Mcl-1 inhibitor 6 is an orally active, selective myeloid cell leukemia 1 (Mcl-1) protein inhibitor with a Kd of 0.23 nM and a Ki of 0.02 μM. Mcl-1 inhibitor 6 possesses superior selectivity over other Bcl-2 family members (Bcl-2, Bcl2A1, Bcl-xL, and Bcl-w, Kd>10 μM). Mcl-1 inhibitor 6 is a potent antitumor agent.