Proteasome Inhibitor Treatment in Multiple Myeloma Can Mobilize Hematopoietic Stem Cells in the Absence of G-CSF

Abstract Introduction Proteasome inhibitor (PI) use in patients (pts) with multiple myeloma (MM) has been associated with increased hematopoietic stem/progenitor cell (HSPC) collection yields in both induction and autologous stem cell collection settings (Niesvizky et al., 2013). Animal models have confirmed this observation (Ghobadi et al., 2012). The mechanism remains unclear, but there is suggestion PI treatment affects pathways associated with HSPC anchoring and migration (Niesvizky et al., 2013). The effect of PIs on HSPC migration in the absence of filgastrim (G-CSF) stimulation remains unknown. We sought to characterize the molecular mechanisms of HSPC mobilization in a cohort of pts undergoing active PI treatment. Methods MM pts undergoing treatment with PIs were consented to obtain peripheral blood (PB) under IRB approval. Pts were eligible if they had symptomatic MM and were undergoing treatment with a PI. Pts receiving alkylating chemotherapy (such as cyclophosphamide) in combination with a PI were excluded. Pts were enrolled on the first day of a new cycle containing a PI. PB was drawn prior to administration of the PI (T0) and just prior to the next dose of PI, 24 or 72 hours later (T1), depending on whether the pt was receiving carfilzomib or bortezomib, respectively. PB mononuclear cells were collected and purified with Ficoll-Paque, viably frozen in CS-10 freezing medium and stored in liquid nitrogen. Serum samples were collected after a 1:2 dilution with PBS and stored at -80oF. Cells were later thawed to perform multiparameter flow cytometry and colony forming unit (CFU) assays. Multiparameter flow cytometry was performed using a BD LSR-II and analyzed using FloJo V9.0 software. Cells were gated on CD45dim SSC-lo characteristics. HSPCs were defined as CD34+/CD133+. Pts were stratified into 3 groups (>2, 1-2, Results Twenty-three pts consented and collected at the 2 prespecified time points. Six pts (26%) increased the percentage of peripheral HSPCs>2 fold. Nine (39%) and 8 (35%) pts increased the percentage of HSPCs 1-2 fold and 2fold group were available and revealed TGF-s levels increase 67.24 pg/mL compared to a decrease of 17.67 pg/mL in 5 pts in the 2fold and Discussion Observations have noted increased HSPC yields in animal models and MM pts after treatment with PIs in both induction and mobilizing regimens (Ghobadi et al., 2012; Niesvizky et al., 2013). Here we demonstrate that treatment with PIs is associated with increases in peripheral HSPC percentages in approximately 2/3 of MM pts despite the lack of concurrent G-CSF. Decreased CD90 has previously been observed in peripherally mobilized HSPC products and, similar to TGF-s, plays a role in regulation of Rhokinase GTPase pathways known to affect migration and adherence of many different cell types (Tsuchiya et al., 1997; Kim et al., 2006; Wen et al., 2013; Kim et al., 2014). Our study shows a correlation between decreased CD90 expression and fold increase of peripheral HSPCs. We also found an increase in TGF-s serum levels after treatment in the>2fold group compared to the Disclosures Niesvizky: Onyx Pharmaceuticals: Consultancy, Research Funding, Speakers Bureau; Celgene: Consultancy, Research Funding, Speakers Bureau; Millennium: The Takeda Oncology Company: Consultancy, Research Funding, Speakers Bureau. Mark: Onyx: Research Funding, Speakers Bureau; Millennium: Membership on an entity’s Board of Directors or advisory committees, Speakers Bureau; Celgene: Membership on an entity’s Board of Directors or advisory committees, Research Funding, Speakers Bureau. Rossi: Celgene: Speakers Bureau.