Bioavailability- 1 Bioavailability Metrological aspects Assessment of parameters Update: july /211/2006

Bioavailability- 1

BioavailabilityMetrological aspects

Assessment of parameters

BioavailabilityMetrological aspects

Assessment of parameters

Update: july /211/2006

Bioavailability- 2

• Bioavailability quantifies the proportion of a drug that is absorbed and available to produce its systemic effect• extent

• rate

BioavailabilityBioavailability

Bioavailability- 3


Definition• Absolute

• amount of administered drug which enters the systemic (arterial) circulation and the rate at which the drug appears in the blood stream

• Relative• to compare formulations (bioequivalence)• to compare routes of administration

Bioavailability- 4

Bioavailability vs. absorption


Bioavailability- 5



•Absorption: movement of drug from the site of administration into the blood which drains the site of administration

•Bioavailability refers to the amount of drug which actually gains the access to the systemic (arterial) circulation

Bioavailability- 6

Bioavailability vs. Bioequivalence

Bioavailability vs. Bioequivalence

Bioavailability- 10

Assessment of drug absorption and bioavailability


• In silico• models based on molecular structure• many physicochemical parameters

(H. bounding, MW, LogP, pKa, polar surface area) and solubility can be generated automatically from chemical structures

• many software to achieve these measurements

Bioavailability- 11



• Physicochemical methods• experimental determination of

physicochemical parameters to predict permeability

Bioavailability- 12



• In vitro methods: cell based methods

• Caco2 (human colonic cell lines)– drawback 21 days culture / overexpression of P-

gp do not model paracellular passages (water soluble molecules of low MW)

• 3 days culture Caco-2• Madin-Darby canine kidney (MDCK)

– 3 days correlation with Caco-2 culture

• Caco-2 cells engineering to express CYP3A4

Bioavailability- 13



• In vitro methods: tissue based methods • Ussing chamber technique• Everted gut sac• perfused isolated intestinal segment

unlike Caco-2 cells these models possess an apical mucus layer

possibility to study drug transport in combination with intestinal metabolism

Bioavailability- 15



IN VIVO METHODS

Bioavailability- 16

Bioavailability in man: prediction from rodents, primates

& dogs

Bioavailability in man: prediction from rodents, primates

& dogs

From Grass ADDR 2002 pp433

Bioavailability- 17

Main steps for bioavailability

(oral route)


(oral route)

Bioavailability- 18

Administered dose

Disintegration

Dissolution

Absorption

in vitro dissolution test

F%

Fab%

Fh%

Fp%

: caco-2, everted sac ex vivo

Hepatic first pass effect hepatocytes culture(intrinsic clearance)

Pulmonary first pass effect : in vivo difference A.V.

bioavailable fraction of the dose(reaching the systemic circulation)


(oral route)


(oral route)

Bioavailability- 19

100 8060

60

30

30

30

CD GG

arterial circulation : 3030

20

20

fp= = 0.503060

PO

fh = = 0.756080fabs= = 0.8

80100

Portal

vein

Gut

Vena cava

F = fabs x fh x fp = 0.8 x 0.75 x 0.5 = 0.3

Lung

Heart

Bioavailability: oral route and first pass effect

Bioavailability: oral route and first pass effect

Bioavailability- 20

GutLumen

Gut WallPortalvein

To site of measurement

Gut Metabolism Hepatic Metabolism

To feces

First pass effect (oral route)

First pass effect (oral route)

LiverLiverCYPP-Gp

Bioavailability- 21

F% = fabs x ffirst pass

ffirst pass : fraction escaping the different first pass effects

fabs : absorbed fraction

Bioavailability by oral route

Bioavailability by oral route

Bioavailability- 24

Grapefruit Juice FactsGrapefruit Juice Facts

• GJ or G (not OJ) elevates plasma peak drug concentration, not elimination t1/2

• GJ reduced metabolite/parent drug AUC ratio• GJ caused 62% reduction in small bowel

enterocyte 3A4 and 3A5 protein; liver not as markedly effected (i.v. pharmacokinetics unchanged)

• GJ effects last ~24 h, require new enzyme synthesis

• Effect cumulative (up to 5x Cmax) and highly variable among individuals depending upon 3A4 small bowel basal levels

Bioavailability- 26

Reasons for knowing the absolute bioavailability

Reasons for knowing the absolute bioavailability

• To assess a possible major source of therapeutic variability• if mean F% close to 100% : no inter-

individual variability of AUC

• if mean F% is low (eg 10%) : large interindividual variability due to formulation or physiological condition

Bioavailability- 27

Absolute bioavailability and interindividual variability

Absolute bioavailability and interindividual variability

125

0 25 50 75 100 125 1500

25

50

75

100

CV

(%

)

F%Hellriegel et al, 1996 Clin. Pharmacol. Ther

Bioavailability- 28

Drawback of a low absolute availability

Drawback of a low absolute availability

Dose 1 3 2

Bio

availab

ilit

y

undesiredconcentration

therapeuticconcentration

underexposureof some animals(therapeutic failure, resistance)

overexposure of some animals (side effects)AUC

Bioavailability- 29

Why is an intravenous PK study required ?

Why is an intravenous PK study required ?

• To know absolute bioavailability because:• absolute low bioavailability is generally

correlated with a poor intersubject reproducibility

and

• poor intersubject reproducibility generally leads to a more than proportional increase in dose rate to ensure drug efficacy in animals with the lowest bioavailabilty

Bioavailability- 30

Drawback of a low absolute availabilityDrawback of a low

absolute availability

• Possible interaction with a spurious increase of bioavailabilty• The case of felodipine

Bioavailability- 31

Effect of Grapefruit Juice on Felodipine Plasma Concentration

5mg tablet with juice

without

Review- D.G. Bailey, et al.; Br J Clin Pharmacol 1998, 46:101-110

Bioavailability- 32

• The 3 AUC are equal• The rates of absorption are different

CE1

CE2

BioavailabilityRelevance of the rate of

absorption

BioavailabilityRelevance of the rate of

absorption

Bioavailability- 33

Administered dose

Disintegration

Dissolution

Absorption

First pass effect(Lung, Liver)

Arrival in the systemic circulation

mean disintegration time

mean dissolution time

mean absorption time

mean disposition time

MRToral

The main steps of bioavailability

Importance of the rate of absorption

The main steps of bioavailability

Importance of the rate of absorption

Bioavailability- 34

The absolute bioavailabilityThe absolute bioavailability

Bioavailability- 35


By IV route : 100% by

definition(except if it is a prodrug e.g.

Ramipril)

Bioavailability- 36

Corticosteroid preparationsCorticosteroid preparations

Methylprednisolone (medrol)

CH3

Methylprednisolone sodium succinate

(solumedrol)

Methylprednisolone acetate

(depomedrol)

HOCH3

C = O

CH2 O - CO - CH3

...... OH

O

CH3

CH3

C23H30O6 = 402,5H H

H

HOCH3

C = O

CH2 OH

...... OH

O

CH3

CH3

C22H30O5= 374,5

HOCH3

C = O

CH2 - O - CO - (CH2)2 - COONa

...... OH

O

CH3

C26H33O8Na = 396,5

Bioavailability- 37

Methylprednisolone (MP) and methylprednisolone succinate (MPS)

disposition after an IV administration of MPS or MP (4 mg/kg)

Methylprednisolone (MP) and methylprednisolone succinate (MPS)

disposition after an IV administration of MPS or MP (4 mg/kg)

Toutain, J. Pharm. Sci.480 minutes0 60120 240 360

102

103

105

104

10

plasma concentration (ng / ml)

MP, IV

MPS

MP after MPS

Bioavailability- 38

PrincipleDose IV AUC IV x Cl IV

Dose EV AUC EV x Cl EV

Assumption : Cl IV = Cl EV

F% = = x 100Dose EV

Dose IV

AUC EV

AUC IV

How to measure an absolute bioavailability?


=

Bioavailability- 39



• If the doses are equal

F% = x 100

• If IV and EV doses are different

F% = x x 100

• Other possible methods (metabolite, urinary data, in steady state conditions, without IV, …)

AUCEV DoseIV

AUCIV DoseEV

AUCEV

AUCIV

Bioavailability- 40



• Assumption : Cliv = Clev

• crossover design: risk of carryover effect

• induction / inhibition

•appropriate washout (PK and PD)

Bioavailability- 41

Bioavailability estimation by semisimultaneous drug

administration

Bioavailability estimation by semisimultaneous drug

administration

-20

0

20

40

60

80

100

120

0 100 200 300 400 500

Série1

Karlsson & Breberg; J Pharmacokinet Biopharm 1990 18 pp102

Bioavailability- 44



• Assumption : Cliv = Clev

• correction by the terminal half-life AUCEV

AUCIV

F% = x x 100t1/2IV

t1/2EV

Warning! : illicit correction if flip-flop

Bioavailability- 45

With a metabolite :

F% = x AUC EV,metab

AUC IV,métab

Dose IV

Dose EV

x 100

N.B.1 : the metabolite should not be formed at the administration site or by a first-pass effect

N.B.2 : note 1 does not hold for a relative bioavailability



Bioavailability- 47

Using urinary drug concentrations• Drug itself :

F% = x Xu,EV

Xu,IV

Dose IV

Dose EV

x 100

•A metabolite : Not formed by a first pass effect

Absolute bioavailabilityAbsolute bioavailability

Bioavailability- 56

Relative bioavailabilityRelative bioavailability

Bioavailability- 57

Single dose

F% = AUCA

AUCB

x 100AUCA

AUCB


Bioavailability- 58

Under steady state conditionsFormulation A Formulation B(after equilibrium) (after new equilibrium)

Time

Plasma

AUCA AUCB

Condition : linearity and stationarity


F% = (AUCA / AUCB) 100

Bioavailability- 59

Evaluation of AUC


Bioavailability- 60

If the samples are numerous and appropriately spaced, the AUC is accurately determined

Measurement of AUC : sampling strategy (1)


Bioavailability- 61

Not enough samples in the ascending phase. The AUC is under-estimated



Bioavailability- 62

Not enough samples in the descending phase. The AUC is over-estimated



Bioavailability- 63

BioavailabilityAssessment of the rate of

absorption

BioavailabilityAssessment of the rate of

absorption

Bioavailability- 64

Cmax et Tmax

Cmax

Tmax

BioavailabilityRate of absorption


Bioavailability- 65

Are Cmax and Tmax suggestive of the

absorption rate of the drug ?

Are Cmax and Tmax suggestive of the

absorption rate of the drug ?

this can be very misleading

Bioavailability- 66

•Cmax and Tmax are hybrid parameters

•Cmax •F%, Ka, K10

•Tmax•Ka, K10



Bioavailability- 67

Monocompartmental model

Tmax = Ln Ka - Ln K10

Ka - K10

Ka varies with bioavailability!

! flip-flop situation

K10

Ka

TmaxTmax

Bioavailability- 69

• Monocompartmental model

1 1

0 0.2 1

Tmax = = 2.01 hLn1 - Ln0.2

1 - 0.2Tmax = = 1.27 h

Ln2 - Ln0.2

2 - 0.2

F = 100% F = 50%

0.2

TmaxTmax

Bioavailability- 71

Tmax & bioavailabiltyTmax & bioavailabilty

• Tmax is observed more early in case of low bioavailability ….

Bioavailability- 72

Tmax and Bioavailability of Cefadroxil in foal

Tmax and Bioavailability of Cefadroxil in foal

Age(month

s)

0.5 1 2 3 5

F% 99.6 67.6 35.1 19.5 14.4

Tmax(h)

2.1 1.60 1.60 0.96 0.90

Duffee JVPT 1997 20 427

Bioavailability- 73

Tmax and flip-flop situation


Bioavailability- 74

Drug A : Ka = 1.0 K10 = 0.1 t1/2 abs = 0.693 h

Drug B : Ka = 0.1 K10 = 1.0 t1/2 abs = 6.93 h

Tmax = 2.55 h

K10 of A or Ka of B

Ka of A or K10 of B



Bioavailability- 75

Bioavailability and half-lifeBioavailability and half-life

In case of flip-flop, bioavailability may influence the terminal half-life

Bioavailability- 77

Half-absorption or half-elimination ?

• a rate-limited absorption (flip-flop) must be recognized

EV: rate of elimination

EV: rate of absorption

time

(C)

IV

What is the meaning of the terminal half-life after an extravascular drug

administration?

What is the meaning of the terminal half-life after an extravascular drug

administration?

0.1

1

10

100

0 5 10 15 20 25 30

Bioavailability- 78

(ng/ml)

Terminal half-life and the flip-flop case


F% = 100%Ka1Ka1 + Ka2

Ka1

Ka2

negligible

K12

K21

K10

Time

Ka=Ka1+Ka2 # Ka1 = flip-flop

0.1

1

10

100

0 5 10 15 20 25 30

Slow process of absorption

elimination

Bioavailability- 79

(ng/ml)



F% = = lowKa1

Ka1 + Ka2

Ka1

Ka2

(+++)

K12

K21

K10

Time0.1

1

10

100

0 5 10 15 20 25 30

Ka=Ka1+Ka2 # Ka2elimination

Lack of flip-flop due to low bioavailability not to an increase of the rate of absorption (Ka1)

Bioavailability- 81

The true meaning of Ka


• Remember:

• Ka is the apparent first order absorption rate constant derived from plasma data

• This parameter may also contain processes parallel to the true absorption step such as degradation of drug in the administration site

Bioavailability- 82

Central compartment

0.001

0.01

0.1

1

10

100

0 6 12

Time (h)

Con

cent

ratio

ns

K10

Ka1+Ka2Ka1+Ka2



Ka1

absorption

Ka2

Irreversible loss of drug from the injection site

K10

Elimination from the central compartment

Site of administration

0.001

0.01

0.1

1

10

100

0 2 4 6

Time (h)

Am

ount Ka1+Ka2

Ka1+Ka2

Bioavailability- 84

How to evaluate the rate of absorption

How to evaluate the rate of absorption

Bioavailability- 85

1- Directly from the ascending phase

2- By peeling method

3- Wagner-Nelson, Loo-Riegelman (deconvolution)

4- Statistical moments

How to evaluate Ka accurately

How to evaluate Ka accurately

Documents

Bioavailability- 1 Bioavailability Metrological aspects Assessment of parameters Update: july /211/2006