Covalent BH3-Mimetics and PROTACs: Targeting BCL2 Family for Precision Apoptosis Induction

The BCL2 family of proteins serves as a central regulator of mitochondrial outer membrane permeabilization (MOMP) and intrinsic apoptosis, with anti-apoptotic members such as BCL2, BCL-XL, and MCL1 frequently overexpressed in many cancers to evade programmed cell death. Traditional BH3 mimetics, such as venetoclax and navitoclax, have revolutionized the treatment of hematologic malignancies by competitively inhibiting these pro-survival proteins. Yet, they face challenges, including acquired resistance, on-target toxicities, and limited efficacy in solid tumors due to reversible binding and incomplete target engagement. To address these limitations, covalent BH3 mimetics—such as NA1-115-7 and drimane derivatives— have emerged as irreversible inhibitors that enhance potency and duration of action, particularly against MCL1 and BCL-XL, by forming stable adducts at key residues, such as histidine or cysteine. Complementing this, proteolysis-targeting chimeras (PROTACs), exemplified by DT2216 (BCL-XL-specific) and 753B (a dual BCL-XL/BCL2 degrader), exploit the ubiquitin–proteasome system to catalyze protein degradation. This approach provides enhanced selectivity, less frequent dosing, and reduced platelet toxicity through strategic E3 ligase selection. These strategies enable precision induction of apoptosis by exploiting tumor-specific dependencies, biomarker-guided patient stratification, and synergistic combinations with chemotherapies or immunotherapies. Preclinical data demonstrate enhanced tumor clearance in leukemia and solid tumor models, with clinical trials underway. Despite hurdles such as PROTAC pharmacokinetics and covalent off-target reactivity, these innovations promise to expand BCL2-targeted therapies beyond hematologic malignancies, fostering a new era of targeted modulation of apoptosis in oncology. This mini-review synthesizes recent advances in targeting BH3 mimetics, particularly in combination with PROTACs, to activate apoptosis in cancer cells. Furthermore, it highlights the challenges associated with the application of BH3 mimetics in inducing apoptosis.