Initial testing (stage 1) of the Akt inhibitor GSK690693 by the pediatric preclinical testing program

Abstract

Background: GSK690693 is a small molecule ATP-competitive inhibitor of the pro-survival kinase Akt. Since Akt regulates multiple downstream targets including transcription factors, glycogen synthase 3, the pro-apoptotic protein Bad, as well as MDM2 and mTORC1, it was tested against the in vitro and in vivo panels of the Pediatric Preclinical Testing Program (PPTP).

Procedures: GSK690693 was tested in vitro at concentrations from 1 nM to 10 µM, and against the in vivo panel of xenografts at a dose of 30 mg/kg daily × 5 for 6 consecutive weeks. Three measures of in vivo 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 measure based on the median event-free survival (EFS) of treated and control animals for each xenograft.

Results: GSK690693 inhibited cell growth in vitro with IC(50) values between 6.5 nM and >10 µM. In vivo, GSK690693 significantly increased EFS in 11 of 34 (32%) solid tumor xenografts, most notably in all 6 osteosarcoma models, but not in any of the 8 ALL xenografts tested. No objective responses were observed and only one solid tumor met EFS T/C criteria for intermediate activity.

Conclusions: GSK690693 demonstrated broad activity in vitro, however our results against both the solid tumor and ALL PPTP in vivo panels demonstrate that, as single agent at the dose and schedule used, GSK690693 has only modest antitumor activity.

Figure 1

GSK690693 in vitro activity: (A) 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. Representative dose response curves for Rh41 (B) and NB-1643 (C) cell lines.

Figure 2

Plots depicting tumor growth and survival of mice treated with GSK960693 versus controls. Median tumor volume over time (left) and Kaplan-Meier plot of EFS over time (right) for the xenografts OS-33 (A) and KT-12 (B). Gray lines indicate vehicle-treated control mice and black lines indicate GSK690693-treated mice.

Figure 3

GSK690693 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.

Figure 4

Blood glucose levels of NOD/SCID mice over time (n=3 per group) after a single injection of vehicle (back dashed line), 30 mg/kg GSK690693 in mannitol-saline (red line) or 30 mg/kg GSK690693 in mannitol-acetate (blue line).

Figure 5

Pharmacodynamic evaluation of Akt inhibition. Western blot analyses were performed as previously described with minor modifications [13]. Each lane represents an individual sample of the osteosarcoma xenograft identified above the lanes collected either prior to or at the specified times after treatment with GSK690693. β-tubulin was used as a loading control.