PKPD ACEI London 05 - 1

1
PK/PD and cardiovascular drugsthe case of ACE
inhibitors
NATIONAL VETERINARY S C H O O L T O U L O U S E
P.L. Toutain EAVPT/ECVPT Workshop London, July
2005
2
PK/PD for cardiovascular drugs

• There may be more information on PK/PD
  relationships of cardiovascular drugs than any
  other group of drugs
• Digoxine
• Beta-blockers
• Anti-arrhythmic
• ACE inhibitors
• Why?
• PK/PD relationship easy to establish with
  surrogates

3
Pharmacodynamic biomarker, surrogate and endpoint

• Many biomarkers / surrogates
• Heart rate
• Blood pressure
• Electro-cardiographic parameter (QT)
• Only surrogate
• May be not validated !!

4
ACE inhibitorsPharmacokinetics andPK/PD
See Toutain PL. Lefebvre H.P, JVPT 2004, 27
515-525
5
Goals of heart failure therapy in the symptomatic
patient

• Relieve HF symptoms
• i.e. make patients feel better
• Improve overall clinical status
• Decrease morbidity and mortality
• Slow and/or reverse disease progression
• Identify and treat reversible causes of LV
  dysfunction

6
How do we make heart failure patients live longer?

• With neurohormonal interventions
• ACE inhibitors
• Angiotensin receptor antagonists (in
  ACE-inhibitor intolerant patients)
• Aldosterone antagonists
• Beta-blockers

7
Neurohormonal blockade for the cardiovascular
diseases

• Angiotensin II
• (Renin-Angiotensin System RAS)

• Norepinephrine
• (Sympathetic Nervous System SNS)

RAS Inhibition
?-Blockade
Disease Progression
8
Angiotensin converting enzyme inhibitors (ACEI)

• Drugs considered as first-line treatment for
  several cardiovascular and renal conditions

Rev Toutain PL. Lefebvre H.P, JVPT 2004, 27
515-525
9
ACE inhibitors

• Enalapril Enacard Merial
• Benazepril Fortekor Novartis
• Ramipril Vasotop Intervet
• Imidapril Prilium Vetoquinol
• Are coboxylalkyl di- or tri-peptide drugs

10
Major effects of Angiotensin II
11
Circulating and local (tissue) RAS influence on
the cardiovascular system
Local RAS Long-term effects
Circulating RAS Short-term effects
Intraglomerular hypertension
Sodium/water reabsorption via aldosterone
secretion
ANGIOTENSIN II
Vasoconstriction
Vascular hypertrophy
Positive chronotropic effects/ arrhythmogenic
effects
Myocardial hypertrophy
Heart
Heart
12
Angiotensin II
Altered peripheral resistance
Altered renal function
Altered cardiovascular structure

• Increase Na reabsorption
• release of aldosterone
• ...

• Increase production of growth factors
• increase synthesis of extracellular matrix

• Vasoconstriction
• Release of CA
• ...

Goodman Gillman, p.741
13
The renin-angiotensin system
Angiotensinogen (a-globulin) from the liver
Renin (released by kidney (juxtaglomerular cells)
Angiotensin I
ACE or Angiotensin Converting Enzyme (Kininase II)
Angiotensin II
Aminopeptidases
Angiotensin III
14
ACEI
Angiotensin I
Bradykinin
ACE inhibitor
Angiotensin II
AT1-blocker
Inactive metabolites
AT1Receptor
AT2Receptor
Vasodilation Natriuresis Extracellular matrix
degradation Cough Angioedema
Hypertension Aldosterone Transforming growth
factor ? Plasminogen activator
Vasodilatation Natriuresis Antiproliferative
effects
15
The non-conventional PK of ACR inhibitors
16
The non-conventional PK of ACE-inhibitors

• A long or very long terminal half-life is
  calculated predicting an accumulation of ACE
  inhibitors during multiple dosing

however
thus
t1/2 20 h
Accumulation (predicted)
No accumulation (observed)
Time (days)
17
The unconventional PK/PD relationship for
ACE-inhibitors

• It has been claimed that there is no relation
  between plasma ACE inhibitor concentration and
  effect !
• Usefulness of PK and PK/PD information has been
  questioned
• This is a misconception due to the
• complexities of ACE inhibitors disposition (PK)
• non-linear nature of the concentration/effect
  relationship (PK/PD)

18
ACEI PK and PK/PD issues

• oral administration requirement for prodrugs
• bioconversion prodrug ? drug
• interpretation of disposition curve
• PK/PD relationship
• dosage regimen (selection)
• dosage regimen adjustment
• renal and hepatic failure

19
Administration / Absorption of ACE inhibitors
20
ACEI administration

• Long term therapy (months, years)
• Only the oral route is convenient
• Only drugs having an appropriate bioavailability
  are usable

21
Prodrug vs. drug for ACEI administration

• Benazeprilat, enalaprilat, imidaprilat and
  ramiprilat are diacid ACE inhibitors with very
  poor intestinal absorption
• Prodrugs were developed to circumvent this poor
  absorption
• Benazepril, Enalapril, Ramipril, imidapril
• Ester derivatives more lipophilic, but no
  activity

22
Imidapril / imidaprilat
Imidapril
COOH
CH3
COOC2H5
(s)
N
N
(s)
(s)
H3C
N H
Imidaprilat (active metabolite)
O
O

• Higher lipophilicity (passive absorption)
• Transported by peptide transporters (active
  absorption)

COOH
COOH
CH3
(s)
N
N
(s)
(s)
H3C
N H
O
O
23
ACE inhibitor absorption

• ACEI are lipophilic carboxyalkyl di- or
  tri-peptides

PEPT (I II) Peptide carrier-mediated
transporter Physical carrier for peptides and
other drugs (betalactams)
lipophilic
Passive
Active
The relative contribution of the 2 routes of
absorption is unknown
24
The question of ACE-inhibitorsBioavailability
25
Bioavailability Benazepril vs Benazeprilat

• From radioactivity excreted in urine
• Benazepril 38
• Benazeprilat 4

Waldmeier and Schmid, Drug Res. 1989, 39, 62
26
How to estimate bioavailability

• Classical approach
• F x 100

AUCoral AUCIV
IV
concentration
oral
Time
27
How to calculate an absolute bioavailability
AUCoral AUCIV

• F x 100 Eq.1
• Assumption for Eq.1
• Clearance,oral Clearance,IV i.e.
• F x x 100

AUCoral AUCIV
Cloral ClIV
28
How to calculate an absolute bioavailability

• In the case of ACE inhibitors
• Clearance,IV ? Clearance,oral
• Consequently equation
• F x 100
• is not applicable

AUCoral AUCIV
29
How to evaluate the absolute bioavailability for
ACE inhibitors

• F x 100
• The equation is applicable because
• Cl oral, free concentration Cl IV, free
  concentration

AUCoral, free concentration AUCIV, free
concentration
30
How to evaluate the absolute bioavailability for
ACE inhibitors

• AUC free concentration can be determined either
• by direct measurement
• by modelisation of ACEI disposition
• e.g. Benazeprilat

31
Bioavailability of ACEI

• Benazeprilat
• single dose, dog 2.57 1.23
• multiple dose, dog 3.95 0.83
• Single dose cat 2.5
• Ramiprilat dog 6.7

32
Conversion of the ACE prodrug to its active moiety
33
Imidapril / Imidaprilat
25
imidapril
20
15
Concentrations (ng/mL)
10
Imidaprilat
5
0
0
3
6
9
12
15
18
21
24
Time (h)
34
Where does hydrolysis occur ?

• Hydrolysis of the active diacid occurs mainly in
  the liver although it may occur in plasma and
  other tissues
• Liver first pass effect
• Other tissues Relevance for the active
  moiety distribution

35
The first-pass effect
36
Bioconversion of Enalapril to Enalaprilat
Blood tissue barrier
Blood tissue barrier
Kidney Enalapril Enalaprilat
Aorta (no conversion)
Low lipophilicity
slow uptake of Enalaprilat
Enalaprilat
Enalapril
(First pass effect)
Liver bioactivation non specific carboxyl esterase
Enalapril Administration
Portal system, carrier system
Enalapril Enalaprilat
Elimination (faeces)
Digestive tract
37
Bioconversion of Fosinipril to Fosiniprilat
Blood tissue barrier
Blood tissue barrier
Kidney Fosinipril Fosiniprilat
Aorta (no conversion)
High lipophilicity
Fosinipril Fosiniprilat
slow uptake of Fosiniprilat
Fosiniprilat
Fosinipril
(First pass effect)
Liver
Fosinipril Administration
Portal system, carrier system
Fosinipril Fosiniprilat
Elimination (faeces)
Digestive tract
38
Prodrug and drug elimination

• Prodrug
• metabolic transformation
• Drug
• Mainly kidney
• Dosage regimen adapted in case of renal failure
• Enalaprilat kidney (95)
• Benazeprilat kidney liver 50

39
ACE inhibitorsPharmacokinetics modelling
See Toutain PL. Lefebvre H.P, JVPT 2004, 27
515-525
40
The non-conventional PK of ACE-inhibitors

• A long or very long terminal half-life is
  calculated predicting an accumulation of ACE
  inhibitors during multiple dosing

however
thus
t1/2 20 h
Accumulation (predicted)
No accumulation (observed)
Time (days)
41
The non conventional disposition profile of ACEI

• Ramiprilat 0.25 mg/kg/day

Day 1 day 8 no accumulation
42
The binding of ACE inhibitors

• Non specific
• Specific to the ACE

43
The non-specific binding of ACE inhibitors

• To albumin
• Benazepril 94
• Benazeprilat 93
• Enalapril lt 60
• Enalaprilat 19

44
The non-specific binding of ACE inhibitors

• Therapeutic meaning
• almost none
• displacement (drug interaction) or decreased
  protein concentration (nephrotic syndrome) are
  unlikely to be relevant
• For the ACE PK modelling
• fraction non specifically bound to albumin will
  be considered as "free" i.e. "free from any
  specific ACE binding"

45
The specific binding to ACE inhibitors
46
The Angiotensin Converting Enzyme (ACE)
47
Protein structure of ACE
Cell membrane
zinc binding sites (catalytic centers)
Vascular endothelium
"N-domain"
"C-domain"
C-terminal hydrophobic tail (transmembrane domain)
extracellular
intracellular
"ectopeptidase"
48
Localisation of ACE

• Tissues
• Everywhere but mainly
• Lung
• Kidney (brush border)
• Endothelium surface
• Plasma (circulating)

49
ACE localisation and distribution
Same binding parameters
ACEI
ACEI
BLOOD
Not measurable by analytical technique
Circulating (soluble) ACE
Measurable by analytical technique
Tissue bound ACE
Vascular endothelium
50
Plasma vs tissular ACE

• Consequence for a physiologically oriented
  kinetic model

Blood
Extracellular fluid
Circulating ACE
I
slow exchange
very rapid exchange
Slow exchange
Bound
I
ACE
I
Instantaneous exchange
I
I
I
Tissue
Albumin
51
The modeling of ACE inhibitors
52
The classical compartmental modelling approach
applied to ACE inhibitors
Ka
distribution
K12
thus
Vc
elimination
K21
K10
C(t) -(Y1 Y2)e-Kat Y1e-?1t Y2e-?2t
absorption / bioconversion
53
Problems encountered with the classical
compartmental modeling approach applied to ACE
inhibitors

• A long or very long terminal half-life is
  calculated predicting an accumulation of ACE
  inhibitors during multiple dosing

however
thus
t1/2 20 h
Accumulation
No accumulation
Time (days)
54
Development of a physiologically based model for
ACE inhibitors
55
Rationale for the development of a
physiologically oriented kinetic model for ACE
Bound to Circulating ACE
Bound to tissular ACE
ACEI
ACEI
Alb
ACEI
free
Bound to alb.
1 compartment model
(simplification by merging events linked by rapid
exchange)
56
ACEI The model
Bound (tissular) ACE Kd, Bmax
(1-fcirc)
Alb
Peripheral compartment
circulating ACE
fcirc
Free
Kd, Bmax
Vc
Volume
measurable concentration
K10
Parameters Cl, Vc, Bmax, Kd, fcirc
57
ACEI disposition
Classical compartmental model
Physiologically based model
Absorption/bioconversion
absorption
Elimination (kidney, hepatic failure)
distribution
Elimination (clearance, VD)
Binding phase (Bmax, Kd, K10)
58
The two phases of ACEI disposition

• Phase influenced by renal / hepatic elimination
  processes
• control drug accumulation and time to reach
  equilibrium
• explains possible overexposure

Concentration

• Phase not influenced by renal and hepatic
  elimination
• Influenced by Bmax, Kd and K10
• control effect on ACE

Time
59
Consequence of drug ACE binding on its
pharmacodynamics

• The long t1/2 reflects the high affinity of the
  drug for the enzyme
• The terminal phase is relevant to the PD
  properties

60
The Benazeprilat disposition IV route

• Vcfree 0.2 L/kg (extracellular water)
• Clfree 3.4 mL/kg/min
• t1/2 free 39 min ??no possible accumulation
• Bmax 119 nmol/L (concentration of ACE in dog)
• fcirc 10.5 (most ACE bound to tissue)
• Kd 4.5 nmol/L (drug affinity)

1
0
0
0
0
1
0
0
0
1
0
0
Concentration (nmol/L)
1
0
1
0
1
2
2
4
3
6
Time (h)
61
The model parameters K10
CE Tissue

• K10 (time-1)
• rate constant of elimination
• of the free fraction
• half-life of elimination 0.693/K10

CE Plasma
F
K10
K10
62
The model parameters clearance

• Clfree parameters
• Clfree K10 x Vc ?
• computation of bioavailability allowed with Cfree
• Cltot variable
• Cltot Dose / AUCtot
• computation of bioavailability not allowed with
  Cltot

Dose AUCtot
Dose AUCfree
63
The ACEI model parameters Bmax

• Bmax maximal capacity (nmol/L)
• assumption 1 molecule of ACE inhibitor binds to
  1 molecule of ACE
• thus Bmax is not a property of the drug but
  of the animal
• Bmax is the same for all ACE inhibitors Bmax is a
  measure of ACE pool

64
The model parameters Bmax
Bmax About 100-200 nmol/L
(f) Circulating enzyme ? 10
(1-f) Tissular enzyme ? 90
expressed as of circulating enzyme
65
The ACEI model parameters Kd

• Kd (nmol/L)
• concentration of the free fraction required to
  saturate half Bmax
• measures the affinity of the drug for ACE (Kd
  1/Ka)
• is related to drug potency
• is a property of the drug

• Benazeprilat 4.5 nmol/L
• Enalaprilat 7.1
• Imidaprilat 5.0
• Ramiprilat 0.51

66
PK consequence of the non-linear ACE inhibitor
disposition

• No dose proportionality
• No possible accumulation but possible
  over-exposure if plasma clearance (free) is
  decreased
• Impossible to calculate properly the
  bioavailability using non-compartmental approach

67
AUC / Effect relationship
Benazeprilat (mg/kg)
4.0
Therapeutically relevant phase (non-linear
binding to ACE)
Dose (mgl/kg) Effect (AUIC) 0.5 18653
1.0 17525 2.0 16747 4.0 16007
Concentration
0.5
Time
68
Consequence of drug ACE binding on its
pharmacodynamics

• The long terminal t1/2 reflects the high
  affinity of the drug for the enzyme
• The terminal phase is relevant to the dynamics

69
PK / PD relationships for the ACE inhibitors
70
Objectives of the PK/PD relationship
Effect ()
Emax efficacy
100
50
Slope (n) (sensitivity)
concentrations
EC50 (potency)
71
Endpoints to investigate for ACE inhibitors

• Blood pressure
• Angiotensin II
• Ex-vivo plasma ACE activity on synthetic
  substrates
• Nussberger et al. Am. Heart.J. 1989, 117, 717

72
The ex-vivo endpoint principle

• ACE inhibitors competitively inhibit the action
  of the ACE (conversion of the inactive AI into
  active AII)
• This inhibitory property can be quantified
  ex-vivo from circulating ACE using an artificial
  substrate

73
The ex-vivo end-point
Synthetic substrate
Hippuryl-glycyl-glycine
ACE
Hippuric acid
End product
(a 11 enzyme interaction)
ACE binding site
ACE inhibitors
74
Imidaprilat PK/PD relationship
0
First administration
32
20
ACE inhibition
24
40
Effect ( inhibition)
16
60
8
80
0
100
24
0
6
12
18
Concentrations (ng/mL)
8th administration
0
32
20
ACE inhibition
24
40
Effect ( inhibition)
16
60
8
80
0
100
168
174
180
186
192
198
204
210
216
Time (h)
75
Dose / exposition / effect relationship for ACEI
76
AUC / Effect relationship

• Conventional drug

4
Effect
4
3
3
Concentration
2
2
1
1
Dose
Time
77
Measured AUC of ACE inhibitor an index of drug
exposure and drug efficacy ?
78
Benazeprilat Dose effect relationship
(simulation)
n 10
Dose (mg/kg) 0.1 0.25 0.5 1.0 2.0
0.1 0.25 0.5 1.0 2.0
Effect
150
120
90
Concentration (nmol/L)
60
30
3
1
2
0
0
1
2
3
Time
Time (day)
79
AUC / Effect relationship
Benazeprilat, control dog
4.0
Therapeutically relevant phase (non-linear
binding to ACE)
Dose (mgl/kg) Effect (AUIC) 0.5 18653
1.0 17525 2.0 16747 4.0 16007
doses
0.5
Time
80
How to characterize properly the exposure-effect
relationship for ACE inibitors

• By PK/PD modelling

81
The ex-vivo endoint
Effect (inhibition)
concentration
Effect, Concentration
effect
Emax
IC50 Cfree
Time
!

• free plasma concentration, not the measured
  plasma concentration
• thus modelling is required to determine Cfree

82
Concentration effect modelling for ACEI

• The inhibitory Emax model
• with
• Emax, the maximum ACE inhibition
• Cfree , the free plasma concentration
• IC50,free , the free plasma concentration
  corresponding to 50 inhibition of the maximum
  activities
• n a slope factor (steepness of the
  concentration effect curve)

Effect
83
Imidaprilat in dog pharmacodynamic parameters
Effect
100
Emax / efficacy100
75
Slope 0.67
50
25
0
0.1
1
10
100
Concentrations (ng/mL)
IC50 2.78 nmol/L (?1 ng/mL) (potency)
84
What is the relationship between the IC50
(ex-vivo) and Kd (in-vivo) ?

• with
• IC50 a measure of drug efficacy (functional
  experiment)
• Kd a measure of drug affinity (binding
  experiment)

85
The use of the PK/PD model for the establishment
of a dosage regimen of ACE inhibitors
86
Benazeprilat (10 control dogs) Dose effect
relationship (simulation)
Dose (mg/kg) 0.1 0.25 0.5 1.0 2.0
Effect
150
120
Concentration (nmol/L)
90
60
30
1
2
3
0
0
1
2
3
Time
Time (day)
87
Benazeprilat dose-effect relationship in cat
88
Benazeprilat dose-effect relationship
0.0625 mg/kg/12h x 12
0.125 mg/kg/24h x 6
0.0313mg/kg/12h x 12
10
10
10
8
8
8
6
6
6
Concentrations (nmol/L)
4
Concentrations (nmol/L)
Concentrations (nmol/L)
4
4
2
2
2
0
0
0
0
100
200
2
4
6
0
0
100
200
Time (h)
Time (day)
Time (h)
20
20
20
15
15
15
Effects
Effects
10
10
Effects
10
5
5
5
0
0
0
0
100
200
0
100
200
6
0
2
4
Time (h)
Time (day)
Time (h)
89
CONCLUSIONS

• ACEI PK is required to understand the drug
• ACEI PK/PD helps to determine optimal dosage
  regimen
• ACEI PK/PD allows to adjust dosage regimen (renal
  or hepatic failure)