Direct Effect on the Stroma by the Conventional Anti-Multiple Myeloma Drug Dexamethasone Results In Resistance of Multiple Myeloma Plasma Cells Against Therapy. Sensitisation to Dexamethasone by the Kinase Inhibitor Dasatinib

Abstract 1931

Dexamethasone is a conventional anti-multiple myeloma (MM) drug that effectively induces MM cell death at presentation and in relapsed patients that continues to be used against MM alone or in combination with the new therapeutic agents. However, MM cells develop resistance to Dexamethasone and several changes in MM cells have been proposed to explain the lack of response. Long exposure to Dexamethasone can induce genetic changes in MM cells resulting in downregulation or expression of mutated or truncated forms of the glucocorticoid receptor. Additionally, overexpression of members of the ABC transporter will extrude Dexamethasone and reduce intracellular levels, leading to resistance. Resistance to Dexamethasone can also arise from the interaction of MM cells with the bone marrow (BM) microenvironment. More specifically, the pro-apoptotic effect of Dexamethasone can be blocked by cytokines secreted in the BM microenvironment or the contact of MM cells to the BM stroma leading to cell adhesion mediated drug resistance. We now report new mechanisms of resistance involving a direct impact of Dexamethasone on the BM microenvironment independently of the presence of MM cells. We show using a novel co-culture experimental platform that resistance to Dexamethasone in MM results from the direct pro-proliferative effect of Dexamethasone on fibroblasts in the BM stroma, which involves the Src family of kinases. Using both MM cell lines and BM aspirates from MM patients, we found that MM cells in the presence of fibronectin accelerate Dexamethasone-induced proliferation of fibroblasts. Additionally, we found that the integrity and functionality of osteoclasts is not affected by treatment with Dexamethasone. These results suggest that resistance of MM cells to Dexamethasone can arise from the direct effect of the drug on fibroblasts as well as on the lack effect on osteoclasts. This would result in formation of BM niches that prevent apoptosis of MM induced by Dexamethasone. Our data show that adhesion of MM cells on stromal cells or osteoclasts rather than adhesion on fibronectin promote MM cell adhesion drug resistance to Dexamethasone. The Src and c-Abl kinase inhibitor Dasatinib used at clinically achievable doses (150 nM) blocked adhesion of MM cells on fibroblasts and osteoclasts. Additionally, both fibroblast proliferation and osteoclasts activity can be blocked with Dasatinib resulting in sensitisation of MM cells to treatment with Dexamethasone. However, Dasatinib alone did not affect MM cell proliferation or viability. Our data support the therapeutic use of Dexamethasone in combination with Src inhibitors against MM to target MM cell viability and the protective role of the BM microenvironment, respectively. Our results also show a new mechanism that promotes drug resistance through the direct effect of anti-tumour therapeutic drugs on the stroma independently of the presence of tumour cells. Our data support the use of co-culture experimental platforms of tumour cells with the tumour microenvironment to evaluate and predict more accurately the effect of possible anti-tumoral activity of therapeutic drugs on both tumour cells and stroma as well as to understand the mechanisms of drug resistance.