Superior cellular efficacy of MS645 in cancer-cell growth inhibition. (A) Persistent transcriptional repression of IL-6 by MS645 over other BET inhibitors in a washout study of MDA-MB-231 cells. The cells were treated with a BET inhibitor (1 μM) or DMSO for 2 h then washed with fresh medium twice and cultured for time periods as indicated. The mRNA level of IL-6 was measured after compound-imposed transcriptional inhibition. The data are plotted from one representative experiment and error bars represent SD of technical repeats. (B) Effects of the BET inhibitors of MS645, MS660, MS688, and JQ1 on protein stability assessed in a cellular thermal shift assay and shown by a representative set of Western blot analyses of BRD4. (C) Effects of MS645, MS660, JQ1, and MS417 on cell growth inhibition of cancer and noncancer cell lines, as assessed in an MTT assay. IC50 values are listed in a table. Results presented in B and C were all from at least three independent experiments and error bars designate SEM.