Multiple myeloma (MM), an incurable clonal plasma cell disorder, is the second most common hematologic malignancy in the United States with over 20,000 new cases and 10,000 deaths each year. Although many chemotherapeutic drugs display cytotoxic activity toward MM cells, MM patients inevitably relapse. Much controversy exists surrounding the cells responsible for relapse—the MM tumor-propagating cells—and their relationship to so-called cancer stem cells (CSCs). During the first year of this project, we discovered by high-throughput RNA sequencing (RNA-seq) analysis that a recently described pluripotent stem cell-selective fluorescent dye, CDy1, identified a subpopulation of dimly staining cells in MM cell lines characterized by high level expression of ABCB1 encoding the P-glycoprotein multidrug resistance extrusion pump. We determined that CDy1 is a substrate of ABCB1 and that CDy1 efflux is a highly sensitive assay of ABCB1 transporter activity.
In clinical studies conducted in the 1980’s and early 1990’s, ABCB1-associated drug resistance was routinely observed in MM patients who had received combination chemotherapies containing high doses of vincristine and doxorubicin, both of which are ABCB1 substrates. For this reason, we believe it is especially noteworthy that we also found that ABCB1-overexpressing MM cells are highly resistant to carfilzomib, a second generation proteasome inhibitor that was recently approved by the FDA for the treatment of refractory/relapsed MM patients. Although ABCB1 was discovered more than 35 years ago, its role in clinical drug resistance has remained an open question because most attempts to restore chemosensitivity have failed to demonstrate a survival benefit owing to complications involving normal tissue toxicity. Nonetheless, what has emerged in studies of other cancers is that ABCB1 expression is a strong indicator of adverse outcomes. Thus, our major objective during the second year of this project is to determine whether ABCB1 expression has prognostic or predictive relevance in MM; i.e., can it be used to inform treatment decisions and/or is it an indicator of worse clinical outcome. Toward this end, we propose to evaluate the utility of CDy1 efflux as an assay of ABCB1 expression and functionality in MM patient samples.