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

The bisphosphonate YM529 inhibits osteoblastic bone tumor proliferation of prostate cancer

BACKGROUND

In men, prostate cancer frequently metastasizes to the bones, where it forms osteoblastic lesions with an osteolytic element that cause pain. However, the role of osteoclastogenesis in bone metastasis of human prostate cancer is unknown. Bisphosphonates are already known to be beneficical for treating osteolytic bone metastases, so we employed a model of osteoblastic bone tumor of human prostate cancer to investigate whether a new bisphosphonate (YM529: minodronate) could inhibit both the formation of bone tumors and the progression of established osteoblastic tumors.

METHODS

Human prostate cancer cells (LNCaP) were injected into adult human bone implants in nonobese diabetic/severe combined immunodeficient mice, after which osteoblastic bone tumors developed. YM529 (1 microg/day) was administered subcutaneously every day for 2 weeks, starting either immediately or 2 weeks after implantation of the tumor cells, and the mice were sacrificed at 4 weeks after implantation. The bone tumors were examined histologically and the number of tartrate-resistant acid phosphatase-stained osteoclasts in each tumor focus was counted.

RESULTS

Histomorphometric analysis revealed that YM529 markedly inhibited both the formation of bone tumors and the progression of established tumors, as well as markedly reducing the number of osteoclasts.

CONCLUSIONS

YM529 reduced the tumor burden in bone by inhibiting both the formation of new lesions and the progression of existing tumors, suggesting that osteoclasts are involved in the formation of bone tumors by prostate cancer. Treatment with this bisphosphonate may potentially be beneficial for patients with bone metastases of prostate cancer.

The potential role of bisphosphonates in prostate cancer

Skeletal morbidity secondary to metastases and osteoporosis is common in patients with advanced prostate cancer. Despite the typically sclerotic nature of prostate cancer metastases, osteoclast mediated osteolysis may play a significant role. This review addresses the newly recognised antitumour effects of bisphosphonates in addition to their role in inhibiting osteoclast mediated bone resorption. Both preclinical and clinical evidence of a role for bisphosphonates in the treatment and prevention of bone metastases secondary to prostate cancer is assessed.

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.

Skeletal response to clodronate in prostate cancer with bone metastases

Bone metastases, together with generalized bone resorption, represent the main complication in patients with advanced prostate cancer, and palliative treatments are required to delay the progression of the metastases and improve the quality of life of these patients. For this reason, the bisphosphonate clodronate was administered to 18 patients (clodronate group) from a total of 30, all of whom were receiving complete androgenic blockade; the remaining 12 formed the control group. Transiliac bone biopsies were taken at the beginning of the study and 6 months later to determine the effect of the bisphosphonate on the skeleton. The results were assessed by bone histomorphometry and showed, although without statistical significance between the groups, an antiresorptive effect of the clodronate expressed as the eroded surface/bone surface and as the osteoclast number/bone surface. However, the bone volume also decreased after 6 months of treatment. Similarly, osteoid formation decreased as indicated by the osteoid surface and by the osteoid volume, probably due to the effect of the drug on the osteoblasts. The mineralization rate was apparently slightly retarded in the clodronate group, although to a lesser degree than in the control group. The results confirm the antiresorptive effect of clodronate and its detrimental effect on osteoblast activity.

Rationale for the use of alendronate in osteoporosis

Bisphosphonates are being used in disorders associated with accelerated resorption of bone, particularly Paget's disease of bone and the bone disease of malignancy. Their undoubted biological efficacy and relatively low apparent toxicity make them attractive candidates for the management of osteoporosis. The bisphosphonate alendronate has many characteristics which suggest that it is suitable for use in osteoporosis. It is a potent inhibitor of osteoclast-mediated bone resorption with no adverse effect on the mineralization of bone. Earlier studies have shown it to be one of the most active bisphosphonates in Paget's disease and the hypercalcemia of malignancy. In common with other bisphosphonates tested thus far, alendronate appears to inhibit bone loss in a variety of experimental models of osteoporosis. Long-term studies are needed to determine its steady-state effects on bone mass in man. Most data indicate that alendronate is capable at least of decreasing the rate of bone loss, and might even induce increments in bone mass for many years. Since the experimental studies show that the increase in bone mass observed with alendronate is associated with an increase in bone strength, its use is likely to decrease the frequency of fractures. However, direct clinical evidence for this requires the outcome of well-designed long-term prospective studies.

New bisphosphonates in the treatment of bone metastases

Normal skeletal integrity is maintained by physiological bone turnover through a coupled process of bone resorption, mediated by osteoclasts, followed by new bone formation, mediated by osteoblasts. Major features of the pathogenesis of cancer-associated skeletal destruction are enhanced osteoclast-mediated bone resorption and disruption of normal bone formation. In this article, the literature on the pathogenesis and clinical manifestations of metastatic bone disease is discussed. Animal and clinical trials investigating novel bone targeted agents, emphasizing the bisphosphonates, are critically assessed. The most frequent clinical manifestations of bone metastases are pain, fracture, immobility, spinal cord compression, and hypercalcemia. New treatments under study for patients with bone metastases include agents specifically targeted to the skeleton such as bone-seeking radioisotopes and bisphosphonates. Studies in animal models of metastatic bone disease show that these bisphosphonates are able to inhibit tumor-induced osteolysis and are potentially useful in this condition. Bisphosphonates have been investigated in several clinical trials of patients with skeletal metastases from breast cancer, prostate cancer, and multiple myeloma. Overall, the studies investigating bone targeted radioisotopes or bisphosphonates for the treatment of morbidity due to skeletal metastases have been inconclusive. An improved understanding of the pathogenesis of metastatic bone disease and preclinical studies with bisphosphonates suggest that these agents may have a role in the treatment of this disorder. Additional trials of new generation bisphosphonates, employing a rigorously controlled, randomized study design with adequate numbers of subjects, are needed to demonstrate the safety and efficacy of this class of agents in this setting.

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.

Bisphosphonates. Pharmacology and use in the treatment of tumour-induced hypercalcaemic and metastatic bone disease

The geminal bisphosphonates are a new class of drugs characterised by a P-C-P bond. Consequently, they are analogues of pyrophosphate, but are resistant to chemical and enzymatic hydrolysis. The bisphosphonates bind strongly to hydroxyapatite crystals and inhibit their formation and dissolution. This physicochemical effect leads in vivo to the prevention of soft tissue calcification and, in some instances, inhibition of normal calcification. The main effect is to inhibit bone resorption, but in contrast to the effect on mineralisation, the mechanism involved is cellular. These various effects vary greatly according to the structure of the individual bisphosphonate. The half-life of circulating bisphosphonates is very brief, in the order of minutes to hours. 20% to 50% of a given dose is taken up by the skeleton, the rest being excreted in the urine. The half-life in bone is far longer and depends upon the turnover rate of the skeleton itself. Bisphosphonates are very well tolerated; the relatively few adverse events that have been associated with their use are specific for each compound. Bisphosphonates have been used to treat various clinical conditions, namely ectopic calcification, ectopic bone formation, Paget's disease, osteoporosis and increased osteolysis of malignant origin. The three compounds commercially available for use in tumour-induced bone disease are in order of increasing potency, etidronate, clodronate and pamidronate. Most data have been obtained with the latter two agents. By inhibiting bone resorption, they correct hypercalcaemia and hypercalciuria, reduce pain, the occurrence of fractures, as well as the development of new osteolytic lesions, and in consequence improve the quality of life. In view of these actions, of their excellent tolerability and of the fact that they are active for relatively long periods, these compounds are, after rehydration, the drugs of choice in tumour-induced bone disease and an excellent auxiliary to the drugs used in oncology.

New bone formation and cancer implants; relationship to tumour proliferative activity

The interaction between tumour and bone with respect to the proliferative activity of transplanted tumour cells was studied using five transplantable human urogenital tumours in nude mice. Cells from those tumours were injected subcutaneously over the calvaria of nude mice following disruption of the periosteum. The extent of tumour-bone interaction varied with the type of implanted tumour as shown on X-ray and by histologic examinations of the calvaria. The classic histologic pattern of bone remodelling including the destruction of bone with proliferation of osteoclasts and reactive new bone formation was seen with all five tumours. Tumour proliferative activity determined from the tumour doubling time and the S-phase fraction using bromodeoxyuridine labelling showed that the rate of reactive bone formation appeared to be inversely proportional to the rate of tumour cell proliferation.

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

A new bisphosphonate, 4-amino-1-hydroxybuthylidene-1,1-bisphosphonate (AHBuBP), was compared with 3-amino-1-hydroxypropylidene-1,1-bisphosphonate (AHPrBP) and 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) in terms of its effect on tumor-induced osteolysis using a bladder tumor in mice (MBT-2). Tumor cells were inoculated subcutaneously (SC) over the calvaria in mice, resulting in a local tumor causing fragmentation of the bone. The tumor-induced osteolysis associated with osteoclasts proliferation was accompanied with reactive new bone formation. This osteolysis was evaluated by measuring the increased area of bone resorption in reduced opacity to radiograph and histologic study. The results showed the following sequence of potency: AHBuBP greater than AHPrBP = HEBP. This inhibition was obtained with no apparent effect on the growth of the MBT-2 tumor. The authors conclude that AHBuBP appears to be an interesting new bisphosphonate with possible clinical application.