This document provides an introduction to the topics that will be discussed in a pharmacology lecture, including quantitative drug-receptor interactions, mechanisms of drug action, factors affecting drug effects, and absorption, distribution, metabolism, and excretion of drugs. The introduction defines pharmacology as the study of interactions between chemicals and biological systems, with a focus on how drugs work and are processed by the body. Key concepts covered include drug-receptor binding, concentration-response curves, receptor affinity and potency, agonists and antagonists, and time-action profiles of drugs.

1. Introduction to Pharmacology
and Drug Metabolism
Luke Lightning, PhD

2. 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

3. 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

4. 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)!

5. 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)

6. Effect of Drugs on Organs and Tissues
 Drugs only modify cellular function – do not create effects
DRUG RECEPTOR RESPONSE
– 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)

7. Drugs, Dose, Receptor, and Response
Drugs 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

8. 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

9. 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

10. Drug-Receptor Interactions
LSD is an agonist at the 2-Bromo-LSD is an
5-HT2A receptor antagonist
LSD
LSD
Br
CNS effects

11. 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

12. 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)

13. Concentration-Effect Relationship
100%
EC50 = concentration of drug required
75% to produce half-maximal effect
Drug
Effect
50% At lower concentrations:
drug effect is changing rapidly
25%
EC50
0% At higher concentrations:
0 200 400 600 800 1000 drug effect is changing slowly
Drug Concentration (µM)
- difficult to accurately extrapolate quantitative information
due to the constantly changing slope of the curve log plot
- difficult to compare multiple curves at the low concentrations

14. Concentration-Effect Relationship (cont.)
Relatively linear portion in the curve about its central point  more accurate quantitation
100%
expansion of scale at lower concentrations
75%
Drug compression of scale at higher concentrations
Effect 50%
25% EC50
0%
1 10 100 1000 easier to compare concentration-effect
Drug Concentration (µM) (dose-response) curves graphically
there is no biological significance to this change in graphical presentation

15. 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

16. Homer Simpson and KD
+ beer
low KD
high affinity
very high KD
+ champagne very low affinity

17. Receptor Binding and Drug Concentration
arithmetic scale the drug-receptor log scale the drug-receptor
binding curve is hyperbolic binding curve is sigmoidal
Ratio occupied receptor
1.0 1.0
Ratio occupied receptor
0.9 0.9
0.8 0.8
0.7 0.7 50 % occupancy
0.6 0.6 when [Drug] = KD
0.5 50 % occupancy 0.5
0.4 when [Drug] = KD 0.4
0.3 0.3
0.2 0.2
0.1 0.1
0.0 0.0
0 100 200 300 0.01 0.1 1 10 100 1000
Drug concentration (mM) Drug concentration (mM)
KD is constant for a drug-receptor system

18. 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

19. 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

20. 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 visual
inspection
Foye’s: page 90

21. Homer and Agonists
Agonists

22. Partial Agonist
Remember 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”

23. Partial Agonist full agonist A
A g o n is t E f f e c t
1
A
0.8 partial agonist B
B
0.6
0.6
Drug C
partial agonist C
0.4
0.4
Effect
0.2
0
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1
Log Agonist Concentration
log [Drug]
 does NOT  same maximal effect as a full agonist regardless of the concentration used

24. 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

25. Example of Concepts
 Potency
A C
B  Efficacy
Response
 Agonist
 Partial
Agonist
log (Dose)

26. 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

27. 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

28. Competitive Antagonist (cont.)
100%
A
E ffe c t
A : agonist alone
B
75% B: (+) competitive antagonist
C
Drug C: (+) more comp. antagonist
D ru g
50%
Effect
In presence of comp. antag.:
25%
Higher [ agonist ] required to:
0%
- overcome inhibition
0 100 200 300 400 500
Drug Concentration - produce effect
[Drug]

29. Competitive Antagonist (cont.)
100%
E ffe c t
- increase [ antagonist ] 
75% increase EC50 of the agonist
Drug
D ru g
50% Increasing
Effect [ antagonist ]
- magnitude of the shift is
proportional to [antagonist ]
25%
0%
-6 -5 -4 -3 -2
- potency decreases
Log Drug Concentration - efficacy is unchanged
log [Drug]
EC50

30. 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)

31. 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!

32. Irreversible Antagonist (cont.)
100% curve is shifted to the right
E ffe c t
increasing
75% at high [ irrev. antag. ]:
[ antagonist ]
Drug - max effect decreases
D ru g
50%
Effect - covalent bond is formed
25%
higher [ agonist ] does not:
0% - overcome inhibition
-6 -5 -4 -3 -2
log [Drug]
Log Drug Concentration - produce max. effect

33. Time-Action Curve
Addresses two main questions for every drug:
1
How quickly will the drug act?
How long will the drug effect last?
0.8
D ru g E ffe c t
0.6
0.4
0.2
Minimum Effective
0
0 1 2 3 4 5 6 7 8 9 10
Concentration Time (hr)
Time to onset Duration of action
Time to Peak Effect

34. 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

35. Residual Effects (cont.)
Marijuana Use
1
0.8
1 drug effects
D ru g E ffe c t
0.6
terminated
0.4 residual
effect
0.2
0
0 1 2 3 4 5 6 7 8 9 10
Time (hr)
impaired neuropsychology
(attention, memory, etc.)
women > men

36. Pharmacokinetics (PK) Section
BODILY PROCESSES DRUG
Drug Absorption and Transport
Pharmacodynamics: Drug  Biological Effects
Text: Katzung, Basic & Clinical Pharmacology, chapters 3-4
Foye’s, Principles of Medicinal Chemistry, chapters 7-8

37. Pharmacokinetics
Absorption Distribution
Biological Effect
Drug
Pharmacodynamics
Metabolism Elimination

38. Pharmacokinetics and Pharmacodynamics
ADME:
- Absorption
- Distribution
- Metabolism
- Elimination
Katzung: page 36

39. 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
methadone

40. 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

41. 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

42. 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

43. Drug Absorption:
Routes of Administration

44. 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

45. 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

46. Bulk Flow (cont.)
Absorption Distribution
Environment Plasma
+
+ -
GI +
ORAL
Skin
- +
Lung o
- o
o
-
SC, IM o
epithelium capillary endothelium
(tight junctions) (loose junctions)

47. 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)

48. 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

49. Routes of Administration: Oral
 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)

50. 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

51. 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

52. 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

53. pH in Body Compartments
 Blood 7 pH 1-3
 Mouth 6-7
 Colon 8
 Cerebral spinal fluid 7
 Urine 5-8
5-7
 Sweat 4-7
6-7
note: stomach pH is variable 7-8
SI and LI pH is near neutral
Foye’s: page 144

54. 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

55. 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

56. 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)

57. 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

58. IV vs Oral Administration
 Bioavailability (F) Calculation:
– Amount of drug available after oral administration
compared 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.

59. Time-Action Curve (PK)
1
 Ideal Situation:
Drug Plasma fLevels
PD and PK Time-Action 0.8
D ru g E fe c t
Cmax
Curves are Correlated 0.6
AUC T1/2
0.4
0.2
0
0 1 2 3 4 5 6 7 8 9 10
Time (hr)
Tmax

60. General Scheme of Drug Metabolism
Lipophilic Hydrophilic
Metabolism
increase elimination
decrease biological activity
Parent compound
Phase I Phase II
Metabolites (synthetic)
Conjugated
(oxidative)
Metabolites
polarity
functionality ionization
water solubility

61. Human P450 Isoforms
 major drug metabolizing P450s  % of drugs metabolized by P450s
Foye’s pages 178-179

62. Clinical Considerations of CYP450 Metabolism
Loss of Drug Effect
No Toxicities
Substrate Oxidation
Drug
CYP450 CYP450 + Metabolite Elimination
CYP450 + Drug + electrons  Activated CYP450  CYP450 + Metabolite
(capable of oxidations)

63. NADPH2
P450
Oxidations bound
molecular
oxygen
cytoplasmic
substrate
side
endoplasmic
reticulum
P450 (membrane)
luminal side

64. Aromatic Oxidation
[O]
bioactivation
inactivation vs. bioactivation cellular toxicities

65. MDMA and Cytochrome P450 Metabolism
MDMA (“Ecstasy”) MINOR
H
O N
CH3
MAJOR CH3
O
P450 2D6 P450 1A2
H
H N
O H
HO N
CH3 CH3
O
CH3
HO

66. CYP450s
ISOZYME SUBSTRATES INDUCERS INHIBITORS
CYP1A2 Acetaminophen Barbecue Antibiotics
(2%) Theophylline Smoking Quinolone
CYP2C fam Diazepam Rifampin Fluoxetine
(20%) Phenytoin
CYP2D6 Codeine None known Quinidine
(25%) Imipramine Antidepressants
CYP3A4 Quinidine Phenobarbital Antifungals
(52%) Warfarin Phenytoin Antibiotics
approximate % of drugs metabolized by this CYP450

67. P450-catalyzed reactions:
 Epoxidation - ring (aromatic)
Benzo[a]pyrene – polycyclic aromatic hydrocarbon
present in cigarette smoke, smog, charcoal grilled meat
P4501A
Epoxidation
O
 known carcinogen in fish, insects, humans, and other animals
 epoxide reacts w/ DNA and macromolecules
 LC50: cricket = 15mg/g (oral)

68. Clinical Considerations of Cytochrome P450 Inhibition
Prolonged or Enhanced Effect
Competitive Inhibition Undesirable Toxicities
(Drug-Drug Interaction)
Drug A Drug B
(Inhibitor) (Substrate)
P450 Inhibited P450 Drug B
slow release of inhibitor
Drug-Drug Interaction (DDI)

69. Time-Action Curve – Competitive Inhibitor
1
+ inhibitor
Drug Plasma Levels
0.8
PK and PD
D ru g E ffe c t
are affected or
0.6
0.4
0.2
0
0 1 2 3 4 5 6 7 8 9 10
Time (hr)

70.  Why are we so interested in DDIs??
FDA: 2006

71. 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!!

72. 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.

73. 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 1 drug
7%
2 drugs
12%
 potential DI’s with: 4 or more 3 drugs
drugs 13%
– herbal drugs and illegal drugs relatively unexplored 68%
 very important with elderly patients who are
often taking multiple drugs simultaneously
approx. 1000 patients at
VA Medical Center, Wichita, KS

74. Steps of the Experiment
Combined with tissues of interest
and other reaction ingredients
Mixture undergoes vigorous
shaking for a period of time
Test Articles

75. Purification and Analysis
Centrifuged to precipitate protein
Injected onto the LC/MS for analysis

76. Data Analysis and Next Steps
Go home and let the
Process the data
LC/MS work overnight
disseminate to
the Project Team
I think we
No More
should perform
Bailouts
this experiment
or next!
DDIs!

77. Competitive Inhibition of Cytochrome P450s
(B) coordination to the heme iron
(A) lipophilic and H-
atom - usually through a nitrogen
bonding interactions
(esp. imidazole ring)
Inhibitor A Inhibitor B
N N
N Fe N
N Fe N
N N
P450 P450

78. MDMA and Cytochrome P450 Inhibition
H
O N
MDMA CH3
CH3
O
Contaminants commonly found:
• MDMA structural derivatives: legal, cheaper
• caffeine and ephedrine (“herbal ecstasy”): mimic speedy feeling
• LSD (very rare)
• dextromethorphan (“green triangles”)
anti-tussive (cough medicines)
raises body temp
inhibits sweating

79. Drug-Drug Interaction
CH 3 CH3
MDMA N Dextromethorphan N
H
CH 3
O
O CH3 O
P450 2D6-Dextromethorphan
P450 2D6
cheaper
plasma levels of MDMA drugs

80. Drug-Drug Interactions
• H2 receptor antagonist (anti-ulcer agent)
• general inhibitor of human P450s
Cimetidine (Tagamet) • inhibits hepatic elimination of many drugs:
H
CN warfarin alprazolam
N N
acenocoumarol triazolam
S phenadion theophylline
MeHN N N
phenytoin imipramine
H carbamazepine caffeine
N chlormethiazole propanolol
Fe N diazepam labetalol
N
N chlordiazepoxide metoprolol
lidocaine ethanol
• imidazole ring able to coordinate to the
heme iron atom of several different P450s
undesirable toxicities

81. Drug-Drug Interactions
Ranitidine (Zantac)
HO
-
N O N Me2
S • H2 receptor antagonist
M eHN N O
H • replacement of imidazole w/ furan ring:
circumvents cimetidine drug interactions
Cimetidine (Tagamet) • knowledge of which structural features of a
H drug were important for P450 inhibition
CN
N N
S
MeHN N N
design of a safer drug
H

82. Mechanism-Based Inhibition (Irreversible)
 FDA Draft Guidance
Metabolic activity will not be restored until enzyme is re-synthesized

83. Pathways of Mechanism-Based Inhibition of CYP450
MBI*
N
Fe N
N
N
Fe
MBI
MBI*
N
N N N
Fe N N
N Fe N
N N
N N Cys

84. Mechanism-based Inactivators of CYP450s
Raloxifene (osteoporosis)
Phencyclidine (street drug)
RU-486 (morning after)
Bergamottin (Grapefruit Juice
Component)

85. Ritonavir
 “BOOSTER” for + ritonavir
other HIV drugs
 Mechanism-based
inactivator of CYP3A4

86. Experimental Design: Mechanism-based Inactivation
≥ 20-fold dilution AND
excess substrate to displace MBI
+ NADPH
(now <<< KD)
time time
product analysis
(0-10 min) (e.g. 7-OH coumarin)
• HPLC/fluorescence
• LC/MS
• GC/MS
1˚ rxn 2˚ rxn
• human liver microsomes • CYP450 selective substrate
• MBI (e.g. 8-MOP for CYP2A6) (e.g. coumarin at 2X KD)
• initiate rxn
• initiate rxn with P450 from 1˚ reaction

87. Enzyme-Drug Interaction - Concepts
E I [E + I]
E time
E
E [E + I]
[E-I]
E
KI
[E + I]
S kinact
time [E-I] 20X
Metabolites [E + S]
I [E + S] dilution
KD

88. RU486 and CYP2B6 (2008)
0-25 µM
competitive
inhibition
31% remaining
kinact
KI

89. 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

90. Famous Esters
heroin
aspirin
Esther Rolle
polyester “Good Times!!”

91. General Esterase Activity
esterase
H2O
ester acid alcohol
+

92. Human Carboxylesterases
 Enzymes known to be involved in drug metabolism
– Human carboxylesterases-1 and -2 (hCE-1 and hCE-2)
hCE-1 microsomes
liver cytosol
Two
purified
hCE-2 enzymes
intestine

93. Inhibitors of Esterases:
Biological Weapons
Sarin
Tabun
VX
AChE inhibitor – developed as a pesticide (1952)
most deadly nerve agent in existence
3X more deadly than sarin
300 mg is fatal
"It's one of those things we wish we could disinvent."
- Stanley Goodspeed, on VX nerve agent

94. Factors Modulating Xenobiotic Metabolism
Age 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

95. 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

96. elimination
DRUG Phase II Reactions
Glucuronosyl Transferases
P450 Sulfotransferases
Phase I FMO Glutathione Transferases elimination
Reactions ADH
esterases Amino Acid Transferases
Acetyltransferases
amidases
Methyltransferases
Metabolite
elimination

97. 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

98. 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.

101. 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

102. Methadone Metabolism
CYP2B6: S > R
CYP3A4: S = R
CYP2C19: R >> S
Several DDIs possible
Methadone EDDP
Cmax ~ 0.6 µM (inactive,
renally excreted)

103. The End