AZD-5991 is a potent and selective Mcl-1 inhibitor with an IC 50 of 0.7 nM in FRET assay and a K d of 0.17 nM in surface plasmon resonance (SPR) assay In Vitro The selectivity of AZD-5991 is evaluated against pro-survival Bcl-2 family members using FRET assays. AZD-5991 is selective for Mcl-1 (IC 50 0.72 nM, K i =200 pM) vs. Bcl-2 (IC 50 =20 µM, K i =6.8 µM), Bcl-xL (IC 50 =36 µM, K i =18 µM), Bcl-w (IC 50 =49 µM, K i =25 µM), and Bfl-1 (IC 50 =24 µM, K i =12 µM). MOLP-8, MV4-11, and NCI-H23 cells are treated with AZD5991 (EC 50 =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. MCE has not independently confirmed the accuracy of these methods. They are for reference only. In Vivo A single intravenous (i.v.) dose of AZD5991 leads to a dose-dependent antitumor effect ranging from tumor growth inhibition (TGI) to tumor regression (TR). Ten days after treatment, AZD5991 shows 52% and 93% TGI (p Form:Solid IC50& Target:Mcl-1 0.7 nM (IC 50 ) Mcl-1 0.17 nM (Kd)