The selectivity of AZD-5991 is evaluated against pro-survival Bcl-2 family members using FRET assays. AZD-5991 is selective for Mcl-1 (IC50 0.72 nM, Ki=200 pM) vs. Bcl-2 (IC50=20 µM, Ki=6.8 µM), Bcl-xL (IC50=36 µM, Ki=18 µM), Bcl-w (IC50=49 µM, Ki=25 µM), and Bfl-1 (IC50=24 µM, Ki=12 µM). MOLP-8, MV4-11, and NCI-H23 cells are treated with AZD5991 (EC50=0.033, 0.024, 0.19 µM, respectively).AZD5991 binds directly to Mcl-1 and induces rapid apoptosis in cancer cells, most notably myeloma and acute myeloid leukemia, by activating the Bak-dependent mitochondrial apoptotic pathway. AZD5991 reduces the levels of Mcl-1 protein in AZD5991-sensitive but not in AZD5991-resistant MM cell lines further supporting the notion that activation of caspases by AZD5991 reduces Mcl-1 protein levels in AZD5991-sensitive cell lines.