Abstract #5117: KU135, a novobiocin-derived C-terminal inhibitor of Hsp90, exerts potent antiproliferative effects in human leukemic Jurkat T cells

Hsp (heat shock protein) 90 is overexpressed in a variety of different cancers and is thought to be important for cell survival. It is estimated that Hsp90 interacts with 100-200 \#8220;client\#8221; proteins that are involved in various signaling pathways and include kinases, transcription factors, apoptotic proteins, and oncogenic chimeric proteins. Consequently, there is growing interest in developing chemotherapeutics that target Hsp90. In fact, several natural product, synthetic, and semi-synthetic Hsp90 inhibitors have been developed that target the N-terminal ATP-binding pocket; however, the use of many of these inhibitors is limited due to adverse hepatotoxic effects and solubility issues. More recently, a C-terminal ATP-binding pocket has been identified and the antibacterial agent, novobiocin, was found to bind and inhibit this domain, albeit with low affinity. In the current study, we investigated the extent to which a novel novobiocin-derived analogue, KU135, could induce an antiproliferative response and how the effects of this compound compare to those exhibited by the established N-terminal inhibitor 17-AAG. The results indicated that KU135 exhibits more potent antiproliferative effects than 17-AAG against Jurkat T-cells with IC₅₀ values of 570 nM and 2.7 µM, respectively. 17-AAG and KU135 both caused the degradation of client proteins; however, 17-AAG caused a much greater increase in the protein expression of Hsp70, and only 17-AAG caused an increase in Hsp90 protein expression. The data generated using these two compounds also suggest that KU135 induces both a cytostatic and cytotoxic cellular response, whereas 17-AAG appeared to be primarily cytostatic. Cell cycle analysis showed that 17-AAG treatment caused an arrest in G₁, whereas KU135 caused an arrest in G₂/M. Further investigation of the cytotoxicity triggered by KU135 showed that these effects were due to the induction of mitochondria-mediated apoptosis as evidenced by the fact that the cytotoxic effects could be inhibited through abrogation of the intrinsic pathway by either overexpressing Bcl-2 or silencing Apaf-1. Combined, these data support the conclusion that KU135 inhibits cell proliferation by regulating signaling pathways that are mechanistically different from those targeted by 17-AAG. Because KU135 appears to be both cytostatic and cytotoxic whereas 17-AAG is largely cytostatic, KU135 would be expected to eradicate tumor cells and lower tumor burden where 17-AAG would only inhibit tumor cell division. Overall, these findings strongly support the continued development of C-terminal Hsp90 inhibitors for the treatment of human malignancies. (Supported by NIH grants K22 ES011647 (to J.D.R.), P20 RR016475, and KU Cancer Center Pilot Grant.)

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 5117.