Drug metabolism Flashcards
1
Q
what is absoprtion?
A
absorption is about crossing biological barriers
2
Q
what is adsorption?
A
adsoprtion is the interaction of the drug with a surface
3
Q
what is the d in adme?
A
distribution - drugs get distributed throughout the body hence why side effects occur. however the brain is protected by the BBB preventing many drugs from entering
4
Q
what does the m in adme refr to?
A
metabolism however not all drugs are metabolised - some are excreted unchanged
5
Q
how would a drug be renally excreted?
A
if a drug is water soluble hence its soluble in the blood plasma it will be filtered in the kidneys and excreted in the urine
6
Q
what requirements are there for a drug to be excreted via the bile duct?
A
for a drug to enter the bile duct it must have a molecular weight greater than 500 or a molecular weight greater than 400 if it is a conjugate, from the bile duct it is then excreted in faeces.
7
Q
what does the dosing frequency depend on?
A
the dosing frequency depends on the half life that depends on the excretion and metabolism of the drug.
8
Q
what does polypharmacy require?
A
when patients are on at least 6 drugs we want to make the dosing regimen convenient to improve compliance.
9
Q
what is the major body metabolite?
A
B-D-glucuronide - a sugar acid
10
Q
how is the loss of small endogenous molecules through faeces prevented?
A
endogenous small molecules of biological synthesis are bound to b-d-glucuronide - at the end of the gut this gets stripped off by the enzyme b-d-glucuronidase so that endogenous molecules like vitamins and steroids can be reabsorbed rather than lost in the faeces.
11
Q
how does b-d-glucuronidase affect drugs?
A
if a drug is very lipophilic the b-d-glucuronidase may strip off the b-d-glucuronide (so that its no longer water soluble hence it wont be excreted) and the drug will be reabsorbed. this therefore extends the half life to reduce the dosing frequency, although this will also increase the side effects.
12
Q
what are the two phases of drug metabolism?
A
phase 1 involves introducing or revealing a functional group whilst phase 2 involves a conjugation reaction - forming bonds that will increase the water solubility or increase the molecular weight to above 400Da.
however not necessarily both phases will occur.
how the drug is metabolised effects how it is excreted e.g. if metabolism increases the molecular weight to above 500 it will be excreted in the faeces rather than urine.
13
Q
what is the purpose of phase 1 metabolism?
A
to add functional groups available for bond forming
14
Q
whats the purpose of phase 2 metabolism?
A
phase 2 metabolism involves conjugation of a drug either to functional groups already present or to functional groups created in phase 1.
15
Q
whats a common super family involved in drug metabolism?
A
cytochrome P450
16
Q
how can enzymes be amplified?
A
when a drug is taken for long periods of time it can amplify the gene so that more enzyme is produced in the body - this therefore speeds up the metabolism of the drug by these enzymes (as well as the metabolism of other drugs that are substrates for the same enzymes)
this therefore means future doses will have to be increased
17
Q
what are esterases?
A
a family of enzymes that cleave esters - they are non specific
18
Q
what do amidases do?
A
hydrolyse amides to release the amine and the acid - another phase 1 drug metabolism enzyme.
19
Q
peptidase cleave peptides - what three types of petidases are there?
A
- endopeptidases start at the middle of the peptide working outwards
- exo peptidase starts on the end of the peptide
- some exopeptidases start from the N terminus (amine end)
- other exopeptidases cleave from the c terminus (carboxylic acid end)
20
Q
why are peptides and proteins poor drugs?
A
the body easily metabolises protein/peptide based drugs using peptidases.
21
Q
acids are polar so they cant diffues into target cells across the cell membrane - what can be done to overcome this?
A
administer a prodrug este with no biological activity that can diffuse into the target cell. it can then be broken down by a non-specific esterase to reveal the biologically active carboxylic acid.
22
Q
what are some of the enzymes of phase 2 drug metabolism?
A
glutathione-s-transferase, UDP-glucuronic acid transferase, sulfotransferase, methyl transferase and acetyltransferase.
all of these enzymes increase the weight or polarity of the drug so it can be excreted
23
Q
how can we study the distribution of drugs and its metabolites?
A
through extracting biological fluids like urine and synovial fluid.
24
Q
what processes can be done in what order to identify compounds?
A
gas chromatography to identify the retention time, then mass spectrometry to identify the mass, then further mass spectrometry to study the fragments of the compound so we can identify the isomer. e.g. ortho meta and para would all have the same molecular weight so we can study the fragments obtained when the weaker bonds get broken to identify which isomer it is.
these tests can be used to identify if people have taken illegal drugs e.g. athletes taking steroids. the metabolites of steroidal body building drugs will be present in the hair for a year following consumption.
25
what requirements are there for gas chromatography?what can be used rather than gas chromatography?
compounds undergoing gas chromatography must have a high vapour pressure and low boiling points.
liquid chromatography - this is used for drugs with low viscosity.
26
a gradient solvent is used for baseline resolution, what is this?
gradient solvent ensures each peak is separate and reach the baseline so that we can identify individual peaks. it makes the first peak come out a bit quicker, the middle peak to remain and the thrird peak to be slightly delayed.
27
how can we see how a patients body is metbolising the drug?
as the peaks are proportionate we can see which metabolites are more prevalent etc. if they make more of a particular metabolite they may just be a faster metaboliser or they may have been induced.
28
why cant gunn rats be used for pharmacology studies?
gunn rats lack the glucuronic transferase enzyme hence drugs may have good pharmacology yet the model will not represent how humans metabolise drugs as it lacks glucuronidation pathways.
29
what makes cats good for testing analgesics? although what can be a downside to their use?
cats are sensitive to pain and scientists have a good understanding of their complex nerve systems. however as cats have no glucuronidation pathways the studies are good for pharmacodynamics but do not show the pharmacokinetice of human drugs.
30
what do preclinical studies do?
preclinical studies on animals study the pharmacodynamics and pharmacokinetics of drugs - finding the pharmacokinetics allows us to discover the half life so we can find a dosing regimen
31
what are glucuronides?
glucuronides are the major human metabolite of phase 2 reactions. they add on about 194Da.
32
why do we increase the drugs mw through conjugation?
cojugation reactions of phase 2 metabolism can increase the weight f drugs allowing them to therefore be removed by the biliary/faecal pathway (as this requires MW>500).
33
other than increasing mw what else can conjugation do?
conjugation can add on carboxylic acid groups making it water soluble so its renally filtered.
drugs also gain 4 oxygen molecules once again making it water soluble for renal excretion.
34
whats the purpose of glucuronidation?
glucuronidation is to remove nucleophiles like oxygen, sulphur nitrogen and C-.
o n and s with a lone pair will rarely be in the drug molecule but can get introduced by P450 or revealed by esterase or peptidase in phase 1 metabolism.
35
how does glucuronidation occur?
equatorial B-D-glucose gets converted into UDP-glucuronic acid . the nucleophile can then attack the glucurnoic acid and the OUDP is a good leaving group, froming B-D-glucuronide. the enzyme used here is UDP glucuronic acid transferase. this also occurs with phase inversion hence the B-D-glucuronide is now axial. by carrying out glucuronidation, the nucleophilic drug has now gained a glucuronic acid molecule hence the drug has picked up 3 secondary alcohols, one ether, and one carboxylic acid group, this therefore increases its MW and hydrophilicity hence allowing it to be excreted.
36
whats b-d-glucuronidase?
b-d-glucuronidase is an endogenous enzyme that strips off glucuronide.
it can cause some lipophilic drugs like steroids to be reabsorbed.
37
whats glucuronic acid transferase?
this is an enzyme which helps glucuronidation.
38
what is xanthine oxidase?
xanthine oxidase is a primary metabolic enzyme involved in the metabolism of dna/rna bases within cells. xanthine oxidase also metabolises methylxanthines like thepylline, theobromine and caffeine. metabolic rates show huge variation in humans making it difficult to decide a dosing regien as the half life will vary between individuals.
39
what are some common phase 1 reactions of drug metabolism?
oxidations, hydrolysis, hydration, dethioacetylation, isomerisation and sometimes reduction.
40
where are CYP enzymes found?
cytochrome p450 enzymes are found in the endoplasmic reticulum (microsomes) and are membrane bound
41
what do the reactions with cytochrome p450 require?
NADPH, O2 and NADPH-cytochrome p450 reductase. there are more selective oxygenases in the mitochondria for steroids.
the liver is a particularly important tissue for drug metabolism.
42
whats the most common type of oxidative reaction?
hydroxylation is the most common type of oxidation reaction
43
how are reactions stereoselective?
reactions with cytochrome p450 are usually stereoselective hence the same isomer is produced through metabolism.
44
how are reactions regiospecific?
usually a particular part of the molecule is oxidated, this depends on the enzyme and the drug structure. generelly gives one specific exclusive product
45
proteins and enzymes are built with chiral amino acids - what does this mean?
there is a diosteriomeric situation with enzymes as they will only accept a particular isomer. different stereoisomers of drugs are viewed as different drugs so they will undergo a different metabolic pathway. they will often be metabolised to different products with different kinetics.
46
what is the structure of the cyp enzymes?
the enzymes possess a prosthetic haem group which contains iron - the oxidative state of the iron depends on the part of the catalytic pathway
cyp enzymes are composed of a-helices, B-sheets and irregular coils (as well as the prosthetic haem group).
the haem cofactor is non covalently bound to the enzyme - it consists of an iron atom in a fixed structure with two variable ligands - these ligands determine the reactivity of the haem centre.
47
how do enzymes catalyse their reactions?
through negative catalysis - the enzyme generates a highly reactive chemical species and supresses the reactions that are not required.
48
whats the iron cycle of the haem group like?
the iron is originally fe3+ however when it binds to the drug it gains an electron to become Fe2+. the iron 3 is then restored once the metabolised drug leaves the haem group.
49
how does substrate binding affect the iron of cyp?
substrate binding to the protein changes the arrangement of electrons in iron so it goes from low spin (6 ligands) to high spin (5 ligands) - this means a water molecule is lost so that there is a vacant site for the oxygen to bind to.
50
what is required before dioxygen binds in the catalytic cycle?
reduction by p450 reductase must occur before the dioxygen binds
51
what occurs following dioxygen binding?
a second reduction step will occur after dioxygen binding but before hydroxylation.
52
what are some methods of hydroxylation?
depending on the substrate there may be more than one hydroxylation mechanism
- radical rebound mechanism - a substrate radical is produced during the mechanism
- concerted mechanism - bond breaking and making occurs together
the pathway for hydroxylation depends on the stability of the substrate radical.
53
what is an uncoupled reaction cycle?
dioxygen and nadph are consumed but the substrate is not oxidised. under most conditions uncoupled reactions are a small proportion of catalytic cycles. the product is reduced oxygen (hydrogen peroxide), this can easily fragment causing damage to cells. the source of h2o2 is unclear but results from an abortive catalytic cycle. p450 enzymes can also functionas organic peroxidases.
54
what is a superfamily?
the whole group of enzymes that catalyse the same or a similar type of reaction using a similar mechanism and are related by primary sequence homology or identity.
55
what is homology?
when amino acid sequences are aligned an amino acid in a praticular position is conserved by type.
56
what is identity?
when the amino acid sequences are aligned, an amino acid in a particular position will always be the same
57
what are the 3 main superfamilies of drug metabolism?
cyp enzymes, udp-glucuronosyl transferases and glutathione s transferases.
58
how are cyp enzymes inducible?
cyp enzymes can be induced by small molecule drugs hence they will have increased expression meaning future drugs metabolised by this enzyme will be metabolised faster.
59
how are enzymes classified?
based on amino acid primary sequence identity
60
why should we be careful when giving children drugs?
clinical trials are often carried out on adults and as metabolism will be different in children it is difficult to know how children will metabolise drugs.
61
whats aromatic hydroxylation?
the most common reaction of drugs and xenobiotics as most contain an aromatic ring. it involves addition of a hydroxy group on an aromatic ring - the position depends on the specific enzyme and drug.
62
whats aliphatic hydroxylation?
addition of a hydroxy group to an aliphatic molecule - the position and chirality of the hydroxyl depends on the soecific cyp enzyme and the drug.
hydroxylation is usually sterespecific when a new chiral centre is formed hence the oh will bind at a specific orientation to get an r or an s product.
63
whats aliphatic hydroxylation and dealkyltion?
involves removal of an alkyl group with the formation of a hydroxy group. the reaction can occur for o-me s-me and n-me.
an unstable intermediate (hemiacetal) is formed then the carbonyl compound is eliminated - this will be methanal in demethylations
64
what happens when codeine is taken?
codeine is a prodrug - it will be metabolised in the body by CYP2D6 in a dealkylation reaction to form morphine - the active opioid.
65
whats oxidative deamination?
the loss of an ammoniumion - the leaving group is an amine, with the addition of an oxygen atom
66
whats alcohol hydroxylation?
the addition of a hydroxy group to an alcohol followed by the elimination of water from a hydrated aldehyde.
67
whats epoxidation?
the addition of an oxygen to an aromatic compound to form a saturated three membered ring (nonaromatic)
68
what further metabolises epoxides?
epoxide hydrolase will further metabolise epoxides by forming two hydroxy groups by splitting the three membered ring - cyp enzymes can then further metabolise this to additional epoxides.
69
what happens to benzo 9a) pyrene in the body?
benzo (a) pyrene is found in cigarettes and gets metabolised in the body by cyp into an epoxide, then by epoxide hydrolase and then again by cyp to ultimately form a carcinogen responsible for cancer.
70
why are epoxides bad?
epoxides are electrophiles - epoxide metabolites are often highly reactive and cause toxicities. as they are alectrophiles they will easily react with nucleophiles like DNA hence causimg toxic damage.
71
whats heteroatom oxidation?
oxidation of an atom in the chemical molecule like a sulphur as in chlorpromazine.
72
why are nitro groups uncommon in drug molecules?
nitro groups typically have bad reactions and produce carcinogens in the body.
73
whats dehalogenation?
the removal of halogen atoms - this may be replaced with hydroxy groups or =O.
74
how does halothane produce a highly reactive product?
halothane undergoes dehalogenation, losing a h atom and a bromine atom and being replaced with a double bonded oxygen atom - however this molecule is highly reactive as both the oxygen and the hydrogen are strong electron withdrawing groups giving the carbon atom a partial positive charge allowing it to act as an electrophile.
75
how does age affect metabolism?
the liver in children will express different enzymes to adults leading to different metabolites.
76
what are polymorphisms?
polymorphisms are when there is a substitution in one or more of the amino acids withi the cyp enzyme that affects the reaction product or the reaction rate for at least one drug.
77
x-ray crystal structures provide atomic level detail of binding - what is it good for?
it can explain the functional effects of polymorphisms and allows us to design drugs that will avoid metabolism by certain cyp enzymes.
late stage drug developement usually reuires determination of which cyp enzymes metabolise the drugs.
78
what is an acute drug interaction?
acute administration of two drugs metabolised by the same enzymes usually leads to one drug being metabolised (perpetrator) while the other drug is not metabolised by the enzyme (victim) - the victim drug could rise to harmful levels which may have serious consequences.
79
how does ethanol interact with paracetamol?
ethanol is preferentially metabalised over paracetamol - this means consuming both together can lead to paracetamol poisoning.
80
how might ddis have therapeutic advantages?
ethylene glycol poisoning was once treated with a shot of alcohol - the alcohol would therefore be metaboilsed in place of ethylene glycol so the ethylene glycol would be excreted before it can produce harmful metabolites that would cause damage to the body.
81
how can extrem exposure to drugs affect metabolism?
extreme exposure to drugs/poisons can lead to a generalised loss of metabolism due to tissue damage. e.g. alcoholics get tissue damage meaning they get intoxicated quickly due to the decreased metabolism of ethanol in the liver - this damage will affect the metabolism of other drugs as well as alcohol.
82
what are some common phase 2 metabolism reactions?
glucuronidation, glutathione conjugation, amino acid conjugation, sulfation, acetylation and methylation - methylation is rare as it usually ends up making the drug more lipophilic which is bad for excretion.
83
what is a glucuronidation reaction?
glucuronidation is the addition of a glucuronic acid (an electrophile) to nucleophiles. - nucleophiles must have one lone pair of electrons or a neggative charge.
84
why are amides not nucleophilic?
the lone pair of electrons in an amide is resonance stabilised with the carbonyl group hence it does not make it nucleophilic.
85
how can we tell whether molecules are likely to be in the protonated form or not?
the pka of molecules is determined at physiological ph of 7.4, the molecules with a lower pka value will have a higher percentage in the unprotonated form.
for example the pka of carboxylic acid is 4.5 - this would therefore mainly be present as a carboxylate ion (in the unprotonated form) due to the low pka.
86
what enzyme catalyses the glucuronic acid conjugation?
UDP-glucuronosyl transferase
87
what happens in the glucuronidation reaction?
as udp is a good leaving group it will leave to form an oxycation intermediate that will then react with the nucleophilic drug
88
where is udp-GT found?
UDP-GT is found in the endoplasmic reticulum - the same as cytochromep450 hence phase 1 and 2 metabolism can occur in sequence.
89
what tissues is udp-gt found in?
udp-gt can be found in many tissues including the liver skin intestine kindney lung adrenals and spleen.
90
how are drugs excreted in the urine?
drugs with a molecular weight less than 200Da will be excreted in the urine.
91
why is the half life of some drugs e.g. paracetamol much longer in babies/children?
children have less expression/enzyme activity of the udp-gt enzymes hence they have longer elimination times hence the half life of paracetamol is 2 to 4 times greater than it is in adults.
92
how might elderly obtain higher bioavailability from drugs?
there is decreased first pass metabolism of drugs in the elderly like codeine hence this must be taken into account when deciding a dose.
93
what is enterohepatic recycling?
drugs with a molecular weight greater than 200 will be excreted in bile. the gut has commensal bacteria which produce the enzyme b-d-glucuronidase that cleaves off glucuronic acid moieties so that important biochemicals like steroid hormones can be salvaged. consequently it means some drugs which undergo glucuronidationmetabolism may have the glucuronic acid molecule stripped off in the gut hence it can then be reabsorbed which will extend the apparent half life. this means the therapeutice effects of the drug will be extended but it may also cause increased toxicity.
the drug must be reasonably lipophilic for absorption to occur.
94
other than drug metabolism what are the udp-gt enzymes important for?
the enzymes are also important for the metabolism of steroids derived from cholesterol.
95
udp-gt is derived from several coding regions/exons within the gene, they share exons 2 to 5 but have a variable exon 1. what could happen if polymorphisms develop?
polymorphisms can affect the rate of transcription and translation, the enzyme stability (rate of degradation) and the catalytic activity. polymorphisms can cause variable rates of drug metabolism between and within populations.
96
what happens when red blood cells degrade?
when red blood cells degrade they produce bilirubin (this has a yellow pigment) - this usually undergoes glucuronidation by ugt1a1 to produce the glucuronic acid conjugate which is then excreted in the urine.
97
what happens in gilberts syndrome?
gilberts syndrome is a common ugt deficiency - due to a variation in this enzyme there will be less excretion of bilirubin and higher levels of bilirubin will remain in the blood - this causes a jaundice colour
98
whats a common mutation for caucasions?
a common mutation for caucasions occurs in the promoter region of the gene affecting enzyme expression (enzymes are effective but have lower expression)
99
whats a common mutation in asians?
missense - an amino acid is replaced which reduces the activity of the enzyme
100
what are the consequences of gilberts syndrome?
bilirubin levels in the blood rise above 17uM - rises tend to be cyclical possibly induced by stress, dehydrate etc
jaundice may occur
as symptoms are mild its usually only detected when carrying out lfts for a different reason
gilberts syndrome can affect the metabolism of other drugs like atazanavir, indinavir, irinotecan and paracetamol
101
reduced metabolism can occur from depletion of udp-glucuronic acid by a drug. how is this the case for paracetamol?
in children a paracetamol overdose uses up the glucuronic acid hence there will be an accumulation of bilirubin hence what causes the paracetamol toxicity.
102
you can sometimes get a somplex with chiral drugs and drugs with more than one glucuronidation site. different enantiomers can react with different ugts due to the chirality of the enzymes (a diasteriomeric situation is produced). give an example of this
R-propanolol is metabolised by UGT1B7 and UGT1A1 whilst S-propanolol is metabolised by UGT1B7 and UGT1A9 - UGT1B7 is therefore non specific with respect to chiral configuration whilst UGT1A1 and UGT1A9 are specific for a particular confirguration.
103
what happens when some drugs have multiple nucleophilic groups?
glucuronidation can occur on multiple groups hence multiple products can be formed - the group which gets glucuronidated depends on the nucleophilicity of the groups and the availability and activity of the enzyme.
104
how can morphine be metabolised?
morphine can be metabolised by UGT2B7 into an 02-glucuronide which is inactive, howevere it will also produce the minor product 06-gluvuronide that is a u-opioid receptor active metabolite.
105
what is glutathione important for?
glutathione is improtant for maintaining the reducing environment hence protecting against radicals and reactive oxygen species.
106
how do the forms of glutathione transfer between each other?
oxidation occurs to form the disulfide species whilst reduction occurs to form the sulfhydral, this is mediated by a NADPH-dependent glutathione reductase.
107
what does glutathione react with?
glutathione reacts with electrophiles (whereas glucuronic acid reacts with nucleophiles)
108
glutathione-s-transferases exists as 3 distinct super families in humans, what are these?
microsomal, cytosolic and mitochondrial. the cytosolic and microsomal enzymes are important for drug metabolism.
- cytosolic enzymes are dimers
- microsomoal enzymes are trimers and mostly metabolise arachidonic acid
109
glutathione is used to detoxify electrophiles - what increases the reactivity of the glutathione?
the sh group of the glutathione is usually deprotonated by enzyme to increase the reactivity.
110
what is glutathione important for?
glutathione is important for metabolism of derivatives of polycyclic aromatic compound e.g. epoxides.
111
how does 1,4 conjugate addition of glutathione occur?
micheal addition occurs with electron deficient double bonds - it requires an electron withdrawing group - usually carbonyl. the reaction proceeds via an enolate intermediate.
112
how is paracetamol metabolised?
paracetamol is metabolised nto a reactive intermediate by CYP which then undergoes dehydration this molecule will then undergo a 1,4 conjugate addition reaction with glutathione, however in acute toxicity the glutathione pools may be depleted.
113
oxidation with cyp can form a quinone intermediate, as with morphine. what then happens to this quinone intermediate?
the quinone intermediate will react with glutathione in a 1,6-conjugate addition reaction, restoring the aromaticity by shifting double bonds.
114
what happens to the metabolites bound to glutathione?
the metabolites with glutathione bound will undergo reactions in the liver and kidneys to produce a negatively charged meracpturate. the pathway involves sequential removal of glutamyk and glycl residues and the addition of an acetyl group
115
whats the first step of the mercapturate pathway?
gamma-glutranspeptidase strips off glu leaving the electrophile bound to the cys-gly dipeptide. this occurs in the liver but the product is then sent to the kidneys as its now water soluble.
116
whats the second step of the mercapturate pathway?
in the kidneys cys-glydipeptidase strips off the glycine which can be reused in the body leaving a cysteine conjugate.
117
whats the third step of the mercapturate pathway?
the cysteine conjugate is not very soluble so n-acetyltransferasee will produce mercapturate (n-acetyl-l-cysteine conjugate) by addition of an ectyl group from acetyl-coa to the n position. this is now water soluble so can be renally excreted.
118
what happens when glutathione displaces chloride in a sn2 reaction?
glutathione displaces chloride in a sn2 reaction, there is then removal of the glutamyl and glycyl residues, then pyridoxal phosphate lyase activity via an amine intermediate. further metabolism of the reactive products may lead to toxicities
119
how do enzymes exist?
enzymes exist as dimers - two subunits associated non-covalently.
they can be either homo or hetero with monomer subunits of 20-25kDa
120
there are 20 isoforms of GST in hukmans - how do sub families arise?
from different combinations of monomers - they are alpha, mu, pi, sigma etc
GST is expressed in all organs that have been studied
121
the expression of gst is tissue specific and there is wide variability in basal leels between individuals. whats one way in which induction of cytosolic and microsomal enzymes may occur?
induction of cytosolic and microsomal enzymes can occur through physiological (oxidative) stresss and xenobiotics/drugs.
122
how do many drugs induce gst enzymes?
many drugs induce gst by stimulating receptors responsible for regulating fatty acid metabolism e.g. peroxisomal proliferation activating receptor-gamma
this leads to higher rates of metabolism due to higher enzyme levels and activity.
123
what is over expression of gst enzymes associated with?
treatment resistance in several cancers
124
many polymorphisms in gst proteins have been identified - where can this occur?
polymorphisms can be in the protein sequence (reducing protein levels) or in the promoter region.
125
what is decreased expression of GSTM1 due to polymorphisms associated with?
a decreased risk of of prostate cancer.
126
cancer chemotherapy often uses cytotoxic drugs as therapeutic agents, how do these work?
cytotoxic drugs are often highly electrophilic - they react with DNA as well as other nucleophiles.
127
how are cytotoxic drugs metabolised?
cytotoxic drugs are usually electrophilic hence theyre metabolised by gst enzymes - polymorphisms in GSTA1 and GSTP1 predict survival following chemotherapy, likely due to different metabolic rates of the drug.
128
whats glucuronidation?
glucuronidation is a phase 2 metabolism that conjugates drugs to make them water soluble for renal excretion and to increase their molecular weight above 400 for biliary faecal excretion.
glucuronidation removes nucleophiles
129
how is the glucuronide for glucuronidation formed?
glucose goes to glucuronic acid which goes to udp glucuronic acid - udp is a good leaving group.
130
whats glutathione?
glutathione is a molecule of Glu-Cys-Gly thats found in the lungs liver kidney blood and gi tract.
glutathione is a powerful nucleophile (due to the SH with a lone pair on the sulphur) - it is involved in phase 2 metabolism of electrophiles. electrophiles are very toxic - theyre intrduced in phase 1 metabolism by P450 oxidation.
131
why do we want drugs to have an inhibition constant, ki of 1nM?
we want the drug to activate receptors but we also want it to dissociate from the receptor - if Ki was less than 1 it would have a high affinity for the receptor and may not dissociate easily. it will bind more strongly/for longer duration.
132
lysine is an amino acid that is nucleophilic due to the NH2, what does this mean when electrophiles are present?
electrophiles would react with lysine hence causing cytotoxicity. if there is an abundance of electrophiles cell death will occur quivker than cell production causing fatalities.
electrophiles are toxic as they will react with lysine causin damage to proteins/enzymes and consequently leading to cell death.
133
how can electrophiles damage our dna?
we have lots of guanine in our dna and this is a nucleophile hence it will react with electrophiles if they enter our cells causing mutagenecity hence dna can no longer replicate leading to cancer.
this is why glutathione is important as we must remove electrophiles from our body.
134
what are some common electrophiles produced by phase 1 metabolism?
enone, epoxide and cyanate or thiocyanate
135
glutathione is endogenous - how is it long lasting?
glutathione is not a substarte for non specific peptidase - due to the gamma Glu its not easily hydrolysed.
136
whats the mercapturate pathway?
glutathione conjugation
137
glutathione binds to electrophiles in the protals of entry and then sent to the liver, what happens next?
gamma-glutranspeptidase strips off the cys-gly dipeptide with the electrophile attached, this then goes to the kidneys as its water soluble. cys-glydipeptidase will then strip off the glycine so it can be reused leaving the cystein conjugate. the cysteine conjugate is not very water soluble so it reacts with acetyl-s-coa using nacetyltransferase produces mercapturate (nacetyl-l-cysteine conjugate), this is water soluble so it can be renally excreted.
138
whats sulfation?
a minor metabolic pathway where alcohols react with the biological cofactor PAPS (sulphuric acid) and is catalysed by sulfotransferase. basically the oh of the alcohol reacts hence the h is replaced with so3 hence it becomes oso3-. this makes it available for renal excretion.
139
peptidases cleave amino acids and are good for digestion, what are the types of peptidase?
endopeptidase cleaves proteins from the centre and exo peptidase which cleaves proteins from the end - this can be either from the c terminal or the n terminal - if the drug contains an amino acid the body will recognise it as a protein for digestion
140
how is glutathione-s-transferases an endogenous enzyne?
it adds glutathione to small molecules like leukotriene allowing it to be transported around the body
141
what does it mean that enzymes are amplifiable?
the nore a patient has a drug, the more of the metabolising enzyme will be produced by the patient, amplification occurs at the gene level hence more dna/rna is produced hence more proteins/enzymes are produced.
142
what does it mean that enzymes are inducible?
with repeated exposure to a ligand the enzyme will develop a new shape to increase the binding affinity for that substrate. these mutations lead to super families as you get sub families developing from endogenous enzymes which mutate following exposure to substates.
143
how is alcohol metabolised?
alcohol dehydrogenase is a non speceific or selective enzyme that metabolises all primary alcohols in the same way - for ethanol its first metabolised to an acetaldehyde intermediate which then gets metabolised by acetaldehyde dehydrogenase into acetic acid.
144
what causes alcohol toxicity?
alcohol and acetic acid are not particularly harmful but the acetaldehyde intermediate is very toxic, it must therefore be rapidly metabolised.
145
whats the disulfiram reaction?
disulfiram gets cleaved at the disulfide bond - it is reduced by 2 glutathione molecules. this molecule produced is a powerful blocker of acetaldehyde dehydrogenase - if it is taken with alcohol the harmful metabolite acetaldehyde will build up causing sickness and headaches. disulfiram can therefore be used in alcoholics. if patients are on antabiuse for alcohol addiction you must be careful about giving them other medicines containing alcohol. e.g. tincture is an alcohol extract applied topically but alcoghol absorbed accross the skin would cause a disulfiram reaction.
