Among the components of the tumor microenvironment, osteoclasts (OC) and bone marrow stromal cells (BMSC) play a key role in stimulating multiple myeloma (MM) cell proliferation via cell-to-cell contact and cytokine secretion. In a vicious loop, MM cells promote OC development and impair osteoblasts (OB) differentiation shaping the microenvironment to support tumor growth and chemoresistance. These effects translate in osteolytic lesions that negatively affect patient quality of life and survival. CCL3 is involved in the pathogenesis of MM bone disease. Indeed, high bone marrow plasma levels of CCL3 correlate with osteolytic lesions in MM and CCL3 has a pro-osteoclastogenic effect. We have previously shown that inhibition of CCL3 receptor, CCR1, results in impaired osteoclastogenesis and blocks the proliferative advantage conferred by OCs to MM cells in vitro. (Vallet et al, Blood 2007) Here, we extend our studies and demonstrate in an in-vivo SCID-hu mouse model the anti-OC effects of MLN3897, a small molecule CCR1 inhibitor (Millennium Pharmaceuticals, Cambridge). CB17 SCID mice bearing a human fetal bone implant engrafted with INA6 MM cells were treated orally with MLN3897 for a total of 49 doses. After 4 weeks of treatment the bones were harvested and stained for TRAP activity and hematoxylin-eosin. The number of OC/400x field was significantly reduced in the treated group (2.7 vs 1.9, p<0.05), thus confirming in vivo the anti-osteoclastogenic effect of CCR1 inhibition. Although a 2-fold inhibition of tumor growth in the treated group was noted, this difference did not reach statistical significance. Our previous data suggested that CCR1 inhibition impaired OC-MM cell adhesion with subsequent reduced MM cell proliferation. Here, we observed a specific effect of CCR1 inhibition on MM cell/OC adhesion rather than BMSCs and fibronectin. Interestingly, these effects translated in anti-proliferative effects of the CCR1 inhibitor in OC-MM cells coculture with no effects in BMSCs-MM coculture, suggesting a specific inhibitory effect on the OC compartment. Ongoing studies to characterize the underlying mechanism of CCR1 inhbition on OC-MM cell adhesion will be presented. Combination approaches to study whether forced OB differentiation by the proteasome inhibitor, bortezomib, will improve BMSC sensitivity to CCR1 inhibition are being studied. These data confirm the specific in vivo anti-OC effects of CCR1 inhibition and will highlight the role of novel combinations by using anti-OC agents like MLN3897 and anabolic drugs like Bortezomib.

Disclosures: Veiby:Millennium Pharmaceuticals: Employment. Anderson:Millennium Pharmaceuticals, Celgene, Novartis: Speakers Bureau. Raje: Millennium Pharmaceuticals, Celgene, Novartis: Speakers Bureau.

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