A purine scaffold Hsp90 inhibitor destabilizes Bcl6 and has specific anti-tumor activity in Bcl6 dependent B-cell lymphomas

We report that Heat shock protein 90 (Hsp90) inhibitors selectively kill Diffuse Large B-cell Lymphomas (DLBCL) that are biologically dependent on the Bcl6 transcriptional repressor. Endogenous Hsp90 was found to interact with Bcl6 in DLBCL cells and could stabilize both Bcl6 mRNA and protein. Hsp90 formed a complex with Bcl6 at its target promoters and Hsp90 inhibitors de-repressed Bcl6 target genes. A stable mutant of Bcl6 rescued DLBCL cells from Hsp90 inhibitor induced apoptosis. Bcl6 and Hsp90 were almost invariantly co-expressed in the nuclei of primary DLBCL cells, suggesting that their interaction is relevant in this disease. We examined the pharmacokinetics, toxicity and efficacy of PU-H71, a recently developed purine derived Hsp90 inhibitor. PU-H71 preferentially accumulated in lymphomas compared to normal tissues and selectively suppressed Bcl6-dependent DLBCLs in vivo, inducing reactivation of key Bcl6 target genes and apoptosis. PU-H71 also induced cell death in primary human DLBCL specimens.