While anthracyclines and taxanes are highly effective drugs used in the treatment of breast and other cancers, tumour drug resistance mechanisms limit their clinical effectiveness. Tumours can either be intrinsically resistant to chemotherapy agents, or acquire resistance upon exposure to a previous round of chemotherapy [
1]. Drug resistance, whether intrinsic or acquired, is believed to cause treatment failure in over 90% of patients with metastatic cancer [
2]. Thus, it is critical that clinically relevant mechanisms for drug resistance be elucidated in order to identify approaches to circumvent drug resistance. A wide variety of proteins or protein mutations have been found to play a role in drug resistance
in vitro, but their relevance clinically remains to be established [
2‐
4].
To date, six drug transporters have been shown to play a role in multidrug resistance in tumour cells
in vitro. These include ABCB1 (P-glycoprotein), ABCC1 (MRP1), ABCC2 (MRP2), ABCC4 (MRP4), ABCG2 (BCRP), and the lung resistance protein (LRP). Of these, three are overexpressed in the large majority of tumour cell lines that have been successfully selected for resistance to anthracyclines and taxanes. These include ABCB1, ABCC1, and ABCG2, and all play a role in reducing the ability of tumour cells to accumulate specific chemotherapy drugs [
5,
6]. Although these transporters are unique in their sequences, there is some overlap in the drugs that can be transported by each protein. ABCC1 confers resistance to doxorubicin, daunorubicin, vincristine, etoposide, epirubicin, chlorambucil, methotrexate, melphalan and paclitaxel [
5,
7‐
9]. ABCC2 has been shown to be associated with resistance to doxorubicin, etoposide, methotrexate, irinotecan (SN-38), vincristine, vinblastine, camptothecin (CPT-11) [
9], paclitaxel, docetaxel, etoposide, mitoxantrone [
10] and cisplatin [
11]. ABCC4 is believed to confer resistance to the camptothecins (SN-38, rubitecan, irinotecan), cyclophosphamide [
12], topotecan [
13], methotrexate, and nucleoside analogues [
14]. Numerous studies have been conducted on ABCB1 and its ability to transport chemotherapy drugs. It has been shown to directly transport vinblastine, paclitaxel, docetaxel, daunorubicin, doxorubicin, epirubicin, etoposide, teniposide, and mitoxantrone [
9,
15‐
18]. The final ABC transporter (ABCG2) confers resistance to mitoxantrone, doxorubicin, epirubicin, daunorubicin, vinca alkaloids, paclitaxel, cisplatin, etoposide, teniposide, irinotecan, topotecan, and camptothecin [
9,
19‐
24]. Although not an ABC transporter, lung resistance-related protein (LRP) is a human major vault protein whose expression appears to correlate with resistance to doxorubicin, mitoxantrone, methotrexate, etoposide, vincristine, and cisplatin [reviewed in [
25]]. While the exact cellular function of the major vault proteins (MVP) remains to be elucidated, the majority of these proteins have been shown to interact with cytoskeletal elements or within the nucleus—in particular nucleoli, the nuclear membrane and/or the nuclear pore complex [
25‐
27]. Elevated levels of MVPs have been observed in some drug-resistant cell lines. While there is little direct evidence that the proteins can directly transport chemotherapy drugs, it has been shown that overexpression of LRP alters the subcellular distribution of doxorubicin, such that the drug localizes to cytoplasmic organelles rather than to DNA within the nucleus [
28].
Despite the overwhelming evidence that drug transporters can confer resistance to a variety of chemotherapy agents in tumour cells
in vitro, attempts to use their expression as definitive biomarkers for the identification of drug resistant tumours have met with mixed success [
29‐
32]. In addition, administration of drug transporter inhibitors (in particular for ABCB1) to prevent or reverse drug resistance in cancer patients has largely been unsuccessful, in part due to the toxicity of these compounds [
33,
34]. Given these findings, it is likely that additional mechanisms may play an equal or much greater role in clinical resistance to chemotherapy drugs. Inhibition of these targets may prove more fruitful in combating drug resistance in patients. To rigorously assess the temporal and causal relationships between the acquisition of drug resistance and the induction of drug transporters and drug accumulation defects
in vitro, we selected MCF-7 breast tumour cells for survival in increasing concentrations of paclitaxel, docetaxel, doxorubicin, or epirubicin. We then examined cells during selection for their expression of various drug transporters, their sensitivity to various chemotherapy agents, their ability to uptake drugs, and their sensitivity to a pan-ABC drug transporter inhibitor. Our findings suggest that changes in cellular drug accumulation do temporally correlate with the acquisition of drug resistance at clinically relevant drug doses. However, the onset of drug resistance is not always correlated with the induction of specific drug transporters. Moreover, inhibition of drug transporter function and/or restoration of drug accumulation has only limited to no ability to restore sensitivity to chemotherapy agents. Additional mechanisms which are temporally and functionally correlated with the acquisition of drug resistance are discussed.