Dynamics of Expression of Drug Transporters: Methods for Appraisal

Cellular drug resistance remains a major concern in cancer therapy and usually results from increased expression of ABC drug transporters. Imatinib mesylate (IM), a competitive inhibitor of BCR/ABL1 tyrosine kinase activity, is the current standard therapy for chronic myeloid leukaemia (CML) which is caused by the BCR/ABL1 gene fusion encoding a constitutively active tyrosine kinase. However, up to 33 % of CML patients do not respond to therapy either initially or due to acquired resistance. Usually, IM resistance is due to the presence of BCR-ABL1 mutations but in many cases resistance is far from being completely understood or from being satisfactorily addressed from a therapeutic standpoint. Although second- and third-generation TKIs (e.g., dasatinib (DA), nilotinib, and bosutinib) were developed to override this phenomenon, resistance remains an unsolved problem. Above all, as more patients are treated with TKIs, more cases of resistance are expected and the discovery of biomarkers of resistance acquires a crucial clinical significance. We established a valuable in vitro experimental system that mimics the acquired resistance in the absence of mutations. It was developed by the continuous exposure of K562, a human CML-derived cell line expressing BCR-ABL gene, to increasing concentrations of IM and DA (over 36 and 24 weeks, respectively) allowing us to obtain several cell lines with different resistance levels, and therefore to evaluate drug transporters' role in the dynamic cellular responses allied with resistance evolution. The development of such cell models is fundamental to understand the role of drug transporters in resistance since the majority of previous studies were performed on cell lines engineered to over-express a single transporter. Drug transporters were overexpressed in the majority of resistant cell lines and cell lines from all levels of resistance had increased expression of more than one drug transporter. However, the transporters that attain higher mRNA overexpression (e.g., ABCB1 and ABCG2) did not substantiate a linear relation with the level of resistance. Also, variation in expression of these genes occurs over time of exposure to the same concentration of IM while maintaining resistance, suggesting that resistance mechanisms could vary dynamically in patients as disease progresses. Indeed, we observed that while responding patients demonstrated stable transporters' expression signatures in consecutive samples, in IM-resistant patients they vary significantly over time, advising caution when comparing single-point samples from responsive and resistant patients.