Drug Metabolism and Disposition Fast Forward
First published on July 20, 2005; DOI: 10.1124/dmd.104.001040
0090-9556/05/3310-1488-1494$20.00
DMD 33:1488-1494, 2005
PHYSIOLOGICALLY BASED PHARMACOKINETIC MODEL FOR PRALMORELIN HYDROCHLORIDE IN RATS
Risa Nasu,
Yoichi Kumagai,
Hirokuni Kogetsu,
Masayuki Tsujimoto,
Hisakazu Ohtani1, and
Yasufumi Sawada1
Pharmacokinetics Department, Central Research Laboratories, Kaken Pharmaceutical Co., Ltd., Shizuoka, Japan (R.N., Y.K., H.K.); and Department of Medico-Pharmaceutical Sciences, Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka, Japan (M.T., H.O., Y.S.)
(Received July 9, 2004;
accepted July 15, 2005)
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Abstract
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Pralmorelin hydrochloride (pralmorelin), consisting of six amino acid residues, is a growth hormone-releasing peptide. The aim of this study is to analyze the pharmacokinetics of pralmorelin after intravenous bolus administration to rats, and to develop a physiologically based pharmacokinetic (PB-PK) model to describe and predict the concentrations of pralmorelin in blood and tissues. Pralmorelin (3 mg/kg) was administered intravenously to 24 Sprague-Dawley rats. Groups of three rats were sacrificed by decapitation at each designated time point (up to 4 h), and plasma and tissues (brain, lung, heart, liver, kidney, small intestine, muscle, adipose, and skin) were collected. Bile was also pooled until decapitation. The concentration of pralmorelin in samples was determined by liquid chromatography-tandem mass spectrometry. Plasma concentrations of pralmorelin declined rapidly in a biexponential manner. Biliary excretion of pralmorelin was so rapid that 80% of the dose was recovered unchanged in the bile within 1 h after administration. The distribution parameters in each tissue were obtained by using a hybrid model and an integration plot. They revealed that the distribution of pralmorelin into liver was blood flow-limited, and its distribution was permeability-limited in all other tissues. The PB-PK model developed in this study well described the time courses of pralmorelin concentration in the blood and tissues of rats.
Pralmorelin hydrochloride (GHRP-2), a synthetic growth hormone-releasing peptide consisting of six amino acid residues, was developed by C. Y. Bowers of Tulane University (New Orleans, LA) and Kaken Pharmaceutical Co., Ltd. (Tokyo, Japan) (Bowers, 1993
). It acts through both the growth hormone secretagogue receptor, which is distinct from hypothalamic growth hormone-releasing hormone receptor, and the ghrelin receptor, and it has been developed as both a diagnostic and a therapeutic agent for growth hormone deficiency. Pralmorelin is rapidly excreted unchanged into the bile from the systemic circulation after intravenous administration to rats.
Recently, various biologically active peptides and their analogs have been considered as drug candidates, including somatostatin analogs (Bauer et al., 1982), endothelin antagonist (Nirei et al., 1993; Nishikibe et al., 1993), and renin inhibitor (Ondetti and Cushman, 1981). These small peptides, consisting of 5 to 10 amino acid residues, have common characteristics, in that they are rapidly taken up by the liver and most of the dose is subsequently excreted unchanged into the bile (Berelowitz et al., 1978; Greenfield et al., 1989; Cathapermal et al., 1991). Both in vitro and in vivo studies have demonstrated that active transport systems are involved in the hepatic uptake and biliary excretion of these peptides (Yamada et al., 1997; Akhteruzzaman et al., 1999; Kato et al., 1999). Therefore, pralmorelin may also be excreted by active transport systems. However, few studies have been conducted to elucidate the in vivo tissue distribution properties of small peptides.
Physiologically based pharmacokinetic (PB-PK) models provide a systematic understanding of the pharmacokinetic behavior of drugs based on physiological parameters. PB-PK models are useful for drug development because they are helpful in describing the distribution, excretion, and biotransformation of drugs, can provide interspecies scale-up, and enable us to predict the drug concentration profile for any dose and route of administration under various physiological conditions, such as impairment of liver or kidney (Sato et al., 1987; Hosseini-Yeganeh and McLachlan, 2002).
The aim of this study is to analyze the distribution of pralmorelin and to establish a PB-PK model well describing the profiles of pralmorelin concentration in the blood and tissues of rats.
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Materials and Methods
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Animals. Adult male Sprague-Dawley rats weighing 208 to 289 g were purchased from Charles River Japan, Inc. (Kanagawa, Japan) and housed in a temperature-controlled room on a normal 12-h light/dark cycle, with free access to food and water. All experimental procedures were consistent with those stipulated in the Kaken Pharmaceutical Co., Ltd. Guide for the Care and Use of Experimental Animals.
Materials. Pralmorelin hydrochloride (D-alanyl-3-(2-naphthyl)-D-alanyl-L-alanyl-L-tryptophyl-D-phenylalanyl-L-lysinamide dihydrochloride) was synthesized by Fuji Chemical Industries, Ltd. (Toyama, Japan). The chemical structure is shown in Fig. 1. [3H]Pralmorelin hydrochloride was purchased from Cambridge Research Biochemicals (Northwich, UK). The specific activity and chemical purity of [3H]pralmorelin hydrochloride were 5.61 mCi/mg and >97%, respectively. The dosing solution used for all animal studies was prepared by dissolving pralmorelin hydrochloride and [3H]pralmorelin hydrochloride in saline. The internal standard, 2-aminobutyryl-3-(2-naphthyl)-D-alanyl-L-alanyl-L-tryptophyl-D-phenylalanyl-L-lysinamide dihydrochloride, was synthesized by Kaken Pharmaceutical Co., Ltd. All other reagents were of analytical grade or higher.
Drug Administration and Sample Collection. Under light ether anesthesia, rats were cannulated in the femoral vein, artery, and bile duct with silicone rubber and polyethylene tubing, respectively. The rats were used for the experiment at 30 to 60 min after recovery from anesthesia. Pralmorelin solution (3 mg/kg) was administered via the femoral vein cannula as a rapid infusion. Blood samples were withdrawn from the femoral artery at designated times of sacrifice (5, 15, and 30 min and 1, 2, and 4 h). The plasma was separated by centrifugation at 12,000g for 3 min and kept frozen at -30°C until analysis. For tissue sampling, the rats were sacrificed by decapitation at designated times, and the brain, lungs, heart, liver, small intestine, kidneys, muscle, adipose, and skin were quickly excised. Bile was pooled until sacrifice. Once dissected, the heart was cut open and residual blood was removed. The contents of the small intestine were removed and the tissue was washed with ice-cold saline. All tissues except for the adipose tissue and skin were homogenized in 5 volumes of purified water. The adipose and skin were minced with scissors. All tissues were kept frozen at -30°C until analysis.
Quantitative Analysis of Pralmorelin. Pralmorelin was analyzed by LC-MS/MS. Plasma was spiked with the internal standard, 25% Block Ace (Dainippon Pharmaceutical Co., Ltd., Osaka, Japan), and 0.1 M phosphate buffer (pH 6.0), and vortexed. EmporeDisk plates (96-well, MPC-SD) were used for extracting pralmorelin from rat plasma. Plates were first conditioned with methanol, followed by 0.1 M phosphate buffer. Following the sample loading step, the analytes were eluted with 4% ammonia solution/methanol. Samples were dried and then reconstituted in mobile phase (0.1% formic acid/acetonitrile, 80:20, v/v). The injection volume was 10 µl.
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FIG. 2. Schematic representation of a one-organ model for a noneliminating organ. Indicated are the blood flow (Qt), the arterial blood concentration (Ca), the concentration in the capillary bed (Cb), the tissue concentration (Ct), the tissue weight (Vwt), the membrane diffusion intrinsic clearance (PS · fb), the backflux rate constant (k2), and the distribution volume that consists of blood space and the space in which the drug concentration rapidly equilibrates with blood (Kp0 · Vwt).
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For analysis of pralmorelin in the tissues, samples were extracted with methanol and solid-phase cartridges. Briefly, tissue homogenate aliquots except for the adipose tissue and skin were spiked with the internal standard, 25% Block Ace, and 1% trifluoroacetic acid/methanol, vortexed, and centrifuged. The supernatant was evaporated. The concentrate was dissolved in 25% Block Ace and 0.1 M phosphate buffer and centrifuged. The aqueous phase was applied to solid-phase cartridges for extraction of pralmorelin. The minced adipose and skin tissues were spiked with internal standard and 1% trifluoroacetic acid/methanol, vortexed, and centrifuged. The supernatant was evaporated. The concentrate was dissolved in 25% Block Ace and 0.1 M phosphate buffer. The samples were applied to solid-phase cartridges for extraction of pralmorelin. The eluate was evaporated and then dissolved in the mobile phase. Aliquots (10 µl) were assayed prior to LC-MS/MS.
LC-MS/MS analysis was performed on a TSQ Quantum tandem mass spectrometer (Thermo Electron Corporation, Waltham, MA) equipped with a Nanospace SI-2 HPLC apparatus (Shiseido Co., Ltd., Tokyo, Japan). The positive ion mode was used. Selected reaction monitoring using precursor
product ion combinations of m/z 410170 and 417170 was used for quantification of pralmorelin and the internal standard, respectively. The analytical column was a 5-µm, 2.1 x 50 mm PLRP-S reverse polymer column (Polymer Laboratories Ltd., Shropshire, UK).
Estimation of Distribution Parameters. All the plasma concentration data were converted to blood concentrations using eq. 1.
 | (1) |
where Ca, Cp, and RB are the arterial blood concentration, plasma concentration, and blood-to-plasma concentration ratio (RB = 0.67).
With regard to the tissue distribution of pralmorelin, we assumed a two-compartment model, which consists of a compartment in rapid equilibrium with blood ("rapid compartment") and a deep compartment in slow equilibrium with blood. Figure 2 is the schematic representation of a one-organ model for a noneliminating organ, which describes the distribution of a drug (Sato et al., 1987). Each tissue consists of the capillary bed and two distinct compartments. The first is a blood flow-limited compartment (the rapid compartment) that is in rapid equilibrium with the drug concentration in the blood, i.e., the concentration (Cb) in the capillary bed. In addition, the sum of the distribution volume of the blood flow-limited compartment and the volume of the capillary bed (i.e., the distribution volume rapid compartment) can be described as the product (Kp0 · Vwt) of Kp0 and the tissue weight (Vwt). The other is the compartment in which the distribution is membrane permeability-limited (the deep compartment). In the latter compartment, the membrane permeation clearance from the rapid compartment into the deep compartment and the backflux rate constant from the deep compartment into the rapid compartment are represented as PS · fb and k2, respectively.
We defined Kp and Kp0 as the tissue-to-blood concentration ratio attributable to the drug in the deep compartment and that attributable to the rapid compartment, respectively. Because the Kp values in three early points (5, 15, and 30 min) were constant, the average of these three points was regarded as Kp0.
In addition, a hybrid model (Fig. 2) was fitted to tissue concentrations using the arterial blood concentration-time profile to determine PS · fb and k2.
The hepatic uptake of pralmorelin was so rapid that we could not estimate the PS · fb and k2 values. Therefore, we designed another in vivo hepatic uptake study, as described in the following section.
To investigate the nature of rapid compartment, we attempted to compare the estimated Kp0 value with the Kp value of inulin, a marker of extracellular space. Unfortunately, the Kp value of inulin was not available, so that we compared the tissue-to-plasma concentration ratio (Kp,plasma) of inulin at 5 min with that of pralmorelin instead. Furthermore, we regarded the Kp,plasma value at 5 min as the distribution into the rapid compartment. We measured the Kp,plasma values at 5 min after intravenous administration of pralmorelin at a dose of 3 mg/kg and [3H]pralmorelin at a dose of 100 µg/kg and compared with that of inulin previously reported by Yamada et al. (1997).
Analysis of Initial Hepatic Uptake after Administration of [3H]Pralmorelin. Under pentobarbital sodium (50 mg/kg i.p.) anesthesia, the femoral vein and artery were cannulated. [3H]Pralmorelin solution (3.23 µCi, 100 µg/kg) was administered through the femoral vein, and blood samples were collected at intervals of 10 to 30 s, prior to and at the time of sacrifice. At designated times 30 s to 3 min after administration, the rats were sacrificed, the liver was excised, and a portion of the tissue was weighed and counted for radioactivity. When a tracer amount of pralmorelin was given intravenously and liver uptake was measured within a period short enough to disregard the backflux and biliary excretion of the parent drug and metabolites from the liver, the liver uptake rate of pralmorelin can be described by the following differential equation,
 | (2) |
Integration of eq. 2 and division by Ca gives the following, eq. 3.
 | (3) |
The uptake clearance of the tissue, K1, was obtained from the initial slope of a plot of Kp (ml/g tissue) versus AUC(t)/Ca(t) (min) (Yamazaki et al., 1993).
The uptake clearance of the tissue (K1) was a hybrid parameter of PS · fb (the membrane permeation clearance) and blood flow (Qt). In the liver, we assumed that: 1) each compartment constituting a whole organ is well stirred ("well stirred" model), 2) only unbound pralmorelin can diffuse across the membrane into each tissue, 3) only unbound pralmorelin is subject to metabolism and elimination, 4) binding equilibrium of pralmorelin and the distribution into blood cells are rapid enough so that the processes of binding to and dissociation from blood cells are not rate-determining.
Based on the above assumptions, the relationship between the uptake clearance of the tissue (K1; ml/min/g tissue) and the membrane permeation clearance (PS · fb; ml/min) is given by eq. 4.
 | (4) |
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FIG. 3. A schematic representation of the PB-PK model to predict the time profiles of pralmorelin concentrations. The arrows show the direction of blood flow. Indicated are the blood flows (Q) of the lung (LU), heart (HE), muscle (MU), skin (SK), liver (H), gut (GU), kidney (R), brain (BR), and adipose tissue (AD), and the intrinsic clearance (CLint).
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Therefore, PS · fb is expressed as follows.
 | (5) |
Development of PB-PK Model. Figure 3 represents the developed PB-PK model for the distribution and excretion of pralmorelin in rats. The model consists of nine tissues and blood compartments that reflect real organs or anatomic tissues in rats. These tissues are connected in parallel between the arterial and venous circulations in this PB-PK model. The blood flows from the venous pool via the pulmonary artery into the lung and then out via the pulmonary vein into the arterial pool. Except for the liver and lung, all tissues are supplied from the arterial circulation, and blood coming out of these tissues flows directly into the venous circulation. The liver receives its blood supply from both the hepatic artery and portal vein. In this model, pralmorelin was assumed to be eliminated only by the liver and kidney, but not by other tissues. In the liver, the distribution of pralmorelin was rapid and is assumed to be blood flow-limited.
The mass balance equations for the PB-PK model are shown in the Appendix. The equations were integrated numerically and simultaneously using a Macintosh G4 (Apple Computer, Cupertino, CA) with MLAB (Civilized Software, Bethesda, MD).
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Results
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Total and Hepatic Clearance of Pralmorelin. Plasma concentrations of pralmorelin declined quickly in a biexponential manner after intravenous administration. Pralmorelin was excreted into the bile very rapidly, and 80% of the dose was recovered in the bile within 1 h. The distribution volumes of pralmorelin were small for the skin, adipose, muscle, brain, and heart, and large for the liver and kidney. For analysis, all plasma data were converted to blood concentrations by means of eq. 1.
The total blood clearance of pralmorelin was 419 ml/h. Since previous studies have reported that almost 80% of the dose of pralmorelin is excreted unchanged into the bile, and 8% of the dose is excreted into urine within 24 h after intravenous administration, we assumed that 8% of the dose was eliminated by the kidney and the rest was assigned to the liver in this study. Consequently, the hepatic and renal blood clearances were calculated as 382 and 36.9 ml/h, respectively. Renal blood clearance is almost equal to the product of the glomerular filtration rate (245 ml/h; Yamada et al., 1997) and fb of pralmorelin (0.129). Therefore, we assumed that pralmorelin was not subject to renal secretion/reabsorption and the renal clearance was accounted for only by glomerular filtration. The hepatic blood clearance was converted to the hepatic intrinsic clearance (CLintH) by eq. 6, based on a well stirred model.
 | (6) |
The hepatic intrinsic clearance was calculated to be 830 ml/h.
Estimation of Distribution Parameters. Kp0 values were determined from the average of Kp values in three early sampling points. Kp0 and PS · fb were estimated from fitting the arterial blood concentration-time profile to the tissue concentrations. Kp, Kp0, PS · fb, and k2 are presented in Table 1.
To determine whether the estimated Kp0 value is consistent with the vascular volume in tissue, the tissue-to-plasma concentration ratio (Kp,plasma) of pralmorelin at 5 min after intravenous administration was compared with that of inulin. Figure 4 shows that the relationship between the Kp,plasma of inulin and pralmorelin (100 µg/kg and 3 mg/kg) at 5 min after intravenous bolus administration in rats was linear (y = 1.20x; r = 0.775, p < 0.05).
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FIG. 4. Relationship between the Kp,plasma values at 5 min after intravenous administration of inulin and pralmorelin. The open circles and closed circles indicate the Kp,plasma values of 100 µg/kg [3H]pralmorelin and those of 3 mg/kg pralmorelin, respectively. The Kp,plasma values of inulin are cited from Yamada et al. (1997). The solid and broken lines indicate the regression line and 1:1 correlation, respectively. The regression line is y = 1.20x (r = 0.775, p < 0.05).
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Analysis of Initial Hepatic Uptake after Administration of [3H]Pralmorelin. The time profiles of [3H]pralmorelin concentrations in the blood and liver within short period after administration were investigated to estimate the initial hepatic uptake. The Kp and AUC/Ca were plotted in Fig. 5 based on eq. 3. The slope (K1) was linear up to 3 min after intravenous bolus administration (Fig. 5). The hepatic uptake clearance (K1) of [3H]pralmorelin was thus calculated to be 1.15 ml/min/g liver.
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FIG. 5. Integration plot for pralmorelin in rat liver. The initial slope represents the K1 value for rat liver (K1 = 1.15 ml/min/g liver).
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In the liver, the PS · fb was calculated according to eq. 5 using K1 based on a well stirred model. The calculated PS · fb (99.7 ml/min) was sufficiently larger than QH (11.8 ml/min). Thus, the distribution of pralmorelin into the liver was considered to be blood flow-limited. The Kpss value in the liver was determined by curve-fitting using a hybrid model, with the arterial blood concentration-time profile as the input function (Hosseini-Yeganeh and McLachlan, 2001). The estimated Kpss value for the liver was 18.9 (Table 2).
PB-PK Modeling. Distribution into all tissues except for the liver was considered to be membrane permeability-limited, because the PS · fb values were much lower than the blood flow rate. Physiological parameters reported for a 250-g rat (Hosseini-Yeganeh and McLachlan, 2002) and the estimated parameters are listed in Tables 1 and 2.
Figures 6 and 7 show the simulated concentration-time profiles of pralmorelin along with the observed concentration-time data for blood and tissues after intravenous administration of pralmorelin (3 mg/kg). The developed PB-PK model agreed well with the observed concentrations in blood and tissues over 4 h, indicating that the developed PB-PK model is appropriate to describe the kinetics of pralmorelin.
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FIG. 6. Concentration-time profiles for pralmorelin in rat blood and liver after administration of pralmorelin at a dose of 3 mg/kg (i.v.). The symbols represent experimentally observed concentrations and the solid lines are the simulated concentration profiles using the PB-PK model (n = 3, mean ± S.D.). Concentration-time profiles in semilogarithmic form are shown in insets.
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FIG. 7. Concentration-time profiles for pralmorelin in rat tissues after administration of pralmorelin at a dose of 3 mg/kg (i.v.). The symbols represent experimentally observed concentrations and the solid lines show the simulated concentration profiles using the PB-PK model (n = 3, mean ± S.D.). Concentration-time profiles in semilogarithmic form are shown in insets.
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Discussion
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Pralmorelin is rapidly eliminated from the body via the hepatobiliary route in rats. To clarify the determinant factors of the pharmacokinetics of pralmorelin, we analyzed the tissue distribution profile of pralmorelin using a PB-PK model, which enabled us to predict the profile of the drug concentration in each tissue from that in the blood.
The Kp0 value represents the tissue-to-blood concentration ratio attributable to the drug in the rapid compartment (distribution volume that consists of the blood space and the volume in rapid equilibrium with blood). The Kp0 value often coincides with a fraction of the capillary bed volume in the tissue. However, Kp0 · Vwt values in this study were significantly higher than the capillary bed volumes, suggesting that pralmorelin rapidly distributes into the space other than the capillary bed (Fig. 2). Therefore, we compared the Kp,plasma values of pralmorelin at 5 min after intravenous administration with those of inulin, a marker for extracellular space, because the Kp,plasma value at 5 min is considered to reflect the rapid compartment (Fig. 4). We found that the Kp,plasma values of pralmorelin are linearly correlated with those of inulin for several noneliminating tissues. Thus, the Kp0 · Vwt reflects the distribution to the capillary bed and extracellular space.
Furthermore, the Kp,plasma (3.91) of pralmorelin for the liver was much higher than that of inulin (0.17), whereas the Kp,plasma (1.82) of pralmorelin for the kidney was lower than that of inulin (9.20; Yamada et al., 1997). The membrane permeation clearance (PS · fb; 99.7 ml/min) into the liver, calculated from the K1 value based on the well stirred model, was far larger than QH (11.8 ml/min). Taking into consideration that the distribution of pralmorelin into all tissues except for the liver was membrane permeability-limited, pralmorelin may be taken up into hepatocytes by active transport systems.
On the other hand, the relatively poor distribution of pralmorelin into the kidney may be explained by the difference in the protein binding ratio in blood between pralmorelin and inulin. The unbounded fraction (fb) of inulin in blood is almost 1.0, whereas that of pralmorelin is very low (0.216). Therefore, the glomerular filtration rate of pralmorelin is much smaller than that of inulin. However, the Kp,plasma (1.82) of pralmorelin for the kidney is still higher than 1. This accumulation in the kidney may be attributable to the binding of pralmorelin, the retention of filtered pralmorelin in the renal tubules, or both (Sato et al., 1987).
In this study, we separately examined the initial hepatic uptake by using [3H]pralmorelin, because the hepatic uptake of pralmorelin was so remarkably rapid, to develop the PB-PK model for the liver. As shown in Figs. 6 and 7, the concentration of pralmorelin estimated by the PB-PK model agreed well with the observed data. For drugs that are subject to rapid hepatobiliary excretion, the uptake clearance may not be accurately estimated by use of the sampling schedule for other organs. In such a case, it is essential to estimate accurately the hepatic uptake clearance for developing the PB-PK model.
However, the PB-PK model underestimated the liver in a terminal elimination phase. It may be due to the adsorption of pralmorelin to the endothelium and/or the parenchymal cell in the liver. Cationic macromolecules tend to bind electrostatically to the surface of hepatocytes in a nonspecific manner (Nishida et al., 1991). Since pralmorelin has positive charge at physiological pH, it may be adsorbed electrostatically on the surface of hepatocytes. Indeed, our preliminary experiment using isolated rat hepatocytes indicates that pralmorelin highly binds to the surface of hepatocytes (data not shown). Therefore, the underestimation of pralmorelin concentrations in the liver is conceivably attributable to the adsorption, which provides a compartment with rapid binding and slow dissociation, not described by the present model for the liver with the assumption of rapid equilibrium with blood.
Another possible explanation is the sequestration of pralmorelin into the liver. Since the membrane permeation clearance into the liver was much larger than hepatic blood flow, hepatic distribution of pralmorelin is conceivably attributable to the active transport system, such as transporter or endocytosis, and its passive influx and efflux may be quite limited. Therefore, although pralmorelin taken up into the liver is excreted unchanged into the bile, it may be partially subject to hepatic sequestration.
The concentrations of pralmorelin at 30 min in most tissues were higher than model simulates. However, the blood concentrations were also slightly increased at the same time. Because the time course of blood concentration was obtained by a one-point-per-animal method, the blood concentration at 30 min might be accidentally increased. Therefore, the aforementioned failure in the simulation of tissue concentration may not be due to the flaw of the model.
In conclusion, we have developed a PB-PK model for pralmorelin in rats. The model agreed well with the observed data. The present study also demonstrated that a blood flow-limited compartment and a membrane permeability-limited compartment can account for the distribution of pralmorelin in all tissues except for the liver, and the former represents the distribution to the capillary bed and interstitial fluid. The uptake of pralmorelin into the liver was very rapid, suggesting the existence of active transport systems for pralmorelin in the liver.
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Appendix: Differential Mass Balance Equations for the PB-PK Model
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Qt and Vwt represent the blood flow rate and the tissue weight, respectively. Ct, Ca, Cv, and Cb are the pralmorelin concentrations in the tissue, arterial blood, venous blood, and capillary bed, respectively. Kpss and fb represent the tissue-to-blood partition coefficient and the unbound fraction of pralmorelin. LU, HE, MU, SK, H, GU, R, BR, and AD represent lung, heart, muscle, skin, liver, small intestine, kidney, brain and adipose tissue, respectively. CLint is the intrinsic clearance.
Arterial Blood
 | (A1) |
Venous Blood
 | (A2) |
Pralmorelin Concentrations in Tissues with Blood Flow-Limited Uptake
- Eliminating tissue (Liver)
 | (A3) |
where QH is the sum of the hepatic artery and portal vein blood flow rates.
Pralmorelin Concentrations in Tissues with Permeability-Limited Uptake
- Lung
 | (A4) |
 | (A5) |
 | (A6) |
- Noneliminating tissues
 | (A7) |
 | (A8) |
 | (A9) |
- Eliminating tissue (Kidney)
 | (A10) |
 | (A11) |
 | (A12) |
where Kp0 · Vwt represents the distribution volume that consists of the blood space and the volume in rapid equilibrium with blood, the concentration of which is equal to Cb. The concentration in the whole tissue (Ct) is given as the sum of the concentration in the intracellular space (Ct') and Cb · Kp0 (eqs. A6, A9, A12).
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Footnotes
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Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
doi:10.1124/dmd.104.001040.
ABBREVIATIONS: PB-PK, physiologically based pharmacokinetic; LC-MS/MS, liquid chromatography-tandem mass spectrometry; AUC, area under the blood concentration-time curve; GFR, glomerular filtration rate; CLint, intrinsic clearance.
1 Present address: Laboratory of Drug Informatics, Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Tokyo, Japan. 
Address correspondence to: Prof. Yasufumi Sawada, Graduate School of Pharmaceutical Sciences, The University of Tokyo, 3-14-15 Hongo, Tokyo 113-0033, Japan. E-mail: sawada{at}mol.f.u-tokyo.ac.jp
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Abstract
Materials and Methods
