood-testes barrier etc.
What are some blockages that prevent the drug entering the target cell?
Lipophillic or hydrophilic
Transporters, channels
Multidrug resistance proteins (ABC transporters)
What are the ways to cross the plasma membrane?
Diffusion
Facilitated diffusion
Active transport
What molecules can get across by diffusion?
Highly lipophilic molecules only (not water soluble)
Not specific: Enters any cell
Wide distribution may mean higher dosage and toxicity
High lipophilicity also means increased penetration across blood brain barrier and similar
Absorption through skill may be possible
What is an example of molecules that can cross through diffusion?
Example: steroids
What is facilitated diffusion mediated by?
Transport proteins
What compounds usually pass through in facilitated diffusion?
Hydrophobic compounds
What is specific targeting?
Enters only cells expressing appropriate
What is equilibrative?
Allows the passage across the plasma membrane until there is equilibrium between intracellular and extracellular concentrations
What can intracellular concentration never be higher then in facilitated diffusion?
Plasma concentration
What is active transport mediated by?
Transport proteins
What is active transport depend on?
Energy dependent
What is concentrative?
Continues to ‘pump’ the substrate into the cell even against high concentration gradient
How can intracellular concentration differ to plasma concentration in active transport?
Can grossly exceed plasma concentration
What does the increased intracellular concentration increase?
Specifity and keep required plasma concentrations low
What are transporters characteristics?
Highly specific
Concentrative if energy dependent
Equilibrative if not energy dependent
Cause of resistance if lost
What do transporters define?
What molecules reach the intracellular target
What concentrations are achievable within the parasite/target cell
How fast the drug is taken up
Drug resistance and cross-resistance
What is active transport?
The combination of high affinity and high rates of uptake requires energy
What direction is active transport?
Uni-directional and accumulates substrates in the cell above the gradient
What is an example of active transport?
Pentamidine given at 1 nanometer accumulates to mM inside trypanosomes
What are the 3 types of active transport?
ATP dependent
Using sodium gradient
Using proton-motive force (H+ gradient)
What is ATP dependent active transport dependent on?
Ion pumps
P-glycoproteins
Multi-drug resistance (mdr) proteins
What is sodium gradient active transport dependent on?
High affinity mammalian transporters
Usually expressed in specialised tissues
What is proton-motive force active transport dependent on?
Most protozoan transporters
What does the BBB consist of?
BBB consists of brain capillary endothelial cells, connected by tight junctions, plus astrocytes
What does the composition of the BBB do?
Keeps many unwanted substance and pathogens out but essential nutrients still need to be taken into the brain: specific transporters
What are transporters in the BBB?
Mostly active and uni-directional
What do efflux pumps do in the BBB?
Severely limit the ingress of drugs
What are the most potential causes of drug resistance?
Mutation of the target protein Loss of the target Overexpression of the target Overexpression of pathway bypassing target Loss of enzyme activating prodrug Detoxification with elevated levels of glutathione etc. Expression of ABC transporters Loss of transporters
What does expression of ABC transporters rely on?
MDR
P-glycopoprotein
How can loss of transporters occur?
Mutations
Gene deletions
Loss of expression
What is an example of mutation of target?
Benzimidazoles as anthelminthics
Most widely used anthelmintics
Thiabendazole (1961). Later analogues reduced toxicity
Widely used in human and veterinary medicine
Effective against most intestinal and systemic nematodes (roundworms) and cestodes (tapeworms) and some trematodes (flukes)
How does this mutation occur?
High affinity, but irreversible binding to beta-tubulin
This cases ultra structural changes to intestinal cells of nematodes and to the tegument of cestodes
Disruption of cytoskeleton
Inhibition of microtubule-mediated vesicular transport
What are microtubules?
13 proto-filament strands
What are proto-filaments?
Made up of strings of alpha-beta hetero-dimers
What does binding to beta-tubulin prevent?
Tubule elongation
What is normal tubule elongation?
alphabeta: alphabeta:alphabeta etc
What does benzimidazole treatment lead to?
Tubule capping and degredation
What do macrolides inhibit?
Including erythrocmycin
Inhibit protein synthesis by binding to 23 S ribosomal subunit
How is resistance associated by alterations to target?
Methylation of rRNA
Plasmid-encoded methyl transferase
How is resistance by reduced cellular concentration?
Plasmid-encoded efflux pump
What is arsenical resistance in trypanosomes?
Tyrpanosoma brucei spp cause sleeping sickness
Are transmitted by the tsetse fly
Sleeping sickness is treated with the organo-arsenic compound melarsoprol
What can melarsoprol resistance be the result of?
Detoxificarion plus-extrusion
Loss of uptake into parasite
Combination of both
What does Melarsoprol (MelB) enter the parasite through?
P2 adenosine transporter
What does MelB form?
Adduct with trypanothione (MelT)
What is the adduct exported by?
TbMRPA (ABC transporter)
What is the most common mechanism?
ABC-transporter-mediated efflux
What is non-expression of (uptake) transporters?
Unlikely to be result of loss-of-function mutations
Heterologous population followed by selection → survival of non-expressors → relapse with resistant cancer
Individual differences between people
