Treatment of bone disease in multiple myeloma (MM) has largely focussed on the osteoclast axis while the osteoblast axis has been underexploited. Dkk1, an inhibitor of the wingless int (wnt) pathway, is important in osteoblastogenesis. Increased expression of Dkk1 in a subset of MM patients and its association with lytic bone disease opens up the potential of targeting the osteoblast axis. The aim of this study was to test the effect of a Dkk-1 neutralizing chimeric antibody (Mab B3) on osteoblasts (OB), osteoclasts (OC) and MM cells in the context of the bone microenvironment. First, we tested the expression of Dkk1 in plasma and bone marrow of 16 MM patients and 10 MM cell lines. Dkk1 levels were >18 ng/mL in 2 out of 16 patients; levels were comparable in blood and bone marrow plasma. In contrast, very little Dkk1 (2–9 ng/ml) was produced by bone marrow stromal cells (BMSC). One out of 10 MM cell lines (INA-6) expressed low concentrations of Dkk1 in the supernatant. Next, we tested the effect of Mab B3 on MM cell lines, in the presence or absence of BMSC, and on OB and OC from MM patient derived bone marrow. The effects on OC were evaluated by TRAP staining and pit formation. Effects on OB were assayed by alkaline phosphatase staining and alizarin red assays for calcium deposition. Mab B3 treatment did not demonstrate direct cytotoxic effects on MM cell lines negative for Dkk1. Mab B3, however, enhanced OB differentiation and calcium deposition in a dose dependent manner and inhibited OC differentiation and function, as evidenced by a decrease in number of multinucleated TRAP+ cells and a decrease in pit formation. Ongoing studies are addressing the effect of Mab B3 on MM cells in the context of OC and OB. Mab B3 is also undergoing in vivo testing in a SCID-hu model bearing INA-6 MM cells. These studies and the underlying mechanism of action of Mab B3 will be presented. Our preliminary data suggests that Mab B3 has anabolic bone effects; a corresponding human monoclonal antibody may be useful for the treatment of MM related bone disease. Future studies will evaluate Mab B3 in combination with catabolic agents such as bisphosphonates with the goal of restoring normal bone homeostatsis.

Disclosures: Schirtzinger:Eli Lilly and Company: Employment. Kuhstoss:Ely Lilly and Company: Employment. Anderson:Millenium Pharmaceutical: Speakers Bureau; Novartis: Speakers Bureau; Celgene: Speakers Bureau. Raje:Millenium Pharmaceutical: Speakers Bureau; Novartis: Speakers Bureau; Celgene: Speakers Bureau.

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