Hepatic Drug Metabolism
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Prof. MJ Coffey
CoffeyMJ@cardiff.ac.uk
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Define the different ‘phases’ of drug detoxification and the metabolites that are produced
Learning Outcomes
After working through this section, you should be able to:
Page 1 / 8
Section 1
Describe the role of liver enzymes in detoxifying drugs
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The series of chemical and structural alterations to a drug/toxin that are usually mediated by enzymes to:
(1) Deactivate pharmacological activity
(2) Create a water-soluble metabolite suitable for excretion
Definition
LIVER
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Drug
The liver has evolved as the organ of detoxification in the body to breakdown anything that poses a biochemical threat caused by chemical toxicity. Hepatic enzymes will create drug metabolites ready for excretion that can be trafficked via P-glycoprotein transporters into the bile duct and then into the gut lumen. Alternatively, the metabolites are transported in the bloodstream to the kidneys for excretion into the urine.
Metabolite
To kidneys
Via bile duct to gut
Enzyme kinetics
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DRUG
METABOLITE
ENZYME
Phase I enzymes typically add or create chemically reactive groups to the drug. Depending on the metabolic pathway and the specific drug, a number of Phase I enzymes may sequentially alter and modify the original drug structure. Such enzyme-mediated reactions may include: oxidation, reduction, esterification and isomerisation; all with an aim to facilitate drug deactivation in Phase II.
An important note here: Phase I metabolites may actually be more biologically active than the parent drug molecule. See the next page for the clinical significance of this...
“Functionalisation reactions” expose or create a chemically reactive group on the drug to facilitate further metabolic processing in Phase II
“Phase I” metabolism
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PHASE I
Intermediate may be a shortlived cytotoxic species (dangerous in high doses)
I
ENZYMES
II
PHASE II
Phase 1 enzymes may create ‘intermediate metabolites’ that are more pharmacologically active than the original drug. But the reactive phase 1 metabolites typically have very short half-lives before they are properly deactivated by Phase 2 enzymes or are excreted from the body. Therapeutically, intermediate Phase 1 metabolites can be exploited to switch pro-drugs on. For example, ramipril is a drug used to treat high blood pressure, but it is inert until it reaches the liver where hepatic esterases modify the drug to ramiprilat which is the pharmacologically active drug able to lower BP. However, some Phase 1 metabolites may be very cytotoxic. If these toxic intermediates build up and are present for long enough, extensive damage to tissues can result. We'll look at a specific example of this with paracetamol in Section 3...
INTERMEDIATE
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Intermediates
Intermediate may be the pharmacoligcally active form of the drug (‘an activated pro-drug)
Phase 2 processes often achieve biochemical deactivation and water solubilisation at the same time by chemically swapping out a drug’s pharmacophoric region (i.e., the element that confers biological activity) with a water-soluble component such as a sugar or a sulfo group.
Phase 2 metabolism has evolved to switch off a drug’s pharmacological activity and also render it water soluble, so it can be excreted either in the urine or the faeces… both of which are aqueous media.
WATER SOLUBLE GROUP
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“Phase II” metabolism
LOST IN URINE or FAECES
PHASE II ENZYME
ACTIVE DRUG or METABOLITE
DE-ACTIVATED, WATER SOLUBLE DRUG METABOLITE
Section 1 summary
Now try the SBA question on the next page...
Phase II metabolism can both render a drug or metabolite inert and add a water-soluble group to facilitate excretion in the urine or the faeces.
Phase I metabolism often ‘functionalises’ a drug by adding or exposing a chemically reactive group to facilitate further metabolic enzyme engagement.
Drug metabolism refers to the deactivation of pharmacologically active compounds, usually mediated by enzymes.
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B
C
D
With regard to the ‘Vmax’ of a drug metabolising enzyme- what does this represent in terms of the drug concentration?
Feedback on your choice:
E
The saturating drug dose causing maximum enzyme turnover
The drug dose that corresponds to the Km
The drug dose that inhibits further enzyme turnover
The drug dose that doubles the enzyme turnover
The dose of drug that represents 100% bioavailability
Click
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Quiz
A
Advance
Compare and contrast the roles of cytochrome P450 enzymes and UDP-glucuronosyl transferases
Type the Section 2 password here & click ‘Advance'
Section 2
Discuss the significance of enzyme polymorphisms on drug metabolism
CYP enzymes are the most significant family of Phase I enzymes. They are responsible for the majority of oxidative modifications to drugs, usually in advance of Phase II metabolic processes. CYPs are ‘on demand’ enzymes, induced through the pregnane-X receptor pathway only when the drug or toxin they metabolise is present. The inducibility of these enzymes is an important consideration if your patient is on multiple medications, as we’ll see in Section 3...
DRUG
+
<< >>
+
WATER
Cytochrome P450s
OXYGEN
HAEM
CYP ENZYME
Slider
A: Correct!The enzyme is working at its maximum rate at Vmax. Any higher dose of drug may lead to dose-related side effects.
The Section 2 case-sensitive password is: Beskar_Steel
B: Try again!
The Km concentration represents the drug dose that causes 50% enzyme turnover (i.e., half Vmax). Useful to know when considering how much metabolism is occurring.
C: Try again!
The drug is the enzyme substrate, so should *not* inhibit the enzyme at whatever dose.
D:Try again!
Depending on the shape of the hyperbolic curve, you can calculate what dose of drug/substrate will be required to double the enzyme rate (up to the Vmax dose).
E: Try again!
Bioavailability is a measure of how much pharmacologically active drug reaches its target. So, this option is a factually incorrect statement.
A child presents with mild jaundice that is NOT a result of obstruction/disease. Genotyping reveals a dysfunctional enzyme as the cause. Which enzyme is it?
Porphobilinogen deaminase
Sulfotransferase-2
Cytochrome P450 (3A4)
UDP-glucuronosyl transferase (1A1)
Prostaglandin H-synthase 2
A: Try again!
This enzyme is involved in haem synthesis, *not* haem breakdown.
B: Try again!
SULT enzymes add water soluble sulfo- groups to metabolites, but *not* to bilirubin.
C: Try again!
CYPs are important Phase I enzymes that oxidise drugs and endobiotics. They do *not* cause jaundice if they are dysfunctional.
D:Correct!
Dysfunctional or deleted UGT1A1 may cause Gilbert's or Crigler-Najjar syndromes with hyperbilirubinaemia.
The Section 3 case-sensitive password is: Mos_Espa
E: Try again!
PGHS enzymes create prostaglandin cell mediators. Their dysfunction is *not* associated with jaundice.
Compare & contrast the effects of enzyme induction & inhibition on drug metabolism
Describe how to treat a confirmed paracetamol overdose
Section 3
Type the Section 3 password here & click ‘Advance'
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Drug interactions (1)
Drug B
Enzyme
Decreased A
Drug A induces a quantity of enzyme that metabolises drug A and reduces the dose of drug A at the expected rate.
Drug B will also induce the same enzyme. Not only will drug B be metabolised but the extra quantity of enzyme that has been induced by drug B will also accelerate the metabolism of drug A. This may lead to drug A being metabolised before it can have its therapeutic effect.
Real world example: St. John’s Wort contains hyperforin. Hyperforin will induce enzymes that can metabolise ethinylestradiol (in the combined contraceptive pill) to a sub therapeutic dose, meaning unplanned pregnancy may occur.
Decreased B
Drug A
acetaminophen
n- acetyl cysteine
glutathione
n- acetyl-p-benzoquinone imine (NAPBQI)
acetyl salicylic acid
An acute admission to A&E is confirmed to have overdosed on paracetamol.
What IV treatment are they started on as a result?
For interest only:
Nature
Predicting Drug Metabolism
VOLUME 14 | JUNE 2015 | 387
Feedback & Queries:
Prof MJ Coffey
CoffeyMJ@cardiff.ac.uk
Further resources
(This is the end of the resource)
Textbook:
Pharmacology (Rang & Dale)
Section 1, Chapter 10
Drug Metabolism & Elimination
Zoom Q&A session:
9:00 am- 9:45 am
Weds 18th May
(Check MyTT)
A: Try again!
Acetoaminophen is the alternative name for paracetamol!
B: Correct!
An infusion of NAC will help replenish hepatic glutathione reserves (which can de-activate the toxic paracetamol metabolite that causes liver failure.)
C: Try again!
Glutathione is a natural anti-oxidant produced by the body. It can deactivate toxic paracetamol metabolites but it is *not* able to be given as a drug antidote.
D:Try again!
NAPBQI is the hepatotoxic paracetamol metabolite generated from CYP activity following a paracetamol overdose!
E: Try again!
Acetyl salicyclic acid is the alternative name for aspirin. We would *not* use this as a treatment for paracetamol overdose.
UG transferases
UDP-glucuronosyl transferases (UGTs) are a major class of Phase II enzymes. They conjugate a water soluble glucuronide (sugar) molecule on to the drug or toxin metabolite. This renders the drug/toxin inactivated and suitable for water-soluble excretion either into the urine via renal excretion or into the lumen of the gut via hepatobiliary excretion.
Click to reset
UDP-GA
Bilirubin
Conjugated Bilirubin
UGT 1A1
A type of jaundice...
FAECES
Damage or disease affecting the liver, or a blocked bile duct, may lead to jaundice (a build up of unconjugated bilirubin to toxic levels). Additionally, polymorphisms (or deletions) of UGT1A1 can lead to Gilbert’s or Crigler-Najjar syndromes where bilirubin cannot be glucuronidated and excreted at the required rate and levels. Genotype testing can be helpful in determining the specific type of jaundice a patient presents with.
CYP 3A4
Caffeine
CYP 2D6
Ethanol
Hepatic drug metabolism is comprehensive and efficient. This is due to the diversity of enzyme isoforms within a family. Each isoform has a slightly different substrate specificity, i.e., preference for a particular drug, and is induced in response to the presence of the specific drug.
Because each isoform is a member of the same family, there is a degree of substrate overlap in terms of which enzymes are able to metabolise which drugs. This is useful if a particular enzyme is dysfunctional or mutated: often another enzyme family member can take over the metabolism of the drug, albeit not as efficiently as the other enzyme, but still better than not having the drug metabolised at all.
CYP 2E1
Sertraline
CYP 1A2
The ‘family’ of CYP all catalyse the same reactions (oxidative modification of substrate), but have different substrate preference/specificities...
Omeprazole
Enzyme isoforms
5
Extensive
Poor
Subtle changes in structure and function of enzymes, caused by mutations in their genes, cause polymorphic variations of the enzyme. These polymorphisms mean that some individuals will poorly metabolise a particular drug but others will metabolise it ‘ultra-fast’.
The rate at which enzymes metabolise drugs has big implications for a drug’s half-life and potential toxicity at high doses.
Genotyping patients before starting them on some drug therapies is becoming more routine.
% of population
Frequency distribution for Enzyme ‘X' polymorphisms metabolising drug ‘Y' in a population
Polymorphisms
Ultra-fast
40
20
Cytochrome P450 enzymes (CYP) are the most significant class of Phase I metabolic enzymes, responsible for oxidative modifications to drugs.
Section 2 summary
Polymorphic variations in drug metabolising enzymes lead to differential patterns of drug metabolism within a population.
UDP-glucuronosyl transferases (UGTs) are a significant class of Phase II ‘conjugative’ enzymes, along with sulfotransferases (SULTs)
Drug interactions (2)
Drug A and drug B are both metabolised by the same enzyme. But drug B is the preferred substrate for the enzyme so will occupy the active site more than drug A.
Drug B can said to be ‘competitively inhibiting’ drug A’s metabolism.
If this is not taken into consideration, repeat dosing of drug A may lead to a build up to toxic levels as it is not being metabolised at the expected rate in the presence of drug B.
Real world example: the antibiotic erythromycin will competitively inhibit the metabolism of ethinylestradiol (in the combined contraceptive pill). High doses of unmetabolised oestrogens are linked to the development of thromboses.
Glutathione reserves rapidly depleted from the liver
UGTs
CYP1A2
CYP2E1
Paracetamol overdose
OTC paracetamol is 500 mg / tablet. 4 g - 6 g is a toxic dose. >12 g may be lethal
Click Normal path
> 4 g
UGTs
Click Overdose
path
2 x 500 mg
Glucuronidation pathway rapidly saturated by overwhelming dose of paracetamol
Majority of dose is directly glucuronidated by UGTs, bypassing Phase I enzymes
Excess drug forced down CYP pathway as UGTs are saturated.
Deactivated water soluble paracetamol glucuronide easily excreted from the body
Massive amounts of toxic n-acetyl-p-benzoquinone imine (NAPBQI) generated
No damage to hepatocytes
Significant hepatotoxicity
Tiny amounts of cytotoxic n-acetyl-p-benzoquinone imine (NAPBQI) generated
NAPBQI very rapidly deactivated by hepatic glutathione
- SH
NAPBQI
…… & treatment of OD
Glutathione- conjugated NAPBQI
Replenishes...
The longer the NAPBQI is present in the liver, the more extensive and irreversible the damage caused.
Infusing a patient with IV n-acetyl cysteine (NAC) allows hepatic glutathione levels to be replenished and allows the glutathione to effectively mop up and de-activate any NAPBQI present.
As a rule, the quicker a NAC infusion can be started on a paracetamol OD patient, the better their prognosis.
n-acetyl cysteine
TOXIC
TREATMENT:
Glutathione
NON-TOXIC
Induction of drug metabolising enzymes by a drug may cause another drug to be metabolised to a sub-therapeutic concentration.
Section 3 summary
Paracetamol toxicity is caused by a large dose overwhelming the glucuronidation pathway, with excess drug then being metabolised to NAPBQI.
Inhibition of drug metabolising enzymes by a drug may cause another drug to build up to toxic levels on repeat dosing (or be processed by another enzyme).
Enzyme rate as a function of drug concentration
100
0
25
Increasing Drug (Substrate) Concentration
75
% Enzyme Rate (Turnover)
50
Enzyme
At this drug concentration the enzyme is working at 75% of its maximum rate. It still has a little capacity to metabolise more drug, faster, but it's approaching saturation point. But drug dose is still rate-limiting.
This drug concentration saturates the enzyme. It is working at full capacity and cannot metabolise any more drug any faster. The enzyme is at ‘Vmax’ and is also now the rate limiting factor for metabolism.
The speed at which a drug is metabolised is a function of the enzyme kinetics. A drug is simply a substrate for a particular enzyme and the enzyme will convert that substrate into a product or ‘metabolite’. The graph on this page describes the relationship between the substrate concentration (drug dose) and how fast the enzyme can turn-over as the drug dose increases...
At this drug concentration, the enzyme is only working at 25% of its maximum rate and has plenty of capacity to metabolise even more drug at a faster rate. So here, drug dose is the rate limiting factor.
If the drug dose exceeds the enzyme Vmax concentration, unmetabolised drug may build up to toxic levels and/or the excess drug may be metabolised by other enzymes into toxic species.
This drug concentration represents ‘Km’- the enzyme is turning over at half its maximum rate but still has capacity to metabolise more drug at a faster rate. Again, drug dose is the rate-limiting factor.
SLIDER
drag me
MORPHINE
UGT
MORPHINE GLUCURONIDE
Repeat dosing will increase drug A