Overcoming Venetoclax Resistance in CLL with Dual BCL-2/BCL-XL PROTACs

The abstract discusses the development and testing of a novel therapeutic approach for chronic lymphocytic leukemia (CLL). The current standard treatment involves small molecules that target specific proteins, such as Bruton's tyrosine kinase (BTK) and BCL-2, but resistance to these treatments is a significant issue due to factors like genetic mutations and alternative protein dependencies.

Researchers performed single cell RNA sequencing on CLL samples before treatment and after relapse to identify changes in cellular clusters. They discovered that cells that relapsed had high levels of BCL-2 and BCL-xL, hinting that dual inhibitors could potentially overcome resistance to venetoclax, a BCL-2 inhibitor.

BH3 profiling revealed that a mutation in BCL-2 (G101V) in leukemia cells made them more sensitive to certain peptides, suggesting a heightened vulnerability to BCL-2 and BCL-xL inhibitors. However, previous attempts to use dual BCL-2/BCL-xL inhibitors were limited by platelet toxicity.

To address this, a new type of compound, a BCL-2/BCL-xL PROTAC (PZ18753b and its soluble variant WH2544), was developed. These molecules are designed to utilize the VHL E3 ligase, which is not present in platelets, thus providing a way to target cancer cells without affecting platelets. Both compounds induced key apoptosis pathways in CLL cells, including the activation of BAK and BAX, release of cytochrome C, mitochondrial depolarization, and exposure of phosphatidylserine on the cell membrane.

The effectiveness of these new compounds was found to be intermediate between that of venetoclax and navitoclax. At a certain concentration and time point, PZ18753b nearly completely killed CLL cells and induced significant degradation of BCL-xL and partial degradation of BCL-2, even in the presence of BCL-2 mutations.

The study concludes that CLL cells resistant to venetoclax still rely on BCL-2 and BCL-xL for survival, and this dependency can be therapeutically targeted using the dual targeting PROTACs PZ18753b and WH2544.