| In current cytotoxic chemotherapy of cancer there is only a narrow therapeutic window of exposure resulting in tumor response and manageable toxicity. Reliable dosing of the anticancer drugs, made possible by a thorough understanding of the factors determining their pharmacological behavior, is thus of utmost importance. Active drug efflux transporters such as P-glycoprotein (P-gp), Multidrug Resistance Protein 1 (MRP1) and the Breast Cancer Resistance Protein (BCRP) confer multidrug resistance to tumor cells, and variously affect the oral drug uptake, tissue distribution, excretion and toxicity of an extensive range of anticancer drugs. Previously developed P-gp, Mrp1 and Bcrp1 knockout mice have been instrumental in elucidating the pharmacological and toxicological significance of these transporters for anticancer drugs. With these models we have explored modalities to interfere with the activity of the transporters using pharmacological inhibitors, in order to optimize the pharmacological behavior of anticancer drugs, especially the oral bio-availability. Ongoing clinical Phase I and II trials testing the insights obtained look promising. Recent studies suggest that MRP2, an apical drug efflux transporter in polarized cells, will have considerable functional overlap with P-gp and BCRP in liver, intestine and kidney concerning drug oral bioavailability and excretion, and with MRP1 in protecting tissues from toxicity of various anticancer drugs. MRP2 transports a range of important anticancer drugs (methotrexate and analogues, anthracyclines, Vinca alkaloids, CPT-11 and its active metabolite SN-38, cisplatinum, therapeutic arsenic derivatives and possibly etoposide), resulting in extensive overlap with the drug substrates of P-gp, BCRP and MRP1. The purpose of this project is to generate Mrp2-deficient mice, and to cross these mice with the available P-gp, Bcrp1 and Mrp1 knockout mouse strains. The relatively low transporter background in the resulting single and combined knockout models will allow us to dissect the contributions of each of the four drug transporters to (limiting) oral drug uptake, tissue distribution, excretion and (protection from) toxicity of a range of anticancer drugs. PLAN OF INVESTIGATION: 1. Generation of constitutive and conditional Mrp2-deficient mice and basic characterization of the physiological and pharmacological properties of these mice. 2. Generation of compound Mrp2/Mrp1, Mrp2/P-gp, Mrp2/Bcrp1 and possibly Mrp2/P-gp/Bcrp1 knockout mice by cross-breeding with the existing knockout strains. Characterization of physiological and pharmacological properties of these strains. Comparative pharmacological analyses of wild-type, single, double and possibly triple knockout strains will allow us to assess the relative importance of each of the drug transporters for (protection from) toxicity, (limiting) oral drug uptake, tissue distribution and excretion of a range of anticancer drugs. Drugs of interest to be studied in the various strains include anthracyclines, Vinca alkaloids, methotrexate, arsenic trioxide, irinotecan/SN-38, and possibly etoposide and mitoxantrone. If suitable MRP2 inhibitors emerge, these can also be tested. 3. We will generate cell lines expressing murine Mrp2 to compare the drug resistance and transport characteristics with those of human MRP2, and monoclonal antibodies against murine Mrp2 to assess the exact Mrp2 tissue distribution, and to validate the Mrp2 knockout. POSSIBLE RESULTS: The insights and tools obtained will allow us to better predict the consequences (both therapeutic opportunities and toxicity risks) of various levels of activity of each of the studied drug transporters for cancer chemotherapy, be it as a consequence of induced or repressed gene expression, intentional or coincidental inhibition of the transporter(s) by co-administered drugs, or pharmacogenetic differences. It may for instance turn out to be desirable to develop suitable in vivo MRP2 inhibitors, e.g. for increasing (reproducibility of) drug oral bioavailability. In the long term, these insights will help in optimally 'tailoring' the drug therapy to the individual needs of each patient. |
