Al Dewar

Dysregulation of bone remodeling by imatinib mesylate

Imatinib mesylate is a rationally designed tyrosine kinase inhibitor that has revolutionized the treatment of chronic myeloid leukemia and gastrointestinal stromal tumors. Although the efficacy and tolerability of imatinib are a vast improvement over conventional chemotherapies, the drug exhibits off-target effects. An unanticipated side effect of imatinib therapy is hypophosphatemia and hypocalcemia, which in part has been attributed to drug-mediated changes to renal and gastrointestinal handling of phosphate and calcium. However, emerging data suggest that imatinib also targets cells of the skeleton, stimulating the retention and sequestration of calcium and phosphate to bone, leading to decreased circulating levels of these minerals. The aim of this review is to highlight our current understanding of the mechanisms surrounding the effects of imatinib on the skeleton. In particular, it examines recent studies suggesting that imatinib has direct effects on bone-resorbing osteoclasts and bone-forming osteoblasts through inhibition of c-fms, c-kit, carbonic anhydrase II, and the platelet-derived growth factor receptor. The potential application of imatinib in the treatment of cancer-induced osteolysis will also be discussed.

Inhibition of c-fms by Imatinib: Expanding the Spectrum of Treatment

Imatinib is a selective protein tyrosine kinase inhibitor currently used in the treatment of chronic myeloid leukaemia (CML). It specifically suppresses the growth of bcr-abl expressing CML progenitor cells by blocking the ATP-binding site of the kinase domain of bcr-abl. Imatinib also inhibits the c-abl, platelet derived growth factor receptor (PDGFR), abl-related gene and stem cell factor receptor, c-kit, protein tyrosine kinases. It is through inhibition of c-kit that imatinib is also used clinically in the treatment of gastrointestinal stromal tumours. We have recently demonstrated that imatinib also specifically targets the macrophage colony stimulating factor receptor, c-fms, at therapeutic concentrations. Although this finding has important implications with regard to potential side effects in patients currently receiving imatinib therapy, these results suggest that imatinib may also be useful in the treatment of diseases where c-fms is implicated. This includes breast and ovarian cancer and inflammatory conditions such as rheumatoid arthritis. We also speculate that imatinib may be used in diseases where bone destruction occurs due to excessive osteoclast activity, such as in the haematologic malignancy, multiple myeloma.

The tyrosine kinase inhibitor dasatinib dysregulates bone remodeling through inhibition of osteoclasts in vivo

Dasatinib is a potent tyrosine kinase inhibitor that is used to treat chronic myeloid leukemia in patients resistant or intolerant to imatinib mesylate. While designed to inhibit Abl and Src kinases, dasatinib shows multitarget effects, including inhibition of the macrophage colony‐stimulating factor (M‐CSF) receptor c‐fms. We have shown previously that dasatinib abrogates osteoclast formation and activity in vitro owing, in part, to its specificity for c‐fms. In this study we examined whether dasatinib could significantly alter bone volume in a model of physiologic bone turnover. Sprague‐Dawley rats were administered dasatinib (5 mg/kg/day) or vehicle by gavage or zoledronic acid (ZOL; 100 µg/kg/6 weeks) subcutaneously. Following 4, 8, and 12 weeks of treatment, serum biochemical, bone morphometric, and histologic analyses were performed. Whole‐body bone mineral density and tibial cortical thickness where unchanged in the dasatinib‐ or ZOL‐treated animals relative to controls. However, micro–computed tomographic (µCT) analysis of cancellous bone at the proximal tibias showed that trabecular volume (BV/TV) and thickness (Tb.Th) were increased in dasatinib‐treated animals at levels comparable with those of the ZOL‐treated group. These changes were associated with a decrease in osteoclast numbers (N.Oc/B.Pm) and surface (Oc.S/BS) and decreased serum levels of the osteoclast marker c‐terminal collagen crosslinks (CTX‐1). Mineral apposition rate (MAR), bone‐formation rate (BFR), and levels of the serum osteoblast markers osteocalcin and N‐terminal propeptide of type I procollagen (P1NP) were not altered significantly in the dasatinib‐treated animals relative to controls. These studies show that dasatinib increases trabecular bone volume at least in part by inhibiting osteoclast activity, suggesting that dasatinib therapy may result in dysregulated bone remodeling.

Imatinib inhibits the functional capacity of cultured human monocytes

Imatinib is a tyrosine kinase inhibitor that has been reported to specifically inhibit the growth of bcr‐abl expressing chronic myeloid leukaemia progenitors. This drug functions by blocking the ATP‐binding site of the kinase domain of bcr‐abl, and has also been found to inhibit the c‐abl, platelet‐derived growth factor receptor, ARG and stem cell factor receptor tyrosine kinases. Reports have recently emerged demonstrating that imatinib also inhibits the growth of non‐malignant haemopoietic cells. Here, we demonstrate that concentrations of imatinib within the therapeutic dose range inhibit the function of cultured monocytes (CM) from normal donors. A decrease in the response of CM to LPS was observed morphologically and functionally, with CM grown in the presence of imatinib showing decreased pseudopodia formation and inhibition of IL‐6 and TNF‐α production following LPS stimulation. Imatinib also reduced the ability of M‐CSF and GM‐CSF stimulated CM to phagocytose zymosan particles, with uptake of non‐opsonized zymosan by M‐CSF stimulated CM (M‐CM) being most affected. M‐CM that had been cultured in the presence of imatinib were also impaired in their ability to stimulate responder cells in a mixed lymphocyte reaction. These results demonstrate that human monocytes cultured in the presence of imatinib are functionally impaired, and suggest that imatinib displays inhibitory activity against other kinase(s) that play a role in monocyte/macrophage development.

Acquisition of immune function during the development of the Langerhans cell network in neonatal mice

The immunological function of the Langerhans cell (LC) network in neonatal skin was examined by defining the development of cutaneous immunity relative to the structure, phenotype and function of the epidermal LC network in neonatal, juvenile and adult mice. Analysis of epidermal sheets showed the presence of major histocompatibility complex (MHC) II+, multilectin receptor DEC‐205– cells within the epidermis of 3‐day‐old mice; both cell density and DEC‐205 expression increased until day 14. When visualized with antibodies directed at MHC II, the network was poorly formed in 3‐ and 7‐day‐old mice, as there was a lower cell density and poor MHC II expression on dendritic processes, compared to mice at day14. Application of a fluorescent antigen to 3‐day‐old mice revealed that the LC were inefficient in transporting antigen to the draining lymph node. There was an improvement at day 7 and by day 14 comparable numbers of antigen carrying cells were detected in the lymph nodes of 6‐week‐old mice. The reduced antigen carriage in 3‐ and 7‐day‐old mice correlated with a poor contact sensitivity response. This was not simply due to failure to present antigen, but development of immunosuppression, as transfer of T cells from adult mice that were previously treated with antigen when they were 3 days old, to adult recipients resulted in antigen specific immunosuppression. Analysis of CD80 and CD86 expression showed that LC from day 3 skin expressed CD80, but not CD86 and application of antigen through this skin was inefficient in upregulating CD86. These findings indicate that when the neonatal LC network is poorly developed it is functionally immature and antigen applied through this ‘functionally immature network’ results in antigen specific immunosuppression.

Tumor Angiogenesis Is Associated with Plasma Levels of Stromal-Derived Factor-1α in Patients with Multiple Myeloma

Purpose: Multiple myeloma is an incurable hematologic malignancy characterized by increased bone marrow angiogenesis and extensive lytic bone disease. We have previously shown that elevated levels of stromal-derived factor-1α (SDF-1α) in peripheral blood plasma are associated with osteolysis in multiple myeloma patients. We have now examined whether SDF-1α levels also correlate with angiogenesis. Experimental Design: We examined the contribution of multiple myeloma plasma cell–derived SDF-1α in the stimulation of in vitro angiogenesis using a tube formation assay. We also collected trephine and peripheral blood plasma samples from patients with multiple myeloma to analyze microvessel density and SDF-1α levels, respectively. Results: We show that multiple myeloma plasma cell line–derived conditioned medium containing SDF-1α stimulates in vitro angiogenesis. In addition, in a large cohort of patients with multiple myeloma and its precursor condition monoclonal gammopathy of undetermined significance, we confirm previous findings that plasma cell burden correlates with both angiogenesis and plasma levels of SDF-1α. We now extend these observations and show the novel finding that peripheral blood plasma levels of SDF-1α positively correlate with the degree of bone marrow angiogenesis in multiple myeloma and monoclonal gammopathy of undetermined significance patients. Conclusions: High levels of SDF-1α produced by multiple myeloma plasma cells promote osteolysis and bone marrow angiogenesis. Therefore, we propose that inhibition of SDF-1α may be an effective mechanism by which angiogenesis and osteolysis can be reduced in multiple myeloma patients.