Nuclear factor-kappa B (NF-kappa B) is a crucial transcription factor family involved in the regulation of immune and inflammatory responses and cell survival. The linear ubiquitin chain assembly complex (LUBAC), composed of the HOIL-1L, HOIP, and SHARPIN subunits, specifically generates Met1-linked linear ubiquitin chains through the ubiquitin ligase activity in HOIP, and activates the NF-kappa B pathway. We recently identified a chemical inhibitor of LUBAC, which we named HOIPIN-1 (HOIP inhibitor-1). To improve the potency of HOIPIN-1, we synthesized 7 derivatives (HOIPIN-2 similar to 8), and analyzed their effects on LUBAC and NF-kappa B activation. Among them, HOIPIN-8 suppressed the linear ubiquitination activity by recombinant LUBAC at an IC50 value of 11 nM, corresponding to a 255-fold increase over that of HOIPIN-1. Furthermore, as compared with HOIPIN-1, HOIPIN-8 showed 10-fold and 4-fold enhanced inhibitory activities on LUBAC- and TNF-alpha-induced NF-kappa B activation respectively, without cytotoxicity. These results indicated that HOIPIN-8 is a powerful tool to explore the physiological functions of LUBAC. (C) 2018 Elsevier Inc. All rights reserved.