LIBX-A403 (Compound 21) is a highly selective acyl-CoA synthetase long-chain family member 4 (ACSL4) inhibitor (IC50=0.049 μM) . LIBX-A403 blocks ACSL4-mediated ferroptosis-promoting phospholipid remodeling. LIBX-A403 is promising for research of cancers (e.g., triple-negative breast cancer) and neurodegenerative diseases (e.g., Parkinson’s disease) [1].