We have synthesized a promising class of bis-naphthalimide anti-tumor agents. A representative compound in this series, XB596, exhibits potent in vitro growth inhibitory activity against several human and murine leukemic and solid tumor lines in culture, with IC50 values ranging from 7.2 to 147.5 nM. XB596 was almost as equally growth inhibitory against three doxorubicin-resistant cell lines compared with their parental lines. Using a human tumor colony-forming assay, XB596 demonstrated cytocidal activity against fresh human tumors taken directly from patients, with 23 of 25 evaluable tumors responding to a continuous exposure of 1 microgram/ml of XB596. When L1210 cells were incubated with XB596 for 1 h, the incorporation of uridine and thymidine into RNA and DNA, respectively, was inhibited with IC50 values of 0.14 microM. DNA single-strand breaks, but not double-strand breaks, were detected in XB596-treated L1210 cells. XB596 bound to DNA with guanine-cytosine sequence selectivity as shown by an indirect ethidium bromide displacement assay. XB596 was shown to interact with DNA by a spectrophotometric titration assay, with an estimated binding constant of 4.7 +/- 2.2 +/- 10(6) M-1. XB596 unwound supercoiled DNA as measured by agarose gel electrophoresis. These data are consistent with XB596 being a DNA intercalator. In vivo, XB596 demonstrated good anti-tumor activity against two human solid tumors (DLD-2 colon adenocarcinoma and MX-1 mammary carcinoma) xenografted in nude mice, but has not demonstrated anti-leukemic activity. In summary, XB596 is a pre-clinical anti-cancer agent which interacts with DNA and demonstrates good in vivo anti-tumor activity against human solid tumor xenografts.