9‐nitro‐camptothecin delays growth of U‐937 leukemia tumors in nude mice and is cytotoxic or cytostatic for human myelomonocytic leukemia lines in vitro

The camptothecin derivatives 9-nitro-camptothecin (9NC) and 9-amino-camptothecin (9AC) inhibit similarly growth of HL-60, KG-1, and U-937 cells in vitro, whereas growth of THP-1 cells is inhibited by 9AC, but not by 9NC. Growth inhibition is accompanied by enlargement of cells which contain one (HL-60, THP-1) or more (KG-1, U-937) nuclei. Flow cytometry studies showed that 9NC-treated HL-60 and U-937 cells accumulate in the S and G₂ phases of the cell cycle; then they die by apoptosis, with the HL-60 cells being more sensitive than U-937 cells to 9NC. In contrast, 9NC-treated KG-1 and THP-1 cells accumulate in S and G₂ phases, but resist death by apoptosis. Of the cell lines tested, only U-937 cells xenografted in nude mice generated subcutaneous myeloid tumors, which exhibited a delayed growth in the presence of 9NC. Further, 9NC-treated advanced U-937 tumors regressed temporarily, indicating that U-937 cells consist of 9NC-sensitive and 9NC-resistant populations.