The evasion of apoptosis, or programmed cell death, is a hallmark of cancer, which promotes tumor initiation and progression. The evasion is in part attributable to the over-expression of anti-apoptotic proteins in the Bcl-2 family. In addition, chemotherapy and radiation can upregulate the expression of the Bcl-2 family in cancer cells, which renders them more resistance to cancer therapy. The most common Bcl-2 family member over-expressed in many solid tumor cells and a fraction of leukemia and lymphoma cells is Bcl-XL and its expression is also highly correlated with resistance to cancer therapy independent of p53 status in many cancers. Therefore, Bcl-XL is one of the most important validated cancer cell targets.

Inhibition of Bcl-XL with a small molecule inhibitor has been extensively exploited as a molecularly targeted therapeutic strategy against cancer, resulting in the discovery of several Bcl-2/XL and Bcl-XL inhibitors as promising anti-cancer drug candidates including navitoclax. Unfortunately, these inhibitors failed to become anticancer drugs because platelets are also dependent on Bcl-XL for survival. Therefore, inhibition of Bcl-XL with Bcl-2/XL and Bcl-XL inhibitors causes severe reduction in platelets or thrombocytopenia, an on-target and dose-limiting toxicity, which prevents their use as an effective anticancer drug in clinic. To overcome this problem, we generated a series of novel bifunctional molecules that targeting Bcl-XL to the ubiquitin-proteasome system (UPS) for degradation. These synthetic proteolytic compounds, termed synthetic proteolytics (Syntholytics) or proteolysis targeting chimeras (PROTACs), were rationally designed to recruit the Von Hippel Lindau (VHL) E3 ligase to ubiquitinate Bcl-XL for degradation by the proteasome. Because VHL is minimally expressed in platelets, our Bcl-XL Syntholytics can selectively induce Bcl-XL degradation in various cancer cells but not in platelets. Amongst these Bcl-XL Syntholytics, DT2216 was found to be the most potent in inducing Bcl-XL degradation leading to the loss of viability of Bcl-XL-dependent T-ALL MOLT-4 cells at nanomolar concentrations but did not cause any platelet toxicity. Compared to navitoclax, DT2216 is more potent in induction of apoptosis in a variety of cancer and leukemia cells in vitro in a caspase-dependent manner. Furthermore, our in vivo studies in immunocompromised mice revealed that DT2216 at 15 mg/kg/wk potently inhibited tumor growth in Bcl-XL-dependent MOLT-4 T-ALL xenografts as a single agent whereas navitoclax had no significant effect at the same dosage. Dosing with DT2216 at 15 mg/kg every four days significantly regressed larger established MOLT-4 T-ALL tumors that failed to respond to navitoclax treatment.

To assess the therapeutic potential of DT2216 in combination with other Bcl-2 family inhibitors, we employed the Bcl-2/xl dependent NCI-H146 small cell lung cancer cells and the Mcl1/Bcl-xl dependent multiple myeloma EJM cells. The combination of DT2216 with Bcl-2 inhibitor (ABT199) or Mcl-1 inhibitor (S63845) synergistically reduced the viability of H146 and EJM cells, respectively. DT2216 in combination with ABT199 effectively inhibited tumor growth in H146 xenografts. Collectively, our findings suggest that targeting Bcl-XL using Bcl-XL Syntholytics can selectively kill Bcl-XL-dependent T-ALL cells and various solid tumor cells without causing significant platelet toxicity. Moreover, the combination of Bcl-XL Syntholytics with other Bcl-2 protein inhibitors could be used to effectively target multiple cancer types including both hematological and solid tumors. Therefore, Bcl-XL Syntholytics have the potential to be developed as safer and more potent novel anti-cancer drugs.

Keywords: Bcl-XL, VHL, Protein degradation, T-ALL, Cancer, Apoptosis

Disclosures: S.K., X.Z., D.L., Y.H., P.Z., X. L., G. Z., and D.Z. are inventors of a pending patent application for use of Bcl-xl syntholytics as anti-cancer agents. R.H, G.Z. and D.Z. are co-founders of Dialectic Therapeutics that develops Bcl-xl syntholytics.


Khan:Dialectic Therapeutics: Patents & Royalties. Lv:Dialectic Therapeutics: Patents & Royalties. He:Dialectic Therapeutics: Patents & Royalties. Zhang:Dialectic Therapeutics: Patents & Royalties. Liu:Dialectic Therapeutics: Patents & Royalties. Konopleva:Stemline Therapeutics: Research Funding. Zheng:Dialectic Therapeutics: Consultancy, Equity Ownership, Patents & Royalties.

Author notes


Asterisk with author names denotes non-ASH members.