Department of Metabolism and Pharmacokinetics, Bristol-Myers Squibb
Pharmaceutical Research Institute
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Introduction |
Avitriptan
(3-[3-[4-(5-methoxy-4-pyrimidinyl)-1-piperazinyl]propyl]-N-methyl-1H-indole-5-methanesulfonamide)
is a new indolylpiperazine compound with abortive antimigraine
properties. It is structurally and metabolically distinct from the
antimigraine drug sumatriptan. Avitriptan interacts with vascular
5-HT11-like receptors to
constrict cerebral blood vessels and reduce carotid artery blood flow
by closing AV anastomoses (AV shunts) that have been implicated in
causing migraine pain (1, 2). In a variety of in vitro
preparations that assess 5-HT1-like interactions, avitriptan demonstrates greater potency and comparable efficacy to
sumatriptan, and is devoid of activity in vascular preparations that
reflect interactions at the peripheral vascular 5-HT2
receptor (3, 4). Avitriptan is structurally and metabolically different from sumatriptan. Sumatriptan undergoes oxidation to an indole acetic
acid metabolite mediated by monoamine oxidase. Avitriptan is a
cytochrome P450 substrate and undergoes hydroxylation, followed by
conjugation at several sites in the molecule. The anticipated oral
therapeutic dose of avitriptan in humans is in the range of 50-150 mg.
The SD rat has been used as one of the primary species for
toxicological evaluation of avitriptan. Single dose and multiple dose
pharmacokinetics after oral administration have been evaluated in the
rat. The exposure to avitriptan as measured by
Cmax and AUC increased more than proportionately
to dose in the dose range of 12-400 mg/kg/day in rats (5).
The objective of this study was to assess the pharmacokinetics,
absolute bioavailability, and disposition of avitriptan; its routes and
extent of excretion; and comparison of disposition in rats and humans
after oral and iv administrations of radiolabeled avitriptan.
Materials and Methods
Chemicals.
Radiolabeled [14C]avitriptan as a fumarate salt, labeled
in the piperazine ring (fig. 1), had a radiochemical
purity of 98% and specific activity of 25.6 µCi/mg. Radiolabeled
avitriptan was synthesized at Bristol-Myers Squibb Pharmaceutical
Research Institute (Syracuse, NY). Unlabeled avitriptan as a fumarate
salt (purity of 96.5%) was obtained from Bristol-Myers Squibb
Pharmaceutical Research Institute (New Brunswick, NJ).
Animal Studies.
Male SD and LE rats (~200-350 g) obtained from Charles River, Inc.
(Wilmington, MA) were used in this study. The animals were acclimated
for 3 days before used in the study. The animals were fed with Purina
brand rodent chow. The animals were housed in stainless-steel cages
before dosing. After dosing, they were housed in individual
stainless-steel metabolism cages. Food was withdrawn ~18 hr before
dosing, but the animals had free access to water at all times. Food was
provided again at 4 hr postdosing. Rats used for serial blood
collection had catheters implanted in the jugular vein (for oral
dosing) or both in the jugular vein and carotid artery (ia dosing).
Rats used for bile collection had catheters placed in the bile duct.
After completion of study procedures, the rats were killed by an
overdose of metofane or by cardiac puncture exsanguination under light
metofane anesthesia.
The dosing solutions were prepared in 0.9% sodium chloride. The oral
dosing solution was administered by gavage using a stainless-steel feeding needle. Each rat received ~1 ml of the oral dosing solution (5 mg/ml, 10 µCi/ml). The iv dose was administered as a bolus at a
rate of 1-2 ml/min in the dorsal penis vein under light metofane anesthesia. Each rat received ~0.5 ml of the iv dosing solution (10 mg/ml, 20 µCi/ml). The ia dose was administered through a catheter in
the carotid artery as a 5-min infusion. Each rat received about a 0.5 ml volume of the ia injection (10 mg/ml, 20 µCi/ml).
In the iv/oral disposition study, six SD rats were administered 20 mg/kg (40 µCi/kg) of [14C]avitriptan (as free base) as
an oral solution, and six other rats received the same dose by iv
injection in the dorsal penis vein. Urine and feces were collected from
each rat over 168 hr postdose. Aliquots of urine, feces, and carcass
were analyzed for total radioactivity. Eight additional SD rats were
used for serial blood collection. Four of these rats were administered 20 mg/kg (40 µCi/kg) of [14C]avitriptan as an oral
solution, and four rats received the same dose by ia injection into the
carotid artery. The ia administration of the drug in the rat was chosen
to allow iv sampling of blood from the jugular vein for pharmacokinetic
measurements. The ia dose was infused for 5 min. Blood samples were
collected from the jugular vein for 24 hr postdose. Plasma was
separated and analyzed for total radioactivity and unchanged
avitriptan. For biliary excretion study, two SD rats were administered
10 mg/kg (36 µCi/kg) of [14C]avitriptan as an oral
solution, and two rats received the same dose as a bolus in the dorsal
penis vein. Bile was collected through a catheter in the bile duct at
hourly intervals up to 12 hr postdose. Urine was collected from 0 to 6 and from 6 to 12 hr postdose. Bile and urine samples were analyzed for
total radioactivity.
Urine and feces were collected at 0-6, 6-24, 24-48, 48-72, 72-96,
96-120, 120-144, and 144-168 hr postdose. After each collection interval, the metabolism cages were washed with water and the cagewashes were also analyzed for radioactivity. Serial blood samples
(500 µl each) were collected at 5, 10, and 30 min and at 1, 2, 4, 6, 8, and 24 hr after ia dose, and at 10 and 30 min and at 1, 2, 3, 4, 6, 8, and 24 hr after oral dose. Blood samples were collected through a
catheter in the jugular vein that was flushed with heparinized saline
after each collection. The samples were transferred to labeled
microtainer tubes containing EDTA as anticoagulant. After gentle
mixing, the blood was placed in chipped ice and centrifuged to obtain
plasma. A 50-µl aliquot of each plasma sample was preserved for total
radioactivity determination, and the remaining was used for
determination of unchanged avitriptan by HPLC. Bile was collected from
the bile duct-cannulated rats at hourly intervals up to 12 hr postdose.
Urine was also collected from these rats in two intervals: 0-6 and
6-12 hr postdose. Two additional rats not receiving
[14C]avitriptan were killed to obtain control samples of
plasma, urine, feces, and carcass for determination of background
radioactivity and the LLQ.
In the tissue distribution study, 50 SD and 20 LE rats were divided in
two groups. Rats in the first group received 20 mg/kg (40 µCi/kg) of
[14C]avitriptan as an oral solution by gavage, and rats
in the second group received the same dose by iv injection in the
dorsal penis vein. Five SD rats dosed orally were killed at each of the
following times postdose: 0.5, 2, 8, 24, and 168 hr. Similarly, five SD rats dosed intravenously were killed at 0.08, 2, 8, 24, and 168 hr
postdose. Five LE rats from each group were killed at 2 hr and the
remaining five at 168 hr postdose. Two of the five rats from each group
were frozen in a mixture of acetone and dry ice. One of these two rats
per group was prepared for whole-body autoradiography, and the other
rat was stored frozen for any repeat analysis. From the remaining three
rats, various tissues were removed for quantitation of radioactivity.
Tissues and fluids from three rats at each collection time were removed
for the determination of radioactivity. Blood was collected by cardiac
puncture while the rat was under anesthesia and was placed in a tube
containing K3EDTA and mixed. Aliquots of each blood
specimen were taken for the determination of radioactivity, and the
remaining blood was used to prepare plasma. In addition to blood and
plasma, the following tissues and fluids were removed from each rat:
adrenal glands, aorta, aqueous humor, blood, bone, bone marrow, brain,
CSF, eye, fat, heart, kidneys, liver, large intestine and contents,
lungs, lymph nodes, muscle, skin, small intestine and contents, spleen,
stomach and contents, pancreas, pituitary gland, prostate, salivary
gland, testes, thymus, thyroid, tongue, trachea, urinary bladder, and
vena cava. Two additional rats not receiving
[14C]avitriptan were killed to obtain control samples of
tissues and fluids for determination of background radioactivity and
the LLQ. All study samples and blank specimens were stored at 
20°C before analysis of radioactivity by scintillation counting.
Human Study.
The study was conducted as a randomized, single dose, cross-over design
in 12 healthy male subjects. Each subject received 10 mg avitriptan as
an iv solution infused for 30 min and 50 mg of
[14C]avitriptan (75 µCi) as an oral solution in water.
The two treatments were separated by at least a 2-week washout. Both
treatments were administered after a 10-hr fast. The subjects continued
to fast until 4 hr after dosing, at which time they were served lunch.
The study protocol was approved by the Institutional Review Board and
Radiation Safety Committee at the investigational site. All subjects
gave consent to participate in the study by signing and dating an
informed consent form after the study was completely explained to each
person.
All subjects were in good health based on medical history, prestudy
physical examinations, and clinical laboratory testing. The mean ± standard deviation age of all subjects who entered the study was
29 ± 5.2 years, with a range of 21-38 years.
Serial blood samples were collected at predose, 0.25, 0.5, 0.75, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 10, 12, 24, 36, and 48 hr postoral dose and at
predose, 10, 20, 30, 35, and 45 min and at 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 12, 24, 36, and 48 hr from the start of the iv infusion.
Immediately after collection, each blood sample was gently inverted a
few times for complete mixing with the anticoagulant (K3EDTA) and placed in chipped ice. Within 1 hr of
collection, a 0.5-ml aliquot of blood taken after the oral treatment
was separated and stored frozen for total radioactivity measurement.
The remainder of the blood sample was centrifuged appropriately to
obtain plasma. A 0.5-ml aliquot of the plasma sample was also stored
for total radioactivity measurement, and the remainder was stored
frozen for analysis of unchanged avitriptan. All blood samples
collected from the iv treatment were centrifuged in their entirety, and plasma was stored frozen for analysis of unchanged avitriptan. Urine
was collected over the following time periods: predose, 0-4, 4-8,
8-12, 12-24, 24-48, 48-72, 72-96, 96-120, 120-144, and 144-168
hr after the oral dose. An aliquot of each urine sample was set aside
for total radioactivity measurement. Feces were collected over the
following time intervals after the oral dose: predose, 0-24, 24-48,
48-72, 72-96, 96-120, 120-144, and 144-168 hr. All blood, plasma,
urine, and feces samples were stored at or below 20°C until
analysis.
Analytical Methods.
Rat plasma samples were analyzed for avitriptan by a validated HPLC
assay with UV detection. Briefly, the method involved the addition of
50 µl of 1 M ammonium acetate (pH 5.0) and internal standard
(BMY-46317) to 0.5 ml of plasma. After vortexing, the samples were
loaded onto conditioned carboxylic acid BondElut SPE columns (Varian
Associates, Palo Alto, CA). Avitriptan and the internal standard were
eluted with 2 ml of 1% triethylamine in methanol after rinsing the
column with ammonium acetate (pH 5.0), followed by methylene chloride.
After evaporation, samples were reconstituted in 200 µl of the mobile
phase, and a 100-µl aliquot was injected onto the HPLC column.
Separation was achieved at room temperature on a DeltaBond cyano column
(Keystone Scientific, Bellafonte, PA) (4.6 × 250 mm) using a
mobile phase of acetonitrile:methanol:water (5:5:90), containing 0.01 M
ammonium phosphate dibasic (pH 3.0) and 0.01 M tetramethylammonium
hydroxide (pH 3.0) at a flow rate of 1 ml/min. The HPLC instrumentation
consisted of a Waters model 600E pump, a Waters model 715 sample
processor, and a Waters model 486 UV absorbance detector at 287 nm.
Data were acquired using a model 3357 laboratory automation system from
Hewlett-Packard. Peak heights were used in the calibration curve and in
the calculation of unknown concentrations. The retention times of
avitriptan and BMY-46317 were 6.5 and 9 min, respectively. Human plasma
samples were assayed by a validated HPLC method with electrochemical
detection (5). Spiked QC samples were prepared, before the initiation of the study in control plasma using a reference standard for avitriptan, and stored with the study samples. QC samples were analyzed
with study samples to establish stability, assay accuracy, and
precision.
The standard curves were linear with correlation coefficients of 0.999. The standard curve range was 10-2000 ng/ml for rat plasma and 1-100
ng/ml for human plasma. The LLQ for avitriptan was 10 ng/ml in rat
plasma and 1 ng/ml in human plasma. During analyses of study samples,
the mean observed concentrations of the QC samples were within 13%
from nominal values. The between- and within-day variations were within
11%. These results indicated that the assay methods were precise,
accurate, and reproducible, and that avitriptan was stable in plasma
under sample storage and assay conditions.
Aliquots of plasma (50 µl for rat and 100 µl for human), urine (100 µl for rat and 200 µl for human), and bile (100 µl) samples were
accurately pipetted and digested with an appropriate amount of
Soluene-350 (Packard Instrument Company, Meriden, CT). Samples were
neutralized with 0.1 ml of a mixture of saturated solution of sodium
pyruvate in methanol, glacial acetic acid, and methanol in a 4:3:1
ratio, and mixed with 15 ml of Hionic Fluor (Packard Instrument
Company). Feces and carcass were prepared for scintillation counting by
homogenization with water. The total sample was weighed before and
after adding water, and was homogenized with a Polytron (Brinkmann
Instruments, Inc., Westbury, NY). An accurately weighed sample of the
homogenate (~200 mg) was digested with 1 ml of Soluene-350 and
bleached with 20% solution of benzoyl peroxide in toluene. Blood
samples were also bleached when necessary. Samples were then
neutralized and prepared for liquid scintillation counting by the same
method used for direct solubilization.
In the rat tissue distribution study, samples of the following tissues
were solubilized directly for scintillation counting: adrenal glands,
blood, bone marrow, brain, CSF, eyes (for SD rats only), large
intestine, large intestinal contents, muscle, plasma, small intestine,
small intestinal contents, spleen, stomach contents, aorta, bone, fat,
lymph nodes, pancreas, pituitary gland, prostate, salivary gland,
thymus, thyroid, tongue, trachea, urinary bladder, vena cava, and
testes. The total organ or tissue sample was accurately weighed and
digested with an appropriate amount of Soluene-350. An accurately
weighed amount of the digested sample was then bleached if necessary,
neutralized, and counted as described before. Aqueous humor was counted
directly after mixing with the scintillation cocktail. Rat heart,
kidneys, liver, and lungs were prepared for scintillation counting by
homogenization with water. Eyes of the LE rats and skin of both SD and
LE rats were prepared by oxidation. The tissue sample was accurately
weighed, transferred to a combustion cone, and oxidized with a model
307 biological sample oxidizer (Packard Instrument Company). The total
[14C]CO2 was collected for scintillation
counting. [14C]Avitriptan standards prepared and
combusted along with study samples consistently yielded a mean
combustion recovery of >95%.
Total radioactivity was measured with a Packard Tricarb scintillation
counter using the external standard method. Each fluid or tissue sample
was processed in duplicate and counted 3 times for 10 min each or to a
2% sigma error. Each fecal sample from the human study was processed
in multiples of 5, and mean values were used in the calculation of
recovery of total radioactivity.
For whole-body autoradiography, the frozen rat carcasses were embedded
in a 5% aqueous solution of sodium salt of carboxymethylcellulose. Each frozen block was cut in half with a bandsaw before mounting on a
cryostat microtome stage (model PMV-2250; LKB, Gaithersburg, MD). The
block was shaved until organs of interest were visible. Lateral
sections (40 µm) were cut, taken up on adhesive tape, and left to dry
inside the cryostat. Dry sections were then used for apposition
autoradiography by exposing them on Kodak Xomat AR film (Eastman Kodak
Co., Rochester, NY) for up to 32 days. The exposed film was developed
in a Kodak M35A Xomat mechanical film processor.
Analysis of Radioactivity and Pharmacokinetic Data.
Concentrations of radioactivity were expressed as nanogram-equivalents
(ng-eq) of avitriptan per milliliter of fluid or per gram of tissue.
Control background specimens were prepared from untreated rats for each
type of fluid or tissue. The net cpm were determined as the gross cpm
minus the average background cpm for that particular type of sample.
Samples having a net cpm less than the minimum acceptable value
(determined by a counting error >20%) were considered to contain an
amount of radioactivity below the LLQ. Radioactivity in urine, bile,
feces, and carcass was expressed as a percentage of the administered
dose.
Pharmacokinetic Analysis.
Plasma concentrations vs. time data for avitriptan were
analyzed by noncompartmental methods (6) using SAS programs on a IBM
3083 mainframe computer. Plasma concentration-time profiles for total
radioactivity were also analyzed by noncompartmental methods. The
terminal log-linear phase of the plasma concentration-time curve was
identified by least squares linear regression of data points that
yielded a minimum mean square error. The AUC(INF) was
determined by a combination of trapezoidal and log-trapezoidal methods
plus the extrapolated area. The extrapolated area was determined by
dividing the observed concentration at the time of last nonzero plasma
concentration by the slope (
) of the terminal log-linear phase. The
t1/2 of the terminal log-linear phase was calculated as 0.693 divided by the absolute value of . The peak plasma concentration, Cmax, and the time at
which Cmax occurred, tmax, were obtained from the observed data.
Total clearance (CLT) and steady-state volume of
distribution (Vdss) were calculated as:
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and
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where AUMC is the area under the first moment of plasma
concentration vs. time curve. The absolute bioavailability
of avitriptan (F) was estimated from the plasma
AUC(INF) data of unchanged avitriptan after parenteral and
oral treatments.
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Results |
Excretion of Radioactivity.
Mean percentage recovery of total radioactivity over 168 hr in urine
and feces is summarized in table 1. After iv dosing in
rats, 22.2% of total radioactivity was recovered in urine. After oral
dosing, the recovery of total radioactivity in urine was lower and
amounted to 9.83% of the dose in rats. In the first 6 hr postdose, a
mean of 18.6% and 7.98% of the total radioactivity was recovered in
urine after iv and oral dosings, respectively. Fecal excretion
accounted for 70.3% after iv dosing and 85.2% after oral dosing,
respectively. The majority of the administered dose was recovered in
the first 24 hr postdose. Overall, 93% of the total iv dose and 95%
of the total oral dose was excreted up to 168 hr postdose in rats.
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TABLE 1
Mean (standard deviation) percentage cumulative recovery of total
radioactivity over 168 hr postdose after oral and iv administrations of
[14C]avitriptan in male SD rats (N = 6) and healthy
male subjects (N = 12)
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Biliary excretion of total radioactivity in two rats after iv dosing
accounted for 54% of the total dose in 12 hr. The corresponding biliary excretion after oral dosing was 62% of the administered dose.
More than 90% of the total biliary excretion occurred in the first 4 hr postdose. Urinary excretion in these rats over 12 hr postdose
accounted for 21% and 12% of the dose after oral and iv dosing,
respectively, and was similar to the urinary recovery obtained in the
rats used for assessing urinary and fecal excretion.
Mean cumulative excretion of total radioactivity in urine over 168 hr
postdose was 18.0% of oral dose in the human study. Approximately 90%
of the radioactive dose recovered in urine was excreted over the first
24 hr postdose. Fecal excretion in the human accounted for the 67.4%
of the dose. Overall, 85.9% of the total radioactivity administered
was recovered in urine and feces over 168 hr post-oral dose.
Plasma Time Course of Total Radioactivity and Pharmacokinetics of
Avitriptan.
Mean (standard deviation) plasma concentration-time profiles of total
radioactivity and unchanged avitriptan after ia and oral dosings in
rats are shown in fig. 2. Mean (standard deviation) plasma concentration-time profiles of total radioactivity after oral
administration and unchanged avitriptan after oral and iv administrations in humans are shown in fig. 3. Mean
(standard deviation) pharmacokinetic parameters of total radioactivity
and avitriptan for rats and humans are summarized in tables
2 and 3, respectively. Plasma
concentrations of total radioactivity in rats were considerably lower
after oral dosing in comparison with ia dosing. Mean
Cmax of total radioactivity after oral dosing was one-tenth of the Cmax after ia dosing (2,038 ng-eq/ml vs. 21,077 ng-eq/ml), whereas mean
AUC(INF) after oral dosing was about one-third that after
ia dosing (4,273 ng-eq · hr/ml vs. 11,567 ng-eq · hr/ml). Peak concentrations of total radioactivity occurred
at 0.5 hr after oral dosing in all rats. The
t1/2 of the total radioactivity was ~8 hr
after both routes of administration.
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Fig. 2.
Mean (standard deviation) plasma
concentration-time profiles of total radioactivity and unchanged
avitriptan after ia and oral (po) administrations of 20 mg/kg of
[14C]avitriptan in rats.
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Fig. 3.
Mean (standard deviation) plasma
concentration-time profiles of total radioactivity and unchanged
avitriptan after iv (10 mg) and oral (50 mg) administrations in
humans.
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TABLE 2
Mean (standard deviation) pharmacokinetic parameters for total
radioactivity in SD rats after ia and oral administrations, and in
humans after oral administration of [14C]avitriptan
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TABLE 3
Mean (standard deviation) pharmacokinetic parameters of avitriptan in
SD rats after ia and oral administrations, and in humans after iv and
oral administrations
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In humans, after oral administration, mean Cmax
of total radioactivity was 259 ng-eq/ml, and AUC(INF) was
1,611 ng-eq · hr/ml. Median tmax was 1.5 hr
and the mean t1/2 was 5.76 hr. Concentrations of
total radioactivity were lower in blood than in plasma. The blood-to-plasma ratio of total radioactivity was ~0.7 (data not shown).
Mean Cmax values of unchanged avitriptan after
ia and oral dosings in rats were 21,616 ng/ml and 1,493 ng/ml,
respectively. Mean AUC(INF) values were 6,879 ng · hr/ml
after ia dosing and 1,329 ng · hr/ml after oral dosing, with a mean
absolute bioavailability of 19.3%. After oral dosing, avitriptan was
absorbed rapidly with peak concentrations achieved within 0.5 hr. The
decline in plasma avitriptan concentrations was essentially parallel
with a mean t1/2 of 0.98 hr after ia dosing and
1.18 hr after oral dosing.
In the human study, mean Cmax values after iv
and oral treatments were 432 ng/ml and 142 ng/ml, respectively. Mean
AUC(INF) values were 358 ng · hr/ml and 312 ng · hr/ml,
respectively, with a mean absolute bioavailability of 17.2%. Median
tmax after oral dosing was 0.5 hr, indicating
rapid absorption. Mean t1/2 values were 5.26 and
8.34 hr for iv and oral treatments, respectively, and the decline in
plasma avitriptan concentrations seemed to be essentially parallel
after the two routes of administration.
Mean total clearance of avitriptan in rats was 48.7 ml/min/kg and mean
steady-state volume of distribution was 1.91 liters/kg. Comparison of
the AUC(INF) of total radioactivity and unchanged avitriptan indicates that ~60% of the total radioactivity in plasma was accounted for by unchanged avitriptan after ia dosing and ~32%
after oral dosing. Mean total clearance of avitriptan in humans was
6.44 ml/min/kg, and mean steady-state volume of distribution was 0.875 liters/kg. About 22% of the total circulating radioactivity in human
plasma was accounted for by unchanged avitriptan after oral
administration.
Distribution of Radioactivity in Tissues and Fluids.
Figures 4 and 5 depict the concentration
of total radioactivity in selected tissues in SD rats after oral and iv
administrations. The tissues selected represent organs associated with
absorption (stomach and small intestine), organs associated with
elimination (kidneys, liver, and large intestine), organs associated
with likely adverse effects of 5-HT1-like agonists (heart)
(7, 8), and organs that showed significant concentrations of
radioactivity at the last time of measurement (adrenals, bone marrow,
testes, thyroid, and lymph nodes).
Highest concentrations of radioactivity after oral dosing were observed
at 0.5 hr postdose and 0.08 hr post-iv dose in most tissues, except for
the tissues associated with the gastrointestinal tract and urinary
bladder. In the large intestine and urinary bladder, the highest
concentrations of radioactivity were observed at 8 hr post-oral dose.
Peak concentrations of radioactivity post-iv dose occurred at 2 hr in
the small intestine and contents, and at 8 hr in the large intestine
and contents. Except for aqueous humor, bone, brain, and CSF, all other
tissues had higher concentrations of total radioactivity than plasma.
Concentrations of total radioactivity were detectable in most tissues
up to 24 hr postdose. At 168 hr post-oral dose, radioactivity was
detectable in the kidneys, liver, large intestine and contents, and
small intestinal contents. At 168 hr post-iv dose, radioactivity was
detectable in the adrenals, bone marrow, kidneys, liver, large
intestine and contents, lymph nodes, small intestinal contents, testes,
and thyroid. Concentrations in all other tissues were below the limit
of detection at 168 hr postdose. The approximate half-life for the
decline of total radioactivity from plasma was 6.4 hr after iv
administration and 6.1 hr after oral administration, and was consistent
with the t1/2 values obtained in the disposition
study. The decline in concentration in all tissues except testes seemed
to be parallel to the decline in plasma. Concentration of radioactivity
in testes seemed to be relatively constant over a 24-hr period postdose but, at the 168 hr time point, was about one-half that concentration post-iv dose and about one-fifth that concentration post-oral dose. The
slow elimination of radioactivity from testes was not associated with
any toxicological modifications in repeat-dose toxicological studies.
Concentrations in all tissues except for those associated with
gastrointestinal tract were ~5-10 times higher after iv
administration, compared with those after oral administration.
AUC(O-T) for total radioactivity after oral dosing was
22% of that after iv dosing. Exposure to brain was <10% of the
exposure in plasma, regardless of the route of administration.
Concentrations of radioactivity in blood relative to plasma ranged from
0.62 to 0.98 at various time points.
Figure 6 shows the comparison of radioactivity
distribution between SD and LE rats in selected tissues after iv and
oral administrations. Except for the eye and skin, the total
radioactivity concentrations in the tissues from LE rats and SD rats
were comparable. At 2 hr postdose, the concentration of total
radioactivity in the eyes of the LE rats was nearly 30 times higher in
comparison with SD rats. At 168 hr postdose, radioactivity in the eye
of LE rats showed a 12% decline after iv administration and a 40%
decline after oral administration relative to the concentration at 2 hr postdose. Considerable greater radioactivity remained in the skin of LE
rats at 168 hr postdose after both iv and oral doses, compared with the
skin of SD rats. The significance of high levels of radioactivity in
the eyes of LE rats is difficult to ascertain, because only SD rats
were used in repeat-dose toxicology studies.
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Fig. 6.
Concentration of total radioactivity in
selected tissues from SD and LE rats at 2 hr post-iv (top) and
post-oral (bottom) doses.
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Whole-Body Autoradiography.
Whole-body sections and corresponding autoradiographs obtained at 2 hr
post-iv dose in SD and LE rats are shown in fig. 7. The
results showed a similar distribution of radioactivity in tissues of SD
and LE rats, except for the pigmented tissues of the eye. At 168 hr
post-iv dose, only the wall of the eyeball showed significant
radioactivity in LE rats. The results of whole-body autoradiography
essentially confirmed the results of the distribution of radioactivity
in various tissues as determined by liquid scintillation counting.
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Fig. 7.
Whole-body section (A) and autoradiograph
(B) of a male SD rat, and whole-body section (C) and autoradiograph (D)
of a LE rat killed 2 hr after a single iv dose of
[14C]avitriptan.
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Discussion |
Mean recovery of total radioactivity in urine after oral
administration of [14C]avitriptan to rats was ~50% of
that after iv administration, suggesting incomplete absorption of the
drug. From the bile duct-cannulated rats, ~62% of total
radioactivity was recovered in bile and 12% in urine in a 12 hr
post-oral dose. This indicates that at least 74% of the oral dose
administered was absorbed from the gastrointestinal tract. Despite the
high percentage of drug absorbed, absolute bioavailability was ~20%.
This suggests a significant portion of the orally administered dose is
subject to first-pass metabolism. A major portion of the dose was
excreted in the urine and feces within 24 hr postdose, and >90% of
the total dose was recovered in 168 hr postdose, regardless of the
route of administration. Because the majority of the iv dose was
recovered in feces, it seems that biliary excretion is the major route
of excretion of avitriptan and its metabolites in rats.
Results of urinary and fecal excretion of [14C]avitriptan
in rats are consistent with those observed in healthy normal subjects. Due to accumulation and slow decline of total radioactivity from the
pigmented tissues of the eye, [14C]avitriptan was not
administered intravenously to healthy subjects. After oral
administration, only 18% of the total radioactive dose was recovered
in urine over 168 hr postdose, and the majority of the dose was
recovered in feces. Because avitriptan administered by the iv route was
not radiolabeled, it is not possible to ascertain as to what fraction
of the radioactivity in feces is the unabsorbed dose and what fraction
is the dose excreted in bile. However, based on rat data, it may be
speculated that a significant portion of the radioactive dose in feces
is coming via the biliary route. Biliary excretion seems to
be a predominant route for avitriptan elimination. This is unlike the
antimigraine drug sumatriptan that is mainly renally excreted as the
parent drug and its indole acetic acid metabolite (9). Metabolism of
avitriptan in rats and humans has been characterized and will be the
topic of a future publication.
Absorption of avitriptan in rats and humans after oral administration
was rapid, with peak plasma concentrations occurring in 0.5 hr
postdose. The <100% absorption of [14C]avitriptan in
rats after oral administration is also evident from the dissimilarity
in the plasma concentration-time profiles of total radioactivity after
oral and ia administrations. Maximum plasma concentration of avitriptan
in rats after oral dosing was ~10-fold lower, compared with that
after ia dosing, with an absolute bioavailability of 19.3%. Because
the pharmacokinetics of avitriptan in rats is concentration-dependent,
it may be argued that the systemic clearance at 10-fold higher
concentrations after ia dosing is lower than the systemic clearance
after oral dosing, and thus absolute bioavailability is underestimated.
However, the apparent elimination half-life for the parent drug between
the ia and oral dosings is similar, and there is no obvious indication
that the systemic clearance between the two routes of administration is different. Absolute bioavailability of avitriptan in humans was 17.2%
and was similar to that observed in rats. Total clearance corrected for
body weight was ~8 times higher, and the steady-state volume of
distribution was ~2 times higher in rats, compared with the
corresponding values in humans.
Comparison of total radioactivity and unchanged drug in rat and human
plasma indicates that there are one or more circulating metabolites of
the drug in plasma. In rats, the t1/2 for the
total radioactivity was longer, compared with the parent drug. However, in humans, the t1/2's for total radioactivity
and unchanged avitriptan were similar.
After oral and iv administrations, radioactivity associated with
[14C]avitriptan and metabolites was extensively
distributed in various rat tissues. Concentrations of radioactivity in
tissues associated with the gastrointestinal tract were higher after
oral dosing, but all other tissues had lower radioactivity after oral
dosing compared with iv dosing; thus suggesting incomplete absorption of the drug. Significant radioactivity in intestinal tissues and contents after iv administration also supports biliary excretion of
drug. Penetration across the blood-brain barrier was low, as seen from
low concentrations of radioactivity in the brain relative to plasma.
Moderate penetration of the drug-related material was observed in the
cellular elements of blood.
LE rats were used in the tissue distribution study to assess the
distribution of radioactivity in pigmented tissues. Except for the eyes
and skin, the distribution of radioactivity was similar in all tissues
of the LE and SD rats. Concentrations in the eye of LE rats were 30 times higher at 2 hr postdose, compared with SD rats and declined very
slowly over 168 hr postdose. The radioactive material seemed to be
concentrated in the melanin-containing uveal tract. This finding is
similar to sumatriptan, which also was found to be concentrated in the
uveal tract of the eye and measurable levels of radioactivity remained
in the eye 7 days after dose (9). Further histopathological evaluation
confirmed that the radioactivity was associated with the iris, ciliary
body, and the choroid and retinal pigmented epithelia of the eye, but
not in the lens or other components of the eye. The amount of
radioactive material associated with the eye of LE rats was minimal and
accounted for <0.1% of the dose at 168 hr postdose.
In summary, a number of similarities were observed in the disposition
of avitriptan in rats and humans. [14C]Avitriptan was
rapidly absorbed after oral administration in both species, with
absolute bioavailability of ~20%. The drug is subject to significant
first-pass metabolism after oral administration, and a major portion of
the plasma radioactivity is associated with metabolites of avitriptan.
Biliary excretion is the predominant route of elimination and a
majority of the administered dose undergoes excretion in feces.
[14C]Avitriptan shows extensive distribution in tissues
with a tendency for accumulation in the melanin-containing tissues.
We are grateful to C. Ita, R. M. Fancher, D. A. D'Aprix, and K. Liao
for their technical assistance, and G. M. Luke for the synthesis of
radiolabeled avitriptan.
Abbreviations used are:
5-HT, 5-hydroxytryptamine;
AV, arteriovenous;
SD, Sprague Dawley;
Cmax, maximum plasma concentration;
AUC, area
under the plasma concentration-time curve;
iv, intravenous;
LE, Long
Evans;
ia, intraarterial;
LLQ, lower limit of quantitation;
CSF, cerebrospinal fluid;
QC, quality control;
cpm, counts per minute;
AUC(INF), area under the plasma concentration-time curve
from 0 to infinity;
t1/2, terminal elimination
half-life;
tmax, time to maximum
concentration.
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Abstract
Introduction
Abstract
, adrenals; 
, plasma; 
, kidneys; 
, liver; 
, large
intestine; 
, small intestine; 
, stomach.
