Critical Membrane Concentration and Mass-Balance Model to Identify Baseline Cytotoxicity of Hydrophobic and Ionizable Organic Chemicals in Mammalian Cell Lines

All chemicals can
interfere with cellular membranes and this leads
to baseline toxicity, which is the minimal toxicity any chemical elicits.
The critical membrane burden is constant for all chemicals; that is,
the dosing concentrations to trigger baseline toxicity decrease with
increasing hydrophobicity of the chemicals. Quantitative structure–activity
relationships, based on hydrophobicity of chemicals, have been established
to predict nominal concentrations causing baseline toxicity in human
and mammalian cell lines. However, their applicability is limited
to hydrophilic neutral compounds. To develop a prediction model that
includes more hydrophobic and charged organic chemicals, a mass balance
model was applied for mammalian cells (AREc32, AhR-CALUX, PPARγ-BLA,
and SH-SY5Y) considering different bioassay conditions. The critical
membrane burden for baseline toxicity was converted into nominal concentration
causing 10% cytotoxicity by baseline toxicity (IC10,baseline) using a mass balance model whose main chemical input parameter
was the liposome-water partition constants (Klip/w) for neutral chemicals or the speciation-corrected Dlip/w(pH 7.4) for ionizable chemicals plus the
bioassay-specific protein, lipid, and water contents of cells and
media. In these bioassay-specific models, log­(1/IC10,baseline) increased with increasing hydrophobicity, and the relationship
started to level off at log Dlip/w around
2. The bioassay-specific models were applied to 392 chemicals covering
a broad range of hydrophobicity and speciation. Comparing the predicted
IC10,baseline and experimental cytotoxicity IC10, known baseline toxicants and many additional chemicals were identified
as baseline toxicants, while the others were classified based on specificity
of their modes of action in the four cell lines, confirming excess
toxicity of some fungicides, antibiotics, and uncouplers. Given the
similarity of the bioassay-specific models, we propose a generalized
baseline-model for adherent human cell lines: log­[1/IC10,baseline (M)] = 1.23 + 4.97 × (1 – e–0.236 log Dlip/w). The derived models for baseline
toxicity may serve for specificity analysis in reporter gene and neurotoxicity
assays as well as for planning the dosing for cell-based assays.