Lesson 6 Flashcards
Which are the mechanisms of inhibition
- Competition with the same enzyme or isoenzyme: two drugs which are transformed by the same enzyme and the interaction with one of them determines a winner and a loser
- Direct interaction with the enzyme: there are two different mechanisms according to the interactions between a given substance and CYP:
o Without metabolic activation
o With/after the metabolic activation: mechanism based - Decrease of the protein synthesis or increase metabolic breakdown (much less important).
What is the multitherapeutic approach and what should be taken into consideration?
It’s the use of more than one active principle and it can lead to the competition for the same enzyme since the number of biotransformation enzymes is very limited. Only CYP3A is responsible for half of the biotransfomration of half of the drug on market.
If you administer two competitive drugs, one will have a higher affinity to the enzyme, and it will be biotransformed. The other may accumulate in the liver or other organs and become toxic.
Describe with an example the direct interaction without metabolic activation
Let’s consider two molecules, N-substituted imidazole antifungals, and Nitric Oxide. They can bind the central heme iron of CYP450 in a weak manner.
So, the enzyme is blocked because it is bound to this kind of inhibitors. Since the binding is quite weak, at a certain time the binding will be resolved, the complex compound is released, and the enzymes becomes again functional.
OPEN QUESTION: Describe the direct interaction with metabolic activation with substrates
For this kind of inhibition preferable substrates are macrolides and Tiamulin.
The macrolides bind to the catalytic side of the enzyme (CYP3A family). We have the active enzyme, the active site, the macrolide and (as usual in CYP-dependent reactions) oxygen and NADPH.
A metabolite is formed, and it binds the central heme iron of the CYP.
In fact, there are two methyl groups linked to nitrogen:
1- The first step is the demethylation of one group, but then it will stop.
2- The second step is the formation of a metabolic intermediate complex (nitrosoalkene complex) which is covalently bound to the iron with oxidation number +2.
N.B.: The +2- oxidation number of iron means that the catalytic cycle is not yet completed.
So, there is this metabolic intermediate complex blocking the activity of CYP, because in this case the active site is still occupied by the inhibitor.
In the mechanism of direct interaction with metabolic activation, which is the inhibitor? The metabolite or the parent compund
The metabolite
Explain with an example the suicide inhibition
It’s a quite rare kind of inhibition, it may be considered as a particular form of mechanism-based inhibition.
The most relevant example is the antibiotic chloramphenicol (it has a chlorinated group, CHCl2), the target enzyme is CYP2B11 because it is the isoform in dogs.
The problem is the de novo synthesis: the reaction comes to an end, but the metabolites produced are extremely reactive. Chloramphenicol is bioactivated and the metabolites, which are so much reactive, will kill the enzyme (the same enzyme which has performed the biotransformation).
For every type of inhibition, explain if the inhibitor is a substrate of the inhibited enzyme or not
Competition, Direct interaction with metabolic activation, suicide inhibition: It must be a substrate
Direct interaction without metabolic activation: it may or may not be a substrate
OPEN QUESTION: explain two examples of modulation of a drug
- a dog is treated with phenobarbital (PB) and benzodiazepine. They affect the same enzyme, PB has more affinity. You should provide more bezodiazepine to have the therapeutic effect
- If you treat an animal or human with chloramphenicol, it inhibit CYP2B, which is involved in the biotransfomation of anesthetics.
If you provide anesthesia, this can be dangerous and its effect may be prolonged
What is the role of drug transporters?
They have to transport non-lipophilic drugs across membranes
OPEN QUESTION: Which are the two main type of membrane transportes? Give examples, energy needed, localization and function
1- Solute carriers (SLC): no energy
o Vesicle transporters
o Simple transported
o Ion gradient
o Mitochondrial transporters or transporters for other organelles within the cell
2- ATP binding cassette (ABC) transporters: they use ATP
o Water channels
o Ion channels
o Specific ABC transporters
Both kind are transmembrane proteins embedded in the phospholipidic bilayes.
Functions:
- Transport of exogenous and endogenous compounds
- Creation of an anatomical structure (ex: BBB) for protection
- Excretion and uptake of chemicals
How are connected DTs and DMEs in drug kinetics? (4 phases)
- Phase 0: uptake of xenobiotics not particularly lipophilic
- Phase 1: oxygen is introduced into the drug, metabolites are formed thanks to CYPs and they now have more polar characteristics
- Phase 2: conjugation step mediated by transferases. they conjugate glucuronides and sulphates
- Phase 3: export of metabolites (they’re no longer lipophilic)
Kindey: which are the different excretion according to the localization?
- Basolateral membrane is in contact with vasculature: if a metabolite is excreted in this part of the cell, it will be immediately in the systemic circulation.
- If it is excreted through the luminal membrane, the biliary excretion will take place.
Which is the function of DT in the intestinal epithelium?
DT may perform a double activity (in and out): some will favour the transport of a xenobiotic from the blood to the intestine, other the opposite (from the cell to the blood back again).
Generally speaking, the main function of DTs in this district is to limit the entry. Despite the low number, their main function is to act as a sort of barrier
Which are the main tissues in which there are DT?
Intestinal epithelium, hepatocytes, kidney and BBB
Describe the structure and the role of DT in hepatocytes
For hepatocytes we have a basolateral and an apical part: two hepatocytes facing each other concur to form the biliary canaliculus.
At this level, all the involved DTs concur the excrete a substate in the bile.
Some DTs are involved into the uptake from the blood to the hepatocytes, others in the opposite activity. However, all those expressed in the apical part of the hepatocytes without exception are involved in excretion.
