Pharmacokinetic Models
description
Transcript of Pharmacokinetic Models
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Pharmacokinetic Models
One Compartment Model–IV Bolus–Absorption
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One Compartment Model
• Simplest compartmental model• Body is assumed to behave as if it were a
single, well stirred fluid.
I.V. Bolus Dose
-dX/dt = CL•Cp
-d(X/V)/dt = (CL/V)•Cp = -dCp/dt
Cp = Cp,oe-(CL/V)t
v
CL
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Example: Dose = 300 mg, as iv bolusV = 35 L and CL = 2 L/h
Cp,o = Dose/V = 300 mg ÷ 35 L = 8.57 mg/L
I.V. Bolus , 300 mgV = 35 L; CL = 2 L/h
0.00
1.00
2.00
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7.00
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9.00
0 10 20 30 40
TIME [h]
Cp
[mg/
L]
I.V. Bolus , 300 mgV = 35 L; CL = 2 L/h
1.00
10.00
0 10 20 30 40
TIME [h]
Cp
[mg/
L]
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I.V. Bolus , 300 mgV = 35 L; CL = 2 L/h
1.00
10.00
0 10 20 30 40
TIME [h]
Cp
[mg/
L]One Compartment Model
Cp = Cp,oe-(CL/V)t
log Cp = log Cp,o – (CL/V)t/2.3
Slope = -(CL/V)/2.3
-2.3Slope = CL/V = KE
V = Dose ÷ Cp,o
V = 300 mg ÷ 8.56 mg/L = 35 L
CL = KEV
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I.V. Bolus , 300 mgV = 35 L; CL = 2 L/h
1.00
10.00
0 10 20 30 40
TIME [h]
Cp
[mg/
L]One Compartment Model
-2.303 x Slope = KE
Slope = (y2 – y1)/(x2 – x1)
= (log Cp2 – log Cp1)/
( t2 – t1)
= log (Cp2/Cp1)/(t2 – t1)
= log (7.65/1.10)/(2 – 36)
= -0.0248KE = (-2.303)(-0.0248) = 0.0571 h-1
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One Compartment ModelI.V. Bolus , 200 mg
V = 35 L; CL = 2 L/h; F = 0.77
1.00
10.00
0 10 20 30 40TIME [h]
Cp
[mg/
L]
Half Life:
t1/2 = ln 2/KE = 0.693/0.0571 h-1
= 12 h
t1/2 = 0.693 V/CL
= 0.693•35L/2L h-1
= 12 h
CL = KEV = (0.0571 h-1)(35 L)
= 2 L/h
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One Compartment ModelKEY CONCEPT!
• Half Life and KE depend on both CL and V; a change in either CL or V will cause a change in t1/2 and KE.
KE
t1/2
CL
CL
V
V
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Area Under the Curve
• AUC = the area under the Cp,t profile, from time = 0 to time = , usually for a single dose.
• AUCt1-t2 = the area under the curve from time = t1 to time = t2.
EE
ptKpp KV
DoseKC
dteCdtCAUC E
0,
00,
0
CLDoseAUC
AUCDoseCL
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Summary of Wednesday
v
CL
I.V. Bolus Dose Cp = Cp,oe-(CL/V)t
I.V. Bolus , 300 mgV = 35 L; CL = 2 L/h
1.00
10.00
0 10 20 30 40
TIME [h]C
p [m
g/L]
- 2.3 x Slope = CL/V = KE
V = Dose ÷ Cp,o
CL = KEV
KE
t1/2
CL
CL
V
V
CLDoseAUC
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Calculation of AUCTrapezoidal Method, R&T, p.469
Cp
mg/L
1 2 4 8Time [h]
2 3 2.4 1.8
12
21
22
1ttCCAUC ppt
t
1012
201
0
AUC
5.2122
322
1
AUC
4.5242
4.234
2
AUC
4.8482
8.14.28
4
AUC
63.08.18
Elast KCAUC
AUC = 1+2.5+5.4+8.4+6 = 23.3 mg•h/L
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CL for individual pathways
DB
MB
Durine
Murine
Expired air
CLH
CLR
CLP
CL = CLH + CLR + CLP
CLDoseMCL u
H
CLDoseXCL u
R
CLDoseXCL P
P
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One Compartment Model
• A two-fold change in CL:I.V. Bolus , 300 mg
V = 35 L; CL = 2 & 4 L/h
0.00
1.00
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8.00
9.00
0 10 20 30 40
TIME [h]
Cp
[mg/
L]
I.V. Bolus , 300 mgV = 35 L; CL = 2 & 4 L/h
0.10
1.00
10.00
0 10 20 30 40
TIME [h]
Cp
[mg/
L]
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One Compartment Model
• A two-fold change in V:I.V. Bolus , 300 mg
V = 35 & 70 L; CL = 2 L/h
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9.00
0 10 20 30 40
TIME [h]
Cp
[mg/
L]
I.V. Bolus , 300 mgV = 35 & 70 L; CL = 2 L/h
1.00
10.00
0 10 20 30 40
TIME [h]
Cp
[mg/
L]
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One Compartment ModelAbsorption Input
• Drug enters body by a first-order, monoexponential process.
dX/dt = kaXg - CL•Cp
v
CL
ka
tkt
VCL
a
ap
aee
VCLkV
DoseFkC
tktK
Ea
ap
aE eeKkV
DoseFkC
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Absorption
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 10 20 30Time [h]
Cp
[mg/
L]
0.1
1.0
0 10 20 30Time [h]
Cp
[mg/
L]
The slope of the log-linear phase reflects the smaller of ka and KE.
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Absorption
• Shape parameters– Cmax
– Tmax
– AUC– t1/2
0.0
0.1
0.2
0.3
0.4
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0.7
0 10 20 30Time [h]
Cp
[mg/
L]Use shape parameters to deduce changes in PK parameters: ka, CL, V, F
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Tmax
At the peak Cp, dCp/dt = 0
tktK
Ea
ap
aE eeKkV
DoseFkC
0maxmax
TKE
Tka
Ea
ap Ea eKekKkV
DoseFkdt
dC
Ea
Ea
KkKkT
ln
max
vCL
ka
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Cmax
0 pEGap CK
VXk
dtdC
tkG
aeDoseFX
maxmax CKe
VDoseFk
ETka a
Ea
E
a
a KkKk
k
E
a eVKDoseFkC
ln
max
EaE
KkK
a
E
kK
VDoseFC
max
vCL
ka
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AUC
0
dteeKkV
DoseFkdtCAUC tktK
Ea
ap
aE
CLDoseFAUC
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ka
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 10 20 30Time [h]
Cp
[mg/
L]
ka1 1/hr 0.25
ka2 1/hr 0.75
ka
___Cmax
___Tmax
___t1/2
___AUC
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CL
CL
___Cmax
___Tmax
___t1/2
___AUC
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 10 20 30Time [h]
Cp
[mg/
L]
CL1 L/hr 20CL2 L/hr 40
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F•Dose
F•Dose
___Cmax
___Tmax
___t1/2
___AUC
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 10 20 30Time [h]
Cp
[mg/
L]
F1 1F2 0.66
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V
V
___Cmax
___Tmax
___t1/2
___AUC
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.0 10.0 20.0 30.0Time [h]
Cp
[mg/
L]
V1 L 250V2 L 50
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Shape parameters as functions of PK parameters
Ea
Ea
KkKkT
ln
max
EaE
E
KkK
a
ETK
kK
VDoseFe
VDoseFC
/
maxmax
CLDoseFAUC
CL
Vt 2ln2/1
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Peak Shape Analysis
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Peak Shape Analysis
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PK Parameters from single-dose plasma concentration profile
CLDoseFAUC
AUCDose
FCL
Uncertainty in ‘F’ is transmitted to CL. The ratio Dose/AUC gives the true value of CL only when F=1. If F1, then the calculation gives a CL value that is larger than the true value.
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PK Parameters from single-dose plasma concentration profile
tktK
Ea
ap
aE eeKkV
DoseFkC
0.1
1.0
0 10 20 30Time [h]
Cp
[mg/
L]
-2.3 x slope = KE (usually)
FV
KF
CLss
E
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Pharmacokinetic Models
One Compartment ModelAbsorption RateBioavailability
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Absorption Rate
v
CL
ka
tktK
Ea
ap
aE eeKkV
DoseFkC
XG = Dose•e-kat
Chloramphenicol500 mg po
0.00.51.01.52.02.53.03.5
0 10 20 30
Time [h]
Cp
[mg/
L]
How can the value of ka be determined from the Cp,t profile?
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Determination of ka
XGI XB XE
vCL
ka
Chloramphenicol500 mg po
0.00.51.01.52.02.53.03.5
0 10 20 30
Time [h]
Cp
[mg/
L]
Chloramphenicol 500 mg po
0
100
200
300
400
500
0 5 10 15
Time [h]
Xg [m
g]
XGI cannot be measured; ka must come from Cp,t profile
X = amount
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Determination of ka
1. Computer fit of equation using software such as WinNonlin.
vCL
ka
tktK
Ea
ap
aE eeKkV
DoseFkC
2. Graphical analysis; aka “method of residuals”, “feathering”, “peeling”
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Method of Residuals
When ka > 4KE, e-kat goes to 0 before e-KEt does. After e-kat goes to 0:
tK
Ea
ap
EeKkV
DoseFkC
3.2tK
KkVDoseFkLogCLog E
Ea
ap
Chloramphenicol500 mg po
0.1
1.0
10.0
0 10 20 30Time [h]
Cp
[mg/
L]
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Method of ResidualsSubtract the Cp,t profile from the line:
tktK
Ea
ap
aE eeKkV
DoseFkC
tK
Ea
ap
EeKkV
DoseFkC
tktK
Ea
atK
Ea
ap
aEE eeKkV
DoseFkeKkV
DoseFkC
tk
Ea
ap
aeKkV
DoseFkC
Chloramphenicol500 mg po
0.1
1.0
10.0
0 10 20 30Time [h]
Cp
[mg/
L]
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Method of ResidualsWhat if KE > 4ka?
tktK
Ea
ap
aE eeKkV
DoseFkC
Chloramphenicol500 mg po
0.1
1.0
10.0
0 10 20 30Time [h]
Cp
[mg/
L]
tk
Ea
ap
aeKkV
DoseFkC
tktK
Ea
atk
Ea
ap
aEa eeKkV
DoseFke
KkVDoseFk
C
tK
Ea
ap
EeKkV
DoseFkC
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Bioavailability - F
0
dteeKkV
DoseFkdtCAUC tktK
Ea
ap
aE
CLDoseFAUC
std
stdstd
test
testtest
std
test
CLDoseF
CLDoseF
AUCAUC
test
std
std
test
std
test
DoseDose
AUCAUC
FF
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Bioavailability - F
test
std
std
test
std
test
DoseDose
AUCAUC
FF
When AUCstd is from an i.v. dose, Fstd = 1.00 and the “absolute bioavailability” of the test is determined.
When AUCstd is somethingelse such as the innovator’s product or a solution, “relative bioavailability” is determined.