Bioavailability
Metrological aspects
Assessment of parameters
Update: july /211/2006
Bioavailability- 1
Bioavailability
• Bioavailability quantifies the proportion of a
drug that is absorbed and available to
produce its systemic effect
• extent
• rate
Bioavailability- 2
Bioavailability
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- 3
Bioavailability vs. absorption
Bioavailability- 4
Bioavailability vs. absorption
• 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- 5
Bioavailability vs. Bioequivalence
Bioavailability- 6
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- 10
Assessment of drug absorption
and bioavailability
• Physicochemical methods
• experimental determination of
physicochemical parameters to predict
permeability
Bioavailability- 11
Assessment of drug absorption
and bioavailability
• 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- 12
Assessment of drug absorption
and bioavailability
• 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- 13
Assessment of drug absorption
and bioavailability
IN VIVO METHODS
Bioavailability- 15
Bioavailability in man:
prediction from rodents, primates & dogs
From Grass ADDR 2002 pp433
Bioavailability- 16
Main steps for bioavailability
(oral route)
Bioavailability- 17
Main steps for bioavailability
(oral route)
Administered dose
Disintegration
Dissolution
Absorption
in vitro dissolution test
: caco-2, everted sac
Hepatic first pass effect
Pulmonary first pass effect :
Fab%
F%
ex vivo
hepatocytes culture
(intrinsic clearance)
in vivo difference A.V.
Fh%
Fp%
bioavailable fraction of the dose
(reaching the systemic circulation)
Bioavailability- 18
Bioavailability: oral route and first
pass effect
30
Gut
Portal
vein
100
80
fabs=
= 0.8
100
30
60
Lung
80
20
20
fp= 30 = 0.50
60
fh = 60 = 0.75
80
PO
30
60
CD
GG
Heart
arterial circulation : 30
F = fabs x fh x fp = 0.8 x 0.75 x 0.5 = 0.3
30
Bioavailability- 19
First pass effect
(oral route)
Gut
Lumen
Gut Wall
CYP
P-Gp
Portal
vein
Gut Metabolism
To feces
Liver
To site of
measurement
Hepatic Metabolism
Bioavailability- 20
Bioavailability by oral route
F% = fabs x ffirst pass
fabs : absorbed fraction
ffirst pass : fraction escaping the different
first pass effects
Bioavailability- 21
Grapefruit 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- 24
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- 26
Absolute bioavailability and
interindividual variability
125
CV (%)
100
75
50
25
0
0
25
50
75
Hellriegel et al, 1996 Clin. Pharmacol. Ther
100 125 150
F%
Bioavailability- 27
Drawback of a low absolute
availability
Bioavailability
AUC
overexposure of some
animals (side effects)
undesired
concentration
therapeutic
concentration
underexposure
of some animals
(therapeutic failure, resistance)
Dose
1
3
2
Bioavailability- 28
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- 29
Drawback of a low absolute
availability
• Possible interaction with a spurious
increase of bioavailabilty
• The case of felodipine
Bioavailability- 30
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- 31
Bioavailability
Relevance of the rate of absorption
• The 3 AUC are equal
• The rates of absorption are different
CE1
CE2
Bioavailability- 32
The main steps of bioavailability
Importance of the rate of absorption
Administered dose
Disintegration
mean disintegration time
Dissolution
mean dissolution time
Absorption
mean absorption time
First pass effect
(Lung, Liver)
mean disposition time
Arrival in the systemic
circulation
MRToral
Bioavailability- 33
The absolute bioavailability
Bioavailability- 34
Bioavailability
By IV route : 100% by definition
(except if it is a prodrug e.g. Ramipril)
Bioavailability- 35
Corticosteroid preparations
Methylprednisolone (medrol)
CH2 OH
HO
C=O
CH3 ...... OH
CH3
Methylprednisolone
sodium succinate
(solumedrol)
O
CH3
C22H30O5= 374,5
Methylprednisolone
acetate
(depomedrol)
CH2 O - CO - CH3
CH2 - O - CO - (CH2)2 - COONa
HO
C=O
CH3 ...... OH
HO
CH3
CH3
C26H33O8Na = 396,5
O
CH3
C=O
CH3 ...... OH
O
H
H
H
C23H30O6 = 402,5
CH3
Bioavailability- 36
Methylprednisolone (MP) and methylprednisolone
succinate (MPS) disposition after an IV administration of
MPS or MP (4 mg/kg)
105
plasma concentration (ng / ml)
104
103
MP, IV
102
MP after MPS
10
MPS
0
Toutain, J. Pharm. Sci.
60 120
240
360
480 minutes
Bioavailability- 37
How to measure an absolute
bioavailability?
Principle
Dose IV
AUC IV x Cl IV
=
Dose EV
AUC EV x Cl EV
Assumption : Cl IV = Cl EV
F% =
Dose EV
Dose IV
=
AUC EV
AUC IV
x 100
Bioavailability- 38
How to measure an absolute
bioavailability?
• If the doses are equal
AUCEV
F% = AUC x 100
IV
• If IV and EV doses are different
AUCEV
DoseIV
F% =
x
AUCIV
DoseEV
x 100
• Other possible methods (metabolite, urinary data,
in steady state conditions, without IV, …)
Bioavailability- 39
How to measure an absolute
bioavailability?
• Assumption : Cliv = Clev
• crossover design: risk of carryover effect
• induction / inhibition
• appropriate washout (PK and PD)
Bioavailability- 40
Bioavailability estimation by semisimultaneous
drug administration
120
100
80
60
Série1
40
20
0
-20
0
100
200
300
400
500
Karlsson & Breberg; J Pharmacokinet Biopharm 1990 18 pp102
Bioavailability- 41
How to measure an absolute
bioavailability?
• Assumption : Cliv = Clev
• correction by the terminal half-life
F% = AUCEV x t1/2IV x 100
AUCIV
t1/2EV
Warning! : illicit correction if flip-flop
Bioavailability- 44
How to measure an absolute
bioavailability?
With a metabolite :
F% =
AUC EV,metab
AUC IV,métab
x
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- 45
Absolute bioavailability
Using urinary drug concentrations
• Drug itself :

F% =
Xu,EV

Xu,IV
x
Dose IV
Dose EV
x 100
• A metabolite :
Not formed by a first pass effect
Bioavailability- 47
Relative bioavailability
Bioavailability- 56
Relative bioavailability
Single dose
AUCA
F% =
AUCA
AUCB
x 100
AUCB
Bioavailability- 57
Relative bioavailability
Under steady state conditions
Formulation A
(after equilibrium)
Formulation B
(after new equilibrium)
Plasma
AUCA
Condition : linearity and stationarity
AUCB
Time
F% = (AUCA / AUCB)  100
Bioavailability- 58
Bioavailability
Evaluation of AUC
Bioavailability- 59
Bioavailability
Measurement of AUC :
sampling strategy (1)
If the samples are numerous and
appropriately spaced, the AUC is
accurately determined
Bioavailability- 60
Bioavailability
Measurement of AUC :
sampling strategy (2)
Not enough samples in the ascending
phase. The AUC is under-estimated
Bioavailability- 61
Bioavailability
Measurement of AUC :
sampling strategy (3)
Not enough samples in the descending
phase. The AUC is over-estimated
Bioavailability- 62
Bioavailability
Assessment of the rate of absorption
Bioavailability- 63
Bioavailability
Rate of absorption
Cmax et Tmax
Cmax
Tmax
Bioavailability- 64
Are Cmax and Tmax suggestive of
the absorption rate of the drug ?
this can be very misleading
Bioavailability- 65
Bioavailability
Rate of absorption
• Cmax and Tmax are
hybrid parameters
• Cmax
• F%, Ka, K10
• Tmax
• Ka, K10
Bioavailability- 66
Tmax
Monocompartmental model
Tmax =
Ka
Ln Ka - Ln K10
Ka - K10
K10
!
!
Ka varies with bioavailability
flip-flop situation
Bioavailability- 67
Tmax
• Monocompartmental model
1
0
Tmax =
1
0.2
Ln1 - Ln0.2
1 - 0.2
F = 100%
= 2.01 h
0.2
1
Tmax =
Ln2 - Ln0.2
2 - 0.2
= 1.27 h
F = 50%
Bioavailability- 69
Tmax & bioavailabilty
• Tmax is observed more early in case of
low bioavailability ….
Bioavailability- 71
Tmax and Bioavailability of
Cefadroxil in foal
Age
(months)
F%
Tmax
(h)
0.5
1
2
3
5
99.6
67.6
35.1
19.5
14.4
2.1
1.60
1.60
0.96
0.90
Duffee JVPT 1997 20 427
Bioavailability- 72
Tmax and flip-flop situation
Bioavailability- 73
Tmax and flip-flop situation
K10 of A or Ka of B
Ka of A or K10 of B
Tmax = 2.55 h
Drug A : Ka = 1.0
Drug B : Ka = 0.1
K10 = 0.1
K10 = 1.0
t1/2 abs = 0.693 h
t1/2 abs = 6.93 h
Bioavailability- 74
Bioavailability and half-life
In case of flip-flop, bioavailability may influence
the terminal half-life
Bioavailability- 75
What is the meaning of the terminal half-life after
an extravascular drug administration?
Half-absorption or half-elimination ?
• a rate-limited absorption
(flip-flop) must be recognized
(C)
100
10
EV: rate of absorption
IV
EV: rate of elimination
1
0.1
0
5
10
15
20
25
30
time
Bioavailability- 77
Terminal half-life and the flip-flop case
Slow process of absorption
(ng/ml)
K12
Ka1
K21
Ka2
negligible
100
K10
Ka=Ka1+Ka2 # Ka1 = flip-flop
10
elimination
1
Ka1
F% =
 100%
Ka1 + Ka2
0.1
0
5
10
15
20
25
30
Time
Bioavailability- 78
Terminal half-life and the flip-flop case
K12
Ka1
K21
K10
Ka2
(+++)
(ng/ml) 100
Ka1
F% =
= low
Ka1 + Ka2
Lack of flip-flop due to low
bioavailability not to an increase of
the rate of absorption (Ka1)
10
1
elimination
Ka=Ka1+Ka2 # Ka2
0.1
0
5
10
15
20
25
30
Time
Bioavailability- 79
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- 81
The true meaning of Ka
Central compartment
Site of administration
100
100
Ka1+Ka2
Ka1+Ka2
1
Ka1
absorption
0.1
10
Concentrations
Amount
10
0.1
0.01
0.001
0.001
2
4
6
Time (h)
Ka2
Irreversible loss of drug from
the injection site
Ka1+Ka2
Ka1+Ka2
1
0.01
0
K10
0
12
6
Time (h)
K10
Elimination from the
central compartment
Bioavailability- 82
How to evaluate the rate
of absorption
Bioavailability- 84
How to evaluate Ka accurately
1- Directly from the ascending phase
2- By peeling method
3- Wagner-Nelson, Loo-Riegelman
(deconvolution)
4- Statistical moments
Bioavailability- 85