Identification of breast cancer resistant protein/mitoxantrone resistance/placenta-specific, ATP-binding cassette transporter as a transporter of NB-506 and J-107088, topoisomerase I inhibitors with an indolocarbazole structure

The antitumor drugs NB-506 and J-107088 are potent topoisomerase I inhibito rs with an indolocarbazole structure. To clarify the factors involved in re sistance to these drugs, we established two NB-506-resistant mouse fibrobla st cell lines (LY/NR1and LY/NR2), a human colon carcinoma cell line (HCT116 /NR1), and a lung cancer cell line (PC13/NR1). These cell lines were highly resistant to NB-506 and J-107088, and LY/ NR2 cells showed markedly reduce d accumulation and strong efflux of NB-506, suggesting activation of a drug efflux pump in the resistant cells. To identify the molecules responsible for efflux of NB-506, we compared the gene expressions of the mouse resista nt LY/NR1 cells, LY/NR2 cells, and their parental cells by oligonucleotide microarray, Of 34,020 genes analyzed, we found that an ATP-binding cassette transporter BCRP/MXR/ABCP (BCRP) gene showed the highest increase in the e xpression, 31-fold higher in the LY/NR2-resistant cells than in their paren tal cells. The selective overexpression of this gene was also detected in t he two human resistant cell lines, suggesting the involvement of breast can cer resistant protein (BCRP) in the resistance and afflux of these drugs. F inally, a PC-13 cell line transfected with BCRP expression vector displayed 22- and 17-fold resistance to NP-506 and J-107088 and enhanced efflux acti vity of J-107088. However, the transfectants were not resistant to mitoxant rone or topotecan, the drugs previously thought to be the substrates of BCR P, Thus, our study presents a novel mechanism of drug resistance mediated b y BCRP.