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Cancer Therapy: Preclinical |
Authors' Affiliations: Divisions of 1 Experimental Therapy and 2 Clinical Chemistry, The Netherlands Cancer Institute; and 3 Department of Pharmacy and Pharmacology, Slotervaart Hospital, Amsterdam, the Netherlands
Request for reprints: Alfred H. Schinkel, Division of Experimental Therapy, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, the Netherlands. Phone: 31-20-5122046; Fax: 31-20-5122050; E-mail: a.schinkel{at}nki.nl.
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Abstract |
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Experimental Design: We generated and characterized Mdr1a/1b/Mrp2–/– mice, allowing assessment of the distinct roles of Mrp2 and Mdr1a/1b P-gp in paclitaxel pharmacokinetics.
Results: Surprisingly, the effect of Mrp2 on i.v. administration of paclitaxel was as great as that of P-gp. The area under plasma concentration-time curve (AUC)i.v. in both Mrp2–/– and Mdr1a/1b–/– mice was 1.3-fold higher than in wild-type mice, and in Mdr1a/1b/Mrp2–/– mice, a 1.7-fold increase was found. In spite of this similar effect, Mrp2 and P-gp had mostly complementary functions in paclitaxel elimination. Mrp2 dominated the hepatobiliary excretion, which was reduced by 80% in Mrp2–/– mice. In contrast, P-gp dominated the direct intestinal excretion, with a minor role for Mrp2. The AUCoral of paclitaxel was 8.5-fold increased by Mdr1a/1b deficiency but not affected by Mrp2 deficiency. However, in the absence of Mdr1a/1b P-gp, additional Mrp2 deficiency increased the AUCoral another 1.7-fold.
Conclusions: Thus far, Mrp2 was thought to mainly affect organic anionic drugs in vivo. Our data show that Mrp2 can also be a major determinant of the pharmacokinetic behavior of highly lipophilic anticancer drugs, even in the presence of other efficient transporters. Variation in MRP2 activity might thus directly affect the effective exposure to paclitaxel, on i.v. administration, but also on oral administration, especially when P-gp activity is inhibited.
Paclitaxel is an excellent P-gp substrate that is widely used in treatment of breast and ovarian cancer, non–small cell lung cancer, and Kaposi's sarcoma (3). We showed earlier that P-gp in epithelial cells of the small intestine actively effluxes its substrates, including paclitaxel, directly from the blood into the intestinal lumen. Moreover, using paclitaxel as model substrate, P-gp was shown to drastically limit intestinal absorption of orally administered substrates (4, 5). Based on these findings, numerous mouse studies and clinical trials have been done, showing that the poor oral availability of paclitaxel could be dramatically improved by coadministration of a P-gp inhibitor (6–10). This is of importance because oral administration of paclitaxel would be preferred over i.v. administration, as it is convenient to patients, reduces administration costs, and facilitates the use of more chronic treatment regimes (11).
Despite virtually complete absorption of paclitaxel from the gastrointestinal tract in Mdr1a/1b–/– mice, bioavailability does not approach 100% (5, 6). Similar results were found in patients when paclitaxel was combined with the potent P-gp inhibitors Cyclosporine A or GF120918 (Elacridar; refs. 10, 12). This might be explained by the fact that besides absorption, first-pass metabolism and elimination also affect the bioavailability of a drug. In addition to the P-gp-mediated excretion of paclitaxel from blood directly into the gut lumen (5), excretion into the bile is another important route of elimination, both in rodents and in humans (13, 14). Given its presence in the canalicular membrane of hepatocytes, P-gp seemed to be a good candidate for this elimination pathway. However, studies with Mdr1a–/– and Mdr1a/1b–/– mice (5, 15) failed to show a significant role for P-gp in hepatobiliary excretion of paclitaxel and its hydroxylated metabolites.
We recently identified human MRP2 as a transporter for taxanes in vitro (16), and we hypothesized that MRP2 may also play a role in vivo, affecting absorption, distribution, and/or elimination of paclitaxel. As MRP2 is expressed at the apical membrane of epithelial cells of the small intestine (17), it might limit oral absorption of paclitaxel, similar to P-gp. Furthermore, MRP2 is found at the canalicular membrane of hepatocytes (18) and could thus mediate biliary excretion of paclitaxel and/or its principal hydroxylated metabolites. Thus, absence or reduced activity of MRP2 might increase absorption or decrease elimination of paclitaxel and hence increase overall paclitaxel exposure, potentially influencing therapeutic efficacy and risks of toxic side effects. Involvement of MRP2 in the pharmacokinetics of paclitaxel could be highly relevant for chemotherapy in patients and possible interpatient variability. Many MRP2 polymorphisms have been described in the human population that affect MRP2 transport activity, including fully deficient variants that occur in homozygous form in Dubin-Johnson patients (19). We have recently generated Mrp2–/– mice (20) and crossed them with Mdr1a/1b–/– mice (15) to obtain Mdr1a/1b/Mrp2–/– mice. The availability of these strains allowed us to address the relative effect of Mrp2 and P-gp on paclitaxel pharmacokinetics.
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Materials and Methods |
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-hydroxypaclitaxel were purified from patients' feces as described (21) or purchased from Gentest Corp. (Woburn, MA). Ketamine (Ketanest-S) was from Pfizer (Cappelle a/d IJssel, the Netherlands). Xylazine was from Sigma Chemical Co. (St. Louis, MO). Methoxyflurane (Metofane) was from Medical Developments Australia (Springvale, Victoria, Australia). Heparin (5,000 IE/mL) was from Leo Pharma BV (Breda, the Netherlands). Bovine serum albumin, Fraction V, was from Roche (Mannheim, Germany). The organic solvents methanol, acetonitril [both high-performance liquid chromatography (HPLC) grade], and diethyl ether were from Merck (Darmstadt, Germany). Drug-free human plasma was from healthy volunteers. Animals. Mice were housed and handled according to institutional guidelines complying with Dutch legislation. Animals used in this study were male Mdr1a/1b–/– (15), Mrp2–/– (20), Mdr1a/1b/Mrp2–/–, and wild-type (WT) mice, all with a >99% FVB genetic background, between 9 and 15 weeks of age. Animals were kept in a temperature-controlled environment with a 12-hour light/12-hour dark cycle and received a standard diet (AM-II, Hope Farms, Woerden, the Netherlands) and acidified water ad libitum.
Plasma pharmacokinetics. For oral administration, paclitaxel formulated in Cremophor EL and dehydrated alcohol (1:1, v/v, 6 mg/mL, Taxol) was diluted with saline to 1 mg/mL and dosed at 10 mg/kg body weight (10 mL/kg). To minimize variation in absorption, mice were fasted for 3 hours before paclitaxel was administered by gavage into the stomach using a blunt-ended needle. Multiple blood samples (
30 µL) were collected from the tail vein at 15 and 30 minutes and 1, 2, 4, 6, and 8 hours using heparinized capillary tubes (Oxford Labware, St. Louis, MO). Blood samples were centrifuged at 2,100 x g for 10 minutes at 4°C, and the plasma fraction was collected, completed to 200 µL with human plasma, and stored at –20°C until analysis. For i.v. studies, paclitaxel was formulated in ethanol and polysorbate 80 (1:1, v/v, 6 mg/mL). This solution was diluted with saline to 2 mg/mL and injected as single bolus at a dose of 10 mg/kg (5 mL/kg) into the tail vein. Blood samples were collected by cardiac puncture under methoxyflurane anesthesia. Animals were sacrificed at 7.5, 15, and 30 minutes and 1, 2, 4, and 8 hours after paclitaxel administration, with three to four animals per time point. Blood samples were centrifuged at 2,100 x g for 10 minutes at 4°C, and the plasma fraction was collected and stored at –20°C until analysis.
Fecal and urinary excretion. Mice were individually housed in Ruco Type M/1 stainless steel metabolic cages (Valkenswaard, the Netherlands). They were allowed 2 days to adapt before 10 mg/kg paclitaxel, supplemented with [3H]paclitaxel (0.5 µCi/animal), was injected into a tail vein. Feces and urine were collected over a 24-hour period; urine was diluted 5-fold with human plasma and feces were homogenized in 4% bovine serum albumin (1 mL/100 mg feces). Part of the sample was used to determine levels of radioactivity by liquid scintillation counting; the rest was stored at –20°C until analysis.
Biliary excretion. In gall bladder cannulation experiments, mice were anesthetized by i.p. injection of a combination of ketamine (100 mg/kg) and xylazine (6.7 mg/kg), in a volume of 4.33 µL/g body weight. After opening the abdominal cavity and distal ligation of the common bile duct, a polythene catheter (Portex Ltd., Hythe, United Kingdom), with an inner diameter of 0.28 mm, was inserted into the incised gall bladder and fixed with an additional ligation. Bile was collected for 60 minutes after i.v. injection of paclitaxel. For gall bladder cannulation experiments, 5 mg/kg were used, as 10 mg/kg paclitaxel in combination with anesthesia and surgery can result in cardiac and respiratory insufficiency (5). At the end of the experiment, blood was collected by cardiac puncture and mice were sacrificed by cervical dislocation. Several tissues were removed and homogenized in 4% bovine serum albumin; intestinal contents were separated from intestinal tissues before homogenization. Tissue homogenates, bile, and plasma were stored at –20°C until analysis.
Drug analysis. Amounts of paclitaxel and its hydroxylated metabolites 3'-p-hydroxypaclitaxel and 6-hydroxypaclitaxel in small plasma samples, obtained by sampling from the tail vein, were determined using a previously described sensitive and specific liquid chromatograpy-mass spectrometry/mass spectrometry assay (22). All other samples were processed using liquid-liquid and solid-phase extraction followed by reversed-phase HPLC with UV detection (23), with minor modifications. We adjusted the mobile phase for HPLC analysis of bile samples and tissue and feces homogenate extracts [acetonitrile-methanol-0.2 mol/L ammonium acetate buffer (pH 5.0; 42:65:93, v/v/v)] to obtain successful separation of drug peaks and interfering peaks.
Clinical-chemical analysis of plasma. Standard clinical chemistry analyses on plasma of WT, Mdr1a/1b–/–, Mrp2–/–, and Mdr1a/1b/Mrp2–/– mice (n = 6, males and females) were done on a Roche Hitachi 917 analyzer (Roche Diagnostics, Basel, Switzerland) to determine levels of total and conjugated bilirubin, alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, creatinine, urea, Na+, K+, Cl–, Ca2+, phosphate, total protein, and albumin.
Hematologic analysis. Hemoglobin, haematocrit, mean corpuscular volume, RBC, WBC, lymphocytes, monocytes, granulocytes, and platelets were determined in EDTA blood on a Beckman Coulter (Miami, FL) Ac·T Diff analyzer.
Pharmacokinetic calculations and statistical analysis. Pharmacokinetic variables were calculated by noncompartimental methods using the software package WinNonlin Professional, version 5.0. The area under plasma concentration-time curves (AUC) were calculated using the trapezoidal rule, without extrapolating to infinity. Elimination half-lives (t1/2, el) were calculated by linear regression analysis of the log-linear part of the plasma concentration-time curves. Plasma clearance after i.v. paclitaxel administration was calculated by the formula plasma clearance = dose / AUCi.v. and the oral bioavailability (F) was calculated by the formula F = AUCoral / AUCi.v. x 100%. The two-sided unpaired Student's t test was used for statistical analysis. Data obtained with single and combination knockout mice were compared with data obtained with WT mice, unless stated otherwise. Differences were considered statistically significant when P < 0.05. Data are presented as mean ± SD.
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Results |
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25% increased liver weight (6.1 ± 0.4% of body weight in Mdr1a/1b/Mrp2–/– versus 4.8 ± 0.3% in WT, n = 5-6; P = 0.0003). No other macroscopic or microscopic anatomic abnormalities were evident. The bile flow in Mdr1a/1b/Mrp2–/– mice was reduced to 40% to 50% of WT levels (P < 0.01) and not significantly different from that in Mrp2–/– mice (20).
Mdr1a/1b/Mrp2–/– mice had a moderately increased (3-fold) plasma level of total bilirubin compared with WT mice, which could be attributed to elevated levels of conjugated bilirubin (3.2 ± 1.6 µmol/L in males and 2.7 ± 0.8 µmol/L in females, n = 6). Conjugated bilirubin levels in WT plasma were below the detection limit (<1 µmol/L). Conjugated and total bilirubin levels in Mdr1a/1b/Mrp2–/– mice were not significantly different from those in Mrp2–/– mice. The other clinical-chemical variables measured in plasma (see Materials and Methods) showed no significant differences between WT and Mdr1a/1b/Mrp2–/– mice.
Hemoglobin levels were moderately but significantly decreased in both male and female Mdr1a/1b/Mrp2–/– mice [males, 7.0 ± 0.1 mmol/L in knockout mice versus 7.4 ± 0.1 mmol/L in WT mice, n = 3-4 (P = 0.017); females, 7.2 ± 0.5 mmol/L in knockout mice versus 7.6 ± 0.1 mmol/L in WT mice, n = 5-6 (P = 0.016)]. These results are qualitatively similar to those for Mrp2–/– mice. None of the other hematologic variables measured revealed significant differences between WT and Mdr1a/1b/Mrp2–/– mice.
Mdr1a/1b/Mrp2–/– mice thus appear in many respects very similar to Mrp2–/– mice (20), and they are likely as amenable to pharmacologic analyses.
Effect of Mrp2 and P-gp on plasma pharmacokinetics of paclitaxel. To investigate the relative roles of Mrp2 and P-gp in absorption, distribution, and elimination of paclitaxel, we studied oral and i.v. plasma pharmacokinetics in WT, Mrp2–/–, Mdr1a/1b–/–, and Mdr1a/1b/Mrp2–/– mice. On oral administration of 10 mg/kg paclitaxel, plasma concentrations and AUCoral were not different between Mrp2–/– and WT mice (Fig. 1A
; Table 1
). For Mdr1a/1b–/– mice, the AUCoral was 8.5-fold higher, in line with previous results (5, 6), but the t1/2, el of the drug was not changed (Table 1). Interestingly, however, in Mdr1a/1b/Mrp2–/– mice, the AUCoral was increased another 1.7-fold compared with Mdr1a/1b–/– mice (and 14.2-fold compared with WT mice), the maximum plasma level (Cmax) was 1.5-fold increased, and a 1.4-fold extended t1/2, el was found (P < 0.01 for each variable; Fig. 1A; Table 1). These results confirm that P-gp is a major factor in limiting the paclitaxel AUC after oral administration, but that in the absence of P-gp, Mrp2 also has a marked effect on oral paclitaxel plasma pharmacokinetics.
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Role of Mrp2 and P-gp in plasma and liver levels of 3'-p-hydroxypaclitaxel and 6-hydroxypaclitaxel. Because metabolism is an important detoxification pathway for paclitaxel, we also studied its primary metabolites: 3'-p-hydroxypaclitaxel and 6-hydroxypaclitaxel. Plasma levels of these monohydroxylated metabolites at t = 8 hours after i.v. administration of paclitaxel at 10 mg/kg were below the limits of detection in WT mice (Table 2
). However, substantial levels were detected in plasma of Mdr1a/1b–/– and Mrp2–/– mice, and for Mdr1a/1b/Mrp2–/– mice, the levels were another 3- to 4-fold higher. Similar results were obtained for metabolite levels in liver at t = 8 hours (Table 2), suggesting an interrelatedness of plasma and liver metabolite levels. The same might apply to unchanged paclitaxel, as its accumulation in liver and plasma concentration were also markedly higher in each of the separate and especially the combined knockout strains.
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18% of the dose). Yet, in Mdr1a/1b–/– mice, where Mrp2 is still present, paclitaxel was nearly absent from feces. This suggests that P-gp helps to keep paclitaxel, initially excreted by Mrp2, in the intestinal lumen, presumably by limiting reabsorption of the drug.
Role of Mrp2 and P-gp in fecal and urinary excretion of monohydroxylated metabolites. The fecal excretion pattern of the hydroxylated paclitaxel metabolites was quite different from that of the parent compound (Table 3). WT mice excreted 15% of the dose as 3'-p-hydroxypaclitaxel and 8.5% as 6-hydroxypaclitaxel. In Mdr1a/1b–/– mice, the fecal excretion of both metabolites was moderately but significantly increased compared with WT mice (P < 0.05 for both) and accounted for more than half of the excreted radioactivity. Mrp2–/– mice, however, displayed a reduced excretion of 3'-p-hydroxypaclitaxel and 6-hydroxypaclitaxel to 67% and 61% of WT levels, respectively. In Mdr1a/1b/Mrp2–/– mice, fecal excretion of these metabolites was nearly abolished. The latter result suggests that, in addition to Mrp2, Mdr1a/1b P-gp is also important in the fecal excretion of the hydroxylated metabolites, in spite of their increased excretion in the Mdr1a/1b–/– mice. This may result from strongly increased formation of the metabolites due to the extended residence time of paclitaxel in Mdr1a/1b–/– mice, more than compensating for a partial reduction in their excretion capacity due to P-gp deficiency. Mrp2 seemed to be responsible for nearly all of the fecal excretion of the metabolites in the Mdr1a/1b–/– mice.
In the urine of Mdr1a/1b–/– mice and especially Mrp2–/– and Mdr1a/1b/Mrp2–/– mice, a highly significant increase in excreted radioactivity was found. Paclitaxel and its primary hydroxylated metabolites only represented a minor fraction (Table 3), so other hydrophilic metabolites likely accounted for the majority of this excreted radioactivity.
Effect of Mrp2 and P-gp on biliary and direct intestinal excretion of paclitaxel and its hydroxylated metabolites. We did gall bladder cannulation experiments to clarify the roles of Mrp2 and Mdr1a/1b in biliary and direct intestinal excretion. Previous experiments suggest that P-gp does not primarily mediate biliary excretion of paclitaxel or its hydroxylated metabolites (5, 15). We measured the biliary excretion for 1 hour in anesthetized mice with a cannulated gall bladder and a ligated common bile duct, receiving i.v. [3H]paclitaxel at 5 mg/kg. In WT mice, 3.3 ± 0.8% of the dose was excreted over 1 hour as unchanged paclitaxel (Table 4
). Mdr1a/1b–/– mice did not show a significant reduction in biliary excretion of paclitaxel, in line with previous findings (5, 15). In contrast, in Mrp2–/– mice biliary excretion of paclitaxel was reduced by 80% compared with WT mice, whereas, in Mdr1a/1b/Mrp2–/– mice, the excretion was almost totally abolished (97% reduction). A similar excretory pattern was found for the principal metabolites (Table 4). This indicates that Mrp2 is the predominant factor in the biliary excretion of paclitaxel and its hydroxylated metabolites and that Mdr1a/1b plays a minor role in this process. Furthermore, in Mrp2–/– and Mdr1a/1b/Mrp2–/– mice, very similar and significantly increased levels of paclitaxel in plasma (by 51% and 53%) and in liver (by 38% and 34%) and increased levels of metabolites in liver were found at the end of the cannulation experiment (Table 4). This probably reflects the decreased hepatobiliary elimination of paclitaxel and monohydroxylated metabolites owing to Mrp2 absence. The biliary radioactivity data indicate that the majority of other paclitaxel metabolites was also primarily transported into the bile by Mrp2 because, in WT and Mdr1a/1b–/– mice, 20% of the radioactive dose was recovered in bile, whereas this was only 4% in Mrp2–/– and Mdr1a/1b/Mrp2–/– bile.
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Different results were obtained for the hydroxylated metabolites. Mdr1a/1b–/– mice showed a significantly increased intestinal excretion of 3'-p-hydroxypaclitaxel and 6-hydroxypaclitaxel, presumably owing to higher plasma levels of these compounds. In contrast, clearly reduced amounts of these metabolites were found in the intestinal contents of Mrp2–/– and Mdr1a/1b/Mrp2–/– mice (Table 4). These data suggest that Mrp2 has a predominant function in the direct intestinal excretion of the hydroxylated paclitaxel metabolites.
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Discussion |
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Although we had shown previously that paclitaxel is transported by human MRP2 (16), we were surprised to find that the effect of Mrp2 on the pharmacokinetics of paclitaxel after i.v. administration was at least as great as that of Mdr1a/1b P-gp. Paclitaxel is an excellent P-gp substrate, so we had expected that its pharmacokinetics would be dominated by P-gp, as is indeed the case on oral administration of the drug. However, on i.v. administration, even in the presence of P-gp, Mrp2 has a marked effect on paclitaxel plasma levels and excretion, at least equal to the P-gp effects. As paclitaxel is currently primarily administered to patients i.v., variation in MRP2 activity might directly affect their effective paclitaxel exposure.
The pronounced effect of P-gp on (oral) paclitaxel pharmacokinetics seems to be determined primarily by the capability of P-gp to reduce net (re-)absorption of paclitaxel from the intestinal lumen and, related to this, its capability to mediate direct intestinal excretion (5). Especially on oral administration in P-gp-proficient mice, very little paclitaxel enters the circulation, leaving little room for a significant contribution of Mrp2. We observed earlier that Mrp2 has a more pronounced pharmacokinetic effect at relatively high plasma drug concentrations of methotrexate, presumably because, at lower plasma concentrations alternative, more high-affinity elimination systems dominate drug removal (20). The same might apply for elimination of the comparatively low paclitaxel levels after oral administration in P-gp-proficient animals (Fig. 1).
The results from Tables 3 and 4 indicate that Mrp2 and P-gp have rather complementary roles in hepatobiliary and intestinal excretion of paclitaxel after i.v. administration. Mrp2 is the dominant factor in biliary excretion of paclitaxel, and P-gp contributes modestly. In contrast, P-gp dominates the direct intestinal excretion of paclitaxel, whereas Mrp2 plays a minor role here. Table 3 shows that Mrp2 activity accounts for at least 18% of the dose being excreted in the feces over 24 hours, which must result mainly from hepatobiliary and perhaps some direct intestinal excretion. In spite of this, in the absence of P-gp in the Mdr1a/1b–/– mice, very little paclitaxel is retrieved in the feces (Table 3). This must mean that the paclitaxel initially excreted by Mrp2 into the intestinal lumen of these mice is readily reabsorbed from the gut due to P-gp absence. This continued reabsorption of unchanged paclitaxel results in prolonged metabolism, explaining why very little unmetabolized paclitaxel leaves the body when P-gp is absent.
It is interesting to note that, in spite of the qualitatively different primary functions of P-gp and Mrp2 affecting paclitaxel pharmacokinetics, the quantitative effect of absence of both proteins on the AUCi.v. was very similar (1.3-fold each). The combination of both deficiencies had rather an additive than a synergistic effect on the paclitaxel AUCi.v.. (1.3 x 1.3 = 1.69, corresponding well with the 1.7-fold increased AUCi.v. in the combination knockout mice).
In the past, MRP2/Mrp2 has been considered primarily as an organic anion transporter, and earlier experiments in Mrp2-deficient rats and mice indicated that Mrp2 could have a marked effect on pharmacokinetics of the anionic anticancer drug methotrexate (20, 29). Our data show that Mrp2 can also be a major determinant of the pharmacokinetic behavior of a highly lipophilic anticancer drug, even in the presence of other very efficient transporters for this drug. As it is now clear that several other nonanionic and lipophilic (anticancer) drugs, including docetaxel and etoposide, and various HIV protease inhibitors are markedly transported by MRP2 in vitro (16, 30), it may well be that these other drugs are equally affected in their (i.v.) pharmacokinetics. This could mean that MRP2 activity has a much broader significance for pharmacokinetic behavior of anticancer and other drugs than previously appreciated. This is of importance, as extensive genetic polymorphisms in human MRP2 are known that affect functionality, some even resulting in full homozygous deficiency for MRP2 (19). In a recent study, six known allelic variants in genes involved in paclitaxel metabolism (CYP2C8, CYP3A4, and CYP3A5) and in the gene coding for P-gp (ABCB1) were evaluated but could not explain the substantial interindividual variability in paclitaxel pharmacokinetics (31). It will be of interest to test whether polymorphisms in the ABCC2 gene contribute to these variations.
Furthermore, factors affecting MRP2 expression, such as hepatic diseases, renal failure, or exposure to certain drugs, can result in interindividual differences in disposition of drugs eliminated via MRP2 (19). Such variation in MRP2 activity might thus affect the therapeutic plasma levels and toxic side effects of a much broader range of anticancer drugs than previously realized and this should be taken into account during chemotherapy treatment of patients.
Our study shows that Mrp2 has a marked effect on both i.v. and oral paclitaxel AUC when P-gp activity is absent (Fig. 1). In a variety of clinical trials, highly efficacious P-gp inhibitors, such as PSC-833 (Valspodar), GF120918 (Elacridar), and others, are coadministered with paclitaxel or other MRP2 substrate drugs to counteract multidrug resistance in tumors or to improve the oral bioavailability of the anticancer drug (7, 10, 12, 32). Under these circumstances, variation in MRP2 activity due to genetic polymorphisms might have even more pronounced effects on effective availability of the drug, with implications for therapeutic efficacy and the risk of toxic side effects. It will thus be important to be well aware of the effect of MRP2 on the pharmacokinetic behavior of many anticancer drugs when P-gp is inhibited.
In principle, simultaneous inhibition of P-gp and MRP2 might be used to further increase the oral availability of paclitaxel when desirable. However, to date, no compounds are identified that specifically inhibit the transport of lipophilic amphipathic drugs by MRP2. For instance, although for organic anions several studies in rats show that biliary excretion via Mrp2 can be inhibited by probenecid (33, 34), we found previously that the in vitro transport of lipophilic amphipathic anticancer drugs and HIV protease inhibitors by MRP2 was rather stimulated in the presence of probenecid (16, 30).
The mouse models we have generated will provide useful tools to qualitatively assess the pharmacokinetic effect of MRP2 and P-gp for a variety of drugs. This information can subsequently be used for rational translation of the insights to the (clinical) situation in humans, which may ultimately lead to more constant and reliable chemotherapy regimens.
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Acknowledgments |
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Footnotes |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 6/ 2/06; revised 7/11/06; accepted 7/26/06.
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), Mdr1a/1b–/– (
), Mrp2–/– (
), and Mdr1a/1b/Mrp2–/– (
) mice after oral (A) and i.v. (B) administration of paclitaxel at a dose of 10 mg/kg. Points, mean concentrations for oral (n = 5-6) and i.v. administration (n = 3-4); bars, SD.