Promising new drugs are being evaluated for treatment of multiple myeloma (MM), but their impact should be measured against the expected outcome in patients failing current therapies. However, the natural history of relapsed disease in the current era remains unclear. We studied 286 patients with relapsed MM, who were refractory to bortezomib and were relapsed, refractory, or ineligible, to an IMiD (Immunomodulatory Drug), with measurable disease and ECOG PS of 0, 1 or 2. The date patients satisfied the entry criteria was defined as time zero (T0). The median age at diagnosis was 58 years and time from diagnosis to T0 was 3.3 years. Following T0, 213 (74%) patients had a treatment recorded with one or more regimens (median=1; range 0-8). The first regimen contained bortezomib in 55 (26%) patients and an IMiD in 70 (33%). A minor response or better was seen to at least one therapy after T0 in 94 patients (51%) including >=partial response in 69 (38%). The median overall survival and event free survival from T0 were 9 and 5 months respectively. This study confirms the poor outcome once patients become refractory to current treatments. The results provide context for interpreting ongoing trials of new drugs.

Multiple myeloma is largely incurable, despite development of therapies that target myeloma cell-intrinsic pathways. Disease relapse is thought to originate from dormant myeloma cells, localized in specialized niches, which resist therapy and repopulate the tumour. However, little is known about the niche, and how it exerts cell-extrinsic control over myeloma cell dormancy and reactivation. In this study, we track individual myeloma cells by intravital imaging as they colonize the endosteal niche, enter a dormant state and subsequently become activated to form colonies. We demonstrate that dormancy is a reversible state that is switched ‘on' by engagement with bone-lining cells or osteoblasts, and switched ‘off' by osteoclasts remodelling the endosteal niche. Dormant myeloma cells are resistant to chemotherapy that targets dividing cells. The demonstration that the endosteal niche is pivotal in controlling myeloma cell dormancy highlights the potential for targeting cell-extrinsic mechanisms to overcome cell-intrinsic drug resistance and prevent disease relapse.

Vrije Universiteit Brussels, Brussel, Belgium Key Points• BMSCs and MM cells mutually communicate through exosomes, which carry selective cytokines.• BMSC-derived exosomes favor MM cell proliferation, migration, and survival and induce drug resistance to bortezomib.The interplay between bone marrow stromal cells (BMSCs) and multiple myeloma (MM) cells performs a crucial role in MM pathogenesis by secreting growth factors, cytokines, and extracellular vesicles. Exosomes are membranous vesicles 40 to 100 nm in diameter constitutively released by almost all cell types, and they mediate local cell-to-cell communication by transferring mRNAs, miRNAs, and proteins. Although BMSC-induced growth and drug resistance of MM cells has been studied, the role of BMSC-derived exosomes in this action remains unclear. Here we investigate the effect of BMSC-derived exosomes on the viability, proliferation, survival, migration, and drug resistance of MM cells, using the murine 5T33MM model and human MM samples. BMSCs and MM cells could mutually exchange exosomes carrying certain cytokines. Both naive and 5T33 BMSC-derived exosomes increased MM cell growth and induced drug resistance to bortezomib. BMSC-derived exosomes also influenced the activation of several survival relevant pathways, including c-Jun N-terminal kinase, p38, p53, and Akt. Exosomes obtained from normal donor and MM patient BMSCs also induced survival and drug resistance of human MM cells. Taken together, our results demonstrate the involvement of exosome-mediated communication in BMSC-induced proliferation, migration, survival, and drug resistance of MM cells. (Blood. 2014;124(4):555-566) IntroductionMultiple myeloma (MM) is a deadly hematological malignancy characterized by the uncontrolled growth and accumulation of monoclonal plasma cells in the bone marrow (BM), the presence of a monoclonal immunoglobulin fraction in the serum or urine, 1,2 renal failure, and osteolytic bone lesions.3 MM cells depend on the BM microenvironment for their growth and survival through interaction with the BM stromal cells (BMSCs). BMSCs consist mainly of fibroblasts and can secrete different kinds of cytokines, chemokines, growth factors, and small molecular mediators. 4 These functional components trigger MM growth, survival, and progression through several signaling pathways, such as mitogen-activated protein kinase kinase/mitogen-activated protein kinase, focal adhesion kinase, phosphatidylinositol 3-kinase/Akt, MEK/extracellular signal-regulated kinase, and signal transducer and activator of transcription 3, 5 which will ultimately lead to angiogenesis, bone disease, and drug resistance.Exosomes are small (40-100 nm) membrane vesicles secreted by various cell types, including dendritic cells, B cells, T cells, mast cells, epithelial cells, and tumor cells, 6 through the fusion of multivesicular bodies with the plasma membrane.7 Exosomes mediate local cell-to-cell communication by transferring mRNAs, miRNAs, and proteins. Because of their ability to transfer functional com...

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