Effects of Cabozantinib Alone and in Combination with Bortezomib in the 5TGM1 Murine Multiple Myeloma Model

Introduction: Cabozantinib (cabo) is an inhibitor of tyrosine kinases including MET, VEGFR2, RET, and the TAM family kinases TYRO3, AXL, and MER. Cabo has shown clinical activity in patients with castration-resistant prostate cancer and other solid tumors with bone metastases. Multiple myeloma (MM) is the second most common hematologic malignancy, and represents ~2% of all cancer deaths. MM is a monoclonal B-cell (plasma cell) neoplasia with clinical hallmarks of multiple osteolytic lesions causing bone pain, pathologic fractures, and hypercalcemia. Circulating levels of HGF and VEGF are upregulated in MM patients, and regulation of plasma cell-osteoblast communication by the HGF-MET signaling pathway has been implicated in the development of lytic bone disease in these patients. Thus, the primary objectives of this research were to A): determine the activity of cabo on bone lesions and tumor burden in the syngeneic 5TGM1 mouse MM model (study 1), and B): investigate the impact of cabo on overall survival of these mice when dosed alone or in combination with bortezomib (btz) (study 2).

Methods: Female C57BL/KaLwRij mice were allocated to treatment groups (n=15-16 per group) with equivalent average body weights. Four experimental groups were utilized in each of 2 studies: a vehicle control group, and groups receiving single agent btz (0.5 mg/kg ip twice a week) or cabo (10 mg/kg, PO QD). Study 1 also included a higher dose cabo group (30 mg/kg, PO QD), and study 2 included a combination group: btz (0.5 mg/kg ip twice a week) plus cabo (10 mg/kg, PO QD). In study 2, each single agent group also received the vehicle from the alternate single-agent group via the appropriate route and schedule.

On day 0, animals were inoculated with 5TGM1 mouse myeloma cells by IV administration. Dosing began on day 1 and continued daily until euthanasia at day 35 (study 1) or day 70 (study 2). Body weights were determined twice a week and blood samples were collected on days -1, 15, 22, and 34 for analysis of paraprotein (IgG2b) and TRACP 5b. In study 1 the development of osteolytic lesions was detected by radiography at the end of the study. Some animals were euthanized before the end of the experiment due to paraplegia. Animals euthanized within four days of the end of the experiment in study 1 were included in the analyses.

Results: In study 1, btz reduced serum IgG2b levels, and decreased the frequency of soft tissue lesions, but did not show bone protective properties. Cabo exhibited bone protective effects: mean and total area of osteolytic lesions were reduced at the 30 mg/kg dose, and serum TRACP 5b values and osteoclast counts at the tumor-bone interface were reduced at both the 10 and 30 mg/kg doses. Relative bone area did not differ from control according to histomorphometry. The rise in serum IgG2b started earlier than vehicle control in both cabo-treated groups, but a significant difference was not observed in relative IgG2b at sacrifice. Cabo dose dependently increased the necrotic tumor area in bone, indicating the possibility that the rise in IgG2b may have been due to lysis of plasma cells. Both doses of cabo decreased the frequency of soft tissue lesions.

In study 2, the median survival times were 36 d (vehicle), 43 d (btz), 48 d (cabo), and 55 d (cabo+btz). The prolongation of OS compared to vehicle was statistically significant for the cabo group but not for the btz group. Prolongation of OS in the combination group was significant compared to btz alone, but not when compared to cabo alone.

Conclusions: Cabo showed both bone-protective and anti-tumor effects in this murine model of MM. In addition, statistically-significant prolongation of overall survival was observed with the combination of cabo+btz compared to btz alone, and with single-agent cabo compared to vehicle. Based on these results, further investigation of cabozantinib alone or in combination with other agents in multiple myeloma is warranted.