Mitochondria and Caspase-Independent Cell-Death Triggered by GCS-100, a Novel Carbohydrate-Based Therapeutic in Multiple Myeloma (MM) Cells.

GCS-100 is a carbohydrate-based therapeutic in clinical development for the treatment of cancer. Here we show that GCS-100 directly targets MM cells and their bone marrow (BM) microenvironment. Treatment of various MM cell lines, including those resistant to conventional anti-MM agents such as, dexamethasone, melphalan, or doxorubicin, with GCS-100 triggers growth arrest and apoptosis. Examination of purified patient MM cells demonstrated similar results, without significant toxicity against normal peripheral blood mononuclear cells. Importantly, GCS-100 decreases viability of bortezomib-resistant MM patient cells. GCS-100 blocks MM cell growth induced by both adhesion to bone marrow stromal cells (BMSCs) and related cytokine secretion, thereby inhibiting growth and survival in MM cells conferred by the BM microenvironment. GCS-100 overcomes drug-resistance conferred by anti-apoptotic protein Bcl-2 and heat shock protein-27. Although GCS-100 induces MM cell death evidenced by morphological and DNA fragmentation analysis, no activation of caspases-8, -9 and -3 was noted at the IC50 dose. Conversely, the pan caspase inhibitor Z-VAD-fmk failed to abrogate GCS-100-induced apoptosis. Furthermore, GCS-100 does not alter mitochondrial membrane potential, suggesting a mitochondria- and caspase-independent cell death-signaling pathway. These data provided the rationale for combining GCS-100 with an agent that specifically triggers mitochondrial apoptotic signaling in MM cells. For example, combining a conventional agent dexamethasone, a known inducer of Smac release from mitochondria and activator of caspase-9/3 cascade, with GCS-100 shows additive anti-MM activity; and combining GCS-100 with an antagonist to mitochondrial peripheral benzodiazepine receptor PK-11195 shows synergistic anti-MM activity, associated with profound mitochondrial cell-death signaling and activation of caspase-9 and -3. Collectively, these findings lay the framework for clinical evaluation of GCS-100, either alone or in combination with dexamethasone or PK-11195, to overcome drug resistance and improve patient outcome in MM.