University of Nebraska Medical Center Omaha, Nebraska, United States
Background: Multiple myeloma (MM) is a hematological malignancy that causes severe bone disease in nearly 90% of its patients. Myeloma bone disease (MBD) is due to the ability of MM cells to disrupt normal bone homeostasis and promote excessive bone resorption, and it is associated with an increased risk of mortality and significantly impacts patient quality of life. Current treatments include nitrogen bisphosphonates (NBP) and the monoclonal antibody denosumab, which are known to reduce skeletal related events but also cause adverse side effects (e.g. kidney damage, osteonecrosis of the jaw). Our lab has developed a novel bisphosphonate, RAM2061, which has direct anti-MM activity in vitro and in vivo, and unlike the NBPs, has prolonged systemic distribution. RAM2061 inhibits the enzyme geranylgeranyl diphosphate synthase (GGDPS) preventing the synthesis of isoprenoid geranylgeranyl pyrophosphate (GGPP) necessary for the post-translational modification (geranylgeranylation) of Rab and Rho small GTPases. We found that by inhibiting geranylgeranylation of Rab proteins in MM cells, we disrupt intracellular protein trafficking, inducing ER stress, and subsequent apoptosis. Osteoclasts, similar to MM cells, rely heavily on secretory pathways for bone resorption. Therefore, we hypothesized that disruption of Rab and Rho protein geranylgeranylation in osteoclast cells should disrupt intracellular trafficking and inhibit bone resorption. Methods: We utilized tartrate-resistant acid phosphatase (TRAP) assays to analyze the effects of RAM2061 on osteoclast generation. We also employed a newly developed LC-MS/MS methodology to characterize binding properties of RAM2061 to bone. Results: In mature osteoclasts, we found RAM2061 to disrupt osteoclastogenesis by significantly downregulating expression of osteoclast-specific genes and inhibiting TRAP+ osteoclast formation. We confirmed that RAM2061 accumulates in the long bones of mice, suggesting this agent may have direct activity against MBD. We further assessed RAM2061 binding properties to hydroxyapatite to characterize interactions with bone. Conclusion: Given its direct anti-MM activity and potential to disrupt excessive osteoclast resorption, these studies provide rationale for further investigation into RAM2061 as a novel therapy for MBD,
