Topotecan is currently approved for relapsed small-cell lung cancer and ovarian cancer. Topotecan's efficacy in the second-line setting and novel mechanism of action suggest broad antitumour activity. We utilised a clinically validated, cell-death, ex vivo assay in human tumour explants to examine the activity profile of topotecan alone and in combination with other antitumour agents. Serial dilutions of topotecan alone and in combination with other cytotoxic agents were applied to biopsy specimens of non-small-cell lung cancer (NSCLC) and breast, colon, and prostate cancers. Dose -response curves were interpolated to provide 50% lethal concentrations (LC 50 ). The degree of synergy (by median effect) and normalised Z-scores (raw scores converted to relative activity distributed around the mean) were then computed. Single-agent activity was observed for topotecan in all four tumour types. In 57 chemotherapy-naive specimens, NSCLC revealed the highest activity, demonstrated by the lowest LC 50 value (0.2670.06 mg ml À1 ; P ¼ 0.002). Overall, previously treated and chemotherapy-naive specimens revealed no significant differences in mean LC 50 's. Synergy was observed for several combinations, including topotecan plus cisplatin in prostate and for topotecan plus 5-fluorouracil in breast cancers. The Z-score analyses conducted suggest activity for previously unexplored drug regimens, including topotecan plus 5-fluorouracil, vinorelbine, and mitomycin-C in NSCLC and breast cancer. Phase II studies are underway to determine the degree to which these ex vivo findings will translate into improved clinical results. Topotecan, a semisynthetic, water-soluble derivative of camptothecin, is an inhibitor of topoisomerase I (Kingsbury et al, 1991). This derivative is more stable, has increased solubility, a shorter half-life, and decreased toxicity compared with its parent compound (Verweij et al, 1993;Burke and Mi, 1994;Rothenberg, 1997). Topoisomerase I is a nuclear enzyme that relieves torsional strain on supercoiled DNA and creates single-strand breaks during DNA replication. Topotecan prevents topoisomerase I from repairing the cleaved DNA, which results in double-stranded DNA breaks and eventually apoptosis. The unique mechanism of action of topotecan and lack of clinical crossresistance with other antineoplastic compounds suggest that topotecan has the potential for broad antitumour activity.The activity of topotecan has been demonstrated in several open-label, randomised phase II and III trials. Currently, topotecan is approved for the treatment of relapsed small-cell lung cancer (SCLC) (Eckardt et