Inhibition by a new bisphosphonate (AHBuBP) of bone resorption induced by the MBT-2 tumor of mice

Antitumor effects of bisphosphonates

BACKGROUND

Bisphosphonates are widely used to treat skeletal complications of malignancy. These drugs accumulate in bone where they inhibit osteoclastic bone resorption and reduce the local release of factors that stimulate tumor growth. The mechanism of action of bisphosphonates is dependent on chemical structure: Nonnitrogen-containing compounds (e.g., etidronate, clodronate) are metabolized into cytotoxic analogues of ATP, whereas the more potent nitrogen-containing compounds (N-BPs; e.g., pamidronate, ibandronate, zoledronic acid) inhibit protein prenylation, thus affecting cell function and survival. Because protein prenylation is required by all cells, not just osteoclasts, the possibility arises that N-BPs could also affect the viability of tumor cells.

METHODS

Several groups have investigated the in vitro effects of bisphosphonates, either alone or in combination with other antineoplastic agents, on the viability and metastatic properties of many tumor cell types. Similarly, the effect of bisphosphonate treatment on osteolysis and tumor burden has been studied in a variety of animal tumor models.

RESULTS

In vitro, submicromolar concentrations of N-BPs inhibited tumor cell adhesion and reduced invasion through extracellular matrix. At higher concentrations, antiproliferative and proapoptotic effects have been reported. In animal models of bone metastases, bisphosphonate treatment markedly reduced osteolytic lesions. There is also evidence of a reduction in tumor burden in bone and occasionally in other organs. Survival may be prolonged, but bisphosphonates do not appear to inhibit the growth of primary soft tissue tumors or orthotopic xenografts.

CONCLUSIONS

The cell culture data clearly demonstrated that N-BPs exert antitumor properties and interact synergistically with other antineoplastic agents. As bisphosphonates accumulate in bone, they can also exert cytostatic effects on tumor cells in bone metastases, either directly or indirectly via osteoclast inhibition and alterations in the bone microenvironment. Further in vivo research is now required to optimize the dosing regimen of N-BPs to exploit fully their antitumor potential.

Alendronate blocks metalloproteinase secretion and bone collagen I release by PC-3 ML cells in SCID mice

We have previously shown that alendronate, a potent bisphosphonate compound, can prevent human PC-3 ML tumor cell metastasis to the bone (Stearns and Stearns, 1996, Oncol Res, 8, 69-75). In this paper, tumor cells were injected into the bone medullary cavity of SCID mice femurs both in vivo and following isolation in vitro. ELISAs showed that the amount of collagen I released in the bone marrow (i.e. in in vitro experiments) and the blood plasma (i.e. in in vivo experiments) was a function of the time of incubation or the number of cells injected in the femurs. ELISAs also showed that the levels of matrix metalloproteinase (MMP-2 and MMP-9) secreted in the bone medullary cavity of the femurs directly correlated with the extent of collagen 1 release. In vitro experiments carried out with 'live' and 'devitalized bone' yielded similar results suggesting that the tumor cells (not the osteoclasts) were primarily responsible for the bone solubilization observed. Alendronate pretreatment of the SCID mice (0.1 mg/kg biweekly for 3 weeks) (or the tumor cells) blocked both MMP production by the tumor cells (and the osteoclasts) and collagen I release, providing direct evidence that alendronate might be utilized to prevent bone destruction by metastatic tumor cells. Zymography indicated that MMP-2 activation might be responsible for bone solubilization. In addition, the data suggest that the plasma levels of collagen I might be a marker of bone metastasis and osteolysis.

Alendronate blocks TGF-beta1 stimulated collagen 1 degradation by human prostate PC-3 ML cells

We have previously shown that alendronate can prevent human PC-3 ML tumor cell metastasis to the bone (Wang and Stearns, 1991, Differentiation, 48, 115-25). In this paper, ELISAs and Western blots showed that TGF-beta1 stimulated the secretion of a 72 kDa matrix metalloproteinase 2 (MMP-2) to enhance the solubilization of radiolabeled collagen 1 by metastatic human prostate PC-3 ML cells. A potent bisphosphonate compound, alendronate, inhibited MMP-2 secretion to block solubilization of collagen 1. Alendronate failed to inhibit MMP-2 activity directly, but instead appeared to block cellular secretion of MMP-2. Alendronate failed to inhibit secretion of tissue inhibitor of metalloproteinase-2 (TIMP-2; the inhibitor of MMP-2) and the decrease in collagen 1 solubilization observed may occur, in part, from changes in the molar stoichiometry of TIMP-2 to MMP-2. We conclude that alendronate may be a potent inhibitor of bone resorption based on its ability to block MMP-2 secretion by tumor cells.

Inhibition of antigen-presenting cell function by alendronate in vitro

Bisphosphonates are potent inhibitors of bone resorption in vivo and are emerging as important and widely used drugs for the treatment of a variety of abnormal bone resorptive processes. In the current study we investigated the in vitro effects of 4-amino-1-hydroxybutylidene-1,1-bisphosphonate (alendronate), a recently developed, extremely potent bisphosphonate, on the immune functions of human peripheral blood mononuclear cells (PBMCs). PBMC proliferation induced by lectins, alloantigens, and a nominal antigen (tetanus toxoid) was inhibited in a dose-dependent manner by alendronate. Pretreatment of monocytes, but not T cells, with the compound at concentrations ranging from 10(-4) to 10(-8) M was inhibitory, indicating that alendronate acts selectively on antigen-presenting cells (APCs). Alendronate did not affect the viability of monocytes or T cells or the expression of cell surface molecules known to play critical roles in antigen presentation. Alendronate exhibited dose-dependent inhibition of the production of interleukin-1 beta (IL-1 beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha) by activated monocytes. The inhibitory effect of 10(-6) M alendronate on PBMC proliferation was reversed by 10 U/ml recombinant rIL-1 beta, whereas other cytokines such as IL-6, TNF-alpha, and granulocyte-macrophage colony-stimulating factor (GM-CSF) had no effect. Thus, alendronate acts on monocytes to inhibit their antigen-presenting/accessory cell functions through a mechanism that can be overcome by exogenous IL-1. The inhibitory effect of this agent on cytokine production may contribute to its inhibitory effect on bone resorption.

Effect of the bisphosphonate risedronate on bone metastases in a rat mammary adenocarcinoma model system

Risedronate (NE-58095) is a third-generation bisphosphonate with very potent antiresorptive activity but few toxic effects. The purpose of this work was to evaluate the effect of risedronate treatment on bone metastases produced in a rat breast cancer model. Berlin Druckrey IV rats inoculated with ENU1564 mammary adenocarcinoma cells were treated daily with risedronate or a saline placebo. Survival times, dictated by extraskeletal metastases (lung, heart, and brain), were not affected by risedronate treatment. Risedronate-treated animals had skeletal changes associated with decreased remodeling of bones undergoing endochondral ossification, most prominently affecting the appendicular skeleton. Despite the skeletal alterations induced by the treatment, the distribution of bone metastases throughout the surveyed skeletal sites was similar for treated and untreated animals. Bone metastases were enumerated in histologic sections of distal femur, spine, and skull. Tumor size was estimated from area measurements obtained from histologic lesions in distal femoral metaphyses and vertebral bodies. A greater number of treated rats had no bone metastases in any of the examined sections (30 versus 16.1% of untreated rats). Multiple bone metastases were observed less frequently in treated rats (33.3 versus 71% of untreated rats). Treated rats had fewer observed bone metastases in each examined site than untreated rats (p < or = 0.025). Mean tumor areas in femora and vertebrae were smaller in treated rats (p < or = 0.05), due to the less frequent presence of very large lesions. In untreated animals, osteoclasts appeared to be active at the tumor/bone interface and osseous structures were often completely replaced by expanding tumors. In contrast, metastases in treated animals caused less disruption of skeletal histoarchitecture. The apparent lack of osteoclastic activity and retention of bone within lesions suggested a decreased contribution of osteoclasts to the bone resorptive process. An in vivo immunohistochemical cell proliferation assay failed to reveal differences in the percentage of dividing tumor cells in bone metastatic sites in treated versus untreated animals. The results demonstrate significant effects of risedronate treatment on the incidence and size of observed skeletal metastases in this model.

Progression delay of prostate tumor skeletal metastasis effects by bisphosphonates

Prostate tumor cell metastasis to the axial skeleton was induced in male Copenhagen rats using the intravenous injection of syngeneic metastatic R3327-MATLyLu tumor cells and concomitant caval vein clamping. The proliferation of tumor cells in the lumbar region was monitored by the progression, within 19 days post tumor cell injection, of hindleg paralysis which appeared in these animals. Histology confirmed the presence of tumor cells within the lumbar spine in 100% of cases displaying hindleg paralysis. Treatment with either of the bisphosphonate drugs, Cl2MDP or APD, suppressed and delayed the development of hind leg paralysis. Bisphosphonate treatment may be expected to delay the onset of axial skeletal metastasis effects in prostate cancer patients.

Response of MBT-2 bladder carcinoma-induced osteolysis to various agents

Tumor-bone interactions were experimentally studied using a bladder tumor in mice (MBT-2). The method consisted of subcutaneously inoculating tumor cells over the calvaria in nude mice after the periosteum was disrupted. This resulted in a local tumor that caused fragmentation of the bone. Bone destruction was found to increase in proportion to the number of osteoclasts in the earlier phase. The osteoclasts decreased in number when the tumors had grown large enough to envelop the residual bone. However, bone destruction continued and seemed to be mediated by the tumor cells by a mechanism that did not involve the osteoclasts. The effects of several agents were investigated in this model. High doses of calcitonin and cyclosporine reduced the bone resorption, and these agents may be effective in the early phase of bone destruction. A bisphosphonate derivative (AHBuBP) inhibited bone resorption markedly in the early and late phases of bone destruction. Autoradiography using carbon 14 (14C)-labeled AHBuBP showed that the isotope was concentrated at the surface of the bone adjacent to the MBT-2 tumors. These results suggest that bisphosphonates may make bone less susceptible to the actions of osteoclasts and tumor cells.

Clinical significance of the vertebral vein in prostate cancer metastasis

A total of 75 prostate cancer and 67 lung cancer patients with positive bone scintigrams were studied. The patterns of spread in the axial skeleton and pelvis were different between the groups. The differences in the distribution of bony metastases between prostate and lung are explained by the role of Batson's vertebral venous plexus. We developed an animal model of spinal bone metastasis to prove this route. As suspension of tumor cells was injected into the tail vein of mice with vena caval occlusion. This procedure reproducibly resulted in metastatic tumor growth in the lumbar region of the vertebral column. The prevalence of spinal bone metastasis is attributed to passage of tumor cells via the vertebral venous plexus.

Effects of a new bisphosphonate (AHBuBP) on osteolysis induced by human prostate cancer cells in nude mice

A new bisphosphonate, 4-amino-1-hydroxybutylidene-1, 1-bisphosphonate (AHBuBP), was compared with 3-amino-1-hydroxypropylidene-1, 1-bisphosphonate (AHPrBP) and 1-hydroxyethylidene-1, 1-bisphosphonate (HEBP) assessing their effects on tumor induced osteolysis using human prostate adenocarcinoma cells in nude mice. The method consisted of inoculating transplantable human prostate cancer cells subcutaneously over the calvaria in nude mice resulting in a local tumor causing fragmentation of the bone. The parameters included assessing the extent of decreased osteolysis in bone as judged by X-ray and histological examination. The results showed the following sequence of potency: AHBuBP greater than AHPrBP greater than HEBP. The compounds were active not only when administered preventively before establishment of bone resorption, but also in an inhibitory fashion once the variables were already under the influence of the tumor. This inhibition was obtained with no apparent effect on the growth of the tumor. AHBuBP appears to be an interesting new bisphosphonate for future clinical use.