Pharmacokinetics: Drug Metabolism Flashcards

1
Q

Why would you want a drug to be lipophilic?

A

So drugs can access tissues – therapeutic effect

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Why would you want a drug to be water soluble?

A

Retained in the blood and delivered to excretion sites

If a drug is water soluble it is less likely to access the tissues and thus is easier to excrete.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Describe the bi-directional movement of drugs

A

Each drug entering a tissue may leave it again- just physics- depends on concentration gradients which direction the NET movement will be

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is important to remember about the movement of lipid soluble and water soluble drugs

A

Some lipid soluble drugs will be excreted (won’t pass through barriers)
Some water soluble drugs will enter the tissues and pass through the barriers.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Summarise what body does to drugs in its metabolism

A

We design relatively lipid soluble drugs.

Body alters the drug to make it less lipid soluble and easier to excrete.

The process of metabolism involves the conversion of drugs (usually quite lipid soluble) to metabolites (usually less lipid soluble and easier to excrete).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Summarise metabolic transformation

A

§ Xenobiotics (drugs/foreign compounds) are usually lipophilic.
§ Metabolism tends to reduce or eliminate pharmacological activity.
§ Metabolism converts lipophilic chemicals to polar derivatives (which are readily excreted).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Summarise the two phases of drug metabolism

A

Drug metabolism involves two kinds of biochemical reaction

Phase 1 – main aim is to introduce a reactive group to the drug (increase polarity)

Phase 2 – main aim is to add a water soluble conjugate to the reactive group

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Describe first phase reactions

A

Phase 1 reactions (oxidation, reduction or hydrolysis) are catabolic, and the products are often more chemically reactive and hence, paradoxically are more toxic or carcinogenic than the parent drug. Phase 1 reactions often introduce a reactive group, such as hydroxyl, into the molecule, a process known as ‘functionalisation’. This group then serves as a point of attachment for the conjugating system (phase 2) to attach a substituent such as a glucuronide.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Where are the microsomal enzymes that take place in phase 1 reactions found

A

In the SER of hepatocytes. They are called microsomal enzymes because, on homogenisation and differential centrifugation, the ER is broken into very small fragments that sediment only after prolonged high speed centrifugation in the microsomal fraction. To access these enzymes in life, a drug must cross the plasma membrane.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the result of the different phase 1 reactions

A

Oxidation/reduction creates
new functional groups

Hydrolysis unmasks them.

Important – functional group
serves as a point of attachment
for phase II reactions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What will phase 1 oxidation reactions produce

A

Electrophiles:
R=O
R-R ( each R joining to an O)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What will phase 1 reduction and hydrolysis reactions

A

Nucleophiles
R-OH
R-SH
R-NH2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the most common reaction in phase 1 metabolism

A

Most common Phase 1 metabolism = oxidation (often start with hydroxylation and then oxidise that compound)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Summarise the cytochrome P450 enzymes

A

S.E.R.
57 enzymes
Different subsets- metabolise different drugs
We all have different variations of these enzymes
Precision medicine- hard to predict how each patient will respond to each drug

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is aspirin oxidised to

A

Salicylic acid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What are the 3 scenarios in phase 1 metabolism

A

Active parent drug Inert metabolite
Examples are cocaine and nicotine

Active parent drug Active metabolite
(prolongs effects)

Examples is cannabis, delta-9-tetrahydrocannibol is the active parent drug, but 11-hydroxyl THC is one of the metabolites and is more powerful- complicates things massively- will only monitor active parent- but metabolite is also active and will produce effects

Inactive parent drug Active metabolite
(prodrug)
Codeine is an example of this- needs to be metabolised to morphine first.

17
Q

Summarise phase 2 metabolism

A

§ The conjugate is almost always pharmacologically inactive.
§ The reaction creates molecules that are less lipid soluble and easier to excrete (as they aren’t reabsorbed in the tubules of the kidneys). Conjugating agent = an agent that attaches a polar group onto the molecule in question.

18
Q

What is the conjugating agent for the electrophiles

A

Glutathione-

19
Q

What are the different conjugating agents for nucleophiles

A

UDP-glucuronic acid — -OH, -COOH, -NH2, -SH

Acetyl coA —- -OH, -NH2

3-phosphoadenosine, 5-phosphosulfate —- -OH, -NH2

Hence the 3 reactions are:
Glucuronidation
Acetylation
Sulfation

20
Q

Describe Glucuronidation of aspirin

A

Glucuronidation is low affinity/high capacity – more likely to occur at high drug dosages.

High capacity means lots will be metabolised in this way

21
Q

Describe the sulfation reaction of paracetamol

A

Sulfation is high affinity/low capacity – more likely to occur at low drug dosages.
40-60% metabolism

22
Q

Describe the reaction of paracetamol with UDP-glucuronic acid

A

Low affinity/high capacity

20-30% metabolism

23
Q

What is required to biotransform a drug into an electrophilic conjugate

A

Drug needs to be electrophilic to be conjugated or biotransformed to an
electrophilic conjugate.

24
Q

How can we convert paracetamol to an electrophile

A

(clue – oxidation can mean gaining oxygen OR losing
hydrogen)
Remove a hydrogen atom from the nitrogen to form NAPQI (phase 1 metabolite).
This can then be conjugated by glutathione

25
Q

What is the issue with the metabolism of paracetamol into NAPQI

A

NAPQI is an electrophile- which is extremely reactive
During a paracetamol overdose- the glutathione system can become overwhelmed and can’t metabolise NAPQI as quickly, leaving NAPQI free to cause liver damage by reacting with proteins in the liver causing massive liver dysfunction

26
Q

What are the 3 other phase 2 metabolic reactions that are less common

A

Other Phase 2 metabolism pathways = acetylation, methylation, amino acid conjugation

Less common than previous 3

27
Q

Describe acetylation

A

§ The acetyl group is transferred to an electron rich atom. Acetyl-CoA is the conjugating agent.
coA SH also formed
Acetyl coA is the donor
Donated to an electron rich atom (N,O or S)
N-acetyltransferase is the enzyme
R usually an aromatic amine

Isoniazid is metabolised in this way

28
Q

Describe methylation

A

§ A methyl group is added to the electron rich atom. S-adenosyl methionine is the conjugating agent.

S-adenosylhomocysteine is also formed
H3C-O-R is produced

Targets R-OH, R-NH2, R-SH, R-NH-R

METHYLTRANSFERASE IS THE ENZYME
Levodopa is metabolised in this way

29
Q

Describe amino acid conjugation

A

Two reactions possible
with carboxyl group of amino acid
with amino group of amino acid

30
Q

Describe the amino acid conjugation using the carboxyl group

A

Acetyl coA is conjugated using acyl-coA synthesase, the coA-SH group is then removed using acyl coA amino acid N acyltransferase

ATP hydrolysed to AMP in first step

31
Q

Describe the amino acid conjugation using the amine group

A

Serine added and ATP hydrolysis- converted to intermediate using seryl- tRNA synthesase

Serine added to this intermediate to make the reactive nitrenium ion

32
Q

Describe the importance of drug metabolism

A

The biological half-life of the chemical is decreased.

The duration of exposure is reduced.

Accumulation of the compound in the body is avoided.

Potency/duration of the biological activity of the chemical can be altered.

The pharmacology/ toxicology of the drug can be governed by its metabolism.

33
Q

What is common about the intermediates

A

§ …involve high energy intermediates such as UDPGA for glucuronidation or PAPS for sulphation.

34
Q

Summarise sulfation

A

§ Sulphotranferases catalyse transfer of sulphate to substrates. 3’-phosphoadenosine-5’-phosphosulphate is the conjugating agent.

35
Q

Summarise glucuronidation

A

§ Drug-OH + UDPGA à Drug-O-D-glucuronide.

§ A high energy phosphate compound is created. UDP-glucuronic acid is the conjugating agen

36
Q

What do the effects of cannabis last longer

A

Both the active parent and metabolite are active and lipid soluble- prolonging its effects- can diffuse into fats and remain as a reservoir in the fats- diffuse back into blood- really complex