pharmacokinetics/ toxicokinetics - university of … 442 pk...mass balance tissue ......

K.L.R. BrouwerPrinciples of Toxicokinetics/Toxicodynamics

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1

Biochemical and Molecular ToxicologyENVR 442/TOXC 442/BIOC 442

Principles of Toxicokinetics/Toxicodynanics

Kim L.R. Brouwer, PharmD, PhD

[email protected]; 919-962-7030

Pharmacokinetics/

Toxicokinetics:

the study of the time course of

xenobiotic absorption,

distribution, metabolism and

excretion

mailto:[email protected]


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ADME

Absorption

– how the xenobiotic enters the body

Distribution

– where the xenobiotic goes in the body

Metabolism

– what the body does to the xenobiotic

Elimination

– how the xenobiotic is removed from the body

Pharmacodynamics/

Toxicodynamics:

the relationship between

xenobiotic concentration at the

site of action and the resulting

effect, including the time course

and intensity of therapeutic and

adverse effects


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Pharmacokinetics/ Pharmacodynamics/

Toxicokinetics Toxicodynamics

Dosage

Regimen/

Exposure

Plasma

Concen-

tration

Site

of

Action

Effects:Pharmacologic

Toxic

Toxicokinetic/ADME Studies

Pharmacokinetics / Bioavailability

Mass Balance

Tissue Distribution

Metabolite Profile

Plasma Protein Binding

Inhibition / Induction


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Clinical TestingPre-Clinical

Testing

In Vitro

PK / PD

Animal

PK / PD

Toxicology

PK-Guided

Dose

Escalation

Safety

Assessment

Phase 1

Dose/Response

Efficacy

Dose Selection

Patient

Variables

Pop’n PK

In Large

Efficacy

Trials

Special

Populations

Phase 2

Phase 3

Bio-

Equivalence

Generics

Product line

extension

Post

Marketing

Terms used in

Pharmacokinetics/Toxicokinetics:

Cmax

Tmax

t 1/2

AUC

AUMC

Vdss

fu

Cl

MRT

MAT

F


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Cmax

maximum concentration of compound

observed in the matrix of interest

Tmax

time of maximum concentration

100

1000

10000

0 5 10 15 20 25

Cmax

Time

Conce

ntr

atio

n

Tmax - time of maximum concentration


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lambda ()

terminal elimination rate constant

(slope from a semi-log concentration vs time plot)

t 1/2

half-life: the time it takes for the

concentration of the compound to

decrease by 50%

100

1000

10000

0 5 10 15 20 25

- slope =

t12

2

ln

Time

Conce

ntr

atio

n


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The Half-life is

Compound- and Subject-Dependent

Compound Name t1/2 (h)

Rosiglitazone Avandia 3-4

Lamotrigine Lamictal 25-33

Buproprion Wellbutrin 21

Fluticasone Flovent n/a

Paroxetine Paxil 21

Data from LexiComp

Half-life

Half-life is used to:

– Determine the time to reach steady-state (5 x t1/2)

– Determine the time to eliminate all the compound from

the body (5 t1/2)

– Calculate dosing intervals

– Determine how much compound accumulates in the body

given a fixed dosing interval or exposure

Half-life is a secondary pharmacokinetic parameter

that is determined by the volume of distribution (V)

and clearance (Cl) of the compound

Cl

xVt

693.0

21


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AUC

area under the

concentration

vs time curve

ln C

on

cen

trati

on

AUC

Time

tau

ln C

once

ntr

atio

n

Time

AUC 0 = AUC tau

Single Dose Multiple Dose to Steady-State

Time


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Steady-state

At Steady-State:

Rate of input = Rate of elimination

AUMC

area under the

first moment

curve

Tim

e * C

on

cen

trati

on

AUMC

Time


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Mean Residence Time (MRT)

The average time one molecule resides in

the body

MRT = AUMC/AUC

Used to express overall persistence of

compound in the body

Clearance (Cl)

– volume of fluid (usually blood) from which compound is removed completely per unit time

Mathematically:

ClDose

AUC

Organs that may be involved in clearance:» GI tract

» Liver

» Kidney

» Lungs

» Other sites (e.g., blood, skin)


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Clearances (Cl) are Additive

Pharmacokinetic Parameters of

GI158104X in Dog

Dosemg/kg/day

ClIV(mL/min/kg)

Clr(mL/min/kg)

9.5 10 0.7

Clother(mL/min/kg)

9.3


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Linear Pharmacokinetics

Linear Kinetics = Dose-Proportional kinetics

AUC (or concentration) changes

proportionally with dose

DoseA

UC

Avandia Package Insert


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Stationary Pharmacokinetics

Pharmacokinetic parameters are

independent of time

Package insert for Tegretol (carbamazepine)

Because Tegretol induces its own metabolism, the half-life is also

variable. Autoinduction is completed after 3-5 weeks of a fixed dosing

regimen. Initial half-life values range from 25-65 hrs, decreasing to

12-17 hr s on repeated doses. Tegretol is metabolized in the liver.

Volume of Distribution

at Steady-State (Vdss)

a parameter that relates

plasma concentration to

total mass of compound in

the body

Mathematically:

VdDose AUMC

AUCss *

2


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The Volume of Distribution is

Compound- and Subject-Dependent

Compound Name V (L)

Rosiglitazone Avandia 18

Lamotrigine Lamictal 70

Buproprion Wellbutrin 140

Fluticasone Flovent 280

Paroxetine Paxil 560

Data from LexiComp

Fraction Unbound (fu)

– fraction of drug that is not

bound to plasma proteins

– the unbound

concentration is in

equilibrium between the

tissues and blood

Blood

Tissue

Bound

Drug

Unbound

Drug

Bound

Drug

Unbound

Drug


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Protein-Binding Changes May be Clinically

Relevant for the Following Types of

Compounds and Routes of AdministrationHigh Extraction

Ratio

Low Extraction

Ratio

IV Administration

Hepatic Clearance Yes No

Nonhepatic Clearance Yes No

Oral Administration

Hepatic Clearance No No

Nonhepatic Clearance Yes† No

† No drugs from a list of 456 met these criteria

Benet and Hoener, Clin Pharmacol Ther 71:115, 2002

Organ Extraction (E)

Clearing

OrganCin Cout

Q Q

Drug Elimination

E =Cin - Cout

Cin


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When Is Protein Binding

Important?

In vitro ADME/preclinical toxicology studies

Scaling pharmacokinetic/pharmacodynamic parameters from animal models to humans

Calculating human doses from in vitromeasures of target concentrations

Therapeutic drug monitoring of blood/plasma concentrations for drugs with a narrow therapeutic index

Bioavailability (F)

fraction of the administered

dose that reaches the systemic

circulation intact

0 < F < 1

Mathematically:

FAUC

AUC

Dose

Dose

PO

IV

IV

PO

*


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Relative Bioavailability

fraction of the dose of a test product

that reaches the systemic circulation

relative to a reference product

Mathematically:

FAUC

testAUC

ref

Doseref

Dosetest

*

Bioavailability is an important

determinant of pharmacologic

response and therapeutic

effectiveness


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Estimation of Oral Bioavailability (F)

After an IV and Oral Dose of

Compound X

Dosemg/kg/day

AUCIV1

(g*hr/mL)

AUCPO1

(g*hr/mL)

5.0 6.25 5.94

25.0 32.4

1 mean, n=4

8.15

F

0.25

0.95

Factors Responsible for Poor

or Variable Oral Bioavailability

Physicochemical Factors

Physiologic Factors


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Influence of Formulation Factors

on the Concentration-Time Profile

Formulations with

different release rates

(absorption kinetics)

Time, h

0 4 8 12 16 20 24 28 32 36Ro

sig

lita

zo

ne P

lasm

a C

on

cen

trati

on

, n

g/m

L

0

50

100

150

200

CC CCCC

C

C

C

C

C

C

C

C

C

CDDDDD

D

D

D

D

D

D

D

D

D

D

D

EE

EE

E

E

E

E

E

E

E

E

E

E

E

E

EFFF

F

F

F

F F F

F

FF

F

F

F

F

GG

G

G

G

G

G

G

GG

G

G

G

G

G

G

H

H

H

H

H

H

HH H

H

H

H

H

H

H

H

Formulation #1CFormulation #2DFormulation #3EFormulation #4FFormulation #5GFormulation #6H

Influence of Formulation on the

Bioavailability of a Compound

Bioequivalence

studies are

performed to select

the formulation

which gives the

greatest oral

exposure to the drug

Serum Concentration vs Time Profile

0

50

100

150

200

250

0 5 10 15

Time (hours)

Co

nc

en

tra

tio

n (

ng

/mL

)

Form. A

Form. B


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Special Studies:

Effects of Food on Exposure

Often the fed/fasted

state of the animal can

influence the

absorption of drugs

Food can enhance or

inhibit absorption

Can impact on clinical

dosing0

500

1000

1500

2000

2500

3000

3500

4000

4500

Fed Fasted

AUC (inf)(ng*hr/mL)

Physiologic Factors Responsible

for Poor or Variable Oral

Bioavailability

Cell Membranes

pH, Volume and Content of GI Fluid

GI Motility

Vascularity and Blood Flow

Enzymatic Degradation / Metabolism

Disease States


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The Gastrointestinal Tract

Presystemic Elimination: (First-Pass Effect)Loss or metabolism of drug between the

administration site and the sampling site


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Liver

Portal VeinGut

Lumen

Gut

Wall

Systemic

Circulation

Metabolism

Feces

Factors Affecting Systemic Availability

Solubility

Chemical Stability

Permeability

Mucosal Metabolism

Transport/Efflux

Protein Binding

RBC Partitioning

Blood/Plasma Stability

Metabolism

Biliary Excretion

First-pass Intestinal and Hepatic

Metabolism of Cyclosporin

Benet et al., J Controlled Release 39:139, 1996

14%51% 8%

27%


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In Vivo Pharmacokinetic

and Bioavailability Studies Resulting Information:

– pharmacokinetic parameters (t1/2, AUC, Vdss,

Cl, F) in the species used for the multiple dose

toxicology studies

– rate and extent of drug absorption

– primary routes of elimination

Utility:

– identifies bioavailability problems

– parameters are used in scaling to humans to

predict the initial human dose

Additional Studies for Compounds

That Exhibit Poor Bioavailability

In Vivo Studies to Assess Sites of

Presystemic Elimination:


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Liver

Portal Vein

Systemic

Circulation

Assessing Sites of Presystemic

Elimination

Duodenum

Jejunum

& Ileum

Dose

DoseStomach

Dose

Additional Studies for Compounds

That Exhibit Poor Bioavailability

In Vivo Portal Vein Administration Studies:

– If FPV approximates 100%, presystemic

elimination occurs prior to the portal vein (gut

lumen or gut wall)

– If FPV approximates FPO, presystemic

elimination occurs primarily in the liver

PV

IV

IV

PVPV Dose

DoseAUCAUCF *


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Liver

Portal Vein

Systemic

Circulation

Assessing Sites of Presystemic

Elimination

Duodenum

Jejunum

& Ileum

Dose

DoseStomach

Dose

Dose

Case Study: Assessing Sites of

Presystemic Elimination

Route of

Administration

AUC

(g*hr/ml)

Bioavailability

(%)

Oral 972 21

Intraduodenal 1378 30

Portal Vein 4386 94

Intravenous 4653 100


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In Situ and In Vitro Models to Assess

Sites of Presystemic Elimination

Models to Assess Presystemic Elimination Prior to

the Portal Vein

– Isolated Perfused Intestinal Segments

– Everted Gut Sacs

– Intestinal Rings

– Cell Monolayers

Models to Assess Hepatic Presystemic

Elimination

– Isolated Perfused Liver

– Liver Slices

– Hepatocytes (freshly isolated/cultured)

Toxicokinetics

Dose Selection

Dose Proportionality

Multiple Dose Pharmacokinetics

Induction/Inhibition

Exposure Verification


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Toxicokinetics: Dose Selection

Determination of the optimum dose

– primarily focused on the relationship between

dose and exposure

» linearity

» saturation

Cmax and AUC Following Escalating

Doses of Compound A

Dosemg/kg/day

Cmax 1

(ug/mL)

AUC 1

(ug*hr/mL)

5.0 1.40 6.25

25.0 9.47 32.4

100.0 10.56 47.2

1 mean, n=4


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Dose Selection

0

2000

4000

6000

8000

10000

12000

0 100 200 300 400

Dose (mg/kg/day)

AUC

(ng*h/mL)

Rat

Dog

Saturation of Absorption

0

5

10

15

20

25

30

35

40

25 500 10 500

Dose (mg/kg)

Bio

avail

ab

ilit

y (

%)

Mouse Dog


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Toxicokinetics: Dose Proportionality

A = AUC increases

proportional to dose.

B = Less than

proportional increase in

exposure: Absorption-

limited exposure.

C = Greater than

proportional increase in

exposure: Capacity-

limited elimination0

200

400

600

800

1000

1200

1400

1600

0 50 100 150 200 250 300

Dose (mg/kg)

AU

C h

r*µ

g/m

L

A

B

C

Interpretation of results demonstrating lower toxicity at higher doses

depends on whether compound exhibits dose-proportional kinetics.

Toxicokinetics:

Multiple Dose Pharmacokinetics Determine how drug distribution is altered after

multiple dosing

– Examine changes in clearance, half-life, accumulation,

linearity

– Assure dose related continuous exposure of the animals

to the test compound

Evaluate:

– Toxicokinetic changes with dose and time

– Effect of advancing age of the animals

– Decreases in lean body mass, total body water, cardiac

output, renal, hepatic and GI tract blood flow


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Terminal t1/2 (mean and SD) on

Days 1 and 30 During a Rat 1-Month

Oral Multiple Dosing Study

Males Females

Half-life (hr) Half-life (hr)

Dose Day 1 Day 30 Dose Day 1 Day 30

(mg/kg/day) (mg/kg/day)

5 2.94 2.31 5 2.15 1.72

0.86 0.29 0.10 0.09

25 2.18 2.06 25 2.45 1.99

0.30 0.25 1.07 0.39

150 15.33 2.14 150 9.76 2.72

3.32 0.14 4.54 0.71

Effect of Age on the Toxicokinetics of

N-Acetylprocainamide HCl After IV

Administration of 100 mg/kg in RatsAge

Parameters

Half-life(hr)

C (0)(ug/mL)

AUC(ug*hr/mL)

Cl(L/hr/kg)

Vd(L/kg)

13 WeeksMean

1.66

19.0

45.3

2.01

4.75

(n=7)CV%

12.1

12.1

18.2

14.8

11.2

26 WeeksMean

1.82

27.7

72.6

1.29

3.35

(n=11)CV%

14.2

21.6

27.0

23.5

22.0

52 WeeksMean

2.29

46.9

156.0

0.664

1.98

(n=7)CV%

42.3

21.8

51.4

71.4

24.9

From: S. Garattini, Drug Metabolism Reviews 13, (3), 1982


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Time (Exposure) Dependence

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25

Time

Inhibit ion

Normal

Induct ion

1

10

100

0 5 10 15 20 25

Time

Inhibit ion

Normal

Induct ion

Induction and Inhibition of

metabolism as examples of

elimination changes

Induction or Inhibition

may occur without

affecting the overall

disposition of the test

compound

Exposure Verification -example

Continuous IV infusion for 14 days in rats

and dogs

Infusion solution at pH = 5

Blood samples taken at various times to

document exposure

Pilot (24 hr) study in both species


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Exposure Verification

1

10

100

1000

10000

0 50 100 150 200

Con

cen

trati

on

(n

g/m

L)

Time (h)

Males

Females

Expected

Special Toxicokinetic Issues

Toxicity / Potency Evaluations

Metabolic Sites

Correlation of Residual Drug with Toxicity

Species Selection

Gender Effects

Placental Transfer

Milk Transfer


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Coverage for

Human Metabolites

From: Ferrero, J.L. and Brendel, K. Advances in

Pharmacology V43, pp131-169, 1997.

Species Selection

confirmation that the

species chosen for

oncogenicity studies

supports the human

metabolism

is there coverage in the

animal species for all

metabolites generated in

human systems

Cumulative

Percent of

Radioactive Dose

in Urine and

Feces of Male

Cynomolgus

Monkeys After a

Single IV Dose of

[14C]GW280430

Collection Percent of Radioactive Dose

Interval Animal Number

(Hours) I04646 I04647 I04650 I04651 I04652 I04653 Mean SD

Urine

0-2 14.5 4.16 25.1 20.3 0.01 ND 10.7 10.80-6 42.6 31.2 40.4 34.0 32.3 10.0 31.8 11.6

0-12 43.3 35.2 45.0 38.6 34.2 10.0 34.4 12.70-24 46.2 37.7 49.1 39.7 35.7 27.8 39.4 7.640-48 48.1 40.0 51.0 42.7 37.4 32.7 42.0 6.780-72 49.2 41.0 51.8 44.1 38.6 35.1 43.3 6.360-96 50.0 42.1 52.4 45.1 39.6 36.3 44.2 6.160-120 50.8 42.9 52.9 45.8 40.4 37.1 45.0 6.050-144 51.9 43.6 53.1 46.5 41.4 37.6 45.7 6.030-168 52.3 43.9 53.5 46.7 41.8 37.9 46.0 6.05

Feces

0-24 15.5 20.5 10.7 7.78 20.8 12.8 14.7 5.290-48 22.2 31.0 28.4 30.8 26.1 30.1 28.1 3.410-72 23.6 32.8 31.2 36.3 28.5 33.2 30.9 4.410-96 23.9 33.4 31.6 37.0 29.2 33.9 31.5 4.510-120 30.6 33.8 32.0 37.5 29.6 34.4 33.0 2.860-144 30.8 34.2 32.3 37.9 30.1 34.7 33.3 2.900-168 31.7 34.5 32.5 38.1 30.3 34.9 33.6 2.78

ND Not detectable.

SD Standard deviation.


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Antiviral

Alone

Antiviral +

GF120918 CSFBrain

CSF

Brain

Blood

Ratio

Antiviral Alone

Antiviral + GF120918

Brain/Blood

CSF/Blood

0.35

0.86

0.06

0.45

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

Ratio

Brain/Blood

CSF/Blood

Antiviral Alone

Antiviral + GF120918

Brain/Blood

CSF/Blood

0.35

0.86

0.06

0.45

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

Ratio

Brain/Blood

CSF/Blood

Brain/Blood

CSF/Blood

Use of WBA to Assess Effect of GF120918

on Antiviral Distribution in Rats

Courtesy of Glaxo, Inc.

Toxicokinetic Modeling

Classic Compartmental Modeling

– 1-Compartment Model

– Multi-compartment Model

Physiologic Compartmental Modeling

– Perfusion-Limited Compartments

– Diffusion-Limited Compartments

– Specialized Compartments


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Disposition of Pafuramidine/Furamidine in Rat

Isolated Perfused Liver (IPL) and Sandwich-Cultured

Hepatocytes (SCH)IPL SCH

0.001

0.01

0.1

1

10

0 8 16 24M

ed

ium

(μ

M)

/ C

ell

s

(nm

ol)

Time (h)

0.001

0.01

0.1

1

10

0 8 16 24

0.001

0.01

0.1

1

10

0 8 16 24

Medium & Cells: n = 3 rats / timepoint

0.001

0.01

0.1

1

10

0 8 16 240.001

0.01

0.1

1

10

0 20 40 60 80 100 120

Furamidine (Perfusate

/ Medium)

Perfusate: n = 3-6 rats / timepoint

Liver: n=1 rat / timepoint

0.001

0.01

0.1

1

10

0 20 40 60 80 100 120

Furamidine (Liver/Cells) M3 (Perfusate/Medium) Pafuramidine (Perfusate/Medium)

Pe

rfu

sa

te (μ

M)

/ L

ive

r (μ

mo

l)

Time (min)

0.001

0.01

0.1

1

10

0 20 40 60 80 100 120

0.001

0.01

0.1

1

10

0 20 40 60 80 100 120

Initial concentration: 10 μM

99%

99%

Yan et al. J Pharmacol Exp Ther, in review, 2010

Time (min) Time (h)

Scheme Describing Disposition of Prodrug and

Metabolite in Isolated Perfused Liver and Sandwich-

Cultured Hepatocytes



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Intrahepatic Binding Markedly Influences

Disposition of Active Metabolite


• Hepatic accumulation of active metabolite was extensive (>95% of total formed)

• Hepatic unbound fraction (fu,L) of furamidine was only 0.3% explaining, at

least in part, low perfusate (systemic) exposure of furamidine.

Schematic Structure of Physiologic Toxico-

kinetic Model for Inhaled Propylene Oxide

Csanády G A , Filser J G

Toxicol. Sci. 2006;95:37-62

pharmacokinetics/ toxicokinetics - university of … 442 pk...mass balance tissue ......

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