How to Overcome Myeloma Stem Cell Resistance to Therapy - Targeting the Stem Cell Niche

Abstract SCI-6

The concept of cancer stem cells has attracted again intense research interest, as the drug resistance attributed to this infrequent subpopulation of tumor cells could explain how patients can relapse even after prolonged complete clinical, biochemical, radiologic or even molecular remissions. In multiple myeloma (MM), several aspects of the cancer stem cell concept remain to be elucidated, including the potential heterogeneity of this cell subpopulation or whether CD138+ expression is incompatible or not with a MM stem cell. As these questions are being resolved, emerging data highlight that the drug resistance of MM cells with clonogenic/stem cell-like features is heavily influenced by interactions with non-malignant accessory cells of the local microenvironment, including bone marrow stromal cells (BMSCs). Indeed, transcriptional signatures of “stemness”, as identified in normal stem cells or cancer stem cells from other neoplasias, are detected in the bulk population of MM cell lines and are upregulated after MM cell interaction with BMSCs. MM cell lines and primary tumor cells contain subpopulations with clonogenic potential, such as the side population (SP) cells. SP cells, detected by low intracellular accumulation of Hoechst 33342 dye (in contrast to the tumor's “main population” (MP), are considered an enriched source of tumor-initiating cells in diverse neoplasias and were detected among CD138− CD20+ CD27+ clonogenic cells in primary MM samples. Interaction with BMSCs increases the viability of SP cells and their percentage within the MM cell population. While interaction with BMSCs or other accessory cells of the microenvironmental niche suppresses the anti-MM activity of glucocorticoids, conventional chemotherapeutics and certain investigational agents, other agents (e.g. immunomodulatory thalidomide derivatives (IMIDs), such as lenalidomide) have increased activity against MM SP cells in the context of this tumor-microenvironment interaction. These observations suggest that MM cells with stem cell-like features exhibit functional plasticity depending on which specific microenvironmental niche they interact with. The Hedgehog, Wnt and Notch pathways, as well as regulators of chromatin remodeling, e.g. histone demethylases, have emerged as putative links between drug resistance, “cancer stemness” and how these functional outcomes are modulated by the local microenvironment in MM. These pathways, as well as embryonic stem cell-associated antigens (e.g. SOX2), represent intriguing targets for investigational therapies. However, clinical translation of such treatments has notable challenges, as conventional criteria for response assessment may not accurately reflect the treatment's impact on clonogenic tumor cells. Progression-free survival is considered a more appropriate endpoint for cancer stem-cell targeting agents, its assessment, however, may be confounded without concomitant treatment that suppresses the bulk of the tumor. Consequently, candidate cancer stem cell-targeting agents may have to be evaluated in combination with regimens (including lenalidomide-bortezomib-Dex (RVD) or other combinations built around the therapeutic “backbone” of proteasome inhibition and IMIDs) which potently target the bulk MM cell population and induce high rates of complete/near complete responses. Further improvements of this dual targeting of clonogenic and bulk tumor cells may be facilitated by recently developed high-throughput platforms (e.g. compartment-specific bioluminescence imaging, CS-BLI) which screen, in the presence vs. absence of stroma or other accessory cells, large numbers of anti-tumor agents and combinations thereof against the bulk tumor cell population or its clonogenic compartments. These new platforms will inform the rational design of regimens that will hopefully improve the long-term outcome of MM patients by suppressing a clonogenic/stem cell-like tumor compartment and circumventing microenvironment-dependent drug resistance.