Introduction: Prostate cancer (PCa) bone metastases have been shown to be more resistant to docetaxel than soft tissue metastases. The proinflammatory chemokine receptor CXCR4 has been shown to confer resistance to docetaxel (DOC) in PCa cells. Balixafortide (BLX) is an epitope mimetic inhibitor of CXCR4. Accordingly, we hypothesized and evaluated if BLX would enhance docetaxel-mediated antitumor activity in PCa bone metastases. Methods: PC-3 luciferase-labeled cells were injected into the tibia of mice to model bone metastases. Four treatment groups were created: vehicle (1% DMSO in DPBS), DOC (5mg/kg), BLX (20mg/kg), and Combo (receiving both DOC and BLX). Mice were injected twice daily subcutaneously with either vehicle or BAL starting on day 1 and weekly intraperitoneally with DOC starting on day one. Tumor burden was measured weekly via bioluminescent imaging. At end of study (29 days), radiographs were taken of the tibiae and blood was collected. Serum levels of TRAcP, IL2, and IFN? levels were measured using ELISA. Harvested tibiae were decalcified and stained for Ki67, cleaved caspase-3, and CD34 positive cells or microvessels were quantified. Statistical analysis was performed using ANOVA or Student’s t-test. Results: Tumor burden was lower in the Combo group compared to the individual treatment groups. The Combo group had fewer mice with large bone lesions and the lowest TRAcP levels compared to the other groups. No significant difference in Ki67 staining was found among the groups; whereas, cleaved caspase-3 staining was lowest in the Combo group and highest in the BLX. The DOC and Combo groups had more CD34+ microvessels than the control and BLX groups. There was no difference between the treatment groups for IL-2, but the Combo group had increased levels of IFN? compared to other groups. Conclusions: Our data demonstrate that that a combination of BLX and DOC has greater anti-tumor activity in a model of PCa bone metastases than either drug alone. The anti-tumor activity was also associated with decreased tumor-induced bone lesions. The mechanism for the anti-tumor activity is not completely elucidated at this time but does not appear to be through impacting proliferation. The observation that IFN? was increased in the Combo group compared to other groups suggests that immune cell activation contributes to the anti-tumor effect. These data support further evaluation of this combination in metastatic PCa. SOURCE OF Funding: This work was supported by Spexis Ltd.
