PR-924, a selective inhibitor of the immunoproteasome subunit LMP-7, blocks multiple myeloma cell growth both in vitro and in vivo.

PR-924 is an LMP-7-selective tripeptide epoxyketone proteasome inhibitor that covalently modifies proteasomal N-terminal threonine active sites. In the present study, we show that PR-924 inhibits growth and triggers apoptosis in multiple myeloma (MM) cell lines and primary patient MM cells, without significantly affecting normal peripheral blood mononuclear cells. PR-924-induced apoptosis in MM cells is associated with activation of caspase-3, caspase-8, caspase-9, BID, PARP and cytochrome-c release. In vivo administration of PR-924 inhibits tumour growth in human plasmacytoma xenografts. Results from SCID-hu model show a significant reduction in the shIL-6R levels in mice treated with PR-924 versus vehicle-control. PR-924 treatment was well tolerated as evidenced by the lack of weight loss. Importantly, treatment of tumour-bearing mice with PR-924, but not vehicle alone, prolonged survival. Our preclinical findings therefore validate immunoproteasome LMP-7 subunit as a novel therapeutic target in MM.

High-dose zoledronic acid impacts bone remodeling with effects on osteoblastic lineage and bone mechanical properties.

PURPOSE: The increasing incidence of osteonecrosis of the jaw and its possible association with high cumulative doses of bisphosphonate led us to study the effects of high doses of zoledronic acid (ZA) on bone remodeling. EXPERIMENTAL DESIGN: Five-week-old C57BL6 mice were treated with saline or ZA weekly for 3 weeks at increasing doses (0.05-1 mg/Kg). Effects of ZA on bone remodeling were studied using standard assays. RESULTS: We observed an increase in bone mineral density and content in treated animals at doses of 0.05 mg/Kg, which was not further enhanced at higher doses of ZA. Trabecular bone volume at the proximal tibia and the distal femur assessed by histomorphometry and microCT, respectively, increased significantly in ZA-treated groups. There was however no difference between 0.5 and 1 mg/kg, suggesting a ceiling effect for ZA. ZA led to decreased numbers of osteoclasts and osteoblasts per bone perimeter that paralleled a significant reduction of serum levels of TRAC5b and osteocalcin in vivo. Effects on osteoblasts were confirmed in in vitro assays. Mechanical testing of the femur showed increased brittleness in ZA-treated mice. CONCLUSIONS: High doses of ZA inhibit both osteoclast and osteoblasts function and bone remodeling in vivo interfering with bone mechanical properties. No dose response was noted beyond 0.5 mg/kg suggesting that lower doses of ZA may be adequate in inhibiting bone resorption. Our data may help inform future studies of ZA use with respect to alternate and lower doses in the treatment of patients with cancer bone disease.

MLN3897, a novel CCR1 inhibitor, impairs osteoclastogenesis and inhibits the interaction of multiple myeloma cells and osteoclasts.

The interaction between osteoclasts (OCs) and multiple myeloma (MM) cells plays a key role in the pathogenesis of MM-related osteolytic bone disease (OBD). MM cells promote OC formation and, in turn, OCs enhance MM cell proliferation. Chemokines are mediators of MM effects on bone and vice versa; in particular, CCL3 enhances OC formation and promotes MM cell migration and survival. Here, we characterize the effects of MLN3897, a novel specific antagonist of the chemokine receptor CCR1, on both OC formation and OC-MM cell interactions. MLN3897 demonstrates significant impairment of OC formation (by 40%) and function (by 70%), associated with decreased precursor cell multinucleation and down-regulation of c-fos signaling. OCs secrete high levels of CCL3, which triggers MM cell migration; conversely, MLN3897 abrogates its effects by inhibiting Akt signaling. Moreover, MM cell-to-OC adhesion was abrogated by MLN3897, thereby inhibiting MM cell survival and proliferation. Our results therefore show novel biologic sequelae of CCL3 and its inhibition in both osteoclastogenesis and MM cell growth, providing the preclinical rationale for clinical trials of MLN3897 to treat OBD in MM.

Honokiol overcomes conventional drug resistance in human multiple myeloma by induction of caspase-dependent and -independent apoptosis.

Honokiol (HNK) is an active component purified from magnolia, a plant used in traditional Chinese and Japanese medicine. Here we show that HNK significantly induces cytotoxicity in human multiple myeloma (MM) cell lines and tumor cells from patients with relapsed refractory MM. Neither coculture with bone marrow stromal cells nor cytokines (interleukin-6 and insulin-like growth factor-1) protect against HNK-induced cytotoxicity. Although activation of caspases 3, 7, 8, and 9 is triggered by HNK, the pan-caspase inhibitor z-VAD-fmk does not abrogate HNK-induced apoptosis. Importantly, release of an executioner of caspase-independent apoptosis, apoptosis-inducing factor (AIF), from mitochondria is induced by HNK treatment. HNK induces apoptosis in the SU-DHL4 cell line, which has low levels of caspase 3 and 8 associated with resistance to both conventional and novel drugs. These results suggest that HNK induces apoptosis via both caspase-dependent and -independent pathways. Furthermore, HNK enhances MM cell cytotoxicity and apoptosis induced by bortezomib. In addition to its direct cytotoxicity to MM cells, HNK also represses tube formation by endothelial cells, suggesting that HNK inhibits neovascurization in the bone marrow microenvironment. Taken together, our results provide the preclinical rationale for clinical protocols of HNK to improve patient outcome in MM.