Relationship between tumor extracellular fluid exposure to topotecan and tumor response in human neuroblastoma xenograft and cell lines

Purpose: We have reported a 6-fold difference in the topotecan (TPT) lactone systemic exposure achieving a complete response in the human neuroblastoma xenografts NB-1691 and NB-1643. However, the relationship between tumor extracellular fluid (ECF) exposure to TPT and the antitumor activity in xenograft and in vitro models has not been established. Methods: TPT was given i.v. to mice bearing NB-1691 and NB-1643 tumors. Prior to dosing, microdialysis probes were placed in tumors of mice bearing NB-1691 and NB-1643 tumors. Plasma and tumor ECF concentrations of TPT lactone were assayed by high performance liquid chromatography. The inhibitory concentration (IC₅₀) was determined for NB-1691 and NB-1643 cell lines in vitro. Results: The TPT AUC(ECF) values determined for NB-1691 (n = 10) and NB-1643 (n = 11) were 7.3 ± 0.84 and 25.6 ± 0.76 ng h ml^-1, respectively (P < 0.05). TPT tumor ECF penetration in NB-1691 and NB-1643 was 0.04 ± 0.04 and 0.15 ± 0.11 (P < 0.05), respectively. The IC₅₀ values recorded after 6 h of TPT exposure daily for 5 consecutive days for NB-1691 and NB-1643 were 2.7 ± 1.1 and 0.53 ± 0.19 ng/ml, respectively (P < 0.05). Conclusions: NB- 1643 was more sensitive in vitro than NB-1691, and at similar plasma TPT exposures, NB-1643 had a greater degree of TPT tumor ECF exposure and penetration as compared with NB-1691. Potential factors affecting tumor TPT ECF disposition include tumor vascularity, capillary permeability, and interstitial pressure. The clinical importance of this study is underscored by the need to select anticancer agents with a high capacity for tumor penetration and to optimize drug administration to increase tumor penetration.