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Vol. 281, Issue 3, 1102-1112, 1997
Dr. Margarete Fischer-Bosch-Institut für Klinische Pharmakologie (A.S.G., G.M., M.E.), Stuttgart, Germany, and Department of Clinical Chemistry (D.R., H.W.), Robert-Bosch-Hospital, Stuttgart, Germany
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Abstract |
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Abstract
Introduction
Methods
Results
Discussion
References
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The pharmacokinetics and pharmacodynamics of the enantiomers of the calcium antagonist gallopamil have been investigated in six healthy volunteers. Each subject was studied on five occasions after receiving, in randomized order: placebo, 25 mg of (R)-gallopamil, 25 mg of (S)-gallopamil, 50 mg of pseudoracemic [25 mg of deuterated (S)- gallopamil and 25 mg of (R)-gallopamil] and 100 mg of (R)-gallopamil HCl orally. After separate administration, the apparent oral clearances of both enantiomers were similar [(R), 15.1 ± 9.9 liters/min; (S), 11.0 ± 6.0 liters/min], indicating that gallopamil first-pass metabolism is not stereoselective. After coadministration, the apparent oral clearance of each enantiomers decreased [(R), 5.9 ± 2.8 liters/min; (S), 5.8 ± 2.66 liters/min], suggesting that a partial saturation of first-pass metabolism occurs because the dose was twice as high than for the single enantiomers. Serum protein binding and renal elimination of gallopamil are stereoselective, favoring (S)-gallopamil. Analysis of urine samples revealed a marked degree of stereoselectivity in the formation of O- and N-dealkyl metabolites. Because these showed opposite stereoselectivity, canceling out each other, the net result was no or only marginal stereoselectivity. Twenty-five milligrams of (S)-gallopamil prolonged the PR interval in all subjects; however, a greater effect was elicited by 50 mg of (RS)-gallopamil. (R)-Gallopamil (100 mg) did not significantly alter the PR interval, although higher concentrations were attained than after the pseudoracemate. Based on a consideration of (S)-gallopamil serum concentrations, a comparable relationship between (S)-gallopamil level and effect occurred after (S)- and (RS)-gallopamil, indicating that the pharmacological effect produced by the racemate could be totally accounted for by the higher concentrations of (S)-gallopamil attained.
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Introduction |
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Abstract
Introduction
Methods
Results
Discussion
References
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Gallopamil
{2-(3,4,5-trimethoxyphenyl)-2-isopropyl-5-[(3,4dimethoxyphenethyl)
methyl-amino]valeronitrile} (D 600) (fig. 1) is a
calcium antagonist with phenylalkylamine structure used in the
treatment of angina pectoris (Brogden and Benfield, 1994
) and reduction
of myocardial damage after infarction (Faria et al., 1990).
It is a methoxy derivative of verapamil, and the small change in
structure results in a 10-fold increase in potency in terms of
vasodilation, negative inotropic action and negative dromotropic
effects (Bayer et al., 1975; Nawrath and Raschack, 1987).
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Gallopamil has a chiral center and is administered as a racemic mixture
of the (+)-(R)- and (
)-(S)-enantiomers. The
enantiomers of racemic drugs can differ in potency and the
spectrum of effects elicited (Eichelbaum and Gross, 1996), and it
is important to establish the contribution of the individual
enantiomers of racemic drugs to the desired and undesired
pharmacological effects of the racemate. In vitro studies
have shown that (S)-gallopamil is a more potent negative
inotropic agent and vasodilator than (R)-gallopamil (Bayer
et al., 1975; Müller and Wilsmann, 1982; Nawrath and
Raschack, 1987; van Amsterdam et al., 1990).
(S)-Verapamil is also a more potent negative dromotropic
agent in vitro than (R)-verapamil and this
difference in relative potency has also been observed in man in
vivo (Echizen et al., 1985a, 1985b). As yet, however,
no data on the relative potencies of the enantiomers of gallopamil in
humans have been published.
Differences in the absorption, metabolism, protein binding and urinary
excretion of the enantiomers of racemic drugs occur (Eichelbaum and
Gross, 1996). It is well established that the pharmacokinetics of
racemic verapamil are stereoselective (Eichelbaum et al.,
1984; Vogelgesang et al., 1984) as metabolism, the major pathway of elimination, favors the (S)-enantiomer. By
analogy with verapamil, gallopamil is eliminated principally by
metabolism (Stieren et al., 1983; Weymann et al.,
1989). In the rat and humans, both O- and N-dealkylated metabolites
have been identified in the urine and in the bile as sulfate and
glucuronide conjugates (Mutlib and Nelson, 1990a, 1990b; Weymann
et al., 1989). The similarity in structure and disposition
of gallopamil and verapamil suggests that the metabolism and,
consequently, disposition of gallopamil may be stereoselective.
However, to date only the disposition of racemic gallopamil has been
reported (Eichelbaum, 1989; Stieren et al., 1983), and the
pharmacokinetics of the individual enantiomers of gallopamil have not
been described.
Gallopamil improves the ratio of myocardial oxygen demand to supply (De
Servi et al., 1987). The influence of racemic gallopamil on
blood pressure, heart rate and exercise stress test electrocardiography has been reported in normotensive subjects (Hopf et al.,
1984; Khurmi et al., 1984; Rettig et al., 1983);
however, in these studies, the relationship between the pharmacological
response and gallopamil serum concentrations has not been investigated.
Furthermore, the influence of gallopamil on peripheral blood flow,
peripheral vascular resistance and plasma renin concentrations has not
been reported. The contribution of the individual enantiomers of
gallopamil to the pharmacological effects observed in humans has not
been examined.
We performed this study to investigate (1) whether the pharmacokinetics of (R)- and (S)-gallopamil differ, (2) which pathways of metabolism are stereoselective on the basis of analysis of the urinary excretion of the major metabolites of gallopamil and (3) the relative effects of (R)- and (S)-gallopamil on cardiovascular and electrocardiographic parameters, peripheral blood flow and plasma renin concentrations.
The disposition and pharmacological effects of single oral 25-mg doses
of (R)- and (S)-gallopamil after separate and
simultaneous administration were compared in healthy volunteers using a
randomized, placebo-controlled study design. (R)-Gallopamil
is less potent than (S)-gallopamil, as demonstrated in
vitro (Bayer et al., 1975); therefore, an additional
100-mg dose of (R)-gallopamil was also investigated. A
pseudoracemate, in which (S)-gallopamil is labeled with two
deuterium atoms (fig. 1) and (R)-gallopamil is unlabeled, was used for simultaneous enantiomer administration to measure the
concentrations of the stereoisomers (Browne, 1990; Eichelbaum et
al., 1982).
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Methods |
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Introduction
Methods
Results
Discussion
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Materials.
Hard-gelatin capsules containing placebo, 25 mg
of ()-(S)-gallopamil HCl, 25 mg of
(+)-(R)-gallopamil HCl and 25 mg of
[2H2]-()-(S)-gallopamil HCl were
used that were >99% isotopically and optically pure. The enantiomeric
composition of the gallopamil capsules was also investigated using a
stereospecific high-performance liquid chromatography technique based
on a method developed for 1,4-dihydropyridine calcium antagonists
(Fischer et al., 1993). Base-line resolution of the
enantiomers of gallopamil was achieved using a Chiralpak AD column
(Daicel Chemical Industries Ltd., Tokyo, Japan) maintained at 40°C
with a mobile phase of hexane/2-propanol (95:5) containing 0.2%
diethylamine pumped at a flow rate of 1 ml/min. Appropriate fractions
eluting from the column at the retention times of (S)- and
(R)-gallopamil (13.04 and 16.5 min, respectively) were
collected and assayed for gallopamil by GC-MS. The
(S)-gallopamil capsules contained 0.11%
(R)-gallopamil, and the (R)-gallopamil capsules
contained 0.14% (S)-gallopamil.
[2H2]-()-(S)-Gallopamil HCl,
labeled with two deuterium atoms at C5, was used when the enantiomers
were coadministered (fig. 1). In an initial study, 25 mg of
(S)-gallopamil HCl and 25 mg of [2H2]-(S)-gallopamil HCl were
coadministered to one healthy volunteer. The serum concentration-time
profiles of [2H2]-(S)- and
[2H2]-()-(S)-gallopamil were
superimposible, and therefore a significant isotope effect can be
excluded.
Subjects. Six healthy male volunteers participated in the study after a thorough physical examination was performed and written informed consent had been obtained. Their age ranged from 24 to 36 years, and their weight ranged from 63 to 86 kg. All volunteers were extensive metabolizers of sparteine and mephenytoin. Subject 5 was a cigarette smoker and abstained from smoking for 12 hr before each dose until the last blood sample was taken.
Protocol.
This study was approved by the Ethics Committee of
the Robert-Bosch-Hospital (Stuttgart, Germany). All gallopamil doses
were administered under medical supervision. Each subject was studied on five occasions with a 
7-day interval between study days. In randomized order, each subject was administered (1) placebo, (2) 25 mg
of (R)-gallopamil HCl (47.98 µmol), (3) 25 mg of
(S)-gallopamil HCl (47.98 µmol), (4) 25 mg of
(R)-gallopamil HCl and 25 mg of [2H2]-(S)-gallopamil HCl (47.98 µmol of each enantiomer)/pseudoracemate [(RS)-gallopamil)] and (5) 100 mg of
(R)-gallopamil HCl (191.9 µmol).
5 hr postdose while pharmacodynamic
effects were monitored. Blood samples (8 ml) were withdrawn through an
indwelling cannula in a forearm vein before gallopamil administration;
and at 0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 2, 2.5, 3, 4, 5, 8, 9, 11, 12, 14 and 15 hr; and via venipuncture at 24 and 25 hr postdose.
One-milliliter aliquots of whole blood were transferred to chilled
tubes containing EDTA and immediately centrifuged, and plasma for
analysis of renin concentrations was stored at -20°C. The remaining
blood sample, which was transferred to a glass tube, was kept at room
temperature for 30 min and then centrifuged at 1500 × g for 15 min. Serum samples were stored in glass tubes at
20°C until assayed. An additional whole blood sample for
determination of blood to serum concentration ratio was taken from each
subject two hr after drug administration and stored at -20°C. The
urine excreted from 0 to 24 hr and 24 to 48 hr post-dose was collected,
the volume determined and a 50 ml aliquot of each sample stored at
-20°C until assayed.
Pharmacodynamic measurements. Pharmacodynamic effects were assessed in each subject before capsule administration and at the time of blood sampling for 5 hr postdose. Blood pressure (systolic, diastolic and mean arterial pressure) and heart rate were measured using an automated sphygmomanometer (Dinamap 1846 SX, Critikon GmbH, Norderstedt, Germany). Electrocardiographic intervals [P wave, PR interval, QRS interval, QT and corrected QT (QTc) intervals] were measured with an electrocardiograph (CS 6/12, Schiller, Baar, Switzerland). Ten measurements were taken at each time point, and the data were transferred, stored and processed with the use of dedicated software. Peripheral blood flow in both legs was determined at the time of each blood sampling using an automatic venous occlusion plethysmograph (Infraton Vasoskript, Boucke, Tübingen, Germany), and the mean value of 10 measurements is reported. The peripheral vascular resistance was calculated by dividing the mean arterial blood pressure by leg blood flow.
Analytical techniques.
Gallopamil and
[2H2]gallopamil were measured in serum using
a specific GC-MS technique with selected ion monitoring (Gross et
al., 1990). The method is very sensitive and has a limit of quantification of 0.192 nM = 1 ng/ml (within-day reproducibility, 4.8%, n = 9; between-day reproducibility, 12.3%,
n = 47). This technique was used without modification
and with comparable accuracy and reproducibility to determine the
concentration of gallopamil and
[2H2]gallopamil in urine and whole blood.
).
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-glucuronidase/arylsulfatase (Helix pomatia, 5.5/2.6
units/ml, Boehringer-Mannheim Biochemica, Mannheim, Germany).
Hydrolysis was stopped after 24 hr at 37°C by refrigeration. After
alkalization of 1 ml of the hydrolyzed sample with 400 µl of 10%
sodium carbonate, 5 ml of diethylether/hexane (50:50 v/v) was added,
and the tubes were mixed for 10 min and centifuged at 4000 rpm for 10 min. The organic phase was transferred to tapered evaporation tubes and
evaporated. Acetic acid anhydride (35 µl) and 5% triethylamine in
acetonitrile (45 µl) were added to the residue. After being heated
for 45 min at 50°C, the derivatization reagent was evaporated under
nitrogen. The residue was reconstituted in 40 µl of acetonitrile, and
an aliquot (2 µl) was assayed by electron impact selected
ion-monitoring GC-MS using a modification of the temperature program
used for gallopamil (Gross et al., 1990). The retention
times of the metabolites were D845 25.3 min, PR53 19.1 min, SZ488 18.1 min, D829 18.9 min and D832 9.4 min. Plasma renin concentrations were
determined by immunoradiometric assay (Renin IRMA Pasteur, ERIA
Diagnostics Pasteur, Marues la Coquette, France).
The serum protein binding of the enantiomers of gallopamil was
determined by equilibrium dialysis at 37°C (Dianorm Equilibrium Dialyzer, Spectrum Instrument Co., Houston, TX). One-milliliter serum
samples taken 2 hr after gallopamil administration were dialyzed for 2 hr across a SpectraPor 2 dialysis membrane against 1 ml of pH 7.4 Sorensen's buffer. Because gallopamil binding is sensitive to pH
(Rutledge and Szlacky, 1988), the pH of all serum samples was adjusted
to pH 7.2 to ensure a pH of 7.4 at the end of dialysis. The
fu of gallopamil was corrected for volume shifts during
dialysis by measuring changes in protein concentration during dialysis
(Lima et al., 1983). The technique was reproducible (fraction bound = 0.919 ± 0.019; CV, 2.1%;
n = 5; racemic gallopamil concentration, 9.6 nmol/liter).
Data treatment and statistical analysis.
Standard equations
were used to calculate model-independent pharmacokinetic parameters
(Rowland and Tozer, 1989). Cmax and tmax were established from the measured serum
concentration-time data. The 
was determined by least-squares
regression of the terminal linear portion of the log serum
concentration-time profile, and terminal half-life,
t1/2, was calculated as 0.693/.
AUC(0-25 hr) was calculated using the linear
trapezoidal rule and extrapolated to infinity by the addition of
C25/. CLo was calculated as dose/AUC. MRT
was calculated as the area under the first moment curve divided by AUC.
CLR was determined from the amount of gallopamil excreted unchanged in the urine (Ae) divided by AUC. The F of
gallopamil was calculated from the relationship between CLo
and F (Somogyi et al., 1982) previously
observed.3
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(1) |
). The model is described by the following
equation, in which E is the observed effect,
Emax is the maximum pharmacological effect,
EC50 is the serum concentration eliciting half the maximum
effect and N is a parameter affecting the slope of the
concentration-response relationship.
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(2) |
).
Gallopamil pharmacokinetic parameters are reported as the mean ± S.D., with the exception of half-life, which is reported as the
harmonic mean. The parameters calculated in each phase of the study
were compared using the Friedman two-way analysis of variance by ranks
(Siegel, 1956). (R)-Gallopamil pharmacokinetic parameters at
the doses of 25 and 100 mg and pharmacodynamic parameters were
calculated using the overall (R + S), and total
or free (S)-gallopamil serum concentrations were compared
using the Wilcoxon matched-pairs signed-rank test (Siegel, 1956). B/P
values were compared using the Mann-Whitney U test (Siegel,
1956). A probability of <.05 was considered significant.
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Results |
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Abstract
Introduction
Methods
Results
Discussion
References
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Pharmacokinetics of (R)- and (S)-gallopamil
administered separately.
In all subjects, similar serum
concentration-time profiles of (R)- and
(S)-gallopamil (25 mg) were observed when the enantiomers were administered separately. Data from a representative subject are
shown in figure 2. Considerable interindividual
variation in the pharmacokinetic parameters of (R)- and
(S)-gallopamil was observed (tables 1 and
2, respectively). After separate administration, Cmax, tmax, AUC,
CLo, t1/2, MRT and F values for
(R)- and (S)-gallopamil were comparable (P > .05). The urinary recovery of (S)-gallopamil was twice
(P < .05) that of the (R)-enantiomer; however, it
accounted for just 0.33% and 0.15% of the dose administered,
respectively. Consequently, the CLR of gallopamil was
stereoselective. The mean fu of (S)-gallopamil
was higher than that of (R)-gallopamil; however, the
difference was not significant. The B/P values for (R)- and (S)-gallopamil were similar (R, 0.63 ± 0.09, n = 5; S, 0.52 ± 0.05, n = 4; P > .05) and substantially lower than
unity.
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Pharmacokinetics of (RS)-gallopamil.
Representative serum concentration-time profiles of (R)- and
(S)-gallopamil after the administration of the
pseudoracemate are shown in figure 3, respectively.
Pharmacokinetic parameters for (R)- and
(S)-gallopamil are given in tables 1 and 2. In all subjects,
the (R) and (S) serum concentration-time profiles were similar; however, there was considerable interindividual variation
in gallopamil pharmacokinetic parameters. No differences in
Cmax, tmax,
t1/2, AUC and CLo values
between (R)- and (S)-gallopamil were observed.
The ratio of the CLo values of the (S)- and
(R)-enantiomers was 1.06 ± 0.37. A small but
significant difference was noted in MRT. The urinary recovery and renal
clearance values for (S)-gallopamil (263.9 ± 152.9 nmol, 0.55% dose; 25.9 ± 9.7 ml/min) were greater (P < .05) than those of (R)-gallopamil (163.7 ± 74.2 nmol,
0.34% dose; 14.5 ± 6.4 ml/min). The fu of
(S)-gallopamil in serum was higher (P < .05) than that
of (R)-gallopamil. B/P value for (R)-gallopamil (0.52 ± 0.05, n = 4) was similar (P > .05)
to that of (S)-gallopamil (0.54 ± 0.02, n = 4). In all subjects, serum
(R)-gallopamil concentrations after administration of the
100-mg dose were higher than after administration of the
pseudoracemate.
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Effect of coadministration of gallopamil enantiomers. The serum concentrations of both enantiomers were higher when coadministered than when the same dose was administered separately (figs. 2 and 3). Consequently, Cmax and AUC for (R)- and (S)-gallopamil were higher after coadministration, and the CLo values for (R)- and (S)-gallopamil were reduced. The F values of both enantiomers were enhanced. The t1/2 and CLR of (R)- and (S)-gallopamil were not altered by administration of the optical antipode. A greater proportion of each dose was excreted as the unchanged drug in urine, reflecting unaltered CLR values and higher serum concentrations of (R)- and (S)-gallopamil. The serum protein binding and B/P for (R)- and (S)-gallopamil were comparable (P > .05) when the enantiomers were administered separately or together.
Gallopamil metabolism. The urinary recoveries of the enantiomers of the gallopamil metabolites after the 50-mg dose of the pseudoracemate are given in table 4. The chemical structures of these metabolites are shown in figure 1. Stereoselective metabolism of gallopamil was observed. N-Dealkylation to form D832 was the major pathway of metabolism and favored (R)-gallopamil. For the N-demethylated (D845) and O-demethylated metabolites at either the aromatic ring adjacent to the chiral center (D829) or the phenethyl aromatic ring (PR53, SZ488), metabolism of (S)-gallopamil was favored. The overall recovery of unchanged gallopamil and metabolites in the 48-hr urine samples was 14.6 ± 2.5% of the dose for (S)- and 17.6 ± 2.9% of the dose for (R)-gallopamil.
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Pharmacological effects.
Gallopamil was well tolerated by all
subjects. Facial flushing was observed in one volunteer (subject 1)
from 30 to 90 min after the administration of (S)-
and (RS)-gallopamil. First-degree AV block occurred
in subject 4 after 25 mg of (S)-gallopamil and in
subject 5 after pseudoracemic gallopamil. After pseudoracemic gallopamil, AV dissociation without loss of rhythm occurred in subject
4 for 75 min, from 0.75 to 2 hr postdose. Predose cardiovascular and
electrocardiographic parameters were similar in each subject on each
study day. In addition, all parameters were within the appropriate
normal range for young normotensives (fig. 4).
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Discussion |
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Abstract
Introduction
Methods
Results
Discussion
References
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Whether administered separately or together, the CLo values of (R)- and (S)-gallopamil are high and not different. Thus, the substantial first-pass metabolism of gallopamil is not stereoselective.
Relative to administration of the separate enantiomers,
coadministration decreases the CLo values and increases the
F values of both (R)- and
(S)-gallopamil. This can be attributed to a
saturation of first-pass metabolism as the gallopamil dose was twice as
high when administered as racemate compared with the dose of the
gallopamil enantiomers administered separately. This is further
supported by the results after the administration of 100 mg of
(R)-gallopamil; a nonlinear increase of the AUC was
observed compared with 25 mg of (R)-gallopamil. From
the F values that were estimated using the equation given above
(1/F = 0.89 × Clo + 0.42), the extraction ratios
of (R)- and (S)-gallopamil were
calculated (ER = 1 F) and are high for both (R)- [25 mg
(R) = 0.90, 50 mg (RS) = 0.76] and
(S)- [25 mg (S) = 0.86, 50 mg
(RS) = 0.77] gallopamil. Enantioselectivity is not
observed when the enantiomers are administered separately or together.
Importantly, the extraction ratio of both enantiomers is diminished
when they are coadministered due to the higher dose. In general,
pharmacokinetic studies undertaken to obtain relevant information on
the disposition of the enantiomers of chiral drugs administered as
racemates must be performed using the racemates or pseudoracemates
because enantiomer/enantiomer interactions have been observed. Studies
using single stereoisomers are invaluable, however, in probing the
contribution of each enantiomer to the pharmacological effects elicited
by the racemate. Furthermore, these studies using the separate
enantiomers have shown that chiral inversion of
(R)-gallopamil to (S)-gallopamil
does not occur in vivo.
No pronounced stereoselectivity was observed in CLo or
first-pass metabolism of gallopamil; however, the individual pathways of biotransformation exhibited stereoselectivity to varying degrees. N-Dealkylation to D832 was the major pathway of elimination monitored and favored (R)-gallopamil (S/R
0.7). In contrast, N-demethylation to D845 favored
(S)-gallopamil (S/R 2.1).
O-Demethylation at both aromatic rings also favored
(S)-gallopamil; however, the enantioselectivity was
more pronounced at the phenethyl aromatic ring (S/R
meta-11.3 and para-9.2) than the aromatic ring adjacent
to the chiral center (S/R para-1.4). In the 0- to
48-hr urine sample, 17.6% of the dose of (R)- and
14.6% of the dose of (S)-gallopamil were
recovered. Therefore, the overall urinary elimination of gallopamil
(S/R 0.8) is less enantioselective than the
individual pathways of metabolism monitored. However, only half the
urinary recovery of oral gallopamil has been accounted for as
identified metabolites (Mutlib and Nelson, 1990a). Therefore, a
significant proportion of the dose is eliminated in urine as
metabolites whose structure has yet to be elucidated. The pathways of
metabolism not measured in this study must favor
(R)-gallopamil because there is no net stereoselectivity in gallopamil clearance (CLo
S/R 1.06).
The stereoselectivity of gallopamil metabolism in vitro by
rat and human liver microsomes has been investigated (Mutlib and Nelson, 1990a, 1990b). However, the authors of the in vitro
study report only the rate of formation of each metabolite at two
gallopamil concentrations. In vivo drug metabolism is
reflected more reliably by the in vitro intrinsic clearance
calculated after measuring the kinetics of metabolite formation
in vitro. For the major metabolite D832, the in
vitro (S/R 0.76) and in vivo data
(S/R 0.66) correspond well. However, for
para-O-demethylation at the aromatic ring adjacent to the
chiral atom, the results differ (S/R in vitro 0.7 vs. S/R in vivo 1.4).
Stereoselectivity in O-demethylation at the phenethyl aromatic ring
favored (S)-gallopamil in both studies, but the degree of stereoselectivity was lower in vitro
(S/R meta-3.5, para-1.5) than in
vivo. In vitro, the formation of D845 was not stereoselective
(S/R 1.09), which differs from the
S/R ratio of 2.14 observed in vivo.
After administration of racemic verapamil, preferential metabolism of
the (S)-enantiomer occurs, and the F value of this
enantiomer is diminished relative to (R)-verapamil
(Vogelgesang et al., 1984), which is in contrast to the
results obtained with gallopamil. It is thus interesting to compare the
stereoselectivity of gallopamil and verapamil metabolism. A large
difference in the stereoselectivity of O-demethylation on the aromatic
ring adjacent to the chiral atom was observed. For verapamil, the
clearance to D703 is more enantioselective
(S/R = 9.0) (Mikus et al., 1990)
than the analogous formation of D829 (S/R 1.38)
from gallopamil. This suggests that the additional meta
methoxy group on this aromatic ring must be a steric hindrance for
interaction with the active site of the enzyme catalyzing
O-demethylation. The stereoselectivity of the O-demethylation at the
phenethyl aromatic ring favors (S)-gallopamil at both
the meta and para positions and although of
negligible significance in vivo (Mikus et al.,
1990), in vitro it also favors (S)-verapamil (S/R 1.2)
(Kroemer et al., 1992). The formation of the major
N-dealkylated metabolite favors (R)-gallopamil (D832 S/R 0.66) but (S)-verapamil
(D617: S/R 5.1 in vivo;
S/R 1.4 in vitro) (Kroemer et
al., 1992; Mikus et al., 1990). Gallopamil and
verapamil, two calcium antagonists with very similar structures, therefore differ substantially in the enantioselectivity of metabolic clearance and consequently disposition.
There was substantial interindividual variation in the CLo
of both enantiomers, as has been reported for other high clearance drugs, including nitrendipine (Mast et al., 1992). In
particular, the diminished clearance of gallopamil in subject 4 in each
phase of the study, relative to the other volunteers, is noteworthy. Higher gallopamil serum concentrations and therefore a greater pharmacological response occurred in each phase of the study; a 20%
PR-interval prolongation was also observed in this subject after 100 mg
of (R)-gallopamil, but this response was
substantially less than that occurring after (S)- or
(RS)-gallopamil. As in other subjects, in subject 4 low concentrations of (S)-gallopamil were measured
after the 100-mg dose of (R)-gallopamil, as a result of optical impurity of (R)-gallopamil, which would
not be expected to elicit a pharmacological response. As the enhanced F
value in Subject 4 was reproducible, first-pass metabolism of
gallopamil must be impaired. The enzymes that metabolize gallopamil
have not been identified. Subject 4 was an extensive metabolizer of both sparteine and mephenytoin, and therefore a deficiency of CYP2D6 (Eichelbaum and Gross, 1990) or CYP2C19 (Wilkinson et
al., 1989) was not responsible for the diminished first-pass
metabolism of gallopamil that we noted.
In contrast to the CLo, the plasma protein binding and
renal excretion of gallopamil are stereoselective. The plasma protein binding of both enantiomers is high, within the range previously reported for racemic gallopamil in healthy volunteers (Rutledge and
Pieper, 1987), and the enantiomers do not compete for protein binding
sites at the concentrations studied. Whole blood concentrations of both
enantiomers were substantially lower than serum concentrations, indicating that gallopamil does not preferentially associate with erythrocytes. The B/P values of (R)- and
(S)-gallopamil were similar and comparable after
separate and simultaneous administration, indicating that erythrocyte
uptake is not enantioselective or influenced by the optical antipode.
Less than 1% of the dose of either (R)- or
(S)-gallopamil was recovered in urine as unchanged
drug, as has been reported previously for racemic gallopamil
(Eichelbaum, 1989; Stieren et al., 1983). The urinary
recovery and renal clearance of gallopamil favored the
(S)-enantiomer, reflecting the higher fu
of (S)-gallopamil. Calculations using the creatinine
clearance and gallopamil fu indicated that net
stereoselective renal tubular secretion of gallopamil occurred but
accounted for only a negligible proportion of the total clearance of
gallopamil, and competition for renal tubular secretion was observed.
The renal clearance of (R)-gallopamil was comparable
after the 25- and 100-mg doses, indicating that net renal tubular
secretion was not saturated at the serum concentrations attained. The
urinary recovery of both enantiomers was greater after
(RS)-gallopamil than when the enantiomers were
administered separately, reflecting the higher gallopamil serum
concentrations.
Of the electrocardiographic parameters monitored, only AV node
conduction was affected by gallopamil. PR-interval prolongation was
observed after 25 mg of (S)- and 50 mg of
(RS)-gallopamil. No change occurred after 25 mg of
(R)-gallopamil or in five of six subjects after 100 mg of (R)-gallopamil. In all subjects, serum
concentrations of (R)-gallopamil were higher after
100 mg of (R)-gallopamil than after 50 mg of
(RS)-gallopamil and, for the pooled data, did not
produce a significant prolongation of the PR interval. Indeed, when the
serum concentration-effect data were analyzed using the sigmoidal
Emax model, the response after the racemate
could be accounted for by the (S)-gallopamil serum concentrations observed. Therefore, (R)-gallopamil
does not contribute to the pharmacological response when the racemate
is administered, and the PR-interval prolongation is elicited solely by
the (S)-enantiomer. These data in human volunteers
in vivo substantiate in vitro studies demonstrating that (S)-gallopamil is a more potent
calcium antagonist than (R)-gallopamil (Bayer
et al., 1975; Müller and Willsmann, 1982; van
Amsterdam et al., 1990). The greater effect observed after
the administration of the racemate was not caused by
(R)-gallopamil directly but rather by the increased
concentrations of (S)-gallopamil, which result from
the saturation of first-pass metabolism by the higher dose of the
racemate administered, relative to the doses of the single enantiomers.
The negative dromotropic effect of (RS)-gallopamil
has been observed in patients who were administered racemic gallopamil (Markus et al., 1992; Rettig et al., 1983;
Scrutinio et al., 1985). No changes in blood pressure were
observed in this investigation. Previous studies in normotensive
subjects have similarly shown that gallopamil does not influence
blood pressure (Brogden and Benfield, 1994). Furthermore, no change in
peripheral blood flow, peripheral vascular resistance or heart rate
were observed. These data confirm previous observations that
phenylalkylamine calcium antagonists do not have a pronounced effect on
the peripheral vasculature and thus are more cardioselective than other
classes of calcium channel blockers. Single-dose gallopamil
administration also did not alter plasma renin concentrations, and
verapamil also does not influence renin release (Chellingsworth and
Kendall, 1988; McTavish and Sorkin, 1989).
In summary, the first-pass metabolism of gallopamil is not stereoselective but saturable as the dose is increased. (S)-Gallopamil is a potent negative dromotropic agent, whereas (R)-gallopamil does not contribute to the prolongation of the PR interval that is observed when the racemate is administered. Studies using both the individual enantiomers and the racemate of a chiral drug are therefore required to fully describe the pharmacokinetics and pharmacodynamics of a drug used clinically as a racemic mixture.
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Acknowledgments |
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The enantiomers and metabolites of gallopamil were generously supplied by Knoll AG (Ludwigshafen, Germany). We thank Ms. Claudia Eser and Mr. Bernd Borstel for their expert technical assistance.
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Footnotes |
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Accepted for publication February 7, 1997.
Received for publication October 15, 1996.
1 This study was supported by the Robert-Bosch-Foundation (Stuttgart, Germany) and Knoll AG (Ludwigshafen, Germany).
2 Present address: Department of Clinical Pharmacology, Royal North Shore Hospital, St. Leonards NSW 2065, Australia.
3 Based on data from 14 healthy normal subjects after intravenous and oral administration; data on file at Knoll AG (Ludwigshafen, Germany).
Send reprint requests to: Dr. Gerd Mikus, Dr. Margarete Fischer-Bosch-Institut für Klinische Pharmakologie, Postfach 50 11 20, Auerbachstrasse 112, 70341 Stuttgart, Germany.
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Abbreviations |
|---|
Cmax, maximum serum concentration;
tmax, time at which Cmax occurs;
, terminal elimination rate constant;
t1/2 terminal elimination half-life, AUC,
area under the serum concentration-time curve;
MRT, mean residence
time;
fu, free fraction;
CLo, apparent
oral clearance;
CLR, renal clearance;
Ae, amount excreted unchanged in urine;
F, bioavailability;
B/P, ratio of
whole blood to serum gallopamil concentrations;
Emax, maximum effect;
EC50, serum gallopamil concentration eliciting 50% of
Emax, N, parameter affecting the slope of
the concentration-effect curve ;
Cn, serum gallopamil
concentration at n hours after drug administration;
ER, extraction ratio;
AV, atrioventricular;
GC-MS, gas chromatography-mass
spectroscopy.
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References |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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)- and (±)-verapamil after intravenous administration.
Br. J. Clin. Pharmacol.
17: 453-458, 1984[Medline].)-desmethoxyverapamil on heart and vascular smooth muscle.
J. Pharmacol. Exp. Ther.
242: 1090-1097, 1987This article has been cited by other articles:
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  A. Suzuki, I. Iida, F. Tanaka, M. Akimoto, K. Fukushima, M. Tani, T. Ishizaki, and K. Chiba Identification of Human Cytochrome P-450 Isoforms Involved in Metabolism of R(+)- and S(-)-Gallopamil: Utility of In Vitro Disappearance Rate Drug Metab. Dispos., November 1, 1999; 27(11): 1254 - 1259. [Abstract] [Full Text]
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) after the administration of 25 mg
of (R)-gallopamil HCl alone and of (S)-gallopamil (
) after the administration of 25 mg
of (S)-gallopamil HCl alone in subject 3.
) and (S)-gallopamil
(
) after the administration of 50 mg of pseudoracemic gallopamil HCl
in subject 3 .
), 25 mg of
(R)-gallopamil (
), 100 mg of
(R)-gallopamil (
).
