The Bcl-xL specific PROTAC, DT2216, increases the sensitivity to target therapy of preclinical tumor models from different origin

The anti-apoptotic proteins of the Bcl-2 family, which canonically act as inhibitors of apoptosis, are frequently deregulated in solid tumors of various origins, thereby promoting tumor progression-associated properties. Specifically, we and others demonstrated that the anti-apoptotic protein Bcl-xL stimulates tumor angiogenesis, metastatization and the establishment of an immunosuppressive niche and fosters drug resistance. In this context, BH3 mimetics have been developed as inhibitors of the anti-apoptotic proteins and some of them have entered clinical trials as single agents or in combination therapy. Nevertheless, Bcl-xL-directed BH3 mimetics show some limitations in clinical applicability, mainly due to the induction of thrombocytopenia. Recently, selective proteolysis targeting chimera (PROTAC) degraders have been identified. Among these, DT2216, a candidate in phase I/II clinical trials, induces Bcl-xL degradation via Von Hippel Lindau E3 ligase, and demonstrates in vitro and/or in vivo antitumoral activity in some preclinical cancer models without inducing thrombocytopenia. Our study aimed to assess the antitumor efficacy of DT2216 in combination with target therapies currently used in clinics or under clinical investigation in different tumor histotypes (i.e. melanoma, colorectal, pancreatic, breast and ovarian cancers). To this aim, we firstly investigated the effect of treatments in terms of cell viability reduction and apoptotic cell death induction, performing cell viability, cytofluorimetric and western blot analyses. Then, we focused on melanoma and investigated more deeply the effect of the combinatorial regimens in in vitro and in vivo experiments. Our results demonstrated that DT2216 was able to induce Bcl-xL degradation with a resulting cell viability loss in a panel of melanoma, colorectal, pancreatic, breast and ovarian cancer cell lines. Moreover, when used in combinational regimen, we proved that DT2216 potentiated the efficacy of: a BRAF inhibitor (Dabrafenib) and a MEK inhibitor (Trametinib) in BRAF V600E/K melanoma cell lines; Trametinib in BRAF wild type melanoma cell lines; Dabrafenib in BRAF V600E/K colorectal cancer cell lines; a KRAS inhibitor (MRTX1133) in KRAS G12D pancreatic cancer cell lines; a PARP-1 inhibitor (Olaparib) in breast and ovarian cancer cell lines. In all cases, the synergistic effect was due to the ability of the combination treatment to reduce cell viability and increase apoptotic cell death compared to single agents. Notably, also in BRAF V600E/K melanoma cell lines resistant to target therapy, DT2216 enhanced the efficacy of Dabrafenib/Trametinib treatment. In melanoma models, DT2216 also induced a potentiating effect on ABT-199, a Bcl-2 specific inhibitor, and S63845, a Mcl-1 specific inhibitor. Finally, we validated our in vitro results by using xenograft mouse melanoma model in which DT2216 potentiated the effect of target therapy, showing a significant reduction of tumor growth and conferring a longer disease control. In conclusion, our results highlighted the relevance of targeting the Bcl-xL protein as a potential therapeutic strategy in combinatorial regimens with target therapies in different tumor histotypes, providing a new therapeutic option for those patients who are unresponsive or evolve resistance to the therapeutic standard of care.