Synergistic cytotoxicity of pyrazoloacridine with doxorubicin, etoposide, and topotecan in drug-resistant tumor cells.

Pyrazoloacridine (NSC 366140, PD115934, PZA) is a new class of acridine anticancer agents under investigation in Phase II clinical trials in patients with advanced cancers. Although poor responses in patients to the treatment with PZA alone have been observed, this class of agents remains of interest because of its distinct mechanism of action from other topoisomerase poisons. Therefore, the combination of PZA with conventional anticancer agents presents an attractive approach to treat drug-resistant human tumors. In the present study, the cytotoxic effects of PZA combined with doxorubicin, topotecan, and etoposide were determined using paired parental and doxorubicin-resistant human colon carcinoma (SW-620 and SW620/AD-300) and breast cancer cell lines (MCF-7 and MCF-7/TH). Cytotoxicity was measured by soft agar clonogenic assays. Dose effect and combination effects were analyzed by the method of Chou and Talalay. The combination of PZA with doxorubicin, topotecan, and etoposide in fixed ratios demonstrated synergistic cytotoxicity on both SW-620 and SW620/AD-300 cell lines. The combination of PZA with doxorubicin also exhibited synergistic cytotoxicity against both MCF-7 and MCF-7/TH cell lines. The mechanism of synergism appeared independent of topoisomerase I and II inhibition, and interference with protein-DNA complexes. Strategies to define optimal drug combinations are proving to be of significant value when considering potential clinical applications of new and established agents.