NDNA4 (compound 17) is a selective Hsp90α inhibitor (IC50: 0.34 μM), characterized by its permanent charge and low membrane permeability. It exhibits minimal cytotoxicity against Ovcar-8 and MCF-10A cell lines (IC50 >100 μM) and maintains hERG channel maturation without eliciting a heat shock response or degrading Hsp90α-dependent client proteins [1].