Transport of 7-ethyl-10-hydroxycamptothecin (SN-38) by breast cancer resistance protein ABCG2 in human lung cancer cells

Overexpression of breast cancer resistance protein (BCRP) ABCG2 reportedly confers cancer cell resistance to camptothecin-based anticancer drugs, such as topotecan and 7-ethyl-10-hydroxycamptothecin (SN-38: the active metabol ite of irinotecan). We have recently shown that SN-38-selected PC-6/SN2-5H human lung carcinoma cells overexpressed BCRP with the reduced intracellula r accumulation of SN-38 and SN-38-glucuronide (S. Kawabata et al, Biochem B iophys. Res. Commun. 280, 1216-1223, 2001). In the present study, we have e xamined whether BCRP transports SN-38 and/or SN-38-glucuronide in vitro, by using plasma membrane vesicles from the parental PC-6 and resistant PC-6/S N2-5H cells, where SN-38 and SN-38-glucuronide accumulation in membrane ves icles was measured by HPLC. Both SN-38 and SN-38-glucuronide were ATP-depen dently transported into membrane vesicles prepared from PC-6/SN2-5H cells, whereas no transport activity was observed in membrane vesicles from PC-6 c ells. The kinetic parameters of the transport observed in PC-6/SN2-5H vesic les were K-m = 4.0 muM, V-max = 714 pmol/mg/min for SN-38 and K-m = 26 muM, V-max = 833 pmol/mg/min for SN-38-glucuronide. These findings suggest that BCRP expressed in PC-6/SN2-5H cells transports both SN-38 and SN-38-glucur onide with a higher affinity toward SN-38. (C) 2001 Academic Press.