Ex vivo analysis of topotecan: advancing the application of laboratory-based clinical therapeutics

Abstract

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 (0.26+/-0.06 microg ml(-1); P=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.

Figure 1

LC₅₀ values for single-agent topotecan in tumour biopsies taken from untreated and previously treated patients in the master database. Tumour biopsies from patients with breast cancer, colon cancer, NSCLC, or prostate cancer were incubated with various concentrations of topotecan, and the LC₅₀ values were calculated. The numbers in parentheses indicate the number of samples for the indicated drug combination.

Figure 2

Z-scores of topotecan combination regimens in four tumour types. Z-scores of topotecan in biopsies from patients with breast cancer (open), colon cancer (dark grey), NSCLC (black), and prostate cancer (light grey). The numbers in parentheses indicate the number of samples for the indicated drug combination. NSCLC=non-small-cell lung cancer; 5-FU=5-fluorouracil; CDDP=cisplatin; DOX=doxorubicin; GEM=gemcitabine; MITX=mitoxantrone; NAV=vinorelbine; NM=nitrogen mustard; TAX=paclitaxel; L-OHP=oxaliplatin; MMC=mitomycin-C.

Figure 3

Relative in vitro activity of topotecan vs irinotecan. A scatterplot of the 50% lethal concentration values for topotecan and irinotecan in 239 tumour biopsy samples. A correlation coefficient of 0.54 indicates significant correlation between the antitumour activity of topotecan and irinotecan in vitro (P<0.0005).