Pharmacokinetics Research Department (K.M., S.H., K.T., S.I.), and
Organic Chemistry Research Department (N.K.), Teikoku Hormone
Manufacturing Company, Takatsu-ku, Kawasaki-shi, Kanagawa, Japan
 |
Introduction |
Osaterone acetate
(17
-acetoxy-6-chloro-2-oxa-4,6-pregnadiene-3,20-dione;
OA1) is a new steroidal antiandrogen (Fig.
1). This compound has been shown to
reduce the volume of the prostate in rats (Honma et al., 1994b
; Mieda
et al., 1994) and dogs (Takezawa et al., 1992, 1993). OA is 5 times
more potent than chlormadinone acetate (Takezawa et al., 1992), which
has been used in the clinical treatment of prostatic hypertrophy and
prostatic cancer (Brennan and Kraay, 1963). Chlormadinone acetate has
been reported to be converted rapidly to 2-hydroxylated and
3-hydroxylated compounds (Honma et al., 1977). OA is an analog of
chlormadinone acetate, 2-oxachlormadinone acetate. Incorporation of an
oxygen atom into the steroid nucleus at C2 might
affect the metabolism.
The pharmacokinetic characteristics of OA in rats have been studied in
great detail (Honma et al., 1994a; Minato and Honma, 1994). The plasma
half-life of OA was 2.0 h after a single intravenous administration. In spite of the incorporation of an oxygen atom into
the steroid nucleus at C2, metabolism could not
be avoided. At 1 h after the administration, unchanged compound in
plasma represented only 40.9% of the total radioactivity, and the
major metabolite
17acetoxy-6-chloro-15
-hydroxy-2-oxa-4,6-pregnadiene-3,20-dione (15-OH
OA) represented 57.7% (Honma et al., 1994a). However, the major
metabolite 15-OH OA is pharmacologically active (Honma et al.,
1994b). The mean excretion of radioactivity in the feces accounted for
90% of the dose, and the enterohepatic recirculation was 75% (Honma
et al., 1994a). On the other hand, the pharmacokinetics in dogs has not
been investigated. The objective of the present study was to
explore the pharmacokinetics and biliary excretion of OA in dogs.
| |
Materials and Methods |
Chemicals.
OA, 15-OH OA,
17-acetoxy-6-chloro-21-hydroxy-2-oxa-4,6-pregnadiene-3,20-dione
(21-OH OA),
6-chloro-17,21-dihydroxy-2-oxa-4,6-pregnadiene-3,20-dione, and
[2,3-18O2,
1-2H]OA were synthesized in the Organic
Chemistry Research Department or the Pharmacokinetics Research
Department of Teikoku Hormone Manufacturing Co.
[17-Acetoxy-14C]OA
([14C]OA) was purchased from Daiichi Pure
Chemicals Co. (Tokyo, Japan). The specific radioactivity was 757 MBq/mmol, and the radiochemical purity (>95%) was determined by thin
layer chromatography (TLC; Kieselgel 60F254;
20 × 5 cm, 0.25 mm thick; Merck, Darmstadt, Germany) using a
developing solvent of chloroform/acetone (20:1, v/v). Other solvents
and chemicals were of analytical or HPLC grade and used without further purification.
Intact Dog Experiment.
Four male beagle dogs weighing 10 to 12 kg were used. Food and water
were provided to the animals ad libitum throughout the study.
[14C]OA (400 kBq/mg, 0.25 mg/ml) dissolved in
saline containing 50% dimethyl sulfoxide (DMSO) was administered (0.25 mg/100 kBq/kg) into the saphenous vein. Plasma samples were obtained at
0.083, 0.25, 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 24, 48, 72, 168, and
240 h, whereas urine and feces were collected every 24 h up
to 240 h. All samples were kept frozen until analyzed. Storage of
OA in plasma, urine, and feces at 
20°C did not result in any
detectable decomposition for more than 6 months.
Biliary Fistula Experiment.
Four beagle dogs (two males and two females) weighing 8 to 10 kg were
used after being anesthetized with sodium pentobarbital (50 mg/kg). The
common bile duct was approached through a median line incision. A
polyethylene catheter (SP-102; Natsume, Tokyo, Japan) was placed in the
common bile duct, and another polyethylene catheter was introduced into
the common bile duct toward the duodenum. The two catheters were
exteriorized on the animal's back. After completion of the surgery,
both catheters were joined. The animals were placed in metabolism
cages, and antibiotics (0.25 mg of chloramphenicol and 0.1 mg of
cephalothin sodium) were given intramuscularly at the end of surgery
and then daily for 5 days. The animals were fitted with a canvas vest
to protect the skin fittings and catheters. The dogs were allowed at
least 7 days to recover from the medical treatment before testing
started. The dogs remained conscious and healthy throughout the experiment.
[14C]OA (400 kBq/mg, 0.25 mg/ml) dissolved in
saline containing 50% DMSO was administered (0.25 mg/100 kBq/kg) into
the saphenous vein. Plasma samples were obtained at 1, 2, 3, 6, 12, 24, 48, 72, 168, and 240 h. The schedule for biliary collection was 0 to 3, 3 to 12, 12 to 24, 24 to 48, 48 to 72, and 72 to 96 h. At 96 h postdosing, both catheters were joined, and the bile was recirculated. Urine and feces samples were collected every 24 h up
to 240 h. All samples were kept frozen until analyzed. Storage of
OA in bile at 20°C did not result in any detectable decomposition for more than 6 months.
Determination of Radioactivity.
Total radioactivity in the plasma, urine, and bile was quantified in a
liquid scintillation counter (LSC-3500; Aloka Co., Tokyo, Japan) after
addition of 5 ml of Insta-Gel (Packard BioScience B.V.,
Groningen, Holland). The counting efficiency was determined by external
standardization. Fecal samples were suspended in an appropriate
quantity of water and homogenized with an Ultra-turrax mechanical
homogenizer (Janke & Kunkel, Stufen, Germany.). Aliquots of homogenate
were dried in vacuo overnight and combusted in an auto-oxidizer
(ASC-113; Aloka Co.). The radioactivity in the combustion products was
determined by trapping the liberated
14CO2 in Carbosorb (Packard
BioScience B.V.) and mixing with Permafluoro V (Packard BioScience
B.V.), followed by liquid scintillation counting.
The radioactivity in plasma was expressed as nanograms of equivalents
of unchanged compound per milliliter. The radioactivity in bile, urine,
and feces was expressed as a percentage of the dose.
Sample Preparation for GC-MS with Selected Ion Monitoring.
To each plasma sample (1 ml) was added
[2,3-18O2,
1-2H]OA (25 ng) as an internal standard,
followed by 1 ml of distilled water and 0.5 ml of 2 M HCl. The
resulting solution was applied to a Bond Elut C18
cartridge (3 ml; Varian, Palo Alto, CA) that had been rinsed with 6 ml
of both methanol and distilled water. The cartridge was washed with 4 ml of distilled water and 2 ml of 30% (v/v) acetonitrile and then
eluted with 2.5 ml of 60% (v/v) acetonitrile. After evaporating the
solvent in vacuo at 50°C with a centrifugal evaporator, the residue
was dissolved in 60 µl of 50% (w/w) dimethylethylsilyl
imidazole in benzene and allowed to stand for 3 h at
60°C. Then, the solution was diluted with 0.5 ml of 25% (v/v) hexane
in benzene, and applied to a Bond Elut CN cartridge (3 ml; Varian) that
had been rinsed with 10 ml of 25% (v/v) hexane in benzene. The
cartridge was washed with 2 ml of 25% (v/v) hexane in benzene and
eluted with 2 ml of ethyl acetate/benzene (1:10, v/v). After
evaporating the solvent under a stream of nitrogen, the residue was
dissolved in 50 µl of acetone and subjected to GC-MS-SIM.
To the urine samples (0.25~1.0 ml) was added
[2,3-18O2,
1-2H]OA (100 ng) as an internal standard,
followed by 1 ml of distilled water and 0.5 ml of 2 M HCl. The solution
was then treated as described above and subjected to GC-MS-SIM.
To each bile sample (0.1 ml) was added
[2,3-18O2,
1-2H]OA (100 ng) as an internal standard,
followed by 1 ml of distilled water and 0.5 ml of 2 M HCl. The solution
underwent extraction using a Bond Elut C18
cartridge, as described above. After evaporating the solvent in vacuo
at 50°C with a centrifugal evaporator, the residue was reconstituted
in 1 ml of 1% NaHCO3, extracted twice with 4 ml
of chloroform, and dried over anhydrous sodium sulfate. Following
filtration, concentration, and evaporation to dryness in vacuo, a dry
residue was obtained to which 60 µl of 50% (w/w) dimethylethylsilyl
imidazole in benzene was added. The resulting solution was allowed to
stand for 3 h at 60°C. Then, the solution was diluted with 0.5 ml of 25% (v/v) hexane in benzene and applied to a Bond Elut CN
cartridge (3 ml; Varian) that had been rinsed with 10 ml of 25% (v/v)
hexane in benzene. The cartridge was then washed with 2 ml of 25%
(v/v) hexane in benzene and eluted with 2 ml of ethyl acetate/benzene
(1:10, v/v). After evaporating the solvent under a stream of nitrogen,
the residue was dissolved in 50 µl of acetone and subjected to
GC-MS-SIM.
GC-MS-SIM.
GC-MS-SIM analysis was conducted on a Hitachi M-80 instrument (Tokyo,
Japan) equipped with a data processor (M-003). The GC column was made
of glass (0.5-m × 6-mm i.d.) packed with 2% OV-17 (GL science,
Tokyo, Japan). The column temperature was set at 247°C. The mass
spectrometer was operated in the electron ionization mode with helium
(30 ml/min) as the carrier gas. The ionization voltage was 20 eV. SIM
was performed on the fragment ions at m/z 303 and
308 for OA and [2,3-18O2,
1-2H]OA, respectively.
Radioluminography.
The radioluminography equipment consisted of an imaging plate (IP;
20 × 40 cm) for 14C, a magazine cassette
14C, a Fuji shield box, and a bioimaging analyzer
system (FUJIX BAS-2000; Fuji Film, Kanagawa, Japan). The IP was used to
record the radioactivity observed on the TLC plates, and the FUJIX
BAS-2000 bioimaging analyzer was used to determine the distribution of radioactivity recorded on the IP. Image analysis of the radioactivity profiles on TLC plates (Kieselgel 60F254; 20 × 20 cm, 0.25 mm thick; Merck) was conducted after two-dimensional TLC
of bile with chloroform/ethanol/formic acid (15:5:4, v/v/v; three
developments) as the first developing system and chloroform/acetone
(20:1, v/v) as the second developing system.
Isolation and Identification of Biliary Compounds.
An oral dose of OA in capsule form (10 mg/animal/day) and an
intravenous dose of [14C]OA (93.2 kBq/kg/day)
in saline containing 50% DMSO were administered to the four dogs with
biliary fistulas for 4 days. Bile was collected up to 24 h after
the final dose. The combined samples from the four dogs were kept
frozen until analyzed. Storage of OA, 15-OH OA, 21-OH OA, and 21-OH
OA-21-glucuronide in bile at 20°C did not result in any detectable
decomposition. At room temperature, 21-OH OA in solution is gradually
changed to
21-acetoxy-6-chloro-17-hydroxy-2-oxa-4,6-pregnadiene-3,20-dione by
intramolecular acetyl rearrangement.
The bile (2 liters) was adjusted to pH 4 with 2 M HCl, loaded onto an
Amberlite XAD-2 column (50 × 7.5 cm; Supelco, Bellefonte, PA),
then washed with 3.5 liters of distilled water, and eluted with 5 liters of methanol. The methanol was evaporated to dryness, and the
residue (54.1 g) was reconstituted in 300 ml of distilled water. The
solution was adjusted to pH 4 with HCl and extracted with 240 ml of
ethyl acetate twice. Compounds A, B, and C were isolated from the ethyl
acetate phase by preparative TLC (chloroform/acetone, 20:1, v/v). The
aqueous phase, containing compound D, was evaporated to dryness; the
residue (38.5 g) was reconstituted in 80 ml of 20% methanol, and the
solution was loaded onto a Sephadex LH-20 column (40 × 5 cm;
Amersham Biosciences AB, Uppsala, Sweden). After washing the
column with 45 ml of 20% methanol, the radioactive fraction was eluted
with 20 ml of 20% methanol. The radioactive fraction was evaporated,
and the residue (29.0 g) was dissolved in chloroform/methanol/water
(35:15:3, v/v/v) and applied to a silica-gel column (55 × 3.5 cm;
Wako-gel C-200; Wako Pure Chemicals, Tokyo, Japan). After washing the
column with 1520 ml of chloroform/methanol/water (35:15:3, v/v/v),
elution was carried out with 1320 ml of mixed solvent. The solvent was
evaporated, and the residue was separated on a Sephadex LH-20 column
(55 × 3.5 cm), as described above, and a radioactive residue
(13.0 g) was obtained. The radioactive residue was dissolved in a small
amount of methanol/acetonitrile/water (3:3:4, v/v/v) and applied to a
C18 column (45 × 2 cm; YMC-Gel ODS-A
60-250/170; YMC Co., Kyoto, Japan) and then washed with 80 ml of mixed
solvent (methanol/acetonitrile/water, 3:3:4, v/v/v). The radioactive
fraction was eluted with 85 ml of mixed solvent (methanol/acetonitrile/water, 3:3:4, v/v/v) and evaporated. The residue
was dissolved in a small amount of distilled water and applied to a
DEAE-Sephadex A-25 column (45 × 2 cm, DEAE-Sephadex A-25;
Amersham Biosciences AB) that had been prepared with distilled water.
The column was washed with 50 ml of distilled water and then eluted
with a linear gradient obtained by mixing 500 ml of 1 M NaCl with 500 ml of distilled water. A flow rate of approximately 15 ml/h was
maintained. The radioactive fraction was chromatographed on a
preparative-HPLC column (YMC-pack SH-343-5; YMC Co.). The obtained
radioactive residue was subjected to HPLC (column, 250 × 20-mm
i.d.; YMC-pack AQ-312; YMC Co.). Compound D (10.8 mg) was obtained
after evaporation of the solvent from the eluate.
An aliquot (0.7 mg) of compound D obtained was dissolved in a mixture
of 10 ml of benzene/methanol solvent (4:1, v/v), and then mixed with 10 ml of trimethylsilyl-diazomethane. The reaction mixture was stirred at
room temperature for 2 h. Volatile compounds were then removed in
vacuo. The residue obtained was treated with 10 ml of pyridine and 10 ml of acetic acid anhydride at room temperature for 48 h. The
concentration in vacuo gave the derivatized compound, which was
subjected to purification by silica-gel chromatography (with
benzene/methanol, 10:1, v/v, as the eluent) to provide the pure
derivative. The assumed structure of compound D was confirmed by
comparison of its mass spectrum with that of the synthetic reference compound.
Characterization of compounds A to C was performed by proton NMR(90
MHz) and mass spectrometry. Proton NMR spectra were recorded on a
Hitachi R-90H instrument or a JEOL GX-500 instrument (Jasco, Tokyo,
Japan). Chemical shifts were expressed relative to the tetramethylsilane used as an internal standard. Mass spectra were obtained using a Shimadzu (Kyoto, Japan) GCMS-QP1000 spectrometer operated in the electron impact ionization mode.
Characterization of compound D was performed by Proton NMR,
13C NMR, and fast atom bombardment-mass
spectrometry. Proton and 13C NMR spectra were
obtained on a JNM-LA 500 spectrometer (Jasco) as solutions in
deuterated methanol or deuterated pyridine. Chemical shifts were
expressed relative to the tetramethylsilane used as an internal
standard. Proton-NMR spectra were recorded at 500 MHz.
13C NMR spectra were recorded at 400 MHz. Mass
spectra and high-resolution mass spectra were obtained using a VG
ZAB-HF mass spectrometer (VG Analytical, Manchester, UK) operated in
the atom bombardment mode. The spectra were obtained using xenon atoms
at 8 kV, glycerol as the matrix, and in the negative ion mode.
Synthesis of
Methyl(17-acetoxy-6-chloro-2-oxa-4,6-pregnadiene-3,20-dione-21-yl-2',3',4'-O-triacetyl--D-glucopyranosid)uronate.
Freshly prepared AgCO3 (50 mg) was added to a
solution of 21-OH OA (30 mg) in anhydrous benzene (5 ml). Then
methyl(1-bromo-1-deoxy-2,3,4-O-triacetyl--D-glucopyranuorate) (70 mg) was added, followed by stirring at room temperature for 72 h. The resulting precipitate was removed by filtration. After evaporating the filtrate under reduced pressure, the oily residue was
subjected to preparative TLC using benzene/methanol (9:1, v/v) as a
developing solvent. The zone corresponding to the target compound was
scrapped from the plate. Extraction with ethyl acetate gave 57.2 mg of
methyl(17-acetoxy-6-chloro-2-oxa-4,6-pregnadiene-3,20-dione-21-yl-2',3',4'-O-triacetyl--D-glucopyranosid)uronate.
Data Analysis.
Model-independent evaluation of the pharmacokinetic parameters from the
plasma OA concentration-time profiles for all individuals was performed
using PAG-CP (ASTA Medica, Osaka, Japan). In biliary fistula dogs, the
area under the plasma concentration time curve (AUC) was calculated by
the linear trapezoidal rule through the point just before restart of
normal bile flow (72 h after dosing). In intact dogs,
pharmacokinetic parameters were calculated through 240 h
after administration (the entire blood collection period). The mean
residence time was calculated as the ratio from the first moment curve
and AUC, and the total plasma clearance was calculated as the ratio of
dose and AUC. All parameter estimates are reported as mean ± S.D.
Differences in AUC0-72 estimates between intact
and biliary fistula dogs were tested for statistical significance with
an unpaired t test. Statistical significance was defined as
p < 0.05.
| |
Results |
Plasma Concentration Profiles.
The plasma concentration profiles of OA and total radioactivity in
intact and biliary fistula dogs are shown in Fig.
2, a and b, respectively. A biexponential
decline was observed in plasma OA concentrations. OA exhibited a very
long half-life of 197.9 ± 109.9 h in intact dogs. There were not
significant differences in the plasma concentration profiles of OA and
total radioactivity. OA accounted for almost all the plasma
radioactivity. Model-independent pharmacokinetic analyses were
performed based on the data of OA. The results of
pharmacokinetic analyses are summarized in Table 1. There was no statistical significance
in AUC0-72 between intact and biliary fistula
dogs.
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TABLE 1
Pharmacokinetic parameters of OA after a single intravenous
administration of [14C]OA (0.25 mg/kg) to intact and biliary
fistula dogs
Values are expressed as the mean ± S.D. of four dogs. Parameters
based on data obtained through 240 h.
|
|
Excretion Studies.
The excretion data after a single intravenous administration of
[14C]OA to intact and biliary fistula dogs are
given in Table 2. In intact dogs,
14.9 ± 2.1 and 23.9 ± 3.6% of administered radioactivity were recovered in the urine and feces over 72 h, respectively. In
biliary fistula dogs, 20.4 ± 8.8, 4.6 ± 0.3, and 30.2 ± 7.0% of administered radioactivity were recovered in the urine,
feces, and bile over 72 h, respectively.
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TABLE 2
Excretion of radioactivity in bile, urine, and feces after a single
intravenous administration of [14C]OA (0.25 mg/kg) to intact
and biliary fistula dogs
Each value represents the mean ± S.D. of four dogs.
|
|
Table 3 shows excretion of OA and
metabolites (total radioactivity OA) and the excretion rate of
total radioactivity in bile and urine in biliary fistula dogs.
Unchanged compound (OA) accounted for about one third of the total
radioactivity in bile and urine over 96 h. The mean biliary
excretion rate of total radioactivity was 0.39 ± 0.06% of the
dose/h. The rate was approximately constant from 12 h after the
administration. Accordingly, the biliary excretion rate of total
radioactivity was slow and steady.
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TABLE 3
Excretion of OA and metabolites and the excretion rate of total
radioactivity in bile and urine after a single intravenous
administration of [14C]OA (0.25 mg/kg) to biliary fistula
dogs
Each value represents the mean ± S.D. of four biliary fistula
dogs.
|
|
Identification of Biliary Metabolites.
Two-dimensional TLC-radioluminography of the biliary metabolites is
shown in Fig. 3. Four main spots
(compounds A-D) were observed. Compounds A, B, and C were identified
as OA, 21-OH OA, and 15-OH OA, respectively, by spectroscopic
analysis and comparison with authentic standard samples. The specific
data for compounds A, B, and C were shown in Table
4. Proton NMR (500 MHz) and MS spectra of
compound D are shown in Figs. 4 and 5,
respectively. Table 5 summarizes the 13C NMR
chemical shifts of compound D and
reference compounds. A comparison of the 13C NMR
spectrum of 21-OH OA with that of
compound D indicated that the C-21 signal was displaced downfield by
6.91 ppm following glucuronidation, while the other carbon resonances
of both compounds appeared at essentially the same positions. A similar
chemical shift following glucuronidation has been reported previously
for 24-nor-5-cholestane-3,7,12,25-tetrol-3-glucuronide
(Kuramoto et al., 1993) and
5-cholestane-3,7,12,25-tetrol-3-glucuronide (Kibe et al.,
1981). Comparison of MS spectra between the methyl-acetyl derivative of
compound D and the authentic glucopyranosiduronic acid of 21-OH OA
indicated that compound D was 21-OH OA-21-glucuronide.
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Fig. 3.
Autoradiograph of silica-gel thin layer
plate after chromatography of bile after intravenous administrations of
[14C]OA to biliary fistula dogs.
The origin is at the bottom left corner of plate; the directions of
development are: D1, upward; D2, to the right. Compound A, OA;
compound B, 21-OH OA; compound C, 15-OH OA; and compound D, 21-OH
OA-21-glucuronide.
|
|
| |
Discussion |
Marked species differences in pharmacokinetics have been found
between rats and dogs. The half-life of OA in intact dogs was very long
(197.9 ± 109.9 h) compared with that in rats (2.04 h) (Honma et
al., 1994a). There were no significant differences between OA and total
radioactivity profiles shown in Fig. 2. OA accounted for almost all the
plasma radioactivity. In rats, unchanged compound in plasma represented
only 40.9% of the total radioactivity, and the metabolite
(15-OH OA) represented 57.7% at 1 h after the administration (Honma et al., 1994a). A lower rate of metabolism in
dogs would be expected to be associated with the species differences in
half-life.
To understand the contribution of enterohepatic recirculation, the
pharmacokinetics of OA was investigated after intravenous bolus
administration to intact and biliary fistula dogs. Within 72 h
after a single intravenous administration of
[14C]OA in biliary fistula dogs, 30.2 ± 7.0 and 20.4 ± 8.8% of the radioactivity is recovered in bile
and urine, respectively. This confirms that bile is a major excretion
route of OA and its metabolites. The biliary excretion rates was much
slower in dogs (0.42 ± 0.10% of the dose/h) than in rats
(0.81 ± 0.23% of the dose/h) (Minato and Honma, 1994). The
difference in the biliary excretion rate would be associated with the
longer elimination half-life of OA in dogs. Furthermore, the fecal
excretion in intact dogs was 23.9%, whereas in biliary fistula dogs
the combined fecal and biliary excretion was 34.8%. The difference
also suggested that a significant amount of biliary recycling occurs in dogs.
OA accounted for almost all the plasma radioactivity. On the other
hand, only one-third of radioactivity in the bile was identified as
being due to unchanged compound. Four radioactive compounds were
observed in the bile. Compounds A, B, and C were identified as
unchanged compound, 21-OH OA, and 15-OH OA, respectively, by
spectroscopic analysis and comparison with authentic standard samples.
The major biliary metabolite (compound D) was identified as a
glucuronide of 21-OH OA. The authentic standard sample
(17acetoxy-6-chloro-2-oxa-4,6-pregnadiene-3,20-dione-21-yl-2',3',4'- O-triacetyl--D-glucopyranosid)uronate)
cannot be hydrolyzed to the glucuronide of 21-OH OA. Because the
aglycone has an ester bond in the A ring,
17-acetoxy-6-chloro-2-oxa-4,6-pregnadiene-3,20-dione-21-yl-2',3',4'-O-triacetyl--D-glucopyranosid)uronate was used as a suitable compound for the direct comparison. It was
believed that the metabolites are excreted into bile passively and do
not diffuse back into the systemic circulation following enterohepatic
recirculation. Therefore, further studies of the pharmacokinetics are required.
The slow and steady excretion of OA may result in slow elimination from
the systemic circulation, and active transport may also be involved. It
is generally believed that most steroid hormones are substrates for
P-glycoprotein (Pgp) (Ueda et al., 1992; Nakayama et al., 1999). Pgp is
expressed in normal tissues and is found on the luminal surface of the
transporting epithelia of liver biliary hepatocytes, kidney proximal
tubules, and the small intestine. Pgp has a relatively broad substrate
specificity (Tanaka et al., 1996; Arimori et al., 1998). The degree of
recognition by Pgp might be highly dependent upon the structure of the
compound involved. The degree of Pgp recognition can be markedly
affected by the presence of only a single oxygen atom in a steroid. For
instance, the presence of a 17-hydroxyl group is associated with a
greater shift. Similarly, there is evidence that progesterone, which
does not have any hydroxyl groups in its structure, is not transported (Yang et al., 1990; Bourgeois et al., 1993). Progesterone has been
shown to be capable of reversing Pgp-dependent drug resistance, inhibiting photoaffinity labeling of Pgp by azidopine, and serving as a
photoaffinity label for the human form of Pgp (Qian and Beck, 1990;
Yang et al., 1990). Therefore, it is evident that progesterone can bind
to Pgp, even if it is not transported. In general, the relative ability
of steroids to reverse drug resistance and inhibit drug binding to Pgp
has been found to be highly dependent upon the hydrophobicity of the
individual steroids (Yang et al., 1989; Barnes et al., 1996).
The present article has described the pharmacokinetics and biliary
excretion of OA in dogs. Biliary excretion is the major excretion
route. The longer elimination half-life of OA in dogs relative to rats
may be due to the slower metabolism, slow biliary excretion, and
enterohepatic recirculation.
Kouichi Minato, Teikoku
Hormone Mfg. Co., Ltd., Pharmacokinetics Research Department and
Organic Chemistry Research Department, 1604 Shimosakunobe, Takatsu-ku,
Kawasaki-shi, Kanagawa 213-8522, Japan. E-mail:
minato-k{at}kw.teikoku-hormone.co.jp
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Abstract
Introduction
) and biliary fistula (
) dogs.
