Drug metabolism Flashcards
what is absoprtion?
absorption is about crossing biological barriers
what is adsorption?
adsoprtion is the interaction of the drug with a surface
what is the d in adme?
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
what does the m in adme refr to?
metabolism however not all drugs are metabolised - some are excreted unchanged
how would a drug be renally excreted?
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
what requirements are there for a drug to be excreted via the bile duct?
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.
what does the dosing frequency depend on?
the dosing frequency depends on the half life that depends on the excretion and metabolism of the drug.
what does polypharmacy require?
when patients are on at least 6 drugs we want to make the dosing regimen convenient to improve compliance.
what is the major body metabolite?
B-D-glucuronide - a sugar acid
how is the loss of small endogenous molecules through faeces prevented?
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.
how does b-d-glucuronidase affect drugs?
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.
what are the two phases of drug metabolism?
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.
what is the purpose of phase 1 metabolism?
to add functional groups available for bond forming
whats the purpose of phase 2 metabolism?
phase 2 metabolism involves conjugation of a drug either to functional groups already present or to functional groups created in phase 1.
whats a common super family involved in drug metabolism?
cytochrome P450
how can enzymes be amplified?
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
what are esterases?
a family of enzymes that cleave esters - they are non specific
what do amidases do?
hydrolyse amides to release the amine and the acid - another phase 1 drug metabolism enzyme.
peptidase cleave peptides - what three types of petidases are there?
- 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)
why are peptides and proteins poor drugs?
the body easily metabolises protein/peptide based drugs using peptidases.
acids are polar so they cant diffues into target cells across the cell membrane - what can be done to overcome this?
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.
what are some of the enzymes of phase 2 drug metabolism?
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
how can we study the distribution of drugs and its metabolites?
through extracting biological fluids like urine and synovial fluid.
what processes can be done in what order to identify compounds?
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.
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.
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.
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.
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.
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.
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
what are glucuronides?
glucuronides are the major human metabolite of phase 2 reactions. they add on about 194Da.
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).
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.
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.
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.
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.
whats glucuronic acid transferase?
this is an enzyme which helps glucuronidation.
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.
what are some common phase 1 reactions of drug metabolism?
oxidations, hydrolysis, hydration, dethioacetylation, isomerisation and sometimes reduction.
where are CYP enzymes found?
cytochrome p450 enzymes are found in the endoplasmic reticulum (microsomes) and are membrane bound
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.
whats the most common type of oxidative reaction?
hydroxylation is the most common type of oxidation reaction
how are reactions stereoselective?
reactions with cytochrome p450 are usually stereoselective hence the same isomer is produced through metabolism.
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
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.
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.
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.
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.
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.
what is required before dioxygen binds in the catalytic cycle?
reduction by p450 reductase must occur before the dioxygen binds
what occurs following dioxygen binding?
a second reduction step will occur after dioxygen binding but before hydroxylation.
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.
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.
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.
what is homology?
when amino acid sequences are aligned an amino acid in a praticular position is conserved by type.
what is identity?
when the amino acid sequences are aligned, an amino acid in a particular position will always be the same
what are the 3 main superfamilies of drug metabolism?
cyp enzymes, udp-glucuronosyl transferases and glutathione s transferases.
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.
