A Stapled BIM BH3 Helix Overcomes the MCL-1-Mediated Apoptotic Blockade of Refractory Hematologic Cancers

MCL-1 is an anti-apoptotic member of the BCL-2 family of proteins and has emerged as a major resistance factor in hematologic cancer. Pharmacologic targeting of anti-apoptotic proteins is an effective strategy for reactivating apoptosis in cancers that evade cell death by neutralizing pro-apoptotic BCL-2 family proteins. Anti-apoptotic proteins contain a hydrophobic groove of differing specificity that binds and sequesters the critical BH3 the BH3-only members such as BID and BIM that transmit signals of cellular stress to that oligomerize upon activation to permeabilize the outer mitochondrial membrane. Small molecules such as ABT-737 that selectively inhibit the anti-apoptotic proteins BCL-2 and BCL-XL induce apoptosis of tumors that are especially dependent upon this subset of survival proteins. However, the compounds are rendered ineffective by cellular expression of alternate anti-apoptotic proteins such as MCL-1, which lie outside their range of binding specificity. We previously developed Stabilized Alpha Helices of BCL-2 domains (SAHBs) that are structurally stable, protease-resistant, and cell-permeable compounds capable of targeting BCL-2 family proteins in situ with high affinity. By inserting a hydrocarbon staple into the BH3 domain of pro-apoptotic BIM, we have generated a broad spectrum BCL-2 family targeting agent that binds all anti-apoptotic proteins with nanomolar affinity and also directly triggers the activation of pro-apoptotic BAX. Here we report that BIM SAHB, but not a point mutant control, induces dosedependent apoptosis of ABT-737-resistant acute myeloid leukemia, acute lymphoblastic leukemia, chronic myelogenous leukemia, diffuse large B-cell lymphoma, and multiple myeloma cell lines. Importantly, we demonstrate by co-immunoprecipitation that BIM SAHB broadly targets BCL-2 family proteins in these MCL-1-expressing hematologic cancers. Thus, by exploiting the native binding spectrum of BIM BH3 for both anti- and pro-apoptotic BCL-2 family members, we demonstrate the therapeutic potential of BIM SAHB in overcoming MCL-1-mediated resistance in hematologic cancers.