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

Abstract

Osteopontin (OPN) is a secreted glycophosphoprotein that derives its name from its high abundance in 33 bone and secretion by osteoblasts. It is also secreted by a number of immune cells and, therefore, is 34 present in human plasma at nanogram per millilitre levels where it affects cell adhesion and motility. 35 These properties implicate OPN as a promotor of tumor progression. Plasma OPN is primarily measured 36 by enzyme-linked immunosorbent assay (ELISA). However, due to the complexity of the various OPN 37 isoforms, conflicting results have been obtained on the use of OPN as a biomarker even in the same 38 disease condition. These discrepant results may result from the difficulty in comparing ELISA results 39 obtained with different antibodies that target unique OPN epitopes.