The Parthenolide Derivative LC-1 Is An Effective Single Agent and Is Highly Synergistic with Existing Therapies in Multiple Myeloma.

Multiple myeloma (MM) remains incurable with conventional therapeutic agents and has a median survival of only 3–5 years. Therefore, there is clearly a need for novel treatment strategies that can change the natural pathology of this condition. The nuclear factor κB (NF-κB) family of transcription factors is constitutively activated in MM cell lines and the majority of MM patients. Since NF-κB has known oncogenic activity in a number of human malignancies, targeted inhibition of this family of proteins may be useful in the treatment of MM. We and others have recently shown that the parthenolide derivative LC-1 has activity in acute myeloid leukaemia (AML) and chronic lymphocytic leukaemia (CLL) cells. Unusually, it induces apoptosis via the activation of both the intrinsic and extrinsic pathways and apoptosis is preceded by marked inhibition of NF-κB. Importantly, LC-1 is more potent against primary AML blasts and CLL lymphocytes than normal bone marrow progenitors and normal B-cells and T-cells. In this study we set out to evaluate LC-1 in MM cell lines and plasma cells derived from MM patients. LC-1 was cytotoxic to MM cell lines H929, U266 and JJN3 and induced apoptosis in a dosedependent manner resulting in an overall LD₅₀ of 3.6mM (±1.8) after 48 hours in culture. Primary myeloma plasma cells, identified by CD38 and CD138 positivity, had a mean LD₅₀ for LC-1 of 5.4mM (±1.6) after 48 hours of in vitro culture. Normal bone marrow cells were significantly less sensitive to the effects of LC-1 under the same conditions (P = 0.0007). Treatment of MM cell lines with LC-1 resulted in a decrease in the nuclear localization of NF-κB, as evidenced by a dose-dependent decrease in the DNA binding capacity of the NF-κB subunit RelA after 4 hours of treatment. To demonstrate whether synergy exists between LC-1 and existing MM therapies, the H929 cell line was treated for 48 hours with LC-1 and doxorubicin (32:1), melphalan (1:1) or bortezimib (1:500) and the combination indices (CI) calculated using the median effect method. A combination index of less than 1 denotes synergy. LC-1 did not show synergy with doxorubicin (CI >1) but was synergistic with melphalan and bortezimib (CI values of 0.53 and 0.59 respectively). Taken together our data clearly demonstrate that LC-1 has activity in MM cell lines and primary MM cells. Its ability to inhibit the nuclear localization of NF-κB is important to its cytotoxic effects. Furthermore, it may also provide an explanation for the synergy demonstrated with melphalan and bortezimib. These results provide a rationale for exploring the potential of LC-1 in clinical studies.