![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
|
|
|
|
|
||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
ABSORPTION, DISTRIBUTION, METABOLISM, AND EXCRETION
Department of Pharmaceutics, Victorian College of Pharmacy, Monash University (Parkville Campus), Parkville, Victoria, Australia (D.M.S., C.J.H.P., W.N.C.); Department of Pharmaceutics, NV Organon, BH Oss, The Netherlands (W.A.F.); Department of Drug Metabolism and Kinetics, NV Organon, BH, Oss, The Netherlands (N.H.); Department of Drug Metabolism and Kinetics, Organon Development GmbH, Waltrop, Germany (H.L.); and Department of Veterinary Sciences, The University of Mebourne, Werribee, Victoria, Australia (C.J.H.P.)
Received April 2, 2003; accepted May 22, 2003.
 |
Abstract |
|---|
 Top Abstract Materials and MethodsResultsDiscussionConclusionReferences |
|---|
).
Conversely, the lipophilic ester prodrug testosterone undecanoate (TU)
demonstrates androgenic activity after oral administration to rats
(Coert et al., 1975) and humans
(Hirschhauser et al., 1975;
Maisey et al., 1981;
Skakkebaek et al., 1981).
Because oral administration of TU results in the appearance of TU (and the
metabolite 5
-dihydrotestosterone undecanoate; DHTU) in lymph of
thoracic duct-cannulated rats (Coert et
al., 1975; Noguchi et al.,
1985) and humans (for whom a thoracic duct cannula was inserted
after neck dissection surgery; Horst et
al., 1976), the androgenic activity of orally administered TU is
generally attributed to T (and 5-dihydrotestosterone; DHT) formed
during the systemic metabolic elimination of TU, which escaped the presystemic
first-pass effect due to intestinal lymphatic absorption and transport
(Coert et al., 1975;
Horst et al., 1976).
Although it is accepted that lymphatic absorption of TU likely contributes
to systemic T exposure after oral TU administration, the extent of that
contribution has not been quantitatively determined. Indirect evidence of
lymph transport of TU in humans was reported where the systemic exposure of T
increased after oral TU administration in the fed state, compared with
administration in the fasted state (Frey et
al., 1979; Bagchus et al.,
2003). However, human studies cannot determine the extent of
lymphatic transport of TU, or the extent to which lymphatically transported TU
leads to an increase in systemic T exposure, because it is not practical or
possible to cannulate the thoracic lymph duct. Therefore, to characterize the
absorption and bioavailability of orally administered TU, we have used a new
lymph duct-cannulated dog model (Khoo et
al., 2001) (a more representative model of the human than
alternative animal models such as the rat;
Edwards et al., 2001) to
conclusively determine whether the intestinal lymphatics are responsible for
the quantitative delivery of TU into the systemic circulation and to test the
hypothesis that lymphatically transported TU is responsible for the majority
of systemic T exposure after postprandial oral administration of TU. The
current study used stable isotope methodology (to minimize intraindividual
differences in drug distribution and elimination and provide the necessary
data in a minimal number of dogs; Heck et
al., 1979) to compare the relative performance of two commercially
available TU formulations (Andriol and Andriol Testocaps) with respect to the
amount of TU absorbed via the intestinal lymphatics and the resulting systemic
exposure of T after oral administration of the TU formulations.
|
Materials and Methods |
|---|
|
TopAbstractMaterials and MethodsResultsDiscussionConclusionReferences |
|---|
Extemporaneous Preparation of an Intralipid Formulation of
[2H]-TU. [2H]-TU for intravenous administration was
incorporated into the Intralipid lipid emulsion by adaptation of a previous
method (Humberstone et al.,
1996). Briefly, 12 mg of [2H]-TU (equivalent to 9.6 mg
of [2H]3-TU) was dissolved in an
N,N-dimethylacetamide/triacetin mixture [3:5 (v/v), 320 µl], and
the resultant solution was added dropwise into 15 ml of Intralipid.
Incorporation of [2H]TU into the emulsion droplets was achieved by
3 x 1-min periods of sonication with a probe sonicator (XL-Series
operated at a power setting of 6; Misonix, Inc., Farmingdale, NY). During
sonication, the temperature of the emulsion was kept below 10°C by cooling
in a bucket of ice water. After the complete addition of [2H]-TU,
the concentration of [2H]-TU in the emulsion was determined before
and after centrifugation to determine the extent of incorporation of
[2H]TU in the lipid droplets. The resulting emulsion was sterilized
before administration by filtration through a sterile 0.22-µm filter.
Immediately after administration in the dog study, a 50-µl aliquot of the
filtered emulsion was collected and stored frozen at -80°C for
determination of the exact [2H]-TU concentration.
Oral Formulations of TU. The two TU formulations assessed in this study were Andriol and Andriol Testocaps. Andriol was formulated as soft gelatin capsules containing a 18.2% (w/w) solution of TU in oleic acid with the individual capsule composition being 40 mg of TU, oleic acid, gelatin, glycerol, Karion 83, sodium ethyl hydroxybenzoate, sodium propyl hydroxybenzoate, titanium dioxide (E171), and iron oxide red (E172). Andriol Testocaps soft gelatin capsules contained a 12.0% (w/w) solution of TU in lauroglycol FCC/castor oil [40:60% (w/w)] with the individual capsule composition being 40 mg of TU, lauroglycol FCC, castor oil, gelatin, glycerol, and sunset yellow (E110).
Surgical Procedures. All surgical and experimental procedures were
approved by the local Institutional Animal Experimentation Ethics Committee.
After induction of surgical anesthesia, the thoracic duct and portal vein of
healthy adult female greyhound dogs (27-30 kg) was cannulated as described
previously (Khoo et al.,
2001). After surgery, the dogs were allowed to recover
unrestrained for a period of 14 to 16 h, and during this time, they returned
to normal ambulatory movement. An intravenous catheter was inserted into the
cephalic vein before drug administration to enable serial sampling of
peripheral blood during the study period. No attempt was made to control dogs
with regard to their position relative to oestrus.
Experimental Procedures. Each dog was fed a standard can of commercial dog food (680 g) containing 5% crude fat (maximum) approximately 30 to 45 min before the intravenous administration of the [2H]-TU/Intralipid emulsion, which was administered at a constant rate infusion (5.0 ml over 10 min) via the cephalic vein cannula. At the end of the infusion period, heparinized saline (2 ml) was rapidly flushed through the cannula to ensure complete delivery of the i.v. dose, and each dog was immediately administered an 80-mg oral dose of TU as either two Andriol capsules (n = 4 dogs) or two Andriol Testocaps capsules (n = 4 dogs). In addition to the dogs having access to drinking water ad libitum, 25 ml of normal saline was administered via the portal vein cannula at hourly intervals to limit any possible dehydration due to continuous collection of thoracic lymph.
Samples (5 ml) of systemic and portal blood were obtained simultaneously via the cephalic vein and portal vein cannulae, respectively, at the following times: -10 (i.e., predose), -5 (i.e., mid-infusion), 0 (i.e., end of infusion), 10, 20, 30, 40, 50, 60, 75, 90, 105, and 120 min, and every 30 min thereafter until 12 h postdose. Blood samples were collected into individual glass tubes containing potassium fluoride [50 µl of 10% (w/v) solution] as an inhibitor of nonspecific esterases. Once the blood had clotted, samples were centrifuged and the resultant serum stored at -80°C before analysis.
Lymph was collected continuously into 50-ml polypropylene tubes containing 75 mg of disodium EDTA (anticoagulant) for the 12-h postdosing period. All lymph samples collected per hour were pooled, and the total volume of lymph that had been collected was determined gravimetrically (assuming a specific gravity of 1 g/ml). Aliquots (50 µl) of the pooled lymph samples from each hourly collection period were dispensed into Eppendorf tubes and stored at -80°C before analysis.
Analytical Procedures. The techniques used to determine the concentration of labeled and unlabeled TU, T, DHT, and DHTU in serum and lymph, the concentration of TU in the intravenous formulation, and the concentration of triglyceride in lymph are described below.
Serum Concentrations of TU/DHTU. TU, [2H]-TU, DHTU, [2H]-DHTU, and the structurally related internal standard TD were isolated from serum by solid-phase extraction before analyte quantification by liquid chromatography coupled to mass spectrometry (LC-MS/MS) using electrospray ionization. Serum samples (200 µl) were mixed with internal standard solution (100 ng/ml in acetonitrile) and acetonitrile (70 µl) in glass tubes and transferred into SPE tubes (Bond Elut C18; Varian Medical Systems, Palo Alto, CA), which had been preconditioned with methanol (3 ml) and water (3 ml) on a vacuum manifold. Samples were introduced dropwise onto the extraction column using minimal vacuum, and the column was then washed once with 3 ml of water. Analytes were slowly eluted from the column into a glass autosampler vial with acetonitrile/tetrahydrofuran [75:25 (v/v), 1.0 ml], and the eluate was then reduced to dryness within the autosampler vial using a vacuum centrifuge. The residue was reconstituted in acetonitrile before LC-MS/MS analysis. The LC system was a Hewlett Packard Series 1100 equipped with an LC analytical column (Supelcosil LC-8-DB, 50 mm x 4.6 mm, 5-µm particle size) running at a column temperature of 40°C. The flow was 0.5 ml/min and an 8-min gradient was used in which the mobile phase composition varied (linearly) from 10% water [0.2% (v/v) acetic acid] in methanol to 100% methanol over 6.9 min. The mass spectrometer was an API 3000 (Applied Biosystems, Foster City, CA) operated at a vaporizer temperature of 200°C and an ionization spray voltage of 5,500 V. In the MS/MS mode the protonated [M + H]+ molecules of TU, [2H]3-TU, DHTU, [2H]3-DHTU, and TD were used as the precursor ion and measured as product ions at m/z ratios of 270.75, 274.09, 254.91, 258.15, and 270.86, respectively. The limits of quantitation (LOQ) for TU and DHTU (and the corresponding related labeled analytes) in serum were 0.5 and 1 ng/ml, respectively.
Lymph Concentrations of TU/DHTU. TU, [2H]-TU, DHTU, [2H]-DHTU, and the structurally related internal standard TD were isolated from lymph by liquid-liquid extraction before quantification by LC-MS/MS using electrospray ionization. Lymph samples (20 µl) were thoroughly mixed with internal standard solution (100 ng/ml in acetonitrile) and 0.1 M HCl solution (20 µl) in glass tubes. A 300-µl aliquot of 2-butanol was added with thorough mixing, after which the samples were centrifuged and the 2-butanol transferred to glass autosampler vials. The eluate was evaporated to dryness using a vacuum centrifuge, and the residue was reconstituted in acetonitrile (25 µl) before analysis. The LC analytical column (Chromolith SpeedROD RP18e, 50 mm x 4.6 mm; Merck, Darmstadt, Germany) was maintained at a column temperature of 40°C, and separations conducted at 1.00 ml/min where an 8-min gradient program was used in which the mobile phase composition remained constant [7% water, 0.2% (v/v) acetic acid in methanol] for 5 min, after which it changed to 100% methanol over 0.1 min where it remained for a further 0.9 min before returning to the starting conditions over a further 0.1 min. The mass spectrometer was an API 3000 (Applied Biosystems), and the operating conditions were identical to those for determination of TU in serum, except that the vaporizer temperature was set at 250°C. In the MS/MS mode, the protonated [M + H]+ molecules of TU, [2H]3-TU, DHTU, [2H]3-DHTU, and TD were used as precursor ion and measured as product ions at m/z 271.10, 274.30, 255.20, 258.10, and 271.20, respectively. The LOQ for TU and DHTU (and the corresponding related labeled analytes) in lymph was 2 and 5 ng/ml, respectively.
Lymph and Serum Concentrations of T/DHT. T, [2H]-T, DHT, [2H]-DHT, and the structurally related internal standard MENT were isolated from dog serum and lymph samples by solid phase extraction. The isolated compounds were converted to their trimethylsilyl (TMS) derivatives, extracted with n-hexane, and quantified using capillary gas chromatography (GC) coupled with mass spectrometry detection in the positive ion chemical ionization mode. Serum (200 µl) and lymph (25 µl) samples were mixed with internal standard solution (50 ng/ml in acetonitrile) and 10% acetonitrile in 0.08 M HCl solution (200 and 450 µl for serum and lymph, respectively) in glass tubes. After a 30-min equilibration period, the solutions were transferred into SPE tubes [Isolute C18 extraction column (Separtis, Grenzach-Wyhlen, Germany), preconditioned with methanol (1 ml) and water (1 ml)] on a vacuum manifold. Samples were introduced onto the extraction columns and each column was washed twice with 1 ml of water. Extraction columns were then placed in centrifuge tubes and centrifuged (4 min at 3500 rpm) before eluting each column with 1 ml of methanol into a 4-ml glass tube. The eluate was evaporated to dryness using a vacuum centrifuge, 25 µl of Tri-Sil derivatizing reagent was added, and the tubes were then mixed and allowed to stand for 15 min at 60°C. Water (0.5 ml) and n-hexane (1.2 ml) were then added, and after thorough vortex mixing and centrifugation, the n-hexane layer was transferred into a glass vial before evaporation to dryness using a vacuum centrifuge. The residue was reconstituted in acetonitrile and an aliquot injected onto the GC. The GC system was a HP 5890 Series II gas chromatograph equipped with a fused-silica capillary column (DB-17, 20 m x 0.18 mm, 0.30-µm film thickness; Agilent Technologies, Palo Alto, CA). Helium was the carrier gas at a column head pressure of 200 kPa, and the injector and transfer line temperatures were set at 260 and 300°C, respectively. A 7-min program was used where the oven temperature remained constant for 1 min before increasing linearly (25.0°C/min) to 300°C over the subsequent 6-min period. The mass spectrometer was a MAT SSQ-7000 (Thermo Finnigan, San Jose, CA) operated with an electron energy of -70 V, an emission current of 200 µA, and an ion source temperature of 200°C. The TMS derivatives of T, DHT, [2H]3-T, and [2H]3-DHT were measured at m/z 361.0, 363.0, 364.0, and 366.0, respectively, whereas the TMS derivative of MENT was measured at m/z 361.0. The LOQ of T and DHT in serum (and the corresponding related labeled analytes) were 0.2 and 0.4 ng/ml, respectively, whereas the corresponding values in lymph were 2 and 4 ng/ml.
[2H]-TU in Intralipid. A 50-µl aliquot of the Intralipid emulsion (n = 5) was diluted in propan-2-ol (950 µl), and the total TU concentration of the resulting solution was determined using an HPLC system consisting of a Spectra System P4000 pump, Spectra System AS3000 autosampler, and Spectra System UV2000 UV detector equipped with an LC analytical column (Kromasil C18, 100 mm x 4.6 mm, 5-µm particle size; Varian Medical Systems) operated at a column temperature of 30°C. The mobile phase was 100% methanol flowing at 1.0 ml/min, the injection volume was 10 µl, and the detection wavelength was 254 nm. The concentration of [2H]3-TU in the Intralipid emulsion was equivalent to 79% of the total TU concentration.
Acceptance Criteria for HPLC and GC Assays. Each analytical run included blank serum or lymph samples, duplicate calibration samples, authentic study samples, and triplicate quality control samples at three different concentrations spanning the calibration range. The results of each analytical run were accepted when the correlation coefficient of the calibration curve exceeded 0.99, when at least two-thirds of the calibration standards were accepted and the accuracy of all accepted calibration points was between 80 and 120%, and when the accuracy of at least two-thirds of the QC samples at each concentration was between 80 and 120%.
Lymph Concentrations of Triglyceride. After a 1 in 10 dilution of lymph samples in Milli-Q water, triglyceride (TG) concentrations were determined using a standardized clinical chemistry analyzer (Roche Cobas Mira, Basel, Switzerland) and a commercial enzyme-based colorimetric assay kit (Roche Diagnostics, Mannheim, Germany).
Data Analysis. The mass of analyte transported into thoracic lymph during each sampling period was calculated as the product of the analyte concentration and total volume of lymph collected during the sampling period. The fraction of the dose transported (Flymph) was calculated as the ratio of cumulative mass of analyte transported and oral dose.
Analysis of Serum Pharmacokinetic Profiles. The maximum serum
concentration (Cmax), and the time to reach the maximum
(Tmax) were noted directly from the individual profiles.
The area under the serum concentration-time profiles from time 0 up to the
last measured concentration
(AUC0-tlast) was calculated by
WinNon-Lin version 4 (Pharsight, Apex, NC) using the linear trapezoidal
method. Where the apparent terminal phase of the serum concentration-time
profiles was evident (i.e., for [2H]-TU and [2H]-T), the
area obtained by extrapolation to infinite time was added to
AUC0-t last to obtain the area from
time 0 to infinity AUC0-
. Serum
clearance of [2H]-TU (CL[2H]-TU) was
calculated as the ratio of intravenous dose and
AUC0-, i.e.,
 | (1) |
The proportion of the dose absorbed into the systemic circulation via the
portal route (Fportal blood) was calculated from the ratio
of the dose-normalized AUC values for TU and [2H]-TU, i.e.,
 | (2) |
The proportion of the dose absorbed into the systemic circulation via the
thoracic lymph (Flymph) was calculated from the ratio of
the cumulative mass of TU transported in lymph and the dose of orally
administered TU, i.e.,
 | (3) |
The predicted systemic serum AUC for TU due to lymphatically transported TU
(AUCTUlymph derived) was calculated as follows:
 | (4) |
The predicted systemic exposure to T, due to lymphatically transported TU
(AUCTlymph derived), was calculated as follows:
 | (5) |
Statistical Analysis. Statistically significant differences in
pharmacokinetic parameters and lymph transport arising from administration of
the Andriol or Andriol Testocaps formulations were analyzed by Student's
t test at a significance level of = 0.05.
|
Results |
|---|
|
TopAbstractMaterials and MethodsResultsDiscussionConclusionReferences |
|---|
|
|
Maximum serum concentrations of [2H]TU were observed at the end
of the infusion period (i.e., Tmax = 0) with maximum
concentrations of [2H]-T and [2H]-DHT being observed at
later times with the mean Tmax values of 26 ± 3 and
49 ± 6 min for [2H]-T and [2H]-DHT, respectively.
The mean Cmax value for [2H]-TU was 1,234
± 65 nM, with lower values observed for [2H]-T (77.7
± 4.1 nM) and [2H]-DHT (7.8 ± 1.1 nM). In the
apparent terminal phase, serum concentrations of [2H]-TU declined
with a t1/2 approximately 2-fold shorter than that of
[2H]-T and [2H]-DHT, the mean t1/2
values being 15.8 ± 0.5, 32.1 ± 2.7, and 37.5 ± 5.8 min
for [2H]-TU, [2H]-T, and [2H]-DHT,
respectively. The calculated AUC0- values were
36,429 ± 2,024, 5,466 ± 264, and 589 ± 99 nM · min
for [2H]-TU, [2H]-T, and [2H]-DHT,
respectively. The mean clearance (CL) and VD values of
[2H]-TU were 0.222 ± 0.012 l/min and 5.08 ± 0.31
liters, respectively.
Lymphatic Transport and Portal Concentrations after Oral Administration of TU. The mean rate of lymphatic transport of TG, TU, and DHTU into thoracic lymph after postprandial oral administration of 80 mg of TU as Andriol or Andriol Testocaps is presented in Fig. 2, A to C, together with the mean cumulative recovery of each compound in thoracic lymph (Fig. 2, D-F). Lymph concentrations of T and DHT were below the LOQ (or where measurable, so low as to be considered negligible) for the entire sampling period. The maximum rate of lymphatic TU transport was observed during the 1- to 2-h postdosing period, whereas the corresponding maxima for TG and DHTU transport occurred during the 2- to 3-h postdosing period. Recovery of TG in thoracic lymph corresponded to >80% of the administered mass of lipid confirming the integrity of each cannulated dog. In terms of drug transport, the cumulative recoveries of TU and DHTU in thoracic lymph accounted for 3 and 0.12% of orally administered TU, respectively. The mean portal serum concentration versus time profiles of TU, T, and DHT are presented in Fig. 3. Portal serum concentrations for DHTU are not available because the concentrations were below the LOQ (i.e., <1 ng/ml) for the entire sampling period in five of the eight dogs.
|
|
Systemic Serum Concentrations of TU, DHTU, T, and DHT. The mean systemic serum concentration versus time profiles of TU, T, and DHT after postprandial oral administration of 80 mg of TU as Andriol or Andriol Testocaps to thoracic lymph duct-cannulated dogs are presented in Fig. 4. Profiles for DHTU are not presented as systemic serum DHTU concentrations were below the LOQ for the entire sampling period in six of the eight dogs. Pharmacokinetic parameters determined for TU and T in each dog are presented in Table 2, together with the mean values for all dogs receiving the same oral formulation. There was considerable intersubject variability in the systemic Cmax, Tmax, and AUC values for TU and T, which gained access to the systemic circulation via the portal blood (i.e., in the lymph cannulated dogs). There was no statistically significant difference (p > 0.05) in any pharmacokinetic parameter in the Andriol or Andriol Testocaps groups.
|
|
Systemic Exposure of TU and T. The mean contribution of intestinal lymphatic transport and portal blood absorption to the systemic availability (expressed as a percentage of the oral dose) of TU administered as Andriol or Andriol Testocaps is presented in Table 3. Irrespective of the administered formulation, the lymphatic route was responsible for the majority of systemic TU exposure (>95%), even though the total mass of TU reaching the systemic circulation represented approximately 3% of the orally administered TU. Andriol and Andriol Testocaps were indistinguishable in terms of either the total oral TU availability or the fractional contributions of the two absorption routes to oral availability. The contribution of lymphatically derived T (i.e., T resulting from lymphatically absorbed TU) and portally absorbed T to systemic T exposure is presented for each dog in Table 4 as the lymph derived and measured systemic AUC values of T, respectively (i.e., AUClymph derived and AUCmeasured, respectively). For all dogs, lymph-derived T constituted the major proportion of systemically available T, and there was no statistically significant difference (p > 0.05) in the total systemic exposure to T (where exposure is evaluated on the basis of AUCtotal) between Andriol and Andriol Testocaps (Table 4). Although the mean AUCmeasured of T after administration of Andriol Testocaps was approximately 30% lower compared with Andriol (i.e., T arising from portal blood absorption), the difference was not statistically significant (p > 0.05) and the absolute values are of minor relevance because the predicted systemic T exposure arising from lymphatically derived TU (which accounts for approximately 85% of total systemic T exposure) was similar for the two formulations.
|
|
|
Discussion |
|---|
|
TopAbstractMaterials and MethodsResultsDiscussionConclusionReferences |
|---|
; Hirschhauser et al.,
1975). Oral administration of TU results in the lymphatic
appearance of TU and DHTU in rat (Coert et
al., 1975; Noguchi et al.,
1985) and humans (Horst et
al., 1976), and it is generally accepted that the androgenic
activity originates from T and DHT formed after systemic hydrolysis. Both TU
and DHTU avoid the hepatic first-pass effect by absorption via the lymphatics
(Coert et al., 1975;
Horst et al., 1976). Over the
past 25 years, there have been numerous reports of the efficacy and safety of
orally administered TU in T replacement therapy
(Luisi and Franchi, 1980;
O'Carroll et al., 1985;
Carani et al., 1990;
Mårin et al., 1992;
Gooren, 1994;
Morales et al., 1997;
Geurts and Coelingh Bennink,
2000); however, few studies have attempted to characterize
efficacy in terms of T bioavailability due to TU administration. Although some
studies have attempted such an assessment, they have not provided direct
evidence of the contribution of lymphatic TU to increased systemic T exposure.
For example, Frey et al. (1979)
compared the relative bioavailabilities of orally administered T (in the form
micronized T, crystalline TU, and TU in arachis oil) and reported an increase
in systemic T exposure when TU was administered in conjunction with a high-fat
meal. Although this result is consistent with enhanced lymphatic drug
transport due to the simultaneous absorption of dietary lipids
(Geurts and Coelingh Bennink,
2000; Khoo et al.,
2001,
2002;
Bagchus et al., 2003), the data
are not conclusive nor quantitatively insightful. Similarly, Tauber et al.
(1986) reported that the mean
absolute bioavailability of T after oral administration of TU to women was
6.83 ± 3.32%, whereas the mean absolute bioavailability of orally
administered T after oral T administration was 3.64 ± 2.45%. Again,
these results suggest that TU leads to increased systemic T exposure (relative
to oral T administration); however, they do not conclusively demonstrate the
role of lymphatic TU absorption because they do not determine the relative
contributions of the lymphatic and portal absorption pathways.
To determine the contribution of lymphatic and portal blood absorption to
the systemic bioavailability of highly lipophilic drugs, our laboratory
developed and validated a thoracic lymph duct-cannulated dog model
(Khoo et al., 2001). The
current study was undertaken to determine the contribution of intestinal
lymphatic transport of TU to systemic availability after postprandial
administration of two TU formulations (Andriol or Andriol Testocaps) and to
examine the hypothesis that lymphatic TU transport provides an advantage in
oral T therapy because it provides for the majority of systemic T exposure
after postprandial administration.
Systemic Serum Concentrations after i.v. Administration of
[2H]-TU. After the i.v. infusion, serum concentrations of
[2H]-TU declined rapidly in a monoexponential manner with a mean
half-life of 15.8 ± 0.5 min with the concomitant formation of
[2H]-DHTU, [2H]-T, and [2H]-DHT arising from
the respective hydrolysis and/or reduction of [2H]-TU due to
systemic esterase and 5-reductase activity. Because the serum clearance
of the relevant TU metabolites was not individually determined in this study,
it was not possible to determine the fractional conversion of
[2H]-TU to [2H]-DHTU, [2H]-T, and
[2H]-DHT. However, the data qualitatively confirm that the
therapeutic benefit derived from TU is likely mediated by T and DHT.
The low variability in the serum concentration versus time profiles of [2H]-TU, [2H]-T, and [2H]-DHT after i.v. administration of [2H]-TU indicated excellent intersubject consistency in the systemic conversion of [2H]-TU to [2H]-T and [2H]-DHT (Fig. 1). Furthermore, there was no significant difference in the i.v. profiles for the two groups that received the different oral formulations, indicating that compositional differences in the oral TU formulations did not affect the systemic pharmacokinetics of TU (Table 1).
Absorption into Lymph after Oral TU Administration. TU and DHTU were
observed in thoracic lymph after oral administration of TU to postprandial
lymph duct-cannulated dogs, consistent with previous reports in rats
(Coert et al., 1975;
Noguchi et al., 1985) and
humans (Horst et al., 1976).
The profile of TU transport in lymph was essentially the same after oral
administration of either the Andriol or Andriol Testocaps formulations
(Fig. 2, B and E). Although of
minor qualitative importance in terms of the mass transported, the profile of
lymphatic TU transport was different compared with that observed for TG and
DHTU transport. For example, although the maximum rate of lymphatic TU
transport occurred 1 to 2 h postdosing, the maximal rate of transport of TG
and DHTU occurred 2 to 3 h postdosing (Fig.
2, A and C). Furthermore, the rate of lymphatic TU transport
declined rapidly after reaching its maximal transport rate, whereas the rate
of TG and DHTU transport declined more gradually
(Fig. 2, A-C). These profiles
provide possible insight into the disposition of TU (and DHTU) within the
gastrointestinal tract and enterocyte before absorption into lymph. For
example, because TU is metabolized within the intestinal lumen and gut wall
(Coert et al., 1975;
Horst et al., 1976), which is
the basis for the high serum concentrations of T and DHT observed in portal
blood in the present study (Fig.
3), the change in the relative rates of TG and TU transport after
the 1- to 2-h postdosing period likely reflects the reduction in the available
mass of TU within the enterocyte for incorporation into chylomicrons. If a
limiting factor in the kinetics of TU lymph transport was rapid hydrolysis
within the intestinal lumen, then the observation of the prolonged transport
of DHTU in concert with TG transport suggests that DHTU (formed by 5
-reductase metabolism of TU) may have a longer residence time within the
enterocyte lipid-processing microdomains which could arise from either
differences in the metabolic stability or enterocyte-based processing
mechanisms for DHTU and TU.
Contribution of Lymphatic and Blood Absorption to Systemic Availability
of TU. In the intact animal, thoracic duct lymph empties directly into the
systemic circulation at the junction of the internal jugular and
brachiocephalic vein. Therefore, the cumulative mass of drug recovered in
thoracic lymph during the experimental period is equivalent to the mass of
drug otherwise systemically available after absorption via the lymphatic
route. In the lymph duct cannulated dog, the apparent bioavailability
calculated from the ratio of the dose normalized systemic serum AUC values
after oral and i.v. administration (determined using a stable TU isotope,
which is not expected to exhibit an in vivo isotope effect; Baba et al.,
1979,
1980; Shinohara et al.,
1980,
1988; Fujioka et al.,
1986,
1989;
Shinohara and Baba, 1990) is
equivalent to the fraction of the dose systemically available after absorption
via the portal vein.
Based on the ratio of the dose-normalized serum AUC values for TU and [2H]3-TU, the mean fraction of the TU dose reaching the systemic circulation via the portal vein was similar and extremely low at 0.054 ± 0.029 and 0.036 ± 0.015% from Andriol and Andriol Testocaps, respectively (Table 3). In terms of the lymphatic transport, the mean fraction of the orally administered dose of TU recovered in thoracic lymph was 3.20 ± 0.46 and 2.85 ± 089% from Andriol and Andriol Testocaps, respectively (Table 3). There was no statistical difference (p > 0.05) in the extent of portal blood or lymphatic transport of TU after administration of either Andriol or Andriol Testocaps.
When the total systemic TU availability is calculated as the sum of the availabilities due to portal and lymphatic TU absorption (i.e., Fportal blood + Flymph), it is obvious that lymphatic absorption was responsible for 93.0 to 99.8% of the TU that ultimately reaches the systemic circulation (Table 3). Because the total systemic availability of TU after postprandial administration of Andriol (3.25 ± 0.48% of the dose) was not significantly different (p > 0.05) compared with Andriol Testocaps (2.88 ± 0.88% of the dose), the two formulations are indistinguishable in terms of their systemic TU exposure.
Factors Contributing to Systemic T Exposure. It is evident from Fig. 1 that [2H]-TU delivered into the systemic circulation is rapidly converted to [2H]-T. However, it was not possible to estimate the fractional conversion of [2H]-TU to [2H]-T without an estimate of the serum clearance of [2H]-T and this precluded calculation of the mass of T reaching the systemic circulation as a consequence of lymphatic transport of TU. This was addressed by using eqs. 4 and 5 to calculate a value for the term AUCTlymph derived to estimate the systemic exposure of T arising from lymphatic transport of TU (i.e., the systemic exposure of T arising from the contents of the thoracic duct emptying into the systemic circulation).
After postprandial administration of 80 mg of TU as Andriol, the mean value of AUCTlymph derived was 4,354 ± 642 nM · min and the corresponding value for Andriol Testocaps was 4,153 ± 1,185 nM · min (Table 4). Because the mean systemic T serum AUC values (AUCmeasured) after administration of Andriol and Andriol Testocaps were 701 ± 110 and 471 ± 132 nM · min, respectively (Table 4), it is clear that T derived from systemic hydrolysis of lymphatically absorbed TU was the major contributor to the total systemic T exposure (i.e., AUCTlymph derived + AUCmeasured). In addition, there was no statistically significant difference (p > 0.05) in AUCtotal between dogs administered Andriol or Andriol Testocaps (Table 4). Consequently, the two formulations were indistinguishable in terms of the systemic exposure to T.
|
Conclusion |
|---|
|
TopAbstractMaterials and MethodsResultsDiscussionConclusionReferences |
|---|
-alkylated testosterone prodrugs via the oral and injected routes)
have previously been applied to circumvent the fact that direct oral T
administration is ineffective in androgen replacement therapy. However, orally
administered testosterone undecanoate is both safe and efficacious upon oral
drug administration (Luisi and Franchi,
1980; O'Carroll et al.,
1985; Carani et al.,
1990; Mårin et al.,
1992; Gooren,
1994; Morales et al.,
1997; Geurts and Coelingh
Bennink, 2000). The results from the current study demonstrate
conclusively that the therapeutic advantage associated with oral TU
administration is a consequence of lymphatic transport of TU to the systemic
circulation. Furthermore, the study demonstrated conclusively that there is no
difference in the systemic exposure of T, or TU, resulting from postprandial
oral administration of Andriol or Andriol Testocaps. |
Footnotes |
|---|
ABBREVIATIONS: T, testosterone; TU, testosterone undecanoate; DHTU,
5-dihydrotestosterone undecanoate; DHT, 5-dihydrotestosterone;
LC, liquid chromatograph; LC-MS/MS, liquid chromatography coupled to tandem
mass spectrometry; LC, liquid chromatography; LOQ, limits of quantitation; GC,
gas chromatography; TG, triglyceride; AUC, area under the curve.
Address correspondence to: Dr. William N. Charman, Department of Pharmaceutics, Victorian College of Pharmacy, Monash University (Parkville Campus), 381 Royal Parade, Parkville, Victoria 3052, Australia. E-mail: bill.charman{at}vcp.monash.edu.au
|
References |
|---|
|
TopAbstractMaterials and MethodsResultsDiscussionConclusionReferences |
|---|
Baba S, Shinohara Y, and Kasuya Y (1979) Determination of plasma testosterone by mass fragmentography using testosterone-19-d3 as an internal standard. Comparison with radioimmunoassay. J Chromatogr A 162: 529-537.
Baba S, Shinohara Y, and Kasuya Y (1980)
Differentiation between endogenous and exogenous testosterone in human plasma
and urine after oral administration of deuterium-labeled testosterone by mass
fragmentography. J Clin Endocrinol Metab
50:
889-894.
Bagchus WM, Hust R, Maris F, Schnabel PG and NSH (2003) Important effect of food on the bioavailability of oral testosterone undecanoate. Pharmacotherapy 23: 319-325.[CrossRef][Medline]
Carani C, Zini D, Baldini A, Della Casa L, Ghizzani A, and Marrama P (1990) Effects of androgen treatment in impotent men with normal and low levels of free testosterone. Arch Sex Behav 19: 223-234.[CrossRef][Medline]
Coert A, Geelen J, de Visser J, and van der Vies J (1975) The pharmacology and metabolism of testosterone undecanoate (TU), a new orally active androgen. Acta Endocrinol 79: 789-800.
Daggett PR, Wheeler MJ, and Nabarro JD (1978) Oral testosterone, a reappraisal. Horm Res 9: 121-129.[Medline]
Edwards GA, Porter CJH, Caliph SM, Khoo SM, and Charman WN (2001) Animal models for the study of intestinal lymphatic drug transport [Review]. Adv Drug Deliv Rev 50: 45-60.[CrossRef][Medline]
Frey H, Aakvaag A, Saanum D, and Falch J (1979) Bioavailability of oral testosterone in males. Eur J Clin Pharmacol 16: 345-349.[CrossRef][Medline]
Fujioka M, Shinohara Y, Baba S, Irie M, and Inoue K
(1986) Pharmacokinetic properties of testosterone propionate in
normal men. J Clin Endocrinol Metab
63:
1361-1364.
Fujioka M, Shinohara Y, Baba S, Irie M, and Inoue K (1989) Endogenous and exogenous testosterone levels after administration of deuterium-labelled testosterone propionate in hypogonadotropic hypogonadism. Chem Pharm Bull 37 : 3100-3101.
Geurts TBP and Coelingh Bennink HJT (2000) Testosterone replacement therapy testosterone undecanoate (Andriol). J Urol Urogenakol Special Edition: 24-35.
Gooren LJ (1994) A ten-year safety study of the oral
androgen testosterone undecanoate. J Androl
15:
212-215.
Heck HdA, Buttrill SEJ, Flynn NW, Dyer RL, Anbar M, Cairns T, Dighe S, and Cabana BE (1979) Bioavailability of imipramine tablets relative to a stable isotope-labeled internal standard: increasing the power of bioavailability tests. J Pharmacokinet Biopharm 7: 233-248.[CrossRef][Medline]
Hirschhauser C, Hopkinson CR, Sturm G, and Coert A (1975) Testosterone undecanoate: a new orally active androgen. Acta Endocrinol 80: 179-187.
Horst HJ, Holtje WJ, Dennis M, Coert A, Geelen J, and Voigt KD (1976) Lymphatic absorption and metabolism of orally administered testosterone undecanoate in man. Klinische Wochenschrift 54: 875-879.[CrossRef][Medline]
Humberstone AJ, Porter CJ, and Charman WN (1996) A physicochemical basis for the effect of food on the absolute oral bioavailability of halofantrine. J Pharm Sci 85: 525-529.[CrossRef][Medline]
Khoo SM, Edwards GA, Porter CJH, and Charman WN (2001) A conscious dog model for assessing the absorption, enterocyte-based metabolism and intestinal lymphatic transport of halofantrine. J Pharm Sci 90: 1599-1607.[CrossRef][Medline]
Khoo SM, Prankerd RJ, Edwards GA, Porter CJH, and Charman WN (2002) A physicochemical basis for the extensive intestinal lymphatic transport of a poorly lipid soluble antimalarial, halofantrine hydrochloride, after postprandial administration to dogs. J Pharm Sci 91: 647-659.[CrossRef][Medline]
Luisi M and Franchi F (1980) Double-blind group comparative study of testosterone undecanoate and mesterolone in hypogonadal male patients. J Endocrinol Investig 3: 305-308.
Maisey NM, Bingham J, Marks V, English J, and Chakraborty J (1981) Clinical efficacy of testosterone undecanoate in male hypogonadism. Clin Endocrinol 14: 625-629.[Medline]
Mårin P, Holmäng S, Jönsson L, Sjöström L, Kvist H, Holm G, Lindstedt G, and Björntorp P (1992) The effects of testosterone treatment on body composition and metabolism in middle-aged obese men. Int J Obesity 16: 991-997.
Morales A, Johnston B, Heaton JP, and Lundie M (1997) Testosterone supplementation for hypogonadal impotence: assessment of biochemical measures and therapeutic outcomes. J Urol 157: 849-854.[CrossRef][Medline]
Noguchi T, Charman WNA, and Stella VJ (1985) The effect of drug lipophilicity and lipid vehicles on the lymphatic absorption of various testosterone esters. Int J Pharm 24: 173-184.[CrossRef]
O'Carroll R, Shapiro C, and Bancroft J (1985) Androgens, behaviour and nocturnal erection in hypogonadal men: the effects of varying the replacement dose. Clin Endocrinol 23: 527-538.[Medline]
Shinohara Y and Baba S (1990) Stable isotope methodology in the pharmacokinetic studies of androgenic steroids in humans. Steroids 55: 170-176.[CrossRef][Medline]
Shinohara Y, Baba S, and Kasuya Y (1980) Absorption,
metabolism and excretion of oral testosterone in humans by mass
fragmentography. J Clin Endocrinol Metab
51:
1459-1462.
Shinohara Y, Fujioka M, and Baba S (1988) Pharmacokinetic studies of testosterone propionate using gas chromatography/mass spectrometry/selected ion monitoring. Biomed Environ Mass Spectrom 16: 241-244.[CrossRef][Medline]
Skakkebaek NE, Bancroft J, Davidson DW, and Warner P (1981) Androgen replacement with oral testosterone undecanoate in hypogonadal men: a double blind controlled study. Clin Endocrinol 14: 49-61.[Medline]
Tauber U, Schroder K, Dusterberg B, and Matthes H
(1986) Absolute bioavailability of testosterone after oral
administration of testosterone-undecanoate and testosterone. Eur J
Drug Metab Pharmacokinet 11:
145-149.[Medline]
This article has been cited by other articles:
|
|
 |
|
  J. K. Amory, S. T. Page, and W. J. Bremner Oral Testosterone in Oil: Pharmacokinetic Effects of 5{alpha} Reduction by Finasteride or Dutasteride and Food Intake in Men J Androl, January 1, 2006; 27(1): 72 - 78. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. K. Amory and W. J. Bremner Oral Testosterone in Oil Plus Dutasteride in Men: A Pharmacokinetic Study J. Clin. Endocrinol. Metab., May 1, 2005; 90(5): 2610 - 2617. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
 |
 |
|
 |
 |
 |
), [2H]-DHTU (
), [2H]-T
(
), and [2H]-DHT (
) after i.v. administration of
[2H]-TU to postprandial lymph duct-cannulated dogs as a 10-min
infusion in Intralipid. Data are presented as the mean of all study dogs as
there was no statistically significant difference in the i.v. pharmacokinetic
parameters for groups of dogs receiving either oral Andriol or Andriol
Testocaps. S.E. error bars are within the size of the individual symbols.
