Week 4 - HPLC and Drug Metabolism Flashcards
(97 cards)
1
Q
What is biotransformation?
A
conversion of free drug into metabolites or excreted product
2
Q
What does ADME stand for?
A
Absorption
Distribution
Metabolism – anabolism, catabolism
Excretion
3
Q
What part of ADME is drug elimination
A
Metabolism – anabolism, catabolism
Excretion
4
Q
WHY IS THE STUDY OF DRUG METABOLISM &
ELIMINATION IMPORTANT?
A
- The route of metabolism of a drug can determine its ultimate
pharmacological or toxicological activity - May impact on dose and frequency
5
Q
Where does a lot of the biotransformation processes happen?
A
liver
6
Q
What is anabolism?
A
adding components
7
Q
What is catabolism?
A
breaking down
8
Q
Characteristics of intravenous drug administration?
A
- active straight away
- likely to ass liver/kidney
9
Q
What is the first pass effect?
A
first thing that happens to drug which is metabolism of drug occurring before entering the systemic circulation
10
Q
What is the drug metabolised by in the first pass effect?
A
- Enzymes in GIT (also not all will be absorbed)
- Liver
11
Q
What are the consequences of the first pass effect?
A
means that there is not 100% bioavailability of the drug and less than 100% of drug enters the circulatory system
- therefore we need more of the drug
12
Q
What type of variation in humans affect drug absorption?
A
metabolic rate and enzyme rate
drugs can also be slowed down when intestinal blood flows changes - during eating etc
having two different drugs at the same time can also cause incomplete drug absorption
13
Q
How does body remove polar/hydrophilic drugs from blood?
A
urine and faeces
14
Q
What needs to be done to non-polar and lipophilic drugs?
A
transform in liver
15
Q
What are the characteristics of most drugs?
A
they are non-polar and lipophilic
16
Q
Where is the primary location where drug metabolism occurs?
A
Primarily in the liver but can occur in all organs
17
Q
What is metabolism?
A
process of converting chemicals to more
polar metabolites
18
Q
What happens to the polarity of drugs/metabolites after phase 1 and 2?
A
polarity increases
19
Q
What are the two phases of drug metabolism?
A
phase 1 and 2
20
Q
What is the purpose of phase one?
A
activation of the drug - to make it more active
21
Q
How does the drug become more active in phase 1?
A
undergoes
oxidation
hydroxylation
dealkylation
deamination
hydrolysis
22
Q
What is formed after phase 1?
A
a derivative
23
Q
What is a derivative?
A
a compound that is derived from a similar compound by a chemical reaction.
24
Q
What is the purpose of phase 2?
A
to form a conjugate from a derivative through conjugation
25
What is a conjugate?
A compound formed by chemically joining two or more different substances
26
What is conjugation?
The process of covalently linking drugs to various natural or synthetic molecule carriers for specific applications
27
What does oxidation do to drug molecules?
increases water solubility
introduces functional group to make it easier to metabolise
28
What is an electrophile?
electron acceptor
29
What is a nucleophile?
provide electron pair to get new covalent bond
30
What process in phase one produces electrophiles?
oxidation of lipophilic molecules
31
What process in phase one produces nucleophiles?
hydrolysis and reduction
32
What is the purpose of phase 1 and 2?
convert lipophilic molecules into hydrophilic
33
What do electrophiles produced from phase 1 undergo?
glutathione conjugation to produce hydrophilic molecule
34
What do nucleophiles produced from phase 2 undergo?
sulfation
acetylation
glucuronidation
35
What type of reactions occur in phase 1?
reactions are catabolic
- Oxidation –most important, catalysed by cytochrome P450 enzymes
- Reduction
- Hydrolysis
36
Where is P450 from?
liver
- lots of genetic variation in cytochrome - which influences drug metabolism
37
What is a feature of the products produced from phase one?
metabolites can be more toxic or carcinogenic than parent drug
- this is when they need extra metabolic processes
38
What occurs in phase one?
introduction of reactive group
- known as a process of functionalisation
- group acts as a point of attack for conjugating system
39
What type of reactions occur in phase two?
synthetic/anabolic
40
What does phase two reactions involve?
conjugation - attachment of substituent group
- Glucuronic acid
- Sulphate
- Amino acids
- Glutathione
- Acetylation
41
What is the aim of phase 2?
to activate end product
42
What are the exceptions of phase 2?
exceptions -active sulphate metabolite of minoxidil (K+channel activator)
43
Where does phase 2 mainly take place?
in the liver
- If drug molecule or Phase 1 product has a suitable ‘handle’, it is susceptible
to conjugation
- Handle -hydroxyl, thiol or amino group
44
What type of drugs are more easily transported?
lipophilic are more easily transported than hydrophilic
45
Why are polar molecules a problem?
they need to be transported actively rather than passively
46
What organ is important in phase 1?
liver
47
Where are drug metabolising enzymes located?
embedded in the smooth ER
48
What type of enzymes are embedded?
microsomal enzymes
49
What happens to the drug to interact with these enzymes?
crosses the plasma membrane
50
What happens to polar drugs?
Polar molecules move less readily unless transport system – polar drugs partly
excreted unchanged in urine
51
What does CYP450 use as cofactor?
heme as a cofactor in their catalytic activity
52
What phase is CYP more involved in?
phase 1 of metabolism
53
Characteristics of CYP450
Superfamily
Differ in sensitivity to inhibition/induction and specificity of reaction they
catalyse
Distinct but overlapping substrate specificities
54
MIXED-FUNCTION OXIDASE REACTION
(CYTOCHROME P450; CYP) EQUATION
NADPH + O2 + RH NADP+ + H2O + ROH
55
Where is P450 found in?
endoplasmic rectilium
liver, kidney, lungs, intestine
56
What does a mixed-function oxidase reaction require?
substrate, enzyme, molecular oxygen
57
What is a mixed-function oxidase reaction catalysed by?
1. Cytochrome P450
2. NADPH-cytochrome P450 reductase
58
What is an example of species differences of P450?
rats and humans developed colon cancer but not monkeys when eating bbq food
- dietary amines and genotoxic products
59
What are characteristics of P450 expression?
polymorphism
environment – enzyme inducers and inhibitors e.g. grapefruit
juice inhibits enzymes, brussels sprouts induce P45. Clinically
important
60
CYP2D6 : GENOTYPE/PHENOTYPE VARIABILITY
poor metabolizer
– little or no CYP2D6 function
intermediate metabolizers
– metabolize drugs at a rate somewhere
between the poor and extensive metabolizers
extensive metabolizer
– normal CYP2D6 function
ultrarapid metabolizer
– multiple copies of the CYP2D6 gene are expressed, so greater-than-normal CYP2D6 function occurs
61
Is P450 the only enzyme involved in drug metabolism?
no there are others
- Suxamethonium hydrolysed by plasma cholinesterase
- Ethanol metabolised by alcohol dehydrogenase + CYP2E1
- Xanthine oxidase inactivates 6-mercaptopurine
- MAO – inactivates biologically active amines – e.g. NA, tyramine, 5-HT
Hydrolysis –
Ester and amide (less readily) bonds susceptible to hydrolytic cleavage
Warfarin – reduction of ketone to OH by CYP2A6
62
PHASE II REACTIONS
Insertion of a chemical group
Glucuronyl
Sulphate
Methyl
Acetyl
63
What happens if there is an enzyme that is missing?
its find because other enzymes have different functional groups that can make up for it
64
What drugs create the R -OH glucuronide conjugation?
paracetamol, aspirin, morphine
65
What drugs create the R- COOH glucuronide conjugation?
clofibrate, nicotinic acid
66
What drugs create the R- NH2 glucuronide conjugation?
dapsone, sulphafurazole, meprobamate
67
What drugs create the R- SH glucuronide conjugation?
2-mercapto-benzothiazole
68
What are the endogenous substrates of glucoronide conjugation?
bilirubin, steroid hormones, thyroxine and catechols
69
Characteristic of glucose
energy rich donor
- replacement of CH for H molecule of carbon 6
70
What are the two molecules glucose is turned into?
glycolysis; glycogenesis
glucuronic acid
71
What is the enzyme required when converting glucose into other molecules?
UDP glucuronosyl transferases
72
GLUTATHIONE (GSH) CONJUGATION:
GLUTATHIONE S-TRANSFERASES - DOES IT REQUIRE ACTIVATION?
no
- they are electrophilic substrates
73
Why is steroselectivity important?
Clinically important drugs e.g. warfarin, cyclophosphamide – mixtures of
stereoisomers
Different pharmacological effects
Differences in metabolism & pathways
Toxicity – linked to one stereoisomer – impact on new drugs
74
Glutathione (GSH) conjugation - What happens to the conjugates?
conjugates can either be excreted or processed further
75
What base is P450?
haem based
76
What can enzyme induction increase?
drug toxicity or carcinogenicity
77
What do phase one drugs tend to be?
reactive - toxic - carcinogenic
78
How can we exploit the fact that phase 1 drugs tend to be reactive?
sometimes we need very reactive phase on metabolites
79
Can some drugs act as inhibitors?
yes
80
What type of inhibition can P450 do?
competitive inhibition
non-competitive inhibition
mechanism-based
81
P450 - Competitive Inhibition Example
eg. quinidine - not substrate
82
P450 Non-Competitive Inhibition
- reversible
- eg. ketoconazole
- complex with Fe 3+ from haem iron of CYP3A4
83
P450 Mechanism-Based
- requires oxidation by P450 enzyme
eg. oral contraceptive gestodene
- oxidation product (epoxide intermediate of gestodene) binds covalently enzyme which destroy itself - suicide inhibiton
84
Microsomal enzyme induction
Rifampicin, ethanol, carbamazepine - incr. activity of enzymes with repeated
admin
Carcinogenic chemicals have this effect e.g. benzypyrene
Can increase drug tox/carcinogenic can be exploited therapeutically
85
ACTIVE DRUG METABOLITES
Pro-drugs
?deliberate design – drug delivery
e.g. aspirin inhibs platelet function and has anti-inflammatory activity but
hydrolysed to salicylic acid (anti-inflammatory not anti-platelet)
Metabolites may have similar effects to parent compound e.g.
benzodiazepines
toxicity eg. methanol and ethylene glycol
86
How can other drugs interact with other drugs?
Slow onset of induction
Slow recovery,
Selective induction
it may affect how fast drug is metabolised and increase tolerance
87
What can enzyme inhibition in the context of drugs?
Slow metabolism, increases action of drugs normally inactivated by enzyme
88
What is a consequence of inhibiton?
therapeutic effects
- eg. disulfiram (aldehyde dehydrogenase inhib)
-> produces adverse reaction to ethanol, also inhibits metabolism of other drugs
89
What does inhibition to the conversion of a prodrug to active metabolite result in?
loss of activity
eg. proton inhibitors (such as omeprazole) and anti platelet drug clopidogrel which is widely co-prescribed
90
What do liver cells do?
transports substances from plasma to bile using transport system
eg. organic cation transporters (OCTs), organic anion transporters (OATs) and P-gylcoproteins (P-gps)
91
Enterohepatic circulation steps
hydrophilic drug conjugates (esp glucuronides) concentrated in bile and delivered to the intestine
glucuronide can be hydrolysed, regenerating active drug;
free drug
reabsorbed
cycle repeated - reservoir of recirculating drug
eg morphine
92
Renal clearance
volume of plasma containing the amount of substance that is removed from the body by kidneys in unit time
93
What are the three processes that account for renal drug excretion?
1. glomerular filtration
2. active tubular secretion
3. passive reabsorption
- diffusion from the concentrated tubular fluid back across tubular epithelium
94
What are the mechanisms by which one drug can affect the rate of renal excretion of another are:
altering protein binding (hence filtration)
inhibiting tubular secretion
altering urine flow and/or urine pH
95
INDIVIDUAL VARIATION
Ethnicity – ethanol, propranolol
Age - hepatic microsomal enzyme activity declines
Pregnancy - Cardiac output increases - increased renal blood flow and
GFR, increased renal elimination of drugs
Disease
Drug interaction
Pharmokinetics interaction
96
GENETIC VARIATION
Plasma cholinesterase deficiency – suxamethonium
Drug acetylation deficiency
Drug targeting
97
What does the entroepatic circulation transport?
different conjugates
