Drug Metabolism

Question 1

Why do drugs get metabolized?

Answer 1

The body needs a process to remove potentially harmful foreign chemicals or chemicals that aren’t doing anything from accumulating.
Plants and some animals produce chemicals that could be toxic.
The body will generally remove all foreign chemical by a process known as metabolism or biotransformation

Question 2

Where do drugs go?

Answer 2

There are many avenues by which drugs can be metabolized: liver (most important; e.g., first pass metabolism), secretions into the bile and elimination in feces, kidneys, lungs, intestinal gut flora (i.e., bacteria metabolize drugs), skin and other organs

Question 3

What happens to hydrophilic drugs?

Answer 3

Very hydrophilic drugs will get excreted unchanged

Question 4

What happens to lipophilic drugs?

Answer 4

Lipophilic drugs are usually metabolized (lipophilic drugs bind to proteins, distribute in the fatty tissue and tend to persist in the body longer than hydrophilic drugs)
They generally avoid excretion by the kidneys

Question 5

What is the general goal of metabolism?

Answer 5

Chemically modify (metabolize) lipophilic drugs to increase their water solubility so that the metabolized drug can be excreted by the kidneys

Question 6

What can metabolism do?

Answer 6

Metabolism can eliminate or reduce the activity of a drug, alter the activity of the drug, increase or decrease the potency of a drug, activate prodrugs (drugs that are designed to undergo metabolism in order for it to be fully active)

Question 7

What happens in phase I metabolism?

Answer 7

A parent drug can be modified (oxidation, hydrolysis, reduction) into a modified drug

Question 8

What happens in phase 2 metabolism?

Answer 8

A (modified) drug can undergo biosynthetic conjugation (glucuronidation, sulfation, glutathione acetylation) to become a drug conjugate

Question 9

Are the phases of metabolism linear?

Answer 9

No, drug can be eliminated by:
No phases (excreted unchanged; hydrophilic drugs)
Phase I only
Phase I then Phase II
Phase II only
Phase I then Phase II then back to Phase I

Question 10

What types of reactions happen in phase I metabolism?

Answer 10

Hydroxylation
Dealkylation
Oxide formation
Desulfuration
Dehalogenation
Alcohol oxidation
Deamination
Reduction

Question 11

What are the most important phase I enzymes?

Answer 11

Cytochrome P450 family aka CYP enzymes

Question 12

What are CYPs?

Answer 12

CYPs are heme proteins that require electrons and oxygen to function (electrons come from NADPH)
Oxygen is reduced while the drug is oxidized

Question 13

Describe the CYP enzyme family

Answer 13

They are grouped by relatedness measured by amino acid similarity
There are at least 30 CYP enzymes, many found in the liver but also found throughout the boyd

Question 14

How are CYPs designated?

Answer 14

CYP#letter# (e.g., CYP3A4)
The first number refers to the family (having >40% amino acid similarity)
The letter refers to the subfamily (having >59% amino acid similarity)
The second number refers to the gene/subtype of the sub family

Question 15

CYPs are generally divided into two groups. What are the groups?

Answer 15

CYP1 through CYP4 are generally known as the drug metabolizing enzymes
CYP11, 17, 19 and 21 are the steroidogenic enzymes (they also participate in drug metabolism)

Question 16

Name some important CYPs

Answer 16

CYP1A1 (found in the lung and placenta induced by smoke)
CYP1A2 (found in the liver induced by smoke metabolizes caffeine)
CYP2B6 (found in the liver and other tissues, induced by some anticonvulsants)
CYP2C8, 2C9, 2C18, 2C19 (found in the liver, metabolizes many drugs)
CYP2D6 (found in the liver and other tissues, metabolizes many drugs)
CYP2E1 (found in the liver, induced by alcohol)
CYP3A3, 3A4, 3A5 (one of the most abundant CYPs in the liver, one of the most important classes of CYPs for drug metabolism)
CYP4A11 (found in the liver and kidney, involved in fatty acid metabolism)

Question 17

What are some CYP reactions?

Answer 17

Aliphatic hydroxylation
Aromatic hydroxylation
Alkene oxidation
N or S oxidation
N, O or S dealkylation
Deamination
Epoxidation
Nitro reduction (uncommon)
Oxidative dehalogenation

Question 18

Where does aliphatic hydroxylation take place?

Answer 18

It happens on the next-to-last or penultimate carbon on an aliphatic chain
The chain has to be 2 or more carbons

Question 19

What does the rate of aliphatic hydroxylation depend on?

Answer 19

The length of the chain; the longer the chain, the higher the rate

Question 20

Where does aromatic hydroxylation take place?

Answer 20

If hydroxylation happens at all, its position of addition depends on R and its position in the ring. Electron donating R groups tend to increase hydroxylation and favour para-hydroxylation. Electron withdrawing R groups decrease hydroxylation

Question 21

How can aromatic hydroxylation be eliminated?

Answer 21

By the addition of electron withdrawing substituents or halogens in the para position to R

Question 22

What happens with regards to aromatic hydroxylation in fused aromatic ring systems?

Answer 22

Usually only one ring will be hydroxylated. Typically it is the ring with no electron withdrawing groups. Typically halogenated rings are also not hydroxylated

Question 23

What happens more often: N oxidation or S oxidation?

Answer 23

S oxidation

Question 24

What happens to epoxides?

Answer 24

Epoxides are very reactive and are hydrolyzed by epoxide hydrolase (EH). This process can also happen spontaneously.

Question 25

What is a consequence of the reactivity of epoxides?

Answer 25

Epoxides are very reactive and can chemically modify proteins (e.g., protein alkylation). This is a potential source of toxicity produced by metobolism

Question 26

What is a trans-diol?

Answer 26

If there is restricted rotation around a bond in an aromatic group, you get a trans diol. Otherwise you just get a idol

Question 27

Can the formation of trans-diols be catalyzed?

Answer 27

Yes. Trans-diol formation can be catalyzed by epoxide hydrolase (EH) or can be non-enzymatic

Question 28

Describe the active site of CYP enzymes

Answer 28

There is a heme group, with Fe in the centre. An oxygen finds to the iron and it is converted into a reactive form (active)

Question 29

Describe CYP enzyme inhibitors

Answer 29

Some drugs are CYP inhibitors and many of these have imidazole groups that bind to the oxygen and inhibit the CYP enzymes. The azole antifungals are the most prominent example (ketoconazole, itraconazole, fluconazole).

Question 30

Explain the importance of esterases and amidases

Answer 30

Esterases and amidases are not CYPs, but they preform hydrolysis reactions and are phase I enzymes. They are found throughout the body and in the blood
Prodrugs are often designed as esters or amides so that they are hydrolyzed by esterases and amidases to produce the active drug

Question 31

Describe phase II enzymes

Answer 31

There are 6 important enzymes that transfer endogenous chemical to the drug.
They require: substrate (can be unmodified drug or drug modified by phase I), activated endogenous chemical (activated because it acts as a good leaving group for transfer of endogenous chemical to drug) and a functional group on the drug to modify

Question 32

What are the 6 types of phase II conjugation?

Answer 32

Glucuronide conjugation (most important)
Glutathione conjugation
Methylation
Acetylation
Sulfate conjugation
Amino acid conjugation

Question 33

Describe glucuronide conjugation

Answer 33

Enzyme: UDP-glucuronosyl transferase (UGT)
Activated endogenous chemical: uridine diphosphate glucuronic acid (UDP-GA)
Target functional groups: OH (most common), SH, NH, COOH, R-NH-OH, R-S-OH
If the product is 350 Da, it is excreted in the feces

Question 34

Describe glutathione conjugation

Answer 34

Enzyme: glutathione S transferase (GST)
Activated endogenous chemical: gamma-glutamyl-cystenyl-glycine (GSH) (exception to the general rule of activated substrate; it is not activated)
Target functional groups: any electrophile, carbonyls, substituted aromatic rings, double bonds
Mostly excreted in the feces

Question 35

Describe glutathione S transferase

Answer 35

A tripeptide that is found in all tissues has a thiol group
Involved in scavenging reactive metabolites and free radicals
Toxic reactive metabolic intermediates of drugs are often detoxified by glutathione
Will work without GST but slower

Question 36

Describe sulfate conjugation

Answer 36

Enzyme: sulfotransferase (ST)
Activated endogenous chemical: phosphoadenosine phosphosulfate (PAPS)
Target functional groups: OH, phenols, catachols, hydroxyl amines, hydroxysteroids (estradiol)
Excreted in the urine

Question 37

Describe methylation

Answer 37

Enzyme: methyltransferase (MTase)
Activated endogenous chemical: S-adenosyl methionine (SAM)
Target functional groups: O, N, S
Excretion: urine or feces

Question 38

Describe acetylation

Answer 38

Enzyme: acetyl transferase (AT; N-acetyl transferase are the most important but there is also O- and S-acetyl transferase) - decreases solubility
Activated endogenous chemical: acetyl co-enzyme A (Acetyl CoA)
Target functional groups: N, sometimes O or S
Mostly excreted in the urine

Question 39

Describe amino acid conjugation

Answer 39

Enzyme: aminoacyl transferase (AA-T; most important is glycine)
Activated drug: Drug-CoA (exception: the drug is activated, usually through COOH, and you end up with a conjugated amino acid)
Target functional groups: COOH
Mostly excreted in the urine