Strategies To Identify Effective Treatments For Proteasome Inhibitor Resistant Multiple Myeloma

Despite the introduction of effective new agents in the treatment of myeloma, the disease is still mostly incurable. Among the most significant issues is the heterogeneity of the disease, with accumulation of multiple genetic abnormalities among patients, resulting in disease refractory to some highly active agents, or the emergence of resistance leading to aggressive relapse. We have focused efforts on modeling drug sensitivity, and generating both genetic and biomarker signatures of response and resistance to the proteasome inhibitors (PIs): bortezomib (Btz), MLN2238 (Takeda) and carfilzomob (Onyx). Using mouse and human myeloma cell lines we dose escalated with bortezomib and selected bortezomib-resistant (Btz-R) lines (that showed cross resistance to MLN2238 and carfilzomib). We previously reported the identification of gene expression signatures that distinguish sensitivity or resistance to bortezomib (Stessman, et al., Mol Cancer Ther 12:1140, 2013). We then took three approaches to identify effective strategies to treat bortezomib resistance: 1) The connectivity map (CMAP) data base contains treatment induced transcriptional signatures from over 1300 bioactive compounds in human cancer cell lines. We queried our Btz-R expression profiles against the CMAP data base and developed a correlation analysis to identify potential drugs that would be predicted to show toxicity to Btz-R lines. We identified HDAC-inhibitor and topoisomerase-inhibitor sensitivity profiles that were able to predict Btz-R tumor responsiveness to these drugs in vitro. 2) We found that Btz-R was associated with loss of PC maturation markers (CD69, CD93, BLIMP, CXCR4, spliced XBP-1), and were able to re-sensitize Btz-R cells to Btz sensitivity by inducing PC maturation. The correlation of expression markers we identified stratified survival outcomes in bortezomib containing clinical trials; thus demonstrating the markers we identified in the in vitro modeling are relevant to patient outcomes. 3) Finally, we designed a cell-based high throughput drug screening approach that led to the identification of several chemical structures with selective activity against Btz-R cells. Topoisomerase-inhibitors were among the top hits, a finding that independently supported our CMAP screening results. We also identified a more novel chemical lead (Velcade Re-sensitizing Compound 2; VRC-2) that showed modest but significant selectivity for Btz-R cells as a single agent, and most notably, exhibited potent Btz re-sensitizing activity when the 2 drugs were combined. The Btz re-sensitizing activity of VRC-2 was confirmed using multiple human and mouse Btz-R cell lines. The initial successes of the in vitro approaches that will be presented demonstrate the value of profiling resistant tumors as a means to identify secondary therapies, and demonstrate there are powerful screening approaches that can be used to effectively treat or reverse drug resistance.