Anti-osteopontin antibodies replicate the tumor suppression phenotype in thrombin cleavage-resistant osteopontin knock-in mice.

Background: The multifunctional proinflammatory matricellular protein, osteopontin (OPN), is a prognostic marker in multiple human cancers. OPN is cleaved by thrombin to reveal a cryptic binding site for α₄β₁ and α₉β₁ integrins at the C-terminus of the N-terminal cleaved fragment (OPN-R). Preventing thrombin cleavage of full-length OPN (OPN-FL) improves outcomes in tumor models.

Objective: Test if anti-OPN antibodies phenocopy the anti-tumor effects of inhibiting thrombin cleavage of OPN.

Methods: A monoclonal antibody that binds to OPN-FL at the thrombin cleavage site (ie, mA6) and one that binds to the C-terminus of OPN-R (ie, mC6R) were tested in both the B16 mouse melanoma subcutaneous implant and metastasis models.

Results: Purified mA6 bound to OPN-FL and mC6R bound to OPN-R with high affinity for their respective target antigens, whether mouse or human OPN. Thrombin cleavage of both mouse and human OPN-FL was inhibited by mA6, but mA6 did not affect coagulation or platelet aggregation assays. The clearance half-life of both antibodies was >7 days in mice. Mice treated with either antibody had reduced B16 melanoma growth in the subcutaneous model, resulting in fewer pulmonary metastases than control-treated wild-type mice. Antibody treatment replicated the tumor suppression phenotype of the thrombin resistant OPN mouse that expresses thrombin-resistant OPN. Growth of CT26, a colon carcinoma cell line, was reduced by treatment with either antibody in BALB/c mice.

Conclusion: Treatment with either antibody reduced B16 melanoma tumor growth in both models and also reduced CT26 colon carcinoma growth. mC6R had similar effects to mA6, suggesting that the active OPN fragment generated by thrombin cleavage of OPN-FL is OPN-R.