Testing of the topoisomerase 1 inhibitor Genz-644282 by the pediatric preclinical testing program

Abstract

Background: Genz-644282 is a novel non-camptothecin topoisomerase I poison that is in clinical development.

Procedures: Genz-644282 was tested against the PPTP in vitro panel (0.1 nM to 1 µM), and in vivo using three times per week × 2 schedule repeated at day 21 at its maximum tolerated dose (MTD) of 4 mg/kg. Subsequently Genz-644282 was tested at 4, 3, 2, and 1 mg/kg in 3 models to assess the dose-response relationship. mRNA gene signatures predictive for Genz-644282 response in vitro were applied to select 15 tumor models that were evaluated prospectively.

Results: In vitro, Genz-644282 demonstrated potent cytotoxic activity with a median IC(50) of 1.2 nM (range 0.2-21.9 nM). In vivo, Genz-644282 at its MTD (4 mg/kg) induced maintained complete responses (MCR) in 6/6 evaluable solid tumor models. At 2 mg/kg Genz-644282 induced CR or MCR in 3/3 tumor models relatively insensitive to topotecan, but there were no objective responses at 1 mg/kg. Further testing at 2 mg/kg showed that Genz-644282 induced objective regressions in 7 of 17 (41%) models. There was a significant correlation between predictive response scores based on Affymetrix U133Plus2 baseline tumor expression profiles and the observed in vivo responses to Genz-644282.

Conclusions: Genz-644282 was highly active within a narrow dose range (2-4 mg/kg), typical of other topoisomerase I poisons. As with other topoisomerase I poisons, how accurately these data will translate to clinical activity will depend upon the drug exposures that can be achieved in children treated with this agent.

Figure 1

Genz644282 in vitro activity. Top panel: The median IC₅₀ ratio graph shows the relative IC₅₀ values for the cell lines of the PPTP panel. Each bar represents the ratio of the panel IC₅₀ to the IC₅₀ value of the indicated cell line. Bars to the right represent cell lines with higher sensitivity, while bars to the left indicate cell lines with lesser sensitivity. The median relative IC₅₀ was 1.2 nM (range 0.2–21.9 nM). Bottom panels: Representative dose response curves for the most sensitive (COG-LL-317) and least sensitive (Rh18) cell lines.

Figure 2

Genz644282 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: (KT-11, KT-13 (Wilms tumors), Rh28 (alveolar RMS); SK-NEP (Ewing sarcoma of kidney). Mice received Genz644282 administered 3 times weekly (M-W-F) for 2 weeks. The cycle was repeated at day 21.

Figure 3

Dose response data for Genz644282. Mice received Genz644282 administered 3 times weekly for 2 weeks only at 1, 2, 3 mg/kg. Graphs show growth curves for individual tumors.

Figure 4

Prediction of activity from expression profiling. The activity of Genz644282 was evaluated against a panel of xenografts to test the predictions derived from expression profiling. Kaplan-Meier curves for EFS, median relative tumor volume graphs, and individual tumor volume graphs are shown for osteosarcoma lines: OS-1, OS-9, OS-17, OS-31. Mice received Genz644282 administered 3 times weekly for 2 weeks.

Figure 5

Correlation of the predicted response of xenografts to the measured response. X-axis represents classes of xenografts based on measured response, Y-axis the predicted probability of Genz644282 sensitivity. (Mann-Whitney U test p<0.02)