Exam 3: Hepatic Clearance Flashcards
The greater the blood flow->
the greater the distribution
If the blood flow is the same
the greater the partitioning into the tissues
- the slower the distribution
If the partitioning is the same,
the larger the organ and the slow the distribution
The net movement of fluid is
into the tissues at the arterial end of a capillary and returned to the capillary at the venous end
What can vary with disease state
Blood flow and permeabiilty of capillary membranes
The primary metabolic organ is
the liver
- 1storgan encountered
•Relatively large organ
•High concentration of metabolic enzymes•High rate of blood flow (~1.5 L/min., ~90 L/hr
The liver
is an adaptive organ
- Can accommodate higher concentrations of drugs
Inactivation
Drug → inactive metabolite
Activation
Pro-drug → drug
• codeine, inactive → morphine, active
• Drug → toxic metabolite
• Meperidine → normeperidine
Quid quo pro
Drug → metabolite with similar activity
- Allegra ( fexofenadine)
Phase 1 Metabolism
- Oxidation
- Reduction
- Hydrolysis
Phase II Metabolism
• Conjugation
- something is added to the molecule
• Glucuronide
• Sulfate
Metabolism generally makes molecules
hydrophilic molecules that are more readily excreted
Which renal process would be most affected
by how hydrophilic a molecule is?
Reabsorption
Phase I Enzymes
• Cytochome P450 enzymes
(CYP450s)
• Flavin-containing monooxygenases
(FMOs)
Examples of Phase I Metabolism
• Oxidation
- N-dealkylation
• Hydrolysis
- Aromatic hydroxylation
• Reduction
- Nitro to amine
Examples of Phase II Metabolism
- Acetylation
- Glucuronidation
- Glutathionylation
- Methylation
- Sulfation
Metabolism Generalities
- Phase I can occur without Phase II and vice versa
* Phase I can occur after Phase II and vice versa
Portal triad
- Hepatic artery (Inlet: ~20% flow)
- Portal vein (Inlet: ~80% flow)
- Common bile duct (Outlet)
Primary function of Hepatic
Serves as a
filter between blood from GI
tract and systemic circulation
• Primary cell type of lIver
Hepatocytes
• Local blood source: sinusoids
Influx
Sink effect for diffusion into hepatocyte
More metabolism
loss of drug (typically)
Efflux
Pump drug out
Other characteristics of Efflux
Return drug to circulation for potential therapeutic action
• Send drug to bile for potential removal from body
• Send drug to bile for potential reabsorption from intestine
Enterohepatic Cycling
• Circulation between liver and intestine via gall bladder
•>90% of bile acids are reabsorbed in the gut and taken back into hepatocytes
-Often glucuronide metabolites
A drug enters the liver via the portal vein and then is returned to the small intestine in the bile via the gall bladder
- or its reabsorbed into portal circulation
- Distributed to systemic circulation via the central vein
Cytochrome P450s
- aka CYP450, CYPs
- Major enzyme superfamily
- Use oxygen and NADPH to carry out reactions
Flavin-containing monooxygenases
- Aka FMOs
- Major enzyme superfamily
- Comparatively minor contributors to metabolism
Cytochrome P450s are responsible for:
- metabolism of dietary and xenobiotics
- Synthesis of steroids and signaling molecules
- Production of bile acids
Cytochrome P450s are primarily found
in the smooth endoplasmic reticulum of hepatocytes
- But, some also in the GI tract, kidney, lungs and CNS (albeit much less)
CYP Nomenclature
More than 50 different CYPs in people
- Grouped based on amino acid sequence similarity
Genetic family:
CYP1
• CYP2
• CYP3
Genetic sub-family:
- CYP2A
- CYP2B
- CYP2C
Gene number:
- CYP2C8
- CYP2C9
- CYP2C19
CYP Specificity
- Some CYPs are very specific
- CYPs have relatively broad specificity
- One CYP can act on many drugs
- And a drug may be metabolized by more than one CYP
Activity and Distribution of Phase I Enzymes
The enzymes present in the largest quantities don’t necessarily equate to those contributing the
greatest activity
CYP 3A4 is responsible for
> 50% of the metabolism of therapeutic drugs
Phase II Enzymes are mainly
‘transferases’
- Variable location (cytosolic, mitochondrial, membrane-bound)
Transferases
- Transfer a functional group/molecule to another
• Conjugate an endogenous molecule onto xenobiotic
Activity and Distribution of Phase II Enzymes
Phase II enzymes normally terminate biological
activity
- One exception to this is morphine which when
glucuronidated becomes more active!
Glucuronidation and sulfation cause log P to
decrease
UGTs: Glucuronyltransferases
40%-70% of therapeutic drugs become glucuronidated
Metabolism usually reduces
biological activity
• Sometimes it can be used to increase biological activity
beneficially
• But sometimes it can form toxic entities
Enzymes exhibit
Michaelis-Menten kinetics
Under sub-saturation conditions, the rate of
conversion is
proportional to [drug]
Once saturated with substrate
an enzyme has a maximum rate (Vmax)
The [substrate] at which v=½Vmax
is the Km
Michaelis-Menten constant
For first order elimination, as the plasma
concentration of drug increases
the rate of elimination increases
For zero order elimination, the enzymes are
working as fast as they can
so the rate of elimination is maximal and at a plateau!
If the plasma drug concentration was high
enough,
all drugs would reach the zero-order range
Most drugs have a therapeutic range
within
the linear portion of the Michaelis-Menten plot
The rate of conversion (v) increases with
[Drug]
V =
Vmax * [C] / Km + [C]
If we have an overdose, we run the risk of saturating the capacity of enzymes to
metabolize the drug and it can build up
Since a drug may be metabolized by more than
one pathway, other pathways may take over
- Some of those pathways may yield toxic
metabolites!
- If we overwhelm ‘good’ pathways, ‘bad’
pathways may prevail!
Extraction Ratio (E)
Efficacy of an organ to remove a drug from the bloodstream
- Includes both metabolism and excretion
Extraction Ratio (E) is expressed as
a fraction from 0-1
O being no removal of drug
1 being complete removal of drug
Eh=
([C]in-[C]out)/[C]in
Clearance by an organ is a function of the
extraction ratio (E) and blood flow (Q) to a given organ
Clearance can be
- flow-limited
- capacity limited
• The organ simply isn’t very efficient at removing the drug
More blood flow means
more clearance
Clearance is also dependent
on the degree of protein-binding
• Only free drug is available to be metabolized or excreted
• The levels of proteins in the blood vary with disease state
Flow-limited:
if the fraction of drug unbound and metabolic activity of the liver is highE≈1), then clearance is limited by blood flow
Capacity limited:
if the blood flow is high, but the fraction of drug unbound and metabolic
activity of the liver is low (E≈0), then clearance is limited by the capacity of the liver to
remove the drug
What determines the Extraction Ratio?
- Blood flow to the organ (Q)
- How much drug is in the unbound form (fu)
- ## “Intrinsic clearance” (Clint)
“Intrinsic clearance” (Clint)
is the enzyme-mediated clearance that would result when in the absence of physiological limitations
When to use Therapeutic Drug Monitoring
Wide interpatient variability • Varied absorption • Varied distribution • Varied metabolism • Disease states • Drug-drug/drug-nutrient interactions
Polymorphisms are
Genetic mutations that cause increased/decreased/loss of activity/presence
- Changes in copy number
• Changes in rate of substrate conversion (Vmax)
• Changes in substrate affinity (Km)
Polymorphisms range from
no activity to ultrarapid conversion! • Poor metabolizers (PM) • Intermediate metabolizers (IM) • Extensive metabolizers (EM) • Ultrametabolizers (UM
CYP2D6
74 known alleles
• Range from no activity to ultrarapid conversion!
• 7-10% of Caucasians are poor metabolizers
• Perhexiline withdrawn from market in 1988 due to toxic effects on PMs
If an individual is a poor metabolizer (PM), would you expect clearance to be higher or lower than average?
A. Higher
B. Lower
B. Lower
drugs can interact in a variety of ways
- Compete for transporters
- Compete as a substrate for metabolic enzymes
- Inhibit metabolic enzymes
- Induce enzyme expression
If clearance reduced, drug concentration may
reach toxic levels
If clearance increased,
drug concentration may not reach therapeutic levels
Enzyme Induction
Increased number of drug-metabolizing enzymes in response to a drug or environmental constituent
- Enzyme synthesis initiated within 24 hours
of exposure
• Increases over 3-5 days
• Decreases over 1-3 weeks after inducing
agent is discontinued
Increased nuclear receptor (NR)-mediated gene transcription
Receptors bind to steroid and thyroid hormones in the cytoplasm and then migrate to the nucleus. Signaling
is very complex and leads to both activation and repression activities.
• Response time: Slow, 30 min. to hours
If we have more an enzyme that acts on a drug, how is k likely affected?
K will increase
If we have more an enzyme that acts on a drug, how is t1/2 likely affected?
t 1/2 will decrease
If we have more an enzyme that acts on a drug, how is E likely affected?
E will increase
- if enzyme goes up we have more ability to extract the drug from circulation
- If E goes up, Cl goes up
If we have more an enzyme that acts on a drug, how is Cl likely affected?
Cl increases
If we have more an enzyme that acts on a drug, how is F likely affected?
F will decrease
What is the relationship between K and t1/2
Inverse relationship
as K goes up t 1/2 goes down and vice versa.
Enzyme Induction
CYP3A, CYP2A-CYP2E are inducible
CYP3A4 inducing agents include
- Phenytoin
- Glucocorticoids
- St. John’s Wort
CYP2D6 inducing agents are
- Dexamethasone
* Rifampin
Enzymes aren’t the only inducible biomolecules
P-glycoprotein is
P-glycoprotein is inducible
what is P-glycoprotein
p-GP is an efflux transporter that removes drugs from cells
• Carbamazepine
P450s
found on CYP3A4 in intestinal wall
• Induced by St. John’s Wort, phenytoin, glucocorticoids
• Inhibited by grapefruit juice, omeprazole, ketoconazole
p-GP
This is on the Apical membrane of our enterocytes
- a efflux transporter
• Induced by St. John’s Wort, phenytoin, rifampin
• Inhibited by erythromycin, ketoconazole
What happens to a drug that is a CYP3A4 substrate if CYP 3A4 is induced?
Plasma Drug conc. will be lower
If we have a substrate and we add more, then we will see more breakdown of that drug
What happens to a drug that is a CYP3A4 substrate if CYP 3A4 is inhibited?
The Plasma Drug Conc. will increase
What happens to a drug that is a p-GP substrate if p-GP is induced?
The Plasma conc. is going to go down
What happens to a drug that is a p-GP substrate if p-GP is inhibited?
The plasma conc. is going to go up
Why cant you take this with grapefruit juice
CYP3A4 can be inhibited by grapefruit juice
CYP3A4 is an
enzyme that metabolizes and breaks the drug down
The apical membrane faces what?
the lumen
If someone is taking St. Johns Wart, they are
increasing their metabolic ability for the drug
- also increasing ability of the drug to get out of the cells
- ultimately their plasma drug conc. is going to be double
- conc. will be much lower bc not only is it being more metabolically acted on but its also having the ability to be shunted out much easier.
Some drugs induce their own metabolism
Carbamazepine is a drug that is metabolized primarily by 3A4
• It also is an inducer of CYP 3A4, 1A2, and 2C9
• Thus, it induces its own metabolism!
Some drugs induce their own efflux
Carbamazepine is also an inducer of p-GP
• Thus, it induces its own removal!
If we know that a drug induces its own metabolism, how will the following PK parameters be affected:
- F will go down
- t1/2 will go down
- Cl will will go up
- Ke will go up
Wide substrate selectivity + many inhibitors
significant potential for
drug interactions
