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ARTICLE
PFISTER
10.1177/0091270004265365
DOSE
PHARMACOKINETICS
SELECTION:
ET AL
VASOPEPTIDASE
AND PHARMACODYNAMICS
INHIBITOR M100240
Optimizing Dose Selection with Modeling and
Simulation: Application to the Vasopeptidase
Inhibitor M100240
Marc Pfister, MD, FCP, Nancy E. Martin, PharmD,
Lloyd P. Haskell, MD, MBA, and Jeffrey S. Barrett, PhD, FCP
Dual inhibition of neutral endopeptidase 24.11 (NEP) and
angiotensin-converting enzyme (ACE) has gained increasing
interest in the treatment of hypertension, heart failure, and
renoprotection. Specifically, M100240, the thioester of the
dual ACE/NEP inhibitor MDL100,173, has been evaluated in
the management of hypertension. A model-based analysis,
including simulations, was employed to characterize the relationship between individual M100240 drug exposure and
neurohormonal response and to optimize the dose selection
for future clinical studies. Sixty-two healthy subjects and 189
hypertensive patients were studied after oral once-daily administration of 2.5, 5, 10, 25, or 50 mg M100240.
Pharmacokinetic-biomarker and blood pressure response
models were fitted to the data with the computer program
NONMEM. A direct inhibitory Emax model adequately described the relationship between MDL100,173 concentration
and ACE activity. No clear concentration or dose-dependent
A
ngiotensin-converting enzyme (ACE) inhibition is
a well-established treatment in hypertension. Recently, inhibition of neutral endopeptidase 24.11
(NEP) in addition to inhibition of ACE has gained increased interest in the treatment of hypertension,1-4
heart failure,5,6 and renoprotection.7-10 NEP catalyzes
the degradation of a number of endogenous vasodilator
peptides, including α-atrial natriuretic peptides
(ANP), brain natriuretic peptide,11 C-type natriuretic
peptide, adrenomedullin,12 substance P, angiotensinFrom Drug Innovation and Approval, Aventis Pharmaceuticals,
Bridgewater, New Jersey (Dr. Pfister, Dr. Haskell); University of New Jersey
Medical and Dental School, New Brunswick (Dr. Martin); and the Children’s Hospital of Philadelphia, Pennsylvania (Dr. Barrett). Submitted for
publication November 8, 2003; revised version accepted March 15,
2004. Address for reprints: Marc Pfister, MD, FCP, Aventis
Pharmaceuticals, 1041 Route 202-206, Bridgewater, NJ 08807.
DOI: 10.1177/0091270004265365
J Clin Pharmacol 2004;44:621-631
NEP or blood pressure responses were evident. Given a target
90% ACE inhibition, simulation reveals that (1) 50 mg
M100240 once daily produces adequate ACE inhibition 24
hours postdose in only 20% of subjects, and (2) higher and/or
more frequent doses on the order of 25 mg three times daily or
50 mg twice daily are required to achieve the target ACE inhibition in at least 50% of patients over 24 hours. Insufficient
blood pressure–lowering effects were observed in healthy
subjects and hypertensive patients due to inadequate ACE
and NEP inhibition with once-daily oral doses of up to 50 mg
of M100240. Divided doses might provide target ACE inhibition in more patients.
Keywords: Vasopeptidase inhibitor; NEP inhibition; ACE inhibition; angiotensin; hypertension; NONMEM;
modeling; simulation
Journal of Clinical Pharmacology, 2004;44:621-631
©2004 the American College of Clinical Pharmacology
(1-7),13 and bradykinin.14-16 NEP is also involved in the
catabolism of vasoconstrictor peptides, including
endothelin-116-18 and angiotensin II.19 NEP inhibitors,
in contrast to ACE inhibitors, are effective in animal
models of low renin activity and have been found to
produce diuresis and natriuresis in animals and humans.1,2,13 Dual ACE and NEP inhibitors, a new class of
agents termed vasopeptidase inhibitors, may show superior efficacy in the treatment of high blood pressure,
especially in low-renin, salt-sensitive hypertensive individuals in whom ACE inhibitors are less effective.2,13
Besides the antihypertensive effect of dual ACE/NEP
inhibition, vasopeptidase inhibitors might be superior
to ACE inhibitors in improving endothelial function in
cardiovascular diseases.17,18
M100240 is the thioester of the vasopeptidase inhibitor MDL100,173. In vivo, M100240 is rapidly converted to MDL100,173. MDL100,173 was previously
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PFISTER ET AL
reported to be a potent and balanced inhibitor of both
ACE and NEP as the Ki appeared to be in the
subnanomolar range and similar for ACE and NEP
(0.08 and 0.11 nmol/L, respectively).20,21 Results of preclinical studies have shown M100240 to be effective at
lowering blood pressure and protecting the heart from
secondary changes in hypertensive, transgenic
((mRen-2)27) rats.22,23 The terminal half-life and duration of effect observed in these preclinical studies suggested that once-daily administration of M100240 in
humans was possible based on animal extrapolations.
In healthy subjects, M100240 exerted a dosedependent ACE-blocking activity, expressed by the inhibition of the pressor responses to exogenous angiotensin I challenges.24 ACE inhibition,25 together with
increased urinary volume, atrial natriuretic peptide
(ANP), and cyclic guanosine 3′5′-monophosphate
(cGMP) excretion after intravenous administration in
healthy volunteers, suggests that M100240 has dual
ACE/NEP inhibition.26
Dose-biomarker response analyses can be useful in
the establishment of the appropriate regimen and efficacy of most drugs.27 Within the context of ACE inhibitors, it was noted that while differences in structural
factors, pharmacokinetic properties, potencies, and
putative differences in tissue penetration have been
observed, few of these have demonstrated to have major clinical relevance.28 Despite such statements, it has
also been contended that both pharmacokinetic and
pharmacodynamic factors play a role in determining
the optimal dosage regimen of an antihypertensive
agent for the individual patient.29 The incorporation of
modeling and simulation techniques to this problem
offers the promise of establishing linkages between
the fundamental assumptions regarding the assignment of clinical benefit to pharmacokinetic and
pharmacodynamic (e.g., biomarker activity, blood
pressure) outcomes. The extension of such relationships to both candidate selection and the design of
more informative clinical trials is the topic of much research across therapeutic areas. The successful deployment of such an approach in diabetes30 and oncology31
both validates the approach and offers promise for cardiovascular disease targets.32
With respect to ACE inhibition alone, there is reasonable precedence for the pharmacokinetic/
pharmacodynamic (PK/PD) properties, which constitute a potentially effective agent. Generally, a trough to
peak blood pressure ratio of greater than 50% is required to support once-daily dosing (ICH-E1 guidelines). Likewise, the studies required to support a claim
in hypertension would have to encompass 300 to 600
patient exposures for 6 months and at least 100 patients
622 • J Clin Pharmacol 2004;44:621-631
for 1 year, for a total dossier of > 1500 patients evaluated. These targets are less certain for an agent
exhibiting both ACE and NEP inhibition.
In an effort to assess the magnitude and duration of
renin-angiotensin-aldosterone modulation as a function of various doses of M100240 administered in
healthy volunteers, we recently conducted a multipledose, parallel-group study with M100240. Dose selection for the clinical program was based on the following three assumptions: (1) bioavailability (half-life
time) of the oral administration of M100240 in healthy
volunteers is 85% (7.5 h),25 (2) approximately 60%
ACE inhibition is observed at the trough (i.e., 24 h
postdose) at oral doses of 10 and 20 mg once daily,25
and (3) M100240 is a balanced inhibitor of both ACE
and NEP with similar Ki for ACE and NEP.20,21 The results of this study were pooled with other recent studies in healthy volunteers and hypertensive patients to
enrich our data set. The main objectives of this modelbased analysis were threefold: first, to characterize the
relationship between individual drug exposure and
neurohormonal response (i.e., ACE and NEP inhibition) across a wide dose range; second, to test the aforementioned PK/PD assumptions; and, third, to apply
simulation to evaluate dose scenarios for potential
clinical studies in hypertensive patients.33,34
MATERIALS AND METHODS
Study Designs and Populations
The designs of the four relevant M100240 phase I/II
studies with available PK data are summarized in Table
I. Biomarkers (i.e., quantifiable surrogate markers measuring in vivo drug effect) for ACE and NEP activity
were measured in study A only. In this study, subjects
were on a fixed-sodium diet (7 g of NaCl per day). The
diet began 3 days before drug administration and was
maintained during the next 12 days. Subjects received
multiple doses of once-daily placebo or 2.5 mg, 10 mg,
25 mg, or 50 mg M100240 during that period. M100240
was administered orally with approximately 240 mL of
water after an overnight fast at the same time every
morning. A light breakfast was provided 30 to 60 minutes after drug intake, and lunch was provided 4 hours
after drug intake. At baseline and on days 1 and 12, serial blood and urine samples were collected for PK and
biomarker profiles.
Pharmacokinetic data and methods. To enhance the
accuracy of PK parameter estimates (e.g., clearance,
volume of distribution, etc.), all available MDL100,173
PK data were pooled and used for the PK analysis. PK
623
Parallel
Sequential
forced
titration
C
D
2.5, 5, 10, and 25 mg
10, 25, and 50 mg
2.5, 10, 25, and 50 mg
50 mg
Dose(s)
0, 1, 2, 4, 8, 12, 24, 48, and 72
0, 0.5, 1, 1.5, 2, 3, 4, 8, 12, 18,
24, 30, 36, and 48
0, 0.5, 1, 2, 3, 4, 6, and 8
Trough
PK Sample
Times (h)
3373
129
468
865
No
No
Yes
No
Number of PK ACE/NEP
Samples
Biomarkers
Cuff
Cuff ABPM
Cuff
Cuff
Blood
Pressure
PK, pharmacokinetic; ACE, angiotensin-converting enzyme; NEP, neutral endopeptidase 24.11; ABPM, ambulatory blood pressure monitoring.
Parallel
Crossover
Design
A
B
Study
Table I Design Summary of M100240 Studies with PK Data
156
33
32
30
Number of
Subjects
Hypertensive patients
Hypertensive patients
Healthy volunteers
Healthy volunteers
Study
Population
PFISTER ET AL
data from four M100240 studies were included in the
analysis: 1333 concentrations from 62 healthy subjects
and 3502 concentrations from 189 hypertensive
patients.
In all four studies, MDL100,173 plasma concentrations were quantified using a validated liquid chromatography tandem mass spectroscopy method (LC/MS/
MS).35
Biomarker data and methods. Biomarker data from
study A subjects (n = 32) included plasma ACE activity,
plasma renin activity (PRA), angiotensin II/angiotensin I ratio, bradykinin, urine ANP, and cGMP. The ACE
inhibition profile was assessed in parallel with the PK
profile (i.e., same sampling times). The NEP inhibition
profile was assessed from urinary ANP and cGMP excretion (daily and fractionated amounts) at baseline
and day 12. Fractional urine was collected over the following time intervals: 0 to 4, 4 to 8, 8 to 12, and 12 to 24
hours. Plasma ACE activity was measured with a validated ACE-trap method.36 PRA, plasma angiotensin I
and II, urinary ANP, and cGMP were measured by
radioimmunoassay as described previously.37-39
Blood pressure data. Three supine blood pressure
measurements were taken at 2-minute intervals after a
rest period of at least 10 minutes. The arithmetic mean
of the three measurements was considered the blood
pressure value at a given time point. A standard mercury sphygmomanometer was used for all blood pressure measurements unless specified otherwise.
In study A, blood pressure was measured before
drug administration, at 2-hour intervals up to 8 hours,
and at 12 hours after drug intake. In study B, serial
blood pressures were collected at 4-hour intervals. In
study C, serial cuff blood pressure assessments were
obtained in parallel with the PK samples. In study D,
24-hour ambulatory blood pressure monitoring
(ABPM) and cuff blood pressure assessments at baseline and at the end of the dosing interval were used.
These readings were assessed, both pooled and independently of the blood pressure measurement
technique.
Pharmacokinetic models. Both one- and twocompartment PK models with first-order absorption
were contrasted. A group effect (healthy subjects vs.
hypertensive patients) on clearance (CL), volume of
distribution (V), and relative bioavailability (F) was
also tested.
The group effect (GE) on PK parameters was modeled as follows:
Pi = (1 + GE) exp(η),
where P is a vector of PK parameters such as V, CL, F,
and the rate constant of absorption (Ka). Pi is a parameter estimate for the ith individual, and η is the
interindividual variance of the PK parameters. The indicator GE is coded as 1 (healthy volunteers) or 0 (hypertensive patients).
Models for humoral response. Individual patient
posterior Bayesian PK parameter estimates are provided by NONMEM for the second stage using the
post hoc procedure.40,41 Emax and sigmoid-Emax models
were contrasted to describe the relationship between
measured MDL100,173 concentrations and measured
biomarker response in plasma and urine.
Possible circadian variation of biomarker response
was described by a cosine function as follows34:
R t = mR + A cos
2π ( t + S )
,
tp
where R t is the biomarker response at a given clock
time t (i.e., where 24 h is midnight), tp is the time period of the cosine function (fixed to 6, 12, or 24 h), mR is
the 24-hour mean value of the measured biomarker for
humoral response, A is the amplitude of the circadian
variation, and S is the phase shift of the variation.
Residual variability for observations. To model residual intraindividual variability for pharmacokinetic
observations (e.g., measurement errors), both additive
and proportional components of error were accounted
for as follows:
y = f + fε1 + ε2,
Data Analysis Methods
A two-step strategy was used for the PK-biomarker response analysis: a PK model was first fit to the
MDL100,173 plasma concentration data. PK parameter
estimates for each individual were derived from this
model (and individuals’ data) and then used in a second step as fixed covariates in a subsequent PKbiomarker response model.
624 • J Clin Pharmacol 2004;44:621-631
where y is the measured MDL100,173 concentration
(or biomarkers in plasma), f is the model for its expectation, and the error ε = (ε1, ε2) is distributed N(0,Σ), where
Σ is diagonal.
Model building. The value of the objective function
at convergence (approximately minus twice the maximized log-likelihood of the data)41 was used to evaluate
model hierarchies—more complex (i.e., more parame-
DOSE SELECTION: VASOPEPTIDASE INHIBITOR M100240
Table II Response Parameter Estimates for ACE Activity
Parameter
Measured ACE Activity
Emax response model
Emax (fmol/mL/min)
EC50 (ng/mL)
Cosine function (12-h time period)
Amplitude (fmol/mL/min)
Phase shift (h)
Angiotensin II/Angiotensin I Ratio
344 (8.8)
0.43 (34)
0.49
0.33
(0.4)
(9.8)
20.1 (—)
1.01 (—)
0.031 (2.4)
1.2
(—)
Data are given as mean (CV%, interindividual variability). Emax, maximum effect; EC50, MDL100,173 plasma concentration eliciting half-maximum effect; CV,
coefficient of variation; ACE, angiotensin-converting enzyme.
ters) over less complex submodels. This was accomplished by computing the log-likelihood ratio, the difference in objective function between the fits of the two
models, and referencing it to a χ2(df) distribution,
where df is the number of free parameters by which the
larger model exceeds the smaller. The selection of an
appropriate PK parameter model was based on (1) a significant reduction in the objective function value with
the likelihood ratio test (for nested models, p < 0.001;
i.e., increase in objective function value of 10.88 for 1
degree of freedom); (2) diagnostic plots, including observed versus predicted values and individual
weighted residuals versus individual predictions; and/
or (3) a decrease in standard error, interindividual
variance of the PK parameters, and residual error.
Computations were performed with the software
NONMEM (version V) for LINUX (NONMEM Project
Group, University of California, San Francisco41) and
S-PLUS 6 for LINUX (Insightful Corporation, Seattle,
WA). Parameter estimates are provided as the mean
(interindividual variability, %).
Simulation for dose-ACE inhibition relationship.
Model-based simulation was used to evaluate candidate dose regimens with regard to a desired biomarker
response. Target biomarker response was defined as (1)
50%, 60%, 70%, 80%, or 90% ACE inhibition in at
least 50% of patients at trough and (2) 90% ACE inhibition in at least 50% of patients over 24 hours. ACE activity profiles in plasma (n = 500) for various dose regimens (higher doses and more frequent administration
schedules than those employed in the clinical program) were simulated with parameters drawn from
estimated parameter distributions.
RESULTS
A two-compartment PK model fit of the MDL100,173
concentration data was superior to a one-compartment
model, based on model selection criteria as previously
PHARMACOKINETICS AND PHARMACODYNAMICS
described. Population estimates (percent standard errors) were 527 L/h (56%) for CL, 0.45 h–1 (7.5%) for Ka,
636 L/h (15%) for intercompartmental clearance (Q),
200 L (51%) for central V, and 8020 L (–) for peripheral
V. Interindividual variability in peripheral V was set to
zero, and interindividual variability in CL, Ka, and central V was estimated to be 42%, 71%, and 105%, respectively. Clearance was estimated to be lower in hypertensive patients than in healthy subjects (231 vs.
527 L/h). There was no group effect on V, the absorption rate constant (Ka), and bioavailability (F).
A sensitivity analysis was performed reestimating
CL with modified healthy volunteer PK data sets (i.e.,
without MDL100,173 concentrations after 8, 12, or 24 h
postdose), as the group effect on CL could be the result
of differences between sampling schemes in healthy
subjects and those in patients. MDL100,173 concentrations were measured up to 48 or 72 hours in healthy
subjects, whereas MDL100,173 concentrations were
measured up to 8 hours or at the dosing interval (i.e., 24
h postdose) in patients. However, the estimate for apparent clearance remained lower in healthy subjects
versus patients with all three modified PK data sets,
and lower apparent clearance could not be explained
by removing PK samples after 8 hours postdose
(reference data on file).
ACE activity. The direct inhibitory Emax model adequately described the relationship between
MDL100,173 concentration and measured ACE activity. There was no hysteresis in these PK-biomarker
data, and adding an effect compartment did not improve the model (i.e., no delay of onset for the inhibitory effect on ACE activity). The calculated Hill response factor (sigmoid Emax model) was close to 1 (i.e.,
0.84); therefore, the simpler Emax model was selected as
the final ACE activity response model. A single cosine
function with a fixed period (tp) of 12 hours with an
amplitude (A) of 5.8% of average maximum effect (i.e.,
344 fmol/mL/min) described the circadian rhythm of
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PFISTER ET AL
ACE activity. Similar results were obtained with measured angiotensin II/angiotensin I ratios. Using
angiotensin II/angiotensin I ratios, the amplitude of the
single cosine function was 6.3% of the average maximum effect (i.e., 0.49). MDL100,173 plasma concentration eliciting the half-maximum effect (EC50) was also
comparable. Average EC50 was estimated to be 0.43 ng/
mL with the ACE activity response model versus an estimated average for the EC50 of 0.33 ng/mL with the angiotensin II/angiotensin I response model. Response
parameters and circadian rhythm parameters for ACE
activity and angiotensin II/angiotensin I ratios are
given in Table II. Figure 1 shows the goodness of fit of
the final PK-response model to the ACE activity data.
Angiotensin I and II. The direct Emax model adequately described the relationship between
MDL100,173 and angiotensin I concentrations as the
Hill response factor (sigmoid Emax model) was estimated to be approximately 1. Maximum drug effect
was estimated to be 300 fmol/mL, and EC50 (i.e.,
MDL100,173 plasma concentration eliciting the halfmaximum effect) was estimated to be 10.6 (33%) ng/mL.
The direct inhibitory sigmoid Emax model described
the relationship between MDL100,173 and angiotensin
II concentrations better than the direct inhibitory Emax
model as the Hill response factor appeared to be greater
than 1 (i.e., 2.9). The maximum drug effect was estimated to be 4.5 fmol/mL, and the EC50 was estimated to
be 2.8 (11%) ng/mL. Allowing a (MDL100,173 plasma
concentration-independent) time effect on angiotensin
II concentrations improved the model significantly.
The concentration-independent increase in mean angiotensin II concentration was estimated to be 0.067
mol/mL per day (i.e., 1.5% of baseline angiotensin II
concentration at day 1).
Renin activity. A dose-dependent increase in plasma
renin activity was only noted at the two highest doses
(i.e., 25 and 50 mg once daily). However, a concentrationdependent increase or decrease in plasma renin activity could not be established.
NEP activity. A transient significant increase in ANP
excretion was observed after the first dose of 25 and 50
mg of M100240. No appreciable increase in cGMP excretion was observed at 2.5-, 10-, and 25-mg doses. A
transient significant increase was observed in subjects
receiving 50 mg, however. The profiles of ANP and
cGMP excretion after the 1st and the 12th doses were
similar. There were no dose- or concentration-dependent
effects on measured biomarkers for NEP activity.
Bradykinin. Measured bradykinin plasma concentrations after M100240 administration ranged from 2 to
626 • J Clin Pharmacol 2004;44:621-631
Figure 1. Goodness-of-fit plots of the final response model for
plasma ACE activity. Left panels: measured plasma angiotensinconverting enzyme (ACE) activity versus population (individual)
predictions PRED (IPRED). Right panels: population residuals (RES;
weighted individual residuals IWRES) versus PRED (IPRED). Fine
dashed lines show smooth through the data. Heavy dashed lines are
the lines of identity in the left upper panel and at the ordinate value 0
in the other panels.
166 fmol/mL. No clear dose-, concentration-, or timedependent change in bradykinin plasma concentrations was found.
Blood pressure response. No appreciable blood
pressure reductions were observed in healthy volunteers (studies A and B) following 2.5, 10, and 25 mg
once-daily administration of M100240. In study C,
only the 25-mg dose gave appreciable diastolic and systolic blood pressure–lowering effects at trough in hypertensive patients. In study D, there was a minimal
stepwise effect on 24-hour mean blood pressure decreases from the 10- to the 25-mg dose. The 50-mg dose,
however, did not add meaningfully to the response in
24-hour mean blood pressure compared to the 25-mg
dose in hypertensive patients.
Dose–ACE inhibition relationship (based on simulation). Figure 2 shows that 50 mg M100240 administered once daily produces greater than 80% and greater
than 90% ACE inhibition in 40% and 20% of subjects
at trough (i.e., 24 h postdose) and indicates that divided
doses (i.e., twice-daily dose regimens) provide > 90%
DOSE SELECTION: VASOPEPTIDASE INHIBITOR M100240
Figure 2. Probability plot of achieving the target angiotensinconverting enzyme (ACE) inhibition at 24 hours with 25 to 125 mg
M100240 once-daily (left panel) or 25 to 125 mg M100240 twice-daily
(right panel). Solid lines represent target ACE inhibition at the 50%,
60%, 70%, 80%, or 90% level. Horizontal dashed lines show a probability of 0.5 (i.e., target ACE inhibition at trough is expected to be evident in 50% of total population). Vertical dashed lines indicate 100
mg total daily dose. Numbers in the plots represent the percentage of
the total population with 90% ACE inhibition at the 100-mg total
daily dose.
ACE inhibition in at least 5% more subjects than oncedaily dose regimens with the same total daily dose (i.e.,
49% vs. 44% of total population with 100 mg/day). To
achieve a target > 90% ACE inhibition in more than
50% of subjects over a desired dose interval (i.e., 24 h),
higher and/or more frequent doses on the order of 25
mg three times daily or 50 mg twice daily may be
required (Figure 3).
DISCUSSION
This report presents the first model-based analysis of
the relationship between individual drug exposure
and biomarker response with the vasopeptidase inhibitor M100240. ACE activity and the angiotensin II/
angiotensin I ratio measured at trough correlated well
on day 1 (R2 = 0.95, p < 0.0001; Figure 4), as previously
reported.42 Mean daily increases in angiotensin II were
estimated to be 1.5% of baseline (predose) values. Similar increases over time in angiotensin II concentrations have been observed with the ACE inhibitor
fosinopril,43 whereas angiotensin II concentrations at
PHARMACOKINETICS AND PHARMACODYNAMICS
Figure 3. Probability of achieving the target > 90% angiotensinconverting enzyme (ACE) inhibition over the entire dosing interval
with two different divided doses. Left panels: 25 mg M100240 three
times daily (i.e., 75 mg total daily dose); right panels: 50 mg M100240
twice daily (i.e., 100 mg total daily dose). Upper plots show expected
ACE activity profiles under treatment. Solid lines represent median.
Shaded areas delimit the 10th and 90th percentiles. Dotted lines in
lower plots show a probability of 0.5 (i.e., target ACE inhibition is expected to be evident in 50% of the total population).
trough after single and multiple administrations of
lisinopril or the ACE/NEP inhibitor omapatrilat remained unchanged.13,43 A direct inhibitory Emax model
adequately described the relationship between
MDL100,173 concentration and ACE activity or the
angiotensin II/angiotensin I ratio. Of importance, there
was no delay of the ACE inhibitory effect as the temporal rise and decline of MDL100,173 plasma concentrations mirror that of ACE inhibition. The circadian
rhythm of both measured ACE activity and the calculated angiotensin II/angiotensin I ratio could be best
described with a single cosine function with a fixed period of 12 hours and an amplitude of approximately
6% of the respective baseline values. This finding is in
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PFISTER ET AL
Figure 4. Measured angiotensin-converting enzyme (ACE) activity
versus the angiotensin II/angiotensin I ratio at the end of the dosing
interval (i.e., 24 h after the first M100240 dose). The dotted line is the
line of identity. The solid line shows the estimated regression line.
The numbers in the plot indicate M100240 doses in mg.
accordance with previous studies reporting that not
only blood pressure but also renin-angiotensinaldosterone biomarkers follow a circadian rhythm.44-46
As expected, the dose-dependent decrease in angiotensin II was mirrored by a dose-dependent increase in
plasma renin activity. Similar results have been reported for lisinopril13,47 and omapatrilat.13,43
Although modest changes in biomarkers for NEP inhibition were observed at the two highest doses, no
concentration-dependent signals for NEP inhibition
(i.e., increase in urinary excretion of ANP or cGMP)
were evident across the dose range evaluated once
daily. This is in contrast to previous studies with intravenous administration of M10024026 or oral administration of omapatrilat,9,43,48,49 in which significant increases in urinary volume, ANP, and cGMP excretion
were observed. Despite the concentration-dependent
changes in ACE inhibition and the nominal NEP inhibition, no overt dose- or concentration-dependent
changes in measured bradykinin were found.
The dose range assessed in this study was selected
based on the proposed balanced in vitro potency of
M100240 for ACE inhibition and NEP inhibition.20
628 • J Clin Pharmacol 2004;44:621-631
However, a recent in vitro study optimizing the stability of the analyte in the test system revealed that the IC50
value for NEP inhibition is significantly higher as compared to that for ACE inhibition (0.0015 vs. 0.000035
µM).50 This finding is consistent with other ACE/NEP
inhibitors, such as gemopatrilat and CGS 30440, with
different concentration-response curves for ACE and
NEP.51,52 That is, at least 10-fold higher concentrations
are required for NEP inhibition than for ACE inhibition. Furthermore, dose selection was based on an oral
bioavailability of M100240 estimated to be not statistically different from 100%.25 Oral bioavailability of
M100240 was, however, overestimated in this study as
ACE inhibition was used as a surrogate for drug exposure by determining its profile after oral and intravenous administration. Indeed, when using a metric of
drug exposure, the absolute oral bioavailability of
M100240 was measured more accurately at approximately 49%.53 Therefore, we used plasma concentrations of the ACE/NEP inhibitor MDL100,17320 to assess
individual drug exposure. Interestingly, area average
drug exposure (i.e., area under the curve) appeared to
be higher in healthy subjects than in hypertensive patients. This could be an artifact based on the measured
sampling scheme as MDL100,173 concentrations were
measured up to 48 or 72 hours in healthy subjects,
whereas MDL100,173 concentrations were measured
up to 8 hours or only at the dosing interval in patients.
However, a sensitivity analysis showed that the estimate for apparent clearance remained lower in healthy
subjects versus hypertensive patients with modified
PK data sets (i.e., without MDL100,173 concentrations
after 8, 12, or 24 h postdose). The apparent difference in
drug exposure between healthy subjects and
hypertensive patients is unlikely the result of a
difference in drug adherence, as tested group effects on
bioavailability were negligible.
It has been shown for ACE inhibitors that the relationship between blood pressure and ACE inhibition is
flat up to 90% ACE inhibition, whereas between 90%
and 100% ACE inhibition, the concentration-effect relationship correlation is steep.54 Most clinically available ACE inhibitors produce 80% to 90% ACE inhibition at trough (e.g., lisinopril,42 cilazapril54), and other
ACE/NEP inhibitors such as omapatrilat and CGS
30440 produce 70% to 90% ACE inhibition at
trough.48,49,52,55 However, it should be noted that different approaches used to assess ACE inhibition may result in slightly different values for ACE inhibition56;
moreover, in vitro and in vivo inhibition of the active
sites of ACE may differ among ACE/NEP inhibitors.57 In
addition, the long terminal phase observed for many
ACE inhibitors typically reflects the dissociation of the
DOSE SELECTION: VASOPEPTIDASE INHIBITOR M100240
active moiety from plasma ACE, with the half-life of
the free drug often much shorter. Hence, MDL100,173
is likely to exhibit a dissociation for ACE that does not
favor once-daily administration.
NEP activity should be additive to the comparable
ACE inhibition of available ACE inhibitors to provide
better clinical outcomes in conditions in which dual
ACE/NEP inhibition ultimately proves to be useful.
Omapatrilat, a balanced ACE/NEP inhibitor with similar Ki values for ACE and NEP,58,59 produces a superior
blood pressure–lowering effect than lisinopril at comparable ACE inhibition (i.e., 70%-90% ACE inhibition
at the end of the dosing interval) due to the addition of
NEP inhibition.49
Simulation reveals that a dose of 50 mg M100240
once daily produces at least 90% ACE inhibition 24
hours postdose in only 20% of subjects. The fundamental assumptions that guided the initial dose selection for M100240 were not valid in the clinical
setting. This most likely explains the suboptimal
blood pressure–lowering effects in hypertensive patients with once-daily doses of up to 50 mg. To achieve
a target 90% ACE inhibition at a dose interval in more
than 50% of subjects, higher total daily doses on the order of 100 mg would be required. As the
neurohormonal response (ACE activity) profile mirrors
the drug exposure profile, divided doses (or an extended-release formulation) should provide better target ACE inhibition in more patients not only at trough
but also over the entire desired dosing interval. These
predictions would be useful to optimize dose and design selection60 for subsequent clinical trials in
hypertension.
The authors gratefully acknowledge the contributions of Drs.
Hans Brunner and Juerg Nussberger, CHUV, Lausanne, Switzerland;
Drs. Anne Floch and Marie-Laure Ozoux, Aventis, Paris, France; Dr.
Jean-Francois Tamby and Ms. Wendy Aspden, Aventis, Bridgewater,
New Jersey; and Ms. Susanna Pfister, Medtranslation, Bridgewater,
New Jersey.
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