Selective HDAC6 Inhibition Via ACY-1215, Either Alone or in Combination with Bortezomib, Restores Osteoblast Function and Suppresses Osteoclast Differentiation in Multiple Myeloma

Abstract 2908

Bone disease in multiple myeloma (MM) is due to the disruption of the delicate balance between osteoblast (OB)-mediated bone formation and osteoclast (OC)-mediated bone resorption. Agents that target both tumor cells and restore normal bone homeostasis can improve long-term disease control and prolong MM patient survival. It has been demonstrated that in vitro pan HDAC inhibitors accelerate OB maturation and suppress OC maturation, while bortezomib triggers OB activation and inhibits osteoclastogenesis. However it has recently been shown that vorinostat (SAHA), a non-selective HDAC inhibitor, causes bone loss in vivo by inhibiting immature OB.

Here, we evaluated effects of a selective HDAC6 inhibitor ACY-1215 (Acetylon Pharmaceuticals, Inc), alone and in combination with bortezomib, on MM cell growth and related bone disease.

ACY-1215 in combination with bortezomib has synergistic cytotoxicity due to simultaneous inhibition of the proteasome and aggresome pathways. We confirm the in vivo anti-MM activity of ACY-1215 in combination with bortezomib in two different xenograft mouse models: human MM injected subcutaneously; and luciferase-expressing human MM injected intravenously (disseminated MM model). Tumor growth was significantly delayed and overall host survival significantly prolonged in animals treated with combined therapy (34 vs 22 days, n=7, p<0.0011) in plasmacytoma model and (40 vs 17 days, n=12, p<0.0001) in disseminated model. Importantly, we show that ACY-1215 alone and in combination with bortezomib overcomes the proliferative effect of bone marrow stromal cells (BMSCs) and cytokines. MM cells stimulate OC formation and function, while inhibiting OB differentiation via both cell-to-cell contact and cytokine secretion. Therefore, osteoclastogenesis is an important therapeutic target in MM. In this context, we evaluated the effect of ACY-1215 (1μM) and bortezomib (2.5nM) on OCs generated from blood mononuclear cells stimulated with receptor activator of nuclear factor kappa B ligand (RANKL). ACY-1215 alone and in combination with bortezomib inhibited OC differentiation, evidenced by a decreased number of TRAP positive multinucleated cells and bone-resorbing activity. In addition, ACY-1215 (1μM) significantly decreased cell growth of mature OC in co-culture with MM cell lines. We next examined the effect of ACY-1215, alone and in combination with bortezomib, on downstream targets in RANKL/RANK signaling. ACY-1215 plus bortezomib inhibits transcription factors implicated in OC differentiation including p-ERK, p-AKT, c-FOS and NFATC1.

Since there is decreased OB function and new bone formation in MM, we next assessed the effect of ACY-1215 on OB differentiation. ACY-1215, alone and in combination, enhanced OB differentiation, evidenced by increased alkaline phosphatase enzyme activity and alizarin red staining. In addition, we show increased mRNA expression of b-catenin, osteocalcin, Runx2 and Sp7 (OB differentiation markers) in immature OB triggered by ACY-1215. Finally, ACY-1215 was not toxic to PHA stimulated PBMCs, suggesting a favorable side effect profile and therapeutic index.

Our studies therefore demonstrate that ACY-1215, alone and in combination with bortezomib, can inhibit osteoclastogenesis enhance osteoblastogenesis, and inhibit MM cell growth. Based upon these studies, ongoing clinical trials are examining the efficacy of ACY-1215 in relapsed MM and associated bone disease.