Chemotherapeutics are the most effective treatment for metastatic tumours. However, the ability of cancer cells to become simultaneously resistant to different drugs--a trait known as multidrug resistance--remains a significant impediment to successful chemotherapy. Three decades of multidrug-resistance research have identified a myriad of ways in which cancer cells can elude chemotherapy, and it has become apparent that resistance exists against every effective drug, even our newest agents. Therefore, the ability to predict and circumvent drug resistance is likely to improve chemotherapy.

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Multidrug resistance in cancer: role of ATP–dependent transporters

Resistance to chemotherapy and molecularly targeted therapies is a major problem facing current cancer research. The mechanisms of resistance to 'classical' cytotoxic chemotherapeutics and to therapies that are designed to be selective for specific molecular targets share many features, such as alterations in the drug target, activation of prosurvival pathways and ineffective induction of cell death. With the increasing arsenal of anticancer agents, improving preclinical models and the advent of powerful high-throughput screening techniques, there are now unprecedented opportunities to understand and overcome drug resistance through the clinical assessment of rational therapeutic drug combinations and the use of predictive biomarkers to enable patient stratification.

Cancer drug resistance: an evolving paradigm

The contribution of tumorigenic stem cells to haematopoietic cancers has been established for some time, and cells possessing stem-cell properties have been described in several solid tumours. Although chemotherapy kills most cells in a tumour, it is believed to leave tumour stem cells behind, which might be an important mechanism of resistance. For example, the ATP-binding cassette (ABC) drug transporters have been shown to protect cancer stem cells from chemotherapeutic agents. Gaining a better insight into the mechanisms of stem-cell resistance to chemotherapy might therefore lead to new therapeutic targets and better anticancer strategies.

/pdf/tumour-stem-cells-and-drug-resistance-210hrtgy2n.pdf

Tumour stem cells and drug resistance

Effective treatment of metastatic cancers usually requires the use of toxic chemotherapy. In most cases, multiple drugs are used, as resistance to single agents occurs almost universally. For this reason, elucidation of mechanisms that confer simultaneous resistance to different drugs with different targets and chemical structures - multidrug resistance - has been a major goal of cancer biologists during the past 35 years. Here, we review the most common of these mechanisms, one that relies on drug efflux from cancer cells mediated by ATP-binding cassette (ABC) transporters. We describe various approaches to combating multidrug-resistant cancer, including the development of drugs that engage, evade or exploit efflux by ABC transporters.

Targeting multidrug resistance in cancer

Membrane transporters can be major determinants of the pharmacokinetic, safety and efficacy profiles of drugs. This presents several key questions for drug development, including which transporters are clinically important in drug absorption and disposition, and which in vitro methods are suitable for studying drug interactions with these transporters. In addition, what criteria should trigger follow-up clinical studies, and which clinical studies should be conducted if needed. In this article, we provide the recommendations of the International Transporter Consortium on these issues, and present decision trees that are intended to help guide clinical studies on the currently recognized most important drug transporter interactions. The recommendations are generally intended to support clinical development and filing of a new drug application. Overall, it is advised that the timing of transporter investigations should be driven by efficacy, safety and clinical trial enrolment questions (for example, exclusion and inclusion criteria), as well as a need for further understanding of the absorption, distribution, metabolism and excretion properties of the drug molecule, and information required for drug labelling.

https://www.njacs.org/wp-content/docs/2010-10-DrugMet-R-Evers.pdf

Membrane transporters in drug development.

Purpose: Flavopiridol is a cyclin-dependent kinase inhibitor currently undergoing human clinical trials. As clinical development is pursued, it becomes important to evaluate resistance mechanisms to flavopiridol. To elucidate the contribution of breast cancer resistance protein (BCRP) to cellular resistance to flavopiridol in acute myeloid leukemia, we studied the relationship between cellular resistance to flavopiridol and mRNA expression of BCRP or P-glycoprotein (P-gp, product of MDR1 gene) in blast cells from adult patients with acute leukemia. Experimental Design: Twenty-one blast cell samples from 20 patients were studied. The expression of BCRP, P-gp, or β-actin mRNA was determined by real-time reverse transcription-PCR, using fluorescent hybridization probes to evaluate codon 482, a known site of mutations in BCRP mRNA. In vitro cell viability and apoptosis were examined after 24 h exposure to flavopiridol. Results: BCRP mRNA expression varied over a 200-fold range. In the blast cell samples with BCRP mRNA expression > 10,000 copies/pg β-actin ( n = 9), BCRP mRNA correlated proportionally with cell viability in the presence of 250 nm flavopiridol ( r = 0.86, P = 0.003) and with apoptosis induced by flavopiridol ( r = 0.71, P = 0.031). In contrast, MDR1 mRNA expression did not correlate with either flavopiridol cytotoxicity or induction of apoptosis. Melting point analysis of the hybridization probes determined that all 21 patient samples had arginine at codon 482 of BCRP mRNA, the wild-type form. Conclusions: These results suggest that unlike P-gp, BCRP may play a role in leukemia cellular resistance to flavopiridol. No mutations at codon 482 were observed in BCRP mRNA in this group of patients.

https://clincancerres.aacrjournals.org/content/clincanres/9/9/3320.full.pdf

Quantitative Analysis of Breast Cancer Resistance Protein and Cellular Resistance to Flavopiridol in Acute Leukemia Patients

We have characterised an etoposide-resistant subline of the small-cell lung cancer cell line, UMCC-1, derived at our centre. Subline UMCC-1/VP was developed by culturing the parent line in increasing concentrations of etoposide over 16 months. UMCC-1/VP is 20-fold resistant to etoposide by MTT assays, relative to the parent line, and is cross-resistant to doxorubicin, vincristine and actinomycin D, but not to taxol, cisplatin, melphalan, thiotepa or idarubicin. Topoisomerase II immunoblotting demonstrates a 50% reduction of the protein in the resistant subline. The UMCC-1/VP subline demonstrates a marked decrease in the accumulation of [3H]etoposide relative to the parent line, as well as a modest reduction in the accumulation of daunorubicin. Reverse transcription-polymerase chain reaction assays demonstrate no detectable mdr1 expression but marked expression of the multidrug resistance-associated protein (MRP) gene in the resistant subline. Northern blotting with an MRP cDNA probe confirms marked overexpression of the MRP gene only in the UMCC-1/VP subline. Western blotting with antisera against MRP peptide confirms a 195 kDa protein band in the UMCC-1/VP subline. Southern blotting experiments demonstrate a 10-fold amplification of the MRP gene in the resistant subline. Depletion of glutathione with buthionine sulphoximine sensitised UMCC-1/VP cells to daunorubicin and etoposide. Our studies indicate that MRP gene expression may be induced by etoposide and may lead to reduced accumulation of the drug.

An etoposide-resistant lung cancer subline overexpresses the multidrug resistance-associated protein.

The multidrug resistance phenotype of human breast carcinoma MCF-7/AdrVp cells is characterized by overexpression of a 95-kilodalton membrane glycoprotein (p95), accompanied by a marked reduction in intracellular anthracycline accumulation, without overexpression of P-glycoprotein or the multidrug resistance protein. We discovered that the mRNA of the H19 gene is overexpressed in MCF-7/AdrVp cells relative to parental MCF-7 cells or drug-sensitive MCF-7/AdrVp revertant cells. H19 is an imprinted gene with an important role in fetal differentiation, as well as a postulated function as a tumor suppressor gene. Another p95-overexpressing multidrug-resistant cell line, human lung carcinoma NCI-H1688, also displays high levels of H19 mRNA. In contrast, several multidrug-resistant cell lines that overexpress P-glycoprotein or the multidrug resistance protein do not have higher levels of H19 mRNA than their drug-sensitive counterparts. This is the first report of H19 gene overexpression accompanying any form of drug resistance. The association of H19 and p95 gene expression in drug resistance warrants further study.

https://cancerres.aacrjournals.org/content/canres/56/13/2904.full.pdf

H19 Gene Overexpression in Atypical Multidrug-resistant Cells Associated with Expression of a 95-Kilodalton Membrane Glycoprotein

Human breast carcinoma MCF-7/AdrVp cells display a novel multidrug resistance phenotype that is characterized by the overexpression of a 95-kDa membrane glycoprotein (p95) and by marked reduction in intracellular anthracycline accumulation, without overexpression of P-glycoprotein or the multidrug resistance protein MRP. p95 is also highly expressed in multidrug-resistant NCI-H1688 cells derived from a human small cell lung carcinoma. Deglycoslyated p95 from NCI-H1688 cells was isolated by two-dimensional gel electrophoresis and then digested with trypsin. The tryptic peptides were analyzed by mass spectrometry and microsequencing. These analyses identified p95 to be identical to NCA-90, the nonspecific cross-reacting antigen related to the carcinoembryonic antigen (CEA). Further confirmation that p95 is indeed NCA-90 was obtained by Northern and Western blot studies using probes or antibodies specific for p95, NCA-90, or CEA family members. Western blot studies also revealed that CEA itself is overexpressed in MCF-7/AdrVp cells compared to parental MCF-7/W cells. The enforced expression of NCA-90 protein in HeLa cells stably transfected with NCA-90 cDNA did not result in increased resistance of the transfected cells to daunorubicin or a decrease in daunorubicin accumulation in the transfected cells compared to cells transfected only with the expression vector. However, a recent report by H. Kawaharata et al. (Int. J. Cancer, 72: 377-382, 1997) of diminished accumulation, retention, and cytotoxicity of doxorubicin in EJNIH3T3 cells in which enforced expression of CEA was accomplished leaves open the possibility that the overexpression of CEA, possibly in combination with that of NCA-90, could account at least in part for the drug resistant phenotype displayed by MCF-7/AdrVp cells.

The 95-kilodalton membrane glycoprotein overexpressed in novel multidrug-resistant breast cancer cells is NCA, the nonspecific cross-reacting antigen of carcinoembryonic antigen.

Background: Human cancer cell lines grown in the presence of the cytotoxic agent mitoxantrone frequently develop resistance associated with a reduction in intracellular drug accumulation without increased expression of the known drug resistance transporters P-glycoprotein and multidrug resistance protein (also known as multidrug resistanceassociated protein). Breast cancer resistance protein (BCRP) is a recently described adenosine triphosphate-binding cassette transporter associated with resistance to mitoxantrone and anthracyclines. This study was undertaken to test the prevalence of BCRP overexpression in cell lines selected for growth in the presence of mitoxantrone. Methods: Total cellular RNA or poly A + RNA and genomic DNA were isolated from parental and drug-selected cell lines. Expression of BCRP messenger RNA (mRNA) and amplification of the BCRP gene were analyzed by northern and Southern blot hybridization, respectively. Results: A variety of drugresistant human cancer cell lines derived by selection with mitoxantrone markedly overexpressed BCRP mRNA; these cell lines included sublines of human breast carcinoma (MCF-7), colon carcinoma (S1 and HT29), gastric carcinoma (EPG85-257), fibrosarcoma (EPF86-079), and myeloma (8226) origins. Analysis of genomic DNA from BCRPoverexpressing MCF-7/MX cells demonstrated that the BCRP gene was also amplified in these cells. Conclusions: Overexpression of BCRP mRNA is frequently observed in multidrug-resistant cell lines selected with mitoxantrone, suggesting that BCRP is likely to be a major cellular defense mechanism elicited in response to exposure to this drug. It is likely that BCRP is the putative “mitoxantrone transporter” hypothesized to be present in these cell lines. [J Natl Cancer Inst 1999;91:429‐33]

Weidong Yang

Papers

Quantitative Analysis of Breast Cancer Resistance Protein and Cellular Resistance to Flavopiridol in Acute Leukemia Patients

An etoposide-resistant lung cancer subline overexpresses the multidrug resistance-associated protein.

H19 Gene Overexpression in Atypical Multidrug-resistant Cells Associated with Expression of a 95-Kilodalton Membrane Glycoprotein

The 95-kilodalton membrane glycoprotein overexpressed in novel multidrug-resistant breast cancer cells is NCA, the nonspecific cross-reacting antigen of carcinoembryonic antigen.

ACCELERATED DISCOVERY Atypical Multidrug Resistance: Breast Cancer Resistance Protein Messenger RNA Expression in Mitoxantrone- Selected Cell Lines