In this study we evaluated whether preventive dose administration of the multi-tyrosine kinase Sunitinib is effective in inhibiting bone metastases arising from disseminated tumor cells. We show that Sunitinib used in the prophylactic setting does not decrease the number of metastatic colonized sites or inhibit subsequent progression of osteolytic lesions. There is however a reduction in tumor growth which is associated with a significant decrease in tumor blood vessels. The findings suggest that Sunitinib alone is not able to prevent colonization and expansion of disseminated tumor cells to bone but may be a useful adjunctive therapy in reducing metastatic tumor burden.
It has been reported that prior administration of Sunitinib at elevated doses (120 mg/kg/day) induce a "conditioning effect" which promote the formation of metastasis by circulating tumor cells . In this study pre-treatment of mice with a dose of 40 mg/kg/day did not lead to enhanced metastasis. This observation is consistent with the findings of others that have used lower therapeutically efficacious doses [34, 35] and support a rationale for application of lower doses of Sunitinib to avoid augmented invasive or metastatic potential.
The concept of the vicious cycle of bone metastases emphasizes the cross-talk between tumor cells and the bone microenvironment in the process of tumor growth and bone destruction [36, 37]. Following colonization to the bone, secretion of factors by tumor cells stimulate osteoclastogenesis via upregulation of receptor activator of nuclear factor kappa B ligand (RANKL) on osteoblast and stromal cells. The increased bone resorptive activity releases growth factors from the bone matrix which in turn stimulate tumor cell growth giving rise to further bone destruction. The observation that Sunitinib did not show any therapeutic efficacy in inhibiting osteolytic lesions despite a reduction in tumor size suggest that osteoclast proliferation and activity was sufficiently stimulated through secretions of growth factors by the tumor. Increased osteolyses associated with tumor shrinkage and decreased vascularization was described previously for the treatment of bone metastases using the histone deacetylase inhibitor Vorinostat . The effect was attributed to off-target effects of the drug on a sub-population of resistant cells giving rise to increased secretion of factors promoting osteolysis. Sunitinib has recognized off-target activity with one report indicating binding to at least 5 off-target kinases with high affinity . A possible off-target effect following Sunitinib therapy is the tumor-independent increase in systemic levels of factors such as granulocyte colony-stimulating factor and osteopontin , factors which are known to promote bone resorption [41–46]. In this regard, the application of second-generation tyrosine kinase inhibitors (TKI's), such as pazopanib and tivozanib, with improved potency and selectivity may provide more effective treatment options [47, 48]. Another possibility which may explain our observation is the adaption of an evasive response by MDA-MB231 cells. The cell line is of metastatic origin and an evasive response to disturbed tumor vasculature may be manifested by increased secretion of certain factors which induce osteolysis. Adaptive evasive responses leading to increased invasiveness and distant metastases have been observed in mouse models of pancreatic neuroendocrine carcinoma and glioblastomas following antiangiogenic targeting of VEGF signaling [49, 50]. The mechanism by which this occurs is not understood but the hypoxia/HIF-1α pathway is implicated.
With an observed decrease in tumor growth in bone there is a compelling biological rationale for the use of targeted combination therapy with an anti-resorptive agent. A retrospective study of patients with bone metastases from renal cell carcinoma revealed an increase in progression free survival and response rate in patients treated concomitantly with bisphosphonate and Sunitinib . Amongst targeted agents which inhibit osteoclast activity and are currently subject to clinical evaluation in breast cancer bone metastases are inhibitors to mTOR, RANKL, Src and Cathepsin K. A recent clinical trial administrating the mTOR inhibitor Everolmus, a rapamycin derivative which inhibits mTORC1, showed beneficial effects on bone turnover and bone progression in postmenopausal women with oestrogen-receptor-positive breast cancer . Denosumab, a human neutralizing antibody to RANKL, has been shown to have a stronger inhibitory effect on bone resorption than bisphosphonates [53–55]. Since long term Denosumab therapy is generally well tolerated, introduction of Denosumab as a second agent for the inhibition of osteoclast activity, may further deprive tumor cells of growth factors sequestered in the bone matrix. In view of the fact that bone remodeling is a complex process requiring orchestrated differentiation of different cell types as well as angiogenesis, establishment of treatment order of agents and dosage may influence therapeutic effects of combination treatment .
Certain limitations of this study are outlined. One is that only a single cell line was employed which is unlikely to reflect the phenotypic properties of all disseminating cells especially those disseminating from the primary tumor at an early stage of the disease. Secondly, the intracardiac injection model does not recapitulate the requirement for cells to escape the primary site and so not all the steps of the metastatic process can be studied. Thirdly, the use of mouse models in itself has the limitation that ethical time points arrive much earlier than for example in a rat model and it is uncertain whether a longer period of Sunitinib treatment may eventually had an effect on osteolytic size.
With an increasing number of targeted therapies entering clinical trials, there is an urgent need to apply these drugs for the prevention of fully developed metastatic lesions arising from disseminated tumor cells. With advances in molecular biomarkers that refine and improve risk stratification in patients, those with a high risk of developing bone metastases may benefit from adjuvant or prophylactic treatment with targeted therapeutics, as long as the cytotoxicity is supportive of long term chronic administration. The targeted therapies that may come into consideration are those that target genes involved in disseminated tumor cells infiltration, survival and colonization of the bone. In the event that metastases ensue, the drug can then be combined with a cytotoxic agent to invoke additive antitumor activity.
The data presented here establish that Sunitinib inhibits tumor bone growth when applied in the preventive setting. Further preclinical studies are warranted to test the potential combination with an anti-resorptive agent in the prophylactic setting, with the aim to improve their overall antitumor efficacy. With the development of more specific and well tolerated second generation TKIs which inhibit VEGF signaling, the prospect of applying these agents in combination with anti-resorptive agents holds significant promise for the prevention of bone metastatic disease.