Leukemia https://doi.org/10.1038/s41375-018-0278-7 BRIEF COMMUNICATION Multiple myeloma gammopathies Preclinical assessment of an antibody–PBD conjugate that targets BCMA on multiple myeloma and myeloma progenitor cells Krista Kinneer 1 ● Matt Flynn 1 ● Suneetha B. Thomas 1 ● John Meekin 1 ● Reena Varkey 2 ● Xiaodong Xiao 2,6 ● Haihong Zhong 1 ● Shannon Breen 1 ● Paul G. Hynes 1,7 ● Ryan Fleming 2 ● Binyam Bezabeh 2,8 ● Cui (Tracy) Chen 1 ● Leslie Wetzel 1 ● Ruoyan Chen 3 ● Nazzareno Dimasi 2 ● Yu-Tzu Tai 4 ● Kenneth C. Anderson 4 ● Ronald Herbst 1 ● Philip W. Howard 5 ● Elaine M. Hurt 1 ● David A. Tice 1 Received: 6 June 2018 / Revised: 27 August 2018 / Accepted: 3 September 2018 © Springer Nature Limited 2018 Although current treatments are effective in controlling multiple myeloma (MM) and prolonging remission, the disease invariably recurs. A possible explanation for the recalcitrance of MM is that a subpopulation of chemore- sistant cells with clonogenic potential, distinct from the bulk of myeloma cells, may be responsible for recurrence [1, 2]. Several studies have shown that some MM cells lack a mature plasma cell phenotype and that these less differ- entiated myeloma progenitor cells (MPCs) can transfer the disease, share immunoglobulin H (IgH) gene rearrange- ments with mature MM cells, show greater clonogenic growth, and predict a poorer patient response to both stem cell transplant and treatment with proteasome inhibi- tors [3–6]. B-cell maturation antigen (BCMA) is absent in non- hematopoietic tissues and is selectively expressed in plas- mablasts, plasma cells, and plasmacytoid dendritic cells [7, 8]. BCMA expression has been linked to hematologic malignancies, most notably MM, where it is expressed in all stages of the disease. Although the expression and functional relevance of BCMA makes it an attractive target for the treatment of MM, its expression in MPCs is unknown. Herein, we characterize the MPC population present in MM patient bone marrow and examine expression of BCMA in these cells. In addition, we describe the preclinical develop- ment of MEDI2228, a novel BCMA-targeting antibody–drug conjugate (ADC) for the treatment of MM. Although the exact phenotype of the MM stem cell remains elusive, many studies have identified MPCs that lack expression of the mature plasma cell marker CD138 [9]. Therefore, we used flow cytometric sorting of the four possible cell populations defined by CD138 and CD19, a marker of less mature B cells, followed by methocellulose assays to identify the clonogenic population(s) present in MM patient bone marrow (see Online Supplement for detailed methods). In all cases (N = 4), the CD19 + CD138 – cells were found to be capable of forming colonies (Fig. 1a). There was no clonal outgrowth of the CD19 – CD138 + cells, which represent the MM and nonmalignant plasma cells. A few colonies were detected in the CD19 – CD138 – fraction, but this fraction does not contain B cells (CD19 + ) or plasma cells (CD138 + ) and may represent a small number of hematopoietic progenitor cells remaining in the purified sample despite fluorescence-activated cell sorting (FACS)-- based depletion. To further investigate the cell phenotypes present in MM patient bone marrow, we conducted flow cytometric ana- lysis with molecules that help define MM cells, such as CD56 [10], kappa and lambda light chains, and cell surface markers that are indicative of different states of B-cell dif- ferentiation, including CD19 and CD138 (see Online Sup- plement for detailed methods). As expected, the MM cells showed a light-chain restriction, with only kappa expression (Fig. 1b, top right plot), whereas the normal plasma cells * Krista Kinneer kinneerk@medimmune.com 1 MedImmune, Oncology Research, Gaithersburg, MD, USA 2 MedImmune, Antibody Discovery and Protein Engineering, Gaithersburg, MD, USA 3 MedImmune, Biosuperiors, Gaithersburg, MD, USA 4 Dana-Farber Cancer Institute, Boston, MA, USA 5 Spirogen, London, UK 6 Present address: Bristol-Myers Squibb, Redwood City, CA, USA 7 Present address: Cell Medica, London, UK 8 Present address: Salubris Biotherapeutics, Gaithersburg, MD, USA Electronic supplementary material The online version of this article (https://doi.org/10.1038/s41375-018-0278-7) contains supplementary material, which is available to authorized users. 1234567890();,: 1234567890();,: