Protein binding capacity in vitro changes metabolism of substrates and influences the predictability of metabolic pathways in vivo

Abstract

Large numbers of lipophilic molecules are attached to fractions of serum protein, e.g. albumin, in vivo. Cell culture medium of most in vitro hepatocyte models for the prediction of metabolism does not contain albumin. Consequently, in vitro availability and metabolism of substrates could differ significantly from the in vivo situation. The influence of albumin on the in vitro metabolism was tested on a new lipophilic compound. Methods: Primary human and rat hepatocytes were cultured in a collagen sandwich configuration and incubated with 14C-labeled compound X127 that is known to exhibit a high propensity to bind to plastic surfaces. Groups contained either 1% (w/v) BSA or none. Substrates as well as metabolism products were determined with radio-HPLC and radioactivity levels in the medium were recorded. Results: Quantitative differences were seen in the distribution of the compound in BSA and non BSA containing groups, thus indicating a substantial binding of the compound to polystyrol surfaces of cell culture dishes. Metabolic radio-HPLC profiles showed different patterns after 24 h of incubation between the two species as well as between the BSA- and non-BSA groups within the species. Conclusions: With addition of albumin the adherance of lipophilic substrates and metabolites to cell culture dish surfaces can be neutralized and in vitro systems can more closely mimic the in vivo situation.