Abstract #1710: A novel pharmacodynamic assay to measure Hsp90 target occupancy by the small molecule inhibitors IPI-504 and IPI-493 in tumors

Heat shock protein 90 (Hsp90) is a protein chaperone important for the stabilization and folding of many oncogenic proteins, and is an attractive target for cancer therapy. Infinity is developing both iv (IPI-504) as well as oral (IPI-493) Hsp90 inhibitors. The most advanced compound, IPI-504, has recently entered a phase III clinical trial. In pre-clinical animal models, IPI-504 and IPI-493 are rapidly cleared from the circulation and normal tissues but accumulate in tumor tissue. Standard measurements used to monitor the biological activity of Hsp90 inhibitors in the clinic therefore may not represent an accurate assessment of the level of target inhibition in tumor tissue. To address this issue, a \#8216;pharmacodynamic activity assay\#8217; was developed for Hsp90 that can accurately quantitate Hsp90 inhibition in tumor samples. Kinetic measurements demonstrate that IPI-504 and IPI-493 exhibit slow Hsp90 off rates, an effect that is exaggerated at low temperatures (t_1/2 4ºC~ 24h). This property made it feasible to develop an assay that measures the fraction of Hsp90 target protein that is occupied (inhibited) with small molecule Hsp90 inhibitors. The method was first validated using purified Hsp90 protein that had been pre-incubated with varying molar ratios of IPI-504 to artificially create different levels of occupancy. Next, the method was applied to cultured cancer cell lines exposed to increasing concentrations of IPI-504. Tissue culture experiments demonstrated a good correlation between the occupancy of Hsp90 and growth inhibition by IPI-504, confirming that the Hsp90 inhibitor indeed affects cell growth through the inhibition of Hsp90. Finally, this assay was applied to xenograft tumor samples from mice treated with different doses of IPI-504 and IPI-493. Importantly, these xenograft studies demonstrated that Hsp90 occupancy was better correlated with Hsp90 client protein degradation and efficacy than either plasma or tumor PK. In conclusion, we developed a novel pharmacodynamic assay which measures the occupancy of Hsp90 by small molecule Hsp90 inhibitors that can be used to directly measure Hsp90 inhibition in tumors from patients treated with IPI-504 or IPI-493.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 1710.