Cellular determinants of resistance to indolocarbazole analogue 6-N-formylamino-12,13-dihydro-1,11-dihydroxy-13(beta-D-glucopyranosyl)- 5H-indolo[2,3-alpha]pyrrolo[3,4-c]carbazole-5,7(6H)-dione (NB-506), a novel potent topoisomerase I inhibitor, in multidrug-resistant human tumor cells.