Irinotecan (7-ethyl-10-[4-(1-piperidino)-1-piperidino]-carbonyloxycamptothecin;
CPT-11) is a widely used potent
antitumor drug that inhibits mammalian
DNA topoisomerase I (
Topo I); however, overexpression of ABCG2 (BCRP/MXR/ABCP) can confer
cancer cell resistance to
SN-38, the active form of
CPT-11. We have recently demonstrated that plasma membrane vesicles prepared from ABCG2-overexpressing PC-6/SN2-5H cells transported
SN-38 and its
glucuronide conjugate in an
ATP-dependent manner (Nakatomi et al., Biochem Biophys Res Commun 2001;288:827-32). In the present study, we have characterized a total of 14 new
camptothecin (
CPT) analogues with respect to both the inhibition of
Topo I and the substrate specificity of ABCG2. All of the tested
CPT analogues, which have different substitutions at positions 10 and 11, strongly inhibited the
Topo I activity in a cell-free system, as did
SN-38. Their antitumor activities in the SN-38-resistant PC-6/SN2-5H2 cell line greatly varied, however, being correlated with intracellular accumulation levels. We have examined
ATP-dependent transport of those
CPT analogues by using plasma membrane vesicles prepared from both PC-6/SN2-5H2 cells and ABCG2-transfected HEK-293 cells. Based on the substrate specificity of ABCG2 thus evaluated, it is strongly suggested that
CPT analogues with high polarity are good substrates for ABCG2 and are therefore effectively extruded from
cancer cells. In this context, to circumvent ABCG2-associated drug resistance, low-polarity
CPT analogues are considered to be potent lead compounds. The present study provides a practical approach to discover new
CPT-based drugs for the
chemotherapy of drug-resistant human
cancer.