Low Expression of CXCR4 in Bortezomib-Resistant Multiple Myeloma Correlates with Extramedullary Disease in a Murine Mouse Model

Abstract 442

The tumor microenvironment is a critical component of cancer growth involved in initiation, progression, metastasis and chemoresistance. This interplay has been described in the plasma cell neoplasm, multiple myeloma (MM), in which the relationship between transformed cells and the bone marrow milieu aids in maintaining cell viability and, in the case of dexamethasone, confers resistance (Abe, M. Int. J. Hematol. (2011) 94: 334). With the advent of new drugs for the treatment of MM, it is critical that we further define what role the MM niche plays in chemosensitivity.

The proteasome inhibitor bortezomib/VELCADE® (Bz) is approved for the treatment of MM and relapsed mantle cell lymphoma and has been widely used, with much success, in patients. However, despite these advances in the clinic, the majority of MM patients treated with Bz eventually relapse, many as non-responders to additional Bz treatments. Chemokine (C-X-C motif) receptor (CXCR4) is a transmembrane, G-protein coupled receptor that is known to be expressed on both normal and malignant plasma cells and is involved in B cell homing to the bone marrow where its ligand SDF-1 is expressed (Nakayama, et al. J. Immunol. (2003) 170: 1136). It was reported that disruption of the MM cell-stroma interaction through chemical inhibition of the cell-surface protein CXCR4 increases Bz sensitivity both in vitro in the presence of bone marrow stromal cells and in vivo(Azab, et al. Blood (2009) 113: 4341). Based on this observation, we hypothesized that genes involved in bone marrow homing would be highly expressed in Bz resistant cells.

To determine whether genes involved in bone marrow homing are differentially expressed in Bz sensitive and resistant cells, we utilized isogenic pairs of Bz sensitive and resistant MM cell lines derived from the genetically engineered iMycCα/Bcl-x_L mouse model of plasma cell malignancy (Boylan, et al. Cancer Res. (2007) 67: 4069). Initial gene expression profiling of three pairs of mouse cell lines in the absence of Bz revealed significantly decreased expression of Cxcr4 in Bz resistant lines (fold change>2, p<0.05). This observation was validated by flow cytometry in both the Bz resistant mouse lines as well as a derived resistant human MM cell line (MM1.S Bz resistant). Furthermore, when we queried human MM patient GEPs (Zhan, et al. Blood (2006) 108: 2020), we found that low CXCR4 expression was associated with both high-risk and relapsed MM. Based on CXCR4 expression alone we could predict better event-free (p=0.018) and overall survival (p=0.012) in patients with high levels of CXCR4 receiving therapy.

To determine whether low CXCR4 expression in the Bz resistant lines correlates with altered bone marrow homing in vivo, isogenic pairs of Bz sensitive and resistant mouse cell lines were transferred back into immunocompetent, syngeneic animals. The Bz resistant cells conferred a more severe phenotype, killing the mice more quickly than sensitive control cells (16 days median survival). Interestingly, FDG-PET and FLT-PET imaging revealed that in contrast to Bz sensitive cells, which showed characteristic homing to the bone marrow, Bz resistant cells resulted primarily in extramedullary disease. These findings strongly suggest that low CXCR4 expression serves as a biomarker for Bz resistance and extramedullary disease in the iMycCα/Bcl-x_L mouse model of human myeloma.