Discovery of Potent and Selective Benzothiazole Hydrazone Inhibitors of Bcl‑X_L

Developing potent molecules that inhibit Bcl-2 family mediated apoptosis affords opportunities to treat cancers via reactivation of the cell death machinery. We describe the hit-to-lead development of selective Bcl-X_L inhibitors originating from a high-throughput screening campaign. Small structural changes to the hit compound increased binding affinity more than 300-fold (to IC₅₀ < 20 nM). This molecular series exhibits drug-like characteristics, low molecular weights (<i>M</i>_w < 450), and unprecedented selectivity for Bcl-X_L. Surface plasmon resonance experiments afford strong evidence of binding affinity within the hydrophobic groove of Bcl-X_L. Biological experiments using engineered Mcl-1 deficient mouse embryonic fibroblasts (MEFs, reliant only on Bcl-X_L for survival) and Bax/Bak deficient MEFs (insensitive to selective activation of Bcl-2-driven apoptosis) support a mechanism-based induction of apoptosis. This manuscript describes the first series of selective small-molecule inhibitors of Bcl-X_L and provides promising leads for the development of efficacious therapeutics against solid tumors and chemoresistant cancer cell lines