Ex Vivo Graft Purging and Expansion of Autologous Mobilized Peripheral Blood Progenitor Cell (PBPC) Products from Patients with Multiple Myeloma (MM).

Introduction: Autologous PBPC transplantation has become the treatment of choice for selected MM patients. Autografts are often contaminated with MM cells which could be a source of post-transplant relapse. We elected to develop a graft manipulation procedure to purge tumor cells from the autograft without compromising its normal reconstituting activity. Previous studies have shown that bortezomib (Velcade, Millenium Pharmaceuticals, Inc., Cambridge, MA) can kill CD138⁺ MM cells, with little effect on CD138⁻ B cells. However, the anti-CD20 monoclonal antibody, rituximab (Rituxan, Genentech, Inc., San Francisco, CA) has been shown to kill CD138⁻ B cells, the putative MM “stem cells” population. Therefore we investigated whether an optimized combination of bortezomib and rituximab might effectively eradicate tumor cells from PBPC products. Previous studies¹ have also suggested that tumor cells can also be selectively purged by ex vivo culture while expanding the normal hematopoietic progenitors. Thus, we incorporated an ex vivo culture step to optimize MM cell depletion and expansion of the reconstituting normal progenitors.

Methods: CD138⁺ cells were depleted from the thawed PBPC of MM patients using the midiMACS device (Miltenyi Inc, Auburn, CA). The CD138⁻ cells were treated for 24 hrs with 10 or 20 μg/ml rituximab followed by 20nM or 80nM bortezomib for 16hrs. Cells were then washed and ex vivo-expanded using an allogeneic normal marrow donor-derived mesenchymal stem cell (MSC) co-culture technique as previously reported.² At the end of culture, cells were evaluated for total viable cells, expression of CD138, CD20, CD19, CD34, CD45 by flow cytometry and colony-forming cell (CFC) content in methylcellulose assays (StemCell Technologies, Vancouver, BC).

Results: CD138⁺, CD138⁻/CD20⁺ or CD138⁻/CD20⁺/CD19⁺ cells were depleted in the MM PBPC products from patients when treated with bortezomib (20nM) and rituximab (20 μg/ml) followed by ex vivo culture.(Fig 1) Compared with input control, TNC increased by 6–72 fold and absolute numbers of CD34⁺ cells increased by approximately 3–9 fold. (See representative data Fig 2)

Conclusion: Treating CD138-depleted MM PBPC products with bortezomib, rituximab and 2 weeks of ex vivo culture depleted CD138⁺ malignant plasma cells and CD138⁻ B cells (MM “stem cells”). An estimated >4 log tumor depletion from a mobilized PBPC product was achieved, while the use of ex vivo expansion culture not only preserved but increased the number of normal hematopoietic progenitors. Further refinements of this procedure are in progress and will be tested clinically.

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