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Multidrug resistance (MDR) is the most frequent type of resistance to anticancer chemotherapy.It has been suggested that overexpression of efflux transporters, such as ABCB 1/P-gp/MDR 1, ABCC 1/MRP 1 and ABCG2/BCRP/MXR play a role in MDR.Multiple studies have linked overexpression of MDR-ABC transporters with MDR using various clinical parameters.However, due to the failure of development of ABC transporter inhibitors reversing MDR, ABC transporters are still considered unvalidated therapeutic targets for conquering MDR.The reason for this failure may be inherent toxicity and inadequate trial design.Key to clinical testing of the role of transporters is a relevant test.Correlation of transporter expression and/or polymorphism with activity has not been unambiguously shown.Therefore, we have developed a MultiDrugQuant (MDQ) kit that has been registered for in vitro diagnostic use in the EU.The MDQ kit employs an improved functional assay system, which can measure multidrug resistance activity of the three, clinically most relevant efflux transporters in tumor cells using flow cytometry.Laboratory validation and preliminary clinical data will be shown.The mechanism of ABC mediated MDR is also debated.According to the original hypothesis efflux transporters reduce intracellular level of cytotoxic drugs leading to unfavorable response and/or prognosis.The actual mechanism is likely more complex as ABC transporters have functions other than just effluxing drugs and evaluation transport function itself is complicated by multimodality of therapies.Nevertheless, mapping transporter interaction of cytostatics is important.MDR-ABC transporter interaction profile of cytostatics will be presented and the value of different in vitro approaches discussed.