Bcrp1 mediates apically directed drug transport, appears to reduce drug bioavailability, and protects fetuses against drugs. We propose that strategic application of BCRP inhibitors may thus lead to more effective oral chemotherapy with topotecan or other BCRP substrate drugs.

Breast cancer resistance protein (BCRP/MXR/ABCP/ABCG2; hereafter ABCG2) is a member of the ATP-binding-cassette family of transporters that causes multi-drug resistance to various anticancer drugs. The expression of ABCG2 in human tumours and its potential involvement in clinical drug resistance are unknown. Recently, two monoclonal antibodies against human ABCG2 were produced, BXP-34 and BXP-21. This study describes an immunohistochemical method using BXP-21 to study ABCG2 expression in formalin-fixed, paraffin-embedded tissues. No staining was seen using BXP-34 with the same protocols. The expression of ABCG2 was then investigated in a panel of 150 untreated human solid tumours comprising 21 tumour types. Overall, ABCG2 expression was frequent. Specificity of immunohistochemistry was confirmed by the detection of a 72 kD band in western blotting. ABCG2 expression was seen in all tumour types, but it seemed more frequent in adenocarcinomas of the digestive tract, endometrium, and lung, and melanoma. Positive tumours showed membranous and cytoplasmic staining. In certain adenocarcinomas, prominent membranous staining was seen. Endothelial cells frequently displayed moderate to strong staining. ABCG2 is widely present in untreated human solid tumours and may represent a clinically relevant mechanism of drug resistance. Future studies in specific tumour types are needed to ascertain its clinical relevance. BXP-21 and the immunohistochemical protocol described here will be of value in these investigations.

No abstract

The multidrug resistance protein BCRP (breast cancer resistance protein) is a member of the ATP-binding cassette family of drug transporters. Overexpression of BCRP caused by exposure of cells to mitoxantrone (MX) or doxorubicin/verapamil resulted in a resistance pattern that is different from what is generally seen in the case of P-glycoprotein and MRP1 overexpression.Recently, the BCRP gene has been described in ovarian, breast, colon, and gastric cancer and fibrosarcoma cell lines. Our human tumor cells T8 and MX3, derived from the ovarian cancer cell line IGROV1 by stepwise increased exposure to topotecan and MX, are resistant to topotecan, CPT11, SN38, and 9-aminocamptothecin as well as MX. Increased energy-dependent efflux of affected drugs was noted. BCRP is a very efficient transporter of topotecan. Our recent studies, using the monoclonal antibody (mAb) BXP34, revealed that BCRP is located in the plasma membrane of the T8 and MX3 cell lines. Preliminary results of staining of human tumor cells showed low or absent levels of BCRP in a panel of solid tumors and acute myeloid leukemia cells.

The reductive activation of mitosene compounds was studied with cyclic voltammetry and HPLC analysis. Reduction of mitosenes, possessing good leaving groups at C-1 and C-10, was shown to result in loss of these groups at pH 7.0 and pH 6.0. The loss of leaving groups from mitosenes occurred faster at lower pH. Mitosenes without good leaving groups were found to be stable upon reduction. In the presence of acetoxy groups at C-1 and C-10, the C-10 site is the most reactive site upon reductive activation. This is opposite to the case of mitomycin C, where the C-1 site is the first to react upon reduction. At pH 6.0 without reduction, acid degradation also caused the loss of leaving groups of mitosenes, although at a very slow rate. In contrast to reductive activation, upon acid degradation of a diacetoxymitosene the C-1 group appeared to be lost faster. Electrochemical as well as dithionite reduction of a bifunctional (diacetoxy) mitosene compound in the presence of calf thymus DNA at pH 5.5 resulted in the formation of DNA interstrand cross-links. Depending on activation method, this diacetoxymitosene was at least as efficient in DNA cross-linking as mitomycin C under comparable conditions.