An inhibitor of induced myeloid leukemia cell differentiation protein (myeloid cell leukemia-1; Mcl-1; Bcl2-L-3), with potential pro-apoptotic and antineoplastic activities. Upon administration, AZD5991 binds to Mcl-1, thereby preventing the binding of Mcl-1 to and inactivation of certain pro-apoptotic proteins, and promoting apoptosis of cells overexpressing Mcl-1. Mcl-1, an anti-apoptotic protein belonging to the Bcl-2 family of proteins, is upregulated in cancer cells and promotes tumor cell survival.