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Comparison of Methods for Estimating Unbound Intracellular-to-Medium Concentration Ratios in Rat and Human Hepatocytes Using Statins

Takashi Yoshikado, Kota Toshimoto, Tomohisa Nakada, Kazuaki Ikejiri, Hiroyuki Kusuhara, Kazuya Maeda and Yuichi Sugiyama
Drug Metabolism and Disposition July 2017, 45 (7) 779-789; DOI: https://doi.org/10.1124/dmd.116.074823

Abstract

It is essential to estimate concentrations of unbound drugs inside the hepatocytes to predict hepatic clearance, efficacy, and toxicity of the drugs. The present study was undertaken to compare predictability of the unbound hepatocyte-to-medium concentration ratios (Kp,uu) by two methods based on the steady-state cell-to-medium total concentration ratios at 37°C and on ice (Kp,uu,ss) and based on their initial uptake rates (Kp,uu,V0). Poorly metabolized statins were used as test drugs because of their concentrative uptake via organic anion-transporting polypeptides. Kp,uu,ss values of these statins provided less interexperimental variation than the Kp,uu,V0 values, because only data at longer time are required for Kp,uu,ss. Kp,uu,V0 values for pitavastatin, rosuvastatin, and pravastatin were 1.2- to 5.1-fold Kp,uu,ss in rat hepatocytes; Kp,uu,V0 values in human hepatocytes also tended to be larger than corresponding Kp,uu,ss. To explain these discrepancies, theoretical values of Kp,uu,ss and Kp,uu,V0 were compared with true Kp,uu (Kp,uu,true), considering the inside-negative membrane potential and ionization of the drugs in hepatocytes and medium. Membrane potentials were approximately −30 mV in human hepatocytes at 37°C and almost abolished on ice. Theoretical equations considering the membrane potentials indicate that Kp,uu,ss values for the statins are 0.85- to 1.2-fold Kp,uu,true, whereas Kp,uu,V0 values are 2.2- to 3.1-fold Kp,uu,true, depending on the ratio of the passive permeability of the ionized to nonionized forms. In conclusion, Kp,uu,ss values of anions are similar to Kp,uu,true when the inside-negative membrane potential is considered. This suggests that Kp,uu,ss is preferable for estimating the concentration of unbound drugs inside the hepatocytes.

Introduction

According to the free drug hypothesis, only unbound drug is believed to interact with metabolic enzymes and pharmacological/toxicological target proteins. Knowing the intracellular unbound drug concentration is essential to estimate accurately the risk of drug–drug interactions involving drug-metabolizing enzymes and canalicular efflux transporters, and efficacy and toxicity of drugs when their targets are intracellular proteins. Extracellular unbound drug concentration is frequently assumed to be equivalent to the intracellular unbound concentration, especially when drugs are neutral or nonionized and freely permeable across the cytoplasmic membrane (Smith et al., 2010). However, this assumption cannot be applied to drugs whose tissue uptake is dominated by active transporters (Smith et al., 2005; Shugarts and Benet, 2009; Giacomini et al., 2010; Niemi et al., 2011; Shitara et al., 2013), in which their intracellular unbound concentration of drugs could be higher than the extracellular unbound concentration. For instance, unbound concentrations of pravastatin and rosuvastatin in the rat liver are 11- to 16-fold (Yamazaki et al., 1993; Nezasa et al., 2003) and 15-fold higher than unbound plasma concentrations, respectively, because of organic anion-transporting polypeptide (OATP)–mediated uptake (Nezasa et al., 2003).

Methods to estimate the unbound hepatocyte-to-medium concentration ratio (Kp,uu) are needed to predict the magnitude of drug–drug interactions involving drug-metabolizing enzymes, efflux transporters, and other intrahepatic target proteins in the liver. Brown et al. (2010) investigated the impact of transporters on the inhibition constant (Ki) values of cytochrome P450 inhibitors by comparing their inhibitory effects using rat liver microsomes and freshly isolated rat hepatocytes. Using the hepatocytes, Ki values of clarithromycin and enoxacin with known hepatic transporter involvement were markedly smaller than those using the microsomes, which was consistent with their high cell-to-medium total concentration ratios (C/M ratios). The International Transporter Consortium published a review summarizing strategies to estimate intracellular drug concentrations (Chu et al., 2013). Among them, a strategy to estimate Kp,uu based on the initial uptake rate (Kp,uu,V0) calculated using active transport clearance (Vmax/Km), and passive diffusion clearance at various concentrations (Yabe et al., 2011) was introduced. In addition, we have proposed an alternative strategy to estimate Kp,uu under steady-state conditions (Kp,uu,ss), which can be calculated by dividing the C/M ratio at 37°C by that at a low temperature (on ice) or in the presence of ATP depletors, when active transport is stopped (Yamazaki et al., 1992; Shitara et al., 2013). Yamazaki et al. (1993) demonstrated that the uptake of pravastatin by rat hepatocytes was more greatly reduced at a low temperature than by ATP depletion.

In the present study, steady-state uptake of typical OATP substrates, pitavastatin, rosuvastatin, and pravastatin in rat and human hepatocytes was investigated at 37°C and on ice to evaluate their Kp,uu,ss. In general, Kp,uu can be described by the C/M ratio of drugs in hepatocytes as follows:

Embedded Image(1)

where fB is the unbound fraction of the drugs in the blood (in vivo) or in the incubation medium (in vitro), and fT is the unbound fraction of the drugs in the hepatocytes. The unbound fraction in hepatocytes at the steady state (fT,cell,ss) obtained using our method should be validated because intracellular binding to cytosolic proteins/cellular organelles might be altered at low temperatures. Therefore, fT,cell,ss were compared with the unbound fraction in liver homogenates (fT,homogenate) measured by equilibrium dialysis using human liver samples. The Kp,uu,ss obtained were then compared with Kp,uu,V0 in both rat and human hepatocytes. The difference between Kp,uu,ss or Kp,uu,V0 and true Kp,uu (Kp,uu,true) is discussed in the context of theoretical equations, considering the membrane potential (ΔΨ) in hepatocytes and the fraction of ionized drugs at the designated pH. Finally, a method to predict Kp,uu,true from Kp,uu,ss and Kp,uu,V0 obtained experimentally is proposed.

Materials and Methods

Chemicals.

[3H]Pitavastatin, [3H]rosuvastatin calcium, and [3H]pravastatin calcium were obtained from American Radiolabeled Chemicals (St. Louis, MO). [3H]Diazepam was obtained from PerkinElmer Life Sciences (Boston, MA). Unlabeled diazepam, pitavastatin calcium, rosuvastatin calcium, and pravastatin sodium were obtained from Wako Pure Chemicals (Osaka, Japan). All other reagents and solvents were purchased from Invitrogen (Carlsbad, CA), Sigma-Aldrich (St. Louis, MO), and Wako Pure Chemicals.

Animals.

Male Sprague–Dawley rats were purchased from Charles River Japan (Shiga, Japan) and acclimatized for >7 days before the experiments. The rats were housed under conditions of controlled temperature and humidity with a 12-hour light/dark cycle with free access to standard laboratory rodent food (CE-2; CLEA Japan, Tokyo, Japan) and water. All animal experiments were approved by the Experimental Animal Care and Use Committee of the Mitsubishi Tanabe Pharma (Saitama, Japan) and conducted in accordance with the Declaration of Helsinki and the guidelines of the ethics committee.

Isolation of Rat Hepatocytes.

Hepatocytes were isolated from male Sprague–Dawley rats (7–9 weeks old) using a procedure described previously (Baur et al., 1975). Isolated hepatocytes were suspended in albumin-free Krebs–Henseleit buffer with 12.5 mM HEPES (pH 7.4), and cell viabilities were determined using a trypan blue exclusion test. Hepatocytes obtained from three independent preparations with >80% viability were used for the uptake studies described below.

Preparation of Human Hepatocytes.

Human biologic samples were obtained ethically, and their research use was in accordance with the terms of informed consent. Cryopreserved human hepatocytes from a donor (Lot Hu8075) were purchased from Life Technologies (Carlsbad, CA). Pooled cryopreserved human hepatocytes from 20 mixed-sex donors (Lot TFF) were purchased from BioreclamationIVT (Baltimore, MD). Pooled cryopreserved human hepatocytes from 50 mixed-sex donors (Lot HUE50C) were purchased from Thermo Fisher Scientific (Waltham, MA). These hepatocytes were suspended in albumin-free Krebs–Henseleit buffer with 12.5 mM HEPES (pH 7.4), and viabilities were determined using a trypan blue exclusion test. Hepatocytes obtained from three independent preparations with >80% viability were used for the uptake studies described below.

Determination of the Intracellular Volume of Hepatocytes.

The intracellular volume of rat hepatocytes (3.68 ± 1.37 μL/106 cells) was estimated using published methods (Supplemental Table 1A) (Baur et al., 1975; Kletzien et al., 1975; Eaton and Klaassen, 1978; Kristensen and Folke, 1984; Yamazaki et al., 1992; Miyauchi et al., 1993; Reinoso et al., 2001; Hallifax and Houston, 2006). In brief, to determine the intracellular volume of human hepatocytes, cryopreserved human hepatocytes (Lot Hu8075) were suspended in Krebs–Henseleit buffer (pH 7.4) at 6.0 × 106 viable cells/mL and preincubated at 37°C for 5 minutes. A reaction was initiated by adding an equal volume of buffer containing [3H]water and [14C]dextran at final concentrations of 2.5 μCi/mL and 0.5 μCi/mL, respectively. After incubation at 37°C for 10 minutes, during which the distribution of [3H]water and [14C]dextran reached a steady state, aliquots were removed and added to a narrow tube containing silicone–mineral oil (density: 1.015; Sigma-Aldrich) over aqueous 2 M sodium hydroxide, followed by centrifugation through the silicone–mineral oil layer to separate the cells from the medium. After the basic bottom layer was neutralized with 2 M hydrochloric acid, radioactivities in both cells and medium were determined using a Tri-Carb liquid scintillation counter (PerkinElmer, Shelton, CT). Thereby, the intracellular volume of human hepatocytes was estimated to be 2.28 ± 0.33 μL/106 cells (Supplemental Table 1B).

Determination of Kp,uu,ss and fT,cell,ss in Rat and Human Hepatocytes Based on Steady-State Uptake.

To determine the incubation time for steady-state uptake into hepatocytes, transport studies were performed by an oil–spin method (Iga et al., 1979) using suspended hepatocytes. Hepatocytes were suspended in Krebs–Henseleit buffer (pH 7.4) at 2.0 × 106 viable cells/mL and preincubated at 37°C for 5 minutes. A reaction was initiated by adding an equal volume of buffer containing each drug (pitavastatin, rosuvastatin, pravastatin, or diazepam previously used as a neutral drug with high membrane permeability for evaluating the uptake into isolated rat hepatocytes) (Ichikawa et al., 1992) at 1 μM. After incubation at 37°C for 0.5, 2, 5, 15, and 30 minutes (rat hepatocytes), 0.5, 5, 15, 30, and 60 minutes (human hepatocytes, Lot Hu8075), or 0.5, 1.5, 30, and 60 minutes (human hepatocytes, Lot TFF), aliquots were removed and added to a narrow tube containing silicone–mineral oil over aqueous 2 M sodium hydroxide and centrifuged through the silicone–mineral oil layer to separate the cells from the medium. To provide low temperature values, the uptake studies were performed on ice. After the basic bottom layer was neutralized with 2 M hydrochloric acid, radioactivity in both cells and medium was measured using the liquid scintillation counter.

The unbound hepatocyte-to-medium concentration ratio (Kp,uu) based on the steady-state uptake ratio at 37°C and on ice (Kp,uu,ss) and the unbound fraction in hepatocytes based on the steady-state uptake (fT,cell,ss) were defined as described in eqs. 2 and 3, respectively, based on the C/M ratio at 37°C and on ice. A part of the method to obtain fT,cell,ss was reported previously (Yoshikado et al., 2016).

Embedded Image(2)Embedded Image(3)

The following assumptions were made in calculating Kp,uu,ss and fT,cell,ss using eqs. 2 and 3: the active uptake in hepatocytes is abolished on ice (i.e., Ccell,unbound,on ice is equal to Cmedium,on ice), and fT,cell,ss is independent of temperature (i.e., fT,cell,ss,on ice is equal to fT,cell,ss,37°C).

Determination of Kp,uu,V0, fT,cell,V0, and Other Kinetic Parameters in Rat and Human Hepatocytes Based on Initial Uptake Rate.

To evaluate the initial uptake rate in pooled human hepatocytes, hepatocytes were preincubated for 5 minutes and then incubated for 0.5–1.5 or 0.5–2 minutes, as shown in the transport studies described above.

The uptake clearance by hepatocytes (CLuptake) was determined by the slope of the plot of C/M ratio versus time, and the initial uptake rate (v) was calculated by multiplying CLuptake with the initial substrate concentration.

According to a method reported previously (Yabe et al., 2011), v can be calculated using eq. 4:

Embedded Image(4)

where Vmax is the maximum uptake rate, Km is the Michaelis constant, PSdif is the passive diffusion clearance, and S is the substrate concentration in the medium. These kinetic parameters were optimized by fitting the equation to observed data using Phoenix WinNonlin version 6.3 (Pharsight Certara, St. Louis, MO). Because CLuptake consists of active uptake clearance (PSact) and passive diffusion clearance (PSdif), assuming that PSdif for the cellular uptake is equal to that for the efflux, Kp,uu and fT,cell based on initial uptake rate (Kp,uu,V0, fT,cell,V0) can be calculated using eqs. 5 and 6 (Yabe et al., 2011):

Embedded Image(5)Embedded Image(6)

where the C/M ratio37°C in rat hepatocytes was obtained at 30 minutes, and that in human hepatocytes was obtained at 60 minutes.

Determination of the Unbound Fraction in Human Liver Homogenates (fT,homogenate) Using Equilibrium Dialysis.

Human liver samples were obtained from the Human and Animal Bridging Research Organization (Tokyo, Japan) with approval of the Ethics Committees of University of Tokyo and Human and Animal Bridging Research Organization. Three lots of liver samples were pooled and homogenized in 66.7 mM isotonic phosphate buffer at 1:3 (w/v) producing 25% homogenates. By diluting these homogenates, 12.5% and 6.25% homogenates were also prepared. Diazepam, pitavastatin, pravastatin, and rosuvastatin (final concentrations: 0.2 μM) were added to the compartment containing homogenates in a Rapid Equilibrium Dialysis plate (Thermo Fisher Scientific) and incubated for 12 hours at 37°C, or on ice.

Calculation of the Ratio of Passive Diffusion Influx Clearance of Ionized Drug to That of Nonionized Drug (λ) Based on the pH-Dependent Permeability Examined in Caco-2 Cells.

Caco-2 cells were obtained from American Type Culture Collection (Rockville, MD). Cells (passages 28–40) were cultivated, as previously described (Neuhoff et al., 2003), with some modifications. Briefly, Caco-2 cells were seeded onto a Millicell-96 Cell Culture Insert Plate (polycarbonate 0.4 μm; Merck Millipore, Billerica, MA) at 2.5 × 105 cells/well, and the culture medium was changed every second day. On day 10, transport experiments were performed; the incubation medium was Hanks’ balanced salt solution (HBSS) buffered with either 20 mM 4-morpholineethanesulfonic acid (pH = 5.5, 6.0, and 6.5), or 20 mM HBSS (pH = 7.4). Before the experiments, the apical side of the Caco-2 cell monolayer was washed four times with HBSS at the corresponding pHs (5.5, 6.0, 6.5, and 7.4). Then, both apical and basal sides of the monolayer were preincubated for 10 minutes at 37°C in the presence of rifamycin SV (100 μM), cyclosporin A (10 μM), and Ko143 (10 μM) as OATP2B1, P-glycoprotein, and breast cancer resistance protein inhibitors, respectively, at corresponding pHs (5.5, 6.0, 6.5, and 7.4 for the apical side, and 7.4 for the basal side). Subsequently, the monolayer was incubated for 10, 30, 60, and 90 minutes at 37°C in the presence of each radiolabeled statin (10 μM, apical side only) and the inhibitors (both sides). Lucifer yellow (300 μM) was used as a paracellular marker to examine the integrity of the monolayer. The radioactivity on the basal side was determined using a liquid scintillation counter.

The PSdif,inf is expressed as follows:

Embedded Image(7)

where the subscripts ion and uion represent the ionized and unionized (nonionized) form of a drug, respectively, and fo,ion and fo,uion are fractions of ionized and nonionized drug outside the cells, respectively. Subsequently, λ was defined as the ratio of passive diffusion influx clearance of the ionized drug to that of the nonionized drug, as follows:

Embedded Image(8)

It is assumed that the λ value is not changed by temperature: the effect of low temperature on PSdif,inf,ion is assumed to be the same as that on PSdif,inf,uion, although these passive diffusion clearances should be affected by the change in membrane fluidity at low temperature (Kanduser et al., 2008). A part of the method to obtain λ was reported previously (Yoshikado et al., 2016). Using λ, eq. 7 can be converted as follows:

Embedded Image(9)

The fo,ion and fo,uion for each statin were estimated using the pH of the medium (7.4) and pKa of the drug obtained from the manufacturer’s Interview Forms, based on the Henderson–Hasselbalch equation (Table 1). Then, λ and PSdif,inf,uion in Caco-2 cells were optimized by fitting eq. 9 to the pH-dependent membrane permeation of statins (Supplemental Fig. 1). Phoenix WinNonlin software version 6.3 (Pharsight Certara) was used to optimize the parameters. The obtained λ values for pitavastatin, rosuvastatin, and pravastatin are shown in Table 1.

View this table:
TABLE 1

Determination of λ based on the pH-dependent permeation data of statins observed in Caco-2 cells

Values are shown as the mean ± S.D.

Measurement of the Membrane Potential Using a Lipophilic Cation.

Tetraphenylphosphonium (TPP+) is a lipophilic cation that has been used as a probe to determine the ΔΨ. Saito et al. (1992) suggested a method to evaluate the ΔΨ with TPP+ correcting for its binding to intracellular constituents and its significant accumulation in mitochondria. By measuring the C/M ratio of TPP+, ΔΨ is calculated as described in eq. 10 (Saito et al., 1992):

Embedded Image(10)

where R, T, and F are the gas constant, absolute temperature, and Faraday constant, respectively. C/Mphys and C/Mabol are the C/M under physiologic conditions and when the ΔΨ is abolished, respectively. The selective abolition of ΔΨ can be achieved by rendering the cytoplasmic membrane permeable with 20 μM amphotericin B (Saito et al., 1992). To measure the C/M ratio of TPP+, transport studies were performed using an oil–spin method (Iga et al., 1979) with cryopreserved human hepatocytes (Lot HUE50C). Hepatocytes were suspended in Krebs–Henseleit buffer (pH 7.4) at 2.0 × 106 viable cells/mL and preincubated at 37°C for 5 minutes. The reaction was initiated by adding an equal volume of buffer containing TPP+ (final: 3 μM) and amphotericin B (final: 20 μM). After incubation at 37°C for 0.5, 15, 30, and 60 minutes, aliquots were removed and added to a narrow tube containing silicone–mineral oil over a 2.5 M ammonium acetate, and then centrifuged to separate the cells from the medium. To examine the effect of low temperature on ΔΨ, uptake studies were also performed on ice. The concentrations of TPP+ in the cells and the medium were determined using a liquid chromatography-tandem mass spectrometry (LC-MS/MS) system equipped with a Nexera UHPLC and a LCMS-8050 triple-quadrupole liquid chromatograph mass spectrometer (Shimadzu, Kyoto, Japan). A Kinetex C18 column (2.6 μm, 3 mm × 100 mm; Phenomenex, Torrance, CA) was used, and UHPLC was conducted under gradient mobile-phase conditions with a mixture of 0.1% formic acid in water and 0.1% formic acid in acetonitrile as solvents (0.3 mL/min; 0–0.5 minute 95:5 v/v, 0.5–3.5 minutes 95:5 to 15:85 v/v, 3.5–5 minutes 15:85 v/v, 5–5.5 minutes 15:85 to 5:95 v/v, 5.5–6 minutes 5:95 v/v, 6–6.1 minutes 5:95 to 95:5 v/v, and 6.1–6.5 minutes 95:5 v/v). The UHPLC eluent was introduced into the MS in positive electrospray ionization (ESI) mode. TPP+ was quantified in multiple reaction monitoring mode (339.10 > 183.10; Q1 prebias: −30.0 V, CE: −54.0, Q3 prebias: −17.0 V), in which chlorpropamide was used as an internal standard (277.20 > 111.05; Q1 prebias: −18.0 V, CE: −–32.0, Q3 prebias: −20.0 V).

Evaluation of the Correlation between Kp,uu Values Examined by the Two Different Methods.

To quantitatively evaluate the correlation between Kp,uu,ss and Kp,uu,V0, commonly employed accuracy test criteria, the average fold errors (AFE), were calculated, as shown in eq. 11.

Embedded Image(11)

Results

Determination of Kp,uu,ss and fT,cell,ss in Rat Hepatocytes.

The time-dependent uptake of diazepam (a lipophilic neutral compound) and pitavastatin, rosuvastatin, and pravastatin (substrates of hepatic OATPs) in rat hepatocytes was monitored. The uptake of diazepam in rat hepatocytes reached a steady state instantaneously because of its high permeability, whereas the uptake of pitavastatin, rosuvastatin, and pravastatin gradually increased over time and reached the steady state within 30 minutes (Fig. 1). On ice, C/M ratios of pitavastatin, rosuvastatin, and pravastatin were significantly smaller than the ratios at 37°C, whereas that for diazepam was not different from that at 37°C (Table 2). The Kp,uu,ss for diazepam was 0.85, and the values for pitavastatin, rosuvastatin, and pravastatin were approximately 11, 13, and 6.7, respectively (Table 2), confirming that these statins were concentrated in the hepatocytes by an active transport system. The obtained fT,cell,ss values for pitavastatin, rosuvastatin, and pravastatin were approximately 0.038, 0.15, and 0.60, respectively.

Fig. 1.
Fig. 1.

Uptake of diazepam (1 μM; A), pitavastatin (1 μM; B), rosuvastatin (1 μM; C), and pravastatin (1 μM; D) by rat hepatocytes measured after incubation at 37°C for 0.5–30 minutes. The data are presented as mean + S.D. (n = 3).

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TABLE 2

Kp,uu,ss and fT,cell,ss in rat hepatocytes

Values are shown as the mean ± S.D. (n = 3).

Determination of Kp,uu,V0 and fT,cell,V0 in Rat Hepatocytes.

Subsequently, based on a method reported previously (Yabe et al., 2011), the kinetic parameters (Vmax, Km, and PSdif) for pitavastatin, rosuvastatin, and pravastatin were determined by fitting eq. 4 to their initial uptake rate. The results are shown in Eadie–Hofstee plots (Supplemental Fig. 2). The obtained Kp,uu,V0 values for pitavastatin, rosuvastatin, and pravastatin were approximately 56, 51, and 8.6, respectively (Table 3). The calculated fT,cell,V0 for pitavastatin, rosuvastatin, and pravastatin were approximately 0.17, 0.65, and 0.69, respectively.

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TABLE 3

Kinetic parameters for the initial uptake rates of pitavastatin, rosuvastatin, and pravastatin in rat hepatocytes

Values are shown as the mean ± S.D.

Determination of Kp,uu,ss and fT,cell,ss in Human Hepatocytes.

We investigated the time-dependent uptake of diazepam, pitavastatin, rosuvastatin, and pravastatin by human hepatocytes prepared from single donor (Lot Hu8075) (Fig. 2, A–D) and pooled human hepatocytes from 20 mixed-sex donors (Lot TFF) (Fig. 2, E–H). The uptake of all these statins increased over time and reached a steady state within 60 minutes, whereas that of diazepam reached a peak instantaneously. On ice, the C/M ratios for pitavastatin, rosuvastatin, and pravastatin were reduced, whereas that for diazepam was not dependent on temperature (Table 4). Although the Kp,uu,ss for diazepam was approximately 1 (1.2 and 0.41), the values were 13 and 6.9 for pitavastatin, 12 and 6.4 for rosuvastatin, and 2.0 and 1.3 for pravastatin in cells from Lot Hu8075 and Lot TFF, respectively (Table 4). The obtained fT,cell,ss values were approximately 0.028 and 0.046 for pitavastatin, 0.22 and 0.23 for rosuvastatin, and 0.55 and 0.48 for pravastatin in cells from Lot Hu8075 and Lot TFF, respectively.

Fig. 2.
Fig. 2.

Uptake of diazepam (1 μM; A and E), pitavastatin (1 μM; B and F), rosuvastatin (1 μM; C and G), and pravastatin (1 μM; D and H) by human hepatocytes measured after incubation at 37°C for 0.5–60 minutes. (A–D) Cryopreserved human hepatocytes from a single donor (Lot Hu8075) were used. (E–H) Pooled cryopreserved human hepatocytes from 20 mixed-sex donors (Lot TFF) were used. The data are presented as mean + S.D. (n = 3).

View this table:
TABLE 4

Kp,uu,ss and fT,cell,ss in human hepatocytes

Values are shown as the mean ± S.D. (n = 3).

Determination of Kp,uu,V0 and fT,cell,V0 in Human Hepatocytes.

Based on a method reported previously (Yabe et al., 2011), the kinetic parameters (Vmax, Km, and PSdif) for pitavastatin and rosuvastatin were determined by fitting eq. 4 to their initial uptake rate by cells from Lot Hu8075 (Supplemental Fig. 3, A and B) and Lot TFF (Supplemental Fig. 3, D and E); the kinetic parameters of pravastatin could be determined in cells from Lot Hu8075 (Supplemental Fig. 3C), but not from Lot TFF, because a saturation of the uptake of pravastatin was not observed clearly within the concentration range of 0.5–300 μM.

Calculated Kp,uu,V0 values were approximately 220 and 20 for pitavastatin; 200 and 3.5 for rosuvastatin in cells from Lot Hu8075 and Lot TFF, respectively; and 55 for pravastatin in cells from Lot Hu8075 (Table 5), which were higher in cells from Lot Hu8075 than from Lot TFF. Calculated fT,cell,V0 values were approximately 0.47 and 0.13 for pitavastatin; 3.8 and 0.13 for rosuvastatin in cells from Lot Hu8075 and Lot TFF, respectively; and 15 for pravastatin in cells from Lot Hu8075, although fT,cell should theoretically be less than 1. Thus, the values of Kp,uu, V0 and fT,cell,V0 exhibited greater differences between lots.

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TABLE 5

Kinetic parameters for the initial uptake of pitavastatin, rosuvastatin, and pravastatin in human hepatocytes

Values are shown as the mean ± S.D.

Comparison of Kp,uu and fT,cell Obtained by Different Methods.

In rat hepatocytes, Kp,uu,V0 values for pitavastatin, rosuvastatin, and pravastatin were, respectively, 5.1, 5.1, and 1.2 times those for Kp,uu,ss (Fig. 3A). By contrast, in human hepatocytes (Lot Hu8075), Kp,uu,V0 values for pitavastatin, rosuvastatin, and pravastatin were, respectively, 16, 17, and 43 times those for Kp,uu,ss (Fig. 3B). The differences between Kp,uu,V0 and Kp,uu,ss in human hepatocytes from Lot TFF were smaller than those in Lot Hu8075: Kp,uu,V0 values for pitavastatin and rosuvastatin were, respectively, 2.9 and 0.55 times those for Kp,uu,ss in cells from Lot TFF (Fig. 3B).

Fig. 3.
Fig. 3.

Comparison of Kp,uu and fT,cell for three statins in rat and human hepatocytes obtained by two different methods. (A and C) Kp,uu,V0 and fT,cell,V0 in rat hepatocytes derived from a previous report (Yabe et al., 2011) are plotted against Kp,uu,ss and fT,cell,ss obtained in the present study as closed symbols. Kp,uu,V0 and fT,cell,V0 obtained in the present study are plotted against Kp,uu,ss and fT,cell,ss as open symbols. (B and D) Kp,uu,V0 and fT,cell,V0 in human hepatocytes from a single donor (Lot Hu8075) are plotted against Kp,uu,ss and fT,cell,ss obtained in the present study as gray symbols. Kp,uu,V0 and fT,cell,V0 in pooled human hepatocytes from 20 gender donors (Lot TFF) are plotted against Kp,uu,ss and fT,cell,ss as open symbols. 1, pitavastatin; 2, rosuvastatin; 3, pravastatin. Solid and dashed lines denote unity and threefold boundaries, respectively. The data are presented as mean + S.D. for the x- and y-axis (n = 3).

The fT,cell,V0 values tended to be higher than fT,cell,ss in both rat (Fig. 3C) and human hepatocytes (Fig. 3D), except for rosuvastatin in human hepatocytes (Lot TFF). Both Kp,uu,V0 and fT,cell,V0 values thus exhibited quite larger interlot and interexperimental variabilities compared with Kp,uu,ss and fT,cell,ss values (comparison between Tables 2 and 3 for rats and Tables 4 and 5 for humans).

Comparison of fT,cell with the Measured Unbound Fraction in Human Liver Homogenates.

For diazepam, pitavastatin, rosuvastatin, and pravastatin, the measured unbound fractions in human liver homogenates (fT,homogenate) were measured using equilibrium dialysis (Table 6). The fT,homogenate values for these drugs obtained at 37°C (fT,homogenate,37°C) were close to those obtained on ice (fT,homogenate,on ice). Moreover, the fT,homogenate,on ice values for pitavastatin and rosuvastatin were comparable to those for fT,cell,ss (Table 4); the difference between fT,homogenate,on ice and fT,cell,ss for pravastatin was within threefold, whereas there was a larger discrepancy between fT,homogenate,on ice and fT,cell,ss for diazepam.

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TABLE 6

fT,homogenate in human liver homogenate

Values are shown as the mean ± S.D. (n = 3).

Measurement of the ΔΨ Using TPP+.

To evaluate whether ΔΨ might affect the ratio of PSdif,inf/PSdif,eff and, accordingly, Kp,uu, the time-dependent uptake of TPP+ was examined in human hepatocytes under physiologic conditions and with amphotericin B, which is reported to abolish ΔΨ specifically by 10-minute incubation with isolated rat hepatocytes (Saito et al., 1992). The C/M ratio of TPP+ gradually increased over time and reached the steady state between 30 and 60 minutes (Fig. 4). On ice, the C/M ratio of TPP+ was significantly smaller than that at 37°C. In addition, in the presence of amphotericin B, the C/M ratio of TPP+ was decreased significantly at 37°C compared with the condition without amphotericin B, whereas the C/M ratio on ice was not significantly changed with amphotericin B (Fig. 4). The C/M ratio (37°C) in the presence of amphotericin B at 60 minutes was lower than that at 30 minutes, suggesting some additional effects of amphotericin B (i.e., cytotoxicity) other than the loss of the ΔΨ. Therefore, based on the obtained data at 30 minutes and eq. 10, ΔΨ values were calculated to be approximately −30 mV at 37°C and 5 mV on ice (Table 7).

Fig. 4.
Fig. 4.

Uptake of TPP+ by human hepatocytes measured after incubation at 37°C (circles) or on ice (triangles) for 0.5–60 minutes. Hepatocytes were incubated with 3 μM TPP+ (closed symbols) or both 3 μM TPP+ and 20 μM amphotericin B (open symbols). Pooled cryopreserved human hepatocytes from 50 mixed-sex donors (Lot HUE50C) were used. The data are presented as mean + S.D. (n = 3).

View this table:
TABLE 7

Theoretical calculation of the ΔΨ using the measured C/M ratios of TPP+

Values are shown as the mean ± S.D. (n = 4).

Using Φ [= exp(z·F·ΔΨ/R/T)] calculated from ΔΨ (Supplemental Equation 5) and physicochemical parameters calculated for statins (Table 1), theoretical values of Kp,uu,true, Kp,uu,V0, and Kp,uu,ss were calculated by Supplemental Equations 6, 8, and 11, respectively (Table 8).

View this table:
TABLE 8

Theoretical calculation of Kp,uu,true, Kp,uu,V0, and Kp,uu,ss using the calculated ΔΨ

Furthermore, we aimed to understand better the quantitative relationship between Kp,uu,true and experimentally obtained Kp,uu,ss and Kp,uu,V0 for statins based on theoretical equations that consider the inside-negative ΔΨ. Using ΔΨ (Table 7) and several physicochemical parameters of statins (Table 1), the ratios of Kp,uu,V0/Kp,uu,true and Kp,uu,ss/Kp,uu,true (RV0/true or Rss/true) can be described as follows:

Embedded Image(12)Embedded Image(13)

where fi,ion and fi,uion are fractions of the ionized and nonionized forms of the drug in hepatocytes, respectively, with the use of intracellular pH (7.2) and pKa. RV0/true was calculated to be 2.2–3.1 for statins, whereas Rss/true was 0.85–1.2 (Table 8). Figure 5 also shows that simulated Rss/true was closer to 1 than RV0/true when λ values for pitavastatin, rosuvastatin, and pravastatin were approximately 0.028, 0.011, and 0.19, respectively (Table 1). Furthermore, experimentally obtained Kp,uu,V0 and Kp,uu,ss for statins were corrected using the calculated RV0/true and Rss/true values (Table 8) and eqs. 14 and 15:

Fig. 5.
Fig. 5.

Theoretical simulation of Kp,uu,ss/Kp,uu,true (= Rss/true: blue) and Kp,uu,V0/Kp,uu,true (= RV0/true: red) using eqs. 12 and 13. (A) The solid line represents the simulation result for pitavastatin. (B) The broken line represents simulation results for rosuvastatin and pravastatin. Arrows represent the λ for statins estimated from our experiments with Caco-2 cells.

Embedded Image(14)Embedded Image(15)

Kp,uu,V0,corrected in rat hepatocytes approached Kp,uu,ss,corrected (Fig. 6A) compared with uncorrected Kp,uu,V0 versus Kp,uu,ss (Fig. 3A): the calculated AFEs are 7.6 in Fig. 3A and 4.4 in Fig. 6A. Similarly, Kp,uu,V0,corrected in human hepatocytes approached corrected Kp,uu,ss,corrected (Figs. 3B and 6B): the calculated AFEs are 132 in Fig. 3B and 41 in Fig. 6B. However, Kp,uu,V0,corrected in Lot Hu8075, a single donor lot selected because of its relatively high uptake ability, remained larger than Kp,uu,ss,corrected.

Fig. 6.
Fig. 6.

Comparison of Kp,uu,ss,corrected and Kp,uu,V0,corrected corrected by eqs. 14 and 15 using Rss/true and RV0/true (Table 8). (A) Closed symbols: original Kp,uu,V0 in rat hepatocytes were obtained from a previous report (Yabe et al., 2011). Open symbols: original Kp,uu,V0 values were obtained in the present study. (B) Gray symbols: original Kp,uu,V0 values were obtained in human hepatocytes from a single donor (Lot Hu8075). Open symbols: original Kp,uu,V0 values were obtained in pooled human hepatocytes from 20 mixed-sex donors (Lot. TFF). 1, pitavastatin; 2, rosuvastatin; 3, pravastatin. Solid and dashed lines denote unity and threefold boundaries, respectively. The data are presented as mean + S.D. for the x- and y-axis (n = 3).

Discussion

We previously proposed a method to estimate Kp,uu,ss for anions in hepatocytes based on their steady-state C/M ratio at 37°C and the C/M ratio after suppressing active transport (Yamazaki et al., 1992; Shitara et al., 2013). Another method reported by Yabe et al. (2011) is to estimate Kp,uu,V0 based on initial uptake rates at various concentrations. Kp,uu,ss values for pitavastatin, rosuvastatin, and pravastatin in rat and human hepatocytes, which were obtained with less interexperimental variation compared with Kp,uu,V0 values in this study, tended to be smaller than the corresponding Kp,uu,V0 values (Fig. 3, A and B). Considering the measured inside-negative ΔΨ (Table 7) and subsequent asymmetric diffusional clearance for influx and efflux, Kp,uu,V0,corrected in rat and human hepatocytes approached Kp,uu,ss,corrected (Fig. 6).

There exists a practical advantage of the method to obtain the Kp,uu,ss value over the method to obtain the Kp,uu,V0 value. Interlot and interexperimental variabilities are much less for Kp,uu,ss than for Kp,uu,V0 (Tables 2–5), because uptake data at longer time (e.g., 30–60 minutes, almost at steady state) are enough to obtain Kp,uu,ss, whereas initial uptake rates (e.g., 0.5–1.5 or 0.5–2.0 minutes) are needed for Kp,uu,V0, and such a method requires quite accurate techniques for rapid samplings.

In the extended clearance concept, Kp,uu,true is described as in eq. 16 (Shitara et al., 2013):

Embedded Image(16)

where PSact,inf, PSact,eff, CLint,met, and CLint,bile represent the active influx clearance, the active efflux clearance, the intrinsic clearance for metabolism, and the intrinsic clearance for biliary excretion in an unchanged form, respectively. Based on eq. 16, a number of explanations can be considered for why Kp,uu,V0 values for pitavastatin, rosuvastatin, and pravastatin tended to be larger than Kp,uu,ss in both rat and human hepatocytes (Fig. 3, A and B).

  • (i) As PSact,inf is estimated by initial uptake rate for the calculation for Kp,uu,v0, it may include not only active transport, but also facilitated diffusion, which is also transporter mediated, but not by active (concentrated) transport. Thus, in this case, Kp,uu,V0 can be larger than Kp,uu,ss. However, in the case of statins, hepatic OATPs are thought to be major uptake transporters, and the impact of facilitated diffusion on their overall uptake may be negligible.

  • (ii) The existence of non-negligible metabolism during the measurement of steady-state uptake (30–60 minutes) of a drug may lead to smaller Kp,uu,ss than Kp,uu,V0 values, because metabolism might be negligible during short incubation times (∼2 minutes) for the measurement of initial uptake rates. Considering that pitavastatin, rosuvastatin, and pravastatin are generally classified as statins metabolized poorly in humans (Shitara and Sugiyama, 2006), it is unlikely that metabolism accounts for the overestimation of Kp,uu,V0. However, a recent report suggested that pitavastatin undergoes lactonization by uridine 5′-diphospho-glucuronosyltransferases more extensively than other statins (Schirris et al., 2015), which may explain, at least in part, the overestimation of Kp,uu,V0 compared with Kp,uu,ss (Fig. 3, A and B). In addition, pentenoic acid derivative was reported to be the major metabolite of rosuvastatin in rats (Nezasa et al., 2002; He et al., 2014). In our experiments using rat hepatocytes, the remaining amount of rosuvastatin after the 30-minute incubation was 87% of the initial amount, whereas little loss of rosuvastatin was observed after the 60-minute incubation in human hepatocytes. Therefore, the contribution of metabolism to the elimination of rosuvastatin in rats might be larger than that in humans, and the metabolism of rosuvastatin in rat hepatocytes might only slightly influence the estimation of its Kp,uu,ss.

  • (iii) PSact,eff and CLint,bile cannot be fully estimated by the short-term uptake of drugs. Thus, if a drug is a substrate of basolateral/apical efflux transporters, Kp,uu,V0 may be overestimated. The expression of apical efflux transporters [P-glycoprotein, multidrug resistance-associated protein 2 (MRP2), and breast cancer resistance protein] and basolateral efflux transporters (MRP3 and MRP4) was partly retained in cryopreserved human hepatocytes compared with fresh liver biopsies (Lundquist et al., 2014), which might contribute, at least in part, to the drug efflux from hepatocytes and lead to a discrepancy between Kp,uu,V0 and Kp,uu,ss.

  • (iv) In most work, PSdif,inf and PSdif,eff are conventionally assumed to be equal for kinetic consideration of the cellular transport (Yabe et al., 2011), although PSdif,eff might be larger than PSdif,inf in the case of anions because of the inside-negative ΔΨ in normal cells. We calculated Kp,uu,ss by taking asymmetric PSdif into consideration with the assumption that ΔΨ is almost abolished on ice, as we confirmed experimentally (Table 7). Conversely, to calculate Kp,uu,V0, PSdif,eff is set to be the same as PSdif,inf, as shown in Supplemental Equation 8. Therefore, theoretically, Kp,uu,V0 should be larger than Kp,uu,ss in the case of anions, which might mostly explain the observed discrepancy between Kp,uu,ss and Kp,uu,V0.

In this study, two assumptions were made. First, fT,cell,ss is not affected by temperature; second, ΔΨ is abolished on ice. To investigate the first assumption, we sought to evaluate whether fT,cell,ss was largely dependent on temperature, as mentioned in previous reports (Sugano et al., 2010; Shitara et al., 2013). We found that values for fT,homogenate,on ice were comparable with those for fT,homogenate,37°C (Table 6), which supports the first assumption, at least for the statins tested. In addition, fT,cell,ss for rosuvastatin and pitavastatin measured by equilibrium dialysis were similar to fT,homogenate, whereas fT,cell,ss for pravastatin was larger than fT,homogenate (Tables 4 and 6), although the mechanism behind this has not yet been clarified. By contrast, fT,cell,V0 for rosuvastatin and pravastatin in human hepatocytes exceeded 1 with large S.D. (Table 5), suggesting that it might be difficult to obtain reliable values for fT,cell based on the rate of initial uptake. Indeed, Yabe et al. (2011) estimated fT,cell,V0 for some drugs, including pravastatin, not by eq. 6, but by using in silico prediction with logD to obtain reasonable values for fT,cell,V0.

The second assumption was investigated using TPP+ uptake studies with and without amphotericin B in human hepatocytes (Fig. 4), according to a method reported previously (Saito et al., 1992). Calculated ΔΨ at 37°C was similar to reported values (−35 to −39 mV) obtained in isolated rodent hepatocytes (Bradford et al., 1985; Edmondson et al., 1985; Fitz and Scharschmidt, 1987; Wondergem and Castillo, 1988; Weinman et al., 1989), and ΔΨ was almost abolished on ice under steady-state conditions at 30 minutes (Table 7). Collectively, our assumptions are valid for anions.

For the estimation of Kp,uu,ss, ATP depletors such as rotenone were also used to stop the active uptake (Yamazaki et al., 1992); however, it is difficult to optimize experimental conditions for the exposure to ATP depletors to abolish the active uptake without affecting cell viability, and the effect of ATP depletion is often required to be insufficient to maintain viability (Yamazaki et al., 1993). Thus, this approach may lead to underestimation of Kp,uu,ss. The use of transporter inhibitors is another strategy to stop the active transport. However, before this, we should understand the major transport mechanism of the drugs to be tested, and confirm that the contribution of other transporters to the overall active uptake is negligible. In the case of statins, because Na+-taurocholate cotransporting polypeptide is known to partly play a role in their hepatic uptake (Bi et al., 2013), the inhibition of Na+-taurocholate cotransporting polypeptide in addition to OATPs is required; otherwise, insufficient inhibition of the hepatic uptake causes an underestimation of Kp,uu,ss.

In conclusion, Kp,uu of statins were estimated in rat and human hepatocytes using two different methods based on steady-state uptake (Kp,uu,ss) and initial uptake rate (Kp,uu,v0). Considering the inside-negative ΔΨ and subsequent asymmetric diffusional clearance for influx and efflux, in theory, Kp,uu,ss should be similar to true Kp,uu (Kp,uu,true), whereas Kp,uu,V0 should be higher compared with Kp,uu,true. This can explain, at least in part, the discrepancies observed between Kp,uu,ss and Kp,uu,V0 for statins in the present study. Using the estimated Kp,uu value and the ratio of diffusional uptake to active uptake determined in vitro, we can mathematically describe the relationship among permeation clearances across the sinusoidal membrane of hepatocytes, which can be applied for the construction of a physiologically-based pharmacokinetic model.

Although our current approach should be valid from the viewpoint of kinetic theory, we will need to practically validate our method for the in vitro–in vivo extrapolation of Kp,uu. One of the approaches is to compare the observed liver-to-blood total concentration ratio of positron emission tomography probes (e.g., [11C]dehydropravastatin) (Ijuin et al., 2012) in humans with that predicted from experimentally estimated hepatic Kp,uu and their unbound fractions in blood and hepatocytes. Another approach is to confirm whether the discrepancy of in vitro Ki values with regard to the medium concentration of inhibitors obtained with hepatocytes and recombinant enzymes can be well explained by in vitro Kp,uu when target proteins are located inside the hepatocytes (e.g., inhibition of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase by statins, inhibition of metabolic enzymes and efflux transporters by inhibitor drugs). Such validations should strengthen our theoretical considerations for in vitro Kp,uu estimation.

Authorship Contributions

Participated in research design: Yoshikado, Toshimoto, Nakada, Maeda, Sugiyama.

Conducted experiments: Yoshikado, Nakada, Ikejiri.

Performed data analysis: Yoshikado, Toshimoto, Nakada.

Wrote or contributed to the writing of the manuscript: Yoshikado, Toshimoto, Nakada, Kusuhara, Maeda, Sugiyama.

Footnotes

  • This work was supported by Grant-in-Aid for Scientific Research (S) and the Scientific Research on Innovative Areas HD-Physiology from the Ministry of Education, Culture, Sports, Sciences, and Technology in Japan [Grants 24229002 and 23136101].

  • T.Y., K.T., and T.N. contributed to this research equally.

  • https://doi.org/10.1124/dmd.116.074823.

  • ↵Embedded ImageThis article has supplemental material available at dmd.aspetjournals.org.

Abbreviations

ΔΨ
membrane potential
AFE
average fold error
CLuptake
uptake clearance by hepatocytes
C/M ratio
cell-to-medium total concentration ratio
fT,cell,ss
unbound fraction in hepatocytes at steady state
fT,cell,V0
unbound fraction in hepatocytes based on initial uptake rate
fT,homogenate
unbound fraction in liver homogenates
HBSS
Hanks’ balanced salt solution
Ki
inhibition constant
Km
Michaelis–Menten constant
Kp,uu
unbound hepatocyte-to-medium concentration ratio
Kp,uu,ss
unbound hepatocyte-to-medium concentration ratio based on steady-state uptake
Kp,uu,true
theoretically true unbound hepatocyte-to-medium concentration ratio
Kp,uu,V0
unbound hepatocyte-to-medium concentration ratio based on initial uptake rate
MRP
multidrug resistance-associated protein
OATP
organic anion-transporting polypeptide
PSdif
influx intrinsic clearance by passive diffusion through sinusoidal membrane
TPP+
tetraphenylphosphonium
Vmax
maximum transport rate

This is an open access article distributed under the CC BY-NC Attribution 4.0 International license.

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Unbound Hepatocyte-to-Medium Concentration Ratio

Takashi Yoshikado, Kota Toshimoto, Tomohisa Nakada, Kazuaki Ikejiri, Hiroyuki Kusuhara, Kazuya Maeda and Yuichi Sugiyama
Drug Metabolism and Disposition July 1, 2017, 45 (7) 779-789; DOI: https://doi.org/10.1124/dmd.116.074823
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