Cellular uptake and interaction with purified membranes of rebeccamycin derivatives

Rebeccamycin is an antitumor antibiotic possessing a DNA-intercalating indolocarbazole chromophore linked to a glycosyl residue. The carbohydrate moiety of rebeccamycin and related synthetic analogues, such as the potent antitumor drug NB-506 (6-N-formylamino-12,13-dihydro-1, 11-dihydroxy-13-(beta-D-glucopyranosyl)-5H-indolo[2,3-a]pyrrolo- [3,4-c]carbazole-5,7-(6H)-dione), is a key element for both DNA-binding and inhibition of DNA topoisomerase I. In this study, we have investigated the cellular uptake of rebeccamycin derivatives and their interaction with purified membranes. The transport of radiolabeled [3H]dechlorinated rebeccamycin was studied using the human leukemia HL60 and melanoma B16 cell lines as well as two murine leukemia cell lines sensitive (P388) or resistant (P388CPT5) to camptothecin. In all cases, the uptake is rapid but limited to about 6% of the drug molecules. In HL60 cells, the uptake entered a steady-state phase of intracellular accumulation of about 0.26+/-0.05 pmol/10(6) cells, which persisted to at least 90 min. The efflux of exchangeable radiolabeled molecules was relatively weak. Fluorescence studies were performed to compare the interaction of a rebeccamycin derivative and its aglycone with membranes purified from HL60 cells. The glycosylated drug molecules bound to the cell membranes can be extracted upon washing with buffer or by adding an excess of DNA. In contrast, the indolocarbazole drug lacking the carbohydrate domain remains tightly bound to the membranes with very little or no exchange upon the addition of DNA. The membrane transport and binding properties of indolocarbazole drugs related to rebeccamycin are reminiscent to those of other DNA-intercalating antitumor agents. The uptake most likely occurs via a passive diffusion through the plasma membranes and the glycosyl residue of the drug plays an essential role for the translocation of the drug from the membranes to the internal cell components, such as DNA.