Initial testing of topotecan by the pediatric preclinical testing program

Abstract

Background: Topotecan is a small molecule DNA topoisomerase I poison, that has been successful in clinical trials against pediatric solid tumors and leukemias. Topotecan was evaluated against the Pediatric Preclinical Testing Program (PPTP) tumor panels as part of a validation process for these preclinical models.

Procedures: In vivo three measures of antitumor activity were used: (1) an objective response measure modeled after the clinical setting; (2) a treated to control (T/C) tumor volume measure; and (3) a time to event (fourfold increase in tumor volume for solid tumor models, or > or =25% human CD45+ cells in the peripheral blood for acute lymphoblastic leukemia, ALL models) measure based on the median event-free survival (EFS) of treated and control animals for each xenograft.

Results: Topotecan inhibited cell growth in vitro with IC(50) values between 0.71 and 489 nM. Topotecan significantly increased EFS in 32 of 37 (87%) solid tumor xenografts and in all 8 of the ALL xenografts. Seventy-five percent of solid tumors met EFS T/C activity criteria for intermediate (n = 17) or high activity (n = 7). Objective responses were noted in eight solid tumor xenografts (Wilms, rhabdomyosarcoma, Ewing sarcoma, neuroblastoma). Among the six neuroblastomas, three achieved a PR. For the ALL panel, two maintained CRs, three CRs, and two PRs were observed.

Conclusions: Topotecan demonstrated broad activity in vitro and in vivo against both the solid tumor and ALL panels, with significant tumor growth delay generated in all the panels. These results further demonstrate the validity of the PPTP panel for preclinical testing of new drugs.

Figure 1

Topotecan in vitro activity. Figure 1A is a dot plot chart that illustrates the relative sensitivity of the cell lines using the IC₅₀ values displayed by histology. The black line indicates the median IC₅₀ (9.13 nM) for the panel. Figure 1B illustrates typical growth inhibition curves for Rh30 and Kasumi-1. Error bars represent standard deviations for each concentration tested.

Figure 2

Topotecan in vivo objective response activity. Left: The colored ‘heat map’ depicts group response scores. A high level of activity is indicated by a score of 6 or more, intermediate activity by a score of ≥ 2 but < 6, and low activity by a score of < 2. Right: representation of tumor sensitivity based on the difference of individual tumor lines from the midpoint response (stable disease). Bars to the right of the median represent lines that are more sensitive (objective responses), and to the left are tumor models that are less sensitive (non-objective response). Red bars indicate lines with a significant difference in EFS distribution between treatment and control groups, while blue bars indicate lines for which the EFS distributions were not significantly different.

Figure 3

Topotecan activity against individual solid tumor xenografts. Kaplan-Meier curves for EFS, median relative tumor volume graphs, and individual tumor volume graphs are shown for selected lines: (A) KT-10 (B) Rh28, and (C) NB-1643. Controls (gray lines); Treated (black lines).

Figure 4

Topotecan activity against individual ALL xenografts. Kaplan-Meier curves for EFS and graphs of median and individual percentages of hCD45 cells, are shown for selected lines: (A) ALL-2, and (B) ALL-16. Controls (gray lines); Treated (black lines).