introduction to pharmacology and drug metabolism
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Introduction to Pharmacologyand Drug Metabolism
Luke Lightning, PhD
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Outline of topics to be discussed:
Introduction Quantitative aspects of drug-receptor interactions Fundamental mechanisms of drug action Drug dose and clinical response Factors modifying effects of drugs ADME
Text: B.G. Katzung, Basic & Clinical Pharmacology, chapters 1 & 2
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Introduction
Pharmacology: study of interactions between chemical compounds and biological systems.
i.e. - how drugs work- where drugs act- how the body processes drugs, etc. (mechanisms of drug action)
The receptor is the cornerstone of pharmacology
Explains how the organism interacts with a drug and initiates a chain of biochemical events that results in observed effects
An agonist is a drug whose interaction with the receptor stimulates a biological response
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Purpose of Drug Therapy
To produce the characteristic effect(s) of the drug being used. The drug must achieve adequate concentrations at its site(s) of action.
To achieve the maximal positive effect of the drug while minimizing undesired effects.
No drug will have only one effect (i.e. adverse effects)!
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Magnitude of Response Following Drug Therapy
Dependent on various factors:– amount of drug administered (dose)– concentration at site of action
» dependent on rate of absorption and blood flow to the site
– amount of time the drug remains at the site of action» dependent on biotransformation (metabolism) and elimination
Appropriate dose of a drug:– amount of drug needed at a given time that results in the appropriate
concentration at the site of action (where biological effect occurs)
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Effect of Drugs on Organs and Tissues
Drugs only modify cellular function – do not create effects
– Pharmacodynamics: Drug Biological Effects– drugs alter the normal biochemical functions of an organ, tissue, or cell
e.g. laxatives increase the activity of the GI tract (i.e. stimulation)
general anesthetics decrease activity of cells in the CNS (i.e. depression)
DRUG RECEPTOR RESPONSE
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Drugs, Dose, Receptor, and ResponseDrugs Dose Target (Receptor/Enzyme) Response
Lipitor 10-80 mg HMG-CoA Reductase Decreases LDL
Singulair 10 mg Leukotriene Receptors Prevents Bronchochonstriction
Lexapro 5-20 mg Serotonin Receptors Relieves Anxiety
Nexium 20-40 mg Proton Pump Decreases Gastric Secretion
Plavix 75 mg Purinergic Receptors Anticoagulation
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Drug-Receptor Interactions
Receptors largely determine the quantitative relationship between dose or concentration of drug and their pharmacological effects.
Receptors are responsible for selectivity of drug action
– binding to the receptor is dependent on the 3-D characteristics of the drug– size, shape (e.g. stereochemistry), and electrical charge of a drug molecule
– changes in the chemical structure of a drug can affect receptor binding– different types of bonds can be formed between drug and receptor (e.g. H-bond)
» explore these 2 aspects in more detail in Dr. Dave’s section of MCMP 407
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Drug-Receptor Interactions (cont.)
Receptors mediate the actions of pharmacologic agonists and antagonists
– Agonists: drugs that bind to a receptor and stimulate a biological response
– Antagonists: » drugs that bind to a receptor but do NOT alter receptor function
(i.e. stimulating a response)» alter the interaction of the receptor with another drug » effect depends completely upon its ability to prevent binding of an agonist to its
receptor and blocking their biological activity» possess affinity, but lack intrinsic activity
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Drug-Receptor Interactions
LSD
CNS effects Br
LSD is an agonist at the 5-HT2A receptor
2-Bromo-LSD is anantagonist
LSD
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Effect of Drugs on Organs and Tissues (cont.) site of drug action: where the drug acts to initiate the chain of events leading to a
biological effect
– extracellular sites:» some drugs do not need to enter the cell to exert their effects » intracellular reactions (i.e. signaling pathways) are responsible» more on these biochemical pathways later
– intracellular sites:» usually involve a lipid-soluble drug that is able to cross membranes
– sites on the cell surface:» usually involve transmembrane receptors
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Concentration-Effect Curves and Receptor Binding of Agonists
Responses to low concentrations of a drug increase proportionally
As the dose increases, the incremental response decreases
Finally, concentrations may be reached at which no further increase in response can be achieved with increasing concentration
akin to Michaelis-Menten kinetics (principles of Km, Vmax)
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Concentration-Effect Relationship
0%
25%
50%
75%
100%
0 200 400 600 800 1000
- difficult to accurately extrapolate quantitative information due to the constantly changing slope of the curve
- difficult to compare multiple curves at the low concentrations
EC50 = concentration of drug required to produce half-maximal effect
At lower concentrations:drug effect is changing rapidly
At higher concentrations:drug effect is changing slowly
EC50
Drug Concentration (µM)
Drug Effect
log plot
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Concentration-Effect Relationship (cont.)
0%
25%
50%
75%
100%
DrugEffect
1 10 100 1000
Drug Concentration (µM)
Relatively linear portion in the curve about its central point more accurate quantitation
EC50
easier to compare concentration-effect (dose-response) curves graphically
expansion of scale at lower concentrationscompression of scale at higher concentrations
there is no biological significance to this change in graphical presentation
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Pharmacological Descriptors of the Receptor
KD:
– describes the interaction between the drug and receptor– drug concentration where drug binding to the receptor is half-maximal– constant for a given drug-receptor system– The lower the KD, the stronger the interaction
Bmax:
– total amount of receptor present in a cell or tissue
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Homer Simpson and KD
+ beer
+ champagne
low KD
high affinity
very high KD
very low affinity
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Receptor Binding and Drug Concentration
50 % occupancywhen [Drug] = KD
0.01 0.1 1 10 100 10000.00.10.20.30.40.50.60.70.80.91.0
Drug concentration ( )mMR
atio
occ
up
ied
rec
epto
r
50 % occupancywhen [Drug] = KD
Rat
io o
ccu
pie
d r
ecep
tor
0 100 200 3000.00.10.20.30.40.50.60.70.80.91.0
Drug concentration ( )mM
arithmetic scale the drug-receptor binding curve is hyperbolic
log scale the drug-receptor binding curve is sigmoidal
KD is constant for a drug-receptor system
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Concept of Affinity affinity: ability of the drug to interact with the receptor
KD is a measure of affinity
affinity is a determinant of potency– lower KD higher affinity more potent
a single drug: different affinities for different receptors
relative affinities among drugs may change from receptor to receptor
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Concept of Potency potency: dose of a drug required to produce a particular effect of given intensity
compare drug doses that produce the SAME effect (usually at ED50)
more potent if less drug is required (higher affinity)
higher KD or EC50 less potent
potency may be over-rated– imperfect: our world of D + R DR response
instead determine efficacy
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Concept of Efficacy efficacy: the biological response resulting from the drug-receptor interaction
– not all DR same amount of response
a strong agonist has high affinity and high efficacy
maximal efficacy is often limited by toxicity– high doses
efficacy is more important than potency as a drug property
log dose-response curves good for visualinspection
Foye’s: page 90
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Homer and Agonists
Agonists
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Partial AgonistRemember LMA: conformational change in R response
k1
[D] + [R] [DR] Effect
k-1
what about this step?
full agonist full occupancy maximal effect
some agonists full occupancy less than maximal effect
effects of these agonists are less efficiently coupled to receptor occupancy
= “partial agonists”
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Partial Agonist
0
0.2
0.4
0.6
0.8
1 A
gonis
t E
ffect
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 Log Agonist Concentration
Drug
Effect
log [Drug]
0.6
0.4
full agonist A
partial agonist B
partial agonist C
A
B
C
does NOT same maximal effect as a full agonist regardless of the concentration used
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Partial Agonist (cont.)
reduced response even at 100% receptor occupancy
may competitively inhibit the response to a full agonist
can have the same affinity for the receptor as full agonists– decreased affinity is not the reason for a less than maximal response
mechanisms complex but probably related to drug binding to inactive form of receptor – receptor can take on two forms (active and inactive)– partial agonist can bind to both forms
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Example of Concepts Potency
Efficacy
Agonist
Partial Agonist
log (Dose)
Response
A
B
C
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Receptor Antagonism a D-R interaction that inhibits the drug response produced by an agonist
binds to the receptor, but does NOT activate it
4 major types of receptor antagonists:
– competitive: almost all antagonists in clinical use are of this type– irreversible: these covalent modifications of the receptor– mixed: we won’t discuss– noncompetitive: we won’t discuss
exhibit very different concentration-effect and concentration-binding curves
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Competitive Antagonist Reversible or equilibrium competitive antagonism:
– antagonist combines with the same binding site on the receptor as the agonist
– can be reversed by increasing the dose of the agonist
– e.g. heroin overdose is treated with competitive antagonist naloxone
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Competitive Antagonist (cont.)
0%
25%
50%
75%
100%
Dru
g E
ffect
0 100 200 300 400 500 Drug Concentration
Drug
Effect
[Drug]
AB
C
A : agonist alone
B: (+) competitive antagonist
C: (+) more comp. antagonist
In presence of comp. antag.:
Higher [ agonist ] required to:
- overcome inhibition
- produce effect
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Competitive Antagonist (cont.)
- increase [ antagonist ] increase EC50 of the agonist
- potency decreases
- efficacy is unchanged
- magnitude of the shift is
proportional to [antagonist ]
0%
25%
50%
75%
100%
Dru
g E
ffect
-6 -5 -4 -3 -2 Log Drug Concentration
Drug
Effect
log [Drug]
Increasing
[ antagonist ]
EC50
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Log Dose-Response Curve in the Presence of a Competitive Antagonist
the shape of the log dose-response curve and the maximal response are not altered by the competitive antagonist
at very high [antagonist], raising the [agonist] should still response
a competitive antagonist has affinity, but lacks significant intrinsic activity (efficacy)
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Irreversible Antagonist
an irreversible antagonist will usually bind to the same site as the agonist, but will not be readily displaced
irreversible inhibition is generally caused by a covalent reaction between antagonist and receptor
inhibition persists even after an irreversible antagonist is removed!
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Irreversible Antagonist (cont.)
0%
25%
50%
75%
100%
Dru
g E
ffect
-6 -5 -4 -3 -2 Log Drug Concentration
curve is shifted to the right
at high [ irrev. antag. ]:
- max effect decreases- covalent bond is formed
Drug
Effect
log [Drug]
increasing
[ antagonist ]
higher [ agonist ] does not:
- overcome inhibition
- produce max. effect
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0
0.2
0.4
0.6
0.8
1
Dru
g E
ffect
0 1 2 3 4 5 6 7 8 9 10Time (hr)
Time-Action Curve
Time to Peak Effect
Addresses two main questions for every drug:
How quickly will the drug act? How long will the drug effect last?
Minimum Effective Concentration
Time to onset Duration of action
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Residual Effects
after the primary effects are terminated, it is possible for a drug to exert a residual effect that is unmasked when another dose of the same drug is given– e.g. impaired psychomotor skills following anesthesia
may not be due to the binding at the receptor responsible for the primary effects
can only be observed if another dose or a dose of another drug is given– e.g. cognitive decline (sleep disorders, impaired memory, etc.) with chronic MDMA use
Can last for long periods of time (months, years)
may also occur when another entirely different drug is given and the phenomenon of antagonism or potentiation is manifested– e.g. 2nd drug bind to receptor responsible for primary effects 1st drug released
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Residual Effects (cont.)
0
0.2
0.4
0.6
0.8
1
Dru
g E
ffect
0 1 2 3 4 5 6 7 8 9 10Time (hr)
residualeffect
1° drug effectsterminated
impaired neuropsychology(attention, memory, etc.)
women > men
Marijuana Use
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Pharmacokinetics (PK) Section
BODILY PROCESSES DRUG
Drug Absorption and Transport
Text: Katzung, Basic & Clinical Pharmacology, chapters 3-4
Foye’s, Principles of Medicinal Chemistry, chapters 7-8
Pharmacodynamics: Drug Biological Effects
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Drug
Absorption
EliminationMetabolism
Distribution
Pharmacokinetics
Biological Effect
Pharmacodynamics
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Pharmacokinetics and Pharmacodynamics
Katzung: page 36
ADME:
- Absorption
- Distribution
- Metabolism
- Elimination
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PK Curve May Not Correlate with PD Curve
Problem:
– PK ≠ PD » average: 6-8 hr activity, 22 hr t1/2» individualized dosing is required
– Prescriptions are increasing– contributed to 3,849 deaths in 2004 (790 in 1999)
» 82% of those deaths listed as accidental
CH3N
CH3
H3C
O*
methadone
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Definitions once thought that the biological response to a drug was due to its pharmacologic activity
– it is now apparent that this is NOT the case
Absorption: movement of a drug FROM the site of administration the circulation
Distribution: movement of drug FROM circulation tissues (e.g. plasma receptor)
Metabolism: biotransformation of drugs into metabolites
Elimination: removal of unchanged drug and metabolites from the body
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Introduction in order for a drug biological activity, it MUST be present at its target site in the body ADME processes occur simultaneously and determine the time course of [drug] at its
target
in combination with the affinity of the drug for its target site:
– ADME processes serve to regulate the pharmacological activity of a drug ADME processes play an important role in the overall drug effect:
– drugs are rarely administered directly to the site of action (e.g. topical administration)
an understanding of cell membrane
properties and structure is required
Foye’s: page 145
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Transport of Drugs: drug transport = movement of a drug molecule across a series of membranes and spaces
most often: drug is given into one body compartment and must move to its site of action in another– requires that the drug be absorbed into the blood and distributed to its site of action
drug action (time of onset and duration) depends on ALL of the rates of ADME processes
elimination can occur by metabolism and/or directly excreted– should occur at a reasonable rate so length of drug effect is appropriate for therapy
the rate of uptake/release by a tissue is a function of:– blood flow to that tissue– affinity (partition coefficient) of tissue for drug
rates of absorption can depend upon the rate of blood perfusion at the site of absorption
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Drug Absorption:
Routes of Administration
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Drug Absorption for most routes of administration, drugs must cross epithelial membranes in order to reach
the blood
– e.g. GI, oral
– but NOT injection (sc, im, or iv)
therefore, (except for injection) drugs must go through the cells in the membrane
– cannot go between cells by bulk flow
drug absorption is usually limited by:
– the rate the drug can cross cell membranes by drug transport mechanisms:
(diffusion, filtration, ion-pairing, endocytosis, facilitated transport, or active transport)
– perfusion (i.e. circulation at the site of absorption) and concentration gradient
– surface area
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Routes of Administration choice will have a profound effect upon the rate and efficiency with which the drug acts
– enteral = drug placed directly in the GI tract (epithelial barriers – stomach)
» oral – swallowing
» rectal – absorption through the rectum
» sublingual – placed under the tongue
– parenteral - BYPASS GI system (endothelial barriers)
» injection - sc, im, iv
– topical - (epithelial barriers - skin)
– inhalation - (epithelial barriers - lung)
remember: no single method of drug administration is ideal for all drugs in all situations
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Bulk Flow (cont.)
+
-o
Plasma
+
-
ocapillary endothelium
(loose junctions)
epithelium
(tight junctions)
Environment
GI
Skin
Lung
Absorption Distribution
+
-o
+-
o
ORAL
SC, IM
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Enteral Absorption formulation: controls the ability of the active ingredients to dissolve and go into solution
– essential 1st step for absorption
– especially important at gastric pH (very low)
– achieve delayed release into small intestine with pH sensitive coatings – avoid stomach
microbial metabolism:
– proteolytic and hydrolytic enzymes of intestinal microflora may metabolize drugs
– altered rate of absorption OR
– altered biological activity (metabolites)
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Enteral Absorption (cont.) FOOD (generally decreases absorption)
– delays gastric emptying
– increases hydrolysis by gastric enzymes
– increases intestinal blood flow and subsequent absorption
– complexes with drugs to retard absorption
» e.g. tetracycline: complexes with Ca2+ in food and milk products
Effect is considerable can reduce absorption of tetracyclines by 80%
Solution: leave a 2 hour gap between eating and taking tetracycline
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Advantages:– convenient: can be self-administered, pain-free, easy to take– absorption: takes place along the entire GI tract– cheap: compared to parenteral routes
Disadvantages:– sometimes inefficient: only part of the drug may be absorbed– 1st pass effect: drugs absorbed orally are initially transported to the liver via the
portal vein– irritation to gastric mucosa nausea and vomiting– destruction of drugs by gastric acid and digestive juices– effect too slow for emergencies– unpleasant taste of some drugs– unable to use in an unconscious patient (patient compliance is a problem)
Routes of Administration: Oral
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1st Pass Effect
drug is absorbed from the gut and delivered to the liver by the portal circulation
enzymes in the liver metabolize the drug to an inactive species before it reaches the systemic circulation– inactive product = metabolite that does not possess the desired pharmacological activity
the greater the 1st pass effect:– the less the drug will reach the systemic circulation
when administered orally
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Routes of Administration: Sublingual
barrier is oral mucosa (epithelial cells) surface area is limited (< 1 m2), but well perfused cell layer is relatively thin absorption is rapid if lipid/water partition coefficient is high
pKa is the major rate limiting factor - saliva pH is 7.0 absorption direct to general circulation - thus bypasses 1st pass metabolism limiting factors: dissolution and transit time in oral cavity
– some drugs are taken as smaller tablets which are held in the mouth or under the tongue
» advantages: rapid absorption, drug stability, avoid 1st pass effect
» disadvantages: incovenient, small doses, unpleasant taste of some drugs
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GI Absorption
size of the absorptive surface of the various parts of the GI tract (in m2):
– oral cavity: 0.02– stomach: 0.1-0.2– small intestine 100– large intestine 0.5-l .0– rectum 0.04-0.07
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pH in Body Compartments
Blood 7 Mouth 6-7 Colon 8 Cerebral spinal fluid 7 Urine 5-8 Sweat 4-7
pH 1-3
7-8
6-7
5-7
Foye’s: page 144
note: stomach pH is variable
SI and LI pH is near neutral
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Other Routes of Administration: Advantages Rectal:
– Bypasses:
» low pH of GI, hydrolytic enzymes in GI, first-pass metabolism
» good for drugs affecting the bowel (laxatives)
– useful for unconscious or vomiting patients or uncooperative patients (children)
Topical:
– generally produces only local effects e.g. dermatology: antibacterial, antifungal, sunscreens, antiviral agents
Lung:
– very highly vascularized and absorption RATE in the lungs is considerably higher than that in the small intestine
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Parenteral Administration
barrier is endothelial cells
can bypass epithelial barriers via injection
subcutaneous (sc): bypass epidermis - only barrier is dermis
intramuscular (im): bypass epidermis and dermis – injected into skeletal muscle
– faster absorption than s.c. due to better perfusion and lateral diffusion
transdermal: diffusion through intact skin
intravenous (iv): bypass ALL barriers (membranes) to absorption
– drug injected directly into the blood stream
– produces essentially immediate response
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Advantages of Intravenous Administration
absorption phase is bypassed (drug is 100% bioavailable)
almost immediate onset of action
obtain precise plasma levels; excellent compliance; fairly pain free
large quantities can be given
good for drugs with narrow therapeutic index (accurate route of administration)
useful for rapidly metabolized or labile drugs – bypass 1st pass and absorption phase
especially good for drugs which are poorly absorbed by other mechanisms
especially good for very large drug molecules (macromolecules that can’t cross membranes)
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Disadvantages of Intravenous Administration
very rapid response potential for overdose (OOPS! factor is high)
non-recoverable – can’t “suck out the poison”
requires skilled administration (costly)
potential for tissue necrosis
potential for embolism – drug or particulate in formulation blocks the flow of blood
potential for microbial or viral contamination in preparation
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IV vs Oral Administration Bioavailability (F) Calculation:
– Amount of drug available after oral administrationcompared to:
– Amount of drug available after IV administration (F = 100%)
– Tells you: » amount of first pass metabolism» if there were absorption problems new formulation?» etc.
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Time-Action Curve (PK)
Ideal Situation:
PD and PK Time-Action
Curves are Correlated
0
0.2
0.4
0.6
0.8
1
Dru
g E
ffect
0 1 2 3 4 5 6 7 8 9 10Time (hr)
Dru
g P
lasm
a L
evel
s
Cmax
Tmax
AUC T1/2
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General Scheme of Drug Metabolism
Metabolism
Lipophilic Hydrophilic
increase eliminationdecrease biological activity
Phase I(oxidative)
Phase II(synthetic)
Parent compound
Metabolites ConjugatedMetabolites
polarityfunctionality ionization
water solubility
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Human P450 Isoforms
major drug metabolizing P450s % of drugs metabolized by P450s
Foye’s pages 178-179
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Clinical Considerations of CYP450 Metabolism
CYP450 CYP450 + MetaboliteDrug
Elimination
Loss of Drug EffectNo Toxicities
Substrate Oxidation
CYP450 + Drug + electrons Activated CYP450 CYP450 + Metabolite
(capable of oxidations)
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bound
molecular
oxygen
substrate
NADPH2
endoplasmic
reticulum
(membrane)P450
cytoplasmic
side
luminal side
P450
Oxidations
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OxidationR R
*H
O*H
R
HO
*H
RHO*H
HO
H2O
Dihydrodiol
Arene Oxide(carcinogen)
:Nu
RHO*H
Nu
Aromatic Oxidation
[O]
inactivation vs. bioactivation
bioactivation
cellular toxicities
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MDMA and Cytochrome P450 Metabolism
O
O
NCH3
H
CH3
MDMA (“Ecstasy”)
P450 1A2
O
O
NH
H
CH3
P450 2D6
HO
HO
NCH3
H
CH3
MAJOR
MINOR
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CYP450s
ISOZYME SUBSTRATES INDUCERS INHIBITORS
CYP1A2(2%)
AcetaminophenTheophylline
BarbecueSmoking
AntibioticsQuinolone
CYP2C fam(20%)
DiazepamPhenytoin
Rifampin Fluoxetine
CYP2D6(25%)
CodeineImipramine
None known QuinidineAntidepressants
CYP3A4(52%)
QuinidineWarfarin
PhenobarbitalPhenytoin
AntifungalsAntibiotics
approximate % of drugs metabolized by this CYP450
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P450-catalyzed reactions: Epoxidation - ring (aromatic)
P450
EpoxidationO
Benzo[a]pyrene – polycyclic aromatic hydrocarbon
present in cigarette smoke, smog, charcoal grilled meat
1A
known carcinogen in fish, insects, humans, and other animals epoxide reacts w/ DNA and macromolecules LC50: cricket = 15mg/g (oral)
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P450 Inhibited P450
Drug A(Inhibitor)
Drug B(Substrate)
Drug B
Prolonged or Enhanced EffectUndesirable Toxicities
(Drug-Drug Interaction)
slow release of inhibitor
Clinical Considerations of Cytochrome P450 Inhibition
Competitive Inhibition
Drug-Drug Interaction (DDI)
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Time-Action Curve – Competitive Inhibitor
0
0.2
0.4
0.6
0.8
1
Dru
g E
ffect
0 1 2 3 4 5 6 7 8 9 10Time (hr)
Dru
g P
lasm
a L
evel
so
r
+ inhibitor
PK and PDare affected
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Why are we so interested in DDIs??
FDA: 2006
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FDA Draft Guidance – Metabolism and DDIs September 2006
– Study design, data analysis methods– Implications for dosing and labeling– Mostly concerned with effects on CYP450
DDIs can be due to metabolism but also:– Changes in PK, transporters, etc.
Does not establish legally enforceable responsibilities Describe the FDA’s current thinking View only as recommendations, not required
– May be best to be running experiments described to stay ahead of or with the rest of the pack
– “Negative findings from early in vitro and early clinical studies can eliminate the need for later clinical investigations.”
– i.e. potentially fewer protocols!!
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Adverse Events Reported to FDA FDA has a website devoted to ADRs:http://www.fda.gov/cder/aers/default.htm
This figure illustrates the patient outcome(s) for reports in AERS since the year 1999 until the end of 2008. Serious outcomes include death, hospitalization, life-threatening, disability,
congenital anomaly and/or other serious outcome.
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Factors Modulating Xenobiotic Metabolism (cont.)
DRUG INTERACTIONS (DI’s):
competitive inhibition by other drugs and xenobiotics can decrease metabolism of drugs
especially important with multiple drug treatments
potential DI’s with:
– herbal drugs and illegal drugs relatively unexplored
very important with elderly patients who are
often taking multiple drugs simultaneously
4 or more drugs68%
3 drugs13%
2 drugs12%
1 drug7%
approx. 1000 patients at
VA Medical Center, Wichita, KS
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Steps of the Experiment
Test Articles
Combined with tissues of interestand other reaction ingredients
Mixture undergoes vigorous shaking for a period of time
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Purification and AnalysisCentrifuged to precipitate protein
Injected onto the LC/MS for analysis
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Data Analysis and Next Steps
Go home and let the LC/MS work overnight Process the data
disseminate tothe Project Team
I think we should performthis experiment
next!
No MoreBailouts or DDIs!
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Competitive Inhibition of Cytochrome P450s
(B) coordination to the heme iron atom - usually through a nitrogen
(esp. imidazole ring)
(A) lipophilic and H- bonding interactions
Inhibitor BInhibitor A
NN
NN Fe
P450
NN
NN Fe
P450
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Contaminants commonly found:
• MDMA structural derivatives: legal, cheaper• caffeine and ephedrine (“herbal ecstasy”): mimic speedy feeling• LSD (very rare)
MDMA and Cytochrome P450 Inhibition
O
O
NCH3
H
CH3
• dextromethorphan (“green triangles”)anti-tussive (cough medicines)raises body tempinhibits sweating
MDMA
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MDMA
CH3O
NCH3
NCH3
H
CH3
O
O
Dextromethorphan
P450 2D6
plasma levels of MDMA
Drug-Drug Interaction
P450 2D6-Dextromethorphan
cheaper
drugs
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N
N
H
SNMeHN
NCN
Cimetidine (Tagamet)
Drug-Drug Interactions• H2 receptor antagonist (anti-ulcer agent)• general inhibitor of human P450s• inhibits hepatic elimination of many drugs:
warfarin alprazolamacenocoumarol triazolamphenadion theophyllinephenytoin imipraminecarbamazepine caffeinechlormethiazole propanololdiazepam labetalolchlordiazepoxide metoprolollidocaine ethanol
H
• imidazole ring able to coordinate to theheme iron atom of several different P450s
NN
NN Fe
undesirable toxicities
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O
N Me2
SNMeHN
N O-
HO
Ranitidine (Zantac)
• H2 receptor antagonist
• replacement of imidazole w/ furan ring: circumvents cimetidine drug interactions
• knowledge of which structural features of a drug were important for P450 inhibition
H
Drug-Drug Interactions
N
N
H
SNMeHN
NCN
Cimetidine (Tagamet)
Hdesign of a safer drug
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Mechanism-Based Inhibition (Irreversible)
FDA Draft Guidance
Metabolic activity will not be restored until enzyme is re-synthesized
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Pathways of Mechanism-Based Inhibition of CYP450
NN
NN
MBI*
MBI
Fe
Fe
NN
N N
MBI*
Cys
NN
NN
FeN
N
NN
Fe
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Mechanism-based Inactivators of CYP450s
Raloxifene (osteoporosis)
Bergamottin (Grapefruit Juice Component)
RU-486 (morning after)
Phencyclidine (street drug)
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Ritonavir
“BOOSTER” for
other HIV drugs
Mechanism-based
inactivator of CYP3A4
+ ritonavir
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Experimental Design: Mechanism-based Inactivation
• human liver microsomes• MBI (e.g. 8-MOP for
CYP2A6)• initiate rxn
+ NADPH
time time
1˚ rxn 2˚ rxn• CYP450 selective substrate (e.g. coumarin at 2X KD)
• initiate rxn with P450 from 1˚ reaction
≥ 20-fold dilution ANDexcess substrate to displace MBI
(now <<< KD)
product analysis(e.g. 7-OH coumarin)
• HPLC/fluorescence• LC/MS• GC/MS
(0-10 min)
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Enzyme-Drug Interaction - Concepts
E
EE
[E + I]
[E + I]E
I
KI
[E-I]
[E + I]E
kinact
time
timeMetabolites
[E-I]
[E + S][E + S] 20X
dilution
S
KD
I
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RU486 and CYP2B6 (2008)
31% remaining
KI
kinact
competitiveinhibition
0-25 µM
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Esterases > 70 different human esterase genes
– Esterases are present in every tissue and blood
a/b hydrolase-fold family (>15,000 members)– Carboxylesterases (hCE-1, 2, 3) – broad substrate specificities– Acetylcholinesterase (AChE) – specific for acetylcholine– Butyrylcholinesterase (BChE) – broad substrate specificity
Others:– Proteases (Chymotrypsin, Trypsin, etc.)– Albumin– Paraoxonases (hPON-1, 2, 3) – broad substrate specificities
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Famous Esters
Esther Rolle“Good Times!!”
O O
OO
n
polyester
OO
O
CH3
O
H3C
O
N
CH3
heroin
OHO
O CH3
O
aspirin
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General Esterase Activity
R1 O
O
R2R1 OH
O
R2OH+esterase
acid alcohol
H2O
ester
+
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Human Carboxylesterases
Enzymes known to be involved in drug metabolism– Human carboxylesterases-1 and -2 (hCE-1 and hCE-2)
hCE-1liver
hCE-2intestine
microsomescytosol
Twopurified
enzymes
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Inhibitors of Esterases: Biological Weapons
P
S
N
H3C
CH3
CH3
CH3H3C
OH3C
O
VX
AChE inhibitor – developed as a pesticide (1952)most deadly nerve agent in existence
3X more deadly than sarin300 g is fatal
F
P
H3C
O
O
CH3
CH3
Sarin
O
P
O
N
CH3
N
H3C
CH3
Tabun
"It's one of those things we wish we could disinvent." - Stanley Goodspeed, on VX nerve agent
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Factors Modulating Xenobiotic MetabolismAge and Ontogeny:
decreased:
– absorption (decreased absorptive surfaces, blood flow, and GI motility)
– tissue perfusion
– general metabolism and liver function
– P450 levels in very young and very old
– different P450 are expressed
altered drug distribution:
– increased % body fat
– decreased: serum albumin (plasma protein), muscle mass, total body water
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Factors Modulating Xenobiotic Metabolism (cont.)PHARMACOGENETICS: sex differences (generally small in humans)
ethnic differences (P450)
– isoniazid - slow vs. fast acetylators
species differences (P450)
– MAJOR problem: drug testing in animals and extrapolation to humans
individual genetic variability (relative amounts of P450s and Phase II enzymes)
organ-specific differences (P450, bioactivation)
individualized drug therapy is the goal
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DRUG Phase II Reactions
Metabolite
P450
FMO
ADH
esterases
amidases
Glucuronosyl TransferasesSulfotransferasesGlutathione Transferases
Amino Acid TransferasesAcetyltransferasesMethyltransferases
Phase I
Reactions
elimination
elimination
elimination
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Drug Elimination Pharmacological activity of drug can be reduced by:
– metabolism
– plasma protein binding
– redistribution to other compartments (i.e. fat)
Elimination:
– required to remove the chemical from the body and terminate biological activity
» especially if drug is minimally metabolized
– necessary to prevent accumulation of xenobiotics in the body
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Major Routes of Drug Elimination: are highly dependent on metabolism:
– KIDNEYS (renal)» represent approx. 1% of of total body weight, » but receive 25% of cardiac output» blood flow rate is approx. 8X more that exercising muscle
– Liver– Intestines– Lungs– Sweat, Saliva, Milk – not really significant
same physiological mechanisms govern drug elimination as absorption– i.e. cell membranes are the barriers.
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Methadone Problems:
– PK ≠ PD (average: 6-8 hr activity, 22 hr t1/2)» F = 36-100%, t1/2 = 5-130 hr» individualized dosing is required
– Lots of interindividual variability– Long t1/2 and high tissue distribution
– DDIs
– Prescriptions are increasing
CH3N
CH3
H3C
O*
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Methadone Metabolism
CH3N
CH3
H3C
O
EDDP(inactive,
renally excreted)
MethadoneCmax ~ 0.6 µM
NCH3
H3CCYP2B6: S > RCYP3A4: S = R
CYP2C19: R >> S
Several DDIs possible
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The End