Stapled Peptides: Leveraging the BH3 α-Helix to Create a New Class of Drugs to Treat Hematological Malignancies

Abstract

AILERON is developing a selective approach to restore programmed cell death in cancer cells by borrowing from nature the BH3 domain alpha-helical “keys” from pro-apoptotic members of the BCL-2 family that fit into the “locks” of other anti- and pro-apoptotic BCL-2 proteins. AILERON has applied its proprietary chemical strategy termed hydrocarbon stapling (J. Am. Chem. Soc., 2000 122:5891) to generate cell-permeable BH3 stapled peptides that modulate the intracellular protein-protein interactions of BCL-2 family members and selectively kill cancer cells both in vitro and in vivo (Science, 2004 305:1466). The lead compounds possess the innate characteristics of the endogenous peptides they mimic, including mechanism of action and target specificity. BID BH3 and BIM BH3 stapled peptides have recently been shown to directly activate the pro-death molecule BAX, a unique property that differentiates BH3 stapled peptides from all BCL-2 small molecule antagonists (Mol. Cell, 2006 24:199). BID BH3 and BIM BH3 stapled peptides were tested for their ability to induce programmed cell death in a panel of 12 lymphoid-derived tumor cell lines. T-ALL derived cell lines were the most sensitive to the compounds, followed by multiple myeloma lines. CML-derived cell lines were the least sensitive. Resting human peripheral blood lymphocytes (hPBLs) and normal human embryonic lung fibroblasts (WI-38) were found to be resistant to BH3 stapled peptides, indicating that the compound may target only cells that are “primed to die” and not normal “resting” cells. The anti-cancer activity of BH3 stapled peptides was further investigated in orthotopic xenograft models and shown to dramatically suppress tumor growth. In a mixed lineage leukemia model (SEMK2), a tumor over control (T/C) of 27% was observed after 13 days of treatment at 30 mg/Kg IV q.d. We also investigated whether BH3 stapled peptides elicit an antibody response in rodents. No antibody titer was detected, indicating that BH3 stapled peptides are non-antigenic in rodents. While peptides are oftentimes unstable in vivo, with half-lives typically in the range of a few minutes, BH3 stapled peptides were 100% stable in both mouse and human plasma ex vivo, and exhibited excellent PK profiles in rats with half-lives greater than three hours. BH3 stapled peptides were well tolerated in all animal models tested to date. In conclusion, we show that BH3 stapled peptides exhibit promising pharmacological properties and represent a novel class of drugs for the treatment of hematological malignancies.