[Cell Cycle 3:12, 1502-1505; December 2004]; ©2004 Landes Bioscience
Pharmacokinetic Resistance to Imatinib Mesylate Extra Views
Role of the ABC Drug Pumps ABCG2 (BCRP) and ABCB1 (MDR1) in the Oral Bioavailability of Imatinib ABSTRACT
*Correspondence to: Kees Nooter; Department of Medical Oncology; Erasmus MC / Josephine Nefkens Institute; RM Be422, PO Box 1738; 3000 DR; Rotterdam, The Netherlands; Tel.: +31.10.4088357; Fax: +31.10.4088363; Email:
[email protected] Received 10/20/04; Accepted 10/22/04
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Previously published online as a Cell Cycle E-publication: http://www.landesbioscience.com/journals/cc/abstract.php?id=1331
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Department of Medical Oncology; Erasmus Medical Center Rotterdam; The Netherlands
Imatinib mesylate is a selective tyrosine kinase inhibitor that is successfully used in the treatment of chronic myeloid leukaemias and gastrointestinal stromal tumours. The drug is taken orally on a daily basis in order to suppress tumour growth. Unfortunately, the vast majority of patients will eventually progress while on therapy. It is generally thought that this acquired unresponsiveness is due to gene amplification or somatic mutations in the drug’s target genes. However, we have now evidence, based on several in vitro and in vivo observations suggesting that pharmacokinetic resistance may also play a definitive role in the ultimate resistance of patients on chronic imatinib. Our findings may have serious implications for the chronic imatinib treatment of cancer patients.
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Herman Burger Kees Nooter*
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imatinib mesylate, pharmacokinetic resistance, gleevec, oral bioavailability, ABC transporters, caco2 cells
Imatinib mesylate (STI571, Gleevec®) is a small synthetic molecule that shows selective inhibition of the tyrosine kinase activity of the BCR-ABL fusion protein, the stem cell factor receptor c-KIT, and the receptor for platelet-derived growth factor. It functions through competitive binding at the ATP-binding site of the tyrosine kinase domain of these enzymes.1 Imatinib is the archetype of a new class of anticancer agents, especially designed for targeted therapy. It is successfully used for the treatment of BCR-ABL-dependent (Philadelphia chromosome positive) chronic myelogenous and acute lymphoblastic leukemia’s, and c-KIT-dependent gastrointestinal stromal tumors (GIST),2-5 tumor types that were hard to treat before the clinical introduction of imatinib. Thanks to a low toxicity profile, patients can take the drug orally, on a daily basis, presumably as long as is necessary to control tumor growth. Evidently, the clinical successes of imatinib have revolutionized our thinking about anticancer drug development, and the era of oncogene-directed targeted therapy has just begun. Although, oral formulation of chemotherapy is very patient friendly and cost-effective, the present use of imatinib might still have its specific concerns. Especially, chronic administration of anticancer drugs might be favorable for the induction of cellular drug efflux by upregulation of ABC drug pumps, analogous to the way cancer cells are made drug resistant in vitro. In general, oral bioavailability is highly dependent on gastrointestinal absorption and first-pass drug metabolism by liver and intestine, two processes that both vary considerably among individuals. So, the apparent clearance of imatinib could theoretically determine the systemic drug exposure in imatinib-treated cancer patients. Several ABC transporters genes, among which the ABCG2 (BCRP) and ABCB1 (MDR1) drug pumps, are expressed at the apical membrane of the small intestine and in the bile canalicular membrane.6–8 This localization suggests an important role of these drug pumps as first-line defense mechanism against xenobiotics. Therefore, in a recent study we investigated whether imatinib is a substrate for the ABCG2 and ABCB1 detoxification proteins. Evidently, in a panel of well-defined ABCG2 overexpressing cell lines, the reduced accumulation of 14C-labeled imatinib as compared to the control, clearly led to the conclusion that imatinib is a good substrate for the ABCG2 drug pump.9 The ABCG2-mediated efflux could be reversed by the fumitremorgin C analogue Ko-143 and imatinib could compete with mitoxantrone for drug export. Although for ABCB1 rather conflicting results have been reported with respect to its capability to mediate the outward-directed transport of imatinib,10–13 we recently found that the ABCB1 protein can indeed use imatinib as a substrate (HB, unpublished results). As quantitated by LC-MS/MS, imatinib accumulation in ABCB1-overexpressing KB8-5 cells was significantly decreased (about 4-fold) as compared to parental KB3-1 cells, consistent with the fact that imatinib is a substrate of the ABCB1 drug pump. These new findings might have far reaching consequences for the clinical use of combination chemotherapy, and the development of cellular and pharmacokinetic (PK) resistance to imatinib.
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2004; Vol. 3 Issue 12
PHARMACOKINETIC RESISTANCE TO IMATINIB MESYLATE
Despite its high oral bioavailability (>97%),14 the PK of imatinib shows a large interpatient variability (coefficient of variation 66 and 51%, respectively for CMax and AUC),15 which might, for a part, be attributable to variations in gastrointestinal absorption and metabolism. In vitro studies have demonstrated that the drug-metabolizing enzyme cytochrome P450 isoenzyme 4A (CYP3A4) is the major human P450 enzyme involved in the microsomal biotransformation of imatinib.15 The large variability of CYP3A4 activity between individuals that is generally found, may certainly contribute to the observed intersubject variation in PK parameters. We hypothesized that in addition to CYP3A4, interindividual variation in gastrointestinal transport activity may add to the variation in PK parameters of imatinib. In particular, the chronic use of oral imatinib may induce enhanced expression of the intestinal drug pumps capable of transporting imatinib (ABCG2 and ABCB1). Figure 1. Induction of the expression of ABC transporters in chronically exposed Caco2 cells with Subsequently, we have studied the in vitro effect imatinib (10 µM). The ABCB1 and ABCG2 mRNA and protein levels in Caco2 cells that have of chronic imatinib exposure (10 µM) of human been exposed to imatinib for 39 consecutive days are shown. The ABCB1 and ABCG2 mRNA, determined by real-time RT-PCR (TaqMan), are expressed as fold induction over untreated control intestinal Caco2 cells on the expression of various cells (5.8-fold and 16.6-fold induction was observed for respectively ABCB1 and ABCG2). Protein genes potentially involved in the transport of levels were determined by Western blot analysis. imatinib. Long-term imatinib exposure appeared to specifically induce the expression of ABCG2 and ABCB1 both at the mRNA and protein level (Fig. 1). The latter indicates that these upregulated drug pumps may have functional significance. In contrast, basal mRNA levels of ABCC1 (MRP1) and ABCC2 (MRP2) were not affected by chronic imatinib treatment. Transcriptional activation of ABC transporters by xenobiotic agents is often mediated through the nuclear orphan receptor SXR/NR1I2.16 However, in our hands imatinib was not able to activate SXR in cell-based cotransfection assays, indicating that SXR is probably not involved in imatinib-induced upregulation of ABCG2 and ABCB1 (HB, unpublished results). The exact nature of the imatinib-mediated transcriptional activation of these drug pumps is not clear yet. As might be expected, the chronically treated Caco2 cells had a significantly reduced intracellular imatinib accumulation, most likely caused by ABCG2- and/or ABCB1-mediated efflux (Fig. 2). Secondary resistance to imatinib is frequently seen in patients who initially responded to the drug, and it is generally believed among clinicians that eventually the vast majority of imatinib-treated patients will progress while on therapy. Obviously, knowledge about Figure 2. Decreased drug accumulation in imatinib-treated intestinal Caco2 cells. Intracellular imatinib concentrations (ng imatinib per mg protein) were the mechanisms of drug resistance towards imatinib is essential to be measured by LC-MS/MS after a 2 h imatinib exposure (steady-state accumuable to circumvent resistance. Although both ABCG2 and ABCB1 lation using 10 µM imatinib) in untreated (Caco2/control) and imatinib-treated can lower intracellular imatinib concentrations, no data are available cells that were drug-treated for 39 consecutive days (Caco2/imatinib). yet on any role of these pumps in the unresponsiveness of tumours Columns and error bars represent means ± S.D. of duplicate measurements in patients. In vitro research on mechanisms of resistance to imatinib of at least two independent experiments. The asterisk (*) indicates a significant has been performed primarily with cell lines overexpressing difference in accumulation (P
