The effects of geranylgeranyl diphosphate synthase inhibitor treatment on osteoblast biology and application in a conditioned media model of myeloma bone disease.

Myeloma bone disease (MBD) is characterized by tumor-induced osteoclast activation with concomitant suppression of osteoblast activity. Nitrogen bisphosphonates (NBPs), including zoledronic acid (ZA), are a mainstay of MBD treatment, due to their anti-osteoclastic effects secondary to high bone affinity and indirect disruption of protein geranylgeranylation through inhibition of farnesyl diphosphate synthase. The development of geranylgeranyl diphosphate synthase (GGDPS) inhibitors (GGSIs) represents a more selective means of targeting geranylgeranylation. The GGSI RAM2061 has direct anti-myeloma activity in vitro and in vivo, achieves both systemic and skeletal distribution, and has anti-osteoclastic activity. However, the effects of this novel therapy on osteoblast activity or in a setting that recapitulates the MBD milieu have yet to be explored. Exposure to RAM2061 or ZA during MC3T3-E1 differentiation resulted in impairment in osteoblast function, including alkaline phosphatase and mineralization activity, however minimal effects were observed in differentiated MC3T3-E1 cells that were subsequently exposed to drug. To evaluate the impact of RAM2061 on osteoblast or osteoclast activity under MBD-like conditions, JJN3 myeloma cell conditioned media (CM) was collected and added to bone cell cultures in order to simulate osteoclast-activating or osteoblast-inhibitory MBD microenvironments. These studies determined that RAM2061 maintains anti-resorptive effects and osteoblast inhibitory effects in undifferentiated precursors while in the presence of JJN3 CM. However, no appreciable effects were detected in osteoblasts exposed to drug post-differentiation. Overall, these studies contribute to the mechanistic understanding of NBP and GGSI effects in the bone and provide support for the continued investigation of GGSIs in MBD.