Ahmed E (Drug metabolism)

Question 1

Drug metabolism

Answer 1

Metabolism is the process by which the body converts xenobiotics (as drugs) usually into a less toxic, more polar form that can be readily excreted (soluble in urine).
Sites of drug metabolism:
- The liver is the primary organ of drug metabolism (75% happen in liver)
- The lung, kidney, intestine, skin and placenta can also carry out drug metabolising reactions

Question 2

Sites of drug metabolism

Answer 2

The largest proportion of drug metabolism takes place in the liver due to:
1. The liver is rich in almost all drug metabolising enzymes
2. The liver is a well perfused organ and so drugs can access the metabolising enzymes
3. Orally administered drugs pass through the liver before reaching the systemic circulation and thus become subject to metabolism

Question 3

Drug mechanism possibilities

Answer 3

Active drug –> inactive metabolite(s). (Most common pathway)

Active drug –> active metabolite(s). (Co-activation). (Prolongs dilation of drugs)

Active drug –> toxic metabolite(s) (metabolic toxicity)

Inactive drugs –> active metabolite(s). (Pro-drugs)

Active parent drug –> active metabolite having a different pharmacological action

Question 4

Drug metabolism- phase I and II

Answer 4

Drug metabolism is carried about through Phase I and/or Phase II.
Phase I (Functionalisation)- add, modify, or expense a polar functional group.
Phase II (Conjugation)- attachment of a hydrophilic substituent.

Cytochrome P450 family are examples of enzymes involved in Phase I, which account for about 75% of the total metabolism.
CYP3A4, CYP2D6, CYP2C9, and CYP2C19 are important for the metabolism of drugs in humans. (Need to know for drug-drug interactions. If patient is on two drugs that are metabolised by the same P450 enzyme there may be competition or if one in an inhibitor it will affect the other drug and its toxicity).
Some drugs may pass directly into Phase II (if they are already functionalised- have polar functional groups)

Question 5

Phases of drug metabolism- Phase I

Answer 5

Oxidation
Reduction
Hydrolysis

Question 6

Oxidation reactions

Answer 6

Addition of oxygen or removal of hydrogen.
The most common reaction for xenobiotics
- P450 uses molecular oxygen- inserts one of its own oxygen atoms into a substrate (RH) and reduces the second oxygen to a water molecule, utilising two electron that are provided by NADPH.
RH + O2 + NADPH + H+ –> R-OH + H20 + NADP+.

5 oxidation reactions:
1. Aromatic oxidation (benzene)
2. Alkene (Olefinic oxidation) (compounds with double bond)
3. Benzylic oxidation (carbon adjacent to benzyl ring)
4. Allylic oxidation (carbon adjacent to double bond)
5. Aliphatic oxidation (straight chain alkyl group)

Question 7

Aromatic oxidation

Answer 7

Produces Arene oxide which is highly reactive. It binds to cellular macromolecules as DNA and proteins. High toxicity, can lead to cell damage, cell death, loss of function, convert normal cells into cancerous cells.
3 different pathways of detoxification of arena oxide:
- NIH shift- converted into Phenolic metabolite
- Epoxide Hydrolase Enzyme- converted into Trans Dihydrodiol metabolite
- Glutathione S Transferase Enzyme- converted into glutathione adduct.

Hydroxylation is more rapid in electron rich rings –> activated rings. Fast metabolism and short half life.
Deactivated aromatic rings (large number of electron withdrawing groups)–> not hydroxylated or hydroxylated slowly. These compounds resist metabolism and have a long half life.
If the drug has 2 aromatic rings hydroxylation occurs in the more electron rich ring and only one ring is attacked.
Hydroxylation occurs at orthodox position if para position is occupied.

Question 8

Alkene oxidation

Answer 8

Intermediate (Epoxide) produced which is readily oxidised by enzyme (epoxide hydrolyse enzyme).
Epoxide of carbamazepine is toxic so can give patient glutathione to detoxify.

Question 9

Allylic C oxidation

Answer 9

Allylic carbon- C is next to double bond.

Question 10

Benzylic C oxidation

Answer 10

First oxidised to primary alcohol. If this primary alcohol is polar enough it can go directly to phase 2. If not then further oxidation to aldehyde and further oxidation to carboxylic acid.

Question 11

Aliphatic oxidation

Answer 11

Only occurs on the terminal carbon (w) or on the penultimate carbon (w-1).
Takes place in drug molecules with straight or branched alkyl chains.
Alcohol metabolites formed -> further oxidation to aldehydes and ketones.
Tertiary alcohols are not oxidised so go straight to phase 2.

Question 12

Alcohols and aldehydes oxidation

Answer 12

Primary alcohol oxidised by alcohol dehydrogenase to aldehyde.
Aldehyde oxidised by aldehyde dehydrogenase to carboxylic acid.
Secondary alcohol oxidised by alcohol dehydrogenase to ketone. (Minor pathway- usually go straight to phase 2).

Secondary alcohols are more polar, so more likely to be conjugated than ketone.

Tertiary alcohols are not oxidised.

Question 13

N and C-N oxidation

Answer 13

Oxidative N-dealkylation occurs twice to form primary amine. Smaller group is removed first. Then oxidative deamination occurs to remove N completely. Only C connected to Hs can be removed.
Tertiary amine to secondary amine by oxidative N-dealkylation. then to primary by oxidative deamination.
Removal of first group is faster than removal of second group.
Catalysed by amine oxidase enzyme.

Question 14

S and C-S oxidation

Answer 14

S-oxidation:
Sulfide oxidised to sulfoxide. Further oxidised to sulfone.

S-dealkylation:
Sulfide to thiol.

Desulphuration:
Oxidative conversion of thio group to carbonyl group. S=C converted to O=C by cytochrome P450.

Question 15

C-O oxidation

Answer 15

O-dealkylation.
Ether to alcohol/phenol.
Small alkyl groups are rapidly o-dealkylated.
If there are several non-equivalent methods groups -> selective dealkylation of only one methoxy.

Question 16

Reduction

Answer 16

Addition of hydrogen

Question 17

Aldehyde reduction reactions

Answer 17

Aldehyde- O=C-H.
Aldehyde reduced to primary alcohol.
Majority of aldehydes oxidised to carboxylic acids but may be reduced to alcohols especially if attached to e-withdrawing groups.
Electron deficient favours reduction not oxidation.

Question 18

Ketone reduction reactions

Answer 18

Ketone- O=C-R.
Ketone reduced to secondary alcohol.
Ketones are generally resistant to oxidation and therefore undergo reduction to secondary alcohols.
They give 2 possible isomers. Can add H from behind or front which gives 2 stereoisomers (R/S enantiomers)

Question 19

Nitro reduction reactions

Answer 19

NO2 -> NH2 (primary amine)

Question 20

Sulfone reduction reactions

Answer 20

2 step reduction
- removal of first O to give sulfoxide
- removal of second O to give sulfide

S(O2)R –> O=S-R –> S-R

Question 21

Azo reductase reactions

Answer 21

Requires 4 H.
Reduced to primary amine.
N=N-R –> NH2 + H2N-R

Question 22

Disulfide reduction reactions

Answer 22

Reduced to Thiol.
S-S-R –> SH + HS-R

Question 23

Hydrolysis

Answer 23

Break down of a compound into metabolites using water

Question 24

Ester hydrolysis reactions

Answer 24

More susceptible to hydrolysis metabolism. C-O bond is broken.
Produces an acid and an alcohol.
O=C-O-R –> O=C-OH + HO-R

Catalysed by esterase which are non-specific and widely distributed in liver, kidney, intestine.
Major pathway for esters is facile hydrolysis.
Products contain functional groups that can be conjugated.

Question 25

Amides hydrolysis reactions

Answer 25

C-N bond is broken.
Produces an acid and an amine.
O=C-N(H)-R –> O=C-OH + H2N-R.

Amides are slowly hydrolysed so have a longer duration.

Question 26

Phase II

Answer 26

Conjugate reactions

Question 27

Phase II reactions

Answer 27

Most phase II reactions are conjugate reactions catalysed by transferase enzymes.
Main purpose of phase II is to make compounds more polar.
Phase II is capable of converting parent xenobiotics or its phase I metabolites to conjugated products which are
- more polar
- water soluble
- readily excretable
- biologically inactive
- non-toxic

Conjugation reactions:
- Glucuronic acid conjugation
- Glutathione conjugation
- Methylation
- Acetylation
- Sulfate conjugation
- Amino acid conjugation

Question 28

Glucuronic acid conjugation

Answer 28

Glucose oxidised to GA. GA activated into UDGA. UDGA conjugated with N, O or S to B-Glucuronide (catalysed by UDP-glucuronyltransferases).
Once bound, solubility and excretion is increased.

Question 29

Sulfate conjugation

Answer 29

Add a sulfate group to a compound.
1. Activation of inorganic sulfate to 3’-PhosphoAdenosine-5’-PhosphoSulfate (PAPS) which is the active form.
2. Binding to the polar group of the compound: phenols, alcohols, aromatic amines
3. N-sulfonation of aromatic amines is a minor pathway.
4. Amount of sulfate available is limited
5. Utilised to conjugate endogenous compounds e.g. steroids, catecholamines, and thyroxin

(Neonates and young children have a decreased glucuronidating capacity because of undeveloped glucuroyl transferases or low levels of these enzymes.
Sulfate conjugation is well developed and becomes the main route of paracetamol conjugation in pediatrics).

Question 30

Glutathione conjugation

Answer 30

1. Glutathione binds to electrophilic drugs by the nucleophilic sulphydryl group present
2. The reaction is catalysed by Glutathione S transferase enzyme

Electrophilic functional groups e.g. epoxides, alkyl halides, disulphides, radical species.
Takes place in most cells, especially in the liver and kidney