review of pharmacokinetics jean d. deupree, ph.d. department of pharmacology university nebraska...

Review of Pharmacokinetics

Jean D. Deupree, Ph.D.Department of Pharmacology

University Nebraska Medical Center3014 DRC559-4565

jddeupre@unmc.edu

2

Helpful Hints• Be able to use the equations on p. 40-41.

– Equations will not be provided on the exam – You will not be allowed to use a calculator on the exam

• Be able to use the pharmacokinetic terms which have been defined

• Be able to calculate the ionization of acid and bases in various biological media

• Understand the different types of drug-drug interactions that can occur

• Be able to apply the information. Exam question are not likely to be asked in the same manner as in this review

• When reviewing old exams make sure you understand why the wrong answers are wrong and the right answers are right

3

What types of molecules (charged, uncharged, chemical structural requirements) can be transported by these different mechanism?

• Bulk flow• Passive diffusion• Facilitated transport• Active transport

What are the differences between these transport processes?

4

Comparison of Transport Processes

Transport Mechanism

Energy Requirement

Saturability and Selectivity

Passive Diffusion

None No

Bulk Flow None No

Facilitated Diffusion

None Yes

Active Transport

Yes Yes

5

Henderson-Hasselbalch Equation

pH - pKa = LogConjugate Base

Acid= Log

IonizedUnionized

Weak acid

Weak base

pKa - pH = LogConjugate Acid

Base= Log

IonizedUnionized

Know and be able to use these equations

6

How much of a weak acid (pKa = 3) will diffuse from the stomach to the plasma?

StomachpH 2

PlasmapH 7

1 A

CB- + H +

A 1

CB- + H +

Log ------ = pH - pKa[CB-][A]

Log ------ =[CB-][A]

Log ------ = pH - pKa[CB-][A]

Log ------ =[CB-][A]

[A] + [CB-] = [A] + [CB-] =

2-3 = -1 7-3 = 4

[CB- ]

[A] = 0.11

0.1

1.1

[CB- ][A]

10,000

10,001=

10,0001

7

How much of a weak base (pKa = 7) will be absorbed from the stomach?

StomachpH 2

PlasmapH 7

1 B + H+

CA+

B + H+ 1

CA+

Log ------ = pKa - pH[CA+ ][B] Log ------ = pKa - pH[CA+]

[B]

Log ------ =[CA+][B] Log ------ =[CA+]

[B]

[CA+][B] =

[CA+][B] =

[CA+] + [B] = [CA+] + [B] =

7 - 2 = 5 7-7 = 0

100,000 =100,000

1

100,000

100,001

1 =11

1

2

8

When is ion trapping most likely to occur in these areas?

Note: You do not need to memorize the pH values for these tissues

Body Fluid Range of pH

Breast Milk 6.4-7.6

Jejunum, ileum 7.5-8.0

Stomach 1.92-2.6

Prostatic secretions 6.5-7.4

Vaginal secretions 3.4-4.2

Urine 5.0-8.0

12

• Will a weak base (pka 8) be more ionized at pH 6 or pH 8?– pH 6

• Will a weak acid (pka 4) be absorbed better from a stomach where the pH is 2 or where the pH is 3?– pH 2

• What percent of a weak acid (pKa 4 ) will be in the ionized form at pH 3– [CB]/[A] = 0.1/1 – % in ionized form = 0.1/1.1*100 = 9%

13

What are the advantages and disadvantages of the different routes of drug administration?

• See review on page 21

14

From which sites of drug administration do you get first-pass metabolism? First-pass hepatic metabolism?

15

What are the factors which affect the distribution of drugs to the different tissues of the body?

• Blood flow• Size of the tissue compartment• Ability of the drug to permeate the tissue• Extent of ionization in the different tissue

compartments• Lipid solubility of the drug versus the lipid

content of the tissue• Extent of plasma protein binding

16

Which are the vessel rich organs and how does blood flow to an organ affect the distribution of drugs?

Vessel-rich organs•Brain•Liver •Heart•KidneyIntermediate group•Muscle •Skin

17

What plasma proteins do drugs bind to?

• Albumin • Albumin• Glycoproteins• Lipoproteins

Acidic Drugs Basic Drugs

18

What are the characteristics of drugs which cross the placenta and enter the brain? •Drugs which are lipid soluble can cross the blood brain barrier and the placenta by passive diffusion. The more lipid soluble the drug the faster it diffuses into the brain and the faster it crosses the placenta•The more aqueous soluble drugs can only enter the brain or the fetus by active transport or facilitated diffusion. Note: Many drugs are not recognized by the transporters.

19

What are the phase I reactions?

• Oxidation• Reduction• Hydrolysis

What are the phase II reactions?

Conjugation

20

What are the consequences of drug metabolism?

•Pro-drug → Drug•Drug → Active and inactive metabolite•Drug → Reactive intermediate•Hydrophobic → Hydrophilic•Exposure of functional groups: -OH, -COOH, -NH2

21

Phase I and Phase II Reactions

Drug

Phase IIPhase I

Conjugation

Drug

Drug

Elimination

Active metabolite

Inactive metabolite

Conjugation

Conjugation

Hydrophobic Hydrophilic

What types of drug-drug interactions can occur?

22

23

What is the principle enzymes involved in Phase I reactions and what are some of the other names for this enzyme?

• Cytochrome P450• Microsomal mixed function oxidase• Microsomal drug metabolizing enzyme

24

• Is there more than one cytochrome P450 isozymes?

• How is the enzyme be regulated?– Induction– Inhibition– Note: Not all drugs will induce or inhibit the

CYP isozymes, only selective isozymes will be inhibited.

– NOTE: COMPETION OF ONE DRUG FOR ANOTHER ONLY BECOMES A PROBLEM IF THE DRUGS ARE IN HIGH CONCENTRATIONS. Remember for most drugs you are very low on the place of rate of elimination versus drug concentration

– Note: Induction or inhibition of a CYP 450 isozyme will affect the metabolism of all drugs metabolized by that isozyme

25

• What CYP isozyme is induced by cigarette smoke?– CYP1A family

• Which CYP isozyme is inhibited by grapefruit juice?– CYP 3A4 in the wall of the intestine

26

• What is the principle location of CYP isozymes?– Liver

• What are the substrates, enzymes and cofactors of the CYP450 complex?

• Which isozyme is responsible for metabolism of over 50% of the drugs?– CYP 3A4

• What is the significance of the CYP 2D6 enzymes?– Numerous polymorphism have been found

27

Cytochrome P450 Enzyme Complex

28

What are the other types of phase I reactions and what is the primary location of these enzymes?• Oxidative reactions

– Flavin monooxygenases (liver)– Amine oxidase (liver and nerve endings)– Dehydrogenase (liver)

• Reductive reactions– Liver– Intestinal microflora

• Hydrolytic– Esterases: Plasma– Amidases: Liver

29

What are the different types of conjugation reactions that can occur?

• Glucuronidation• Sulfate conjugation• N-acetylation• Methylation • Glutathione conjugation• Amino acid conjugation

What are the enzymes involved in glucuronidation and hydrolysis of the glucuronide product?

30

Phase II reactionsGlucuronidation

R-OH + UDP-glucuronic acid

R-O-glucuronide

UDP-glucuronyltransferase

R-OH + glucuronic acid

Beta-glucuronidase

COOH

O O-R(Liver)

Intestinal mucosa Intestinal bacteria

Bile Kidney

Intestine Excretion

31

• Age– Less cyc P450 in

• Very young• Elderly

– Phase II reactions are usually not affected in the elderly

– Decreased blood flow to liver in elderly

How Does Age Affect Drug Metabolism?

32

• Pharmacogenetic– Genetic differences in the activities of many

metabolic enzymes• Altered activity of CPY enzymes and enzymes

involved in conjugation– 2D6 enzymes– N-acetyltransferase: Slow vs fast acetylators

Factors Affecting Drug Metabolism

33

What factors determining the amount of drug excreted in the urine?

Nephron

Glomerular Filtration

Tubular Reabsorption

TubularSecretion Excretion = glomerular filtration

- tubular reabsorption + tubular secretion - passive reabsorption +passive secretion

34

• How do you speed up the renal elimination of a weak acid?– Make the urine alkaline

35

How are drugs or metabolites excreted from liver to bile?• Passive diffusion• Transporters for

– anions

– bile salts

– cations

– neutral organic compounds

• How do you enhance biliary excretion? – polar groups

– high molecular weight

– Conjugation add polar groups and molecular weight

36

What drugs go through the enterohepatic recirculation pathway?

Drugs which enter the bile and are: •Lipid soluble enough to be reabsorbed from the wall of the gut •Hydrolyzed in the gut and then reabsorbed•Transported by transporters across the wall of the gut

37

• What is the major route by which drugs are eliminated from the body?– Kidney

• What are the minor routes?– bile– skin– lungs– sweat glands– saliva– breast milk

38

What are the basic types of pharmacokinetic drug- interactions?

• Involving metabolism– Induction of cyc P450 enzymes– Inhibition of cyc P450 enzymes– Competition of two drugs for the same enzyme– Depletion of endogenous compounds used in

conjugation reactions• Displacement from albumin• Competition for transporters• Blood flow to an organ• Changes in pH of a body of fluid

39

Time After Drug Administration (hr)

0 2 4 6 8 10 12

5

4

3

2

1

0

Ser

um

Co

nce

ntr

atio

n (g

/ml)

Peak Drug Level

Time to Peak Drug Level

40

Time After Drug Administration (hr)

Ser

um

Co

nce

ntr

atio

n (g

/ml)

0 2 4 6 8 10 12

5

4

3

2

1

0

Therapeutic Threshold

Duration of Therapeutic Effect

41

Therapeutic Window

Time After Drug Administration (hr)

Ser

um

Co

nce

ntr

atio

n (g

/ml)

0 2 4 6 8 10 12

5

4

3

2

1

0

Therapeutic Effect

Toxic Effect

42

What is the difference between therapeutic window and therapeutic index?• Therapeutic window is the plasma

concentration range where therapeutic effect occurs without toxic effects

• Therapeutic index is: TD50/ED50

43

How do you determine bioavailability?

F = AUC for route being studiedAUC after IV administration

What factors will alter bioavailability?

44

What is the Volume of Distribution of a drug?

Vd (L) = Xo mgCp (mg/ml)

45

How do you measure the concentration of drug in the plasma at time T = 0?

0 10 20 30 40 50 60

10

8

6

4

2

Time (Hours)

Pla

sma

Dru

g C

on

cen

trat

ion

(μg

/ml)

Cp

46

What does it mean if a drug has the following Vd in a 70 kg person?

• 500 L

• 10 L

• 35 L

• 42 L

48

• What is meant by t1/2?• How many half-lives does it take for the

drug concentration to reach steady-state?– 4 to 5 half-lives

• How many half-lives does it take for the drug to be eliminated from the body?– 4 to 5 half-lives

• What is kel? – Rate constant for elimination

• What are the units for kel? – 1/time

• How does kel change with plasma concentration? – It remains constant at low concentrations of

drug

49

Is this drug being eliminated by first order or zero order kinetics?

What is the half-life of this drug? Half-life changes with plasma concentration

If you double the dose how long does it take to eliminate the drug from the body? Twice as long

What happens to steady state plasma levels if you double the dose? Greater than 2x plasma level

Time (Hrs)

Pla

sma

Dru

g C

on

cen

tra

tion

(μ

g/m

l)

2 4 6 8

100

8060

40

20

Zero order

51

8 16 24 32 40 48 56 64 72 80 88 96

20

15

10

25

Time (hrs)

Pla

sma

Co

nce

ntr

atio

n (

mg

/L)

Effects of Doubling DoseZero Order Kinetics

NOTE: Drug is entering the body faster than it leaves the body

52

Is this drug being eliminated by first order or zero order kinetics?

What is the half-life of this drug? 10 hoursIf you double the dose how long does it take to eliminate the drug from the body? 4 to 5 half-lives or 40-50 hours --- time is independent of plasma levelsWhat happens to steady state plasma levels if you double the dose? Plasma levels double

0 10 20 30 40 50 60

10

6

42

Time (Hours)

Pla

sma

Dru

g

Co

nce

ntr

atio

n (

μg

/ml)

8

1st order

53

8 16 24 32 40 48 56 64 72 80 88 96

6

4

2

8

Time (hrs)

Pla

sma

Co

nce

ntr

atio

n (

mg

/L)

Effects of Doubling Dose1st Order Kinetics

Note: Drug enters body at the same rate it is leaving the body

54

Is this drug being eliminated by first order or zero order kinetics?

What is the half-life of this drug?

1st order

How long will it take to reach steady state plasma concentration if this drug is given by continual IV infusion?

10 hours

40 to 50 hours

57

1st Order Zero Order

Elimination Exponential Linear

Half-life Constant with changing plasma concentrations

Changes with plasma concentration

Eliminated per unit time

Constant Fraction

Constant Amount

Clearance Rate of elimination/Cp

Maximum rate of elimination process

Time required to eliminate drug

4-5 Half-lives Based on amount of drug given

Double the dose Double plasma concentration

Greater than twice plasma concentration

58

What is meant by the clearance of a drug from the body, how is it calculated and what are the units?

• Theoretical volume of fluid from which a drug is completely removed in a given period of time.

• For a drug eliminated by first order kinetics

Clearance (CL)(ml/min) = Rate of elimination (g/min)

Concentration (g/ml)

59

How is clearance related to Vd and t1/2?

CLT = 0.7 X Vd

t1/2

If CL changes but not Vd what happens to the t1/2 ?

Half-life is inversely proportional to clearance. If volume of distribution does not change and clearance decreases then the half-life will increase.

60

How does rate of elimination change with drug concentration for a drug eliminated by zero order kinetics?

• V = Vmax

• Rate of elimination of the drug from the body is based on the maximum rate of the rate limiting step in the elimination process (metabolizing enzyme, transporter ect.)

62

What is meant by saturation kinetics?

• Low doses first order kinetics occur• As dose increases the half-life increases and

elimination is by Michaelis-Menton kinetics• At high doses rate of elimination is dependent on

the maximum rate of the elimination pathway

• Rate of clearance is dependent on the rate of blood flow through the organ

What is meant by flow dependent elimination?

63

Are drugs A or B being eliminated by first or

zero order kinetics?

Why isn’t the curve for Drug B linear?

Time (min)

0 25 50 75 100 125 150Pla

sma

Co

nce

ntr

atio

nµ

g/m

l10.0

1.0

0.1

A

B

1st order

Two compartment distribution

Zero order

64

Two Compartment Model

Time (min)

0 25 50 75 100 125 150Pla

sma

Co

nce

ntr

atio

nµ

g/m

l

10.0

1.0

0.1

One compartment

TwoCompartment

1st Order

What is determines the two phases of the curve for Drug B? Alpha-slope is due to redistribution of the drug from the vessel rich compartment to other compartments. Beta-phase is due to metabolism and/or excretion

65

Two Compartment Model for Drugs Acting on the CNS

• Note: The more lipid soluble the drug the faster the rate of on set of action– More lipid soluble the faster the drug crosses

the blood brain barrier• Note: `The more lipid soluble the drug the

shorter the duration of action– The faster the redistribution phenomenon

66

How do you calculate the maintenance dose for IV administration?

Dosing rate (g/min/kg) = CL X Css

How do you calculate loading dose?

LD = Vd x Css

How do you calculate the oral dose?

Dose =

(CLp (ml/min) x Cpavg (g/ml)) x Dosing interval (Hr)F

67

How do decreases in body weight affect the following parameters?

• t1/2

• Vd

• CL

• Loading dose

• Oral dose

• IV maintenance dose

No change

Decrease

Decrease

Decrease

Decrease

Decrease

68

Given the following properties of a drug for a 70 kg man:

Bioavailability = 0.8CL = 3.5 L/hrHalf-life = 8 hoursEffective plasma concentration = 1 g/ml

What is the Vd for a 70 kg male?

0.7 x Vd

t1/2

CL =

Vd =CL x t1/2

0.7

Vd = 3.5 L/hr x 8 hr

0.7

Vd = 40 L

69

Given the following properties of a drug for a 70 kg man:

Bioavailability = 0.8CL = 3.5 L/hrHalf-life = 8 hoursVd = 40 LEffective plasma concentration = 1 g/ml

What is the t1/2 for an individual whose CL is ½ the normal rate?

0.7 x Vd

t1/2

CL =

t1/2 = 0.7 x 40 L1.75 L/hr

t1/2 = 16 hours

=0.7 x Vd

CLt1/2

NOTE: If CL is decreased by 50%, t1/2 will increase by 50%

70

Given the following properties of a drug for a 70 kg man:

Bioavailability = 0.8Vd = 40 LCL = 3.5 L/hrHalf-life = 8 hoursEffective plasma concentration= 1 g/ml

What is the loading dose for a 70 kg male?

LD = Vd X Css

LD = 40 L x 1 g/ml

x 1000 ml/L x 1 mg/1000 g

LD = 40 mg

LD = 40 L x 1 mg/L = 40 mg

LD = 40 mg/70 kg = 0.57 mg/kg

71

Given the following properties of a drug for a 70 Kg man:

Bioavailability = 0.8Vd = 40 LCl = 3.5 L/hrHalf-life = 8 hoursEffective plasma concentration= 1 g/ml

What is the dose required for IV maintenance in a 100 kg male?

Dose = CL/70 kg x Css x Body Weight

Dose = 3.5 L/hr x 1 mg/L x (100 kg/70 kg)

Dose = 5 mg/hr

72

Given the following properties of a drug for a 70 kg man:

Bioavailability = 0.8Vd = 40 LCl = 3.5 L/hrHalf-life = 8 hoursEffective plasma concentration= 1 g/ml

What would the oral dose be for a 100 kg if you wanted to give the drug every 8 hours?

CL x Cavg x Dosing interval x body weightF x 70 kg

Dose =

3.5 L/hr x 1 mg/L x 8 hr x 100 kg0.8 x 70 kg

Dose =

Dose = 50 mg every 8 hours

73

8 16 24 32 40 48 56 64 72 80 88 96

20

15

10

25

Time (hrs)

Pla

sma

Co

nce

ntr

atio

n (

mg

/L)

Fluctuations in Plasma Concentration with Dosing Intervals

5