Identification of N, C-capped di- and tripeptides as selective immunoproteasome inhibitors

A series of N, C-capped di- and tripeptides were designed as selective immunoproteasome inhibitors based on the known inhibitor 4-CA. Forty-eight new compounds were synthesized and evaluated, and the structure-activity relationship (SAR) of this compound class as β5i selective inhibitors were explored. Most of these compounds showed significant inhibition against the β5i subunit of the immunoproteasome and the most potent β5i inhibitor (15) showed an IC₅₀ of 0.94 nM. A selective β5i inhibitor (54) with over 500-fold β5i/β5c selectivity was identified. Three of the inhibitors were found to selectively inhibit β5i and β5c, and showed no noticeable inhibition against the other four subunits. Six inhibitors with significant inhibitory activity against the HCT-116 cells were recognized, and the most active inhibitors, 14 and 50, showed IC₅₀ values of 0.46 μM and 0.16 μM, respectively. Some selective β5i inhibitors exhibited significant inhibitory effects on the release of the cytokines TNF-α and IL-6. The results not only afford effective chemical tools to elucidate the relationships between subunit selectivity and pharmacological profiles, but also offer useful clues for further optimization and development of selective immunoproteasome inhibitors.