DT2216

Anti-apoptotic protein BCL-XL plays a key role in tumorigenesis and cancer chemotherapy resistance, rendering it an attractive target for cancer treatment. However, BCL-XL inhibitors such as ABT-263 cannot be safely used in the clinic because platelets solely depend on BCL-XL to maintain their viability. To reduce the on-target platelet toxicity associated with the inhibition of BCL-XL, we designed and synthesized PROTAC BCL-XL degraders that recruit CRBN or VHL E3 ligase because both of these enzymes are poorly expressed in human platelets compared to various cancer cell lines. We confirmed that platelet-toxic BCL-XL/2 dual inhibitor ABT-263 can be converted into platelet-sparing CRBN/VHL-based BCL-XL specific degraders. A number of BCL-XL degraders are more potent in killing cancer cells than their parent compound ABT-263. Specifically, XZ739, a CRBN-dependent BCL-XL degrader, is 20-fold more potent than ABT-263 against MOLT-4 T-ALL cells and has >100-fold selectivity for MOLT-4 cells over human platelets. Our findings further demonstrated the utility of PROTAC technology to achieve tissue selectivity through recruiting differentially expressed E3 ligases.

B-cell lymphoma extra large (BCL-XL) is a well-validated cancer target. However, the on-target and dose-limiting thrombocytopenia limits the use of BCL-XL inhibitors, such as ABT263, as safe and effective anticancer agents. To reduce the toxicity of ABT263, we converted it into DT2216, a BCL-XL proteolysis-targeting chimera (PROTAC), that targets BCL-XL to the Von Hippel-Lindau (VHL) E3 ligase for degradation. We found that DT2216 was more potent against various BCL-XL-dependent leukemia and cancer cells but considerably less toxic to platelets than ABT263 in vitro because VHL is poorly expressed in platelets. In vivo, DT2216 effectively inhibits the growth of several xenograft tumors as a single agent or in combination with other chemotherapeutic agents, without causing appreciable thrombocytopenia. These findings demonstrate the potential to use PROTAC technology to reduce on-target drug toxicities and rescue the therapeutic potential of previously undruggable targets. Furthermore, DT2216 may be developed as a safe first-in-class anticancer agent targeting BCL-XL.