A targeted isotope dilution mass spectrometry assay for osteopontin quantification in plasma of metastatic breast cancer patients.

Osteopontin (OPN) is a secreted glycophosphoprotein that derives its name from its high abundance in bone and secretion by osteoblasts. It is also secreted by a number of immune cells and, therefore, is present in human plasma at nanogram per millilitre levels where it affects cell adhesion and motility. OPN is involved in several normal physiological processes; however, OPN dyregulation leads to overexpression by tumor cells leading to immune evasion and increased metastasis.