Validation of a prediction model for estimating serum concentrations of chemicals which are equivalent to toxic concentrations in vitro.

The objective of the present study was to evaluate the validity of a recently developed extrapolation model for the prediction of concentrations of chemicals in serum which are equivalent to in vitro effective nominal concentrations. Necessary input data are in vitro toxic concentrations and distribution relevant system and substance specific parameters, e.g. lipid volume fractions and albumin concentrations, octanol/water partition coefficients and specific binding to albumin. It was investigated whether the influence of human and bovine serum, respectively, on nominal cytotoxic potencies (EC(50)-values) of selected chemicals in vitro can be properly predicted using this algorithm. Cytotoxicity was determined as growth inhibition of proliferating Balb/c 3T3 cells after exposure for 72 h. Concentration-effect relationships were measured in the presence of 2% foetal bovine serum (FBS) and, additionally, 18% FBS or human serum (HS), or 1% (w/v) bovine (BSA) or human (HSA) albumin, respectively. Addition of HSA and BSA increased the EC(50)-values of the different chemicals by factors of 2.1 - 22 and 1.7 - 29, respectively. From these measurements values for the specific binding of the test compounds to BSA and HSA were derived. Addition of 18% HS increased the EC(50)-values by factors between 4.2 and 52, while addition of 18% FBS resulted only in 1.5 - 10.4-fold increases. A comparison of experimentally determined and calculated EC(50)-values revealed that the differing influence of human and bovine serum was quite well predicted by the extrapolation model. Deviations did not exceed the factor 3 and were in most cases lower than 2. It is concluded that the extrapolation model is quite well suited to predict equivalent concentrations in serum from in vitro effective concentrations.