Two SPRi biosensors for the determination of cathepsin S in blood plasma

Cathepsin S is an emerging marker for ovarian cancer. Two 'analytically specific' SPRi biosensors for the determination of Cath S have been developed. The reception part of one of the biosensors consists of the rat monoclonal antibody specific for cathepsin S attached to the gold surface via covalent bonds with cysteamine linker, while the second biosensor consists of the inhibitor LY3000328 attached via hydrophobic interaction with the 1-octadecanothiol linker. Under optimized conditions, in terms of pH and receptor concentration, both biosensors have linear response ranges between LOQ (0.14 ng mL^-1) and 2.5 ng mL^-1, which is suitable for the determination of Cath S in blood plasma samples of ovarian cancer patients and healthy individuals, after corresponding dilution with 0.15 M PBS buffer. Precision and recoveries are quite acceptable: below 7% and 98-101% respectively for the biosensor with antibody, and below 12% and 101-103% for the biosensor with inhibitor. The biosensors were validated by the determination of Cath S in series of plasma from ovarian cancer patients and healthy volunteers using both biosensors and ELISA, giving Pearson coefficients close to 1. Plasma Cath S concentration can be used as an ovarian cancer marker, in view of the highly elevated concentrations detected.