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.

Probenecid alters topotecan systemic and renal disposition by inhibiting renal tubular secretion.

Topotecan is primarily eliminated by the kidneys, with 60 to 70% of the dose recovered as topotecan total in the urine. To elucidate the mechanisms of topotecan renal clearance, we evaluated the effect of probenecid on topotecan renal and systemic disposition in mice. Topotecan lactone or hydroxy acid (1.25 mg/kg i.v.) was administered alone or in combination with probenecid (600 or 1,200 mg/kg) given by oral gavage 30 min before and 3 hr after topotecan. Serial blood samples (three mice per time point) and urine samples (five mice per treatment arm) were collected during a 6-hr period. Compared with topotecan alone, coadministration of topotecan lactone or hydroxy acid with probenecid (600 mg/kg) decreased topotecan lactone, total, and hydroxy acid systemic clearance, and total renal clearance. The predominant effect of probenecid was to increase hydroxy acid area under the plasma concentration time curve after administration of topotecan lactone (238.8 vs. 109.9 ng.hr/ml alone, P < .05), or hydroxy acid (1297.2 vs. 355.0 ng.hr/ml alone, P < .05). By inhibiting renal tubular secretion, probenecid decreased renal and systemic clearance which led to an increase in topotecan systemic exposure. These data suggest that probenecid primarily inhibited secretion of the anionic hydroxy acid form, and by direct or indirect mechanisms increased topotecan lactone systemic exposure. Topotecan elimination through renal tubular secretion may have clinical relevance for the use of topotecan in patients with altered renal function.