Zelavespib formic (PU-H71 formic) is a potent Hsp90 inhibitor with an IC50 value of 51 nM for Hsp90 in MDA-MB-468 cells.
References:
[1]. Caldas-Lopes E, et al. Hsp90 inhibitor PU-H71, a multimodal inhibitor of malignancy, induces complete responses in triple-negative breast cancer models. Proc Natl Acad Sci U S A. 2009 May 19;106(20):8368-73.