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Department of Pharmaceutics, Hoshi University, Tokyo, Japan (T.Y., M.M., K.T.); Capsugel, a Division of Pfizer, Colmar, France (H.B.); and the Graduate School of Pharmaceutical Sciences, the University of Tokyo, Tokyo, Japan (H.K., Y.S.)
(Received December 7, 2006; Accepted April 16, 2007)
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Abstract |
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; Meerum Terwogt et al., 1999). During the past two decades, several inhibitors of efflux transporters have been developed to enhance the bioavailability of their substrates (Hyafil et al., 1993; Dantzig et al., 1996; Wacher et al., 2001; Stewart et al., 2004). Although effective in animal experiments, their clinical applicability has been limited (Breedveld et al., 2006) and, consequently, more effective and safer inhibitors are needed.
Excipients, such as surfactants, are extensively used in "passive" pharmaceutical formulations to improve dissolution of poorly soluble drugs. Previously, it has been found that several excipients can inhibit P-gp. Nerurkar et al. (1996) reported that Cremophor EL and Tween 80 enhanced the absorptive transport of a model peptide by inhibiting the secretory directed transport of this peptide in Caco-2 cells. Yu et al. (1999) also reported that vitamin E-TPGS (D-
-tocopheryl polyethylene glycol succinate) inhibited the efflux system and enhanced the permeability of amprenavir in Caco-2 cells. Lo (2003) reported that Tween 20 markedly enhanced the intracellular accumulation of epirubicin in Caco-2 cells and enhanced mucosal-to-serosal absorption of epirubicin in the rat jejunum and ileum. Previous reports suggest that coadministration of excipients enhanced oral absorption of P-gp substrates not only because of improved solubilization of drugs, but also inhibition of P-gp-mediated efflux (Yu et al., 1999; Martin-Facklam et al., 2002; Varma and Panchagnula, 2005). These excipients are commonly used in human pharmaceutical formulations and are considered safe and relatively nontoxic. Therefore, the use of excipients as the inhibitors of efflux transporters may lead to important pharmacotherapeutic benefits.
Breast cancer resistance protein (BCRP/ABCG2) belongs to the ATP-binding cassette (ABC) family of drug efflux transporters (Doyle et al., 1998; Allen and Schinkel, 2002). BCRP is expressed in various normal tissues, such as liver, kidney, brain, placenta, intestine, and colon (Allen and Schinkel, 2002; van Herwaarden and Schinkel, 2006). BCRP shows broad substrate specificity including anticancer drugs, food carcinogens, antibiotics, HMG-CoA reductase inhibitors, and conjugated metabolites, and can act as an active secretion system by transporting substrates from the cells (Jonker et al., 2000; Litman et al., 2000; Suzuki et al., 2003; van Herwaarden et al., 2003; Hirano et al., 2005; Merino et al., 2005). In the gastrointestinal tract, BCRP is localized in the apical membrane of the intestinal epithelia and shows quite high levels of mRNA compared with other ABC transporters, including P-gp in the human intestine (Jonker et al., 2000; Maliepaard et al., 2001; Taipalensuu et al., 2001). It has been reported that BCRP has a marked effect on the oral bioavailability of its substrates. The oral availability of topotecan, 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine, ME3277, and nitrofurantoin, typical substrates of BCRP, was dramatically reduced by BCRP (Jonker et al., 2002; van Herwaarden et al., 2003; Kondo et al., 2005; Merino et al., 2005). BCRP also has been shown to extrude glucuronide and sulfate conjugates formed in enterocytes into the intestinal lumen (Adachi et al., 2005). Furthermore, concomitant use of GF120918, a BCRP and also a P-gp inhibitor, and topotecan effectively increased the oral bioavailability of topotecan in a clinical study (Kruijtzer et al., 2002). This cumulative evidence suggests that inhibition of BCRP will improve the oral absorption of drugs.
The purpose of this study was to examine whether excipients could inhibit BCRP function and improve the oral absorption of its substrates. We used Pluronic P85 and Tween 20 as test excipients. Pluronic P85 consists of hydrophilic ethylene oxide and hydrophobic propylene oxide blocks, and Tween 20 contains a sorbitan segment between the polyoxyethylene and fatty acid groups. Both excipients are acceptable for pharmaceutical use as solubilizing agents, suspending agents, and emulsifying agents and inhibit P-gp function (Batrakova et al., 2001a; Lo, 2003). In this study, we examined their effect on the oral absorption of topotecan as a typical BCRP substrate drug, because its oral absorption is limited by BCRP (Jonker et al., 2000, 2002).
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Materials and Methods |
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Animals. Female Bcrp (–/–) mice and age-matched wild-type control mice were produced as reported previously (Jonker et al., 2002). All mice (8–20 weeks) were housed in rooms maintained at 23°C and 55 ± 5% relative humidity, and allowed free access to food and water during the acclimatization period. The animal work was performed at Hoshi University and complied with the regulations of the Committee on Ethics in the Care and Use of Laboratory Animals.
In Vivo Oral and Intravenous Administration of Topotecan. Oral and intravenous administration of topotecan was performed as described previously (Jonker et al., 2000). Pluronic P85 and Tween 20 were dissolved in phosphate-buffered saline (137 mM NaCl, 2.6 mM KCl, 6.4 mM Na2HPO4, and 1.4 mM KH2PO4, pH 7.4). Animals, lightly anesthetized with ether, were given Pluronic P85 (100, 250, or 500 mg/kg; 10 µl of drug solution/g body weight), Tween 20 (50, 100, or 250 mg/kg; 10 µl of drug solution/g body weight), or a corresponding amount of buffer by a Sonde needle into the stomach. At 15 min after oral administration of Pluronic P85 and Tween 20, mice received topotecan orally or intravenously at a dose of 1.0 mg/kg body weight (5 µl of drug solution/g body weight). For intravenous administration, topotecan was injected into the tail vein of mice lightly anesthetized with ether. Blood was collected from the tail vein at predetermined time intervals. Plasma was separated by centrifugation at 13,000g for 1 min and kept under refrigeration until analysis. Heparinized plasma was mixed with 7 volumes of methanol. The total topotecan levels (lactone plus carboxylate form) in plasma were determined by high-pressure liquid chromatography with fluorescence detection (RF-10AXL; Shimadzu Science East Co., Tokyo, Japan) as described previously (Rosing et al., 1999). Topotecan was detected using a fluorescence detector at an excitation wavelength of 380 nm and an emission wavelength of 520 nm. The area under the plasma concentration-time curve (AUC) from 0 to 4 h was calculated by the linear trapezoidal rule.
Preparation of Everted Sacs. Everted sacs were prepared by a modification of the procedure described previously (Sakamoto et al., 2006). Mice were anesthetized with ether and sacrificed by exsanguination from the abdominal aorta. Then, the jejunum and ileum were immediately removed and rinsed in ice-cold Krebs-Ringer-Henseleit bicarbonate buffer (118 mM NaCl, 4.75 mM KCl, 2.50 mM CaCl2, 1.19 mM KH2PO4, 1.19 mM MgSO4, and 25 mM NaHCO3, pH 6.5). Five-cntimeter segments of intestine were isolated to perform the intestinal absorption and intestinal secretion studies. The intestinal segments were everted using a stainless steel rod. For the absorptive transport (mucosal-to-serosal) study, polyethylene tubes were inserted into both ends of the everted segments and tied. For the secretory transport (serosal-to-mucosal) study, one end of the everted segment was ligated with silk thread, and a polyethylene tube was inserted into the other end and tied.
In Vitro Transport Study in Everted Sacs. For absorptive transport (mucosal-to-serosal) studies of topotecan, 5-cm segments of the everted ileum were placed in 50 ml of Krebs-Ringer-Henseleit bicarbonate buffer gassed with O2/CO2 (95:5) at 37°C. The everted ileum was initially filled with 1.5 ml of Krebs-Ringer-Henseleit bicarbonate buffer and perfused with the buffer at 0.1 ml/min using an infusion pump (syringe infusion pump; KD Scientific Inc., Holliston, MA) throughout the transport study. After preincubation in the buffer for 15 min, topotecan was added to the mucosal side to give a final concentration of 10 µM. Then, the outflow perfusate was collected for 5 min at 15-min intervals up to 90 min. The length of the ileal segment was measured at the end of the experiments. The concentration of total topotecan in the outflow perfusate was measured by high-pressure liquid chromatography. The absorption rate of topotecan in the everted sac was determined according to the following equation: Absorption rate = Cout x Q/L, where Cout represents the topotecan concentration in the outflow solutions, Q is the perfusion rate (0.1 ml/min), and L is the length of the intestinal segments.
The absorption rates of topotecan in the absence and presence of Pluronic P85 or Tween 20 were also determined by using the above-mentioned absorption transport method. In this experiment, the everted ileum was preincubated in 50 ml of Krebs-Ringer-Henseleit bicarbonate buffer containing 20 µM Pluronic P85 or 250 µM Tween 20 for 2 h at 37°C, and then topotecan was added to the mucosal side to give a final concentration of 10 µM. During the experiment, the everted sac was perfused with Krebs-Ringer-Henseleit bicarbonate buffer at 0.1 ml/min. The absorption rates of topotecan in the absence and presence of the excipients were determined from the concentration of total topotecan in the outflow perfusate at 60 min.
For the secretory transport (serosal-to-mucosal) studies of rhodamine 123, 5-cm segments of everted jejunum and ileum were placed in 10 ml of Krebs-Ringer-Henseleit bicarbonate buffer gassed with O2/CO2 (95:5) at 37°C. After preincubation in Krebs-Ringer-Henseleit bicarbonate buffer for 15 min, rhodamine 123 (1 µM; 70 µl/cm tissue) was added to the serosal side. Aliquots (100 µl) of mucosal medium were collected at designated times and then replaced by the same volume of Krebs-Ringer-Henseleit bicarbonate buffer. The length of the segment was measured at the end of the experiments. The concentration of rhodamine 123 in mucosal medium was determined by fluorometry (Mithras LB940; Bertold Japan Co. Ltd., Tokyo, Japan). The secretion rate of rhodamine 123 in the everted sac was determined according to the following equation: Secretion rate = dCm/dt/L, where Cm represents the concentration of rhodamine 123 in mucosal medium, and L is the length of the intestinal segments.
Western Blot Analysis. For Western blot analysis, crude membrane was prepared from mouse intestine as described previously (Ogihara et al., 1996). The crude membrane was suspended in phosphate-buffered saline, then frozen in liquid N2 and stored at –80°C until analysis. The protein concentrations in the crude membrane vesicles prepared from mouse intestine were determined by the method of Lowry (Lowry et al., 1951) with bovine serum albumin as a standard. The membrane fraction was dissolved in 3x SDS sample buffer (New England Biolabs, Ipswich, MA) with ß-mercaptoethanol and loaded onto a 7% (P-gp or villin) or 8.5% (Bcrp) SDS-polyacrylamide electrophoresis gel with a 3.75% stacking gel. The molecular weight was determined using a prestained protein marker (New England Biolabs). Proteins were transferred electrophoretically to a polyvinylidene difluoride membrane (Pall Corporation, East Hills, NY) using a blotter (Trans-blot; Bio-Rad, Hercules, CA) at 15 V for 1 h. The membrane was blocked with Tris-buffered saline with 0.05% Tween 20 (TTBS) and 5% skimmed milk overnight at 4°C. After washing with TTBS, the membrane was incubated at room temperature for 1 h in TTBS with 100-fold diluted monoclonal antibody against P-gp (C219; Signet Laboratories, Inc., Dedham, MA), 500-fold diluted monoclonal antibody against Bcrp (BXP53; Signet Laboratories, Inc.), or 100-fold diluted monoclonal antibody against villin (C19; Santa Cruz Biotechnology, Inc., Santa Cruz, CA). For the detection of each protein, the membrane was placed in contact with 500-fold diluted anti-mouse IgG (P-gp), 500-fold diluted anti-rat IgG (Bcrp), or 500-fold diluted anti-goat IgG (villin) conjugated with the horseradish peroxidase (Amersham Biosciences Inc., Piscataway, NJ) for 1 h in TTBS. The band was detected using an ECL Plus Western blotting starter kit (Amersham Biosciences Inc.), and its intensity was quantified in a luminescent image analyzer (LAS-3000 mini; Fuji Film Co., Tokyo, Japan).
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Effect of Oral Doses of Pluronic P85 and Tween 20 on Intestinal Absorption of Topotecan in Wild-Type and Bcrp (–/–) Mice. After administration of 250 mg/kg Pluronic P85 or 100 mg/kg Tween 20 orally to wild-type and Bcrp (–/–) mice 15 min before oral administration of topotecan, the plasma concentration of topotecan was determined at designated times (Figs. 2 and 3; Table 2). In Pluronic P85-treated wild-type mice, the AUC of topotecan was statistically higher than that in non-excipient-treated wild-type mice (2.0-fold). In contrast, no significant effect of Pluronic P85 on the AUC of topotecan was observed in Bcrp (–/–) mice. Similarly, in Tween 20-treated wild-type mice, the AUC of topotecan was statistically higher than that in non-excipient-treated wild-type mice (1.8-fold), whereas no significant effect of Tween 20 on the AUC of topotecan was observed in Bcrp (–/–) mice.
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Effect of Oral Doses of Pluronic P85 and Tween 20 on Topotecan Pharmacokinetics in Wild-Type and Bcrp (–/–) Mice after Intravenous Administration. After administration of 250 mg/kg Pluronic P85 or 100 mg/kg Tween 20 orally to wild-type and Bcrp (–/–) mice 15 min before intravenous administration of topotecan, the plasma concentration of topotecan was determined at designated times (Figs. 2 and 3; Table 2). No significant effect of Pluronic P85 on the AUC of topotecan was observed in wild-type and Bcrp (–/–) mice. Similar to Pluronic P85, no significant effect of Tween 20 on the AUC of topotecan was observed in wild-type and Bcrp (–/–) mice.
Intestinal Absorptive Transport of Topotecan in Everted Sacs of Wild-Type and Bcrp (–/–) Mice. The intestinal absorption rate of topotecan was measured using everted sacs and compared between wild-type and Bcrp (–/–) mice. Topotecan was applied to the mucosal side, and the serosal side of the sacs was perfused with drug-free buffer. The concentration of topotecan in the outflow was determined. Compared with wild-type mice, the intestinal absorption rate of topotecan was higher in Bcrp (–/–) mice. The intestinal absorption rates of topotecan in wild-type and Bcrp (–/–) mice reached a plateau at 45 min during constant infusion (Fig. 4A). The intestinal absorption rates of topotecan at 60 min, at which steady state had been reached, in wild-type and Bcrp (–/–) mice were 0.43 ± 0.08 and 0.83 ± 0.14 pmol/min/cm tissue, respectively. The steady-state intestinal absorption rate of topotecan in Bcrp (–/–) mice was statistically higher than that in wild-type mice (2.1-fold; P < 0.01).
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Effect of Pluronic P85 and Tween 20 on Intestinal Absorptive Transport of Topotecan in Everted Sacs of Wild-Type and Bcrp (–/–) Mice. The steady-state intestinal absorption rate of topotecan in the presence of 20 µM Pluronic P85 or 250 µM Tween 20 was determined (Fig. 4B). Each excipient was added to the mucosal side. A Pluronic P85 concentration of 20 µM statistically increased the intestinal absorption rate of topotecan in wild-type mice (2.1-fold), whereas no significant effect of Pluronic P85 was observed in Bcrp (–/–) mice. Similarly, 250 µM Tween 20 statistically increased the intestinal absorption rate of topotecan in wild-type mice (2.5-fold), whereas no significant effect of Tween 20 was observed in Bcrp (–/–) mice.
P-gp Expression and Transport Function in the Intestine of Wild-Type and Bcrp (–/–) Mice. After Western blot analysis, no significant difference in P-gp expression in jejunum was observed between wild-type and Bcrp (–/–) mice. In the ileum, the P-gp expression in Bcrp (–/–) mice slightly increased compared with that in wild-type mice (Fig. 5A). In contrast, Bcrp was detected in the jejunum and ileum of wild-type mice, but not in Bcrp (–/–) mice (Fig. 5A). The villin content remained relatively unchanged (Fig. 5A).
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The secretory transport of rhodamine 123, a typical P-gp substrate, was measured using everted sacs of wild-type and Bcrp (–/–) mice. Rhodamine 123 was applied to the serosal side, and the mucosal efflux of rhodamine 123 was determined. In wild-type and Bcrp (–/–) mice, the mucosal efflux of rhodamine 123 in the jejunum and ileum was linear up to 60 min (Fig. 5B). No significant difference was observed in the secretion rates of jejunum and ileum between wild-type and Bcrp (–/–) mice (Fig. 5B), which were 0.29 ± 0.01 versus 0.28 ± 0.06, and 0.64 ± 0.16 versus 0.64 ± 0.12 pmol/min/cm tissue, respectively.
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Discussion |
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Both Pluronic P85 and Tween 20 significantly increased the AUC of topotecan after oral administration in a dose-dependent manner, but at a high dose, they were less effective in wild-type mice (Fig. 1; Table 1). In contrast, neither Pluronic P85 nor Tween 20 given orally affected the AUC of topotecan after intravenous administration in wild-type mice (Fig. 2; Table 2), implying that these excipients given orally did not affect the systemic clearance of topotecan, probably because of the low absorption of these excipients. These results suggest that excipient-mediated enhancement of the AUC of topotecan after oral administration may be due to an increase in its intestinal absorption. The ability of excipients to enhance oral drug absorption can be possibly ascribed to increasing the solubility of drugs in the intestinal lumen and/or inhibition of efflux transporters (Yu et al., 1999; Varma and Panchagnula, 2005). To investigate the contribution of increasing solubility of drugs and inhibition of BCRP, the effect of Pluronic P85 and Tween 20 was examined in Bcrp (–/–) mice. Neither Pluronic P85 nor Tween 20 exhibited any enhancement of the AUC of topotecan after oral and intravenous administration in Bcrp (–/–) mice (Fig. 3; Table 2). These results suggest that inhibiting BCRP by excipients mainly contributes to the enhancement of oral topotecan absorption, and increasing drug solubility was, at most, minimal. These conclusions were further supported by an in vitro study using everted mouse intestine. The steady-state intestinal absorption rate of topotecan was significantly greater in Bcrp (–/–) mice than that in wild-type mice (Fig. 4A). Consistent with in vivo observation, both Pluronic P85 and Tween 20 significantly increased the steady-state intestinal absorption rates of topotecan in everted sacs from wild-type mice, whereas the effect was reduced in sacs from Bcrp (–/–) mice (Fig. 4B).
Topotecan is also a substrate of P-gp (Chen et al., 1991; Jonker et al., 2000), and it has been demonstrated that P-gp also plays a significant role in the pharmacokinetics of topotecan after oral administration (Jonker et al., 2000). Whether the expression and the transport activity of intestinal P-gp are changed as a result of the impairment of Bcrp activity remains in question. This was investigated by Western blot analysis and an in vitro transport study using everted sacs. Western blot analysis showed that P-gp expression was similar in the jejunum, but there was a slight increase in the ileum of Bcrp (–/–) mice (Fig. 5A). There was no significant change in the transport function of P-gp between wild-type and Bcrp (–/–) mice (Fig. 5B). Namely, there was no significant difference in the intestinal P-gp transport function between wild-type and Bcrp (–/–) mice. Therefore, it can be concluded that the increase in the AUC of topotecan after oral administration in Bcrp (–/–) mice is due to impairment of Bcrp in the intestine.
Taken together, the present study elucidated that Pluronic P85 and Tween 20 inhibit intestinal efflux by Bcrp and can improve the oral absorption of topotecan. The mechanism underlying the inhibition of BCRP by these excipients remains unknown. Previous reports have suggested that a change in the microviscosity of the plasma membrane due to integration of excipients and/or a reduction in the intracellular ATP are the underlying mechanisms (Dudeja et al., 1995; Batrakova et al., 2001a; Hugger et al., 2002). However, the effect of a change in the microviscosity remains questionable since cholesterol and benzylalcohol, known as membrane fluidity modulators, did not affect the efflux transporters (Rege et al., 2002). In addition, although Pluronic P85 has been shown to inhibit P-gp and reduce the intracellular ATP, there is no report of other excipients examining the reduction in intracellular ATP. It is likely that these excipients may competitively inhibit substrate binding to the efflux transporter, although further studies are needed to clarify the mechanism.
For clinical application, it should be noted that there was an optimal oral dose of excipients to enhance the oral topotecan absorption (Fig. 1; Table 1). Similar tendencies have been reported from in vitro drug transport studies (Nerurkar et al., 1996; Batrakova et al., 1999; Yu et al., 1999; Rege et al., 2002; Shono et al., 2004; Varma and Panchagnula, 2005). Possibly, this is due to the micelle formation of the excipients at the highest doses. Incorporation of topotecan into the micelle might reduce the effective concentration of topotecan in the intestinal lumen; thus, no enhancement of oral topotecan absorption might be observed. Therefore, doses of excipients such as surfactants need to be designed to achieve their appropriate concentration in the intestine to improve oral drug absorption. In addition, if excipients are administered intravenously, they might increase the penetration of drugs into the brain by inhibiting efflux transporters in the blood-brain barrier. In fact, it has been reported that the intravenous coadministration of Pluronic P85 with digoxin markedly increases the penetration of digoxin into the brain by inhibiting P-gp function at this critical point (Batrakova et al., 2001b). Thus, an increase in the accumulation of drugs in the brain by intravenous coadministration of excipients might lead to toxicity.
In conclusion, this study demonstrated that the excipients Pluronic P85 and Tween 20 can improve the oral bioavailability of topotecan, a BCRP substrate, by inhibiting BCRP function in the small intestine. Thus, some excipients are not as inert as initially thought but are indeed functional.
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Acknowledgments |
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Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
ABBREVIATIONS: P-gp, P-glycoprotein; BCRP (Bcrp), breast cancer resistance protein; AUC, area under the plasma concentration-time curve; ABC, ATP-binding cassette; TTBS, Tris-buffered saline with 0.05% Tween 20; GF120918, N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide; ME3277, sodium hydrogen [4-[(4,5,6,7-tetrahydrothieno[3,2c]-pyridine-2-yl)carbonylamino]acetyl-o-phenylene] dioxydiacetate; ANOVA, analysis of variance.
Address correspondence to: Dr. Mariko Morishita, Department of Pharmaceutics, Hoshi University, Ebara 2-4-41, Shinagawa, Tokyo 142-8501, Japan. E-mail: morisita{at}hoshi.ac.jp
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