Continuously monitoring cell cultures is essential for both controlling critical parameters and improving understanding of key processes. An ideal technique in this context is surface plasmon resonance (SPR) spectroscopy, which essentially exploits changes in the angle of incident light that occur when molecules bind to a surface. It provides the ability to monitor real-time changes in small concentrations of various molecules, with no need for additional labels or sample preparation. Here we present an SPR-based immunoassay for monitoring concentrations of human serum albumin (HSA), and compare its sensitivity when used in conjunction with a Biacore platform and the cheaper, smaller liSPR system. In conjunction with either system, the immunoassay can detect HSA (a hepatocyte viability marker) at concentrations typically present in three-dimensional hepatocyte cultures mimicking the liver used to evaluate effects of drug candidates before exposure to humans or animals. Furthermore, in conjunction with the liSPR system, it is sufficiently sensitive to measure the much lower HSA levels present in skin–hepatocyte co-cultures.

We used the interaction between human serum albumin (HSA) and a high-affinity antibody to evaluate binding affinity measurements by the bench-top ^liSPR system (capitalis technology GmbH). HSA was immobilized directly onto a carboxylated sensor layer, and the mechanism of interaction between the antibody and HSA was investigated. The bivalence and heterogeneity of the antibody caused a complex binding mechanism. Three different interaction models (1:1 binding, heterogeneous analyte, bivalent analyte) were compared, and the bivalent analyte model best fit the curves obtained from the assay. This model describes the interaction of a bivalent analyte with one or two ligands (A + L ↔ LA + L ↔ LLA). The apparent binding affinity for this model measured 37 pM for the first reaction step, and 20 pM for the second step.