Combination of Proteasome Inhibitor PS 341 (Velcade®) with Histone Acetylase Inhibitor (HDAC) PXD 101 Shows Superior Anti-Myeloma Activity and Inhibits Osteoclastogenesis.

Background: PS 341 has demonstrated activity in multiple myeloma (MM), and PXD 101 is a new HDAC inhibitor, currently being evaluated in Phase II clinical trials for MM. In addition to the anti-myeloma activity of PS 341 and PXD 101, HDAC inhibitors are strong inhibitors of osteoclast formation. Therefore, the combination of the two agents, targeting both the tumor and the development of osteolytic lesions should lead to an optimal treatment strategy for MM.

Methods: MM cell lines (MM.1S, OPM 2, RPMI 8226) were exposed to serial dilutions of PS 341 (1–1000 nM) and PXD 101 (10–1000 nM) for 48 hours. Thymidine uptake was measured to assess inhibition of proliferation. To evaluate the effects of PS 341 and PXD 101 on the formation of osteoclasts from mononuclear hematopoietic precursors, we used a human bone marrow culture system from healthy donors and MM patients to generate osteoclasts using in vitro RANKL/M-CSF stimulation.

Results: Dose-dependent inhibition of proliferation was achieved in all MM cell lines after a 48-hour treatment with PS 341 and PXD 101. For further experiments we used PS 341 at a fixed low concentration (10 nM), which showed no significant inhibition of proliferation of MM cell lines. PXD 101 was used at 100 nM and demonstrated median inhibition of 46% (range 0–48%). The combination of PS 341 with PXD 101 induced synergism with inhibition of proliferation of median 73% (range 44–73%). Although 100 nM PXD 101 and 50 nM PS 341 used as single agents were required to inhibit the proliferation of MM cells, combination of 25 nM PXD 101 and 1 nM PS 341 was sufficient to inhibit OCL formation completely, indicating that the drug combination results also in a highly synergistic inhibition of osteoclast formation.

Conclusions: These studies provide the framework for clinical evaluation of PS 341 combined with PXD 101 in patients with MM and supports the hypothesis that the drug combination targets both the tumor and osteoclastic activity. (Drugs provided by NIH/NCI; Support: UO1 CA099168)