Although Hsp90 is a rational therapeutic target in cancer, the clinical use of the available quinone Hsp90 inhibitors has been challenged by their liver toxicity. We developed a non-quinone Hsp90 inhibitor named PU-H71. Relatively little is known about the efficacy of Hsp90 inhibitors in diffuse large B-cell lymphomas (DLBCL). The BCL6 transcriptional repressor is the main oncoprotein in DLBCL. The continued presence of the BCL6 is required for survival of DLBCL. Considering that Hsp90 is overexpressed in DLBCLs, we hypothesized that sustained BCL6 expression in DLBCL could be regulated by Hsp90 chaperone activity. In that case, Hsp90 inhibition would affect the maintenance of the malignant phenotype by BCL6. When screened against 12 DLBCL cell lines, PU-H71 selectively killed DLBCLs that are biologically dependent on the BCL6 activity. Cell uptake and Hsp90 inhibition were similar among the cell lines. Treatment of DLBCL cells with PU-H71 resulted in apoptosis and destabilization and subsequent degradation of BCL6. The destabilization of BCL6 occurred at the mRNA and protein levels. The BCL6 protein half-life decreased 5 times in the treated cells. PU-H71-induced downregulation of BCL6 was partially blocked by treatment with a proteasome inhibitor. By co-IP we showed that BCL6 and Hsp90 interact at the nuclear level. Moreover, ChIP assays revealed the presence of both proteins at the critical BCL6 target genes ATR and TP53. In addition, PU-H71 increased the BCL6 mRNA decay rate by 32%. PU-H71 was also active in 19 out of 21 primary DLBCL cells obtained from patient biopsies. To evaluate the in vivo effect, PU-H71 75mg/kg/day or vehicle were administered to Farage, Ly7 and SUDHL4 xenografts (n=5 per treatment and per cell line). By day 10, a 76%, 95% and 95% inhibition of tumor growth was observed in Farage, Ly7 and SUDHL4 mice respectively (p=0.002, p<0.0001 and p=0.0002). There was also a significant increase in survival for PU-H71 treated mice (n=15) compared to controls (n=15) (Cox\#8217;s F test p<0.0001). PU-H71 also induced depletion of BCL6 protein and mRNA in the tumor xenografts. No toxicity was observed during treatment in these mice as well as in 50 additional normal mice as evidenced by a lack of microscopic organ damage and biochemical panels (including liver enzymes) and CBC. The plasma and tissues of xenografted mice were analyzed for PU-H71 concentration by HPLC-MS. After a single dose of PU-H71 75mg/kg, pharmacologically relevant levels were found in tumors even 24 h later, while in normal tissues PU-H71 was undetectable at 12 h. Moreover, the level in tissues were at all-times 10 to 125 times lower than in tumors. This preferential tumor retention could also be observed by PET imaging in living animals using a radiolabeled PU-H71. Due to its potent antitumor activity and favorable toxicity profile, PU-H71 is scheduled to enter early clinical trials in 2009.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2937.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO

American Association for Cancer Research
2009