Multiple myeloma (MM) is an incurable plasma cell malignancy characterised by the accumulation of neoplastic plasma cells in the bone marrow. Dysregulation of BCL2 family proteins (BCLXL, MCL1, and BCL2) contributes to MM drug resistance and relapse. However, the clinical translation of conventional BCLXL inhibitors is limited by on-target thrombocytopenia. Aims: To overcome thrombocytopenia, a proteolysis-targeting-chimera (PROTAC), called DT2216, was developed that couples navitoclax (BCLXL binder) to a von Hippel–Lindau (VHL) E3 ligase-recruiting ligand to selectively degrade BCLXL in tumour cells, while sparing platelets that lack VHL. Our goal is to use ex-vivo BH3-profiling and BH3 mimetic sensitivity to identify MM patient samples that rely on BCLXL for survival and test vulnerability to DT2216, as a precision-medicine approach. Building on this, we are developing a next-generation, in-cell self-assembling PROTAC, termed a Clip-TAC, that forms a hetero-bifunctional degrader inside MM cells via click chemistry.

Protein degradation, cell viability, and apoptotic dependencies were evaluated by western blotting, flow cytometry, co-immunoprecipitation, and BH3-profiling in a panel of MM cell lines. DT2216 was tested ex-vivo in MM patient samples.

DT2216 induced dose- and time-dependent degradation of BCLXL across the MM cell line panel. Degradation was rescued by pre-treatment with A-1331852 (BCLXL inhibitor) or MLN-4924 (NEDD8-activating enzyme inhibitor), confirming on-target, proteasome-dependent activity. BH3-profiling and co-immunoprecipitation after treatment revealed a compensatory increase in MCL-1 dependence. Co-targeting this compensation, either directly with MCL-1 inhibitor AMG-176 or indirectly with cyclin-dependent kinase inhibitors (CYC065, THZ1, Samuraciclib), synergistically enhanced DT2216-induced cell death. MM patient samples, identified as BCLXL-reliant, were sensitive to DT2216 ex-vivo, and BCLXL degradation was confirmed by flow cytometry. In collaboration with Prof. Griffith, we have developed a novel BCLXL Clip-TAC and confirmed in cell click chemistry inducing degradation at nanomolar concentrations; efficacy studies are ongoing in the chicken chorioallantoic membrane (CAM) model and we are testing toxicity to human platelets.

MM exhibits heterogeneous dependence on BCL-2 family proteins. BH3-profiling can identify BCLXL-reliant tumours. Targeted degradation of BCLXL with DT2216, or a novel Clip-TAC, shows therapeutic potential in MM.