Metabolism Flashcards
A drug that has high Cl is eliminated ____
faster
Metabolism is considered what types of pathways?
elimination
deactivation
detoxification
Drug metabolism can result in…
- form metabolite more hydrophilic
- deactivate active into inactive
- detoxify toxic xenobiotics
- activate inactive to active
- activate inert into more reactive and toxic
Phase I metabolism includes what types of reactions?
oxidation
reduction
hydrolysis
Oxidation enzymes
CYPS, P450s, MAO, ADH, ALDH
Reduction enzymes
CYPs
CYP450s are responsible for metabolism of ___% of drugs
75%
Where are CYPs?
mostly microsomal in ER
How do CYPs work?
transfer O atom by aid of NADPH CYP450 reductase (POR)
transfers 2 electrons from NADPH
Which CYP is involved in metabolism of 50% of all drugs?
CYP3A4/5
Which CYP metabolism 90% of CYP substrates?
1A2 2D6 2C9 2C19 2B6 3A4/5
Which CYP metabolism of 70% of all drugs?
1A2 2D6 2C9 2C19 2B6 3A4/5
Induction and inhibition of CYPs is a major source of what?
DDI and drug food interactions
CYP2D6, 2B6, 2C9 and 2c19 expression is significantly influenced by what?
polymorphisms to give rise to poor and extensive metabolizers
CYP2D6 polymorphisms
need to screen before you take the drug
- need to know how you metabolize for dose
Metabolizing enzymes are located where?
- membrane
- microsomal via ER
- mitochondria
- nuclear
- cytosolic
What are the 4 outcomes from metabolism of a drug?
- active –> inactive
- non-toxic –> toxic
- inactive prodrug –> active drug
- active –> active metabolite
What are the enzymatic processes of drug metabolism?
Phase I: structural modification
Phase II: conjugation
What enzymes are involved with hydrolysis?
esterases
amidases
epoxide hydrolases
CYPs must contain
heme containing proteins
P450 reductases
NADPH
What factors influence variability between CYP isoforms?
- polymorphisms
- gender
- age
- DDI
- pathological conditions (liver disease)
Drugs primarily eliminated intact in urine are ____ suspected to inter and intra individual variation or DDI
less
Conjugation with polar moieties create what?
glucuroinic acid or sulfate groups
Conjugation with what decreases polarity?
alkyl or acetyl groups
Conjugation and kinetics
some have limited capacity (Vmax), cause non linear kinetics
- saturation
UGTs
transfer of glucuronic acid from UDPGA
UGT characteristics
- low affinity
- high capacity enzymes
- polar and ionizable metabolites (increase MW)
- facilitates renal and biliary excretion
SULTs
transfer of sulfate group from PAPs
SULTs characteristics
- high affinity
- low capacity enzymes
- inactivation of steroids and bile acids
NATs
transfer of acetyl group from acetyl CoA
NATs characteristics
- metabolite less polar than parent
- polymorphism
GSTs
- transfer GSH to reactive electrophilic species (ROS)
- eventually transformed into mercapturic acid derivatives
What is the major source of bacterial metabolism?
gut microflora
What are examples of spontaneous biotransformation?
hydrolysis of labile bonds
What reactions are catalyzed by gastric acid?
hydrolysis of esters
Significance of intestinal metabolism
- orally administered drugs have higher concentrations in enterocytes
- long resident times
- in liver, there are alternative pathways if CYP3A4 is inhibited; in intestine no alternatives!
T/F Liver metabolism is more extensive than intestinal metabolism of drugs
False; as extensive
Vmax
- max reaction speed at which all enzyme molecules are being used to catalyze the reaction
- defines capacity of the reaction, related to enzyme expression levels (E) and its kcat
Kcat
catalytic constant = turnover number
number of substrate molecules that one molecule of enzyme can convert to product per time
Km
- substrate concentration at which reaction rate (v) is half that of Vmax
- defines affinity between substrate and enzyme
Decreasing Km = _____ affinity
more
Higher Vmax = _____ capacity
higher
When [S} «_space;Km…
Cl is concentration independent
1st order
linear
At higher [S]…
Cl is concentration dependent
0 order
nonlinear
PK approaches saturation
Homotropic cooperativity
- biphasic kinetics
- sigmoidal
- substrate inhibition
Heterotrophic acitvation/cooperativity
effector, a different chemical entity than substrate, increase reaction velocity of substrate without affecting enzyme expression
Effector molecules
same or different chemical entity bind simultanesouly with substrate molecule to enzyme at distinct sites and affect rate of substrate binding
IC50
concentration of inhibitor to reduce enzyme activity toward substrate by 50%
What does IC50 depend on?
substrate concentration
What is Ki independent of?
substrate concentration
Ki or IC50: which can you compare inhibitors regardless incubation conditions?
Ki
Competitive inhibition
Vmax = no change Km = increased Vmax/km = decreased
**extent of inhibition decreased or completely reversed by increasing substrate concentration
Noncompetitive inhibition
Km = no change Vmax = decreased Vmax/km = decreased
** cannot be reversed by increasing substrate concentration
Uncompetitive inhibition
Km = decreased Vmax = decreased Vmax/km = no change
Mixed inhibition
Km = increase or decrease Vmax = decreases
ratio of the 2 increases or decreases
Mechanism based inhibition
- AKA suicide inhibition
- not instantaneous; time dependent
- quasi irreversible non covalent coordination with heme group
- catalytically formed reactive intermediates cause permanent/irreversible inactivation of enzyme
Mechanism based inhibition effects
- reaches max effect slowly
- lasts for a long period of time until a new functional enzyme synthesized
Structural alerts may be considered as red flags for what type of inhibition?
mechanism based
What does enzyme induction typically affect?
enzyme concentration rather than Kcat (activation/cooperativity)
What are some examples of enzyme inducers?
barbiturates, carbamazepine, phenytoin
rifampicin
St John’s wort
phenobarbial
Autoinduction
carbamazepine induces its own metabolism with time and/or higher doses
Enzyme induction effects
slower onset and longer effect
How does enzyme induction work?
- activates nuclear receptor
- translocates to nucleus
- binds to response element in promotor region of target gene
- induces its expression
T/F only the protein-unbound fraction can be extracted by heptaocytes
True
Intrinsic hepatic clearance
- measure pure metabolic capability inherent to hepatic enzymes activity independent of other extra hepatic factors
- sum of all enzymes involved
Well stirred model
assumes liver is a single homogenous compartment
- concentration of unbound drug in effluent is in equilibrium with unbound drug in liver fluid
What are the factors that affect hepatic clearance?
- hepatic blood flow (Q)
- protein binding (fu)
- intrinsic clearance (Clint)
High E
> 0.7
- sensitive to changes in Q
- flow or perfusion limited drugs
- insensitive to changes in fu or Clint
Low E
<0.3
- sensitive to changes in fu and Clint
- capacity limited drugs
- insensitive to Q
Within low E drugs, are highly protein bound drugs or low protein bound drugs more affected by changes in fu?
highly protein bound drugs (>90%)
Well stirred model and AUC
- Not affected by Q in low or high ER drugs in oral drugs!
High E and AUC IV
- AUC increase with decreasing Q
- no change in Clint
Low E and AUC IV
- AUC not affected
- AUC decrease with increasing Clint
Which type of drugs are most vulnerable to metabolism based DDI?
highly permeable compounds (perfusion limited)
Which type of drugs are most vulnerable to transporter based DDI?
Poorly permeable compounds but good substrates for uptake transporters
Which type of drugs are not vulnerable to DDI?
poorly permeable compounds and are not substrates for uptake transporters (diffusion limited)
In poorly permeable compounds but good substrates for uptake transporters, hepatic clearance is primarily determined by what?
rate of uptake rather than metabolism
