Initial testing of the MDM2 inhibitor RG7112 by the Pediatric Preclinical Testing Program

Abstract

Background: RG7112 is a selective inhibitor of p53-MDM2 binding that frees p53 from negative control, activating the p53 pathway in cancer cells leading to cell cycle arrest and apoptosis. RG7112 was selected for evaluation by the Pediatric Preclinical Testing Program (PPTP) due to the relatively low incidence of p53 mutations in pediatric cancers compared with adult malignancies.

Procedures: RG7112 and its inactive enantiomer RG7112i were evaluated against the 23 cell lines of the PPTP in vitro panel using 96 hours exposure (1 nM to 10 µM). It was tested against the PPTP in vivo panel focusing on p53 wild-type (WT) xenografts at a dose of 100 mg/kg daily for 14 days followed by 4 weeks of observation. Response outcomes were related to MDM2 and p53 expression datasets (http://pptp.nchresearch.org/data.html).

Results: RG7112 demonstrated cytotoxic activity with a lower median IC(50) for p53 WT versus p53 mutant cell lines (approximately 0.4 µM vs. >10 µM, respectively). RG7112 induced tumor growth inhibition meeting criteria for intermediate activity (EFS T/C > 2) in 10 of 26 (38%) solid tumor xenografts. Objective responses included medulloblastoma, alveolar rhabdomyosarcoma, Wilms, rhabdoid and Ewing sarcoma xenografts. For the ALL panel, there was one partial response, five complete responses and one maintained complete response. The ALL xenografts expressed the highest levels of p53 among the PPTP panels.

Conclusions: RG7112 induced tumor regressions in solid tumors from different histotype panels, and exhibited consistent high-level activity against ALL xenografts. This high level of activity supports prioritization of RG7112 for further evaluation.

Figure 1

RG7112 in vitro activity. A) The Relative in/out values (as percentage) for RG7112 are shown for the in vitro cell panel. Values of −100%, 0%, and 100% are consistent with complete cell kill, cytostasis, and no treatment effect, respectively. Solid black bars indicate cell lines with wild-type p53 and hatched bars indicate cell lines with mutant p53. B) Ratios of rIC₅₀ for RG7112i versus rIC₅₀ for RG7112 for the in vitro cell line panel. Solid black bars indicate wild-type p53 cell lines and hatched bars indicate mutant p53 cell lines.

Figure 2

RG7112 activity against individual solid tumor xenografts, Kaplan-Meier curves for EFS and graphs of median and tumor volumes (controls, gray and treated, black lines), are shown for selected lines: BT-50 (medulloblastoma), Rh65 (alveolar rhabdomyosarcoma), KT-12 (kidney rhabdoid) and CHLA258 (Ewing sarcoma).

Figure 3

RG7112 activity against individual ALL xenografts, Kaplan-Meier curves for EFS and graphs of median and individual %hCD45 (controls, gray and treated, black lines), are shown for selected lines: ALL-4, ALL-7, ALL-8 and MLL-7.

Figure 4

RG7112 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, and to the left are tumor models that are less sensitive. 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.