Division of Pharmaceutics and Biopharmaceutics, Faculty of
Pharmaceutical Sciences (S.K., H.W., S.A.Y., K.W.R., F.S.A.) and
British Columbia Research Institute of Children's and Women's Health,
Department of Obstetrics and Gynecology, Faculty of Medicine
(D.W.R.), The University of British Columbia, Vancouver, British
Columbia, Canada
Separate 24-h maternal and fetal infusions of valproic acid
(VPA) were administered to five pregnant sheep at 125 to 138 days gestation (term ~145 days) to determine maternal-fetal disposition. The pharmacokinetics of VPA were also investigated in five newborn 1-day-old lambs after a 6-h drug infusion. Plasma, urine, and amniotic
and fetal tracheal fluid samples were analyzed for VPA using gas
chromatography-mass spectrometry. During maternal drug infusion,
the average steady-state fetal/maternal unbound VPA plasma
concentration ratio was 0.81 ± 0.09. Unbound maternal-to-fetal VPA placental clearance (69.0 ± 20.2 ml/min/kg) was similar to that in the other direction (61.9 ± 24.2 ml/min/kg); this
indicates passive placental diffusion and intermediate placental
permeability of VPA in sheep. Newborn unbound VPA clearance (0.66 ± 0.28 ml/min/kg) was much lower than in the mother (5.4 ± 2.7 ml/min/kg) or the fetus (62.1 ± 22.4 ml/min/kg), and exhibited
pronounced Michaelis-Menten characteristics. The elimination half-life
of the drug was much longer in the newborn (18.6 ± 2.6 h)
relative to the mother (5.6 ± 1.4 h) and the fetus (4.6 ± 1.9 h). Thus, VPA elimination in newborn lambs is much slower
as compared with adult sheep, a situation similar to that in humans.
Plasma protein binding of VPA was saturable, with similar VPA binding
capacities and affinities in maternal and fetal plasma. VPA was
extensively displaced from binding sites in the newborn lamb during the
first 1 to 2 days of life, possibly because of increased plasma free
fatty acid concentrations at birth. Thereafter, newborn plasma appeared
to have a similar VPA binding capacity but lower affinity compared with
the mother and the fetus.
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Introduction |
Valproic
acid (2-propylpentanoic acid, VPA)2 is a
low-molecular-mass (144.2 Da) antiepileptic drug with a unique
branched-chain fatty acid structure (Davis et al., 1994
). There are
approximately 12,000 births per year to epileptic women in the U.S.
alone, and ~95% of these women are on antiepileptic therapy during
pregnancy (Vorhees et al., 1988). VPA is one of the four major drugs
(phenytoin, phenobarbital, carbamazepine, and VPA) used to treat
epilepsy in the pregnant population (Lindhout and Omtzigt, 1994; Malone and D'Alton, 1997). The use of all these drugs is associated with an
increased risk of numerous major birth defects, including
cardiovascular and neural tube defects, orofacial clefts, genitourinary
defects, and dysmorphic syndromes (Lindhout and Omtzigt, 1994; Malone
and D'Alton, 1997). In addition to the teratogenic effects associated with the use of VPA and other antiepileptic drugs, prenatal exposure to
these compounds may also result in alterations in cognitive function
and behavior during postnatal life (Trimble, 1990; Koch et al., 1996).
VPA undergoes extensive placental transfer in animals as well as humans
(Dickinson et al., 1979, 1980; Ishizaki et al., 1981; Nau et al., 1981,
1984; Nau and Krauer, 1986; Kondo et al., 1987). In humans,
cord-to-maternal blood VPA concentration ratios at birth range from 0.5 to 4.6 (Dickinson et al., 1979; Ishizaki et al., 1981; Nau et al.,
1981, 1982, 1984; Nau and Krauer, 1986; Kondo et al., 1987). Although
cord-to-maternal blood ratios of drug concentrations are highly
dependent on the timing of drug administration and sampling (Rurak et
al., 1991), the above data indicate a high degree of fetal VPA exposure
after maternal administration in humans. We have also previously
examined the placental transfer of VPA in chronically catheterized
pregnant sheep during late gestation after i.v. bolus administration
(Gordon et al., 1995). After maternal i.v. bolus administration, VPA
appeared rapidly in plasma (within 2 min), and the average fetal drug
exposure index based on fetal-to-maternal arterial plasma area under
the curve of arterial plasma concentration-time profile (AUC)
ratio was 0.41.
In humans, the plasma protein binding of VPA gradually decreases in the
mother over the course of gestation, whereas that in the fetus
gradually increases, such that at birth, fetal VPA plasma protein
binding exceeds that in the mother (Nau and Krauer, 1986). Also, there
is an additional reduction in maternal plasma protein binding of the
drug at birth due to elevated plasma free fatty acids (Nau et al.,
1984; Nau and Krauer, 1986). These phenomena result in fetal
accumulation of the drug and a greater than unity cord-to-maternal
blood VPA ratio at birth (Nau et al., 1984).
The reported elimination half-life of VPA in less than 1-month-old
human newborns ranges from 15.1 to 80 h, which is ~2- to 8-fold longer than the values in adults (Dickinson et al., 1979; Ishizaki et al., 1981; Nau et al., 1981, 1984; Irvine-Meek et al.,
1982; Kondo et al., 1987; Gal et al., 1988). Similar findings have been
reported for newborns of other species such as rats and guinea pigs (Yu
et al., 1987; Haberer and Pollack, 1994). This indicates a much reduced
elimination capacity for the drug during the immediate newborn period
in all species studied.
In spite of the considerable body of data described above, there have
been very few studies to systematically examine the fetal and newborn
disposition and metabolism of VPA in animals or humans, and the exact
reasons behind the impaired VPA elimination in human or animal newborns
are not known. Thus we undertook a series of studies in chronically
catheterized pregnant sheep and newborn lambs to examine the
steady-state placental transfer, comparative pharmacokinetics and
plasma protein binding (this article), as well as renal excretion and
metabolism (accompanying article, Kumar et al., 2000b) of VPA in the
mother, fetus, and newborn. The specific issues that we have addressed
in the current manuscript are the mechanism of VPA placental transfer
(active or passive), the extent of fetal drug exposure after maternal dosing, and the role of different factors determining fetal VPA exposure, such as placental permeability, drug lipophilicity, and fetal
drug clearance capacity. In addition, we have quantitatively evaluated
and compared the VPA elimination capacity in maternal, fetal, and
newborn sheep so as to assess the development of VPA elimination routes
during late gestation and early newborn period in this species.
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Experimental Procedures |
Animals and Surgical Preparation.
All studies were approved by the University of British Columbia Animal
Care Committee, and the procedures performed on sheep conformed to the
guidelines of the Canadian Council on Animal Care.
Pregnant sheep.
Five pregnant Dorset Suffolk cross-bred ewes, with a maternal body
weight of 77.5 ± 10.6 kg (mean ± S.D.), were surgically prepared between 121 and 125 days gestation (term ~145 days) using procedures described previously (Kumar et al., 1997). Briefly, surgery
was conducted under halothane (1-2%)-nitrous oxide (70%) anesthesia
after induction with i.v. pentothal (1 g) and used sterile
techniques throughout. After a midline abdominal incision to the ewe,
access to the fetal head and hindquarters was gained via two separate
uterine incisions in the areas devoid of major blood vessels and
placental cotyledons. Polyvinyl catheters (Dow Corning, Midland, MI)
were implanted in both fetal femoral arteries and lateral tarsal veins,
a fetal carotid artery, fetal trachea, amniotic cavity, and in four
animals, in the fetal urinary bladder (via a suprapubic incision).
Amniotic fluid lost during surgery was replaced with warm sterile
irrigation saline, and the uterine and abdominal incisions were closed
in layers. Catheters were also implanted in a maternal femoral artery
and vein. After a recovery period of at least 3 days, the sheep
were moved to a monitoring pen adjacent to and in full view of the
holding pen for experimentation purposes.
Newborn lambs.
For newborn lamb studies, five additional pregnant sheep were prepared
surgically as above at 121 to 132 days gestation. Lambs were allowed to
undergo spontaneous delivery at term (139-143 days) along with their
intact catheters. Experiments on the lambs were started the day after
birth. On the day of experiment, the lambs were placed in small pens in
full view of the mother, and were fed mother's colostrum at intervals.
After completion of the drug infusion period (6-h), the lambs were
returned to their mother. Cumulative newborn lamb urine samples were
collected in the following manner. A sterile bag was attached to the
bladder catheter, then bag was placed in a small plastic housing that was bandaged to the abdomen of the animal. This permitted free movement
of the lambs within the pen and also allowed them to nurse on their
mother ad libitum.
Pregnant Sheep Experiments.
All experiments on pregnant sheep were completed between 125 and 138 days gestation (term ~145 days). Two sets of experiments were carried
out on all five pregnant sheep in a randomized manner and with an
appropriate washout period in between.
Maternal administration.
A bolus loading dose of VPA (sodium valproate; Sigma Chemical Co., St.
Louis, MO) equivalent to 20.1 mg VPA/kg maternal body weight was
administered to the ewe via the maternal femoral venous catheter over 1 min; this was followed immediately by a 24-h continuous infusion of the
drug at 138.3 µg/min/kg via the same route.
Fetal administration.
The fetal experimental protocol was similar to that for the maternal
experiments described above, except that doses were administered via
the fetal lateral tarsal vein and were reduced to one-fourth the
maternal doses (i.e., 5.0 mg/kg bolus and 34.6 µg/min/kg infusion rate based on maternal body weight).
Newborn Lamb Experiments.
As mentioned above, the newborn lamb experiments were begun the day
after birth. Drug administration involved a 10 mg/kg bolus administered
over 1 min via the lateral tarsal vein, followed immediately by a
continuous 6-h infusion at 138.3 µg/min/kg via the same route. VPA
was infused to newborn lambs only for a period of 6 h for two
reasons. Firstly, the drug caused marked sedation in lambs so
much so that it interfered with their normal feeding behavior.
Secondly, long-term infusion necessitated separation of the lambs from
their mothers for at least the infusion duration, and hence restriction
of their free movement, nursing, and maternal-newborn bonding.
All doses were prepared in sterile water for injection and were
sterilized by filtering through a 0.22-µm nylon syringe filter (MSI,
Westboro, MA) into a capped empty sterile injection vial.
In all pregnant sheep experiments, serial blood samples were collected
from the fetal (2 ml) and maternal (3.0 ml) femoral arterial catheters
at 5 min, and 0.5, 1, 3, 6, 9, 12, 20, 22, and 24 h during the
infusion, and at 0.5, 1, 3, 6, 9, 12, 24, 36, 48, 60, and 72 h
postinfusion. Fetal femoral arterial samples (0.5-ml) were also
collected at the same time intervals for blood gas analysis and
measurement of glucose and lactate concentrations. All fetal blood
removed for sampling during the experiment was replaced, at intervals,
by an equal volume of blood obtained from the mother before the start
of the experiment or from another ewe (after the first day). Samples of
maternal and fetal urine, amniotic fluid, and fetal tracheal fluid were
also collected at predetermined intervals.
During the newborn lamb experiments, serial femoral arterial blood
samples (2-ml) were collected at 5 min, and 0.5, 1, 2, 3, 4, 5, and
6 h during the infusion, and at 0.5, 1, 2, 4, 6, 18, 30, 42, 54, 66, 78, and 90 h postinfusion. Cumulative urine samples were also
collected for 96 h.
All maternal, fetal, and newborn blood samples were placed into
heparinized Vacutainer tubes (Becton-Dickinson, Rutherford, NJ) and
centrifuged at 2000g for 10 min. The plasma supernatant was
removed and placed into clean borosilicate test tubes with polytetrafluoroethylene-lined caps. Samples were stored frozen at
20°C until the time of analysis.
Determination of VPA Plasma Protein Binding.
The unbound plasma concentrations of VPA were measured ex vivo in all
fetal, maternal, and newborn plasma samples by an ultrafiltration procedure at 1000g for 30 min using Centrifree
micropartition devices (Amicon, Inc., Danver, MA). Briefly, ~0.75 ml
of plasma was placed into Centrifree micropartition devices, followed
by centrifugation at 1000g for 30 min and measurement of
ultrafiltrate VPA concentrations as described below. Plasma samples for
the determination of unbound VPA concentrations were stored in separate aliquots so as to avoid repetitive thawing that could result in lipolysis and release of free fatty acids, and, hence, competitive displacement of bound VPA from plasma binding sites (Haberer and Pollack, 1994).
Drug and Metabolite Assay.
The concentrations of VPA and its metabolites in all biological fluids
and plasma ultrafiltrate were measured using a previously developed gas
chromatographic-mass spectrometric analytical method (Yu et al., 1995).
Validation studies demonstrate that the variability and bias of this
assay for all compounds does not exceed 15% (Yu et al., 1995). In this
article, only the VPA concentrations will be reported. The metabolite
data are presented in the companion article (Kumar et al., 2000b). The
lower limit of quantitation of this assay for VPA was 25 ng/ml using
0.1 ml of plasma.
Physiological Recording and Monitoring Procedures.
Fetal
blood pH, Po2, and Pco2 were measured using an
IL 1306 pH/blood gas analyzer (Allied Instrumentation Laboratory,
Milan, Italy). Blood O2 saturation and hemoglobin
concentration were determined using a Hemoximeter (Radiometer,
Copenhagen, Denmark). Blood glucose and lactate concentrations were
determined with a 2300 STAT plus glucose/lactate analyzer (Y.S.I. Inc.,
Yellow Springs, OH).
Pharmacokinetic Analyses.
For the analysis of VPA protein binding data in maternal, fetal, and
newborn plasma, plasma protein-bound concentrations of the drug were
first calculated from the difference between corresponding experimentally measured total and unbound concentrations. Plasma protein binding parameters were calculated separately for maternal, fetal, and newborn plasma using the respective pooled bound and unbound
plasma concentration data from all animals. Rosenthal plots
(bound/unbound concentration versus bound concentration) were first
constructed to identify multiplicity of the binding sites. The bound
versus unbound concentration data were then fitted to the appropriate
binding model using the nonlinear least-squares regression program
ADAPT II to estimate the binding parameters (D'Argenio and Schumitzky,
1997). The best binding model was identified based on the reduction in
sum of squared residuals and Akaike's Information Criterion (AIC).
Net steady-state maternal and fetal clearances of the unbound
drug (CLum(net) and
CLuf(net), respectively) were
calculated by dividing the respective VPA infusion rate by the
corresponding steady-state arterial plasma unbound VPA concentration
(Gibaldi and Perrier, 1982). The net maternal and fetal clearances of
the total drug (CLm(net) and CLf(net), respectively) were calculated in an
analogous fashion, except that total instead of unbound maternal and
fetal arterial plasma concentrations were used.
Placental and nonplacental clearances of unbound VPA in the ewe and the
fetus were calculated from the maternal and fetal steady-state unbound
arterial plasma VPA concentrations according to the two-compartment
model described earlier (Szeto et al., 1982b). This model is based on
separate steady-state maternal and fetal drug administration and
assumes bidirectional placental transfer and drug elimination from both
maternal and fetal compartments. Thus, the model is defined by four
clearance parameters, including maternal-to-fetal placental clearance
(CLmf), fetal-to-maternal placental clearance
(CLfm), maternal nonplacental clearance
(CLmo), and fetal nonplacental clearance
(CLfo), all clearances of the total drug.
Maternal and fetal total clearances of the total drug (CLmm and CLff,
respectively) equal the sum of their respective placental and
nonplacental clearances. All these clearance parameters were calculated
from the following equations (eqs. 1-6) as described previously (Szeto
et al., 1982b).
The symbols ko and
ko' denote the drug infusion rates to the
mother and the fetus, respectively. Cm and
Cf are the steady-state plasma drug
concentrations in the mother and the fetus after maternal VPA
administration, respectively, and Cm' and
Cf' are the steady-state maternal and fetal
plasma drug concentrations, respectively, after fetal VPA administration.
The net maternal and fetal clearances described above sum the overall
rate of drug elimination from the maternal-fetal unit via maternal and
fetal nonplacental routes after maternal or fetal drug administration,
respectively. These clearances are related to the corresponding
two-compartment estimates of total maternal or fetal clearance by the
following relationships (Szeto et al., 1982b; Wang et al., 1986):
and,
The presence of saturable plasma protein binding presented a
complexity in the estimation of placental and nonplacental clearances for VPA using the above method. The two-compartment model assumes unaltered linear pharmacokinetics between the maternal and fetal drug
administration experiments. However, this assumption was violated for
VPA because of significantly different steady-state maternal plasma
unbound fractions during maternal and fetal dosing (0.35 ± 0.13 versus 0.18 ± 0.08, respectively; Table
1); this would result in proportional
alterations in total (not unbound) maternal VPA clearance during the
two experimental periods for this low-clearance drug (Wilkinson and
Shand, 1975). Thus, steady-state unbound plasma VPA concentrations were
used for the estimation of maternal and fetal placental and
nonplacental clearances. The "effective" placental and nonplacental
clearances of total VPA in the mother and the fetus were calculated as
the product of respective unbound clearance parameter and the
steady-state maternal (in case of maternal clearances) or fetal (in
case of fetal clearances) plasma unbound fraction.
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TABLE 1
Mean steady-state maternal and fetal plasma concentrations of total and
unbound VPA, maternal-fetal plasma VPA concentration ratios, and
steady-state plasma unbound fractions in five pregnant ewes
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All other pharmacokinetic parameters were calculated by the equations
described below (Gibaldi and Perrier, 1982). Maternal and fetal
parameters were calculated using the data from maternal and fetal
infusion experiments, respectively.
Mean residence time (MRT) of VPA (total or unbound):
where ko, 
, and
Dbolus are the infusion rate, infusion duration,
and initial bolus dose of VPA, respectively. Plasma
AUMC0-
and AUC0-
values of the unbound and total drug were calculated by the linear trapezoidal rule.
Total body clearance (CLtb) of VPA (total
or unbound):
Area-weighted unbound fraction of the drug
(fp):
Steady-state volume of distribution of the unbound drug
(Vduss):
Steady-state volume of distribution of the total drug
(Vdss):
Steady-state volume of distribution of the total drug corrected
for the effects of saturable plasma protein binding
(Vdss'):
For drugs with saturable plasma protein binding,
Vduss is constant only for a
particular fp value and can be used to
relate steady-state plasma concentrations to the amount of the drug in the body if the steady-state unbound fraction of the drug is equal to
fp (McNamara et al., 1983). Also for such
drugs, the Vdss of the total drug obtained using
eq. 13 overestimates the "true" Vdss.
Instead, Vdss' provides a more reliable estimate
of true Vdss (McNamara et al., 1983). As with
Vduss above, this
Vdss' parameter is also constant only for a
particular fp or a steady-state plasma
unbound fraction equivalent to fp.
Maternal and fetal terminal elimination half-life
(t1/2
) of the total and unbound drug was
obtained from a two-compartment model fitting of the data using the
nonlinear least-squares regression software WinNonlin (Scientific
Consulting, Inc., Apex, NC). Maternal t1/2 values were calculated from the
data obtained during maternal VPA infusion, whereas fetal
t1/2 values were calculated from those
obtained during fetal infusion. The terminal plasma t1/2 of the total and unbound drug
in the newborn lamb was approximated using a one-compartment model.
These model fittings were carried out using a weighting factor of
1/predicted (y2).
Statistical Analysis.
All data are reported as mean ± S.D. unless otherwise indicated.
The achievement of steady state for total and unbound VPA concentrations in maternal and fetal plasma was established according to two criteria: 1) the slope of the plasma concentration versus time
curve should not be significantly different from zero, and 2) the
coefficient of variation of the measured concentrations should be
<10%. Maternal and fetal steady-state plasma concentrations measured
during the same infusion experiment were compared using the paired
t test. All maternal versus fetal pharmacokinetic parameters including net clearances, two-compartment clearances (placental, nonplacental, and total), overall clearances, volumes of distribution, half-lives, and MRT values were also compared using the paired t test. Amniotic and fetal tracheal fluid VPA concentrations
were also compared with fetal plasma unbound concentrations from the same experiment using the paired t test. Newborn
pharmacokinetic parameters were compared with those in the ewe and
fetus using the unpaired t test. The significance level was
P < .05 in all cases. Fetal weight in utero at the
time of experimentation was estimated from the weight at birth and the
time interval between the experiment and birth (Koong et al., 1975).
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Results |
The average maternal body weight was 77.5 ± 10.6 kg and
estimated fetal body weights on the day of maternal and fetal VPA infusion were 2.93 ± 0.21 and 2.98 ± 0.34 kg, respectively.
The mean gestational age on the day of maternal and fetal experiments was 129.8 ± 4.3 and 130.2 ± 2.2 days, respectively, and
these were not statistically different (paired t test,
P > .05). During maternal experiments, the control
period fetal femoral arterial blood pH, Po2,
Pco2, O2-saturation, and
hemoglobin, glucose, and lactate concentrations were 7.344 ± 0.047, 20.8 ± 5.9 mm Hg, 50.4 ± 1.8 mm Hg, 41.6 ± 15.4%, 12.6 ± 3.2 g/dl, 0.54 ± 0.08 mM, and 0.91 ± 0.40 mM, respectively. Likewise during fetal administration, the
control values for these variables were 7.313 ± 0.067, 20.0 ± 4.1 mm Hg, 49.9 ± 2.6 mm Hg, 38.0 ± 13.3%, 12.8 ± 3.6 g/dl, 0.95 ± 0.48 mM, and 1.27 ± 0.80 mM, respectively.
Apart from a small increase in fetal blood lactate concentrations
during both maternal (1.40 ± 0.54 mM) and fetal (1.40 ± 0.25 mM) infusions, there were no systematic changes in any of these
variables during the experimental period. Because appropriate control
saline infusion experiments were not carried out, a more detailed
analysis of these data was not possible.
Placental Transfer and Maternal and Fetal Plasma Protein Binding of
VPA.
Figure 1A shows representative total as
well as unbound maternal and fetal arterial plasma concentration-time
profiles of VPA during and after a 24-h maternal VPA infusion. Figure
1B shows similar profiles during and after a 24-h fetal VPA infusion.
Total as well as unbound VPA concentrations in maternal and fetal
plasma were at steady state during the 6- to 24-h infusion period
according to the criteria described in the data analysis section.
Placental transfer of VPA was rapid, with significant concentrations
detected on the other side of placenta at the first sampling time (5 min) after drug dosing (Fig. 1).
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Fig. 1.
Representative concentration versus time
profiles of total and unbound VPA in maternal and fetal plasma during
and after a 24-h steady-state VPA infusion (E5108).
A, maternal infusion; B, fetal infusion.
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Table 1 presents the steady-state maternal and fetal femoral arterial
unbound as well as total plasma concentrations and unbound fractions of
VPA during separate maternal and fetal 24-h infusion experiments.
During maternal drug infusion, the fetal steady-state unbound and total
plasma concentrations (Cuf and
Cf) were significantly lower compared with the
corresponding maternal values
(Cum and
Cm). Thus, the indices of fetal drug exposure
based on both unbound and total steady-state fetal/maternal plasma
concentration ratios (0.81 ± 0.09 and 0.70 ± 0.10) were
significantly less than unity. Similarly, during fetal drug infusion,
the steady-state maternal unbound as well as total plasma
concentrations (Cum' and
Cm') were significantly lower than the
corresponding fetal values (Cuf'
and Cf'). Also, the
Cm'/Cf' (0.60 ± 0.08)
and
Cum'/Cuf'
(0.38 ± 0.13) ratios were significantly lower than unity.
Plasma protein binding of VPA was saturable (or nonlinear) in both the
mother and fetus, and thus varied inversely with total VPA
concentration. The range of plasma-unbound VPA fractions in maternal
and fetal plasma during maternal drug administration were 0.09 to 0.45 and 0.12 to 0.44, respectively. Similarly, these ranges during fetal
drug infusion were 0.10 to 0.21 and 0.12 to 0.53, respectively. VPA
plasma unbound fraction in both maternal and fetal plasma at steady
state was constant during individual administration experiments because
maternal and fetal plasma total as well as unbound plasma
concentrations were relatively constant (Fig. 1). Table 1 also presents
steady-state maternal and fetal plasma unbound fractions during both
maternal and fetal drug infusions. The average steady-state maternal
plasma unbound fraction of VPA during fetal drug infusion was
significantly lower compared with that during maternal drug infusion
(0.18 ± 0.08 versus 0.35 ± 0.13). However, the average
steady-state fetal plasma unbound fractions during maternal and fetal
infusion experiments were not significantly different (0.39 ± 0.12 versus 0.29 ± 0.08).
Because of insufficient data points for individual animals to
characterize plasma protein binding at a wide enough range of concentrations, binding parameters were estimated separately for the
mother and the fetus using pooled maternal and fetal data from all
experiments. Figure 2 shows VPA binding
characteristics in maternal and fetal plasma using pooled bound and
unbound VPA concentration data from all experiments. Rosenthal plots in
both maternal and fetal plasma (Fig. 2, A and C, respectively) show a
biphasic curvilinear relationship. The initial steep declining portion
of the Rosenthal plots suggests the presence of a high-affinity but
low-capacity (saturable) binding site, whereas the relatively flat
portion of the curves suggests a linear (nonsaturable) or another
saturable binding site with a low affinity but high binding capacity.
Statistically better fits (lower AIC and sum of squares, smaller
c.v. values for fitted parameters) were obtained when the bound
versus unbound concentration data were fit to a two-site binding model
with one saturable and one nonsaturable binding site (eq. 15) as
compared with a one-site binding model.
where, Cb and Cu are
the corresponding bound and unbound plasma concentrations.
Bmax1 and Bmax2 are the
maximal VPA binding capacities of the first and second binding site,
respectively. Kd1 and
Kd2 are the equilibrium dissociation
constants of VPA at the first and second binding site, respectively.
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Fig. 2.
Maternal and fetal plasma protein binding
characteristics of VPA; pooled data from all maternal and fetal VPA
infusion experiments.
Cb and Cu are the maternal or fetal plasma
concentrations of the bound and unbound VPA. A and C, the Rosenthal
plots of the data in maternal and fetal plasma, respectively,
demonstrating a biphasic relationship. B and D, the relationship
between Cb versus Cu in maternal
and fetal plasma, respectively; actual data (scatter points) and
model-predicted line obtained from the fit of the data to a
two-site binding model are depicted. Different scatter plot symbols
denote data from different animals.
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Figure 2, B and D, shows the scatter plots of the pooled bound versus
unbound VPA concentration data in maternal and fetal plasma,
respectively, from all the experiments. The model-predicted lines based
on eq. 15 are also depicted, and indicate excellent fit of the data to
the above model. From Fig. 2, B and D, it appears that the data from
one animal (E105x), which had higher VPA plasma concentrations compared
with the other four ewes, could unduly influence the final maternal and
fetal VPA binding parameter estimates. Hence, maternal and fetal VPA
plasma protein binding parameters were also estimated after excluding
the data from this animal. Table 2
presents the estimates of binding parameters of VPA in maternal and
fetal plasma obtained from the entire pooled data as well as those
obtained after excluding the data from E105x. The estimates of VPA
binding parameters for maternal plasma were similar using the entire
data or after excluding E105x from analysis. However, data from E105x
do appear to significantly influence the binding parameters for fetal
plasma (Table 2). Using the entire data, the maximal VPA binding
capacity of maternal and fetal plasma at the high-affinity saturable
binding site (Bmax1) is similar (62.8 versus 65.0 µg/ml); however, after excluding the data from E105x, the capacity of
fetal plasma at this site appears somewhat lower (46.4 µg/ml).
Similarly, the affinity of this binding site toward VPA appears
substantially lower in fetal plasma relative to that in maternal plasma
using the entire pooled data (Kd1 of 7.6 versus 13.0 µg/ml in maternal and fetal plasma, respectively).
However, there is no difference in the VPA binding affinity of maternal
and fetal plasma after the data from E105x are excluded
(Kd1 of 6.8 versus 7.2 µg/ml,
respectively). Thus, it appears that in E105x, fetal plasma had a
substantially higher capacity and lower affinity for VPA binding as
compared with the remaining four ewes. Although the exact reasons
underlying this remain unclear, it may result from the presence of
higher plasma protein concentrations in combination with significant
concentrations of certain endogenous substances in fetal plasma that
can compete with VPA for binding sites (e.g., bilirubin, free fatty
acids).
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TABLE 2
Estimated plasma protein binding parameters of VPA in maternal, fetal,
and newborn sheep obtained by fitting the pooled data to a one- or
two-site binding model
Data are presented as estimated mean value ± S.E.
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Placental and Nonplacental Clearances of VPA in the Mother and the
Fetus.
Table 3 presents the weight-normalized
steady-state maternal and fetal clearances of VPA based on unbound as
well as total drug concentrations. Maternal net, total, and
nonplacental clearances are normalized to maternal body weight, whereas
all other clearances are normalized to the estimated fetal body weight.
Normalization of maternal as well as fetal placental clearance to
estimated fetal body weight allows for an easier comparison of the
bidirectional placental transfer efficiency. The net maternal and fetal
clearances are lower compared with the corresponding two-compartment
estimates of total maternal or fetal clearance because of the
relationships described in eqs. 7a and 8a (Szeto et al., 1982b; Wang et
al., 1986). Fetal net
(CLuf(net)) and total
(CLuff) clearances of the
unbound drug were significantly greater compared with the corresponding
maternal clearances (CLum(net)
and CLumm, respectively).
However, there was no significant difference between maternal and fetal
placental clearances of the unbound drug
(CLumf and
CLufm, respectively). Also,
CLufo was significantly greater
than CLumo.
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TABLE 3
Mean steady-state maternal and fetal placental and nonplacental
clearances of unbound and total VPA in five pregnant ewes during late
gestation
Maternal net, total, and nonplacental clearances are expressed as
milliliters per minute per kilogram of maternal body weight; all other
clearances are expressed as milliliters per minute per kilogram of
estimated fetal body weight.
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The maternal clearances of the total VPA
(CLm(net), CLmm,
CLmf, and CLmo) presented
in Table 3 are calculated as the product of steady-state maternal
plasma unbound fraction during maternal infusion and the corresponding
maternal clearance of the unbound drug. Similarly, the fetal clearances
of total VPA (CLf(net), CLff, CLfm, and
CLfo) are the product of steady-state fetal
plasma unbound fraction during fetal drug infusion and the
corresponding fetal clearance of the unbound drug. As with the unbound
clearances above, CLf(net),
CLff, and CLfo values were
significantly higher compared with CLm(net),
CLmm, and CLmo values,
respectively; however, CLmf and
CLfm values were not significantly different.
The average contribution of maternal nonplacental clearance to net
maternal drug elimination
(%CLmo/CLm(net)) based
either on total or unbound concentrations was 79.6 ± 13.2%, with
placental elimination accounting for the remainder (20.4 ± 13.2%). The contribution of fetal nonplacental clearance to net fetal
drug elimination (%CLfo/CLf(net)) was
significantly lower compared with that in the mother (41.7 ± 19.6%; P < .005). Thus, there is a significantly greater contribution of fetal placental clearance to net fetal drug
elimination as compared with the mother (58.3 ± 19.6%).
Amniotic and Fetal Tracheal Fluid Disposition of VPA.
In all animals, VPA was detectable in amniotic and fetal tracheal
fluids at the earliest sampling time, i.e., 5 min after the start of
maternal or fetal VPA infusion. As with maternal and fetal plasma, the
amniotic and tracheal fluid VPA concentrations were also at an apparent
steady-state during the 6- to 24-h period of infusion (Fig.
3). In general, tracheal fluid
concentrations exhibited somewhat larger fluctuations compared with
amniotic fluid. The steady-state VPA concentrations in amniotic fluid
during maternal and fetal infusion were 17.3 ± 13.8 and 11.0 ± 4.7 µg/ml, respectively, whereas those in tracheal fluid were
2.2 ± 1.4 and 1.0 ± 0.5 µg/ml, respectively. The tracheal
fluid VPA concentrations were lower compared with the fetal plasma
unbound concentrations during both fetal (statistically significant)
and maternal (not significant) drug administration. Similarly, the
amniotic fluid VPA concentrations were lower than the fetal plasma
unbound VPA concentrations in all but one animal; however, the
difference between means was not statistically significant during
maternal or fetal drug administration. During the postinfusion period, the concentrations in these fluids appeared to decline in parallel with
maternal and fetal plasma concentrations (Figs. 1 and 3).
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Fig. 3.
Representative amniotic and fetal tracheal
fluid concentration versus time profiles of VPA in E4241, showing
apparent steady-state concentrations in these fluids during the 6- to
24-h infusion period and subsequent rapid decline in concentrations
during the postinfusion phase.
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Overall Maternal and Fetal Pharmacokinetics of VPA.
Table 4 presents the overall
pharmacokinetic parameters of VPA in the mother and the fetus. The mean
maternal and fetal total body clearances of the unbound as well as the
total drug presented in Table 4 are similar to the corresponding net
steady-state clearances presented in Table 3. Like the steady-state net
fetal clearance (Table 3), the fetal CLtb and
total body unbound clearance (CLutb) values were
significantly higher than the corresponding maternal values (Table 4).
Fetal terminal elimination half-life of the unbound drug
(t1/2u)
was significantly shorter than the corresponding maternal t1/2u
value (3.1 ± 1.3 versus 5.5 ± 1.9 h). However, there
was no significant difference between maternal and fetal
t1/2 values based on total plasma drug
concentrations (t1/2) (5.6 ± 1.4 versus 4.6 ± 1.9 h). Maternal
t1/2 values based on maternal plasma
unbound
(t1/2u)
and total drug (t1/2) concentrations
were not statistically different (5.5 ± 1.9 versus 5.6 ± 1.4 h). In contrast, fetal
t1/2u
was significantly shorter than fetal
t1/2 (3.1 ± 1.3 versus 4.6 ± 1.9 h). There was no significant difference between MRT of the
drug in maternal and fetal circulation based either on total or unbound
drug concentrations. However, in both the mother and the fetus, the MRT
of the unbound drug was significantly shorter compared with that for
the total drug. All fetal steady-state volume of distribution
parameters were significantly greater than the corresponding maternal
values.
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TABLE 4
Comparative maternal and fetal pharmacokinetic parameters of the
unbound and total VPA in five pregnant ewes during late gestation
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Pharmacokinetics of VPA in Newborn Lambs.
Figure 4A shows representative plasma
concentration versus time profiles of the total as well as unbound VPA
in a newborn lamb over the entire sampling period. Unbound and total
VPA concentrations did not reach steady state in newborn plasma during
the 6-h infusion period. As illustrated more clearly in Fig. 4B, VPA
continuously accumulated throughout the infusion period and resulted in
a plasma Cmax value of 136.9 ± 30.6 µg/ml
(range 107.5-173.3 µg/ml). This accumulation behavior of VPA in
lambs is different from the VPA pharmacokinetic profile in maternal and
fetal plasma during the initial 6 h of the 24-h infusion period
after maternal or fetal dosing (Fig. 4C). Also, in contrast to the
mother and the fetus (Fig. 1), a typical convexity was evident during
the postinfusion portion of the unbound and total plasma concentration
versus time pharmacokinetic profile in the newborn lamb (Fig. 4A). This
convexity is characteristic of Michaelis-Menten nonlinear
pharmacokinetic behavior (Gibaldi and Perrier, 1982). Pharmacokinetic
parameters of unbound and total VPA in four newborn lambs are presented
in Table 5 (plasma samples were not
available from the fifth lamb due to catheter failure). Although VPA
appears to exhibit pronounced nonlinear pharmacokinetics in lambs,
valid comparisons of newborn pharmacokinetic parameters with those of
the mother and the fetus can be made because of a similar range of
plasma concentrations observed. The average
CLutb as well as
CLtb values in newborn lambs was significantly
lower than the corresponding values in the mother and the fetus. The terminal t1/2 and MRT of the unbound as
well as total VPA were significantly longer in newborn lambs compared
with the corresponding values in the mother and the fetus. All
steady-state volume of distribution parameters
(Vduss,
Vdss, Vdss') in the newborn
lamb were significantly lower compared with the fetus but were not
statistically different from those of the mother.
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Fig. 4.
Plasma VPA concentrations during a 6 h i.v.
infusion protocol in newborn lambs and the first 6 h of a 24-h maternal
i.v. infusion protocol in pregnant sheep.
A, representative plasma concentration versus time profile of
total and unbound VPA in the newborn lamb NL0123z; B, unbound and total
VPA plasma concentrations in NL0123z during the 6-h infusion period,
showing continuous accumulation of the drug in newborn plasma; C,
typical plasma concentration versus time profiles of total and unbound
VPA in maternal and fetal plasma during the initial 6-h period of the
24-h maternal VPA infusion in E5108, showing lack of any drug
accumulation. Profiles similar to (C) were also observed in all animals
after fetal VPA infusion.
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Similar to the mother and the fetus, the plasma protein binding of VPA
in newborn lambs was saturable over the concentration range encountered
in these studies, with plasma unbound fractions ranging from 0.08 to
0.92. However, as opposed to the mother and the fetus, no distinct
relationships were apparent when the entire pooled newborn bound and
unbound VPA plasma concentration data were analyzed in a fashion
similar to Fig. 2 (plot not shown). In contrast, there were two clear
patterns of VPA binding in newborn plasma samples. These are depicted
in Fig. 5, which shows the newborn VPA
plasma protein binding characteristics in pooled data from the four
lambs. There are profound differences in VPA binding characteristics of
newborn plasma sampled on day 1 (0-24 h; Fig. 5, A-B) and that
sampled after day 1 (24-96 h; Fig. 5, C-D) of our experiments. On day
1 (Fig. 5A), the bound/unbound concentration ratio is positively
related to the bound concentration as opposed to an inverse
relationship between these two variables in the mother and the fetus
(Fig. 2, A and C). Also, in contrast to the mother and the fetus, there
is no obvious relationship between bound and unbound concentrations
(Fig. 5B). However, in the newborn plasma samples obtained after day 1 (Fig. 5, C and D) the situation appears to be similar to that of the
mother and the fetus (Fig. 2). As in the mother and the fetus, a
biphasic relationship appears to exist in the Rosenthal plot of the
data (Fig. 5C). However, enough data points were not available in each
phase to adequately model the data according to a two-site binding
model. Hence, these data were modeled according to a
one-saturable-binding-site model and the estimates of binding
parameters are presented in Table 2. Fit of the data to this one-site
binding model resulted in a lower AIC as compared with the fit to a
two-site binding model.
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Fig. 5.
Plasma protein binding characteristics of
VPA in newborn lambs during day 1 (A-B) and after day 1 (C-D) of
experiments.
Cb and Cu are the plasma concentrations of the
bound and unbound VPA. A and C, the Rosenthal plots. B and D, the
relationships between Cb and Cu. The data in
(D) were fitted to a one-site binding model and the model-predicted
line is also shown. Different scatter plot symbols denote data from
different animals. Striking differences in VPA binding characteristics
between the two groups of data are evident.
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Discussion |
Placental Transfer and Maternal and Fetal Plasma Protein
Binding of VPA.
The steady-state maternal and fetal total plasma concentrations of VPA
during maternal as well as fetal drug administration were near the
therapeutic range in humans (40-100 µg/ml, Davis et al., 1994).
During maternal drug administration, the lower fetal steady-state
plasma-unbound VPA concentrations compared with the mother indicate
irreversible drug elimination by the fetus and/or the placenta (Szeto
et al., 1982b; Wang et al., 1986). Fetal-to-maternal steady-state
plasma concentration ratios (0.70 ± 0.10 and 0.81 ± 0.09 based on total and unbound drug, respectively) indicate high fetal VPA
exposure after maternal administration. The average unbound VPA
Cf/Cm ratio is
higher than that of a number of other drugs studied in pregnant sheep
at this stage of gestation. These include diphenhydramine (0.50, Kumar
et al., 2000a), morphine (0.14, Szeto et al., 1982a), methadone (0.42, Szeto et al., 1982a), and metoclopramide (0.67, Riggs et al., 1990).
This is in spite of the fact that VPA placental clearance (and
permeability, see below) is lower compared with many drugs including
diphenhydramine, methadone, and metoclopramide. However, the VPA
Cf/Cm ratio is similar to that of acetaminophen (0.77, Wang et al., 1986). Because at
steady state, Cf/Cm = CLmf/(CLfm + CLfo) (Szeto et al., 1982b), it is important to
note that VPA and acetaminophen also have the lowest values for
CLfo among all these drugs. This emphasizes the
importance of fetal drug elimination capacity in determining fetal drug exposure.
The estimates of plasma protein binding parameters after excluding the
data from E105x, which had much higher VPA plasma concentrations as
compared with the remaining four ewes, indicate a similar VPA binding
affinity for maternal and fetal plasma. In contrast, the fetal plasma
VPA binding capacity appears to be somewhat lower than that of the
mother; this may be related to lower fetal plasma protein
concentrations (Kwan et al., 1995). The steady-state unbound fraction
in maternal plasma during maternal drug administration (0.35 ± 0.13; Table 1) appears to be somewhat higher than the range
(0.05-0.15) observed in epileptic nonpregnant patients (Levy and Shen,
1995); however, it is in reasonable agreement with that measured in
serum obtained from pregnant mothers at birth (0.27 ± 0.06) (Nau
et al., 1984; Nau and Krauer, 1986). The pregnancy-related increase in
VPA-unbound fraction in humans appears to be due to reductions in
plasma albumin concentration and displacement by increased free fatty
acid plasma concentrations during pregnancy (Nau et al., 1984; Riva et
al., 1984; Nau and Krauer, 1986). We did not observe a higher fetal VPA
plasma protein binding compared with the mother, as demonstrated in
humans at birth; however, this may be related to the fact that our
experiments were conducted 1 to 2 weeks before term.
Placental and Nonplacental Clearances of VPA in the Mother and the
Fetus.
The placental clearance of unbound VPA in the two directions is similar
(69.0 versus 61.9 ml/min/kg; Table 3). With the exception of
acetaminophen, this is in contrast to all other drugs studied in
pregnant sheep where CLfm is significantly higher
than CLmf. Previously, we have demonstrated that
a significant proportion of the drug transferred from the maternal
circulation across the placenta may be taken up by the fetal liver
before reaching the fetal circulation, thus leading to an
underestimation of CLmf relative to
CLfm (Kumar et al., 1997). Because VPA is a low
clearance drug, a significant fetal hepatic first-pass uptake of the
drug would not be expected and accurate estimates of
CLmf are likely obtained. The equal magnitude of
placental clearance in both directions across the placenta also
indicates a passive diffusion mechanism for placental transfer of VPA
in sheep.
The CLfm value of unbound VPA is lower compared
with that of diphenhydramine (Kumar et al., 1997), methadone (Szeto et
al., 1982a), metoclopramide (Riggs et al., 1990), and compounds with blood flow-limited placental diffusion (antipyrine and ethanol, ~200
ml/min/kg) (Wilkening et al., 1982); however, it is greater than that
of morphine (Szeto et al., 1982a), labetalol (Yeleswaram et al., 1993),
and acetaminophen (Wang et al., 1986). This indicates an intermediate
placental permeability for VPA in sheep. Molecular size of VPA is
smaller relative to all of the above compounds, except ethanol, and
hence, its intermediate placental clearance is likely related to its
polarity (octanol/water logP = 2.6) and high degree of ionization
at the physiological pH (pKa = 4.8).
The clearance data in Table 3 also indicate that fetal total and
nonplacental clearances are larger compared with the corresponding maternal clearances, with CLfo being remarkably
high. One limitation of the two-compartment model is that the
uptake/metabolism of the drug by the placenta, if present, is
calculated as part of CLfo (or
CLmo) estimates (Wang et al., 1986). VPA, due to
its branched-chain fatty acid structure, enters fatty-acid
-oxidation pathways (Baillie and Sheffels, 1995). Various lipid
metabolism pathways also exist in the placenta of sheep and many other
species (Coleman, 1989). Moreover, keto-acid compounds that are
structurally similar to VPA (e.g., acetoacetate and
-hydroxybutyrate), are metabolized by the sheep placenta at slow
rates (~30 µmol/min for -hydroxybutyrate and ~6.0 µmol/min
for acetoacetate) (Carver and Hay, 1995). Use of VPA by the placenta at
a more conservative rate of 6.0 µmol/min during both maternal and
fetal infusion experiments would introduce an error of ~15.5
ml/min/kg (~150%) in CLfo, and of ~0.27
ml/min/kg (~7%) in CLmo. Although we do not
have direct evidence for placental metabolism of VPA, this may be a
plausible explanation for the high CLfo value
observed in our studies. This is especially true in light of the fact
that VPA clearance in newborn lambs immediately after birth is much
lower compared with the estimated CLfo (Table 4).
Amniotic and Fetal Tracheal Fluid Disposition of VPA.
One feature of amniotic and tracheal fluid disposition of VPA is a lack
of its significant accumulation in these fluids relative to maternal
and fetal plasma. This is in contrast to a number of amine drugs such
as metoclopramide (Riggs et al., 1987), diphenhydramine (Riggs et al.,
1987), labetalol (Yeleswaram et al., 1993), and ritodrine (Wright et
al., 1991), which accumulate in these fluids at concentrations ranging
from ~4 to 15 times compared with fetal plasma, and
subsequently decline at a slow rate. Excretion of the drug into fetal
urine and its passage from the fetal systemic circulation across the
fetal membranes play an important role in drug transfer into and out of
amniotic and allantoic fluids (Szeto et al., 1979; Rurak et al., 1991).
Low fetal urinary excretion of VPA (see Kumar et al., 2000b) appears at
least partly responsible for the small amounts of VPA detected in
amniotic fluid.
Overall Maternal and Fetal Pharmacokinetics of VPA.
The adult human unbound and total VPA clearances (1.0-3.0 and 0.1-0.3
ml/min/kg, respectively) are somewhat lower compared with maternal
sheep (Davis et al., 1994; Levy and Shen, 1995). Similarly,
t1/2 of unbound and total VPA in sheep
is much shorter compared with that in nonpregnant and pregnant humans (9-18 h) (Nau et al., 1982; Davis et al., 1994; Levy and Shen, 1995).
The Vdss' of VPA in maternal sheep (0.24 ± 0.10 liters/kg) is similar to that in humans (0.13-0.20 liters/kg)
(Davis et al., 1994; Levy and Shen, 1995). A generally large fetal
Vdss value for VPA and many other drugs may in
part be due to the fact that it includes a maternal volume component
due to rapid distribution of a portion of the administered fetal dose
to the ewe via placental transfer.
Pharmacokinetics and Plasma Protein Binding of VPA in Newborn
Lambs.
Newborn lambs possess a much lower VPA elimination capacity
compared with the mother and the fetus as reflected in a lower clearance and longer elimination half-life (Tables 4 and 5). Fetal-newborn VPA elimination differences are at least partly related
to the loss of placental route of drug elimination at birth. As
discussed in Kumar et al. (2000b), glucuronidation and renal excretion
of the unchanged drug are the two most important routes of VPA
elimination in adult sheep. Both these routes are significantly
underdeveloped in newborn lambs, thus leading to a slower clearance of
VPA relative to the mother. The estimated VPA clearances in newborn
lambs are very similar to the range observed in human neonates less
than 1 month of age (total: 0.15-0.48 ml/min/kg; unbound: 0.7-4.2
ml/min/kg) (Irvine-Meek et al., 1982; Gal et al., 1988). Similar to our
results in sheep, apparent elimination half-life of VPA in these human
neonates (15.1-80 h) is much longer compared with that in human
epileptic adults (9-18 h) (Ishizaki et al., 1981; Nau et al., 1981,
1984; Irvine-Meek et al., 1982; Gal et al., 1988; Davis et al., 1994;
Levy and Shen, 1995).
The data presented in Fig. 5 elucidate a possible reason for the
difference between the VPA binding properties of newborn versus
maternal or fetal plasma. A positive relationship of bound/unbound concentration ratio with bound concentration in Fig. 5A indicates that
with an increasing VPA plasma concentration, the bound concentration increases to a larger extent than the unbound concentration. This could
occur if significant concentrations of a competitive VPA plasma protein
binding displacer were present in the newborn lamb plasma. In this
situation, an increase in VPA plasma concentration will lead to
competitive displacement of the displacer from the binding sites and to
an increased binding of VPA. This would then result in an increase in
the bound/unbound concentration ratio with increasing VPA concentration
until all of the displacer has been displaced from the binding sites.
The plasma unbound fractions of VPA correlate inversely with plasma
concentrations of free fatty acids (Nau et al., 1984; Riva et al.,
1984). Hence, increased plasma free fatty acids during pregnancy and
the newborn period have been suggested as potential competitive
displacers of VPA binding to plasma proteins (Nau et al., 1984; Riva et
al., 1984). Plasma concentrations of free fatty acids rise dramatically
soon after birth in sheep as well as in the human newborn (Noble, 1980; Nau et al., 1984) and this may be responsible for the anomalous VPA
plasma protein binding characteristics in newborn lambs. Also, the
plasma concentrations of free fatty acids begin to decline after the
initial 2 to 3 days of life both in the lamb and the human newborn
(Noble, 1980; Nau et al., 1984). In parallel with this, the VPA binding
characteristics of newborn lamb plasma after day 1 of our experiments
tend to approach those of maternal and fetal plasma (Fig. 5, C and D).
Somewhat lower VPA binding affinity of newborn plasma may be related to
the fact that the free fatty acid concentrations in newborn plasma may
still be higher as compared with the mother and the fetus until a few
days after birth. The increased plasma-unbound concentrations of VPA in
the immediate newborn period due to displacement from plasma binding
sites could have important pharmacological or toxicological
implications, including increased penetration of the drug across the
blood-brain barrier and an augmented possibility of acute as well as
long-term central nervous system adverse effects.
In summary, VPA undergoes rapid placental transfer via passive
diffusion and results in a high degree of fetal exposure in sheep. The
drug exhibits an intermediate placental permeability, possibly due to
its polarity and high degree of ionization at physiological pH.
Although there is evidence of fetal VPA elimination, the remarkably
high fetal nonplacental clearance estimates may indicate possible
placental metabolism of the drug. Similar to the human newborns, VPA
exhibits a slower Michaelis-Menten elimination and longer elimination
half-life in newborn lambs as compared with adult sheep. In addition,
VPA appears to be extensively displaced from the binding sites in
newborn lamb plasma during the initial 1 to 2 days of life, possibly as
a result of elevated plasma free fatty acids at birth. The maximal VPA
binding capacities of the maternal, fetal, and newborn plasma (after
the initial 2 days of life) are similar, whereas the binding affinity
of newborn plasma appears to be somewhat lower.
These studies were supported by funding from the Medical
Research Council of Canada. S.K. was the recipient of a University of
British Columbia Graduate Fellowship. H.W. is supported by a
Pharmaceutical Manufacturers of Canada/Medical Research Council studentship. D.W.R. is the recipient of an Investigatorship award from
the British Columbia Children's Hospital Foundation.
Abbreviations used are:
VPA, valproic
acid;
AIC, Akaike's Information Criterion;
AUC, area under the curve
of arterial plasma concentration-time profile;
AUMC, area under the
first-moment curve;
CLm(net), net maternal clearance of the
total drug;
CLmm, maternal total clearance of the total
drug;
CLmf, maternal-to-fetal placental clearance of the
total drug;
CLmo, maternal nonplacental clearance of the
total drug;
CLf(net), net fetal clearance of the total
drug;
CLff, fetal total clearance of the total drug;
CLfm, fetal-to-maternal placental clearance of the total
drug;
CLfo, fetal nonplacental clearance of the total drug;
CLtb, total body clearance;
Cm, maternal plasma
steady-state concentration after maternal drug administration;
Cf, fetal plasma steady-state concentration after maternal
drug administration;
Cm', maternal plasma steady-state
concentration after fetal drug administration;
Cf', fetal
plasma steady-state concentration after fetal drug administration;
MRT, mean residence time;
t1/2, terminal
elimination half-life;
t1/2u, unbound
drug;
Vdss, steady-state volume of distribution.
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Abstract
Introduction
![<UP>CL<SUB>mm</SUB></UP>=<FR><NU>k<SUB><UP>o</UP></SUB></NU><DE>[<UP>C<SUB>m</SUB></UP>−<UP>C<SUB>f</SUB></UP> · (<UP>C<SUB>m</SUB>′/C<SUB>f</SUB>′</UP>)]</DE></FR>](/content/vol28/issue7/fulltext/845/img001.gif)
